Unloader Valve: Its Purpose

Posted on: June 28, 2023

A reciprocating air compressor is a positive displacement mechanical device that uses a cylinder and crankshaft-driven piston to produce compressed air. An unloader valve is a small and essential part that ensures the effective, efficient operation of a reciprocating compressor.

What Is an Unloader Valve?

A compressor unloader valve is a small part, usually sized between 4 and 5 inches. The valve releases trapped air from inside the compressor’s tank, which allows the motor to restart. If the unloader valve cannot release the air, the built-up pressure might prevent the motor from starting.

An air compressor’s valve type 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 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 tries to restart, the trapped air produces a load that restricts its ability to switch on. By venting the air, the unloader valve minimizes the load across the piston, which encourages the repowering process.

Where Is the Unloader Valve Located?

Unloader valves are located on top of or inside the pressure switch in smaller reciprocating air compressors. 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 usually contains a larger unloader valve. It’s controlled by air pressure that’s released 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 within the compressor tank arrives at the point where it cannot deliver any more air, the valve switch automatically disrupts the power supply to the motor. After the power supply turns off, the unloader valve opens and vents 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.

How to Replace an Unloader Valve

A malfunctioning unloader valve is one of the most common issues that prevents a compressor from operating properly. When the air cannot escape from the tank, the built-up pressure often hinders the unit’s ability to restart.

When the unloader valve malfunctions, it becomes unable to open or close. Open valves release a continuous stream of leaking air. Closed valves result in significant pressure buildups that can restrict the pump’s internal components. When the interior components can’t move, this prevents the engine shaft from opening.

You should replace faulty unloader valves as soon as possible before they impact other critical compressor components. Learn more about how to replace an unloader valve below.

What You Need to Get Started

Various air compressors have different types of unloader valves. Whether you’re working with a toggle switch on the side of the pressure controller or a valve that mounts directly to the unit, both types require the same materials for replacements:

  • A Phillips or flathead screwdriver
  • An adjustable wrench
  • Adjustable pliers
  • Snipping pliers
  • Thread lubricant
  • The new unloader valve
  • Safety glasses

Unloader Valve Replacement Steps

First, open the drain valve to depressurize the tank. After the pressure releases, identify the unloader valve. It’s connected to a hose that transfers water to the recirculation pump.

Next, it’s time to remove the old valve. Follow these steps:

  1. Unplug the compressor.
  2. Remove the line going into the valve.
  3. Use the adjustable pliers or a wrench to remove the quick-connect fitting if the valve has one.
  4. Disconnect the wires from the pressure switch.
  5. Detach the valve.

After removing the old valve, you can install the new one with these instructions:

  1. Apply thread lubricant to the new valve.
  2. Cut a small amount of the line with the snipping pliers to ensure a clean connection.
  3. Connect the line.
  4. Reattach the quick-connect fitting.
  5. Attach the new valve.
  6. Adjust the tension on the new unloader valve according to the pump manufacturer’s specified pressure.

Find Reciprocating Compressor Unloader Valve Parts at Quincy Compressor

A new unloader valve can save you significant time and expenses. Proper maintenance steps protect your compressor components from excessive repairs.

The air experts at Quincy Compressor can assist with any questions about unloader valves and replacements. Connect with us online today to learn more.


Last Updated on June 28, 2023 at 11:40 am