Alum. water tank failure

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I got a call to repair an aluminum water tank on a truck that was used to pressurize water to @ 40psi to work a hose.3/4" hairline crack.so i tigged it up no problem.next day the tank looked like the pictures!!any ideas of what happened??when i repaired it it looked just like a small stress fracture.i could see the crack when i was welding as it opened up a little with the arc.I did not notice any other cracks. I did a 1 1/4" long row of 3 beads and thought i surrounded the crack! Attached ImagesG
Reply:Chances are if it was cracking, the aluminum had reached its fatigue life, welding just annealed the area that was already failing. Thats the fun of aluminum, it WILL fail, its just a matter of time, and there is no resetting the "clock".
Reply:That sure isn't a pressure tank! You could weld forever on a flat ended tank and it will still fail as a pressure vessel.tractor,loader.dozer,backhoe,and all the tools to keep em movin
Reply:No reputable manufacturer would make a pressure tank with flat ends.  As miner said it was going to fail no matter what.  In theory the ends of a properly constructed pressure tank are shaped so that all the pressure is transferred to the end so that the plate is in tension and there are no secondary bending stresses.  That flat plate is similar to a clothes line pulled tight and then some clothing is hung in the middle.  The line sags and the load at the poles skyrockets to many times the weight of the clothing.
Reply:All of the above.Thin aluminum, cyclically-loaded aluminum means probably fatigue failure, welding on some aluminum made a weaker area because the HAZ lowered the material strength down to 'DangSoft-n-Weak', a flat-ended tank used as a pressure vessel.Yes -some- aluminum alloys can be welded.  Some can't.Yes, aluminum has a finite fatigue life.  It -WILL- fail if there is cyclic loading, unlike steel which can be designed to pretty much last forever from a fatigue standpoint.Just be glad that the 'tank' was pressurized with water and not a compressible media like say air.  Time for a new REAL pressure tank.  You know, like one with domed ends.    The best laid schemes ... Gang oft agley ...
Reply:Good picture.  Based on the deformation of the metal being most exaggerated around your (former) weld bead, it does appear that the failure started right there (at the weld bead) initially, paused while the rushing out water deformed the metal around the bead, and then the crack started spreading out above and below and finally stopped when the water level got low enough to allow air to escape and relieve the pressure.Also interesting is that the original cracking failure, and also all the subsequent tearing occured pretty much fixed about 1" or so away from the original weld.  Too far away to be a heat affected zone failure I think... it is probably at the point where the material was flexing back and forth the most, as others have said, fatiguing the material over time and probably numerous flexing cycles.Last edited by jakeru; 07-22-2010 at 02:00 PM.
Reply:That tank needs caps, not flat ends.UA Local 598
Reply:gordfraser,I'm not sure, because the images aren't meant to show the tank ends as clearly as the failure, but there are a series of tanks made with ends that are press formed so there is a curve at the 'break' or turn or 'edge' of the cylinder. These are very low domed, almost flat ends, but they hold fairly well at low pressures because they're formed by a bead rolling, rubber female die in a press (cold forge in some terminology) or stretch formed; however they get formed- they've been "stamped" into a radius edged end panel.These ends are welded along a seam that is an inch or several down along the sides of the cylinder to avoid a HAZ at the critical turn between the end cap and the side wall.They flex at the line of the press forming, die bending/bead rolling line because they're thinned here and stressed and some alloys harden or work harden in the process.  These panels' sides or edge cylinders resist pressure as a 'hoop' - or tension ring; but the ends flex like diaphragms so the location where these two movements meet is the failure point.  The hoop is strong in tension the panel not as stiff in deflection- so the curved edges get 'worked'. As the tank pressures up and down (pressurized, emptied and filled cycle) this area is the flexed countless times until the forming line that made the tank so affordable becomes the weak point.It looks like that might have happened here?  I'd say there had been some crevice corrosion at this point, resulting from the stress of constant flexure and mineralized water over time.  That point leaked first, the welding allowed the crevice line to be thinned over a wider area and the (as welded) alloys were brittle compared to the formed end. (?) The failure line and 'unzipping' along a long term flexure/failure line began along the forming curve, where the part was thinned and developed by the cycling of tank pressure.I've worked on similar tanks on equipment, fuel, air brakes, no liquids, but they all seemed to give up the like this one has.  One trucking company ordered piles of new ends and we cut the seams along the sides using a skill saw set into a sheet of plywood, scored the welds and cleaned the joints and re-welded new ends onto the old tanks.  The sides had truck steps, fill and draw and lots of plumbing built into them.  In that case evidently the ends were cheap compared to new tanks?  Cheers,Kevin Morin
Reply:If that tank has a 12 inch radius, the area of the end is 12 x 12 x 3.14 which is 452 square inches. With 40 lbs per square inch that is 18,000 lbs on each end.
Reply:Dadgum...did that wrong.2 pie R.......2 x 12 x3.14 = 75 x 40lbs = 3,000lbs.. That makes more sense.
Reply:Originally Posted by gow589Dadgum...did that wrong.2 pie R.......2 x 12 x3.14 = 75 x 40lbs = 3,000lbs.. That makes more sense.
Reply:Thanks!
Reply:did you cut the crack out before you welded it up? If you just welded over the crack I highly doubt you got to the ends of it and stopped it in its tracks. There is almost ALWAYS more than you can see. You need to cut it ALL out and go way beyond where you THINK the crack ends. In your defense I believe the tanks to be poorly designed, cheap products with a relatively short life span.
Reply:I can't understand why anyone would use aluminum for a water pressure vessel anyway.  Stainless would have worked better and lasted about 100 times longer.  Sometimes I think we humans just enjoy trying to do things that we know we shouldn't do.... so we can watch the failure when it inevitably occurs.Miller Syncrowave 350Millermatic 252/ 30A spoolgunMiller Bobcat 225g w/ 3545 spoolgunLincoln PowerArc4000Lincoln 175 Mig  Lincoln 135 Mig Everlast 250EX TigCentury ac/dc 230 amp stickVictor O/AHypertherm 1000 plasma
Reply:40 psi water pressure is plenty enough to rupture that aluminum tank, which looks as if it were originally designed to hold fuel without any pressure.If you really want to see what can happen when a pressure vessel ruptures, watch Mythbusters the next time they show the exploding electric hot water heater.  They seal off the relief valve and disconnect the thermostat then leave the electricity on until she blows. Somewhere upwards of 300 psi it heads for the moon like a Saturn V.  Then they repeat the demonstration with the unit launching through two floors of residential style platform construction with a roof.  The tank still dissappears off camera on it's way towards the stratosphere.The moral to this story is don't weld on pressure vessels until you have been certified to weld on pressure vessels.  I am pleased no one was injured when this one ruptured.- MondoMember, AWSLincoln ProMIG 140Lincoln AC TombstoneCraftsman Lathe 12 x 24 c1935Atlas MFC Horizontal MillCraftsman Commercial Lathe 12 x 36 c1970- - - I'll just keep on keepin' on.
Reply:I've seen plenty of pressure vessels made from aluminum and plenty of vessels with flat heads.  There's nothing wrong with either of them.  This, however, is not a pressure vessel.  If it were *designed* for 40 psi, the flat head on the end would have to be about 5/8" thick, and that's if you put a good weld on the *inside* too.  It would likely have to be thicker.  Not to mention, a pressure vessel on a vehicle would be required to have a U (or UM) stamp and nameplate (or a T stamp, but since there are 0 states that have a T stamp program, that's not going to be a situation you encounter).Where I'm at, in Kentucky, there's a pretty sizable fine for operating a pressure vessel without a state ID number on the vessel.  I wouldn't report the customer, but I also wouldn't weld on their tank.Thanks for the pictures and story so that everyone here can learn about the job.Fegenbush/Also, the above poster is correct, this is a atmospheric pressure fuel tank.
Reply:would it have made a difference if he had drilled say a 1/8 holes on the very edges of the crack, then welded it? any kind of crack, I always drill out the ends, then weld
Reply:Worked a job building a hydroelectric powerplant.They built a sixteen foot diameter three hundred eighty foot long inch and half thick wall penstock.Put pressure caps on the ends to hydro test to 250psi. The fools had it pumped up to thirty psi before anyone asked them if they filled it with water. "Water?what do we need water for?"You never saw people scrambling so fast to open valves!I had this picture in my mind of an air powered space shuttle.tractor,loader.dozer,backhoe,and all the tools to keep em movin
Reply:Where were the project superintendent and chief engineer when this was going on?  A major test like that needs competent supervision and it is obvious that there wasn't any around before the test was initiated. - MondoMember, AWSLincoln ProMIG 140Lincoln AC TombstoneCraftsman Lathe 12 x 24 c1935Atlas MFC Horizontal MillCraftsman Commercial Lathe 12 x 36 c1970- - - I'll just keep on keepin' on.
Reply:The two engineers were right there.They were Chinese,couldn't speak a word of English.they had a young girl for an interprater that knew maybe fifty english words.One time they wanted me to pull test some 50' by 1 1/4" rock bolts....At five million psi(thats right,six zeros) I pumped to five thousand and explained that in America,workers are pretty stupid so the pressure gauge manufacturers don't put all the zeros on the gauges as it will confuse us.They bought it!tractor,loader.dozer,backhoe,and all the tools to keep em movin
Reply:Originally Posted by Shadillwould it have made a difference if he had drilled say a 1/8 holes on the very edges of the crack, then welded it? any kind of crack, I always drill out the ends, then weld
Reply:There is no way that tank is designed as a pressure vessel....... Looks like it was gonna fail either way bud, hopefully your not stuck with the bill though.Tons of hand tools mostly KingchromeArc welderMiG weldersO/A setupMany grindersair compressorsmany air toolsIndustrial drill pressTwo Forkliftsmany more toolsMan has his will, but woman has her way.- Oliver Wendell Holmes
Reply:Better water and air than GASOLINE!!!! All of the above have correct points. I'd say add them up, and chalk this one up to a learning experience.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:heres the new one.looks just like the old one (before the rupture) but newerthe stickers came on the tank.I was doing other repairs inside the garbage truck (trying not to remember how methane was made) when i tried to repair the tank so it might cost me an hour, but i still had several other hours i can bill forDah! wonder if i drilled out the ends would it have held?? Attached ImagesG
Reply:Notice how the ends on the new one are domed... it doesn't take much but that dome helps to make tension loads on the ends rather than bending loads.  Much better :'))  Make sure that the steel straps do not make direct contact with the aluminum otherwise you will have electrolysis and corrosion.  Strips of rubber does the trick... some outfits use layers of duct tape which is better than nothing.seemingly didnt come out in the first set of pics as im sure that the old tank had the same "slightly" domed ends. looks like an exact replacement for the old one.ill see if it is still around.G