Summary of Functions and Fault Causes of Compressor Valves
As complex mechanical equipment, compressors are assembled orderly with hundreds of different components. Smooth and efficient operation relies on the reliable performance and coordinated cooperation of all parts, especially key components, among which valves play an important role. If practitioners fully understand the working principle, function and common faults of each valve, it will provide theoretical support to guarantee normal operation of the compressor.
Due to differences in compression principles, design concepts and brands, the types of valves adopted by compressors vary. Although it is impossible to cover all types of compressor valves, we summarize the main valve types as follows.
1. Intake Valve
The intake valve is a component for controlling air pressure inside the air receiver, also known as an integrated intake control valve. It realizes intake control, load/unload control, capacity regulation, blowdown, and prevents oil ejection during unloading or shutdown. Its operating rule can be summarized as: Energize for loading, de-energize for unloading.
Compressor intake valves mainly adopt two structures: 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 air inlet closes; when pressure drops to the low-limit set value, the air inlet reopens.
Capacity regulation mode: When the air receiver pressure rises to a certain value, the inlet is slightly closed under the control of the proportional valve. As the pressure rises further, the opening of the air inlet shrinks; when the pressure drops slightly, the inlet opens wider to stabilize pressure within a certain range. If pressure fails to reach the capacity regulation setpoint, the disc of the intake valve remains fully open.
The intake valve is normally closed, which prevents massive gas from entering the airend during startup and increasing the motor starting current. An intake bypass valve is installed on the intake valve to avoid forming 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 movement or blocked exhaust port; deteriorated lubricating oil increases friction between the intake valve piston seal and cylinder and prevents reset; capacity regulating valve closes the intake valve.
Solutions: Inspect the circuit to ensure normal solenoid valve operation; replace the solenoid coil or the entire valve body; disassemble and remove internal impurities of the solenoid valve; clear oil sludge inside the intake valve and cylinder wall, and adopt high-quality lubricating oil; 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 the solenoid valve; blocked throttling hole inside the intake valve; excessively fast blowdown speed.
Solutions: Inspect the circuit and restore normal operation of the solenoid valve; replace the solenoid coil or valve body; disassemble and clean impurities inside the solenoid valve; disassemble the intake valve to clear blockage in the throttling hole; adjust blowdown speed, enlarge pipe diameter or install an elbow.
(3) Oil Ejection from Intake Valve
Main causes: Defective oil separator; blocked oil return check valve; poor filtration performance of air filter, impurities adhering to the sealing surface of intake valve spool resulting in sealing failure; harsh operating environment causes wear between intake valve piston and spring seat.
Solutions: Inspect the oil separator and replace the separator element; remove the oil return check valve and clear internal impurities; inspect and replace the air filter, thoroughly clean the intake pipeline; shorten maintenance intervals and advise users to improve the compressor operating environment.
2. Minimum Pressure Valve
Also called pressure maintaining valve, the minimum pressure valve is installed at the outlet above the oil separator, with the opening pressure generally set at about 0.45 MPa. It consists of valve body, spool, spring, sealing rings and adjusting screws.
Functions in the compressor:
Rapidly build up circulating pressure required for lubrication during startup to avoid wear caused by insufficient lubrication.
Buffering function: control the 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 material.
Non-return function. When the compressor stops or enters no-load state, the pressure inside the oil separator tank drops, and the minimum pressure valve prevents compressed air in the air receiver from flowing back into the oil separator tank.
Common Faults
Numerous compressor failures are triggered by defective minimum pressure valves:
Safety valve vents air during operation: The minimum pressure valve fails to open, leading to overpressure and safety valve relief protection.
Motor overload protection is activated: The minimum pressure valve fails to open, system overpressure causes motor overload and shutdown by thermal relay.
Compressor startup failure: Poor sealing of the minimum pressure valve leads to backflow of pipeline compressed air, resulting in failed startup under pressure.
High oil pressure and increased energy consumption during unloading: Poor sealing of the minimum pressure valve causes pipeline air backflow during unloading, raising pressure inside the compression chamber and oil pressure.
Causes of poor sealing, damage and failure of the minimum pressure valve:
Poor air quality or external foreign matter entering the unit; particle impurities impact the valve under high-speed airflow, damaging components or being trapped between sealing surfaces and causing valve failure; excessive oil filling leads to oil viscosity resistance inside the valve, delaying opening or closing of the valve disc; the minimum pressure valve is designed for specific working conditions, large or long-term deviation from design parameters results in premature failure; after long shutdown, moisture contained in lubricating oil and air accumulates inside the unit, corroding valve components and forming oil adhesion.
3. Safety Valve
Also known as relief valve, the safety valve provides safety protection for the compressor system. When the system pressure exceeds the specified value, the safety valve opens and discharges part of the gas to atmosphere, ensuring the system pressure does not exceed the allowable value and preventing accidents caused by overpressure.
Common Faults
(1) Gas leakage or failure to pop open. Insufficient sealing is mostly caused by overdue maintenance, excessive dirt or component wear, which requires timely cleaning or replacement. Incorrect setting is another possible cause for failure to open.
(2) Continuous vibration. Mismatched matched spring with improper stiffness, which requires adjustment.
Compressor safety valves belong to automatic valves. According to the Safety Technical Supervision Regulation for Safety Valves, safety valves shall be calibrated periodically at least once a year. Standard calibration methods include offline calibration on test bench, on-site instrument calibration and pressure rise pop test. A simple inspection method: gently lift the valve cover when the compressor operates under full load; the safety valve is regarded functional if air exhaust occurs. Note that high-temperature oil mist will be discharged together with air, so safety precautions must be taken.
4. 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 lubricating oil emulsification. During cold startup, the thermostatic valve diverts lubricating oil 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 leading to the oil cooler, and all lubricating oil flows through the cooler before being injected into the airend.
Since lubricating oil entering the thermostatic valve is unfiltered, its complex internal passages easily accumulate contaminants. It has a high failure rate after long-term operation and frequently triggers 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 for hot oil flowing to the airend. The temperature of lubricating oil 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.
5. Oil Stop Valve
The oil stop valve serves 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, preventing lubricating oil 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.
6. 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).
7. 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 2-position 3-way 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-position 2-way 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 very small (less than 0.008 mm). 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.
8. 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, open or close the intake valve accordingly, 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
Due to differences in compression principles, design concepts and brands, the types of valves adopted by compressors vary. Although it is impossible to cover all types of compressor valves, we summarize the main valve types as follows.
1. Intake Valve
The intake valve is a component for controlling air pressure inside the air receiver, also known as an integrated intake control valve. It realizes intake control, load/unload control, capacity regulation, blowdown, and prevents oil ejection during unloading or shutdown. Its operating rule can be summarized as: Energize for loading, de-energize for unloading.
Compressor intake valves mainly adopt two structures: 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 air inlet closes; when pressure drops to the low-limit set value, the air inlet reopens.
Capacity regulation mode: When the air receiver pressure rises to a certain value, the inlet is slightly closed under the control of the proportional valve. As the pressure rises further, the opening of the air inlet shrinks; when the pressure drops slightly, the inlet opens wider to stabilize pressure within a certain range. If pressure fails to reach the capacity regulation setpoint, the disc of the intake valve remains fully open.
The intake valve is normally closed, which prevents massive gas from entering the airend during startup and increasing the motor starting current. An intake bypass valve is installed on the intake valve to avoid forming 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 movement or blocked exhaust port; deteriorated lubricating oil increases friction between the intake valve piston seal and cylinder and prevents reset; capacity regulating valve closes the intake valve.
Solutions: Inspect the circuit to ensure normal solenoid valve operation; replace the solenoid coil or the entire valve body; disassemble and remove internal impurities of the solenoid valve; clear oil sludge inside the intake valve and cylinder wall, and adopt high-quality lubricating oil; 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 the solenoid valve; blocked throttling hole inside the intake valve; excessively fast blowdown speed.
Solutions: Inspect the circuit and restore normal operation of the solenoid valve; replace the solenoid coil or valve body; disassemble and clean impurities inside the solenoid valve; disassemble the intake valve to clear blockage in the throttling hole; adjust blowdown speed, enlarge pipe diameter or install an elbow.
(3) Oil Ejection from Intake Valve
Main causes: Defective oil separator; blocked oil return check valve; poor filtration performance of air filter, impurities adhering to the sealing surface of intake valve spool resulting in sealing failure; harsh operating environment causes wear between intake valve piston and spring seat.
Solutions: Inspect the oil separator and replace the separator element; remove the oil return check valve and clear internal impurities; inspect and replace the air filter, thoroughly clean the intake pipeline; shorten maintenance intervals and advise users to improve the compressor operating environment.
2. Minimum Pressure Valve
Also called pressure maintaining valve, the minimum pressure valve is installed at the outlet above the oil separator, with the opening pressure generally set at about 0.45 MPa. It consists of valve body, spool, spring, sealing rings and adjusting screws.
Functions in the compressor:
Rapidly build up circulating pressure required for lubrication during startup to avoid wear caused by insufficient lubrication.
Buffering function: control the 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 material.
Non-return function. When the compressor stops or enters no-load state, the pressure inside the oil separator tank drops, and the minimum pressure valve prevents compressed air in the air receiver from flowing back into the oil separator tank.
Common Faults
Numerous compressor failures are triggered by defective minimum pressure valves:
Safety valve vents air during operation: The minimum pressure valve fails to open, leading to overpressure and safety valve relief protection.
Motor overload protection is activated: The minimum pressure valve fails to open, system overpressure causes motor overload and shutdown by thermal relay.
Compressor startup failure: Poor sealing of the minimum pressure valve leads to backflow of pipeline compressed air, resulting in failed startup under pressure.
High oil pressure and increased energy consumption during unloading: Poor sealing of the minimum pressure valve causes pipeline air backflow during unloading, raising pressure inside the compression chamber and oil pressure.
Causes of poor sealing, damage and failure of the minimum pressure valve:
Poor air quality or external foreign matter entering the unit; particle impurities impact the valve under high-speed airflow, damaging components or being trapped between sealing surfaces and causing valve failure; excessive oil filling leads to oil viscosity resistance inside the valve, delaying opening or closing of the valve disc; the minimum pressure valve is designed for specific working conditions, large or long-term deviation from design parameters results in premature failure; after long shutdown, moisture contained in lubricating oil and air accumulates inside the unit, corroding valve components and forming oil adhesion.
3. Safety Valve
Also known as relief valve, the safety valve provides safety protection for the compressor system. When the system pressure exceeds the specified value, the safety valve opens and discharges part of the gas to atmosphere, ensuring the system pressure does not exceed the allowable value and preventing accidents caused by overpressure.
Common Faults
(1) Gas leakage or failure to pop open. Insufficient sealing is mostly caused by overdue maintenance, excessive dirt or component wear, which requires timely cleaning or replacement. Incorrect setting is another possible cause for failure to open.
(2) Continuous vibration. Mismatched matched spring with improper stiffness, which requires adjustment.
Compressor safety valves belong to automatic valves. According to the Safety Technical Supervision Regulation for Safety Valves, safety valves shall be calibrated periodically at least once a year. Standard calibration methods include offline calibration on test bench, on-site instrument calibration and pressure rise pop test. A simple inspection method: gently lift the valve cover when the compressor operates under full load; the safety valve is regarded functional if air exhaust occurs. Note that high-temperature oil mist will be discharged together with air, so safety precautions must be taken.
4. 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 lubricating oil emulsification. During cold startup, the thermostatic valve diverts lubricating oil 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 leading to the oil cooler, and all lubricating oil flows through the cooler before being injected into the airend.
Since lubricating oil entering the thermostatic valve is unfiltered, its complex internal passages easily accumulate contaminants. It has a high failure rate after long-term operation and frequently triggers 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 for hot oil flowing to the airend. The temperature of lubricating oil 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.
5. Oil Stop Valve
The oil stop valve serves 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, preventing lubricating oil 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.
6. 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).
7. 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 2-position 3-way 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-position 2-way 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 very small (less than 0.008 mm). 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.
8. 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, open or close the intake valve accordingly, 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
