If flange leakage is caused by damage to the gasket surface or a partially corroded gasket surface, the flanges can normally be repaired by weld build-up and/or skim cutting. Flanges that are close to ASME tmin should be built up with weld material before skim cutting the surface in order to avoid losing flange thickness.
Flanges that are designed in accordance with Appendix G can be skim cut down to tmin as determined by Appendix G without a problem because these flanges are usually much thicker than ASME flanges. However, skim cutting to thicknesses below Appendix G tmin may result in leakage.
The type of skim cut depends on the gasket selected. For example, composition asbestos gaskets work best with a 250 RMS surface because the deeper grooves “bite into” the gasket better. Refer to Section 540 and Figure 500-15 for the surface requirements for various types of gaskets.
Surface Designation Appearance
250 RMS Relatively deep grooves
125 RMS Fairly smooth
63 RMS Smooth appearance
It is very important that all repairs be accompanied by a welding procedure if needed and a technical drawing showing the dimensions and tolerances that must be met after the repair is complete.