SIP compressor not building pressure

[carrera]

My SIP Nimbus Airmate 25050 is playing up and no longer building pressure. Despite extensive reading around I cannot decide what is wrong with it,so I need some help. This is also my first post after being registered for 4 years (I have alsways found the answers on the forum before) . Sorry for what I think will be a long one.

I have had it about 7 years, and in that time has been reliable, until recently. It was mounted in a belly locker in a truck so is not so accessible

I noticed that when switching on from cold it used to take 10 seconds or so before a loud hissing stopped.
I now understand that this could be the compressor building pressure up to 20psi in the unloader valve ??

One day, instead of using the 240v generator in the truck I used an extension cable. I now know this may not have been a great idea.
This was made worse by the fact that unknown to me the socket I was using had poor connections.
The result was that the compressor did not run properly and stalled.
There was also an electrical burning smell. I therefore switched off at the mains and investigated what could be wrong with the compressor, in the process changing the oil on the advice in this forum.
I used SAE30 non detergent oil.

After plugging back into a good mains supply all was well, until a couple of weeks later when it would not start.
I diagnosed a faulty run capacitor (again, from the posts on here), and on testing it had gone open circuit.
There were no markings on the capacitor so I have fitted a 50uf / 450 volt cap, on the advice of the very helpful people at capacitor sales.
It is physically somewhat larger than the original one.

The motor ran again, but it didn't seem quite as fast as before and the compressor now doesn't build tank pressure.

It did have 50psi pressure, but would not go beyond that. I then drained the tank and it will not build any real pressure. It is not losing what little pressure it does build

What have I done ?
Checked the non return valve - no obvious damage
Checked the safety valve - the seat seems fine, it is free to move
Checked the unloader valve seals when the pin is pulled out - it seals when the pin is fully extended
Removed the head and inspected the head gasket and cleaned the reed valves at the same time - there doesn't seem to be any blowback through the intake valve
Checked for leaks from the delivery tube and the unloader valve.

This is where I have spotted a problem. When the pump is running air is escaping from both the safety valve and the unloader valve.
I believe (but this is why I am asking the question) that after a couple of secconds the pressure in the unloader pipe should cause that to close and pressure to build in the tank.
I don't think that the safety valve should be opening at all, but I really don't know

Here are some pics

The NRV and safety valve

The Unloader and tank pressure gauge

The motor - I am a little concerned by the condition of the windings

I'm confused as to whether the pump is not displacing enough air for the unloader to work properly, or the unloader valve is not working properly, or something else is wrong

Help appreciated. If it's just junk now I would rather sell for spares rather than waste money on components that won't fix the problem. If I can get it working properly again then that would be great

Many thanks

 

[carrera]

a minor update after more testing .......

I just tried running the pump while having my finger over the safety valve. When I did this I could hear the tone of the motor change as the load increased.

After about 15 seconds or so the unloader valve closed and pressure slowly started to build in the tank. Hovever, the safety valve was still leaking air while the pump was running. I had tried this before and the unloader didn't close previously so maybe the reed valves are working better since I stripped and cleaned them last night ?

The safety valve closes when the compressor pump is switched off and does not leak.

 

[rtbcomp]

The safety valve is in the wrong place.

It should be fitted to the tank directly (usually to one of the ports on the pressure switch) so it checks the pressure in the tank.

Where it is at the moment it is checking the pressure coming out of the pump, which is quite a bit higher than tank pressure.

 

[carrera]

I know you're one of the experts on here, and I think I can why you are saying that but the damn thing has worked for 7 years like that.

By way of additional background when the compressor was fitted into the truck the motor unit was dismounted and installed alongside the tank. I can't see any reason why the guy who did it would have changed the piping arrangement - and I respect his engineering knowledge.

Looking at the picture again could the solution be as simple as swapping over ports A and B.... if they are the same thread.

That way the safety valve will be "direct" onto the tank, as you suggest. But won't it still see the higher pressure from the pump as the air is pumped past the NRV into the tank??

Or could it even simpler and the spring on the safety valve has gone weak, and is now opening due to pump pressure which previously was insufficient to open the valve. ??

 

[rtbcomp]

If it worked like that before then the valve spring could be weak or broken, can you lift the pip in the centre of the valve? It should have quite a strong spring pulling it down.

Looking at the diagrams here of an NRV

http://www.mig-welding.co.uk/forum/album.php?albumid=900

There isn't a port for a safety valve, so your looks like it's designed with that in mind. Unscrew it and have a look inside (drain the tank first!)

 

[carrera]

There's not enough of the pip showing to get a grip on it with fingers or pliers to try and pull it upwards, and it's dark now.

I dismantled the NRV last weekend and all seemed in order, though I did think that the spring for the pressure relief looked a bit thin and weak to provide 11 bar of force.

Looking again at the compressor is there a PRV on the pressure switch as well ? It's to the far right of picture 2 above. If that's the "proper" PRV, what's this one on the NRV valve for.

I'll have another look in the morning and see if the item on the pressure switch is a manual relief valve. Unfortunately, with everything being exposed for 7 years it's generally held in place by rust.

Thanks for the help so far

 

[rtbcomp]

I think you've hit on it.

There are two types of relief valves, delayed and instant, that can be fitted to the pressure switch.

When an instant type is fitted an extra valve is provided in the delivery pipe to delay the buildup of pressure.

This usually closes at about 20 psi.

Once you've managed to build up pressure, press the pip in, if it releases air then it is a delay valve

 

[carrera]

Hmm, you might think I've hit on it, but I'm not there yet.

I believe the control unit has a delay type unloader valve. The control unit is a Condor MDR1/11 with an AEV1S unloader. More information is available at the links below

http://www.condor-werke.de/en/produ...essor_version/?con_temp=detail&products_id=30

http://www.condor-werke.de/en/produ...ssories_mdr_1/?con_temp=detail&products_id=31

http://www.condor-benelux.nl/nederl...rs_wissel/mdr1/info_mdr1.pdf/mdr1_2013_gb.pdf

So I understand that the delay unloader empties the feed pipe when the compressor switches off, so that when the motor restarts it has a chance to get up to speed without having to work against tank pressure. The unloader valve closes at about 20psi or 2 bar, depending on your data source, 2 bar is the Condor spec.

That said, I have yet to work out the function for the secondary safety valve on the NRV valve (shall I call it the USV for unloader safety valve)??

It's presence means that the unloader valve doesn't see a build in pressure as quickly as it otherwise would, and (at the moment) once the unloader valve has closed feed air is being vented to atmosphere.

Now the obvious thing to do is to simply block the valve off, (which I could do through buying a standard NRV valve), but that's just not the right thing to do is it. The USV also works the opposite from what you have described above, if I understand correctly. When the pump is off the spring pressure holds the valve closed and the pip is flush with the flange. When operating the pump pressure is opening the valve and the pip is raised out of the flange.

Can we shed any more light on what the USV does, and even where I can get a replacement. The new SIP site is pretty useless for finding parts.



EDIT: Could the USV be properly called a cold start valve, as illustrated and described on http://www.ereplacementparts.com/cold-start-valve-p-72672.html?osCsid=lle4jkeeo23mf3f41mnv36pn54

I have removed the USV again and taken a photo. I think that it is open by default, and when the pressure in the feed gets up to a certain value ( 2 bar?) the pin moves outwards and the rubber o-ring seals against the back of the threads. I removed it without releasing pressure in the tank as I believed the NRV valve would stop air leaking from the tank. It did

If this is passing air then I assume either there is insufficient pressure in the feed pipe (not enough air from the pump), or the seal is not being made ??

I said earlier that I didn't think that the motor was spinning as quickly now. yesterday when testing there were one or two occasions when the motor seemed to speed up for a fraction of a second. Is it perhaps the centrifugal swirches not engaging, or could the wrong value capacitor cause this problem ?

 

[rtbcomp]

That's definitely a cold-start valve (for want of it's proper name!). I have the same arrangement on my Clarke compressor, the valve is fitted to the aftercooler though.

It's function must be to extend the time the motor is allowed to spin up before it is put under load.

It may be worth trying a plug in its port for the purpose of experiment, although I'm thinking the problem may lie with the motor or the electrics. Do the contacts on the pressure switch get hot?

 

[carrera]

Thanks for that, that's real progress for my understanding.

I don't think the pressure switch gets hot, but the motor does. In 7 years of ownership the overheat trip never operated, but since the capacitor failure the motor has stopped working even though the tank control presssure had not been reached. After a few minutes the motor will restart automatically.

This might be a chicken / egg problem. Did the capacitor fail and cause a problem or is there some other problem which caused the capacitor to fail ?

My guts are telling me that the poor power supply caused a problem in the motor which caused the original capacitor to fail.......but I do have a tendency to diagnose the complex rather than the simple problem

I'm going out now so will be offline most of the day. For testing purposes I think I'll remove the spring and perhaps tape the pin in the out/closed position.

I tried to start dismantling the motor already, but I think the screws are rusted in so tight they will just snap. That's what happened on the first one anyway. I'll soak with some more release fluid in the meantime to improve my chances.

Would you mind looking at the picture of the motor, and let me know if what looks like burned paper / severed securing wires look normal or damaged.

Cheers

 

[rtbcomp]

There was also an electrical burning smell.

I think this says it really, windings tend to smell when they fail, capacitors just tend to burst.

It's difficult to tell from the photo, but I would be looking for brittle insulation and discoloured windings.

I suppose using the extension cable may have prevented the motor getting up to speed which may have caused the centrifugal switch to arc/overheat which in turn may have damaged it causing the current symptoms.

 

[carrera]

I removed the spring this morning, and as anticipated the valve closed and all the output of the pump was filling the tank.
The unloader valve also emtied the pipe when switched off.

After running for 30 seconds, more slowly than is normal, the motor started to smoke. I switched off immediately.

With application of some releasing spray and heat I managed to remove the motor clamping screws.
I have tried separating the motor, hoping that the end plate or centre magnet section would separate, but they seem to be solid.
I don't think anything else needs to be removed before spearation ??

Anyway, it seems the problem is definitely in the motor, here's some more photos of the visible part of the windings.
Do you think the problem will be in the switch, and if that is repaired the compressor will work as normal, or are the windings now cooked and the motor junk ??

 

[Flapdisk]

Some of those windings look like they have been extremely hot but as to whether they are still ok I'm afraid I don't know, there are some guys on here who are expert with electric motors hopefully one will be along shortly good luck with it mate

 

[rtbcomp]

If it's got hot enough to burn the lacing cord I'd say it was well and truly done for.

In the top picture you can see where the shellac has been burnt off. It will have got hotter still in the centre of the bundle.

 

[carrera]

Well, I've spent a bit of time, but no more money, and now have a much better understanding of how a compressor works, thanks for all the education and help in getting there.

I spent most of the day fitting a new compressor into my truck, it's a Sealey 3hp V twin so should give me a bit more oomph for the air tools. Only job left to do is to source a new pipe or an extension pipe to go between the dismounted motor and the tank.

I suppose I now have a spare receiver tank, and will just have to keep a look out for a cheap SIP motor to bring this one back into use

I'm still intrigued as to why mine failed. Was it the capacitor, the power supply, the valve unit or the centrifugal switch that failed first ?