ASME B31.3 Weld Spacing: What the Code Sets and What Governs Instead

Minimum distance between welds in ASME B31.3

There is no minimum. B31.3 contains no paragraph that sets a required distance between two butt welds, between a butt weld and a fitting weld, or between consecutive girth welds on a straight run. A short piece of pipe between two fittings, often called a pup, is acceptable to the code provided every weld meets the welding, heat treatment and examination requirements that apply to it.

Where a spacing figure appears on a project, trace it before enforcing it. It comes from the owner's piping class or fabrication specification, from a contractor's standard, or from a vessel or structural rule that someone has carried across. Those documents can legitimately require a minimum, commonly expressed as a multiple of pipe diameter or a fixed length, whichever is greater. The figure then binds the job through the contract, not through B31.3, and a deviation is approved by whoever owns that specification.

What B31.3 does say that affects spacing

Three parts of the code bear on how close welds can sensibly be. The branch connection rules in para. 304.3 require a branch that relies on reinforcement to find that metal within a defined zone around the opening, and when two branches sit close together the designer has to make sure the same metal is not counted twice. The heat treatment rules in para. 331 apply to every weld that needs postweld heat treatment, and a heated band laid over one weld can reach the next. The examination rules in Chapter VI set the type and extent of examination for each fluid service, and that examination has to be physically possible on every weld it selects.

Sampling matters too. Under normal fluid service B31.3 examines a percentage of welds by radiography or ultrasonics and requires further examination when a sampled weld is rejected. A weld that cannot be examined because its neighbour is in the way is not a usable sample, and the usual outcome is a change to the spool rather than a waiver of the examination.

How a Level III works out a practical minimum

When an owner asks what spacing to specify, the useful answer comes from the examination, because it is usually the tightest physical constraint. For angle-beam ultrasonic examination the probe has to move far enough from the weld to put the beam through the whole weld volume. The distance from the probe's beam exit point to the point where the beam returns to the scanning surface is the full skip, and for a wall of thickness t examined at a refracted angle θ it is 2 × t × tan θ.

Take a 12.7 mm (0.5 in.) wall examined at 60 degrees. The full skip is 2 × 12.7 × 1.732, about 44 mm (1.73 in.). Add the probe and wedge length in front of the exit point and half the width of the weld cap, and the clear surface the technician needs on that side of the weld comes to several times the wall thickness. If the toe of the next weld sits inside that zone, the probe rides over a second cap, couplant contact is lost and part of the volume goes unexamined.

Phased array and TOFD change the numbers but not the logic. A sectorial scan covers more of the volume from a fixed position, which helps where space is tight, while TOFD needs both probes of the pair on clear surface either side of the weld. Radiography is rarely the limiting method, although film or detector placement, location markers and image overlap still need planning on closely spaced welds. The scan plan, written for the actual geometry, is the evidence that a spacing works.

Why close spacing causes trouble beyond the examination

Each weld puts a thermal cycle into the material around it. When two welds are close, the second cycle reheats the heat-affected zone of the first, and in hardenable steels or sour-service piping that can move hardness or toughness away from what the procedure qualification demonstrated. Heat treatment bands are sized for one weld; a band laid over a neighbouring weld that has already been treated gives it a second cycle, which the heat treatment procedure then has to allow for.

Repairs add a further problem. A repair excavation next to a closely spaced weld can reach into the neighbouring weld or its heat-affected zone, and the repair procedure has to account for both. Fit-up and distortion are harder to control on short pieces, and short pieces are harder to hold while welding. None of this is prohibited by B31.3; all of it is a reason owners write a minimum into their specifications.

Common mistakes

Citing a spacing figure as a B31.3 requirement in a nonconformance report when the figure lives in the owner's specification. Measuring centreline to centreline when the specification means toe to toe, or the reverse; the specification has to say which, because on a wide weld cap the difference is significant. Applying a straight-pipe spacing rule to branch connections without checking the reinforcement zones. Forgetting attachment welds such as support lugs and trunnions, which bring the same heat and access problems as a second girth weld.

Accepting a spool whose scan plan was never checked against its geometry, then discovering at examination that one side of a weld cannot be scanned. Recording limited coverage without telling the engineering designer, so the gap in examination never reaches the person who decides whether it is acceptable.

What to write into the specification and the inspection plan

State the minimum, the reason for it and how it is measured, toe to toe being the most useful basis for examination. Apply it to butt welds, branch and fitting welds and attachment welds, and say whether pup pieces have their own minimum length. Tie the figure to the examination technique: if the UT scan plan needs a given clear distance for the wall thicknesses on the job, the specification should not allow less. Give the fabricator a written deviation route that sends the request to the engineering designer and the owner's inspector, and record approved deviations against the weld numbers they cover.

For an independent check of a piping specification or a fabricator's scan plans against these points, Atlantis NDT's ASNT Level III consulting reviews procedures and technique sheets against the governing code and contract. The ASME B31.3 summary covers the code's wider examination requirements.

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