Assessment of PVST in accordance with IEC 61508:2010

For many years, diagnostics have been implemented for field devices (smart devices and final elements) comprising shutdown valves, actuator and solenoid valves, using online measurement and testing technologies. Process plants strive for maximum efficiency and in doing so look to ensure continual operation avoiding sudden shutdown, very often reliant upon condition monitoring to optimize maintenance activities. Online product diagnostic is one of the measures that is used to identify the critical conditions where maintenance can be scheduled reliably, to extend the operating life of the plant and reduce cost. Partial stroke testing (PST) is widely used in the final element to ensure product availability, extend the proof test interval and improve the probability of failure on demand (PFD), but not the safe failure fraction. This PST can be integrated internally or implemented externally to the final element. Before the release of issue 2 of the IEC 65108:2010, many final elements were certified and achieved an architecture constraints of SIL 3 on the basis of having PST as a tool for dangerous undetected without looking deeper to the indirect effects to the overall structure of the type of the product and subsequently to the overall safety instrumented function. The paper is discussing some of the changes made in IEC 61508:2010 (edition 2) which may affect the assessment of the final element with diagnostic, implication of these changes on product certification and its impact on the currently certified products assessed as per edition 1. Example is reviewed with and without PST using Fault Tree analysis and recommendation made when reviewing the overall SIL as a result of these changes.