angel14;
The minimum distance should be such that NDT of each butt weld can be performed with no interference, and the separation distance should be far enough apart to avoid overlapping weld heat affected zones.
 

I worked at a major engineering company and we always used a "rule of thumb" of 3" minimum distance between welds.

From my recollection BS2633 calls for 40mm between the edges of the welds as a minimum. Obviously dependant upon the pipe thickness also.

I've learned as a rule of thumb:
50mm(2") or 7x wallthk. whatever is greatest.

This question has been asked many times in this forum.

From the replies I surmise

No piping code defines a minim separation distance (expect maybe AS4041 but I can not find it in the code)

Rules of thumb and company specification range from 2t to 2d.

Best reason for separation is to not have a stress riser in the gap between HAZs. The change in hardness in HAZ causes the stress riser.