Robert E. Gross

Analysis of safety relief valve proof test data to optimize lifecycle maintenance costs

Proof test results were analyzed and compared with a proposed life cycle curve or hazard function and the limit of useful life. Relief valve proof testing procedures, statistical modeling, data collection processes, and time-in-service trends are presented. The resulting analysis of test data allows for the estimation of a probability of failure on demand (PFD). Extending maintenance intervals to the limit of useful life as well as methodologies and practices for improving relief valve performance and reliability are discussed. A generic cost-benefit analysis and an expected life cycle cost reduction concludes that

Extending Pressure Relief Valve Inspection Intervals by Using Statistical Analysis of Proof Test Data

90 million maintenance dollars might be avoided for a population of 3000 valves over 20 years.

Probability of Initial Failure for Spring Operated Relief Valves

This paper correlates as-received relief valve test results with current inspection intervals and presents conclusions based on statistical analysis. During the past three year period over 500 used valve proof test records from a site population of 3500 safety relief valves were acquired and reviewed. Collection and analysis of spring-loaded relief valve test data continues with the goal being to increase the test intervals within guidelines, reduce costs, and maintain safety margins. Based on current test intervals of 1–7 years, time in service appears to have a minimal effect on valve performance. Seat material and inlet size are identified as having a statistically significant impact on valve performance. An increase in TP/SP of 1–2% per year was noted for soft seated, small inlet sizes. Photographs of failed valve internals and discussion of failure causes are included.Copyright

Hybrid Power: A 2003 Perspective for the Decade

We present clear and convincing evidence that, for new spring operated relief valves (SORV) that are not proof tested by the user shortly before installation, there is a non-trivial probability that the SORV will be installed in the fail-to-open (stuck shut) failure mode. Using the results of over 4800 new ASME Boiler and Pressure Vessel Code Section VIII SORV proof tests, we estimate the probability of initial failure (PIF) due to manufacturer/assembly anomalies, as well as PIF due to in-storage aging of SORV based on their material composition. We indicate how PIF can be reduced by various preinstallation activities that may be undertaken by the user. We show how to compute values of PIF to be used in calculating the average probability of fail danger (PFDavg) (as required by IEC61508 and similar safety standards in order to determine a safety integrity level (SIL)) which accounts for both the SORV material composition and the pre-installation activities undertaken. For four typical SORV of different material compositions we show how pre-installation activities influence the achievable SIL. We discuss the implication of these findings for estimating PIF for used (previously installed) SORV. We close with recommendations to further address PIF.Copyright

The Adhesion Failure Mode in Stainless Steel Trim Spring Operated Pressure Relief Valves

Hybrid power generation systems are evolving as attractive candidates for high-efficiency production of electric energy and a near-zero emission of primary pollutants. Hybrids, by definition, are a combination of two or more power generation technologies that create a synergistic value greater than the sum of the individual parts. Examples include fuel cells integrated with gas turbines, fuel cells integrated with photovoltaics, and heat engines integrated with wind generation. This paper describes the evolution of hybrid systems for both distributed power generation and central power generation as anticipated for this decade, the development requirements and application opportunities, and strategies that utilize both fossil and renewable fuels to achieve the dramatic increase in electrical efficiency that hybrid systems portend.Copyright

Validation of spring operated pressure relief valve time-to-failure†

This paper addresses dangerous failures of stainless steel (SS) trim spring operated pressure relief valves (SOPRV) due to a particular failure mode (SS-to-SS adhesion) which is not currently being included in SOPRV failure rates. As a result, current methods for estimating or predicting failure rates for SS trim SOPRV significantly underestimate these failure rates and, consequently, overestimate the safety provided by the SOPRV as measured by its average probability of failure on demand (PFDavg) or its corresponding safety integrity level (SIL). The paper also illustrates the critical importance of root cause analysis (RCA) of dangerous SOPRV failures in understanding the impacts of various failure modes.Over 1300 proof test results for both new and used SS trim SOPRV from the Savannah River Site (SRS) were identified. RCA was used on the failed valves to classify those failed due to SS-to-SS adhesions. Statistical analysis of the data convincingly demonstrates adhesions, previously assumed to be only an in-storage failure phenomenon, are also an in-service failure mode which needs to be included in SOPRV failure rates. The paper discusses the factors which potentially influence the adhesion failure mode and suggests a possible approach to including this mode in failure rate predictions. An example illustrates how current failure rate models overestimate SS trim SOPRV safety by one or two orders of magnitude.Copyright

The Effects of Maintenance Actions on the PFDavg of Spring Operated Pressure Relief Valves

The Savannah River Site operates a relief valve repair shop certified by the National Board of Pressure Vessel Inspectors. Local maintenance forces perform inspection, testing, and repair of ∼1,200 spring‐operated relief valves each year as the valves are cycled in from the field.

Statististical Performance Evaluation of Soft Seat Pressure Relief Valves

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.

Investigation of Adhesion Formation in New Stainless Steel Trim Spring Operated Pressure Relief Valves

Risk-based inspection methods enable estimation of the probability of failure on demand for spring-operated pressure relief valves at the United States Department of Energys Savannah River Site in Aiken, South Carolina. This paper presents a statistical performance evaluation of soft seat spring operated pressure relief valves. These pressure relief valves are typically smaller and of lower cost than hard seat (metal to metal) pressure relief valves and can provide substantial cost savings in fluid service applications (air, gas, liquid, and steam) providing that probability of failure on demand (the probability that the pressure relief valve fails to perform its intended safety function during a potentially dangerous over pressurization) is at least as good as that for hard seat valves. The research in this paper shows that the proportion of soft seat spring operated pressure relief valves failing is the same or less than that of hard seat valves, and that for failed valves, soft seat valves typically have failure ratios of proof test pressure to set pressure less than that of hard seat valves.

Estimated SIL Levels and Risk Comparisons for Relief Valves as a Function of Time-in-Service

Examination of proof test data for new (not previously installed) stainless steel (SS) trim spring operated pressure relief valves (SOPRV) reveals that adhesions form between the seat and disk in about 46% of all such SOPRV. The forces needed to overcome these adhesions can be sufficiently large to cause the SOPRV to fail its proof test (FPT) prior to installation. Furthermore, a significant percentage of SOPRV which are found to FPT are also found to “fail to open” (FTO) meaning they would not relief excess pressure in the event of an overpressure event. The cases where adhesions result in FTO or FPT appear to be confined to SOPRV with diameters less than or equal to 1 in. and set pressures less than 150 pounds per square inch gauge (psig) and the FTO are estimated to occur in 0.31% to 2.00% of this subpopulation of SS trim SOPRV. The reliability and safety implications of these finding for end users who do not perform pre-installation testing of SOPRV are discussed.