Pressure drop after air compressor shutdown is a common phenomenon, but the rate of pressure drop and the final value have normal and abnormal ranges. If the pressure in the air tank returns to zero shortly after shutdown, it may indicate that the minimum pressure valve is not closing tightly; in a normal temperature environment of 25 to 30 degrees Celsius, the normal range of static pressure is usually 0.7 MPa to 0.9 MPa. Understanding these patterns helps to quickly determine the equipment status and avoid safety issues in production caused by abnormal pressure.
Five Major Reasons for Pressure Drop Explained:
The compressibility of compressed air is a fundamental physical characteristic. Unlike the incompressibility of hydraulic oil, compressed air gradually releases pressure after shutdown due to the restoration of inter-molecular distance. This characteristic determines that pneumatic systems cannot maintain a constant pressure like hydraulic systems, especially when pressure maintenance devices are not installed.
Minimum pressure valve malfunction is a common cause. This valve consists of a valve body, valve core, spring, and other components. It should automatically close during normal shutdown to prevent compressed air back-flow. If the spring fails or the sealing element wears down, the pressure will drop sharply from 0.8 MPa to below 0.2 MPa within 10 minutes. This needs to be confirmed by disassembling and checking the valve core closure status.
Model differences affect pressure maintenance. Reciprocating air compressors have residual volume, and the expansion of residual air after shutdown will accelerate the pressure drop; while screw air compressors, through their continuous rotor meshing design, can extend the pressure maintenance time by more than 30%, making them more suitable for scenarios requiring a stable air supply.
Ambient temperature changes pressure values through thermal expansion and contraction. In a summer environment of 35℃, the static pressure of the same system after shutdown may be 0.15 MPa higher than in a winter environment of 5℃. This change is a normal physical phenomenon, but it is necessary to ensure that the pressure gauge has been calibrated for temperature compensation.
Pipeline leaks are a more concealed cause. A 2mm diameter hole can leak approximately 0.3m³ of air per hour at 0.7MPa pressure. Air bubbles can be detected at connections by applying soapy water, or a small leak can be located using an ultrasonic leak detector.
Judgment and Handling of Abnormal Pressure Drops：
The rate of pressure drop is a key indicator. After normal shutdown, the pressure should decrease linearly and slowly. If the pressure drop exceeds 50% within 30 seconds, caution is advised. It is recommended to install a pressure sensor and set an alarm threshold. When the pressure drops from 0.8 MPa to 0.4 MPa in less than 2 minutes, trigger the maintenance procedure.
Minimum pressure valve checks require three steps: First, observe whether the pressure gauge pointer quickly returns to zero after shutdown; second, disassemble the valve to check the spring preload (standard value is 15-20 N); finally, verify the valve core sealing with an airtightness tester. The leakage rate should be less than 0.1 Pa/min.
Pipeline system inspection should use a segmented troubleshooting method. Starting from the gas tank outlet, close each valve sequentially and monitor pressure changes. If the pressure stops dropping after closing a valve, the leak point is downstream of that valve. Focus on checking stress concentration areas such as elbows and flanges.
Model baseline values should be referenced from the manufacturer’s manual. One hour after shutdown, the pressure of a reciprocating air compressor should not be lower than 0.5 MPa, while that of a screw compressor should remain above 0.65 MPa. For oil-free screw compressors, the pressure drop rate may be 15% faster than that of oil-injected models, but the final value should not be lower than 0.6 MPa.
Correct Shutdown Operation Guidelines:
The inlet valve should only be closed after the pressure has reached the set value. Specifically: When the pressure gauge reads 0.75 MPa, slowly close the inlet valve until the exhaust pressure drops below 0.1 MPa before disconnecting the power. This operation avoids residual high-pressure air in the pipeline and reduces condensation.
Zero-pressure operation requires the equipment to run unloaded for 3-5 minutes. Observe the ammeter reading to ensure that compressed air in the system has been completely discharged by reducing it to the unloaded value (usually 20%-30% of the rated current). This step prevents residual pressure from causing lubricating oil emulsification.
Drain valve maintenance should be a regular procedure. It is recommended to open the drain valve at the bottom of the air tank before leaving get off work each day and continuously drain for 30 seconds until no water flows out. For automatic drainers, the reliability of the solenoid valve operation should be checked monthly to prevent back-flow due to blockage.
Filter replacement cycles directly affect pressure stability. Air filters should be replaced every 2000 hours, oil filters every 4000 hours, and lubricating oil every 8000 hours. Use a laser particle counter to check oil cleanliness; replace the filter when the ISO code exceeds 18/15.
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Post time: Jul-02-2026
