Unloader Valve: Its Purpose

Posted on: November 10, 2020

A reciprocating air compressor is a positive displacement mechanical device that relies on a cylinder and crankshaft-driven piston to produce compressed air. An unloader valve is one of the many small but essential parts and components that ensure the effective, efficient operation of a reciprocating compressor.

What Is the Unloader Valve?

A compressor unloader valve is a small part, typically measuring approximately 4-5 inches in length and width. It performs the critical function of releasing trapped air inside the tank, enabling the motor to restart. An unloader valve malfunction is one of the more common air compressor issues. If the trapped air cannot escape, the accumulated pressure may be enough to keep the motor from restarting.

The type of unloader valve found in an air compressor depends on the machine’s make and size. Some operate via a toggle switch on the side of the unit’s pressure switch. Others have an integral valve installed under the pressure switch.

What Is the Purpose of an Unloader Valve?

During the operation of a typical reciprocating air compressor, the machine’s electric motor-driven tank fills with air. The device contains a pressure switch that responds to the air accumulation by shutting off the motor’s power supply. The compressor pump, which is attached to the motor shaft, also stops.

When the motor attempts to restart, the air trapped inside the cylinder generates a load that makes it more challenging to execute the task. By venting the air, the unloader valve reduces the load over the piston, facilitating the restarting process.

Where Is the Unloader Valve Located?

If you own a smaller reciprocating air compressor, you’ll likely find the unloader valve mounted on or inside the pressure switch. When the switch shuts the compressor off, it actuates the valve. A small tube or pipe runs from the check valve, a device that keeps the entire tank from draining, to the unloader valve.

A larger air compressor often features a more sizeable unloader valve controlled by air pressure emanating from a smaller pilot valve. In this arrangement, the valve typically sits next to the compressor.

How Does an Unloader Valve Work?

When the air pressure inside the compressor’s tank reaches the pressure switch’s cut out pressure (the point when air is no longer delivered), the switch automatically trips off and interrupts the power supply to the motor. This process causes the unloader valve to open and vent the accumulated air.

When the tank pressure drops back down to the predetermined setting on the pressure switch, it causes the switch to trip again. This action results in the release of the unloader valve’s pin, preventing the escape of compressed air into the atmosphere.

Contact Quincy Compressor to Learn More About Compressor Unloader Valves

As a leading provider of high-quality compressed air equipment for companies in a wide range of industries, Quincy Compressor can answer all your questions about unloader valves, their purpose and how they work. Feel free to contact us today for more information.