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

3-92 API RECOMMENDED PRACTICE 581
5.8.4.3 Fireball Radiated Energy
The amount of energy radiated by the fireball (often referred to as surface emitted heat flux) is a fraction of its
total combustion power. The fraction of the total combustion power that is radiated, , is often quoted in the
range of 0.25 to 0.4; see Equation (3.162).
β
fb
Qrad
fball
C14
⋅β
⋅mass ⋅HC
=
π ⋅
fb fb l
2
Dmax
fb
⋅t
fb
(3.162)
The fraction of combustion power that is radiated from a fireball can be calculated from a correlation by [24] :
β = C ⋅ P
(3.163)
fb
25
0.32
B
The burst pressure used above for determining the radiation fraction depends on the consequence being
calculated. If the calculation is for pressurized fixed equipment where the concern is for rupture during normal
operation, the normal operating pressure is used. When the calculation is to be performed at elevated
pressures such as the case when the COFs of PRDs are being evaluated, the likely overpressure that results
from the failure to open upon demand should be used.
5.8.4.4 Fireball Safe Distance and Consequence Area
The amount of the radiated energy that actually reaches a target at some location away from the fireball is a
function of the atmospheric conditions as well as the radiation view factor between the source and the target.
The received thermal flux can be determined as before:
= τ ⋅ ⋅ (3.164)
fball
fball
Ith
atm
Qrad Fsph
For a fireball, the spherical model for the geometric view factor is used:
where
( Dmax ) 2
fb
Fsph = (3.165)
4C
2
fb
C
fb
2
fball
2
⎛Dmax
fb ⎞ ⎛ xs ⎞
= ⎜ ⎟ + ⎜ ⎟
⎝ 2.0 ⎠ ⎝ 2.0 ⎠
(3.166)
Note that the atmospheric transmissivity and the geometric view factor are functions of the distance from the
fball
flame source to the target, xs . An iterative approach is required to determine the acceptable distance away
from the fireball.
This procedure is carried out for personnel injury as well as component damage for the rupture case. Once the
fball
fball
safe distances, xscmd
and xs , are determined, Equation (3.167) and Equation (3.168) are used to calculate
inj
the fireball consequence areas.
CA
fball
f , cmd
fball
( xs ) 2
cmd
= π ⋅ (3.167)