Is it good practice to weld HDPE pipe to a big poly tank? Details to follow:

What I have is an aboveground polyethelene water tank vertical, cylindrical, 95" diameter, 92" high. There are two 1.5", untapped holes at the top of the tank, no other openings. The tank is white like hdpe, but I am not sure of the composition of the plastic.

http://www.greertank.com/polyethylene.html

This tank is to be used as the buffer tank in my GSHP system. There will be two loops, one will be the loop supply and return from the GSHP and the other loop will supply hot water to house domestic and in-floor (heating sominated climate here)

I think I need two dip tubes with four total fitings on the top of the tank. Do you pros drill or weld HDPE pipe to the tank? Would you weld pipe to the tank and tap the end of the hdpe pipe?

I have looked around for how this is done and didn't come up with a good answer. Any help is much appreciated!

I'd T the GSHP supply and the radiant return and run a tube to the bottom of the tank. I'd T the GSHP return and the radiant and domestic supplies and take from the top of the tank.

I've seen many such tanks, none of them with pipes welded to them.

The dip tube is what is especially giving me trouble, can you help me with a

description of

what/how the tube attaches to the pipe and gets through the fitting?

Is there a flare, a weld, a bulkhead fitting welded/bolted on to the top of the tank?

I've seen loose pipes just sitting in a hole and bulkhead fittings. But I'm not sure what your concerns are - a drawing would help.

Picture in your minds' eye a pressurized polyethelene tank with no manholes but with two untapped holes on the top only.


A dip tube is to be installed in one hole. How is it done?


Picture a pipe with a backing ring welded through the top of the top of the tank. Is that good practice? Or is there a standard dip tube that I can use if I thread the holes in the tank?


A bulkhead fitting is not possible to install with one side of the fitting on the inside of the tank. Loose pipes cannot work with the pressurized tank.

Just some random thoughts here...

Is there enough material around those 1.5" holes to allow taping them?

What kind of temperatures and pressures are we talking about here?

Could the dip tube be made of PVC, ABS, or higher temperature rated CPVC?

If taping the 1.5" hole is acceptable and PVC or CPVC dip tube is acceptable, then you could likely find a solvent weld fitting that could be solvent welded to the pipe then threaded into the tank.

The pressures will be hydronic system pressure, lets say 10psi and temperature up to 110f - 120f. The tank is not designed as a pressure vessel, but nine feet of water is already 4psi, the temps are not great; the tanks should hold up to the pressure and temperature.

It turns out the untapped holes are 2". Lots of room around them and the tank top material is about 1/2" thick.



I haven't found a solvent weld fitting that I could thread into the tank. Could a slip socket x socket that was threaded on one end work? My preferred material for piping was to be PEX or HDPE pipe.

Why pressurize, especially when the tank isn't designed for it and you are operating around max temperature for PE?

I imagine something like this (bulkhead fitting with barbs on both sides) would work.

If you can't find the right size, you can also seal the holes you have with plates and make new ones (perhaps further down so that normal bulkhead fittings would work).

goin' pressurized :}

A Uniseal slip fitting might work.

http://www.injectionplasticsmfg.com/products/uniseal/

I guess I just need to thread the inside of a 2" bushing.

My dip tube hangs off the new inside female threads and nipple off the other side too!

Posted By arkie6 on 16 Aug 2014 05:39 PM
A Uniseal slip fitting might work.

http://www.injectionplasticsmfg.com/products/uniseal/

Is there engineering specs on that item? There wasn't information on what kinds of pressures those seals are good for.

It seems a machined part might be required. I see a bushing that is threaded inside.
not sure why there is a lack of info on this - perhaps someone wants to take the need and make a product?

Another possibility is to combine the above Uniseal with a couple of barb type fittings like in the link below. You would need to determine the OD of the smooth portion of the barb and get a Uniseal for that size. Attach your PEX or HDPE pipe to the barb for the dip tube, then solvent weld a short piece (~12") of PVC pipe inside the barb fitting. The short piece of PVC pipe would fit loose inside the Uniseal. Slip the Uniseal over the short section of PVC pipe and insert the whole assembly inside your tank per the Uniseal instructions. Lubricate the Uniseal with silcone sealant and pull the barb fitting up inside the Uniseal using the short 12" piece of PVC pipel. Now cut off the short piece of PVC pipe so that another barb can be solvent welded on the outside with the barb pointing away from the tank. Cut the PVC pipe short enough so that the outside barb fitting fits tight against either the Uniseal or the top of the inside barb fitting.

http://www.irrigationdirect.com/irrigation-products-and-supplies/fittings/pvc-insert-fittings/fitting-adapters/fitting-adapters-spig-x-insert/pf-1432-010

Here is another option using a flange fitting:

http://www.assmann-usa.com/LitPDFs/Flange_&_Bulkhead_Fittings.pdf

The assmann link doesn't work as there is no access to the inside of the tank. The seal needs some datum to show that it can hold pressure. Would it work for the long term w/o leaking?

With the Assmann (what a name for a company, huh?) flange, I was thinking you could drill the 4 holes for the flange bolts, then drop the bolts through the 2" opening with some curved needle nose pliers and then pull them up through the bolt holes securing them in place temporarily with some type of spring loaded clamp or clip until the flange could be lowered and nuts installed. It would be a little tricky and the possibility of dropping the bolts would exist. You would need a recovery plan like a magnet on a string, but I bet the bolts are stainless and may not be sufficiently magnetic to pick them up.

Another option would be to enlarge your two holes to 4" or 6" so that you could get your hand inside and then get the appropriate sized Assmann flanges. Say you went with the 4" flange. You could get a 4" steel pipe nipple to fit inside the flange and then have a steel plate welded across one end of the nipple with two 3/4" or so holes drilled in it with two steel 3/4" double barbed pipe nipples welded into the flange. You could go with stainless steel if necessary. That would give you two sealed pipe penetrations per each 4" opening. Heck, you could forgo the Assmann flanges and 4" pipe nipples altogether and just make some 1/4" stainless steel flat plate 6" diameter with two welded pipe nipples installed. Drill eight 1/4" holes equally spaced 1/2" in from the outside edge of the plate. Lay the plate on the tank and mark your holes for the tank. Drill the corresponding bolt holes in the tank and use 1/4" x 1-1/2" stainless steel bolts with 1" OD stainless steel flat washer under the head of the bolts. Then install the bolts and flat washers by reaching inside the tank through the 4" hole. You could either secure the bolts temporarily with spring clips or just slightly under size the holes (13/64") and tap them with a 1/4"x20 tap from the outside then thread the bolts through the plastic tank from the inside to hold them in place until you can drop the flange and nuts on them. Drilling and tapping the bolt holes in the plastic tank would probably be the better option to insure they hold in place while you install and tighten the nuts.

A threaded bushing with each side of the fitting's interior having right hand threads

starts sounding a lot better:}r

I'm still surprised there isn't an off-the shelf part.