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Question about piston 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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Question about piston valve

Unread postAuthor: rc dude » Fri Aug 15, 2014 3:13 pm

I am in the process of making a piston valve in a three inch tee. I have everything I need, except the 3" PVC I have for the part circled in the attached photo it DWV PVC. I know that using it for a chamber is a big no-no, but would it be OK to use for the part I circled? It shouldn't have any pressure since it will be inside the coupler and tee, but it will have tensile force. (I believe it is tensile). Thanks for any help
The attachment mypistonvalve.jpg is no longer available
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Re: Question about piston valve

Unread postAuthor: jackssmirkingrevenge » Sat Aug 16, 2014 8:08 am

As long as it's properly sleeved I see no reason why it shouldn't work safely.
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Re: Question about piston valve

Unread postAuthor: DYI » Sun Aug 17, 2014 5:25 pm

All that's really relevant here is whether or not the "DWV PVC" is cellular core or not. If it's solid, its behaviour under these loading conditions will be similar to that of a pressure rated section. If it is cellular core, it is very likely to stand up to the static pressure in the valve body, but may fail on actuation of the valve. The axial component of the stress under static pressure is relatively small, even by PVC standards - about 700psi, if you run the valve at 100psi. Note that this cannot be compared directly with the tensile strength to determine safety factor - the stress has other components (the most significant here being hoop stress from the internal pressure, at twice the axial stress). The Von Mises equivalent stress, which is a function of the stress field, is the number which CAN be directly compared with the yield stress.

By far the largest component of the loading here is the impact of the piston into the housing when the valve opens. Assuming 1" of piston travel and a vacuum behind the piston, the impact force would generate an axial stress in the housing on the order of 6kpsi, similar to the tensile strength (roughly twice that where the two sleeving sections meet and there is effectively only one layer). This does not happen in reality because the pressure on the pilot side of the piston is substantially different from zero, but it is generally the case that piston valves fail on opening due to the impact load.
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Re: Question about piston valve

Unread postAuthor: Technician1002 » Mon Aug 18, 2014 10:56 pm

Some sort of proper padding in the impact zone is required to prevent failure. My Mouse Musket broke pistons on a very regular basis. They even would fail in areas other than the impact face due to the shock wave. Two common faiure points were the face of the piston and right behind the face as shown in this example.

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