air-to-open or air-to-close
air-to-open or air-to-close
(OP)
What is the conventional procedure to show control valves' O and C positions on P&ID drawings, on (actuators') air action or on air failure ?
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air-to-open or air-to-close
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RE: air-to-open or air-to-close
RE: air-to-open or air-to-close
If I'm understanding your question correctly, from my experience, a control valve will be indicated with a fail position as either FC (fail closed) or FO (fail open) notation on a P&ID. The valve would be expected to go to the fail position upon loss of air to the actuator.
The information from ISA standard S5.1 is consistent with this but maybe one of the instrument engineers can comment.
From ISA S5.1, Section 6.7, Symbols for actuator action in event of actuator power failure....
The failure modes indicated are those commonly defined by the term "shelf-position".
So an air-to-open valve would be indicated as FC, air-to-close valve would be indicated as FO.
RE: air-to-open or air-to-close
RE: air-to-open or air-to-close
The conventional procedure to show control valves is FO for fail open, FC for fail closed, FL for fail last (as-is) etc. on the P&ID's.
John
RE: air-to-open or air-to-close
In control valves some positioners can be programmed to go into "alarm state" before it's supply pressure decays below a level which the actuator needs to move the valve. A balance trim design (ball valve)would be required to maintain the valve's position. The DCS control system could be monitoring the plant air pressure and close valves before the pressure decayed.
Is the fail position based on loss of air, loss of electricity or loss of both?
Double acting actuators are always pressurized to move. Spring return actuators "inhale" what ever is around them when they are de-pressurized. Dirt or corrosive vapors, moisture that freezes in the North.
Sorry, I just think spring return actuators are over sold in North America due to a lack of consideration making us less competative to install a process.