How thick should my HDPE holding tank be? - SailNet Community

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  #1  
Old 07-12-2007
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How thick should my HDPE holding tank be?

I am having a high density polyethylene holding tank made...

THey quoted me 3/8" thick material, then I saw something on this board where it was made of 1/2" material.

Well, they want like $100 extra for the thicker 1/2" material.

The tank is 19" tall and about 15x15" at the top.
It tapers thinner as it goes down. So it will only hold about 12-15 gallons.

DO I REALLY NEED 1/2 INCH MATERIAL? Or will the 3/8 be enough?
The tank will sit on a small shelf and won't have much support on its sides.

Roy
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Not any kind of expert but I would guess 3/8 is more than adequate. 1/4 would probably work but thicker is prefered for better for odor control.
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Search for tensile strength or physical properties. Call a company that makes both and ask them.

I think you'll find that it's about half the strength of PVC (7500 psi per ASTM D638). My tank is 1/2" PVC but it covers a large area and not very deep. You'll only be dealing with about 100#.
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I would go with the 1/2" tank, since it is a holding tank, and if it bursts or leaks, you'll really be sorry... better to be safe with a heavier wall tank. 1/2" is probably overkill, but you're talking about 12-15 gallons of sewage if the sucker leaks... your call...
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Yeah. I hate to spend it, but it might be $100 well spent.

Steve, you sound a lot like a mechanical ingineer, and it doesn't sound like 3/8" is a slam dunk to you....

Any other opinions are appreciated.
thx,
gh
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Old 07-13-2007
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In my experience with such tanks, carrying them in the back of pick 'em up trucks, abrasion will kill you sooner than anything. I carry 400 gallon tanks and they seem to be plenty strong and, while I have not measured wall thickness, they are nowhere near 1/2" thick. Secure the tank well, perhaps gasketing the bottom and contact points for abrasion.

Avoid excessive chlorination as chlorine is an oxidizer and will "dry out" the plastic causing it to become brittle with age, and cracking. UV light does the same, but I don't think that's going to be a factor in your situation.
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Thanks Sailaway,

So the tanks in your pickup are HDPE?

Also, you can get molded ones that have no seams, and custom welded ones, which is what I am getting. THe seamless ones are more reliable as I understand. It's the seams that you worry about on the welded ones.
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Many plastics have a good strength to weight ratio, but lack durability to such things as abrasion or chafing, and UV light. In my opinion, the key to successful use is not demanding something out of the plastic it is poor at doing. Laying down a rubber mat, removing any surface imperfections in the bilge, floors, or stringers, and avoiding oxidizers are a few that come to mind.

I can assure you that one 1/2" long quarter-twenty bolt under your tank will make it's presence known, probably sooner than later. Extra thickness may make it more later, but no less inevitable. Mounting/preperation is everything.
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Old 07-14-2007
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Sailaway,

So.. are your tanks are welded or molded?

I am encouraged if you have 400 gallon tanks sloshing around if they are welded. I am not encouraged if they are molded and have no seams.

I appreciate the advice on the abraision. I will take precautions to make sure that my tank is buffered from any nearby bolts (and there are a few sticking out nearby).

Thanks,
Roy
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I'll have to look more closely. I'd imagine welded, but wish to be sure before I tell you anything. PM me if you don't hear back shortly.
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