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Flat plates on a pressure vessel

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Flat plates on a pressure vessel

Unread postAuthor: DYI » Fri Nov 16, 2007 4:54 pm

I'm planning a pressure vessel for an upcoming design after I recently recovered some 4" ID seamless steel tube with 1/4" thick walls, and I had two questions.

1. What would be the approximate bursting pressure of this tube, assuming it is made of relatively low grade steel?

2. Would 1.5" steel plate welded on the ends by a good welder be relatively safe if the maximum working pressure was 500 psi? 1000 psi?

Please don't post useless comments, or respond saying you don't know either answer. Thank you.
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Unread postAuthor: Novacastrian » Fri Nov 16, 2007 5:23 pm

The flat plates won't be a problem *if you trust the boily that welds them*.
If my memory serves me correct, in the past i learned that the burst and or working pressure of 304 stainless at the same dimentions you plan on using is 2200psi, i cant tell you if normal steel is stronger or not, i would think it would be about the same.

A better option would be to have the ends of vessel threaded as the plates,
That way you can continue to check the inside condition of this potential bomb :wink:
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Unread postAuthor: cdheller » Fri Nov 16, 2007 7:12 pm

googel barlows formula.
or use this online calc. page
http://www.dixiepipe.com/barlows.html

http://www.engineeringtoolbox.com/barlow-d_1003.html


a bunch on this one
http://www.engineeringtoolbox.com/pipes ... -t_40.html

it's been used for a while and is proven
it'l give you a safe working pressure for most any pipe .
your wall thickness should be .237 not .25 though

and ,,um threading cut's it pressure rating in halve.
and thats if you can find someone that has the special threader for 4"pipe(for grins call around)
proper weld fittings don't make much difference in pressure ratings.
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Last edited by cdheller on Fri Nov 16, 2007 7:41 pm, edited 2 times in total.
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Unread postAuthor: DYI » Fri Nov 16, 2007 7:16 pm

It will be a hybrid chamber, so the welds won't be under constant strain. I also don't plan on being particularly close to this thing when it fires, for the simple reason that I have had an endcap blowout before on a hybrid, and it managed to hit me. An ABS cap is one thing, but a 9 pound chunk of sharp steel plate is quite another, so I'll have some shielding between me and it. Probably a partial box made of 1/4" steel plate that I can place over it for firing. That, and I'll run nice long wires on the stungun.

Threading the ends would be nice if I had the threaded caps, but unfortunately I don't, so welding seemed like a good option. I will have a 2" plug threaded into one of the end plates to check the internal condition of my bomb errr *hybrid*.
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Unread postAuthor: frankrede » Fri Nov 16, 2007 9:01 pm

What size plate would you be using?( I would recommend 3/8") but 1/4" would be fine.

Who will be welding it? How good are they?
What are they going to be welding it with?
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Unread postAuthor: DYI » Fri Nov 16, 2007 9:39 pm

I have some 1.5" plate hanging around in just the right size, but if I can't find the rest of it, I'll probably just order some thinner stuff (i.e. 3/8" to 3/4") from the welding shop. I have no intent of connecting it with threaded fittings, mostly for the reasons that cdheller mentioned.

The person I'm thinking of to weld it runs a welding shop, and has worked there for decades. He is a real expert. If he doesn't feel confident doing this, he'll probably know someone who will.
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Unread postAuthor: Killjoy » Fri Nov 16, 2007 11:10 pm

I used a 3/4" steel plate on the end of Fear which I welded myself and I have gone to 6x mixes with no problems. The front end was a 1/2" plate as well, so I think a 1.5" plate would be a tad overkill and could increase weight significantly. A 1/4" or 1/2" thick steel plate should be fine.

Also the pressure rating of your pipe is roughly 400 psi with a safety factor of ten, and a 1000 psi with a safety factor of 4 (your choice as to what you want.) This is with a a 35000psi yield strength by the way.

Hope this helps and good luck.
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Unread postAuthor: cdheller » Sat Nov 17, 2007 11:46 am

IIRC, the flat end of a pipe needs to be a smiggen thicker than the wall thickness of the pipe for a perfect balance of bursting strength.

deforms differently.

If your pipe is 4"sch 40 or4" standard weight pipe its .237,
if you can get 3/8 plate cheap . it fits the bill.

my chamber is 8"standard weight with .322 wall thickness .
I went with 1/2 " plate .

like your project it'll never get close enough to bursting strength to matter.

I can not remember where ,what or who but there is a formula for this somewhere.
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Unread postAuthor: boilingleadbath » Sat Nov 17, 2007 5:47 pm

Flat plates are weak in pressure-bearing applications; overkill isn't a bad idea.

Plus, in the event of a failure, a heavy endcap will be endowed with less kinetic energy.
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Unread postAuthor: DYI » Sat Nov 17, 2007 6:17 pm

Flat plates are weak in pressure-bearing applications; overkill isn't a bad idea.

Plus, in the event of a failure, a heavy endcap will be endowed with less kinetic energy.


Exactly my point. Unfortunately, I seem to have permanently misplaced the second plate, so I'll be using 1.5" for the front (to tap the 2" threads into), and 3/4" for the back (to tap ignition, fuel, vent, etc...). Anyone see any problems with that setup? If not, welding will likely commence in the coming week.
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Unread postAuthor: frankrede » Sat Nov 17, 2007 6:26 pm

You have a 2" tap?
How are you going to make the threads?
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Unread postAuthor: Novacastrian » Sat Nov 17, 2007 6:31 pm

frankrede wrote:You have a 2" tap?
How are you going to make the threads?


Thread cutting tool in a lathe i would imagine :wink:
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Unread postAuthor: DYI » Sat Nov 17, 2007 6:52 pm

You have a 2" tap?
How are you going to make the threads?


You raise a good point. I could get the threads machined, or, more cheaply, I could simply cut off the threaded end of some 2" pipe, and get it welded on the outside of the plate.
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Unread postAuthor: Killjoy » Sun Nov 18, 2007 2:12 am

Probably just be easier to weld a piece of threaded 2" pipe onto the plate
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Unread postAuthor: Novacastrian » Sun Nov 18, 2007 2:26 am

I would recommend boring or drilling a hole in the plate and welding the inner and outer of the fitting, just leave a bit inside so you don't screw* the thread when you weld it.

*ruin
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