Unloader Valves

Rebuilding The Flow-Actuated Unloader Valve


The K-5 and K-7 designs are typical of flow unloaders. A number of manufacturers make unloaders of this type including Suttner, Giant, Cat and others. This unloader type can be rebuilt using a vise and minimal tools. Care should be taken to not scratch or damage the unloader during rebuilding. Special care should be taken to not damage internal surfaces and parts.


The bypass port on many unloader designs has a female thread and a male thread. The male thread is used to assemble the unloader and is generally not intended as a bypass hose connection. However, Landa has used the male thread to fit into a jig fitting in some of its equipment designs. Flow-actuated unloaders may be mounted in any position vertically or horizontally.


Flow Unloader Orifice Sizes
Flow unloader outlet nipples and orifices are precisely sized for the flow range of the equipment. If either the orifice or the nipple is to be replaced, make sure that the orifice and nipple are the correct size to handle the rated water flow for the system being serviced.

The Flow Range


Changing the orifice and nipple will change the operating characteristics of the unloader. If an unloader is needed for a cleaner in a different flow range, changing the nipple and the orifice in an unloader of this type will usually be just as suitable as using a new unloader appropriate for the flow range. Unloader manufacturers supply information on appropriate orifices and nipples for different flow ranges.

Different Pressures, Same Orifice
Unloaders rated for different pressures may use the same orifice size. When the pressure increases, the lower limit of the flow range the unloader is suited for rises as well. The top limit of the flow range does not change. The same flow unloader housing and parts may be used at a variety of flows and pressures. Changing the orifice and nipple, allows you to change the flow. Changing the main springs can allow for different pressure applications.

Sample Flows And Pressures
For example, the same orifice is used in the Giant flow unloaders for a pressure range should be used for a particular cleaner, the lower range should generally be selected.

Secure The Unloader Body In A Vise
The unloader should be clamped in a vise. The valve body may be deformed if the valve is clamped too tightly. Wood blocks may be used to protect the unloader body. If no blocks or other protection for the valve body is available, excessive tightening of the vise should be avoided. Clamp the body so that bypass and outlet ports are accessible. In the field an unloader may often be serviced while shill installed on the system. If this is done, all hose and lines should be disconnected from the unloader. If the unloader is removed from the system in the field and disassembly is called for, the unloader may be braced in a manner appropriate to the situation.

Remove The Outlet Nipple
The outlet nipple may then be removed from the outlet port using a 7/8" wrench to provide access to the orifice seated in the outlet port. There is an o-ring, which fits on the backside of the outlet nipple. This o-ring is damaged, leaking may occur at the outlet port.

Remove And Check The Orifice
The orifice may be removed using a pair of needle-nosed pliers. Care should be taken to avoid damaging or deforming the orifice. Check the orifice to see if it is distorted, worn or clogged with debris. If wear or damage is apparent, the orifice should be replaced. Debris or other clogging should be cleared. If the unloader is clogged with chemical scale, it may be soaked in descale solution. On some flow unloaders there is a small, thin, pointed metal diffusion plate behind this orifice to help control the returning flow and pressure caused when the trigger gun is closed. This plate helps direct the flow and pressure to travel up the channel into the valve body.

Remove The Adjusting Bolt


The adjusting bolt and lock nut may then be removed from the end of the valve body. The adjusting bolt threads are left-handed. Turning clockwise loosens the bolt. This is exactly the opposite of the normal thread direction. Care should be taken to see that the threads are not damaged. Thread damage may require replacement of the entire unloader valve body.

Remove The Bypass Seat
In order to remove the spring, piston, sleeve and other internal parts, the bypass seat must be removed. A 1 1/8 inch wrench should be used to remove the seat. The internal assembly consisting of the piston, piston rod, helper spring and bypass cone will come out with the bypass seat.

Remove The Sleeve
The sleeve may be removed by driving it gently out the bypass opening. A wood dowel or other soft object may be inserted in the adjusting screw opening. Use the dowel to gently push the sleeve out of the valve body. Do not use a pointed object to remove the sleeve. Care must be taken to not distort or damage the small opening in the center of the sleeve.

Free The Piston From The Piston Rod


The piston assembly is held on the piston rod with a snap ring just above the spring retainer. The helper spring holds the spring retainer against the snap ring. The spring retainer should be held securely with one hand when the snap ring is removed or the spring retainer will fly off. The piston, spring, helper spring and spring retainer will now slide off the piston rod. The piston rod and cone valve is equipped with a flow balancer in the bypass seat housing, the balancer will have to be removed prior to sliding the rod out of the bypass seat. The flow balancer may simply be unscrewed using a large, flat-blade screwdriver.

Clean And Oil
When reassembling the unloader, make sure all parts are clean and free of any scale buildup. Lubricate all moving parts with a light oil. Lubrication before reassembly protects the parts and eases reassembly as well as protecting the unloader from damage during initial operation.

Inspect The Rod And Cone Valve
The piston rod and cone valve should be inspected for signs of wear. If wear is apparent, the worn item should be replaced. Most often, wear will occur only to the cone valve. A round depression on the cone valve is caused by wear from the seat into which the cone valve is forced every time the gun trigger is squeezed.

Remove The Cone Valve
To remove the cone valve the piston rod should be held carefully to prevent damage and the nut removed using a 7mm wrench. The manufacturer recommends using a rubber gripper to hold the piston rod to prevent damage to the rod. With the nut removed, the cone valve can be pulled off the end of the piston rod.
Note: Do not clamp the piston rod in a vise without providing adequate protection for the rod. Vise marks or distortions on the rod will cause the piston to stick. The piston rod must be smooth and straight for the piston to slide freely.

Replace The Cone Valve
When replacing the cone valve, remember that the flat side of the cone valve faces the threaded end of the piston rod. Holding the piston rod carefully so as not to damage it, tighten the nut firmly but firmly but not too tightly on the piston rod. The manufacturer recommends torquing to 20 inch pounds. If a little Lock-Tite is used, less torque is necessary.

Remove The Seat
The stainless steel seat may be unscrewed from the seat housing. If scale is present, the seat should be soaked in descaling solution. The seat may be carefully removed by inserting two screwdrivers in the round holes on each side. The seat will distort easily if excessive force is used. Penetrating oil to help loosen the seat should be tried before attempting to force seat removal. If the seat will not budge and there is not apparent wear, assume the seat is in usable condition and continue rebuilding. If wear is apparent, the seat will have to be removed and replaced.

Inspect The Seat
Inspect the bypass seat for signs of wear or pitting. Clean and remove any scale deposits. Replace the seat if wear is apparent. Inspect the o-ring on the bypass seat housing for wear or damage. Lubricate the o-ring upon reassembly.

Insert The Piston Rod
Slide the piston rod through the bypass seat. The piston rings should be inspected for wear and replaced if necessary. Slide the piston over the piston rod. The piston should have the flat side facing the bypass seat and the grooved side facing the spring. The spring will slide down into the groove on the piston. Then slide the spring, helper spring and spring retainer over the end of the piston rod. The spring retainer has a groove for the spring to fit into.

Seat The Snap Ring
With the piston rod pushed all the way into the bypass seat, press the spring retainer against the helper spring until the spring retainer passes the snap ring groove on the piston rod. Install the snap ring with the flat side facing away from the springs. If the snap ring is properly installed, it will make an audible snap when it is seated on the piston rod.

Check The Spring Clearance Gap
At this point the spring clearance gap should be checked. This is the gap between the main adjusting spring retainer after the snap ring is installed. Measure the gap with the piston rod assembly sitting upright. The helper spring will push the spring retainer against the snap ring and gravity will hold the adjusting spring against the bypass seat. If properly assembled, the spring clearance gap should be between 1/64 and 1/32 inches or about the thickness of four pieces of typing paper. If paper can be pulled through the gap with some resistance, the gap is probably in the correct range.

Install A Shim If Gap Is Too Wide
A shim should be installed between the snap ring and the spring retainer if the gap is greater than 1/32 inch (that is, if there is little or no resistance when paper is pulled through the gap). The snap ring must be removed for shim installation. Once the shim has been installed the spring clearance gap should be measured again. If the spring clearance gap is too wide, the system will tend to stay in bypass and will not come up to operating pressure.

Too Little Clearance
The unloader will work if the spring clearance gap is less than 1/64 inch. However, there may be some system pressure retained in the bypass mode. If necessary, the spring may be ground or filed if more clearance is necessary. The shim may also be filed down to allow for more of a gap. Normal wear during the life of the spring will cause an eventual shortening of .015 to .030 inches.

Inspect The Sleeve And O-Rings
The sleeve should be inspected for wear, especially at the piston rod bore. Inspect the two sleeve o-rings for wear or damage. The internal sleeve o-rings may be removed using a straight pin or small probe. Be careful to not scratch the internal surface of the sleeve. When the internal o-ring is reinstalled, make sure it is completely seated in the groove. O-ring damage or excessive o-ring wear will cause leaking around the adjusting bolt.

Reinsert The Sleeve In The Body
Make certain the valve body is not worn or deformed. The sleeve may now be pushed into the valve body through the bypass port. Use the same dowel or other mans used to extract the sleeve. Push the sleeve gently but firmly into the valve body. Make sure it is seated all the way into the sleeve bore.

Reinstall The Piston Rod Assembly
Lubricate the piston rod assembly. With great care insert the assembly in the bypass port. Do not force the assembly against the seat threads in the valve body. Lubricate the piston rings before inserting in the valve body. Piston ring damage could result and the unloader will not function properly. The bypass seat should be tightened until it is snug with the valve body. Do not over tighten. A restrictor or flow balancer may be installed in the bottom of the bypass seat. This is simply a plate-like plug with a hole in the center. This hole in the plug restricts the flow of water in bypass and helps prevent unloader cycling when the system is in bypass.

Reinstall The Orifice And Outlet Nipple
The orifice and outlet nipple may now be reinstalled. Make sure the outlet o-ring is in good condition. Press the orifice into the outlet port with the funnel-like or conical portion pointing outward.
Note: Unloaders with diffuser plates require the plate be inserted prior to orifice seating. Tighten the outlet nipple until it is snug with the valve body.

Replace The Adjusting Bolt
The adjusting bolt and lock nut assembly may now be installed in the adjusting bolt opening at the top of the valve body. The adjusting bolt should be well lubricated to ease future removal or adjustment and screwed all the way into the body but not tightened. The final adjustment will be made when the unloader is tested. When this adjustment is made it will be set with the lock nut.

Reinstall The Unloader
The unloader is now ready to be reinstalled in the system and adjusted to the proper pressure. The pressure gauge should be located on the pump outlet manifold or between the pump and unloader for the most accurate adjustment. The gauge may, however, be installed anywhere in the discharge or high-pressure side of the system.

Adjust The Unloader To Minimum Pressure
The unloader should be installed and adjusted so that the cleaner operates with about 10% of the unloader’s capacity in bypass. Before adjusting the unloader, make sure the adjusting bolt is screwed all the way into the valve body. At this setting the unloader will allow the minimum pressure it is capable of producing at that flow. Check to make sure that the high-pressure spray nozzle is properly sized for the desired system output.

Start With The Gun Open


Start the cleaner with the trigger depressed and water flowing through the nozzle. Check the pressure gauge. High system operating pressure should not occur at this point in the adjustment. The unloader is set to develop minimum pressure and any pressure above 1000 psi at this minimum adjustment may indicate unloader malfunction.

Check For Proper Operation
To check to see if the unloader operates properly, simply release the trigger gun while watching the pressure gauge for an excessive pressure build up. If system pressure rises rapidly, the unloader is not working. Depress the trigger immediately to prevent a rupture in the system from overpressure and shut down the system. All high pressure cleaning systems should be equipped with a pressure relief valve to prevent problems with overpressure.

Service Hint:
Installation of a pressure relief valve in the test gauge assembly will protect against this hazard. When system pressure rises above the relief valve setting, water is released to atmosphere and the pressure is relieved.

Adjust With Gun Shut
Adjustments to the adjusting bolt should be made with the gun shut and pressure readings taken with the gun open. Once the system has developed a stable operating pressure and demonstrated the ability to unload or bypass the water, release the trigger and back off the adjusting bolt one full turn in a clockwise direction. The adjusting bolt limits the travel of the piston, which in turn regulates the amount of water left in bypass. The more water in bypass, the lower the operating pressure. Backing out the adjustment bolt allows the piston to travel higher, further reducing bypass volume and increasing operating pressure.

Maximum Allowed Pressure
The system pressure will rise proportionately each time the adjusting bolt is backed out one turn until the highest pressure the unloader will develop is reached. At this point, each additional turn of the adjusting bolt will only increase the pressure spike. Do not continue to adjust the unloader valve past the point where allowed system pressure will not increase.

The Adjustment Process
Each time the bolt is adjusted, open the gun and allow the system to reach a stable operating pressure. Read the pressure from the gauge. To increase the pressure, release the trigger, back off the bolt another turn and take a pressure reading. When the desired pressure is reached, tighten the lock nut. The lock nut should be tightened while the gun trigger is open and the system is under pressure.

The Wrong Nozzle
If this maximum pressure the unloader will allow is less than the rated system operating pressure then the high-pressure spray nozzle orifice is the wrong size. If the correct unloader orifice is installed, the spray nozzle size may be reduced one step (a 4.5 to a 4.0, for example).

If the unloader cycles under pressure, discharge side flow is either not at the proper level or is not regular. The unloader may not have the proper orifice for the system flow, or there may be air in the system. If the unloader cycles while in bypass, the system flow, or there may be air in the system. If the unloader cycles while in bypass, the system is allowing enough flow to briefly activate the unloader. A flow unloader should not be installed with a weep gun, or cycling or other problems will result.

If the unloader body or fittings leak, it is likely that an o-ring was damaged in installation or the threads were inadequately taped with Teflon tape. Damage to the adjusting bolt o-ring will result in leaking around the adjusting bolt, for example.

Low Pressure
If the unloader valve will not allow the cleaner to achieve the rated system pressure, the orifice may not be the proper size for the flow, insufficient flow is diverted to bypass or the spray nozzle may be worn or the incorrect size.

Pressure Spikes
Extreme pressure spikes are usually the result of improper unloader adjustment. However, a restriction in the bypass line can cause excessively high-pressure spikes.

Installation Wish Accumulator For Pulsation Dampening
If an accumulator is used with a flow unloader, it should be positioned on the downstream side of the unloader. Using an accumulator installed between the pump and the unloader can result in unloader malfunction and possible system damage.

The Right Spray Nozzle
Although the correct spray nozzle size for the system should always be used, in actuality, pressure unloaders allow considerable latitude in spray nozzle sizing. If a larger spray nozzle orifice size is used, system pressure will simply drop. If a smaller nozzle size than specified is used in the system, the pressure will be maintained at the unloader setting but flow will be bypassed and the pressure will be produced at a lower flow than the rated system flow that would be produced with the proper or specified spray nozzle orifice size.
Note: In either case of incorrect nozzle sizing, the system will function but performance will be below the level rated for the equipment in that either output pressure or flow will be reduced.

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