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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-63
c) STEP 4.3—Determine the adjustment factor, F s , using Section 6.3.2 g.
d) STEP 4.4—Determine the environmental adjustment factor, F env , using Table 6.7.
e) STEP 4.5—Calculate the modified characteristic life, η mod , using Equation (5.111) and the factors
obtained in STEP 4.3 and STEP 4.4.
f) STEP 4.6—Assemble the PRD’s inspection history. Grade each record using the inspection
effectiveness table, Part 2, Annex 2.C, Table 2.C.3.1. Record the results of each inspection record; NO
LEAK/LEAK and determine the confidence factors, CF i , as applicable, for each inspection history based
on the results of the test. Determine the time duration, t dur,i , of each inspection cycle.
g) STEP 4.7—Starting at the earliest inspection record, update the modified characteristic life, η mod ,
determined in STEP 4.5 as follows.
1) STEP 4.7.1—Calculate the prior probability of leakage using Equation (5.95). Note that for the first
inspection record, the modified characteristic life, η mod , is used. Subsequent inspection records will
use the updated characteristic from STEP 4.7.5 below.
STEP 4.7.2—Calculate the prior probability of passing using Equation (5.96).
STEP 4.7.3—Determine the conditional probability of leakage and conditional probability of pass using
Equation (5.97) and Equation (5.98), respectively.
STEP 4.7.4—Calculate the weighted probability of leakage,
6.10.
prd
P l ,wgt
, using the appropriate equation from Table
STEP 4.7.5—Determine the updated characteristic life, η upd , using Equation (5.99) and the weighted
probability of leakage,
prd
P l ,wgt
, established in STEP 4.7.4.
STEP 4.7.6—Repeat these steps for each of the inspection records available for the PRD until a final
updated value for the characteristic life, η upd , is determined.
h) STEP 4.8—Calculate the probability of leakage, P
prd
l ,wgt
, for the specific PRD in accordance with Equation
(5.112). Adjust the probability of leakage for operating close to the set pressure using Equation (5.113).
Consequence of PRD Failure to Open
General
The consequence calculations for event outcomes such as fires, explosions, and toxic exposure are
described in Part 3. For PRDs, failures to open upon demand will likely result in the protected equipment
being exposed to significantly higher pressures than during normal operations. This methodology calculates
the consequences for each PRD failing to open at sometimes significantly higher overpressure than the
normal operating pressure of the equipment.
Table 6.12 shows the expected potential consequences of an overpressure event in a pressure vessel.
Table 6.12 is only provided for a qualitative discussion of the potential risks to equipment due to
overpressure and is not intended to indicate any specific event outcome. The methodology accounts for the
effects of overpressure on protected equipment by increasing the probability of loss of containment. At an
overpressure equal to the burst pressure (estimated to be the design margin times the MAWP), the
probability of loss of containment is conservatively assumed to be equal to 1.0; see Section 6.2.4 j.