【2025 At a Glance】Structural Differences Between Air-Open Valves and Air-Close Valves
Thread Content
The figure below shows the structural differences between air-open valves and air-close valves.
The following response is for reference only. Air-on valves and air-off valves are two common types of pneumatic control valves; the main difference lies in the opening and closing state of the valve under the action of the pneumatic signal pressure (such as compressed air), as well as its safe position in the event of a failure (loss of air supply). Their structure and working principle are closely related to the design of pneumatic actuators.
Pneumatic control valves typically consist of an actuator and a valve body component, with the key difference lying in the design of the actuator.
1. Common structural elements of actuator mechanisms (1) Diaphragm (or piston): Senses the pressure of the pneumatic signal and converts it into mechanical thrust.
(2) Spring: Provides a restoring force to return the valve to a safe position in the event of air loss.
(3) Valve stem: Connects the actuator to the valve body, transmitting thrust to control the movement of the valve core.
(4) Positioner (optional): Used to precisely adjust the valve opening, with feedback signals enabling closed-loop control.
1. The air-operated valve (AO) operates in an open state when there is air pressure and in a closed state when there is no air pressure. (1) When a pneumatic signal pressure (such as 4–20 kPa) is applied to the top of the diaphragm in the actuator, the air pressure pushes the diaphragm downward, compressing the spring below and causing the valve stem to move downward; as a result, the valve core gradually opens, increasing the flow area.
(2) When the air pressure signal is lost, the spring’s restoring force pushes the diaphragm upward; the valve stem then causes the valve core to close the valve, returning it to a safe position.
Typical applications: Used in scenarios that require \"fail-safe shutdown\" (such as pipelines carrying flammable media, where the flow is interrupted in case of air loss to prevent leaks).
2. The air-operated valve (AC) operates in such a way that it closes when there is air pressure and opens when there is no air pressure. (1) When the pneumatic signal pressure is applied to above the diaphragm of the actuator, the air pressure pushes the diaphragm downward, compressing the spring above it; this causes the valve stem to move downward, and the valve core gradually closes the valve (reducing the flow area).