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Piston valve question

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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Piston valve question

Unread postAuthor: Gun Freak » Tue Aug 03, 2010 2:47 pm

I am designing a coaxial piston valve and (I'm gonna sound like a noob here lol :lol: ) the chamber is 1" and the barrel is airsoft sleeved in half inch pvc, which is the seat. So, I know the OD of 1/2" is bigger than the ID of 3/4" (at least sch 40) so that would make the piston have a slightly smaller OD than the barrel. BUT, if I make the 3/4" thin walled, then the piston will be wider than the barrel, which I think is necessary but I am not sure. So what I'm asking is do I need to use thin walled 3/4" for the piston pipe?
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Unread postAuthor: Gaderelguitarist » Tue Aug 03, 2010 3:04 pm

Are you asking what material would be best for constructing the piston, or what would best be used as the valve seat?
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Unread postAuthor: ramses » Tue Aug 03, 2010 3:07 pm

The piston does need to be larger than the seat. Why don't you play it safe and use the 1" pipe as the piston pipe? I mean, it's even there already! you just have to make it 1-2" longer.
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Unread postAuthor: Gun Freak » Tue Aug 03, 2010 3:19 pm

ramses wrote:The piston does need to be larger than the seat. Why don't you play it safe and use the 1" pipe as the piston pipe? I mean, it's even there already! you just have to make it 1-2" longer.

Because it has to fit inside 1.25" PVC pipe, so I have to use a 3/4" female adapter not 1".
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Unread postAuthor: SpudFarm » Tue Aug 03, 2010 4:21 pm

Could you draw a simple diagram? I can't quite understand what you are asking about from what I read.
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Unread postAuthor: Technician1002 » Tue Aug 03, 2010 6:44 pm

What is required is the valve seat diameter (the seal area) must have a smaller diameter than the OD of the piston. I think the issue is the OD of the piston and the sealing surface is too close to 1:1 so the piston may not fire even if the ID of the barrel is too small for the piston to fit inside. I think this is the issue the OP is trying to address.

To answer the question to make sure the valve seat OD is smaller than the piston OD to prevent a 1:1 or very close to 1:1 ratio valve, the valve seat can be grooved to provide piston support so the seat doesn't cut the seal by being too narrow, but still provide the seal near the ID of the barrel diameter.

Use a small triangle file and notch the outside of the barrel valve seat so the OD area does not provide a seal, but does provide a resting surface. This can allow the piston to settle on the valve seat and have the sealing surface diameter near the ID of the barrel diameter. I think this may fix any issues of the seal surface being too large in diameter in relation to the piston OD.

As an example of this, the valve seat in my Mouse Musket is not sealing at the OD of the valve seat. It seals at the O ring diameter where it touches the piston face, not the OD of the seat assembly.
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Unread postAuthor: Gun Freak » Tue Aug 03, 2010 6:50 pm

Technician, I'm not totally following what you're saying but I'm sure it's something along the lines of my current knowledge of piston valves.

@SpudFarm, here is a pic.
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Unread postAuthor: Technician1002 » Tue Aug 03, 2010 7:43 pm

One way to illistrate what I was saying is to use a hypothetical piston valve. The OD is 1 inch, It seals on a 1/2 inch ID barrel with 1/4 inch thick walls.

For finding the opening force of the piston, the valve seat diameter needs to be known. If the valve seal is really good, and seals at the OD of the valve seat, the 1 inch diameter valve seat would mean the piston won't open when the pilot pressure reaches 0 PSI if there is any friction on the piston at all due to the seat diameter to piston OD diameter is a 1:1 ratio.

If the valve seat is rough near the outside and it only makes a good seal near the ID of the barrel, then the valve seat diameter is effectively 1/2 inch. This makes the area inside the valve seat less than 1/2 the piston face area for a valve with an effective area ratio exceeding 1:2 which will lift the valve when the pilot is still above 1/2 the chamber pressure.

By designing your valve seat, you have some control over the piston valve effective valve seat diameter and thus the valve performance.
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Unread postAuthor: Insomniac » Wed Aug 04, 2010 1:33 am

Tech, I'm a bit confused with what you're saying... I can't think of a situation in which a piston would not pilot with the pilot totally vented (barring of course a piston with such high friction that the pressure can't move it from the barrel).

Even if the seat diameter is larger than the piston's OD, I think it would still pilot. I may be wrong, but I can't see there being any 'suction' on the piston to the barrel... if the pilot is totally vented, then there is still an unbalanced force pushing the piston backwards. Even if you had a three inch barrel, with a piston and chamber a few mm larger in diameter, I believe it would pilot (assume perfect seals and no friction in this case, as the forces would be rather small).

I may be wrong, but what you're saying seems counterintuitive (or I'm mistaken in what you're saying).
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Unread postAuthor: Technician1002 » Wed Aug 04, 2010 5:32 am

For a piston to move, there must be a force on it in the direction of the desired motion. This force must exceed the friction (there is always some friction). As the seat becomes larger, the force becomes less. When the ratio of seat diameter to piston OD reaches 1:1, the force to open the piston goes to zero. Friction remains, and often the pilot area never vents to zero PSI due to the EQ port.

It is true that the force holding the seal closed works to push the piston open, which can allow some chamber pressure in that space so the piston may pop open when the pilot pressure aproaches zero, but in that condition, the ratio is no longer 1:1. The area exposed to chamber pressure increases. This is why it seems counterintuitive.

A common problem with large seat piston valves is the won't fire issue where the opening force is less than the friction and low pilot pressure that is holding the valve closed. A common troubleshooting help request is the pilot just hisses and it won't fire, please help. The hiss is a hint the pilot area still has pressure in it. Often the EQ port or piston leakage is too great to allow the pilot valve to drop the pressure low enough to zero to open the valve.

I hope this helps.

For reliable opening, and yet still high performance, I like the piston designed to open when the pilot pressure is at about 20% of the chamber pressure. A closer ratio tends to be unreliable.

A wider ratio opens too soon and tends to slam back closed during the shot robbing power as the chamber pressure drops fast and the pilot pressure remaining pushes the piston back closed early. Ideally you do not want the piston to open when the pilot still has high pressure in it.
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