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Too large of a pilot valve?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Too large of a pilot valve?

Unread postAuthor: deathbyDWV » Thu May 21, 2009 3:45 pm

I was wandering if anyone knew if the pilot valve for my four inch co-axial could possibly be too big? At first when i was making it it wouldn't work but then i constricted the airflow of the pilot by putting on a 1" x 1/2" 90 degree elbow on and it worked. That is how it has been working ever since i finished it but i was wondering if i might be decreasing the preformance. The cannon can be found here so you can take a closer look at it.

http://www.spudfiles.com/forums/4in-coaxial-cannon-t18237.html
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Unread postAuthor: jrrdw » Thu May 21, 2009 3:59 pm

I've never heard of to much flow from a pilot valve, but anything can happen. Try it again without the elbow and see what happens. Sometimes it takes a few tries to get a piston working.
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Unread postAuthor: dewey-1 » Thu May 21, 2009 4:15 pm

I agree with jrrdw.

I have used a 1 inch sprinkler to pilot a 1.5 inch sprinkler valve.
I had to because the blow gun was to restrictive on the larger valve.

Look here;

http://www.spudfiles.com/forums/viewtop ... tml#127106
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Unread postAuthor: Technician1002 » Thu May 21, 2009 5:48 pm

dewey-1 wrote:I agree with jrrdw.

I have used a 1 inch sprinkler to pilot a 1.5 inch sprinkler valve.
I had to because the blow gun was to restrictive on the larger valve.

Look here;

http://www.spudfiles.com/forums/viewtop ... tml#127106


I don't.

Here is why. It is not the size of the pilot that is the problem. The problem is something about the sprinkler valve used. They only snap open if when they open, the air pressure rise slams the diaphragm out of the way. The diaphragm may be cracking and riding a volume on the spring which is just enough to keep up with the piston leakage, but not enough drop to unseat it.

The added restriction on the sprinkler valve causes it to pop open against the spring.

To fix it, either reduce the piston leakage, lube the piston, reduce the sprinkler valve spring tension, or use a ball valve. The pressure is simply not dropping enough to unseat the piston. The blow gun was the hint the equalization leakage is high. Too high.
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Unread postAuthor: jrrdw » Thu May 21, 2009 6:24 pm

I wasn't going to type this out, but...

1" x 1/2" 90 degree elbow
All that is really doing is redirecting the air. I doubt that it's putting enough of a restriction to make that kind of difference.

Like I said in my 1st post, "Sometimes it takes a few tries to get a piston working." Which is probly the case being as how rarely does any piston work the 1st time.

It's just par for course. :P
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Unread postAuthor: dewey-1 » Thu May 21, 2009 6:37 pm

Technician1002 wrote:
dewey-1 wrote:I agree with jrrdw.

I have used a 1 inch sprinkler to pilot a 1.5 inch sprinkler valve.
I had to because the blow gun was to restrictive on the larger valve.

Look here;

http://www.spudfiles.com/forums/viewtop ... tml#127106


I don't.


Tech;

Are you disagreing about too large of a pilot valve or the fact that he has to restrict his pilot valve by adding a 90 degree elbow?

Maybe I am not comprehending something.
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Unread postAuthor: Technician1002 » Thu May 21, 2009 7:36 pm

dewey-1 wrote:
Technician1002 wrote:
dewey-1 wrote:I agree with jrrdw.

I have used a 1 inch sprinkler to pilot a 1.5 inch sprinkler valve.
I had to because the blow gun was to restrictive on the larger valve.

Look here;

http://www.spudfiles.com/forums/viewtop ... tml#127106


I don't.


Tech;

Are you disagreing about too large of a pilot valve or the fact that he has to restrict his pilot valve by adding a 90 degree elbow?

Maybe I am not comprehending something.


I'm saying the pilot size is fine, so yes, I am disagreeing. The sprinkler valve is needing a little back pressure to operate. The valve is not snapping open to operate the main piston.
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Unread postAuthor: jrrdw » Thu May 21, 2009 7:50 pm

The sprinkler valve is needing a little back pressure to operate.


Back pressure in a sprinkler valve used for piloting???

Please explain how there is back pressure to begin with?
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Unread postAuthor: mark.f » Fri May 22, 2009 8:29 am

I think we need a better look at your piston.

If it leaks at a significant portion of your pilot valve's flow, you will get slow actuation and possibly bouncing, but you also have the chance that it may not actuate at all.

A sprinkler valve is a good pilot for a 4" pistoned valve. I think you're most probably having problems with the piston itself.
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Unread postAuthor: Technician1002 » Fri May 22, 2009 11:32 am

jrrdw wrote:
The sprinkler valve is needing a little back pressure to operate.


Back pressure in a sprinkler valve used for piloting???

Please explain how there is back pressure to begin with?


In simple terms, when the sprinkler valve starts to open, more of the diaphragm is exposed to the inlet side pressure, so it tends to have more opening force as it opens. When used as a pilot for another piston cannon, often the pressure drops rapidly before the sprinkler valve produces much pressure on the outlet side.

This drop on the inlet side coupled with little rise on the outlet side may simply make the sprinkler valve hold pressure (in the piston pilot area) and maintain it with the spring in the sprinkler valve. This pressure may be high enough to be above the pressure required to hold the piston valve closed so it doesn't fire.

Adding a restriction (added elbow) to the sprinkler valve adds restriction to the flow from the sprinkler valve, enough to pop it open. Once it pops open, the inlet side is permitted to drop to a new lower value, opening the piston valve.

The problem wasn't too big a pilot area. The problem was the sprinkler valve working as a "pressure relief" instead of popping open.

I hope this helps.
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Unread postAuthor: psycix » Fri May 22, 2009 1:31 pm

I think technician is on the right path. The sprinkler diaphragm empties the pilot area of the gun faster then the blowgun can pilot the sprinkler. Result: the diaphragm slams back.
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Unread postAuthor: deathbyDWV » Fri May 22, 2009 7:00 pm

Yeah the piston is a 3" cap but it had room on the sides so i glued a strip of velcro around it and then it fit fairly tightly. That is how i have hade it ever since i finished it. I did build it where i can take out the piston so any ideas on improvement are welcome. :)

EDIT: check it out i got a damage video up!!! :D
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Unread postAuthor: mark.f » Fri May 22, 2009 9:10 pm

Why not try something a little more seal-worthy to wrap the piston in, like duct tape. May sound like a slap-together, but that's how all my piston valves worked up until earlier this year...
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Unread postAuthor: Technician1002 » Fri May 22, 2009 10:17 pm

mark.f wrote:Why not try something a little more seal-worthy to wrap the piston in, like duct tape. May sound like a slap-together, but that's how all my piston valves worked up until earlier this year...


Even a wide strip of Velcro, anything to provide friction to the leakage will help. A very weakened spring in the sprinkler valve may help it avalanche.

Anything to keep friction low on the piston will help. Even filling the Velcro with grease will help both the leak rate and friction. The bummer is the grease will keep the Velcro from sticking.
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Unread postAuthor: dewey-1 » Sun May 24, 2009 7:13 am

Technician1002 wrote:
dewey-1 wrote:I agree with jrrdw.

I have used a 1 inch sprinkler to pilot a 1.5 inch sprinkler valve.
I had to because the blow gun was to restrictive on the larger valve.

Look here;

http://www.spudfiles.com/forums/viewtop ... tml#127106


I don't.

The blow gun was the hint the equalization leakage is high. Too high.


Tech this is why I am confused.

I made the statement of a restrictive blow gun on my cannon.
It has nothing to do with resrictive blowgun on his! He never even stated that.

I thought you were stating that I also had a bad or messed up main valve
because of your reference to "blow gun was the hint"

I could have used a ball valve but they are inconvenient and ugly for the purpose I wanted. That is to have a trigger handle.
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