Detailed procedures for repairing the bulge in the shell of an HRT steam boiler

All repairs must be acceptable to the applicable code jurisdiction (such as NBIC), therefore all repair methods must have the written approval of the Chief Inspector or Authorized Inspector. Guidelines for flushing the patch can be obtained from RD-2060a Flush Patches of NBIC.

Perform a hardness test (DT) on the protruding area to determine the extent of the defect. Mark the boundary line of the defective section to be cut based on the acceptable tensile strength obtained. Use the Vickers Hardness (HV) test to test the metal strength of carbon steel. The repair method is called “flush patching”, which means that the casing is trimmed around the failure in an elliptical or rectangular shape, and a piece of new, rolled repeating plate is inserted to correct the curvature with the edge of the original casing and butt-welded patch. For the rectangular shape, provide a precise radius as square corners will be avoided.

Consult the manufacturer’s drawing for shell material specifications, and heads must be made from material that is equal in quality and thickness to the original material. The border will be aligned without overlap. Weld preparation to be double “vee” with an included angle of 60o to 70o. Gas Tungsten Arc Welding (GTAW) will be used for the initial step joining the head to the shell. The remainder of the weld shall be a manual shielded metal arc weld using low hydrogen electrodes confirmed to ASME Code Section IX.

Solder the inside first, then gouge the outside again. Grind sound metal before continuing with exterior welding. Grind down to level the accessible surface and, after welding is complete, visually check for weld undercuts and repair if necessary. Refer to the manufacturer’s drawing for the NDT used during construction, and typically the weld on the shell flush patch is 100% radiographed. Weld acceptance is in accordance with ASME code or BS 2970 code. Post Weld Heat Treatment (PWHT) should be performed to relieve residual stress in the Heat Affected Zone (HAZ). Perform hydrostatic test on the boiler. The test pressure will be 1.5 times the design pressure. The waiting time is 10 minutes.

Leave a Reply

Your email address will not be published. Required fields are marked *