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.
Why would you go with a piece of used cylinder of unknown composition and history when you could buy a piece of bran new, known grade pipe for not a lot more cost given the foot or so needed?
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.
'nother thought - springs from last weekend visited the "HMS Belfast", a World War 2 era cruiser moored on the Thames in London opposite the Tower of London. It has "Yarrow" type (?) 3-drum water-tube boilers. And one of the lower drums was opened up to show the inside with the pipes coming in. And the way that could be done - if I remember rightly, it had an oval access plate on the inside, so that when there is no internal pressure and you push it inwards, by rotating it bit in space you can bring it out of the vessel narrowest-axis of panel to longest axis of opening - the classic boiler access plate.
So can you get "stock item" a tube dome end with an oval access panel built in, like you would use in the drum of a water-tube boiler?
No welding required for the bottom. The pipe is likely to be welded and drawn. Gas cylinders are made from a billet and the top spun if I recall correctly. Someone here is likely to know what steel is used in making gas cylinders. It is probably some sort of alloy so would need to use a low hydrogen method of welding. TIG qualifies.
Sure. The environment at some tire shops is complacent and haphazard even though tire seating doesn't provide any opportunity for prior testing. Workers in such situations are generally considered hapless victims, and society has deemed them worthy of protective regulation. BTW, I have 4 two-piece rims due for my attention. I use a home made slide hammer for busting them although it's obvious that most of the previous guys used a poorly aimed sledge instead. I fill them with a long hose from about 10' back and out of the line of fire. I don't even like walking by the damned things no matter how long they've been in service.
Anyway, I have a better analogy for you than tires - homebuilt aircraft. They're generally designated "experimental", as opposed to "CERTIFIED". ;-) Even if the builder starts with a kit, he is officially the manufacturer and therefore responsible for all design, construction, and ratings. He is assumed to have used some care in making his decisions. For example, it's generally his choice where he uses AN fasteners and where he substitutes butter bolts. The ship usually receives a single regulatory inspection before its maiden flight, and there's no way for that inspector to take the plane apart looking for every potential problem even if he was qualified to judge. The inspector will generally look for the obvious such as lock wire etc., but he's in no position to say if the spar can take 1G or 10, or if control surfaces were accurately balanced. All of that is expected to be accomplished and tested to the limits of his choosing by the manufacturer (AKA hobbyist), who generally gets the job done more by sheer determination than through any credentialed expertise. And yet it's undeniable that some pretty impressive ships come out of normal folk's backyards and basements. Mosey on over to Oshkosh if you want to see thousands of examples
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Many of them are more advanced and better built than certified models. So you might try thinking of the OP's project in that light.
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
Here's another similar analogy: DIY submarines. I'm not saying that an airplane, a submarine, or a pressure vessel is beyond the abilities of a given do-it-yourselfer. I found a web site by an Austin, Texas couple who built an impressive submarine.
Nevertheless, we do these things at our peril. However, I lift my cup to the guy who does his homework, acquires the necessary skill, takes a deep breath, and plunges forward.
However, I personally would not undertake any of these projects, given my current skill level. Were I to want to do so, and inquire here, I would hope the advice would be: "You're ill advised to do this at your current skill level. But learn this and consider that. Then, if you still want to do it, go for it.
On the other hand, I would not consider it to be charitable or brotherly to simply say, "Go for it, dude!".
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
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(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
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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
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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.
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