Control Valve · Glossary

What is Fail Safe Position?

FO, FC, FLControl Valve

Fail safe position is the position a valve goes to on loss of air or signal: fail open, fail closed or fail last (in place).

Fail Safe Position Explained

The fail action is chosen from the process hazard: a fuel valve fails closed, a cooling water valve often fails open. Spring return actuators provide it naturally.

Fail last needs lock up valves or a double acting actuator with a volume tank, and is less certain than spring action.

Fail Safe Position: Typical Values

FC	fail closed, air to open
FO	fail open, air to close
FL	fail last or fail in place

Calculators for Fail Safe Position

CalculatorVolume Tank Sizing CalculatorFree online calculator with formula and worked example.CalculatorActuator Stroke Time CalculatorFree online calculator with formula and worked example.

Free Downloads

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Frequently Asked Questions

What is Fail Safe Position?

Fail safe position is the position a valve goes to on loss of air or signal: fail open, fail closed or fail last (in place).

Does the positioner set fail action?

No. Fail action comes from the actuator spring or lock up; the positioner only sets action on signal.

What if power fails but air remains?

The positioner loses signal and vents or holds according to its design; check that it matches the required fail action.

Learn more on the blog

Diaphragm actuator vs piston actuator

A practical guide on instrumentationblog.in with more detail on fail safe position and how it is applied on real plants.

Read Full Article

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