The FMEDA technique is performed on a specific device (e.g., ball valve, pressure transmitter, temperature sensor, electronic module, etc.) specified down to the manufacturer and series/model. Based on the specifics of the design, the parts used to execute the design, the design margins, any automatic diagnostics, the specific use and the environment in which the device will be deployed, the FMEDA produces predictions for the dangerous detected and undetected failure rates, the safe detected and undetected failure rates, the diagnostic annunciation failure rates, the no effect failure rates, the dangerous and safe diagnostic coverages (for devices with self-diagnostic capabilities), and the useful life.

The analysis is FMEDA application-specific because a particular failure mode may be dangerous in one FMEDA application but safe in a different FMEDA application; for example, consider the difference when a valve opens on trip (OOT) vs closes on trip (COT). Currently six different environmental profiles for equipment may be used in the FMEDA method. These profiles include cabinet mounted or climate controlled, general field mounted (two versions with differences in internal temperatures), off shore subsea, off shore topside and process wetted. FMEDA analysis requires a validated database of failure modes and failure rates for the parts which comprise the various devices. This database slightly skews the part failure rates to be certain that the device failure rates will be conservative. However, currently the FMEDA analysis does not account for the effects of site-specific end-user activities. Essentially FMEDA predicts the inherent failure rate of a specific product in a specific FMEDA application and environment assuming that all end-users will take all appropriate end-user actions to insure that

  • the equipment is appropriate to the task, properly installed and calibrated, and correctly functioning when installed,
  • all in-service maintenance is correctly and completely performed on schedule, the equipment is maintained so that no ageing occurs prior to proof testing which is completely and correctly performed, and
  • the equipment is replaced when its useful life can no longer be extended by maintenance and refurbishment.

Over the years, a large number of devices have been subjected to FMEDA analysis and all FMEDA results have been collected and retained in a FMEDA database (separate and different from the validate parts failure mode and rate database used by FMEDA). FMEDA’s in the database are continuously calibrated against FFD as such data become available. FMEDA’s are updated if the parts database changes significantly. Based on this FMEDA database, generic failure rates are being compiled [12] giving the minimum, maximum, mean and 25, 50, 75 and 90 percentile of all failure rates for devices of a common type assuming a specific use. Other percentiles can be extracted from the FMEDA database but are not published.

Note that the percentiles are not confidence intervals because the FMEDA data does not represent a statistical sample of failures. Rather it provides the known range of failure rate predictions for similar devices which differ by design and manufacture. For example, FMEDA’s have been performed on a total of 106 specific ball valves (manufacturer and series/model). These 106 ball valves include, for example, 31 different floating ball valves, 37 different trunnion-mounted ball valves, etc. The total number of different FMEDA’s for ball values is 999 which is large because a specific ball valve may have been subjected to multiple analyses under different uses, environments, levels of diagnostic testing, etc.

Tagged as:     Loren Stewart     FMEDA  

Other Blog Posts By Loren Stewart