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

3-18 API RECOMMENDED PRACTICE 581
ignition goes up significantly. (The probabilities of ignition and other event tree probabilities for the Level 1
COF are presented in Annex 3.A). As a result, there is an abrupt change in the Level 1 consequence results
between a continuous release and an instantaneous release. An instantaneous release is defined as any
release larger than 4,536 kg (10,000 lb) in 3 minutes, which is equivalent to a release rate of 25.2 kg/s
(55.6 lb/s). A continuous release of 25.5 kg/s would have a much lower consequence than an instantaneous
release at 25.2 kg/s of the same material. Therefore, the Level 1 COF includes a blending of the calculated
results of the continuous and instantaneous releases (see Section 4.8.7).
4.8.2 Consequence Area Equations
4.8.2.1 Generic Equations
The following equations are used to determine the flammable consequence areas for component damage and
personnel injury. The background for development of these generic equations is provided in Annex 3.A.
a) Continuous Release—For a continuous release, Equation (3.15) is used. Coefficients for this equation for
component damage areas and personnel injury areas are provided in Table 4.8 and Table 4.9,
respectively.
CA
CONT
f , n
( )
n
b
= a rate
(3.15)
b) Instantaneous Release—For an instantaneous release, Equation (3.16) is used. Coefficients for this
equation for component damage areas and personnel injury areas are provided in Table 4.8 and Table
4.9, respectively.
CA
INST
f , n
( )
n
b
= a mass
(3.16)
4.8.2.2 Development of Generic Equations
Equation (3.15) and Equation (3.16) were employed to calculate overall consequence areas following a threestep
process.
a) An event tree analysis was performed by listing possible events or outcomes and providing estimates for
the probabilities of each event. The two main factors that define the paths on the event tree for the release
of flammable material are the probability of ignition and the timing of ignition. The event trees used are
provided in Figure 4.2 where event probabilities were set as a function of release type (continuous or
instantaneous) and temperature (proximity to the AIT). These probabilities are provided in Annex 3.A.
b) The consequence areas as a result of each event were calculated using appropriate analysis techniques,
including cloud dispersion modeling. Additional background on the methods used for these calculations
are provided in Annex 3.A.
c) The consequence areas of each individual event were combined into a single probability weighted
empirical equation representing the overall consequence area of the event tree (see Annex 3.A).
4.8.2.3 Threshold Limits
Threshold limits for thermal radiation and overpressure, sometimes referred to as impact criteria, were used
to calculate the consequence areas for a particular event outcome (pool fire, VCE, etc.).
a) Component damage criteria:
1) explosion overpressure—34.5 kPa (5 psig);
2) thermal radiation—37.8 kW/m 2 [12,000 Btu/(hr-ft 2 )] (jet fire, pool fire, and fireball);