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Pilot Volume - not really variable

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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Pilot Volume - not really variable

Unread postAuthor: sanaka » Wed Oct 24, 2007 7:29 am

Hi.

Check out the beautiful gun in this thread:

http://www.spudfiles.com/forums/shock-a ... 11308.html

A comment was made that this gun's performance might be increased by decreasing the "pilot volume," which it was said was made bigger by the length of hose between the sprinkler valve and the ball valve trigger. Someone even suggested that smaller diameter hose be used to decrease pilot volume.

It should be pointed out the the pilot volume of a diaphragm sprinkler valve is not the volume captured between the back of the diaphragm and the eventual opening to atmosphere. It is instead simply the volume displaced by the travel of the diaphragm. A fixed quantity. The effect of the lines and/or valves downstream has to do with flow, not volume.

To visualize what I mean, imagine what would happen if the hose and trigger valve on this gun were giant, like 8" diameter by 10’ long, so that the volume captured behind the diaphragm was enormous. Like a small room. It would work just fine. Excellent, in fact. It would be like opening directly to atmosphere. The moving diaphragm doesn't push all the air captured behind it to open. It only needs that air to nudge along easily enough not to impede its own travel from closed to open position. So, a hose is only bad if it can't accommodate the required flow. i.e. if it is too small.

Taking the case of this gun with its huge 2" commercial sprinkler valve: Let's say the diaphragm itself is 3" diameter and travels 1.5" when it opens (the numbers are probably less than this). A cylinder of these dimensions is ~10.6 cubic inches. The diaphragm displacement will be something less than this, let's call it 8 cubic inches. With no resistance, the force of 100 psi on this diaphragm would theoretically push it the 1.5" to open position in about .0008 seconds, let's round to .001 or 1 ms. A 1/4" diameter line can flow ~12 cubic feet per minute at 100 psi, hence could only exhaust the 8 cu. in. pilot volume in 23 ms. 1/2" line gets you to around 5ms. You'd need a 1.5" line to have almost zero impedance to the pilot exhaust.

However, I doubt one could even squeeze a blowgun in 23ms, much less rotate a ball valve open. So the size of diminishing returns is probably pretty small - 1/2" line will probably get you all you can get.

The big point being don’t use a smaller diameter pilot exhaust line size trying to get the “pilot volume” down. Go for flow.

Peace,
Sanaka
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Unread postAuthor: experament-u2 » Wed Oct 24, 2007 7:53 am

welcome to the forums :lol:
let me be the first to say your knowledge wont go astray around here
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Unread postAuthor: BC Pneumatics » Wed Oct 24, 2007 9:45 am

I can see what you are saying, and it is a perfect explanation of a system that uses a liquid. However the gas isn't simply being displaced, it is undergoing depressurization. The smaller the pilot volume, the quicker the pilot gas will drop in pressure. This increased drop means the diaphragm will have less pressure acting on it as it opens. (Thus, opening faster)
It is really just a matter of how fast you can get that air out. Smaller pilot volumes are the way to do this if you used a fixed valve. (As most people do. A blowgun.)
Of course a larger pilot valve helps, we have always known this, but with a static valve smaller pilots are directly linked to performance.
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Re: Pilot Volume - not really variable

Unread postAuthor: jackssmirkingrevenge » Wed Oct 24, 2007 12:08 pm

sanaka wrote:Check out the beautiful gun in this thread:

http://www.spudfiles.com/forums/shock-a ... 11308.html

A comment was made that this gun's performance might be increased by decreasing the "pilot volume," which it was said was made bigger by the length of hose between the sprinkler valve and the ball valve trigger.


Are you sure this is the right launcher? The pilot valve is mounted directly on the sprinkler valve, there's no hose to be seen.

Also, agreed with BC re pilot volume affecting performance - there are those who will testify that shorter hose = more power.
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Unread postAuthor: spanerman » Wed Oct 24, 2007 12:50 pm

^like me....i belive it would....as the faster the drop in pressure the faster the valve will open...
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Unread postAuthor: jimmy101 » Wed Oct 24, 2007 1:02 pm

Sanaka: welcome to the forums. It is really nice to get a new member who's first post isn't "How do u make ... can someone give me instructions".

And, it is not only written in real English but it raises interesting points.

As you can see from the posts, a lot of times disagreements come up because people use different meanings for words. Or, someone will post a comment like yours which assumes "everything else except what I am talking about is held constant" and the responses will be "but if everything else isn't constant (which is usually the real world situation) then such and such will happen".

I would think that you are correct, the pilot volume is simply the volume displaced by the diaphragm and not the entire volume between the diaphragm and the vent valve. But the dynamics of that displacement are dependent on the volume between the diaphragm and the vent valve (like BC and Jack said) and the character of that volume.

Since time is a critical variable in the performance of a valve, you really need to look at the dynamics of the system and not just a static quantity like the pilot volume.


It is so cool that someone posted with a concept as complex as "impedance" as it relates to air moving through a system!
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Unread postAuthor: sanaka » Wed Oct 24, 2007 1:26 pm

Thanks for the welcome, experament-u2 :) I look forward to building some launchers and sharing the experience here. Just got some parts from McMaster last night.

Hey, I think you're right BC Pneumatics. I didn't think too much about the whole volume behind the diaphragm being under pressure. At the scale usually concerned, I guess it's more of a chamber situation than a piping system. Still, going overboard with too small of a line would make it like an orifice and constrict the pilot output.

Thanks for the Kudos Jimmy101. BTW, kud yew tel mee how 2 mak a spugun 2 blo up mi naburs howse?:wink:

And...

OOPS! jackssmirkingrevenge is right, the gun I was thinking of with the pilot volume issue was this QEV rifle:

http://www.spudfiles.com/forums/new-qev ... 11296.html

I don;t know what i was thinking typing so much after 2am last night :shock: Same issues apply, though. I really like the tennis ball gun in the first link I gave.


Peace,
Sanaka
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Unread postAuthor: BC Pneumatics » Wed Oct 24, 2007 4:30 pm

I know a lot of the newer members here, people that have been around under a year or thereabout, will think what I am about to say is trivial, but oh well.

The truth is, this thread is pretty amazing. We have a new member here that has a good bit of knowledge and has written a coherent, well composed post. Moreover, Sanaka has shown an ability for actual conversation and collaboration. Everyone that has written a reply was civil, and had good input or at least a kind word.

Jimmy said it pretty well with:
you really need to look at the dynamics of the system and not just a static quantity like the pilot volume.


This thread boils down to something that people do not make direct mention of, the Pilot Volume:Vent Cv ratio. This statistic is as relevant to the performance a valve as the C:B ratio is to an entire cannon system.

Of course there are many other dynamic factors included in Jimmy's statement. The forces acting on the chamber-side portion of the valve (As opposed to the pilot-side portion) will have an effect. These forces show up in differences in surface area, etc. Of course, the stiffness of the spring on the pilot-side portion of the valve also introduce a new variable into the equation.

It is apparent that the performance of our valves include many more complexities than people take into account. Before we go much further, I would think we have to figure out which of these dynamic factors we can consider negligible in common applications.
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Unread postAuthor: ALIHISGREAT » Wed Oct 24, 2007 4:50 pm

hmm i can see your point Sanaka but BC is right.
also i agree with you BC, certainly didn't post intelligent well written topic or replies when i was new, infact i still don't post intelligent well written topics or replies :lol:

but another thing to think about is than Sanaka could have been a 'lurker' and could have been reading the forum for months gathering knowledge, but he/she is probably just an intelligent person.

and what are you planning to make first?
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Unread postAuthor: clide » Wed Oct 24, 2007 10:09 pm

Good to see a new member thinking for themselves.

Despite what is commonly said, an increased pilot volume can often increase the opening speed of a valve . The valve will begin to open at a certain pilot pressure no matter how big the pilot volume. If the moving piston is decreasing the pilot volume faster than the exhaust valve can vent then the pressure behind the piston will increase as the piston moves back. The smaller the pilot volume, the more the pressure will rise as the piston moves back; this translates into a slower opening speed.

However it is still suggested that you minimize pilot volume. First there is the obvious loss of efficiency by having more wasted air. Second, a larger pilot volume takes longer to exhaust the pilot to the firing pressure. Since most piston and diaphragm valves have air that leaks back into the pilot volume from the chamber, the increased time required to vent the pilot area translates into decreased chamber pressure. In many cases the small drop in chamber pressure would have more of an affect on performance than the increased opening speed of the valve.
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Unread postAuthor: octane89 » Wed Oct 24, 2007 10:43 pm

I can understand how less pilot volume will increase the speed of a valve opening, but does the amount of the distance the valve (or in some cases a piston) travels matter then? For example, in a 3" tee valve you could have about 1" of travel for the piston, but then you have roughly 7 cubic inches of pilot volume.
Now I have seen posts where some have had the piston only move 1/4", but would that sacrifice the "flow" and not let the chamber depressurize fast enough? And here's a thought that has came across my mind- The piston has that 1" to travel, but it may open to just as fast to 1/4" as another cannon might, but then the idea of "how long it will take to open the valve" comes into play. I'd like to understand how one can figure the opening speed of a valve like that.
Or is there some equilibrium between ? What sanaka and BC said both have me thinking about this more and more, and I can't seem to wrap my head around it.

I may just need to test a few things out to completly understand, so if you read this and think "newb", no harm will be done.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 24, 2007 11:16 pm

Now I have seen posts where some have had the piston only move 1/4", but would that sacrifice the "flow" and not let the chamber depressurize fast enough?


The whole idea of the 1/4th of the barrel diameter for piston travel (so in the case of a 2 inch barrel, half an inch of travel) is that at that point, the flow would be equivalent to what can flow through the barrel, so opening the piston more than that still won't create better flow because of the barrel constriction.
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Unread postAuthor: octane89 » Wed Oct 24, 2007 11:20 pm

Thank you-that solves the pain in my head. Now to finish my valve.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 24, 2007 11:28 pm

if you want to see the math it's here.
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Unread postAuthor: SpudBlaster15 » Thu Oct 25, 2007 12:49 am

jackssmirkingrevenge wrote:The whole idea of the 1/4th of the barrel diameter for piston travel (so in the case of a 2 inch barrel, half an inch of travel) is that at that point, the flow would be equivalent to what can flow through the barrel, so opening the piston more than that still won't create better flow because of the barrel constriction.


The piston travel really should depend upon the valve's dimensions. For a coaxial launcher, where no T fitting exists to impede airflow, the opening distance should be lateral surface area = cross sectional surface area, or 2pi*r*h = pi*r^2, rearranged and simplified to be h = 1/2r (which is the same as d/4) to solve for h. It is instantly obvious that the d/4 equation will be accurate only when used for a launcher with no flow restrictions around the valve (coaxial).

However, a conventional barrel sealing T valve (assuming the barrel port is similar in diameter to the inside of the T) does not have optimal flow efficiency through all of it's geometry, and modeling would likely be more accurate if the barrel port was assumed to be a rectangle (the actual geometric shape would vary depending on the configuration of the valve). In this case, the equation would be l*w = pi*r^2, rearranged to l = pi*r^2/w.

Assume a barrel sealing T valve with a T diameter of 2", and a port diameter of 1.5". Should it be modeled with travel = d/4, we would have the piston travel optimized at 0.375". Should it be modeled with travel = pi*r^2/w, we would have the piston travel optimized at ~0.88". This means that depending on your exact valve configuration, if you allow your piston to move only 1/4 of the port's diameter, you may be achieving less than half of your valve's potential flow.
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