Each time we add automatic valves in a control loop it is very important to identify what can happen if the valve fails.
Valve failure is related to the loss of power to open, close or modulate an auto valve moving its actuator. This power usually is air, but can be electricity, other gases or liquid.
Fail position can be OPEN, CLOSE or KEEP POSITION. Choose the correct safe mode will depend on your process and the control purpose.
For instance, in a control system to keep an exothermic reaction under temperature setpoint, a control valve at the cooling system usually is Fail Open. If the valve goes to 100% open because of the loss of air, it is safer than going to 0% open.
Fail position is reviewed during HazOp however chemical process engineers must anticipate all process conditions as possible during conceptual design.
It interesting to mention that the auto valve fail position is only part of the safety system. The design team needs to guarantee that controller, I/P transducer, transmitters and operation screen converge to the same purpose and information.
Sometimes that is very trick in control loop that uses reverse mode, but that is a subject to another post 🙂
The picture shows the representation of the fail mode shown in the P&ID.
