SpudFiles

Hello All, new to the forum. Built a few simple hair spray combustion guns in my time, and looking to build a pneumatic gun now. Been working on a fairly simple design (2" barrel about 60" long with a 1 inch sprinkler valve to a 4" chamber about a foot long - still playing with GGDT) but I'm still going back and fourth about pipe material.

Where I live (Portland, Oregon) ABS is the name of the game for large pipe. Home Depot, Lowes, etc.. don't even carry 4" PVC - it's *all* ABS. That's what I've made my simple combustion guns out of in the past, but since I want to get this one up to 100 PSI while hand holding, I'm much more concerned about safety.

From what I can tell, it comes down to this:

PVC:
Pro: Pressure rated, actually designed to be used under pressure.
Con: Highly brittle, fails catastrophically, specially illegal for compressed air use in construction as a result. (PVC manufacturers even prohibit the use of compressed air for simple testing!)

ABS:
Con: Not designed to carry pressure at all. Lightly whipped plastic intended for nothing more than gravity drains.
Pro: Flexible, fails more gracefully and more resistant to breakage from drops, etc..

My gut feeling is to go with PVC given that it's actually intended to be used under pressure, even though it's a significant added expense for me. I keep flip flopping though.

What are your expert opinions on the subject? Thanks for your time!

Welcome to spudfiles, nice to see a well thought out first post from someone who has done their research.

NathanF wrote:a fairly simple design (2" barrel about 60" long with a 1 inch sprinkler valve to a 4" chamber about a foot long - still playing with GGDT)

I am almost certain that a pneumatic built to the above specifications used at 100 psi will have significantly lower performance than a combustion of the same chamber length and barrel volume (assuing you're using metered fuel such as propane), especially with refinements like a chamber fan and decent ignition.

Think of the valve restriction for example, a 1" valve on a 2" barrel gives you just 25% of the flow of a 2" valve.

As combustions only have to resist the pressure spike on firing, I'd be much more comfortable using ABS for a combustion than a pneumatic.

It depends on what you want to achieve ultimately, for larger bores and good performance, combustions are infinitely cheaper and easier to build than pneumatics.

My personal advice is this:

Don't expect your cannon to fail. Pressure ratings are there for a very good reason. If you find some pressure-rated PVC and don't run it past the pressure rating you should be perfectly safe. PVC is certainly a lot more dangerous upon failure than ABS, but that's only if you expect what you build to fail. If plastics worry you, make something out of metal.

jackssmirkingrevenge wrote:As combustions only have to resist the pressure spike on firing, I'd be much more comfortable using ABS for a combustion than a pneumatic.

Hear hear! For a combustion I'd go for ABS, since is is unlikely either will fail, the ABS wins based on a preferred failure mode.

I've done some pressure testing of the not rated for pressure ABS. Since the two layers on the foam core are both thicker than the skin of a 2 liter pop bottle, I was wondering if it was indeed as tough as a pop bottle. For my testing results, see the link in my sig to the ABS cannon. For lower pressure such as below 100 PSI, it should be fine.

As mentioned above, use a larger valve if possible. I'm near Portland if you would like to see the ABS cannon in person.