Function of The Unloader Valve in Reciprocating Compressors - SAMCO Enterprises

Function of The Unloader Valve in Reciprocating Compressors

Reciprocating compressors are machines used in a wide range of industries. For example, you will always find a reciprocating in a painting garage, a dentist’s clinic (to operate tiny clinical tools), an automobile garage, a workshop, etc. This is because economically efficient equipment compresses air or gas for running their appliances or devices. The unloader valve is one of the most vital components that make a reciprocating compressor function effectively and efficiently. 

This article covers the unloader’s primary function or purpose in a reciprocating compressor and other crucial information about this remarkable component.

What is an Unloader Valve?

A reciprocating compressor’s unloader valve is a minor component measuring about 4 to 5 inches in length and width. This component releases trapped air within the compressor tank, enabling the motor to restart.

One of the most common issues that plague reciprocating compressors is the malfunction of the unloader valve. If trapped air within the tank cannot escape, accumulated pressure prevents the motor from restarting.

The type of valve used in a particular compressor depends significantly on the equipment’s size and make. Some compressor valves come with integral parts installed right beneath the pressure switch, while others function via a toggle switch embedded on the side of the unit’s pressure switch.

What is the Primary Function of the Unloader Valve in a Reciprocating Compressor?

When a reciprocating compressor operates, its tank – driven by an electric motor – fills with air. A pressure switch is embedded in the equipment, responding swiftly to the accumulation of air by shutting off the motor’s power supply.

When this happens, the compressor pump – connected to the motor shaft – stops immediately.

If the compressor motor attempts to restart, the trapped air within the cylinder suddenly generates a load that makes it very challenging to accomplish this vital task. This is where the unloader valve’s primary function comes to the fore.

The #1 function of the unloader valve in a reciprocating compressor is to vent the air within the cylinder. This action considerably reduces the load over the piston, and this facilitates the motor-restarting process.

How an Unloader Valve Works in a Reciprocating Compressor

If you still don’t fully understand how the unloader valve within a reciprocating compressor works, here’s how:

Once the air pressure within the reciprocating compressor’s tank reaches a stipulated pressure, it triggers the switch’s cut-out pressure is initiated. This is the point at which air is not delivered to its intended destination, and the control swings into action by automatically tripping off and interrupting the power supply to the motor.

This process causes the compressor unloader valve to open and rapidly vent the accumulated air.

Once the tank pressure goes back down to the pre-determined pressure switch setting, it causes the pressure switch to trip again. The ‘trip’ action of the pressure switch causes the unloader valve to release its pin, preventing the escape of already-compressed air into the atmosphere.

The Location of the Unloader Valve in a Reciprocating Compressor

Knowing the functions of the compressor’s unloader valve is vital, but so is knowing precisely where it is located. You will always find the unloader valve mounted inside or on the pressure switch. But you can only pinpoint this in small reciprocating compressor units.

The action automatically actuates the valve when the pressure switch shuts off the compressor. A small pipe or tube runs from the check valve, i.e., a device that prevents the entire tank from draining to the unloader valve.

Large compressor units mostly feature a much more sizeable unloader valve controlled by the air pressure from the smaller pilot valve. The valve always sits right next to the compressor in this particular arrangement.

The Other Functions of an Unloader Valve

The unloader valve in a compressor also has other functions. For instance, it ensures the reciprocating compressor operates efficiently. This efficiency helps eliminate halt-in-motion issues.

The valve also makes the pressure switch trip again as soon as the air passes. This process enables an engine to regain its switch process.

Overall, an unloader valve helps users perform as an appropriate regulatory system within a compressor. This is part of the basics of learning regulations that technicians must adhere to when working with a reciprocating compressor.

Other Vital Components of a Reciprocating Compressor

Asides from the unloader valve, other vital components of the reciprocating compressor that ensure its seamless operation include:


The piston is a component driven directly by the crankshaft via a crosshead or connecting rod as well as the piston rod. These piston rods are formed from cast iron.

Connecting rod

The connecting rod connects the piston – or crosshead – to the crankshaft. It is fabricated from steel forging.


Two types of valves exist within a reciprocating compressor:

  • A suction or inlet valve – Air or gas enters the compressor cylinder through this valve.
  • A discharge or outlet valve – Air or gas is discharged from the compressor cylinder through this valve.

Both valves are one-way, spring-loaded valves.


The cylinder is connected to both the suction and discharge valves, allowing the suction and compression of the gas. The material used in making the cylinder depends on the operating pressure reciprocating compressor.

Cast iron is the typical choice for pressures as high as 1,000 PSI, while nodular iron is used in making a cylinder that holds pressures from 1,000 to 1,500 PSI. Forged steel and cast steel are ideal for pressures higher than 1,500 PSI.


The crankshaft of a reciprocating compressor is made from steel forging and is usually driven by an internal combustion engine or electric motor.

Distance piece

The distance piece separates the cylinder of the compressor from the compressor frame.


Reciprocating compressors are vital machines used extensively in nearly every industry. Becasue of this, compressor repair is extremely important to be able to fix machines as quickly as possible. One of the significant components that get faulty from time to time but is crucial to the practical function of the compressor is the unloader valve.

It is vitally essential for technicians handling reciprocating compressors to know every inch of an unloader valve. Having a basic knowledge of this crucial component makes it easy to identify and fix any issue that may result in breakdowns during operations.

From industry standards like Ariel compressor parts to Ingersoll Rand and Cooper Bessemer compressor parts, we carry compressor parts you can trust and the service you can count on.