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

5-70 API RECOMMENDED PRACTICE 581
repair a leaking PRD without shutting the unit down. Calculate the costs of lost production for the stuck
so
open case, C prod , using Equation (5.124).
i) STEP 6.9—Calculate the consequence associated with mild leakage,
stuck open PRDs,
mild
C l
so
C l , using Equation (5.125) and Equation (5.126), respectively.
j) STEP 6.10—Calculate the final leakage consequence,
Risk Analysis
Risk from Failure to Open
prd
C l
, using Equation (5.127).
, and the consequence of
The calculation of risk for a PRD failing to open at a specified inspection interval, t insp , is calculated for each
applicable overpressure demand case using the POF of the PRD,
the demand case,
prd
C f ,j
, as follows:
prd
P f ,j
, and the calculated overall COF for
prd prd prd
Risk f ,j Pf ,j C f ,j
= ⋅ (5.128)
The overall risk for the fail to open case is then determined by summing the individual risks associated with
the applicable overpressure demand cases as follows:
ndc
prd prd prd
f
= ∑ f ,j
⋅
f ,j
(5.129)
j=
1
Risk P C
In Equation (5.129), j represents each of the number of applicable overpressure demand case, ndc.
Since a PRD may protect multiple pieces of equipment, the above calculations are repeated for each piece
of equipment protected by the PRD. The resulting risk for the PRD is the maximum risk calculated for each of
the pieces of equipment protected by the PRD.
Risk from Leakage
The calculation of the risk associated with PRD leakage is obtained by multiplying the probability of leakage,
, and the consequence of leakage, C , in accordance with Equation (5.130):
prd
P l
prd
l
prd prd prd
Riskl Pl Cl
= ⋅ (5.130)
Total Risk
The total risk associated with PRD failure to open and leakage is obtained as follows:
Risk prd
Risk prd prd
f Risk l
= + (5.131)
Calculation Procedure
The following summarizes the calculation procedure for the failure to open case.