quarter turn pneumatic actuator, wether direct acting or spring return, has a given output torque based upon the air supply pressure provided. This output torque can be fairly linear, or very non-linear depending on the style of the actuator (rack and pinion vs. scotch yoke) or if the application calls for springs to be used for opening or closing.
It is critically important to properly select and match the design and torque requirements of the pneumatic actuator and the valve in any given process. Improper selection will result in failure, much higher installed cost, not to mention lost production.
There are two main ways an improperly sized actuator costs money. If the actuator is too small (underpowered at a given line pressure to provide ample seating and unseating torque), it will fail and need to be replaced. If an actuator is severely oversized, the cost of the actuation package is higher, and space is wasted accommodating the larger physical size.
Many actuator failures result before the valve and actuator are even installed because of improper sizing. Proper application engineering calls for proper use of safety factors.
A safety factor is applied when sizing a pneumatic actuator. These recommended safety factors vary slightly from manufacturer to manufacturer. However, applying a safety factor is not enough. Special consideration for the valve torque curve from fully closed to fully open must be evaluated. After the valve full-stroke torque is known, then the actuator torque curve should be evaluated side by side to assure that the safety factor exceeds all point of the valve torque curve. Otherwise, an accidental "dead-zone" where the actuator torque curve is inadequate to move the valve at a certain part of the valve stroke can happen.
Obviously, its not a good idea to apply an actuator with too small a safety factor. These actuators can add maintenance costs due to erratic operation or a higher likelihood of failing all together.
Quarter turn valves and actuators are often used in demanding application requiring high cycle rate, exposure to harsh environments and / or control critical processes such as hazardous chemicals. For these reasons a great deal of attention and care must be given to proper sizing. If you're unsure about any given application, make sure you contact an expert who can help you.