Vaporization

“If a vessel containing a superheated liquid under pressure ruptures, a proportion of the liquid vaporizes. This initial flash fraction is determined by the heat balance, the latent heat of vaporization being supplied by the fall in the sensible heat of the liquid. The rapid formation of vapor bubbles also generates a spray of liquid drops so that typically much, or all, of the remaining liquid may become airborne, leaving little or no residue in the vessel.

In considering the generation of a vapor cloud from the liquid spillage, the following situations can be distinguished:

1.      A volatile liquid at atmospheric temperature and pressure, i.e., acetone.

2.      A superheated liquid

a.      At ambient temperature and under pressure, i.e., butane.

b.      At high temperature and under pressure,i.e., cyclohexane.

3.      A refrigerated liquefied gas at low temperature but at atmospheric pressure, i.e., cold methane.

The vaporization of the liquid is different for these three cases. In the first case the liquid after spillage is approximately at equilibrium and evaporates relatively slowly. In the second case the liquid is superheated and flashes off when spilt, and then undergoes slower evaporation. The first category of a superheated liquid, at ambient temperature but under pressure, is that of a liquefied gas, while the second, at high temperature and under pressure, is that of a liquid heated above its normal boiling point. The third case is that of a refrigerated liquefied gas that on spillage evaporates rapidly at first and then more slowly.”

Reference: Mannan, Sam. (2012). Lees’ Loss Prevention in the Process Industries, Volumes 1-3 – Hazard Identification, Assessment and Control (4th Edition) – 15.10.1 Vaporization situations and 15.8.1. Vaporization.Elsevier.

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