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[chem101]

Hi everyone:

why don’t we use bypass for fail open control valve. I have talked to my senior engineers
they mention that because of low cost.
Thank you very much in advance

[joerd]

I don't quite understand. The failure position of a control valve has nothing to do with providing a bypass or not.
Normally, you provide a bypass if you want to continue operating the plant while the control valve is out of service, for maintenance or because of a failure.
The fail position of a control valve is selected so that the system goes to a safe state should the actuator or instrument air fail. For example, usually reflux control valves fail open so that cooling is maximized in the event of a failure. Likewise, most steam supply valves fail closed.
Hope this helps,