THICK-WALLED PRESSURE VESSEL REPAIR BY EVALUATION OF STRESS CONCENTRATIONS AT CRACK GRIND-OUTS

摘要

Cracks may develop at weld locations or at stress concentrations in thick-walled pressure vessels operating at high pressure and temperature. When cracks are detected by periodic inspections, they must be removed from the pressure vessel. Cracks are removed by grinding out the metal up to the root of the crack. Repair of the vessel by welding may be required to retain the integrity of the pressure boundary. The weld replacement process causes problems related to possible adverse heat treatment. For example, the weld and the surrounding material may become brittle, more susceptible to corrosion and will experience residual stresses that may cause cracks to form again. The weld replacement process is also costly and time consuming. In this paper, we investigate an alternative to weld replacement by analyzing an aging boiler drum with a circumferential crack near the cylinder to head transition weld. The alternative is simply to grind-out the crack up to the root without replacing the material lost. This alternative may be acceptable for shallow grind-outs where a trade-off between adverse effects of weld replacement is balanced by stress concentrations due to the grind-out geometry and the reduced wall thickness. We investigate this alternative through a case study where the stress concentrations at various grind-out depths are evaluated using a finite element model. We then propose a criterion to determine whether weld replacement may be avoided. The criterion is based on the results of our model and on specific requirements of the ASME Boiler and Pressure Vessel Code and is presented in simplified format. Utility engineers are often required to select and specify a repair method at short notice when a crack is found in a pressure vessel during an in-service inspection of an operating power plant. The detailed design calculations are generally not available from the Original Equipment Manufacturer. The analytical approach presented in this paper will provide a cost effective and practical tool for such emergency situations.