What are the standard requirements on the proximity of welds?

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

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BS 2633 ‘Class I arc welding of ferritic steel pipework for carrying fluids’ (Section 11), states 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 which 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), states 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), states 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), states 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).

ASME Boiler and Pressure Vessel Code Section VIII Division 1, the only mention of proximity of welds is in UG-82 ‘Lugs and fitting attachments’:

All lugs brackets, saddle type nozzles, manhole frames, reinforcement around openings, and other appurtenances shall be formed and fitted to conform reasonably to the curvature of the shell or surface to which they are attached.

1. When pressure parts, such as saddle type nozzles, manhole frames and reinforcement around openings extend over pressure retaining welds, such welds shall be ground flush for the portion of the weld to be covered.
2. When nonpressure parts, such as lugs, brackets, and support legs and saddles, extend over pressure retaining welds, such welds shall be ground flush as described in (2) above, or such parts shall be notched or coped to clear those welds.