While some engineers swear by such a three-tiered safety approach for high pressure pump system defense, 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.
Aside from the unloader, a second line of high pressure pump system 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.
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.
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 trigger high pressure spray gun is one of the two main components of the most common safety system for high pressure cleaning equipment. The other is the unloader.
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 releasing the trigger, or in “dump gun” systems, system pressure can be drastically reduced to safe levels.
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.
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.
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.
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.