Views: 21 Author: Site Editor Publish Time: 2022-12-12 Origin: Site
Valve actuators, used for operating valves for opening, closing or modulating service, come in a variety of configurations and power sources. Two of the most recognizable types based on these criteria are pneumatic and hydraulic. Each has positives and negatives depending in part on the specific application, space requirements, operating pressures and speeds needed for safe Valve operation.
Even on a single application, there can be a combination of both pneumatic and hydraulic Valve actuators. In remote locations, where neither compressed air nor electricity is available, hydraulic is often the choice.
In refineries, chemical plants and pipeline compressor stations, where compressed air sources are available, the features and capabilities of a pneumatic actuator are usually more favorable.
Pneumatic actuators are designed to operate a piston within a closed cylinder. Pressure from an external air or gas source powers the actuator, moving the cylinder to fully open or close the Valve, or partially modulate its open/close movement.
Depending on the valve type, the pneumatic actuator valve can either operate in a 90-degree horizontal direction or in a linear up and down motion.
Pneumatic actuators are divided into two working modes of double acting and single acting.
Working principle of double-acting mode:
When the compressed air from the air port (2) into the middle chamber between the two pistons of the cylinder, the two pistons are separated to move to the two ends of the cylinder, and the air at both ends of the air chamber is discharged through the air port (4). At the same time, the two piston rack synchronously drive the output shaft (gear) counterclockwise rotation. On the contrary, when the air source pressure enters the air chamber at both ends of the cylinder from the air port (4), the two pistons move to the middle direction of the cylinder, and the air in the middle air chamber is discharged through the air port (2). At the same time, the two piston rack synchronously drives the output shaft (gear) clockwise rotation. (If the piston is installed in the opposite direction, the output shaft turns into a reverse rotation, that is, a double-acting reverse transformation).
Working principle of single acting mode:
When the air source pressure from the air port (2) into the middle chamber between the two pistons of the cylinder, the two pistons separate to the direction of the two ends of the cylinder, forcing the two ends of the spring compression, the air at both ends of the air chamber through the air port (4) discharge, while the two pistons rack synchronous drive output shaft (gear) counterclockwise rotation. After the air source pressure is reversed by the solenoid valve, the two pistons of the cylinder move in the middle direction under the elasticity of the spring, and the air in the middle chamber is discharged from the air port (2), and the two piston rack synchronously drives the output shaft (gear) clockwise rotation. (If the piston is installed in the opposite direction, the output shaft turns into reverse rotation when the spring is reset, which is the single-acting reverse transformation)
It can be seen from the above respective working principles that the double-acting pneumatic actuator valve opening and Valve closing need the thrust of compressed air to execute. In other words, the pneumatic actuator will remain in a certain position in the case of loss of air.
The single-acting actuator belongs to the automatic reset type. After losing the continuous air pressure, the actuator will drive the Valve to return to the initial set position under the action of the tension of the spring.