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A pop-off 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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A pop-off valve?

Unread postAuthor: Marffy » Tue Feb 02, 2010 3:52 pm

I've searched google and the search thing on spudfiles.

What is a pop-off valve used for?What were they meant for? i've seen some cannons use them along with a QEV to make a full auto but i really don't know how they are made and what they do. I think they look like the picture i attatched. All i know is that they open at a certain pressure. I'd like to know where to buy one or how to make one.(preferably make one)How could i make an adjustable one if i did make one? this[/url] cannon uses a pop-off valve as a primary valve. could it get more power if it had a QEV. This whole post it for me and others to get a better understanding of a pop-off valve. Posting links to informational threads would be nice.
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Unread postAuthor: deathbyDWV » Tue Feb 02, 2010 4:09 pm

They are really meant for (in spudding) venting pressure in a pneumatic cannon so it doesn't accidentally over pressurize and explode. More recently the have been adapted for other use. One example is the qev pop off full autos. An inlet is filled on the pilot side of a qev. The pop off is attached to the pilot side also. The pressure from the inlet goes into the chamber. Once it is at full pressure, the pop off pops and the qev is piloted, and the gun fires. Then the process is repeated.

I think someone recently made a how to on homemade pop off valves. Check in the how to section.
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Unread postAuthor: Marffy » Tue Feb 02, 2010 4:12 pm

deathbyDWV wrote:They are really meant for (in spudding) venting pressure in a pneumatic cannon so it doesn't accidentally over pressurize and explode. More recently the have been adapted for other use. One example is the qev pop off full autos. An inlet is filled on the pilot side of a qev. The pop off is attached to the pilot side also. The pressure from the inlet goes into the chamber. Once it is at full pressure, the pop off pops and the qev is piloted, and the gun fires. Then the process is repeated.

I think someone recently made a how to on homemade pop off valves. Check in the how to section.


k, thanks. i'll check. i found a thread about a hammer valve. is it the same as a pop-off valve?
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Unread postAuthor: deathbyDWV » Tue Feb 02, 2010 4:22 pm

No... They are kind of backwards of each other. In a pop off,
the pressure pushes the sealing face out. In a hammer valve, the pressure tries to push it out but the hole is too small. To make it open it has to be hit with some force (a hammer) to push the sealing face in and make it open.
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Unread postAuthor: Marffy » Tue Feb 02, 2010 4:24 pm

deathbyDWV wrote:No... They are kind of backwards of each other. In a pop off,
the pressure pushes the sealing face out. In a hammer valve, the pressure tries to push it out but the hole is too small. To make it open it has to be hit with some force (a hammer) to push the sealing face in and make it open.


cool. good to know.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 02, 2010 5:26 pm

After experimenting with commercial pop-offs, I made my own valvebased on the concept most pop-off valves use, and better versions, works great for full auto ;) especiallywith HPA :D
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Re: A pop-off valve?

Unread postAuthor: Technician1002 » Tue Feb 02, 2010 7:11 pm

Marffy wrote:I've searched google and the search thing on spudfiles.

What is a pop-off valve used for?What were they meant for?


Pop off valves are designed to protect pressure systems. They are similar to a relief valve with one major difference. A relief valve opens at a set pressure and closes at the same pressure. As the pressure rises, it opens further.

A Pop-off valve has a sleeve on the piston in it. When they open, the sleeve restricts the flow so pressure builds under the larger diameter of the piston, so it pops all the way open and remains open until the pressure drops enough so it can't remain held open. At this time the spring returns the piston back into the sleeve and the valve seat seals off the flow so the valve closes suddenly and tight. This feature when used with a QEV can make a full auto cannon.

Liquid systems such as a water heater use a relief valve. Steam and air systems use a pop off valve.

By designing the space between the larger diameter of the piston and the sleeve, the point where the valve pops closed can be engineered into the valve design. Pop-off valves used to protect air compressors typically close at a pressure 20-60% lower than the opening pressure. For example a 100 PSI pop off will open near 100 PSI and will stay open until the pressure drops below 80 PSI.

I hope this helps.
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