UNDER PRESSURE: UNDERSTANDING THE UNLOADER VALVE

Hawk Pumps expert reiterates safety first

When a high-pressure pump operates with a closed outlet, the pressure builds up to dangerous levels, risking potential damage or accidents. To prevent such hazards, the unloader valve serves as a vital safety measure, directing water flow and maintaining a stable maximum pressure. However, many users mistakenly attribute pressure problems to the unloader valve when the real culprit is often the worn or incorrectly sized nozzle. Understanding the roles of these components is essential for harnessing the power of high-pressure pumps while prioritising safety.

The Danger of Pressure

A fundamental aspect of using a piston pump is limiting the maximum pressure right from the start. Over pressure can be incredibly dangerous for anyone in the vicinity of the pump, as an explosion could produce shrapnel. They can also burn out the motor, or destroy certain components, leading to the failure of the entire machine.

The Unloader Valve

To prevent an over pressure situation and the associated effects, an unloader valve must be fitted to the pump. This valve protects both the unit and the operators.

Like a traffic officer directing a busy intersection, the unloader valve directs the flow of water, either to the gun, or with the gun closed, to a by-pass pipe back to the inlet of the pump or to a header tank. This will happen when the limit of the pressure is reached, until the gun is opened again. Importantly, this redirection keeps the maximum pressure stable.

As pumps come in many volume and pressure sizes, it is absolutely critical that the correct unloader valve is fitted and is correctly set. This is something that the trained experts at Hawk Pumps do inside the factory. All Hawk pumps fitted to cleaning machines come with a standard limiting sleeve installed to the shaft under the adjusting knob to avoid over pressuring.

The Common Mistake

Unfortunately, many pump users incorrectly attribute pressure problems to this unloader valve when the usual problem is caused by a worn nozzle. It is a mistake to screw the unloader valve even tighter to create more pressure.

“Under no circumstances should users tamper with the unloader valve. This creates a dangerous situation for the pump, the user, and anyone in the vicinity,” says Hawk Pumps expert Pravanand Naicker.

The Wrong Fix

Equally important is understanding that pressure has little to do with the unloader valve. The valve exists to keep pressure at the same rate while the pump is on, but the hose is not in use. Pressure, on the other hand, is something that is set in the factory and is determined by the pump size and power and a correctly sized nozzle.

The Nozzle

“In almost all pressure-related cases, we find that the nozzle is to blame,” says Pravanand.

The size of the nozzle orifice is very important. Too big and the desired pressure won’t be achieved. Too small, and while the desired pressure will be achieved, the rated volume of water flow will be reduced.

Nozzles are ‘wear-and-tear’ items. Over time, the sides of the hole will wear, and as it gets bigger, the pressure decreases. Sometimes there may be blockages or chips that make the hole smaller, resulting in poor water flow. Nozzles are inexpensive to replace and should be checked regularly for chips, blockages, and wear.

Important to Understand

The unloader valve knob is not to be turned to increase pressure to the system.

In most pressure-related cases, the nozzle is the offending component; very rarely does the unloader valve have anything to do with pressure problems. 

Safety First

In order to improve the safety of the pump, there is also a safety valve option that you can purchase from Hawk Pumps. This safety valve creates a release channel for spikes in pressure, allowing the pressure to escape until it has returned to normal. These safety valves come standard on pumps above 500 bars but are easy to install on smaller versions too.

“We are strong defenders of the ‘safety-first’ approach. Tampering with the unloader valve could be very dangerous, and we want our users to fully understand their machinery and how best to operate it,” says Pravanand.

Visit the Hawk Pumps website or contact one of the Hawk Pumps distribution offices:

Durban: (031) 274-8555
Johannesburg: (011) 822-4039
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

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Issued By: The Lime Envelope
On Behalf Of: Hawk

For Media Information: Bronwyn Levy
Telephone: 076 078 1723
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

About Hawk
Hawk is a manufacturing company that primarily creates industrial high-pressure pumping and cleaning equipment. They also manufacture spraying solutions, including industrial nozzles as well as eyewash and safety showers. One of Hawk’s biggest strengths is that they have the capacity to manufacture, and custom design machines tailored to specific customer needs.

Hawk serves a wide range of customers from virtually every industrial sector, including processing plants, factories, workshops, contract cleaners, drain and sewer cleaners, car, truck and bus washers, mines, agricultural and more.