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On a reciprocating, rotary screw, and even fossil fuel air compressors, compressed air can be trapped over the piston when the compressor reaches the cutout pressure and stops.
If that air cannot escape, a significant additional load is created for the start-up of the compressor motor when the pressure switch turns the compressor back on and that pressure build-up may be enough to prevent the compressor from starting.
This highlights the importance of an air compressor pressure switch unloader valve.
If you’re interested in the broader principles of how an air compressor works – not just the unloader valve, you should check out our guide explaining how each type of air compressor works.
Now on with how unloader valves work, break, and how to adjust them!
Table of Contents
- What is an Unloader Valve?
- Where is an Unloader Valve Located?
- How Does an Unloader Valve Work?
- Unloader Valve Differences
- Types of Unloader Valves
- Continuous Run Valve
- Pilot Unloader Valve
- Installation and Adjustment of the Unloader Valve
- FAQs (Frequently Asked Questions)
- Unloader Air Hissing Continuing
- The Pressure Switch Has Reached Set Pressure – Unloader Valve Needs Adjustment?
- Adjusting the Unloader Valve on a Reciprocating Compressor 75hp
What is an Unloader Valve?
Gas air compressor unloader valves are one of the numerous small, common and essential parts of an air compressor and the whole pressure maintenance process. They are a critical function in ensuring that the air compressor is able to restart and operate sufficiently.
Somewhere in the garage, the workshop or the plant, the “kathumping” of a reciprocating air compressor echoes throughout. Suddenly the thumping stops, and sometimes there is an audible… “pssssssschhhht”, the sound of air escaping, but just for a second or so. That is the unloader valve at work and the sound you would expect to hear!
Where is an Unloader Valve Located?
The unloader valve will most likely be located inside the pressure switch or attached to it on most air compressors. During the operation of the air compressor, the switch turns the compressor off which activates the unloader valve. The air compressor unloader check valve keeps the entire tank from draining and has a pipe running to the unloader valve.
Above is an unloader valve diagram where the valve is attached to the pressure switch. However, the unloader valve can also be located next to the compressor on larger-scale air compressors. Here, a pilot unloader valve is used to control the emerging air pressure into the unloader valve.
How Does an Unloader Valve Work?
On a reciprocating compressor, compressed air can be trapped over the piston when the compressor reaches the cut-out pressure and stops. If that air cannot escape, a significant additional load is created for the start-up of the compressor motor when the pressure switch turns the compressor back on. That pressure build-up may be enough to prevent the compressor from starting.
Compressed air captured in the cylinder over the piston after the compressor shuts off, increases the load against which the electric motor would have to work to start. If the load on the motor increases too much the compressor motor may fail to start completely, or it may pull too many amps, working hard against that additional and unneeded load, and fry a fuse or pop a breaker.
When the compressor shuts off the typical unloader valve opens and unloads the air that may be trapped over the piston into the atmosphere, and that motor overload problem is resolved.
Unloader Valve Differences
An important thing to note is that unloader valves do not always look or work in the same way.
The unloader valve is very often part of the pressure switch assembly. It will, in many cases, be opened by a toggle extending from the side of the pressure switch as the switch operates, or from an internal mechanism inside the switch that opens and closes the unloader valve that is built inside the pressure switch.
As previously mentioned, larger more industrial air compressors may have an entirely different unloader valve setup. Yet, if the compressor is reciprocating, and has pistons, you will find one or more unloaders somewhere in the plumbing on that air compressor.
Very small, fractional HP air compressors, may have no visible unloader valve. Some of these have a small hole in the line from the pump to the tank that is often under the cover. That hole is bleeding air all of the time the compressor is running, and then, when the compressor stops, that hole, the unloader for that compressor, continues to bleed air until all the air over the piston is gone. Then the bleeding of air should stop as long as the tank check valve is working!
You need to ensure when selecting an unloader valve that you find the right one for your applications, this youtube video below can help point you in the right direction!
Types of Unloader Valves
Continuous Run Valve
Some compressors are not able to stop and start frequently enough to supply compressed air to high-volume demand plants. If the cycle time between high and low air pressure set points is too frequent, this increases compressor wear and can reduce the life expectancy of some of the compressor parts.
If your air compressor cycles on and off more than 15-20 times per hour, you have reached the frequency benchmark for that type of compressor. It’s time to consider moving from a Stop & Start type of air compressor to a continuous run style.
Pilot Unloader Valve
Pilot unloader valves are necessary for the proper functioning of most the continuous run types of compressors. When an air compressor runs continuously, the air is continuously compressed into the tank causing the pressure to rise exponentially. This pressure level could rise past the pressure-safety limit which could cause potentially catastrophic results. This is where the pilot unloader valve comes into play, to allow the release of air pressure from the compressor head when the pressure reaches a set point.
This component allows for the activation of engine control and reduction in oil consumption. It does so between the air compressor unloader check valve and pump valve by leveling the pressure. This leveling includes diverting and discharging air pressure when the air compressor tank top pressure setting is reached and when the maximum air pressure setting is reached within the air compressor respectively.
Installation and Adjustment of the Unloader Valve
A general guide to installing a pilot unloader valve onto your air compressor can be understood here:
- Place the valve ball into the pilot valve hole in the air compressor body and hand tightens the pilot cap assembly into the pilot hole body. Tighten the differential locknut.
- Turn the unloader screw assembly into the pilot cap assembly suitably to set the cut-out pressure. Tighten the pressure screw lock nut.
- Ensure you check the operation of the pilot valve and the resulting pressures, adjustments can be made if necessary.
It is important to understand that this is a general guide for installing a pilot unloader valve and that it will certainly vary depending on the manufacturer and type of valve you have. Refer to the instructions provided with your type of valve or contact the manufacturer.
The cut-in and cut-out pressure inside an air compressor can be controlled by making adjustments to the unloader valve. A few important steps to follow can be understood here:
- Ensure that the air compressor is turned off.
- Turn the differential screw clockwise to loosen up the differential locknut. When the pilot seat of the differential screw is in contact with the ball, secure the locknut.
- Adjust the air compressor by setting the load pressure to a minimum and then turning it off.
- Slowly screw the pressure screw back into its place until you feel the air coming out the top of the valve. This then sets the bottom pressure so you must secure the pressure screw locknut.
- Investigate adjusting the compressor until you are able to take not of your minimum pressure setting. Turning the pressure screw clockwise will help increase pressure and turning the screw counterclockwise will help decrease the pressure.
- Repeat the above steps for the maximum pressure value.
- Ensure all locknuts are secured thereafter.
Again, it is important to understand that this is a rough guide to adjusting unloader valves and you should seek to follow the instructions provided to you and/or contact the manufacturer.
FAQs (Frequently Asked Questions)
What does an unloader valve do on an air compressor?
An air compressor unloader valve is a component used on air compressors tohelp to release the trapped air inside the compression chamber and tank discharge line when the motor comes to a halt. This allows, therefore, allows the motor to start up easier when it next needs to.
Why unloader is used in compressor?
A compressor unloader valve is a small component, typically measuring around 4-5 inches in length and width, which performs the critical function ofreleasing trapped air inside the tank. By doing so, it enables the motor to restart. An unloader valve malfunction is one of the more common air compressor issues.
Additional valve reading:
- Types of Compressed Air Valves – Guide To Pneumatic Valves
- Pneumatic Flow Control Valves – What Are They, How Do They Work?
- Air Compressor Air Line Non-Return valves/Inline (In The Air Line) Air Check Valves Explained
- Check Valve Sizes
- What is Check Valve Cracking Pressure
- Air Compressor Troubleshooting Check Valve
- Unloader Valves On Twin V Piston Compressor Guide
- 5 3 Valves Explained
- 5-2 Air Valves
- 4-2 Compressed Air Valves
- 3-2 Air Valves
- Drawing a 5/3 compressed air valve
- Draw A 5/2 Air Valve
- Pneumatic Soft Start Valves
- Solenoid Pilot Air Valves
- Compressed Air Solenoid Valve Guide
- Air Compressor Auto Drain Valves Guide
- Needle Valve vs Ball Valve
- Globe Valve vs Ball Valve
- Globe Valve vs Gate Valve
- Butterfly Valve vs Gate Valve
Reader Questions & Responses
Unloader Air Hissing Continuing
When the unloader valve operates, the relatively small amount of air trapped over the piston is voided. Usually, one or two seconds of air escaping is as long as is needed for all the air trapped over the piston to escape. Sometimes though, the air evacuating from the unloader valve does not stop the unloader valve from continuing leaking air after the compressor is stopped.
If there was no way to prevent it, when the unloader valve opened, all of the air already compressed into the tank would escape out of the unloader valve which is open to the atmosphere the whole time the compressor is off.
To prevent that from happening a tank check valve is installed. Often in the fitting where the line from the compressor pump head enters the air compressor tank. This check valve (also known as a one-way valve) keeps the compressed air in the tank when the unloader valve opens up.
If air is bleeding from the unloader valve continuously, it is a good bet that the tank check valve has either failed or has not seated properly. The compressed air in the tank is bleeding back up out of the tank to the unloader valve, and out to the atmosphere.
When the tank check valve isn’t sealing tightly or at all, compressed air will continue to bleed out until the tank pressure reaches the cut-in pressure setting on the pressure switch, and then the air compressor will start to pump up the pressure in the tank again.
This cycle will continue until the check valve has been repaired or replaced.
The Pressure Switch Has Reached Set Pressure – Unloader Valve Needs Adjustment?
I have built compressors for several years but I am having a problem with the electric motor restarting after the pressure switch has reached set pressure and shuts the motor off.
I have seen some compressors which have a pressure valve that keeps the intake valve open until the motor restarts, and some that allow the motor to run free until the pressure drops.
I would like to find a supplier for one of these valves. I would like your help in this matter, thanks.
Thanks for your question.
What I think you need is an unloader valve, which automatically discharges the air pressure over the piston after the compressor has reached cut-out pressure.
If you don’t unload that pressure over the cylinder, it makes it very hard for the electric motor to start. Since the electric motor already has an inrush requirement greater than many electrical supply circuits, without the ‘capacitor’ (I think that’s the name) to provide a boost when the motor starts, the circuit breaker or fuse would pop each time the motor went to start.
The unloader valve mechanism is commonly part of a compressor pressure switch assembly. You can get a combo pressure-switch / unloader valve from a compressor parts supplier.
Further, all-electric motors have an inrush current when they start, and then once they are going, draw fewer amps. Some motors come with starters that are basically (if I understand my electrician friend correctly) that allow a quick boost of energy to the motor when it goes to start. This reduces the inrush current and helps prevent blowing fuses or popping breakers when the motor starts.
By not “unloading” the piston-type compressor, you are adding load to the start-up current requirements, and that may be why you are overloading the motor on start-up and blowing the fuse.
The compressors have an unloader valve to bleed off the compressed air over the piston when the compressor stops so that there’s less load when it goes to start.
Your electric cutouts are shutting off the motor, but not unloading the compressed air over the piston. You should be using a pressure switch that’s built for compressors, and when you do, it comes complete with the unloader valve.
Or, you can use the cutout to operate a small 2/2 valve that will perform the same function, but putting that into the circuit will probably cost more in materials and frustration than buying the pressure switch and plumbing the compressor that way.
Adjusting the Unloader Valve on a Reciprocating Compressor 75hp
I have a two-stage compressor with an unloader valve, after several years of work the unloader valve got full of gummy residue, I think from the little amount of oil leaking from the compressor into the tank.
One day I noticed that the compressor was running for too long to reach the stop pressure.
I checked the compressor while it was running, and it seemed like it was running at idle.
Then noticed a small leak at the unloader valve, and when I put my finger over the unloader valve, just where it was leaking, the compressor sound changed and started pumping really well again and reached the stop pressure in less than 3 minutes.
Took apart the unloader valve and clean it really well with solvent, then I put it back together, after checking that all the o-rings were sealing.
After installing the unloader valve all hell broke loose… it is totally out of adjustment, it has 3 points were it can be adjusted.
1.- A knob on top, a real adjustment intended by the manufacturer. How does it adjust?
The valve has 3 sections
2.- So the top and middle sections can be adjusted and set with a locknut.
to adjust what? and how to set it ?
3.- The middle and bottom sections, can also be adjusted and set with a locknut.
to adjust what? and how to set it ?
when I change the adjustment on the bottom parts it chatters. Fiddling with it I can make it to unload the compressor but then it doesn’t pump until I move the adjustment of the valve again… I feel really lost with this.
HOW TO ADJUST A UNLOADER VALVE 3 Sections and top knob?
Gosh, this is impossible to do without having the switch here and playing with it on the compressor.
You don’t indicate the brand of compressor, so I can’t even refer you to the factory if you could find it.
Go to your local “big box store” that sells compressors. They probably won’t have a compressor repair depot, but what they will have is a local company that they use for warranty repair of the compressors they sell. Find out who that company is.
Take your pressure switch to them and either ask them to show you how to set it or given that you’ve taken it completely apart, it might be a better idea to ask them for a replacement for it.
Do you remember the pressure settings of the compressor before you started having problems? You want to set the OEM replacement switch to those same levels when you reinstall it.
If you have any questions regarding air compressor unloader valves, please leave a comment below, with a photo if applicable, so that someone can help!
How does an unloader valve work? ›
Essentially, an unloader valve is a safety switch. It sends pressurized water into a by-pass by cycling it from the outlet side of the pump to an inlet or water tank. This process depressurizes the water and prevents pressure from building up while the pressure washer machine is idling.What is an air unloader valve? ›
Pressure switch. It's a very popular part and very common problem when needing to replace thisWhich valve does a compressor unloader keep open on an air compressor? ›
Piloted unloader valves combine a pilot valve and an unloader valve to run an air compressor continuously. While this type of valve is most commonly used on gas-powered air compressors, it can also be used on electric models.What does an unloader do on a pressure switch? ›
The unloader valve responds to the pressure switch as the switch reacts to the compressor tank pressure. It is designed to "unload" the pump and air lines after motor shutdown. The mechanical action which opens the contacts inside the switch to shut off the compressor motor opens the unloader valve.How does an air compressor unloader work? ›
An air compressor unloader valve is a device used by air compressors to release the trapped air inside the compression chamber and tank discharge line when the motor stops. This allows the motor to start up easier. In general there are two types of air compressor unloader valves, electrical and mechanical.How do you know if your air compressor unloader is bad? ›
You can have two problems. Air leaking from an "unloader" valve on the pressure switch continuously (after the motor shuts off) is an indication of a leaking tank check valve, not a bad pressure switch. Air leaking from the unloader while the motor is running is an indication of a bad unloader valve.How do I know if my unloader valve is bad? ›
If the valve does not work correctly, the pressure washer pump will get hot, and more power is asked from the engine. If you have a gas pressure washer, you can hear more engine noise due to the higher load. Water leaks and inconsistent output pressure are also signs that there is a problem with the unloader valve.What is the effect of the unloader valve is in open position? ›
If the unloader stays open, you will hear a continues leak of compressed. I usually sounds like the pressure switch is leaking, because the unloader is often located on/near/inside the pressure switch.Can you adjust an unloader valve? ›
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- Make sure the engine or motor is spinning at the proper rpm. ...
- Use a new high power washer spray nozzles of the proper size orifice for your pressure washer.
- Remove the nylock nut off the top of the unloader valve and spin the black plastic pressure adjusting knob off.
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Electrical unloader valves – These valves open as a result of an electric current being applied to a solenoid coil, which produces a magnetic field and initiates a mechanical process in response to an increase in pressure inside the cylinder head assembly or discharge line.How do you adjust a pilot unloader valve? ›
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The relief valve protects the low-volume/high-pressure pump from pressure above 3000 psi. The unloading valve is set at 500 psi to divert flow from the high-volume/low-pressure pump to tank when system pressure climbs above this setting.What happens if unloader valve is bad? ›
A failed unloader valve can cause high pressure at the nozzle. The unloader valve recirculates water to the pump inlet when the pump is running but the spray wand isn't in use.How do you clean a compressor unloader valve? ›
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The most common reason behind a reciprocating compressor's inability to reach sufficient pressure is a defective reed valve — which can exhaust air out from the air inlet on single stage models or, through the intercooler safety valve in the case of a defective reed valve on a two-stage piston compressor instead of ...Should drain valve be closed on air compressor? ›
Drain tank of moisture after each day's use.
If unit will not be used for a while, it is best to leave drain valve open until such time as it is to be used. This will allow moisture to completely drain out and help prevent corrosion on the inside of tank.
Conrader Piloted Unloader Check valves are an all-in-one combination valve to run an air compressor for continuous operation. The valve includes a pilot valve, discharge unloader valve, and check valve. This can be used on gas engine or electric motor driven air compressors.
What is a pilot valve on an air compressor? ›
The pilot valve opens, actuating the unloading device and allows the compressor to run in an unloaded mode. When the receiver pressure drops to the preset cut-in pressure, the pilot valve closes allowing the unloading device to close and the compressor once again pumps into the receiver.What is the valve on the bottom of an air compressor? ›
A condensate drain valve is a small device typically located at the bottom of a compressor's air receiver. Whether the air compressor drain valve is open or closed determines if water flows out of or remains inside the compressor tank.