The pressure cleaner's unloader valve functions as a safety device. Unloader valves control the direction of the flow of water exiting the pump. A positive displacement pump always delivers water, regardless of whether the spray gun is open or closed. When the spray gun is closed, the unloader valve redirects the flow of water back to the inlet side of the pump. This prevents the build up of dangerously high pressure and guards against the failure of component parts. Pressure cleaners are installed with either a Trapped Pressure Unloader or a Flow Actuated Unloader.
Spray guns start, stop, and direct the flow of water from the pressure washer’s nozzle. Squeezing the trigger releases pressure. Releasing the trigger interrupts the flow of water to the nozzle, although the pump continues to run. Without an outlet for releasing this water, pressure would continue to build. A very dangerous situation would result.
The Unloader Valve
“Trapped Pressure” unloader valves are the most commonly used. These valves sense and are activated by building pressure at the pump output. Trapped Pressure valves initiate bypass mode in direct response to building pressure in the hose between the pump output and the spray gun. They are often simply called “Pressure” valves. A spike of pressure results when cycling is interrupted and Trapped Pressure valves re-release water to the spray gun. Operators should be prepared for the “kickback” effect on the spray gun or wand to avoid loss of control or injury.
“Flow Actuated” unloader valves respond to the interruption of water flow to the nozzle. These valves sense any decrease in flow from the valve to the spray gun and initiate the bypass loop in response. Unlike Trapped Pressure valves, no pressure is trapped, so no “kick back” occurs when water is re-released. With Flow Actuated unloader valves, operators cannot regulate pressure by downsizing the orifice size of nozzles. Flow Actuated valves detect the loss in flow, and react by cycling repeatedly.
