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Max psi of metal

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: Novacastrian » Fri Dec 14, 2007 4:44 pm

*5*STAR*GENERAL* wrote:there is really no telling you first have to find out what guage the metal is then do the math i forgo the formula for psi. length x width x guage=...
then sivided by air consumtion something liek that. :wink:


Oh my lord. Words escape me.
That has to be a new, all time stupid ass comment
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Unread postAuthor: paaiyan » Fri Dec 14, 2007 4:50 pm

Novacastrian wrote:
*5*STAR*GENERAL* wrote:there is really no telling you first have to find out what guage the metal is then do the math i forgo the formula for psi. length x width x guage=...
then sivided by air consumtion something liek that. :wink:


Oh my lord. Words escape me.
That has to be a new, all time stupid ass comment


I quite heartily agree with your ever-so-subtle statement. There's no formula to find the maximum pressure rating as far as I know. It's more of a matter of them testing multiple pipes of the same type, finding the burst or deformation pressure, and saying we'll call the safe operating pressure X% of that average. Every pipe is different. Metals are all different even at the same gauge, the diameter and gauge make a difference.
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Unread postAuthor: DYI » Fri Dec 14, 2007 4:51 pm

PCGUY needs to install a dumbass filter on this forum...

Congratulations 5stargeneral, you have just been awarded the title of official forum idiot.
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Unread postAuthor: paaiyan » Fri Dec 14, 2007 4:56 pm

DYI wrote:PCGUY needs to install a dumbass filter on this forum...

Congratulations 5stargeneral, you have just been awarded the title of official forum idiot.


I think we created a thread and voted on that title last time actually. Someone even found a picture of a trophy for it.

And I agree with Crowley, piston valve is definitely a good choice for a cannon of these pressures. Ball valves open very slowly, and you're not likely to find a sprinkler valve that can handle that pressure.
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Unread postAuthor: MrCrowley » Fri Dec 14, 2007 5:00 pm

bigbob12345 wrote:Ive never made a metal piston valve but Ill try it


That's the spudding spirit :P

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Guys i've PM'd 5* on his incoherent posting but I can't force him to post like all the other members on the forum, but I did say if we can't understand his post, it's going, going, gone.

But for now, you're going to have to keep the flaming to a minimum and just PM me with a link to the thread if he makes another incoherent or stupid post.
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Unread postAuthor: paaiyan » Fri Dec 14, 2007 5:02 pm

MrCrowley wrote:
bigbob12345 wrote:Ive never made a metal piston valve but Ill try it


That's the spudding spirit :P


Indeed it is. I'm sure if you search around and gather some information, most members here would be more than happy to assist you with the finer details.

P.S. Crowley: Can't we flame him just a little? I mean, you know we all like fire...
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Unread postAuthor: SpudBlaster15 » Fri Dec 14, 2007 6:56 pm

You can estimate the burst pressure of a pipe using the formula Pressure<sub>Burst</sub> = Strength<sub>Fatigue</sub> * (2*Thickness<sub>Wall</sub>)/Diameter<sub>Internal</sub>.

Your question is, "What is the maximum pressure I can use in Sch40 steel pipe". I am going to assume you mean hardware store grade fittings, of which 2" is the largest nominal size (and will thus have the hgihest bursting pressure). These fittings are generally made of relatively low grade steel, with a fatigue strength of around 15,000psi.

Pressure<sub>Burst</sub> = (15,000)*((2*(0.154))/(2.067)
Pressure<sub>Burst</sub> = ~2235PSI

Simply put, you will not be able to burst the pipe, short of using unregulated HPA or Nitrogen, or causing DDT within the pressure vessel. However, I would not use them above 1,000PSI.
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Last edited by SpudBlaster15 on Fri Dec 14, 2007 7:26 pm, edited 1 time in total.
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Unread postAuthor: DYI » Fri Dec 14, 2007 7:16 pm

You wrote the burst pressure formula in two different places, entirely dissimilarly.
The first one : FS x (2 x ID) / WT gives a burst pressure of 402 662 psi,
while the second one : FS x (2 x WT) / ID gives a burst pressure of 2235 psi.
Which one is correct? The first one, the second one, or neither? (and I know its not the first one)
Didn't you calculate this before, and give us a burst pressure of 1300 psi?
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Unread postAuthor: SpudBlaster15 » Fri Dec 14, 2007 7:25 pm

This is what happens when I post while in a state of near unconciousness.

The second one is correct, and if I have ever calculated the burst pressure of 2" Sch40 steel pipe to be 1300PSI, I was probably in a similar state to my current.
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Unread postAuthor: DYI » Fri Dec 14, 2007 7:42 pm

Ah. I try to avoid posting under the influence of anything very mind-altering...
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Unread postAuthor: Ragnarok » Fri Dec 14, 2007 8:13 pm

Wait... yield stress, fatigue strength and ultimate tensile strength are all three very different things.

Fatigue strength relates to repeated stress cycles, not failure under a single loading. However, the figure you have quoted is a yield stress for steel.
In this case, the UTS will indicate the burst pressure, not yield stress.
However, yield stress is the indicator of the safe working region.

And all of this assumes no micro fractures and perfect geometry - a minute change could lead to the fracture toughness of the material being exceeded and the whole structure failing under fast fracture. It's a lot more complicated than can be calculated with a single simple equation, so you really need to slap a large safety factor on that.

Apologies, but that's the engineer kicking in there.
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Unread postAuthor: SpudBlaster15 » Fri Dec 14, 2007 10:00 pm

DYI wrote:Ah. I try to avoid posting under the influence of anything very mind-altering...


Well, I count 3 hours of sleep during the past 48 hours as very mind altering. Hence the "near unconciousness" description.

Ragnarok wrote:Wait... yield stress, fatigue strength and ultimate tensile strength are all three very different things.

Fatigue strength relates to repeated stress cycles, not failure under a single loading. However, the figure you have quoted is a yield stress for steel.
In this case, the UTS will indicate the burst pressure, not yield stress.
However, yield stress is the indicator of the safe working region.


"Repeated stress cycles" is exactly why I choose to use fatigue strength (15,000psi figure provided by D_Hall of the Spudtech forums, who has a MSc ME degree) in the calculation. You are absolutely correct in stating that the material's ultimate tensile strength will determine the failure pressure of the pipe under a single loading, but a launcher's pressure chamber undergoes many pressurization/depressurization cycles during its life cycle. The figure given by that equation is the minimum pressure that could potentially rupture the pipe, even if it occurs after a relatively large time period.

Ragnarok wrote:And all of this assumes no micro fractures and perfect geometry - a minute change could lead to the fracture toughness of the material being exceeded and the whole structure failing under fast fracture. It's a lot more complicated than can be calculated with a single simple equation, so you really need to slap a large safety factor on that.


Yep, which is why I suggested not using the pipe over 1000PSI or so. Too much potential for imperfections to exist within the grain of the steel, which will compromise the vessel's pressure holding capacity. The equation simply gives a baseline value to base our safety factors off of.

Ragnarok wrote:Apologies, but that's the engineer kicking in there.


No problem there. Its good to have intuitive, logical, knowledgeable individuals such as yourself on these forums. Too many threads around here make me feel as if I have jumped back to a 1st grade classroom.
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Unread postAuthor: Ragnarok » Fri Dec 14, 2007 10:26 pm

SpudBlaster15 wrote:"Repeated stress cycles" is exactly why I choose to use fatigue strength ... in the calculation. You are absolutely correct in stating that the material's ultimate tensile strength will determine the failure pressure of the pipe under a single loading, but a launcher's pressure chamber undergoes many pressurization/depressurization cycles during its life cycle. The figure given by that equation is the minimum pressure that could potentially rupture the pipe, even if it occurs after a relatively large time period.

Fatigue doesn't normally become a major concern in a system such as a spudgun working in reasonable pressure ranges for thousands of cycles - even tens of thousands.
Most launchers will see the scrap pile before fatigue is a worry. It should also be borne in mind that other materials such as copper and aluminium do not have a fatigue limit like steel.

Some non-ferrous launchers that see heavy use and high pressures (like I do with HEAL) will eventually require replacement parts and in some cases, complete rebuilds after time. I'm keeping track of the use of each of my spudguns, and as and when it is needed, the various parts will be retired and replaced.
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Unread postAuthor: Novacastrian » Fri Dec 14, 2007 11:26 pm

SpudBlaster15 wrote:
DYI wrote:Ah. I try to avoid posting under the influence of anything very mind-altering...


Well, I count 3 hours of sleep during the past 48 hours as very mind altering. Hence the "near unconciousness" description.

Ragnarok wrote:Wait... yield stress, fatigue strength and ultimate tensile strength are all three very different things.

Fatigue strength relates to repeated stress cycles, not failure under a single loading. However, the figure you have quoted is a yield stress for steel.
In this case, the UTS will indicate the burst pressure, not yield stress.
However, yield stress is the indicator of the safe working region.


"Repeated stress cycles" is exactly why I choose to use fatigue strength (15,000psi figure provided by D_Hall of the Spudtech forums, who has a MSc ME degree) in the calculation. You are absolutely correct in stating that the material's ultimate tensile strength will determine the failure pressure of the pipe under a single loading, but a launcher's pressure chamber undergoes many pressurization/depressurization cycles during its life cycle. The figure given by that equation is the minimum pressure that could potentially rupture the pipe, even if it occurs after a relatively large time period.

Ragnarok wrote:And all of this assumes no micro fractures and perfect geometry - a minute change could lead to the fracture toughness of the material being exceeded and the whole structure failing under fast fracture. It's a lot more complicated than can be calculated with a single simple equation, so you really need to slap a large safety factor on that.


Yep, which is why I suggested not using the pipe over 1000PSI or so. Too much potential for imperfections to exist within the grain of the steel, which will compromise the vessel's pressure holding capacity. The equation simply gives a baseline value to base our safety factors off of.

Ragnarok wrote:Apologies, but that's the engineer kicking in there.


No problem there. Its good to have intuitive, logical, knowledgeable individuals such as yourself on these forums. T
1st grade classroom.


Glad that you cleared that up, i was beginning to think that you loved Horse :lol:
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Unread postAuthor: SpudBlaster15 » Sat Dec 15, 2007 12:39 am

Ragnarok wrote:Fatigue doesn't normally become a major concern in a system such as a spudgun working in reasonable pressure ranges for thousands of cycles - even tens of thousands.
Most launchers will see the scrap pile before fatigue is a worry.


Yeah, of course fatigue will not be an issue at reasonable operating pressures. However, my point was that, because such a large difference exists between steel's ultimate tensile strength and its fatigue strength, it is possible to operate the launcher for some time at pressures above its fatigue stress threshold. However, doing so would result in eventual failure.
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