Any control valve actuator is a device that uses energy to drive a valve. This device may be a manually operated gear set used to open or close the valve, or an intelligent electronic component with sophisticated control and measuring devices that enables continuous modulation of the valve. With the development of microelectronics, actuators have become increasingly complex. Early actuators were no more than motor-gear assemblies with position-sensing switches. Today’s actuators feature many advanced functions: they not only open or close valves but also detect the operating status of valves and actuators, providing various data for predictive maintenance.
What Is an Actuator?
The most widely accepted definition of an actuator is: a drive device capable of producing linear or rotary motion, which operates using a specific energy source and under the action of a control signal.
Actuators use liquid, gas, electricity, or other energy sources and convert them into driving force via motors, cylinders, or other devices. Basic actuators are used to drive valves to the fully open or fully closed position. Actuators for control valves can precisely position the valve at any intermediate point. Although most actuators are used for on-off service, modern actuator designs go far beyond simple switching functions. They integrate position sensing, torque sensing, motor protection, logic control, digital communication modules, and PID control modules—all housed within a compact enclosure.
As more plants adopt automated control, manual operation is being replaced by mechanical or automated equipment. Actuators are required to serve as the interface between control systems and the mechanical motion of valves, with enhanced safety and environmental performance. In hazardous applications, automated actuators reduce personnel exposure to risk. Certain special valves require emergency opening or closing under abnormal conditions; valve actuators prevent further hazard escalation and minimize plant losses. For high-pressure, large-bore valves, the required output torque is very high, so actuators must improve mechanical efficiency and use high-output motors for smooth operation.
Valves and Automation
To successfully implement process automation, it is critical to ensure that the valve itself meets the specific requirements of the process and pipeline medium. Generally, the process and process fluid determine the valve type, plug/trim design, and the structure and materials of internal parts and the valve body.
Once the valve is selected, the next consideration is automation requirements—namely, actuator selection. Actuators can be categorized according to two basic valve operating types:
Rotary valves (quarter-turn valves)These include plug valves, ball valves, butterfly valves, and dampers. They require actuators that deliver 90° rotary motion with specified torque.
Multi-turn valvesThese may have non-rotating rising stems or rotating non-rising stems, requiring multiple revolutions to drive the valve open or closed. Examples include globe valves, gate valves, and knife gate valves. Alternatively, linear-output pneumatic or hydraulic cylinder/diaphragm actuators can also drive these valves.
There are currently four main types of actuators, using different energy sources to operate various valves:
1. Electric Multi-Turn Actuators
Electrically driven multi-turn actuators are among the most common and reliable types. Single‑phase or three‑phase motors drive gears or worm gears, which in turn drive the stem nut to move the stem and open/close the valve. Multi-turn electric actuators can quickly drive large-size valves. To protect the valve, limit switches at the ends of travel cut off motor power; torque-sensing devices also cut power if safe torque is exceeded. Position switches indicate valve status, and handwheel mechanisms with clutches allow manual operation during power failure.
Main advantages: All components are housed in a single, waterproof, dustproof, explosion-proof enclosure integrating basic and advanced functions.Main disadvantage: During power loss, the valve remains in its last position. A backup power system is required to achieve fail-safe (fail-open or fail-closed) operation.
2. Electric Quarter-Turn Actuators
Similar to electric multi-turn actuators, the key difference is that they deliver a final 1/4 turn (90°) output. New-generation electric quarter-turn actuators incorporate many complex functions of multi-turn models, such as non-intrusive, user-friendly interfaces for parameter setting and diagnostics.
They are compact, suitable for small valves, typically with output torque up to 800 kg·m. Due to low power demand, batteries can be installed for fail-safe operation.
3. Fluid-Driven Multi-Turn or Linear Output Actuators
These are commonly used for globe valves and gate valves, operating pneumatically or hydraulically. They feature simple structure, reliable performance, and easy implementation of fail-safe modes.
Electric multi-turn actuators are generally preferred for gate and globe valves; hydraulic or pneumatic actuators are considered only where electric power is unavailable.
4. Fluid-Driven Quarter-Turn Actuators
Pneumatic and hydraulic quarter-turn actuators are widely used, requiring no electric power and offering simple, reliable construction. Output torque ranges from a few kg·m to tens of thousands of kg·m. They use cylinders and mechanisms to convert linear motion to 90° output: scotch yoke, rack-and-pinion, or lever. Rack-and-pinion provides constant torque over full travel, ideal for small valves. Scotch yoke offers high efficiency and high torque at the start of travel, suitable for large-bore valves. Pneumatic actuators are typically fitted with solenoid valves, positioners, or limit switches for control and monitoring.
Fail-safe operation is easily achieved with this type.
Key Factors for Actuator Selection
When selecting the proper type and size of valve actuator, the following factors must be considered:
1. Drive Energy
The most common sources are electric or fluid power. For large valves, three-phase power is typical; for small valves, single-phase. Electric actuators often support multiple power options, including DC, which allows battery-powered fail-safe operation.
Fluid sources include compressed air, nitrogen, natural gas, hydraulic oil, etc., at various pressures. Actuators are sized to deliver required force or torque.
2. Valve Type
Valve type dictates the correct actuator category: some require multi-turn, some quarter-turn, some reciprocating linear drive. Pneumatic multi-turn actuators are generally more expensive than electric multi-turn, but reciprocating linear pneumatic actuators are cheaper than electric multi-turn.
3. Torque Magnitude
For 90° rotary valves (ball, butterfly, plug), obtain torque values from the valve manufacturer, typically tested at rated pressure. For multi-turn valves, distinguish between:
Reciprocating (rising) motion – stem non-rotating
Reciprocating motion – stem rotating
Non-reciprocating – stem rotating
Stem diameter and connection thread size determine actuator sizing.
4. Actuator Sizing
Once actuator type and required driving torque are determined, selection can be done using manufacturer data sheets or sizing software. Valve operating speed and cycle frequency must also be considered.
Fluid-driven actuators allow adjustable stroke speed, while three-phase electric actuators have fixed stroke times. Some small DC electric quarter-turn actuators offer adjustable speed.
On-Off Control
A major benefit of automatic control valves is remote operation, allowing operators to control processes from a control room rather than manually adjusting valves on-site. Pipelines connect the control room to actuators; energy is supplied via lines, and 4–20 mA signals typically provide valve position feedback.
Continuous Control
When actuators control process variables such as level, flow, or pressure, they operate frequently. A 4–20 mA signal serves as the control input, changing as rapidly as the process. For high‑frequency operation, special modulating actuators designed for frequent starts and stops are required. For multiple actuators, digital communication systems reduce installation costs and enable fast, efficient command transmission and data collection. Common protocols include FOUNDATION FIELDBUS, PROFIBUS, DEVICENET, HART, and PAKSCAN (designed specifically for valve actuators). Beyond cost savings, digital systems collect extensive valve data valuable for predictive maintenance.
Predictive Maintenance
Using built-in data memory, operators record torque values measured during each valve operation. This data monitors valve condition, indicates maintenance needs, and supports diagnostics.
Diagnosable data includes:
Valve seal or packing friction
Friction torque of stem and valve bearings
Seat friction
Friction during valve operation
Dynamic forces on the plug/trimmer
Stem thread friction
Stem position
Most of these apply to all valves but with varying emphasis: friction during operation is negligible for butterfly valves but significant for plug valves. Different valves have distinct torque curves. For wedge gate valves, opening and closing torques are very high; during intermediate travel, only packing and thread friction exist. When closing, hydrostatic pressure increases seat friction, and wedging causes a sharp torque rise to full closure. Torque curve changes predict potential failures and provide valuable information for predictive maintenance.
