Fail Open & Fail Closed Control Station Assembly

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You can see Fail Closed (FC) or Fail Open (FO) control valves in P&ID. What does it mean? What to do on this situation as Piping Designer. Before going into action we need to know exactly what happens when the actuating medium fails(most of the cases Instrument Air). See the snap below. It is a control valve assembly extracted from a P&ID           If the actuating medium fails may be

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