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BPR/PMV Rebuild

Posted: Mon Apr 02, 2018 9:21 am
by SurfLung
Rebuilding my Back Pressure Regulator (BPR/PMV)
- Last Fall when I disconnected the filter array from my compressor (so I could bring the compressor indoors for the winter), I noticed that there was air coming out of the BPR when I turned on the compressor. It's supposed to hold back air until the pressure reaches 1800 psi... This is in order to condense nearly all of the water out of the air before it reaches the filter array. Leaking here is not good. So, I ordered a BPR rebuild kit from August Industries on line. Complete illustrated instructions are included. Here's a "BEFORE" photo before I did the re-build.
- Here's another photo that shows the original configuration of air inlet, gauge and air outlet on the bottom of the BPR. Note the label on the side of the BPR says "1500 psi". The manual says 1800 psi and that's what it was actually running on when I did the BPR video last year. The rest of the parts you see to the left of the BPR are a check valve, a 3600 psi safety release valve, and a small bleeder valve.
- I have been bleeding some water out of the line using the little bleed valve at the lower left... So now I'm looking at the air outlet on the bottom and thinking, "Why should the air outlet be at the bottom where water is likely to drain?" Hopefully the reason there was still water in the line is because the BPR was leaking and after re-building, it shouldn't have water in the line anymore. Still, I resolved to put the air outlet on top of the BPR when I put it all back together. Okay so I disassembled the BPR and this is what I found (below).
- As you can see above, the old BPR valve seat has a lot of corrosion and is missing a backing washer for the grease packed O-Ring that seals it. The O-Ring is supposed to be packed with silicone grease because the seat actually moves in and out on its shaft. I cleaned up the inside surfaces of the housing and installed the new seat parts. I re-arranged the plumbing to put the gauge where I can read the pressure coming out of the compressor. And while I was at it, I changed the air outlet position to the top of the BPR as described above.
- The last step in the rebuilt process was to adjust the BPR to hold back air until the pressure reaches 1800 psi. So, I turned on the compressor and watched the needle go up to 500 psi and stop. Air was coming out. Turned in the BPR adjustment screw and it shut the air off until the needle climbed a few more hundred psi. I repeated this until it held back air up to the 1800 psi RIX specification. I then tightened the lock ring on the adjustment screw, turned off the compressor and drained the pressure with the drain valves. Closed everything up and turned on the compressor. It held back the air with no leaks right up to the 1800 psi mark and then released the air... maintaining the 1800 psi pressure level. I repeated this test a couple of more times to be sure it worked and the 1800 psi setting was consistent.

- Final Thoughts... The RIX SA3 and SA6 are named "SA" for "Sweet Air". If I understand correctly, the BPR and two water condensation towers removed water well enough that the Navy divers filled tanks right off the compressor with no additional filtration. The air was "Sweet" because the oil-less compressor had no oil induced taste or smell AND the air was slightly moist and comfortable for breathing... i.e. No dry mouth. This is why (I think) the pressure gauge was mounted on the BPR... Because the whip came off the BPR and the gauge read the tank pressure. Probably also why that check valve is there in front of the BPR... So that tank pressure wouldn't feed back into the compressor.
- I don't think there's any reason for me to put the gauge on the BPR because air goes from the BPR to my filter array, which has its own tank filling pressure gauge and check valve.

Re: BPR/PMV Rebuild

Posted: Mon Apr 02, 2018 2:14 pm
by ScubaLawyer
Hi Eben, nice write up.

As you may or may not know, I know less than zero about compressors. Yeah, I use to fire up a small gas portable on a friend's boat and also fill from banks at a friend's shop years ago. I've read compressors 101 in the manual section about 50 times but I can sense large gaps in my knowledge. I'm the kind of guy that needs to conceptually visualize each step and process therein from intake to tank fill, how the various stages and filters work, etc... I guess I could just buy a used one, tear it down, and figure it out that way but I'd like to get some book-learning in first. Anyone have any recommendations as to what I should read? Preferably something with lots of pictures and diagrams for my simple mind. :D Thanks, Mark.

Re: BPR/PMV Rebuild

Posted: Mon Apr 02, 2018 3:30 pm
by SurfLung
Antique Diver gave me a link to the instructions for servicing and/or rebuilding my Back Pressure Regulator. These are the same instructions that came with the re-build kit:
http://valvesandregulators.aquaenvironm ... 20906R.pdf

Mark, I think the sophistication of compressor knowledge presented on this forum is beyond what most compressor owners and operators actually know. I've visited with a few private compressor owners and I think for the most part, they just fire the thing up and pump air. Most don't have a deep understanding of the BPR/PMV function, for example. Changing oil and filters is all they do. Servicing and repairing they leave to professional technicians. I think the same goes for most dive shops.

The only reason I've had to learn so much technical detail is because I bought a used compressor and had to re-build it. Somewhere along the way, I got fascinated and began to enjoy learning more advanced aspects of compressor ownership. Fortunately, I had a heck of a lot of help from Antique Diver and others. But I think if you buy a new compressor you probably won't have to know the thing inside out anymore than you're expected to know your car or your boat motor.

BPR = Back Pressure Regulator
PMV = Pressure Maintenance Valve
I think they both accomplish the same thing but they may look different.

Re: BPR/PMV Rebuild

Posted: Wed Apr 11, 2018 7:53 pm
by SurfLung
I did a test run and filled a steel 72 from zero to 2250 psi. Drained the moisture towers every 10 minutes and they gave water. Drained the bleeder and moisture tower after the BPR and they gave nothing but air. That's the way it is supposed to work so I'm happy.

Re: BPR/PMV Rebuild

Posted: Sun Apr 15, 2018 11:05 am
by couv
Eben, et al,

Agree....very nice write up. Never-the-less I still have (and always will have) a few questions. Please talk slow.

The Compressor 101 article mentions, "To remove 99.3% of the water present in the air, the compressor MUST have a PMV, and it must be set to at least 140bars/2,000psi."

1. Why would Rix recommend a lower pressure?

2. 101 also mentions, " the Compressor Pressure Maintaining Valve (PMV) located after the Main MS/AC air filter" is this a separate PMV/BPR?

3. The moisture separators: Are they simple mechanical devices that swirl, whirl, and twirl the air to remove water or do they contain an element or MS that requires periodic changing?



Re: BPR/PMV Rebuild

Posted: Mon Apr 16, 2018 3:45 pm
by SurfLung
Good Questions Couv!
1- Why only 1800 psi in the BPR? I suspect it might have something to do with what's best for the compressor mechanism itself. The 3rd stage has a floating piston that stays in contact with the piston rod thru nothing more than the air pressure. That, plus the rather high RPM of 2300 might mean less wear and tear at the 1800 psi setting. The BPR is the same one used by Bauer compressors and the Bauer spec is 1800 psi as well.
2 Where should the filter tower be located? I looked up my RIX SA3 to see where the factory specs the filter tower should go and found this diagram: It shows a top view with the two moisture separators at the top left. Then it shows the fill whip coming out of the 3rd stage separator and going directly to the rectangular BPR (Back Pressure Regulator). The BPR has a pressure gauge on one outlet and the fill connector on the other. The fill connector has a bleeder valve between it and the BPR. And in this case, the BPR is also serving as a check valve to keep tank pressure from entering the compressor. In THIS configuration, the air goes directly from the moisture separator to the tank with NO FILTER TOWER!
RIX Moisture Removal Air Purification System
- This is an Add-On offered by RIX in a few difference sizes, "The RIX Moisture Removal Air Purification System is designed to further remove moisture and purify the air downstream of the final moisture separator on the compressor." And the drawing indicates that it goes before the BPR in the arrangement of separators and filters.
No. 6 is a Check Valve...
- Note that there is a check valve before the Filter tower (and after the 3rd stage moisture separator). So, separating the filtration tower(s) from the moisture separator with a check valve or PMV or BPR is an accepted way to go at least from the RIX SA3 manual.

3 - There is nothing in the moisture separator to change. My separators have a kind of cyclonic air direction intended to help separate out the water by centrifugal force. The air then goes out through a screen. I cleaned the screens when I was replacing the 3rd stage separator but I don't remember it being all that dirty and I don't think cleaning the separators is considered regular maintenance like changing filters and lubrication.

4. BTW - The Scuba Pro 209 that you upgraded to GP250 for me is working great. It's my wife's regulator and she loves it. :)

Re: BPR/PMV Rebuild

Posted: Thu Feb 07, 2019 11:28 am
by SurfLung
Why 1500 psi and not 1800 or 2000 psi on the RIX BPR setting?
- I've been having an interesting correspondence with Eric Connally, the tech Support engineer at RIX, and it answers Couv's question as well as clearing up some things for me. It may also begin to explain the difference between the BPR and PMV.
- Eric explained that the main function of the Back Pressure Regulator in the RIX is NOT to knock out moisture but instead, to quickly build pressure in the compressor so as to minimize hammering the 3rd stage piston rod against the floating 3rd stage piston. And for this purpose, the 1500 psi setting is "sufficient"... No need for a higher setting.
- Now here's where it gets interesting. Eric says the amount of moisture knock out before and after the BPR depends on the pressure difference between the BPR setting and the starting pressure of the scuba tank. It has to do with the condensation that occurs from the amount of expansion of the compressed air into the scuba tank you're filling.
- If I understand this right, you will get more expansion, cooling, and condensation after the BPR when you are filling a totally empty tank than you would from filling a tank that is partly filled.
- Taking it one step further, Setting the RIX BPR for 1500 psi rather than 1800 or 2000 psi reduces the difference between the before and after pressures at the BPR and therefore reduces the expansion, cooling, and condensation after the BPR.
- In other words, the 1500 psi setting is the MOST EFFICIENT SETTING for a BPR on the RIX SA3e

This does NOT contradict the RIX SA3E Instruction Manual: "... The regulator will improve moisture separator efficiency and filter life as much as 450%. This is done by maintaining pressure in the separator and filter at 1500 psi or more when tank pressure is lower."

Finally, the way I have my filter array set up between the BPR and the scuba tank puts a PMV after the filter array, so there's no pressure after the BPR whenever I start up my compressor... It's as if I'm always starting with an empty tank. So, there's nothing wrong if I have moisture to drain from the BPR when I'm filling tanks.

Re: BPR/PMV Rebuild

Posted: Sat Feb 09, 2019 12:30 am
by couv
Thanks for the follow up Eben.