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Looking for help on importing valve design into GGDT

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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Looking for help on importing valve design into GGDT

Unread postAuthor: Solar » Tue Jan 22, 2008 7:52 am

Here are the basics.

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Unread postAuthor: Hotwired » Tue Jan 22, 2008 9:09 am

To the printer! Then the machinist!

Only joking :)


My printers broken :D


Not entirely sure GGDT will work properly as this valve works in the opposite way to the valves intended for it but here goes:

So thats a 1" hole through which the air escapes from the chamber. Taking into account theres a 0.5" piston rod also sharing that space I get 0.785 - 0.196 = 0.589"sq. The 'seat diameter' of a circle of that area is 0.866".

The piston diameter would be the large disk which is about 1.5" after the o-ring is on.

The vent diameter is where you need to put the diameter of the pilot hole.

The piston mass is for the whole moving section of the valve.

The pilot volume is whatever volume the compressed air has to fill before it gets to that 1.5" piston face and the volume created by the piston moving forwards.

Once you've got all that in you can find the flow co-efficient using the GGDT Cv/Kv/Eff Calculator which is in GGDT but thats a help page for it.

Hope that helps.
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Unread postAuthor: Solar » Tue Jan 22, 2008 12:58 pm

Interesting... The vent/exhaust is really an input with this valve. It is not shown properly on this drawing, but is a 1/8 inch npt. I suppose it will still be fairly accurate even with the piston body entering into the air mass instead of away from. I will have to add the spindle and volume chamber volumes as well then and weigh the piston assembly. Thanks for the starter info.
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Unread postAuthor: clide » Tue Jan 22, 2008 5:52 pm

Yeah it should still give you a decent model since many of the dynamics of the valve are fairly similar to a normal piston valve. Some things will be thrown off a bit by the different design, but flow coefficient is far more important to most models than opening time, which is what most of those dimensions are for.

Also Hotwired didn't say what valve type setting should be used. Even though your valve more closely resembles a chamber sealing valve, the way the forces work is closer to a barrel sealing, so I believe the "Barrel Seal"setting should be used for the valve type.

As Hotwired said you would probably be best off using the Cv calculator to find performance since the valve is pretty unique, but provided the downstream areas from the valve to the barrel aren't very restrictive I would expect you would probably be looking at a Cv around the high 8's or low 9's (~40% efficiency). If you use the calculator and get values much higher or much lower than that something is probably wrong with your model.
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Unread postAuthor: Hotwired » Tue Jan 22, 2008 7:30 pm

Aha?

Theres always something to be forgotten.

I had actually put in chamber seal originally but it went missing during much editing of the post.

Much could be discussed regarding which category it is more likely to fit into but in reality it is neither: it's using a positive action to force a piston into the chamber to vent it as opposed to using a negative action to result in a piston being pushed away from the chamber to expose the barrel port.

No one really wants to go into the realms of "generic valve" though and to be honest, if you switch between chamber and barrel sealing valve presets there is only a small difference, a chamber seal being very slightly more powerful.

Flip a coin? :wink:
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Unread postAuthor: clide » Tue Jan 22, 2008 9:31 pm

Your right there is hardly a difference especially in this case, but I would go with barrel sealing because of how the valve behaves. If you make the seat diameter bigger then it will open faster because there you need more pressure behind the valve to push it open. This increased pressure creates more force once it actually opens and the force on the seat is removed.

It is the same way in barrel sealing valves except we would be talking in terms of pressure decreases in a standard piston valve.

In a chamber sealing however the opposite is true, an increase in seat diameter will decrease the opening time.
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Unread postAuthor: Hotwired » Tue Jan 22, 2008 10:06 pm

I'm not digging how increasing seat diameter in a chamber valve makes it slower...?

The reason why I originally filed it under chamber valve was the layout.

Chamber and barrel sealers are all T valves

In a chamber valve the pilot and chamber are facing each other across the cross and the barrel is the branch. The piston directly seals the chamber.

In a barrel sealer the pilot and barrel face each other across the cross and the chamber is the branch. The piston directly seals the barrel.

Albeit the reverse action on the piston it does fit the chamber sealing profile.
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Unread postAuthor: clide » Tue Jan 22, 2008 11:28 pm

Ok picture your typical chamber sealing valve with a fixed piston size. What really makes a piston valve open fast is the increased surface area exposed to pressure once the valve starts to open. Lets say for all cases the pressure and surface area exposed once the piston opens will be the same. Which will provide a certain backwards force to the front of the piston no matter what size of the seat is.

However you also have a forward force on the rear of the piston due to the pressure in the pilot area. The pressure remaining in the pilot depends partially on the size of the exhaust, but mainly by the size of the seat. The way to find roughly what pressure will be in the pilot when the valve starts to move back is to balance forces on the piston. If you think about that for a minute you can see that with a larger seat for a chamber sealing valve there will be more pressure in the pilot when the piston starts to move, so a higher forward force slowing down the valve.

If you do that same analysis with this type of valve you will see that it performs more like a barrel sealer despite being built like a chamber sealer, and that performance is what you are after with a GGDT model.
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Unread postAuthor: Solar » Wed Jan 23, 2008 1:00 am

Ok, I am having my scale returned tomorrow and will be able to get the rest of the data ready as well. I'm curious what the differences will be with the different construction types now. GGDT seems more complex AND more simple then I had expected at the same tgime. Thanks for all the advice guys. Any other members know this software well enough to add anything?
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Unread postAuthor: Hotwired » Wed Jan 23, 2008 7:17 pm

Oh I finally get what you're getting at.

The barrel sealer needs a much larger pressure drop on the pilot side before the piston moves because of the initial area imbalance favouring the pilot side.

Yeah I dig.

For a chamber sealer of equal piston face dimensions almost the same surface area as pilotside has been present before the pilot even popped which means it reacts faster than a barrel seal when the pilot starts to fall.

The chamber seal makes up for it's slimmer dimensions by requiring better piloting.

But thats going off tangent.

In a barrel sealer the force suddenly increases chamberside of the piston the moment the valve cracks open enough to expose the area covered by the seat.

In a chamber sealer opening speed is much less dependant on the area covered by the seat and more on the pressure difference of pilot and chamber.

In Solars valve there is no movement of the piston at all until the 1.5" disk has enough force to overcome the 1" disk on the other end of the spindle and when it does, there is no sudden increase of force from extra area being exposed but a comparatively slower decrease of opposing force as the chamber vents.

It's not really classifiable as either since what it's really dependant on to open is an area difference between pilot and chamber which neither barrel or chamber sealer has as they are both negative action valves while Solars is a positive action one and the pilot can have a greater area than the chamber side because it's not pressurised to start with.

My personal opinion would be just to do both, only takes two clicks to change between them, if theres a significant difference we can start arguing about it ^_^
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