Actuation: manual, gear, electric, pneumatic, hydraulic

A lever handle gives direct quarter-turn operation of ball, butterfly and plug valves and provides clear visual position indication (in line with the pipe for open; perpendicular for closed). Levers are suitable for small bores and low-torque valves up to approximately DN50 or DN80 depending on the seat type and pressure rating.

Handwheels give multi-turn operation for gate, globe and needle valves where the stem requires many turns from fully open to fully closed. Handwheels are sized so the rim effort to operate the valve at maximum differential pressure does not exceed approximately 250–300 N (roughly 25–30 kg force), which is the sustained effort a person can apply for a reasonable operating time.

Manual operation is always the simplest and most reliable option where it is physically possible and the valve does not need to be operated frequently or remotely. It requires no external power, no control wiring, no limit switches and no maintenance of powered components. Specify manual operation by default and upgrade to actuated only when manual is not feasible.

Gear operators (bevel gearboxes) are mounted between the handwheel and the valve stem to multiply the operator's applied force by the gear ratio. They are standard on butterfly valves above approximately DN300 and on gate valves from DN200 upwards, where the torque required to open or close the valve exceeds comfortable direct manual operation.

The gear ratio increases the number of handwheel turns required to fully open or close the valve: a 1:60 ratio on a quarter-turn valve means 15 full handwheel turns for 90 degrees of disc travel. This is acceptable for infrequent operation but impractical if the valve must be opened and closed frequently.

Gear operators can be weatherproofed for outdoor service and can be fitted with locking devices to hold the valve in the open or closed position. They are also used as the torque interface between an electric actuator and the valve stem where the actuator output torque and speed must be converted to the valve shaft speed and torque requirements.

Electric actuators use an electric motor, gearbox and limit switch assembly to open, close or modulate a valve under remote electrical control. They are the standard choice for remote operation from a control room, for automation in process plants, and for valves that must operate on a timed or programmed basis.

Electric actuators are specified by output torque (Nm), quarter-turn or multi-turn operation, supply voltage (typically 24 VDC, 110 VAC or 230 VAC), and control signal type: on/off (simple open/close), modulating (4–20 mA or 0–10 V signal for proportional positioning), or bus-controlled (Modbus, PROFIBUS, HART, Foundation Fieldbus for digital integration).

The fail mode of an electric actuator must be specified: fail-in-last-position (motor stops on power loss, valve stays where it is), or fail-open/fail-closed using a spring return mechanism or battery backup. Spring-return electric actuators are larger and more expensive than standard designs but provide a safe position on power loss without external power or compressed air.

Pneumatic actuators use compressed air to move the valve. They are faster than electric actuators, simpler mechanically, and inherently safe in hazardous areas (no electrical equipment in the hazardous zone with a direct-acting pneumatic). They are the preferred actuator for process control valves, high-cycle duty and applications requiring fast response.

Double-acting pneumatic actuators use air pressure on both sides of the piston to open and close. Single-acting (spring-return) designs use air to move in one direction and a spring to return: spring-to-close means the valve closes on air loss (fail-closed); spring-to-open means the valve opens on air loss (fail-open). Specify the fail mode based on the process safety requirement.

Pneumatic actuators require a clean, dry, regulated instrument air supply, typically at 5–7 bar. Contaminated or wet air causes positioner and solenoid valve failures. A pre-mounted filter-regulator on the actuator supply is good practice for any pneumatic installation.

Hydraulic actuators use pressurised hydraulic fluid to generate high forces from a compact cylinder. They are specified for very large torque requirements (large-bore pipeline valves, subsea valves), for underwater or high-pressure environments where pneumatic supply is not available, and for applications requiring very fine position control under high loads.

Offshore subsea valves are typically hydraulically actuated using the platform hydraulic power unit (HPU) or a dedicated subsea HPU. Hydraulic fluid pressure of 200–350 bar is common in subsea systems, giving force output impossible to achieve with pneumatic at instrument air pressures.

Hydraulic actuators are heavier, require a hydraulic power unit and return line, and the hydraulic fluid must be maintained (water content, contamination level). They are not a first choice for onshore applications where electric or pneumatic actuation is feasible.

The actuator must generate torque in excess of the valve's breakaway torque at maximum differential pressure, accounting for seat friction, stem friction and packing load. The actuator datasheet should state the output torque at minimum supply pressure, not at nominal supply: a pneumatic actuator at 5.5 bar supply with a 5 bar instrument air minimum must be sized on the 5 bar output.

For modulating duty, the actuator must also satisfy the running torque across the full travel range and the seating torque at close. Specify a safety factor of at least 1.25 on valve maximum torque when sizing the actuator, and verify the sizing calculation is documented in the procurement package.

Limit switches, position transmitters (4–20 mA feedback), torque limiters and partial stroke testing (PST) capability are specified as options depending on the control system requirements. PST allows a safety-critical valve to be exercised through part of its travel without fully closing, verifying operability without interrupting the process.