The Effects of Maintenance Actions on the Average Probability of Failure on Demand of Spring Operated Pressure Relief Valves Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manus

Abstract

The safety integrity level (SIL) of equipment used in safety instrumented functions is determined by the average probability of failure on demand (PFDavg) computed at the time of periodic inspection and maintenance, i.e., the time of proof testing. The computation of PFDavg is generally based solely on predictions or estimates of the assumed constant failure rate of the equipment. However, PFDavg is also affected by maintenance actions (or lack thereof) taken by the end user. This paper shows how maintenance actions can affect the PFDavg of spring operated pressure relief valves (SOPRV) and how these maintenance actions may be accounted for in the computation of the PFDavg metric. The method provides a means for quantifying the effects of changes in maintenance practices and shows how these changes impact plant safety. In a properly operating SOPRV, a spring exerts a downward force/pressure on the disk pressing the disk against the seat. The seat is the top surface of the wall of the nozzle. The green circles in During normal plant operation, the SOPRV is in the closed position. If the process pressure increases beyond that of the spring set pressure, the disk will be lifted allowing process fluid to flow through the outlet thereby relieving excess process pressure. When the process pressure returns to the closing pressure of the SOPRV, the disk once again closes against the seat to provide a fluid tight seal and the process proceeds normally. The SOPRV can fail in a number of ways. If the SOPRV either spuriously opens or fails to form a fluid tight seal when the process pressure is within normal ranges, the valve is said to leak. These failure modes usually are considered to be safe failures (provided that the unintended pressure relief and fluid release do not themselves induce a safety hazard). On the other hand, if the SOPRV does not open under conditions of excessive process pressure, the valve is said to be "fail to open" (FTO) or to be "stuck shut," and this is a dangerous failure. PFDavg measures the average probability of being in this dangerous failure mode when excessive process pressure needs to be relieved. In a process, the occurrence of excessive pressure is called a demand on the SOPRV hence the metric, PFDavg. Because the SOPRV is normally closed, it is not possible to observe the FTO dangerous failure mode during normal operation. Consequently, safety standards, such as Refs. Journal of Pressure Vessel Technology DECEMBER 2015, Vol. 137 / 061601-1 Copyright V C 2015 by ASME identified. Implementation of these maintenance actions will reduce the failure rate and impact PFDavg. The remainder of this paper: • provides background information about the computation of PFDavg relevant to the study • describes the source of data, rationale for data choice, and summarizes the relevant data • presents an analysis of the proof test failure data for a particular group of SOPRV • provides examples of categorizing the FTO and using the results to calculate the necessary parameters for computing PFDavg • shows the impacts of three different levels of maintenance actions on PFDavg under two different assumptions about infant mortality failures and compares these to the ideal case • closes with a discussion of the results and conclusions