I would like to know the basis for the Industry standard of keeping minimum 50 mm gap between welds.

To my knowledge :

1) This is certainly not based on HAZ as the typical length for HAZ even for high heat input welding like SAW or EGW is within 5 mm of the weld line.I have spoken to some metallurgists also, who confirmed the same.

2) This is also not based on the minimum length to properly move the welding rod ( which are typically in the size rage 2.5 mm or 3.2 mm).

If 1 and 2 is correct, what then is the reason behind this requirement and is there any industry standard which suggests this 50 mm requirement?

Regards

I assume there are many reasons. Here are couple I can think of:
- NDT. If the welds are too near each other you cannot test them properly especially if UT is used. 50 mm is not actually sufficient for that. (x ray film positioning, cleaning for dye pen, jaws of MPI equipment...)
- access. If you have 2 butt welds normally it would not be a problem but if you have a bracket and butt weld next to each other welder may have a problem.
- fitting aids. Fitters use all kinds of temporary brackets, which are tack welded. You do not want these on HAZ of the other weld
- stress concentration?

There is a website Pipe fabrication institute, I am sure you are aware of

and the web site, the address is given below.

http://www.pfi-institute.org/Product.htm

There is reference given in B31.3 Appendix "E". I assume this can throw some light on your subject.

I feel it could be the reason of either heat zone overlapping or due to X-ray limitations. There could be another reason of replacement of spool in future. My answer could be vague but these are my thoughts.

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

The distance (Rt)^0.5 appears in various piping and vessel codes as a minimum distance between discontinuities or stress raisers. For example PD5500 states for Primary Local Membrane Stress:-
"A stressed region may be considered as local if the distance over which the stress intensity exceeds 1.1f does not extend in the
meridional direction more than 0.5(Rt)^0.5, and if it is not closer in the meridional direction than 2.5(Rt)^0.5 to another region where the limits of general primary membrane stress are exceeded."
On this basis the requirement for keeping welds at least (Rt)^0.5 apart has a logical basis of keeping stress raisers like welds sufficently far apart for the effects to be regarded as localised stresses. I think the 5-6t requirement is a rough guide and would probably be pretty sensible for 'average' pipe sizes with 'average' wall thicknesses. Where it becomes less workable is in the thick wall situation in my opinion. This is because, not only is 6t [toe to toe] a very large dimension in many cases, but the thick wall acts like a heat sink to pull heat away from the weld area reducing the HAZ in relation to the wall thickness. This is especially true in materials like duplex SS where the heat input is very carefully limited, and the HAZ may only be 1-2mm thick.

Anindya,
The aspects of Testing and HAZ/Stress raisers are discussed appropriately. The other view which restricts this dimension could be practical aspect of permorming the weld so close at the field.

Consider that the nearby branches are flanged (assume two weldoflange), then it may result in maintenace issue(bolting and unbolting to name one).

So looking at layout point of view apart from inspection and fabrication at site, it is suggestable to keep in general 50 mm or 6 * Thickness of pipe, whichever is greater is a wise choice.

However if you have problem in a particular it should be reviewed as stand alone basis and distance may be lowered down to 6*thk.

Regards