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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-71
a) STEP 7.1—For the PRD failure to open case, calculate the risk associated for each of the applicable
prd
overpressure demand cases, Risk f ,j
, using Equation (5.128).
b) STEP 7.2—Sum up the individual risks associated with the applicable overpressure demand cases to
prd
get a total risk for the failure to open case, Risk f
, using Equation (5.129).
c) STEP 7.3—Calculate the risk for the PRD leakage case,
d) STEP 7.4—Calculate the total risk using Equation (5.131).
Inspection Planning Based on Risk Analysis
RBI Intervals
prd
Risk using Equation (5.130).
l
The inspection intervals for PRDs are determined by the probability and consequence of the event outcomes
that can result from PRD failure to open or leakage. The probability side of the equation relates to the
probability that the PRD fails to perform its function, either failing to open upon demand or leaking. The
consequence side relates to loss of containment from the piece of equipment protected by the PRD or to
leakage through the PRD.
Risk increases as a function of time since the POF of the PRD increases and the probability of leakage
through the PRD increases with time. Additionally, the risk of PRD failure increase since the protected
equipment damage increases over time, which increases the probability that loss of containment occurs. The
recommended interval is determined for a PRD by calculation of the risk as a function of time and
determination of the time at which the risk is equal to the risk target. See Part 1, Section 4.4.2 for a
discussion of risk targets.
Effect of PRD Inspection, Testing, and Overhaul on Risk Curve
Figure 6.7 shows the effect of testing, inspection, and repair of the PRDs. The figure also illustrates the effect
of the risk target. For the example presented in Figure 6.7 a risk target of $25,000/year resulted in inspection
intervals of 5 years. Alternatively, if the risk target were $10,000/year, the resulting inspection interval would
have been every 3 years.
A critical assumption is made with the risk curve shown in Figure 6.7. The approach assumes devices are
normally overhauled to an as-new condition or replaced at the time of testing. The risk of failure goes to zero
just after the test.
Effect of PRD Testing Without Overhaul on Risk Curve
Many PRD inspection and test plans are set up such that when a PRD is inspected and tested, the
opportunity is taken to overhaul the PRD and return it to service in an as-new condition. In other inspection
and testing plans, however, a PRD is inspected and/or tested without overhaul. An example would be
performing a pop test in the shop in the as-received condition and returning the PRD to service without
overhaul. Another example would be performing an in situ pop test while the PRD remains on the unit.
In these situations, confidence that the PRD is in working condition is gained, but the PRD has not been
restored to an as-new condition. Inspection is credited by adjusting the POF and leakage curves (adjustment
of Weibull η parameter). If the test were successful, the test interval will be increased, marginally. However,
the risk does not drop back down to zero as would be the case if the PRD were overhauled. Therefore, the
full benefit of the increased test interval will not be realized.