high 2nd stage pressure

[neutrino]

Hi have another compressor question - this time it is for a friend.
He has a fairly new (couple of months I believe) Nuvair Mini Tech compressor.

When running it keeps popping the 2nd stage safety valve.
I had him add a gauge in its palace for a quick test and the pressure reads about 85bar when the final thrust stage is at 200bar. The book says it should be about 45bar. The safety valve is apparently set for 65bar.

What would cause the 2nd stage pressure to get so high? Is there anything I can get him to check? Is he still able to fill tanks with the high 2nd stage pressure?

He is in the middle of his busy season out here in Honduras so down time will be an issue.

I have not seen the compressor as we have only spoke on the phone.

Any help and suggestions are appreciated

Cheers
Rich

 

[iain/hsm]

Frankly you gave your friend bad advise with your quick test idea.
Especially on a new and I presume in warranty compressor.

1. Taking off a safety relief valve and replacing it with a pressure gauge is a dumb idea, better to use a run tee fitting and fit the gauge together with the relief valve. That way you can use both safely, you can safely read the interstage pressure and with the relief valve still in place you can safely investigate up to the maximum pressure lift setting.

2. If 45 barg is the nominal interstage pressure by increasing this up to the relief valve setting 65 barg you increase rod loading by 40% and the manufactures maximum design pressure and thus maximum rod loading.

Now by removing the only safety device you have and this resultant jacking of pressure up to 85 barg your now approaching double the design rod loading for that model.

Now keep this up for a minute or so and the protective oil film between either the second stage con rod bearing or the lower main crankshaft journal bearing will begin to break down and eventually the rod will seize or snap, that or the cylinder head tie rods fail and you attempt putting up another satellite to orbit the planet.

3. This high interstage pressure your seeing is being caused by a leaking valve assembly on the inlet side of the third stage.
It needs the third stage inlet valve repairing (valve seat, valve plate or spring) or replacing

4. If down time is an issue out there I guess your friend will have to hand a complete set of rings, valves and service kit.
If not and if its critical kit with as you say, down time being an issue then a more robust industrial compressor may have been more applicable to this application.

 

[neutrino]

Thanks Iain,

It was only run for a very short period to see what the pressure ran to to make sure it wasn;t the safety valve blowing at a lower pressure. Stuck out here in a remote location in Honduras there is a distinct lack of parts/bits and pieces and there was no T available.

With further research online I came to the conclusion that it was the 3rd stage inlet valve. I know he had some spares sent out with the compressor but is missing the tool to remove the valve. Last I spoke with him was that he was getting the tool from a neighboring island.

I will touch base with him today.

Cheers
Rich

 

[iain/hsm]

Good points, also I had forgotten about needing a special key tool to remove the intake valve on that brand, pretty irksome having to find the darn thing every time you want to make a valve inspection, It's not like they couldn't have made it with a simple retention ring or similar so most general workshop tools would work.

Why make a job easy when with a key tool you can make a little more money I guess.
Besides its not like they couldn't supply one free of charge after you spend so much to buy a new compressor for all they cost to make.

Always a good option to have intake, interstage and if possible oil pressure gauges as they pretty much always pick up most valve and piston problems early and you can identify cause before failure.
Also a good idea to have a full set of spares. Glad your on top of this, pulling the valve is a cinch (with the tool) All the best. Iain

 

[neutrino]

Thanks Iain,

I try and keep abreast of what they are doing and post updates here.

Cheers
Rich

 

[scubastevesa]

Hi - you are correct in that faulty 3rd stage valve will lead to increased pressure in 2nd stage - it can also manifest in 3rd stage not building enough pressure