anindya,
Piping fab specs that I have used, generally have a dual limitation, ie. the minimum distance to be the greater of 50mm and 5 or 6 times wall thickness. The HAZ is affected by pipe wall thickness, ie. thicker the pipe the greater the HAZ. Whereas the 50mm requirement as I recall, relates to the minimum distance required for inspection. In the past, I have also seen the limitation of square root of the sum of pipe radius times wall thickness ie. (R.t)^0.5, but the 5 or 6 times wall thickness criteria is what I see these days.
Regards
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Mike Kowal