Piston valves explained visually

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Postby Navigator7 » Thu Jun 15, 2006 10:53 pm

Clide...That would be great.
I'm a Mac guy so ggdt is off limits to me.

The C:B ratio for a combustion cannon has been beat to death pretty good but I don't hear much discussion on piston ratios.
The other thing not talked about is how far the valve really needs to open....
and....
Fluid dynamics around the barrel entrance.

There was an article I read about deep well drillers and their discovery with the flow of fluid through pipes.
One of the worst flows possible available is into any square cut pipe....the end of a pipe exactly like that in most every spudgun.

They had pictures of vortexes and back eddies that form during high flow periods and discussed how much flow actually chokes the pipe off...and it is really significant! I would think especially with piston valves.

The fix is really easy which is like a venturi insert into the pipe....which actually makes the pipe inside diameter smaller but because the insert direct the flow...it stops the back eddies and allows much more flow.

Basically I am saying most cannons could benefit from installing a simple insert.

We purchased carbide nozzle inserts used for deep well oil drilling...only we used the nozzles for another application. In one the boxes we ordered was some literature about flow into pipes. I think it was Smith International or Smit International, a company in Texas that makes all sorts of drilling applied tools and such. One thing they were offering was these weld on inserts for pipe ends to increase flow in a pipe.

So...after writing this...it occurs to me the the chamber sealing valve has an advantage over say a coaxial.

Sure like to view others thoughts on piston ratios.
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Postby clide » Fri Jun 16, 2006 11:47 pm

GGDT pretty much confirms what I was thinking as far as changes in sealing port size. The limit is highly dependent on exhaust valve size (and probably other things as well).

How far the valve needs to open is easy in a coaxial, ideally it would be 1/4 of the barrel diameter, a little more to be safe wouldn't hurt much. A tee valve makes things more difficult, but my guess would be that 1/2 of the barrel diameter would work in most cases.

Fluid dynamics around the barrel entrance I can't help you much with, maybe a D_Hall or Freefall type person could give you some insight on that.
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Postby Navigator7 » Sat Jun 17, 2006 11:41 am

Tks Clide!

Sounds like the piston ratio thing I was talking about can be a huge variable depending on the application?

For debate sake.....
Any thoughts if say .....you were to build a chamber sealing piston valve with a 2" seal on the chamber but a 4" diameter piston on the exhaust side?
Pro's / cons?


Regarding pipes....
Where I see the problem in "choking" is in co-axial and barrel sealing piston valves.
The squared edge pipe was the problem the oil guys were dealing with.

What I don't know is at what flows and pressures the choking begins. Like I said, I got my info from the deep well drilling.

I think Freefall commented on the phenomenon of "choking". I can't remember if he said we got to those pressures and volumes spudding or not.

I learned about QEV valves in 1996 during an unintended on the job training SNAFU.
Marine weather had rotted out the old one and the company no longer existed.
We needed to replace a valve that would release pressure and volume on an adjustable dial in range from 2 PSI to about 80 psi.
It worked really cool:
You dial in 2-80 psi on a regulator and when the hand valve was activated it would dump at 2-80psi, whatever you set the psi....no more or less. If you then dialed the pressure down...the valve leaked off to the new pressure setting.

The idea was to pressurize a long clamp with a moving track.

Replacing valves long extinct and not being a pneumatics engineer, I ordered and installed a 2" QEV valve that worked like this:
You preset the activation pressure to 2-80 psi on a regulator and when that pressure was reached the valve would dump all the air at full system pressure!

The valve was connected to 5ea, 12" dia air cans that pushed down a track used to grip large power cable and push or pull it.

In action, the unit honked like a sprinkler but sounded like the T-Rex in Jurassic Park...the unit was alive and insane as we had no control over it!
;-) It was kinda funny.

There was no taking the valve back so I disassembled it to learn it's secrets. What amazed me was how little the valve disk actually moved!
The valve that did the sealing had multiple sealing surfaces....It looked like a martian flying saucer, a frisbee or cross between an automotive exhaust valve and a spool valve. It had multiple sealing surfaces which the literature suggested guaranteed very fast opening times and little opportunity to be jammed by foreign material.

It was called an adjustable piloted QEV valve.
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Postby clide » Sun Jun 25, 2006 12:19 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">
For debate sake.....
Any thoughts if say .....you were to build a chamber sealing piston valve with a 2" seal on the chamber but a 4" diameter piston on the exhaust side?
Pro's / cons?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Sorry for the late reply, been on vacation.

Since the piston is so much larger than the sealing port it would allow you to use a small pilot valve without slowing the opening speed down a lot. It will also be easy to seal against the port since there is a lot of force to hold it closed.

But you are also wasting quite a bit of space having a 4" tee with only 2" of flow. You could move down to a 3" piston and still have plenty of opening speed for most applications.

Or you could move up to a 3" sealing port and have a lot more flow, and since it's still not at an extreme as far as piston size vs sealing port size, the increase in flow will trump any decrease in opening speed you will get.



Some of my tests would suggest that a chamber sealing valve actually has more issues with flow choking than a barrel sealing valve. Although the test was very limited and not enough to say that conclusively.
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Postby Navigator7 » Sun Jun 25, 2006 4:35 pm

Well...dang...let's get more specific
http://s76.photobucket.com/albums/j18/N ... Waterdawg/?

The link above is to a slideshow of a 9" piston valve I'm building.

<img src="http://i76.photobucket.com/albums/j18/Navigator7/Waterdawg/WDa7.jpg">

The critical measurements as I see it are the minor and major measurement of area acting on the piston.

The widest spot that internal air pressure will be trying to force the sealing piston out the barrel is 9.12......ahhhh...lets just say 9".

Inside the 12" chamber is a 10" sleeve. The OD of sch 40 pipe is 10.75.
The ID is 10.02. This will be machined probably to 10.2.
Two sliding pistons ...say 10" have air pressure acting on them. One sealing the barrel shut and the other...trying to race out the ass end when the air is dumped.

If the air is dumped cleanly I calculate 1492 Lbs more pressure trying to force the piston off the barrel.What is a good ratio?
100 pounds or 5000 lbs?
My question is....how can you tune this to make the valve act faster or slower?
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Postby Navigator7 » Mon Jun 26, 2006 10:23 am

I found and then lost a webpage that showed the flow into pipes.
Thought I could find it again and share....but naught.
Essentially it was establishing a coefficient rating for different pipe entrances.
Some very real figures for us spudders.
The very worst coefficient was a cut off square pipe! ....It's coefficient was something like .50 or .58 something like that.
The very best was a pipe with an inside radius. It had a coefficient of like .89 or .90!
This link below is not the same...but the principle is the same.

Notice the water tube showing the pressure differential.

My point is....any of us using barrel into chambers might have beneficial performance simply by a radius in the inside barrel entrance!

I knew this before building my last cannon. Using a router, I performed this simple circumcision of every one of my pipes in the WaterDawgâ„¢ and believe it benefits performance.

http://www.aeromech.usyd.edu.au/aero/fp ... ode43.html
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Postby clide » Mon Jun 26, 2006 10:57 pm

Well since that is a barrel sealing valve, the 1492 lbs shouldn't really come into play. 6360 lbs is the number you need to be looking at. That's how much force is shoving it against the seat and how much force is going to be shoving it towards your endcap once the valve begins to open. Needless to say, that piston is going to be hauling ass towards that endcap.

The only ways I can think of to slow it down much would be to minimize pilot volume as much as possible. That way the piston has a chance to compress the air in the pilot to a high pressure and slow the piston. Other than that, have a good bumper and I wouldn't recommend standing directly behind it ;).

Yeah I try to bevel all the edges of my pipe when I am making valves. Can't say for sure that it does anything, but it certainly can't hurt anything.
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Postby Navigator7 » Tue Jun 27, 2006 6:11 pm

Oh yeah...I didn't show a UHMW ring between the piston and the end cap. It was on a different layer and I left it off to keep from complicating the jpeg.
Good observation...Tks!
The ring spans the outer diameter of the piston and is hollow in the center to allow for incoming and outgoing air.
The piston travel is 2''.

This is exactly the discussion I wanted.

The differential pressure is needed to make the piston even lift off or the valve stays stuck on the barrel.
If the differential pressure is too close...the piston ...IMHO would move slowly...the projectile would poop out the cannon barrel and THEN you would then have a big rush of air. Is this an error in thinking on my part???

The ratio too high and you damage the piston or break off the end cap...which in this case is a 300# steel flange welded inside and out. 12 ea (1 1/8") bolts holding the end cap on. An ugly place to be in the event of failure.

At 300 PSI the flange should be able to withstand 33,912lbs of force...but not shock loading. I don't remember if the safety factor on end flanges is 3 to 1 or 5 to 1. If 3 to 1, I'm guessing good to go.

My computer says the moving piston assembly weigh 140 Lbs. Part steel and part UHMW. I'm not exactly sure that is true ...I'd guess in th 60 Lb range from all the steel and UHMW I've machined.

I've seen, heard and felt water hammer in 12" pipes and it will make the toughest set of tough guy union nuts shrink to the size of raisins. I'm sure the weight of a water piston weighs way more!

I think radiused pipe edges on the barrel is like balancing and blueprinting an engine. The performance gains...at least in the 60's and 70's....when I cared about that stuff ....was huge. It's all about airflow. If we are choking off airflow at all...we are choking our spudguns exactly when we need the most flow....and the higher the flow the more the restriction. I'm sure our air cannons are 60's technology 4 decades too late! LOL.

I'm really interested, Clide, in what you have to say about...for lack of a better word...the "pressure differential" created by area blocking the barrel and the area of the piston forced back when the air is dumped. Does GGDT call it something? (Mac user...no GGDT)
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Postby Zorrowannabe9 » Tue Jun 27, 2006 10:19 pm

will piston valves opperate correctly with a bike pump?
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Postby clide » Tue Jun 27, 2006 10:54 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">
The differential pressure is needed to make the piston even lift off or the valve stays stuck on the barrel.
If the differential pressure is too close...the piston ...IMHO would move slowly...the projectile would poop out the cannon barrel and THEN you would then have a big rush of air. Is this an error in thinking on my part???

The ratio too high and you damage the piston or break off the end cap...which in this case is a 300# steel flange welded inside and out. 12 ea (1 1/8") bolts holding the end cap on. An ugly place to be in the event of failure.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
You need to remember that the pressure in the pilot chamber isn't completely vented when the piston starts to move. As long as it's enough force to overcome friction of the piston won't really see a difference in initial opening force because its going to move once a certain force is reached no matter what.
Whats important is how much of a jump in force the piston has once it starts opening.

Also, for a valve this size, once it starts opening, there is a 63 lb increase in force for every 1 psi increase in pressure in the barrel, I don't think you'll have any problems with the projectile pooping out the barrel [:p]

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">
I'm really interested, Clide, in what you have to say about...for lack of a better word...the "pressure differential" created by area blocking the barrel and the area of the piston forced back when the air is dumped. Does GGDT call it something? (Mac user...no GGDT)
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
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.

Edit: Zorrow: Maybe, what kind of size are you looking at? I was able to seal my first coaxial with a 4" piston with a bike pump, but it took some furious pumping. And I have heard several people report that theirs won't seal with a bike pump.
If you build one and it doesn't work then adding a spring to reset the piston, or maybe making a small chamber to pressurize and then release behind the piston would be a couple of possible solutions.
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