It's the least complicated to use. This valve responds to the amount of pressure exerted on the bypass valve. The unloader uses a seat, a ball and a spring. These components control the flow of water to the bypass port.
When the operator has the trigger gun open, water flows from the unloader inlet port to the outlet port, not overlooking just a small amount. When the trigger is closed, the pressure increases as the pump continues to expel water. When the pressure exceeds the resistance of the spring, the ball moves away from the seat and allows water to pass through the bypass port. The bypass valve opening is aided by a check valve in the outlet port that quickly diverts water flow to the ball and the discharger seat.
The check valve at the outlet also traps pressure in the hose, causing the characteristic pressure blow when the trigger gun is opened, which is associated with a pressurized type unloader. An adjustment knob or bolt allows you to adjust the tension of the spring in the ball by compressing the spring downward to create more resistance or by expanding the spring, which causes it to exert less pressure on the ball. When the spring is adjusted to create the maximum outlet pressure, little water enters the bypass port. When the spring expands, lower water pressure can push the ball out of the way, allowing more water to be diverted even when the trigger is open.
This reduces the flow of water to the gun and, of course, to the nozzle. If less water is forced through the nozzle, the pressure in the nozzle is reduced. Pressure-type dischargers are less sensitive to flow restrictions than flow-type dischargers. This allows various hole sizes to be used without cycling problems.
Reconstruction kits are usually very simple and inexpensive, since they contain the ball, the spring and the seals or gaskets. A disadvantage of this type of relief valve for pressure washers is the trapped pressure that creates the burst of pressure when the gun is opened. It can be dangerous when the operator is working from a ladder; it also increases the pressure inside the equipment. The type of relief valve found in an air compressor is determined by the model and size of the machine.
Some are controlled by a toggle switch on the side of the pressure switch. Others have an integrated valve underneath the pressure switch. With the trigger activated, you won't get more working pressure if you push harder on the spring, but if you release it, you'll get a dangerously high pressure peak that could destroy your pump. If the nuts are too low, you'll reach a point where you can reach maximum pressure with the gun on while you squeeze the spring, but the pressure will only increase by 6 to 9% when you release the gun trigger.
Recheck the pressure and pressure on the peaks by rotating the spring until it hits the bottom with the two nuts. For example, when you release the trigger after setting the operating pressure to 3500 PSI, the pressure should not exceed 3850 PSI. The discharger redirects water flow from the outlet side of the pump to the inlet side, causing water to flow in a circuit back to the pump with almost no pressure instead of to the nozzle at working pressure. Since there is no check valve to prevent pressure from returning to the unloader from the outlet, the pressure in the entire system is relieved each time the unloader cycles.
In addition, adequate spring tension must be achieved in the valve to ensure maximum pressure when the spray gun is activated and a minimum pressure increase when released.