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Old 07-01-2016, 10:30   #31
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Re: Leaking fitting on polyethylene holding tank

Installing the UnisealŽ is a simple process. After cutting a hole in the wall of a tank or other surface, the seal is inserted. Then the pipe is coated with a slippery detergent and pushed into the seal. The Du Pont AlcrynŽ is the key to the liquid-tight seal. When the pipe is pushed through the rubber-like UnisealŽ from the outside, the inside AlcrynŽ wall becomes thin enough to allow the pipe to squeeze through and then forms a liquid-tight seal. The coefficient of friction with the plastic pipe is roughly three times higher than any other material which allows it to create a better seal than with any other material. In static conditions the seal is good up to 65 PSI. UnisealŽ can withstand pressure, and suction, acids, oils, greases, gasoline, sewage, waste, heat and cold. It offers long life, easy installation and reduced labor and material costs.
UnisealŽ Pipe-to-Tank Seals | U.S. Plastic Corp.

If, in order to get a "clean" hole you must ream/hole saw the exist'g hole out; you can insert a short nipple in the larger size, and use a "reducer" to get back to your exist'g pipe size.
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Old 07-01-2016, 11:03   #32
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Re: Leaking fitting on polyethylene holding tank

These plastic tanks usually have female threaded fittings spin-welded to the tank. Any male adapter is simply screwed into the female fitting. If the welded on fitting gets damaged, the best fix would be to re-drill the tank and spin in another fitting.
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Old 08-01-2016, 14:10   #33
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Re: Leaking fitting on polyethylene holding tank

I also have the thread problem. The Headmistress suggested if I have enough of the fitting protruding from the tank I should try a hose clamp. IT WORKED! I've plugged the 2 side 1 1/2" holes and will use Uniseal fittings to bring both in and out pipes through the top of the tank.
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Old 13-04-2016, 20:53   #34
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Re: Leaking fitting on polyethylene holding tank

The common cause for this type of failure with poly fittings would be due to someone trying to stop a leak by nipping up the fitting a bit tighter.

Sometimes the weld holding the fitting in the tank will break but more often the thread will strip and very rarely the fitting will break and only then because it was totally inferior (junk) to start with.

A decent fitting will quite happily hold over 200psi in a pressure application for many years.

Threads in Polyethylene (plastic) have to be treated with kid gloves although when properly install will be incredibly efficient.

The torque required should be finger tight with a nip at the end.

Thread tape is paramount.

If a fitting leaks after doing up repeat the process but use more thread tape. Use two spanners when doing up so that the torque used isn't being transferred into the weld of the fitting in the tank.

Female threaded fittings are the worst and should be avoided as they expand when anything is threaded into them. There are fittings with stainless steel rings around the outside of the threaded part of the fitting which stop the thing expanding. They work well.

Welding polyethylene is relatively easy but as with most things in life preparation is everything.

All polyethylene oxidizes over time and this oxidized layer needs to be removed back to fresh raw material before welding.

The easiest way to explain this is with black poly as it's the easiest to
see say using a pipe. The outside surfaces develop a greyish look to them but if you lightly scrape it you'll find a jet black glossy material underneath.

That's fresh raw poly.

Whatever you're trying to weld including new fittings should all be washed (water is fine but leave to dry before continuing) and then scraped/ground before welding and these surfaces you've prepared should be kept pristine including not putting your fingers on them.

There are solvents that can be used for cleaning but are not readily available from the local hardware store. Acetone, Metho etc. are not appropriate solvents as they all leave a residue when they dry. If something gets dirty again lightly re scrape it.

Unless you are certain of the quality of an old fitting I would toss it and get a new one and if the old one is broken I'd replace it every time.

Check out the fitting in the store as it will have printed on it some where what it's made from ie. PE - Polyethylene, PP - Polypropylene. If it doesn't it's probably made from 10 year old ground up milk crates (junk). Many will also have a standards mark or number.
They are not expensive and the best fittings are most easily found in the
local plumbing, irrigation or pump shop.

Try to get something with a high pressure rating like SDR 11 or PN16 which will be the easiest to find anyway as they are easier to weld and will distort less (thicker wall) when the required amount of heat is put into them in the welding process

You can't weld dissimilar materials ie PE can't be welded to PP.

Such a fitting could be socket, butt, wired or extrusion welded onto a tank.
For a Rolls Royce job I'd socket weld it and then beef it up with an extrusion weld. This allows for a full thickness (tank wall) together with a mass I suppose of gusseting externally onto the tank and fitting.

Butt welding would require some skill in probably heating the tank with a hot air gun and the fitting on a hot plate before bringing together. This wouldn't be very strong by itself as it'd be just a surface weld (on the tank)

Wire welding wouldn't be very strong either although the hole could be bevelled to get better penetration into the tank wall.

Extrusion and wire welding are basically the same thing except that a wire weld produces a slug of approx. 4mm while an extrusion slug could easily be 5 times the size increasing the size of the welded area by the same ratio.

Would this be a diy job.

Probably not.

The tank will have been roto moulded and additives will have been included into the raw material to make the molten poly flow better in the mould and these additives won't be present to the same extent in the fitting. Applying equal heat into both with for this case an equal wall thickness would see the material in the tank weld zone turn to mush before the fitting was heated sufficiently.

Dissimilar polyethylene's ie. PE100 or PE80 can be welded together as can MDPE (medium density) or HDPE (high density). Your local water authority would have a heart attack to hear that but for the purpose of a low or no pressure application on a boat any half decent weld will easily out live the owner and boat combined.

The diy'ers will be fuming but many plumbers these days would have the necessary gear to fix a new fitting into a tank and it won't be a long process. Most of their time will be spent waiting for various things to heat up. In a workshop situation such a weld once everything was hot enough would probably take about 15 minutes so you won't be paying all the college fees for the plumbers kids.

I tried to get some pics into this but I'm afraid it's beyond an old bloke.

As a heads up when buying something like a tank try to get a black one as they will have the greatest UV resistance. The black colour is caused by the addition of carbon black to the raw material which is the only reason poly has any UV resistance at all. Natural poly will break down very quickly when exposed to UV. The pretty coloured ones also have this added but as has been discovered their protection isn't as good. Probably still last in excess of 50 years yet black poly's working life span still hasn't been determined but it's in excess of 100 years.

Also supporting whatever is hanging off a fitting is very important especially if theirs any vibration involved as poly will fatigue break just like most things if burdened by un unequal weight or being allowed to move back and forth over time.

School's out
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Old 14-04-2016, 04:26   #35
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Re: Leaking fitting on polyethylene holding tank

Thank you very much for this excellent education on welding PE tanks, I have to change the location of a fitting on one of the tanks I'm preparing to install, all the other tanks I've installed I've managed to use the existing fitting locations. I am a DIYer with just about everything, but it sure helps to have the education before I tackle the problem instead of during the many attempts to solve it.
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Old 14-04-2016, 05:37   #36
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Re: Leaking fitting on polyethylene holding tank

Greetings and welcome aboard the CF, Tbonem.

Thanks for the detailed exposition.
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Old 14-04-2016, 07:51   #37
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Re: Leaking fitting on polyethylene holding tank

Quote:
Originally Posted by jheldatksuedu View Post
Thank you very much for this excellent education on welding PE tanks, I have to change the location of a fitting on one of the tanks I'm preparing to install, all the other tanks I've installed I've managed to use the existing fitting locations. I am a DIYer with just about everything, but it sure helps to have the education before I tackle the problem instead of during the many attempts to solve it.
A tip for you would be to cut out your new hole as neatly as possible and use the piece you have cut out to fill the old hole. Probably best to use a hole saw and then weld the piece into the old hole.
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Old 14-04-2016, 09:01   #38
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Re: Leaking fitting on polyethylene holding tank

Excellent post Tbonem!! Thanks.
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