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The Role of the Unloader

October 8, 2015

The trigger gun and unloader valve are the two main components of the most common safety system for high pressure cleaning equipment.

Essentially, depressing the trigger allows water to flow out of the system through the nozzle under pressure while releasing the trigger diverts water flow back to the pump. The pump, however, continues to run.

The gun, with the unloader valve, forms a valve system which gives the operator immediate control of water flow or, in some types of systems, pressure. Water flow can be turned on or off by depressing or leasing the trigger, or in "dump gun" systems, system pressure can be drastically reduced to safe levels.

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.

Using the Gun Avoids a Walk to the Switch

In most pressure cleaning equipment, the pump motor is turned on and off at the machine. The operator, however, may be cleaning as far as 50 feet away. When he wants to stop spraying water he needs a way to stop water flow without walking back to the machine to stop the motor or engine.

The trigger gun and unloader provide that kind of control literally at the operator's fingertips. The valves in the trigger gun and unloader can be thought of as a pair of traffic cops, directing the "traffic flow" of water in the cleaning system.

Diverting to ByPass

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.

As Simple as Making a Fist

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.

Acting in Combination

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.

How the Gun Works

Guns are fairly simple mechanisms designed around a trigger-operated valve. Generally a ball valve is used in trigger guns but piston valve may be used in a dump gun.

In one of the most common configurations for a trigger gun, the valve ball is held in a closed or forward position by the flow of water and blocks the flow of water through the gun to the nozzle.

When the trigger is depressed it pushes a pin against the ball, forcing the ball out of its seating and opening a path for water to flow to the nozzle. When the trigger is released again a spring returns the ball to its seating and the flow is again blocked.

Flow blockage actuates the unloader on the outlet or high side of the pump and water is diverted back to the pump's inlet or low side, reducing pressure at the gun.

The "Pressure Spike"

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.

Factors Contibuting to Pressure Spike Severity

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.

The Dump Gun

A dump gun, on the other hand, avoids the pressure spike by simply rerouting the flow of water at the gun.

The dump gun design avoids the need for an unloader by simply diverting the flow around the pressure nozzle and to the atmosphere instead of back to the pump. In other words, it directs the flow of water to reduce pressure of the outflow rather than stopping it altogether. In some cases the flow is diverted to a tube surrounding a smaller tube the nozzle is mounted on. Other dump guns use double-barreled wands.

In superficial appearance the dump gun is very similar to the trigger gun used with an unloader.

Trigger Gun Types

There are two main types of trigger guns, flow-through where water passes all the way through the gun and the valve is above the trigger, and a front-fitted or front entry gun - designed so that the inlet and outlet fittings and valve are all forward of the trigger.

This design trades off increased operator fatigue - the weight and the front of the gun throws it off balance - for the potential for handling higher temperatures because the water never passes through the handle. Guns come in a wide range of flow, pressure and temperature ratings. A fairly standard heavy duty gun design will generally be rated for 3,000+ psi at 300 degrees Fahrenheit and a 10 gpm flow.

Overheating in ByPass

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.

How the Unloader Works

The pressure-actuated unloader is a simple valve and spring arrangement set in a meal (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 imply responds to a reduction or stoppage of flow.

Using the Unloader to Regulate Pressure

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 et 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.

Or Changing the Nozzle

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.

The Quick Fix

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.

Not Necessarily the Safe Solution

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.

Other Uses of the Unloader

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.

The Second Line of Defense

Aside from the unloader, a second line of defense is the safety or pop-off valve, which is set to release water to the atmosphere when a set pressure is exceeded. However, this valve can be adjusted as well, usually with a screw driver.

The valves are built for a certain range of pressures, say from 2,000 to 4,000 psi and a safety valve can easily be adjusted, or improperly adjusted, to the point where it no longer functions as a safety device.

Instead of the safety valve functioning as .an artificial weak point in the machine, over pressurization may cause a rupture at a real weak point, perhaps with disastrous results.

And the Third Line

Consequently, some engineers suggest a third safety measure - a rupture disk. This disk is engineered to burst at a certain pressure, usually a bit above the machines rated operating pressure to allow for normal pressure spikes. In other words, the disk, which can be screwed into a port on the high pressure side, is a non-adjustable artificial weak point in the system.

Overheating Protection

Disks which are heat sensitive - the solder melts at high temperatures, are also available as safety devices for hot water machines.

This system of preventing coil overheating was long used by a pioneer steam cleaner manufacturer.

The Three-Tiered Approach

While some engineers swear by such a three-tiered safety approach, others feel this much redundancy is unnecessary and a safety valve and unloader in combination is sufficient.

Nevertheless, it is important to remember that thwarting any valve or other safety device is courting disaster and should never be done.

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