Flame and Gas Detection Handbook

ICEweb acknowleges the submission of this fire and gas technical handbook by AMPAC INDUSTRIES

CONTENTS

2.1 Infrared Single Frequency Flame Detectors
2.2 Multi Spectrum Flame Detectors
2.3 Ultraviolet Flame Detectors
2.4 Ultraviolet / Infrared Flame Detectors
2.5 What Do I Connect A Flame Detector To?
2.6 Flame Detector Installation
2.7 Flame Detector Selection

3.0 Gas detectors

3.1 Catalytic Gas Detectors
3.2 Infrared Gas Detectors
3.3 Electrochemical Gas Detectors
3.4 What Do I Connect A Gas Detector To?
3.5 Gas Detector Installation
3.6 Gas Detector Selection

1.0 INTRODUCTION

The purpose of this handbook is to provide information on the various types of flame and gas detectors available.

Although Ampac has created the handbook, and represents Det-Tronics, it does not relate specifically to our products and it is intended to be used as a guide for all manufacturers’ equipment and systems.

It should be noted that the information is to be used as a reference only and that when specifying equipment for a project, it is best to consult with the end user and the manufacturer to ensure the most appropriate equipment is selected.

Additional copies of the handbook are available on request, as are in house presentations, or just call one of our customer service offices for more information.

	HEAD OFFICE
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Website: www.ampac-ind.com.au

We acknowledge Jim Russell, Principal Instrument Engineer for Woodside Energy Perth Western Australia and Mark Sutherland, Product Manager for Detector Electronics Corporation for their assistance in creating this handbook.

2.0 FLAME DETECTORS

Most fire detection technology focuses on detecting heat, smoke (particle matter) or flame (light) – the three major characteristics of fire. All of these characteristics also have benign sources other than fire, such as heat from steam pipes, particle matter from aerosols, and light from the sun. Other factors further confound the process of fire detection by masking the characteristic of interest, such as air temperature, and air movement. In addition, smoke and heat from fires can dissipate too rapidly or accumulate too slowly for effective detection. In contrast, because flame detectors are optical devices, they can respond to flames in less than a second. This optical quality also limits the flame detector as not all fires have a flame. As with any type of detection method its use must match the environment and the risk within the environment.

Typical applications for optical flame detectors are;

Wherever highly combustible materials are involved
Where there is a need for instantaneous response to flame
Wherever unsupervised areas require automated fire protection
Where there is a large capital investment to be protected

Examples of actual installations are;

Gasoline transport loading terminals
Pipeline pumping stations
Refineries
Aircraft hangers
Automotive paint spray booths
Munitions production facilities
Jet engine test cells
Offshore drilling and production platforms

There are three types of flame detectors currently available. They are Infrared (IR), Ultraviolet (UV), and a combination of UV and IR. The spectrum below shows the relationship between these frequencies and visible light.

2.1 INFRARED SINGLE FREQUENCY FLAME DETECTORS

Infrared detectors have been available for many years, however, it has only been in recent times that technology has allowed for stable, accurate detection to occur. There are two types of Infrared detectors, single frequency and multi spectrum.

The basic principle of operation for a single frequency IR detector is;

The detector is sensitive to a narrow band of radiation around the 4.4 micron range which is a predominant emission band for hydrocarbon fuelled fires. Additionally, the sun’s radiation at this band is absorbed by the earth’s atmosphere, making the IR flame detector solar blind. Single frequency detectors use a pyroelectric sensor, which responds to changes in IR radiation intensity. In addition they incorporate a low frequency band pass filter, which limits their response to those frequencies that are characteristic of a flickering fire. In response to a fire signal from the sensor, electronic circuitry in the detector generates an output signal.

Strengths of the single frequency IR detector are;

Highly immune to optical contaminants like oil, dirt, and dust
High speed response under 30 milliseconds for some brands
Insensitive to solar, welding, lightning, X-rays, sparks, arcs and corona

Limitations of the single frequency IR detector are;

Generally not suitable for non-carbon fires
Some brands will respond to modulated infra-red sources
Rain, ice and water vapour on the detector lens will inhibit detection

2.2 INFRARED MULTI SPECTRUM FLAME DETECTORS

The basic principle of operation for a multi spectrum IR detector is;

The detector has three sensors, each sensitive to a different frequency of radiation. The IR radiation emitted by a typical hydrocarbon fire is more intense at the wavelength accepted by one sensor than the other two. Electronic circuitry in the detector translates the difference in intensity of the three sensors to a ratio, that along with a synchronous flicker must be present before a fire signal is produced. This allows the detector to reject high intensity flickering black body radiation sources since these sources will not meet the proper ratio criteria.

Strengths of the multi spectrum IR detector are;

Virtually immune to false alarms
Fire response in the presence of modulated infra-red black body radiation with some brands
Long detection range (60 metres to some fires)

Limitations of multi spectrum IR detector are;

Typical response time is longer when compared to single frequency detectors

IR detectors are sensitive to most hydrocarbon fires (liquids, gases, and solids). Fires such as burning metals, ammonia, hydrogen and sulphur do not emit significant amounts of IR in the detector's sensitivity range to activate an alarm. IR detectors are suitable for applications where hydrocarbon fires are likely to occur and high concentrations of airborne contaminants and / or UV radiation sources may be present. The detector should be used with caution when the presence of hot objects and the potential for ice build up on the detector are likely.

2.3 ULTRAVIOLET FLAME DETECTORS

A UV detector uses a sensor tube that detects radiation emitted in the 1000 to 3000 angstrom (one ten billionth of a metre) range. It is important to note that ultraviolet radiation from the sun that reaches earth starts at 2800 angstrom. If the detector's sensor has a wide range then it will be triggered by the sun’s rays, which means it is only suitable for indoor use. There are sensors available with a range of 1800 to 2500 angstroms. Virtually all fires emit radiation in this band, while the sun’s radiation at this band is absorbed by the earth’s atmosphere. The result is that the UV flame detector is solar blind. The implication of this feature is that the detector can be used indoors and outdoors. In response to UV radiation from a flame that falls within the narrow band, the sensor generates a series of pulses that are converted by the detector electronics into an alarm output.

Strengths of the UV detector are;

Responds to hydrocarbon, hydrogen and metal fires
High speed response – under 10 milliseconds
Solar insensitive

Limitations of the UV detector are;

Will respond to welding at long range
May respond to lightning, X-rays, sparks, arcs, and corona
Some gases and vapours will inhibit detection
Some UV sensors have a wide detection range resulting in solar false alarms

UV detectors are sensitive to most fires, including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulphur, hydrogen, hydrazine and ammonia. The UV detector is the most flexible general purpose optical fire detector available. They are fast, reliable, have few false alarm sources and respond to virtually any fire.

2.4 ULTRAVIOLET / INFRARED FLAME DETECTORS

A UV/IR detector consists of an UV and single frequency IR sensor paired to form one unit. The two sensors individually operate the same as previously described, but additional circuitry processes signals from both sensors. This means the combined detector has better false alarm rejection capabilities than the individual UV or IR detectors.

Strengths of the UV/IR detector are;

Virtually immune to false alarms
High speed response – under 500 milliseconds
Solar, welding, lightning, X-rays, sparks, arcs, and corona insensitive

Limitations of UV/IR detector are;

Not recommended for non carbon fires
Some gases and vapours will inhibit detection due to blinding of the UV sensor

Since the UV/IR detector pairs two sensor types, it will typically only detect fires that emit both UV and flickering IR radiation. UV detectors will respond to virtually all fires including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulfur, hydrogen, hydrazine and ammonia. IR detectors typically only respond to hydrocarbon fires. Since the IR detector is not sensitive to burning metals, ammonia, hydrogen and sulfur the combined unit will not respond to these fires.

The detector is suitable for applications where hydrocarbon fires are likely and other sources of radiation may be present (X-rays, hot surfaces, arc welding). They maintain constant protection while arc welding takes place. The UV/IR detectors are highly reliable with fast response times and low propensity to false alarms.

2.5 WHAT DO I CONNECT A FLAME DETECTOR TO?

Flame detectors can be connected in 4 different ways to provide varying degrees of information.

Stand Alone – the detector is fitted with internal relays that provide alarm and fault outputs. When the detector senses a fire it activates warning devices and some method of fire suppression. This is the simplest method of connection and while the detector does have LED status there is not any remote indication in the event of a fire or if the detector fails.
Fire Alarm Panel – the detector is connected to a Fire Alarm Panel (FAP) as part of an overall site detection system. Warning devices and suppression systems can be operated, the advantages are that the power supply to the detector is monitored, and indication of the detector status is centralised.
Control Panel – the detector is connected to a dedicated flame detector control panel, this is used when the site does not have a Fire Alarm Panel. This system offers the same advantages as a FAP.
Monitoring System – the detector provides a 4-20mA output that connects to a site monitoring system. The output provides multiple alarm and fault conditions. The advantage of this system is that the flame detectors can be incorporated into a system that is monitoring other functions on the site such as air conditioning.

2.6 FLAME DETECTOR INSTALLATION

As with all fire detectors the placement of flame detectors is determined by the environment that they will be operating in. What appears to be a good place to locate a flame detector on paper may be a poor location in reality. Some of the factors to consider are;

The viewing angle of the detector
The detection range
Obstructions such as girders, beams, supports, hoists, air conditioners and other solid objects will block the cone of vision and / or hinder access for service
All high risk fire ignition areas must be covered by at least one detector
Adequate detector coverage will ensure that ‘voids’ in the optical coverage do not occur
Optimum detector mounting height is a function of the height of the most likely point of fire ignition

When designing a system we recommend that a manufacturer be contacted as details can be provided on previous installations of a similar nature. This will ensure that the correct number of detectors is provided to ensure the most suitable detection.

2.7 FLAME DETECTOR SELECTION

When selecting which type of flame detector to use there are 6 questions to be answered;

What is the area that I’m protecting (aircraft hanger, storage tank, turbine enclosure etc)?
What are the dimensions of the area that I’m protecting?
What are the anticipated sources of fire?
Each type of fuel, when burning produces a flame with specific radiation characteristics. The detector must be chosen for the type of fire that is probable. For example, a UV detector will respond to a hydrogen fire but an IR detector will not.

What other sources of radiation will be present?
Radiation sources other than fire are present in many applications. For example, arc welding is often performed in an industrial area. IR or UV/IR detectors will ignore arc welding where a UV will false alarm. Each application must be assessed to determine if any such sources are present before choosing a detector.

What will prevent the detector from detecting a fire?
Industrial environments often contain elements that inhibit the ability of a detector to ‘see’ a fire. For example, a build up of ice on an IR detector will reduce the detector's range. A build up of oil on a UV detector will reduce its range. Other obstructions such as pipes, partitions, air conditioners etc will block the optical viewing area. If a fire started on the other side of a partition it would not be detected.

How fast must the detector respond to a fire?
UV detectors can respond to a fire as fast as 10 milliseconds. Other detector types such as IR and UV/IR typically take between one and five seconds to respond.

Once these questions have been answered the type of detector required will become evident. As previously stated we recommend that the manufacturer be contacted for verification and further site specific information.

It should be noted that not all flame detectors available offer the same features and level of protection, important considerations are;

Frequency Band – a wide band will initiate more false alarms
Range – at what range will the detector detect a fire
Viewing Angle – at what distance and angle will a fire be detected
Cone of Vision – will the detector have the same range over a 90 degree span
Optical Integrity – how does the detector monitor the sensor and lens
Serviceability – can the detector be serviced on site or does it need to be returned to the manufacturer
Construction / Mounting – is the detector construction suitable for hazardous areas, is there sufficient movement in the mounting bracket to ensure the sensor will be aimed at the source
Indication – does the detector have on-board visual indication
Outputs – does a fault condition over ride an alarm trigger
Heating – does the detector have heated optics to prevent ice build up
Discrimination – does the detector have electronic capacity to distinguish between black body emission, flickering phenomenon, and flame

3.0 GAS DETECTION

Gas detection is an important element in an overall protection plan for life and property. It is often used in conjunction with other forms of fire detection such as smoke, heat and flame detectors.

Typical applications for gas detectors are;

Wherever highly combustible gases are involved
Where there is a need for instantaneous response to gas
Where there is a large capital investment to be protected

Examples of actual installations are;

Mines
Refineries
Ship Hulls
Sewage Plants
Manufacturing Plants
Pipeline pumping stations
Gas transport loading terminals
Offshore drilling and production platforms

There are three types of gas detectors currently available. They are Catalytic, Electrochemical and Infrared (IR).

3.1 CATALYTIC GAS DETECTORS

Catalytic sensor technology uses a pair of computer matched elements in a Wheatstone bridge circuit. Combustible gases and vapours are detected as they oxidise on the active catalytic element. This reaction produces a differential voltage that is proportional to the gas concentration present up to its lower explosive limit (LEL).

Strengths of the catalytic detector are;

Comparative low cost
Responds to virtually any flammable / combustible gas

Limitations of catalytic detector are;

Routine calibration is required for effective protection
Sensors do ‘wear out’ over a period of time
Will not sense non combustible gases
Sensors can be poisoned

3.2 INFRARED GAS DETECTORS

Infrared gas detection is used to detect combustible levels of hydrogen gases and vapours, based on the absorption of energy by hydrocarbons. There are two types of detection, point and open path. A point type detector is located as close to the risk as possible and samples around the risk eg a storage tank. In the detector a beam of IR energy is emitted between a source and detector, any attenuation caused by hydrocarbons in the short beam being electronically processed to give a reading in LEL. Commonly a reference beam is utilised to overcome any reduction in beam intensity due to of the optics being impaired eg fog, and temperature changes.

Strengths of the point infrared detector are;

Responds to many hydrocarbon gases
Highly resistant to poisoning and etching
Limited maintenance required
Long sensor life – some manufacturers offer a 5 year warranty
Minimal drift
Unaffected by oxygen depleted or enriched environments

Limitations of the point infrared detector are;

Can not measure non hydrocarbons eg hydrogen

Open path or line of site detectors have a similar principle of operation to that of the point type detector. However, it is a two part detector (transmitter and receiver), the IR beam is transmitted over a long distance (up to 150m) and they give a reading in LEL meters. Open path detectors can be used as an electronic fence around the perimeter of a facility to detect an escaping gas cloud.

Strengths of the open path infrared detector are;

Ideal for open areas without obstructions
Covers large areas – minimising wiring and multiple detectors
Limited maintenance required
Fast response time

Limitations of the open path infrared detector are;

Can not measure non hydrocarbons eg hydrogen

3.3 ELECTROCHEMICAL GAS DETECTORS

Electrochemical sensors are designed to be highly selective and are capable of detecting concentrations in the parts per million range. Gases detected include oxygen, hydrogen sulphide, carbon monoxide, nitrogen dioxide and sulfur dioxide. The sensors utilise multiple electrodes immersed in an electrolyte. As gas diffuses into the sensor an electrochemical reaction occurs, which produces a current that is proportional to the gas concentration.

Strengths of the electrochemical detector are;

Cost effective protection
High sensitivity

Limitations of the electrochemical detector are;

Require a certain amount of humidity to correctly function
Sensors ‘wear out’ over time
Sensors can be poisoned by foreign material

3.4 WHAT DO I CONNECT A GAS DETECTOR TO?

Gas detectors can be connected in 3 different ways to provide varying degrees of information.

Stand Alone – the detector is fitted with internal relays that provides alarm and fault outputs. When the detector senses the set level of gas it activates warning devices. The unit can have LED indication, however, the disadvantage of a stand alone configuration is that in the event of an emergency or fault there is no central remote indication.
Control Panel – the detector is connected to a dedicated gas detector control panel. This panel may contain single cards with analogue and digital displays so as to provide centralised monitoring of multiple detectors. Warning devices can be operated from the panel, another advantage is that the power supply to the detectors is monitored.
Monitoring System – the detector provides a 4-20mA output that connects to a site monitoring system. The output provides multiple alarm and fault conditions. The advantage of this system is that the gas detectors can be incorporated into a system that is monitoring other functions on the site such as air conditioning.

3.5 GAS DETECTOR INSTALLATION

As with all detectors the environment that they will be operating in determines the placement of gas detectors. What appears to be a good place to locate a gas detector on paper may be a poor location in reality. Some of the factors to consider are;

The gas to be detected, some gases are heavier than air, therefore, the detector must be located near the floor.
Obstructions such as girders, beams, supports, hoists, air conditioners and other solid objects may interfere with the flow of gas to the detector.
All high risk areas must be covered by at least one detector
Airflow within the area may prevent the gas from reaching the detector.

3.6 GAS DETECTOR SELECTION

When selecting which type of gas detector to use there are 6 questions to be answered;

What is the area that I’m protecting (storage tank, pumping station, manned or unmanned, etc)?
What are the dimensions of the area that I’m protecting?
What are the anticipated gases?
What type of gas is most likely to cause an emergency, The detector must be chosen for the type of gas that is probable.

What other sources of gas will be present?
Interference/background gases can effect the sensor‘s reading

What will prevent the detector from detecting gas?
Industrial environments often contain elements that inhibit the ability of a detector to ‘smell’ gas. For example, obstructions such as pipes, partitions, air conditioners etc can prevent detection taking place.

What are the long term plans of the area?
This will assist in determining if the detector will be effected by future changes in the environment and confirm the service requirements.

It should be noted that not all gas detectors available offer the same features and level of protection, important considerations are;

Display – what information is displayed at the detector and in what format
Poisoning – is the sensor resistant to foreign materials
Serviceability – can the detector be serviced on site or does it need to be returned to the manufacturer
Calibration – can calibration take place without declassifying a hazardous area
Construction – is the detector construction suitable for hazardous areas
Outputs – how many outputs are available for connection to monitoring / warning systems – low, high, auxiliary and fault
Options – are options available to protect the sensor against, rain, dust, etc