Accurate valve actuator sizing is important to guarantee proper operation of automated valves under process conditions in the process industry. In selecting an electric valve actuator, various factors need to be considered in order to select a proper actuator for a process application.
The actuators are responsible for moving and holding a position in a valve’s closure member. If the actuator does not meet process requirements and design, it may affect valve operation negatively. Evaluating requirements for actuators is important in the initial stages of selecting a proper valve actuator for a process application.
The High Cost of Improper Sizing: Safety Risks and Component Wear
Actuators often get overlooked during valve system design, yet they directly control valve motion and final position for balls, discs, or plugs. Proper valve actuator sizing ensures the actuator can deliver the required torque to move and seat the valve under all operating conditions.
If actuator selection does not match system requirements, valves may fail to open or close fully. This can create safety hazards such as leaks from incomplete sealing, overpressure events, or uncontrolled flow. Undersized actuators may stall under load, while oversized units can introduce excessive stress on valve components.
Improper sizing also accelerates wear on internal parts. Gears may experience abnormal loading, leading to tooth wear or misalignment. Repeated overload conditions can increase friction, raise torque demand, and shorten component life.
Selection should consider the full system, not just the valve. A proper evaluation supports reliable operation, reduces safety risks, and limits premature wear by ensuring the actuator delivers controlled, adequate torque throughout the valve’s operating range.
Step 1: Matching Actuator Action to Valve Type (Quarter-turn vs. Multi-turn nuances)
The process of selecting actuators starts with an evaluation of the application needs of the system. Industrial valves are employed for a variety of functions in a system, for example, for flow control, pipeline isolation, or for protecting equipment.
For the functions of the industrial valves to be realized, the actuators, including multi-turn electric actuators used for gate or globe valves, must be able to control the movement of the closure members of the valves. This is because the closure members, whether a ball, a plug, or a disc, must be responsive to the movement of the actuators for the proper control of flow in the system.
The process conditions, for example, pressure, temperature, and flow rates, affect the performance of the industrial valves in the system. These conditions also affect the forces acting on the closure members of the valves.
Step 2: Calculating Torque Requirements with a Safety Factor (Breakaway vs. Running torque)
Process conditions have a significant impact on the performance of actuators. Pressure, temperature, and flow rate may affect the amount of force required to move and hold the valve closure member in place, which must be considered during electric actuator torque calculation.
If process conditions are not taken into consideration in selecting the actuator, it may not provide sufficient torque to move the valve.
Two torque conditions are normally used in selecting actuators.
Breakaway torque is defined as the amount of torque required to move a valve from a fully closed or seated position. In this position, it is normally more challenging to move a valve due to friction between surfaces and internal moving parts.
Running torque is defined as the amount of torque required to continue moving a valve after it has started moving from a fully closed or seated position.
These two torque values may vary depending on process conditions. In some cases, high pressure and high temperature may affect the amount of torque required to move a valve.
The following conditions may affect the amount of torque required by a valve:
| Evaluation Factor | Description | Importance for Actuator Selection |
| Process pressure | Pressure acting on the valve within the system | Can increase the force required to move the valve |
| Operating temperature | Temperature conditions within the process | May affect actuator components and valve friction |
| Flow rate | Fluid movement through the valve | Can influence valve positioning forces |
| Valve function | Role of the valve (flow control, isolation, protection) | Determines operational demands on the actuator |
These conditions need to be evaluated to ensure that the chosen actuator will be able to provide adequate torque to move and position the valve within the operational environment while maintaining an appropriate actuator safety factor.
Step 3: Determining Duty Cycle & Power Requirements (Avoiding motor overheating)
Actuator selection must consider control signals, power supply, and valve duty cycle. The actuator should support relay, analog, and digital protocols such as Modbus, HART, and Profibus, with position feedback for accurate control.
Valve duty cycle defines usage as operations per hour or per day. For example, 12 open/close actions per hour equals a high-duty application. Higher duty increases start-stop frequency, current draw, and heat generation.
Duty cycle directly drives motor selection. High-duty service requires motors with higher thermal capacity, suitable insulation class, and start frequency ratings to avoid overheating. Low-duty service can use standard motor configurations.
Most electric actuators include thermal protection that trips at 120°C ± 5°C and resets after cooling. Common designs use squirrel cage asynchronous motors with Class H insulation and supply options such as 110/220V AC single-phase and 380/440V AC three-phase.
Correctly matching valve duty cycle to the motor prevents overheating, extends component life, and supports reliable multi-turn electric actuator operation.
Step 4: Evaluating Environmental Conditions (Industrial, Marine, or High-Temperature Applications)
Another factor for consideration during the selection of actuators is the reliability of the assets. Reliability is a crucial factor, especially for valves that are performing critical functions in a system.
When the failure of an actuator is deemed to impact the stability of a system, then the reliability of the actuator is a key factor for consideration during the selection of actuators. The importance of the valve in the system is another factor for consideration when selecting an actuator.
Conclusion: Avoiding Costly Selection Errors with a Professional Checklist
Successful valve actuator sizing requires considering application needs, system compatibility, reliability, and cost. By taking this into account, it is ensured that the actuator functions effectively in an automation system.
It is necessary to examine process needs, system compatibility, and reliability prior to selecting a valve actuator to avoid sizing mistakes.
