Air Tank Dump
Just experienced air tank dump in my condor. Install Tony’s top-hat with the med insert shooting 18gr JSB and the gun shot beautiful. Decided to up the power with some 24.4 gr and some enjin 32gr so I took out the insert, turned up the power wheel to 8 and shot the heavier pellets. After about 7/8 shots the tank dumped. Turned it back down to 4 on the power wheel and was shooting 25.4 and it dumped again. Never had that experiance with the stock top hat. Maybe need a 0 ring behind Tony’ top hat?? I really don’t know what to do for sure. Did I damage something when it dumped the first time. After the dump the bolt was still closed but it marked a scratch where the bold cams over in the bolt lock slot.
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Putting a o ring is similar. Problem is the o ring is going compress so you really don’t know what it’s set at. O ring is not adjustable. With a plastic washer it is adjustable. I make the spacers every .005″ when trying to find out what gap to run.
On .257 there is no tank dump and the spacer is there so the valve doesn’t waste air.
Just because the gap is there doesn’t mean you need all of it. On my regulated guns I have spacers also. Shot count goes up and noise goes down.
Knife,
Sounds like a good explanation. Also consider that the O ring seals in the breech and on the valve generate really big clamp loads when the chamber is pressurized. This clamp load would tend to hold the valve in place as long as the pressure is high.
One of the tricks used in high pressure pumps with O ring seals is to put a teflon backer/washer next to the O ring. This keeps the O ring from extruding into the gap between valve and housing. This makes the clamp load lower and more consistent.
Sorry for the excessive engineer-speak, sometimes I can’t help it..
-x356b
To Dyotat100 As I read it that is that putting an O ring behind the top hat is the same thing that you are doing with a washer right??
Knife: I like your simple illustration of what is happening. So the only solution is to go .25 cal to be able to run Tony’s tophat wide open with no restricor and have a dependable air flow without dump at the higher pressure for heavy pellets?
On the other hand I just bought a FX Bobcat in .30 cal for heavy duty hunting so I will leave the Condor in a state of a more mellow tune so I don’t have that problem. You members are great and I am learning a lot…..Thank you..Gary
All these “high brow” 😆 explanations 😳 are making my head hurt!
For we simple folk, another way to look at it is with a larger or thicker barrel with a smaller opening, the valve is seeing what is tantamount to an obstruction. Even as the projectile travles down the barrel, the pressure is not dropping as one would expect, as the valve opening is still seeing a blockage. Or what would be similar to a “Stuck larger Dia. Bullet”.
With that being the case, the back pressure causes the valve to stay open much longer. It simply can’t depressurize fast enough. Result? Ker-Blam!!! The valve continues to flow due to the back pressure due to the blockage caused by the larger, (Thick) breach end of the barrel.
It is effectively trapped as it cannot escape fast enough to allow the pressure to drop quickly enough for the valve to close. The Breach pressure is too high, for too long to over come the return spring and tank pressure.
Such a situation could cause a lesser quality breach to fail as well in extreme cases. Learned this the hard way!
Knife
The way to adjust it is to put a shim under the top hat to limit the travel. All my valves are that way.
Here is my .308 tank with its spacer. Only has .070″ of travel. If I set it to .085″ it does the same thing yours does. I didn’t lose any power. Just stopped wasting air.
Have Tony’s top hat in with different restrictors and as far as I know there is no adjustment other than changing the restrictor. The trouble came when I left the top hat wide open with no restrictor. Medium one is in there now and everything is fine.
So have you tried the heavier pellets?
What is the gap now on the tophat?
Put the medium restrictor back in, put a washer behind the top hat and lubed it with silicon grease. Shot probably 75 pellets at 910 fps w/18.4 JSB and got my accuracy back with no dump. The dump happened with Edjun 32 gr and 25 gr HN which fit a lot tighter in the breech. JSB from now on!!………Thanks guys…….gary
Dyotat,
The commonest chromoly steel, SAE 4130, yields at over 70000 psi. 4130 can be heat treated up to somewhat higher yields. If the barrel is from a powderburner barrel application it might be 4150, which has a yield strength similar to 4130. All of the common SAE chromoly grades have similar yields.
The other thing to consider is that loading from a tight fitting slug may increase the load on the thin section of barrel, probably not an issue.
So, drifting totally off topic here, where I live in the souther half of lower Michigan, we have restrictions on weapons used for deer hunting. Just recently, the state DNR has allowed PCP airguns greater than 35 cal for deer hunting. It’s just another application for a PCP powerplant with more grunt, although we’d be looking for a slightly larger slug…
-x356b
Barrel is hammer forged chromoly.
Funny how you put in 4000 psi. I’m in the process of coming up with a 4000-4200 psi setup up for my guns. I want to run a 7 mm and .308 on that pressure.
Like I said earlier, the way I got around the tank dump was making the tophat gap smaller. Doesn’t take much. I went from .085″ to .070″ on the .308 and lost no power. Now the gun is usable. Before the noise and the recoil was really bad.
I use a delrin spacer to change how much gap I have on my custom valves.
Dyotat,
Got it, for tank dump, thinner wall should be better. Can’t wait to hear the results of the 0.032″ setup.
Is the barrel material strong enough for the thin wall? Just ran the numbers, for the 0.373″ OD, .032″ wall, 4000 psi gas pressure, the yield strength of the material should be above 30000 psi. (Barlows formula web site).
Had to ask, once upon a time we had a problem with a lightweight howitzer barrel coming apart in testing.
-x356b
Lovely tech-filled thread about tank dumps! Sticky???
Experienced similar results as Doug on a .25 -breech at .375″ with a sleeved condor valve. It fully dumped on the first go from 220 bar to 50 bar!!!
Dough has a lot of experience under his belt. In the end it seems that a thinner breech wall works best taking into considerations all the other parameters of the valve design.
Dough let us know how the thinner walled .308 performs when you are done. 😀
I don’t think it matters on od. I think the factor is how thick the edge of the barrel is. Because .177 is the same od as .25 but it has a smaller hole and requires a different return spring. .25 has the thinnest wall.
We will see here soon when I take my .308 from .428″ to .373. I’m looking to shoot 194 gr slugs without tank dump. Right now I shoot 158 gr with no problems and can shoot 171gr. That would make the barrel .032″ thick.
Dyotat,
I think this all adds up. The larger barrel OD means a larger O ring clamp area. The smaller barrel ID means a smaller chamber volume and faster pressurization. Bigger clamp area and higher pressure would add up to greater clamp force, which is not good.
Did I get it right that higher durometer O rings worked better? I have a vague memory of reading this in one of your earlier posts.
-x356b
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No doubt that valve lift control with appropriate spacers is crucial.
I was referring to something a little different:
An overloaded O ring creates a lot of drag on the tophat or breech structures.
To avoid the gap-filling, overloaded condition, use a teflon “backer” to close the gap. Backers come in a number of different types. To install in the ID of a breech or valve housing probably requires a split-ring type.
The backer goes in the positions shown here.
Reference: Zatkoff/Parker O Ring Master Catalog.
-x356b