Need for an industry standard for ESD valves from engineering and safety point of view

Abstract
Since the Piper Alpha disaster in the North Sea, design of ESD valves has been given top priority and remains to be of great concern for plant safety management. Constant improvements have been made to ensure the integrity of the ESD valves. Essentially, ESD valves should perform their duty (usually closure of valves) under plant demand condition. To meet the production bottom-line, these valves are required to remain open for months, even years, which leads to build up or corrosion in the valve internals. Final control element is the weakest link in the SIS. It contributes 50% of total PFDavg for a SIF. To meet the desired ‘Availability’ figure for ESD valves, different tools have been devised. Partial Valve Stroke Test, Valve – Actuator signature curve, Testing Intervals (TI) are buzzwords in the industry. However, there is no industry standard for ESD valves which covers both engineering and safety aspects together.

KeywordsESD valves (Emergency Shutdown Valve), SIS (Safety Instrumented System), SIF (Safety Instrumented Function), Test Interval (TI), Maximum shut off Differential Pressure (Max. shut off DP), CAPEX (Capital Expenditure), Partial stroking, PFDavg (Probability of Failure on Demand), Dynamic Torque, Torque Safety Margin, PHA (Process Hazard Analysis), HAZOP (HAzard and OPerability study ), FMEDA (Failure Mode Effect and Diagnostics Analysis)IntroductionSelection of maximum shut off DP for ESD valve torque calculation and associated actuator sizing sometimes become subject for debate during the design stage. Too much conservative value leads to ‘oversized‘ actuator and increased CAPEX.Furthermore, there is not a standard guideline for the selection of the valve safety factor amongst the valve/actuator manufacturer (service factor) and actuator-valve torque safety margin. Many valve manufacturers utilize their own safety contingency in addition to the user‘s specification.As per the study, final control element contributes 50% to the SIS failure. Special care should be given while designing the ESD valves. However, ESD valves should not be oversized.

With the introduction of SIS, “Partial Stroke”, valve/actuator signature facilities, questions may be raised whether high design contingency is still required to cater the life time torque requirement. Essentially, all put together, elevates the CAPEX.This paper addresses the following:· Basis of maximum DP selection from process application and recommendation for a suitable maximum DP for SIS application · Cost analysis at different shut off DP & different actuator-valve torque safety margin · Type of actuator mechanism to be fitted with the valve · Dynamic torque specifications · Valve opening & closing time -design and safety concern · Safety valve on the actuator supply line - weakest link in actuator circuit · Partial stroke testing , Valve-Actuator signature curve, Test Interval (TI) · Valve-Actuator assembly PFDavg and SIL levels · ESD valve leakage, fire test specification · Miscellaneous design issues

Basis of maximum DP selection from process application and recommendation for a suitable maximum DP for SIS applicationSizing of an ESD valve actuator becomes a very easy task for an actuator manufacturer if all the necessary information is made available from the valve manufacturer. Valve torque is a function of maximum shut off DP. There is no standard guideline for deriving the maximum differential pressure for actuator sizing. In practice, philosophy varies from operator to operator, from one asset to another for a particular operator, and widely varies from one design consultant to another. Essentially, this happens because of lack of proper analysis during the design stage. In m