API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
5-60 API RECOMMENDED PRACTICE 581STEP 2.7.5—Determine the updated characteristic life, η upd , using Equation (5.99) Weibull parameters βfrom STEP 2.2, and the weighted POF,prdPf ,wgt , established in STEP 2.7.4.STEP 2.7.6—Repeat these steps for each of the inspection records available for the PRD until a finalupdated value for the characteristic life, η upd , is determined.h) STEP 2.8—For each overpressure scenario, determine the adjustment factor, F op,j , using Equation(5.108). Note that to perform this step, an estimate of the overpressure given a failure of the PRD toopen upon demand is required. This is covered in more detail in Section 6.4.6, STEP 5.2.i) STEP 2.9—For each overpressure demand case, determine the initiating event frequency, EF j , usingTable 6.1 or based on owner–user experience for the particular overpressure demand case.j) STEP 2.10—Determine the demand rate reduction factor, DRRF j , which accounts for any layers ofprotection in the process that may reduce the probability of overpressuring the protected piece ofequipment, see Section 6.2.3 b and Table 6.2 for guidance.k) STEP 2.11—For each overpressure demand case, determine the demand rate placed on the PRD, DR j ,using Equation (5.90).l) STEP 2.12—Determine the MAWP of the protected equipment.m) STEP 2.13—If an RBI study has been completed for the protected equipment, calculate its damageadjusted POF, P f , using Equation (5.89). Since the DF for the protected equipment is a function of time,the DF must be determined at the PRD inspection interval, t insp , specified in STEP 2.1. If a risk analysisfor fixed equipment has not been completed, a DF can be estimated using the values in Table 6.11.n) STEP 2.14—Calculate the POF of the protected equipment at the elevated overpressure, P f,j , usingEquation (5.109). Use the overpressure determined in STEP 5.2 of Section 6.4.6, the MAWP of theprotected equipment and the POF determined in STEP 2.13.o) STEP 2.15—Calculate the POF,prdP f ,j, using Equation (5.89) using P fod,j from Equation (5.94).p) STEP 2.16—Repeat STEP 2.1 through STEP 2.15 for each piece of equipment protected by the PRD.Probability of Leakage (POL)OverviewThe leakage case is different than the fail to open case since the POF is not a function of demand rate butrather is based on failure during continuous operation. The industry data associated with the probability ofleakage, P l , data are in per year units (i.e. failures/year). No multiplication by any demand rate is necessary.Probability of Leakage Calculation ProcedureThe probability of leakage for a PRD is determined using the following steps.a) STEP 3.1—Determine default Weibull parameters, β and ηdef, based on category of Service Severityand type of PRD; see Section 6.3.2 f through Section 6.3.2 j.b) STEP 3.2—Apply an adjustment factor, F s , to account for the presence of soft seats; see Section 6.3.2 k.c) STEP 3.3—Apply an adjustment factor, F env , for environmental factors; see Section 6.3.2 l.
RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-61The result of the procedure outlined above will be a modified characteristic life, η mod , as defined inEquation (5.111).ηmod = Fs ⋅Fenv ⋅ ηdef(5.111)The modified characteristic life, η mod , is updated based on the PRD’s inspection history, identical to themethod presented in Section 6.2.4 i) for the failure to open case. This updated characteristic life, η upd , isthen used to calculate the probability of leakage for the specific PRD in accordance with Equation (5.112).d) STEP 3.4—The modified characteristic life, η mod , is updated based on the PRD’s inspection history,identical to the method presented in Section 6.2.4 i) for the failure to open case. This updatedcharacteristic life, η upd , is then used to calculate the probability of leakage for the specific PRD inaccordance with Equation (5.112).prdlP⎡β⎛ t ⎞ ⎤= 1−exp⎢ ⎥⎢−⎜ ⎟⎝η⎥upd⎢⎠⎣ ⎥⎦(5.112)e) STEP 3.5—The probability of leakage needs to be adjusted based on the closeness the system isoperating near the set pressure with Equation (5.113). The set pressure factor, F set , is dependent on thePRD type, operating pressure, P s , and set pressure, P set (see Table 6.7 for equations).P prd prdl P l F setf) Categories of Service Severity= ⋅ (5.113)Guidance on selecting the proper service severity for the leakage case is provided in Table 6.12.Ideally, the owner–user’s experience with a PRD in a particular service will provide guidance as to thechoice of service severity for the leakage case.g) Default Probability of Leakage Rates vs Time in ServiceAn additional set of Weibull curves tracks the leakage failure case. In this case, the curves are not basedon a per demand failure rate but are taken from data of PRDs in continuous service (i.e. a continuousdemand, unlike the failure to open case, which is on a per demand basis). The data are collected in unitsof failures/year and do not have to be multiplied by a demand rate. Table 6.13 provides the default PRDprobability of leakage vs time information using a Weibull function to describe the three types of service:MILD, MODERATE, and SEVERE. These data are currently based on a limited amount of industry dataand should be supplemented by owner–user data where available.As an example, the default cumulative failure (leakage) distribution curves for spring-loaded conventionalPRVs using the Weibull function to describe the three categories of service severity—MILD,MODERATE, and SEVERE—are provided in Figure 6.3.h) Default Weibull Parameters for Balanced Bellows PRVsThe Weibull parameters for the probability of leakage curve for balanced bellows PRVs provided inTable 6.13 match the industry failure rate data. These data reflect a minor increase in the probability ofleakage compared to conventional valves.
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5-60 API RECOMMENDED PRACTICE 581
STEP 2.7.5—Determine the updated characteristic life, η upd , using Equation (5.99) Weibull parameters β
from STEP 2.2, and the weighted POF,
prd
P
f ,wgt , established in STEP 2.7.4.
STEP 2.7.6—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 2.8—For each overpressure scenario, determine the adjustment factor, F op,j , using Equation
(5.108). Note that to perform this step, an estimate of the overpressure given a failure of the PRD to
open upon demand is required. This is covered in more detail in Section 6.4.6, STEP 5.2.
i) STEP 2.9—For each overpressure demand case, determine the initiating event frequency, EF j , using
Table 6.1 or based on owner–user experience for the particular overpressure demand case.
j) STEP 2.10—Determine the demand rate reduction factor, DRRF j , which accounts for any layers of
protection in the process that may reduce the probability of overpressuring the protected piece of
equipment, see Section 6.2.3 b and Table 6.2 for guidance.
k) STEP 2.11—For each overpressure demand case, determine the demand rate placed on the PRD, DR j ,
using Equation (5.90).
l) STEP 2.12—Determine the MAWP of the protected equipment.
m) STEP 2.13—If an RBI study has been completed for the protected equipment, calculate its damage
adjusted POF, P f , using Equation (5.89). Since the DF for the protected equipment is a function of time,
the DF must be determined at the PRD inspection interval, t insp , specified in STEP 2.1. If a risk analysis
for fixed equipment has not been completed, a DF can be estimated using the values in Table 6.11.
n) STEP 2.14—Calculate the POF of the protected equipment at the elevated overpressure, P f,j , using
Equation (5.109). Use the overpressure determined in STEP 5.2 of Section 6.4.6, the MAWP of the
protected equipment and the POF determined in STEP 2.13.
o) STEP 2.15—Calculate the POF,
prd
P f ,j
, using Equation (5.89) using P fod,j from Equation (5.94).
p) STEP 2.16—Repeat STEP 2.1 through STEP 2.15 for each piece of equipment protected by the PRD.
Probability of Leakage (POL)
Overview
The leakage case is different than the fail to open case since the POF is not a function of demand rate but
rather is based on failure during continuous operation. The industry data associated with the probability of
leakage, P l , data are in per year units (i.e. failures/year). No multiplication by any demand rate is necessary.
Probability of Leakage Calculation Procedure
The probability of leakage for a PRD is determined using the following steps.
a) STEP 3.1—Determine default Weibull parameters, β and ηdef, based on category of Service Severity
and type of PRD; see Section 6.3.2 f through Section 6.3.2 j.
b) STEP 3.2—Apply an adjustment factor, F s , to account for the presence of soft seats; see Section 6.3.2 k.
c) STEP 3.3—Apply an adjustment factor, F env , for environmental factors; see Section 6.3.2 l.
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