GAS DETECTION OVERVIEW                                         

The storage or distribution of any gas involves the gas being contained in vessels or pipes under pressure. Any leak in these systems will therefore result in significant amounts of gas (probably invisible) escaping into the surrounding air. What happens to the gas then depends on the nature of the gas and the immediate environment, but can pose a threat to life in three ways:

1. Combustible gas can gather and reach a density at which it can ignite and cause a fireball. 

2.  Toxic gas can cause illness, paralysis or death if inhaled.

3.  Any gas in sufficient quantities will displace oxygen and therefore pose a serious risk to life.

TYPES OF GAS DETECTION:

Electrochemical Detectors

These detectors contain various components designed to react with a specific toxic gas. This reaction generates a current which is measured and converted into a concentration value (PPM-parts per million).

Advantages: Low cost devices that allow continuous monitoring ‘at source’ so leaks are detected quickly and monitoring is continuous, (so no gaps in coverage for sampling sequences.) There are no moving parts that can cause mechanical failure.

Disadvantages: Some sensors respond to gases other than those they are required to detect. Sensors require quarterly calibration and do not have a long life.  

          Catalytic Detectors

    These sensors ‘burn’ combustible gases on a small catalytic bead. The sensor measures the resulting increase in resistance and translates that into a percentage of LEL, the Lower Explosion Limit for the target gas.

Advantages: Like electrochemical sensors these are low cost devices that allow continuous monitoring ‘at source’ so leaks are detected quickly and monitoring is continuous, (so no gaps in coverage for sampling sequences.) There are no moving parts that can cause mechanical failure.

Disadvantages: Some sensors respond to gases other than those they are required to detect. Sensors require quarterly calibration and do not have a long life.

  Paper Tape Instruments

These use a chemically-impregnated tape to detect toxic gases. The tape changes colour when exposed to a given gas and this change is detected by a photocell, analyzed and translated into a concentration value.

Advantages. As a result of the color change reaction, paper tape instruments provide physical evidence of a gas leak (versus electrochemical, catalytic, solid state, and FTIR instruments, which only send out a 4-20 mA signal). Typically, they are also somewhat less prone to interference from other gases than electrochemical and solid state instruments. In addition, paper tape devices typically can detect more gases than electrochemical instruments.
Disadvantages. Paper tape instruments can only be used for toxics — they cannot detect combustible gases such as hydrogen. Because of their high cost, paper tape instruments are typically kept in a central location and connected to multiple detection points through sample tubing; samples are pumped from each individual point in sequence. As a result, significant lag times can exist between a leak and its detection, and sequencing can cause the instrument to overlook some leaks. In addition, reactive gases (such as HF, Cl2, HCl, and NH3) are easily adsorbed on tubing, which can prevent the instrument from "seeing" a leak. Mechanical failure is always an issue with paper tape detectors (the cassette drive can jam, the optics can foul, pumps can fail, filters can plug, and flows can become unbalanced), and regular preventative maintenance is required. In addition replacing the tapes every 2-4 weeks as recommended can prove expensive.

             Solid State Instruments

These instruments are made of a metal oxide material that changes resistance in the presence of gas. This resistance change is converted into a reading of concentration.

Advantages. Solid state sensors have a very long lifetime, typically 10 years. They can detect a wide range of gases, including many that electrochemical and paper tape instruments are unable to see. Because they are fairly inexpensive, solid-state instruments typically are used to detect gas at the source, so response to leaks is quick and monitoring is continuous. In addition, they have no moving parts that can cause mechanical failure.
Disadvantages. While solid state sensors can detect a wide range of gases, they have very low selectivity — so the possibility of "false alarms" is significantly higher than with other technologies. In addition, when they have not been exposed to gas for some time, some solid state sensors oxidize and become dormant, meaning that they will not respond to real gas leak. Solid state sensors also provide a non-linear output, so calibration is more difficult and time-consuming than it is with electrochemical sensors (which have a linear output).

 

       FTIR Instruments

Fourier-transform infra-red instruments use spectrophotometric techniques to detect gas. Infra-red light is shined through a sample, and the resulting absorbance spectrum is analyzed to determine its constituents.
Advantages. FTIR is the most accurate gas technique commonly used, providing good sensitivity and low risk of false alarms. No consumables are involved, so ongoing maintenance costs are less than with other technologies.
Disadvantages. Because of their high cost, FTIR instruments are typically kept in a central location and connected to multiple detection points through sample tubing; samples are pumped from each individual point. As a result, significant lag times can exist between a leak and its detection. In addition, reactive gases (such as HF, Cl2, HCl, and NH3) are easily adsorbed on tubing, which can prevent the instrument from "seeing" a leak. Mechanical failure is also an issue with FTIR instruments — the rotating shutter can wear and/or jam, and the pump can fail.

 POSITIONING GAS DETECTORS

The positioning of gas sensors (or sampling points) is determined by considering the general principles outlined below. However a site survey is invaluable and should be carried out in consultation with a process engineer familiar with the plant and the Health and Safety Officer who has the responsibility for protecting personnel in the area.

 

Density of gas

If the gas has a similar density to air (1.29g/cc) then the detector should be placed at breathing level or at the likely leak point.

If the gas is lighter than air then the detector should be placed near the ceiling. 

If the gas is heavier than air the detector should typically be placed 18 to 24 inches above the floor.

 Consideration should be given to the effects of temperature on the density of the gas. Heating or cooling a gas by 30⁰C will change the density by roughly 11%.

 Air Movement

The movement of air will affect the distribution of gases and therefore consideration should be given to placing the detector where the greatest amount of gas will be carried.  

It should be remembered that air-handling will not always be operational.

Consideration should also be given to the fact that if a gas leaks under pressure the location of the leak will determine the distribution of the gas. If a gas leaks under pressure a jet may carry the gas away from a sensor positioned locally, e.g. directly above a valve. Point detection should be considered as an extra to area detection, not as a replacement.

Common locations for leaks include pump and compressor seals, valve seals, instrumentation sources, gaskets, and sample points.

 Ignition Source 

The possible sources of ignition should be identified when detecting combustible gases. The detector should be placed between the potential source of the gas and the likely ignition source.

 Personnel

The likely location of personnel should be identified when detecting toxic gases. The detector should be placed between the potential source of the gas and the likely location of personnel.

Environment

Consideration should be given to the possibility of an inhibitor gas in the area which would interfere with the detectors ability to detect the target gas or a ‘poisonous’ gas which might poison the detector.

Mounting

Detectors should be securely mounted to guard against vibration, and positioned so as to avoid damage. Consideration should be given to access for maintenance.

 SERVICING GAS DETECTORS

As with all safety equipment it is vital that the detection will operate correctly when called upon to do so and to this end regular maintenance as directed by the manufacturer is essential.

In particular calibration of the detector should be regularly checked. Some detectors have a finite life-span after which they should be replaced. In this case the life span is dependent on the environment and its end will become apparent as calibration becomes more difficult. Some detectors can be saturated after exposure to a large amount of gas and may need to be replaced if they do not recover.

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