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Results of Statistical Analysis of Pressure Relief Valve Proof Test Data

The purpose of this document is to report on our successful efforts to validate
statistically certain random equipment failure rate data used in a mechanical
parts failure rate and failure mode database and, by extension, to validate the
techniques used to derive the data. To accomplish this, a Failure Modes,
Effects, and Diagnostic Analysis (FMEDA) is initially used to predict the usefullife
failure rate for the fail-to-open condition of a particular pressure relief valve
(PRV) using the failure rates from the mechanical parts database. Next, this
prediction is statistically tested against three independent data sets consisting of
proof test data for PRV provided by Fortune 500 operating companies. The data
sets all meet the intent of the quality assurance of proof test data as documented
by the Center for Chemical Process Safety (CCPS) Process Equipment
Reliability Database (PERD) initiative. By applying the quantal response method
to the results of these PRV proof tests, it is demonstrated that the proof test data
are consistent with the predictions of the FMEDA. Specifically, all of the data
sets support the FMEDA result at a 95% confidence level. All analyses lead to a
useful-life PRV failure rate between 10-8 and 10-7 failures/hour.

It is very important to note that the results of this study cannot be used to
justify extension of proof test intervals beyond the useful life of the PRV. The
small value of the failure rate derived from the FMEDA applies only to the useful
life of the PRV which depends not only on the equipment’s specifications but also
on other factors, such as the ambient and process environment in which the PRV
is used and the levels and frequency of any on-line maintenance performed.
Data analyses place useful life in the range of 4 to 5 years.

Finally, we note that the results of the statistical analyses of the three
independent data sets predict an initial failure probability of approximately 1% –
1.6%. This initial failure probability is extremely significant as it accounts for the
vast majority of failures observed in proof test. This emphasizes the value of
careful installation and thorough commissioning procedures. When
commissioning testing cannot be done after installation, as is the case with a
PRV, both the initial probability of failing to open, as well as the PFD based upon
the random failure rate must be taken into account in the risk analysis.

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