【2025 At a Glance】Key Differences Between Air-Open Valves and Air-Close Valves
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The figure below shows: the key differences between air-on valves and air-off valves.
The following reply is for reference only. 1. Air-operated valve (AO): opens when air is present and closes when air is lost. (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 of gas isolation valves: Used in scenarios that require \"fail-safe shutdown\" (such as pipelines carrying flammable media, where the flow is interrupted in the event of a loss of gas to prevent leaks).
2. Air-operated shut-off valve (AC): closes when air is present and opens when air is absent. (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).
(2) When the air pressure signal is lost, the spring’s restoring force pushes the diaphragm upward; the valve stem then moves the valve core to open the valve, returning it to a safe position.
Typical applications of air shut-off valves: used in scenarios where \"fail-safe flow\" is required (such as cooling systems, where maintaining the flow prevents the equipment from overheating in the event of a loss of air pressure).
1. Key basis for selection: Process safety requirements: (1) If the medium is a hazardous substance (such as flammable gases or corrosive liquids), gas-operated valves should be preferred, as they cut off the flow when there is no gas supply.
(2) If continuous flow needs to be ensured (such as for preventing condensation in pipelines or cooling equipment), gas-operated shut-off valves should be preferred, as they maintain the flow path even when there is no gas supply.
Control system logic: It must match the signal direction of the controller (such as PLC/DCS) to ensure the stability of closed-loop control.
2. Typical application scenarios: Air-operated valves: feed valves for chemical reaction vessels (to prevent leaks), shut-off valves for gas boilers, solenoid valves in fire sprinkler systems (shut off in the absence of air to prevent accidental spraying).