Welding a small pressure test vessel...

I'm not a certified welder in any way, shape or form. Mostly self taught
though I did take an evening class at a tech school once. I do 95% TIG
on steel with my Syncrowave 250.
I have a project that I need to do which is a small pressure test
vessel. Max operating pressure is 150 PSI, air only, at least two safety
valves. Rough design is about a 12" length of 8" dia .5" wall steel
pipe, fixed welded on base plate and a removable top cap.
Figure the top cap should have say six bolts to secure it, most likely
hand knob style on a loosen and swing to the side out of slot in cap
type arrangement for convenience. The top cap will also need to have a
view port, presumably 1" lexan well reinforced with a couple steel cross
bars.
Recommendations on how to do this safely? Am I crazy to attempt it? It's
fairly low pressure and small enough to be overbuilt without excessive
cost. Were I able to do this as a hydrostatic chamber I'd be perfectly
comfortable, but as an air type chamber the potential catastrophic
failure mode makes me nervous.
Thanks,
Pete C.
Reply to
Pete C.
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Let's try to do the math. The 8" ID circle is 50 square inches or area exposed to air pressure. At 150 PSI, it means 7,500 lbs. The outer circle (9" diameter) is about 28 linear inches of weld seam. That means about 7,500/28 = 267 lbs of force for every inch of the weld. That's not much if your weld is half decent. I would not hesitate to weld the bottom plate to the pipe provided you have enough amps to make penetrating welds.
Same 7,500 lbs will apply to the top cover (cap). For your six bolts, it would amount to about 1,300 lbs of pressure (pull) per bolt, assuming that the top cover is not distorted and all bolts are equally tightened. Looks like 3/8" bolts would carry about 14,000 PSI, which is acceptable for good bolt grades.
Make a grease fitting adapter and test it with oil and grease. i would have a pressure gauge and a safety valve on it.
No comment on Lexan.
Do your math etc.
i
Reply to
Ignoramus3938
Sounds like a remote pressurized paint pot, probably easier to buy a certified unit than to fab your own. YMMV
Reply to
Private
You are not crazy to build it. But if it is going to be used by lots of other people I would over build it. How about using threaded rod and running the six bolts from the base to the top cap. No welding involved. Top cap and bottom identical.
Dan
Reply to
dcaster
Depending on where you are located in the world it will be illegal for you to build it. Might not be a big deal if it's for your own use but if you sell it or put it in commercial use you'd be in legal trouble in many jurisdictions in the U.S.A.
JTMcC.
Reply to
JTMcC
I have to get in line behind JT. If ever there was a dimension where "code" welding is justified, it is pressure vessels.
V
Reply to
Vernon
I would suggest that you keep the area of the Lexan small. Look at other plastics and compare strengths. As the other poster suggested, doing allthreads with two caps would allow you to sandwich the plastic in there as one layer, and you could make the hole in the top cap smaller in diameter making the surface area of Lexan (whatever) smaller, thus a little beefier in area/thickness ratio.
Interesting project.
Keep us posted.
Steve
Reply to
Steve B
You are talking about 8" schedule 80 pipe. I did a quick calc and if the material strength is 18,000 psi, which seems conservative, and using a safety factor of 4, the working pressure would be 600 psi and the calculated burst pressure is 2400 psi.
Normally pressure vessels have either domed heads or a standard flange and blind so to be technically correct you should either locate a 8" schedule 80 dome or flange and blind.
Having said that I might comment that you are grossly over designing for a 150 psi system.
8" schedule 40 has a wall thickness of 0.322 and using a conservative strength of 18,000 psi and a safety factor of 4 the working pressure is 400 psi.
8" schedule 10 with a wall thickness of 0.165 with other data the same has a working pressure of 200 psi.
The fixed and removable ends are another story. If I were building this for my own use I'd probably use schedule 10 pipe and a 0.5" plate for the closed end, which would be the base of the chamber. For the end that is removable I would try to locate a manufactured flange (I don't know where you are but here in Asia you can buy low pressure weld neck flanges. Check the pressure rating to be sure what you are buy though.
As you want a viewing port I would make the flange blind out of 0.5" material also and machine a stepped hole in it for the lemon: Top______ __ | _______|
Since the lemon is held in by pressure I'd just mount it with some kind of sealing gunk.
When I had the thing finished, if I felt nervous at all, I'd hydrostat it. Fill the vessel completely full of water, clamp on the lid and pump it up to 300 psi. Close the valve and come back tomorrow. You will probably have some pressure variance because of temperature but if you've still got about 300 psi in the pot your good to go.
Bruce in Bangkok (brucepaigeatgmaildotcom)
Reply to
Bruce
It'll be easy enough to test. Here's a photo of a quick and dirty test pot I made for testing small underwater camera housings to 500psi
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In use it's clamped to a car hoist so I can get some leverage on that 3" pipe bushing. Spare port at the top is used to fill the assembly completely with water. Plunger has been removed from a HF $10 grease gun. I squirt perhaps a quarter inch of oil into the grease tube via the plunger shaft hole (cheap gun wouldn't function with water). It only takes a few squeezes of the grease gun handle to get the pressure up. It works so well I wish I'd made it pretty.
As for lenses, I've used two different types on my housings - 1/4"X1.5" diameter polycarbonate, and a 1/4"X 2" diameter glass round from McMaster Carr. Both with about .200" seating around the rim, and sealed with an Oring.
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Both tested fine at 500psi. No worry of flying glass in my application though.
Wayne
Reply to
wmbjk
Your chamber is a hydrostatic one so it doesn't have the same issues as the air one I need to make. I've got a really good design for a hydrostatic chamber, but for this application it needs to be an air type since you want to avoid filling the unit you are leak testing with water. It's not going to 1,000' depth so 150 PSI is sufficient. It also needs to be larger and reasonably quick to open and close.
Pete C.
Reply to
Pete C.
A tie rod cylinder type design would be somewhat inconvenient in use. I'm thinking of a loosen knob and swing bolt out to side to open setup, similar to what you see on a trash pump where you need to be able to open and close conveniently to clear a clog. Some of the commercial units use a multi lug bayonet type locking setup on the lid, but I'm not really equipped to do that kind of machining on a piece this size.
Pete C.
Reply to
Pete C.
Considering the operating pressure is in the same range as a normal shop compressor and not all of those have "code" tanks I'm not overly concerned on that front. I also don't plan to produce and sell them, it's a one or two off thing (one for me, one for a friend) though it will be used in a commercial environment.
Pete C.
Reply to
Pete C.
The window does not need to be large, just enough to keep an eye on the unit under test. The real testing is watching the pressure gauge after shutting off the air supply to look for a drop in pressure indicating air is leaking into the "sealed" device under test.
The tie rod design is not ideal from an operational standpoint as it would require full removal of the hand knobs to remove the lid vs. loosening and swinging to the side like the fasteners used on many trash pump housings.
Pete C.
Reply to
Pete C.
Grossly over designing makes me more comfortable :) If I could do this as a hydrostatic chamber I wouldn't be concerned since that doesn't have a potential for a catastrophic failure mode.
Pete C.
Reply to
Pete C.
Easier yes, but way too expensive, like 5X what it should cost to build one. Buying one simply isn't in the budget.
Pete C.
Reply to
Pete C.
It will have a pressure gauge since that is necessary for the testing anyway, and two safety valves. Presumably if the clamp knobs are not tightened evenly the failure will be a leak at the seal. The lexan window does not need to be very big, just enough to keep an eye on the device being tested.
Pete C.
Reply to
Pete C.
great.
Depends on thickness of the seal, not necessarily true.
Having to compress the seal adds extra strain to the bolts.
Failure of one bolt can possibly make nearby bolts fail also, amke sure that you are way below their limits.
Reply to
Ignoramus6365
"Pete C." wrote
I always like making stuff AT LEAST 3x stronger than it needs to be.
Steve
Reply to
Steve B
If you are in the U.S. they most certainly do.
JTMcC.
Reply to
JTMcC
If 5:1 safety ratios work for elevators I figure they're good for me...
Pete C.
Reply to
Pete C.

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