Extent of Zone

The extent of the zone depends on the estimated or calculated distance over which an explosive atmosphere exists before it disperses to a concentration in air below its lower explosive limit with an appropriate safety factor. When assessing the area of spread of gas or vapour before dilution to below its lower explosive limit, expert advice should be sought. Consideration should always be given to the possibility that a gas which is heavier than air may flow into areas below ground level (for example, pits or depressions) and that a gas which is lighter than air may be retained at high level (for example, in a roof space).

Release Rate.

The greater the release rate, the larger the extent of the zone. The release rate depends itself on other parameters, namely

Geometry of the source of release:

This is related to the physical characteristics of the source of release, for example, an open surface, leaking flange.

Release velocity:

For a given source of release, the release rate increases with the release velocity. In the case of a product contained within process equipment, the release velocity is related to the process pressure and the geometry of the source of release. The size of a cloud of flammable gas or vapour is determined by the rate of flammable vapour release and the rate of dispersion.

Concentration:

The release rate increases with the concentration of flammable vapour or gas in the released mixture.

Volatility of the flammable liquid:

This is related principally to the vapour pressure, and the enthalpy (―heat‖) of vaporisation. If the vapour pressure is not known, the boiling point and flashpoint can be used as a guide.

Liquid temperature:

The vapour pressure increases with temperature, thus increasing the release rate due to evaporation. The temperature of the liquid after it has been released may be increased, for example, by a hot surface or by a high ambient temperature. Lower Flammable Limit (LFL): For a given release volume, the lower the LFL, the greater the extent of the zone.

Ventilation:

With increased ventilation, the extent of the zone will normally be reduced. Obstacles which impede the ventilation may increase the extent of the zone. On the other hand, some obstacles, for example, dykes, walls or ceilings, may limit the extent.

Density of released material:

If a flammable material is released, it is important to know whether the material will rise or fall in the atmosphere. The different flammable materials are compared with air and allocated a number to denote their relative density with air. Since air is the reference, its relative density is 1 so that for a material twice as heavy as air, its relative density will be 2. Therefore, materials with a relative density less than unity will rise in the atmosphere, and those greater than unity will fall in the atmosphere.

Materials which rise in the atmosphere can collect in roof spaces, and those which fall, such as butane or propane, can drift along at ground level and possibly into a non- hazardous location, or may collect in locations lower than ground level without ever dispersing. Such locations should be well ventilated in order to avoid ignition due to a stray spark or a discarded cigarette.

Knowledge of where a flammable material will collect ensures that gas detectors when fitted will be located at the correct level and ventilation is directed accordingly.