Hello

Some fabrication drawings provide dimensions of nozzles from Weld Line rather than Tan Line. Some people say, Weld line is visible, while Tan line cannot be exactly located.

Which is correct?

Both are "corect", but the most common one is to give the dimension from the TL. The reason is that usually the data sheet specify this.
In both cases it is important to specify the dimension betweeen the TL and the WL.

Dimension always to be shown TL to TL and TL Is not visible that's why it will not going to change- so your dimensions will be maintained as per drg. but if give dimensions WL to WL then would be problem some time

variation of lenght of plate
variation in dish height
Varition in Weld contour

So TL - TL always prefrable

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Niraj