[dave 21445]

Help please, I need to build a pressure vessel for air over water / solvent. I need to be able to pressure up to 200 PSI. I would like to build this vessel with aluminum plate but do not know the ratings of the steel plate and aluminum plate. This vessel is 24 high" x 48" wide x 120" long. In other words, long and flat.

I know that air compresser tanks, round, are capable of 200 PSI using a 3 / 16" thick wall steel metal. When the structure is changed from round to square it obviously affects the need for a thicker metal. So, if steel, what thickness ? If Aluminum, what thickness ? Is there a caluclation method of steel vs aluminum that I can use. Or a rule of thumb. Or can someone just give me some anecdotal information on what would be the size of the plate needed to construct this unit.

Dave Glassel

[erase42]

wow thats seems a pretty scary one to me. Im pretty sure Id be looking deeper than a DIY forum for the answer for that one. not that there arent some brilliant minds here.
but pressure tanks are round for exaclty this reason. your large panel will be 5760 sq inches. the stresses of 200 psi on that area are massive. if it is possible with plate, im sure its going to involve all sorts of internal gussets and webbing to try to reduce the stress on the right angle edges and corners which are going to try to pry open as the vessel swells. I think id turn this one over to an engineer, especially if its going to be pressurized and depressurized repeatedly. airplanes cycle between much lower pressures, and even they have to have rounded windows due to stress fractures that would occur at the corners. in fact they lost a few early comet jets i believe to this very matter, squares and pressure. gook luck, be safe.

[Old Megawatts]

I would caution you to be extremely carefull when fabricating any kind of air or steam tank/drum. Especially one of an rectangular shape. As the other gentleman stated, there are 5,760 square inches of surface on the large flat side and each one has 200 lbs. pressing on it. That is 1,152,000 lbs. total. That shape would require many stay bolts to hold it together. There is a very good reason why most pressure vessels are round and that is because the stresses are distributed uniformally. Should a vessel containing 200 psi. rupture in an enclosed area and expand to atmospheric pressure, it would likely destroy the building. Please be carefull.

[erase42]

what is the actual use going to be, maybe there is another way? how are devices that accomplish this normaly made? im just curious, you called it air over solvent? give me a hint

[dave 21445]

Thanks for the tips guys. I appreciate it. I was hoping to short cut a mobile pressure vessel for wood treatment. I built several of these but was trying to come up with a flat rectanagular unit that could accomodate 4x8 sheets of plywood and more than 2 railroad ties. The smaller unit I built is 9" X 9" x 36 inch, from a piece of 5/8 inch wall square tubing. It worked so well with that configuration I thought perhaps I could expand on the design and keep from going round which complicates loading and limits treatment area in relationship to the total volume of fluid I have to carry on the unit. I guess I will have to rethink my strategy.

Thanks for the help

[erase42]

yeah thats a tough one. i bet you could do it, you would have to band the whole thing every few inches with some sort of channel or I beam. the more liquid, and the less volume of air you have inside would make the whole thing safer since there is less expansion involved if it ruptured, but i think you could still be in pretty grave danger.

[Geof]

Here is an idea. Make your rectangular chamber just using thin plate then put this chamber inside a cylindrical pressure vessel. The cylindrical vessel would need to be around 5 feet ID.

Fill your inner chamber with the wood and the preservative; fill the volume between the inner chamber and the cylindrical pressure vessel with water. Pressurize everything equally; there will be no pressure differential across the inner chamber.

The access hatches for loading and unloading may be a bit complex and the reloading sequence could take a while because it involves emptying the water out of the cylindrical vessel. But I think the combined weight would probably be much less than a rectangular vessel built to safely handle the pressure you are dealing with.

[erase42]

what if you built it upright, then you wouldnt really loose or have to drain any of either liquid. just load wood from the top end. In fact would you need to have water in the outer chamber at all? it would be safer if it ruptured but if it were rated for that pressure, it should be fine the inner square chamber would need no hatch or seal, just an open end inside the larger tank. this sort of ceases to be a do it yourself project at this point tho. a tank that size with a removable endcap is gonna cost someone big bucks im sure.

[Geof]

....a tank that size with a removable endcap is gonna cost someone big bucks im sure.

Get hold of a junked liquid carbon dioxide tank trailer.

[fpworks]

I highly recommend that you test your vessel with a pressurized liquid before throwing pressurized air to it. I would hydro test it on a regular basis while in use. Nobody is going to get hurt if it ruptures under water pressure.

I would definately make this of a steel or cast iron ROUND pipe, absolutely not rectangular. Depending on material availability, you may have to weld together short sections. I like the idea of standing it up for top loading. Weld on a flange to your loading end with a circular bolt pattern.

Done.

[Zumba]

Some people are saying that the liquid is safer than the gas. That would be true if you were using water. But you're pressure treating lumber... I assume you're using the stuff that turns lumber green? Doesn't that contain arsenic?

I would recommend doing this out in the desert. Kind of like those high-powered rocket launches but more dangerous. Wear one of those biohazard suits and maybe a hardhat for kicks.

Anyway, as other members have said, you're dealing with over a million pounds of pressure. Even if you changed your design to a cylinder, it would still have to be 4 feet in diameter, and you would need half-spheres for the ends. The bottom end could be welded completety shut... the top would need heavy flanges with massive bolts lining the perimeter. What is your budget? You could probably buy a nice HMC for what it'd cost to properly fabricate this thing.

There's probably a reason why pressure treated plywood costs 3 times as much as untreated ply of the same grade, whereas pressure treated 2x4s are only double.

P.S. Just to give you an idea of what you're dealing with... a typical gas tank... SCUBA, welding gas, etc....has a flat bottom. That flat bottom is what.. 8" in diameter? Pi*r squared times 3000 PSI = ~151,000lbs of force. If you built a 48" cylinder, flat end caps would have to support ~362,000 lbs. And you can't merely double the thickness because the span is six times as long. Think "beam deflection calculation" but more complicated.

[unterhaus]

aluminum is a bad material for this since it doesn't really have a fatigue limit and it doesn't fail nicely like steel does. There are ASME codes for pressure vessels.

The reason you pressure test with water is that if you have a failure, the tank will not explode. The water pressure will be relieved more quickly.

[steele]

Look, this is very do-able. Just use some basic engineering formulas & some common sense. I probably would not weld on the edges & in the corners where possible. If I were making a weldment I would use thick angle iron for the edges (a frame) & thin plate inbetween to reduce tension on the welds. Or better yet try to find a fabricator who will bend the edges on a press brake. Don't try to save money by keeping the chamber size minimized; you'll definitely have to leave some room for curved edges & corners. I might also consider using a bolted design using 26" to 30" long all thread studs. If it fails it willn't be catastophic. If you are going to draw a vacuum before backfilling w/ treatment fluid you will have to worry more about the vacuum of 14 psi colasping than the 200 psi of expansion.

[Zumba]

Look, this is very do-able. Just use some basic engineering formulas & some common sense. I probably would not weld on the edges & in the corners where possible.

"This is do-able"... you're basing this assumption on... what? Gut instinct? You mention common sense, but clearly you're not using it because you're making suggestions on how to "properly" assemble a rectangular tank design!

The safest way to do this is to use cylindrical steel for the length of the tank, place it vertically, and if flat end caps have to be used, so be it. First of all, this will reduce the number of joints from 6 to 2 (and you could probably consider the cylinder indestructable). Second, it'll reduce the maximum loading on the highest-loaded joint from 1.1 million lbs to about 360,000. Third, when was the last time you saw a cuboid gas tank or submarine? Even a soda can for that matter?

[Zumba]

BTW, to the OP, take what you read here with a grain of salt. You should really hire an engineer for this job if it matters to you. If you don't do it properly, I can see it ending very very badly.

[Mcgyver]

I'm with the naysayers on this one. 'strength of materials' or such will show you how to do the calcs but you want to be really really sure of a pressure vessel design - either build to tried and true designs or get engineering help. pressure vessel design is size dependent - the wall size of a smaller tank is meaningless and everything changes if you go rectangular. to build a rectangular tank you have to be able to figure out the frequency and size of staybolts before you can determine, or at least do them in concert, the material thickness. think locomotive firebox to get a sense of how complex the assembly of rectangular pressure vessels taking hundreds of pounds are; staybolts everywhere!

[kevincnc]

Wow, I agree with hiring an engineer with working pressure vessel experience. If there's any liabilty involved (and there must be), make it a PE. You have the potential to kill people if you do it wrong. If you do it on your own, I would pressure test it to at least 3X the working pressure. Sounds like a fun project, let us know how it goes.

[Geof]

BTW, to the OP, take what you read here with a grain of salt. You should really hire an engineer for this job if it matters to you. If you don't do it properly, I can see it ending very very badly.

But he should first get liability insurance to cover giving the engineer a heart attack when the idea is described .

[steele]

I stand by what I said. It's not that difficult. Looking for a used pressure vessel to do the job is still a great idea. If liability is a concern, then yes, you need to go to a fab. shop w/ all the necessary ASME certifications & pay the big bucks. If you're only concerned about killing youself it's another matter. Tank mfgrs. use special grades of steel that rip slowly not fail catastrophically-don't know the grade. A 5ft. dia. tank (big enough to hold a 2'x4' tray has over twice the volume of a 2'x4' cross-section tank. Now you have to make up w/ lots of dangerous air or tremendous amount of usless fluid. The absolute last thing you should do is weld a round pipe to a thick solid plate end cap. Welding a flange around the outside of a cylinder & bolting it to a solid plate is O.K. though. Stop & think about it: how do the big guys do it the right way? They would use a horizontal clynidrical tank w/ a full size swinging door w/ about 30 latches of thick bars in pure shear. Both the door & the back of the tank have eliptical heads w/ a wall thickness similar to the cylinder's walls. They buy their autoclave from a reptuable, experienced manufacturer and the vessels are inspected during use by industry accepted guidelines. They would draw a vacuum to make room for the treatmen fluid. They would fill a pan the wood is sitting in w/ fluid, pressurize & heat to reduce fluid viscosity. If you're going to compete w/ Koppers that's it. If this is an experiment & you're only going to use it a few times get some 3/4" plate, a big hoist, a torch or magnetic base drill, a bunch of all-thread, nuts, I-beams, & some tubes of Silicone caulk & go for it.

[Zumba]

I stand by what I said. It's not that difficult.

I'm not trying to convince you to change your mind. I'm trying to convince Dave not to listen to you.

Do a quick google image search for "pressure vessel". I get 7000+ hits. How many of these hits shows a rectangular structure? None, because "it's not that difficult". People must use cylinders because they're trying to make their lives as hard as possible... obviously. (nuts)