Pneumatic Actuator Classification and Selection

　　Actuators are classified into three major categories based on their power source: pneumatic, electric, and hydraulic. Each type has its own unique characteristics and is suited to different applications. Pneumatic actuators are one of these categories. Pneumatic actuators can further be divided into two types: single-acting and double-acting. In a double-acting actuator, both the opening and closing movements are driven by compressed air; this is referred to as DOUBLE ACTING. In a spring-return (single-acting) actuator, only the opening movement is powered by air pressure, while the closing movement is accomplished by the spring’s return force.

　　The purpose of this reference material on selecting actuators is to help customers make the right choice when selecting actuators. Before installing pneumatic or electric actuators onto valves, the following factors must be taken into consideration: * The operating torque of the valve, plus the manufacturer’s recommended safety factor—based on the specific operating conditions. * The air supply pressure for pneumatic actuators or the supply voltage for electric actuators. * The actuator type—double-acting or single-acting (spring‑return)—as well as the output torque under a given air supply pressure or the rated output torque under a specified supply voltage. * The actuator’s rotation direction and failure mode (fail‑open or fail‑close). Choosing the correct actuator is critically important: if the actuator is too large, the valve stem may be subjected to excessive stress; conversely, if the actuator is too small, it may not generate sufficient torque to fully operate the valve. In general, we believe that the torque required to operate a valve primarily stems from the friction between the valve’s metal components—such as the ball core and valve disc—and the sealing elements, namely the valve seats. Depending on the valve’s application, operating temperature, duty cycle, pipeline and differential pressure, as well as the nature of the fluid being conveyed (lubricating, dry, or slurry), numerous factors can influence the operating torque. The structural principle of a ball valve is essentially based on a polished ball core—complete with flow passages—positioned between two valve seats (upstream and downstream). As the ball rotates, it either blocks the flow of fluid or allows it to pass through the ball core. The pressure differential between the upstream and downstream sides exerts a force that presses the ball core tightly against the downstream valve seat (in a floating-ball design). Under these circumstances, the torque required to operate the valve is determined by the friction between the ball core and the valve seats, as well as between the valve stem and the packing. The maximum torque occurs when there is a pressure differential and the ball core rotates from the closed position toward the open position—in the case of butterfly valves.

　　The structural principle of a butterfly valve is fundamentally based on the butterfly disc, which is fixed to the valve stem. When the valve is in the closed position, the butterfly disc forms a complete seal with the valve seat; as the disc rotates (around the valve stem) and aligns parallel to the fluid flow, the valve reaches its fully open position. Conversely, when the butterfly disc is perpendicular to the fluid flow, the valve is in the closed position. The operating torque of a butterfly valve is determined by the friction between the butterfly disc and the valve seat, as well as between the valve stem and the packing. Additionally, the force exerted by the pressure differential acting on the butterfly disc also influences the operating torque—typically, the torque is at its maximum when the valve is closed, but after even a slight rotation, the torque decreases significantly. The structural principle of a plug valve is essentially based on a plug that is sealed within a conical body. A passage is provided in one direction along the plug. By rotating the plug into the valve seat, the valve can be opened or closed. The operating torque is generally not affected by the fluid pressure; instead, it is determined by the friction between the valve seat and the plug during the opening and closing process. The torque is at its maximum when the valve is closed. Due to the influence of pressure, the torque remains relatively high throughout the remainder of the operation.