Partial Stroke Test

A very useful method of detecting possible faults of failing of On-Off-Valves is the “Partial Stroke Test” (PST).

If certain On-Off-valves are used very rarely, such as safety valves which have to open or close only in emergency situations of the plant, they may increase their friction in the course of time, especially the initial break off friction, because of corrosion or increasing encrustation of the plug stem. If the emergency event happens and the ability of valve operating is urgently needed, often the safety valve is then blocked. To prohibit this malfunction a “Partial Stroke Test” in defined time intervals is required.

The philosophy and the realization of this test is as follows:

The valve is moved by the actuator from time to time for a small amount only, partially, which is related to the expression “partial stroke”.

This partial stroke movement doesn’t influence the running process in a severe manner, because the valves mostly are in series or parallel to control valves, which make this very small reaction on the flow performance of the fluid to vanish, that the resulting effect is negligible. During this short lasting PST the reaction of the valve is measured and the result is signalized by the three coloured wrenches.

If the result of the test was successful, two important facts can be manifested:

• The performance of the valve is O.K. for the next future
• Possible existing corrosion or encrustation before the test have been removed, as the valve has moved itself. So the stem and plug are “free” again.