Although flammable and toxic gas hazards are generally well understood by operators, technicians and safety personnel working within the petrochemical industry, continuous training and refreshment of knowledge is essential to avoid potential incidents linked to complacency or misguided 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 in the petrochemical industries.
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 (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, 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 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 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 petrochemical industry the raw materials are a mixture of chemicals which 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 represented by many substances on a typical petrochemical refining plant.
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’.
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.
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 are currently defined in three standards: ISO10156, EN61779-20 and IEC60079. The ‘original’ ISO standard lists LELs obtained when the gas is in a static state. LELs listed in the EN and IEC standards were obtained with a stirred gas mixture; this resulted in lower LEL’s in some cases (i.e. some gases proved to be more volatile when in motion).
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.
Gases and vapours released from 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 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 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.
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.
Crowcon Gas Detection Systems
Both flammable and toxic gases pose serious hazards in petrochemical processing facilities. There can be a very diverse range of gases depending on the process application, including methane (CH4), hydrogen sulphide (H2S), nitrogen oxides (NOx), chlorine (Cl2) and oxygen (O2). 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 fitted with portable gas detectors.
Crowcon offers a very wide range of portable and fixed gas detectors for virtually any application in the petrochemical industries. The company also offers control systems for monitoring multiple arrays of fixed detectors, and gas sampling systems which use pumps or compressed air-driven vacuum generators to extract air/gas samples from the area to be monitored and present the samples to one or more gas sensors.
Most gas detectors, including Crowcon’s, 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, such as Crowcon’s Checkbox, also enable simple day-to-day testing of portable gas detectors and easy management of maintenance cycles.
All Crowcon’s instruments are designed with for a minimum 10 year life-span. Sensor life depends on technology, with 2-3 years typical for electrochemical cells and oxygen sensors and 5 years+ for IR sensors.
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 life-times 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.
Crowcon is leading the way in this new technology. Its new IR flammable gas detector, the IREX, was specifically designed to replace pellistor type flammable gas detectors and results in significantly faster response times and greatly reduced zero drift compared to pellistor detectors. Capable of detecting methane, butane, propane and many other hydrocarbons, the IREX is specifically designed for applications such as offshore platforms, refineries, gas storage and distribution networks, sewage treatment plants and certain manufacturing processes (such as aerosol production).
Crowcon, a subsidiary of Halma p.l.c., is a world leader in portable and fixed gas detection instruments. Formed in 1970, the company is based in Oxfordshire in the UK and has branch offices in the Netherlands, the USA, Singapore and China. It specialises in developing, manufacturing and marketing innovative, reliable and cost-effective flammable and toxic gas detection equipment and has constantly led the field with products designed for safety and environmental monitoring.
Crowcon’s products are sold throughout the world, serving oil, gas and petrochemical companies, public utilities, clean water and sewage treatment companies, fire brigades, construction companies and other organisations where accidental leakage of gas or vapour can become a toxic or explosive danger. The company is represented in India by Detection Instruments Pvt. Ltd. in Mumbai (www.detection-india.com).
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