Tim,
I must say I like Bill Van der Hoof's flanges' solution posted on Nov. 24 much better than Niekie's one. Two main reasons are the following:
1st. If the flanges are, say, three inches wide, your welding will be that far away from the transformer. By the time the heat arrives to the transformer tank wall, it'll have greatly dissipated away to the atmosphere. Why three inches wide? Well, that's a good number. That flange isn't a structural one, so its dimensions can be figured out according to your good judgement.
On the other hand, Niekie's lips make part of the transformer tank, so when welding them the heat will be in direct contact with the transformer inside.
Now, if you're going to weld the two flanges together you don't need any gasket between them. Gaskets are used for connections which are wholly flanged to seal them up, and after all this is a welded one, where the weld is the seal.
2nd. At the time of putting the top and bottom sections together, it'll be much eastier to do that if the connection is flanged instead of lipped.
Now, let me tell you that, unfortunately, you won't get rid of a bolted connection in your transformer. Which connection is that? A manhole, for someone to be able to get into the tranformer if necessary. As I told you in my first reply, in 30 years of life expectancy sooner or later some maintenance will be necessary, and someone must enter the transformer (after it's emptied from oil, of course) to see what happened and what's to be done.
In these days I went to take look at a substation near where I live which comprises three huge transformers 132/13,2 kV. Not only the transformers tanks are bolted, as I suggested you since the beginning, but they have a bolted manhole on the cover. Why? As I said, to allow someone to get into them, see what's going on and repair the trouble.
Does the manufacturer swear with his hand on the Bible that his transformer won't need any maintenance over 30 years? Tell him to remember Murphy's laws.
If you don't you know Murphy's laws, here they are:
1st. If something can go wrong, it will go wrong. (The transformer, in this case)
2nd. If several things can go wrong, the one that will is the one that will produce the most damage.
3rd. Nature always sides with the hidden flaw.
4th. Let to themselves, things always go from bad to worse.
5th. If everything seems to be going fine, you've obviosly overlooked something (in this case the manufacturer, who thinks that his transformer is going fine and doesn't need maintenance)
6th. No matter what goes wrong, there will be always someone who'll exclame that he knew about it since a long time (in this case that "someone" is me, who, the first time the transfomer goes out of service, will yell: I told that !!)
Tim, you're a young guy still in his highschool years. Listen to the advice of an "old fox" like me. Never forget Murphy's laws for ther rest of your life.
Giovanni S. Crisi
As Prof. Crisi explained, a gasket is not usually used in a welded joint. I however assume you want to use a gasket to try the "quick seal" suggestion that I made, to reduce moisture contamination in the tank. If this is the case, then you need a gasket that will seal the roof under its own weight. (You could possibly add some weights if needed.)
This means that you need to make sure the gasket is soft, so that it can "give". In addition, it should be narrow, so that the resulting pressure is high. As this gasket will be staying in the transformer for the proposed 30 years service life, it will also need to be rather resistant to whatever could attack it in that time.
For such a once-off application, I would suggest a PTFE gasketing material that is marketed by a company called GORE. The exact trade name of the product I do not know, but it is a type of rope that you get on a roll. You merely roll it off and place it along the flange surface. The two ends you merely overlap, as the weight of the roof will squash it flat and give you the seal required.
I know this sound very "Heath Robinson", but they use this technique to even seal turbine casings, so it is actually a "done thing". If you contact them, they will give you further info.
Regarding the flange shape, (lip or flat) I believe that you should be guided by your application. The criticism that the lip will lead to more heat on the transformer body is valid, but then I believe that this is a non problem if you have a reasonably thick tank wall. (e.g. 20 mm) It will be kept even cooler if you already have the oil inside the transformer to cool it down. (Will the oil be in already?) To get the heat away from the gasket, simply make the lip longer. At any rate, the design constraints should guide you on the most appropriate flange design.
Hope this helps
Niekie Jooste
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