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

5-66 API RECOMMENDED PRACTICE 581
The consequence calculations should be performed in accordance with Part 3 for each of the overpressure
demand cases applicable to the PRD and for each piece of equipment that is protected by the PRD. The
prd
resultant consequence is C f ,j
expressed in financial terms, ($/year).
Calculation Procedure
The following procedure may be used to determine the consequence of a PRD failure to open.
a) STEP 5.1—Determine the list of overpressure scenarios applicable to the piece of equipment being
protected by the PRD under evaluation. Table 6.2 provides a list of overpressure demand cases
specifically covered. Additional guidance on overpressure demand cases and pressure-relieving system
design is provided in API 521 [22] .
b) STEP 5.2—For each overpressure demand case, estimate the amount of overpressure, P o,j , likely to
occur during the overpressure event if the PRD were to fail to open. Section 6.4.3 and Table 6.3 provide
guidance in this area.
c) STEP 5.3—For installations that have multiple PRDs, determine the total amount of installed PRD
orifice area, A
prd
total
, including the area of the PRD being evaluated. Calculate the overpressure
adjustment factor, F a , in accordance with Equation (5.114).
d) STEP 5.4—Reduce the overpressures determined in STEP 5.3 by the overpressure adjustment factor in
accordance with Equation (5.115).
e) STEP 5.5—For each overpressure demand case, calculate the financial consequence,
prd
C f ,j
, of loss of
containment from the protected equipment using procedures developed in Part 3. Use the
overpressures for the demand cases as determined in STEP 5.4 in lieu of the operating pressure, P s .
f) STEP 5.6—Using the values as determined above, refer to Section 6.6 to calculate the risk.
Consequence of Leakage
General
Even though the consequences of PRD leakage are typically much less severe than that of a loss of
containment from the protected equipment as a result of a PRD failure to open, the frequency of leakage
may be high enough that the PRD may be ranked as a high priority on a leakage risk basis.
The calculation of leakage consequence from PRDs,
prd
C l
, is estimated by summing the costs of several
items. The cost of the lost inventory is based on the cost of fluid multiplied by the leakage rate (see
Section 6.5.5) and the number of days to discover the leak (see Table 6.14). Regulatory and environmental
costs associated with leakage should be considered as well. Next, the cost of downtime to repair or replace
the device is estimated if it is determined that continuous operation of the unit with a leaking or stuck open
PRD cannot be tolerated. If a shutdown is required to repair the leaking PRD, then the cost associated with
lost production will also be added.
The consequence of leakage,
prd
C l
prd
Cl
Costinv Costenv Costsd Cost prod
, is calculated using the following equation:
= + + + (5.116)