Simplified Balanced Valve

[PikeP]

One issue with balancing a poppet valve is the duration of the valve cycle having such a tiny window @ ~1ms, that you either get it perfectly right, or not at all.

Ideally you want the most air flow through the transfer plenum which causes the exhaust side of the poppet to see the highest pressure rise in this configuration resulting in a valve that wants to "blow open" much like a Cothran. I think combatting that with a pressure chamber is a very delicate balancing act between orifice sizes.

Maybe the solution to that is balancing some in the throat like shorty is suggesting, attack the problem at its roots.

[shorty]

Change yellow highlighted cells. Green are results. It may not be correct but sure gets you thinking.

[shorty]

Simple Cobra.

[shorty]

This is Lloyds example that gave me the idea.

I also did the step poppet (SS valve ) and Cothran but will not share unless they say go ahead.

To me,
This was an eye opener (even if it has errors). Just knowing the "real" specs on any of these valves and running a DOE in minitab could possible map these valves with the least amount of work.

I hope you guys don't mind me using your diagrams.

[rsterne]

Tim, please explain the term in the closing force equation (D-F)…. When the valve is open even slightly there is HPA surrounding the poppet on all sides, and "D" is only a hole in the valve body, it exerts no force on the poppet.... I think that term (which is the force pushing the poppet away from the seat, if I understand you correctly, hence the negative sign) should be (B-F)…. This means that the diameter of the stem F cancels out, and the equation (assuming no air passes through the vent E) becomes....

Closing force = (B-C) - (B-F) + (A-F) = B - C - B + F + A - F = A - C …. which works out to 90.49 lbs....

I realize that you have not included any pressure change in the balance chamber C, through vent E.... but during the time the valve is open, the pressure in C is rising from atmospheric to some value nearing 3000 psi.... If we assume that the pressure becomes 3000 psi, then the forces change completely.... There is no net force across the diameter of the balance chamber at C, so the force there is zero.... This means that the entire poppet, with the exception of where it is penetrated by the stem F, is surrounded by 3000 psi.... This results in a closing force on the poppet, using the same equation as above, but letting C = 0, of....

Closing force = (B-C) - (B-F) + (A-F) = B - C - B + F + A - F = A - C = 184.75 - 0 = 184.75 lbs....

If there is a partial pressure rise in the balance chamber C, then the closing force will be some value between 90.49 and 184.75 lbs.... If the pressure is 1500 psi, then C = 47.13 lbs, and the closing force = (184.75 - 47.13) = 137.62 lbs....

Forget the concept of a balanced valve for a minute, and examine the effect of a larger valve stem.... If you have a 1/8" stem, the closing force is only 36.82 lbs.... With a stem diameter of 0.280", you have increased that 5 times to 184.75 lbs.... That means you will need 5 times the hammer momentum to get the same dwell.... You have vastly increased the hammer strike required because you have increased the diameter of the stem.... The same thing happens with a balanced valve, a bigger stem means more hammer strike is required.... What possible reason can you have for wanting to increase the closing force of the valve?.... All that does is increase the hammer strike required.... The whole point of a balanced valve is to reduce the hammer strike, not increase it.... You need only to substitute an MRod poppet in a Disco, changing nothing else, to see the advantage of a smaller stem on hammer strike, because you are reducing the stem from 5/32" to 1/8"....

Bob

[rsterne]

Your post labelled "Simple Cobra" is a conventional valve.... and the same problem exists, the valve "throat" has nothing to do with the forces.... If you use the minus sign to represent forces "away" from the seat and the plus sign to represent forces "towards" the seat, you will find it much easier to understand what is going on.... JMO....

Bob

[PikeP]

I'll interject by saying I've tested bigger and smaller stems and have found in my tests that the majority of the hammer energy goes into cracking the valve opened as it has to overcome anywhere from 100 to 400 static lbs holding the valve closed. It takes more energy to overcome 100 to 400 lbs static lbs of force on the poppet while closed and creating lift opposed to rising pressure against the stem OD. Pressure equalization in the transfer plenum happens faster than full lift can be generated so the valve stem OD does help greatly in closing a valve, which requires more hammer energy to counter after valve is cracked off its seat...the question I have is will a poppet with just enough pressure on it to seal operate with less than conventional hammer energy when balancing at the stem opposed to variable pressure at the poppet.

It only makes sense that if the valve stem is an integral part of closing a poppet valve that one should consider an approach revolving around just that...that is if closing the valve is your issue...especially in a timely manner. Just my 2c...if one had all the answers we wouldnt be here asking questions.

[shorty]

"please explain the term in the closing force equation (D-F)…. When the valve is open even slightly there is HPA surrounding the poppet on all sides, and "D" is only a hole in the valve body, it exerts no force on the poppet...."

I thought as the poppet opens and allows pressure to enter between the exit port and the pellet that "that" area increases in pressure. Now there is pressure on both sides of the poppet (HPA side "high" and exit port to pellet side "lower"). I thought that's how the mechanism worked.

In the simple revisions, since the valve stem is "knotched ?" I would think that opening force would not be affected since it's not on the HPA side until the valve is opened (creating a lower cracking force and faster closing in a balance design and just creating a faster closing in a cobra design).

I know some of the math is wrong  But, I want the same thing as you.

Is there any chance you can put together a spread sheet similar to mine to help and post it?

[PikeP]

Generating minimal lift ie cracking a conventional poppet valve takes quite a bit of energy, once that is achieved you enter your tuning range where adding tiny bits of preload to the hammer spring which slightly increases hammer energy starts to have great effects with said tiny adjustments. Therefore its without question the pressure holding the valve shut and creating minimal lift absorbs a lot of energy before the transfer plenum even comes close to equalizing or we would have very wide ranges of spring preload required for tuning.

Remove most the above via reduced reference of poppet to pressure and exchange that for valve stem pressure nearly equal and the valve should operate...but the question is how well..and can a balance there be achieved. Just my 2c on shorty's idea. I have no clue if it will not work as I have yet to put enough thought into it and pen on paper ie data models dont carry as much weight as does prototyping and tangible models.

[PikeP]

Example from my data.

.125" stem turned at 20 fpe. Reduced stem bore and installed new poppet and stem with od of .096". No hammer spring or weight change resulted in no change in fps whatsoever...boggled I tried a different tune..tuning at 60 fpe on the same configuration, the. 096" stem took approximately 20% less hammer energy to achieve 60 fpe... which meant my tuning range narrowed, yet the valve was no easier to open on the low end.. the 20 fpe tune was 685 fps with 20.06gr and the 60 was 900 with 33.95gr. Due to the narrower tuning range (cut in half) I experienced double the extreme spread...which resulted in me going back to a .125 stem.

That to me says the valve stem od is very important in valve closure in the latter half of valve operation, and negligible in the former.

Therefore IMO valves with too narrow or no tuning range need more closing force in the latter part of valve operation. Also too narrow of a tuning range results in higher extreme spreads and a wider one will result in lower the further from velocity plateau you are.

[rsterne]

Tim, you are correct, the diameter of the valve stem makes no difference to the cracking force of the valve.... However, that force is only applied during the brief time the poppet (seat) material is decompressing, and for a few microseconds after that, while the port volumes (between seat and pellet base) are rising to (nearly) reservoir pressure.... A Delrin poppet at 3000 psi may be compressed 0.010" and a PEEK poppet half that, so the hammer energy required, while significant, and worthwhile reducing by using a balanced valve, is only part of where the hammer energy is used up.... Most of the remainder goes into overcoming the force from the pressure in the exhaust port acting on the valve stem.... Increasing that force will slow the hammer more while the valve is opening, reducing the lift and dwell.... and the slam the poppet closed faster as well, further reducing the dwell.... I can assure you that increasing the area of the valve stem by a factor of 5 will drastically increase the hammer energy required to release a given amount of air from a valve....

Bob

[shorty]

Ohhh, The "input" numbers supplied in the spread sheet mean nothing. I only supplied the spread sheet so that if someone wants to change a few of the balancing and closing forces, they can do it to see what happens.

And in hopes, see and improve on it. Like you I hope.

By all means,
None of those "input" numbers I put in are legit at all.

[mann]

I think if I were to use derlin I maybe able to get the valve to seal all the way  curious are you use peek or derlin Bob

[rsterne]

Mark, I am using PEEK.... I have been giving this valve a lot of thought (since I can't seem to get any shop time ATM)…. and just like the SS Valve, I think we want as large a vent as possible through the poppet, between the valve exhaust port and the balance chamber.... With a large vent in the SS Valve, the pressure rise in the balance chamber is very fast, and the valve transitions from a balanced valve while cracking to a conventional valve during the rest of its cycle.... This makes it more responsive to tuning with hammer strike, and avoids it "blowing open" to any significant degree.... Yes, it requires more hammer strike than with a tiny vent through the poppet.... but the tuneability increase is huge and well worth it....

BTW, none of my SS Valves use a restrictive front jet.... I use a 1/16" hole through the thimble mounting screw.... I want to control the dwell with hammer strike to the greatest degree possible, and combined with the large plenum I always run, that means the pressure on the front of the poppet will always be reservoir pressure....

Back to your original question, yes, since you have a large front section on the poppet, and therefore little sealing force available.... a Delrin poppet would be a good idea.... 

Bob

[rsterne]

Tim, IMO you can remove the small part of the stem diameter, F, from your spreadsheet.... It works in both directions all the time, and so cancels out.... In addition, where you were using the throat diameter, D, you should just use the poppet seat diameter B.... Once the poppet is open, B also cancels out, of course....

HTHs....

Bob

[WyoMan]

As an aside, but kind of on-theme with this topic is getting hard sealing surfaces to seal with the reduced contact pressure on the seal/seat.

I’ve had reasonable success by simply using enough mechanical force on the seal and seat, and then letting it sit (somewhere warm) for a day or so. No lapping/abrasion necessary as it will cold-form to the seat. It can also correct an alignment issue (i.e. stem not perfectly square to the seat).

This is a 13xx valve with a Delrin exhaust valve. I’m using the wooden spacer so I can apply mechanical force on the seal by screwing the valve halves together:



This is the bottom of the Delrin seal after a day or so:


I can get them to seal in a 13xx which is pressurized from empty at about 150 psi per pump. The air-tight seal may be due to the squaring of the stem, or the pressing-in of the new sealing surface, or the increase in the seal contact-area (the sides of the groove)… or all of the above…. don’t really know but I got the idea from Don Cothran… it works for me.

Great stuff guys… this has been an informative thread. Thanks-

Wyo

[mann]

I will give derlin a try . As far as blowing open the valve seems to open easy but also slams shut fairly fast I suppose it could be blowing open to some degree . It doesn't stay open though I tried quite a few hammer strikes and it seems to want to shut. I will get a few different bits from the hardware store and open up the hole some more I'll go up to .050 also with a derlin poppet

[rsterne]

Mark, that is good news indeed.... I suspect that there will be some vent diameter above which you can no longer find a difference in the way the valve cycles.... Even my .457 cal Hayabusa functions fine (500 FPE! ) with just a 0.040" vent.... I know this is a different beast, but I would be surprised if you need to go past that diameter with your much smaller valve....

Bob

[skorec]

Wow
 I am just  thinking dummy way  haw to improve the SS balance valve and here is it already solved.

Bob , Probably shorter moving of valve popped  is compensating with larger area D  ? 
         I hope that  repairing our mistakes  at thinking may précising your  ideas.

Lloyds simplifying idea of vent hole insidev the stem  may decrease the price.   

[Brian W Cook]

Here is my version of a balanced valve i built almost 17 years ago.   If i was to build new version it would probably be a little prettier as i built the first one while working night shift .  It was a bit overkill for a .452 barrel that i mostly shot round balls out of .   I vented it through the stem to a relief cut up in the valve body .   I had a vent into the port on the first version but can’t remember what the differences were  if any .