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:
| CH4 methane (gas) | C6H14 hexane (liquid) |
| C2H6 ethane (gas) | C7H16 heptane (liquid) |
| C3H8 propane (gas) | C8H18 octane (liquid) |
| C4H10 butane (gas) | C9H20 nonane (liquid) |
| C5H12 pentane (liquid) | C10H22 decane (liquid) |
Alkenes are similar but their molecular structure includes double bonds (examples are ethylene and propylene). Alkynes contain triple bonds (example is acetylene). The above compounds are all known as aliphatics. Aromatic hydrocarbons such as benzene have a ring molecular structure and burn with a smoky flame.
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) and ‘cracking’. Some of the substances produced.
Explosive Risk
In order for gas or vapour to ignite there must be an ignition source typically a spark, or flame or hot surface. For ignition to take place there must be an explosive mixture. This means 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 old 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.
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.
Inerting
In order to prevent explosions during shutdown and maintenance operations many industrial processes employ an inerting procedure. For example nitrogen may be used to purge a vessel (for example a fuel tanker, or storage tanks on an oil tanker) of hydrocarbons before carrying out maintenance or repair work. Before entry by personnel, the vessel is then purged with air. Crowcon has special instrumentation to monitor this process to ensure efficient inerting and alert operators to the presence of potentially dangerous mixes of air, nitrogen and hydrocarbons during maintenance operations.
Standards defining LEL concentration
Safety procedures are generally concerned with detecting flammable gas before it reaches its lower explosive limit. There are two commonly used standards which define the ‘LEL’ concentration for flammable substances: ISO10156 (also referenced in the superseded standard EN50054), and IEC60079-20:2000 (also referenced in BS EN61779-1:2000). The IEC (International Electrotechnical Commission) is a worldwide organization for standardization. Historically, the flammability levels have been determined by a single standard: ISO10156 (Gases and gas mixtures- Determination of the fire potential and oxidizing ability for the selection of cylinder valve outlets).
Recent IEC and EU (European) standards (IEC60079 and EN61779) define LEL concentrations measured using a ‘stirred’ concentration of gas (as oppose to the ‘still’ gas method employed in ISO10156). Some gases/ vapours have proven to be more volatile when stirred, and the resultant LEL’s vary between the two standards for some gases/vapours.
The table on the following page shows some of the notable differences in LEL values between the two standards. It can clearly be seen that 50% LEL of methane in EN61779 calculates to a 2.2% volume concentration in air, as oppose to 2.5% volume as stated in ISO10156. Therefore if a detector is calibrated according to EN61779 using a mixture of 50% LEL methane made to ISO 10156, a 13.6% error would occur potentially invalidating the calibration. The error could even be greater for non-linear infrared detectors.
The European ATEX Directive (covering the certification and use of equipment in flammable atmospheres), stipulates that manufacturers and users comply with the EN61779 standard. Crowcon’s policy is to move to the new values of LEL in Europe and territories that adhere to European Standards. However as the old standard is still used in the US and other markets we will continue to calibrate to ISO 10156 in these territories. ATEX/IECEx certified Crowcon products will be supplied calibrated according to the IEC60079/EN61779 standards (ie methane sensors will be calibrated such that 100% LEL = 4.4% Volume). UL/CSA certified products will be calibrated according to the ISO10156 standard (ie methane sensors will be calibrated such that 100% LEL = 5% volume) unless a customer stipulates otherwise.
Alarm Levels
Flammable gas detection systems are designed to create alarms before gases/vapours reach an explosive concentration. Typically the first alarm level is set at 20% LEL (although there are industries that prefer 10%LEL; particularly Oil and Gas companies). Second and third alarm levels vary according to the type of industry and application, but are commonly set to 40% LEL and 100% LEL respectively.