This is the main safety component of a high pressure cleaning system where trigger operated gun is being used. It along with the trigger gun literally controls the traffic flow of water in the system. Without an unloader valve when gun is shut off, the pressure will continue to rise until either the motor - engine stalls or pump, hose or gun ruptures. Unloader divert all or part of the flow to a supply tank or the inlet of pump (should not connect without manufacturers’ permission) to prevent pressure from building up. When gun is opened water moves from outlet of pump through hose to gun and nozzle. When trigger is released the valve closes and the unloader is activated either by an increase in pressure (pressure actuated type) or a reduction in flow (flow actuated type). Water flows into channel and pushes piston down which opens bypass valve. The unloader diverts flow of water from outlet side of pump back to inlet side causing water to flow back to pump virtually under no pressure.
Advantages of flow actuated unloader is that hose and pump pressure is reduced in unload mode. This is safer for hot water coil and when operator does not want fast kick back when gun trigger is depressed. It however, cannot be used on multi gun systems and is generally more expensive and sensitive to adjust.
Pressure actuated type is the common. It traps pressure in hose during unload mode so immediate pressure is available when gun is opened.
Pressure of system can be controlled to some extend by adjusting the tension on the spring holding the piston in the valve in place. Changing nozzle is preferred method of controlling pressure and unloader should only be adjusted to fine tune the system.
If unloader valve goes on/off when gun is shut off either the spring is not properly adjusted or there is a leak in hose, gun or connections. Too tight a spring tension can create a safety problem due to high-pressure spikes before it unloads. Sleight bypass of liquid prevents valve erosion.