To fix a water tank.

Why not a patch!!

They are for a water tank David.

A water tank that may have pressure on it? Your op is pretty vague. I have done minor repairs like that on thin bottom plate on tug boats when they are in the water. They work good when you have a hole or crack. Put that weld on flange over it, weld it on, then goop up the threads on a plug and throw a plug in. Holds great until they can drydock it, and it works much better than trying to weld up a hole in a hull of a boat when there is water pressure on it. That can be done too, but it's tricky..

You can’t drain it down, so you weld those on, then install a plug?

Why not use a bulkhead fitting?

I'll try to take some pictures that will explain better.

I don't know if you posted this for a human interest story or info...in any event, when you weld on a bulkhead flange ...always...install a pipe plug with anti sieze snugly into the fitting. Don't remove the pipe plug until it has cooled enough to handle without gloves. Over and out.

This is the tank I'd like to repair.

Another tank that needs work.

OK, I see what you are wanting to do. The upper tank with the bung torn clear out...it is customary to weld a "doubler" over the hole. Fancy talk for a large rectangular patch, 100% fillet weld all around the circumference of the doubler....weld your new bulkhead fitting onto your doubler...then weld the doubler over the nasty hole. Don't get fancy with welding rod, just use plain old 6011, it takes rust, paint and dirt in stride. Lots of overlap where you stop and start the puddle. Not sure what I'm looking at on tank number 2, but same principles apply.

I understand now

The idea is to use 3" plastic pipe to fill a water tank for cattle.
Not glue the pipe for a hinge .