SpudFiles

Hi all,

I built a combustion potato cannon a few weeks ago strictly out of abs pipe. It ended up being about 7 foot tall and launched potatoes further than I could see. Im now interested in making a pneumatic cannon for reliability purposes, but have a question. No where around my house has pvc pipe larger than 1 1/2" diameter. However, there is abs pipe up to 4". Would abs pipe be safe to use up to 100 psi? That is the absolute limit I plan on firing at.

Thanks in advance,

Anthony

A properly built combustion should be totally reliable every shot as long as the operator does his part.

Technician1002 has used it to make his first qdv, and he pressure tests it once a year. so far its held up fine, of course it wont be as safe as pressure rated pipe but will be fine for under 100 psi

That reminds me that I have not done the test this year. I should have new test photos up later this month for all the cannons in use.


For under 100 PSI, it seems that ABS may be safer than PVC after finding that video of the shop with a hole in the roof. That 4 inch PVC cap blew at only 70 PSI.

Be aware that the threaded clean out cap may have a bad habit of blowing off the breech with high force. A reducer to 2inch and a 2 inch clean out is recommended to reduce the cross threads or blow out of the soft cap. With the wrong bumper in my ABS cannon, the piston used to remove the cap. I used a much longer bumper to reduce the impact and that fixed the problem.

I loved the comment, "Do you think 70 PSI was too much?"

If you find 4 inch PVC, make sure all the pipe and fittings are pressure rated and not DWV.

Technician1002 wrote:For under 100 PSI, it seems that ABS may be safer than PVC after finding that video of the shop with a hole in the roof. That 4 inch PVC cap blew at only 70 PSI.

I think it's also important to note that it was DWV they were using and they also failed to use primer (and by the looks of it, not enough cement either). Also, I believe that was 6", which really means he should have chamfered the pipe ends as well. It took me and my friend the better part of a morning to properly chamfer and glue together his 6" combustion chamber. Still firing fine, even during those hair-standing moments when he decides to whip it out in freezing temperatures to shoot. We used PW fittings and well casing (pressure rated) pipe.

It is true that they did a lot wrong on that launcher. I used it for an example as many builders here are not professional plumbers and may make the same mistakes. The failure mode was the focus of the comment. I think you are right that was 6 inch pipe.

ABS is pretty much slop on the glue and shove it all the way together and hold it until it sets. Even one of my joints that didn't get shoved all the way together has held and is free of leaks. It is one area I check real close during pressure testing. Any sign of a crack or leak and that cannon gets introduced to a saw.

I found some Polyethylene water pipe. I'm looking into doing some heavy duty testing of that. It is one plastic that is rated for use with compressed air. It requires heat bonding instead of solvent welding (glue)

Polyethylene Pipe – The ideal choice
The outstanding physical and performance characteristics of High Density Polyethylene Pipe (HDPE) have made it the ideal piping choice for many applications. Outstanding reliability, excellent chemical and environmental stress crack resistance combined with permanent leak free joints and a 50+ year life expectancy have made polyethylene the preferred piping for transporting natural gas in the United States for over 25 years. The benefits of using HDPE are now being realized in other markets including Municipal and Rural water systems, Forced Main Sewer installations, Geothermal Heat Exchangers and Industrial applications such as Landfills, Mining, Chemical and cooling water transport lines. Let Charter Plastics help you determine the right polyethylene pipe for your application.

Quote from http://www.charterplastics.com/

Tech,

HDPE is pretty common here in diameters 32mm and under so I've also often thought about using it but never have. I know one Aussie member did make a pneumatic out of the stuff, think he took it up to 200PSI. Anyway, had this .pdf file bookmarked if you haven't already seen it, has some good information although is only relevant for sizes above 4".

I haven't seen that one. It does have an obvious bias due to the sponsor of the document.

Not mentioned in the document was that HDPE pipe due to flexibility survives freezing. My dad re-plumbed his motor home because of frost damage.

I replaced the waterline to my house because of galvanic rust corrosion, also not mentioned in the report. An Iron to Copper connection at the water meter will fail over time.

Both were replaced with HDPE to overcome the shortcomings of Iron Pipe.

My intention wasn't to discredit HDPE. While the document is clearly biased in favour of using Iron, I linked it because it had some good information on HDPE itself. I think I remember reading about very large diameter HDPE rated for only about ~200PSI bursting at over 500PSI in hydrostatic tests.

Do you know the failure characteristics of HDPE? Would be interesting to know and an important deciding factor.

I do know that excess sunlight makes it a brittle mess. Old HDPE pails are easy to shatter with marshmallows.

When it is in good shape it will split much the same way ABS does. Due to the failure mode it is used in Natural Gas. In seismic areas the flexibility reduces broken pipe and pulled apart fittings. In wet areas, it does not rust.

Plastic wrap and HDPE bags in a landfill seem to last forever. HDPE under extreme impact does have a crack failure mode.