How to Properly Size an Actuator - Part 2

By Carlos Gamero, Engineering Manager, MRC Global
July 06, 2017 Tags: Valve Automation, Actuation Sizing, Maximum Supply Pressure, Minimum Supply Pressure
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Choosing the Right Actuator Type

In our business, after a customer has selected the specific valve and actuator package, that’s when my team steps in to size the actuator properly. Selecting the correct actuator for the size and purpose of your valve is key but it’s only the first step. If the actuator isn’t sized by a knowledgeable and technically trained expert, the results can be disastrous or expensive.

To size an actuator correctly, we need to understand:

  • Minimum and maximum supply pressure

  • Actuator type

  • Fail mode

  • Valve torque

In my previous post, Understanding Minimum and Maximum Supply Pressure, I explained the importance of selecting an actuator that can safely operate at both the maximum and minimum supply pressure for the selected valve. Today, I’ll explain the next step in properly sizing an actuator: choosing the appropriate actuator type.

There are subtle differences between the common actuator types and understanding the differences can help guide you when selecting the best one for your application.

Rack and pinion actuators are more commonly used on small valves (<4”) due to its small size and relatively low output torque. The torque output throughout the stroke is a straight line for rack and pinion actuators.

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These actuators are most commonly used in process industries such as refining, chemical and power generation.

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Scotch yoke actuators are commonly used on bigger valves (>/=4”) because they have the capability to output higher torques.

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The torque output throughout the stroke is a concave up parabola for a scotch yoke actuator. This parabola is a result of a variable moment arm. This is a unique characteristic of a scotch yoke actuator. Since, in general terms, the torque is the force generated by the pressure pushing the piston times the moment arm, the torque at the mid position is lower than at the end positions because the moment arm is minimum at the mid position and maximum at the end positions.

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Spring Return actuators use pressure to create a driving force in one direction and a spring to return to the “home” position. Spring return actuators are usually selected when the automated valve does not need to depend on an external motive power to go to its home (open or close) position. For example, safety applications like emergency shutdown valves would mostly likely need a spring return actuator.

Double Acting actuators use pressure to create a driving force to move in both extend and retract directions. Double acting actuators are usually selected when the valve needs to stay in place (fail last) after the control signal or the supply pressure has been lost. They also tend to be less expensive than spring return actuators.

As with any technical decision, there are many factors that can impact which actuator is the safest and most efficient option for your specific need. MRC Global has experienced technical inside sales representatives and an engineering department that can help you with sizing and selecting the right actuator for your unique application. Contact us with your valve automation questions and let one of our experts help you get the most out of your valve system.


About the Author

Carlos Gamero holds a Mechanical Engineering degree from the University of Houston and is a licensed professional engineer on control systems. He joined MRC Global in 2006, where a mentor introduced him to valve automation. Carlos immediately fell in love with the opportunity to solve new problems every day. That love has not changed. In his current role, Carlos enjoys mentoring other valve automation engineers and working with his team to create valve automation solutions for our customers on a daily basis.