Hydrostatic Test Decision on a Repaired Mud Drum
A water-tube boiler at 25 bar(g) design pressure has a mud drum with a localized corroded area found during internal inspection. The area is 150 mm by 80 mm, and ultrasonic thickness readings show 6.1 mm remaining against a nominal 9.5 mm and a calculated minimum of 6.4 mm. The owner wants to weld-repair the area and return the boiler to service within 48 hours. The repair procedure is available, but the welder's qualification for the specific material and position has expired. The boiler has a valid operating certificate, and the jurisdiction requires notification before major repairs. The plant has a hydrostatic test pump and the means to isolate the boiler.
1. Decide whether the boiler can be returned to service in 48 hours. What must happen before the repair proceeds, and what does the thickness result mean for the repair decision?
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The boiler should not be returned to service in 48 hours as proposed. The remaining thickness of 6.1 mm is below the calculated minimum of 6.4 mm, so the area is not acceptable for continued service and must be repaired or the drum replaced or derated per engineering evaluation.
Before the repair proceeds: (1) the repair procedure must be reviewed and accepted by a qualified engineer and, where required, the jurisdiction; (2) the welder must hold a current qualification for the material and position, or be requalified by test before welding; (3) the jurisdiction must be notified before the major repair, as required; (4) the repair area must be prepared, the extent of corrosion mapped, and the weld inspected after completion, typically by nondestructive examination such as radiographic or ultrasonic testing plus surface examination.
The 48-hour target is unrealistic once requalification, notification, weld inspection, and the hydrostatic test are included. A realistic plan is to schedule the repair properly, complete the welder qualification, notify the authority, perform the repair, examine it, and then hydrotest before return to service.
2. Outline the post-repair testing and documentation needed before the boiler is returned to service, including the hydrostatic test and the code basis.
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Post-repair sequence: complete the weld and any heat treatment required by the procedure; perform nondestructive examination of the repaired area, including surface and volumetric methods as specified; visually inspect the drum interior for cleanliness and any remaining corrosion; then hydrostatically test.
Hydrostatic test: fill with treated water, vent air, and pressurize to the test pressure required by the code for the design pressure, holding for the specified time and inspecting all joints, the repair, and the drum for leakage or distortion. Use a calibrated gauge and record the pressure and duration. The test pressure for a repair is generally based on the code's hydrostatic test rules, and the jurisdiction may specify or witness it.
Documentation: the repair procedure, welder qualification records, NDE reports, heat treatment records, hydrostatic test record with gauge readings, and the inspector's acceptance. Notify the jurisdiction and obtain any required acceptance before return to service. Update the boiler records and the operating certificate as required. The code basis is the ASME Boiler and Pressure Vessel Code (Section I for power boilers) and the local jurisdictional rules for repairs and inspections. Return to service only after all records are accepted.