In one of the most common configurations for a trigger gun, the valve ball is held in a closed or forward position by the valve spring. The flow of the water forces the valve ball to seat more tightly and blocks the flow of the water through the gun to the nozzle.
When the trigger is depressed it pushes a control pin against the ball, forcing the valve ball out of its seat and opening a path for water to flow to the nozzle. When the trigger is released again a spring returns the ball to its seat and the flow is again blocked. Flow blockage actuates the unloader on the outlet or high side of the
The Unloader Valve’s Role and Operation
Unloader valve operation is simple. The bypass cycle actually begins at the trigger gun. 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 and out of the spray nozzle.
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 change in flow (a flow-actuated unloader).
The unloader valve diverts the flow of the water from the outlet side of the unloader back to the inlet side of the pump or to the float tank, causing water to flow in a loop back to the pump under virtually no pressure rather than to the nozzle under operating pressure.
The "Pressure Spike"
However, when the flow stops initially, there is an immediate increase in pressure or a "pressure spike" at the gun and in the high-pressure hose and coil because the pump is continuing to move water into the discharge line but the gun is closed.
The severity of this pressure increase at the trigger gun depends on a number of variables including the length of hose between the gun and the pump, the pump type and speed of operation, and the unloader valve type and adjustment.
Flow-Through and Front-Entry Trigger Guns
There are two main types of trigger guns, flow-through guns with water flow passing all the way through the gun and with the valve located 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. The front-entry 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. Front-entry guns are generally less expensive than flow-through guns.
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 or more at 300 degrees Fahrenheit and a 10 gpm flow.
Other Gun Types
The truck wash gun is a special type of trigger gun designed in a horizontal configuration instead of the traditional pistol grip style. This design allows for ease of use in washing tall vertical surfaces because water is expelled vertically when the gun is held in the orientation as a pistol grip gun. Operator fatigue may be reduced.
A foot valve may be used instead of a trigger gun. Foot valves are usually found only on very high-pressure systems. It operates just like a trigger gun but is controlled by the operator’s foot. When the operator steps on a pedal the valve opens and water flow is allowed. When the pedal is released, flow stops. Two-step washing systems are generally equipped with a special gun with switches for the solenoid valves controlling chemical flow. Another gun design uses a radio remote control to give the operator almost total control over the equipment.
The Dump Gun
A dump gun, on the other hand, avoids the pressure spike by re-routing 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. This system directs the flow of water away from the pressure nozzle to reduce pressure of the output rather than stopping output altogether. Instead of flowing through the pressure nozzle, water flows through a large opening at the end of the lance. The dump gun simply chooses whether water flows through the larger, low-pressure orfice or through the smaller, high-pressure nozzle. The operator has a simple choice between high pressure and low-pressure output.
In some cases the flow is diverted to a tube surrounding a smaller tube the high-pressure nozzle is mounted in. Other dump guns use double barreled lances. In superficial appearance the dump gun is very similar to the trigger gun used with an unloader. With a dump gun, releasing the trigger relieves the output pressure but to stop the flow the machine must be turned off.