Replacing Plastic Water Tank Fittings - How is it done?

These fittings are usually spin welded. That is the best repair. Agree that a successful repair will involve providing support for the pipe/valve to prevent the spin weld from cracking again.

http://spinwelding.com/products.html

Depends on where the leak is. In order to determine exactly where the leak was I removed the tank -- and discovered that the tank was fine but the old fitting had cracked. My fitting just screwed out and I found a replacement at the hardware store.

Here are a couple options:
https://www.amazon.com/dp/B004RCPA2U/?coliid=I17PJAQ8FPZRN6&colid=KNDITYI6UJ2H&psc=0&ref_=lv_ov_lig_dp_it

https://www.amazon.com/gp/product/B007XIMYX8/ref=oh_aui_search_detailpage?ie=UTF8&tag=foreforums-20&linkCode=as2&camp=1789&creative=390957

x2 and it most likely cracked because theres no support on that pipe hanging out there. pull the tank and be done with it.

Plastic welding may work, or it may not. The issue is the limited amount of surface area of the fitting to weld to, its not the same as welding metal. The bead of melted plastic from welding along the edge of the fitting flange will be much more prone to cracking than spin welding. The issue is the thin plastic tank wall and tank flex. You also have to be good at it, because one wrong move and you just melted a hole through the tank. Its a tricky art and takes practice, even with a good air regulated setup. The HF el cheapo is even tougher to work with.

There is a reason they spin weld fittings to MDPE tanks. In your case you would remove that fitting and spin weld a patch disc over the hole, then spin on a new fitting in a different location. A patch disc is a larger diameter than the flange on the fitting. It spins and melts into virgin plastic around the perimeter of where the original fitting was. You can't pull a fitting off and spin a new one in the same place, The tank is now too thin from the original spinweld and the new one will probably blow right through it.

The collets needed for spin welding are expensive, you would need two, one for the patch disc and one for the fitting. I had a machinist friend make the set I have. Easy to make if you have a mill, most people don't. You also need a stout high rpm router. Definitely need to remove the tank to spin fittings.

A lot of times is just cheaper to buy a new tank. Shop around. I put a 2nd 43 gallon FWT on my Rockwood and found the tank on Amazon for $90, fittings already installed, free shipping. Drop a couple hundred on 2 collets you'll use once?

You can get mixed results by shoemakering on some type of epoxy or glue sealant concoction and cross fingers. That's probably not an option for how yours has cracked on the fitting itself.

The simple fact is there is no glue, adhesive, or epoxy known to man that bonds to Poly, and no solvent to solvent weld it like PVC.

Its a tough nut.

Harbor Frieght plastic welder, the cheap one, basically a soldering iron with a big flat tip.

If you don't support that valve hanging out there, you will need to repair again and again.

What if you took out that white part of the equation?

From there you can probably tell if it's the threads that are leaking or the white thing is cracked or?

Good luck!

Mitch