Purpose of Welding proximity requirement is:

1 – To avoid the overlapping HAZ, the lowering of the welding HAZ mechanical properties which is not covered in WPS is approved for welding configuration. 

2- To avoid the duplicate of residual stress between two joints which may combine with the load during service, this leads to structural failure due to the total combined local stress exceeding allowable stress.

Weld proximity is covered by a number of international standards. Extracts from these standards are shown below and summarized in the following table:

Standard	Applies to	The minimum distance between weld toes	Notes
BS 2633:1987	Ferritic steel pipework	4t	t= nominal thickness of the pipe See below for notes on attachment welds.
BS 4515:2009	C-Mn pipelines	4t	t= pipe thickness
BS 2971:1991	Carbon steel pipework	Agreed by parties	–
PD 5500:2012	Pressure vessels	4e or 100mm whichever is greater	e= design thickness Refers to the staggering of longitudinal welds
ASME B&PV	Boilers and pressure vessels	Not specified	–

A B31.1 Interpretation says 

Interpretation:  B31.1-22-7
Subject:         B31.1, 127.4, Welding Requirements
Date Issued:     December 10, 1992
File:            B31-92-027
Question:  Does the B31.1 Code require that a minimum distance be maintained between adjacent piping welds.
Reply:  No.  However, refer to paragraph 104.3.1(G.7) for rules regarding the reinforcement of multiple openings.

A B31.3 Interpretation Says 

Interpretation:  B31.3-7-02
Subject:         B31.3-1987, Paras.  304.3.3 and 328; Spacing of Welds
Date Issued:     May 24, 1988
File:            B31-87-039
Question:  In accordance with B31.3, are there requirements or guidelines on the minimum distance between welds in a pipe?
Reply:  No, except as provided in paragraph 304.3.3(e).
Para 304.3.3(e)refers to the overlapping of reinforcement zones.

In the oil and gas project, the code normally not requires about this issue, but it’s necessary to notify in the project’s Specification on welding & inspection.

Example:

Because of the HAZ overlapping, thus the weld-zone mechanical will be affected with no support documents that proving the soundness of weld especially in welding DSS – high corrosion material (weld decay).

Because of the design capability and actual narrow arrangement, this issue can’t be avoided 100%. In such a case, a Technical query has to be issued and supported by a production test. (Hardness test is normally to be presented for closing this issue).