Please help diagnos an old worthington air compressor problem

pmtool · Nov 27, 2015

This is an old 20 HP Worthington reciprocating compressor. It starts up fine and seem to run well but after it is cut off by the pressure switch it leaks are back from the receiver tank and back out the compressor. As you can see in the photo there is the large compressor outlet hose that fills the tank, and on the top of the tank there is a small line that goes back to the compressor. What does this line do? It sounds like all the air that is leaking out in going through the small tubing. I imagine I have a sticky check valve somewhere? The compressor has been sitting for a while. You can also see from the photos that I probably have a oil leak somewhere as well. I am sure that to someone that knows compressors this is a simple problem but I have little experience with compressors.

Thanks!

malcolm rypauf · Nov 27, 2015

The large line going from the compressor to the tank needs a check valve in it (usually at the tank end) so that when the compressor stops, the air in the tank does not force it's way back thru the compressor outlet valves (one on each head) which are spring-loaded valves that open under pressure from inside the cylinder. You need to confirm that the check valve is not allowing pressure to back-flow when the pump is not actually pushing air into the tank.
The small line running from the tank back to the compressor is called the unloader line. It supplies pressure to hold the valves on the cylinder heads open when the pump is stopped if the pump is controlled by a pressure switch (start-stop mode) or if the pump is running constantly but not pumping (unloaded) because the air pressure in the tank is up to the set pressure. Also, there are diaphragms in the unloader valves on top the heads that can leak.
Hope this helps.
Larry

pmtool · Nov 27, 2015

Thanks Larry,

That gives me an idea of what to look for.

Should the unloader line also have a check valve at the tank?

Justin

dkmc · Nov 27, 2015

Being that that line looks recently replaced, it seems suspicious that there
may have been a check valve, and it's been left out. Maybe it was
defective or got broken during removal?

malcolm rypauf · Nov 28, 2015

pmtool said:
Thanks Larry,

That gives me an idea of what to look for.

Should the unloader line also have a check valve at the tank?

Justin

The unloader line brings tank pressure to the unloader valves, therfore a check valve in this line is NOT desirable.

pmtool · Nov 28, 2015

Ok, I will do some more investigating today. It sure sounds like the air is flowing back from the tank to the compressor continually after shutoff through that smaller line. This line goes into a "t" at the tank and the other side goes to a pressure switch near the electrical panel. If there was a check valve before this "t" then the pressure switch would not see any pressure. Currently it is draining from about 150 psi to 100 psi out of two 80 gallon tanks and a fair bit of pipe in about 5 minutes. There are no other leaks in the system.

kineticmx · Nov 28, 2015

You have the right idea, the small line supplies pressure to the unloader mechanism, which holds the valves open and makes starting the pump easier.

There are several types of unloaders available but I'm guessing the line feeds into a hydraulically actuated valve and from there flows to unloader diaphragms on the pump. When the pump starts spinning the oil pump builds pressure and vents pressure going to the unloader diaphragms.

So its probably the valve leaking or one of the diaphragms.
Hope this helps, Teddy

pmtool · Nov 28, 2015

kineticmx said:
You have the right idea, the small line supplies pressure to the unloader mechanism, which holds the valves open and makes starting the pump easier.

There are several types of unloaders available but I'm guessing the line feeds into a hydraulically actuated valve and from there flows to unloader diaphragms on the pump. When the pump starts spinning the oil pump builds pressure and vents pressure going to the unloader diaphragms.

So its probably the valve leaking or one of the diaphragms.
Hope this helps, Teddy

Interesting. I did take some photos. Perhaps this is part of the mechanism your are speaking of. There is a gage here but it only goes to 60psi so it must be oil pressure?

kineticmx · Nov 28, 2015

Excuse the poor job but I circled the two components I'm referring to.

Highlighted in blue should be a crank driven oil pump (Which looks oil soaked, and is probably the source of the oil leak.) It feeds into the unloader valve which is highlighted in red.

When the compressor is shut off the oil pressure is zero and the valve allows air to flow to the two caps on the top of each cylinder, holding the intake valves open.

Once the compressor starts and gets up to speed the oil pressure will build and it will trigger the unloader valve, which in turn vents the pressure to the two caps and allows air pressure to build.

So check the oil pump for oil leaks, and check the unloader valve, the caps, and all connecting plumbing for leaks. Sometimes the copper lines can crack or the compression fittings will leak.

To anyone more knowledgeable please excuse my vocabulary, I totally made up all the terms I just used. Feel free to correct me so I don't sound like a jackass.
Let me know what you discover

Best of luck, Teddy

pmtool · Nov 28, 2015

Thanks for your effort. Things are making more sense now. I will let you know what I find out....

Justin

john.k · Nov 29, 2015

Its a 3cyl/2stage,so only the HP cylinder is connected to the tank.Therefore concentrate your efforts fixing the delivery valve in that cylinder.The unloaders look to be piston type,which is good,as by simply freeing stuck piston rings,leakage should stop.Diaphrams are often ultra expensive.Depending on how much air you will be using[cap should be 80cfm plus with 20hp]a rubber /polymer delivery line may be unsatisfactory.Flexible all metal lines are expensive though.I would take off all the heads and repair valves and decarbonize.The intercooler may also need to be chemically cleaned as it may be choked with carbonized oil.Regards John.

pmtool · Dec 2, 2015

Looked into this a little further and there is no check valve in the main line between the receiver tank and the compressor. This is obviously not the original configuration of this compressor so I am hoping someone left one out as a oversight and that adding one will fix my problem. Thanks for all the help!

john.k · Dec 2, 2015

With continuous air delivery the check valve will get very hot,so it will need to be all metal,and sized big enough not to impede flow.A cheap simple cure for leaking unloaders is to put an electric solenoid in the line,so air is shut off when the motor stops.The valves will lift as soon as motor starts,so it starts unloaded,if its meant to do that.You should be aware that leaking delivery valves will cost serious money on the power bill,and create a great deal of extra heat in the heads.Regards John.

M. Moore · Dec 3, 2015

I have an old worthington 7.5 hp compressor and just replaced the main line to the tank. By law in Canada you have to use the proper compressor flexible pipe, it is expensive, if you don't have it then your insurance will probably be invalid. There is a lot of heat generated and most hydraulic lines are not rated for that much heat.
I had to install a check valve on my tank as I had a similar problem as you do, once I installed a big enough check valve it has been working well ever since.
I paid $350 for the compressor nearly 10 years ago and it works hard nearly every day. It has been working harder the last three years as I installed a cabinet sand blaster and the compressor can't quite keep up to the demand. These are really good compressors, as one old fellow I met said "the cadillac of compressors". \

Good luck,

Michael

john.k · Dec 3, 2015

Agreed on the flex pipe.Yours is the blue type which is used on flooded screw compressors,where the delivery temp never exceeds 110degC[auto shutoff temp].You might get around it by mounting the air tank on the compressor base and then using a copper pipe with two or three complete turns as expansion and flexing takeups.Continuous delivery temp would set poylmer tube afire,or decompose teflon tube.The blue tube goes rock hard after a while on screw compressors and gets brittle,and thats at 110degC.Regards John.

pmtool · Dec 4, 2015

I fixed the problem for the time being with a check valve. It sounds like it will need more attention in the future as time permits. I will look into a better output hose. Maybe worth having a good tech come out and look at it and see what needs to be done in the long term. Thanks for all the help....

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