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Manual Piston Equalization Safety Benefit

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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Manual Piston Equalization Safety Benefit

Unread postAuthor: mark.f » Tue Mar 01, 2011 11:34 am

Just a thought to throw out there.

I've only seen it done a handful of times, but, with a completely sealed piston, equalization can be accomplished by simply running a piece of pipe or tubing from the pilot area to the chamber, with a ball valve in between.

So, I got to thinking. With the ball valve there, you would have to open to fill, and then close the ball valve before firing.

If the flow between the pilot area and chamber with the ball valve open is greater than the flow of your pilot valve, this would also act as a safety. As long as the ball valve was open, your pilot valve couldn't drop the pilot pressure enough to actuate the piston.

Of course, you could always add an inline check valve instead of the ball valve for no safety and automatic equalization.

Not the most bulletproof safety, but I believe it should work.
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Attachments
floatpiston.png
This is a standard piston with one-way equalization accomplished by a floating o-ring.
floatpiston.png (4.81 KiB) Viewed 771 times
modpiston.png
This is a completely sealed piston with equalization accomplished with a ball valve and some 1/8" pipe.
modpiston.png (6.58 KiB) Viewed 773 times
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Unread postAuthor: Gaderelguitarist » Tue Mar 01, 2011 11:46 am

I don't think that shape could be made with threaded fittings. It is a solid method of creating a safety though.
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Unread postAuthor: Technician1002 » Tue Mar 01, 2011 12:44 pm

It will work. Too large of a leakage path past the piston is often a noob reason some air cannons won't fire.

The ratio of the OD of the piston to the diameter of the barrel seal will play a large role in how much lower the pressure behind the piston must be before the piston will move. The closer the valve seat is to the OD of the piston, the lower the pressure in the pilot must drop before it will fire on a barrel sealing valve.

The drawing above shows a good example of a large diameter seat in relation to the OD of the piston. It won't take much bypass leakage to prevent that piston from opening.

This WiKi page has more information on the relationship of the valve seat to diameter and the pilot pressure drop required to open it.
http://www.spudfiles.com/spud_wiki/index.php?title=Piston_valve#Common_traits
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 01, 2011 2:20 pm

My first piston launchers used separate chambers and airtight pistons, but they were a bother. Don't you think having a "safety" is a little redundant on a spud cannon though?
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Unread postAuthor: mark.f » Tue Mar 01, 2011 4:19 pm

Gaderelguitarist wrote:I don't think that shape could be made with threaded fittings. It is a solid method of creating a safety though.


Good point, although in practicality terms I would have used tubing anyways.

Technician1002 wrote:The ratio of the OD of the piston to the diameter of the barrel seal will play a large role in how much lower the pressure behind the piston must be before the piston will move. The closer the valve seat is to the OD of the piston, the lower the pressure in the pilot must drop before it will fire on a barrel sealing valve.


I build most of my valves using this principle. My early valves needed a fairly large pilot valve since I used no o-rings. The valve I've made most frequently is a 2" piston 1-1/2" barrel size (in PVC), and this one is no different. (My copper launcher used a 1" piston and 5/8" OD seat, though, which is why it was much easier to pilot with a small ball valve.)

jackssmirkingrevenge wrote:My first piston launchers used separate chambers and airtight pistons, but they were a bother. Don't you think having a "safety" is a little redundant on a spud cannon though?


Well I think it would be a bother, having to fill from two points. :wink: For this one: 1.) Make sure ball valve is closed. 2.) Open fill valve. Piston will seat. 3.) Open equalization valve. Fill. Close equalization valve when ready to fire.

Safety's are a little redundant but I almost put a large chunk of chocolate through my face once.
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Unread postAuthor: FighterAce » Tue Mar 01, 2011 4:40 pm

mark.f wrote:Well I think it would be a bother, having to fill from two points. :wink: For this one: 1.) Make sure ball valve is closed. 2.) Open fill valve. Piston will seat. 3.) Open equalization valve. Fill. Close equalization valve when ready to fire.

Safety's are a little redundant but I almost put a large chunk of chocolate through my face once.


You don't need to fill from two points for a 100% sealing piston. This is how I did it.

For safety its a good idea to have a ball valve between your main valve and the barrel. Yes it adds dead space but with high performance piston valves who cares for a little bit of dead space... its like asking whats more important... your life or extra 10 fps
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Unread postAuthor: mark.f » Tue Mar 01, 2011 5:57 pm

I was commenting on jack's early designs, which required filling from two points.
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Unread postAuthor: Gun Freak » Tue Mar 01, 2011 6:38 pm

FighterAce wrote:You don't need to fill from two points for a 100% sealing piston. This is how I did it.

He never said that... :lol: The first picture he posted was of a piston with built in check valve achieved by floating o-ring.

@ Mark, I've seen it done before, but it is a good idea. Still I don't think it's worth it when you can just make a built in check valve with floating o-ring, which is what I always so, but sure it's another option.
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