In this blog I will discuss minimum weld spacing as well as reason for it. During welding, base metal will be affected with heat and that zone is called heat affect zone. Due to high temperature, some area in heat affected zone will be partially melted which cause changes in the material microstructure .Typical heat affected zone will occur 1″ from weld toe, and it is shown on the figure below.
Due to uneven cooling rate after welding, residual stress will also be present.
Please see the figure below.
Residual stress become problem in thin plate (3/8″ and smaller) since entire plate can be deformed. Please see the figure below.
Changes in material structure and presence of residual stress in heat affected zone will result in material weakness. Typical crack will occur at the weld toe, between base material and weld. Please see the figure below.
It is worth to mention that heat affected zone is susceptible for fatigue, and cracks will usually occur prematurely at the weld toe. If heat affected zone is overlapped welds might become brittle, residual stress in base metal will be larger and material structure will be more deteriorated. In addition, weld examination will be issue if heat affect zone is overlapped. Applying post weld heat treatment will remedy these issues, but not to the full extent. Post weld treatment for economical reason is applied only when pipe thickness exceeds certain value and piping codes specified requirement for post weld treatment depending of wall thickness. In manufacturing environment, post weld treatment of spools is usually carried out in a furnace.
If post weld treatment is required in the field, than partial or local post weld heat treatment is applied. Please see the video below.

Based on the discussion above, it is evident that welds toes distance must limited to a certain minimum value. The minimum weld distance is given on the figure below.
Piping code B31.3 and ASME code does not specify minimum weld distance, but some other codes such as Australian do specify minimum weld distance of 3 times the shell thickness between weld toes or 40 mm, whichever is smaller. Visiting an engineering forums and reading discussions I found that many engineers agrees on having minimum distance between weld toes 2.5″ or 4t whichever is greater, where ‘t’ is nominal pipe thickness. It is worth to mention that pipe welding has been very well documented in the Piping codes.