Fail Open vs Fail Closed Valve: Setting the Safe Default

For any valve with an actuator, one of the first questions on the datasheet is the failure position: what does the valve do when the air supply, power, or signal is lost? The answer — fail open or fail closed — is a safety decision, not a preference. Choosing it correctly means imagining what the process does when the valve stops working, and that depends entirely on the application.

The Short Answer

Fail closed (fail-to-close, FC) means the valve closes automatically when the actuating energy is lost; it is used where losing flow is the safe condition — for example, cooling water, fuel shutoff, or isolation of a hazardous process. Fail open (fail-to-open, FO) means the valve opens on loss of energy; it is used where keeping flow moving is the safe condition — for example, vent paths, purge lines, or cooling circulation that must not stop. The fail position is typically achieved with a spring, a counterweight, or a stored-energy system, and it must be stated explicitly in the spec.

What Each Failure Mode Is

Fail closed

With fail closed, the actuator spring or stored energy drives the valve to the closed position when supply is lost. Typical services include fuel or gas shutoff valves, chemical isolation, and cooling lines where a closed valve protects equipment or people. The valve must also seal tightly at the fail position, so seat tightness at zero supply is part of the design.

Fail open

With fail open, loss of supply moves the valve to the open position. Typical services include relief and vent paths, purge and blanketing lines, and cooling or lubrication flow that must continue even if control is lost. The fail-open valve must be sized so that full open does not create an unsafe flow condition.

Key Differences

Characteristic	Fail closed (FC)	Fail open (FO)
Position on energy loss	Valve closes	Valve opens
Safe condition	No flow	Full flow
Typical services	Fuel shutoff, cooling isolation, hazardous chemical isolation	Vents, purge lines, cooling circulation, blanketing
Actuation	Spring-to-close or stored energy	Spring-to-open or stored energy
Seat requirement	Tight shutoff at fail position	Full-open flow capacity at fail position

Which to Choose

Run the failure scenario: if the energy is lost right now, is the process safer with the valve open or closed? If a loss of flow would overheat, starve, or isolate a system dangerously, choose fail open. If a loss of containment or continued flow would create a hazard, choose fail closed. Document the reasoning on the datasheet, because the fail position is part of the safety case.

Common Pitfalls

• Copying the fail position from a similar valve without checking the actual process consequence.
• Assuming an air-to-open valve is automatically fail open; the fail position depends on the spring, not the normal operating direction.
• Ignoring the fail-closed seat leakage class — a fail-closed valve that does not seal at zero supply defeats the purpose.
• Forgetting that loss of instrument air, electrical power, and hydraulic pressure can fail differently; verify each energy source.

Practical Recommendation

State the fail position explicitly in the valve specification and the P&ID, and confirm it with the process engineer before ordering. For each energy source the valve relies on, verify the failure position is the intended one. When the consequence of the wrong choice is severe, consider a position indicator or a safety-instrumented shutdown review rather than relying on the actuator alone.

Conclusion

Fail open and fail closed are safety settings decided by the process, not by the actuator. Imagine the loss of energy and ask which position protects the system. Name the fail position on the spec, verify it with the process team, and the valve will do the right thing when it matters.

This article provides general educational knowledge only and is not professional engineering advice.

Tags: Actuators