Minimum distance or proximity between two circumferential pipe weld joints

What is the minimum distance between two welds?

This is the most common welding design-related issue faced by almost every Welding Engineer, Design Engineer, QA-QC Engineer & welding Inspectors. Many people try to co-relate it as overmatching HAZ, reduction in weld strength if welds seams are in close proximity. But does all really a concern?

Lets find out in this post.

What should be Weld toe to toe distance?

BS 4515 ‘Specification for welding of steel pipelines on land and offshore’ (Section 11), states that the proximity of weld toe-to-toe distance shall not be less than four times the pipe thickness. In API 650 and 653 the minimum weld toe to toe distance requirement is 250mm.

What is there in various codes & standards?

ASME B31.3 or ASME Section VIII doesn’t specify any requirements related to the distance between two weld seams. The rationale is simple, welding is an integral part of the pipeline joining. Each weld qualified using a qualified Welding Procedure Qualification is supposed to offer minimum required mechanical properties such as tensile strength, yield strength, toughness, or hardness, etc. Having multiple weld joints which meet the minimum specified inspection requirements, will not going to affect the integrity of the pipe joints.

So what can be disadvantages of two close proximity seams?

Obviously, too close welds seams will make the welding inspection more complicated. For example, having two circumferential seams in pipe joints will affects the root run the internal inspection. Close weld seams can hinder the full coverage of the root run, can mask or merge in a double-wall radiograph, or will not offer access for UT probing.

Minimum distance between two weld seams as per ASME B 31.3

To your disappointment, ASME B31.3 or ASME Section VIII doesn’t specify any such distance. Also, there is no such reference in AWS D1.1 Structural Welding Code.

ASME B31.3 also clarified by the code interpretation No. 14-03(b) & No. 7-02 as:

Question (4): Does ASME B31.3-1993 Edition, Addenda b, have requirements for the minimum lengths of pipe sections between welds?

Reply (4): No.

Which standard cover minimum distance between two weld seams?

Well, for those who are looking for a standard reference to satisfy client query, or to insert any such reference to the contractors, there is a relief for them in BS codes such as BS 4515-1:2009 (Specification for welding of steel pipelines on land and offshore. Carbon and carbon-manganese steel pipelines), BS 2633-1987, BS 2971-1991 & PD 5500:2012. The table is given below summarizes the requirements from these standards.

In BS 2633, Section 11, specify that the ”toes of adjacent butt welds shall, whenever possible, be no closer than four times the nominal thickness of the pipe”.

It also states that attachments of non-pressure parts by welds that cross existing main welds or for which the minimum nominal distance between the toe of the attachment weld and the toe of the existing main welds or branch welds is less than the smaller of twice the thickness of the pressure part or 40mm should be avoided.

If such welds cannot be avoided, they should cross the main weld completely rather than stop abruptly near the main or branch weld in order to avoid stress concentrations in these areas. Non-destructive testing of main welds should be carried out before attachment welds which cross are deposited.

BS 4515 (Specification for welding of steel pipelines on land and offshore), Section 11 specify that the proximity of weld toe-to-toe distance shall not be less than four times the pipe thickness

BS 2971 (Class II arc welding of carbon steel pipework for carrying of fluids), Section 10, specify that if design factors are such that the meeting of more than two welded seams cannot be avoided, then appropriate precautions shall be taken which shall be agreed between the contracting parties.

PD5500 (Specification for Unfired fusion welded pressure vessels), Section 4.1.3, specify that where any part of a vessel is made in two or more courses, the longitudinal seams shall be completed before commencing the adjoining circumferential seam(s) and, where practicable, the longitudinal seams of the adjacent courses shall be staggered by 4e or 100mm, whichever is the greater, measured from the toe of the welds. (where e = nominal thickness).

How two close weld seams affects the weld joint Quality?

As discussed earlier, two close weld seams can have a structural integrity as long as they don’t hinder in the inspection of each other. Most common misconception that two HAZ will overlap and lower the mechanical properties is totally out of scope when you are dealing with carbon steel or low alloy steels. Practically, the new HAZ is going to create the tempering of the previous weld HAZ if they are really close. And, we know tempering do improve the mechanical properties of the weld not degrade them.

This can be an potential issue in the welding of super duplex stainless steel or modified low alloy steels, where such overlapping of the HAZ can change the phase balance or give rise in the grain growth in the microstructure. Such cases, the weld joint integrity will be compromised due to decreased corrosion resistance or decrease in mechanical properties.

Recommended distance between two weld seams

Most of engineering specifications require minimum 2 inch distance between two weld seams. Some specifications call for 2T, where T is the thickness of pipe. The rational is same as stated above, to allow proper inspections & avoid HAZ overlapping.

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