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Using Vacuum pressure to speed up a sprinkler 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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Using Vacuum pressure to speed up a sprinkler valve?

Unread postAuthor: bullrees » Fri Jan 22, 2010 11:16 pm

First off I did search the forum and could not find anything related.It may be because it is not reasonable but I have to ask because it may be effective in some other application.

Question:If a blow gun/mod blowgun makes a big difference in the valve open time. What if you had a vacuum chamber to help evacuate the pilot pressure on the end of the blowgun.

My first concerns are that the eq valve in the sprinkler would let to much pressure into the vacuum chamber for it to be effective for long. I'm not sure if the diaphragm would seal it off enough when it was triggered.

I don't expect this pump to do hi pressure vacuum but if the volume is enough it may work?

This is my BASIC idea.
LAYOUT (ignore the connections for now)
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The pump is from a "food saver" the wife will never know :P
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Closer on the pump
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even closer
Image
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Video of pump with 3x 9 volt
[youtube]http://www.youtube.com/watch?v=wIIaRKHwWE4[/youtube]
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Unread postAuthor: inonickname » Fri Jan 22, 2010 11:29 pm

For this kind of effort, it would be quicker and easier to build a piston valve which would have inherently higher performance.

The vacuum chamber would need to be gigantic. A perfect vacuum will draw around 14.7psi. So an equal volume of air at 15 psi will bring it back up to atmospheric pressure. At 10 bar, you'll need a perfect vacuum chamber 10 times the size of the pilot (which in a sprinkler valve can be substantial) to have any effect. Any smaller and you'll hurt performance. The pump wont draw a perfect vacuum so it will have to be larger again. The size you have it now will likely be big enough.

If your happy to wait for it each shot, then it's fine. (and your willing to lug around the extra weight, and replace the batteries). Will it improve opening time? Maybe.

Do you have any idea how much vacuum the pump can draw? If it's fair then you could make a ping-pong ball cannon with it.
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Unread postAuthor: D_Hall » Fri Jan 22, 2010 11:35 pm

A vacuum won't help. At all.

Google for "choked flow." Read and learn.
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Unread postAuthor: bullrees » Fri Jan 22, 2010 11:46 pm

Well before I devised some type of mythbuster type of experiment I wanted to see if the whole idea was useless.

I see what you mean about the pilot chamber and the pressure.
Same pilot volume but more pressure means more vacuum that may not be possible because of pump or vacuum chamber.

Edit:@D_Hall why would a vacuum not help at all?
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Unread postAuthor: D_Hall » Fri Jan 22, 2010 11:56 pm

bullrees wrote:Edit:@D_Hall why would a vacuum not help at all?

If you understand the concept of "choked flow" the answer is obvious. If you don't, I'm wasting my breath. Google is your friend (and I bet, wikipedia).

Here, I'll even spoon feed you: http://en.wikipedia.org/wiki/Choked_flow
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Unread postAuthor: hi » Sat Jan 23, 2010 12:03 am

well Wikipedia only talks about fluids, so we are good... :lol:
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Unread postAuthor: btrettel » Sat Jan 23, 2010 12:06 am

Above a certain pressure drop ratio (0.528 for air in an isentropic flow) the velocity in the valve reaches the sonic velocity (the speed of sound). This means that information about the downstream pressure can't travel upstream so the pilot chamber doesn't know what the pressure on the other side is other than that it's lower than the value that reaches sonic velocity.

In other words, the gas flows out faster than the information that the pressure on the other side is lower can travel, so that information never reaches the pilot chamber.

I remember reading that belief that the mass flow rate is choked is a common misconception. The velocity is choked, not the mass flow rate. A number of books apparently get this wrong. An increase in upstream density will increase the mass flow rate. This is being pedantic however; you won't see any serious increase in performance.

I could also be wrong, but I don't think I am. Edit: A Google search for choked flow misconception seems to verify what I remember.
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Last edited by btrettel on Sat Jan 23, 2010 12:09 am, edited 1 time in total.
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Unread postAuthor: D_Hall » Sat Jan 23, 2010 12:09 am

btrettel wrote:An increase in upstream density will increase the mass flow rate.

Indeed. But a vacuum pump won't increase upstream density.
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Unread postAuthor: btrettel » Sat Jan 23, 2010 12:11 am

Doh. You're correct. :wink:

Edit: Checking out the Wikipedia link brings up something interesting on the same line: http://en.wikipedia.org/wiki/Choked_flo ... e_orifices

The mass flow rate through the [thin plate] orifice continues to increase as the downstream pressure is lowered to a perfect vacuum


This doesn't make sense mathematically but I can't argue with it if it's been measured. I'll have to track down the paper referenced.
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Unread postAuthor: bullrees » Sat Jan 23, 2010 12:25 am

So the gas at the valve is the smart gas but the gas at the end of the tube is dumb/slow gas because sound can't travel there(end) fast enough to tell it to hurry it's ass up?

Simplified..I didn't gogle :(
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Unread postAuthor: inonickname » Sat Jan 23, 2010 12:31 am

I think that the general idea is that without heating or other properties a gas wont exceed it's own speed of sound. This is why air powered pneumatics often have a limited speed.
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Unread postAuthor: hi » Sat Jan 23, 2010 12:32 am

so basically the gas in the valve is like a bat out of hell and the gas in the vacuum is like some big dude that kicks the bat in the face and tells him to get back in there?
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Unread postAuthor: bullrees » Sat Jan 23, 2010 12:37 am

So a sprinkler valve wil open just as fast with a blowgun to the atmosphere and using a vacuum will not help the speed?
Just making sure.before I start making anything.
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Unread postAuthor: D_Hall » Sat Jan 23, 2010 3:32 pm

bullrees wrote:So a sprinkler valve wil open just as fast with a blowgun to the atmosphere and using a vacuum will not help the speed?
Just making sure.before I start making anything.

Correct.
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Unread postAuthor: jimmy101 » Mon Jan 25, 2010 2:53 pm

bullrees wrote:So a sprinkler valve wil open just as fast with a blowgun to the atmosphere and using a vacuum will not help the speed?
Just making sure.before I start making anything.

In most instances yes. But in some cases the vacuum would help, in particular if you are operating at low chamber pressures. If the chamber is say 10 PSIG or less then the vacuum would help the valve open a bit faster.
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