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Will a Partial Valve Stroke Testing lead to a higher SIL? - Hazardex 2016

Author : Hassan EL-Sayed, CSA Group

28 October 2015


**This paper will be presented at Hazardex 2016. Contact us for details of the available delegate packages & offers**

Diagnostics may form part of smart devices and final elements as an enhancement for the safety integrity levels, and to offer improvement in maintenance programs to ensure continual operation and detecting of hiding faults. Diagnostics are widely used to identify critical conditions, parts not performing reliably, hidden faults where demands for safety functions may not be fulfilled.  

Partial valve stroke testing (PVST) of the final element is sued to ensure safety function availability without interrupting operation and extend the recommended full proof test interval.. This also improves the probability of failure on demand (PFD) while the safe failure fraction (SFF) remains unchanged. As seen in some of the current certified products to IEC 61508, SFF is affected because the PVST is considered as a diagnostic tool and used in the derivation of the failure mode and effect analysis (FMEA) which yields to increase in SFF. This conflicts with the practice of the IEC 61508 which eventually affects the final calculation of the intended safety loop. This PVST function is designed to be integrated internally or implemented externally to the final element.
 
Before, and even after the release of issue 2 of the IEC 61508:2010, many final elements were certified and achieved safety integrity level (SIL 3) based either on route 2 H  or route 1 H . These claims were based on having PVST as a tool for diagnosing dangerous undetected components without examining the rest of the overall safety related structure and whether it meets the general requirements of the safety integrity level (SIL) as specified in IEC 61508-2, clause 7.4.4.1. 

The paper reviews a few examples of already certified products in the market using PVST capability, and shows the impact of PVST in the SIL analysis for which a higher SIL capability is being claimed. The paper will discuss a newly certified final element based on the analysis published in the IET conference in 2013. It shows that when a PVST tool is used as an internal part of the element, the final element becomes a type B as stated in IEC 61508-2, clause 7.4.4.1.3, and the improvement made is noticeable to the PFD values while SFF remains unchanged.  

This paper looks at the past, the current practice of using PVST and highlights the issues that lead to misunderstanding the main purpose of PVST. It will provide guidance for proper use of the PVST which is based on an independent point of view with no vested interest, tangible or intangible, from the certification bodies. 


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