Troubleshooting Air cannon

[andyM96]

He there new to this forum

I am a newbie to pneumatic air cannons and so have just built my first one, which unfortunately isn't working. It uses a 2" chamber and 2 1/2 barrel connected to a 'U' shaped 1" rain bird ASV 100 valve. When I activated the solenoid I get more of a 'horn' type noise rather than a lot of force and so the projectile simply falls out the end.
Any ideas how I could sort this problem?
Ive heard about modifying the valve with a blowgun etc. would this help with the problem..?
Any answers would be much appreciated.

[Boomer58cal]

Mod the valve, it will probably fix the problem. Read this --> http://www.spudfiles.com/how-to-database/topic305.html

U shape valves generally don't flow as much as straight through valves.

[jackssmirkingrevenge]

What sort of pressure are you using?

Are you triggering the solenoid with enough voltage?

As per above, modifying it for direct pneumatic actuation will give you the best performance.

[andyM96]

Okay thanks I will look at modifying the valve. Hopefully that solves the problem!

And to the other question, the pipe and valve is rated at at 150psi.
When testing I had put in 60psi and the tennis ball simply fell out the end and went a metre or two.
And Using 24v dc which seems to operate the solenoid fine. Although when operating at 60psi I was able to open the valve twice and get two air horn sounds with the one load, which makes me think the valve isn't opening fast enough to release it all.

[jackssmirkingrevenge]

Basically what happens is that the pilot area of the valve is dumped, but refills quickly, hence the "honking" - modifying the valve for pneumatic actuation should do the trick.

[GonzoInferno]

Your chamber is 2". Your barrel is 2 1/2". You are flowing the air charge into a larger space removing a bit of back pressure. Couple that with a sluggish response from the valve and that can give your results. Remember that sprinkler valves are designed for water which has a lot more mass than air. A large and violent rush through the valve is not desired for irrigation. That's why we mod them. The valve body and internals can handle 150psi, but beyond 200psi they start coming apart.
I try not to make multiple changes in design if I can so I can learn which modification has which effect.
Keep us posted!!

[andyM96]

Thanks for all the tips guys! Il go ahead and purchase the parts for the mod and hopefully that'll do the trick! Il you keep you posted with my results! Cheers

[Technician1002]

Basically what happens is that the pilot area of the valve is dumped, but refills quickly, hence the "honking" - modifying the valve for pneumatic actuation should do the trick.

In many cases this is almost correct. The real reason for honking in unmodified valves is the rate the pressure drops in the pilot vs the pressure the valve opens and the fast rate the main chamber drops pressure.

Example, A sprinkler valve due to the diaphraghm will open when the pilot is at 80% of the chamber pressure and will close when the pilot is at 90% of the chamber pressure just to pull numbers out of thin air for the example.

Start at 100 PSI. Pilot and chamber are at 100 PSI. Pull the trigger and the pilot bleeds to 80 PSI and the valve opens. Chamber quickly changes to 70 PSI while the pilot drops from 80 PSI to 78 PSI as the pilot drop was much faster than the pilot is falling. Valve closes. Pilot continues to drop. At 80% of 70 PSI or 56 PIS, the process repeats. See a trend down to zero? Open Close, open close, etc. HONK!. The solution is to increase the speed the pilot pressure drops so it does not close again at highter pressures. Closing at 10 PSI is fine. Shot is over. Closing during the shot steals power from the shot.

The reason I chose 70 PSI for valve closing is simply due to the valve is too slow to close precisly at the point the vavle begins to close. It has some overshoot while closing.

[andyM96]

Parts arrived today and it's been assembled and tested.
Tennis ball shot out pretty reasonable at 50psi, not great but a big improvement. A soda can works very well and when shot vertically it went at least 10m.
Heading down to the park later for some proper testing so will get better results then.

[Boomer58cal]

Parts arrived today and it's been assembled and tested.
Tennis ball shot out pretty reasonable at 50psi, not great but a big improvement. A soda can works very well and when shot vertically it went at least 10m.
Heading down to the park later for some proper testing so will get better results then.

As long as you used sch 40 or better pipe you should be able to go as high as 100 or 120 psi safely. Then you should get some good range.