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 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 (ESDVs). With constantly increasing safety demands, more actuator buyers are taking the extra step of requesting actuator sizing documentation.
For an actuator to be reliable, it must:
- 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; 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 must be able to develop sufficient torque to operate the valve at the minimum supply pressure. This will ensure 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 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 must be able to handle that pressure safely as well.
If it’s determined 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 straightforward and cost-effective solution. The more pressure, the more torque the actuator has the potential to output; therefore, a regulator and a relief valve also will reduce the probability the valve stem will be damaged 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. Equally important is to choose the right type of actuator.
Choosing the Right 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 inches) due to their small size and relatively low output torque. The torque output (see Figure 1) throughout the stroke is a straight line for rack and pinion actuators.
These actuators are most commonly used in process industries such as refining, chemical and power generation.
Scotch yoke actuators are commonly used on bigger valves (/=4 inches) because they have the capability to output higher torques.
The torque output throughout the stroke (see Figure 2) is a concave up parabola for a scotch yoke actuator. This parabola is a result of a variable moment arm and 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.
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 ESDVs most likely would 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, many factors can impact which actuator is the safest and most efficient option for your specific need. By working with engineers and qualified supplier specialists, you will get the most out of the right valve system for your unique application.