Power-generation facilities rely on electric valves to control cooling water, fuel-support systems, ventilation lines and water-treatment processes. When the external power supply fails, a standard electric actuator may stop at its last position. If that position does not match the process requirement, isolation, equipment cooling or system recovery can be affected.
A fail-safe electric actuator uses stored energy to move the valve to a predefined position after detecting a power failure. However, the correct safety position may be fully open, fully closed or the last commanded position. It must be determined through a process risk assessment rather than applying one rule to every valve.
Fail-close is appropriate for many isolation duties, but it is not a universal requirement in a power plant.
Typical examples include:
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Valves isolating hazardous media may need to fail closed;
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Cooling-water or fire-water valves may need to fail open;
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Some modulating valves should remain in place to avoid a sudden process change;
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Ventilation dampers may require different positions for exhaust, smoke control or equipment cooling.
The project specification should therefore define fail-open, fail-close or fail-in-place before the actuator model and energy-storage system are selected.
During normal operation, the internal control circuit charges a supercapacitor or battery. When it detects that the external supply has been disconnected, it transfers the motor supply to the stored-energy device and drives the valve to the preset safety position.
The DCL fail-safe module operates with a DC24V supply. Depending on actuator size, smaller models use a supercapacitor while higher-torque models use a battery. The product documentation specifies stored energy for at least four full-stroke operations.
When external power returns, the actuator exits the fail-safe state and resumes normal operating mode. Whether the valve automatically returns to its previous command should be defined by the control logic and plant interlocks.
The DCL design uses a supercapacitor or battery to operate the electric motor. It is not a mechanical spring-return actuator. These technologies differ in return time, available torque, maintenance requirements and energy-storage method, so “spring return" and “battery backup" should not be used interchangeably.
Stored-energy systems also require inspection of battery voltage, trigger records and energy-storage condition. A fail-safe actuator does not replace the plant emergency power system, interlocking logic or safety-instrumented design.
During a power failure, the stored-energy system must still overcome the valve’s actual operating torque. Sizing should use the valve manufacturer’s maximum torque and consider differential pressure, seat design, deposits and higher breakaway torque after long idle periods.
DCL documentation recommends selecting approximately 1.1 to 1.3 times the valve test torque. The actuator operating time must also meet the process response requirement. Higher torque alone does not confirm that the valve will reach its safety position within the required time.
An on-off fail-safe configuration can use passive switching signals for open and close commands and provide passive contacts for fully open and fully closed status. Modulating configurations support 4–20 mA input and output, with 2–10 VDC or 1–5 VDC available on selected versions.
Modbus RTU can support valve-position monitoring, battery-voltage reporting and fail-safe trigger records. Engineers should still determine which signals remain available during a complete site power failure.
Outdoor cooling-water, pumping and treatment systems may use the DCL weatherproof range, which is IP67 as standard with IP68 available on selected configurations. Fuel-handling areas or locations containing flammable gases require a separate hazardous-area assessment. The DCL explosion-proof range includes an IP68 enclosure and Class I, Division 1 configurations.
Ingress protection, explosion protection and fail-safe operation are separate selection requirements. One does not replace another.
The DCL weatherproof range covers standard output torque from 16 to 3000 Nm, while the explosion-proof range covers 50 to 3000 Nm. Both product families can be configured with fail-safe return, on-off control, 4–20 mA modulation and Modbus RTU, depending on the selected model.
The fail-safe module also lists 1 kV surge protection and a 4 kV EFT interface. These parameters describe electrical immunity features, but they do not constitute a SIL certification or approval for use as the final element of a safety-instrumented system.
Selecting a fail-safe electric actuator for a power plant should begin with the required safety position, followed by valve torque, operating time, energy-storage method, control signals, environmental rating and maintenance plan. The actuator can support the plant safety strategy only when these requirements are defined together.