Study on Life Extension of Steam Turbine Casing Material by Weld Repair in Actual Equipment

Thermal Power Research Section

Our thermal power plants, which account for about 70% of all power plants of Hokkaido Electric Power Co., Ltd. (HEPCO), have been operating for over 100,000 hours since the start of the service. At these "aged thermal power plants," great costs are required for equipment maintenance and replacement. As liberalization of the power industry is being promoted, the reduction in such expenses by extending the life for aged thermal power plant equipment becomes increasingly important for lower the cost of power generation.

To meet this financial challenge, HEPCO has established weld repair techniques for cracks on steam turbine casing materials and life assessment technology for a weld zone in actual equipment, aimed at cutting costs by extending equipment replacement period.

A turbine casing used for a long term often has defects such as cracks. The defective parts are removed, and then, if its remaining wall thickness goes below the requirement, the equipment will be replaced. We conducted experimental researches to establish weld repair techniques for defective parts and life assessment technology for a weld zone.

A conventional weld method requires Post Weld Heat Treatment (PWHT) to remove residual stress after weld repair. In the meantime, weld repair for large parts like turbine casing needs a long construction period and enormous costs for delivering the damaged equipment to a factory for proper heat treatment. Also, the generation of strain in the casing by thermal effect is a great technical concern. Consequently, in the temper bead weld method, known as a method requiring no PWHT, HEPCO tried to establish application tests and an optimum weld method for turbine casing material and life assessment technology for a weld zone and took some technological measures for applying these techniques to actual equipment..

The following achievements in the technical application to actual equipment are provided.

1. We found out the damage mechanism by thermal stress analysis in finite element method for actual damaged parts and selected an optimum weld method.
2. Using scrapped casing materials, we made a comparison of the optimum method developed in this study and the conventional weld method. This comparison found that residual stress in the optimum method is significantly lower than that of the conventional weld method and this weld method is more favorable in the use of actual equipment.