Strengthening the Weak Link: the Shutdown Valve

The advent of IEC 61511 has been a good start in improving availability and reliability of safety instrumented loops. Unlike earlier standards like ANSI/ISA S84 and API 14 C, IEC 61511 facilitates mandatory performance based analysis of safety-instrumented functions based on failure data. It is observed that failure data of shutdown valves have been extremely high compared to other elements of safety-instrumented loop. Increasing number of valves in a loop also will drastically affect availability; maintainability, space and cost. Objective of this paper is to improve safe availability of shutdown valve by employing complete stroking instead of partial stroking, increasing diagnostic coverage by using continuous position monitoring instead of limit switches, eliminating the air regulator and selection of proper communication protocol. Some buffering capacity in process is also suggested to facilitate full stroking instead of partial stroking. It is envisaged that SIL 3 level with single shutdown valve in a safety instrumented loop can be achieved with these modifications