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

3-108 API RECOMMENDED PRACTICE 581
A BLEVE can occur upon rupture of a vessel containing a superheated but pressurized liquid that flashes to
vapor upon release to atmosphere. The classic example of a BLEVE is when an LPG storage vessel is exposed
to fire. As a vapor space is created in the vessel, the vessel metal in the vapor space, if it is exposed to flame
impingement, can fail at a pressure well below the MAWP of the vessel. If the vessel ruptures, the remaining
superheated liquid will expand significantly, causing an overpressure blast wave. Quite often, a BLEVE will be
followed by a fireball (see Section 5.8.4). Since a BLEVE can only occur from a rupture, the consequence area
for BLEVEs is equal to zero for all hole sizes except the rupture case.
BLEVEs can also occur for nonflammable fluids, such as high-temperature pressurized water.
5.10.3.2 TNT Equivalency Method
Similar to VCEs (Section 5.8.5) and physical ruptures (Section 5.10.2) of gas-filled vessels, the TNT
equivalency method can conservatively be used to estimate the blast pressure wave and the resultant
consequence area. The energy associated with the BLEVE of a vessel containing superheated liquid can be
estimated using Equation (3.195).
⎡ P ⎤
s
WTNT = C30 ⋅nvRTs
⋅ln ⎢ ⎥
⎣Patm
⎦
(3.195)
For cases where the vessel contains liquid and vapor just prior to the rupture, the released energy can be
calculated by using Equation (3.192) to calculate the energy released from the vapor portion stored in the
vessel and adding to that the energy released as calculated using Equation (3.195) for the expanding liquid
portion.
5.10.3.3 BLEVE Safe Distance and Consequence Area
At this point, the calculation of the consequence area as a result of a BLEVE from a vessel rupture is identical
to that described earlier for VCEs. The calculation of the blast overpressure uses blast curves as described in
Section 5.8.5.5 c). The calculation of the consequence area is identical to Section 5.8.5.5.
In general, the procedure results in a safe distance for both component damage,
bleve
xs
cmd
, and personnel injury,
bleve
xs , from which a consequence area can be calculated per Equation (3.196) and Equation (3.197).
inj
bleve
f , cmd
bleve
( ) 2
cmd
CA = π ⋅ xs for rupture case only
(3.196)
bleve
f , inj
bleve
( ) 2
inj
CA = π ⋅ xs for rupture case only
(3.197)
5.10.4 Steam Leaks and Chemical Spills
The consequence calculations for steam leaks or chemical burns, such as mild acids or caustic, are calculated
in the same way as performed in the Level 1 consequence analysis (see Section 4.10).
5.10.5 Nonflammable, Nontoxic Event Tree Probabilities
Based on the consequence analysis event trees (Figure 5.3 and Figure 5.4), nonflammable, nontoxic events
are taken into account when released fluids fail to ignite. Therefore, the probability for a nonflammable,
nontoxic event is the non-ignition frequency for the event (i.e. 1− poi ).
n