A Practical Overview of Level Continuous Measurement Technologies

Introduction

There are multiple technologies available on the market to measure level. Each and every technology works, when applied appropriately. This paper will discuss the strengths and weaknesses of RF Admittance, Displacers / Floats, Capacitance, Ultrasonic, Radar, Nuclear, Differential Pressure, and Bubbler level measurement technologies. Level measurement for liquids, granulars, slurries and interfaces can be accomplished with several different level technologies. There are over 20 different technologies being offered on the market today. But how do you know which technology to specify for your application? Obviously all technologies have (or had) their place, otherwise companies would not be in business today offering them. The truth is that every technology for level measurement works, when used in the specific circumstances where they have a high probability of success. This paper will discuss the most popular level measurement technologies used in industry, outline their advantages and disadvantages, and tell you where they are best applied. The Application Box Rating Included with the description of each technology is an application box rating for the four categories of level measurement (Liquids, Granulars, Slurries and Interfaces). This gives you an at-a-glance overview of where the technology in question is best used .

Liquids
A green box indicates that this technology is suitable for this category of level measurement.

Granular
A red box indicates that this technology is not practical for this category of level measurement. This does not mean that people have not used this technology for measurement, but just that it is not practical. This could mean that there is excessive maintenance, excessive error, unreliable outputs, or it just simply doesn’t work in this application based on the characteristics of the level measurement.

Solids
A yellow box indicates that this technology can sometimes be applied to this application. But be careful. There are usually special considerations or additional maintenance needed.

Example: The Application Box Rating below indicates that the technology in question works on liquids. It needs special considerations or additional maintenance when used on slurries, and it is not practical for granulars or interfaces.

Liquids Granulars Slurries Interfaces
EXAMPLE ONLY

Differential Pressure

Liquids Granulars Slurries Interfaces

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Theory

Perhaps the most frequently used device for the measurement of level is a differential pressure transmitter. Using DP for level is really an inferential measurement. A DP is used to transmit the head pressure that the diaphragm senses due to the height of the material in the vessel multiplied by a density variable.

Advantages

The primary benefit of DP’s is that it can be externally installed or retrofitted to an existing vessel. It can also be isolated safely from the process using block valves for maintenance and testing. There are certain measurements such as  total level in separator vessels that due to wide variations in material composition of the upper phase DP is the only viable if not ideal option.

Disadvantages

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Typical Differential Pressure Configuration

D/P transmitters are subject to errors due to changes in liquid density. Density variations are caused by temperature changes or change of product. These variations must always be compensated for if accurate measurements are to be made. DP’s are mainly intended for clean liquids and require two vessel penetrations. One is near the bottom of the vessel where leak paths are the cause of the majority of problems. D/P’s should not be used with liquids that solidify as their concentrations increase. An example is paper pulp stock.

Practical Notes

Fluid density must be stable if readings are to be accurate. If liquid density is subject to change a second d/p transmitter is required to measure density and then used to

compensate for any changes. To accommodate the measurement of light slurries, differential pressure transmitters are available with extended diaphragms that fit flush to the side of the vessel. However, if the d/p transmitter diaphragm becomes coated, it may require recalibration, which can be impractical and will add to the "cost of ownership". Frequently, the measuring device is only one consideration in the total installation of the job. Although a D/P transmitter is often less expensive than other types of level sensors, there is usually considerable additional hardware and labour required to make a practical installation. The implementation of a stable, low-pressure leg and 3 / 5 valve manifolds will add considerable cost to the installation.

Displacers / Float

Liquids Granulars Slurries Interfaces

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Theory

Although float and displacer devices are often similar in appearance they have differing theories of operation. Float devices operate on the buoyancy Principle, as liquid level changes a (predominately) sealed container will, providing its density is lower than that of the liquid, move correspondingly. Displacers work on the Archimedes Principle, when a body is immersed in a fluid it loses weight equal to that of the fluid displaced. By detection of the apparent weight of the immersed displacer, a level measurement can be inferred. When the cross sectional area of the displacer and the density of the liquid is constant, then a unit change in level will result in a reproducible unit change in displacer weight.

Advantages

Both floats and displacers work well with clean liquids and are accurate and adaptable to wide variations in fluid densities. Once commissioned, however, the process fluid measured must maintain its density if repeatability is required, this is particularly true of displacers. Float Switches are available with a glandless design and are capable of fail safe operation in extreme process conditions, unlike displacers, which if the torque tube fails can provide a leak path.

Disadvantages

Displacers are affected by changes in product density since the displacement of the body (its weight loss) is equal to the weight of the fluid displaced. If the specific gravity changes, then the weight of the displaced material changes, thus changing the calibration. This is especially problematic in interface measurements, where both liquids increase or decrease density, while the signal is proportional to the density difference. Because the displacer is emersed in the process fluid it will be vulnerable to

particulate deposition. This will change the displacer mass and the effective displacement causing a calibration shift. Glandless float systems provide more reliable readings under difficult conditions, however in situations of a coating media the moving parts may seize and the unit will no longer function.

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Float Level Switch & chamber on a Cryogenic

Ethylene Application at Exxon, Mossmorran.

Practical Notes

Displacers and floats should only be used for relatively non-viscous, clean fluids and provide optimal performance in switch applications and over for short spans. Spans of up to 12m are possible, but they become prohibitively expensive. Cost of installation for displacers is high and many refineries are now replacing them due to the inaccuracies experienced under process density changes especially on interface duties. High quality float switches still provide reliable and repeatable performance. Even with todays array of level technologies, if a 100% process seal is required under fail conditions for a Cryogenic application the only technique available, other than nucleonic’s, is a magnetically coupled float switch.

Bubblers

Liquids Granulars Slurries Interfaces

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Typical Bubbler Configuration

Theory

This simple level measurement has a dip tube installed with the open end close to the bottom of the process vessel. A flow of gas, usually air or nitrogen passes through the tube and the resultant air pressure in the tube corresponds to the hydraulic head of the liquid in the vessel. The air pressure in the bubbler tube varies proportionally with the change in head pressure.

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Advantages

Simplicity of design and low initial purchase cost are frequently given as advantages of bubblers, but this is somewhat misleading. The system consists of a pipe, an air supply, a pressure transmitter and a differential pressure regulator. The regulator produces the constant gas flow required to prevent calibration changes.

Disadvantages

Calibration is directly affected by changes in product density. It is frequently also necessary to periodically clean this device. The tip of the pipe can collect material from the process, solidify, and plug the hole. Bubblers are not suitable for use in non-vented vessels.

Practical Notes

Instrument air lines should be trace heated if there is a frost risk. Calibration of a bubbler system should be at maximum temperature to avoid overfills. Accuracy depends on a stable air supply and is limited by the regulator, which may be + 10% of full scale. In applications where the purge air is exposed to a hazardous substance, additional steps must be taken to contain any possible contamination.

Ultrasonic / Sonic

Liquids Granulars Slurries Interfaces

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Theory

Ultrasonic transmitters work on the principle of sending a sound wave from a peizo electric transducer to the contents of the vessel. The device measures the length of time it takes for the reflected sound wave to return to the transducer. A successful measurement depends on reflection from the process material in a straight line back to the transducer.

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Ultrasonic Level Measurement for bulk storage vessels

Advantages

The main advantages of ultrasonic level instrumentation are that the transducer does not come into contact with the process material, they have no moving parts and a single top of vessel entry makes leaks less probable than fully wetted techniques.

Disadvantages

There are various influences that affect the return signal. Things such as powders, heavy vapors, surface turbulence, foam and even ambient noise can affect the returning signal. Temperature can also be a limiting factor in many process applications. Ultrasonic devices will not operate on vacuum or high pressure applications.

Practical Notes

Successful measurement depends on the transmitter being mounted in the correct position so that the internal structure of the vessel will not interfere with the signal path. To ignore obstructions in the vessel, tank mapping has been developed. Tank mapping lets the operator take a "sonic snapshot" of an empty vessel. The transducer transmits a sound burst and the echo is recorded as a signature of the tank. Any obstructions in the vessel will send an echo and create a profile. Later on, this signature or profile is locked into the ultrasonic unit’s memory so it will not respond to echoes created by these obstructions.

Radar

Liquids Granulars Slurries Interfaces

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Theory

The two technologies on the market are frequency modulated continuous wave (FMCW) or pulsed wave time of flight. Pulsed Wave systems emit a microwave burst towards the process material, this burst is reflected by the surface of the material and detected by the same sensor which now acts as a receiver. Level is inferred from the time of flight (transmission to reception) of the microwave signal. Microwave "echoes" are evaluated by sampling and building up a historical profile of the echoes. FMCW systems, however, continuously emit a swept frequency signal and distance is inferred from the difference in frequency between the transmit and receive signals at any point in time. FMCW is therefore the only method that is suitable for the high accuracy’s demanded for tank gauging.

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FMCW Radar Gauge complete with Data Acuisition Unit

Advantages

This non-contact technology produces highly accurate measurements in storage tanks and some process vessels. Radar is an excellent, but fairly expensive technology (£1k to £5k per measurement) for continuous level measurements. Several manufacturers have reduced the cost/price of the technology with various process radar offerings. These systems do not have the accuracy (and associated cost) of radar used for inventory control. In conclusion, radar can be highly accurate, is immune to most vapours / physical characteristics of the measured media, other than, in some cases, dielectric constant.

Disadvantages

It’s primary disadvantage is cost, which can be justified for tank gauging and inventory control. The pressure ratings on radar antenna are limited and these devices cannot measure interfaces. Pulse radar has difficulty making accurate measurement when the media is in close proximity to the antenna because the time difference between send and return signals is too fast to measure accurately.

Practical Notes

In the case of hydrocarbons, an accurate water bottoms measurement must be made for precise inventory control. Typically, another technology, such as RF Admittance is used to make the interface measurement between water and hydrocarbons. Some installations, such as floating roof tanks, require the installation of a stillpipe. Inconsistencies on the internal surface of the stillpipe can cause erroneous echoes, these can have an adverse effect on the accuracy of some vendor's equipment.

Nuclear

Liquids Granulars Slurries Interfaces

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Theory

Nucleonic level controls are used for point and continuous measurements, typically where most other technologies are unsuccessful. The radioisotopes used for level measurement emit energy at a fairly constant rate but in random bursts. Gamma radiation, the source generally used for nucleonic level gauging is similar to microwaves or even light (these are also electromagnetic radiation, but of lower energy and longer wavelength). The short wavelength and higher energy of gamma radiation penetrates the vessel wall and process media. A detector on the other side of the vessel measures the radiation field strength and infers the level in the vessel. Different radioisotopes are used, based on the penetrating power needed to "see" the process within the vessel. With single point gauges the radiation provides a simple on/off switching function, whereas with continuous level measurement the percentage of transmission decreases as the level increases.

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Nucleonic Scintillator Configuration to give linear output of a dished vessel at Cynamid

Advantages

As no penetration of the vessel is needed there are a number of situations that cause nucleonic transmitters to be considered over other technologies. These applications generally involve high temperatures / pressures or where toxic or corrosive materials are within the vessel. Placing the source and / or detector in wells within the vessel can reduce source sizes. An extension of this is to use a moving source within the vessel; this facilitates the unique ability to combine density profiling with accurate tracking of a moving interface.

Disadvantages

It would appear that nucleonic gauges provide a truly universal "fit and forget" level measurement technology. Although when the "cost of ownership" is calculated nuclear level measurement is often more expensive than conventional systems. Hidden costs include initial licensing and periodic surveying. These services are usually provided by external authorities or by the equipment supplier, assuming they have appropriately qualified staff. If no longer required, the nucleonic gauge must be disposed of through appropriately licensed, external organizations, which again can be a costly exercise.

Practical Notes

From a psychological standpoint, the radiation symbol found on these controls is frequently the cause of unfounded concern with uninitiated plant personnel. Plant Management is usually required to ensure that appropriate education is given to any staff likely to be involved with this measurement technology. Source size regulations can often be met in difficult applications by placing the source and / or detector in wells within the vessel if necessary.

Capacitance

Liquids Granulars Slurries Interfaces

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Theory

As the level rise’s and material begins to cover the sensing element the capacitance within the circuit between the probe and the media (conductive applications) or the probe and the vessel wall (insulating applications) increases. This causes a bridge misbalance, the signal is demodulated (rectified), amplified and the output is increased.

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Capacitance Level System in on a Test Rig

Advantages

Capacitance techniques are capable of operation at extremes of temperature and pressure. They work well for materials that won’t leave a coating. Usually only a single tank penetration is required.

Disadvantages

Capacitance systems are intrusive. Have problems with varying dielectric materials and those media’s, that coat the sensing element. Thus users are normally limited to water-like media. Even acids and caustics that don’t appear to coat the sensing element are so conductive that the thin film they leave can cause serious errors in measurement.

Practical Notes

Most users’ realise the limitations of Capacitance level measurement, such as the large errors caused by coatings. This has led to a decrease in the number of these systems in operation. Other technologies such as FMCW radar and in particular RF Admittance have now gained acceptance due to high levels of reliability and accuracy.

RF Admittance

Liquids Granulars Slurries Interfaces

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Theory

The theory of operation for an RF Admittance level transmitter is similar to that of Capacitance transmitters, but with two important circuit additions. The oscillator buffer and chopper drive circuits permit separate measurement of resistance and capacitance. Since the resistance and capacitance of any coating are of equal magnitude (by physical laws), the error generated by a coating can be measured and subtracted from the total output. The result is an accurate measurement regardless of the amount of coating on the probe.

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RF Admittance level monitoring of HF Acid & Caustic Soda Levels at BP Grangemouth

Advantages

RF Admittance is next generation capacitance as such it is by far the most versatile technology for continuous level measurement. It can handle a wide range of process conditions anywhere from cryogenics to approximately 850 o C and from vacuum to 10,000 psi pressure. Aside from the electronic circuit technology, sensing element design is very important to handle these process conditions. There are no moving parts to wear, plug, or jam. As with capacitance systems there is only a single tank penetration, usually at the top of the tank, above the actual process level.

Disadvantages

RF admittance is intrusive. Insulating granular measurements require special considerations, such as the moisture range and location of the sensing element to minimize errors caused by probe movement.

Practical Notes

Admittance technology and nucleonic measurement provide the only practical methods for level measurement in coating applications. For insulating materials with changing dielectric constants, the measurement can only be made reliably if the material being measured is homogeneous. A reference sensor is added to monitor the dielectric constant and then compensate the calibration based on this information. Smart RF transmitters are available providing superior levels of stability and accuracy as well as remote communication. Knowledge of the approximate electrical character of the process material is key to optimum system selection and performance.

Summary

The many level technologies available in the market place today all work. If they did not work, companies would not be able to stay in business manufacturing them, and other companies would not be buying them. The important thing to remember is that all technologies work when applied properly. Special consideration must always be given to the principle of operation, and where the limitations of each technology lie. The correct selection of a technology and then the subsequent correct application of the technology will make the level measurement a successful one. Below is a summary list of the general application of the technologies discussed in this paper.

  Liquids Granulars Slurries Interfaces
RF Admittance        
Ultrasonic        
Radar        
Differential Pressure        
Displacers        
Bubblers        
Nuclear        
Code Key O.K. Use Caution Not practical

To enable us to provide you with the most accurate and relevant information please contact us with any feedback, both positive and negative, on the content and the method in which you receive forum bulletins. To encourage problem analysis we would particularly like to hear from forum members who have current level measurement problems and those members who have solved difficult applications.

Absolute Pressure

Absolute pressure is defined as the pressure measured with a pure vacuum as its reference point. That is zero gauge pressure (see gauge pressure defined below) will be approximately 1bar/14.7psi absolute.

Whilst not used often engineers and technicians must be aware that pressures and transmitters may sometimes be defined/specified in these units. It is a real trap that one can fall into assuming that the pressure is "gauge" or absolute when in fact it is the reverse. A barometer is a typical absolute pressure gauge which is used for measurement of local atmospheric pressure.

Differential Pressure

Differential pressure is defined as the difference between two pressures.

Measurement commonly used throughout industry, especially in conjunction with flow metering devices such as Orifice Plates, Venturies, Pitot tubes etc.

Equal Column Liquid Manometer

This is one of the oldest methods of measuring differential pressure. In its most basic configuration it consists of two identical vertical tubes partly filled with a liquid. These tubes are connected together at the bottom. Pressure is measured by determining the change in height between the two tubes and is scaled according to the density of the liquid. Thus different fluids will give a change in the pressure range.

Typical fluids used are water and mercury.

The method is very accurate however is limited to lower ranges of pressure. It is rarely used today, however it used to be used quite extensively as a calibration standard for low pressure instruments and control valves. This role has now been taken over by highly accurate calibration tools. The following diagram depicts an equal column liquid manometer. It shows a glass tube bent into a "U" shape. This tube is partly filled with the fluid and a pressure scale is located between both tubes.

The Principle of an equal column "U" tube manometer

The liquid column produces a pressure which is proportional to the height of the liquid in the leg. The pressure developed is equal to hr g (h=height/r =density/g=the acceleration due to gravity. The instrument measures the difference in pressures exerted on the liquid in each of the columns.

With higher pressure p1 being applied to column A and lower pressure p2 to B the liquid in column A is forced down until the pressure p1 is equalised by the pressure in column B. thus the difference in pressures is determined by the height of the liquid h. The scale has been calibrated as a result of the selection of the fluid used, as different fluids at different densities will give a different change in height for the same pressure applied.

That is

p1 - p2 = h

r g

Units are important, if for instance:

  • p1 and p2 are in pascal r must be in gram/cm3 and h in meters

Flow Instrumentation and Measurement

Ultrasonic Flow Metering

A full page of Ultrasonic Flowmetering technical links - Thanks to Zedflo Australia.

V-Cone and Wet Gas Flow Metering

This is a swag of information on this innovative technology - Thanks to McCrometer.

Subsea Flow Measurement - Marcus Davis - Flow measurement in subsea production systems, modules and templates is a challenge for flow meters. The complexity of subsea production systems ranges from simple satellite wells with one line to complex multiple well sites with a network of lines. Several pipe lines join together below the surface and will eventually extend to a fixed platform, Floating Production Storage and Offloading Vessel (FPSO), or perhaps a pipeline running to a land-based operation. Flow measurement is required in all phases of these operations, especially at well heads and where lines merge - Thanks to McCrometer.

The following general links are - Thanks to Coleparmer.

General Flow Links

LegendsofFlow.com - The purpose of this website is to provide information and knowledge about those people who have made substantial contributions to the field of flow measurement. There are three levels or tiers of people on this site including Pioneers of Instrumentation - Historical figures who have developed theories or made important contributions to flow measurement. Legends of Flow - Living or contemporary people who have developed theories, founded companies, or otherwise made important contributions to flow measurement and Movers and Shakers of Flow - Important people and decision makers in flow who have not yet achieved legendary status, but who nonetheless play an important role today in the field of flow measurement and instrumentation. One can also find plenty of technical information on this site.

An Hour with Doctor Flowmeter - Walt Boyes - How do you pick a flow meter? Picking a flow meter takes both knowledge of the kinds of meters available and the kind of empirically derived knowledge that comes from experience. If you are like most people, you start by thinking about kinds of meters, their features and their hardware and software. You might start to build the meter from the application requirements. There are lots of flow devices, for both liquids and gases, and closed and open channels. What we’ll do here is to look at some of the most common, and try to home in on some simple ways to select and use them. Note that there are at least 10 different types…and they don’t all work on all applications - from the excellent Spitzer and Boyes website.

Fundamentals of Gas Measurement - Pat Donnelly - This is a super paper on the basics of Gas Flow Measurement. It covers the history, measuring volume, types of flowmeters and temperature correction - from CEESI.

The American School of Gas Measurement Technology - This is a GREAT website with loads of flow instrumentation technical links.

Flowmeter Selection Strategies for Liquid Measurements Part 1 - How to Choose the Right Technology for Liquid Measurements - John Frederick - With so many different flow measurement technologies available, optimal flowmeter selection can be difficult and confusing. In an effort to provide some valuable strategies to simplify the flowmeter selection process, here we provide the first installment of a two-part article addressing flow measurement. The article describes common types of liquid flowmeters and provides guidelines for their selection and usage. It also discusses the influence of fluid properties and installation effects on meter performance, and examines methods and equipment for accurate calibration - from www.flowcontrolnetwork.com.

Flowmeter Selection Strategies Part 2 - How to Choose the Right Technology for Gas Measurements - John Frederick - Covers the topics typically covered in a comprehensive gas flow measurement course. The following describes common types of gas flowmeters and provides guidelines for their selection and usage. It also discusses the influence of basic gas properties on meter performance and examines methods and equipment for accurate calibration - from www.flowcontrolnetwork.com.

Flowmeter Selection Strategies for Gas Measurement Applications - In this second installment of a two-part article, we address the topics typically covered in a comprehensive gas flow measurement course. The following describes common types of gas flowmeters and provides guidelines for their selection and usage. It also discusses the influence of basic gas properties on meter performance and examines methods and equipment for accurate calibration - from www.flowcontrolnetwork.com.

Flow & Measurement Articles - This super website contains an absolute "swag" of published articles on flow and measurement written by Dr. Jesse Yoder. These articles have been published in various trade journals beginning in 1995.

Focus on Liquid Flow Measurement - Controlling the flow rate of liquids is a key control mechanism for any chemical plant. There are many different types of devices available to measure flow. This article from cheresources covers some of the basics.

Pioneers of Flow Measurement - Founding the Technologies of Today - Jesse Yoder, Ph.D - It is easy to forget in today’s fast-paced world the importance of the many thinkers and pioneers who have made modern-day technologies possible. This is as true in the area of flow measurement as it is in other technology segments. The following looks at some of the pioneers of flow measurement, who formulated many of the principles that underlie the sophisticated flow instruments of today - An excellent article from flowresearch.com.

The Effect of Ambient Conditions on Flow Measurement - David W. Spitzer, P.E - The entire flow measurement system should be operated within its constraints in order to provide accurate flow measurements. This means that each component of the flow measurement system should be considered to ensure that it is operated properly - from David W Spitzer and Flow control Network.

Top Tips for flowmeter selection - The huge array of flow technology options on offer can make selecting the correct flowmeter for an application a bewildering task. A broad range of factors can influence flowmeter selection, of which cost is just one. Dr Bryan Franklin, Flow Products Manager, ABB Limited recommends a list of top tips for selecting the best all round flow system for an application.

Let it flow - Measuring the flow of liquids in industrial plants is critical - In some operations, the ability to conduct accurate flow measurements is so important it can make the difference between making a profit and taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious or disastrous results. Includes a flowmeter selection guide - Thanks to ISA and InTech.

The Water Measurement Manual - US Bureau of Reclamation - A terrific 14 chapter US Government Water Resource Research Laboratory reference site with absolutely everything you need to know about water flow measurement.

Calibration and Standards in Flow Measurement - Richard Paton - Calibration is defined as follows: ‘The set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system and the corresponding values realized by standards.’ It is important to recognize at the outset the definition of a ‘calibration’ and to note that the ‘comparison’ applies only to the conditions at the time of the calibration. The purpose of calibration is to increase the confidence in the reading obtained from the flowmeter in service - from IDC.

Flow and Level Calibration Information Notes (INX Inc)

Flowmeters, Liquid - Omega.com, a very nice tutorial on flow meters.

Flow Measurement information Notes (INX Inc)

Flowmeters for water and wastewater applications - (Controlotron)

Pipe Flow Rate Calculation Calculate Pipe Pressure Drop - This excellent Pipe flow calculation web site offers fluid flow calculators for pressure drop calculation, pipe diameter calculation, control valves sizing, air flow calculation, Venturi tube and orifice flow, Reynolds number calculation, pressure drop in natural gas line, lpg pipe sizing calculator, Prandtl probe, thermal energy calculator and more... from Pipeflow Calculations.

The following technical articles are from the excellent publication Flow Control Network:

Straight-Run Requirements for Flow Accuracy - David W. Spitzer - Have you ever seen the inside of a carbon steel pipe used for sodium chloride brine service? The inside pipe wall has ridges, mountains, boils and other strange shapes. How can you accurately measure flow in such a pipe? The upstream and downstream piping might be straight in your installation. However, the inside of the pipe can be far from straight/smooth. In this application, the strange shapes on the inside of the pipe can affect the velocity profile and, in turn, the flow measurement - from www.flowcontrolnetwork.com.

Which of the following Flowmeters can Measure both Forward and Reverse Flow? - David W. Spitzer - A. Differential-Pressure B. Magnetic C. Turbine D. Ultrasonic - The answers from www.flowcontrolnetwork.com can be found in this article.

Flowmeter Turndown Overkill - Is 1,000-to-1 Really Better Than 100-to-1? - David W. Spitzer - It would be fair to say that the overwhelming majority of instrument suppliers make valid claims about the instruments they sell. For example, a supplier may claim that its flowmeters perform within a specified accuracy, such as 0.5 percent of flowrate. The supplier has presumably tested the flowmeter and found that its performance falls within this specification. The user can then decide whether this particular flowmeter meets the needs of the application - from www.flowcontrolnetwork.com.

Pressure & Temperature Tap Placement - David W Spitzer - Deciding Between Upstream and Downstream Location for a Flow Metering Application - from www.flowcontrolnetwork.com.

Fluid Density’s Effect on a DP Flow Control Loop - A short but useful article on how an orifice-plate flowmeter measurement in a flow control loop is affected by an increase in fluid density - from Spitzer and Boyes.

Flowmeter Piping Requirements How Much Straight Run Is Enough? - Greg Livelli - A very useful technical paper detailing straight run requirements for various types of flowmeters.

A Primer on Primary Elements - Understanding a Key Aspect of DP Flow Measurement - Jesse Yoder - A general description of some flowmeter types.

Calculating for Calibration & Conversion - Factor Inconsistency - David W. Spitzer - Accurate flow measurement often entails careful attention to detail. Errors can be introduced when calculations in different parts of the flow measurement system are performed using different standard conditions - from P.E and flowcontrolnetwork.com.

Coriolis Flow Meters

A page full of great technical information

Custody Transfer Flow Meters

Calculating and Optimizing Accuracy & Repeatability of Natural Gas Custody

Dall Tube Flow Meters

New Page Coming Soon

Elbow Flow Meters

New Page Coming Soon

Flare Flow Meters

Measuring Flare Gas within the European Union Emissions Trading Scheme - Simon Harwood and Jack Koeken, Sr - Prior to the European Union Emissions Trading Scheme, the measurement of flare gas on oil and gas production facilities in the North Sea was driven mainly by statutory regulations that required operators to simply report emissions to the Environment Agency. Consequently, there was never an economic incentive to install metering equipment. Importantly, depending on the Industry and size of the facility, the Trading Scheme stipulates different levels of accuracy for the instrumentation used to measure both fuel gas and flare gas. For the oil and gas industry, flow meters used to report emissions from flares fall within the Tier 3 accuracy level which, means they must have a degree of uncertainty (accuracy) better than ±7.5 percent of the measured value - from Fluid Components.

Flow Conditioning

Dealing with Unpredictable Flow Profiles - Eliminating Transitional Flow Effects via Flow Conditioning - Mike Bess & Don Lundberg - Flow conditioners are widely recognized and applied in order to correct flow profile distortions caused by upstream flow disturbances. These unpredictable flow profile variations can be neutralized by a well-designed flow conditioner. Such flow conditioners are capable of presenting a consistent and predictable outlet flow profile to the flowmeter, resulting in accurate and repeatable flow measurements. However, often overlooked in wide-turndown flow measurement applications with Reynolds number ranges that go below 4000, are the effects of three naturally occurring transitional flow profile phenomena. These transitional flows have dramatic differences in the flow velocity profile that disrupt flowmeter accuracy and repeatability. A well-designed flow conditioner can also efficiently neutralize transitional flow effects - from www.flowcontrolnetwork.com.

Flow Metering Mass Balance

Improve Material Balance by Using Proper Flowmeter Corrections - S Peramanu and J. C Wah - Process plants frequently encounter mass imbalances. These can be attributed to various factors, but often they lead back to inappropriate measuring devices, improper calibration, incorrect installation or incorrect interpretation of the measured flows. There are well-established guidelines available to ensure appropriate flowmeter selection based on the process conditions and control requirements - from Hydrocarbon Processing.

Flow Meters for Leak Detection

Non-Intrusive System Detects Leaks Using Mass Measurement - By Don Bloom, Vice President, Controlotron.

Leak Detection and prevention - Joseph Baumoel, Controlotron, USA, provides an overview of the ongoing problem of pipeline spills and evaluates the technologies available for the identification and control of pipeline leaks.

Pipeline Integrity - New Innovations in Leak Detection for Hydrocarbon Applications - Frank Fromm - In order to ensure the integrity of their pipelines and remain in compliance with new and tougher laws, a greater number of businesses are now making the conscientious decision to install advanced leak-detection systems. These systems are capable of detecting both the presence and location of a leak, allowing a pipeline operator to address the situation quickly and effectively - before the leak can harm property or people. A variety of leak-detection systems are currently available and corporations must thoroughly evaluate the benefits of each. A leak-detection system should demonstrate consistently high levels of sensitivity, accuracy, reliability, and robustness. It should be easy for a pipeline operator to learn, understand and use. Ideally, the system should also integrate software with instrumentation in order to enhance performance and simplify maintenance. After reviewing several recent U.S. and international laws pertaining to leak detection for pipelines, this article takes a detailed look at one innovative leak detection system and demonstrate how a Taiwanese petrochemical company put this system to good use - from www.flowcontrolnetwork.com.

Selecting and Installing a Flowmeter

New Page Coming Soon

Impact Flowmeter

New Page Coming Soon

Magnetic Flowmeters

5 Keys to Selecting a Mag Meter - When choosing an electromagnetic flow meter, it’s important to consider a selection process that will provide the best possible solution for your flow meter application. Electromagnetic flow meters are typically the meter of choice when considering cost, accuracy, and longevity. Here are some tips that can assist with ensuring that the electromagnetic flow meter is the right choice. This e-Book is from McCrometer.

The following articles are from Emerson Process Management:

Installation and Grounding of Magmeters in Typical and Electrolytic Process Applications - Proper installation and grounding of magnetic flowmeters is important for accurate, reliable measurement performance. Stray AC or DC currents through the fluid or instrument can produce noise signals that may in turn interfere with the relatively low flow signals generated in today's modern pulsed DC magmeter.

Magnetic Flowmeter Material Selection Guide - A general resource for the selection of materials for the magnetic flowmeter. One of the advantages of the electromagnetic flowmeter is that it is suitable for a wide variety of applications including severely corrosive chemicals and highly abrasive slurries. The key to this versatility is that there is a large choice of materials available for the electrodes and the flowtube liner.

Permeation of Teflon Flowtube Liners and Installation Techniques to Reduce the Rate of Permeation - Ways to Reduce Permeation including Liner Quality, Liner Thickness, and Temperature.

Magnetic Flowmeter Applications

Next Generation Mag Meter Solves Water Treatment Plants Tight Installation Dilemma - Michelle Pawlowicz - The flow meter selected is ideal for the needs of the Lake Huron WTP because it is economical for large line sizes, features a compact insertion design for ease of installation in compact spaces with limited access points and can be installed and maintained without shutting down flow - from McCrometer.

Flow Measurement in a Pump Station - Michelle Pawlowicz - Water losses within the distribution system can raise costs for both plants and customers. As a result, many municipal plants are being forced to run their plants and operations as efficiently as possible. Water supply issues, budget constraints and environmental concerns have raised the importance of understanding plant production and losses. Acquiring the ability to measure flow effectively at various points within the distribution system will provide data regarding the quantity of water pumped into a treatment facility, and the quantity of water pumped from the treatment facility throughout the distribution system. Measuring flow in each pump station will provide a reference point to these quantities of water flowing through the pump station. This will assist with maintenance planning, provide data references for potential loss analysis and accurately determine how much water is transported for billing and revenue processes - from McCrometer.

Mass Flowmeter

New Page Coming Soon

Metering

New Developments in Metering Technology

Multiphase Flow Metering

Exceptional Multiphase Metering technical information on this dedicated page.

Open Channel Flow Meters, Weirs and Flumes

Flow like an Egyptian - The Basics of Open Channel Flow - Walt Boyes - There are two basic ways to measure flow in an open channel or conduit. The first is to use one of the formulas like Manning’s Equation, which have been developed for open channel flow in ditches, rivers, and canals. You must calculate a wetted perimeter, and know the slope of the conduit, and guess at the roughness of the bottom of the conduit, or pipe. The second method is to measure the height of the flowing stream at a known distance behind (upstream) of a predictable hydraulic jump, such as a flume or weir - from the excellent Spitzer and Boyes website.

Orifice Plates

A page full of great technical papers/articles on Orifice Plates, associated standards and associated equipment.

Pitot Tubes and Annubars

Annubar® Flow Handbook - This excellent reference book provides all of the information necessary to accurately measure fluid flow using the Rosemount 485 Annubar primary element.

Positive Displacement Meters

New Page Coming Soon

Rotary-vane and Nutating disk meters

New Page Coming Soon

Ultrasonic Flow meters

A full page of Ultrasonic Flowmetering technical links - Thanks to Zedflo Australia.

Steam Flow Measurement and Meters

Behind the Scenes of Steam Flow Measurement - Jesse Yoder - Steam, or vaporized water, is an important part of our daily lives. It is easy enough to experience steam; simply boiling water will produce it. But steam has equally important uses as a source of power for ships, in paper production, and for cooling and heating buildings. Probably the most important use of steam is as a source of power in electricity production. Steam is difficult to measure accurately, mainly because of its sensitivity to changes in temperature and pressure. Steam is often classified into three types, which reflect the different pressure and temperature conditions that steam is subject to - from ISA and InTech.

Using Meters to Measure Steam Flow - Jesse Yoder - When people hear about fluid measurement, they usually think of checking either liquid or gas flow. This belief is natural, since about 90% of fluid measurements are for these two applications. In a worldwide survey of flowmeter users, 68% measured liquids, 22% gas, and only 10% steam. While 10% may not seem like a large percentage, it represents a significant number of measurements. Many companies measure steam for purposes of internal custody transfer and for utility applications in power generation and chemical plants - from www.flowresearch.com and Automation Research Corp.

Understanding Steam Flow Meters - Steam flow measurement is important in this context because steam is widely used as a source of power in the production of electricity. As the costs of energy rise, many companies are looking to increase efficiencies in their energy and power generation. As a result, more attention is being paid to steam flow measurement - Thanks to Chemical Professionals.

Target Meters

New Page Coming Soon

Thermal Flow Meters

The following links are from  Austral Powerflo Solutions and Magnetrol:

Are Your Gas Flow Rates Too Low to Use Conventional Measurement Technologies? - by Wayne Shannon.

Tracking Natural Gas with Flowmeters - Wayne Shannon - Thermal mass flowmeters provide advantages over other options for metering the consumption of natural gas by individual combustion units throughout the facility.

Thermal Dispersion Mass Flow Measurement Handbook - Accurate mass flow measurement of gas is difficult to obtain. Thermal mass flow technology is a method of gas flow measurement that does not require correction for changes in process temperature or pressure. Thermal mass flow technology also has a benefit of measurement at low velocities and greater turndown capabilities than those obtainable with other gas flow measurement devices.

A New Paradigm for Thermal Dispersion Mass Flowmeters - John G. Olin - The ASME Standard and Recent Technology Advances - Since the publication of the American Society of Mechanical Engineers (ASME) standard on thermal dispersion mass flowmeters in 2011, there have been major advancements in the technology. A review of that standard and a discussion of technology advancements provide the background to understanding ground breaking innovations in sensor design. That, along with a comprehensive algorithm facilitated by current hyper-fast microprocessors, has created a new paradigm for the measurement of the mass flowrate of gases by means of thermal dispersion technology - from Sierra Instruments.

A Standard for Users and Manufacturers of Thermal Dispersion Mass Flowmeters - Dr John G Olin - This 49 page standard establishes common terminology and gives guidelines for the quality, description, principle of operation, selection, installation and flow calibration of thermal dispersion flowmeters for the measurement of flowrate and to a lesser extent, the volumetric flowrate of the flow of a fluid in a closed conduit. Multivariable versions additionally measure fluid temperature. Thermal dispersion mass flowmeters are applicable to the flow of single-phase pure gases and gas mixtures of known composition and, less commonly, to single-phase liquids of known composition - from Sierra Instruments.

Flowmeter Selection for Improved Gas Flow Measurements: a comparison of DP and Thermal Dispersion Technologies - As the costs of fuels and consumables continue to rise, the ability to accurately measure the amount used in a process becomes significant in controlling costs and determining bottom line profits. Therefore, it is important to implement a strategy of adding cost-effective, accurate, gas flow measuring devices to heaters, boilers and cogeneration equipment - from AMS Instrumentation & Calibration Pty Ltd and www.processonline.com.au.

Turbine Meters

Diesel Fuel Flow Measurement - Avoiding Common Pitfalls - As the price of fuel increases and environmental regulations tighten, it is more important than ever to measure the amount of fuel used by diesel engines and standby power generation systems. However, accurate monitoring of diesel fuel consumption presents a significant challenge for end users in various industries. The following white paper addresses a number of common pitfalls to help ensure the best flow measurement solution for diesel fuel applications - from Flow Technology.

Flowmeter Spin - History & Evolution of Turbine Flow Measurement - Josse Yoder - While the Greeks and Romans had their own means of measuring flow, the first flowmeter of the modern era was invented by Reinhard Woltman. Woltman was an engineer who came up with the idea for the turbine meter while studying the loss of water in open canals. This makes the turbine flowmeter the oldest type among the group of new and traditional technology flowmeters. Woltman’s original design, along with his name, persists in the Woltman turbine meter. This meter is used mainly to measure water in bulk amounts. The idea behind turbine meters is quite intuitive. Turbine meters have a spinning rotor with propeller-like blades that is mounted on bearings in a housing. The rotor spins as water or other fluid passes over it. The rotor turns due to the force of the current. Flowrate is proportional to the rotational speed of the rotor. A variety of methods are used to detect the rotor speed, including a mechanical shaft and an electronic sensor - from Flow Research.

Variable Area Flow Meters - Rotameters

Introduction to Rotameters - The rotameter is an industrial flowmeter used to measure the flowrate of liquids and gases. The rotameter consists of a tube and float. The float response to flowrate changes is linear, and a 10-to-1 flow range or turndown is standard. In the case of OMEGA® laboratory rotameters, far greater flexability is possible through the use of correlation equations. The rotameter is popular because it has a linear scale, a relatively long measurement range, and low pressure drop. It is simple to install and maintain - from Omega.

Variable Area Meter Sizing - This useful technical data sheet details sizing calculations for both Gas and Liquid VA meters - from King Instrument Company.

V-Cone and Wet Gas Metering

This is a swag of information on this innovative technology - Thanks to McCrometer.

Flow Venturi and Flow Nozzles

New Page Coming Soon

Velocity Flow Meters

New Page Coming Soon

Vortex Shedding Flow Meters

Vortex Shedding Tutorial - Wade Mattar and James Vignos - This excellent Tutorial covers; Principle of Operation, Calculation of Mass Flow and Standard Volume, Flowmeter Construction, Application Considerations, Meter Selection, Meter Installation and Meter Configuration - from Invensys Foxboro.

Vortex Shedding Flowmeters - Thanks to Caldon.

Why Your Vortex Meter Shows Zero - Liquid flow of a process fluid into a vessel was visually confirmed. However a vortex-shedding flowmeter in the feed pipe measured zero flow. The flowmeter was removed from service and was found to function properly on the flow bench in the instrument shop. Which of the following problems could cause this issue to occur? From David W Spitzer P.E and Flow Control Network.

Gauge Pressure

Gauge pressure is defined as the pressure measured with local atmospheric pressure as its reference point. That is Pressure gauge equals Pressure absolute minus Atmospheric pressure.

Gauge pressure measurement is commonly used in Industry, if the data-sheet does not define "psia/MPaa" or "psig/MPag" then it usually pretty safe to assume that the measurement is being made in gauge.

Inclined Limb Manometer

This is another variation where the construction is based on a glass reservoir connected via its base to the glass tube which is set at an angle rising to the maximum level which can be in the cistern. Again it is basically a differential pressure measurement instrument.

Whilst use is diminishing today, it is still used in low pressure applications, usually about 20 inches wg maximum (4.981Kpa)

Typical example of Inclined Limb Manometer

Manometers come in many different forms and variations.

Level Measurement and Instruments

The Measurement of Level involves a Comprehensive Knowledge of many Level Measurement Techniques and the Associated Level Instrumentation. This page which is just full of free Information on Level Instrumentation is a Fantastic Technical Resource.

Go to Specific Subject: General Level Measurement Theory | Level Measurement Standards | Level Measurement Applications | Level Measurement by Industry | Accoustic Volume Measurement | Capacitance Level Measurement | Displacer Level Measurement | Float/Magnetic Level Measurement | Radar Level Measurement | Resistance Tape Technology Level Measurement | Ultrasonic Level Measurement | Tapping Point Blockage Tools | Thermal Dispersion Level Measurement | Vibrating Level Switches | Boiler Level Control | Bubble Tube Level Measurement | Level Gauge Glass | Hydrostatic (Head Pressure) Level Measurement | Interface Level Measurement | Laser Level Measurement | Nucleonic Level Measurement | Penetrating Pulse Technology Level Measurement | Level Measurement using Profile Concepts | Radio Frequency Level Measurement | Level Sight Glasses | Maintenance of Level Systems | Standards for Tank Level Gauging

 

General Level Measurement Theory

The following Level Measurement Technical Papers are provided by Powerflo Solutions Pty Ltd and Magnetrol.

  • Technical Handbook from Magnetrol - This comprehensive publication provides important conversions, chemical properties, equivalents and pipe data.

  • A Guide to Level Instrumentation for Power Generating Plants - This guide serves as an introduction to level sensing and control products for Power Gen applications.

  • Comparing Displacer Transmitters with DP Transmitters - There have been many technologies over the years that have helped the process industry with level measurement. From the early days of simple mechanical float-operated level switches, the process instrumentation industry has been innovating new technologies to make its customers’ lives simpler. One of these older technologies commonly used in the process control industry is the differential pressure (DP) level transmitter, which first was introduced in the 1950s. It measures the hydrostatic head pressure of a liquid in a vessel and converts this to a level measurement, based on an input specific gravity/density of the liquid. A newer technology that is also dependent on specific gravity is the displacer level transmitter. There are significant differences between these two products, which affect the installation, maintenance and accuracy of the level transmitter chosen. These are discussed in this white paper.

  • Understanding Tank Bridle Level Measurement - A bridle is a vertical pipe connected to the side of a storage tank or process vessel, typically with side/side or side/bottom connections. Because the fluid inside the bridle will rise and fall equally with the level of fluid inside the tank or vessel, the bridle has been adapted for level measurement on a broad scale.

  • Magnetrol Level White Papers - Magnetrol have a swag of white papers covering their range of level instrumentation. Just select the Level icon.

Other Links
Level Measurement Information Notes - Inx Inc

Level Calibration Notes - Inx Inc

A Practical Overview of Level Measurement Technologies - Martin Bahner - There are multiple technologies available on the market to measure level. Each and every technology works, when applied properly. This presentation discusses the strengths and weaknesses of RF Admittance, Capacitance, Ultrasonic, Radar, Nuclear, Differential Pressure, and Bubblers level measurement technologies - from Gilson Engineering Sales.

Level Overview - a comparison of technologies - (supplied by Able Instruments and Controls Limited UK)

The Principles of Level Measurement - Gabor Vass, Princo Instruments, Inc and sensorsmag.com

Level Measurement selection Criteria - from omega.com

Water Level Accuracy and Correcting for Errors due to Gravitational Acceleration and Liquid Density - Ronny D. Harris, Ph.D -from In-Situ Inc

Improving Differential Pressure Diaphragm Seal System Performance and Installed Cost - Best practice diaphragm seal installation to compensate errors caused by temperature variations - from Emerson Process Management.

Choices in Automated Level Detection-Part 1 and Part 2 - This article provides a useful overview of level instrumentation techniques and technologies - from the excellent processonline.com.au.

 

Level Measurement Standards

Safety Standards of Level Control Devices - Malfunctioning level controls allegedly contributed to the 1986 Chernobyl meltdown and the 2005 Buncefield depot explosion north of London, to name just two of the more notorious incidents. For decades, the industrial firewall against safety incidents as they relate to level controls has been governmental and professional association standards that require manufacturers to make their products according to safety guidelines. The International Standards Association, however, lists some 180,000 varieties of international standards. The key health and safety standards that can affect level control devices and applications fall into three categories: (1) Instrument and Component Standards, (2) Safety Integrity Levels, and (3) Hygienic Standards - from Powerflo Solutions Pty Ltd and Magnetrol.

 

Level Measurement Applications

The following Level Measurement Technical Papers are provided by Powerflo Solutions Pty Ltd and Magnetrol:

  • Magnetrol have a Comprehensive Library of Level Applications which detail possible level measurement solutions. 

  • How well do you Understand Safety Integrity Level (SIL)? - This brochure targets safety applications and installations like Emergency Shutdown Systems, however it is worth noting that more than 90% of all applications are not safety-related. Some engineers are now using the SIL data as an indicator for reliability, i.e., the better the numbers, the more reliable the instrument.

  • The Impact of Instrument-Induced Errors on Feedwater Heater Heat Rate - A key way to reduce a power plant’s heat rate - and fuel costs - is to ensure accurate level control within its feedwater heaters. Although many physical anomalies can degrade feedwater heater performance, many older power plants use outdated level technologies. Older level technologies simply cannot achieve a performance level sufficient to manage controllable losses due to instrument-induced errors.

  • The Importance of Application Level Controls for Steam Generation Systems in Coal-Fired Power Plants - This blog discusses how level control technology can help you achieve accurate level measurement for critical applications within steam generation systems, including the condenser hotwell, condensate storage tank and deaerator.

  • Level and Flow Solutions for Gas Compression Skids - Modular skid systems are increasing in popularity in the process industry as a way for owner/operators, OEMs, and plant engineers to fabricate their unit operations. This post discusses level and flow instrumentation for gas compression skid systems.

 

Level Measurement by Industry

Magnetrol have a Comprehensive Interactive List of Level Solutions by Industry.

 

Accoustic Volume Measurement

Acoustic Volume Mapping - An ideal solution for bulk solids and powders - acoustic volume mapping can help you make informed inventory control and usage decisions. This system measures the volume of bulk solids and powders in any size or shape of storage vessel. It accurately measures bulk solids and powders in any type of container, silo or open bin - regardless of the type of material or product characteristics - to provide continuously reliable volume and inventory information Also it measures challenging applications that were previously not possible, including buildup loads and random irregularities that can occur over time - Powerflo Solutions Pty Ltd and Magnetrol.

 

Capacitance Level Measurement

Capacitance Level Control - The end of an Era? Is it time to replace your troublesome RF Capacitance/Admittance Liquid Level Transmitters? - Boyce Carsella - RF Capacitance/Admittance is one of the most flexible level measurement technologies ever developed. RF devices can be used in almost any type of media, and the probes are actually mechanical devices that serve an electronic function in the system. The probes can be made of almost any material and, therefore, they are very flexible devices. They can handle very high temperatures and pressures and corrosive media. Why then do some people refer to capacitance level, as “the level technology some people love to hate”? It seems that with this flexibility comes extreme application sensitivity as well. It's not difficult to find people who have had a capacitance application that did not work properly - from Powerflo Solutions Pty Ltd and Magnetrol.

Other Links
RF/Capacitance Level Instrumentation - from omega.com

Capacitance Level Measurement Technology - David W. Spitzer - Capacitance level measurement sensors are probes that are partially covered by material in the vessel. Rising level tends to cover more of a probe inserted from the top of the vessel - from Spitzer and Boyes.

 

Displacer Level Measurement

The following Displacer Level Measurement Technical Papers are provided thanks to Powerflo Solutions Pty Ltd and Magnetrol:

  • Electronic Displacer Transmitter Liquid Level Measurement Basics and Displacer Level Measurement, Displacer Controller Technology - A good introduction from Magnetrol.

  • Displacer switch operation - This is based upon simple buoyancy, whereby a spring is loaded with weighted displacers which are heavier than the liquid.

  • Comparing Displacer Transmitters with DP Transmitters - There have been many technologies over the years that have helped the process industry with level measurement. From the early days of simple mechanical float-operated level switches, the process instrumentation industry has been innovating new technologies to make its customers’ lives simpler. One of these older technologies commonly used in the process control industry is the differential pressure (DP) level transmitter, which first was introduced in the 1950s. It measures the hydrostatic head pressure of a liquid in a vessel and converts this to a level measurement, based on an input specific gravity/density of the liquid. A newer technology that is also dependent on specific gravity is the displacer level transmitter. There are significant differences between these two products, which affect the installation, maintenance and accuracy of the level transmitter chosen. These are discussed in this white paper.

  • Dont take the Risk - Is your Bulk Liquid Storage Tank Protected? - Do you have bulk liquid storage tanks containing petroleum products, ammonia, caustic chemicals, acids, ethanol, bitumen, pharmaceuticals, beverages, tallow where overflow could occur. This article describes how an high integrity, independent level switch, preferably of different technology to that used for the tank gauging system, is required to monitor any failure of the auto tank gauging system and shut down product transfers - thanks to Powerflo Solutions Pty Ltd.

 

Float/Magnetic Level Measurement

The Following technical information are provided by Powerflo Solutions Pty Ltd and Magnetrol:

  • The Magnetic Level Indicator: A Technology Overview - Demands for increased safety and improved efficiency in processing facilities have made the magnetic level indicator an indispensable level control device. With the ability to perform reliably under extreme process conditions, as well as offer dual-technology redundancy for safety critical applications, magnetic level indicators, or MLIs, such as those manufactured by Orion® Instruments, can make a smart alternative for a wide range of level measurement and control needs. This blog provides a closer look at the applications and operating principle of MLI technology.

  • Selecting the Right Magnetic Level Indicator - Companies in the process industry need the ability to visually monitor liquid levels in vessels (boilers, storage tanks, processing units, etc.). Traditionally, armored glass sight gauges have been used. However, many companies want an alternative to sight gauges to avoid problems such as breakage, leaks, or bursting at high pressures and extreme temperatures. In addition, the visibility of the sight glass can be poor and often affected by moisture, corrosion, or oxidation.Magnetic level indicators (MLIs) do not have the shortcomings of glass sight gauges and are suitable for a wide variety of installations.

  • Float - Details of float measurement techniques explained.

  • Atlas MLI vs. Gage Glasses.

  • Evolution of Magnetic Level Indicators.

  • Replacing DP Transmitters with Magnetic Level Indicator.

  • MLI Branch Connection Comparison - Full-Bore / Full Penetration vs. Extruded Tee - This application note provides a comparison of Orion’s full-bore/full penetration (conventional) branch connections and extruded tee branch connections as used on Magnetic Level Indicators.

  • External Caged Floats - the basics.

  • Float Level Switches - the basics.

  • Magnetic Level Indicators - the basics.

  • Advantages of the Magnetic Level Indicator - A magnetic level indicator is often used in applications where a sight glass (or glass sight gauge) is either ill-suited based on process variables or is under performing based on plant requirements. These can include enhanced safety for personnel; environmentally risky situations including media leakage or fugitive emissions; need for maintenance reduction; or need for high visibility from a distance.

Other Links
Typical Specific Gravities - from the Engineering Toolbox
Resistive Magnetic Level for LPG tanks - Thanks to Bintech

 

Radar Level Measurement

The Following technical information are provided by Powerflo Solutions Pty Ltd and Magnetrol:

  • Choosing the Right Guided Wave Radar Product - Kevin Martyn.

  • Guided Wave Radar vs. Differential Pressure Transmitters for Liquid Level Measurement - Over the years, however, newer level measurement devices have emerged and are consistently capturing market share from older technologies which utilize mechanical and pressure-based measurement-including DP transmitters. Technologies such as Non-Contact Radar, RF Capacitance, Ultrasonic, Magnetostrictive, and GuidedWave Radar employ the latest microprocessor-based digital electronics. By incorporating internal diagnostics, these devices have improved the control, analysis, communication, and overall reliability of fluid level management systems - from www.controlglobal.com.

  • Eclipse® GWR vs. RF Capacitance - Boyce Carsella -Is it time to replace your troublesome RF Capacitance/Admittance Liquid Level Transmitters?

  • Non-Contact Radar Measurement - An examination of the various types - Kevin Martyn.

  • Radar/Magnetic - This unit incorporates Magnetrol’s advanced Eclipse guided wave radar technology with the time proven and time tested magneticlevel indicator.

  • Comparison of Guided Wave Radar against other level Measurement Technologies - This is a neat comparison tool.

  • Guided Wave Radar - The basics.

  • Pulse Burst Radar - The basics.

  • Replacing Old Technology - Revitalising level measurement accuracy at a Soviet-era plant.

  • Sanitary Liquid Level Measurement - Drug-makers are now choosing solid-state, non-mechanical devices that deliver higher accuracy, require less frequent calibration, offer enhanced validation schemes, and are installed in such a way that an instrument failure won’t compromise process uptime or derail a batch in process.

  • Gauge Your Level Instrumentation - Keith Larson - Technology advances and downtime avoidance steer drug makers toward top-mount, non-intrusive methods for continuous liquid level measurement.

  • Level Indicators Reach New Heights - Exactly how much is in there? It’s an easy question if you can see into a tank, but what if you’re basically blind, or your tanks must be sealed? Sure, traditional solutions may work for awhile, but they’re historically hard to maintain and sometimes lack accuracy.

  • Radar Level Gauges - Frequently Asked Questions - Bob Botwinski.

  • Pulsar Radar Level Up to the Challenge at Chemical Plant - This facility offered numerous challenging level measurement applications, including overfill and spill protection requirements. This is a large 7.0 MB download.

  • Eclipse®705 receives SIL3 Certificate from Exida - Magnetrol International, Incorporated has announced that exida, an accredited global functional safety certification company, has certified the product reliability and the engineering change processes for the Eclipse® Model 705 Guided Wave Radar Transmitter as Safety Integrity Level (SIL) 3 capable per IEC 61508. SIL certification is obtained through analysis based on quantitative data and tests indicating the length of time between failures and expected performance in the field. A Failure Mode Effect and Diagnostic Analysis (FMEDA) confirmed that the Magnetrol® Eclipse Model 705 has demonstrated a solid field use history, includes sound engineering processes, and is designed with capable self-diagnostics. Download the IEC61508 Functional Safety Assessment here.

  • 6 Key Features to Look for When Considering a Guided Wave Radar Technology Solution - All process industries today require the ability to safely and accurately measure level in critical applications. This task, however, can be complicated by a variety of factors, including process media that are very low dielectric or contain corrosive vapors, foam, steam, build up, agitation, bubbling or boiling. Guided wave radar (GWR) technology, which is the operating principle used by the Eclipse® Model 706 transmitter from Magnetrol®, provides true level and interface control for applications across many process industries, including petroleum refining, electric power generation, chemical manufacturing, water and wastewater, pulp and paper, pharmaceutical processing and food and beverage. To ensure that you implement an optimal solution for your company, it is important to learn about some of the innovative features that the most reliable GWR solutions offer. The remainder of this post will discuss 6 features that you should look for when you are considering GWR technology as a possible solution for level measurement.

  • The Impact of Signal-to-Noise Ratio on Guided Wave Radar Transmitter Performance - In recent years, much has been said in the industry about the importance of the amplitude (size) of the guided wave radar (GWR) transmit pulse. While the size of the transmitted radar pulse is certainly important, it is a fact that pulse amplitude alone will not always yield accurate level measurement under all process conditions. A far more important parameter in reliable level measurement in difficult applications is the signal-to-noise ratio (SNR), which essentially describes the difference between the desired signal and the unwanted noise. If the amplitude of the noise approaches that of the level signal, loss of accuracy or linearity is the first observed effect due to distortion of the level signal as it passes through and interacts with the noise. Worse yet, if SNR is bad enough, the adverse signal interaction can actually result in a loss of the level signal. While it would be desirable to eliminate all the unwanted impedance discontinuities, it is simply not possible. The good news is that today’s most advanced GWR solutions address this critical design issue.

  • Investigation of New Level Technologies in Single Use, Disposable Systems - David Ladoski and Dan Klees - This article presents guided wave radar level measurement as an acceptable, less expensive alternate to load cell systems.

  • Reliable Foam Measurement Within Liquid Process Media is a Challenging Application - Process media susceptible to foaming are particularly challenging to accurate liquid level measurement. Foam’s lower density, as compared to a foam-free liquid, will absorb or deflect a substantial portion of the return signal, diminishing the all-important reflectivity required by non-contact measurement technologies.

  • Accurate Measurement of Water in Feedwater Heaters - Boyce Carsella, Jr - The accurate measurement of liquid levels in power plant operations is key to efficient operation. Although water is a liquid that can be easily measured by numerous measurement technologies, detection in applications like feedwater heaters, for example, takes on a range of complexity that stresses even the most robust devices.

  • Guided Wave Radar Instrumentation in Steam Drum and Feedwater Heater Applications - Many operators have questions and concerns about level instrumentation and how it can improve the efficiency and safety of their plants. Magnetrol® expert Donald Hite recently answered questions about guided wave radar as part of a power industry-focused webinar. Read his answers in this blog post.

  • Guided Wave Radar Provides Accurate, Cost-Effective Level Measurement for Single Use Systems - Guided wave radar (GWR) level measurement provides a cost-effective control solution for single use bioprocessing systems. Get the recent technical article that investigates the use of GWR technology in a wide range of single use system environments.

  • Six Reasons Why Guided Wave Radar Technology is Preferred Over Differential Pressure Level Control - Discover why differential pressure technology has lost ground to guided wave radar transmitters as the level control solution preferred by process industries.

  • Reliable Foam Measurement within Liquid Process Media is a Challenging Application - Foam Measurement and Liquid Level Instrumentation: A Magnetrol Applications Study Process media susceptible to foaming are particularly challenging to accurate liquid level measurement.

  • Level Measurement Techniques: Minimising Guided Wave Radar Probe Buildup - Learn simple, yet effective, level measurement techniques for minimizing guided wave radar probe buildup. Helpful information for natural gas, condensate and crude processing level control applications.

Other Links
Radar Level Measurement Best Practice - Sarah Parker, Applications Manager, Emerson Process Management, Rosemount division - The emergence of radar has been an important advance in the level measurement field. Radar represents a cost effective, accurate solution that is immune to density and other process fluid changes as well as most vapour space conditions - from Emerson Process Management and the Read-Out Instrumentation Signpost.

Non-Contact Radar Level Measurement - David W. Spitzer - Some basic technical information.

Innovation in Level Measurement - End-of-Probe Algorithm - From automation.com.

Guided Wave Radar Transmitters:Meeting the Challenge for Level-Detection Under Harsh Conditions - From automation.com.

Proper Employment of Guided Wave Radar in Steam Loops - Keith Riley and Ravi Jethra - This white paper discusses how guided wave radar can be used to measure level in steam applications such as feedwater tanks, high pressure preheaters, hotwells and drums from www.automation.com. and Endress+Hauser.

 

Resistance Tape Technology Level Measurement

Principle of Operation - Resistance Tape Level Measurement is remarkably different from other level sensors; in principle, appearance, and performance. The sensor consists of a gold plated, stainless steel base strip, partially insulated from a gold plated wire that is wound around the base strip to form a helix. This helix is sheathed in either Mylar or HastelloyTM to insulate the sensor's inner elements from the liquid to be measured. Sliding the sheathed helix into a protective channel that leaves the front of the sensor exposed then completes the sensor. The sensor is generally contained within a still pipe suspended from the top of the tank. However, other mounting options are available - from Jowa.

 

Ultrasonic Level Measurement

The following technical information are provided by Powerflo Solutions Pty Ltd and Magnetrol:

Principal of Operation :

  • Ultrasonic Contact - The basics.

  • Ultrasonic Non-Contact - The basics.

  • Look for Advanced Performance and Safety Benefits When Choosing an Ultrasonic Level Switch - Contact ultrasonic level switch technology was first applied to process control in the 1960s - and continues to provide accurate and reliable liquid level measurement in virtually every process industry today. Specific design features of ultrasonic level switches are intended to enhance safety in process environments.

  • Flexibility, Powerful Performance from Magnetrol Ultrasonic - Magnetrol’s new Echotel Model 355 Transmitter Can Handle Applications That Were Unthinkable Just a Few Years Ago - As advances in digital signal processing and low-power operation circuits make their way into the design of field instruments, ultrasonic level transmitters have gotten smarter and more effective. Thanks to www.controlglobal.com.

  • Level Sensing with Ultrasonic Contact Technology - Reliable level instrumentation is required to improve process efficiency and ensure the safety of your facility and your employees. Whether you’re specifying liquid level controls for a paper mill or a facility that processes natural gas, chemicals, petrochemicals, power or water and wastewater, finding high-quality instrumentation that can accurately measure level in your clean liquid applications is a mission-critical requirement. This blog discusses ultrasonic contact technology to provide an overview of how the technology works, application considerations and a comparison to other level control technology, specifically tuning forks.

  • Application Considerations When Specifying Ultrasonic Contact Technology for Liquid Level Sensing - Ultrasonic contact level measurement technology plays a key role in the safe and effective measurement of liquids across all process industries. To ensure that you implement an optimal solution for your facility, it’s important to learn about the different applications where an ultrasonic gap switch can be used - and the advantages and limitations of using this technology.

Other Links

Sound Advice - The measurement of level and flow is crucial to effective and efficient waste water treatment. Ultrasonic technology promises that and some more. From AIA.

No Contact Means Ultra Level Measurement - In level-measuring applications where it’s undesirable to have contact of the measuring instrument with the liquid in the process, a sonic or ultrasonic device may be an option. These types of level-measurement instruments really measure the distance from one point in the vessel (usually a reference point) to the level interface with another fluid. The general operating principle of both sonic and ultrasonic devices is similar - from ISA and InTech.

 

Tapping Point Blockage Tools

Tapping Point Blockage - Automatic Cleaning tool, see the advantages of this advanced technology on the level applications page.

Thermal Dispersion Level Measurement

Thermal Dispersion - The basics, thanks to Powerflo Solutions Pty Ltd and Magnetrol.

Liquid Level/Interface Monitoring in Flocculant/Sludge Control - Steven Craig - Precisely measuring and controlling the mixture of wastewater effluent, flocculants and sludge is essential to efficient water treatment. The level/interface instruments that support the process liquid measurements must be capable of distinguishing between liquids with varying properties in order to detect the levels where the different liquids interface in the pond or tank - from Fluid Components International (FCI).

 

Vibrating Level Switches

Vibrating Switches - The basics - thanks to Powerflo Solutions Pty Ltd and Magnetrol.

 

Boiler Level Control

Boiler Drum Level Transmitter Calibration - Steam Drum Level is both a critical and difficult measurement to make. Control of the water level in the drum must be precise. Thanks to Emerson Process Management.

The Boiler Drum Level Measurement Guide - Engineers must be ever vigilant to insure the integrity of the equipment and designs they employ when dealing with critical process applications. Some processes have been studied quite thoroughly and necessary control requirements are already well defined. Hundreds of thousands of boilers have been placed in service and the elements needed for proper drum level measurement are well understood. Years of experience have gone into determining satisfactory designs. Unfortunately, a failure to compile and distribute this information has resulted in many engineers going “back to the drawing board” for every new project. Isn’t this wealth of experience available somewhere. New products and methods are being constantly introduced. But how do these new products combine with existing equipment? Are there regulations which these new methods must meet? Is this really a new product, or a research experiment? This excellent 28 page guide is intended to help in the engineering effort. A sampling of “Tried and True” designs enhanced with the latest developments in equipment and methodology are presented. Hopefully it will provide safer operation of the boiler, save time for the design engineer, and simplify the selection of components that are required to efficiently and safely monitor and control the boiler drum level - Thanks to The Clark-Reliance Corporation.

Boiler Drum Level Control - Drum level control is critical to good boiler operation, as well as safe boiler operation - Jerry Gilman -The drum level must be controlled to the limits specified by the boiler manufacturer. If the drum level does not stay within these limits, there may be water carryover. If the level exceeds the limits, boiler water carryover into the superheater or the turbine may cause damage resulting in extensive maintenance costs or outages of either the turbine or the boiler. If the level is low, overheating of the water wall tubes may cause tube ruptures and serious accidents, resulting in expensive repairs, downtime, and injury or death to personnel. A rupture or crack most commonly occurs where the tubes connect to the drum. Damage may be a result of numerous or repeated low drum level conditions where the water level is below the tube entry into the drum - from ISA and InTech.

Boiler Drum Level Measurement and Control - David C. Farthing - Improved efficiency can have many connotations everything from fuel savings, improved equipment operation and useful life span, to labour and manpower savings. This paper will focus on the effects of boiler drum level and feedwater control as a means of energy savings by thermal/mass balancing the boiler. This paper will also discuss the effects of steam drum pressure and feedwater temperature on the overall efficiency of the boiler - from Federal Corporation.

Cascade, Feed Forward and Boiler Level Control - Allen D. Houtz - One common application of cascade control combined with feed forward control is in level control systems for boiler steam drums. The control strategies now used in modern industrial boiler systems had their beginnings on shipboard steam propulsion boilers. When boilers operated at low pressure, it was reasonably inexpensive to make the steam drum large. In a large drum, liquid level moves relatively slowly in response to disturbances (it has a long time constant). Therefore, manual or automatic adjustment of the feedwater valve in response to liquid level variations was an effective control strategy. However as boiler operating pressures have increased over the years, the cost of building and installing large steam drums forced the reduction of the drum size for a given steam production capacity. The consequence of smaller drum size is an attendant reduction in process time constants, or the speed with which important process variables can change. Smaller time constants mean upsets must be addressed more quickly, and this has led to the development of increasingly sophisticated control strategies - from the controlguru.

All in One Glass - Alternate approach to gage-glass maintenance leads to more accurate drum level measurement - Dale P. Evely - Southern Company, a producer of electricity, fiber optics, and wireless communications, developed an alternate approach to address gage glass maintenance issues as well as measurement uncertainties associated with water-column-type measurements - from ISA and InTech.

Boiling Water is easy as One, Two, Three, Four - according to David W. Spitzer. There’s more to boiler level control than measuring level and adjusting a feedwater valve. Improved measurements and inverse response are just a few of the influences on operating boilers in a reliable manner. Thanks to ControlGlobal.com.

 

Bubble Tube Level Measurement

Bubble Tube Installations for Liquid Level, Density, and Interface Level Measurements - The bubble tube principle of hydrostatic measurement is a convenient, low-cost method of measuring liquid level, density, or interface level in an open tank. It is particularly applicable for those installations where:

  • Process liquid could crystallize in transmitter lines.
  • Process temperature exceeds temperature limit of flange-mounted transmitter.
  • Process tank does not have side connections for flange-mounted transmitter.
  • Process liquid is corrosive and cannot have direct contact with transmitter.

from Invensys.

The Simple Bubble Tube - With all the new-fangled multifunction instrumentation available nowadays, it is easy to forget about the old tried and true measurement techniques. I believe the old adage, “the simplest solution is usually the best solution.” For example, I like the simplicity and elegance of a bubble tube. A bubble tube is a tube (typically ¼ inch to ¾ inch tubing or pipe) that is inserted into a tank a fixed distance from the bottom. The liquid is pushed out of the bubble tube with air or nitrogen, which is metered through a purge meter. The resulting backpressure is proportional to liquid level or density and is usually measured with a differential pressure transmitter. Bubble tubes can be used to measure liquid level, interface level, and density in open tanks- from Dex Automation.

Bubble-Tube Level System - In a Bubble-Tube Liquid Level System, the level is measured in a vented vessel by measuring the pressure required to force a gas into the liquid at a point beneath the surface. This method allows for level measurement without liquid entering the piping or the instrument. A pressure regulator and constant-flow regulator combine to establish a consistent flow of clean air or gas to a bubble pipe immersed a fixed distance in the tank. The flow is regulated to a very low level, building up pressure in the end of the bubble pipe. Thereafter, pressure is kept at this value by the escape of air bubbles through the liquid. Changes in the measured level cause the gas pressure to build or drop - from ControlAir Inc.

Purge Controls - A downpipe is the simplest type of liquid level sensor, often referred to as a “bubbler." This is merely a length of open ended pipe that extends downward into the tank. LiquiSeal™ or Purge Control differential pressure air flow regulators are highly refined versions of the basic bubbler air control.

More details on Bubbler Tubes can be found in the General Level Theory Section above.

 

Level Gauge Glass

All in One Glass - Alternate approach to gage-glass maintenances leads to more accurate drum level measurements - Dale P. Evely - Southern Company, a producer of electricity, fiber optics, and wireless communications, developed an alternate approach to address gage glass maintenance issues as well as measurement uncertainties associated with water-column-type measurements - from ISA and InTech.

Glass Liquid Level Gauges - There are numerous types of level detection devices that incorporate transducers, transmitters, sensors, or indicator instrumentation to monitor and regulate industrial systems. Most level gages rely on the principles of pressure differentials, conductivity, or capacitance and their operations can involve a range of different techniques, such as optical, electromagnetic, microwave, and ultrasonic detection methods. Liquid level gauges are designed for relatively straightforward level regulation and usually provide direct indications through visual, magnetic, or transduction properties. They typically consist of a measuring chamber connected to the vessel being monitored, with gage levels matching the changing levels in the vessel. There are a variety of different liquid level gage designs and each one features distinct operational characteristics and performance requirements. For example, a glass gage has a transparent design that allows for visual readings of the process level, while a magnetic level gage consists of an opaque metal measuring chamber. A floating device equipped with a permanent magnet is suspended upon the fluid in the chamber and it moves an indicator or a transducer through magnetic coupling to produce a level reading. The design differences between these types of liquid level gages determines their effectiveness in various applications, as well as the operating parameters for individual gage units - from www.thomasnet.com.

 

Hydrostatic (Head Pressure) Level Measurement

Level Measurement - Transmitters with Seals - Transmitters with remote seals allow the transmitter to be removed from direct contact with the process fluid - from Emerson Process Management.

Multivariable Approach to Liquid Level - Density Compensated Leveling Eliminates nearly all Error - from ISA and InTech.

 

Interface Level Measurement

Liquid Interface Level Measurement - Five leading interface measurement technologies in use today are discussed in this technical bulletin. Grouped by their operating technologies, these include Buoyancy (Floats and Displacers), RF Capacitance, Thermal Dispersion, Radar, and Redundant Technologies (those combining two measurement technologies in one instrument) - from Powerflo Solutions Pty Ltd and Magnetrol.

Technologies for Liquid Interface Level Measurement - Liquid level interfaces are used in a variety of processing applications. The need for interface level measurement arises whenever immiscible liquids-those incapable of mixing-reside within the same vessel. This blog post discusses available technologies for liquid interface level measurement - from Powerflo Solutions Pty Ltd and Magnetrol.

Measuring Level Interfaces - Measuring the Interface between Two Liquids in a Tank presents Unique Challenges - Gene Henry - A common measurement in the oil & gas, chemical, and petrochemical industries is detecting the interface level between two liquids in the same tank or vessel, such as oil and water. The dissimilar density or specific gravity of the two liquids means the lower density liquid will float on top of the higher density liquid. In some cases, the two liquids will entirely separate, resulting in a “clear” interface that will be easier to detect. In other cases, an emulsion or “rag” layer will exist between the two liquids. Other interface situations include multiple interfaces between more than two products, or the interface between a liquid and a solid. In some cases, it may be necessary to measure the thickness of the upper layer - from the ISA, InTech and Endress+Hauser.

 

Laser Level Measurement

Comprehensive Page on Laser Level - Thanks to ZEDFLO.

 

Nucleonic Level Measurement

Radiation Based Level Measurement - from www.omega.com.

 

Penetrating Pulse Technology Level Measurement

Principals of Penetrating Pulse Technology - From HiTech-A safe, economical, non-contact, non-invasive, “through the wall” digital technology for accurate level measurement - from HiTECH

 

Level Measurement using Profile Concepts

Profile Concepts -These are relatively new very accurate techniques with particular relevence to the oil and gas industry.
Technology Associated with Profiler Level Systems

 

Radio Frequency Level Measurement

RF Level Controls - The basics-thanks to Powerflo Solutions Pty Ltd and Magnetrol.

RF Level Measurement in Lined Vessels with Grounded Shell - from www.omega.com.

Dielectric Constants from Delta Controls Corporation.

 

 

Level Sight Glasses

The following Level Sight Glass Engineering resources are from L.J. Star Inc.

  • Are You Gambling with Your Sightglass? - Selection Strategies for When Failure Is Not an Option - Andrew Obertanec -Sightglasses are sometimes called “the weakest link” in a processing system because of the fragility of the glass. Actually, that’s not necessarily true. A sightglass need not be a weak link in a processing system because glass, when uniquely designed, is not necessarily fragile.

  • Technical Information and Guidelines for the Specification, Installation and Maintenance of Sight Glasses in Chemical and Pharmaceutical Processing Plants - This illustrated "Chemical and Pharmaceutical Sight Glass Application Handbook" spans the full scope of the subject from descriptions of common glass formulations to design factors, to a step-by-step checklist for installation and maintenance.You will have to register to get a copy.

  • Critical Standards for Specifying Sight Glasses - Sight glasses are highly engineered products. Although brands look alike, differences in their specs have tremendous importance for worker safety, sanitary processes, and maintenance costs.

  • Metaglas Sightglass Design - Andy Obertanec - Mechanically Prestessed Gas/Steel Sight Windows Resist Pressure and Impact Failure - The most effective approach to the use of conventional sight glasses has proven to be the simplest…and the most difficult: proper design, proper installation and proper maintenance, or the use of the new mechanically prestressed window design where applicable. Given proper application and maintenance of conventional borosilicate glass windows and, especially, with the availability of new technology, sight glasses no longer need to be the weakest link in a process / fluid system.

  • Step-by-Step Guide to Sight Glass Selection - Sight glass components allow operators to safely observe processes inside tanks, pipes, reactors and vessels. When it comes to selecting a sight glass, a wide variety of factors should be considered in order to ensure optimal performance. The guidelines that follow give tips and step-by-step advice on how to best select the ideal sight glass for a specific application. Start by defining the process to be observed, with considerations given to temperature, pressure, impact, physical characteristics of the process media, flow, and whether or not the process has sanitary requirements. With these factors in mind, one can begin the process of selecting the type of sight glass that best suits the needs of the process.

  • Video - LED Lighting for Sight Glass Applications - This video compares LED Technology with Halogen and outlines the advantages / applications.

  • Sight Glass Selection and Maintenance Tips Webinar - Glass is a critical element in process observation equipment, and understanding the properties of glass is important for proper specification. This streaming video webinar covers the physical characteristics of glass, ways to maximize its strengths and minimize its weaknesses, plus sight glass safety and maintenance - You will have to register to access this Webinar.

  • Important Guidelines for Mounting and Use of Sight Glass Fittings, Toughened Glasses and Luminaires - These commissioning and servicing instructions are critical to ensure safety.

  • How to Spec Lighting for Sight Glass Applications - Chances are pipelines and vessels are too dark for level detection and to view important stages of a process through a sight glass. Flashlights cannot supply sufficient lighting, and may cause a glare on the sight glass, making visual inspection virtually impossible. If the view port is small, there may not even be enough room to combine viewing and lighting. Illumination may be supplied by adding lights (also called “luminaires”) on sight glasses. When deciding upon a proper light for an application, be sure to consider all of the properties of the light. The size, weight, voltage, wattage, materials of construction, mounting configuration, and light pattern are all very important factors.

  • Sight Glass and Sight Glass Lighting Tutorial Video Library - L.J.Star Inc’s collection of tutorial videos created to help you learn about sight glass applications such as flow indicators in chemical and pharmaceutical processes. Learn how sight glass lighting allows for even more effective monitoring. Also learn how thermal shock and pressure affect sight glasses, as well as how to properly select a sight glass for your application. Videos also cover sight glass safety as well as energy savings considerations of LED lighting.

 

Maintenance of Level Systems

Testing, Inspection and Maintenance of Your Overfill Prevention System - A key point of the updated API 2350 recommended practices includes a provision for proof testing the equipment involved in an overfill prevention system (OPS). The new guidelines prescribe that all OPS equipment required to terminate receipt must be tested annually, while the High-High sensor alarm must be tested semi-annually. Additionally, continuous level sensors should be tested quarterly and point level sensors, semi-annually - thanks to Powerflo Solutions Pty Ltd and Magnetrol.

A Comparison of Recommendations for Overfill Prevention - The Buncefield Report (MIIB) & API RP 2350 - Back in 2005, there was a dangerous accident that occurred at the Buncefield Oil Depot, which was the largest fire in Europe since World War II. This fire was caused by an overfill of an outdoor storage tank, causing a release of a flammable vapour that was ignited. The overfill safety system for Tank 912 in bund A failed to operate and shut off the supply of petrol to the tank. When API 2350 was released, it was based on the events of the Buncefield Oil Depot overfill back in 2005. Both API and the MIIB (Major Incident Investigation Board) released new revisions and reports respectively to their standards after reflecting on what went wrong at Buncefield. API RP 2350 was released in 2012 and helped establish good practices. The Buncefield final report was released in 2008 and helped lay out recommended practices for primary, secondary and tertiary containment of a potential overfill situation. These recommended practices covered a wide range of overfill prevention areas from having systematic assessments of SIL requirements to creating a culture where high performance and leadership are expected. The Buncefield reports and API 2350 cover very similar topics relating to overfill prevention - thanks to Powerflo Solutions Pty Ltd and Magnetrol.

 

Standards for Tank Level Gauging

The following links are from Powerflo Solutions Pty Ltd and Magnetrol:

  • Automated Tank Gauging Redundancy through Disparate Level Instrumentation Technologies - Anyone who specifies, operates or maintains instrumentation for critical applications knows that tank gauging redundancy is one of the most effective methods of reliable level detection. In fact, the updated API RP 2350 standards recommend redundant sensors for Category III - or unattended - tanks. Ideally, redundant sensors will be of disparate technologies to avoid multiple or simultaneous failures due to an intrinsic behavior or “weakness” of a given technology.

  • Storage Tank Categories: API RP 2350 Definitions - With the American Petroleum Institute’s API RP 2350 fourth edition guidelines having been updated earlier this year, tank storage facility operators are using the recommendations to safeguard against potential life- and environment-endangering overfill accidents. One aspect operators need to understand is tank categories, upon which overfill prevention methodologies are based. API 2350 categorizes storage tanks by the level of staffing present during receiving operations.

  • Calculating Levels of Concern Ensures a Successful Tank Overfill Protection System - Many tank inventory operators have begun adopting the recommended practices set out in the American Petroleum Institute’s API RP 2350 fourth edition - which features safety measures to prevent tank overfill hazards. A key recommendation of API RP 2350 is the development of Levels of Concern, or LOCs, for each tank in an inventory. Simply put, LOCs are calculated product levels in the tank, upon which all alarm and alert positions and response times are based. Careful calculation of tank LOCs ensures the success of an overfill prevention system that complies with the API recommended practices. The API 2350 guidelines define five LOCs, starting from the top of the tank.

  • Factors to Consider When Assessing Tank Overfill Risk - With increased focus on improved safety measures in the petroleum refining industry, the American Petroleum Institute has addressed tank overfill prevention with its recently published API 2350 4th edition update. It is a requirement of the recommended practices and the responsibility of the terminal owner/operator to develop a written risk assessment procedure. The procedure should incorporate a process for determining the probability of overfill release, evaluating the consequences of an overfill occurrence and identifying the means of reducing risk. The API RP 2350 guidelines do not specify how risk assessments should be conducted, due to the specific nature of site risk and stakeholder values. It recommends that, where possible, owner/operators and transporters conduct risk assessments jointly.

  • The Impact of Roof Configuration on Tank Overfill Protection: Level Instrumentation Considerations - The API RP 2350 4th Edition Update provides details about the type of storage tank, class of liquid, level of concern and attendance category that are affected by the new recommended practices. Additionally, it requires sensors on tanks with floating roof configurations to detect both the roof and the liquid level, should the liquid cover the roof. Beyond this recommendation, however, the API 2350 does not detail level instrumentation standards for external versus internal floating roofs - with or without an instrument well - or for fixed roofs. Yet, certain level instrumentation technologies work better than others, depending on the roof configuration.

  • New API 2350 Overfill Prevention Standards: What You Need to Know About the Fourth Edition Update - In the wake of the Buncefield Oil Depot and other significant tank overfill incidents in recent years, the American Petroleum Institute has revised its API 2350 recommended practices to address malfunctioning or insufficient tank level gauging. While not mandatory, the API 2350 standards are being used by many facilities to improve tank storage safety. This is a quick summary of what you need to know about the new guidelines.

Other Links

Standards for Tank Gauging - Some of the reference standards used for liquid level or mass tank gauging worldwide - thanks to Gauging Systems Inc.

The Art of Tank Gauging - This excellent 26 page technical reference from Enraf gives an excellent overview.

This email address is being protected from spambots. You need JavaScript enabled to view it.Level measurements info notes:

Basics.

Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid. Level is simply a measure of height. It defines the position of the interface, that is, the surface where the two phases meet with respect to a reference point. This measurement is often converted to a volumetric or gravimetric quantity.

· Direct Level Measurement

Direct methods employ physical properties such as fluid motion and buoyancy, as well as optical, thermal, and electrical properties. Direct level measurement does not require compensation for changes in level caused by changes in temperature. Direct level measurements show the actual level of the interface.

· Indirect Level Measurement

Indirect level measurement involves converting measurements of some other quantity, such as pressure to level by determining how much pressure is exerted over a given area at a specific measuring point, the height of the substance above that measuring point can also be determined. For example, the formula used to determine the height of water in an open tank is:

h = P / .433 psi

where:

h = height,

p = pressure indicated on a gage,

.433 psi = pressure exerted by one square inch of water, one foot high.

For substances other than water, the liquid's specific gravity (the ratio of the liquid's density to water's density) must be factored into the level calculation:

h = P / .433 psi (G)

where:

G = specific gravity

Temperature can also affect the accuracy of indirect level measurement. Substances have a tendency to expand when heated and contract when cooled. Gases are greatly affected by changes in temperature, while solids are affected very little. Because indirect level measurement is sensitive to specific gravity and the effects of temperature, it is necessary to compensate for these factors to ensure accurate measurement.

· Continuous Level Measurement

In many processes, continuous level measurement is required because it is necessary to know at all times the exact position of the interface in relation to one or more specific reference points. A gage or sight glass, can be used to continuously observe the position of the interface.

· Point-to-Point Level Measurement

Certain processes require only that the level of a substance be maintained between two points. Frequently these two points are a high level and a low level. When this is required, a point-to-point level measurement system is used. Such a system activates control devices only when predetermined levels are reached.

· Selecting Measurement Devices

Some level measurement methods and devices are better suited to point measurement. When selecting a measuring device, it is important to consider the operating parameters and the physical and chemical properties of the process materials.

Visual sensors.

· Dip Sticks and Lead Lines

A dip stick is essentially a stick or rod that is calibrated to indicate level. The dip stick is lowered vertically into a tank or vessel until it reaches a reference point. Usually the bottom of the tank is used to ensure that the dip stick is inserted to the correct depth. The dip stick is then withdrawn and the level is ready by determining where the interface last made contact with the dip stick. Reading the scale on the dip stick indicates the level measurement. A lead line acts in the same way as a dip stick. A steel measuring tape with a weight attached, the lead line can be used in most places that the dip stick can. Since the lead line can be rolled up into a smaller, compact unit, it is often easier to handle than a dip stick.

· Sight Glasses and Gage Glasses

The sight glass is an important method for visually determining level. The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of liquid in the process tank.

In process systems that contain a liquid under high pressure a reflex sight glass is used. This device is armored, to permit it to tolerate higher temperatures and higher pressures. Gage glasses are typically glass covered ports in a vessel that make it possible to observe the level of the substance in the vessel. Many gage glasses will have a scale mounted on the tank that allows the level to be read.

· Float Devices

These devices operate by float movement with a change in level. This movement is then used to convey a level measurement. An object of lower density than the process liquid is placed in the vessel, causing it to float on the surface. The float rises and falls with the level, and its position is sensed outside the vessel to indicate level measurement.

· Magnetic-Type Float Devices

Floats can also be used with magnets to detect and indicate level. This type of measurement system uses the attraction between two magnets to follow the level of a process liquid.

Variable displacement sensors.

When a body is immersed or partly immersed in a liquid, it loses weight equal to the liquid weight displaced. Variable displacement level devices utilize this principle by measuring the weight of the immersed displacer.

· Archimedes' Principle

Archimedes' Principle states that a body immersed in a liquid will be buoyed up by a force equal to the weight of the liquid it displaces. This upward pressure acting on the area of the displacer creates the force called buoyancy.

· Principles of Variable Displacement

The float displaces its own weight in the liquid in which it floats. It will sink into the liquid until a volume of liquid is displaced that is equal in weight to that of the float. When the specific gravity of the liquid and the cross-sectional area of the float remain constant, the float rises and falls with the level. So, the float will assume a constant relative position with the level and its position is a direct indication of level. The amount of liquid displaced by variable displacers depends on how deeply the device is submerged in the liquid. With variable displacement devices, the amount of displacement varies with the level of the liquid.

The span of the displacer is the distance that the displacer will respond to the forces of buoyancy. Buoyant force depends on the amount of liquid displaced and the density of the liquid. It is important to note the relationship of specific gravity to the change in weight of the displacer as the level changes. Displacers used in liquids with lower specific gravity will not change weight as dramatically as those used in liquids with higher specific gravity. This is why displacer level measuring systems are not used in applications where they could be immersed in liquids of varying specific gravities.

· Liquid-Liquid Interface Measurement

An advantage of variable displacers is that they are capable of detecting liquid-liquid interfaces as well as liquid-gas interfaces. When a displacer is used to determine the level of an interface between two liquids, it is always completely submerged.

· Variable Displacement Level Measuring Devices

A displacer must be connected to a measuring mechanism which, when sensing the changes in buoyant force, converts this force into an indication of level. A displacer body can be suspended directly in a tank, or installed in a float chamber on the outside of the vessel. Torque tube displacer level instrument is suspended from an arm that is attached to a torque tube or torque rod. A knife-edge bearing supports the movable end of the torque tube. This type of bearing provides an almost frictionless pivot point. The torque tube must be sufficient strength to support the full weight of the displacer in the absence of buoyancy, or when the level is at minimum. It is a solid or hollow tube that transfers displacer motion to an electronic instrument or a pneumatic instrument that will produce a signal proportional to the changes in the weight of the displacer. Spring balance displacers are devices similar to torque tube displacers. In these devices, the torsional spring of the torque tube is replaced by a conventional range spring. The motion of the displacer is transferred to the indicator by means of magnetic coupling.

· Applications

Variable displacement level devices are most often used for local level indication or control. Because displacers are immersed in process fluids, their material of construction must be compatible with the process. Displacers are also extremely sensitive to changes in the density of process liquids. Provisions must be made to measure and compensate for such changes in density when variable displacers are used.

Pressure sensors.

Since level can be determined by pressure, or head, many pressure measuring devices are used for indicating level.

· Hydrostatic Pressure

A liquid at rest in a vessel exerts a pressure on the walls of the vessel. At any given point the pressure on the wall of the vessel is proportional to the vertical distance between that point and the surface of the liquid, and varies with the height of the liquid. The relationship between the weight produced by the vertical height of a column of water and the pressure exerted on the supporting surfaces of the vessel can be used to determine level. The relationship between pressure and level makes it possible to convert hydrostatic measurements directly to level in feet or inches. In the following equations, "WC" stands for water column and is usually omitted from equations as understood in discussions of hydrostatic pressure.

1 lb./in.2 = 2.31 feet water

= 27.7 inches water (WC)

1 psi = 2.31 feet

= 27.7 inches

· Open-Tank Head Level Measurement

If level is to be determined and indicated by measuring pressure, the specific gravity of the liquid must be known. The specific gravity of water is 1.00. If the liquid has a lower specific gravity, the pressure exerted by the column of liquid will be less than that exerted by a column of water of the same height. For liquids with a specific gravity greater than 1.00, the pressure exerted by the column of liquid will be greater. To compensate for the difference in specific gravity, the following equation is used:

h = (p (2.31 ft.)) / G

where:

h = height in feet

p = pressure

G = specific gravity

· Diaphragm Box

The diaphragm box is submerged in the process liquid and connected to a pressure gage by a gage line. The hydrostatic head produced by the level of the liquid in the tank exerts pressure on the bottom of the diaphragm causing it to flex upward. This action compresses the gas in the box and the gage line. The pressure is applied to a gage or other pressure element that is part of an indicator assembly calibrated to indicate liquid level units.

· Air-Trap Sensors

As the liquid level rises, the hydrostatic head forces liquid up into an air trap sensor, or inverted bell. As the level of the liquid rises, it compresses the air trapped in the bell and the gage line until an equilibrium between the air pressure and the pressure exerted by the hydrostatic head is reached.

· Air Bubble or Surge Tube

Known by various names, including an air bubble, a surge tube, an air purge and a dip tube, this type of system uses a continuous air supply that is connected to a tube that extends into the tank to a point that represents the minimum level line. An air regulator controls the air flow. It increases air flow to the tube until all liquid is forced from the tube. At this pressure and flow rate, the air begins to bubble out of the bottom of the tube. This indicates that the air pressure forcing the liquid out of the tube is equal to the hydrostatic head produced by the height of the process liquid being forced into the tube. The air pressure acting against the hydrostatic head provides the pressure indication to the gage.

This is most useful for applications such as underground tanks and water wells. However, as with other hydrostatic pressure systems, the major limitation of these systems is that they are generally limited to open-tank applications.

· Closed-Tank Applications

In open tanks, measurements are referenced to atmospheric pressure. At atmospheric pressure, the pressure on the surface of the liquid is equal to the pressure on the reference side of the pressure element in the measuring instrument. When atmospheric pressure changes, the change is equal on both the surface of the liquid and the reference side of the measuring element. To compensate for the effects on level measurement caused by such pressure variations in closed-tank applications, a differential pressure (d/p) cell is often used to measure and indicate level. The d/p cell only responds to differences in pressure applied to two measuring taps. One pressure tap is the measuring point on the tank, which is usually below the minimum level point for the liquid. The other tap is usually located near the top of the tank. The tap in the liquid region of the tank is referred to as the high-side; the other tap, located above the level of the liquid, is referred to as the low-side. System pressure is sensed by both the high and low sides. In addition to system pressure, the high side also senses the pressure exerted by the height of the liquid. Since both sides are exposed to the same system pressure, the effects of system pressure are canceled and the differential pressure cell only indicates liquid level.

An instrument can be calibrated to compensate for the additional static pressure created by the condensed liquid. This compensation or adjustment is called zero elevation. Other means are also available to eliminate inaccuracies due to wet leg problems. For instance, in what is referred to as a wet-leg installation, the low pressure leg is deliberately filled with liquid. Another method involves the use of a device called a pressure repeater or one-to-one relay. The repeater is installed at the top of the tank and linked by pipe to an air relay. The pressure in the tank actuates the air relay, which is connected to an air supply. When the pressure in the tank increases, the relay increases the air pressure on the low-pressure leg. The relay regulates the air pressure so that it is equal to that of the tank pressure. When the pressure in the tank decreases, the relay vents air from the low pressure leg to maintain the equilibrium. Zero suppression, is the correction adjustment required to compensate for error caused by the mounting position of the instrument with respect to the level measurement reference.

Electrical sensors.

· Capacitance

A capacitor consists of two plates separated from each other by an insulating material called a dielectric. In applications involving capacitance measuring devices, one side of the process container acts as one plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel. The dielectric varies with the level in the vessel. This variation produces a change in capacitance that is proportional to level. Thus, level values are inferred from the measurement of changes in capacitance, which result from changes in the level.

Capacitance type level measurement devices offer many advantages. Simple in design, they contain no moving parts and require minimal maintenance. The availability of corrosive resistant probes is also an advantage. Measurement is subject to error caused by temperature changes affecting the dielectric constant of the material. If the probes should become coated with a conductive material, errors in measurement may occur.

· Conductivity

A material's ability to conduct electric current can also be used to detect level. This method is typically used for point measurement of liquid interfaces of relatively high conductivity. Conductivity applications are usually limited to alarm devices and on/off control systems. A common arrangement is two electrodes positioned at the top in a tank. One extends to a minimum level and the other is positioned so that its lower edge is at the maximum level. The tank is grounded and functions as the common, or third electrode. Usually, a stilling well is provided to ensure that the interface is not disturbed and to prevent false measurement.

There are limitations to the conductivity method. The first is process substance must be conductive. Second, only point detection measurements can be obtained. The possibility of sparking also makes this method prohibitive for explosive or flammable process substances.

Advantages include low cost and simple design, as well as the fact that there are no moving parts in contact with the process material. These advantages make this type of system an effective method of detecting and indicating level for many water-based materials.

· Resistance

Resistance type level detectors use the electrical relationship between resistance and current flow to accurately measure level. The most common design uses a probe consisting of two conductive strips. One strip has a gold-plated steel base; the other is an elongated wire resistor. The strips are connected at the bottom to form a complete electrical circuit. The upper ends of the strips are connected to a low voltage power supply. The probe is enclosed in a flexible plastic sheath which isolates the strips from the process material. As the level of the process material rises, the hydrostatic pressure forces the resistance strips together up to the interface. This action shorts the circuit below the interface level, and total resistance is reduced proportionately. Resistance sensing devices can be used for liquid-gas interfaces and for slurries or solids. As with the other electrical level sensors discussed, resistance-type level detectors require relatively little maintenance.

Pressure Instrumentation and Measurement

Theory

What is the Difference between Absolute, Gauge and Differential Pressure? From Instrumentation and Control Net.

The following excellent articles are from Emerson Process Management.

Pressure Best Practices -  Plants are continuously challenged to be more and more efficient - deliver more with less.  How can you reduce initial capital costs, improve plant quality, throughput and availability, while decreasing operating costs?  Pressure Best Practices shows you how to use innovative technologies and new field practices to increase your plant efficiency

Density Measurement - Pressure and differential pressure transmitters are often used to measure the density of a fluid. Both types of transmitters measure level based on the principle that pressure (P) is proportional to the level of the liquid multiplied by its specific gravity.

Interface Measurement - Differential pressure (DP) transmitters are used to measure the interface of two fluids that have different specific gravities (S). To make an interface measurement, the overall level must be at or above the low pressure tap at all times. It is important that the level be large enough to create a reasonable DP between the two specific gravity extremes. This measurement can be done with or without remote seals. However, from a maintenance standpoint, it may be easier to use a remote seal assembly; keeping the wet leg at a constant height can be difficult in some applications.

Eliminating the Need for Methods of Additional Process Sealing - According to installation code found in both NEC 501.5(f)(3) and API 14F, process connected device installed in hazardous locations are required to have dual process seals in order to prevent flammable or combustible process fluids from entering the external electrical system - this technical note discusses a solution for this.

Transmitter Diagnostics Reduce Maintenance Costs - A comprehensive article which discusses methods for reducing maintenance costs.

Specify the Right Solution for Vacuum Applications - When a vessel is under a vacuum pressure, it is important to specify the correct transmitter remote seal system to measure level accurately and reliably. Failure to do so will result in output drift or complete system failure. The combination of high process temperature and vacuum process pressure conditions creates additional requirements when specifying the transmitter remote seal system.

Safety Tip

Risks associated with the use of Mercury

Pressure Conversions

Pressure Conversions from ICEweb.
Pressure Unit Conversion - Look up conversion factors or select a conversion table for a particular pressure unit.

Pressure Measurement

Pressure Measurement Glossary - Find an explanation here for pressure measurement terms listed in alphabetical order - from SensorsOne.

Pressure Instruments

Manometers

  • Equal Column liquid Manometer
  • Inclined Limb Manometer
  • Parallax Error - This is an error that can occur if the reading is not read in line with the level of the liquid.
  • Capillary Effect - When a small tube is used, it should be noted that the level of the fluid is read at the centre of the tube.

Pressure Gauges

Principal of Operation - from hydraulicspneumatics.com.

Pressure Gauges - A useful article from OMEGA Engineering.

Seven Steps to Pressure-Gauge Selection - Many factors affect proper gauge selection, and if you follow the seven-step process, the task can become simpler and more exact - Thanks to IMPO and Richard Jankura - Dresser Instrument.

Seven Steps to Select a Pressure Gauge - Covers those important considerations required when selecting a pressure gauge - from Ashcroft.

Pressure Gauge Selection Strategy - 7 Key Considerations to Ensure Application Success - Chikezie Nwaoha - In order to maintain the efficiency of pressure gauges in general (bourdon tube, bellows, helix, etc.), a thorough selection strategy should be employed. As such, end-users would be wise to carefully consider pressure gauge selection, as failure to select the correct pressure gauge for an application can result in significant plant downtime. Further, in a worst-case scenario, an improperly specified pressure gauge can fail to measure pressure during an overpressure event, compromising plant safety and potentially resulting in costly damages. In order to improve pressure gauge operating efficiency and service life, there are seven basic selection criteria end-users should consider. Thanks to www.flowcontrolnetwork.com.

Pressure Gauge Selection - When selecting a pressure gauge consideration should be given to a number of parameters which have an effect on gauge accuracy, safety, and utility - from McDaniel Controls.

Demystifying the Pressure Gauge Spec Sheet - How to Calculate the Real - World Accuracy of a Pressure Measurement Device - Tom Halaczkiewicz and Patrick Klima - There is no such thing as an exciting product specification sheet. Filled with rows of numbers and figures and formatting that switches between manufacturers, it is no wonder why a spec sheet makes informed decisions difficult. Under actual operating conditions, it is easy to be unsure of a pressure device's performance and accuracy. One central challenge is that no industry rules govern how pressure gauge manufacturers write their product specs and data sheets. Some companies advertise their performance under specific, controlled conditions. Others bury their actual figures inside dense spreadsheets and complicated formulas. And sometimes, important numbers are not published at all. This article outlines how to cut through the confusion and read a specification sheet to determine the real-world accuracy of a pressure measurement device - from Flow Control.

Pressure Regulators

ICEweb's specific page of everything that you wish to know about Pressure Regulators.

Pressure Transmitters and Sensors

Pneumatic Pressure Transmitter - Pauline Gill - Pneumatic transmitters and pneumatic positioners represent two of the four distinct elements in pneumatic process control systems. Pneumatic transmitters sense process variables such as temperature, flow, pressure, or liquid level and transmit proportional three to 15 pounds per square inch (psi) process signals corresponding to 0 to 100 percent of full range to control instrumentation. Pneumatic positioners receive output signals from pneumatic controllers to position the control valve's opening percentage to keep a process variable at a desired set point - from www.ehow.com. 

Pressure Sensing Technology - This useful link from Wikipedia gives a good description of the various sensing technologies.

Pressure Sensor Technologies - Peter Welander - Pressure transducers are everywhere, but do you know what’s inside? Choosing the right technology can be critical, especially in extreme applications - From Control Engineering

Ceramic Sensors Perform well in Tough Applications - John Van Nostrand and Ravi Jethra - The purpose of this paper is to: facilitate the advantages of selecting ceramic pressure measuring cells in applications where plant operations and maintenance personnel may realize optimum process performance and stability through the use of ceramic pressure sensors. Identify and demonstrate the inherent problems of using metal diaphragm pressure cells particularly in vacuum and chemical service. Bring awareness of enhanced performance and stability in pressure measurement to plant maintenance, quality assurance and operations personnel in order to improve their operation and the bottom line - from Endress+Hauser.

Pressure Measurement Advances and Applications - Wally Baker - Improvements in pressure-measurement technologies are opening up new application areas, leading to operational improvements - from the ISA and InTECH.

Identifying Pressure Sensor Problems - How Barometric Pressure, Installation, and Drift Affect Sensor Performance - Elden Tolman - It is common for a pressure sensor to perform well at first and then later fall outside of the acceptable range of performance. Frequently such errors are classified as drift, when in reality there are other forces at play. This article reviews the influence of barometric pressure, installation and drift on sensor performance - from Flow Control.

How Temperature Affects Pressure Sensors - Temperature has very little effect on pressure measurement when it has been properly calibrated and falls within the specified range. In fact, pressure sensors are less prone to temperature variables than other types of sensors, particularly when used for level measurement. However, when the temperature swings beyond a certain point, depending, of course, on the capabilities of the sensor itself, things can get a little dicey.Temperature variations outside the norm can cause accuracy errors and physical failures depending on the circumstances. It is important to choose a sensor that is designed for the temperature gradient for your application - from APG Sensors.

Checklist for Pressure Sensor Selection - 15 Tips to Ensure Performance Meets Your Expectations - Elden Tolman - We have all experienced the frustration of a problematic sensor, even when the supplier tells us everything seems to be working properly. What we often find is that while the sensor may be “working,” it is often not the right sensor for the application. Successful pressure measurement usually comes down to one thing—selecting the right sensor for your application. There are so many considerations that it is easy to overlook a key element until it’s too late. Nearly a decade of working with pressure sensors in various applications has convinced me that the most common reason for problematic pressure measurements is the failure to select the right sensor. Even industry veterans can fall into this fine spun trap. This article presents 15 fundamental considerations when selecting a pressure-sensing device- from Flow Control Network.

Differential Pressure

Differential Pressure Transmitter Performance - An excellent guide from http://www.spitzerandboyes.com.

Guide to Impulse Lines for Differential-Pressure Flowmeters - Good practice in the design and installation of impulse lines (small bore pipes) that connect a differential pressure flow meter to the instrument for measurement of the pressure difference is provided. The guide is intended to assist the designer to avoid known problems with impulse lines that can lead to incorrect measurement - from IDC and NEL.

Comparing Differential Pressure Transmitter Accuracy - Ted Dimm - Differential pressure transmitters are extremely versatile instruments fitting a broad range of applications in various process industries. Accuracy is a key performance measure for any process-measuring device, and is an important factor for proper device selection and maintenance. Differential pressure devices are very versatile, but it is not always easy to understand, calculate, or compare accuracies between devices. This document is intended to help the reader better understand what manufacturers’ accuracy statements mean, what specifications are important for a given application, and how to properly compare various product capabilities - From Honeywell Process Solutions.

Specifying Pressure Transmitters and Transducers

Specifying A Pressure Transducer - How To Select Pressure Range And Device Type - Mitch Berkson and Dave Field - When choosing a pressure transducer for a particular application, usually the first question which arises is: “For what pressure range should the transducer be rated?” This simple question begets a bevy of related ones, namely: “What is the pressure range in which the device typically operates? Does the device occasionally need to measure pressures outside this range? What pressure must the device withstand and still operate within specification when returned to its normal range? What pressure must the device withstand without failing even if it will function properly after returning to the normal operating range?” Closely related to the pressure specification is the type of device to choose - absolute, gage or sealed gage. The following sections will first address pressure selection and then device type - from Sensata.

Profile Type Level Measurement

This type of level measurement profiles the level in a vessel measuring the various mediums, eg., sand, water, oil in a oil and gas production separator. The technique is very accurate and utilises principals such as nucleonics and capacitance.  The accuracy is however reflected in the cost in that the units are expensive,  however when the business case is examined on these devices they have the potential to make Oil and Gas facilities more efficient.  This is based on optimising production/control,  improved potential for minimising oil in produced water, reduced chemical use and determining sand accumulation.

Details of the technology can be found at the following supplier links:

http://www.sentech.no/technology.htm (Capacitive Method)

http://www.synetix.com/services/oilandgas-topside-separator.htm (Nucleonic Method)

Risks Associated with the use of Mercury

Mercury is a poison hazard and thus contact has to be avoided. In the "old days" where Mercury manometers were used extensively special "mercury bays" were utilised to mitigate the hazard. Contact with Mercury can cause brain damage. I suppose you have heard the term "As mad as a hatter", well this was the result of the hatters utilising mercury to put the sheen on top hats. Mind you I have experienced a few mad Instrument Technicians who worked in the Mercury Bays as well!

Static Pressure

Static pressure is defined as the pressure of a fluid that is independent of kinetic energy . Pressure exerted by a gas at rest, or pressure measured when the relative velocity between a moving stream and a pressure measuring device is zero.

Sounds confusing? What it means is when the process fluid is "static" the pressure measured at that point is the static pressure. When fluid is in motion however the meaning is less clear. Lets look at an example of a vessel in which the fluid is in motion. If a tapping point (the measuring instrument point of entry) is installed into the vessel at a low level, the static pressure is the pressure indicated at the instrument.

This term is used in the measurement of level by pressure techniques.

Temperature Instruments and Measurement

Go to Specific Subject: Basics of Temperature Measurement | Temperature Controller | Temperature Conversion Tool | Bi-Metallic Thermometer | Emissivity | Mercury in Steel Thermometers | Temperature Instrument Measurement Standards | Selecting a Temperature Sensor | Selecting a Temperature Transmitter | Temperature Sensor Types | Temperature Sensor Uses, Measurements or Applications | Temperature Problem Solvers | Resistance Thermometers | Radiation/IR Thermometers/Pyrometers (Non Contact Temperature Measurement) | Thermocouples | Thermistors | Thermometers | Thermal Imagery | Thermowells | Temperature Scales and Conversions | Temperature Regulators |

 

Basics of Temperature Measurement

Industrial Temperature Measurement - Basics and Practice - This absolutely fantastic 280 page technical engineering resource from ABB is just about the best document of its type. Well done to the authors.

The Isotech Journal of Thermometry - This brilliant five part journal is an excellent collection of technical articles dedicated specifically to thermometry. It includes articles related to temperature scales, different methods to realize them and practicle techniques used to calibrate interpolation devices such as SPRTs, IPRTs, Thermocouples, Thermistors and Pyrometers.

Welcome to the Isotech Journal Of thermometry - Fundamentals of Thermometry, Part 1: Temperature Scales - Practical Calibration of Thermometers on the ITS-90 - Platinum Resistance Thermometers as Interpolation Standards for the ITS-90.

Fundamentals of Thermometry, Part 2: Fixed Points - Standard Platinum Resistance Thermometer calibrations on the ITS-90, How to specify & order - International Equivalence of thermometer calibration, testing & certification - Improvements in Metrological Apparatus - Evaluation of the Gallium melting Point by a two cell comparison - An MS-DOS computer program for the interpolation of ITS-90.

Fundamentals of Thermometry, Part 3: Standard Platinum Resistance Thermometers - The Platinum Resistance Thermometers of C.H. Meyers (Historic Reprint) - Coiled Filament Platinum Resistance Thermometers - Open Cell, Sealed Cells, & Slim Cells - The Heat Pipe and its use in Thermometer Calibration - Isothermal Heat Pipes and Press Controlled Furnaces.

Fundamentals of Thermometry, Part 4: Standard Thermometers, Bridges & Measurements - Cost of Calibration to ITS-90 at various institutions - Uncertainties in Temperature Measurement - The Gallium Watchdog (in quality assurance of measurement) - New Developments & Discoveries - High Temperature Platinum Resistance Thermometers - Review: About Words - available metrology vocabularies.

Fundamentals of Thermometry, Part 5: Industrial Platinum Resistance Thermometers - Common Errors in Industrial Temperature Measurement - I Blame the Mother-in-law (Evolution of a Fluid Bed Calibration Bath) - The Water Triple Point and Gallium Point in Secondary Laboratories in Germany - Comparison Calibration at the Boiling Point of Nitrogen or Argon - A Caution from Phil Metz - News of a new Metrology Society (Slovak Metrological Society) - Melting Point and Triple Point Measurements of Gallium on the IPTS-68 (Historic Reprint).

Temperature Measurement and Calibration: what every Instrument Technician should know - Temperature may be the most commonly measured physical parameter. Yet there have never been so many ways to measure it as there are today. With so many options it’s natural to have a few questions. How do I measure temperature? How accurate is my measurement? What temperature range is required? What type of device best measures temperature? These are very common questions when confronted with the need to measure temperature. A variety of measurement devices may be used for temperature: liquid in glass thermometers (LIG), thermocouples (TCs), thermistors, resistance temperature detectors (RTDs), platinum resistance thermometers (PRTs) and standard platinum resistance thermometers (SPRTs) - Thanks to Fluke and www.processonline.com.au.

Challenges of Temperature Sensing - Measuring each of the "big four" process variables has its specific peculiarities, but temperature seems particularly controversial. In fact, this apparently simple task often gets complicated. This tutorial explains why- from Control Engineering Magazine and Moore Industries-Pacific, Inc.

Trends in Process Temperature Measurement - An Evolving Technology Segment Changes Focus to Meet End-User Needs - Mike Cushing - This article also covers the basics about resistance thermometers, thermocouples and the use of temperature transmitters - from www.flowcontrolnetwork.com.

NIST Launches New Website to Educate Industry About Alternatives to Mercury Thermometers - As part of a larger effort to reduce the amount of mercury, a potent neurotoxin, in the environment, the National Institute of Standards and Technology (NIST) has launched a new website to help industry scientists and engineers decide the best temperature measurement alternative for their purposes. The website also includes information about myths pertaining to mercury and temperature measurement and how to safely package and recycle mercury-containing products.

Selecting Temperature Measurement and Control Systems - Steve Byrom - How to get accurate data and perform reliable control from systems designed for the rigors of industrial applications - Measuring and controlling temperature is undoubtedly the most common measured parameter because it is critical to so many operations and tasks. Accurate temperature measurement and control is vital to the quality of manufactured goods, such as finished metal components, and to the efficient and safe operation of a process or system. In today’s market, there are myriad devices for monitoring and controlling temperature, ranging from simple temperature controllers to complex distributed control systems. Most temperature measurement and data acquisition products are well-suited for the job for which they are intended, but care must be taken when applying them in harsh industrial environments - from ISA and InTech.

Industrial Temperature Measurement Engineer's Guide - This 420 page guide from Emerson Process Management is excellent. From the basics to engineering design and white papers it is very comprehensive. You will have to register to get it but it is worth the effort!

A Practical Guide to Improving Temperature Measurement Accuracy - Gary Prentice - For many temperature applications, getting a high level of accuracy is vital. “A Practical Guide to Improving Temperature Measurement Accuracy” highlights how plant and site engineers can ensure the most accurate temperature measurement for critical applications. This article details steps that can also help end users improve the stability of their measurements and reduce calibration costs - from Moore Industries.

Engineered Solutions for Temperature Measurement - Keith Riley and Tim Schrock - Difficult temperature measurements in a process require engineered solutions, such as profiling and surface sensing, and applications in hazardous or harsh environments - from ISA and InTECH.

 

Temperature Controller

Buying a Temperature Controller? - Understand the Specification Before you Order - Arthur Holland - A review some of the features and specifications of the commonly used discrete panel-mounted controllers. A review of all makes and features is impossible here, so to supplement this column, my best advice is, extend your reading to catalogs, operation manuals, FAQs and web sites of the top manufacturers. Technology help lines are so overloaded that they become impenetrable and direct you to existing sources of help. Rightly so - but be prepared to quarry your way through some hard to read material. With product knowledge in your brain and an eye on your process you can make a sound and economical choice of controller.

 

Temperature Conversion Tool

A useful degree C/F/Kelvin conversion tool - from Raytek.

The Majority of the following links are compliments of http://www.temperatures.com , this website is excellent and provides comprehensive technical information on all temperature related instrumentation. ICEweb congratulates temperatures.com on the development of such a great resource.

 

Bi-Metallic Thermometer

The principle behind a bimetallic strip thermometer relies on the fact that different metals expand at different rates as they warm up.
Bimetallic Thermometers and Thermostats

 

Emissivity

Emissivity - Emissivity: a mystery to some? But not to all! You can't live with it. You can't live without it - Emissivity is linked to Infrared Radiation Thermometry (or, if you prefer, pyrometry) . It's a mystery to many people, however, even to some who sell non-contact temperature sensors and thermal imagers.mPart of the mystery of emissivity is its spelling, it gets mangled more often than consistant; emmissivity, emistivity, emystery and emisomething are just a few. Seriously, it is the often misunderstood parameter that is always associated with IR temperature measurement and radiation heat transfer ("consistent" is the correct spelling, BTW and emissivity has always had only one 'm'). Heat transfer people have no problems with their emissivities. Are they better educated than some of the users of IR thermometers?

 

Mercury in Steel Thermometers

Mercury in Steel - Basic principles from the Glossary of Meteorology.

 

Temperature Instrument Measurement Standards

Standards for Liquid in Glass Thermometers - Glass thermometers are among the oldest and still the most widely type of thermometer used in laboratory work and in households to determine fever temperature in humans.The first standard issued by ASTM on thermometery was standard E1.

 

Selecting a Temperature Sensor

Choosing the Right Temperature Sensor - Mick Carolan - Temperature sensors are an effective way to measure temperature, but which should you use, and for which application? Thanks to www.pacetoday.com.au.

Selecting a Temperature Sensor - Choosing a temperature sensor can often be very straightforward, sometimes tricky, but always worth doing well. That's because these sensors, especially in science and engineering uses, can spell the difference between repeatable results and nonsense numbers. The name of the game in measurement is to measure with an amount of inaccuracy or uncertainty that is acceptable. So, the first thing you need to know is how well you need to know the value of the temperature numbers you expect to get. A simple series of questions, when answered, will usually get you started.

How To Select And Use The Right Temperature Sensor - Ron Desmarais - This paper answers the question "How do I determine which sensor to use in my application?” After a brief review of how RTD’s and thermocouples are constructed and used to measure temperature, it discusses what differentiates these sensors from one another. It covers the topics of temperature range, tolerance, accuracy, interchangeability and relative strengths and weaknesses for each type. After reviewing these topics you will have a better understanding as to when each type of sensor should be used and why. From Pyromation, Inc.

A Comparison of Thermocouple and RTD Temperature Sensors - Many users simply look to fill the basic needs of their application and do not worry much about their choice of temperature sensing technology. That is, they will make a selection based simply on temperature range and their own bias, perhaps based on their familiarity with a particular sensor type. At a minimum, an informed sensor choice should first consider (a) Measurement range, including the range extensions of shutdown, startup, and process upset (b) The response time and (c) The sensor stability, accuracy, and sensitivity in the application environment. The optimum choicebetween thermocouple and RTD can be difficult. There is a lot of overlap between these sensors at the more popular lower end of the operating temperature range. So for sensors that cover the same operating range, and applications where response time is not a driving issue, plus stability, accuracy, and sensitivity are acceptable, it is necessary tocompare characteristics between sensors to find the best fit for a given application - from Acromag.

 

Selecting a Temperature Transmitter

Choosing a Temperature Transmitter - From Moore Industries International - While there are many practical and economic advantages to using temperature transmitters, the most basic are to ensure measurement integrity and to convert a temperature sensor’s low-level (ohm or millivolt) signal to a standard 4 to 20 mA current signal that can be readily accepted by a monitoring and control system. Advancing technology has made the use of temperature transmitters affordable even in cost-sensitive applications. Here are few things to consider when choosing one. Thanks to Process Heating Magazine.

 

Temperature Sensor Types

Sensor Types - Big differences exist between different temperature sensor or temperature measurement device types. Using one perspective, they can be simply classified into two groups, contact and non-contact. The two links in this article take you to descriptive pages on each type with a breakdown by more specific, detailed types.

 

Temperature Sensor Uses, Measurements or Applications

Temperature Sensor Uses, Measurements or Applications - Measurements are where temperature sensors meet the "real world" where results prove that one understands their properties and has selected a sensor well enough to do the job within the desired measurement uncertainty. This link is where the real fun begins for anyone trying to make a serious temperature or dewpoint measurement. It highlights web sources for this information.

Temperature Measurement Applications in Process Plants - Ravi Jethra - Temperature is one of the most common measurement parameter used for monitoring and control in process industries. This paper covers some of the basics of temperature measurement, and leads into some of the technical advances that impart higher a degree of safety and reliability to process plant operation. These advances are based on some of the latest and innovative technologies that are being implemented in process instrumentation. Irrespective of the type of process plant, temperature measurement remains high on the list for operational excellence throughout the plant. Implementation of some of the new technologies results in improved safety and lower installation and maintenance costs. Incorrect measurement information due to temperature effects, non linearity or stability can result in major equipment getting damaged. Ensuring instruments that have minimal downtime from a maintenance standpoint, not just devices that have been evaluated to provide safety integrity level service in safety instrumented systems, is crucial for daily operations in a power plant from Endress+Hauser Inc and Control Design.

Improving Temperature Measurement in Power Plants - Ravi Jethra - Temperature is one of the most widely measured parameters in a power plant. No matter the type of plant, accurate and reliable temperature measurement is essential for operational excellence. Incorrect measurement because of electrical effects, non linearity or instability can result in damage to major equipment. Using advanced diagnostics, modern temperature instrumentation can inform a plant's maintenance department that a problem exists, where it is and what to do about it long before anyone in operations even suspects that an issue exists. This article covers some of the basics of temperature measurement in power plants and discusses technical advances that impart higher a degree of safety and reliability. These advances are based on innovative technologies that are being implemented in process instrumentation. Implementation of these new technologies can result in improved safety along with lower installation and maintenance costs - from Endress+Hauser Inc and Power Engineering.

 

Temperature Problem Solvers

Temperature Sensors, Transmitters and Assemblies from Moore Industries - These are REALLY excellent.

Corrosion Causes Inaccurate Measurement
Differential Temperature in a Heat Exchanger
Enhance Accuracy Using Transmitters
False Spike Leads to Expensive Shutdowns
Get the Average of Three RTD Signals
High Accuracy Clean Room Monitoring
Interfacing Temperature Sensors to a DCS
Mass Flow Temperature Compensation
Prevent False Shutdowns
Temperature Calibration Made Easier
Total Sensor Diagnostics Cuts Time and Cost
Universal Temperature Transmitters Cut Costs
Why Use Temperature Transmitters Instead of Direct Wiring?

 

Resistance Thermometers

Electrical Resistance Temperature Sensor (RTDs and Thermistors) - Kamal Siddique - This report gives a brief description of Electrical Resistance Sensors for the measurement of temperature. The main focus of this report is on “Resistance Temperature Detectors RTDs” and “Thermistors”. The main body of the report includes definition, working principle, construction, different types, wiring configuration, advantages and disadvantages of RTD and Thermistor. All these topics have been explained in such a way that their role in process industries for the measurement of temperature, probably the most important variable in process industries, becomes crystal clear and their selection by a process engineer for a specific duty becomes easier - from Engineering Resource.

Resistance Thermometers - What are RTD's? -Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance.

Standard Platinum Resistance Thermometers - Frequently Asked Questions - From ISOTECH.

The Basics of Temperature Measurement Using RTDs - This paper provides information for choosing a Resistance Temperature Detector (RTD) sensor type. After a review of the basic construction of an RTD it looks at an RTDs Temperature Coefficient of Resistance (TCR), its sensitivity, accuracy, interchangeability, repeatability, stability and drift, corrosion and contamination effects, shock and vibration effects, insulation resistance, lead-wire resistance, self-heating effects, meter-loading, packaging and thermalransfer considerations, response time, and thermoelectric effects - from Acromag.

 

Radiation/IR Thermometers/Pyrometers (Non Contact Temperature Measurement)

Questions and Answers on Infrared Thermometers - A useful technical information sheet from Zedflo Australia.

Radiation/IR Thermometers/Pyrometers.

Infrared Thermometers - from Omega.com.

Principles of Non-Contact Temperature Measurement - This manual focuses on the practical operations of non contact temperature measurement devices and IR thermometry, and answers important questions that may arise - from Raytek.

Emissivity - This is the measure of an object's ability to emit infrared energy. Emitted energy indicates the temperature of the object. Emissivity can have a value from 0 (shiny mirror) to 1.0 (blackbody), this technical article provides further information on this including emissivity tables for metals and non metals - from Raytek.

 

Thermocouples

Thermocouple Millivolt Tables

Thermocouple - A good introduction from Wikipedia, the free encyclopedia.

Thermocouple Tables

Thermocouple Theory and Practice - W. Dhavepatana Co., Ltd, a neat site this one.

Thermocouple Extension Cable Tables - Data and colours from Raychem Thermocouple Wire - Some of the questions answered by Omega.

Traps and Colour Confusion in Thermocouple Wiring - A useful reference from Arthur Holland and Eurotherm.

Thermocouples - Greg Passler answers the following questions: What is a thermocouple and how does it work? Why do we use thermocouples? What is thermocouple extension cable? From Shawflex.

The Care and Feeding of Thermocouples - Richard D. Smith, P.E - This is a absolutely superb paper.

How to Prevent Temperature Measurement Errors When Installing Thermocouple Sensors and Transmitters - This paper reviews thermocouple behaviour and outlines some of the problems that people encounter when connecting to thermocouples to measure temperature. It contains helpful information for minimizing system error so that you can get the best possible performance from your thermocouple temperature measurement system. It is written primarily for industrial users of thermocouples and thermocouple transmitters, but much of this information can be extended to any thermocouple instrument - from Acromag.

The Basics of Temperature Measurement Using Thermocouples - This paper looks at important characteristics the thermocouple. There are important points about thermocouples that must be understood and this white paper will help you to make an informed selection between sensor types and avoid potential problems in your application - from Acromag.

Get Rid of Rigid - The WORM Flexible Temperature Sensor - Flexible temperature sensors are the new frontier in accurate temperature measurements and easy maintenance. The WORM’s mission: to fit nearly everywhere, to be quickly cut to the correct length, and to reduce the number of spare parts a plant has to keep on hand. The WORM is a flexible sensor for thermowell temperature assemblies. It was designed to replace restrictive, rigid, straight sensor probes with a universal strategy that saves time and money. When it comes to flexible and rigid temperature sensors, both can be inserted into thermowells or protection tubes, welded into place on boiler tubes or other objects, or clamped down for surface measurements. Both types of sensors are rugged, durable, and can measure a wide range of temperatures in industrial applications. So, why replace rigid, straight sensors? From Moore Industries. Additionally see a Video outlining the features here.

 

Thermistors

Thermistor Resistance Table
Thermistors As Accurate Temperature Sensors - Introduction and Methods - Darren O'Connor, and Kasandra O'Malia - This two-part article describes how to use a simple voltage divider circuit with a thermistor to achieve high-accuracy temperature readings over broad measurement ranges. Part 1 discusses the circuit and various temperature estimation methods. Part 2 of this two-part article shows a method to a real-world application and how, by combining estimation methods with thermistor characterization data, high accuracy measurements can be achieved over a wide temperature range using the simple voltage divider circuit - from Sensors Magazine.

Electrical Resistance Temperature Sensor (RTDs and Thermistors) - Kamal Siddique - This report gives a brief description of Electrical Resistance Sensors for the measurement of temperature. The main focus of this report is on “Resistance Temperature Detectors RTDs” and “Thermistors”. The main body of the report includes definition, working principle, construction, different types, wiring configuration, advantages and disadvantages of RTD and Thermistor. All these topics have been explained in such a way that their role in process industries for the measurement of temperature, probably the most important variable in process industries, becomes crystal clear and their selection by a process engineer for a specific duty becomes easier - from Engineering Resource.

 

Thermometers

Liquid in Glass Thermometers

 

Thermal Imagery

 

Thermowells

Following are a number of technical articles on the new Thermowell Design Standard ASME PTC 19.3 TW-2010, the correct design of thermowells is very important especially in respect of stress failures - this standard addresses this.

New Standard for Thermowell Design - ASME PTC 19.3 TW - 2010 - The long awaited PTC 19.3 TW-2010 is a completely new standard that establishes the practical design considerations for thermowell installations in power and process piping. This code is an expanded version of the thermowell section contained in the PTC 19.3-1974, and incorporates the latest theory in the areas of natural frequency, Strouhal frequency, in-line resonance and stress evaluation. ASME responded to changing industry demands for a more comprehensive set of thermowell evaluations. Key enhancements over the 1974 edition include:

  • Expanded coverage for thermowell geometry;
  • Natural frequency correction factors for mounting compliance, added fluid mass, and sensor mass;
  • Consideration for partial shielding from flow;
  • Intrinsic thermowell damping;
  • Steady state and dynamic stress evaluations;
  • Improved allowable fatigue limit definition

PTC 19.3 has been the standard used by piping designers since it’s release and has been highly successful in the industry. The new, expanded PTC 19.3 TW edition—developed by end users and manufacturers—builds on decades of industry and research data to make it the new worldwide authority for thermowell design safety. Intended for piping designers, instrument engineers, instrument designers and plant I/C engineers/designers, plant engineers, plant safety engineers, process engineers, thermowell manufacturers, instrument manufacturers, anyone who assembles thermowell bids or design specifications, and regulatory agencies.

Thermowell Calculations - A white paper from Emerson Process Management - Dirk Bauschke, David Wiklund. Andrew Dierker and Alex Cecchini - Thermowells are essentially a circular cylinder installed like a cantilever into the process piping. They allow a temperature sensor to be located within a process flow while providing a process seal and protecting the sensor from the process fluid. As a process fluid passes around the thermowell, low pressure vortices are created on the downstream side in both laminar and turbulent flow. The combination of stresses, generated by the static in-line drag forces from fluid flow and the dynamic transverse lift forces caused by the alternating vortex shedding, create the potential for fatigue-induced mechanical failures of the thermowell. Until recently ASME PTC 19.3-1974 has been the standard by which most thermowells are designed. For the most part, though, ASME PTC 19.3-1974 was used successfully in both steam and non-steam applications. Several key factors caused ASME to re-form the committee in 1999 to completely rewrite the standard; advances in the knowledge of thermowell behavior, a number of catastrophic failures (Monju among them) and the increased use of Finite Element Analysis for stress modeling. When combined, these factors caused many in the industry to move away from the rudimentary methods and simplified tables laid out in ASME PTC 19.3-1974 in favor of more advanced methods for predicting the thermowell natural frequency and calculating the forced frequency. Rather than simply update the existing version of ASME PTC 19.3-1974, the committee decided to release a new standard due to the significant changes associated with the effort. The thermowell calculation portion of ASME PTC 19.3-1974 was 4 pages. By comparison, the new standard, known as ASME PTC 19.3TW-2010 (“TW” for thermowell), is over 40 pages due to the explanations of theory and the sheer complexity of the process. By February 2010, ASME PTC 19.3TW-2010 was approved through all applicable committees and it was finally released in July 2010.

Thermowell Design Standard ASME PTC 19.3 TW-2010 - An explanatory video - from Emerson Process Management.

Typical Thermowell Calculations Report per ASME PTC 19.3TW-2010 - From Emerson Process Management.

Velocity Collars: No Longer Best Engineering Practice - A velocity collar is a metal ring machined into the shank of a thermowell and installed tightly in to the standoff of a pipe. Due to the nature of thermowell vibration behavior, installation practices, and ASME’s position, Emerson does not recommend velocity collars as a best practice for means of reducing vibration-related failure. Emerson also feels that other installation methods that attempt to reduce unsupported length in a similar fashion to velocity collars, such as DIN Weld-in style thermowells, are not a best practice.

Do Your Thermowells Meet the ASME Standard? - Thermowell design has become more conservative in the age of ASME/ANSI 19.3TW-2010 - Mitchell P. Johnson, J.D. and Allan G. Gilson, P.E. - Depending on process conditions there are a number of other factors that can cause a thermowell to suffer mechanical failure at insertion lengths less than one-third of the pipe. These include flow-induced vibration (wake frequency failure), dynamic (oscillating) and steady state stress, pressure, corrosion, erosion, material selection, and improper installation technique. All of these must be considered in properly designing a thermowell for a given installation. So who is responsible for making the judgment call here? The 19.3TW standard makes plain that ultimate design responsibility for a thermowell well rests with the engineer designing the system into which the well is being installed: Specification of a thermowell, including details of its intended installation and all intended operating conditions, is the responsibility of the designer of the system that incorporates the thermowell. The designer of that system is also responsible for ensuring the thermowell is compatible with the process fluid and with the design of the thermowell installation in the system - from Flow Control.

Basics of Thermowell Design and Selection - Do not underestimate the importance of thermowells in temperature measurements - When planning for a temperature measurement application, a fair amount of consideration is typically given to sensor selection (e.g., thermocouple vs. RTD) and wiring of the output (e.g., transmitter vs. direct wiring), and how these factors will affect the measurement. Often, by comparison, relatively little consideration is given to the mechanical components of the sensor assembly, particularly the thermowell. Of all the components in a typical temperature assembly, a thermowell would seem to be the simplest and least critical. In reality, the thermowell is fundamentally important because it directly and significantly affects the life span of the sensor and accuracy of the measurement. It also protects the closed process, providing plant and personnel safety - from ISA and InTech.

Safety Notice!

Chloride Induced Stress Corrosion Cracking of Stainless Steel Thermowells: Potential for Ingress Of Atmospheric Moisture - This safety notice from the HSE describes a specific degradation mechanism found inside stainless steel thermowells operating where the external atmosphere contains halides, as is typical in coastal locations or near to cooling towers. Thermowells can ‘breathe’ during normal operation as vessels heat up and cool down, drawing in the external atmosphere through non gas tight fittings. If the atmosphere contains halides this can leave any stainless steel susceptible to Chloride Stress Corrosion Cracking (CISCC).

Other Thermowell Links

What is a thermowell? From - Trerice.

About Thermowells - A super page from Temperatures.com covering most aspects including the one most forgotten related to velocity constraints.

Thermowell Wakes, Vortices, Vibrations & More - Another top rate page from Temperatures.com covering most aspects.

Introduction to Thermowells - From Newport.

Thermowell Materials Selection Guide - From Azom.com.

 

Temperature Scales and Conversions

Calculate temperature and millivolt tables for Thermocouples based on NIST Monograph 175 - From MINCO.

Calculate temperature and resistance tables for Minco's RTD Elements - From MINCO.

 

Temperature Regulators

Temperature Regulators - The characteristic feature of self-operated temperature regulators is their compact design, including a sensor, a valve and a capillary tube. Their simple operating principle is based on fundamental mechanical, physical and thermodynamic laws. Thanks to Samson Controls.

Vacuum Pressure

Vacuum Pressure is defined as the pressure measured with local atmospheric pressure as its reference less the amount of vacuum being drawn.

Selecting Instruments for Vacuum Service - David W. Spitzer - I recently read an Internet posting by a well-respected instrumentation engineer about selecting instruments for vacuum service. His comments are informative in that he mentions that instruments can be damaged when operated under vacuum. Vessels can also be damaged. The plant in which I worked in a previous professional life a tank that held somewhat less volume after it encountered a vacuum. In this case, pumping liquid out of the tank while the inlet and vent valves were closed caused a vacuum to be created in the tank. The tank partially collapsed, but fortunately no one was hurt and no liquid escaped - from www.flowcontrolnetwork.com

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