API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-93
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5.8.5 VCEs
5.8.5.1 General
When a sizable amount of flammable fluid is suddenly released into the air and is not immediately ignited,
three things can happen: the cloud can encounter an ignition source and explode, producing a VCE; the cloud
can encounter an ignition source and flash back, producing a flash fire (Section 5.8.6); or the cloud can safely
disperse. For a VCE or flash fire to occur, the released material must form a partially mixed vapor cloud that
contains vapor concentrations above the LFL. The cloud then encounters an ignition source and either
explodes or flashes back. Since VCEs produce devastating effects on plants if they occur, significant research
on their causes has been performed. From research on VCEs that have occurred, Lees [25] has identified
several parameters that affect VCE behavior:
a) quantity of material released,
b) fraction of material vaporized,
c) probability of ignition of the cloud,
d) distance traveled by the cloud,
e) time delay before ignition of the cloud,
f) probability of explosion rather than fire,
g) existence of a threshold quantity of material,
h) efficiency of the explosion,
i) location of ignition source with respect to the release.
VCEs can occur as a result of a delayed ignition of a vapor cloud. The source of the vapor cloud could either
be from a vapor or two-phase jet release or evaporation off the surface of an un-ignited liquid flammable pool.
Dispersion modeling of the cloud is required to evaluate the extent of a vapor cloud, since the amount of
flammable material in the cloud is needed. (See the general discussion on cloud modeling presented in Section
5.7.4.) A VCE is a deflagration (not detonation) that produces significant overpressure (blast wave) and occurs
when the flame propagation through the cloud travels at extremely high velocities. If the flame propagates at
a relatively slow velocity, a VCE, with the resulting overpressure, does not occur. In this case, a relatively low
consequence, low energy, flash fire is the outcome (see Section 5.8.6).
5.8.5.2 Source of Vapor
The source of flammable vapor for the VCE could either be from a jet release or from an evaporating liquid
pool release. For the jet release case, the source rate is simply the jet release rate as discussed in Section
5.7.3.
For an evaporating pool, the vapor rate used as the source for the VCE is dependent on whether the pool is a
boiling or non-boiling, as discussed in Section 5.7.4 and shown in Figure 5.1.
5.8.5.3 Amount of Flammable Material
The first step in evaluating the effects of a VCE is to determine the amount of flammable material that is in the
source cloud. The mass is a function of the release rate, the atmospheric dispersion of the cloud, and the time