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Wye piston valve - good idea?

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: noname » Thu Jul 12, 2007 9:54 am

The valve in my avatar has the same configuration as a sprinkler valve, except it was cheaper to make and has bigger parts (higher flow rate).
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Unread postAuthor: dongfang » Thu Jul 12, 2007 1:16 pm

Hi,

So there's an elbow coming from one port to the center .. and its OD is somewhat smaller than the tee's ID? And it seals with a piston?

Regards
Soren
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Unread postAuthor: noname » Thu Jul 12, 2007 4:14 pm

Yeah, that's all it is, except it uses a diaphragm instead of a piston (more compact, pilots faster).
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Unread postAuthor: clide » Thu Jul 12, 2007 6:33 pm

dongfang wrote:Wait wait that's actually a good idea. I did design like that originally. The problem is that the 45 deg barrel cut becomes sqrt(2) times longer than it is wide.
With a 67.5 degree cut, the length:width of the piston and the barrel end becomes the same ---> better fit, smaller piston.
With a 45 degrees barrel cut, one will then need a piston diameter quite a lot larger than the barrel diameter - but ok that brings us back to:

I drew up the situation. Maybe there is a single solution for both problems.

But ok it WILL be a somewhat large piston for the barrel. Something like 71 mm piston OD for a 50 mm barrel OD. On the other hand, construction is simple (90 degrees piston end).

What is a good piston surface area : sealed face area for barrel sealers?

Maybe the best cut is somewhere in between 67.5 degrees and 45 degrees.


Good point, If you do it that way you are going to have to use a pipe that is about 1/1.4 the size that you could use on a normal valve, and that would only give you 1/2 the flow since it increases as a square of diameter. Considering that you only get about 1.5 times the flow (in theory) out of the strait configuration, you may actually lose performance over a standard tee valve. Although one thing that you point out in the drawing is that the widest dimension of the 45° cut can be about the same size as the piston. Still, now that you mention that, you may be better off making the piston and sealing face at equal angles, although it may depend on the size of pipe you have available.

If you have a good piston (o-rings, a check valve, and low friction) then you can get the sealed face area very close to the piston surface area in a barrel sealing valve.

Here is the valve that I made awhile back
http://www.spudfiles.com/spudtech_archi ... hp?t=13120
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Unread postAuthor: dongfang » Fri Jul 13, 2007 6:06 pm

Hi,

I was out at that koi-breeder place, and although they had beautiful PN10 (or was that even 16??) wyes and also 45 degree elbows, I ended up shopping for a regular tee barrel sealer.

Can somebody with experience in this tell me:
- Am I on the right track with the sealing ring diameter vs. the piston diameter? (49 mm : 60 mm) ? Larger seal diameter? The next O-ring I can get (apart from metric, which I didn't consider) is 46.99 mm ID; that will be 57.65 mm OD, which will leave very little wall after a groove is cut in a 60mm OD fitting. On the other hand, a larger (and maybe thinner) ring could help me eliminate the reducer in the valve seat.
The O-ring is quite hard; NBR 90. I also have an NBR 70 to try with.
- Is there a fair chance of a reliable seal with this stuff? Or will the O-ring not be able to withstand the pressure of the whole piston on it?

The tee has the usual two different diameters: 63.1 mm where pipes are supposed to be glued in, and it then reduces to 60 mm where the pipes are supposed to stop. That is the beauty of it: The valve seat and the piston can just be hammered into the 60 mm part. I might not even have to glue the seat (a 63 to 50 mm reducer outside it will hold it in), but may need to machine down the piston a little. The back of the piston will stick out into the 63 mm part of the tee, and I will cut a groove in very back of the piston for an O-ring that will serve as a guide. I could get a real piston ring too (how cool!), but the piston and the tee are too short for that.

OK I hope I can get it to work! Otherwise, sgort, don't worry, I'll get your Mauler then ;)

Oh, and Clide, where did you get that 1.5 times the flow coefficient figure from? And how does one get about finding a flow coefficient for a valve at all? And, what did you make your (inline valve) diaphragm of?

Regards
Soren
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tee-valve1.jpg
Seat and piston.
tee-valve2.JPG
Line-up for a 63 mm ID (about 2.5 inches) tee barrel sealer.
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Unread postAuthor: noname » Fri Jul 13, 2007 8:06 pm

If you can secure it well, the O-ring will be a great seat for the piston. Looks good, I love that dark gray Schedule 80 (even if that isn't) color.
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Unread postAuthor: clide » Fri Jul 13, 2007 10:27 pm

Looks like you're on the right track for the standard piston valve.

dongfang wrote:Oh, and Clide, where did you get that 1.5 times the flow coefficient figure from? And how does one get about finding a flow coefficient for a valve at all? And, what did you make your (inline valve) diaphragm of?


I use the efficiency setting for flow coefficient in GGDT since it scales well with size. A standard tee valve will usually be around 40% efficiency. I've measured this number by using chronograph results. I believe a strait configuration would get around 60% efficiency. This number comes from several sources: quasi-educated guess, various flow coefficients given my manufacturers for various kinds of valves, and vague memories of something said by someone who knows what he is talking about (the maker of GGDT). I'm not extremely confident in the figure, but I am confident that you won't get more than 80%, so it would still be questionable gains. But that is just for the 45° cut. In theory your original idea would allow you to get the same port size as you would with a standard tee valve, so you would get a significant benefit assuming the valve opened properly.

Also this is all highly dependent on the pipe sizes available, so it could be that the 45° cut would work for the pipe you planned on using for the seat anyway, if that were the case then the 45 would be easiest

As for the inline valve, noname used a diaphragm. I used a piston made of PVC rod with a .25" neoprene sealing face and a custom made PVC washer to keep the piston from sliding off the piston face.
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Unread postAuthor: dongfang » Sat Jul 14, 2007 12:58 pm

Hi,

Oops, seems I mixed up noname and Clide, asking about that diaphragm. But it seems nobody got offended ;)

Thanks for the input. Clide, you sure must have build quite some valves, to determine efficiency empirically ....

OK I might build the valve of the original plan one day. I will have to find exactly the right thing to use as a piston first. Lathe access is very limited for me here .. ultimately maybe I have no choice but to pay someone to make the bare minimum number of cuts...

I'm less enthusiastic about the 45 degree cut valve .. a 70 mm wye is bulky and expensive (3 times the price of a tee, and 3 times that if a 63 mm wye), and I will have to fill it up with reducers and only get 50 mm piping. I could make 2 regular tee barrel sealers for the same price and mass (well ok I will need a third tee to join them but still...)

Meanwhile I found the ultimate stuffing for the wye: A DFTV. This design is going to kick ... well GBs!!! I have some refinements for rod guidance and damping that are still classified, so they are not on the drawing. With a DFTV, the long piston that a wye requires is not a problem - on the contrary it allows for some piston acceleration before release (all one has to take care of is not to get KO'ed by the rod as it exits!). Notice that dead space in the barrel is negative - the projectile gets sucked backwards before making a sudden turnaround.

For those who did not hear about the DFTV, it was the result of an old exchange on the spudtech forum -- google for "trident antenna launcher DFTV" or something like that and you will find an explanation.

On the other hand, maybe there is a reason that we have heard so little of the DFTV since then?

PS: That gray color is "PN10/16 gray".

And: What does a check valve do in a piston? Connects chamber and pilot chamber?

Regards
Soren
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Wye DFTV
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Unread postAuthor: clide » Sat Jul 14, 2007 8:03 pm

The main reason you don't hear much of the DFTV is that is takes up more space and requires more machining than a regular piston valve. Most people don't have access to lathes or don't want to take the extra time to make it.

While it is really a great valve design, I feel that it is typically unnecessary. There isn't usually a lot of performance difference between a valve that opens very very fast (a normal piston valve) and a valve that opens very very very fast (DFTV). That being said, if you have proper machining equipment then it doesn't really add that much work.

A check valve can be used to replace the equalization hole. It is especially useful if you have a high input flow and/or a small pilot valve. It allows flow from the pilot area to the chamber when filling, but blocks it from flowing back when you start exhausting the pilot area. Personally I build one into every valve that I can. I could fire my 2" valves by leaking air out of the schrader valve if I wanted to.

Good work on doing research for your project. The fact that you dug up the DFTV shows that you've been looking around quite a bit.
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Unread postAuthor: Gepard » Sat Jul 14, 2007 8:07 pm

Sorry, what's a DFTV?

Michael
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Unread postAuthor: clide » Sat Jul 14, 2007 8:26 pm

dongfang said how to find info about it in his last post, but since I'm nice I'll post the link :)

http://antennalaunchers.com/dftv19/
LONG spudtech thread where it was developed http://www.spudfiles.com/spudtech_archi ... php?t=2851
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Unread postAuthor: Gepard » Sat Jul 14, 2007 8:29 pm

I saw that but didn't realise it was an explanation.

Cheers,

Michael
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Unread postAuthor: dongfang » Sun Jul 15, 2007 3:59 am

Hi Clide,

Thanks for making up for my laziness w the links.

OK I can see what you mean with the down side of the DFTV. Its inventor used it with with a light projectile and a short barrel (his 1st design parameter seemed to be the measurements of the case that the the gun should fit into!). For that application, very very very fast is much better than just very very fast. For GBs in a long barrel, it does not really matter.

I suggest the DFTV for the wye because it has two pistons between the barrel and the pilot chamber, and is easy to stretch long enough for a wye. A single piston would become heavy.

Edit: I also like the idea of triggering a monster size cannon with a blowgun alone.

The piston assembly in the original DFTV plans seems to be made up of an awful lot of parts. I think less could do it (sacrificing some adaptability for experimentation, but that has already been done).

Regards
Soren
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