The unloader valve for high pressure pumps is one of the two main components of the most common safety system for high pressure cleaning equipment. The other is the trigger gun.
Today, a number of automatic systems have been developed which allow for complete pump and power shutoff when the trigger gun is released. However, none of these systems are used as widely as the unloader.
The pressure-actuated unloader is a simple valve and spring arrangement set in a metal (usually brass) body. A channel drilled through the body gives water access to the back side of the valve ball. When pressure against the valve increases to a point where spring tension is overridden and water is diverted by bypass.
Not all of the flow necessarily goes into bypass as the amount the unloader opens depends on how much pressure is exerted against the valve.
The flow-actuated unloader simply responds to a reduction or stoppage of flow.
The gun and unloader make up a two-part valve system that directs water into bypass back to the pump or float tank. The gun shuts off the flow of water, causing the unloader to re-circulate the water back into the inlet side of the pump. This is called sending the water into bypass. Generally, unloader valves are pressure-actuated and are opened by the increase in pressure occurring when the valve in the gun closes. Flow-actuated valves are available as well.
Unloader operation is simple. When the trigger is depressed a valve in the gun opens, and water moves from the outlet side of the pump through the hose to the gun.When the trigger is released the valve closes and the unloader is activated, either by an increase in pressure (a pressure-actuated unloader) or a reduction in flow (a flow-actuated unloader).
The unloader diverts the flow of water from the outlet side of the pump back to the inlet side, causing water to flow in a loop back to the pump under virtually no pressure rather than to the nozzle under operating pressure. Flow may be directed back to the pump inlet or to a float tank or other type of reservoir.
The unloader and trigger gun, in combination then, act as both a safety and a convenience feature. Should the operator lose control of the gun, it automatically shuts off the flow of water to and through the nozzle.
The unloader, or pressure regulating unloader, in combination with the gun, are a prime example of how safety and control functions can be combined in a single subsystem of a high pressure cleaning system.
However, when the flow stops initially, there is an immediate increase in pressure or a “pressure spike” at the gun because it takes a moment for the unloader to be actuated.
The severity of this pressure increase at the gun depends on a number of variables, including the length of hose between the gun and the pump and the pump type and speed of operation.
For example: a pump is operating at a fairly low rpm is moving more water with each stroke than a pump rated for the same flow and pressure but operating at a higher speed.
Consequently, each stroke of a slowly-operating pump forces more water against the closed valve and places more stress on the fittings in the gun and the valve seating. A duplex or two cylinder pump will create more stress at the closed gun than would a four cylinder pump operating at the same flow and pressure.
A machine should not be run in bypass mode continuously. When a machine is in bypass mode the temperature of the water in the closed loop increases rapidly. It is undesirable for most pumps to handle water in excess of 140 degrees Fahrenheit, but occasional discharge at the gun replaces water in the loop and prevents pump damage from high temperatures. A temperature-sensitive valve or pump protector on the bypass line can provide even more permanent pump protection.
In the specific case of the pressure-actuated unloader, pressure can be controlled to a certain extent by adjusting the tension on the spring holding the piston in the valve in place. (Remember, this is called a pressure regulating unloader).
An unloader is not an absolute, on-or-off device. Rather, it can allow a certain amount of flow to bypass the hose to the gun and return to the inlet side of the pump. The amount of by pass is set by tightening or loosening spring tension with a nut or hand grip on the end of the unloader opposite the outlet port.
This means that the flow of water to the nozzle, and consequently, the amount of pressure produced at the nozzle, can be regulated to some extent by adjusting a pressure-actuated unloader.
However, it is important to remember that the unloader is not always primarily a pressure-regulating device. That is, you can but do not generally, control the amount of pressure produced at the nozzle by tightening or loosening the unloader.
Pressure can best be controlled by changing the nozzle to a larger orifice size (less pressure) or a smaller orifice (more pressure). This is a matter of some disagreement in the industry since some operators, and others have long used unloaders as a handy way to increase pressure.
What generally happens is that the operator notices a decrease in pressure at the nozzle. Under normal circumstances this is a sign of nozzle wear and means the nozzle should be replaced.
However, a quick fix can be to simply tighten the adjusting nut on the unloader to decrease the amount of water in bypass. But if no water is being diverted from the nozzle, then there is no way for pressure to increase and the whole exercise is futile.
Some unloaders manufactured today have a small hand grip for unloader adjustment instead of the nut. In the case of at least one manufacturer this hand grip not only makes adjustments which can be made on the unloader.
To most manufacturers of unloaders, the ability to adjust spring tension is simply a fine-tuning feature and the use of a set screw is representative of that approach.
Safety is the reason most often cited by engineers in opposing the use of the unloader as a pressure controller. An unloader valve under maximum spring tension may require up to twice the pressure to go into bypass mode as a properly set valve.
This can become critical in a situation where a machine designed for 2000 psi is subjected to up to an additional 2000 psi pressure spike before entering bypass mode.
Although most pressure-containing components used in the industry conform roughly to Society of Automobile Engineers’ standards for hydraulic components, or, in other words, have a burst pressure four times rated operating pressure, an operator cannot be guaranteed the machine he is working with conforms to those totally voluntary standards.
In fact, the gun is one of the components which is not necessarily designed with a four-to-one safety factor and guns have been known to burst at less than double rated pressure.
Some components manufacturers recommend always having a little water in by pass if only to cushion the valve seat. Without some circulation the ball is slammed into the valve seat and more rapid wear results.
Additionally, by pass flow can compensate for nozzle wear. As the nozzle orifice erodes, water is diverted from by pass to the gun and operating pressure is maintained.