Types and Fault Summary of Air Compressor Valves (Comprehensive Version)
As complex mechanical equipment, compressors consist of hundreds of different components assembled in order. Smooth and efficient operation relies on the proper functioning and coordinated cooperation of all parts, especially key components, among which valves play a vital role.
Due to differences in compression types, design concepts and brands, the types of valves adopted by compressors vary. Although it is impossible to cover all compressor valves, we summarize the main types as follows.
01 Intake Valve
The intake valve is a component that controls air pressure inside the air receiver, also known as an integrated intake control valve. It features functions including intake control, load/unload control, capacity regulation, blowdown, and preventing oil ejection during unloading or shutdown. Its operating rule can be summarized as: Energize for loading, de-energize for unloading.
Compressor intake valves generally adopt two mechanisms: rotating disc and reciprocating valve plate.
There are two control modes for intake valves:
On-off mode: When the air receiver pressure reaches the high-limit set value, the inlet closes; when the pressure drops to the low-limit set value, the inlet reopens.
Capacity regulation mode: When the air receiver reaches a certain pressure, the inlet is slightly closed under the control of a proportional valve. As the air receiver pressure rises further, the inlet opening shrinks; when the pressure drops slightly, the inlet opens wider to stabilize pressure within a certain range. If the pressure fails to reach the capacity regulation pressure, the intake disc remains fully open.
The intake valve is normally closed to prevent massive gas from entering the airend during startup, which would increase the motor starting current. An intake bypass valve is fitted on the intake valve to avoid forming a high vacuum inside the airend during startup and no-load operation, which would impair lubricating oil atomization.
Common Faults
(1) Failure to Load
Main causes: Solenoid valve not energized; damaged solenoid valve; impurities inside the solenoid valve causing poor action or blocked exhaust port; deteriorated lubricating oil increasing friction between the intake valve piston seal and cylinder, preventing reset; capacity regulating valve closing the intake valve.
Solutions: Inspect the circuit to ensure normal solenoid valve operation; replace the solenoid coil or the entire valve body; disassemble and clean internal impurities of the solenoid valve; clear oil sludge inside the intake valve and cylinder wall, adopt high-quality lubricant; adjust the capacity regulating valve (module) of the intake valve. Non-professional personnel are not recommended to perform adjustment.
(2) Failure to Unload
Main causes: Solenoid valve remains energized; damaged solenoid valve; blocked small orifice inside solenoid valve; blocked throttling hole inside intake valve; excessively fast blowdown speed.
Solutions: Inspect and restore circuit for solenoid valve; replace solenoid coil or valve body; disassemble and clean impurities inside solenoid valve; disassemble intake valve to clear blockage in throttling hole; adjust blowdown speed, enlarge pipe diameter or install an elbow.
(3) Oil Ejection from Intake Valve
Main causes: Faulty oil separator; blocked oil return check valve; poor filtration performance of air filter, impurities adhering to sealing surface of intake valve spool leading to sealing failure; harsh operating environment causing wear between intake valve piston and spring seat.
Solutions: Inspect the oil separator and replace the separator element; remove and clean internal impurities of oil return check valve; inspect and replace air filter, thoroughly clean intake pipeline; shorten maintenance intervals and advise users to improve compressor operating environment.
02 Minimum Pressure Valve
Also called pressure maintaining valve, the minimum pressure valve is installed at the outlet above the oil separator. It consists of valve body, spool, spring, sealing rings and adjusting screws.
Functions in compressor system:
Rapidly establish circulating pressure required for lubrication during startup to avoid wear caused by insufficient lubrication.
Buffering function: control gas flow velocity passing through the oil separation element, prevent high-speed airflow from deteriorating oil separation efficiency and carrying lubricant out of the system, and avoid excessive pressure difference damaging the filter media.
Non-return function. When the compressor stops or enters no-load status, the pressure inside the oil separator tank drops; the minimum pressure valve prevents compressed air from the air receiver flowing back into the oil separator tank.
Common Faults
Many compressor failures are caused by defective minimum pressure valves:
Safety valve blows air during operation: Minimum pressure valve fails to open, resulting in overpressure and safety valve relief protection.
Motor overload protection triggers: Minimum pressure valve fails to open, system overpressure leads to motor overload and shutdown by thermal relay.
Compressor startup failure: Poor sealing of minimum pressure valve allows backflow of pipeline compressed air, leading to failed startup under pressure.
High oil pressure and increased energy consumption during unloading: Poor sealing of minimum pressure valve causes pipeline air backflow during unloading, raising pressure inside compression chamber and oil pressure.
Reasons for poor sealing, damage and failure of minimum pressure valve:
Poor air quality or external foreign matter entering the unit; particles impact the valve under high-speed airflow, damaging components or being trapped between sealing surfaces; excessive oil filling causes oil viscosity resistance inside the valve, delaying opening or closing; the valve is designed for specific working conditions, large or long-term deviation from design parameters leads to premature failure; moisture contained in lubricant and air accumulates inside the unit after long shutdown, corroding valve components and forming oil adhesion.
03 Safety Valve
Also known as relief valve, the safety valve provides safety protection for the compressor system. When system pressure exceeds the specified value, the safety valve opens and discharges part of gas to atmosphere, ensuring pressure does not exceed allowable limits to prevent accidents caused by overpressure.
Common Faults
(1) Gas leakage or failure to open. Insufficient sealing is mostly caused by overdue maintenance, excessive dirt or component wear, which requires timely cleaning or replacement. Failure to open may also result from incorrect pressure setting.
(2) Continuous vibration. Mismatched spring with improper stiffness; adjustment is required.
Compressor safety valves belong to automatic valves. According to the Safety Technical Supervision Regulation for Safety Valves, regular calibration shall be conducted at least once a year. Standard calibration methods include offline calibration on test benches, online instrument calibration and pressure rise pop testing. A simple inspection method: gently lift the valve cover when the compressor operates under full load; the safety valve is considered functional if air exhaust occurs. Note that high-temperature oil mist will be discharged together with air, so safety precautions must be taken.
04 Thermostatic Valve
The thermostatic valve controls airend discharge temperature. Working principle: Based on thermal expansion and contraction, the thermostatic element extends and retracts to adjust the oil passage formed between valve body and housing, controlling the proportion of lubricant entering the oil cooler, so as to maintain rotor temperature within the set range.
If the airend discharge temperature is too low, moisture precipitates inside the oil separator tank and causes lubricant emulsification. During cold startup, the thermostatic valve diverts lubricant to bypass the oil cooler and flow directly into the airend. When oil temperature rises to a certain value, the thermostatic valve fully opens the passage to the oil cooler, and all lubricant flows through the cooler before being injected into the airend.
Since lubricant entering the thermostatic valve is unfiltered, the complex internal passages easily accumulate contaminants, leading to high failure rates after long service and frequent high-temperature protection shutdown. In addition, even when the passage to the oil cooler is fully open, the thermostatic valve cannot completely cut off the direct passage of hot oil to the airend. The temperature of lubricant entering the airend is always higher than the outlet temperature of the oil cooler, reducing cooling efficiency. For this reason, some manufacturers omit thermostatic valves and control oil temperature by starting and stopping cooling fan motors. The fan activates when airend discharge temperature rises to 90°C and stops below 70°C to stabilize temperature.
The working status of the thermostatic valve can be judged by checking four connected oil pipelines:
Oil separator tank → Thermostatic valve; Thermostatic valve → Oil cooler; Oil cooler → Thermostatic valve; Thermostatic valve → Oil filter.
Normal temperature status: High, High, Low, Slightly High.
05 Oil Stop Valve
The oil stop valve acts as a switch controlling main oil supply to the screw airend. Its core function is cutting off oil supply to the host upon compressor shutdown to prevent lubricant ejection from the airend inlet and oil backflow at the moment of shutdown.
Common Faults
During operation, tiny impurities in air inevitably mix with cooling oil and may deposit and block the oil stop valve. Consequently, the valve cannot open upon startup and stays closed or partially closed, reducing cooling oil flow. This causes rapid high-temperature shutdown, high-temperature alarm or oil back ejection upon shutdown.
Therefore, regular cleaning and maintenance of the oil stop valve are required. Optimize the operating environment: deploy the compressor in well-ventilated spaces away from volatile gas and suspended debris. Replace air filters strictly in accordance with differential pressure specifications.
06 Check Valve
Also called non-return valve. In compressed air systems, it mainly prevents sudden reverse ejection of compressed oil-gas mixture into the host during emergency shutdown, which would cause rotor reverse rotation.
The check valve sometimes fails to close tightly. Main causes: Detached rubber sealing ring, broken spring (requires replacement); foreign matter trapped under the sealing ring (requires impurity removal).
07 Solenoid Valve
Solenoid valves belong to the control system, including load solenoid valves and blowdown solenoid valves. In compressors, solenoid valves are used to adjust flow direction, flow rate, speed, on-off status and other medium parameters.
(1) Load Solenoid Valve
It is a 3/2 normally closed solenoid valve. Under PLC instructions, it controls the on-off of the pipeline from oil separator tank to the intake valve actuator cylinder. When energized, compressed air from the oil separator tank enters the actuator cylinder to open the air inlet. When de-energized, the passage is cut off and residual air inside the cylinder is vented to close the inlet.
(2) Blowdown Solenoid Valve
It is a 2/2 normally open solenoid valve. Its nominal diameter must match the intake bypass valve to ensure the pressure inside the oil separator tank stays around 0.2 MPa under no-load operation. When the compressor unloads or shuts down, the blowdown solenoid valve de-energizes and releases gas inside the oil tank to the air filter.
Common Faults
a. Loose or fallen wiring terminals lead to no power supply for the solenoid valve; tighten the wiring to solve the problem.
b. Solenoid valve air leakage. Leakage results in insufficient air pressure and difficult valve switching, caused by damaged sealing gaskets or spool wear leading to cross air leakage.
c. Burnt solenoid coil. Disconnect wiring and measure with a multimeter; open circuit indicates coil burnout.
d. Solenoid valve jamming. The clearance between the spool sleeve and spool is small. Jamming easily occurs when mechanical impurities enter or lubricant is insufficient.
Solutions: Insert a steel wire through the small hole at the top to reset the spool. The thorough solution is to disassemble the valve, take out the spool and sleeve for cleaning to ensure flexible movement. Record assembly sequence and external wiring positions during disassembly for correct reassembly. Also check whether the oil mist injector orifice is blocked and confirm sufficient lubricant supply.
08 Proportional Valve (Capacity Regulating Valve)
The proportional valve, also known as capacity regulating valve, only activates when pressure exceeds the set value. It is generally used with butterfly intake control valves. When system pressure rises due to reduced air consumption and reaches the set value of the proportional valve, the valve actuates and reduces output control air flow, lowering compressor intake to balance system air demand.
Turn clockwise to increase intake volume; counter-clockwise to reduce intake volume. Adjustment procedure is simple, yet multiple tuning attempts are needed to achieve sensitive response. The adjustment target is to trigger the proportional valve when pressure approaches the upper/lower limits of the pressure switch, so as to adjust intake volume in advance, reduce frequent load/unload cycles of the compressor.
Summary
Compressors are equipped with a wide variety of valves with different working principles and service environments. Therefore, the root causes of valve faults and corresponding compressor symptoms differ greatly. Only by fully understanding valve construction and failure mechanisms can we guarantee safe, long-term and stable compressor operation and improve production efficiency.
Due to differences in compression types, design concepts and brands, the types of valves adopted by compressors vary. Although it is impossible to cover all compressor valves, we summarize the main types as follows.
01 Intake Valve
The intake valve is a component that controls air pressure inside the air receiver, also known as an integrated intake control valve. It features functions including intake control, load/unload control, capacity regulation, blowdown, and preventing oil ejection during unloading or shutdown. Its operating rule can be summarized as: Energize for loading, de-energize for unloading.
Compressor intake valves generally adopt two mechanisms: rotating disc and reciprocating valve plate.
There are two control modes for intake valves:
On-off mode: When the air receiver pressure reaches the high-limit set value, the inlet closes; when the pressure drops to the low-limit set value, the inlet reopens.
Capacity regulation mode: When the air receiver reaches a certain pressure, the inlet is slightly closed under the control of a proportional valve. As the air receiver pressure rises further, the inlet opening shrinks; when the pressure drops slightly, the inlet opens wider to stabilize pressure within a certain range. If the pressure fails to reach the capacity regulation pressure, the intake disc remains fully open.
The intake valve is normally closed to prevent massive gas from entering the airend during startup, which would increase the motor starting current. An intake bypass valve is fitted on the intake valve to avoid forming a high vacuum inside the airend during startup and no-load operation, which would impair lubricating oil atomization.
Common Faults
(1) Failure to Load
Main causes: Solenoid valve not energized; damaged solenoid valve; impurities inside the solenoid valve causing poor action or blocked exhaust port; deteriorated lubricating oil increasing friction between the intake valve piston seal and cylinder, preventing reset; capacity regulating valve closing the intake valve.
Solutions: Inspect the circuit to ensure normal solenoid valve operation; replace the solenoid coil or the entire valve body; disassemble and clean internal impurities of the solenoid valve; clear oil sludge inside the intake valve and cylinder wall, adopt high-quality lubricant; adjust the capacity regulating valve (module) of the intake valve. Non-professional personnel are not recommended to perform adjustment.
(2) Failure to Unload
Main causes: Solenoid valve remains energized; damaged solenoid valve; blocked small orifice inside solenoid valve; blocked throttling hole inside intake valve; excessively fast blowdown speed.
Solutions: Inspect and restore circuit for solenoid valve; replace solenoid coil or valve body; disassemble and clean impurities inside solenoid valve; disassemble intake valve to clear blockage in throttling hole; adjust blowdown speed, enlarge pipe diameter or install an elbow.
(3) Oil Ejection from Intake Valve
Main causes: Faulty oil separator; blocked oil return check valve; poor filtration performance of air filter, impurities adhering to sealing surface of intake valve spool leading to sealing failure; harsh operating environment causing wear between intake valve piston and spring seat.
Solutions: Inspect the oil separator and replace the separator element; remove and clean internal impurities of oil return check valve; inspect and replace air filter, thoroughly clean intake pipeline; shorten maintenance intervals and advise users to improve compressor operating environment.
02 Minimum Pressure Valve
Also called pressure maintaining valve, the minimum pressure valve is installed at the outlet above the oil separator. It consists of valve body, spool, spring, sealing rings and adjusting screws.
Functions in compressor system:
Rapidly establish circulating pressure required for lubrication during startup to avoid wear caused by insufficient lubrication.
Buffering function: control gas flow velocity passing through the oil separation element, prevent high-speed airflow from deteriorating oil separation efficiency and carrying lubricant out of the system, and avoid excessive pressure difference damaging the filter media.
Non-return function. When the compressor stops or enters no-load status, the pressure inside the oil separator tank drops; the minimum pressure valve prevents compressed air from the air receiver flowing back into the oil separator tank.
Common Faults
Many compressor failures are caused by defective minimum pressure valves:
Safety valve blows air during operation: Minimum pressure valve fails to open, resulting in overpressure and safety valve relief protection.
Motor overload protection triggers: Minimum pressure valve fails to open, system overpressure leads to motor overload and shutdown by thermal relay.
Compressor startup failure: Poor sealing of minimum pressure valve allows backflow of pipeline compressed air, leading to failed startup under pressure.
High oil pressure and increased energy consumption during unloading: Poor sealing of minimum pressure valve causes pipeline air backflow during unloading, raising pressure inside compression chamber and oil pressure.
Reasons for poor sealing, damage and failure of minimum pressure valve:
Poor air quality or external foreign matter entering the unit; particles impact the valve under high-speed airflow, damaging components or being trapped between sealing surfaces; excessive oil filling causes oil viscosity resistance inside the valve, delaying opening or closing; the valve is designed for specific working conditions, large or long-term deviation from design parameters leads to premature failure; moisture contained in lubricant and air accumulates inside the unit after long shutdown, corroding valve components and forming oil adhesion.
03 Safety Valve
Also known as relief valve, the safety valve provides safety protection for the compressor system. When system pressure exceeds the specified value, the safety valve opens and discharges part of gas to atmosphere, ensuring pressure does not exceed allowable limits to prevent accidents caused by overpressure.
Common Faults
(1) Gas leakage or failure to open. Insufficient sealing is mostly caused by overdue maintenance, excessive dirt or component wear, which requires timely cleaning or replacement. Failure to open may also result from incorrect pressure setting.
(2) Continuous vibration. Mismatched spring with improper stiffness; adjustment is required.
Compressor safety valves belong to automatic valves. According to the Safety Technical Supervision Regulation for Safety Valves, regular calibration shall be conducted at least once a year. Standard calibration methods include offline calibration on test benches, online instrument calibration and pressure rise pop testing. A simple inspection method: gently lift the valve cover when the compressor operates under full load; the safety valve is considered functional if air exhaust occurs. Note that high-temperature oil mist will be discharged together with air, so safety precautions must be taken.
04 Thermostatic Valve
The thermostatic valve controls airend discharge temperature. Working principle: Based on thermal expansion and contraction, the thermostatic element extends and retracts to adjust the oil passage formed between valve body and housing, controlling the proportion of lubricant entering the oil cooler, so as to maintain rotor temperature within the set range.
If the airend discharge temperature is too low, moisture precipitates inside the oil separator tank and causes lubricant emulsification. During cold startup, the thermostatic valve diverts lubricant to bypass the oil cooler and flow directly into the airend. When oil temperature rises to a certain value, the thermostatic valve fully opens the passage to the oil cooler, and all lubricant flows through the cooler before being injected into the airend.
Since lubricant entering the thermostatic valve is unfiltered, the complex internal passages easily accumulate contaminants, leading to high failure rates after long service and frequent high-temperature protection shutdown. In addition, even when the passage to the oil cooler is fully open, the thermostatic valve cannot completely cut off the direct passage of hot oil to the airend. The temperature of lubricant entering the airend is always higher than the outlet temperature of the oil cooler, reducing cooling efficiency. For this reason, some manufacturers omit thermostatic valves and control oil temperature by starting and stopping cooling fan motors. The fan activates when airend discharge temperature rises to 90°C and stops below 70°C to stabilize temperature.
The working status of the thermostatic valve can be judged by checking four connected oil pipelines:
Oil separator tank → Thermostatic valve; Thermostatic valve → Oil cooler; Oil cooler → Thermostatic valve; Thermostatic valve → Oil filter.
Normal temperature status: High, High, Low, Slightly High.
05 Oil Stop Valve
The oil stop valve acts as a switch controlling main oil supply to the screw airend. Its core function is cutting off oil supply to the host upon compressor shutdown to prevent lubricant ejection from the airend inlet and oil backflow at the moment of shutdown.
Common Faults
During operation, tiny impurities in air inevitably mix with cooling oil and may deposit and block the oil stop valve. Consequently, the valve cannot open upon startup and stays closed or partially closed, reducing cooling oil flow. This causes rapid high-temperature shutdown, high-temperature alarm or oil back ejection upon shutdown.
Therefore, regular cleaning and maintenance of the oil stop valve are required. Optimize the operating environment: deploy the compressor in well-ventilated spaces away from volatile gas and suspended debris. Replace air filters strictly in accordance with differential pressure specifications.
06 Check Valve
Also called non-return valve. In compressed air systems, it mainly prevents sudden reverse ejection of compressed oil-gas mixture into the host during emergency shutdown, which would cause rotor reverse rotation.
The check valve sometimes fails to close tightly. Main causes: Detached rubber sealing ring, broken spring (requires replacement); foreign matter trapped under the sealing ring (requires impurity removal).
07 Solenoid Valve
Solenoid valves belong to the control system, including load solenoid valves and blowdown solenoid valves. In compressors, solenoid valves are used to adjust flow direction, flow rate, speed, on-off status and other medium parameters.
(1) Load Solenoid Valve
It is a 3/2 normally closed solenoid valve. Under PLC instructions, it controls the on-off of the pipeline from oil separator tank to the intake valve actuator cylinder. When energized, compressed air from the oil separator tank enters the actuator cylinder to open the air inlet. When de-energized, the passage is cut off and residual air inside the cylinder is vented to close the inlet.
(2) Blowdown Solenoid Valve
It is a 2/2 normally open solenoid valve. Its nominal diameter must match the intake bypass valve to ensure the pressure inside the oil separator tank stays around 0.2 MPa under no-load operation. When the compressor unloads or shuts down, the blowdown solenoid valve de-energizes and releases gas inside the oil tank to the air filter.
Common Faults
a. Loose or fallen wiring terminals lead to no power supply for the solenoid valve; tighten the wiring to solve the problem.
b. Solenoid valve air leakage. Leakage results in insufficient air pressure and difficult valve switching, caused by damaged sealing gaskets or spool wear leading to cross air leakage.
c. Burnt solenoid coil. Disconnect wiring and measure with a multimeter; open circuit indicates coil burnout.
d. Solenoid valve jamming. The clearance between the spool sleeve and spool is small. Jamming easily occurs when mechanical impurities enter or lubricant is insufficient.
Solutions: Insert a steel wire through the small hole at the top to reset the spool. The thorough solution is to disassemble the valve, take out the spool and sleeve for cleaning to ensure flexible movement. Record assembly sequence and external wiring positions during disassembly for correct reassembly. Also check whether the oil mist injector orifice is blocked and confirm sufficient lubricant supply.
08 Proportional Valve (Capacity Regulating Valve)
The proportional valve, also known as capacity regulating valve, only activates when pressure exceeds the set value. It is generally used with butterfly intake control valves. When system pressure rises due to reduced air consumption and reaches the set value of the proportional valve, the valve actuates and reduces output control air flow, lowering compressor intake to balance system air demand.
Turn clockwise to increase intake volume; counter-clockwise to reduce intake volume. Adjustment procedure is simple, yet multiple tuning attempts are needed to achieve sensitive response. The adjustment target is to trigger the proportional valve when pressure approaches the upper/lower limits of the pressure switch, so as to adjust intake volume in advance, reduce frequent load/unload cycles of the compressor.
Summary
Compressors are equipped with a wide variety of valves with different working principles and service environments. Therefore, the root causes of valve faults and corresponding compressor symptoms differ greatly. Only by fully understanding valve construction and failure mechanisms can we guarantee safe, long-term and stable compressor operation and improve production efficiency.
