Valve and Actuator Power Failure Scenarios and Solutions
Updated: Aug 24
What Should a Valve Do When Power or Air Is Lost?
A valve’s safe response to loss of electricity, instrument air, or hydraulic pressure must be defined by the process hazard analysis. The required action may be fail-closed, fail-open, or remain in position; the actuator, stored energy, controls, accessories, and proof-test plan must then be designed as one final-element system.
Valve Failure Modes: Fail-Open, Fail-Closed, Fail-In-Place
The failure mode defines what the valve does when power (electric, pneumatic, or hydraulic) is lost: Fail-Closed (FC, also called 'air fail-close' for pneumatic actuators, 'spring fail-close' for electric actuators with spring return)—the valve moves to the closed position upon power loss. Used when closing the valve is safer (e.g., isolating a hazardous fluid). Fail-Open (FO, opposite of FC)—the valve moves to the open position upon power loss. Used when opening the valve is safer (e.g., allowing cooling water flow, relieving pressure).
Fail-In-Place (FIP, also called 'fail-last' or 'fail-as-is')—the valve stays in its current position when power is lost. Used when moving the valve upon power loss could cause more harm than staying put (e.g., a control valve in a sensitive process where the current position is acceptable). Note: FIP requires a mechanism to hold the valve position without power (e.g., a mechanical brake, a capacitor bank, or a battery backup).
Pneumatic Actuator Power Failure (Loss of Instrument Air)
Pneumatic actuators are widely used in process plants because instrument air is generally reliable and pneumatic actuators are intrinsically safe. However, instrument air can be lost due to: compressor failure; air line break; or power failure to the air compressor.
For spring-return pneumatic actuators: loss of air pressure causes the spring to move the valve to the fail position (open or close). For double-acting pneumatic actuators (no spring): loss of air pressure causes the valve to fail-in-place (unless a spring-return option is added, or a separate air reservoir is provided to drive the valve to the fail position upon air loss).
Solution for double-acting actuators: install an air reservoir (receiver tank) sized to provide at least one full stroke upon air loss; or add a spring-return mechanism. Also, consider a local air supply (small compressor or nitrogen bottle) for critical valves.
Electric Actuator Power Failure (Loss of Electricity)
Electric actuators require electric power (24 VDC, 110 VAC, 220 VAC, or 380 VAC). Power failure can be caused by: plant blackout (total loss of electric power); local circuit breaker trip; or power cable damage.
For electric actuators: if power is lost, the valve fails in place (the motor stops, and the valve stays where it was). To achieve fail-open or fail-closed, the electric actuator must have: a capacitor bank (stores energy to move the valve to the fail position upon power loss—typically provides enough energy for one full stroke); or a battery backup (provides power for multiple strokes or extended time—more expensive but more reliable).
Another option: mechanical spring return for electric actuators (rare, because electric actuators typically have high torque requirements, and springs would be very large). More common is to use a 'fail-safe' electric actuator with capacitor or battery backup.
Hydraulic Actuator Power Failure (Loss of Hydraulic Power)
Hydraulic actuators use pressurized oil to generate force. Hydraulic power can be lost due to: hydraulic pump failure; power failure to the pump; or hydraulic line rupture.
For hydraulic actuators: similar to pneumatic, they can be spring-return (fails to open or close upon hydraulic pressure loss); or double-acting (fails in place upon pressure loss). Solutions: hydraulic accumulator (stores pressurized fluid to drive the valve to fail position upon power loss); and hand pump (allows manual operation upon hydraulic failure).
Uninterruptible Power Supply (UPS) for Valve Actuators
A UPS (Uninterruptible Power Supply) provides temporary electric power upon mains power failure. For electric actuators, a UPS can keep the actuator powered for a specified time (enough to move to the fail position, or to keep the valve in position until power is restored).
For pneumatic actuators with smart positioners (which require electric power), a UPS can keep the positioner powered during mains failure. However, the actuator itself will still fail to the spring position upon air loss—the UPS only protects the positioner electronics, not the air supply.
UPS sizing must include actuator starting and running demand, power factor, required stroke or hold time, control and accessory loads, ambient temperature, aging, and design margin. Inspection and replacement intervals should follow the manufacturer’s instructions and the site maintenance strategy rather than a universal calendar value.
Capacitor and Battery Backup for Electric Actuators
Many modern electric actuators have built-in capacitor or battery backup: Capacitor backup (stores energy in capacitors; upon power loss, the capacitor discharges to drive the motor to the fail position. Typically provides enough energy for 1-2 full strokes. Lower cost than battery, but limited energy storage and shorter life). Battery backup (stores energy in rechargeable batteries; upon power loss, the battery powers the motor. Can provide multiple strokes and longer backup time. Higher cost, but more capable. Requires battery maintenance/replacement).
When specifying electric actuators, state the required failure mode and backup energy source. Also specify the required number of strokes upon power loss (one full stroke is typical; some applications require two or more).
Design Considerations for Power Failure Scenarios
When designing a plant, consider power failure scenarios: what happens if the entire plant loses power? Which valves should be fail-open, fail-closed, or fail-in-place? Perform a failure modes and effects analysis (FMEA) to systematically evaluate the consequences of valve failure upon power loss.
Also consider: redundant power sources (some critical actuators may have dual power feeds—if one fails, the other takes over); and local power generation (some plants have diesel generators that start upon mains failure—critical actuators should be on the emergency power bus).
Testing Valve Failure Modes
It is important to test valve failure modes periodically (as part of proof testing for safety valves, or as part of maintenance): simulate power failure (cut power to the actuator and observe the valve movement—does it move to the correct fail position?); and test backup power (UPS, capacitor, battery—disconnect mains power and verify the actuator can move to the fail position).
For an ESD final element, test the complete safety function at the interval established by the safety requirements specification and functional-safety lifecycle. Document valve travel, fail action, diagnostics, bypasses, test coverage, results, and restoration to service.
Common Design Mistakes
Specifying the wrong failure mode (valve fails to the wrong position upon power loss—can cause safety incidents). Not providing backup power for electric actuators (if fail-open or fail-closed is required, the actuator must have capacitor, battery, or UPS). Using double-acting pneumatic actuators without air reservoir (valve fails in place, which may not be acceptable).
Also: not testing failure modes periodically (backup power (UPS, capacitor, battery) degrades over time—test regularly to ensure it works when needed); and ignoring the failure mode of ancillary equipment (e.g., the positioner requires power—if it fails, can the actuator still move to the fail position?).
Conclusion
Power failures are inevitable—valves and actuators must be designed to fail safely. Understanding failure modes (fail-open, fail-closed, fail-in-place) and providing appropriate backup (spring-return actuators, capacitor/battery backup, UPS, air reservoirs) ensures that valves move to the correct position upon power loss. Pneumatic actuators with spring-return are the simplest and most reliable for fail-safe applications. Electric actuators require capacitor or battery backup to achieve fail-safe operation. Regular testing of failure modes is essential to verify that backup systems work when needed. By carefully considering power failure scenarios during design and testing them periodically, plants can ensure valve reliability and process safety.
Frequently Asked Questions
What is the difference between fail-closed and fail-open?
Fail-closed (FC) means the valve moves to the closed position upon power loss. Fail-open (FO) means the valve moves to the open position upon power loss. The choice depends on process safety (which position is safer upon power loss).
Can a double-acting pneumatic actuator fail to a safe position?
Not by itself—without air pressure, it fails in place. To make it fail-open or fail-closed, add a spring-return mechanism, or install an air reservoir that provides air to drive the valve to the fail position upon air supply loss.
How long does a UPS need to power a valve actuator?
Size the backup source for the specified safe action, actuator starting and running load, stroke time, controls, temperature, battery or capacitor aging, required margin, and the time until an alternate supply becomes available.
How often should I test valve failure modes?
Use the proof-test interval defined by the safety requirements specification, SIL verification, operating experience, manufacturer instructions, and applicable IEC 61511 lifecycle procedures. There is no universal annual interval for every ESD valve.
Official Technical References
Contact Us
For inquiries about our valve products, custom solutions, or technical support, please reach out to our team. We supply valves with electric, pneumatic, and hydraulic actuators, and can provide guidance on failure mode selection and backup power options.
Ted Wang
Wechat/Whatsapp: +86 18267833722
Email: sales@wofervalve.com
Web: www.wofervalve.com
Wenzhou Wofer Valve Co., Ltd.
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