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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-91
This procedure is carried out for personnel injury as well as component damage for each of the release hole
sizes selected as described in Section 4.2. Once the safe distances, jet
jet
xs and
cmd , n
xs , are determined,
inj,
n
Equation (3.155) and Equation (3.156) are used to calculate the jet fire consequence areas.
CA
= π ⋅ xs
(3.155)
jet
jet 2
f , cmd , n
cmd , n
CA
jet
jet 2
f , inj, n
= π ⋅ xs
(3.156)
inj,
n
5.8.4 Fireballs
5.8.4.1 General
Fireballs result from the immediate ignition of a flammable, superheated liquid/vapor. Fireballs always occur in
combination with a physical explosion or a BLEVE. The effects of fireballs need to be evaluated for
instantaneous releases (or ruptures). Continuous releases do not result in fireballs.
CCPS [17] provides a suitable methodology for determining the effects of fireballs. Four factors have to be
considered to determine the heat flux of a fireball: the mass of the flammable fluid, the fireball’s diameter,
duration, and thermal emissive power. The main parameter needed is the mass of flammable fluid in the stored
equipment prior to rupture. The flammable mass for the fireball, mass , is the fraction of the released mass
that contains flammable material and can be determined using Equation (3.157).
flam
mass
fb
mfrac massavail,
n
= ⋅ (3.157)
fb
The maximum mass available for release, mass , is defined in Section 4.4.2 [see Equation (3.11)].
avail,
n
Once the flammable mass of the fireball is known, the diameter, duration, and height of the fireball can be
readily calculated. The effects of thermal radiation on personnel and equipment can be determined in much
the same way as has been previously done for jet fires and pool fires.
5.8.4.2 Fireball Size and Duration
The diameter of the fireball is a function of the flammable mass as follows:
Dmax = C ⋅ mass
(3.158)
fb
22
0.333
fb
The center height of the fireball is assumed to be:
H
fb
= 0.75⋅ Dmax
(3.159)
fb
The duration of the fireball is also a function of the flammable mass as follows:
t = C ⋅mass for mass ≤ kgs lbs
(3.160)
0.333
fb 23 fb fb
29,937 [66,000 ]
t = C ⋅ mass for mass > kgs lbs
(3.161)
0.167
fb 24 fb fb
29,937 [66,000 ]