Atlas Copco Compressor problem – please help guys

[Manchecd]

I have a 1968 Atlas Copco, not sure what model but there are some photos of it below.

I have a problem with the pump occasionally sticking (i.e. not turning) and jamming which requires the compressor to be switched off pretty quickly.

The pump always seems to work fine after I start the compressor when it has an empty tank. It turns over freely and pumps 120 psi into the tank, at which point the pressure switch turns it off.

The problem arises when the pressure falls to 80 psi and the pressure switch turns the motor back on. There seems to be a strong resistance at this point preventing the pump from turning over freely.

I have tried turning it by hand at this point with the compressor switched off and it is very hard to turn it.

Any ideas gratefully received.

Thanks a lot.

Colin

 

[rtbcomp]

Does it hiss when the pump turns off?

If not then the unloader valve is probably sticking, or the pipe to it blocked &c.

What it does is de-pressurises the pipe from the pump to the tank so it doesn't start under load.

Not sure what arrangement your compressor has but this is a typical setup;

 

[tigler]

As above, the unloader valve or something associated with it is most likely the problem. On that compressor it's going to be the part with the pressure gauge attached or something near to it.
Lovely old Atlas though, well worth sorting out.

 

[Manchecd]

Thanks for the quick response.

There is no audible hissing when the pump switches off. However, I have got up close to the pump to listen when there is pressure in the tank and it does sound as though there is some slight hissing from inside the pump. If I stop using the compressor when there is pressure in the tank and come back say a few hours later the pressure will have gone.

As regards checking the unloader valve and pipe, the only two pipes that come out of the pump are the large 'L' shaped pipe with air fins on it that goes from silver cylindrical tube to the pressure guage below the pressure switch (best seen in photo 2) and a thin silver pipe from the top of the right hand cylinder (seen in photo 2) to the top of the pump housing (seen in photo 3).

Is it possible to tell from the photos which is the unloader pipe?

If it is the thin one, it may be that there is an unloader valve only on one of the cylinders. I did a bit of dismantling last weekend and removed the large 'L' shaped pipe and the silver cylindrical tube in order to loosen the cylinder head on the right hand cylinder, which I think is the one causing the problem. I discovered that there is something a few inches long threaded/screwed into the left hand cylinder (which could be a valve) but nothing at all threaded into the right hand cylinder.

The valve (if it is a valve) projects from the left hand cylinder through the smaller diameter tube at the back of the silver cylindrical tube. If you look at photo 3 it is inside the flanged tube joining the cylinder head to the large silver cylindrical tube.

I hope this makes sense and is some help!

Many thanks.

Colin

 

[Manchecd]

Hi Tigler

Thanks for your response, which arrived whilst I was preparing a reply to rtbcomp's comments which I have now posted up. This was before I read your comments.

From my reply, does it look as though I have an unloader valve missing on the right hand cylinder (which must have been like that since I have had the compressor, which is nearly 25 years!!). Or is the item I mention nothing to do with this?

You mention it may be the part with the pressure guage attached. Is the unloader valve the silver bit with three fins, into which the pressure gauage screws?

Although I do not use it very often, it's been a great compressor and I would like to get it fixed if I can.

Many thanks.

Colin

 

[rtbcomp]

Are the outputs of the two cylinders connected together? If so you only need one unloader valve.

Usually the unloader valve is inside the pressure switch, but I'm sure I read somewhere that some unloaders work off centrifugal force, with a device connected to the pump crankshaft. It may be worthwhile doing a search of "copco" in this section of the forum.

 

[Manchecd]

Hi rtbcomp

I think they are connected together - by the silver cylindrical tube.

There is a single ouput pipe (with air fins) from this tube, which goes to the bit beneath the pressure guage, so it looks to me as though they are connected.

I will try a search for 'Copco' as you suggest.

Thanks.

Colin

 

[Davek0974]

I can't see a non return valve or unload pipe in those pictures. It may have a mechanical unloader, I have seen an old copco that was built to run permanently, when pressure was reached it simply released compression and just run idle until pressure was needed again, it's a very simple system but very power inefficient.

If its hissing when off and losing pressure, the non return valve is leaking or stuck as well.

 

[rtbcomp]

The slight hissing could be air escaping slowly from the delivery pipe. If the pressure in the tank is OK when it stops then the non-return valve is OK.

 

[shenion]

I think there may be a centrifugal unloader in the pump. Probably right under cover beneth where the small hose goes to the crankcase,

 

[Manchecd]

Hi guys

Thanks to all of you for your responses.

I have not been able to locate an unloader valve around the pressure switch but have posted a few photos of this area to see if anyone can identify one.

Re the hissing, this is not audible when the compressor is off. I can only hear a faint hiss if I put my ear against the pump and the pressure in the tank seems to be OK when it is switched off.

Re a centrifugal unloader in the pump, would I be right in thinking that this is where the thin pipe is screwed into the top of the pump (see photo 3 above)? Is it likely to be inside the pump, on the other end of the external thread onto which the pipe screws?

I will remove the silver cylindrical tube tomorrow and take a photo of whatever it is that projects from the left hand cylinder through the smaller diameter tube at the back of silver cylindrical tube (see photo 3 above and my post of 10.50 am today). This might help to identify whether it is an unloader valve, or not as the case may be.

Many thanks for all your help which is much appreciated.

Cheers.

Colin

 

[rtbcomp]

In the top left picture there is a thin pipe with a nut on the end, attached to the pressure switch. This looks like the unloader pipe, the unloader valve being the bit it screws into with the hole on its side.

When the pressure switch goes "off" a plunger opens this valve and the air in the delivery pipe exhausts through the side hole. There is a definite hiss, lasting about a second, when this happens.

Check to see the valve or the thin pipe isn't blocked and that the plunger inside the switch hasn't broken off.

 

[Manchecd]

Hi rtbcomp

Thanks for this suggestion which I have followed up.

On removing the pipe and blowing through it, there was a little of bit of resistance and some water came out but there is no blockage in the pipe. Once the resistance was overcome I could blow through the pipe freely.

I removed the unloader valve and the plunger is OK and moves freely.

I have run the compressor to see whether the bit of resistance in the delivery pipe was preventing a hiss from occurring but there is still no hiss. The pump remains difficult to turn over.

There is still a hissing when I place my ear to the pump, so I removed the oil dipstick and there is definitely air finding its way back into the pump. There is a constant slow hiss.

This suggests to me that the non-return valve (NRV) is allowing air to leak past it and the air is going back to the pump. Could the leaking of the air be preventing the unloader valve from going off with a hiss?

If I have understood things correctly, the NRV is the rectangular-shaped item into which the bottom of the delivery pipe screws and has the two large pipes going in at each end (see last photo above).

If I remove this, do you know if it is possible to check whether the valve is sticking partially open or will I have to try to find a replacement?

Thanks again for your help.

Colin

 

[rtbcomp]

Have you checked to see if the bit in the switch that depresses the valve plunger is working?

The unloader valve shifts quite a lot of air and you'll hear the hiss a couple of metres away from the compressor. It should cope with a slight leak from the NRV. A point worth noting is that these valves usually don't close until about 20psi is reached in the delivery pipe.

As I mentioned earlier, this slight hiss might just be the delivery pipe unloading back through the valves in the head of the pump.

Run the pump with the unloader pipe in place but disconnected from the unloader valve, you should get quite a lot of air through it.

If I have understood things correctly, the NRV is the rectangular-shaped item into which the bottom of the delivery pipe screws
Looks like it to me.

 

[Davek0974]

Yes if you run the pump with the unloader disconnected, a fair bit of air should come out of the small pipe, if you then turn it off, the air should stop but the pump should be easy to restart as the air pipe will be empty. This would all point to a duff unloader valve.

 

[Manchecd]

Hi rtbcomp and Davek0974

Thanks for your comments.

I have run pump with the unloader disconnected and there is quite a bit of air coming out of the pipe.

I have had a look inside the switch and am not sure whether the bit inside that depresses the valve plunger is working (in the main because I am not sure what should happen).

I removed the unloader valve again to make sure the plunger moved freely and gave it a clean up. The end of the threaded section on the valve has a small hole in the middle to allow the air through and it is shut off by being spring loaded (very similar to grease nipples on older cars). Pushing the spring in, however, does not seem to move the plunger at all when the plunger is in place - the spring pushes and comes out (but not sufficiently far to make a complete seal at the end of the valve) but there is no movement at all on the plunger when this is done.

On reassembling, I ran the pump up to about 40 psi and then switched off. The good news was that that air came out of the unloader valve whereas previously it had not done so. The bad news is that it did not come out with the real hiss and then stop, as (I think) it should do, but continuously leaked a bit of air. When I touched the top of the plunger where the pin end locates in the pressure switch with the end of screwdriver and applied gentle pressure, that was enough to stop the leak but on releasing the screwdriver the leak continued.

I have uploaded a couple of photos of the unloader valve and the plunger (sorry they are a bit blurred, probably as a result of trying to get too close) and one photo of where the pin end of the plunger locates in the pressure switch. This is on the right side at the top of the pressure switch box just above the switch with the dab of red paint on it.

The unloader valve does not seem to be doing what it should be, so I guess replacement would be a first step if I can get a replacement part. Any thoughts on where I might get one?

Thanks again for your help with this - much appreciated.

Colin

 

[brightspark]

its a danfoss pressure switch doubtfull getting any parts for that just a new unit

 

[rtbcomp]

In the switches I've seen the valve is held open by the spring and closed by air pressure. The spring is there to allow a buildup of pressure to about 20psi before it closes.

When the switch opens to stop the motor a plunger or tab on the switch presses the plunger in the valve to open it against air pressure.

Have you tried running the pump with the valve connected to the pipe but removed from the switch? From an empty tank it should emit air and then close. Switch the pump off and try operating the valve with a screwdriver.

 

[Manchecd]

OK, thanks - thought that might be the case!!

Not being at all familiar with pressure switches, are unloader valves an integral part of all of them or only some of them?

In other words, can I get a pressure switch with an unloader valve to which I can connect the unloader pipe on my compressor (or a replacement pipe if I need to have one)?

I am happy to buy a new pressure switch if this will solve the problem.

Are there any ones that you would recommend?

Colin

 

[Manchecd]

Thanks, rbtcomp, I will give your suggestion a try tomorrow.

I am a little unclear how the air pressure will close the valve. If the air is coming up the unloader pipe and the valve in front of it is in the open position, will the air not push the valve open further rather than closing it? I could see that the air would close it if it was going in the opposite direction. Apologies if I do not know what I am talking about here (this is a steep learning curve for me, so please bear with me).

Thanks again.

Colin