Sizing a direct-acting pneumatic actuator is relatively simple. A fixed air supply can only power the actuator. You're good to go if you have enough actuator torque at a given supply pressure to open and close your valve (plus a safety factor).
Spring-return actuators, on the other hand, present an additional sizing challenge. The supply air energizes the actuator in one direction (open/close) and the internal springs in the opposite direction (close/open) in a spring-return actuator. Because there are two energy sources, you must consider four torques and compare them to the required valve torque:
- Air Start - naturally has a higher value because it works opposite a decompressed spring.
- Air End - naturally has a lower value because it's working opposite a compressed spring.
- Spring Start - naturally provides a higher value because it starts compressed and then expands.
- Spring End - naturally has a lower value because it loses "strength" as it decompresses.
Most actuators also include a variety of "spring packages," which are different spring combinations that produce more or less spring torque. But don't forget about the inverse relationship between air and spring.
A spring torque value may be sufficient at first, but as the spring "relaxes," the torque will decrease and may not be adequate to overcome the valve torque threshold. In contrast, the torque provided by the air supply at startup may be sufficient to overcome the valve and the relaxed spring, but it may fail as the spring compresses. Because of the spring/air torque relationship, you may need to switch between several actuator model sizes to provide the required torque.
This post is, of course, a simplified explanation of the considerations when sizing pneumatic actuators. There are many other factors you must consider for proper sizing. Before using any pneumatic actuator, always seek an expert's professional installation and application advice.