In July this year, an air compressor unit suffered repeated difficulty loading during operation. Judging from the sound during loading, faults occurred in the loading process. However, the customer had urgent production tasks and uninterrupted air supply was required. Without disrupting production air consumption, maintenance personnel only replaced the loading solenoid valve temporarily.
Nevertheless, within two days, the maintenance department received an emergency call: the air compressor failed to load and build pressure. The unit ran in an unloaded state with no compressed air output. To analyze this issue, let’s first understand the working principle of the air compressor loading valve.
The inlet valve stays closed when the air compressor starts, enabling the motor to drive the main unit under no-load condition. The inlet valve opens when loading is required. Generally, an air pipe with diameter greater than 5 mm is led out from the top cover of the oil separator tank to control the inlet valve via the solenoid valve (normally open solenoid valve).
When the solenoid valve is energized: No compressed air enters the inlet valve. The inlet valve opens under suction, the unit loads and starts generating compressed air.
When the solenoid valve is de-energized: Compressed air flows into the inlet valve, pushing the piston to close the inlet valve, and the blow-off valve opens simultaneously.
Air pressure is divided into two paths: one flows to the blow-off valve for exhaust, and the other is drawn into the compressor. The blow-off valve is equipped with an adjustable connector to regulate exhaust flow and control the pressure inside the oil separator tank. The pressure is normally adjustable to 0.3 MPa. Turn clockwise to increase pressure and counterclockwise to reduce pressure. Tighten the lock nut after adjustment.
Air Volume Regulation Principle of Loading Valve
When the customer’s air consumption is lower than the rated exhaust capacity of the unit, pressure in the customer pipeline system rises. When the pressure reaches the preset unloading pressure, the solenoid valve is de-energized, cutting off the control air supply to the built-in combination valve of the inlet controller. The piston closes under spring force, the blow-off valve opens at the same time, and compressed air inside the oil separator tank flows back to the air inlet until pressure drops to a certain value.
At this moment, the minimum pressure valve closes to isolate the customer pipeline from the unit, and the compressor operates under no-load condition. As pipeline pressure gradually falls to the preset loading pressure, the solenoid valve is energized, connecting the control air supply to the built-in combination valve. The piston overcomes spring force and opens under air pressure, the blow-off valve closes, and the unit resumes loaded operation.
From the operating cycle of the compressor, we know the loading valve is a component subject to frequent loading and unloading cycles. For fixed-frequency units, air output is controlled by loading/unloading pressure. Every loading and unloading cycle relies on the opening and closing of the loading valve, so sufficient attention shall be paid to this component.
1. Inlet Valve Fails to Load
Causes
No power supply to the solenoid valve
Damaged solenoid valve
Impurities inside the solenoid valve causing malfunction or blocked exhaust port
Capacity regulating valve keeps the inlet valve closed
Deteriorated lubricating oil increases friction between piston sealing ring and cylinder of the inlet valve, preventing piston reset
Solutions
Inspect the circuit and restore normal power supply to the solenoid valve
Replace the solenoid coil or the whole solenoid valve
Dismantle the solenoid valve and clear internal impurities
Dismantle the inlet valve, remove oil stains and sludge on cylinder inner wall, and install new stable lubricating oil
Adjust the capacity regulating valve (module) of the inlet valve
2. Inlet Valve Fails to Unload
Causes
Solenoid valve remains energized
Damaged solenoid valve
Small orifice inside the solenoid valve blocked
Throttle hole inside the inlet valve blocked
Excessively fast blow-off speed
Solutions
Inspect the circuit and restore normal operation of the solenoid valve
Replace the solenoid coil or the whole solenoid valve
Dismantle the solenoid valve and clear internal impurities
Dismantle the inlet valve and remove impurities in the throttle hole
Adjust blow-off speed, enlarge pipe diameter or install an elbow fitting
3. Oil Spitting from Inlet Valve
Causes
Poor performance of oil separator
Clogged oil return check valve
Low filtration efficiency of air filter; impurities adhered to the sealing surface of inlet valve spool resulting in poor sealing
Severe operating environment; wear on the matching pair between inlet 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 pipeline of the inlet valve
Replace the inlet valve, shorten the maintenance cycle of the air compressor, and advise the customer to improve the operating environment
Maintenance
Regular maintenance of the loading/unloading valve is essential to ensure stable operation of the air compressor unit. Maintenance requirements are listed below:
Operators shall observe the loading pressure of the unit daily.
Judge abnormal mechanical noise during unloading by listening to loading and unloading sounds.
Conduct formal inspection every 10,000 operating hours according to the maintenance plan. Replace the valve promptly if it fails to meet operating requirements.