Ventilation systems, passive protection

In general, containers of pressurized flammable gases should be stored in outdoor open air areas with overhead protection from the elements. If outside storage is not an option, controlling the buildup of flammable gases or vapor can be accomplished, up to a limit, with either a passive or active ventilation system. However, a large release of gas or vapor may exceed the capacity of a practical ventilation system resulting in a hazardous concentration. 

Industrial fire protection and safety engineers attempt to eliminate hazards at their source or to reduce their intensity with protective systems. Hazard elimination may typically require the use of alternative and less toxic materials, changes in the process, spacing or guarding, improved ventilation or, spill control or inventory reduction measures, fire and explosion protective measures - both active and passive mechanisms, protective clothing, etc. The level or protection is dependent on the risk prevalent at the facility versus the cost to implement safety measures. 

Hartless (2004) has demonstrated that in most cases for methane to reach explosive levels within a passively ventilated void there would need to be a significant failure of the passive ventilation (for example if all the vents become blocked) or the entry rate of gas would have to increase by orders of magnitude. For simplicity he assumed that the probability of such an occurrence was 0.01. One approach that is less subjective is to estimate the failure rate using the probability density method described previously (Box 6.4, see colour section). An example is shown in Box 6.8. However, it is difficult to allow for human factors when estimating the probability of failure for elements of a protective system and an element of professional judgment will always be required. Studying examples and guidance from other industries can also assist in this respect (e.g. chemical and oil industries). 

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