Transformer welding

Date 11-19-2001 20:13

Hi, I'm a high school student engaged in an engineering project which requires us to develop a method for welding the seam between the tank and cover of a high voltage power transformer.
At present the tank and cover of transformers are bolted in position. However, there is environmental concern that the insulating oil from inside the transformer may leak out. So we are planning to replace the bolted seal with a welded joint.
We are planning to do Gas Shielded Metal Arc Welding or Manual Metal Arc Welding.
The problem is that the weld has to be strong enough to withstand the stresses imposed by a vacuum of 3 milllibars and the lifting of the 80 ton transformer. The weld has to last about 30 yrs and retain a leakproof seal.
The most important thing is to ensure that during the process of welding the fumes do not contaminate the inside of the transformer or the assembly area around it. The core and windings cannot be allowed to contain more than 0.5% moiture. To stop it taking up too much moisture, welding has to be completed as quickly as possible (approx 8 hrs) and a drying agent has to be used.
Can anyone please help me tackle this problem and advice me in how to stop moisture uptake and contamination of the transformer? Any help will be much appreciated.

By G.S.Crisi (****)

Date 11-22-2001 19:38

Dear Timbuktu,
Let's use our common sense (or good judgement, if you prefer) and look at the problem from a practical point of view.
You say that the transformer should last for 30 years. Chances are (strong chances, by the way), that sometime sooner or later during those years you will need to take the transformer internals (i.e. windings and core) off the tank for maintenance. How are you going to do that if the cover is welded to the tank? Will you cut it off with a torch or plasma? Impossible. Will you cut it off by means of a hacksaw? A hell of a job. That's why transformer covers are bolted to the tank: to have access to the windings and core.
To prevent oil from leaking off the tank a good gasket should be used in the flanged joint between the tank and cover. Rubber gaskets are not indicated because rubber is soluble in transformer oil. Asbestos gaskets are forbidden in the USA and other countries. Get in touch with a reputable gasket manufacturer and ask for material recommendations (John Mansville is one, but I don't know if he's still in business).
Once the gasket is in place, it should be tightened properly, i.e., until the proper thickness reduction has been achieved. Again, ask the gasket people for recommendations.
Also, discuss this matter with your professor.
Giovanni S. Crisi
Sao Paulo - Brazil