Where flammable liquids are escaping from their container, stopping any leak will limit the hazard presented by the liquid pool. Various equipment may be available to manage leaks and reduce the flow, including equipment provided by the environmental agencies.
Similar to the hierarchy of control for environmental protection (refer to Environmental protection), when containing or controlling a flammable liquid, the following hierarchy should be considered. The smaller the surface area the less vapour will be produced, reducing the risk of the vapour finding an ignition source whilst within its flammable range.
| Tactical Options | |
|---|---|
| At source |
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| Close to source |
|
| On the surface |
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| In the drainage system |
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| In the watercourse |
|
A number of the techniques in the chart above rely on the flammable liquid being immiscible with water and lighter than water (floating).
In addition to controlling the spread of a flammable liquid, suppressing or controlling the rate of evaporation will be necessary to prevent an ignitable vapour cloud forming. The primary method to reduce vapour production would be to cover the spill, although container cooling may be beneficial in certain situations. Providing the material used does not react with the liquid, numerous options may have a beneficial effect, such as salvage sheets, plastic sheeting or foam.
An appropriate foam can put a barrier between the flammable liquid and the supporter of combustion to inhibit the formation of an ignitable fuel/air mixture. Foam blankets can limit the size of any ignitable plume and slow the fire growth if an ignition does take place.
A key consideration when selecting foam is whether the flammable liquid is miscible with water. Alcohols are water miscible and will react with the water content of general foams, causing them to break down and be less effective. Selecting an alcohol resistant foam will counter this effect.
Although termed ‘alcohol resistant’, this type of foam will act effectively on other miscible solvents such as acetone or organic acids.
When a pressurised liquid escapes from its container, not only will large quantities of vapour be released, but also, if the leak is below the liquid level, a pool of very cold liquid at sub-zero temperatures will accumulate. This pool will vaporise rapidly. It will also pose cryogenic hazards and will boil rapidly if water or foam is introduced to the pool. This additional hazard should be considered when deciding on any tactics when dealing with leaks of flammable pressurised liquids, for example liquefied petroleum gas (LPG).
For pooled spillages of liquefied pressurised gases such as propane, medium expansion foam suppression may prove effective due to its reduced water content.
Flammable liquids will produce vapours based on their vapour pressure (see definition). The higher the vapour pressure, the more volatile the liquid and the greater the volume of flammable vapours that will be produced. Temperature is another important factor in determining the quantities of vapours produced; as the temperature of a liquid increases, larger quantities of vapours will be produced.
As many flammable liquids are immiscible with water, cooling the liquids directly with water will spread the hazard. However, in situations where flammable liquids are present, removing or reducing any heat sources will reduce the potential for a flammable vapour cloud to develop. As a ‘rule of thumb’ a 10C reduction in temperature will reduce the vapour pressure by about 50 per cent.