SpudFiles

Im planning to build my first piston valved potato (nature friendly) cannon. I have some experience with piston valves from my dart gun build which some of you may remember (see avatar). As of now I am not sure on the exact design as I am pricing it out and trying to see what pressure rated parts I can even obtain or justify the price of. I did manage to run into a Master Plumber at Menards who said he could order me pretty much anything and has some large diameter scrap that I could have/buy cheap. My motto as always is that bigger is better but as you all know PVC jumps up fast in price in the larger diameters.

At the moment im thinking 5'+ long 2-2.5" barrel coaxial design with a 6-8" chamber. I also plan to use a modified sprinkler valve as the pilot.

My questions are:

1. What is the max safe PSI for a pressure rated cannon?

2. What barrel:chamber volume ratio is ideal?

3. Are all reduction bushings pressure rated? I was reading the wiki on PVC and it seemed to hint as to such but the ones I found were in a rack labeled DWV.

4. What is the ideal piston travel distance to allow good flow but not excess pilot volume?

5. I have read that some people claim that coaxials dont flow as well as T style valves. Is there any truth to this? Its seems counter intuitive as the t style needs to turn 180 degrees as well and the coaxial can have air enter the barrel from 360 degrees around it.

Thats all for now. Thanks all in advance!

if your going big chamber might as well go big barrel unless you want the chamber really "squat".

I would go with a barrel 1/2 the diameter of your chamber as for your questions:

1) i use about 60% the pressure rating of your weakest part

2) depends your definition of "ideal", i would fiddle around with ggdt a little bit, but i usually use a 1 1/2 to 2 c:b ratio

3)try to keep the answer simple, no, always check for a pressure rating or an NSF:PW mark

4)it is said diameter/4 is perfect but i usually go slightly over that to make up for any mistakes or flow constrictions that i don't think about

5) some people say this because a coaxial has a very abrupt turn, not a smooth continuous turn like a t-valve has, but i am not an expert in this area so i will let someone else claim this one

1) Most people on this forum refuse to take PVC launchers above ~100psi. PVC is quite brittle and can crack easily due to the repetitive strain of a piston slamming against it. This can be rectified with a good bumper, such as a small section of air hose. Sprinkler valves are also only rated to about 120psi, so you won't be able to take the pressure higher than that.

3) While DWV fittings can apparently take about 100psi of pressure, I would highly recommend finding something that is in fact rated. You wouldn't want the reducer along with the barrel to unexpectedly burst out and fly downrange.

4) As stated above, diameter/4 is considered the optimum amount for maximum flow. What are you using as a piston? If your using something like an end cap you will have more pilot volume due to the space in the cap, but if the piston is solid you will have less pilot volume. Also what size sprinkler valve? For a 6-8" chamber you will need something fairly large to vent the pilot area and pilot the piston.

5) It is true that coaxial configurations have less flow than an equivalent barrel sealing piston launcher, however not by much and the difference in power should be negligible for a lower pressure launcher such as this one. Both configurations work, and thats all that really matters I think.

Hey man, just a couple of things to say:

If your definition of ideal c:b ratio is efficient then a 1.5-2 is way overkill. In MOST cases a .8 will give most efficiency. note NOT power; Efficiency.

A piston in a O.D of 6-8" is going to be mighty heavy as you will need ~ atlesst need 3.5" lengh in it so it doesn't jam. This weight will put very large strain on pvc parts with out a very good bumper.

Take the cannon to the least rated part, assuming there's no reason to believe the strength has been compromised. A.K.A it's been dropped.

tghhs wrote:If your definition of ideal c:b ratio is efficient then a 1.5-2 is way overkill. In MOST cases a .8 will give most efficiency. note NOT power; Efficiency

No. Efficiency in pneumatics is usually achieved with a far lower ratio than that, but it depends on pressure. 0.8:1 only really applies as a minimum ratio to use on a combustion...

To counter rikukiakuchiki777:

1) Not quite sure where you got this. People are generally happy in the 100-150 psi range with PVC launchers.. 100 psi is about the "minimum maximum" - in other words, generally the highest pressure most people are confident with.

4)D/4 is not "the ideal". It's the calculated absolute minimum for the flow area opened by the piston to be the same as the valve flow area. As a value, I would place little worth on it other than "do not use less than this value".

5) Co-axials do not automatically have lower flow. In their case, while the air has to turn a sharp 180 rather than two more leisurely 90 bends (so less ideal), it can indeed flow into the valve from all angles with less restriction.

I personally rate the two as pretty identical.

And that's why i try to make my answers vague and based on what i use not what is perfect, shall i get the wrath of Ragnarok