Fail Open & Fail Closed Control Station Assembly

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 intentionally or accidentally will impact the position of the control valve depends on the type of the control valve.

 

 

Control Valve Fail action may be classified as

• Fail Closed (FC)
• Fail Open (FO)
• Fail Indeterminate (FI)

Fail Closed (FC)

Say a P&ID calls for FC. That means when Instrument air goes off, the control valve will be in closed position. To remove the control valve for maintenance or replacement, we have to open the bye-pass valve and close two block valves one bye one. After closing the valve the fluid will be trapped between  first block valve and the control valve and also between control valve and the second block valve (remember the control valve is now in closed position). To remove the fluid and also depressurize the system we need to drain the fluid both upstream and downstream of the control valve.

So providing drains on both upstream and downstream will solve the problem

 

The below snapshot show the Fail Closed control valve.

 

 

FAIL OPEN (FO)

Say a P&ID calls for FO. That means when Instrument air goes off, the control valve will be in Open position. To remove the control valve for maintenance or replacement, we have to open the bye-pass valve and close two block valves one bye one. After closing the valve the fluid will be trapped between  first block valve and the second block valve (remember the control valve is now in open position). To remove the fluid and also depressurize the system we need to drain the fluid both upstream and downstream of the control valve.

 

So providing drain on upstream or downstream will solve the problem

The below snapshot show the Fail Open Control valve.

 

FAIL Indeterminate (FI)

Say a P&ID calls for FI (Fail Indeterminate). That means when Instrument air goes off, the control valve will be in the position. It will not change its position. To remove the control valve for maintenance or replacement, we have to open the bye-pass valve and close two block valves one bye one. For the worst case scenario if the control valve in closed position when the power goes off. To remove the fluid and also depressurize the system we need to drain the fluid both upstream and downstream of the control valve.

So providing drain on  both upstream and downstream will solve the problem