Welding a small pressure test vessel...

"Pete C." wrote:


http://www.howitismade.net
Program 3 - Series #7
High-Pressure Cylinders
Worthington Cylinders (Tilbury, Ontario)
www.worthingtonindustries.com
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On Sun, 13 May 2007 02:58:48 GMT, "JTMcC"

Are you assuming that the only way to do it is to weld the pipe to a oversize plate with a fillet on the outside? That could certainly be strong enough, although if it were me I think I'd cut the plate to the pipe ID, leave the root a bit open, and fill the corner. Assuming a flat fillet, that would make the thinnest part of the joint about .35"

I qualify.

It's 150 psi, and 8.625" OD. About 323 pounds per inch of weld. Are you saying that there's some reason that the best welders you know can't build up a one inch long fillet you'd trust to hold 323 pounds? Then how come my own little quicky test tank, made from a 3" coupling with plating barely ground off, and single-pass fillet welded to 1/4" plate, can hold 500 psi, which is about 475 pounds per inch of weld?

Why write about soda bottles when somebody is prepared to build the actual item, spot you 50 psi, and let you make an easy 2 grand off your opinion? I'm sure we could run the thing through a couple hundred cycles in less than 2 hours including setup. Where else can you make $1000 an hour for doing nothing? Would it help if I upped the wager? I think you're as exactly sure as I am that your money would be moving into my wallet, but just aren't willing to admit it.
Wayne
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I would have the plate oversize and bevel the pipe. Also leave a gap at the root. That would end up with a flange which I think would help keep the plate from flexing. From what I can deduce, a flat plate causes concentrated stresses. A hemispheric dome can be made of much thinner material because there is no bending force.

The formula I found for the stress in a flat plate end on a tank is:
f = d^2/ t^2 3/16 p Where f is the stress d is id t is thickness of plate p is pressure
So 3^2/ .250 ^2 equals 9 / 1/16 or 9 * 16 or 144
And 8^2 / .5^2 equals 64/ 1/4 or 64 * 4 or 256 so your test tank is about twice as robust as the bigger tank. But both tanks ought to be fine according to what I could find. The strength of the weld is not as significant as the thickness of the plate from what I could find. But I would not want to use a fillet weld on the outside of the pipe as you would then have a larger diameter on which the pressure is acting.
Please note that I am not a mechanical engineer and don't rely on my calculations.
Dan

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On 14 May 2007 19:49:11 -0700, snipped-for-privacy@krl.org wrote:

Makes sense, and very little more prep work.

Sure, but the OP's original idea was to make the thing as simple as possible. There's obviously a thickness that will do, and I'm looking forward to reading what that should be.

The second half isn't clear to me. Is the source available online?

I get the idea, and the ratio makes sense. But we don't really know if the small one had a margin or was at its limit. The existing gauge is pegged at 500. I think I'll get a 1000 psi gauge and crank the thing up to that or failure, whichever comes first. :-) It would be interesting to know how much the quicky single-pass fillet can take, as well as how much pressure a cheap pistol grip grease can produce.

One could place a round bar across the plate, shimmed off by say .030", tack weld the ends, and then use a feeler gauge to measure the deflection (if any) while testing.
I found this in a quick search, but couldn't determine what material the calculator is for http://www.epcserver.com/Structural/Analysis/plates/plate8.asp 8" square plate, .5" thick, 150 psi.= .0023 deflection. There's other interesting stuff at the site, and despite the $5 warning, no request for payment appeared. This doc might interest you http://www.powerengineering.ca/errata/PE2_%201_01_revised.pdf . Flat plate calcs on page 31.

My thoughts as well. I can't think of any practical reason to use an outside fillet in the application anyway. Maybe if it was Sunday and one was out of grinding discs? :-)
Wayne
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Pete, just a thought, but fatigue testing can be done with a air valve (I think that 3 way will work), and a timer. Assuming a 10 second cycle, after just one day you will have tested 8,640 cycles.
i
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wmbjk wrote:

Actually it wasn't a matter of simple, it was a matter of mostly cloning the design of an existing commercial product.
Pete C.
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wrote:

I can't figure exactly how the lid seal works on it
http://eopen.gmcscuba.com/eOrder/assets/product_images/48300.jpg . Can you describe it?
Wayne
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wmbjk wrote:

I've not seen it personally so I'm not 100% positive. As near as I can tell from the pics and the test in the catalog PDF the lid is a disk of 1" thick plexiglass with some O ring arrangement for sealing. I'm not clear on whether the O ring is on the perimeter of the plex. or the steel, but it does not appear to be on the bottom pressing on any sort of flange. This makes some sense due to the bayonet locking lugs not appearing to have enough travel to apply any sort of downward pressure against a seal.
This plex. is attached to the steel locking ring which looks to be about 1" square plus the reinforcing bar across the middle. The whole lid assembly has six locking lugs that drop down in the six slots of the pipe housing and rotate to the lock position.
Pete C.
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My guess is the plastic is Lexan. I would put an O ring groove in the plastic as a piston seal. Easier to machine, smaller part to chuck in a lathe, and does not depend on clamping pressure to seal.
Dan
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snipped-for-privacy@krl.org wrote:

The GMC catalog specifically states "Plexiglas". I'm not sure on the differences between acrylic and polycarbonate for the application.
Pete C.
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wrote:

Yesterday I substituted the gauge from my Oxygen tank. Pumped it up to 1100 psi. About then a bead of water appeared on the 3/8" ball valve shaft and I noticed that the shaft appeared to have shifted outward. So I called that the limit. The grease gun was definitely bypassing. Thicker oil might be better, or maybe 1100 is all that can be expected. The gun is more effective with grease but that gets in the fill tube and is a pain to clean before refilling for successive tests.
I looked around and realized that I had every single thing I needed to build a little larger and better tank, and that the turntable I'd made for a previous project would make the job easier and nicer. I used some scrap 1/4" pipe and plate, and a piece of truck inner tube for a gasket. Inside dimensions are 4" diameter and 6" tall. 1/2" bolts because that's what I had. Squared up the ends in the lathe. Used a bendable rest for the MIG gun and supported my elbow on the drill press while cranking the winch handle. Overdid the welding quite a bit. Winch handle turned smoothly for the cutting, but got somewhat draggy when welding. Probably arcing at the pivots and gear teeth.
Cleaned the tank up this morning and assembled. First I tightened the bolts until the lid just started to deform. Gasket was evenly compressed, but every squeeze of the grease gun after 400 psi just seeped water past the gasket. Let it sit for a while at 400, no drop. Relieved the pressure and tightened down the bolts some more which caused a permanent doming of the lid. Repressurized and got the same effect at 700 psi. Working pressure for my application is 500, so I won't bother making a thicker lid or doubling up on the bolts. 7 hours total including testing, 6.9 of it grinding. ;-) Even though the seal doesn't hold as well as the previous threaded one, it's faster and nicer to use than having to dope and crank down a 3" pipe bushing. Photos here http://www.citlink.net/~wmbjk/testtank.htm . All equipment solar powered.
Wayne
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A highly impressive and entertaining set of photos.
V
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Likely not for this one Pete -
But the group might be interested.
I have a plasma machine and it had been in and out of moisture in the lines. Not good for plasma.
I put two dryers in-line and it cleaned up. But I noticed I keep getting water in the tank (80-90$ RH here most of the time ) and I figured a little money was worth a rusted out tank and lots of bad cuts.
I would empty the tank when I could think of it - then one day I didn't get the valve as tight as I should. A slight (no sound) leak drives out of the bottom. It keeps the bottom dry or at least nothing comes out when opened up!
I noticed the tank turns on once in a while and I get the tank running to fill before I use it on the machine. There are two sets low and high - I like to start with the tank full to allow less pump on time. I pump faster than use anyway.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
Pete C. wrote:

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For those not paying attention, this question (asked in a rather "odd' manner) has been addressed in eng-tips by experienced vessel designers.
JTMcC.

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JTMcC wrote:

I've not been paying attention the last few days. Where is this "eng-tips"?
The HF paint pot looks like it will work ok, within it's 80 PSI limitations with just the requisite plumbing changes.
Pete C.
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wrote:

This link will take you to a sample thread http://www.eng-tips.com/viewthread.cfm?qid 5280&pageg. I searched the forum but didn't see any directly applicable info. If you have the time perhaps you'll have better luck. Failing that, this pdf http://www.powerengineering.ca/errata/PE2_%201_01_revised.pdf (pg 29) appears to have the appropriate formula but I couldn't find any online copies of the referenced tables. By doing some substituting I estimated that my 4.5" tank with a 1/4" head and an 8" version with a 1/2" head might both be rated at about 250 psi using material with an allowable stress of 13000 psi, which should represent a substantial safety margin for a low use item. Note that if you break down the equation, using a thicker than needed cylinder wall reduces the cap thickness. Presumably making the cap oversize as I did is worth something as well, perhaps someone can put a number on that.
This site http://www.risacher.org/rocket/testing.html has a more basic formula for minimum wall thickness - that it should equal pressure in psi (1.5X recommended) times tank OD in inches, divided by two times the allowable stress in psi. That's about .075" for 8.625" diameter, 150 psi operating pressure and 13,000 psi allowable stress (they used 20k for seamless), which jives with the estimated .105" of a cheap, larger diameter portable air tank. The writer used a cap about 6X the minimum wall thickness for his 4.5" diameter project, and the other link noted 5X in their 30" diameter example.
Meanwhile I cranked my own tank up to 1200 http://www.citlink.net/~wmbjk/testtank.htm . I might drill for more, smaller bolts and a hole circle with a shorter arm, to see how much pressure a simple gasket can seal.

Cool, can't beat the price.
Wayne
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