Gas hazards in the oil, gas and petrochemical industries

Although flammable and toxic gas hazards are generally well understood by operators, technicians and safety personnel in the oil, gas and petrochemical industries, continuous training and refreshment of knowledge is essential to avoid potential incidents caused by complacency. New personnel are often assigned work activities in potentially hazardous areas with only very limited training about gas hazards and the use of gas detection equipment.

This article offers a basic introduction to gases and associated hazards in the oil, gas and petrochemical industries.

A worker at a petrochemical plant

A worker at a petrochemical plant

What is Gas?
Whilst different gases have different densities, they do not totally separate into layers according to their density. Heavy gases (e.g. hydrogen sulphide) tend to sink and light gases (e.g. methane) tend to rise, but their constant motion means that there is continuous mixing.

So, in a room where there is a natural gas (methane) leak, the gas will tend to rise because it is lighter than air, but the constant motion means that there may be a considerable concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, mixing will be increased.

Air is a mixture of gases, but because its composition is reasonably constant it is usually considered as a single gas, which simplifies the measurement of toxic and flammable gases for safety and health applications.

Combustion of Gases
Most organic compounds will burn. Burning is a simple chemical reaction in which oxygen from the atmosphere reacts rapidly with a substance, producing heat. The simplest organic compounds are hydrocarbons, which are the main constituents of crude oil and gas. Hydrocarbons are composed of carbon and hydrogen, the simplest hydrocarbon being methane, each molecule of which consists of one carbon atom and four hydrogen atoms. It is the first compound in the family known as alkanes. The physical properties of alkanes change with increasing numbers of carbon atoms in the molecule: those with one to four being gases, those with five to ten being volatile liquids, those with 11 to 18 being heavier fuel oils and those with 19 to 40 being lubricating oils. Longer carbon chain hydrocarbons are tars and waxes.

When hydrocarbons burn they react with oxygen from the atmosphere to produce carbon dioxide and water (although if the combustion is incomplete because of insufficient oxygen, carbon monoxide will result as well).

More complex organic compounds contain elements such as oxygen, nitrogen, sulphur, chlorine, bromine or fluorine and if these burn, the products of combustion will include other compounds as well. For example, substances containing sulphur such as oil or coal will result in sulphur dioxide whilst those containing chlorine such as methyl chloride or polyvinyl chloride (PVC) will result in hydrogen chloride.

In most industrial environments where there is the risk of explosion or fire because of the presence of flammable gases or vapours, a mixture of compounds is likely to be encountered. In the oil, gas and petrochemical industries the raw materials are a mixture of hydrocarbons and chemicals, some of which may be being altered by a process. For example crude oil is separated into many materials using processes referred to as fractionation (or fractional distillation); fractions are further converted using processes such as ‘cracking’ or ‘catalytic reforming’. Flammable hazards are therefore likely to be represented by many substances on a typical petrochemical refining plant.

Explosive Risk
In order for gas to ignite there must be an ignition source, typically a spark (or flame or hot surface) and oxygen. For ignition to take place the concentration of gas or vapour in air must be at a level such that the ‘fuel’ and oxygen can react chemically. The power of the explosion depends on the ‘fuel’ and its concentration in the atmosphere. The relationship between fuel/air/ignition is illustrated in the ‘fire triangle’.

Fire Triangle

Fire Triangle

The ‘fire tetrahedron’ concept has been introduced more recently to illustrate the risk of fires being sustained due to chemical reaction. With most types of fire the original fire triangle model works well – removing one element of the triangle (fuel, oxygen or ignition source) will prevent a fire occurring. However, when the fire involves burning metals like lithium or magnesium, using water to extinguish the fire could result in it getting hotter or even exploding. This is because such metals can react with water in an exothermic reaction to produce flammable hydrogen gas.

Fire Tetrahedron

Fire Tetrahedron

Not all concentrations of flammable gas or vapour in air will burn or explode. The Lower Explosive Limit (LEL) is the lowest concentration of ‘fuel’ in air which will burn and for most flammable gases it is less than 5% by volume. So there is a high risk of explosion even when relatively small concentrations of gas or vapour escape into the atmosphere.

LEL levels for gases and vapours are defined in various international standards. The original long-established standards measured the LEL points using a static concentration of gas. More recent European and international standards list LEL levels measured using a stirred gas mixture: some substances are more volatile when in motion and represent an explosive risk at lower concentrations than indicated on previous ‘static’ tests. Methane is the most commonly occurring flammable gas in industry: the long-established Lower Explosive Limit is 5% in air, the ‘new’ LEL recognized in Europe and other territories is 4.4%LEL and calibration practices has been changed accordingly.

The propane vapour LEL is affected to an even greater degree: the ‘old’ LEL value being 2-2.2% in air (depending on which standard is referenced), the ‘new’ LEL being 1.7% in air. A more comprehensive list of affected gases and vapours can be viewed at’.

Flammable Gas Risk
Flammable gas detection equipment is generally designed to provide a warning of flammable risks before the gas reaches its lower explosive limit. The first alarm level is generally set at 20% LEL, with a second-stage alarm at 40-60%LEL. In some applications such as gas turbine monitoring alarms may be set as low as 5%LEL.

Toxic Gas Risk
Gases and vapours released from oil, gas and petrochemical processing activities can, under many circumstances, have harmful effects on workers exposed to them by inhalation, being absorbed through the skin, or swallowed. People exposed to harmful substances may develop illnesses such as cancer many years after the first exposure. Many toxic substances are dangerous to health in low ‘ppm’ (parts per million) or even ppb (part per billion) concentrations.

Given that 10,000 ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.

It is worth noting that most flammable gas hazards occur when the concentration of gases or vapours exceed 10,000ppm (1%) volume in air or higher. In contrast, toxic gases typically need to be detected in sub-100ppm (0.01%) volume levels to protect personnel.

Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Hydrogen sulphide for example is particularly hazardous; although it has a very distinctive ‘bad egg’ odour at concentrations above 0.1ppm, exposure to concentrations of 50ppm or higher will lead to paralysis of the olfactory glands rendering the sense of smell inactive. This in turn may result in the assumption that the danger has cleared. Prolonged exposure to concentrations above 50ppm will result in paralysis and death.

Definitions for maximum exposure concentrations of toxic gases vary according to country. Limits are generally time-weighted as exposure effects are cumulative: the limits stipulate the maximum exposure during a normal working day.

Alarm Levels
It is important to note that whereas portable gas detection instruments measure and alarm at the TWA (time-weighted average) levels, instantaneous alarms are also set at the same numerical values to provide early warning of an exposure to dangerous gas concentrations.  Workers are often under risk of gas exposure in situations where atmospheres cannot be controlled, such as in confined space entry applications where alarming at TWA values would be inappropriate.

Gas Detection Systems
Both flammable and toxic gases pose serious hazards in oil, gas and petrochemical processing facilities. There can be a very diverse range of gases depending on the application. Multi-gas mixtures are also a common danger, especially in confined spaces. Fixed gas detectors can be positioned in strategic zones, and operatives undertaking maintenance or cleaning work, for example in confined spaces or ‘hot work’ areas, should always be equipped with portable gas detectors.

Most gas detectors should be calibrated every six months to ensure optimum operation. However, a new range of IR (infrared) detectors allow users to extend maintenance checks to once every 12 months – and this only requires a ‘gas test’, not full re-calibration, which is more time consuming. ‘Bump-test’ stations and intelligent instrument management hubs also enable simple day-to-day testing of portable gas detectors and easy management of maintenance cycles.

Crowcon's Tetra 3 four gas portable gas detector

Crowcon's Tetra 3 four gas portable gas detector

Future Trends
It is likely that both portable and fixed hydrocarbon gas detectors will use IR sensors rather than the traditional catalytic bead (pellistor) sensors currently used in most detectors. IR sensors provide increased reliability, more dependable operation and increased lifetimes when compared to pellistors. The cost of IR sensors has fallen in the past few years, and a commercial case can easily be made for switching to IR technology.

Sensor technologies such as PID (photo-ionisation) are being used more commonly as requirements for monitoring levels of VOCs (volatile organic compounds) in industry increase. Optical sensing developments and solid-electrolyte sensors will provide solutions in toxic gas and oxygen sensing applications where traditional electrochemical cells have operating limitations.

Portable gas detectors will include features appropriate to a diverse range of applications with extended battery life and connectivity to other types of portable devices and control systems.

Fixed detection systems will also continue to utilise a variety of technologies (such as point-type detectors, open-path detectors, acoustic sensors and even gas cameras) for the most comprehensive coverage. Wireless connectivity will replace cables in some applications.

6 Responses to “Gas hazards in the oil, gas and petrochemical industries”

  1. AKIO HOPE says:

    Thanks foe this brief but educating article. I will really need some more detailed materials on this subject, am presently working as a SAFETY HEAD of my firm.



  2. damian says:

    Dear Akio

    Thanks for your response. I will ask someone from Crowcon to contact you with more information.

    Kind regards

    Damian Corbet

  3. YOUSAF IMRAN says:


    dear i am working with Drilling and Exploration company in pakistan,
    here i need your guidness kindly tell me in detail that which type of fire extingushir here in processing area and for which type of fire and as well as for gas also,
    which type of prcation we will take here ??

    Yousaf Imran.

  4. damian says:

    Dear Yousaif

    Thanks for your response. I will ask someone from Crowcon to contact you with more information.

    Kind regards

    Damian Corbet

  5. ganesh says:

    good day, i would like to know the list of toxic gases in oil & gas sector and its properties & TLV. thank you.


  6. damian says:

    Dear Ganesh

    Thanks for your enquiry. You can find more information about toxic gases in the oil and gas sector here: .

    Best regards

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