Hydrostatic Test Decision on a Repaired Superheater Header
A shop has completed a weld repair on a superheater outlet header of a 12.4 MPa design-pressure boiler. The repair replaced a 300 mm section of the header shell. Post-weld heat treatment was performed and hardness readings are within the agreed range. Radiography of the new longitudinal and circumferential seams shows no rejectable indications. The authorized inspector must decide whether a hydrostatic test is required before return to service, at what pressure, and what else must be verified. The jurisdiction adopts the ASME Boiler and Pressure Vessel Code. The owner asks whether radiography alone is sufficient and whether the test can be done at operating pressure to save time.
1. Explain the role of the hydrostatic test relative to the radiographic examination, and whether radiography alone can substitute.
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Radiography and hydrostatic testing answer different questions. Radiography interrogates the weld volume for internal discontinuities - porosity, lack of fusion, cracks - and the films here show none rejectable. It says nothing about the strength and leak-tightness of the completed assembly under pressure, fit-up stresses, or the integrity of the whole pressure boundary including the transition to the existing header.
A hydrostatic test is a proof test of the assembled boundary. It verifies that the repair and its heat treatment produced a sound component that holds pressure without leakage or permanent deformation. It is a system-level check that radiography cannot provide.
The Code's construction rules require a hydrostatic test as part of the completed pressure part unless a specific exemption applies, and the authorized inspector makes the call. Radiography alone is not a substitute for the required test where the Code and jurisdiction require it. The owner's convenience does not override the Code requirement; the inspector should confirm the applicable rule and document the basis.
2. Address the test pressure question and list the pre-test verifications the inspector should require.
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The test pressure must follow the Code's rule for the component - typically a defined multiple of the maximum allowable working pressure, corrected for temperature, applied at the header and referenced to the highest point. Testing at operating pressure to save time is not acceptable; operating pressure is well below the proof pressure the Code specifies, so the test would not demonstrate what it is meant to demonstrate.
Pre-test verifications: confirm the repair procedure and welder qualifications, the post-weld heat treatment records, and the radiographic reports are complete and accepted. Verify the test pressure calculation, the gauge calibration and range (the gauge should read in the upper part of its span at test pressure), and that the test medium and temperature are suitable - water temperature above the brittle-fracture concern for the material, with the metal temperature documented. Confirm temporary supports, isolation of safety valves and instruments that cannot take test pressure, and venting at high points. Check that the test can be held long enough to inspect all joints, and that a documented examination for leakage and permanent deformation follows. The inspector witnesses and signs off.