Understanding how flammable gases and vapours behave when released is fundamental to hazardous area classification under ATEX and DSEAR. Several key physical and chemical properties determine whether a flammable atmosphere can form, how often it may occur, and how far it may extend.

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The principal parameters considered are flash point, flammability limits, auto-ignition temperature, gas group, and vapour density.

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Flash Point (Tf)

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The flash point of a liquid is the lowest temperature at which it produces sufficient vapour to form an ignitable mixture with air in the presence of an ignition source.

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Flash point is a key indicator of a liquid’s volatility and its potential to generate flammable vapours following a release. Liquids handled below their flash point are significantly less likely to form flammable atmospheres, whereas handling at or above the flash point increases the likelihood of vapour generation.

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It is important to note that the flash point applies to liquids only. Gases such as methane and hydrogen are capable of forming flammable mixtures at any ambient temperature and are therefore treated separately in hazardous area assessments.

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Limits of Flammability (LEL and UEL)

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Flammable gases and vapours are only ignitable when their concentration in air lies within a specific range, defined by:

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• Lower Explosive Limit (LEL) – the minimum concentration capable of flame propagation once ignition occurs

• Upper Explosive Limit (UEL) – the maximum concentration capable of flame propagation

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Below the LEL, the mixture is too lean to ignite. Above the UEL, it is too rich. However, high-concentration releases still present a hazard because dilution with air can bring the mixture back into the flammable range.

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The LEL is particularly important in hazardous area classification, as it influences zone boundaries and extents. Increasing temperature or pressure widens the flammable range, increasing risk.

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Auto-Ignition Temperature (Tig)

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The auto-ignition temperature is the lowest temperature at which a gas or vapour will ignite spontaneously, without the presence of an external ignition source.

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Auto-ignition temperature is not an absolute constant. It varies depending on:

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• Test method

• Surface material (e.g. steel, rust, glass)

• Environmental conditions

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This parameter is crucial for selecting ATEX equipment, as it determines the maximum permitted surface temperature of equipment used in hazardous areas. Equipment temperature classes (T1 to T6) are selected so that surface temperatures remain safely below the minimum ignition temperature of the relevant gas or vapour.

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Temperature Classes

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Electrical and mechanical equipment used in hazardous areas is assigned a temperature class indicating its maximum surface temperature. The selected class must always be lower than the auto-ignition temperature of the flammable substance present.

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Typical temperature classes range from T1 (450 °C) to T6 (85 °C), with increasing levels of protection as the class number increases.

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Gas Groups

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Flammable gases and vapours are categorised into gas groups based on their ignition characteristics and flame propagation behaviour:

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• Group IIA – least easily ignited (e.g. propane, petrol vapour)

• Group IIB – intermediate ignition sensitivity (e.g. ethylene)

• Group IIC – most easily ignited and most severe (e.g. hydrogen, acetylene)

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Equipment certified for Group IIC is also suitable for IIB and IIA, but not vice versa.

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Vapour Density

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Vapour density describes the weight of a gas or vapour relative to air, with air having a reference value of 1.

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• Vapours with a density greater than 1 tend to sink, spread at low levels, and accumulate in pits, ducts, or low-lying areas

• Vapours with a density less than 1 tend to rise and may accumulate at high levels, such as under roofs or ceilings

• Vapours with a density close to 1 disperse more readily and are influenced by air movement and convection

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Vapour density is not a flammability property, but it is essential for estimating gas movement, layering, and zone shape.

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Release conditions can also affect vapour density. Rapid expansion or cooling during release may increase density compared with published reference values.

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Oil and Flammable Fluid Classification

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Flammable liquids are commonly classified based on their flash point and handling temperature. Further categorisation considers whether a release would:

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• Vaporise rapidly

• Boil on release

• Form a flammable mist or spray

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These classifications support the calculation of hazardous zone extents and help determine whether additional controls are required.

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Summary

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For gas and vapour hazardous area classification, the most important parameters are:

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• Flash point – likelihood of vapour formation

• Explosive limits (LEL/UEL) – concentration range for ignition

• Auto-ignition temperature – equipment surface temperature limits

• Gas group – ignition severity and equipment suitability

• Vapour density – dispersion, layering, and accumulation behaviour

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Together, these properties determine whether a flammable atmosphere can form, how often it may occur, and how far it may extend, forming the technical basis of ATEX and DSEAR risk assessments.

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