Rix SA6 Rebuild / Blowby

[robmwpropane]

Good day all. I just got done rebuilding my Rix SA6. Everything came apart fine, everything went back together fine, followed the manual. Fired right up, builds pressure no problem. But when I turn it off I get a good amount of blowby from the 3rd stage piston. This is a brand new piston, brand new rings, installed with one of their proprietary 3rd stage tools.

The only thing I can figure is maybe it takes some time for the teflon to "seal" the walls? I did clean the cylinders with soapy water during the rebuild. I've let it run for 15 minutes, but still get the blowby after shutdown. I'm a little nervous to continue in case there's a chance I'm damaging something.

Any thoughts? It seems to build pressure just fine (I have interstage gauges on 2nd and 3rd), so I don't think I'm getting blowby during operation, just when it shuts down?

Any thoughts??

[robmwpropane]

So after some back and forth it seems like my 3rd stage head got dirty with stuff coming through the compressor. I have to take the heads back off and see what I find. Already removed all the pistons and they look good. 3rd stage head looks really dirty. It's bleeding back into the 2nd stage. Maybe an o ring got messed up during install, not sure.

[SurfLung]

If your 3rd stage is leaking back into the 2nd stage, it could be the reed valve in the head is dirty. I had this on my RIX SA3. The instructions say to clean the reed valve with Joy dish washing soap but this didn't get it clean enough... It still leaked. So, I got some 1500X emory paper and laid it flat on a glass widow pane. I very gently wet polished the reed valve until the carbon was gone. This solved the leakage from 3rd stage to 2nd stage.

BTW... I am paying this solution forward... Bill AntiqueDiver Tucker solved this for me.

P.S. I re-ringed my RIX SA-3 a few times without replacing the "Rider" rings. I have since learned that the Rider rings are very important for keeping the the actual piston rings aligned and sealed. Those teflon piston rings are very delicate and unforgiving. So, don't forget to replace the rider rings when you replace the piston rings.

[robmwpropane]

Gosh @SurfLung, that was a great piece if information. Thank you very much for passing it along.

Iain had posted on Scubaboard about swapping the reed valves around so they're acting the opposite from when it was put together. I had not read that before bu my it makes sense. I'll try that as well.

I'm still a little confused as to why I'm getting so much blow by from the 3rd stage piston but maybe when I fix the head it'll seal and I won't get as much. Maybe I always got blowby before but it was just a small puff as the cylinder emptied..

[SurfLung]

Regarding Blow-By... I mentioned the fragility of the teflon rings and the need for new rider rings... Here's my experience and procedure:

When I did my first re-ring, I skipped replacing the rider rings. I just didn't understand the purpose for them and figured I'd save money by not replacing them. I actually re-ringed the 3rd stage a couple of times and always found most of the rings stuck in the compressed state... In other words not sealing. I developed a "refresh" process where I simply removed the whole piston and washed it and gently got the rings moving and expanding again. This then had to be blown dry and allowed to completely dry before putting the piston back into the compressor. At some point, I realized that the O-rings under the teflon rings were not applying very much resiliency to the seal. And I suddenly realized that the piston was tipping with each piston stroke... causing the teflon rings to lose and regain their seal with every stroke. Probably causing more wear and heat... and teflon dust to gum them up and make them stick in the compressed state. And I realized how important the rider rings are to keeping the piston aligned... so that the teflon rings maintain their seal throughout the piston stroke. This is seemingly a simple concept but it took me several years of re-ringing and even replacing one 3rd stage piston that had broke in half... to finally understand this aspect of the RIX design.

Still, the teflon rings are far more delicate and far less springy than the metal rings in oil lubed compressors. So, I don't leave them under pressure any more than necessary. When I turn the compressor off, I immediately relieve the pressure by opening the drains. That's my shut off procedure.

My start up is also worth noting. The 3rd stage is a floating piston (as you know). And it doesn't come to rest on the end of the piston rod until the pressure is high enough to hold it there. It can get hammered back and forth until it settles. So for my start up procedure, I close the drains before turning the compressor on. This forces the pressure to build quickly and settle the 3rd stage piston onto the piston rod quickly with the least amount of hammering.

[robmwpropane]

Thanks for that. My 3rd stage piston is brand new, loaded by Rix themselves and shipped inside of the tool. The 1st time it ever saw daylight was being inserted into the machine.

I still have my old piston, and actually added new rider rings and teflon rings, but then I got the new piston so now the old one just sits.

I need to pull the 3rd stage head apart and clean it again and see what that does for me. I really hope that's it.

I also run mine the same way, start with towers closed and empty asap after turned off.

[SurfLung]

The teflon rings DO get scummy and sticky. I think my "refresh" idea is a good process to do once in awhile to keep the rings free and clear to expand and seal reliably. I learned the hard way to make sure they're completely dried off before reinserting into the cylinders. (moisture makes them scum up faster).

One other thing I detected... My 1st stage piston had allowed the big teflon ring to compress so far that the ends overlapped one too many times. This created a too thick section that made the big ring stick in the piston groove in the compressed state... compromising the 1st stage seal. I was hearing leakage when I turned off the switch and initially thought it was the 3rd stage. I forget how I traced it to the 1st stage. But sure enough that's where it was coming from. I had to shorten and bevel the ends of the 1st stage ring slightly to remove the overlap and stop the ring from sticking in it's groove.

I have found the oil-less teflon ring system to be delicate and sometime finicky. But when it works, it DOES work brilliantly. And, compared to old style oil lubricated scuba compressors, it was definitely a step up in air quality and purity. But the modern improvements of filtration media and PMV valves have raised the air quality and purity of oil lubricated compressors so much as to render the RIX Oil-less system obsolete (IMHO) or at least not necessary.

For example, I bought an Alkin W31 Verticle compressor a couple of years ago and it rates it's filtration life at up to 50 hours. I think it's alot tighter so that less oil gets into the condensate. But it also has lot's more intercooler capacity so that the 1st and 2nd condensation towers remove virtualy 100% of moisture and oil vapor. The final (3rd) condensation tower houses the filtration media. Virtually nothing is left to drain out the bottom of this tower and the air gets purified and dried to as good or better than the oil-less RIX compressors... without all of the fussing over delicate teflon rings.

I still love my RIX SA-3 and use it for my PP Nitrox blending. But I think the Oil-less advantages are nowhere near what they used to be.