Control Valve

What is an unloading valve and how does it work

What is unloading valve and what does it do

Unloading valves are used in hydraulic circuits, these valves would stop or remove the flow of the pump back to the tank when the machine is not operating. So by this reduction of heat and energy saving can be achieved because the prime mover load is reduced. Unloading valve can be remotely operated and it is a great advantage. With the help of an unloading valve, the pump can be operated at minimum load. So the power or energy used will be less. The pump can be unloaded by using an unloading valve. These valves are pressure controlled device which would transfer the excess fluid to the tank. Unloading valves are mostly used in high-low pump circuits. They are also used to transfer the excess flow from a cylinder to the tank.

What is a pump unloading

The unloading valve operation is similar to a pilot valve. So the unloading valve would respond to a pressure that comes from a remote source. This valve would allow the pump to build-up pressure to a normal rate and then allows it to discharge it to the tank. When the preset pressure is reached the unloading valve will transfer the fluid back to the tank. It does the same function as a pressure relief valve but unlike the pressure relief valve, it doesn’t need pressure from its own line to activate. It would be activated when the fluid approaches a certain pressure. These valves would unload the pump when the system pressure is reached, the unloading of the pump is done by directing the pump output to a reservoir at low pressure.

How does an unloading valve work

A force will be exerted on the valve by the spring and thus the valve would stay close. The valve spool will only shift if the external force is greater than the spring force. This external force would be acted on the opposite side of the valve spool so when the valve spool shifts then the output will be diverted.

The unloading valve would open if the cylinder reaches the end of its extension stroke. In the pilot of the unloading valve, there would be high-pressure oil. During the retraction of the cylinder, the pressure in the pilot of the unloading valve would reduce due to the motion of the piston. During this point, the unloading valve would be reset till the cylinder retraction, at this time the pump would be unloaded by the unloading valve. The pump would be unloaded by the unloading valve after the retraction strokes.

Why unloading valves are used in accumulator circuits

These valves are used in accumulator circuits so that when the accumulator is charged then the pump can be unloaded. The unloading valve would stay close when the pump charges the accumulator. So the valve would open after the accumulator charging, the pump would unload at low pressure and the accumulator supplies the fluid to the system. When the accumulator charge decreases below the preset level then the unloading process will be repeated.

What are the types of unloading valves

Direct-acting unloading valves

In this type of unloading valve, a spool will be held in the closed position by a spring. During normal conditions, the spool will block the flow from the inlet to the tank. If the high-pressure fluid from the pump enters the external pilot port. Then it would exert a force against the pilot piston. If the system pressure is more than the spring pressure then the fluid will be diverted to the tank.

Pilot operated unloading valve

In order to control the main valve unloading valves are made up of pilots. The plunger in the main valve has a port and it would transfer the system pressure on both ends of the plunger. The valve would be closed because of the pressure exerted by the spring. If the pressure on the system is dropped to a preset value, then the valve would close. The pump flow through the main valve spool port will close the valve. A pilot-operated unloading valve can control the opening and closing pressure.

How to install an unloading valve

These valves must be installed in a branch of the high-pressure pump line. It must be mounted directly at the pump discharge or behind the discharge pulsation damper. It should be mounted vertically so that contamination can be reduced. The gas regulators must be installed close to the unloading valve.

How to do the maintenance of an unloading valve

Unloading valves can perform well for a long period of time, but it needs routine inspection and servicing so that it can last long. These valves must be used in an emergency flow situation and they must not be used for continuous fluid flow. if abrasive fluids are allowed to flow through the valve for a long time then it could damage the valve and it won’t reseal properly.


post-graduate in Electronics & communication.

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