What are the methods for detecting air leakage in pneumatic valves?

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\"Air leakage\" in pneumatic valves refers to abnormal airflow between different chambers within the valve’s air circuit system (such as the upper and lower diaphragm chambers of the actuator, or between the positioner and the actuator), or between the air supply source and the atmosphere. This results in abnormal valve operation (such as sticking, failure to move fully to the on or off position, or fluctuations). The methods for making judgments mainly involve observing phenomena, conducting segmented tests, and performing air circuit tests, as detailed below: First, a preliminary judgment is made based on the operating conditions. Air leakage can cause abnormal valve operation, and it can be identified through the following signs: 1. Slow or incomplete valve operation: If the valve should be fully open but isn’t, or if there is still leakage when it should be fully closed, it may be due to air leakage in the actuator’s diaphragm chamber (compressed air leaking from one side of the diaphragm to the other, resulting in insufficient thrust). 2. Mismatch between actuator output and valve operation: The air pressure output by the actuator is normal (for example, a 4-20mA signal corresponds to an air pressure of 0.02–0.1 MPa), but the valve does not respond or responds with delay; this may be due to air leakage in the air path from the actuator to the valve (air pressure loss). 3. Abnormal drop in air supply pressure: If the pressure in the air supply pipeline continues to decrease even when the valve is not activated, or if there is still a consumption of air from the supply source after the valve is closed, it may be due to air leakage between the air supply pipeline and the atmosphere (such as leaks in the valve position feedback circuit). 4. Abnormal noises from the actuator: A continuous hissing sound of air leakage is heard when the valve operates; this is usually caused by a damaged diaphragm or aging seals, resulting in air leakage between the chambers. II. Segmented detection method (core judgment approach) 1. Determine whether there is air leakage between the positioner and the actuator. Step 1: Disconnect the air pipeline connecting the positioner and the actuator (e.g., the pipeline from the positioner’s output port to the upper/lower diaphragm chamber of the actuator). Step 2: Supply a fixed signal to the positioner (such as a 50% signal, corresponding to an output pressure of 0.06 MPa), and observe whether the output pressure of the positioner remains stable. If the pressure is stable, it indicates that there is no air leakage in the positioner ； If the pressure drops rapidly, it indicates air leakage inside the locator (such as worn valve elements, or a connection between the air supply and the exhaust port). Step 3: Apply a fixed air pressure (such as 0.1 MPa) separately to the actuator diaphragm chamber. After shutting off the air supply, check whether the pressure remains constant; if the pressure drops rapidly, it indicates that there is air leakage in the actuator diaphragm chamber (for example, the diaphragm is damaged, allowing communication between the upper and lower chambers) ； If the pressure drops slowly, it may be due to a slight leak in the membrane chamber or aging of the seals. 2. Determine whether there is air leakage between the upper and lower chambers of the actuator. This method applies to double-acting actuators (cylinder type) or single-acting actuators (diaphragm type). For single-acting actuators: shut off the air supply, manually move the valve stem to the fully open/fully closed position; if the valve stem rebounds automatically along with audible air leakage sounds, it indicates that the diaphragm is damaged and there is air leakage between the air supply side and the spring side. Double-acting actuator: Air pressure (such as 0.1 MPa) is applied to the rodless chamber and the rod-bearing chamber respectively, while the air supply to the other chamber is shut off. If the pressure drops, it indicates that there is air leakage between the two chambers (for example, due to a damaged piston seal). 3. Check whether there is air leakage from the valve accessories (such as solenoid valves and limit switches). For solenoid valves: power the solenoid valve on and off, and observe whether air is continuously leaking from its exhaust port (under normal conditions, air is only released during operation); if leakage persists, it indicates that the seal of the solenoid valve’s spool is poor, allowing air to leak from the air supply source to the exhaust port. Limit switch (pneumatically controlled): It should supply or cut off air only when the valve is in the correct position; if air leaks when the valve is not in position, it may be due to cross-connection in the internal air circuit. III. Tool Inspection Method 1. Soap water test: Apply soap water to areas such as joints where air leakage is suspected, membrane chamber flanges, and valve stuffing boxes. If bubbles appear, it indicates a leak point; the larger the bubbles, the more severe the leakage. 2. Pressure gauge inspection: Install pressure gauges at various points in the gas circuit (such as the output terminal of the positioner, the inlet of the actuator, and the gas supply lines) to monitor whether there are any abnormal pressure fluctuations when the valve operates. If the pressure in the upper diaphragm chamber of the actuator increases, the pressure in the lower diaphragm chamber also increases simultaneously, which indicates that air is leaking between the two diaphragm chambers ； If the input signal to the positioner remains unchanged while the output pressure continues to drop, it indicates that there is air leakage in the pipeline from the positioner to the actuator. 3. Flow meter detection: A small flow meter is installed in the gas supply pipeline; if the flow rate is greater than 0 when the valve is closed, it indicates air leakage (under normal stationary conditions, there should be no gas consumption). IV. Typical air leakage failure points and characteristics – Summary: The key to identifying air leakage in pneumatic valves is \"segmental isolation and pressure retention testing\"; by combining observation of symptoms with instrumental detection, it is possible to locate the source of the air leakage (whether it is the actuator, executor, accessories, or pipelines). Air leakage can reduce the precision of valve control and even cause process fluctuations; once detected, it is necessary to replace the seals, diaphragms, or any damaged components promptly to ensure proper sealing of the gas circuit.