This Is What Can Happen.....

[Gerard]

Possible, but seems unlikely, as I've always maintained my greases in a very clean condition. Still, stuff happens, so it could be that some particle of just the right size, shape, and density snuck into the grease I applied and clogged the path preventing air leakage. I suppose if all the conditions were just right such a particle could jam up sufficiently to block 2,200psi, given that it'd be in a very confined space to the friction against the threads needed wouldn't be nearly so great as, say, an O-ring on a 3/4" tube wall. Interesting. Well pending further, perhaps better theories, this seems plausible enough to allow for now.

Still doesn't explain the biggest problem though - the damming of air pressure at the valve body. That's the really sticky one for me. Even with what is perhaps a 0.002" to 0.003" gap, how could it block air flow completely? Especially considering that with the valve face seal blown out and the front half of the tube then pressurized (with perhaps a 5% pressure drop allowing for the small volume of air around the small end of the HiPAC) it would seem there would be as much pressure acting on at least the front half of the valve body's outside wall as anywhere inside the valve. It would have to be a very specific, consistent contact just behind the threaded section of the valve wall expanding to fill that void so perfectly. A head-scratcher.

[aluminumfetish]

I hate to see a guy get hurt but he did that to himself.

[WHITEFANG]

For others sake the word Grease keeps coming up?
I know lubes and what and where to use them but others may need  clarification of the word.
 This post have covered many pages and much display of great talents in the root cause.
Sorry if I offended anyone. JMO Wrong grease / lube and we will be reading another one of these. JMO

FANG

[Gerard]

In my experience, a high grade white lithium grease such as I'm using for much assembly work, Campagnolo brand (high end Italian bicycle component maker - but no doubt they just re-packaged someone else's white lithium grease with their brand on the jar) is non-combustible under the sort of conditions encountered with HPA. It's actually somewhat popular with Americans as an ablative in firearm suppressors, where a small amount is placed into the first few chambers of a suppressor where it is diffused by the blast into the remaining baffle chambers, and helps in absorbing/reducing noise, while also assisting in keeping baffle surfaces free of carbon build-up so that cleaning is not so difficult. With the kinds of temperatures and pressures involved there, many times higher than anything encountered with high pressure air, I figure it's a safe enough grease to use in airguns. So far I've had no problems with it. Makes disassembly for maintenance very easy.

(oops, guess I typed a 'bad word' in offering an alternate to 'suppressor' - edited out)

[Gippeto]

According to measurements I took a while back...valve od is .737" and tube id is .748"...a difference of .011".

I have seen tubing (<1" od) "grow" a couple thousandths while subjected to high pressure...have never seen anything made of metal <1" grow by .011" without failing utterly.

I'm sure there's a way to calculate expected growth...just haven't come across it yet.


Al

[Gerard]

Okay, well there's another interesting point. Seems Crosman's tolerances for the valve OD must be quite broad, as mine's a smooth but not a drop-in fit at that point in the tube. It'll slide with a push, but I can tip the tube upright in either direction and the valve might shift a bit, but not a lot, certainly not out the end. I didn't measure the OD as I had no need to do so, and won't now, because the thing's set up so nicely I don't feel like going to all that trouble and risking throwing something out of whack. It'd glitch up the paint job too.

[rsterne]

I have measured Crosman tubes all the way from 0.745" (which is the nominal diameter) to 0.752".... Could they be smaller than 0.745", possibly.... I have a 22XX valve here right now that is 0.737" OD, so that would leave a 0.004" gap, 3/4" diameter, which is over 0.010 sq.in.... the same as the area of a 0.114" diameter hole.... I have never seen a valve that won't fall easily right through a tube unless it hits a burr on a slot or hole.... Disco valves are even smaller OD than the 22XX valves.... If your valve won't do that, it is indeed oversize.... Whether it started that way, or grew during your "experience" is another matter.... I have had lot's of valves, both brass and aluminum, turned to larger than the root diameter of the 9/16" threads inside (I typically use 0.60" ID on my bored valves), and have never seen any permanent expansion even at 3000 psi (valve secured with three 10-32 screws for safety at that pressure)....

Bob

[Gerard]

Well that pushes things back into the more puzzling direction, as I slightly counter-sunk and filed smooth the edges of each of the 3 holes after drilling and tapping for 10-32 bolts. I didn't use the recessed socket head method though, as I hadn't stumbled across that yet. Makes sense, but since I'm only running 800psi behind the regulator and since I ran 3,000psi for months before installing that without problems or notable deflection of the tube at the set screws I don't see a need to retro-fit to stronger valve restraint at this point. 800psi is less than CO2 so no real concerns there.

[Gippeto]

Curious if you measure tubing id in more than one spot Bob...rarely truly round in my experience.

Measured two valves just now... .737" and .735".

Also measured up three hipac units at the point where the one failed. I'm a curious bugger and had the caliper in hand and the hipacs discharged.


For the sake of information only...insufficient information to draw any accurate conclusions regarding strength.



Original hipac that I hydro tested has fine threads only and a hex machined into the end. od of the oring groove is .700" x id of tube .604" ( .052" wall)

Second tube (purchased second hand) has the hex and coarser threads. od of the oring groove is .688" x tube id of .613" (.0375" wall)

Third tube was purchased new. Coarse threads and no hex. od of the oring groove is .682" x tube id of .615" (.033" wall)

Don't know what the material is or heat treat if any. Yield strength of original tube has to be =/> 48000psi at a minimum.

For the sake of comparison only...1018 (common cold rolled steel) has a yield strength of 53700psi.

1144 free machining steel has a yield strength of 100,000psi.

Would not have noticed that change in spec without measuring.....in the face of changed specs...I've no choice but to consider my hydro test as no longer valid.

Al



 

[Gerard]

Not just changing specs either, but add in the randomizing element brought about by the fact that the insides are machined rather roughly, leaving some tool passes with several thousandths of an inch thinner walls here and there. Curious if your tubes showed this rough inside boring, or if you got lucky and the tubes all looked just about as smooth inside as they do outside? Both mine and the one I sent to Rick were pretty darn rough inside... I'd intuit an approximate fudge factor of maybe 15% wall strength on account of that roughness. Again, stress risers. Nasty fact of life with metals, though some more so than others.

[rsterne]

If I did the math right, an ID of 0.604" with an OD of 0.700" is a wall thickness of 0.048".... Using a yield of 48K, with the original dimensions of 0.700" OD x 0.048" wall (OD is the bottom of the O-ring groove), I get a pressure to yield of 7629 psi, which is a safety margin of 2.5:1 at 3000 psi.... Using 1018, with a yield of 53,700, that goes up to 8535 psi and 2.8:1.... If it's 1144, that jumps to 5.3:1, but of course we have no idea what the material is....

With the third (smallest) tube, using a yield of 48K, and an OD of 0.682", ID of 0.615", so a wall thickness of 0.033", I get a pressure to yield of 5143 psi, and a safety margin of only 1.7:1 at 3000 psi, or 2.6:1 at 2000 psi.... If the material is stronger, the safety margin is higher....

At 3000 psi, with an ID of 0.615", the end force is 894 lbs.... The cross sectional area of the wall below the O-ring groove, with an OD of 0.682" and a wall thickness of 0.033", is 0.0707 sq.in.... If we use a 48K yield, that works out to 3392 lbs. of force required to cause it to stretch, which is a safety margin of 3.8:1.... However, if the tube was pushed sideways at the end, or impacted something, the force would be multiplied by the ratio of the length to the diameter.... For example, if the tube was 10" long ahead of where it broke, a side force at the end of 100 lbs. would result in an increase of the force stretching the wall by (10 / 0.615) x 100 = 1626 lbs.... Add that to the 894 lbs. from the pressure, and we are creeping pretty close to the 3392 lb. yield point.... These numbers are just a talking point, not indicative of what happened in this case.... but if you fell on a pressurized HiPac, with the impact just right to buckle the tube at the O-ring location, it might be a concern?....

Bob

[chekbone]

Talking points? Most of us can only read what you wrote. To heck with trying to be part of the numbers discussion.

Bottom line. Don't fall on your hipac. Don't even sleep with it.

[Gerard]

I would expect that any tube of that small diameter and thin wall would be dangerous to fall on while pressurized, even without threaded connections in the middle. But the HiPAC would undoubtedly fail at those connections owing to the dramatically thinner walls in the threaded locations. That's not a failing of the HiPAC, as of course it is being made to be light enough for practical carrying while also strong enough to avoid catastrophic failures in normal use. Falling on a long, thin pressure vessel is just a bad idea. Though I suppose the odds would favour the break happening on the side away from the guy falling on it... so perhaps not too serious an injury, more just a big hole in the ground underneath him.

[Nick_59]

Just a quick question, as I have two of these Hipacs.  I am using the stainless steel valves sold by Powermax, these are retained by a single 8/32 stainless screw.  Is this sufficient if filling to 2200psi?

[Gerard]

Just a quick question, as I have two of these Hipacs.  I am using the stainless steel valves sold by Powermax, these are retained by a single 8/32 stainless screw.  Is this sufficient if filling to 2200psi?

Might be. Might not be. You've seen what a single screw did at 2,000psi in the video, right? The expert conclusion seems to be fairly solidly stating that a single screw is not a safe bet at that kind of pressure. Alex has many times stated in many forums that a HiPAC can't blow a valve out the back of the tube, there are too many things in the way... and in the case of the video which started this discussion, that would seem to hold true, as the objects in the way of the valve had been removed by the user, leaving a clear bore down which the valve could travel, very quickly, through his leg. But what about the case last month at an FT event in, where was it, ah yes, Didsbury Alberta:
 http://www.airgunforum.ca/forums/topic69359.html
No one was hurt, but a valve blew out the back of a tube taking all the steel in its path out of the way on the trip. Sheared steel and zinc. Bits blown as far as the valve at least, at 38 yards from the gun. And the owner swore he'd never filled it anywhere NEAR as high as 2,000psi. Said it was at 500psi at the time of the failure. That valve was secured by a single screw. Do you want to bet your safety on that little screw with that much pressure potentially driving it out of the gun? What about the safety of the kid behind you in a lawn chair? What about the guy next to you on the firing line?

[dv8eod]

If I did the math right, an ID of 0.604" with an OD of 0.700" is a wall thickness of 0.048".... Using a yield of 48K, with the original dimensions of 0.700" OD x 0.048" wall (OD is the bottom of the O-ring groove), I get a pressure to yield of 7629 psi, which is a safety margin of 2.5:1 at 3000 psi.... Using 1018, with a yield of 53,700, that goes up to 8535 psi and 2.8:1.... If it's 1144, that jumps to 5.3:1, but of course we have no idea what the material is....

With the third (smallest) tube, using a yield of 48K, and an OD of 0.682", ID of 0.615", so a wall thickness of 0.033", I get a pressure to yield of 5143 psi, and a safety margin of only 1.7:1 at 3000 psi, or 2.6:1 at 2000 psi.... If the material is stronger, the safety margin is higher....

At 3000 psi, with an ID of 0.615", the end force is 894 lbs.... The cross sectional area of the wall below the O-ring groove, with an OD of 0.682" and a wall thickness of 0.033", is 0.0707 sq.in.... If we use a 48K yield, that works out to 3392 lbs. of force required to cause it to stretch, which is a safety margin of 3.8:1.... However, if the tube was pushed sideways at the end, or impacted something, the force would be multiplied by the ratio of the length to the diameter.... For example, if the tube was 10" long ahead of where it broke, a side force at the end of 100 lbs. would result in an increase of the force stretching the wall by (10 / 0.615) x 100 = 1626 lbs.... Add that to the 894 lbs. from the pressure, and we are creeping pretty close to the 3392 lb. yield point.... These numbers are just a talking point, not indicative of what happened in this case.... but if you fell on a pressurized HiPac, with the impact just right to buckle the tube at the O-ring location, it might be a concern?....

Bob

Out of curiosity, that math would still give a large safety margin for use with CO2, which the hipacs were originally designed for, correct? Or were they designed for HPA first? I'm thinking when people started bumping up the pressures, the original math was never updated to reflect the changed conditions. I am also wondering what the longevity of these devices is, since they are apparently rough milled without further treatment, being subjected frequently to those pressures.

[Nick_59]

I think they were always intended for air, i'm assuming the name means hpa conversion.
@Gerard  wow another one,  you are right to advise caution,  it seems Disco valves definitely need the extra screws.   
As previously mentioned though, there is less pressure on the valve with the Hipac sealing method,  which neither of these failures were using, I was wondering if from a technical standpoint a single 8/32 stainless is good for 3k psi in this application as claimed.  .  '  Might be. Might not be.' is making me nervous   I don't use my 22xx much anymore,  I've got a Disco and p rod now, and these events provide even less incentive, but they are part of the collection and I need to know they are safe to have around.
Obviously I don't know much about fasteners, but I also wondered is there any point using higher tensile end cap screws as extra precaution or would they shear under impact more easily?

[mad187]

i do use a hipac tube on a 2250 but i have a stock valve and filling the tube whit a hand pump .i have tried up to 3k psi with ease.did this guy i the video fill whit a scuba tank ?

[Gippeto]

Bob, good catch and you are correct...was rushing things to get out the door and made a mess of it...my bad.

At a wall thickness of .048" and knowing that it did not yield at 5000psi...that puts minimum yield strength at =/> 37000psi. Really a moot point...most steels are above this number.

Without knowing the actual material strength, wondering what value you place in calculating safety factors that have no basis?? Would be nice to know for sure, but not the kind of information ANY manufacturer releases.

Gerard...really? Apples and oranges...

Suntan Sid...There's a thread on pinning the valve...might want to give it a read. Tried to make it easy to follow and understand but never know what another set of eyes sees. If it gives anyone problems I would like to know so that can be sorted.

Dv8ed...hipac was developed for hpa. Longevity is unknown, but product has been around and in use for a good while now.

mad187...fill method doesn't matter in that case...gun was already filled and was being worked on while pressurized.

Al

[mad187]

okey not smart to work on the gun with with pressure in the tube .i always empty the tube before working on it