Causes of Overpressure – Internal Explosions

If sufficient air, oxygen, or other compound capable of supporting combustion can infiltrate an equipment item containing flammable gases, vapors, or dusts, an explosive mixture may result inside the equipment. Ignition of such a mixture can result in either a deflagration or a detonation, which differ in the speed of propagation of the combustion flame front. In the event of a deflagration, venting the overpressure generated is possible, although the methods for designing such a vent are somewhat dependent on the precise configuration of the equipment involved. These methods are discussed in NFPA 68, Guide for Venting Deflagrations. NFPA 69, Explosion Prevention Systems, provides a description of methods for controlling deflagrations other than venting – for example, total containment, suppression, and oxidant concentration reduction.

Venting of a detonation, in which the propagation speed of the flame front exceeds the local speed of sound, is generally not possible. For cases that hold the possibility of a detonation, prevention of the creation of the flammable mixture and/or of a source of ignition becomes of great importance.

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