Simplified Balanced Valve

[PakProtector]

hey-Hey!!!,
It could be that you can shrink the balance chamber volume with some solid packing( like a pill of delrin). should be able to get it a wee bit smaller, and enough reduced to see if that is going to help...

Douglas

[rkr]

There is enough flow in pcp valve to $uck O-rings from "half gland" and yet still I have been told that it doesn't matter that much in closing the valve?

Martyring doesn't work so well as some might think.

It's really simple to try how much flow affect closing, make a really streamlined valve and see how much difference you'll get when adding a corner or two retaining the same valve area.
Studying fluid dynamics and dry flow can reveal interesting things.

And nature really knows how things should be done, just take a look at some of the birds flying or the fish swimming , wondering how the shape became like that.

Marko

The flow will create low pressure area in front of the "sail" of the poppet so it should help in closing the valve. How much force that creates is IMO impossible to calculate but we could experiment with that phenomenon in order to get some educated guesses.

[PikeP]

Something like this might help? If interested let me know and I'll share the excel file. Would imagine once fully functional if its not already to be worth the sweat and tears.

The second pic shows what happens when you change vent size or use the suggested chamber volume.


The last pic is same as first but only changed chamber reduction from 70 to 60

[PakProtector]

On the ss valve, we have two areas that can force the valve shut. Small piston( which is always at tank pressure), and large-small which is at rest vented to the outside, and provides the reduction in seat pressure when closed. Given the results Bob has so far gotten, it *SEEMS* that the vented balancing chamber is not coming up to throat pressure when fired, and the valve knocks open. I'd count that as a vote for the large stem behind the throat, which when pressurized during firing can deliver a significant close force...

I wish I had some time to try building one...but at least one other is on the way. Looking forward to seeing the results.

Douglas

[Brian W Cook]

I had kinda forgotten about my old experiment until i had seen some post about Cothran valves and the balanced valves .  I had to go dig the reservoir tube out of a pile of airgun stuff and disassemble it to see what exactly i made .   Working nights for 20 years will make a guy forget stuff.     
I think on mine the airflow around the poppet does most of the closing action.   
Speaking of air , if you could magically put your hand in a tank of 3000 psi air what would it feel like viscosity wise ?

[rsterne]

Marco, thank you for the suggestion of streamlining the poppet to reduce the closing force on it.... Could you please explain how I can do that further when the front and sides of the poppet are already inside a streamlined thimble?.... Or, are you suggesting that I lose the lower part of the thimble in an attempt to have the flow help close the poppet faster?.... Some specific suggestions instead of broad generalizations or criticisms would be most welcome....

Douglas, I could perhaps extend the spring seat on the poppet into an internal guide (stuffing) to get rid of some volume....

Mike, I will need to study your spreadsheet further (there is a lot of information there)…. but it would seem that drilling out the front cross vent so that the whole vent is 0.047" (or larger) should help, no?.... I also don't understand the "percent chamber reduction" (70 or 60), is that the small end of the poppet divided by the large end (diameter ratio), and if not, what is it?....

Bob

[MJP]

It was meant as an example to try on a normal non balanced valve without any valvespring.
Just to demonstrate how much there is closing force just by in flow alone.

Brians valve is definitely aided by flow over the valve head directed by the slots.

Marko

[rsterne]

Brian, I don't know what 3000 psi air would "feel" like, but perhaps this representation (molecules not to scale, but spacing is) would be a good visual....



The point is that the molecules of nitrogen and oxygen are much closer together in compressed air.... the molecules themselves (which are incredibly tiny) don't compress, what happens is that you reduce the space between them.... In a liquid, they are touching but still free to move.... and in a solid they are in what amounts to a crystalline structure, each molecule is vibrating in place, but fixed relative to its neighbours.... The density and viscosity of air at 1 bar are: Density= 1.189 kg/m^3, Dynamic Viscosity= 18.23, Kinematic Viscosity= 15.33.... at 200 bar they are: Den= 232.02, Dyn.Vis= 23.99, Kin.Vis= 0.1029.... Sorry, but I don't know viscosity relates to "feel".... Compressed air at room temperature, above about 550 psi, is considered a "supercritical fluid".... 

Bob

[rsterne]

It was meant as an example to try on a normal non balanced valve without any valvespring.
Just to demonstrate how much there is closing force just by in flow alone

.

OK, got it, thank you.... Can you give me some feel for how much difference the shape of the poppet makes?.... For instance, no spring, two poppets both the same diameter, one a cylindrical shape and the other streamlined (eg parabolic nose, or whatever you find best)…. With no other changes, are you talking a couple of turns on a preload adjuster?.... or "WOW, I have to change to a MUCH lighter spring" ?....

Bob

[Brian W Cook]

Was just throwing that question out there .  Sitting here at lunch i can wave my hand around and feel the slight resistance of the air .   Was wondering what 200 times that would be like .   Sometimes things that can’t be felt or seen are hard to imagine.   
Shortly after my piston valve stuff i figured out knocking a conventional.437 poppet open really wasn’t that hard and just went with that setup.   
If i were to revisit the piston valve it would be to get a .750 port to open for my 1” rifled barrel i whipped up a few years ago . 
Thanks for the technical data Bob !

[MJP]

It closer to the WOW than couple of turns.
In my 9mm pistol the streamlined would dump the tank vs stepped poppet pressure drop was around 50bar from 250bar.
On a same spring and hammer.
Cylinder is around 65cc

Marko

[TPL]

Sorry if we don't have a formula or graph to estimate the closing force by flow but it definately is there as we can consider it. What we have got so far is no - there is no meaning, it is common missunderstanding. Ok, that'll be it.

Where is our doctor of physics now? His calculations incuded temperature changes too?

PS: Is it that hard to test?

[rsterne]

OK, Mike, I printed out those screen shots of your spreadsheet so that I could study them.... In a nutshell, compared to the first one, where the chamber fill time was 0.96 mS, the other three options reduced it to 0.73-0.76 mS.… You only changed 1 thing at a time (thank heavens, or I would have gotten lost)…. One option was increasing the vent from 0.040 to 0.045", which moves the air faster, filling the balance chamber sooner.... Another option was reducing the height of the balance chamber from 0.20" to 0.15", with a corresponding reduction in the volume of air to move to fill the balance chamber, again filling it sooner.... The last option was to reduce the diameter of the front portion of the poppet from 0.28" (70%) to 0.24" (60%), which increases the force holding the poppet on the seat, and also the volume of the balance chamber, again filling it sooner.... (I noticed that you increased the hammer spring rate to knock open the now harder to open valve, completely appropriate)…. OK, so I got how those changes work.... However....

I am complete baffled by the cells labelled "Reduced lbs." and "Lbs Valve Closed".... I have no idea what they are.... Using your first photo, the poppet OD is 0.40" and the front of the poppet is 0.28" (70%)…. When the valve is closed, there is no stem force.... The force on the poppet towards the seat is:

(0.40^2 x PI/4 x 3000) - (0.28^2 x PI/4 x 3000) = 377 lbs - 185 lbs = 192 lbs.... I don't see that number anywhere.... This means, of course, that I don't understand the "xx% Equalized Force" cells either.... HELP !!!

Bob

[rsterne]

Marco, thanks for that, it gives me some idea of the magnitude of what you found....

Timo, please don't just throw fuel on the fire, either help or don't bother.... Generalized statements may work for you, because you understand, and have seen the results first hand.... However, without quantifying them, it is impossible for others to understand the importance of these effect.... If you have already done the tests, why would you not share the results, instead of saying "is it that hard to test" (in other words, do it yourself)?....

Bob

[PakProtector]

Viscosity of air at 300K:
1 Bar 184.5 N-s/m^2
100 bar 205 N-s/m^2
1000 Bar 545 N-s/m^2

Get air at 1 Bar up to 1300K and its viscosity is 496 N-s/m^2

Did not find a value for 200 Bar/300K yet, but I'd bet on something OTO 230-240 N-s/m^2.
cheers,
Douglas

[TPL]

Bob, unfortunately I cannot quantify it any better than "remarkable".

But if we can discuss about it as existing phenomena, we have come over some threasold now anyway.

Again, I am sorry for not saving all the measurements on scientific basis but am still quite sure myself.

Why would I need to be the one who is quantifying it? I'm just saying it is real.

[oldpro]

https://www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601.html

[rsterne]

Mike, I have a spreadsheet also (compliments of Lloyd) that calculates the closing force on the poppet at various stages of the valve cycle.... The chart below is for my current valve (7/16" poppet with 5/16" balance chamber).... with three stem diameters, 1/8", 3/16 and 1/4".... A conventional valve of the same poppet diameter, but no balancing, is shown in black for comparison....



Numbers above the "zero" axis indicate a closing force.... while those below it indicate an opening force, trying to blow the valve open before the balance chamber fills with air.... The horizontal axis is the percent of 3000 psi in the balance chamber and is NOT the complete valve cycle.... Mike says that is about 1 mSec.... To get some feel for what the forces are over the entire valve cycle, you must extend the final value of each line, which is the closing force on the stem area that penetrates the valve body, over to the right for the entire length of the valve cycle, which is 2-3 times (or more) the time it takes for the balance chamber to fill to 3000 psi....

I hope this will allow all of you (as it did me) the opportunity to visualize the forces acting on the poppet, and why the current version is blowing open....

Bob

[rsterne]

Thanks for those graphs, Travis.... It appears that the Dynamic Viscosity varies little at room temperature below 500 bar.... However, nearly all the decease in Kinematic Viscosity occurs between 1 bar and 50 bar, and above that it changes little.... Dynamic Viscosity is the difficulty in "shearing" the liquid as something passes through it.... ie the resistance felt by the moving object.... That changes little for air below 500 bar.... Kinematic Viscosity is that value divided by the Density.... Since the Dynamic (shearing) viscosity doesn't change much, but the Density increases rapidly with pressure.... the Kinematic Viscosity drops rapidly as the pressure increases.... For our purposed, logic tells me that the Dynamic Viscosity would be what we would "feel".... and below 500 bar it doesn't vary that much.... It increases by about 1/3 at 3000 psi....

Bob

[Brian W Cook]

Looks like a pretty slow increase till it’s beyond 8 to 10000 psi .  That helped clear that up .  Thanks !