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| Need wall thickness for aluminum tank . . . |  | ||
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| Posted by: Bruce L. ® 11/04/2009, 22:14:54 Author Profile eMail author Edit |
I did a lot of searching before I found this place, and I'm hoping someone here has an answer. I want to build an aluminum (6061) water tank to preheat the water going into my hot water heater. It will sit behind my woodstove and will never get above 100 F. Would a 7" OD, .125 wall tube, with caps welded on each end be strong enough for a 50 PSI pump cutoff pressure? The caps will probably be something like 3/4" thick. My gut instinct tells me it will but I'd like a more educated answer before I shell out any money. Thanks! Modified by Bruce L. at Wed, Nov 04, 2009, 22:16:27 |
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| : Need wall thickness for aluminum tank . . . -- Bruce L. | Post Reply | Top of thread | Engineering Forum |
| Posted by: jboggs ® 11/05/2009, 08:50:25 Author Profile eMail author Edit |
Instead of designing around what "should" happen, design around what "could" happen. That's why relief valves are used. The designer hopes they are never needed but Murphy's Law prevails. What would happen if you did get an over-temp or an over-pressure? Without a relief valve you have a potentially hazardous bomb or missile. Others on this forum can add more, but I think you are getting into an area of local code requirements and possibly even ASME pressure vessel code. |
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| Posted by: Marky ® 11/05/2009, 07:45:33 Author Profile eMail author Edit |
Hi and welcome to the forum. You should get some good feedback. If you do design a preheater...the design should mimic your regular hot water heater with the proper Over Pressure Relief Valve design....you don't want to design a bomb. Do some research before you spend any money. I googled "Hot Water Tank for Wood Stove" and found several sites Here's one link.. http://www.hearth.com/econtent/index.php/articles/domestic_dhw Modified by Marky at Thu, Nov 05, 2009, 07:52:40 |
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| Posted by: Bruce L. ® 11/05/2009, 08:24:51 Author Profile eMail author Edit |
Thanks for the reply, Marky. I was hoping, with a maximum temperature of 100 F., I wouldn't have to mess around with a relief valve. Not true? The tank would sit vertically, about 2' away from my woodstove where in the coldest part of winter it only gets around 95 degrees or so. |
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| Posted by: Marky ® 11/05/2009, 08:45:52 Author Profile eMail author Edit |
Spend 10 bucks on a valve. |
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| Posted by: zekeman ® 11/05/2009, 10:10:51 Author Profile eMail author Edit |
Agree with all of the above and for a 7 inch tube with a .125 wall, the stress in the aluminum is
pD/2t=50*7/2/.125= 1400 psi which is very safe, since the design stress needed for 6061 aluminum is about 5,000 to 10,000 psi. However, you must be very careful not to isolate the tank ( i.e. valve it off) since the heated water expands much more than the tube. In any case you MUST have a pressure relief valve installed at the tank for safety. |
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| Posted by: Bruce L. ® 11/05/2009, 10:18:05 Author Profile eMail author Edit |
"Agree with all of the above and for a 7 inch tube with a .125 wall, the stress in the aluminum is pD/2t=50*7/2/.125= 1400 psi
which is very safe, since the design stress needed for 6061 aluminum is about 5,000 to 10,000 psi. However, you must be very careful not to isolate the tank ( i.e. valve it off) since the heated water expands much more than the tube. In any case you MUST have a pressure relief valve installed at the tank for safety." ----------------------------------------------------------- Thanks for the info on the wall thickness! I will put a relief valve in also, not a big deal, however originally I didn't think it would be necessary because of the low temperatures involved, and because I am on a well and always have an air pocket in the water tank. But thanks to all for the replies, and I will take your advice. |
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| Posted by: Bruce L. ® 12/11/2009, 14:51:24 Author Profile eMail author Edit |
Hey guys, I got this thing up and running. I'd post pix, but it looks like we can't do that on this forum. :( Plumbing it into my water system was a lot of work. I put a relief valve on it as suggested, set at around 60 PSI. I also painted the entire thing flat black to help it absorb heat. When we welded the caps on, we leak tested it to 75 PSI and all was well. Thanks to all for the tips! |
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| Posted by: Bruce L. ® 12/11/2009, 22:33:05 Author Profile eMail author Edit |
I should have asked about this first, but we live and learn. I used CPVC fittings into the aluminum end caps and tightened them as much as I dared, and used a good teflon paste on them but I have a very slow leak on the bottom fitting. I used plastic because I was worried about galvanic action had I used iron. Would this have been a concern? If it helps any, my water is softened.
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