stainless steel tube burst pressure

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bravootome
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Sun Dec 27, 2020 1:17 pm

hy there, i cant figured out how to calculate the burst pressure of my airtank as i got a hillpump capable of 300 bar. i used to charge it to a maximum of 60-70 bar with a fridge compressor. so far i stopped to 100 bar....

airtank is a regular stainless steel pipe used in constructions( the one you hold on when go up the stairs , or down ) . 40 mm diameter , 1 mm wall ( 1,20 in fact ) , and 60 cm long.
there is this formula but i only know the parameters above. https://www.engineersedge.com/calculato ... t_calc.htm

i am pretty tempted to go to 150 bar ....... 8-)
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Sun Dec 27, 2020 4:37 pm

This should help

https://eicac.co.uk/Pipe-Bursting-Pressure-Calculator

With your given parameters it looks like a burst pressure of roughly 4438psi.
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Mon Dec 28, 2020 3:58 am

Watch out with that. I'm thinking your tube might have a weld seam since it's just a structural steel tube, right? Only seamless tubes should be used with high pressure and only then might the calculations provide some value. You often can't see the seam unless you look closely, and even then you might not see it. If you aren't sure, then assume it's not seamless. I don't even know what to say about the pressure calculations for a welded tube. Research it before putting high pressure into a welded tube.
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Mon Dec 28, 2020 5:16 am

There's also the question of how it's capped, etc etc. Pressure vessels aren't, perhaps, rocket surgery, but there's still a lot of hard theory involved, and major consequences if you get it wrong.

The only way to be sure, without putting yourself in danger, is hydrostatic testing. Fill it *completely* with water and use a hydraulic pump or grease gun to get it to the pressure you're wanting to cap at. At least that way, if it fails, it won't fail in an explosive manner. Don't run your fingers over it looking for leaks, though.
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farcticox1
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Mon Dec 28, 2020 6:26 am

Some welded tube on Mcmaster Carr has pressure ratings, but not that high.
https://www.mcmaster.com/tubing/metal-t ... el-tubing/
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Mon Dec 28, 2020 12:31 pm

Moonbogg wrote:
Mon Dec 28, 2020 3:58 am
Watch out with that. I'm thinking your tube might have a weld seam since it's just a structural steel tube, right? Only seamless tubes should be used with high pressure and only then might the calculations provide some value. You often can't see the seam unless you look closely, and even then you might not see it. If you aren't sure, then assume it's not seamless. I don't even know what to say about the pressure calculations for a welded tube. Research it before putting high pressure into a welded tube.
it is welded.......
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bravootome
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Mon Dec 28, 2020 12:49 pm

PracticallySane wrote:
Sun Dec 27, 2020 4:37 pm
This should help

https://eicac.co.uk/Pipe-Bursting-Pressure-Calculator

With your given parameters it looks like a burst pressure of roughly 4438psi.
tks, it looks it is rated at 2393 psi .......as it is welded.....
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Mon Dec 28, 2020 2:41 pm

Disaster averted? Also, you need to know the yield strength to make a calculation. Without knowing the grade of steel, assume it's the weakest stainless in existence, plus the weld seam.
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Mon Dec 28, 2020 10:27 pm

Mr Foo hit on the major point...the ends are where most failures occur first, and it can be pretty dramatic...even deadly.

Wasn't too long ago that a fellow was working on a modified Benjamin Discovery and had the valve blow nearly through his thigh...barely missing the femoral artery. Another fellow closer to home had the valve blow out the back on another Discovery build...no one was hurt that time.

In both cases, the root cause of the failure was improperly pinning the valve. In the leg instance, Mr dum dum was working on the rifle while it was still charged, the other fellow had it laying on a table while refilling it.

If you want help, post pictures of the reservoir and exact details of it's construction.

Al

Edit; Looking back over your post, 304 is the most likely material (not guaranteed though) with a yield strength of 31200Psi. Your tube is 1.57" od and .047" wall. This gives a burst pressure of 1868 Psi (12,879Kpa)

I build to a minimum 3x safety factor at yield, so would consider this to be 600 Psi MAX use...providing it passes a 5/3 hydrotest...meaning test to 1000Psi and use it at 600Psi. The welded tube would give me the willies...I use 4130 seamless from Aircraft Spruce and Specialty.

Should really look at the rest of the construction though...
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bravootome
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Tue Dec 29, 2020 11:13 am

Gippeto wrote:
Mon Dec 28, 2020 10:27 pm

Should really look at the rest of the construction though...
Image
that black line inside looks to be the weld........ the walls are 1.2 mm but could be 1.5 not sure, my measurement tool is kind of inexact ....

forgot to mention that the main valve system is a fire extinguisher bronze head.... i did some mods like pine valve, a filling probe. it should be strong enough as it is massively constructed .....
i could buy a pcp AL bottle but it means i have to reconstruct the entire "thing". the bottle is the only legal part( as its a paintball part) to find to buy in this country.
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Gippeto
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Tue Dec 29, 2020 12:20 pm

I realize that there may be some issues obtaining "preferred" materials in some countries. Can you show and give details of "how" you plugged each end of the tube?

Al
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bravootome
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Tue Dec 29, 2020 12:38 pm

Gippeto wrote:
Tue Dec 29, 2020 12:20 pm
I realize that there may be some issues obtaining "preferred" materials in some countries. Can you show and give details of "how" you plugged each end of the tube?

Al
i just welded the part i cut of from the fire extinguisher that the bronze head screw into ( hope i have express myself correctly ) so i just screw the head into airtank ......, and at the other end i welded a 2,5 mm stainless piece.
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Tue Dec 29, 2020 1:22 pm

bravootome wrote:
Tue Dec 29, 2020 12:38 pm
Gippeto wrote:
Tue Dec 29, 2020 12:20 pm
I realize that there may be some issues obtaining "preferred" materials in some countries. Can you show and give details of "how" you plugged each end of the tube?

Al
i just welded the part i cut off from the fire extinguisher that the bronze head screw into ( hope i have express myself correctly ) so i just screw the head into airtank ......, and at the other end i welded a 2,5 mm stainless piece.
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Tue Dec 29, 2020 2:01 pm

Your English is MUCH better than my Romanian, so no worries...you express yourself just fine.

There is not much to be gained by examining the welded ends, far too many variables.

Are you familiar with F=P*A ? The weld holding your end plug is holding approximately 1719lbs of force (781Kg) at 1000Psi/6895Kpa.

About the best thing to do is what Mr Foo recommended... do a hydrotest on it. Paintball, scuba and scba tanks are taken to 5/3 of the max service pressure.

Regards,
Al
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bravootome
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Tue Dec 29, 2020 2:59 pm

Gippeto wrote:
Tue Dec 29, 2020 2:01 pm
Your English is MUCH better than my Romanian, so no worries...you express yourself just fine.

There is not much to be gained by examining the welded ends, far too many variables.

Are you familiar with F=P*A ? The weld holding your end plug is holding approximately 1719lbs of force (781Kg) at 1000Psi/6895Kpa.

About the best thing to do is what Mr Foo recommended... do a hydrotest on it. Paintball, scuba and scba tanks are taken to 5/3 of the max service pressure.

Regards,
Al
i understand you " understand romanian ?
about pressure , i tested various things as airtanks to high pressures, during the years of "constructions" , i mean sort of all kind of tubes, tanks i could find at scrap yard .... just one has blown up a door cuz it flew out from the gun into it at about 30 bar - 1 liter...like a rocket .

i will make another airtank like this one and do the test . i will film it and post it here.

anyhow i just try to replicate a performance with a gun i made a couple of years ago ( i mean 20 m hole to hole groupings shots with airsoft bbs) but i just cant figure out what, why was so accurate ..... spent lots of money on barrels, bbs, scopes, air supply, etc.....
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