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.
Also, some companies' standards are to color the "bow-tie" black for a closed valve and not to fill the symbol for an open valve. So a FC valve would be colored, and a FO valve would only be outlined. Still there is less room to screw up if you write FC or FO next to the symbol.
You found an old controversy. Some consider a liability issue. What if a mechanical problem occurs and the valve does not travel to the failure position?
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.
Although the indication of "FO", "FC" and "FL" are excelent on a P&ID, we hope that further clarification is made on a "Control Valve Data Sheet" so this action can be further defined by the loss of what energy source. Double acting actuators which are smaller and less expensive than single acting can also have a fail position on loss of electricity with the solenoid valve. Some times on the loss of air pressure, it is better for the process to remain in last position.
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.
