All repairs must be acceptable to the governing code jurisdiction (such as NBIC), therefore all methods of repair must have written approval from the Chief Inspector or Authorized Inspector. The guideline to flush patch can be obtained from RD-2060a Flush Patches of NBIC.
Carry out hardness test (DT) on the bulge area to determine extent of defect. Mark out the boundary line of the defective section to be cut out based on acceptable tensile strength obtained. Use Vickers Hardness test (HV) to test for the metal strength of carbon steel. The method of repair is called “flush patch”, which means the shell is cutout around the failure in elliptical or rectangular, and a piece of new boilerplate rolled to correct curvature is inserted with the edge of original shell and patch butt-welded. For rectangular shape, provide accurate radius as square corners shall be avoided.
Refer the manufacturer’s blueprint on the material specifications of the shell and the patches shall be made from the material that is equal in quality and thickness to the original material. The edge shall align without overlap. Weld preparation to be double “vee” with an included angle of 60o to 70o. The gas tungsten arc welding (GTAW) shall be used for the initial pass joining the patch to shell. The balance of the weld shall be manual shielded metallic arc welding using low hydrogen electrodes confirmed to ASME Code Sect IX.
Weld the inside first, then back gouge on the outside. Grind the sound metal before continuing the welding on the outside. Grind flush the accessible surface, and after completion of welding, visually check for weld undercut and repair if necessary. Refer the manufacturer’s blueprint on the NDT used during construction, and normally the weld on flush patch of shell is 100% radiographed. Acceptance of weld is in accordance with ASME Code or BS 2970 code. Post weld heat treatment (PWHT) is to be done to relieve the residual stress on the heat affected zone (HAZ). Carryout hydrostatic test on the boiler. Test pressure shall be 1.5 times the design pressure. The holding time is 10 minutes.