It is a function of a Process Hazard Analysis (PHA) to identify those inherent properties, conditions or equipment that could cause injury to personnel, the environment, or facilities. Gases and vapors have one property that can make them particularly problematic. They readily mix with air. This means that loss of containment incidents can quickly expose processes and personnel to fire or explosion hazards if the gas or vapor is combustible, the results can be catastrophic. What properties are of most concern? Because gases and vapors can quickly mix with air, short-term effects, or acute properties are of greatest safety interest. The ability of gases and vapors to form ignitable mixtures with air or other oxidants can lead to several hazardous conditions including flash fires, pool fires, jet fires, vapor cloud explosions and detonations. As the concentration of a released gas or vapor increases a point is reached when a flame can propagate away from an ignition source. This concentration is called the Lower Flammable Limit (LFL) for the material. This concentration varies with system pressure and temperature and may need to be measured under other than standard conditions. If the concentration of the gas or vapor continues to increase it will ultimately reach a point at which there is insufficient amount of oxidant to allow flame propagation. This concentration is called the Upper Flammable Limit (UFL) and forms the upper end of the flammable range. If ignition of a gas/air or vapor/air cloud occurs rapid burning is initiated and several possible outcomes are possible. If the cloud is unconfined, a flash fire will be the result. The high temperature flame and combustion products pose a danger to personnel and facilities. If the flash fire is confined, or if conditions are right for flame acceleration, then a vapor cloud explosion (VCE) will occur. A VCE can cause extensive facility and equipment damages and threaten personnel over a wide area. The vapors from a flammable liquid can flash back and ignite the liquid pool resulting in dangerous radiation levels and spreading the fire. Flammable liquid fires are difficult to extinguish. The release of gases or vapors under pressure can cause jet fires. These fires are fuel rich and burn very hot. Besides the obvious hazard associated with the torch, jet flames pose an extreme hazard to nearby storage tanks. If the storage tank is pressurized and contains a super-heated liquid, such as a propane tank, and the flame "impinges" on the unprotected shell a catastrophic stress failure is likely. Such a failure can lead to a Boiling Liquid Expanding Vapor Explosion, or BLEVE. The effects of a BLEVE include missiles, blast and if the contained material is combustible, a fireball with extreme radiation hazard. The contained material does not have to be flammable for a BLEVE to occur. It is the pressure and rapid phase change that causes the physical damage. Materials such as chlorine and anhydrous hydrogen chloride can BLEVE. If you store or process flammable gases or liquids and need to access your fire and explosion risks, Chilworth Global has experienced Process Safety Engineers who can help. The Chilworth Global Lab can provide you with all of the critical flammable property data needed to assess risk at both ambient and process conditions.
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