What Are the Main Actuation Mechanisms in Air Actuated Valves?
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In the field of modern industrial automation, fluid control is a very critical link. Whether it is liquid or gas, precise control of its flow, pressure and direction is inseparable from the valve system. Among many valve types, air actuated valves use compressed air as a power source to drive the opening and closing of the valve. They have the characteristics of rapid response, safety, relatively simple structure and easy maintenance. So, how exactly does the core drive mechanism inside these pneumatic valves work? This article will delve into the main driving methods of pneumatic valves and reveal the working principles behind them.
Core drive method one: piston actuated valve
Piston actuated valves are one of the most common drive mechanisms in pneumatic valves. The basic principle is similar to a simple cylinder: compressed air is introduced into a chamber of the actuated valve, pushing a sealed piston to move linearly in the cylinder. This piston is directly or indirectly connected to the closing parts of the valve (such as valve core, valve plate) through a connecting rod (valve stem). When compressed air enters one side, the pressure generated overcomes all resistance (including medium pressure, friction and possible return spring force), pushing the piston to move, thereby driving the valve to open or close.
Depending on the design, the piston actuated valve can be linear, directly driving gate valves, globe valves and other valves that require linear motion; it can also convert the linear motion of the piston into rotary motion through an internal conversion mechanism (such as the rack gear or fork structure mentioned below) to drive ball valves, butterfly valves and other rotating valves. Piston actuated valves can produce larger thrust or torque, so they are suitable for valves with larger sizes and higher opening and closing resistance.
Unlike piston actuated valves, which utilize a rigid piston, diaphragm actuated valves rely on one or more flexible rubber or metal membranes. Compressed air is introduced into the chamber above the membrane, and the air pressure acts on the effective area of the membrane, causing it to deform and generate a downward thrust. This force is transmitted to the valve stem through the central push rod, thereby driving the valve action. Diaphragm actuated valves are usually used in applications that require smaller thrust but higher control accuracy, such as regulating valves.
For valves that need to be rotated 90 degrees (or other angles) to open and close, such as ball valves and butterfly valves, the linear motion of the piston actuated valve needs to be converted into rotational motion. The two main conversion mechanisms are the Rack and Pinion and Scotch Yoke mechanisms.
In the rack and pinion design, the piston is machined with a rack. When the piston moves linearly, the rack drives a pinion connected to the valve stem to rotate, thereby driving the valve to open and close. This structure can provide constant output torque and rotate smoothly throughout the entire stroke. In the yoke mechanism, the end of the piston rod usually has a roller or slider, which is embedded in the groove of a yoke that is fixed to the valve stem. When the piston moves linearly, the roller slides in the fork groove and forces the fork to rotate around the center of the valve stem. The characteristic of the fork mechanism is that its output torque is not constant. It usually generates greater torque at the initial and final positions of opening and closing, which is very helpful for overcoming the sealing force of some valves (such as butterfly valves) in the closed position and the static friction when opening.
According to the action mode of the actuated valve after losing the air source power, pneumatic actuated valves can be divided into single-acting and double-acting. Single-acting actuated valves, often called spring-return actuated valves, have one or more sets of springs inside them. During operation, compressed air enters the cylinder or membrane chamber and overcomes the spring force to drive the valve action. When the air source pressure disappears (whether it is a normal command cut-off or an unexpected failure to cut off the air), the pre-compressed spring will release its stored energy and push the piston or diaphragm back to the initial position, thereby allowing the valve to automatically return to the preset safe state.
Action Mode: Double Acting Actuated Valve
Double-acting actuated valves do not have a built-in return spring. Its action depends entirely on compressed air: air needs to be supplied to one port of the actuated valve (such as port A) to make the valve move in one direction (such as opening), and then the air source needs to be switched to supply air to another port (such as port B) while port A is evacuated to make the valve move in the opposite direction (such as closing). This means that a double-acting actuated valve requires an air source to provide power in both directions of movement. The advantage is that for the same output torque or thrust, double-acting actuated valves are usually smaller and lighter than single-acting actuated valves (because there is no spring), and the valve can be made to stay in any intermediate position when the air source is stable.
