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

5-54 API RECOMMENDED PRACTICE 581
In general, the adjustment of the POFOD is based on the results of the inspection of the PRD itself
(e.g. bench test results). This could lead to nonconservative results if the inlet or outlet piping plugs
during operation and could affect the reliability of the PRD system. For each inspection date
entered, the inspected condition of the piping should be documented. If the piping is determined to
be plugged, the methodology assumes the inspection/test to be a FAIL, regardless of the results of
the bench test or inspection method used on the PRD. Good engineering practice would suggest
that if the piping is plugged by more than 25 %, the piping should be defined as being plugged,
since this would then drive down the inspection interval. This methodology adjusts the reliability of
the PRD system to reflect excessive pipe plugging.
4) Effectiveness of Inspection Programs in Confirming Failure Rates
Inspection programs vary in their effectiveness for determining failure rates. The definitions for
inspection and testing effectiveness for PRDs are provided in Part 2, Annex 2.C, Table 2.C.3.1.
The inspection’s effectiveness is based on its ability to adequately predict the failure (or pass)
state of the PRD being inspected. Limitations in the ability of the program to improve confidence in
the failure rate result from the inability of some test methods to detect and quantify damage.
For PRDs, an inspection and testing program should track the effectiveness of the inspection and
the testing performed for each PRD. The concept of inspection effectiveness is similar to the
concept that is described in Part 2, Section 4.4.3 of this document for fixed equipment. For
inspection effectiveness of PRDs, a measure of confidence in the pass/fail/leak result of the
inspection effort is used.
Table 6.9 provides default confidence values based on expert opinion. The confidence values are
an indication that the inspection will result in an accurate representation of actual PRD
performance during an overpressure demand case. For example, the 90 % effectiveness
associated with passing a “highly effective” bench test means that there is a 90 % probability the
device would have opened upon demand in its installed service. Therefore, it also carries a 10 %
probability that the PRD would have failed upon demand during operation. The values shown in
Table 6.9 are called conditional probabilities.
The conditional probabilities listed reflect the confidence that an inspection result will predict the
device’s performance upon demand. For passing PRDs, the highest confidence is assigned when the
PRD is bench tested without any prior cleaning (i.e. as-received condition). Bench testing where the
devices are cleaned prior to testing, in situ testing, and visual inspections provide some information
about PRD performance, but are not considered as reliable as the as-received bench test.
The philosophy is different for PRDs that fail an inspection. In the case of a “highly effective” bench
test failure, the 95 % confidence translates to a 95 % chance that the PRD would have failed upon
demand in service. Unlike the passing test case, the “usually effective” in situ test, or bench test
where the device has been steamed out prior to testing, is assumed to have the same 95 %
confidence for failure upon demand in actual service.
An ineffective test does not provide any information to predict PRD performance upon demand and
therefore the PRD does not receive any credit for the test/inspection date. The inspection still will
get some credit if an overhaul was performed in that the device is assumed to be returned to
service in like-new condition, and the in-service duration is calculated from the ineffective
inspection date.
5) Inspection Updating
As previously discussed, Weibull parameters for the failure on demand curves have been
determined based on the analysis of a sample set of data. Initially, these values are default
(suggested) parameters for the listed fluid services. As inspection data are collected for each PRD,
these parameters may be adjusted for each device based on the inspection results.