The function of unloading valve with a diagram.

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Unloading valves are pressure-control devices that are used to dump excess fluid to the tank at little or no pressure.

Unloading valves are similar to Pressure Relief Valve. Both types of valves send fluid back to a tank when a preset pressure is reached. However, unlike a Pressure Relief · Valve, which is activated using pressure from its own line. The Unloading Valve dumps the fluid when the fluid from an external line reaches a certain pressure.
A common application is in high-low  pump circuits where two pumps move an actuator 
at a high speed and low pressure. The circuit then shifts to a single pump providing a high pressure to perform work ,It is used in application in which the first portion of cylinder stroke requires - High Flow (Speed) and Low Pressure (Force) for the reminder of the
cylinder stroke which requires- Low Flow and High Pressure.





A pilot-operated  unloading relief valve is the same as a pilot-operated relief valve with
the addition of an unloading spool. Without the unloading spool, this valve  would function just like any pilot-operated  relief valve.
Pressure buildup in the pilot section would open some flow to the tank and unbalance the poppet, allowing it to open and relieve excess pump flow.
In a pilot-operated  unloading valve; the unloading spool receives a signal through the remote-pilot port when pressure in the working circuit goes above its setting. At the same time, pressure on the spring-loaded ball in the pilot section starts to open it. Pressure drop on the front side of the unloading spool lowers back force and pilot pressure from the
high-pressure circuit forces the spring-loaded ball completely off its seat. Now there is  more flow going to the tank than what the control orifice can keep up with. Te main
poppet opens at approximately  20 psi. Now, all high-volume pump flow can go to the
tank at little or no pressure drop and all horsepower can go to the low-volume pump to do the work. When pressure falls approximately 15% below the pressure set in the pilot section, the spring-loaded ball closes and pushes the unloading spool back for the next cycle.


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