Erosion-fatigue of steam turbine blades
Introduction
Brazil has 57.5 GW of installed hydroelectric power (ranking 4th in the world) and the world largest reserves of hydric resources (8.2 km3/year). Although impressive, these figures may be misleading: the recent growth in demand for electricity led to energy rationing during the dry season of 2001, revealing the lack of infrastructural investment in the supply and distribution of electrical energy. As a direct result, the Brazilian Government created an emergency plan to expand the energy supply, which included the construction of thermoelectric plants. Thermoelectric plants usually generate electricity from heat, which is released during the combustion of fossil fuels. Currently, Brazil emits 4.5 million tonnes of carbon per year into the atmosphere and, with the construction and operation of these new thermoelectric plants, this could increase to 16 million tonnes per year, worsening the profile of the Brazilian electrical energy matrix, which is currently composed of 75% renewable resources [1], [2], [3], [4].
The most versatile thermoelectric plants operate with more than one type of fuel, whose combustion generates steam at high temperatures and high pressures. The passage of high-pressure and high-temperature steam through the various stages of the turbine causes the rotation of the blades, which are submitted during working conditions to severe static and dynamic loading, usually associated with mechanisms of microstructural degradation, such as corrosion, wear and creep [5], [6].
Tu et al. [7] studied the premature failure of martensitic stainless steel (12% Cr) blades belonging to the last stage of a steam turbine. The blades broke in the leading edge, which also featured erosion marks, and the fractographic investigation indicated that the crack was propagated by fatigue. The mechanical test results showed that the results for elongation, reduction of area and toughness resistance were below the specifications and they concluded that the crack nucleation was promoted by improper tempering treatment associated with the presence of stress raisers. Mazur et al. [8] also investigated the premature failure of martensitic stainless steel blades and observed during the visual inspection that some of the mechanical stabilizer segments, which were present on the tip of the blades, were missing. FEM calculations indicated that the lack of the stabilizers generated a drastic increase in the vibration of the blades, especially during the transient operation of the turbine, which is more critical to vibration, causing their premature failure by fatigue. Illescas and Rodriguez [9] observed fatigue cracks in the trailing edge of the blades, concluding that the growth of the fatigue cracks was also associated with the operation of the steam turbine in transient regimes.
The drag of solid particles by the gas/steam flow is another major problem during the operation of turbines. These foreign particles can either be deposited over the turbine trail and blades (see Fig. 1), causing wear and reducing the efficiency of the turbine, or be launched at high velocities against the blade surfaces, promoting the formation of erosion pits in preferential areas, which can act as stress raisers [10], [11], [12], [13].
The present investigation analyzes the premature failure of the rotor blades of the last stage of a steam turbine (generating unit of 140 MV A) belonging to a thermoelectric plant. The last stage was composed of 65 blades, of which 17 blades were broken. The martensitic stainless steel 12% Cr–NiMoV blades were manufactured by forging and heat treatment.
Section snippets
Experimental procedure and results
Twenty-five damaged blades from a total of 65 blades of the last stage of the steam turbine were sampled for a more detailed laboratorial investigation. The following tests were carried out: visual inspection; non-destructive tests to identify presence of cracks (magnetic-particles and fluorescent liquid penetrant inspections); macrofractography (stereomicroscope); microfractography (JEOL 5200 and JEOL 6300 scanning electron microscopes); metallography (JEOL 5200 and JEOL 6300 scanning electron
Discussion
Chemical analysis and mechanical testing results (see Table 1, Table 2) along with the microstructural characterization (see Fig. 11, Fig. 12) and hardness measurements (see Table 5) confirmed that the material of the blades was in accordance with the manufacturer’s specification for 12% Cr–NiMoV martensitic stainless steel, indicating that the premature failure of the blades was not related to the material.
Visual inspection and macrofractographic examination (see Fig. 2, Fig. 3, Fig. 4, Fig. 5
Conclusions
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Chemical analysis and mechanical testing results along with microstructural characterization and hardness measurements confirmed that the material of the blades was in accordance with the manufacturer’s specification, indicating that the premature failure of the blades was not related to the material.
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Some blades presented a considerable proportion of stable crack growth by fatigue, with visible presence of beach and striation marks.
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Fractographic and microstructural characterization indicated
Acknowledgment
The authors would like to thank Mr. G. Spera and J. Belotti, from Instituto de Pesquisas Tecnológicas do Estado de São Paulo.
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2021, EnergyCitation Excerpt :Among all the components, the stage has the most significant influence on unit performance [6]. After a long-term operation in a harsh environment, the steam turbine stage blades are prone to scale, and the steam seals are prone to wear [7,8]. These lead to a decline in the thermal-power conversion capability [9] and then affect the unit’s performance.