Is fail lock control valve work based on air pressure loss/failures or DCS controller failures (or output controller signal loss) or on both failures?

Regards,
Sigit

 

I think it is due to air supply failure

Regards
saravanan.m

 

The rest of us think the originator didn't provide enough information, and that it depends on the design of the valve and the system and the philosophy required for the application.

 

When we talk about "Fail Open", "Fail Close" or "Fail Lock" action of valve it generally applies for air failure condition only and hence are normally termed as AFO, AFC or AFL. This is normally indicated in P&ID and is a licensor requirement. In earlier days of pneumatic instrumentation this information was good enough as the other dimension of electronic "signal failure" was not existing.

Signal failure creates the same condition as air failure to reach end position of "fail Open" or "Fail Close" if the valve is simple, single acting spring return type with globe/ball/butterfly etc construction.

If the valve is double acting (by air), air failure may lock the valve (as valve may stay put in absence of the driving force if the valve do not have volume bottle & trip valve) but signal failure may take the valve to open or close or stay put depending upon how the SOV or other accessories are connected to valve.

Regards.