Gas hazards in the oil and gas industry

By Andy Avenell, Product Manager – Fixed Systems, Crowcon Detection Instruments Ltd

“It’s 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”

Although flammable and toxic gas hazards are generally well understood amongst operators, technicians and safety personnel working within the oil and gas industry, continuous training and refreshment of knowledge is essential to avoid potential incidents linked to complacency or mis-guided actions.

New personnel may be assigned work activities in potentially hazardous areas with only very brief training in the basics of gas hazards and the operation of detection equipment. The following is a basic introduction to gases and associated hazards.

1. What is Gas?

The name gas comes from the word chaos, which neatly summarises the main feature of the simplest state of matter. A gas is a swarm of particles moving randomly and chaotically, constantly colliding with each other and the walls of any container. The real volume of the particles is minute compared to the total space which they occupy and this is why gases fill any available volume and are readily compressed. The average speeds of gas molecules are of the order of 100's of metres per second and they are colliding with each other billions of times per second. This is why gases mix rapidly and why they exert pressure.

This constant motion is easily demonstrated by releasing a small amount of odorous gas in a room. Within seconds the gas can be smelt in all parts of the room. These properties apply to substances, which are normally gaseous, and to vapours from evaporated liquids.

A volume of any gas at the same temperature and pressure contains the same number of molecules irrespective of what the gas is. This means that measuring gas by volume is very convenient. Gas measurements at higher levels are in % (volume) and at lower levels parts per million, ppm (volume).

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

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 will be a considerable concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, the mixing will be increased.

Air is a mixture of gases, typically:

Because its composition is reasonably constant, air is usually considered as a single gas, which simplifies the measurement of toxic and flammable gases for safety and health applications.

2. Combustion of Gases and Vapours

Most organic chemical 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 those known as hydrocarbons and these are the main constituents of crude oil/gas. These compounds 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 number 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. The first ten alkanes are:

When hydrocarbons burn they react with oxygen from the atmosphere to produce carbon dioxide and water (although if the combustion is incomplete because there is 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 petrochemical industry the raw materials are a mixture of chemicals, many of which are decomposing naturally or are being altered by the processes. 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 presented by many substances on a typical oil refining plant.

Explosive Risk
In order for gas or vapour 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.

The Fire Tetrahedron concept has been introduced 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.

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 and vapours 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.

Note: the LEL of a substance may also be referred to as the 'LFL': Lower Flammable Limit.

The Upper Explosive Limit (UEL) is the maximum concentration of 'fuel' in air which will burn. Concentrations above the UEL will not burn because there is insufficient atmospheric oxygen available.

LEL levels are defined in three standards currently: ISO10156, EN61779-20 and IEC60079. The 'original' ISO standard lists LEL's obtained when the gas is in a static state. LEL's listed in the EN and IEC standards were obtained with a stirred gas mixture; this resulted in lower LEL's in some cases (ie some gases proved to be more volatile when in motion).

Example LEL levels and other gas properties are shown in the following table.

Alarm Levels
Flammable gas detection equipment is generally designed to provide a warning of flammable risks before the gas or vapour 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.

3. Toxic Risk

Gases and vapours released from oil and gas production and processing activities can under many circumstances have harmful effects to workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Persons exposed to harmful substances may develop illnesses (for example; cancer) many years after the first exposure. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). 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's 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 of concentrations above 50ppm will result in paralysis and death.

Definitions for maximum exposure concentrations of toxic gases vary according to country; common examples and their meanings are summarised below. Limits are generally time-weighted as exposure effects are cumulative: the limits stipulate the maximum exposure during a normal working day.

UK, Health & Safety Executive (HSE):

WEL: Workplace Exposure Limit, toxic gas exposure limits are defined as:

STEL (short-term exposure limit): 15 minute exposure limit

LTEL (long-term exposure limit): 8 hour exposure limit

USA- Occupational Safety and Health Administration (OSHA)

PEL (permissible exposure limit): 8-hour time weighted average (TWA)

Note: WELs can be expressed as parts per million (ppm) and milligrams per cubic metre (mg/m³) if the substance exists as a gas or vapour at normal room temperature and pressure. Compounds that do not form vapours at room temperature and pressure are expressed in mg/m³ only.

Alarm Levels
It is important to note that whereas portable gas detection instruments measure and alarm at the TWA (time-weighted alarm) 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.

The following data has been extracted from the HSE document: EH40 and OSHA standard 29 CFR 1910.1000 for some common toxic gases:

Workplace Exposure Limits:

Note 1: NO alarm levels currently advised in the UK are based on those defined for tunnelling operations.

Note 2: NO₂ and SO₂ TWA levels are currently subject to temporary Chemical Hazard Alert Notices (CHAN's) issued by the UK Health and Safety Executive.

*TLV-C- Ceiling: the concentration that should not be exceeded during any part of the working exposure.

References:
• Health and Safety Executive: EH40/2005 Workplace exposure limits
• Occupational Safety and Health Administration: standard 29 CFR 1910.1000 Table Z-1
• www.talkinggas.com

Author:
Andy Avenell, Product Manager – Fixed Systems
Crowcon Detection Instruments Ltd
T: +44 (0)1235 557700
E: sales@crowcon.com
www.crowcon.com