Meeting Torque Requirements
As I have noted before, selecting the correct actuator for the size and purpose of the valve is important, but it’s only the first step in selecting the best overall valve automation solution. If the actuator isn’t sized by a knowledgeable and technically trained expert, the results can be disastrous and/or expensive.
To size an actuator correctly, we need to consider four things:
In my previous posts, I explained the importance of selecting the right type of actuator that can safely operate at both the maximum and minimum supply pressure for the selected valve. Today, I’ll explain the final step in properly sizing an actuator: meeting valve torque requirements.
By definition, torque is a measure of how much a force acting on an object that causes that object to rotate. Applying this concept to a valve and actuator, we can say that the valve is the object and the actuator is the device that produces the force that will act on the object (the valve). To make sure that an actuator will operate on a valve, the actuator has to output more torque than what the valve requires to be moved (rotated). The actuator output torques have to be greater than the valve torques through all the travel and in both directions open and close. And, as described in my first blog, the actuator maximum torque should not exceed the valve maximum allowable stem torque (MAST).
When sizing an actuator, always request the torque values and the recommended safety factor from the valve manufacturer. When requesting the valve torques, it is important to request all six point values; break, run and end to open and break, run and end to close. All of these point values are required to properly size the actuator.
If the end user allows you to use reduced differential pressure torques, request those torques from the valve manufacturer. This will allow you to decrease the size of your actuator because you will have lower valve torque values. Occasionally end users specify a safety factor that is different from the valve manufacturer, in this case MRC Global always uses the highest value when sizing the actuator. Again, be careful not to exceed the valve maximum allowable stem torque when selecting the actuator with a high safety factor. The higher the safety factor is, the higher the valve torque values will be and the bigger the actuator will be. The bigger the actuator, the higher the maximum actuator output torque will be, which can damage the valve stem or drive train.
Sizing and selecting the right actuator depends on many factors but you don’t have to navigate them on your own. MRC Global has experienced technical inside sales representatives and an engineering team to help guide you through selecting the best actuator and controls to operate your automated package. Contact us today, we would be happy to help.
How to Properly Size an Actuator
Part One: Understanding Minimum and Maximum Supply Pressure
Part Two: Choosing the Right Actuator Type
Part Three: Identifying Fail Mode Specifications
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.