Welding a small pressure test vessel...

I've considered using a unit like that as a base to modify from. Building from scratch with heavier materials would allow more safety margin however.

None of that applies to a non commercial project. I'd be building a whopping two of these, neither to be sold.

I was not planning on using bolted flanges and caps, just a piece of sch

Were are these "LOT" of weak links? What I'm proposing has only a few components. The welded base plate has a lot of area relative to the pressure applied. The sch 80 pipe is pretty well known. The cover is largely the same as the base, but bolted instead of welded.

How the heck do you come to that conclusion? Did you read anything I wrote? I specifically indicated the potential catastrophic failure mode, which is why I'm asking for suggestions to suitably overbuild with an adequate safety margin. This isn't a reactor vessel, or a high pressure storage tank, we're talking 150 PSI, not 3,000 PSI.

Now that is total BS.

First off, the design is largely based on a commercial unit so it is within reasonable design parameters.

Second off, we're talking TIG welding steel, not stainless, aluminum or some exotic alloy.

Third off the welds in question will be under quite low tensile load due to the large weld area relative to the modest

You asked, "Recommendations on how to do this safely?"

I answered with (what I hoped would be a helpful) recommendation to use an existing engineered and certified product, but after reconsideration, I am not sure will be rated for the load which you wish to apply. YMMV

My suggestion to check out 150# rated flanges and caps was so you could see the size of material that is used in engineered design but I should have specified flanges and caps for gaseous and not liquid pressure containment as 150# water flanges can be much smaller than those for energy containment.

In My Humble and respectful Opinion, hopefully helpful, and not intending any flame or insult,

YOU JUST DON'T KNOW WHAT YOU DON'T KNOW.

You also asked "Am I crazy to attempt it?"

My answer, (becoming much more obvious) is YES!

PLEASE, GET QUALIFIED ENGINEERING ADVICE.

just my .02, YMMV

You should be totally fine on the welded side, based on my own calculations, with the synchrowave etc. Plus, if your vessel is not too deep, you can stick weld it on the inside seam also, not just on outside. I would stick weld it myself, with 7018, for convenience more than strength. Would be faster and just as strong as TIG, and the weld would lend itself to a very comfortable welding position if you rotate your vessel by hand as you weld.

The bolted side, is an area of much greater concern than the welded side. I would like to ask, as a challenge, that you prove that it has a large safety margin under realistic conditions (use of gasket, bolt torque in ft-lbs, relation of bolt thickness to pipe wall thickness, etc). 3/8" thickness pipe (you mentioned 1/2" originally, but then the discussion shifted to 3/8 as schedule 80 pipe was mentioned) does not leave able to use large bolts parallel to the axis of the pipe, so your requirement of 6 bolts only can subject them to considerable stress. (I may be mistaken, but you seemed not wanting to use flanges, unless I missed something) Can you do some convincing math?

As I indicated, I *am* using an engineered commercial design, I'm essentially cloning a commercial product. The material types, sizes, wall thickness' etc. are all known. The commercial product is for the exact same application at the same working pressure. The commercial product is some $1,500 which is not in the budget. I expect to build my own not-for-sale version for a few hundred. Labor costs don't count since it's for my own use and a friend's use.

Pete C.

With respect, You are not using an engineered commercial design, you are attempting to reverse engineer a product without access to the material specifications and without material analysis or quality control.

Just my .02, YMMV

IMHE, Flat plate is not generally accepted practice as a method of capping a pipe for pressure containment. If nothing else the stresses resulting from the shrinkage and cooling of the weld will place the weld under shear stress that may cause failure. Normal practice requires the use of domed caps or a curved bull nose and installed with full penetration welds. The use of flat plate is also prone to failure caused by failure of the plate itself, it is very common for even good quality plate to have internal failures similar to a lamination failure, (I cannot recall the correct term immediately but this defect can be introduced in the rolling phase, IIRC it is called lamellar tearing). It is somewhat counterintuitive, but the likelihood of this type of failure is even greater with larger welds and even more so with less than full penetration welds on both sides of the pipe and or welds made with multiple small beads. The presence of this type of failure will give no surface indication and will progress further with each cycle of pressure or thermal loading.

Your caution regarding the bolted side is well placed and you are to be commended for considering the effect of the use of bolts which are not parallel with the expected load.

I agree with your use of math to estimate loads but there is no way to estimate or account for reduction in performance due to operator error or material failure. Simple calculations are only valid for fabrication in heaven. On earth we use quality control including NDT to compensate. I suspect that the OP has no QC program.

Just my .02 YMMV

Ok, more exact info re: the commercial unit:

"Its diameter is 10? inside with 11? of usable height; outside dimensions are 10.8? in diameter, 15? high, with a weight of 100 pounds. The steel alloy pipe body, with a 0.45? thick wall."

"A steel ring that engages into the body by means of a cog and slot system contains a 1.0? thick, O-ring sealed, Plexiglas viewport reinforced by a narrow, transverse metal bar."

"Although hydrotesting is not required, each unit undergoes multiple inspections and pressure tests to 350 psi."

"All chambers possess two overpressure relief valves: one set at each model?s working pressure and a second set at the factory test pressure."

Basically the only thing I'm looking at doing differently is the cover attachment. Their setup has six bayonet lugs latching into slots milled in the pipe body. While I do have a 10" tilting rotary table I think it would be easier to build with the bolt down setup though it is slower to open and close. I suppose I could try to clone it more exactly. The unit in question is not patented as far as I've been able to find and I'm not planning on selling them.

Pete C.

So, what bolts will you be using and what are your calculations of bolt tension.

My point was that you could use the same basic method I used to safely pressure test your vessel before use. I wasn't suggesting 500psi for your test, but that might be a fair safety margin. The welding shouldn't be an issue anyway, and once you're satisfied that the lens won't blow out at 3X operating pressure and that the lid stays sealed, then you should be pretty confident about filling the thing with air and putting your eyeball (or mirror) up to the lens.

Wayne

The commercial unit does not use domed ends. The base is flat plate welded to the pipe body and the top cap is 1" thick plexiglass retained with what looks to be about a 1" thick steel ring 10" OD x 8" ID with a cross bar to reinforce the plex.

I'm not sure what you're both talking about here, my intention is that the bolts will be fully axial to the load, just not full length "tie rod" style, but rather connected to very oversized pivots to allow them to swing outward once loosened to allow removal of the lid. The commercial unit uses six lugs that appear to be about 5/8" dia in a shear configuration. The commercial unit is 10" ID and tested to 350 PSI, so over 27,000# load.

QC programs are not applicable to one-off items.

Pete C.

"Pete C." wrote in message news: snipped-for-privacy@snet.net...

There are exemptions that allow some small air recievers (and a few other things) to be not considered an ASME vessel. Outside of those limited exemptions any envelope that contains 15 psi or more is legaly considered a pressure vessel. You mentioned in your original post the term pressure vessel. Depending on where you live, vessels may or may not be regulated. The number of places they aren't is diminishing due in part to the new treatment osha is giving vessels. If your jurisdiction does regulate vessels, the BPV Code carries the force of law and it's then illegal for you (without holding the required stamps and conforming to all other inspection, material tracking, and testing, ect. requirements) to build a vessel and expose the public to it. Vessel construction involves quite a lot more than just using materials that will calc out to the load. That's why the BPV code is so stinking huge and cost's over $10,000 in it's entirety. I have quite a few sections of the code, and I first started building vessels over 15 years ago. I'm no design expert but I am familiar with a FEW of the many, many considerations that go into vessel design. Vessel failure in the pressure range you mention can range from a simple hissing sound to catastrophic failure and there are quite a few cases of people being seriously injured or killed at pressures even lower than yours, loss of arms being the most common I've seen documented. The welding requirements in the BPV code are extensive and are there to compensate for a lot of different failure mechanisms, fatigue induced cracking, corrosion, built in notches, the list goes on for quite a bit. I haven't commented either way on what you intend to do but I have pointed out a few of the many factors those of us that have build vessels for a living (I don't anymore, I do still work in pressure piping) have to take into consideration. I consider it an endeavor well beyond anyone describing themselfs as you have. Not to mention the large amounts of liability insurance one has to carry to weld on either new construction or repair of any pressure containing device. I have no interest in argueing the point with you, build what you want but you no longer have the excuse if ignorance should your vessel cause damage in the future.

JTMcC.

Offhand, 1/2" grade 8 which I'm pretty sure are vastly stronger than necessary, but a convenient size. If I'm reading the specs correctly they're good for something like 21,000# tensile strength.

I expect the worst case tension would be equivalent to 1/3 of the total load or just under 9,200# based on the commercial units dimensions and test pressure. I don't believe there is a scenario that would apply worse loading since anything else would not be able to produce a seal to apply load at all.

Pete C.

My plan for testing was to fill with hydraulic fluid and use a Porta-Power type hand pump to pressurize.

Pete C.

I've concluded that the testing that has to be done in an air environment can be done passably using a modified Harbor Freight pressure paint pot. It's limited to 60 PSI, but that should be sufficient for what has to be done in air.

For testing that requires higher pressures it will simply have to be done hydrostatically. I'll build the chamber with the same basic specs as I indicated, but limit it to hydrostatic (hand pump) testing so there is no catastrophic failure mode potential.

Pete C.

IMHO this is a wise decision. 'Safety before all else.'

'You should always think about how your actions will appear in the accident report.'

I would caution you to resist the urge to switch to air after many successful hydrostatic cycles as failures due to lamellar tearing or other metallurgical fault are cumulative and may give no warning before failure.

Good luck and continue to work safely.

Why not? A lot of stuff I do NDT on is exactly that - a one off item.

Then don't you need some sort of approval for it?

Holey smokes! Now I've heard it all! Think of the tens of trillions of dollars the industry could of saved if only they'd of known that!

JTMcC

WOW! For someone who entered with,

I'm not a certified welder in any way, shape or form. Mostly self

you know an awful lot. If you're so sure of yourself, go ahead and build it. Why even stop here for advice?

Please keep us posted. (or have your next of kin post something)

Have you ever stayed at a Holiday Inn Express?

Steve