MCH-6 low fill rate and O2%

[zider]

My apologies for the long post but I wanted to give as much information as possible. Thanks in advance for reading and any advice you might have!


I bought a maxair 35 with no motor for next to nothing a while back, and I’ve been working on trying to restore it to working order. The unit turned hard from day one but I replaced the oil and installed an electric motor to see how it ran. Things went down hill quick and the unit seized after about 5 minutes. Upon disassembly I found metal fibers and needle bearings in the crankcase. Turns out the bearing on the second stage rod was trashed.


Since I had to tear the unit down completely to fix the bearing, I decided to purchase a 500 hour kit for a clean start on the rebuild. I replaced the bearing in the 2nd stage rod and completely cleaned the crankcase if old oil and metal shavings. I cleaned the cooling tubes, water separator, and filter housing to remove old oil and gunk. Everything went back together well with the only exception being the 3rd stage piston. Compressing the rings to install the piston into the cylinder was WAY more difficult than shown in the coltri video and it took two sets of hands to complete. I finished the assembly and ran with clean oil under no pressure for about 20 minutes. I then changed the oil again and installed a filter cartridge.


I was hoping that I would be ready to go but no such luck. The unit seems to run fine and gets up to 3000psi in just a few minutes. The issue I’m having is that the fill rate is crazy slow at around 0.75cfm. Additionally, my O2 meter shows the output gas at 19.9%. My CO content shows 0.2ppm, so I know I’m good there. I did a leak test on the whole unit with soapy water and found no leaks. Temperature readings on the 1st-4th are 95, 81, 92, and 89 respectively (ambient ~35). The gas output had a slight oil smell at first but that went away after about 20 minutes of flow


Does anyone have some advice on what could be wrong? I’m out of ideas about what could be causing the problem. Thanks.

 

[tbone1004]

what RPM is the pump turning now? and how did you calculate the fill rate? Can you get interstage pressures between the stages? With such a low second stage pressure I'd question whether the first stage pressure is high due to a faulty check valve between the 1st and 2nd stages

 

[zider]

what RPM is the pump turning now? and how did you calculate the fill rate? Can you get interstage pressures between the stages? With such a low second stage pressure I'd question whether the first stage pressure is high due to a faulty check valve between the 1st and 2nd stages

@tbone1004 The pump is running at just under 2800 RPM. I measured the fill rate using an old 6cf tank. I ran the pump up to the PMV pressure (~1900) and then timed the fill. After 4 minutes the tank was at 1500 so I estimated 3cf over 4 minutes is 0.75 cfm.

Is there a standard process for testing the interstage pressure? That’s not something I’ve done before. I sure hope the valves are good as they are all brand new from the 500 hour kit I used during reassembly.

 

[tbone1004]

@zider try tracking the fill from start to finish, ideally on a bigger tank, and ideally going over the PMV pressure. Everything after the PMV has to be filled as well and while it doesn't sound like much, it adds up quick. At 1500psi, there is about 1cf of gas per linear foot in a 1/4" line. I.e. if you have a 3ft long fill whip immediately after the PMV, there is 3cf of gas that it has to fill in addition to the ~3cf that you calculated in the tank. That's where filling a much larger tank like an AL80 helps to balance everything out in terms of those losses.

Measuring interstage pressure just means putting a pressure gauge on the lines. Since the stages after the 2nd usually have a water trap on them, you can usually find a 1/4" NPT port to put a pressure gauge and check. I don't know what those interstage pressures are supposed to be for that pump though.

 

[zider]

@tbone1004 I’ll try filling an 80 tomorrow and report back what I find. Thank you very much for your feedback!

 

[NAM001]

for inter statge pressures you may have a connection on the moisture separators you can fit a gage to depending on what separators you have. I did this on my bauer to get interstage pressures. You can also remove the drain valve and replace it with fittings thta can accomodate a gage to get those pressures. It is possible you damaged the 3rd stage rings also and they leak. Find the crank case vent tube and measure the flow through it. It should be minimal. In the case of my Bauer it is normally about .7 cu ft / min. If yours is say 2-3 then you have blowby from probably the 3rd stage. As a rule of thumb frm my bauer guy low fioll rate is normally a final stage problem because of the type of rings used on the final piston. Most low fill rate problems are fixed by 3rd poiston / ring replacement. Again per teh Bauer tech I used valves are normally good and seldom need replacing compared to final piston ring assy.

 

[tbone1004]

@zider make sure it gets up to final pressure as well, what @KWS is talking about could certainly be an issue if you struggled with the final stage and the rings are leaking. It may not show up until you get up to max pressure though. You can put the fill whip on a tank valve and just leave it closed to let the pressure build to full pressure if you don't have any HP bottles.

 

[zider]

I filled an Al80 today and it took close to two hours to do so. The pressure would increase about 100psi every 5minutes. I closed off the valve on the filling adapter to test the pressure and was able to get the system up to 3500 before I released the buildup into the tank. Things definitely are not running normal.

 

[tbone1004]

You are likely going to have to redo the final stage rings...

 

[zider]

You are likely going to have to redo the final stage rings...

Are you referring to the 3rd stage rings? The 4th stage piston does not have rings.