Abstract

Following a bearing failure in a steam turbine, internal inspection of the unit post-failure additionally uncovered cracking in nearly all of the 76 High Pressure (HP) Stage 1 blade T-hooks (at the base of each blade) used to retain the blades in the rotor. Considering the similar design, manufacture, installation, and operation of a sister turbine, it was assumed that similar cracking was likely present in that unit’s blades. A fracture mechanics assessment using procedures from the fitness-for-service standard API 579-1/ASME FFS-1 was performed to analyze the stability of the assumed cracking and the susceptibility to fatigue crack growth caused by continued operation. The second unit’s rotor was to be replaced in 30 months during which the blades would also be replaced. Continued operation of the second turbine using the existing blades until the new rotor was ready represented a significant financial gain by reducing the unit’s outage time minimizing the loss of power generation as well as saving on the cost of replacement blades. A cyclic symmetric finite element model of the full rotor and HP1 blades was used to model the two primary load cycles of interest: cold start through shutdown and a daily load swing. Mechanical and thermal loads provided by stream temperature and pressure data and rotational forces due to the spinning rotor were applied to coupled heat transfer and static stress analyses. Although fully transient processes, static analyses at specific, relevant “snapshots” over the transients were performed to analyze the turbine’s behavior over the two load cycles. Using custom 3-D crack meshes, a 9 mm deep crack was inserted into the trailing edge of the HP1 Blade’s T-hook. From the finite element analysis results, stress intensity values along the crack front were compared against material fatigue data to determine if the particular load cycle was susceptible to fatigue crack growth. It was determined that the daily load swing was unlikely to cause crack growth as the change in stress intensity over the cycle was less than the fatigue crack growth threshold, but a small amount of growth was predicted to occur during each cold start/shutdown cycle. However, due to the relative infrequency of the cold start/shutdown load cycle (typical operation saw the unit shutdown on average three times per year) and size of the predicted growth, it was determined that continued operation blades was acceptable for at least 30 months assuming a maximum of 10 cold start/shutdown cycles per year. The analysis results supported continued operation of the turbine until the replacement rotor was available and helped provide operating guidelines to minimize potential crack growth in the turbine blades.