NDTnet 1998 Aug, Vol.3 No.8

Abstract

Turbine rotor discs are known to be susceptible to stress corrosion cracking (SCC). This paper outlines the strategy developed by Magnox Electric to manage the threat from SCC and then concentrates on the inspection of the discs and on the defects found.

The assessment of disc integrity relies on detailed knowledge of operating conditions, material properties, stresses and information on the presence of defects. A strategy has been developed to ensure that safety risks are as low as reasonably practicable (ALARP) and that commercial risks are controlled. A fully probabilistic assessment procedure has been devised to define the probability of rotor failure occurring during each year of operation. Locations at risk from SCC were identified as highly stressed regions which operate in wet steam. These include the blade fixings on disc rims, the disc bores and the button recesses in disc hub faces. Ultrasonic inspections have been used to determine the incidence of cracking in all of these areas. The inspections provide one of the key inputs for the assessments of rotor failure probability and it is important that the inspection capabilities at each location are understood and that the procedures are correctly implemented.

To meet the inspection demands, most discs have been inspected with a purpose-built automated system and permanent recording of the data. The disc areas covered by the automated system have been the disc rims, the disc bores/keyways and the dowel recesses.The inspection techniques applied to the main disc areas and the results obtained are discussed in some detail. Over 450 turbine discs have been inspected to date and cracks have been detected and confirmed in plain bores, keyways, button recesses and most kinds of blade fixings. The most significant cracking has been found in disc rims. In a number of disc rims the cracks were of such substantial depth that continued operation could not be justified and repair or replacement had to be undertaken.

Assessments of the inspection capability have been made, based mainly on comparisons of inspection results with destructive examinations of genuine service induced defects. Detection capabilities are quoted for the main disc areas inspected. Further work aimed mainly at improving the sizing capability is under way.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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