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This report documents a study performed on the set of Common Cause Failure (CCF) events of Check Valves (CVs). The events studied here were derived from the International CCF Data Exchange (ICDE) database. Organizations from Canada, Finland, France, Germany, the Netherlands, Sweden, Switzerland and the United States contributed with data to this data exchange.
This study examines 94 CCF events of CVs reported in the ICDE database by tabulating the data and observing trends. The database contains general information about event attributes like root cause, coupling factor, detection method and corrective action taken. As part of this study, most of these events were reviewed in more detail and characterized by failure cause and failure symptom categories.
The study itself begins with an overview of the entire data set in chapter 5. Charts are provided for each of the abovementioned event attributes. This chapter forms the baseline for chapter 6. The intention of chapter 6 is to give the reader a deeper qualitative insight in the database content beyond that obtained from using the event coding only. Chapter 7 contains the summary of the study results and the conclusions derived from.
Approximately 8% of all ICDE events of CVs were complete CCFs (all redundant components had failed). The number of partial CCF events (at least two of the redundant components failed) accounted for 24%. In the remaining 68% of the ICDE events, less than two components had failed completely, and the other components of the observed group only suffered from small defects, incipient degradation or were not affected at all. However, it was found that for more than 75% of the ICDE events the causal factors had a high probability to be shared by all the
redundant components.
88 of the 94 reported ICDE events were reviewed in some more detail in Section 6 of this report with respect to failure causes, failure symptoms and failure mechanism. All events classified with a low "shared cause factor" were screened out for that review.
The most common failure mode of CVs is "failure to close" (includes internal leaking). Deficiencies in operation were responsible for about 50% of the failure causes, mainly due to "deficient maintenance procedures". In several cases test and maintenance intervals were too long, which prevent timely detection of the failure mechanism. The other 50% of failure causes were mainly due to "deficiencies in design of hardware". Two dominant failure symptoms have been identified: valve movement impeded by deposition of dirt or oxidation products and valve leakage due to disk/seat surface degradation. Other failure symptoms are disk/seat misalignment and problems with loose or broken piece parts. The dominant failure mechanism are mechanical wear, (in particular disk/seat surface degradation causing the valve to leak), and chemical wear (in particular corrosion products impeding valve movement).
