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plasticfan · Novice Joined: 10 Aug 2006 Posts: 43

After months of searching for a heater tank on freecycle, craigstlist, ebay etc, Ive all but given up. Im thinking of dropping about 100 clams on a propane tank. However the one I saw at Lowes, seems a bit thin.

Anyone here use a propane tank, and if so, what kind?-- ie, make, model, lbs, etc.

Anyone use the 100 lb tank at Lowes? I saw it last night. Seemed kinda thin.

thanks
Plasticfan

jegner · Posted: Wed Oct 25, 2006 7:44 pm Post subject:

I don't know how much the propane tank you refer to costs, but have you considered using a 30 gallon hot water tank? Used ones would do fine as long as the did not leak. Cheap too. 40 gallon is more comon.

drcrash

If it seems thin, I would not risk it.

People often think that if a tank can hold several atmospheres of pressure, it should be able to hold one atmosphere of vacuum. That is not true in general, maybe especially if you're talking about steel.

In general, the crush strength of a tube with a given thickness of wall drops off rapidly with increasing diameter, much more rapidly than pressure handling does.

Positive pressure handling depends primarily on tensile strength of the walls, and not directly on wall thickness.

Crush strength depends on thickness, because thickness increases rigidity. (Thicker is better.) It also depends on curvature, so it depends on diameter. (Smaller is better.)

A material with very high tensile strength can be thin and still handle a lot of pressure, because when it's under pressure, the walls are in tension like a ballon.

Those thin walls will have much lower crush strength for a big tube, both because they're thin and because they're not very curved. Thin metal is much more flexible than thick metal, and flat metal is much more flexible than bent metal. (In the direction at right angles to the bend, i.e., inward.)

That means that at some sufficiently large diameter, a thin-walled steel tube that can handle any given number of atmospheres of pressure will nonetheless crumple under one atmosphere of vacuum.

Unfortunately, I don't know how to figure out what that diameter is, for a given pressure-handling rating. (And it probably depends somewhat on what alloy is used, details of construction, etc.)

I only trust my little compressed air carry tank as a vacuum tank because it's little.

I wouldn't even trust large household hot water tanks, except that everybody seems to use them, even pros, and they don't seem to implode on anybody.

(The Plexiglas (TM) forming manual suggests using them as vacuum tanks. I suspect that if hot water tanks were prone to imploding under vacuum, they'd have heard about it and stopped recommending them. That gives me more confidence than Thurston James's recommendation alone, or the fact that nobody I know of has had a problem.)

jegner · Posted: Thu Oct 26, 2006 7:27 am Post subject:

The general rule of thumb is the max obtainable vacuum, [as it releates to tank stress] exerts the equivalent of about 15psi. Tony, can give you the exact specifics, but it seems like he and I have talked about this before. Compressed gas cylinders, and water tanks usually have enough strength to hold a vacuum without imploding. I've even seen those plastic farm tanks used as vacuum resivors.

drcrash · Posted: Thu Oct 26, 2006 6:16 pm Post subject:

plasticfan · Novice Joined: 10 Aug 2006 Posts: 43

So Im not sure where that leaves me.

What I do know is general guage strength. The propane tank I saw at lowes sounded thin when I tapped on it, so my judgement was hardly scientific but rather based on a hunch that this tank was no more than about 16 guage thick.

My reason for seriously looking into propane tanks is because I simply cannot located a water heater. Tried ebay, craigstlist, etc. Bubkiss.

I would be more than willing to travel and pay a reasonable fee for a good conditioned one. Trade too.

drcrash · Posted: Sat Oct 28, 2006 10:48 am Post subject:

Here's somebody giving away a 50-gallon on DC Craigslist

http://washingtondc.craigslist.org/doc/hsh/226479619.html

rockwerks · Posted: Sat Oct 28, 2006 5:13 pm Post subject:

co2 tanks and liquid fuel tanks are rated for 1650 psi or 3000 depending on the MFGR.

drcrash · Posted: Sun Oct 29, 2006 5:28 am Post subject:

plasticfan · Novice Joined: 10 Aug 2006 Posts: 43

I agree the rust issue....or better put the oxidation issue via humidity which is invariably part of many areas in the US, is a serious concern for an otherwise thinner tank.

As for the Craigstlist finds, most are for modern tanks...nothing more than fiberglass and copper.

plasticfan · Novice Joined: 10 Aug 2006 Posts: 43

A few more thoughts:

--Lets see if we can determine the guage thickness of a water heater tank and a propane tank. My gut is telling me they must be close---I cant possibly see why a water heater would be that thick....er, rather thicker than a propane tank.

--If the advantage is merely that the water tank is presumably internally coated with something, I would like to know what....zinc? tin? copper?

--and if it is coated, that implies that any tank other than a water tank is subject to oxidation, so Id then be curious to know what other people are using if not water heater tanks. ..and how long theyve been using them.

drcrash · Posted: Mon Oct 30, 2006 12:12 am Post subject:

drcrash · Posted: Mon Oct 30, 2006 12:30 am Post subject:

rockwerks · Posted: Mon Oct 30, 2006 9:05 am Post subject:

My personal thhoughts on the subject are that if one can safely use a beer keg the propane tank will surely be safe. LOL

drcrash · Posted: Mon Oct 30, 2006 10:38 am Post subject: