The AirGun Guild

General Category => General Airgun Forum Discussion => Topic started by: Christopher on August 29, 2017, 02:28:50 PM

Title: Sealing thread for HPA
Post by: Christopher on August 29, 2017, 02:28:50 PM
Been toying around with the idea of making a plenum extension for my 25 Mrod and will have to seal up some threads against the high pressure air. Anyone have a product or technique that they recommend. Tried just regular teflon tape and it was a no go so I'm reaching out to perhaps someone with some successful experience.

Thanks,
Chris
Title: Re: Sealing thread for HPA
Post by: rsterne on August 29, 2017, 02:49:36 PM
You will need to use an O-ring.... Parallel threads won't retain HP Air, because when you tighten them up, there is a spiral gap along the threads.... That is why pipe threads are tapered, to eliminate that gap when tight.... Teflon tape only acts as a lubricant while tightening (in theory).... although I think it helps seal pipe threads as well.... However, it won't work on straight threads.... There are some hydraulic Loctite products (purple?) that might work, if you let them cure long enough.... but you might never get it apart again without heat....

The normal method for sealing an extension is to have a portion go past the threads, and seal with an O-ring, just the way the fill fitting does.... That way the threads are not under pressure.... One thing to watch is the thickness of the tubing you use below the threads.... too thin and you will have a chance of the extension blowing off the end from the end force on it.... which will be well over a ton.... It will have a tendency to fail at the root of the first thread if the wall is too thin.... and the extension will become a projectile driven by that ton of force....

Bob
Title: Re: Sealing thread for HPA
Post by: Christopher on August 29, 2017, 04:01:56 PM
All the parts that i have gathered for my plenum extension are all high pressure stainless rated to operate at 5k psi. They are all pipe threads too (guess it would have been helpful to state that in the beginning). It would hold to about 800-1000 and that was it. I may try the loctite.  Thanks for the input Bob,

Chris
Title: Re: Sealing thread for HPA
Post by: int3man on August 29, 2017, 04:45:30 PM
All the parts that i have gathered for my plenum extension are all high pressure stainless rated to operate at 5k psi. They are all pipe threads too (guess it would have been helpful to state that in the beginning). It would hold to about 800-1000 and that was it. I may try the loctite.  Thanks for the input Bob,

Chris
Hi Chris,
               Humm  5,000 PSI rated Pipe Threads??  Well I have 30+ years in Hydraulics and I would be interested in seeing those, and the test support documentation to support them. 

If I were going to do this I would use specialty parts supplied from reputable vendors for this specific purpose.  As Bob stated the successful ones in the past have been straight threads backed up with an O-Ring seal, similar to the fill ports.  The Main tube is tapped with straight threads how would you interface with that?

Good Luck and Be careful!!

Michael :- )
Title: Re: Sealing thread for HPA
Post by: Christopher on August 29, 2017, 05:43:19 PM
Here's the link to the part I got https://www.mcmaster.com/#48805k98/=195ovi8 (https://www.mcmaster.com/#48805k98/=195ovi8)

Chris
Title: Re: Sealing thread for HPA
Post by: rsterne on August 29, 2017, 06:06:34 PM
Chris, I think we both misunderstood you.... I thought you meant a tubular reservoir extension, similar in diameter to the reservoir, that made it longer.... For that fitting I would use yellow Gasfitters Teflon tape.... You should have no problem with that, I have used similar fittings (in 1/8" NPT) at 4500 psi.... never had a problem getting them to seal.... The white plumbing tape is too thin, IMO....

Bob
Title: Re: Sealing thread for HPA
Post by: Christopher on August 29, 2017, 06:49:41 PM
Well, I planned on dropping out of the Marauder where the gauge goes with the fitting I posted earlier. Then size up with this bushing https://www.mcmaster.com/#48805k254/=195plga (https://www.mcmaster.com/#48805k254/=195plga) followed by a 6" stainless nipple https://www.mcmaster.com/#4830k178/=195poxd (https://www.mcmaster.com/#4830k178/=195poxd) then with a cap https://www.mcmaster.com/#48805k26/=195pn58 (https://www.mcmaster.com/#48805k26/=195pn58). Then of course put the gauge back in at the bottom of the tee. May not be real practical and certainly not economical, but I was just really itching to see how much power I can get out of my Mrod when I regulate it as opposed to the small plenum of a regularly installed Huma regulator.

Thanks for the responses,
Chris
Title: Re: Sealing thread for HPA
Post by: rsterne on August 29, 2017, 06:54:30 PM
I'm nervous about the pressure rating of the 6" pipe nipple.... The wall thickness at the bottom of the thread roots in a Sched.40 pipe isn't than much....

Bob
Title: Re: Sealing thread for HPA
Post by: caniborrowsomeammo on August 29, 2017, 08:58:16 PM
Bob is correct, that is a low-pressure pipe nipple. I wouldn't chance anything over 300 psi.
Title: Re: Sealing thread for HPA
Post by: Christopher on August 30, 2017, 02:34:36 AM
Ok everyone, thanks for the responses and warnings. Mission aborted.......

Chris
Title: Re: Sealing thread for HPA
Post by: Christopher on August 30, 2017, 02:51:19 AM
What about a CO2 cartridge? What is the safe operating pressure for 12, or better yet a 16 gr cartridge? Also would anyone happen to know the capacity of a 16 gram cartridge?

Thanks again.
Chris
Title: Re: Sealing thread for HPA
Post by: rsterne on August 30, 2017, 08:36:10 AM
That can work.... They are certainly OK up to 1900 psi, because CO2 runs at that at 120 *F.... I used a 16 gr. as an auxiliary reservoir on my .25 cal Carbine Pumper prototype.... Here is the data on it.... Note the internal volume is just over 20 cc, and it uses 3/8" - 24 NF (straight) threads....

(http://i378.photobucket.com/albums/oo221/rsterne/Important/CO2%2016%20gr_zpslpmokahj.jpg) (http://s378.photobucket.com/user/rsterne/media/Important/CO2%2016%20gr_zpslpmokahj.jpg.html)

A "Test Pressure" of 250 bar would indicate a working pressure of 3/5 x 250 = 150 bar = 2175 psi.... The "Burst Pressure" of >500 bar would mean a working pressure of > 500 / 3.5 = > 143 bar, or "more than 2070 psi".... I would not use more than 2000 psi....

Note that the neck can only be drilled safely to about 3/16" or the wall will be too thin for strength.... The same applies to a 1/8" NPT fitting, of course (and if smaller, you should not drill it out).... That is not much larger than the exhaust / transfer port on an MRod (and smaller than it can be drilled to).... so while the extra air in the reservoir will help keep the pressure up at the valve, it won't have a large enough exit to work as a 100% free-flowing plenum....

Bob
Title: Re: Sealing thread for HPA
Post by: Christopher on August 30, 2017, 02:25:54 PM
Hey Bob, hate to go back and kick the dead horse, but back to my original plan. Providing all other parts are the same with the exception of the nipple, what if I replaced it with this thick walled piece https://www.mcmaster.com/#46755k74/=1964s6s (https://www.mcmaster.com/#46755k74/=1964s6s)
Again all just ideas right now but wanting your input and along with any others. The CO2 cartridge probably wouldn't work as I would need a higher set point than what that would be rated for.

Appreciate the responses,
Chris
Title: Re: Sealing thread for HPA
Post by: int3man on August 30, 2017, 02:50:26 PM
I saw a posting somewhere where the gentleman hollowed out his synthetic stock and placed a 16cc reservoir in the void, internal to the stock.  He t'd it off the gauge port.  The issue I see is that the stock gauge port is BSPP thread.  A custom block could be machined with pipe threads and then a standard adaptor used.  You may want to Google Reservoir extensions Benjamin Marauder.

Michael :- )
Title: Re: Sealing thread for HPA
Post by: rsterne on August 30, 2017, 03:28:10 PM
Chris, from what I can see, that Sched.80 Stainless Pipe Nipple should be strong enough (but really heavy).... However, the ID of the 1/8" pipe fittings will still not be large enough to permit the HPA to flow freely into the valve.... because they won't have a through hole any bigger than 3/16" in most cases.... I have a couple of 1/8" NPT 5000 psi fittings here right now, and the through hole is only 11/64".... While you would probably be OK drilling them to 3/16", you WOULD reduce the pressure rating by doing so....

You will get a partial increase in plenum effect with either plan.... but if you are producing big power (likely the reason you want a big plenum) you may still have to bump the setpoint slightly to achieve your goals....

Bob
Title: Re: Sealing thread for HPA
Post by: int3man on September 10, 2017, 10:18:10 AM
Hi Chris,
               Well I tripped over it.  Here is a link to the site where the gentleman installed the Aux reservoir in the stock.

http://www.marauderairrifle.com/forum/viewtopic.php?f=24&t=13168&sid=a0e18e6a67bca3df3a17c7d4f1972f49

I hope that helps you.
Michael :- )
Title: Re: Sealing thread for HPA
Post by: Christopher on September 10, 2017, 07:12:55 PM
Thanks Michael.......

Chris