4inch welded pipe wall thickness 1/4 inch how much allowance should be made for two slip on flanges with 3/16 thickness

What is the purpose? Measure the OD and ID and fit accordingly.

I have done plumbing work for 55 years and never saw a welded pipe, what is this?

I have seen seen some welded gas pipe systems in Europe. However, we don't want to guess. We will need the poster to give us more details before we offer our opinions

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Milo