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). They have more energy per molecule and so burn hotter. They are also more valuable in the manufacture of other chemicals including plastics. Alkynes contain triple bonds (example is acetylene), used in welding of metals. The above compounds are all known as aliphatics, which means the carbon atoms are all stretched out in a line. Aromatic hydrocarbons such as benzene have a ring molecular structure, hence less hydrogen per carbon atom and thus burn with a smoky flame.

When hydrocarbons burn, they react with oxygen from the atmosphere to produce carbon dioxide and steam, although if the combustion is incomplete because there is insufficient oxygen, carbon monoxide will result aswell.

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 additional compounds. 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 decompose naturally or can be altered by processing. For example, crude oil is separated into many materials using fractionation (or fractional distillation) and ‘cracking’. Fractionation is where highly volatile gases are removed at temperatures where they alone are volatile, then higher temperatures where heavier compounds are volatile then hotter still for larger hydrocarbons. Cracking is where big hydrocarbon molecules are broken up by heat and catalytic action to form smaller hydrocarbon molecules.

Inerting

In order to prevent explosions during shutdown and maintenance operations many industrial processes employ an inerting procedure. Fill a container of hydrocarbon gas with air and at some point, the mixture will become explosive and dangerous. Use a 2-stage process where the hydrocarbon is replaced by nitrogen and then the nitrogen is replaced by air, and at no stage do you risk explosion.  This is called purging a vessel (for example a fuel tanker, or storage tanks on an oil tanker). Purging of hydrocarbons is common practice before carrying out maintenance or repair work. Before entry by personnel, the vessel must be purged with breathable air. Crowcon has special instrumentation to monitor this whole 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 IEC60079-20-1:2010. 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).

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 been shown to be able to sustain a flame front at lower fuel concentrations when stirred than when still. Small differences in the 100%LEL volume results. It is caused by the average distance of a burning molecule from an unburned molecule being a little less when the gas is being stirred. The resultant LEL’s vary a small amount 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 EN60079 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 EN60079 using a mixture of 50% LEL methane made to ISO 10156, a 13.6% sensitivity 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 apply 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 (i.e., methane sensors will be calibrated such that 100% LEL = 4.4% volume). UL/CSA certified products will be calibrated according to the ISO10156 standard (i.e., 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.