Vapour slumping

The density of a vapour or gas at eonstant pressure is proportional to its relative moleeular mass and inversely proportional to temperature. Sinee most gases and vapours have relative moleeular masses greater than air (exeeptions inelude hydrogen, methane and ammonia), the vapours slump and spread or aeeumulate at low levels. The greater the vapour density, the greater the tendeney for this to oeeur. Gases or vapours whieh are less dense than air ean, however, spread at low level when eold (e.g. release of ammonia refrigerant). Table 6.1 ineludes vapour density values. 

On release, vapours heavier than air tend to spread (i.e. to slump ) at low level and will aeeumulate in pits, sumps, depressions in ground, ete. This may promote a fire/explosion hazard, or a toxie hazard, or eause an oxygen-defieient atmosphere to form, depending on the ehemieal. 

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