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DWV-Like error in hybrid construction?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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DWV-Like error in hybrid construction?

Unread postAuthor: Moonbogg » Wed Jan 11, 2012 10:50 am

So, I need to humble myself a bit to post this, but I figured id get your opinions. I realized a while ago that the aluminum pipe and tube I used for my venom 2x hybrid was NOT seamless! Oops? Yeah, id say. The website I bought it from at the time said all their tube and pipe was seamless. That was a severe LIE, as the ASTM spec for the tube they sell is indeed structural and not seamless. The aluminum is split and flows around the die when its is extruded, and rejoined in a pressure and heating chamber and welded back together to form a hard to see and very smooth seam or seams, where as seamless is extruded around a special die resulting in a single, non seamed piece.
The seams on properly made aluminum tube and pipe are not visible to the eye, so it looked good to me. Upon further research I realized that there is indeed a seamless variant for "reliable" high pressure use.
Anyway, the question is, would you still use an aluminum 2x hybrid if the tube and pipe was structural rather than seamless? 2X pressure isn't THAT much higher than 1X and many people use DWV plastic for 1X without issue, so maybe it will continue to be fine. What do you think?
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Unread postAuthor: Gun Freak » Wed Jan 11, 2012 11:02 am

Its hard to say when we don't know the size... Like diameter, wall thickness, and stuff. I'd probably say yes though.
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Unread postAuthor: Moonbogg » Wed Jan 11, 2012 12:03 pm

Gun Freak wrote:Its hard to say when we don't know the size... Like diameter, wall thickness, and stuff. I'd probably say yes though.


If you click "venom" in my sig, you will see a drawings of the cannon. Chamber is 4 1/2" OD x 3/16" wall. Barrel is 1.87ID x .065 wall.

6061-T6
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Unread postAuthor: SpudBlaster15 » Wed Jan 11, 2012 12:49 pm

Given that the pipe was presumably annealed and artificially age hardened to a grade of T6 following forming and welding, I wouldn't expect a significant loss of strength over a seamless extruded piece.

With the dimensions you've provided, you're looking at a burst pressure somewhere over an order of magnitude higher than the maximum adiabatic combustion pressure at 2X, so there's nothing at all to worry about.
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Unread postAuthor: jrrdw » Wed Jan 11, 2012 1:10 pm

Can you get the data sheets for the pipe you have? If you bought it from/in the USA and it's labeled for sale as "structural" the seller has to maintain and provide them upon request of the customer. You can get the ratings from the data sheets.

To answer your question, it really depends on the data and what the parts look like after inspection after use. Clean the pipe up and look for stress cracks, chipping/pitting around the edges of the exhaust port. Check it for 'round' still holding it's shape or getting oblong/oval shaped. Is the pipe growing any tumers/bulging any where etc...
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Unread postAuthor: mark.f » Wed Jan 11, 2012 1:25 pm

SpudBlaster15 wrote:With the dimensions you've provided, you're looking at a burst pressure somewhere over an order of magnitude higher than the maximum adiabatic combustion pressure at 2X, so there's nothing at all to worry about.


x2, people have used welded pipe at much higher mixes.
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Unread postAuthor: Gun Freak » Wed Jan 11, 2012 5:14 pm

Galvanized is has a seam and it holds thousands :)
I would not be worried at all.
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Unread postAuthor: mobile chernobyl » Wed Jan 11, 2012 6:07 pm

I use 3" welded Sch10 in my 10X

This may not hold true for all pipes, but take a look at this document.
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Unread postAuthor: Moonbogg » Wed Jan 11, 2012 10:36 pm

Thanks for the feedback. I will continue to use it. Even if it did fail, I couldn't immagine it fragging like PCV. That document uses an 80% value for pressure rating compared to seamless. If that is anything like true for my tube, then the cannon should still have a reasonable safety factor. Don't worry though. If it explodes i'll totally take pics and show you guys.
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Unread postAuthor: MrCrowley » Wed Jan 11, 2012 11:03 pm

The barrel I use isn't seamless and it's the same wall thickness as yours (IIRC). Although it is steel and not aluminium, it's burst pressure is over 4000PSI, I believe.

You'll remember I used ABS, rated for something like 220PSI, as a barrel at 10x mixes with heavy projectiles. I wouldn't be too worried about using your barrel, even at 10x mixes with heavy projectiles.
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 12, 2012 1:24 am

PVC cartridge at 6x;

[youtube]http://www.youtube.com/watch?v=w8MZ5aS2-JA[/youtube]

You'll be fine ;)
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Unread postAuthor: Labtecpower » Thu Jan 12, 2012 4:25 am

this tube has survived a 12x mix, it was the connector that exploded.
It has an OD of 30 mm, and walls of 1 mm.
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