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. The flush patch guide can be obtained from NBIC’s RD-2060a Flush Patches.
Perform a hardness test (DT) on the area of the lump 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 patch,” which means that the carcass is cut around the fault in an elliptical or rectangular shape, and a piece of rolled new boiler plate is inserted to correct the curvature with the edge of the original shell and butt welded patch. For rectangular shapes, provide a precise radius as square corners should be avoided.
Consult the manufacturer’s drawing for shell material specifications and patches will be made of a material that is equal in quality and thickness to the original material. The edge will align without overlap. Preparation of the double “V” welding with an included angle of 60o to 70o. Gas Tungsten Arc Welding (GTAW) will be used for the initial pass joining the patch to the carcass. The remainder of the weld will be manual metal arc shielded welding using confirmed low hydrogen electrodes per ASME Code Sect IX.
Weld the inside first, then gouge back on the outside. Grind up the healthy metal before continuing to weld on the outside. Grind accessible surface and, after completion of welding, visually check for weld cuts and repair if necessary. Consult the manufacturer’s drawing on the NDT used during construction, and normally the weld on the flush-to-shell patch is 100% radiographed. Weld acceptance is in accordance with ASME code or BS 2970 code. Post weld heat treatment (PWHT) must be performed to relieve residual stress in the heat affected zone (HAZ). Carry out hydrostatic test on the boiler. The test pressure will be 1.5 times the design pressure. The waiting time is 10 minutes.
