The piston-operated actuator has few moving parts, so maintenance is minimal. Both rack-and-pinion and scotch-yoke design piston actuators have been successful. Rack-and-pinion actuators are preferable because of their compact size, but they are typically limited to valve sizes of six inches and smaller.
Though both the spring-return and double-acting piston actuators can be made failsafe, the spring-return actuator is preferred. The spring moves the valve into the fail-safe position when the control signal fails. To make the double-acting piston fail-safe, add a pressure switch and a reserve tank with sufficient capacity to move the valve to the fail-safe position.
Piston-operated actuators can actuate ball and butterfly valves, ranging from NPS 2 to 48, and ANSI Class 150 to 2500 lb.
One disadvantage of a scotch-yoke piston actuator is it’s size; e.g., a spring-return actuator is 72 inches long, and a double-acting actuator is 60 inches long for a 14-inch ANSI Class 300 ball valve, sized using the MAWP. This size could be a problem when trying to install an EBV in an existing plant with space limitations.
Another disadvantage of any piston-operated actuator is its tendency to corrode if wet instrument air is used. Make sure that the internals have a corrosion-resistant surface, such as nickel-plating.
The most common brands of piston actuators for quarter-turn process valves are: