Understanding Gas Detection

As we live and breathe, we live at the bottom of an invisible ocean called the atmosphere (remember the word), a layer of gases surrounding the earth.

Earth’s atmosphere is one of the main components of its interdependent physical systems, composed of 78% nitrogen, 21% oxygen and 1% other gases. These gases are found in the various layers (troposphere, stratosphere, mesosphere, thermosphere, and exosphere) and are defined by unique features such as temperature and pressure. In the 1% of gases, we have carbon dioxide, a gas we hear a lot of in the news given its role in driving climate change, argon (a gas that is used in welding), helium, and neon.

The hazard

Given the sectors in operation across the globe, from oil and gas to industrial processing, the utility sector, to mining, to waste, to the emergency services; all who work within those industries recognise the criticality of the risks of various gases, or indeed the lack of needed gases. These industries are likely to expose professionals to toxic atmospheres, flammable atmospheres, deliver oxygen deficiency or enrichment; all of which may be fatal to those exposed, unless these can be detected.

The basics of gas detection

Some time ago (approx. eight years) I wrote an article on gas detection through the ages and identified that since the early days of mining, we have understood the risk posed by a substance that you cannot hear, see, taste, rarely smell or touch. That same threat is still there today but like in many areas we have come a long way from relying on canaries to alert us to the presence of a harmful gas.

Reliance on the canary was basic enough in terms of gas detection. If the canary, who is much more susceptible to gases, and react more quickly and visibly than us humans, swayed or fell from its perch, it alerted the miners to the presence of gas, allowing them to evacuate to safety. Thus the beginnings of an industry that has moved from animals to automated detection was born.

The basic idea seemed simple enough, though there is always that lingering issue, is there sufficient gas present to trigger a form of response, and subsequently today’s modern detection systems use different reactions to determine the presence of gases. Subsequently, not all gas detection is created equal, and why would they when each serves a different purpose. One gas detector may be designed to specifically identify one gas and would be useless for detecting others. Therefore, gas detection has evolved to meet the needs of the industries which they serve.

“we have come a long way from relying on canaries”

Types of detectors

As stated earlier, gas detectors are not equal, they serve various purposes within the various industries in which they operate. Many industries will often have a good idea of what gases they’re likely to encounter at a given location, so when this is the case operatives can use ‘single-gas’ meters geared for the specific gases to be encountered.

However, what happens if we are likely to encounter more than one type of gas? Do you expect operatives to carry more than two, three or even four gas detectors? Not really a practical solution this is it?

The answer to this is ‘multi-meters’, which detect the presence of gases in an area typically before workers enter. They monitor the air quality and allow personnel to properly vent spaces before entering. Additionally, multi-meters also detect oxygen deficiency and enrichment, whilst also detecting potentially explosive and toxic gases.

Due to the sensitivity of the multi-meter, using it prior to entry, its activation prevents operatives entering an area until the atmosphere is safe and allows the workers to remain safe by constantly monitoring levels during the conduct of activities.

Whilst there are the above personal gas detection systems, there are also mobile and static detection, which operate in a simple way, but provide extra levels of protection for plant and personnel, they can be monitored remotely, and provide alarms within plants.

So there you have it, fixed, portable or transportable systems, all have their own key point of safety. A ‘fixed’ detector is permanently installed in a chosen location to provide continuous monitoring of plant and equipment. It is used to give early warning of leaks from plant containing flammable gases or vapours, or for monitoring concentrations of such gases and vapours within plant. Fixed detection is particularly useful where there is the possibility of a leak into an enclosed or partially enclosed space where flammable gases may accumulate.

“fixed, portable or transportable systems, all have their own key point of safety”

A portable detector usually refers to a small personal, handheld device that can be used for testing an atmosphere in a confined space before entry, for tracing leaks, or to give early warning of flammable gas or vapour when hot work is conducted.

Transportable detection is equipment that is not intended to be hand-carried but readily moved from one place to another. One of its main purposes is to monitor an area while a fixed gas detector is undergoing maintenance.

Going further, you have Point or Open-Path detectors, with the point detectors measuring the concentration of the gas at the sampling point of the instrument, normally measuring:

• percentage volume ratio
• percentage lower explosion limit (LEL) for flammable gases
• ppm or mg/m³ for low level concentrations (primarily for toxic gases)

Open-Path detection, also called beam detectors, typically consist of a radiation source and a physically separate, remote detector. The detector measures the average concentration of gas along the path of the beam.

Portable and transportable detectors are always ‘point’ detectors, while fixed gas detectors can be either type.

Standards and regulations

With gas detectors used to detect potential hazards in the atmosphere, be they oxygen deficiency, toxic gas build-up or the accumulation of potentially explosive gases, it is imperative that explosion protection is as important when dealing with flammable gases and vapours, and this especially applies not only to the equipment use in such areas, but also applies to the gas detectors themselves. Detectors are categorised as electrical equipment, they must fulfil the relevant requirements for operating in potentially explosive areas (within the European Union (EU) this is regulated by using harmonised European Directives).

The EU, potentially lead the way in the development of standards and regulations aimed at protecting its workforce. For gas detection, this is a critical element and there are several aspects which need to be identified either on the equipment or indeed in its documentation. Such markings consist of (not exhaustive):

• CE marking (a legal marking indicating the equipment conforms to the requirements of the European directives)
• Equipment group – identifies whether use is for mining or other industry
• Equipment group and category – this element identifies the hazardous area characteristics and potential combustible area, e.g. are gases present continuously, are likely to occur during normal operation, or not likely to occur
• Type of explosive atmosphere –gas, mist, vapour, dust
• Types of protection – primary/secondary protection measure; field of application and the safety concept, along with all the relevant standards associated
• Equipment protection levels – identifies the level of protection assigned to the equipment and for what atmosphere, e.g. gas or dust
• The representative aspect of type of gas or dust

Detectors in use

If any specified gas concentration or set point is exceeded, then the detector system should trigger an alarm. The alarm should not stop or reset unless a deliberate action is taken (BP’s Texaco City disaster in 2005 is a primary example, whereby deficiencies in BP’s mechanical integrity programme resulted in the ‘run to failure’ of process equipment at the refinery¹).

Any alarm should be audible and, or visible, and again, within the EU there are performance standards for such alarms. In portable detection, the alarm is a component of the detector; whilst additionally an ‘at-fault’ alarm is also important because, if a detector fails, it could falsely indicate a ‘safe condition’, e.g. showing a zero reading. Furthermore, there is typical a low-battery alarm on portable instruments, with details of battery life provided in the technical data as supplied by the manufacturer.

When should the detector alarm?

Detectors are set to alarm at a low enough level to ensure the health and safety of the operatives engaged, but high enough to prevent false alarms. In determining the required levels for fixed gas detection systems the following is taken into account:

• Industry standards and recommendations
• Lower explosive limit of the gas/vapour
• Size of potential leak and the team to reach a hazardous situation
• Whether area is occupied
• Time required to respond to alarm
• Actions to be taken following the alarm
• Toxicity of gas/vapour

Actions for gas detector alarming

The purpose of detection is to provide warning of an issue. The actions to be taken if an alarm sounds need to be considered prior to the system being put to use. An emergency plan should be documented detailing the procedure on requirements for evacuating, rescuing or other condition. This should be supported with training of all operatives, and refresher courses.

It is considered ‘best practice’ to ensure that your gas detection has two alarm settings, which provides an advantage. If the alarm is set with two levels, the lower level could be for personnel to stop their work and put on respiratory protective equipment (RPE), investigate the issue and determine if a solution is possible and that it is easily rectified. With any alarm at the higher-level, this should be to conduct the emergency procedures e.g. plant shutdown, evacuation etc.

Fixed detection can be designed to automate plant/process shutdown, increase ventilation etc., though these are not sufficiently reliable.

Inspection, maintenance and calibration

As with anything within the health and safety domain, it needs to be subject to inspection, periodic maintenance, and calibration. The performance of detectors deteriorates with time, with rates depending on the sensor and the operating conditions it encounters. These factors will influence any inspection frequency, maintenance and calibration needs.

Pre-use inspection by the operative is also a necessity for portable detectors. They should only be used if they are in good condition and functioning correctly. They should be inspected for damage, like dents, kinks, bends, blockages and holes in probes. Damaged batteries, cracks to housing could make the instrument unsafe or unreliable, or indeed both. Contamination is also another element which can lead to false readings or false alarms.

“contamination can lead to false readings or false alarms”

Detection in domestic use

In addition to industry, gas detection systems are valuable allies for our and our family’s safety in the home. Many homes now use natural gas as a convenient source of energy (typically across Europe and the United States). Gas is usually safe, but like anything else, under certain circumstances such as improperly installed or poorly maintained gas lines, gas leaks can turn into major issues for health and the home.

With natural gas a fossil energy source that comes from the earth’s surface, it is made of methane, as well as non-hydrocarbon gases and other natural gas liquids. The dangers of a leak and the presence of methane can cause a host of problems, and although natural gas is non-toxic, in certain conditions, natural gas can result in:

• Dizziness or asphyxiation when high concentrations gather in confined spaces
• Flammable mixtures can explode when exposed to open air
• Deaden vegetation, houseplants and trees
• Property damage

The most efficient way to find out if the home has a gas leak or fires are burning inefficiently is to use gas detection in the home, but it is critical where you place them. Typically for the home environment you want to look at elements like:

• Propane and natural gas detection
• Carbon monoxide detection
• Carbon monoxide and explosive gas detection
• Radon detection

Conclusion

There are many different types of industrial and domestic gas detectors, and each has advantages and disadvantages. It is important to apply the correct technology for the selected application. Point detectors are the most common types and are widely used across industry. There are lots of manufacturers that manufacture these detectors and certify them to the various industry standards.

Equally, it is important to consider the power requirements, mounting requirements, standoff distances from other units, operating temperature ranges, gas calibration limitations, accuracy, and the response time at different concentrations. There is little general guidance regarding the design and placement of detection system, with many facilities relying on experience-based designs. It has been shown that designing detection systems based on geographic coverage may result in gaps. Thus an ideal quantitative detection design would be scenario based, and examine such factors as release location, release orientation and wind direction, size of release from process, trajectory, detector elevation, detector uncertainty (potentially under FMEA) and weather effects. However, the latter approach would require extensive dispersion modelling, therefore, designing or auditing detection systems in conjunction with a facility siting study or quantitative risk assessment is the practical approach.

References

1 www.csb.gov/file.aspx?DocumentId=5596