TL;DR: Fail safe valves are specified by what they do during a loss of power/air: fail open, fail closed, or fail last. “Normally open” and “normally closed” only make sense when you define the normal energy state and the failure scenario.
Misunderstanding fail-safe behavior is one of the fastest ways to create a safety incident or an expensive outage. This guide clarifies normally open vs normally closed, when to use spring-return, and how to write a spec that prevents ambiguity.
Normally open vs normally closed: what “normal” means
- Normally open (NO): the valve is open in the de-energized state (no power/air), and closes when energized.
- Normally closed (NC): the valve is closed in the de-energized state, and opens when energized.
The critical detail is: what energy source is being lost (electric power, instrument air, or both) and what the valve should do when that happens.
Fail positions (the spec you actually need)
- Fail closed: stops flow on loss of power/air. Common for chemical isolation and spill prevention.
- Fail open: maintains flow on loss of power/air. Used where continuous flow is safety-critical (cooling, purge).
- Fail last: stays in last position. Common for motorized valves unless spring-return is added.
How fail-safe is implemented
- Spring-return actuators: drives valve to fail position when energy is removed.
- Double-acting actuators: often fail last unless paired with a separate fail-safe mechanism.
- Solenoid valves: coil energization can be NO or NC; pilot supply and pressure also matter.
Common mis-specs (and how to avoid them)
- Ambiguous wording: “normally closed” without stating fail condition.
Fix: specify “fail closed on loss of power and loss of air.” - Ignoring process consequence: fail closed when a purge must continue.
Fix: define the hazard scenario and choose fail position accordingly. - Ignoring media compatibility: wrong seats/seals for chemical.
Fix: specify full wetted BOM and temperature range.
Spec template (copy/paste)
- Valve function: isolation/control/check
- Actuation: solenoid/motorized/pneumatic
- Fail action: fail open/fail closed/fail last (define energy loss condition)
- Power: voltage and control method
- Media: chemistry, concentration, temperature, solids
Related engineering resources
Frequently Asked Questions
Not always. “Normally” describes the de-energized state for a specific actuator/solenoid configuration. “Fail closed” describes what happens during a defined failure (loss of power, loss of air, etc.). Always specify the failure condition and desired action.
Many motorized valves fail last (stay where they are) unless they include a spring-return or battery/backup system designed to drive them to a safe position.
Often fail closed to prevent spills, but it depends on the hazard. Some systems require fail open for cooling or purge. Decide based on the consequence of stopping flow vs continuing flow during a failure.
