Failsafe Actuator: How to Choose Fail Open vs Fail Close

A failsafe actuator is selected for one critical event: loss of normal electrical power. At that moment, the valve must move to the position that creates the lowest process risk. The safest position may be open in one system and closed in another.

What Fail Open and Fail Close Mean in a Failsafe Actuator

Fail Open: The Valve Moves Open When Power Is Lost

A fail open electric actuator drives the valve to the open position after a defined power failure. It is appropriate when maintaining flow is safer than stopping it.

Cooling-water, ventilation, drainage, or controlled depressurization paths may need to remain available during an outage. However, an actuated isolation valve should not be presented as a substitute for a code-compliant pressure-relief device.

Fail open does not describe the valve’s usual position. It describes the emergency position required after main power is lost.

Fail Close: The Valve Moves Closed When Power Is Lost

A fail close electric actuator moves the valve to the closed position when the specified failure occurs. Its purpose is commonly to isolate the medium, stop an incoming flow, or prevent uncontrolled transfer.

Examples include shutting off a chemical feed, preventing tank overflow, or isolating a process line. Closure is not automatically safest if the process depends on cooling, lubrication, drainage, or pressure control.

Fail-Safe Position vs Normal Operating Position

Normal position describes how the valve usually operates. Fail-safe position describes where it must move during the defined emergency.

A normally closed valve may require fail open action, while a normally open valve may require fail close action. The emergency position should therefore be stated directly instead of being inferred from “normally open” or “normally closed.”

Fail Open vs Fail Close: Key Differences at a Glance

Comparing Process Outcome, Flow Continuity, and Isolation

The real comparison is not simply open versus closed. It is the consequence of each position under actual process conditions.

Selection factor	Fail open	Fail close
Position after power loss	Valve opens	Valve closes
Main objective	Maintain a necessary flow path	Stop or isolate flow
Typical concern	Overheating, blockage, pressure buildup	Leakage, overflow, contamination, uncontrolled transfer
Key question	What happens if flow stops?	What happens if flow continues?

The selected position should remain consistent across the safety review, valve data sheet, actuator specification, and commissioning procedure.

What Happens During Power Loss and Power Restoration

A supercapacitor failsafe actuator normally works through three stages:

1. During powered operation, it follows opening or closing commands.

2. When the supply fails, the stored energy powers the specified emergency movement

to the configured safe position.

3. When power returns, the storage system recharges. Normal control should resume only according to the defined restart logic, permissives, and interlocks.

Restart behavior must also be defined. The actuator may hold its safe position, return to the previous command, or wait for a new signal. This decision belongs in the system logic.

How to Choose the Right Safe Position for Your Valve

Compare the Risk of Continued Flow and Stopped Flow

Start with two questions:

1. What is the worst credible result if flow continues?

2. What is the worst credible result if flow stops completely?

Consider personnel safety, equipment protection, environmental release, pressure, temperature, and restart requirements. A hazardous medium may suggest isolation, but the complete process still matters.

Selection case: In a water-treatment inlet line, fail-close action may be appropriate if continued inflow could cause downstream overflow, provided that hydraulic transients, pump protection, and upstream pressure are also addressed. On a cooling-water line, fail-open action may be appropriate only when a cooling source remains available during the outage; opening the valve alone does not guarantee circulation if the pump also loses power. One facility may therefore require different fail actions for different services.

Match the Failure Action to Ball and Butterfly Valve Applications

A fail safe valve actuator should be selected as part of a complete valve assembly. Ball valves are often used for quarter-turn shutoff, while butterfly valves are common in water, HVAC, utility, and larger-pipeline services.

For either type, the failure direction must match the installed valve orientation and stem rotation. Mounting position, coupling, mechanical stops, and full travel should be checked before commissioning.

At AOITEC, we develop quarter-turn electric actuation solutions for ball and butterfly valves. Our supercapacitor technology supports emergency positioning across shutdown, oil and gas, chemical, water, and power applications.

Verify Torque, Stroke Time, Voltage, and Installation Conditions

The actuator must complete the emergency stroke under the highest expected load. Valve size alone is not a sufficient selection basis.

Confirm:

1. Breakaway/unseating torque, running torque, and seating/end-of-travel torque at maximum differential pressure and under worst-case service conditions

2. Required torque safety margin

3. Travel direction and mechanical stop settings

4. Maximum acceptable emergency stroke time

5. Supply voltage and control interface

6. Mounting standard, stem dimensions, and available space

7. Temperature, moisture, corrosion, vibration, and enclosure requirements

Torque should come from valve-manufacturer data and actual service conditions. Deposits, seal aging, pressure changes, and long idle periods can increase the force needed to move the valve.

How a Supercapacitor Failsafe Actuator Reaches the Safe Position

How Stored Supercapacitor Energy Completes the Emergency Stroke

A supercapacitor failsafe actuator stores electrical energy while normal power is available. When the supply fails, the stored energy drives the motor and moves the valve to its preset open or closed position without external backup power.

Compared with battery-based emergency power, supercapacitor storage offers faster charging, longer service life, and lower maintenance requirements. Its purpose is to complete the required safety movement rather than provide continuous backup operation.

Stored energy, valve torque, travel time, and environmental conditions still need to be evaluated together.

When an AOITEC SuperCap Failsafe Actuator Fits the Application

At AOITEC, we use built-in supercapacitor energy storage to provide automatic valve positioning during loss of power. The solution is intended for quarter-turn applications where a ball or butterfly valve must reach a predefined safe position without an external battery system.

Our engineering is supported by more than three decades of experience in electric valve actuation, motor technology, compact design, and operational safety. We support standard and application-specific requirements for valve manufacturers, OEMs, and system integrators.

A supercapacitor fail-safe actuator is especially relevant when the application requires:

1. A specified emergency movement after power loss

2. Reduced battery inspection and replacement

3. Compact electric valve automation

4. Integration into an emergency or controlled shutdown strategy

5. Repeatable quarter-turn positioning

Common Selection Mistakes and the Final Specification Checklist

What to Confirm Before Ordering a Fail Safe Electric Actuator

Many specification errors begin with a request that says only “failsafe actuator.” This does not define the required action, trigger, load, or response time.

Before ordering, confirm:

1. Required position after power loss

2. Exact trigger, such as loss of power or control signal

3. Valve type, size, stem, mounting, and rotation direction

4. Operating and emergency torque

5. Medium, pressure, differential pressure, and temperature

6. Emergency stroke time

7. Electrical supply and control requirements

8. Installation environment and available space

9. Position feedback and interlocks

10. Restart behavior after power returns

Do not confuse actuator movement with valve shutoff performance. A fail-close actuator may reach the closed limit, but actual seat leakage depends on the valve design, seat material and condition, specified shutoff or leakage class, seating torque, differential pressure, process medium, temperature, and test conditions.

Frequently Asked Questions

Q: What is the difference between a fail open and fail close failsafe actuator?

A: A fail-open unit moves the valve open after the defined power failure, while a fail-close unit moves it closed. The correct choice is the position that produces the lower overall process risk.

Q: Does a fail safe electric actuator always close when power is lost?

A: No. It may be designed for opening or closing action. The required position depends on the consequences of continued flow and stopped flow.

Q: How does a supercapacitor failsafe actuator work during a power outage?

A: It charges its built-in supercapacitor during normal operation. When main power is lost, the stored energy powers the configured fail-safe movement so that the valve reaches its designated safe position.

Q: Can a fail safe valve actuator be used with ball and butterfly valves?

A: Yes. It can operate quarter-turn ball and butterfly valves when torque, travel direction, mounting, stem dimensions, stroke time, and operating conditions are correctly matched.

Q: What information is needed to select a fail safe electric actuator?

A: Selection requires the fail position, failure trigger, valve type and size, torque, pressure, medium, supply voltage, stroke time, mounting details, environmental conditions, control logic, and restart behavior.