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Pop off valves

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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Unread postAuthor: Fnord » Fri Feb 10, 2012 9:33 am

usdiver63 wrote:numerous times and then they reseat themselves, you can either pull them manually or they go off at a preset pressure.


That and your avatar confused the hell out of me for a couple seconds. I didn't remember posting it so I thought I might have been really, really drunk yesterday.

If you decide to go with a design similar to the diagram, don't solder it together. Use threaded fittings to make it serviceable. Besides, you risk damaging your orings without careful temperature control.
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Unread postAuthor: Technician1002 » Fri Feb 10, 2012 11:16 am

An actual pop off valve has a stepped or tapered valve core. The reason is to make it a POP valve instead of a relief valve.

A relief valve is a spring loaded valve that uses pressure to open the valve. It can open a little or more depending only on the pressure. It has a set point where the opening and close pressure are at the same point, such as 100 PSI. At above 100 PSI the valve is open and below, the valve is closed.

A pop off valve is similar to a relief valve with one important difference. The step in size on the valve body acts like a parachute and is placed in the air flow path to the exhaust. These valves have 3 important values.

1 Pop off pressure
2 Closing pressure
3 Flow when open.

Pop off pressure is the value at which the valve remains closed below this value and pops open at this value. This is the maximum pressure the system should see.

The closing pressure is the value at which the flow can no longer hold the valve open allowing the valve to close. As it closes, flow holding it open vanishes and the valve pops closed. This value is always significantly lower than the pop off value. For most valves this is about 20% lower, so a 100 PSI valve will re-close at about 80 PSI.

Flow when open. This value is the capacity rating of the valve. To protect a steam boiler or compressor, it must always flow a higher value than the source boiler or compressor can provide.
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Unread postAuthor: Major Collins » Fri Feb 10, 2012 9:53 pm

al-xg wrote:Yes there is a problem, the piston isn't balanced.

The air from the tank will be pushing the piston to the left helping the spring.
Because it has a larger area to push on than the pilot side , the pop-off valve will never fire.


back to the drawing board ...

Fnord wrote:If you decide to go with a design similar to the diagram, don't solder it together. Use threaded fittings to make it serviceable. Besides, you risk damaging your orings without careful temperature control.


hrmm i did not take that into account , but i cant even go with making the valve now :(

after all this i was thinking on putting a pop off on the exhaust and using a spring loaded piston valve for full auto but now after seeing JSR's link i think i could add his custom popoff valve to the front of of the piston valve instead and i could still achieve full auto... is this correct ?
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