Sometimes, even when the equipment is in the unload state, the current can be slightly high. For example, a 22KW compressor has a current of 31A when unloaded. We suspect that the inlet valve is not fully closed. When there's no air demand, the machine unloads, but the pressure still slowly rises, sometimes triggering a high-pressure alarm and causing the machine to shut down. After shutting down, there is still some residual pressure (around 6 bar) inside, which causes the main engine to overload during the next startup.
In screw air compressors with normally open inlet valves, the solenoid valve is open when the machine is not loaded. When the compressor loads, the solenoid valve closes. When the solenoid valve is open, air is directed to close the inlet valve, but it cannot close completely. A small space remains open to maintain the minimum pressure needed for compressor lubrication, usually around 1.5 bar. After the solenoid valve closes, the compressor works normally until it reaches the set working pressure, depending on user settings.
The inlet valve has two pipes: one for venting and the other for releasing the remaining air in the valve, ensuring the compressor loads correctly. If the current is too high during unloading, it means the tank pressure is too high, and the inlet valve is open too wide. You need to adjust the vent screw to prevent the tank pressure from becoming too high.
After shutting down, the tank pressure should drop to zero in about 60 seconds. If the pressure remains at 6 bar, it indicates a problem with the inlet valve's vent line. Possible issues include:
- The pipe diameter may be too small.
- The combination valve orifice may be too small or blocked by debris.
After unloading, the pressure may slightly rise and fall, but this is just a balancing process and should last only a short time. It should not rise to the point of causing a high-pressure shutdown. If it does, the solenoid valve may have a problem, so check if it's functioning properly. Any inlet valve should keep the tank pressure between 1.0 and 2.0 bar during unloading. Higher pressure is a significant waste of energy for the screw compressor.
Reasons and Solutions for a Screw Compressor Inlet Valve Failing to Open
The screw compressor inlet valve controls the air pressure inside the air tank. Common inlet valves include rotating disc and reciprocating valve plate structures. These valves control airflow entering the machine head by opening or closing the inlet.
There are two control methods:
- On/Off: When the air tank pressure reaches the high set limit, the valve closes the inlet. When the pressure drops to the low limit, the valve reopens.
- Proportional: When the air tank reaches a certain pressure, the rotating disc partially closes the inlet under the control of a proportional valve. As the pressure increases, the inlet narrows further. When the pressure drops, the inlet opens wider, keeping the pressure stable within a set range.
If the inlet valve closes during operation, here are some common causes and solutions:
- Excess moisture in control pipes – Clean the control pipes daily.
- Faulty inlet valve – Check the cylinder, valve sleeve, etc., and replace parts if needed.
- Stuck inlet valve – Inspect the valve, valve sleeve, or shaft, and replace if necessary.
- Broken inlet valve spring – Replace the spring.
- Solenoid valve failure – Repair or replace it, but check the wiring before replacing.
- Damaged reciprocating valve – Repair or replace it.
- Loose solenoid valve wiring – Check and tighten the wiring terminals.
All these issues can cause the screw compressor inlet valve to malfunction. It’s important to handle these carefully during production. In general, inlet valves are normally closed to prevent large amounts of air from entering the machine head when starting, which would increase the motor starting current.
There is a bypass valve on the screw compressor inlet valve, which prevents a vacuum from forming inside the machine head during startup or when it’s unloaded. This vacuum could affect the atomization of the lubricating oil.