Unconfined vapor cloud explosion (UVCE)

On June 1, 1974, at Flixborough, Lincolnshire, a temporary cyclohexane pipe fails at a caprolactam plant. The released cyclohexane forms a cloud that finds an ignition source: twenty-eight dead, damage kilometers away. The mechanism has a name, UVCE, unconfined vapor cloud explosion. A liquid or gas escapes, mixes with air, reaches its flammable concentration, finds a spark, and the flame front accelerates through the obstacles of the installation. The more crowded the site is with piping and structures, the higher the overpressure climbs. The underwriter therefore models the overpressure rather than the fire, by radius, around each possible leak point. The calculation sets the site's probable maximum loss (PML) and dictates the separation distance between a unit and its neighbors, including the control room, the administration building and the contractor trailer that Texas City made famous. That is also why industrial underwriters look at neighboring sites: a cloud does not stop at the fence. The loss is generally covered under the property policy, but liability to third parties, residents included, is what drives severity. Not to be confused with a dust explosion, nor with the BLEVE, which is the rupture of a pressurized vessel.