Functional Testing of Valves: A study of possible improvements that can be achived by Partial Stroke Testing at Statoil Kollsnes
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Summary and Conclusion Emergency Shutdown (ESD) valves and High Integrity Pressure Protection System (HIPPS) valves are final elements in Safety Instrumented Systems. Proof or functional testing are performed at specific intervals in order to ensure the valves’ function. For these type of valves, such tests may require costly shutdowns. For a final element, like a valve, the interval between proof tests are by performance based standards like IEC61505/ 61511, related to the Probability of Failure on Demand (PFD) for the element’s complete function. Partial Stroke Testing (PST) can reduce the time between such tests. PST is performed by allowing the valve to start closing, and to start reopening before the process is interrupted. By performing a partial test at a shorter interval, a fraction of the dangerous undetected failures, the PST coverage, are tested more often. This will reduce the PFD and imply that the proof test interval can be extended, still achieving the same Safety Integrity Level (SIL). Three systems for PST have been evaluated: The Emerson FieldVue DVC 6000 SIS, the Drallim Smart Valve Monitoring and the Crane ValveWatch II. ValveWatch and Smart Valve Monitoring provides leak testing in open and closed position, respectively. Leak testing on an open valve is treated as a partial test and is included in the PST coverage. FieldVue can not be used on hydraulic operated valves. Three ESD valves and one HIPPS valve on the Kollsnes gas processing plant have been analyzed. Two systems have been evaluated for each valve. A Failure Mode, E.ects and Diagnostic Analysis has been performed on the tested parts of each valve’s complete function. Based on this analysis, the PST coverage and increased test intervals are calculated. The PST coverage found for the hydraulic operated ESD gate valves are 66% and 74% for ValveWatch and Smart Valve Monitoring, respectively. For the pneumatic operated ESD ball valve, a PST coverage of 21% has been found for FieldVue and 56% for Smart Valve Monitoring. The calculated intervals between functional tests to achive a SIL 2 rating, are between 0, 54 and 0, 89 years for these ESD valves. SIL 2 corresponds to a PFD < 10-2 . For the ESD valves, new control circuits with lower failure rates should be designed in order to improve the results. Today’s one year proof test interval on ESD valves seems not to be in accordance with the standard used in this project, i.e to achieve SIL 2 the proof test interval should be reduced. A PST coverage of 100% and 0% has been found on the HIPPS solenoid valve for Smart Valve Monitoring and FieldVue, respectively. The PST coverage for the valve and actuator is 53% for both systems. Based on an inital annual test interval for the overall pressure protection system, is PST, by the system with best coverage, roughly estimated to increase the test interval by 76%. The results are highly dependent on the choices and assumptions made in this project. A closer study of experience data could have increased the quality of the analysis. Such data is not found easily available. The results are assumed to be relatively conservative. Deeper involvement of professionals with operational experience from relevant areas would probably also have improved the quality. The Drallim Smart Valve Monitoring has achieved the best results in this analysis. ValveWatch and FieldVue would improve their PST coverage if ESD solenoids were tested. They are also able to monitor actual shutdowns and may contribute to document a shutdown as proof test. These systems also make more measurements and may be better suited for predictive maintenance purposes. ValveWatch has achieved the highest PST coverage for a valve separately, because of its leak test. All the evaluated systems will give increased knowledge about the valve’s condition, and thereby improve safety. The PFD will be substantially reduced. Future needs at Kollsnes should be analyzed further, and an eventual installation of such systems should be done according to these needs.