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New advances in emerging technologies: Steam turbine casing cracking at several power plants
The average age of utility steam turbines in the United States was approximately 40 years as of 2006. It is anticipated that most of these units will be required to provide reliable peaking or baseload service for a number of decades beyond their original design life. An understanding of possible turbine component damage mechanisms and related inspection and repair processes is vital to ensure reliable long-term operation.
Steam turbine casings typically are massive steel castings that encase the rotating and stationary components of a steam turbine. They function to contain the steam pressure of the turbine and to support and maintain alignment of the internal components. Under baseload conditions, the high temperatures and operating pressures combine to result in long-term degradation of component materials. Accelerated startup or shutdown cycles can further aggravate the problem by increasing the thermal differentials
Five case studies were reported in the EPRI study. The units had in-service dates ranging between 1952-1960, with unit ratings in the 80-180 MW range. The reported operating hours at the times in which the cracks occurred varied between approximately 150,000 and 350,000 hours. The reported cracks occurred in HP, HP/IP and IP shells. All cracks had been discovered during planned outages/inspections. One of the crack occurrences had led to a steam leak. Visual and liquid penetrant inspection methods were used on all of the casing cracks with wet fluorescent magnetic particle inspection being used on 60 percent of the crack cases.
Typically, casing cracks are relatively large. Crack dimensions reported were in the order of 5′-15′ long with depths in the order of 5′ to casing through-wall thickness. In all reported cases, the cracks were repaired. Some cracks were repaired on initial discovery while others had been monitored for a number of inspection cycles. In all reported cases, the repairs had been performed by non-OEM organizations. The cracks were ground out and weld repaired. The majority of the reported repairs used a preheat and temper bead repair process with PWHT (stress relief).
Subsequent to the repair, in the majority of cases subsequent inspections only occurred at normally scheduled turbine outages/inspections. In one case, particular care was taken to monitor casing temperatures, hot reheat temperatures, blowdown piping temperatures and inner vs. outer shell temperatures during operation. The durability of the repairs was queried. Based on this information, the average age of the repairs was approximately 11 years.