Water droplet erosion continues to pose a serious potential threat to the critical path of maintenance outages. Despite advances in techniques and processes to mitigate erosion, combined cycle arrangements still result in an unexpected level of wetness within the steam, which can produce extensive damage to the last stage blade rows. A procedure is discussed that has been used successfully to manage the critical reliability issues that are raised once conditions of water droplet erosion are found. Erosion rate is estimated from (1) empirical models obtained from the literature, (2) for the steam flow conditions specific to the unit, and (3) with stresses obtained from analysis of the damaged blades. Two applications are presented to demonstrate how the erosion rates were used to provide a basis for two very different strategies, both of which were able to avoid the prohibitive cost of extending the immediate outage to obtain and install new replacement rows.
The tie-lugs on a 46” last stage blade (LSB) operated in a power plant first commissioned in the early 1960s experienced a long service history of cracking problems. A root cause investigation was performed, which identified the third mode of the bladed disc as operating near resonance. The original configuration is a 4-blade group structure with two tie-lugs. The third mode is a group torsional mode in which maximum stress occurs in the outer lugs. Dynamic stress throughout the rotating stage was calculated using a FE model. Based on the results of the investigation features of a retrofit were identified to detune the resonant mode and change the brazed lug to a more reliable forged design. A probabilistic analysis was used to demonstrate how the risk of cracking problem would be dramatically reduced.
Stress corrosion cracking (SCC) is a common problem found on aging low-pressure turbine (LP) rotors that operate in a wet/dry stream environment. While much has been published on the growth rate of SCC in turbine rotor-disk materials, incubation time is rarely addressed. Since no effective way has been demonstrated to prevent disk rim SCC from occurring other than to replace the damaged rim with a weld repair of higher chromium content, a better understanding of incubation time could provide operators with a means to treat SCC before cracks are large enough to start to grow. This paper discusses the critical mechanisms involved in the SCC incubation, process and describes a probabilistic approach to make meaningful assessments of incubation time. Data published for General Electric turbine rotors is used to test the model.
The demand for increased performance from combustion turbines has pushed firing temperatures to the point where the life consumption of the hot section components may no longer be accurately monitored based on traditional engineering practices. For rotating blades in particular, creep rupture criterion such as strain limits, tend to be arbitrary, subjective and prohibit the use of statistical data available from material tests. This inhibits the application of a probabilistic treatment as a basis to answer the fundamental question of an operator responsible for the life cycle management of these costly parts, i.e. how does the chance of an in-service failure increase over the lifetime of the component. Presented in this paper is a more direct engineering approach that has been used to predict the risk of crack initiation and propagation due to creep in hot section parts. Results are compared first against the traditional method of designing against creep rupture, and then conclude with the risk analysis of an actual first stage gas turbine blade.
Described within this paper are the results derived from an investigation into the root cause of cracks which are prematurely forming within the aft end rotor rim attachments of large combustion turbine compressors. The overall objective for this program was to use these results to develop a means for projecting the remaining service life of similarly affected rotors based on non-destructive measurements which identified the degree of fatigue damage sustained in the critical disc rim fillets. Projected rotor retirement was identified by systematically calculating the change in stress intensity for a crack as it propagated from a corner of the attachment fillet through the disc rim. Risk of catastrophic failure was identified when the stress intensity along the front of a given crack size and depth exceeded the fracture toughness of the rotor material, risking failure. Estimates of both crack initiation and propagation relied on properties derived from fatigue tests of specimens obtained from a retired rotor and stress results from a transient thermal analysis of the stages. When compared, projections of crack growth correlated well against field measurements of damaged rims.
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