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Abstract: A last stage (L-0) turbine blades failure was experienced at a 28MW geothermal unit after seven years of operation period. This unit has one flow intermediate/low-pressure turbine composed of nine stages with 25-in./3600rpm last stage blades. The last stage row contains 62 free standing blades. Visual examination indicated that the 37 L-0 blades had cracks in their airfoils initiating at the trailing edge, near the blade platform. Laboratory evaluation of the cracking indicates the failure mechanism to be high cycle fatigue (HCF), and the cracks initiation was accelerated by erosion picks on the blade surface due to steam recirculation flow and corrosion. A last stage blade failure evaluation was carried out. The investigation included a metallographic analysis of the cracked blades, natural frequency analysis, blade stress analysis, unit operation parameters, history of events analysis and crack initiation and propagation analysis. This paper provides an overview of the failure investigation, which led to the identification of some operation periods with low load as the primary contribution to the observed failure. [Copyright &y& Elsevier]

Abstract: The failure analysis of the 70MW gas turbine first stage nozzle made of cobalt-base alloy FSX-414 is presented. The nozzle experience cracks in different vane airfoil sections assisted by base alloy degradation due to an operation at high temperature. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; grain growing (coarsening) and vane airfoil stress level. It was found that the vane crack initiation and propagation was drived by thermal fatigue mechanism. The vane crack initiation and propagation were due to fatigue mechanism which was facilitated by grain boundaries brittleness caused by formation of continuous film of carbides on grain boundaries, grains coarsening, and high thermal stress level. [Copyright &y& Elsevier]

Abstract: The failure analysis of a four-stroke 3000kW off-shore operating diesel engine is presented. The failure occurred during engine normal operation period in the leak-off oil pipe from the injection valves, which experienced a fracture through the pipe wall and a diesel engine fire as a result. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely leak-off oil pipe vibration level, pipe stress level, presence of corrosion pits on the pipe external surface under the zinc coating and engine components temperature distribution in the failure zone. It was found that the crack initiation and propagation of the leak-off oil pipe from injection valves was driven by a fatigue mechanism, which was facilitated by loose pipe supports (excessive pipe vibration) and corrosion pits on the pipe surface that acted as stress concentrators. The contact of leak-off atomized oil due to the pipe through wall fracture with the hot engine exhaust muff (390°C approximately) caused local fire of the diesel engine. [Copyright &y& Elsevier]

Abstract: A last stage (L-0) turbine blades failure was experienced at the 110MW geothermal unit after 1 year of operation period. This unit has two tandem-compound intermediate/low-pressure turbines (turbine A and turbine B) with 23in./3600rpm last-stage blades. There were flexible blades continuously coupled 360 degrees around the row by loose cover segment at the tip and loose sleeve and lug at the mid-span (pre-twist design). The failed blades were in the L-0 row of the LP turbine B connected to the generator. The visual examination indicated that the group of 12 L-0 blades of rotor B on the generator side was bent and another group of 5 blades at 140 degrees from the first damaged group was also bent. The cover segments were spread out from the damaged blades and had cracks. Laboratory evaluation of the cracking in the cover segments indicates the failure mechanism to be high cycle fatigue (HCF), initiating at the cover segment holes outer fillet radius. The L-0 blades failure investigation was carried out. The investigation included a metallographic analysis of the cracked cover segments and bent blades, Finite Element Method (FEM) stress and natural frequency analysis (of blades/cover segments), fracture mechanics and crack propagation analysis. This paper provides an overview of the L-0 blades failure investigation, which led to the identification of the blades vibrations within the range 250–588Hz induced due to unstable flow excitation (stall flutter) as the primary contribution to the observed failure. [Copyright &y& Elsevier]

Abstract: The failure analysis of the 70 MW gas turbine first stage blade made of nickel-base alloy Inconel 738LC is presented. The blades experience internal cooling hole cracks in different airfoil sections assisted by a coating and base alloy degradation due to operation at high temperature. A detailed analysis of all elements which had an influence on the failure initiation was carried out, namely: loss of aluminium from coating due to oxidation and coating phases changing; decreasing of alloy ductility and toughness due to carbides precipitation in grain boundaries; degradation of the alloy gamma prime (γ′) phase (aging and coarsening); blade airfoil stress level; evidence of intergranular creep crack propagation. It was found that the coating/substrate crack initiation and propagation was driven by a mixed fatigue/creep mechanism. The coating degradation facilitates the crack initiation due to thermal fatigue. The substrate intergranular crack initiation and propagation were due to a creep mechanism which was facilitated by grain boundary brittleness caused by formation of a continuous film of carbides on grain boundaries, the degradation of γ′ due to elongation (rafting) and coalescence, and high thermomechanical stress level. [Copyright &y& Elsevier]

We apply the Hurst exponent idea for investigation of DJIA index time-series data. The behavior of the local Hurst exponent prior to drastic changes in financial series signal is analyzed. The optimal length of the time-window over which this exponent can be calculated in order to make some meaningful predictions is discussed. Our prediction hypothesis is verified with examples of 1929 and 1987 crashes, as well as with more recent phenomena in stock market from the period 1995 to 2003. Some interesting agreements are found. [Copyright &y& Elsevier]