Control Valve Fail Action


Thread Starter



i am working with instrumentation engg group. we are come across control valve datasheets in which the fail action is fail locked drift close and fail locked drift open. What we know are Fail open, Fail close and Fail Last. now in all datasheet they have use lock up relay (Fisher 164A), which will locked the actuator in last position in the case of air supply interrupted, so in this condition fail. position called Fail Last. now question is what kind of fail action is this (fail locked drift close or Fail locked drift open) and how does it work in real time? What kind of process application is this? Your efforts will be highly appreciated

Thank you

Tanmoy Majumder


Failed locked / Drift Open or Close will work based on Fisher lock up relay, which is a pneumatic relay by function. On long run (i.e 2~3 hours), because of leakage of air through the relay, valve will go to its spring driven position, as they are not using any volume bottle (possibly not required by Process to be locked at the last position for longer hours). Many times Process do not require valve to be locked for longer hours as the loops are not critical or can be handled within 2~3 hours period.