It can be done, and we have done similar things in the past, but it would be very difficult to get a weld sufficiently smooth on the inside for CIP.
The fitting must be perfect--no filler rod. The poster above describes the fit very well, but I'd go for an ID-ID fit--the ferrule resting perfectly on the outside of the tank. You'll need to fill the tank with an inert gas--we usually use Argon for TIG welding. A slight pressure on the tank will insure that no O2 can infiltrate, and will help support the weld, which must have 100% penetration and will sag if the pressure is too low, or bulge if it's too high. The weld must be perfectly flush with the inside of the tank for good CIP. We use tape to cover an opening, then punch a few small holes--using the tip of the tungsten--to control the pressure.
You would want to run a few practice welds using tubing of the same thicknesses as the ferrule and tank walls first, which could be cut open to reveal the quality of the welds.
If you go with the 8" ferrule, this gets much easier, as the weld can be made from the inside.
In the end, unless you hire a certified pressure-vessel welder and have the welds inspected--probably using x-rays--you will not have a legal pressure vessel.
Finding a good cylindro-conical fermenter and using this tank for something else would probably be the best way to go.
Last edited by TGTimm; 07-31-2015 at 09:16 AM.
Terminal Gravity Brewing,