Welding a small pressure test vessel...



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
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Steve B wrote:

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.
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wrote:

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)
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Bruce wrote:

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.
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I always like making stuff AT LEAST 3x stronger than it needs to be.
Steve
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Steve B wrote:

If 5:1 safety ratios work for elevators I figure they're good for me...
Pete C.
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wrote:

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
http://www.citlink.net/~wmbjk/images/camtestpot.jpg . 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.
http://www.citlink.net/~wmbjk/images/colorcam.jpg . Both tested fine at 500psi. No worry of flying glass in my application though.
Wayne
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wmbjk wrote:

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.
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wrote:

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
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wmbjk wrote:

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

Aren't those SIP scraps handy?
--Andy Asberry-- ------Texas-----
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wrote:

Yup, but I can't figure out what you think I used them for on this project. Maybe I forgot. ;-)
Wayne
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wrote:

I just looked like one in the background of that shot.
--Andy Asberry-- ------Texas-----
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wrote:

I see what you mean now. Yes, it sure does look like the edge of a SIP. But it's actually one edge of a recess in the concrete that allows the hoist to be lowered below floor level.
Wayne
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"Pete C." wrote:

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.
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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.
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You might consider using a " Joy " test. Instead of pressurizing to do a leak test, immerse the item in water with some Joy ( or Dawn ) dishwashing detergent and pull a vacuum. It used to be called out on aerospace drawings as a leak test. The detergent is so you can see very small leaks that only generate a bubble every 5 minutes or so.
Dan
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wrote: snip

Good suggestion, and nice to see somebody thinking 'outside the box'.
Sometimes we go to great effort to implement an obvious solution when an alternate is simpler and safer and less work.
just my .02, YMMV
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wrote:

If that's more practical, then by all means. But if it's because of the safety angle then I'm sorry to hear it. Whenever these sort of things come up on Usenet, there's a always a flock of "better safe than sorry" posts. It's discouraging but tolerable when the project in question might involve the safety of the unsuspecting. But it's hard to take when the only person at risk is the one who's willing to take it. At the rate the scope of home projects is being limited by safety concerns, we're going to need licenses to sharpen our pocket knives before long. Your plan as you described it was certainly doable with reasonable skills and common sense. A conservative builder could test the thing to say, 5 times operating pressure. I expect that's a higher margin than was used for HF's paint pot, or countless portable air tanks etc. which everybody seems to have faith in.

Well, at least it'll be easy to tell if the test part leaked. :-)
Wayne
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Your plan as you described it was certainly doable with

You're missing a lot here. To put it as simply as possible, this is a field where there are literaly hundreds of possible failure modes that will happily appear days or years after the vessel has passed a pressure test. A pressure test is one small portion of the overall picture and does not tell you you have a suitable vessel. There are dozens of red flags in the op's idea, number one being he has no way of knowing (and therefore duplicating) the original material specs, weld procedure, any pre/post heat treatment, ect. And, I can say with confidence that when you desire to build a vessel with flat plate, your engineering bill will turn straight up with many engineers not even willing to work it, and the thought of putting a fillet weld on the plate to pipe juncture would give your vessel engineer nightmares, you just can't design in a more wicked notch than that. I think thats a guaranteed fatigue failure down the road. This is the world of 100% weld penetration. Not an area hobby welders are going to excell. We could go on and on. Anyone who thinks this is a simple, solve it in the backyard type endeavor is showing how little they know about a very complex topic.
JTMcC.
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