Isometric Discrepancy on a Stainless Sanitary Spool
An 80 mm nominal bore stainless steel sanitary discharge spool is prefabricated for a passenger ship from an approved isometric. The drawing shows a 45° branch at elevation +2,450 mm, with a centerline dimension of 1,200 mm from the bulkhead penetration, and a butterfly valve with the handwheel to port. In the field, the bulkhead penetration is 150 mm lower and 200 mm outboard of the drawing. The spool has already been welded, pickled and pressure-tested at the shop. The fitter proposes to cut 250 mm from the branch, re-weld it at the new location, and use a flexible coupling at the bulkhead to take up the misalignment. The pipe is in an accommodation area and passes through an A-class bulkhead; the penetration is fitted with an approved fire-rated sleeve. The shipyard's QA manager asks for a technical evaluation before the spool is installed, because the owner's representative has threatened to reject the work. The vessel is at the outfitting stage with delivery in three weeks.
1. How should the discrepancy be resolved using the drawing set and change-control process?
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Resolve the conflict by comparing the approved isometric with the P&ID, line list, GA, bulkhead penetration drawing and the owner's specification. The field dimension is a nonconformance: raise an RFI/NCR and have the design engineer confirm which dimension governs. Do not let the fitter "make it fit" on the deck. If the penetration is correct and the isometric is wrong, revise the isometric and obtain approval before fabrication changes. If the spool is already fabricated, check whether the 150 mm/200 mm offset can be absorbed by an approved spool piece or whether a new spool is required. Document the as-found dimensions with photos and survey. Change control must include the owner, yard QA and class if the change affects the fire boundary or approved arrangement.
2. Evaluate the proposed cut-and-re-weld and flexible coupling at the A-class bulkhead.
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The proposed cut-and-re-weld is a major modification to a pressure-retaining stainless spool. It requires an approved WPS/PQR, qualified welder, purge and post-weld cleaning/passivation. Re-welding in the field may be difficult to control and may distort the spool. A flexible coupling at an A-class bulkhead penetration is not acceptable as a general misalignment fix: it may compromise the fire rating, hygiene and support, and the approved fire-rated sleeve must be maintained. The sanitary line also requires smooth internal surfaces and no crevices. Do not use a rubber flexible coupling in an accommodation fire boundary unless it is approved and fire-tested for that location. The correct solution is an engineered spool piece or revised penetration detail, with the fire sleeve and support details approved.
3. What documentation and tests are required after an approved modification?
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After an approved modification: NDT the welds (visual, PT and/or RT per class), passivate stainless steel, hydrotest at 1.5 × design pressure with calibrated gauges, and perform a sanitary flush/chlorination if required. Verify the fire-rated penetration is intact and the pipe is supported and aligned. Update the isometric/as-built, weld map, line list and inspection records. Obtain owner and class acceptance before closing the NCR. The QA manager should not release the spool for installation until the revised drawing and procedures are approved.