All About Unloader Valves


The Unloader Valve in Action

The unloader valve diverts water flow into bypass when the trigger gun valve is closed. Unloaders may be designed to respond to either an increase in pressure or a change in flow. The unloader valve may also be used for variable pressure adjustment.

The trigger gun and unloader valve make up a two-part valve system that directs water into bypass back to the pump inlet or the float tank. The trigger gun shuts off the flow of water, causing the unloader valve to re-circulate the water back into the inlet side of the pump. This is called sending the water into bypass. Pressure-actuated unloaders are opened by the increase in pressure occurring when the valve in the gun closes. Flow-actuated unloader diverts water to bypass when there is a sufficient reduction or stoppage of water flow.

How The Pressure-Actuated Unloader Works


The pressure-actuated unloader responds to an increase in pressure, which generally indicates a stoppage of water flow through the system.

The pressure-actuated unloader is a simple valve and spring arrangement set in a metal (usually bronze) body. A channel drilled through the body gives water access to the backside of the piston cup. When pressure against the piston cup increases to a point where spring tension is overridden, flow is ported past the valve ball and into bypass. If the trigger gun is closed, all of the water will flow into bypass.

How the Flow-Actuated Unloader Works


The flow-actuated unloader simply responds to stoppage of flow, which will also be manifested as an increase in internal pressure. Where the pressure-actuated unloader has the check valve in the discharge port, which helps divert water to press the piston down and open the valve ball allowing water to bypass, the flow-actuated unloader has an orifice.


When the trigger gun valve closes and water flow stops, system pressure is increased rapidly at all pints in the system past the orifice. A small channel in from of the orifice allows the increased pressure to travel up the channel to the piston assembly, pushing it down. The rest of the water flow is diverted through the bypass port and back to the float tank or pump inlet.

Normal Bypass Volume
Many unloaders are designed for operation with 5% to 10% of flow in bypass at all times. This is particularly true of pressure-actuated unloaders, which generally require about 10% of water capacity in bypass. This small amount of water will provide a cushion of water between the seat and valve ball to lessen the shock resulting when the two metals are forced together during valve operation.

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 due to friction in the crankcase, which is passed on to the water. It is undesirable for most pumps to handle water in excess of 140 degrees Fahrenheit. Occasional discharge at the gun replaces water in the loop and prevents pump damage from high temperatures.

A temperature-sensitive switch or pump protector on the bypass line can provide even more permanent pump protection.

The flow unloader’s diversion to bypass, functions on a pressure differential between the inlet flow and discharge flow across the balance orifice. Flow goes in the direction of least resistance or through the port where the least amount of effort is required. This will be the port with the least pressure.

Flow Variations Will Cause Problems with Flow-Actuated Unloaders
Using a flow unloader with a weep gun will cause unloader cycling. Do not use a flow unloader in this system configuration. A clogged nozzle that is too small or inadequate flow through the orifice being used will also cause problems with a flow-actuated unloader. An accumulator used as a pulsation dampener will not coexist well in a system with a flow-actuated unloader.

This tendency to cycle is one reason many service personnel do not like to work with flow-actuated unloaders. The system must be functioning properly and maintaining the correct flow for the unloader to function smoothly. However, since the flow unloader does not trap pressure in the outlet side of the system, it has safety advantages over the pressure-actuated unloader.

Using the Unloader Valve to Regulate Pressure
Pressure and flow-actuated unloader valves can both control pressure to a certain extent by adjusting the tension on the spring holding the piston in the valve body in place. The difference is that tightening the spring on a flow unloader reduces pressure while the same action increases pressure on a pressure unloader.

When the adjusting bolt on the flow-actuate unloader is loosened, this causes the piston controlling the cone valve to right, allowing more flow through the spray nozzle and a consequently higher operating pressure.

When the adjusting bolt on the pressure-actuated unloader is loosened, this causes the spring tension on the piston assembly to lessen, allowing less flow to be forced out of the spray nozzle and a lower operating pressure results.

Adjusting the Pressure-Actuated Unloader


Pressure unloaders must be adjusted with the system operating and the trigger depressed so that water is flowing through the spray nozzle. The initial adjusting bolt position should be loose or screwed away from the body and with very little tension on the spring.

Release the trigger to see if the unloader is going to work at all. Depress the trigger, allowing the system to develop operating pressure at that unloader adjustment. Note the pressure gauge reading.

Depress the trigger and tighten the adjusting bolt about one turn. Repeat until the desired pressure is reached. Use the gauge to monitor the system pressure rise with each adjustment.

All adjustments must be made with water flowing and pressure in the system. Release the trigger only to check spike pressure. This adjustment procedure is exactly the opposite of the procedure for adjusting the flow unloader.

When the desired pressure is reached, tighten the lock nut to make the adjustment permanent.

Adjusting the Flow-Actuated Unloader


Flow unloader adjustments must be made with the cleaner operating and the trigger gun released. The flow unloader allows the system to develop the lowest pressure when the adjusting bolt is screwed all the way into the valve body.

When the trigger is first released, monitor the pressure gauge to make sure the unloader valve is operating at all. If pressure continues to rise rapidly, open the gun and shut off the cleaner.


Start the flow unloader adjustment procedure with the adjusting bolt screwed all the way into the valve body and the system developing minimum pressure.

To check pressure when adjusting the unloader, simply open the trigger gun. Allow full system pressure at that adjustment to be achieved. The unloader should be adjusted gradually until the desired pressure is achieved. Turn the adjusting bolt one turn, then open the trigger gun and wait till even pressure gauge.

When opening the gun for a pressure check, allow a moment for system pressure to stabilize before noting the pressure. If pressure is not high enough, release the trigger and turn the adjusting bolt again. Squeeze the trigger and check the pressure again. If more pressure is desired, repeat this step, checking pressure and spike each time. When finished, tighten the lock nut to prevent loosening and make the adjustment permanent.

Gradual Pressure LossT

Usually gradual pressure loss is the result of high-pressure nozzle orifice wear. Replacement of the worn spray nozzle with a new high-pressure nozzle of the same orifice size should restore any pressure lost through nozzle wear. Replacement of the worn spray nozzle with a new high-pressure nozzle of the same orifice size should restore any pressure lost through nozzle wear.

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 a no way for pressure to increase. All that happens in this instance is that the pressure spike increases.

Some pressure unloaders manufactured today have a small hand grip for unloader adjustment instead of the nut. In the case of a least one manufacturer this hand grip not only makes adjustment easy but hides two set screws that limit the high and low pressure 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 adjustment-limiting set screws is representative of that approach. Some component manufacturers recommend always having a little water in bypass if only to cushion the valve seat. Without some circulation the check ball is slammed into the valve seat and more rapid wear results.

Additionally, bypass flow can compensate for nozzle wear. As the nozzle orifice erodes, water is diverted from bypass to the gun and operating pressure is more closely maintained.

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. In most cases doubling of pressure will not occur. This can become critical in a situation where a machine designed for 2000 psi may be subjected to up to an additional 2000 psi pressure spike before entering bypass mode.

The Second Line Of Defense

Aside from the unloader valve, a second line of defense is the safety, pressure-relief 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 an Allen screw.

The pressure relief valve functions as a sort of a circuit breaker for water. 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.

When properly adjusted, the valve should be set at just over the normal spike pressure so the valve will not be actuated every time the unloader valve sends water into bypass.

The Third Line of Defense
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 machine’s 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. If all other pressure relief options fail, the rupture disc will provide a final, non-mechanical line of defense.

The Three-Tiered Approach
While some engineers swear by such a three-tiered safety approach, others feel this much redundancy is unnecessary; many believe a safety valve and unloader valve in combination are sufficient. Nevertheless, it is important to remember that leaving off; forgetting, by-passing or defeating any valve or other safety device is courting disaster.

Unloader Problems
Diagnostic procedures are slightly different for flow and pressure unloaders. The procedures outlined here will take you through a logical process for pinpointing unloader-related problems.

Pressure-Actuated Unloader Troubleshooting:

Look at the output from the end of lance to judge amount of discharge water volume if flow is smooth but the water volume is low:
· Adjust the unloader valve for a higher pressure.
· Check to see if the spray nozzle is clogged. An irregular spray pattern will indicate clogging.
· Check to see if the spray nozzle is the right size. Too small a nozzle will cause a reduction in output volume. (Refer to the nozzle chart).
· If the unit is equipped with a downstream chemical injector, check to see if the injector orifice is blocked.
· The pump may not be delivering its rated output volume.
If pressure is normal but increases when trigger is released:
· Check to see if the unloader piston and shaft assembly are stuck or frozen in the unloader valve body.
If flow is normal but pressure is low:
· Adjust the unloader valve for a higher pressure.
· Check to see if the spray nozzle is the wrong size (too large) or not in place. (Refer to the nozzle chart).
· The pump seals may not be capable of producing the desired pressure.
If the water leaks from under the main pressure spring or adjusting knob:
· The shaft o-ring in the valve body is worn.

Flow Unloader Troubleshooting:

If flow is smooth but volume is low:
· Adjust the unloader valve for a higher pressure.
· The pump may not be delivering its rated output volume.
· Check to see if the unloader valve is stuck in bypass. This may be caused by a restriction at the spray nozzle or downstream injector orifice, an incorrectly sized unloader orifice or improper internal spring tension or clearances.
If the unloader cycles while system is under pressure:
· Check to see if the spray nozzle is the wrong size or clogged (Refer to the nozzle chart).
· Check to see if the downstream injector orifice is clogged.
· Check to see if the unloader orifice is properly sized or blocked.
· The pump may not be delivering the necessary output volume.
If the unloader cycles when the system is in bypass:
· Check to see if there is a restrictor in the bypass port, one may be needed.
· Check to see if a weep type trigger gun is installed.
· Check for excessive downstream leakage.
· Check to see if an accumulator is installed downstream of the unloader. Remove the accumulator from its downstream installation and reinstall upstream if desired and practicable.
· If the pump is direct fed (no float tank), check the water source to see if supply pressure appears to be excessive.
If the volume of flow is normal but pressure is low:
· Adjust the unloader for a higher pressure.
· See if the nozzle is sized too large (Refer to this nozzle chart).
If the pressure is normal but only increases when the trigger is released:
· Check to see if the unloader piston and shaft assembly are stuck or frozen in the unloader valve body.
· Check to see if the bypass port is clogged or restricted.
· Check for excessive tension on main spring and shaft assembly.
If water is leaking out of the adjusting bolt:
· Check to see is the internal sleeve o-ring is worn. Replace the o-ring if necessary. This is the most common cause of leaking at the adjusting bolt.
· Check to see if the external sleeve o-ring is worn.

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