Welding a small pressure test vessel...

Amen, JT!

I was all in favor of Iggie building his trailer and emitting flatulence in the general direction of detractors and doomsday naysayers. That was because in a trailer, steel can reasonably be expected to YIELD befoe it fails.

With a pressure vessel it can reasonably be expected to EXPLODE before it fails.

I consider myself to be a moderately advanced and competent hobbyist welder. But that doesn't mean you'd want me welding in your nuke generating plant.

V

By the way, Vernon, the trailer is quite well functioning, I recently used it to carry a 2,200 lb Bridgeport mill and it did not even budge. After about 50 miles, there was not a single trace of the mill ever having been transported.

Also, some concern was expressed that the drop axle's top was too close to the frame fo the trailer -- but even with the mill on the trailer, and some bumps on the way, the U-bolts never touched the frame.

I hope that you mean "soon after it fails".

i

JTMcC, very impressive actually. I am sure that those welders are very highly trained pros and have to endure a lot, so they deserve every dime that they get.

i

In one your posts you mentioned that you could point a person to some info on pressure vessels and the stresses/dynamics/requirements. Without over loading me (I think you mentioned something about providing more informatiion than any human could want), do you have a favorite site(s) that explains the issues involved?

Thanks,

Peter

Great. I've generally enjoyed your posts, and can't remember ever disagreeing with you. I doubt I was alone in believing that reliably capping some low pressure pipe was a piece of cake for you. So I was disappointed that you didn't offer any "this is how I'd probably do it" advice here, and a bit baffled that you haven't responded to direct questions about deflection and strength.

It's not a case of being "untrue". It's about spinning advice to the point of being needlessly discouraging to the OP and others who might have been willing to share their own experience. Despite all your posts to this thread, I still can't figure how you're able to reconcile "literally hundreds of failure modes" with your obvious lack of confidence that even one of those modes would manifest itself in testing that's well beyond the planned use of the subject item.

Well, not directly at least. But don't you think it would be fair to say that you did everything you could to scare him off the project? It's a bit ironic that a recent thread discussed how every welder is only as good as his last proven project, yet IIRC, three of you have pronounced the OP unfit without ever seeing his work.

Nobody said it did. My point was that one need not necessarily be an IA to successfully work on aircraft, just as one need not necessarily be an automotive engineer to successfully change wheel bearings, or need not necessarily be a pressure vessel designer to successfully build every type of pressure vessel. For example - imagine a reasonably competent farmer with a buzz box, capping off an 8" diameter, 12'' long, 1/2" wall pipe with 1/2" plate. One cap with a fitted and filled joint, the other with a heavy outside fillet. We'll further say that he was a bit lazy with the brushing and chipping between passes, that he left some undercut here and there, but had a minimum of 3/8" thickness at the thinnest part of the joints. Now we charge his work with water to 150 psi, and drag it behind a car for a while. I'd be willing to bet that it'll stay pressurized. How much would *you* wager that an ASME certified portable air tank would survive the same treatment? Can you not acknowledge any possibility that a home built version could be overbuilt to the point that it's not only acceptable for its application, but better in some ways than the factory model? How about the subject item with a steel lid, fixed fasteners, and a small view port, versus the commercial item with a full polycarbonate lid and hinged fasteners?

Yes, but everybody has their own idea of how far we should go in making others "aware", and too many can't see the downside of moving the safety line until *everybody* is on one side of it, no exceptions no matter what. We've met the enemy and he is us, and the cumulative effect is that we've already reached the point where store clerks wear unfastened back braces, and some folks are forced to pay professionals to pump their fuel.

Much like what's happened in this thread, there are no doubt experts in NJ who cite flaming secretaries as proof positive that amateur fuel pumpers must be protected from themselves, even if their day job is driving a fuel tanker.

Respectfully, I'd like to trade your lame characterizations of my position, and your constant reminders that you're in the biz, for some answers to direct questions, and helpful comments on the best joint design, the fastest lid fastener scheme, the most secure gasket, the easiest view port arrangement, etc. How about it, do you have anything to offer?

Wayne

FWIW the commercial version can be found here:

the 48300 and 48310 test chambers.

I would also be interested. I pulled out the only book I have that has anything on pressure vessels, but it does not explain things clearly enough for me. The formula it gives for stress in a flat end gives 7200 psi for a 1/2 inch thick bottom on a 8 inch id tank with

150 psi pressure. But does not indicate if this is at the edge or center of the bottom.

Dan

I don't really know of any one overview, such as a "pressure vessels for dummies" type thing (I'd have it if there was ; )), mainly because there is so much ground to cover. But here are some resources that have an awfull lot of information available to those interested. I'm not sure but I think the closest thing to an overview might be "Applying the ASME Codes Vol 2". I don't have the book, I'm just guessing here.

A) That and a lot of other information is available at the ASME website,

I don't think asme has anything that's free tho.

B) The National Board website,

also has tons of information available. Some of it is free, they have quite a few "technical articals" on the site that are pretty brief, but interesting. Both of those sites have a lot of books and publications at a cost.

C) There are many thousands of sometimes very interesting and enlightening posts on vessels, piping and related topics here:

The forums most on topic are:

1) Boiler and Pressure Vessel Engineering 2) Piping and Fluid Mechanics Engineering 3) Mechanical Engineering Other Topics 4) ASME (Mechanical) Code Issues

The forums that are related are:

1) Corrosion Engineering 2) Material Engineering Other Topics 3) Metal and Metalurgy Engineering

If I think of anything else that might be helpfull I'll post it but there is mucho info in the above three.

JTMcC.

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

I'm sure some publisher will catch on to the vast untapped market of people interested in pressure vessels. I'm sure they could sell... dozens.

This is a great site. It also has a number of forums on subjects in which I am more normally interested.

Thanks,

Peter

Matter of curiousity...

Suppose you had a blacksmith's forge, or a muffle furnace at say

1050C, and you forged a dome from a flat plate over a "mooring bollard" shaped object - going round and round with a hammer like you do for drawing copper to a bowl, until the ends of the plate had made it round 90deg at the furthermost point, normalised the plate a couple of times from 900C (that is, air-cooled it from 900degC) - then did a full-penetration butt weld - say 70deg included angle single-sided groove - between the end of the done and the end of the tube?

Would that get you anywhere in the right direction???

Richard Smith

BINGO, we have a winner, or at least somebody that is actually listening.

The only change that I would make is to just buy one from an industrial plumbing supply house. They are carefully engineered and CERTIFIED and manufactured under good production quality control. They come nicely finished in primer and with a prepared bevel edge that will just need a little work to clean up ready for welding. They are available in all standard sizes and are generally

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

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 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

Because plate will be the cheapest solution, and if it's thick enough it will be as strong as a dome. The top is almost certainly going to be a plate regardless.

Humble? LOL Officious and pigheaded, but hardly humble.

Speak for yourself. Check this quote from pg 29 here

"Flat plates... are used extensively in boilers and pressure vessels. When a flat plate or cover is used as an end closure or head of a pressure vessel...". (calculations follow)

Is it your position that the ASME are demonstrating "monumental ignorance"?

Your points about the fish-mouth bottom and the availability of commercial flanges are well taken and much more useful than your insults and caps.

Wayne

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

I'll note again that the commercial pressure test chamber with the flat plate base and flat plex top can be seen at:

as items 48300 and 48310. Pictures and all...

I picked up the $40 HF 2.5 gal paint pot today. Haven't unpacked it yet, but it's a rather heavy box.

Pete C.

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

Pete C.

Yesterday as I was working in the yard, I thought of how I might approach making a pressure test chanmber. And will post the idea here to see what more conservative people think.

I would go to the local scrap yard and look for a large steel argon or oxygen bottle. Even though it is out of hydro or even if it failed hydro, I suspect it would still be good for 150 psi. The local scrap yard has a band saw so would have them cut off the bottom and the top. A 12 inch by 12 inch granite floor tile would work for a surface plate and I don't think it would take long to get the cut surfaces flat to within 5 or 10 thousandths. Now the problem is to figure out the best way to secure the top piece to the bottom. I don't know what alloy is used for the compressed gas bottles. So I am not sure about welding lugs on. Anyone know what steel is used?

Dan

I can't figure exactly how the lid seal works on it

Can you describe it?

Wayne