Tie-In Misalignment in Cold Weather
Two 12.75-in. OD, 0.375-in. wall X52 pipe ends meet in a bell hole for a tie-in. The upstream end is on skids and has been in direct sun; the downstream end is held by 800 ft of already-welded pipe. The fitter reports 3/16-in. hi-lo at the top, a 1/8-in. root gap, and 1.5% out-of-round. The two ends are also about 2° out of parallel. Ambient temperature is 28°F with a 15-mph wind. The line-up clamp can pull about 8 tons. The crew's workmanship limit is 1/8-in. hi-lo. The welder proposes to fill the gap with a root pass and grind the inside smooth. The inspector is concerned about the cold, the misalignment, and whether the pipe will pass a pig later.
1. Is the 3/16-in. hi-lo acceptable? What are the practical options?
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No. The 3/16-in. hi-lo exceeds the crew's 1/8-in. workmanship limit and is likely to create a stress concentration, poor root fusion, and an internal profile that could interfere with pigging. The 8-ton clamp might pull the joint into tolerance, but 3/16-in. hi-lo plus 2° out of parallel may require enough force to induce bending stress in the already-welded line. Filling the gap with weld metal is not a fix; the 1/8-in. root gap is not the main problem. The practical options are to cut back one end and install a pup joint that fits without forcing, re-round the end if out-of-round is the cause, or obtain an engineering variance supported by fitness-for-service analysis and increased NDT. In the field, cut and pup is the safer choice unless engineering specifically approves another method.
2. What cold-weather and welding controls apply?
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At 28°F with wind, the qualified WPS must be followed for preheat and wind protection. Typical practice is to preheat the joint to at least 150°F or to the temperature specified by the WPS, verify it with a contact thermometer or temperature-indicating crayon, and shelter the weld from the wind. Keep snow and ice off the pipe, maintain interpass temperature, and do not use the root pass to bridge misalignment. The welders must be qualified for the process and position, and the completed joint needs the required NDT. If the WPS does not cover ambient below 32°F, stop and qualify the procedure or obtain engineering approval before welding.
3. How does the 1.5% out-of-round affect the decision, and what check should be made before welding?
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A 1.5% diameter variation on 12.75-in. pipe is about 0.19 in. That amount of out-of-round can make line-up difficult and can reduce the internal drift diameter. Before welding, check the end with an internal sizing plate or pig gauge to confirm that the completed tie-in will pass the intended pig. If the end cannot be re-rounded within tolerance without cracking or excessive force, cut it back. Also check that the line-up clamp is not ovalizing the pipe further. After welding, verify the internal root profile and consider running a pig through the section before commissioning.