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Quick Opening Valves?

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: Technician1002 » Wed May 13, 2009 4:31 am

Moonbogg wrote:How do you think a chamber sealing piston valve would perform if it was released mechanically without any resistance behind it at all? No pressure equalization and the piston housing could be a strong, well vented cage. What do you think?


I think it would be very fast, but the mechanical trigger would have quite a bit of pressure on it in the larger sizes. Using a sear on a 2 inch piston with 100 PSI would require a strong sear that can still move while holding over 300 LBS of force. It would open faster than the QDV. In the sub millisecond valves GGDT shows very little performance changes in any valve under about 2 ms opening time.

The chamber sealer would have a disadvantage of a larger dead space if it was built in a T for a full around vent, or it would need a longer stroke if built in a small T with just one side of the piston opening to the barrel. It would be worth modeling in GGDT both ways.

I think it would be easier to build and trigger the QDV for about the same performance to take advantage of a smaller dead space, simple trigger, and if coax, higher efficiency.

inonickname wrote:Jack did a barrel sealer that was released mechanically. He stated it showed improved performance over an equivalent valve but the opening time on a proper piston valve with a powerful pilot is not slow by any means.

As for chamber sealing, I'm unsure.


On the QDV with the sliding trigger rod the initial opening is basicaly a hammer blow to the piston. It appears to fully open in under 1 ms.
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Unread postAuthor: Moonbogg » Wed May 13, 2009 5:31 am

Technician1002 wrote:[ Using a sear on a 2 inch piston with 100 PSI would require a strong sear that can still move while holding over 300 LBS of force.


I had thoughts about using a turn release shaft with thrust bearings to eliminade axial load. You could have 500psi in the chamber and trigger it loose with your pinky finger.
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Unread postAuthor: Technician1002 » Wed May 13, 2009 7:26 am

Moonbogg wrote:
Technician1002 wrote:[ Using a sear on a 2 inch piston with 100 PSI would require a strong sear that can still move while holding over 300 LBS of force.


I had thoughts about using a turn release shaft with thrust bearings to eliminade axial load. You could have 500psi in the chamber and trigger it loose with your pinky finger.


I haven't figured out the logistics of such a trigger. The thought of a spear chucked by a multi hundred pound force out the back of the piston chamber does not appeal to me. :shock: You will need some kind of safe decelleration for the piston and trigger so it stops safely. It will pick up some speed. The release shaft needs to be out of harms way.

Care to do up a drawing?
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Unread postAuthor: jeepkahn » Wed May 13, 2009 9:44 am

I just posted a ROUGH drawing of a sear released valve the other day... Here it is again... It's actually a sear released pilot, that uses a dump valve similar to tech's, for an extremely large pilot vent(relative to pilot volume)... but also allows for light trigger pull even with HIGH chamber pressures...
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Unread postAuthor: jeepkahn » Wed May 13, 2009 9:50 am

double post to keep pictures and description from getting mixed up...

And here is a method of using a stepdown for a 1500+psi chamber volume and a 500psi< pilot volume, so that you could build a LARGE, VERY HIGH PRESSURE, cannon that could still be piloted by a $12 qev...

The descriptions in the picture is for a large copper cannon, but the principles would still apply for a large bore steel cannon...
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Last edited by jeepkahn on Wed May 13, 2009 10:01 am, edited 1 time in total.

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Unread postAuthor: Technician1002 » Wed May 13, 2009 10:00 am

jeepkahn wrote:I just posted a ROUGH drawing of a sear released valve the other day... Here it is again... It's actually a sear released pilot, that uses a dump valve similar to tech's, for an extremely large pilot vent(relative to pilot volume)... but also allows for light trigger pull even with HIGH chamber pressures...


Now I understand. The pilot is the only part you are talking about. The main cannon valve is still a traditional barrel sealer piston valve. I was trying to see how you would trigger the main valve that way.
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Unread postAuthor: jeepkahn » Wed May 13, 2009 10:09 am

Technician1002 wrote:
jeepkahn wrote:I just posted a ROUGH drawing of a sear released valve the other day... Here it is again... It's actually a sear released pilot, that uses a dump valve similar to tech's, for an extremely large pilot vent(relative to pilot volume)... but also allows for light trigger pull even with HIGH chamber pressures...


Now I understand. The pilot is the only part you are talking about. The main cannon valve is still a traditional barrel sealer piston valve. I was trying to see how you would trigger the main valve that way.


Egg-Zactly.... the performance is going to come from the large ventability relative to pilot volume...I'm not sure the efficiency rating of the QDV but, I do know that a QEV is only about 40-45% ....

And I just did the math, on a 2"barrel/4"chamber filled to 1500psi, using a 1" pilot connector and a 4" pilot disc, you would only need 100psi in the pilot chamber to hold the valve closed if using the stepped pilot....

Either way, this would definitely be the trick for piloting high pressure guns without worry of qev failure, or having to use a ballvalve to fire...
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Unread postAuthor: Technician1002 » Wed May 13, 2009 12:04 pm

jeepkahn wrote:
Technician1002 wrote:
jeepkahn wrote:I just posted a ROUGH drawing of a sear released valve the other day... Here it is again... It's actually a sear released pilot, that uses a dump valve similar to tech's, for an extremely large pilot vent(relative to pilot volume)... but also allows for light trigger pull even with HIGH chamber pressures...


Now I understand. The pilot is the only part you are talking about. The main cannon valve is still a traditional barrel sealer piston valve. I was trying to see how you would trigger the main valve that way.


Egg-Zactly.... the performance is going to come from the large ventability relative to pilot volume...I'm not sure the efficiency rating of the QDV but, I do know that a QEV is only about 40-45% ....

And I just did the math, on a 2"barrel/4"chamber filled to 1500psi, using a 1" pilot connector and a 4" pilot disc, you would only need 100psi in the pilot chamber to hold the valve closed if using the stepped pilot....

Either way, this would definitely be the trick for piloting high pressure guns without worry of qev failure, or having to use a ballvalve to fire...


All this time I thought you were trying to use it for the main valve. :(

:D Great job on the math. The piloting of the high pressure valve looks like a workable solution. I like it. :D

I don't know the efficiency of the QDV, but to take my measurements of in barrel projectile positions, I have to be optimistic in the valve efficiency to get it to match the GGDT graphs. I may have some bad data due to the lack of scales so the piston weight and projectile weight was a best guess.
The thread on the projectile in barrel positions is here;
http://www.spudfiles.com/forums/ggdt-and-in-tank-quick-dump-valve-performance-t18090.html
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Unread postAuthor: jeepkahn » Wed May 13, 2009 12:14 pm

the efficiency of a QDV will be pushing 75% or better depending on vent diameter/chamber diameter ratio... And remember that your dealing with a working percentage of venting area, and the QDV in comparisin to a QEV has a much larger area to start with so the efficiency percentage will have a multiplied effect(ie, the valve will perform better) than a similarly "sized" QEV... 45% of 3/4 inch is the equivelant(roughly) of venting through a .198" hole while the QDV at 75% of 3/4" is the equivelant of venting through a .331" hole... and that .133" differance in effective vent diameter is HUGE in relationship to each other...
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Unread postAuthor: Technician1002 » Wed May 13, 2009 12:24 pm

jeepkahn wrote:the efficiency of a QDV will be pushing 75% or better depending on vent diameter/chamber diameter ratio... And remember that your dealing with a working percentage of venting area, and the QDV in comparisin to a QEV has a much larger area to start with so the efficiency percentage will have a multiplied effect(ie, the valve will perform better) than a similarly "sized" QEV... 45% of 3/4 inch is the equivelant(roughly) of venting through a .198" hole while the QDV at 75% of 3/4" is the equivelant of venting through a .331" hole... and that .133" differance in effective vent diameter is HUGE in relationship to each other... .


That is about the numbers I was getting, but I didn't know why. I just assumed it had to do with the large diameter tank feeding the valve with no plumbing between the chamber and the valve or my weights were way off. The hammer blow to the piston by the trigger rod assembly to quick start it may have also contributed to this. Because the pin used to push the valve closed retracts out of the way through the "Vent", it is large into the long space in the tank behind the valve. In addition to the "vent" the trigger rod goes through, the stopper assy is not sealed in place. Air is free to pass over the outside as well as the "Vent" in the middle.
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Unread postAuthor: jeepkahn » Wed May 13, 2009 12:48 pm

Technician1002 wrote: tank feeding the valve with no plumbing between the chamber and the valve .


Your tank isn't feeding your valve... That's my point... Your "valve" isn't inline with the flow once opened, your chamber is feeding the barrel directly, when opened your valve effectively no longer exists....Much like a burst disc, a piston valve or QEV or ballvalve(unlss fully ported) is still inline with the flow and therefore offers restrictions and reduces efficiency...
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Unread postAuthor: Technician1002 » Wed May 13, 2009 12:54 pm

jeepkahn wrote:
Technician1002 wrote: tank feeding the valve with no plumbing between the chamber and the valve .


Your tank isn't feeding your valve... That's my point... Your "valve" isn't inline with the flow once opened, your chamber is feeding the barrel directly, when opened your valve effectively no longer exists....


Isn't a coaxial in the chamber barrel sealer piston valve the same in this regard?
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Unread postAuthor: jeepkahn » Wed May 13, 2009 1:07 pm

Technician1002 wrote:
jeepkahn wrote:
Technician1002 wrote: tank feeding the valve with no plumbing between the chamber and the valve .


Your tank isn't feeding your valve... That's my point... Your "valve" isn't inline with the flow once opened, your chamber is feeding the barrel directly, when opened your valve effectively no longer exists....


Isn't a coaxial in the chamber barrel sealer piston valve the same in this regard?


Theoretically, yes... But depending on chamber to barrel gap and piston to barrel gap, the efficiency can vary from very high to very low, a larger chamber with small barrel(4" chamber and 1.5" barrel) will have a better flow characteristic than a larger chamber with a slightly larger barrel(4" chamber and 2.5" barrel), as well as a piston that only moves .25 of barrel diameter (will be more efficient in overall air usage but will flow less) than a piston that moves .5 of barrel diameter( will theoretically waste chamber volume but will flow more)... And before everyone jumps on me about .25 of diameter being optimum for flow, remember that with higher pressures the air has the ability to exceed static flow rates for a given orifice/area...
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Unread postAuthor: Technician1002 » Wed May 13, 2009 1:17 pm

jeepkahn wrote:
Technician1002 wrote:
jeepkahn wrote:
Technician1002 wrote: tank feeding the valve with no plumbing between the chamber and the valve .


Your tank isn't feeding your valve... That's my point... Your "valve" isn't inline with the flow once opened, your chamber is feeding the barrel directly, when opened your valve effectively no longer exists....


Isn't a coaxial in the chamber barrel sealer piston valve the same in this regard?


Theoretically, yes... But depending on chamber to barrel gap and piston to barrel gap, the efficiency can vary from very high to very low, a larger chamber with small barrel(4" chamber and 1.5" barrel) will have a better flow characteristic than a larger chamber with a slightly larger barrel(4" chamber and 2.5" barrel), as well as a piston that only moves .25 of barrel diameter (will be more efficient in overall air usage but will flow less) than a piston that moves .5 of barrel diameter( will theoretically waste chamber volume but will flow more)... And before everyone jumps on me about .25 of diameter being optimum for flow, remember that with higher pressures the air has the ability to exceed static flow rates for a given orifice/area...


Thanks. I'm using about and 8 inch diameter chamber and feeding into a 2 inch coaxial valve. That must help.

In regards to the "Optimum" opening distance, I never liked the math of matching the opening to the barrel area. It seemed to me like stacking 2 stop signs on a road near each other. Removing one restriction to flow and letting the other provide the only major choke point to flow seemed like a good idea. For this reason I ported my 2 inch valve with 1 inch tall ports so the total port area is considerably larger than the valve seat area instead of equal to it.
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Chamber diameter to Valve outlet diameter of the 2 inch QDV cannon. The valve is inside the chamber.
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Unread postAuthor: jeepkahn » Wed May 13, 2009 1:41 pm

One of the reasons that practice overrides theory is because you've got to look at all aspects of layout/design/interaction... You have to not only look at the valve, but also how much proximal volume is available to the valve/barrel.. theoretically your gun would actually benefit from the chamber to barrel ports being centered(frt to rear) in your chamber, as opposed to being rearward or forward... a coax with a closer ratio of chamber to barrel diameter than your setup would be limited because of the inability to maintain pressure at the breech due to the frictional losses of pressure in a smaller chamber as the air flows out the barrel, so you see less effective pressure applied to the projectile due to inability of the air(even under pressure) to keep up...

these crappy drawings help illustrate... the coax on top has limits due to frictional losses as the pressure exits the barrel and the air further up the chamber has to overcome those frictional losses tokeep the breech area pressure up, the coax on the bottom has more proximal volume available before it has to rely on the breech area to be "refilled"...
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