How Much Do You Know About Air Compressor Intake Valves?

The air compressor intake valve can also be referred to as an integrated intake control valve. It integrates multiple functions: intake regulation, load/unload control, capacity modulation, 

pressure venting, and preventing oil spray during unloading or shutdown. Today we will learn about the working principle of the intake valve of screw air compressors, 

as well as the causes and solutions for intake valve failure to open.

Working Principle of Screw Air Compressor Intake Valve
The general rule for screw compressor intake valves: Energized for loading, de-energized for unloading.

Sometimes the operating current remains relatively high under unloaded conditions. For a 22 kW unit, the unloaded current may reach 31 A. A preliminary judgment is that the intake valve is not fully closed. 

When no air is consumed and the compressor enters unload status, system pressure still rises gradually, which may trigger high-pressure alarm and unit shutdown.

In addition, residual pressure (around 6 bar) cannot be vented completely after shutdown, resulting in main motor overload upon restart.
This type is a normally open intake valve. The solenoid valve opens when the screw compressor is unloaded and closes when the compressor is loaded.

When the solenoid valve opens, a control air circuit acts to close the intake valve, yet full closure cannot be achieved. A small intake clearance must be retained to maintain the minimum pressure required for compressor lubrication, 

normally 1.5 bar. After the solenoid valve closes, the compressor enters normal operation until reaching the user-defined working pressure.

The intake valve is equipped with two pipelines: one for air venting, and the other for exhausting residual air inside the valve to ensure normal compressor loading.
Excessively high current during unloading indicates high separator tank pressure caused by an overlarge opening of the intake valve. Adjust the bleed screw to prevent excessive tank pressure.
If the separator tank pressure remains at 6 bar after shutdown, there is a fault in the vent pipeline of the intake valve.
Possible causes: undersized pipe diameter, too small orifice of the integrated valve, or blockage by foreign debris.

After unloading, slight pressure fluctuation is normal for pressure balance and lasts only a short period. It should not rise to trigger high-pressure shutdown. 

High-pressure shutdown under such circumstances is usually caused by a faulty compressor solenoid valve. Check whether the solenoid valve is intact and functioning properly.

Regardless of intake valve type, the separator tank pressure shall be maintained between 1.0 ~ 2.0 bar during unloading. Excessively high pressure will lead to substantial energy waste for the screw air compressor.
Causes and Solutions for Screw Air Compressor Intake Valve Failing to Open

The intake valve controls air pressure inside the air receiver tank. Two common structures are adopted: rotary disc type and reciprocating valve plate type. 

The disc or valve plate opens and closes the air inlet to regulate airflow entering the air-end.

Two control modes are available:
On-off control: When receiver pressure hits the high setpoint, the air inlet closes; when pressure drops to the low setpoint, the inlet opens again.

Modulation control: When receiver pressure reaches a certain threshold, the rotary disc partially closes the inlet under the control of a proportional valve. 

The inlet closes further as pressure rises and opens slightly as pressure drops, stabilizing pressure within a certain range. Before pressure reaches the modulation setpoint, the intake disc remains fully open.

Let us analyze the possible causes and corresponding solutions if the intake valve fails to close during operation:
Excessive moisture inside control piping; perform routine maintenance on control pipelines every day.
Malfunction of intake valve assembly; inspect the servo cylinder, valve sleeve and other components, replace parts if necessary.
Intake valve jamming; inspect the intake valve, valve sleeve and shaft, replace components as required.
Fractured intake valve spring; replace the spring.
Solenoid valve failure; repair or replace as required, check wiring before replacement.
Damaged reciprocating valve; repair or replace it.
Loose wiring terminals of solenoid valve; inspect and fasten all cable terminals.
All above faults will cause intake valve malfunction and shall be handled carefully during production.
The intake valve is normally closed by design, preventing massive air from entering the air-end during startup and avoiding excessive motor starting current.
An intake bypass valve is fitted on the intake valve. It prevents extreme vacuum inside the air-end during startup and no-load operation, which would otherwise impair lubricant atomization.