Why Use Gas Detectors?
Published: 01st Jan 2006
Gas detectors perform a valuable role in the workplace. Emerging technologies can enhance their effectiveness. Gas detectors form an important part of safety systems to help protect users from the effects of explosion, fire or ill-health (acute, i.e. short term and chronic, i.e. long term) arising from flammable, toxic or asphyxiant gases.
They are predominantly used to trigger alarms if a specified concentration of gas is exceeded and measure workers’ exposure to gases. This can provide early warning of a problem and help ensure worker’s safety and health, and protect equipment. However, a detector does not prevent leaks occurring or indicate what action should be taken. It is not a substitute for safe working practices and maintenance.
The primary driver for the use of gas detectors in the workplace is legislation. In the EU, member states must enact directives through their own regulations. Examples of directives most relevant to gas detection, although they might not directly refer to gas detectors but indirectly through equipment safety, control and monitoring, are:
- ATEX (“product”) 94/9/EC manufacture and distribution of equipment and protective systems intended for use in potentially explosive atmospheres
- ATEX (“worker protection”) 1999/92/EC installation and use of equipment covered by the above directive and requires amongst other things that employers should prevent and provide protection against explosions
- Chemical Agents Directive 98/24/EC Protection of the health and safety of workers from the risks from chemical agents, eg risk assessments for chemical agents; prevent/control exposure; eg where required by the safety and health document, monitoring devices measuring gas concentrations at specified places automatically and continuously, automatic alarms and devices to cut off power automatically from electrical installations and internal combustion engines must be provided
- Carcinogens Directive 90/394/EEC Protection of workers against risks arising specifically from exposure to carcinogens and mutagens, eg to establish exposure limit values and to take preventive measures
- IOELV Directives (1st 2000/39/EC, 2nd to be published) Indicative Occupational Exposure Limit Values (IOELVs) are intended to be health-based. EU member states are now required to set national limits which must take into account the IOELV
- Surface and underground mineralextracting industries 92/104/EEC improving the safety and health protection of workers, eg where required by the safety and health document, monitoring devices measuring gas concentrations at specified places automatically and continuously, automatic alarms and devices to cut off power automatically from electrical installations and internal combustion engines must be provided
- Mineral-extracting industries (drilling) 92/91/EEC improving the safety and health protection of workers, eg where required by the safety and health document, monitoring devices measuring gas concentrations at specified places automatically and continuously, automatic alarms and devices to cut off power automatically from electrical installations and internal combustion engines must be provided
- Personal protective equipment Directive 89/656/EEC assessment, selection and correct use of personal protective equipment
Other relevant regulations which have been put in place in various countries include those for working in confined space.
Standards and guidance
The above EU directives define the “essential requirements”, e.g., protection of health and safety, that must be met as part of workplace activities or when goods are placed on the market. Products manufactured in conformity with harmonised standards2 are presumed to be conformant to these essential requirements. But standards are not mandatory, they remain voluntary. Alternative paths for conformity are possible but there is an obligation to prove conformity to the essential requirements. In recent years the harmonisation of standards throughout Europe has led to the publication of many new standards covering performance and use of gas detecting instruments and systems.
For gas detectors, in addition to the performance standards, there are associated guides for use which help the user to select the right equipment, install it if appropriate, use it properly and maintain it so that it remains fit for purpose.
Gas detection in confined spaces
One of the most critical uses of a gas detector is for working in confined spaces. Typically multigas instruments are deployed for flammable gases (eg methane, LPG, petrol), oxygen, and various toxic gases including carbon monoxide, hydrogen sulphide, carbon dioxide, volatile organic compounds (VOCs). Gas in a confined space arises from various sources:
- Gas may remain from previous processing or as a result of previous storage, or arise from sludge or other deposits disturbed, for example during cleaning
- VOCs may be present under scale even after cleaning
- Gas may enter the space from adjoining plant that has not been effectively isolated
- Gas can build up in sewers, manholes, contaminated ground or leak from behind vessel linings, rubber, lead, brick etc
- Vapour can be produced by work inside the confined space, for example, welding, flame cutting, lead lining, brush and spray painting, or moulding using glass reinforced plastics (GRP), use of adhesives or solvents, or from the products of combustion
- Vapour can occur inside a compartment or space by hot work taking place on the exterior surfaces or enter the space from equipment in use outside the space, such as exhaust fume from mobile plant, especially on construction sites
- Plant failure can also cause problems; for example, by the build-up of ammonia if refrigeration plant fails or the potential for accumulation of carbon dioxide in some pub cellars following leaks from compressed gas cylinders
Oxygen deficiency may result from, for example:
- Purging of the confined space with an inert gas, eg nitrogen argon, steam
- Naturally occurring biological processes consuming oxygen
- Leaving a vessel completely closed for some time (particularly one constructed of steel) since the process of rust formation on the inside surface consumes oxygen
- The risk of increased levels of carbon dioxide from, for example, limestone chippings associated with drainage operations when they get wet (acid leaching), anaerobic action
- Burning operations and work such as welding and grinding which consume oxygen
- Displacement of air during pipe freezing, for example, with liquid nitrogen
- A gradual depletion of oxygen as workers breathe in confined spaces and where provision of replacement air is inadequate
Oxygen enrichment may be caused by, for example, a leak from an oxygen cylinder forming part of welding equipment or in hyperbaric applications, eg tunnelling from oxygen decompression and treatment chambers, diving.
Confined spaces and areas adjacent to landfill sites or underground coal strata have the potential for both harmfully low oxygen levels and high carbon dioxide levels. There is not however a general, convenient relationship between oxygen deficiency and carbon dioxide enrichment which allows the sole use of an oxygen monitor to provide warning of a oxygen deficiency and carbon dioxide hazard. It may not be obvious when entering a confined space, landfill site etc how any oxygen depletion may have occurred and whether there are potentially dangerous levels of carbon dioxide. Measurement of both oxygen and carbon dioxide should be carried out in order to provide warnings of both oxygen deficiency and high carbon dioxide levels over short and long term exposure periods (Greenham and Walsh, 2003; HSE, 2003).
Personal, portable and fixed monitoring
Personal, portable and fixed monitoring provide different types of information on gas concentration levels and are used for different purposes. Personal monitoring is used to establish the timeweighted average (TWA) concentration of an airborne substance within the breathing zone of the worker; for convenience the monitor is usually located on the upper torso. Measurement by comparison with long term exposure limits (8-hour) should be used as a means of assessing adequacy of control over a typical 8 hour working day. Measurement by comparison with short term exposure limits (15-min) should be used as a means of assessing effects which may occur following exposure for a few minutes.
Portable monitoring is used to protect a worker from flammable, asphyxiant and toxic hazards. The detector is designed to be readily carried from place to place and to be used while carried, typically by a mobile workforce. Multigas detectors are the predominant instruments of choice, providing an alarm in case of emergency.
Fixed (static) monitoring is used for various purposes:
- As a continuous monitoring alarm system for flammable, asphyxiant and toxic gas leaks
- To check the effectiveness of control measures, eg ventilation
- To identify sources of emission
- To determine background workplace contaminant concentrations, but note that it does not accurately reflect the amount that could be inhaled by workers and therefore cannot be used to calculate personal, time-weighted average exposures or indicate peak exposure levels
- Where there are no suitable personal monitoring methods available
- When the wearing of personal monitoring equipment may introduce additional hazards
- For certain chemicals if specifically required by legislation (eg vinyl chloride, which is a carcinogen)
Point and openpath detectors
The use of a light source and detector for measuring pollutant concentration allows measurements to be made either over a small, enclosed pathlength, ie effectively at a point, or over a large distance over an open path. Point detectors measure the concentration of gas at the sampling point of the instrument: in a portable or personal detector (eg for flammable gas, carbon dioxide, nitrous oxide); or fixed installation (eg flammable gas). The unit of measurement can be % volume ratio, % LEL (or LFL) for a flammable gas, and ppm for low level concentrations for toxic gases or leaks of flammable gas. Units of mg/m3 are not very common in workplace gas measurements and are typically used for environmental concentrations.
Open-path detectors, also called beam detectors, measure the average concentration of gas along the path of the radiation beam (mainly infrared but also ultraviolet), typically between 5 and 200 m long. They commonly consist of a radiation source and a physically separate, remote detector. But sometimes a remote mirror is used and the beam is reflected back to the detector which is co-located with the source. The unit of measurement is concentration multiplied by pathlength, ie %LEL.m, %.m or ppm.m. The openpath monitor is unable to distinguish between a high concentration along a small part of the beam or a lower concentration distributed over a longer length. However, with appropriate sensitivity and alarm levels, they are able to provide greater coverage than point detectors over an area where flammable or toxic gas may be present.
Pressurised flammable gas leaks
One effective way to minimise major flammable gas leaks is to detect and then control releases before they develop into larger more hazardous ones. A recent review of gas turbine incidents both offshore and onshore showed that the overall risk of a gas leak igniting is 16%. However, this risk is reduced to 3% if the leak is detected, compared to 57% if it is undetected. This is despite the fact that the undetected leaks are likely to include a much larger proportion of small leaks that are physically harder to ignite.
If there are high pressure pipelines containing gas, as in gas/oil production, then acoustic or ultrasonic detectors are sometimes used to detect leaks. The principle of operation is based on the emission of ultrasound (in the region of 25-70 kHz) from escape of gas from a high pressure pipeline or other pressurised system (typically 10-200 bar). The level can provide a measure of the leak rate, which is detectable down to around 0.1 kg/s. This is a nonconcentration based detector for high pressure leaks. Theoretically, it provides 360° coverage and does not require transport of gas to the sensor. However, care is needed in placement and false alarms may occur due to other ultrasonic emissions.
Some degree of signal processing, eg duration of emission, can be used to minimise some types of interferent emissions. Offshore platforms therefore can have fixed point and open-path, portable and ultrasonic detectors. Furthermore, recent developments have now made it possible to image gas leaks using gas imagers. These monitors can operate in a passive (ie using background radiation not a source), open-path mode, and are able to superimpose the gas concentration along the path over a visual image of the area under surveillance.
Oil mists are generated by the release of flammable liquids under pressure, eg in crankshafts, generators, turbines. Oil mists are very flammable and can ignite at a lower temperature than most hydrocarbon gases. The current evidence is that gas detectors do not seem able to detect oil mist releases by detecting the associated vapour. Most oil mist detectors are based on scattering or absorption of light.
Current issues - Oxides of nitrogen and sulphur dioxide
Exposure to oxides of nitrogen (NOx) commonly arises from diesel engine exhaust emissions and from the use of explosives. In the UK, Workplace Exposure Limits (WELs) for nitric oxide (NO - nitrogen monoxide), nitrogen dioxide (NO2) and sulphur dioxide (SO2) have been withdrawn since 2002 and are currently under review, pending EU directions. While their exposure limits are highly likely to be reduced from previous values it is not yet clear to what levels, though they are likely to be around the 1 ppm mark. This presents a measurement challenge, particularly for nitric oxide, which previously had occupational exposure limits of 25 ppm (8-hr TWA) and 35 ppm (15-min short term exposure limit). HSL has been and is undertaking projects evaluating emerging nitrogen monoxide sensors particularly for use in the tunnel construction industry.
Emerging applications - Alternative fuels
The EU has set the target of 20% use of alternative fuels in road transport by 2020. Three main types of alternative fuels have been identified: biofuels (short-term), natural gas (mid-term) and hydrogen (long-term). There is therefore, a large market potential developing covering not only these fuels and the motive or power generation units that will utilise them, but, perhaps more importantly, the supporting infrastructure for which safety-related issues may well be paramount. The safe transport, storage, distribution and use of new fuels, is therefore of great importance.
Hydrogen in particular has several unique features suggesting that it presents a more serious safety challenge than carbon-containing fuels and some bio-fuels, particularly when used by the general public. It is a colorless, odorless, tasteless and nonpoisonous gas under normal conditions. Some of its important safety related properties are:
- Very wide flammability range (4-77%)
- Very low ignition energy (less than one tenth that of methane, LPG, petrol)
- High energy content per weight (nearly 3 times as much as petrol)
- Burns with a pale blue, almost invisible flame, making hydrogen fires difficult to see
- Possibility of detonation (which causes more damage and is more dangerous than an ordinary explosion)
- Low viscosity (easily leaks from pipes)
- High diffusivity and very much lighter than air (stratifies/layers easily)
- Causes embrittlement of some metals
- Condensation of oxygen-rich liquid air on cryogenic storage systems
Alternative fuels based on carboncontaining fuels can be detected using existing technology (eg catalytic, infrared sensors). It is not envisaged that there would be any major difficulties applying this sensor technology to fuels such as LNG, LPG, methanol and ethanol as the technology is already used to detect these gases/vapours.
For hydrogen, the situation is a little different to that for carbon-containing fuels as current technology has its limitations and hydrogen cannot be detected by infrared. New technologies have, however, emerged and some of these are commercially available and in use, but relatively untried (Castello and Salyk, 2005). There is still much developmental work on these new techniques but existing technology, eg catalytic, has also been adapted to be more specific to hydrogen.
Smaller size, lower power, lower cost, longer life, greater functionality and intelligence of existing sensors can be and are attained by incremental improvements to construction materials and processes. And this has certainly been the case with industrial gas detectors. Over a longer timescale, however, new technologies appear and either replace the older technology or find new niches. Such emerging techniques include optical-based sensors where developments in optical sources, detectors and measurement techniques, particularly in the infrared region, are the source of much activity. For example, LED and laser sources, while still expensive compared to incandescent lamps, are coming down in price and offer greater functionality, ie greater sensitivity and selectivity, particularly for laser sources, with considerably reduced power consumption. Also, optoelectronic sensors which transduce chemical or physical interactions on a receptive surface by means of an optical measurement via fibre optics are receiving much attention, not least because they obviate the need for electrical cabling.
Sensor networks are a current hot topic with researchers and developers. The focus is on the benefits of monitoring over wide areas with wireless networks of low cost sensors, linked to a GPS (global positioning system). On a smaller scale, a local positioning system for the workplace has recently been described (Lee et al., 2005). Indeed, there is a commercial system recently available which has a GPS option, including the ability to track and display readings from up to 32 remotely located gas detectors on a GPS map. Finally, the ability to couple networks of sensors with intelligent processing could allow more rapid detection of leaks, which may otherwise be ignored by a simpler system based on an alarm threshold from essentially independent sensors.
Castello P and Salyk O (2005) Testing of hydrogen safety sensors in service simulated conditions. Proc. Int. Conf. on Hydrogen Safety. Pisa. Sep 8-10, 2005
1 Greenham, L and Walsh, P. (2003) Int. Environmental Technology. Gas detectors in the workplace - The UK Confined Space Regulations, 13, (July 2003), 20-22
2 HSE (2003). Measurement of Oxygen and Carbon Dioxide in Confined Spaces. Toxic Substances Bulletin. Issue 50, Jan 2003. http:// www.hse.gov.uk/toxicsubstances/issue50.htm
HSE (2004) The selection and use of flammable gas detectors. http://www.hse.gov.uk/pubns/ gasdetector.pdf
HSL (2004) http://www.hsl.gov.uk/case-studies/ visual.htm
Lee LA, Soderholm SC, Flemmer MM, Hornsby- Meyers JL (2005). Field test results of an automated exposure assessment tool, the local positioning system (LPS). J. Environmental Monitoring, 7, 736-742.
Rosén G, Andersson I-M, Walsh PT et al (2005) A review of video exposure monitoring as an occupational hygiene tool. Annals Occup. Hyg. 49, 201-217
SIRA (2005). Gas Detector Selection and Calibration Guide. First Edition. Witherbys Publishing.
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Published: 01st Jan 2006 in Health and Safety International