Analysis and management of operating risk created by turbine operation under flexible regimes

Abstract

The new conditions in which coal-fired power plants, especially 200 MW units, have to operate require a considerable increase in the dynamics of their operation. The power unit start-up frequency increases and so does the frequency of changes in loads. This intensifies some wear processes, such as low- cycle fatigue and crack propagation in particular. Therefore, further operation of power units which have already been in service for a long time has to be supplemented with results of analyses and tests taking account of the intensification of wear processes. The paper presents a proposal for an extension of standard diagnostic testing of turbines by adding small punch tests (SPT) of the rotor material micro specimens. The SPT method enables a fast quasi non-destructive assessment of changes in mechanical properties, especially rotor steel embrittlement due to the turbine previous operation. The other element of the proposed testing is the analysis of the propagation rate of potential cracks in the rotor and assessment of the rotor failure probability for different scenarios of the power unit further operation.

Risk-Based Planning of Diagnostic Testing of Turbines Operating with Increased Flexibility

“…This can be done using the quasi-nondestructive small punch test (SPT) method [40][41][42][43][44][45][46]. An example application of the method in the procedure for planning diagnostic testing of turbine elements is presented below [47].…”

“…After that, using appropriate correlations, it is possible to determine the temperature of the fracture appearance transition temperature (FATT). The details of the analyses are given in [47]. By conducting these tests of specimens sampled from the rotor inlet and outlet part, it is possible to assess the decrease in the rotor steel fracture toughness due to previous operating conditions.…”

“…The results obtained from the testing of rotors operated for more than 200,000 h indicate that the decrease in material toughness may reach from a few to about fifteen percent of the initial value [47]. Such a change may have an essential impact on the safety of the turbine with further operation at increased intensity.…”