Understanding Minimum and Maximum Supply Pressure
A common misconception in our industry is that actuating a valve is as simple as putting the most cost efficient actuator on top of your valve of choice, but the process is actually much more complicated than that. Selecting the correct actuator for the size and purpose of the 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 and/or expensive.
When sizing an automated valve, the actuator must operate properly when on demand, especially on Emergency Shutdown Valves (ESDV’s). With constantly increasing safety demands, more and more of our customers are taking the extra step of requesting actuator sizing documentation.
For an actuator to be reliable, it has to:
- be capable of operating at the minimum supply pressure available
- develop sufficient torque to overcome the valve torque throughout the complete stroke
- meet the service factor and safety factor established by the end user or what is recommended by the valve manufacturer (whichever is higher).
An undersized actuator will be unable to operate the valve; but, on the other hand, an oversized actuator has the potential to damage the stem of the valve as well as being higher priced.
To size an actuator correctly, we need to understand:
- Minimum and maximum supply pressure
- Actuator type
- Fail mode
- Valve torque
Let’s look at what is required concerning the minimum and maximum supply pressure. The normal pressure, which is often provided alongside the minimum and maximum, should never be used to size the actuator.
Minimum Supply Pressure
The actuator has to be able to develop sufficient torque to operate the valve at the minimum supply pressure. This will ensure that the actuator will work even when the supply pressure is at its lowest.
Can an actuator be sized for less than the minimum available supply pressure? Yes. This practice is usually done when actuators are oversized and the maximum torque of the actuator at the minimum supply pressure is exceeding the maximum allowable stem torque. For this specific situation, a regulator and a relief valve need to be added. The regulator will reduce the operating pressure even further, below the minimum supply pressure, ensuring that the actuator will not achieve a torque output that could damage the valve. The relief valve will ensure that in the event of a regulator failure, the output torque of the actuator at the minimum supply pressure will not damage the valve stem. It’s recommended to set the relief valve at 10% or 15% above the regulator setting.
Maximum Supply Pressure
The maximum supply pressure is important because the actuator has to be able to handle that pressure safely as well.
If it’s determined that the actuator cannot handle the maximum supply pressure, there is a way to adjust the package to perform safely. Installing a regulator and a relief valve is a relatively straight-forward and cost effective solution. The more pressure, the more torque the actuator has the potential to output; therefore, a regulator and a relief valve will also reduce the probability to damage the valve stem due to the excess torque from the actuator.
Properly sizing an actuator is an important step to realizing a valve’s full potential safely and efficiently. 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 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.