Piston valves explained visually

Post questions and info about pneumatic (compressed gas) powered launchers here. This includes discussion about valves, ratios, compressors, and anything else relevant to launchers powered by compressed gas.

Postby Zorrowannabe9 » Tue Jun 27, 2006 11:09 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by clide
[br][quote]maybe making a small chamber to pressurize and then release behind the piston would be a couple of possible solutions.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I was actually considering that possibility, maybe a small ballvalve controlled mini chamber...
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Postby Navigator7 » Wed Jun 28, 2006 7:25 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by clide
Is this the pressure differential that I addressed above or are you talking about something different? If it's something different I think I need you to re-explain what you are talking about because I'm not quite sure what you're asking about.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

I'll make a drawing and post either tonight or tomorrow...to show you what I mean.

Tks.

Edit....ahhhh! Have had no time to make a drawing.
Coming tonight or Sat afternoon.

Edit2;
Here we go:
<img src="http://i76.photobucket.com/albums/j18/Navigator7/Waterdawg/NavQuestion.jpg">

What I want to discuss is the "Ratio" between the maximum sealing diameter of the barrel or chamber and the acting surface area of the piston.

If you divide the sealing area of the piston by the camber diameter you get a ratio.

.5 means the area of the piston sealing the barrel or chamber is equal to the area available to force the piston off the sealing face.

I'm thinking smaller ratios...to a point work better.

Thougts?
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Postby Boomer » Fri Jul 07, 2006 3:20 pm

wow thank you this has given me exactly what i need to actually build one of these, they looked too complicated and dangerous but now they dont look so bad.
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Postby markfh11q » Sat Jul 08, 2006 12:10 pm

OKAY, I deleted my last post. I thought we were talking about chamber sealing valves for a second...

All of these simulations are run at 100 PSI.

With design #1...

Force pushing piston against barrel: 706 net pounds
Force pushing piston away from barrel: ... need barrel outside diamter. I'm gonna go with 2-3/8", though, so: 263.2 net pounds
Sealing force: 442.8 net pounds

With this design, you would need a drop of 62.72 PSI for the piston to fire, (sealing force to equal zero). NOTE: these are in ideal conditions with equalization flow area and pilot venting flow area not taken into consideration.

This means that there will still be 37.28 PSI in the pilot volume for the piston to compress against, (once again, pilot venting flow area not taken into consideration).

On design #2...
Force pushing piston against barrel: 706 pounds
Force pushing piston away from barrel: I will increase the O.D. of barrel proportionally: 155.6 net pounds

This means with this design, you would need a drop of 77.96 PSI in order for the piston to fire, leaving 22.04 PSI in the pilot volume for the piston to compress.

So, as you can see, in design #2, the piston unseats slower, but has less force going against it in the pilot volume, which gives it less of a chance of bouncing/resealing/failing than piston design #1.

There is probably a relation between equalization flow, pilot venting flow, piston diamter, and sealing area, (area in front of piston around O.D. of barrel exposed to the 100 PSI used in these tests as an example), that can allow you to make a ratio or equation to calculate the effectiveness of a barrel sealing coaxial valve.
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Postby clide » Sat Jul 08, 2006 1:10 pm

Thanks mark.

In a barrel sealing valve, the valve will actually fire faster as the ratio goes up. Now there is probably a practical limit where there won't be enough force to overcome friction of the piston and the valve will just leak open or not open at all, but that would be a very extreme ratio.

As mark showed, increasing sealing port size like that will decrease the pressure left in the pilot area when the piston begins to move. It will also increase the jump in force that the piston experiences once the valve begins to open, and that jump in force is what is really important in quickly opening the piston valve.

So as a more direct answer to your questions.
<i>As the ratio goes down, I think the cannon fires faster and harder.</i> False
<i>As the ratio goes up, there is a point where the cannon ceases to fire</i>
This one is a little more difficult. At a ratio of 1 the cannon will cease to fire. Below that it will fire fine in theory, but friction may cause some issues, but for most cases I would say that statement is false.

Now just remember that this is reversed in the case of chamber sealing. Your first statement would be true for chamber sealing valves. Modifying your second one slightly would also make it true for chamber sealing valves.
<i>As the ratio goes up, there is a point where the cannon ceases to fire <b>well</b></i>
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Postby Navigator7 » Sun Jul 09, 2006 12:11 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by markfh11q
[br]OKAY, I deleted my last post. I thought we were talking about chamber sealing valves for a second...<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">

Please Mark, I'd be interested in your comments on chamber sealing valves!
I like both species!
;-)


[quote]All of these simulations are run at 100 PSI.

With design #1...

Force pushing piston against barrel: 706 net pounds
Force pushing piston away from barrel: ... need barrel outside diamter. I'm gonna go with 2-3/8", though, so: 263.2 net pounds
Sealing force: 442.8 net pounds

With this design, you would need a drop of 62.72 PSI for the piston to fire, (sealing force to equal zero). NOTE: these are in ideal conditions with equalization flow area and pilot venting flow area not taken into consideration.

This means that there will still be 37.28 PSI in the pilot volume for the piston to compress against, (once again, pilot venting flow area not taken into consideration).
~Mark<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

This reads like you are taking the information from some calculator? GDDT?
I'm missing some key knowledge on how you arrived at the 62.72 PSI drop for the piston to fire.
Can you explain?

I'd also like your ideas on variations of what you just suggested.
For example.....Let's say you had the ability to control the dump pressure behind the piston? (Throwing out pilot flow, drag, piston leakage etc)
What would happen if you brought the pilot pressure just below the firing point...say 1 lB less ...as opposed to total evacuation of the pilot area.
Does the piston dump equally well at any pressure below the firing point?

[quote]~Mark...
There is probably a relation between equalization flow, pilot venting flow, piston diamter, and sealing area, (area in front of piston around O.D. of barrel exposed to the 100 PSI used in these tests as an example), that can allow you to make a ratio or equation to calculate the effectiveness of a barrel sealing coaxial valve.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

I agree. I think it is a subject worth much discussion for two reasons:
1. Performance.
2. Reliability
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Postby Navigator7 » Sun Jul 09, 2006 2:30 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by clide
In a barrel sealing valve, the valve will actually fire faster as the ratio goes up. Now there is probably a practical limit where there won't be enough force to overcome friction of the piston and the valve will just leak open or not open at all, but that would be a very extreme ratio.

As mark showed, increasing sealing port size like that will decrease the pressure left in the pilot area when the piston begins to move. It will also increase the jump in force that the piston experiences once the valve begins to open, and that jump in force is what is really important in quickly opening the piston valve.

So as a more direct answer to your questions.
<i>As the ratio goes down, I think the cannon fires faster and harder.</i> False
<i>As the ratio goes up, there is a point where the cannon ceases to fire</i>
This one is a little more difficult. At a ratio of 1 the cannon will cease to fire. Below that it will fire fine in theory, but friction may cause some issues, but for most cases I would say that statement is false.

Now just remember that this is reversed in the case of chamber sealing. Your first statement would be true for chamber sealing valves. Modifying your second one slightly would also make it true for chamber sealing valves.
<i>As the ratio goes up, there is a point where the cannon ceases to fire <b>well</b></i>
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

I'm missing something with regard to barrel sealing but get your point on chamber sealing.
Great explanation.
I see the piston trying to get out the barrel up until the area of the chamber and the pressure differential causes it to move the piston off the barrel.

I'm trying to pin down that grey area where it's not exactly no pressure behind the piston but less pressure behind the piston causing the firing.

Can you rephrase some of your comments?
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Postby markfh11q » Sun Jul 09, 2006 8:06 pm

No, no simulation program, just a sheet of scrach paper and MS Calculator.

I'm getting the pressure drop to fire by multiplying the piston surface area by a variable to equal the force produced by the area in front of the piston, (surrounding the barrel), at 100 psi. This will give me the pressure required in the pilot to equal force from the front, which, when subtracted from 100, will give me the drop required.

I think that the less pressure behind the piston when firing would be better. That's just my opinion right now though. If I could look up some pneumatic theories, (from Google), on how fast a certain flow area can evacuate a specific volume at a certain pressure, I could maybe throw out a few ideas or questions, but I'm still looking so far. Probably something obvious I'm missing.:(
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Postby Navigator7 » Sun Jul 09, 2006 9:53 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by markfh11q
[br]No, no simulation program, just a sheet of scrach paper and MS Calculator.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Ok...cool...that's the way I do it...only with a Mac.
;-)
<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">I think that the less pressure behind the piston when firing would be better. That's just my opinion right now though. If I could look up some pneumatic theories, (from Google), on how fast a certain flow area can evacuate a specific volume at a certain pressure, I could maybe throw out a few ideas or questions, but I'm still looking so far. Probably something obvious I'm missing.:(
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I'm not suggesting you are missing anything.
I agree the less pressure behind the piston the better.....almost to the point of...if the whole back end fell off so much the better.

I'm trying to dial in the piston sealing area versus the chamber sealing area....like everyone else does with C:B ratios on combustion guns.
I don't think this area has been covered very well.

I'm looking for the optimum ratio for a chamber sealing valve and a piston sealing valve.

Regardless, I think I am missing some fundamental logic so I'm promoting the topic.
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Postby Freefall » Mon Jul 10, 2006 1:06 pm

Maybe I can sum this up with bullet points:

Inlet sealing valve (aka chamber sealing):
As port area goes up:
- Piston closing force goes down (more difficult to get a good seal)
+ Piston begins to move sooner (requires less pressure drop in pilot chamber)
- Piston does not move as fast due to reduced port-side area ratio
- Piston does not move as fast due to higher pilot pressure when piston unseats
- Piston may bounce earlier and thus not open as far, due to higher pilot pressure

Outlet sealing valve (aka barrel sealing):
As port area goes up:
+ Piston closing force increases (easier to get a good seal)
- Piston takes longer to begin moving (requires more pilot pressure drop to unseat the piston)
+ Piston moves faster due to increased port-side area ratio
+ Piston moves faster due to reduced pilot pressure as piston unseats
+ valve may open farther before bouncing due to reduced pilot pressure when piston unseats

In terms of performance, I see many more advantages to an outlet sealing valve.
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