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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: Crna Legija » Tue Feb 08, 2011 10:17 pm

go JSR on its ass and cast it in a solid block of epoxy :lol:
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Unread postAuthor: Jimmy K » Thu Feb 10, 2011 9:12 pm

Technician1002 wrote:Due to the issues of transonic flow in the valves (how air turns the corners at high speed) I have to disagree on the performance of one vs 2 valves. 2 valves will still have a fairly high loss even though the port area of the combined valves is slightly larger in area than the barrel. The dual valves will have a considerable pressure drop across them limiting flow to about 1/2 that of a burst disk with no bends between the valve and barrel.


Could you elaborate on this? Sounds kinda interesting.
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Unread postAuthor: Technician1002 » Thu Feb 10, 2011 10:11 pm

Study the CV values of valves. Here is some info in the Wiki.

What it boils down to in simple terms is in high flow some paths have more resistance to the free flow of gasses and fluids. This resistance provides a pressure drop across the valve. The two sprinkler valves in parallel will have more resistance to flow and a higher pressure drop than the barrel, even though the two valves have a combined orifice area that is greater than the barrel. In addition to the two valves there is additional pressure drop on the elbows and t's to connect them in parallel. Wikipedia has an excellent explanation on this also. Link below.
At high speeds, a high loss valve has a high pressure drop severely limiting the maximum flow from the chamber into the barrel.

http://www.spudfiles.com/spud_wiki/index.php?title=Flow_coefficient
http://en.wikipedia.org/wiki/Flow_coefficient

This PDF file provides even more in depth reading. It is highly recommended reading.
http://www.swagelok.com/downloads/webcatalogs/EN/MS-06-84.PDF

The real important stuff is on page 3 of the PDF.

The basic orifice meter illustrates
the difference between high and low
pressure drop flow conditions.
In low pressure drop flow—when outlet
pressure (p2) is greater than half of inlet
pressure (p1)—outlet pressure restricts
flow through the orifice: as outlet
pressure decreases, flow increases,
and so does the velocity of the gas
leaving the orifice.

When outlet pressure decreases to half
of inlet pressure, the gas leaves the
orifice at the velocity of sound. The gas
cannot exceed the velocity of sound
and—therefore—this becomes the
maximum flow rate. The maximum flow
rate is also known as choked flow or
critical flow.

Any further decrease in outlet pressure
does not increase flow, even if the
outlet pressure is reduced to zero.
Consequently, high pressure drop flow
only depends on inlet pressure and not
outlet pressure.


Having high pressure in your chamber and then having low pressure behind the projectile is not much help. You want great flow.

The valves in parallel and their plumbing to connect them affect the overall CV of the assembly.
The valve flow coefficient (Cv) takes into account all
the dimensions and other factors—including size and direction
changes—that affect fluid flow.
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Unread postAuthor: Fnord » Fri Feb 11, 2011 4:51 pm

Could you elaborate on this? Sounds kinda interesting
.

For a real-world demonstration, try blowing into 20' or so of 1/4" coiled tubing. It's hard! Whereas you can blow a lot of air through a 2' section easily.
Now, your valve obviously has better flow than this, but there's a lot more high-density air and it's changing directions a lot faster. If you want to get scientific about it you could put a pressure transducer in front of the valves. You said you'll have a laptop with you right?
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Unread postAuthor: Jimmy K » Sat Feb 12, 2011 10:40 am

Most likely microprocessors, not a laptop, if I even still do electronic controlled stuff. It all depends on the money left over from all the essential parts.
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Unread postAuthor: Jimmy K » Sun Feb 20, 2011 8:57 pm

First off, I am sorry for the double post.
Secondly, this will pretty much be a build log now, as I have no more questions on theory or on specifications (for now).

For starters, here are a few pictures.
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Attachments
2011-02-20 18.22.39-1.jpg
Pilot valve (1") next to one of the main valves (3")
2011-02-20 18.22.39-1.jpg (21.77 KiB) Viewed 492 times
2011-02-15 17.31.36-1.jpg
Chamber
2011-02-15 17.31.36-1.jpg (38.48 KiB) Viewed 492 times
2011-02-15 17.31.11-1.jpg
Barrel - 20'
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Unread postAuthor: Technician1002 » Sun Feb 20, 2011 9:05 pm

Nice.. Is that barrel 3 inch?
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Unread postAuthor: Jimmy K » Sun Feb 20, 2011 9:22 pm

Not quite.. It's a four inch diameter.
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Unread postAuthor: Lockednloaded » Sun Feb 20, 2011 9:23 pm

I'm glad this project is getting off the ground, t'would of been a shame if you lost intrest/ran out of funds
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Unread postAuthor: Jimmy K » Sun Feb 20, 2011 9:29 pm

I could not agree more.

I should have enough money to at least get it fully operational. I already made the most expensive purchases.

Believe it or not, the most expensive part thus far was a 24 inch long, 3" diameter black iron nipple... Set me back over $100. Utterly ridiculous.
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Unread postAuthor: Lockednloaded » Sun Feb 20, 2011 9:30 pm

what do you need a two foot long pipe nipple for? was there no possible replacement?
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Unread postAuthor: Jimmy K » Sun Feb 20, 2011 9:38 pm

It is going to be welded through the width of the air tank to provide two 3" ports for the valves.
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Unread postAuthor: Technician1002 » Mon Feb 21, 2011 1:19 am

Are you planning to eliminate as many short radius elbows as possible? Using a long radius sweep has much lower loss.
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Unread postAuthor: Jimmy K » Mon Feb 21, 2011 5:29 pm

Where possible, yes. From the air tank to the barrel there are three 90 degree turns, including the tee into the barrel. I will be able to use long radius elbows on the first turn, as the main valves will act as an elbow in between the tank and the tee.
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Unread postAuthor: Fnord » Tue Feb 22, 2011 7:27 pm

Wow, I'm glad to see this might actually happen! Not many people start large build logs and actually keep up with them.

Can't you weld the nips to the front of the tank, rather than the top? You would save a turn by having the ports already facing forward. If you find a pressure rated wye you may even be able to use two 45 degree elbows instead. That would improve your flow patterns.

How do you plan on actually attaching the valves to the tee? You may be building yourself into a corner unless you do the (metal)welding operation last. Or find some big unions.
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