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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-101
1) Determine the flammable mass of the fluid contained in the equipment using Equation (3.157), the
flam
mass fraction of flammable material, mfrac , obtained in STEP 8.1, and the inventory mass
available for release, mass , determined in STEP 4.7.
avail,
n
2) Calculate the maximum diameter, Dmax , and the center height,
fb
H , of the fireball using Equation
fb
(3.158) and Equation (3.159), respectively.
3) Calculate the duration of the fireball, t fb
, using Equation (3.160) or (3.161) based on the mass of the
fireball.
fball
4) Calculate the amount of energy radiated by the fireball, Qrad , using Equation (3.162).
5) For the component damage consequence area, API 581 uses a radiation limit of 37.8 kW/m 2
(12,000 Btu/hr-ft 2 ). For personnel injury, 12.6 kW/m 2 (4,000 Btu/hr-ft 2 ) is used. These radiation limits
are used to determine the safe distances, xs and xs , from the fireball using the following
four-step iterative procedure.
fball
cmd
fball
inj
fball
i) Guess at an acceptable distance from the fireball, xs .
ii)
iii)
iv)
τ
Calculate the atmospheric transmissivity,
atm
, and the spherical view factor, Fsph , using
Equation (3.140) and Equation (3.165). Both of these parameters are functions of the distance
from the fireball chosen above,
fball
xs .
fball
Calculate the received thermal heat flux, Ith , at the distance chosen using Equation (3.164)
and compare it to the acceptable radiation limit [37.8 kW/m 2 (12,000 Btu/hr-ft 2 ) for component
damage and 12.6 kW/m 2 (4,000 Btu/hr-ft 2 ) for personnel injury].
Adjust the distance, xs
fball
thermal heat flux equals the allowable limit.
, accordingly, and repeat the above steps until the calculated received
6) Calculate the component damage consequence area, CA
,
fball
f cmd
fball
consequence area, CA , using Equation (3.167) and Equation (3.168).
f , inj
, and the personnel injury
vce
n) STEP 8.14—For each of the hole sizes, calculate the component damage consequence area, CA ,
f , cmd , n
vce
and the personnel injury consequence area, CA , of a VCE.
f , inj,
n
Using the vapor source rate and source area determined in STEP 7.7, perform a cloud dispersion analysis
in accordance with Section 5.7.4 and determine the mass of flammable material, mass
vce
, in the vapor
cloud. This is the portion of the cloud that has concentrations between the LFL and the UFL of the fluid
being released. The LFL and UFL were obtained in STEP 1.2.
1) Determine the amount of potential energy in the vapor cloud expressed as an equivalent amount of
TNT, W
TNT
, using Equation (3.181). Note that the energy yield factor, η , is equal to 1.0 when the
mass used in this step is based on the flammable mass of the cloud between the LFL and the UFL.