Risk assessment of failure by stress corrosion cracking in shrunk-on disks of low pressure turbines
- Structural Integrity Associates, San Jose, CA (United States)
- Tennessee Valley Authority, Chattanooga, TN (United States)
Several large LP turbines in the TVA system utilize shrunk-on disks which are keyed to the shaft. The 7th, 8th, and 9th stage disks experience wetness in operation which renders them potentially subject to stress corrosion cracking (SCC) in the keyway of the shrink fit area. To minimize SCC concerns in the disk keyway, TVA has embarked on a phased approach to refurbish the LP rotors with a fat shaft and a tab design replacing the rectangular keyways. Nondestructive examinations have been performed on rotors being refurbished and those continuing in service to assure that the schedule for refurbishment does not place operating units with the original design at undue risk. Many variables are involved in the quantification of the risk of disk failure from SCC: incubation time, stress intensity distribution as a function of crack size, material SCC rate as a function of strength and environment, fatigue crack growth rate, loading history (especially overspeed events), and fracture toughness. For a given set of assumptions and parameters, a deterministic model may be used to assess the risk of failure. Unfortunately, if mostly conservative assumptions are made for the random variables, it can be concluded that small initial flaws will grow to critical sizes in two years of additional operation. As an alternative to the deterministic approach, a probabilistic Monte Carlo simulation was performed using ten critical variables. Variable distributions were examined by binning data values within three standard deviations above and below the mean. One million iterations were performed for each disk.
- OSTI ID:
- 550003
- Report Number(s):
- CONF-961006-; ISBN 0-7918-1796-2; TRN: IM9753%%78
- Resource Relation:
- Conference: 1996 international joint power generation conference, Houston, TX (United States), 13-17 Oct 1996; Other Information: PBD: 1996; Related Information: Is Part Of 1996 international joint power generation conference: Proceedings. Volume 2; PWR-Volume 30; Kielasa, L. [ed.] [Detroit Edison Co., MI (United States)]; Weed, G.E. [ed.] [Eastman Kodak Co., Rochester, NY (United States)]; PB: 936 p.
- Country of Publication:
- United States
- Language:
- English
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