For shutdown valves used in safety instrumented systems it is essential to know that the valve is capable of providing the required level of safety performance and that the valve will operate on demand.
The required level of performance is dictated by the Safety Integrity Level (SIL). In order to adhere to this level of performance it is necessary to test the valve. There are 2 types of testing methods available being
Proof test – A manual test that allows the operator to determine whether the valve is in the “as good as new” condition by testing for all possible failure modes and requires a plant shutdown
Diagnostic Test – An automated on-line test that will detect a percentage of the possible failure modes of the shutdown valve. An example of this for a shutdown valve would be a partial stroke test. An example of a mechanical partial stroke test device.