The 20.65 MW gas turbine experienced catastrophic damage. The failure occurred at the first stage buckets and resulted in damage of the all buckets of this stage. Five rotor disc grooves were also seriously damaged. Additionally, all second stage buckets, first and second stage nozzles, shroud segments, the No 2 bearing casing (turbine side), compressor moving blades, and other elements were damaged. Due to urgent power generation needs, it was decided to repair a seriously damaged stage 1 rotor disc in-situ, and replace all the other damaged parts. The development of a propietary welding technology for the in-situ repair of the five damaged disc grooves without disc disassembly, and of in-situ disc grooves’ mechanized machining is fully described. The repair process included the removal of damaged grooves, method of groove restoration by welding deposition, stress relief and groove machining to recover their original geometry. After rotor disc repair and assembly, the rotor was put back into service. The approach to the repair of the rotor disc damage has been successful. It enabled significant reductions in expenditure on replacement parts and a reduction of outage time to be achived.

A major failure event was experienced at a 44 MW plant powered by four aeroderivative gas turbines arranged in two units, property of the Federal Commission of Electricity (CFE). The failure consisted of total fracture in the shaft coupling between the generator and free-turbine. Unit 2 has a twin pack configuration with two aero derivative Pratt&Whitney 20 MW gas turbines coupled to one generator at both end sides. The “A” side generator coupling was completely damaged as well as the coupling configuration at the free turbine. Failure analysis showed as root cause, an abnormal configuration of the coupling systems between the free turbine to rotor generator at side “A”. This side had an additional shaft component to compensate a longer coupling distance between the turbine and generator. This was longer than the original distance, generating additional dynamic forces during operation leading to a fatigue failure mechanism. The replacement coupling configuration for the rotor generator was different than the Original Equipment Manufacturer (OEM). The new (non-OEM) spare rotor generator was shorter in the longitudinal direction than the original one, forcing the addition of a new shaft in one side of the generator. This work describes the rehabilitation process of the generator coupling by the replacement of the old configuration by a new redesigned coupling. This was done keeping the original configuration distances and components for both end shaft sides of the rotor generator. The paper includes the redesigned couple analysis by finite element method and the in-situ activities for the installation of the new couple in the rotor generator.

This paper presents creep damage evaluation in the service exposed air cooled first stage blade 1100 °C class of a gas turbine after 24000 operation hours. The blade is made of Inconel 738 LC Nickel based superalloy. The gas turbine inlet temperature (TIT) is 1100 °C. To get blade operational load, a thermomechanical analysis was performed using the Finite Element Method (FEM) including centrifugal stresses and thermal stresses. Blade airfoil temperature distribution obtained from previous Computational Fluids Dynamics (CFD) analyses was used for thermal stress determination in the blade. The effect of multi-axial stresses has been taken into account. Using the thermomechanical stress level value obtained and its distribution on the blade airfoil, some creep life prediction models were evaluated including the Norton-Bailey, Dorn-Bailey and Larson-Miller Parameter, comparing them to real bucket life. On the basis of results obtained, a new analytical model for gas turbine blade creep life prediction is proposed, which includes the influence of blade material ultimate tensile strength to reflect heat-to-heat variation in strength. The results obtained were validated to real bucket life and found in a good concordance to experimental creep data for an Inconel 738 LC super alloy.

A compressor blade failure was experienced on a 69 MW gas turbine of a combined cycle (C.C.) unit after four years operation since its last overhaul (January 2005). The unit accumulated 27,000 service hours and 97 start-ups since the last overhaul. This unit consists of four gas turbine stages and 19 compressor stages and operates at 3600 rpm. In 2006, the unit was equipped with a fogging system at the compressor air inlet duct to increase unit power output during high ambient temperature days (hot days). These fog water nozzles were installed upstream of the compressor inlet air filter without any water filter/catcher before the water spray nozzles. Three unit failure events occurred within a small time period, which caused a forced outage. The first failure occurred in December 2008, a second event in March 2009 and the third event in May 2009. Visual examination carried out after the first failure indicated that the compressor vanes (diaphragms) had cracks in their airfoil initiating at the blade tenons welded to the diaphragm outer shroud at stages 3, 8, 9, 10 and 11. Also, a number of stationary vanes and blades at each stage of the compressor showed foreign object damage (FOD) and fractures at the airfoil. Visual examination performed for the second failure event after 60 unit operating hours indicated that many compressor vanes (diaphragms) and blades had FOD at the airfoil. This was attributed to fractures caused by the fogging system. The water spray carried over in the compressor flow path at high velocity causing the FOD damage. Visual examination completed upon the third failure event after two unit startup attempts indicated damage of compressor stationary vanes and blades, principally at stages 12 to 16, and also stages 17 to 19. The damage consisted of airfoil fracture in the stationary vanes and blades, FOD, blade tip rubbing, and bending of the stationary vanes, blades and diaphragm shrouds. A laboratory evaluation of stationary vane tenon fracture indicated a high cycle fatigue (HCF) failure mechanism, and crack initiation was accelerated by corrosion pitting on blade surfaces due to high humidity air generated by the fogging system. Stationary vane damage was caused by a rotating stall phenomenon, which generates vibratory stress in stationary vanes and blades during unit start-ups. During the third failure event, the stationary vane HCF damage was highly accelerated due to pre-existing partial fractures in the tenons generated during previous failure events which had not been detected by non-destructive tests. Stationary vane and moving blade failure was also influenced by high tenon brittleness in stationary vanes and blades generated during manufacture by welding the diaphragms, and repair welding the blades without adequate post-weld heat treatment (stress relieving). A compressor stationary vane and blade failure evaluation was completed. This investigation included cracked blade metallographic analysis, unit operation parameter analysis, history-of-events analysis, and crack initiation and propagation analysis. This paper provides an overview of the compressor failure investigation, which led to the identification of the HCF failure mechanism generated by rotating stall during unit start-ups, highly accelerated corrosion generated by the fogging system, and high brittleness in the stationary vanes and blades as the primary contribution to the observed failure.

A compressor blade failure was experienced at the 69 MW gas turbine of a combined cycle (C.C.) unit after four years operation since the last overhaul (January 2005). The unit accumulated 27,000 service hours and 97 start-ups since the last overhaul. This unit consists of four gas turbine stages and 19 compressor stages and operates at 3600 rpm. In 2006, the unit was equipped with a fogging system at the compressor air inlet duct to increment unit power output during high ambient temperature days (hot days). These fog water nozzles were installed upstream of the compressor inlet air filter without any water filter/catcher before the water spray nozzles. Three unit failure events occurred at small periods, which caused forced outage. The first failure occurred in December 2008, a second event in March 2009 and the third event in May 2009. Visual examination carried out after the first failure event indicated that the compressor vanes (diaphragms) had cracks in their airfoils initiating at blade tenons welded to the diaphragm outer shroud at stages 3, 8, 9, 10 and 11. Also, many stationary vanes and moving blades at each stage of the compressor showed foreign object damage (FOD) and fractures at the airfoil. Visual examination performed for the second failure event after 60 unit operation hours indicated that many compressor vanes (diaphragms) and moving blades had FOD at the airfoil. This was attributed to fractures of the fogging system water spray nozzle, which were then induced to the compressor flow path channel at high velocity causing the above-mentioned damage. Visual examination completed upon the third failure event after two unit startup attempts indicated damage of compressor stationary vanes and moving blades principally at stages 12 to 16, and also stages 17 to 19. The damage consisted of airfoil fracture in stationary vanes and moving blades, FOD, moving blade tip rubbing, and bending of stationary vanes, moving blades and diaphragm shrouds. A laboratory evaluation of stationary vane tenon fracture indicated a high cycle fatigue (HCF) failure mechanism, and crack initiation was accelerated by corrosion picks on blade surfaces due to high humidity air generated by the fogging system. Stationary vane damage was caused by a rotating stall phenomenon, which generates vibratory stresses in stationary vanes and moving blades during unit start-ups. During the third failure event, stationary vane HCF damage was highly accelerated due to pre-existent partial fractures in tenons generated during previous failure events, which had not been detected by non-destructive tests. Stationary vane and moving blade failure was also influenced by high tenon brittleness in stationary vanes and moving blades generated during manufacture by welding (diaphragms) and repair welding (moving blades) without adequate post-weld heat treatment (stress relieving). A compressor stationary vane and moving blade failure evaluation was completed. This investigation included cracked blade metallographic analysis, unit operation parameter analysis, history-of-events analysis, and crack initiation and propagation analysis. This paper provides an overview of the compressor failure investigation, which led to identification of the HCF failure mechanism generated by rotating stall during unit start-ups, highly accelerated by corrosion generated by the fogging system and influenced by high stationary vane and moving blade brittleness as the primary contribution to the observed failure.

During a major overhaul of a 30 MW geothermal turbine for the electricity sector of Mexico, a new rotor and new diaphragms were provided to recovery the power rate of the unit. Recuperation of the horizontal joint of upper and lower casings was required, as well as the inner diameter for diaphragm lodgings in both casings. The Geothermal steam turbines in Mexico suffer of accelerate degradation due mainly to high levels of sulphidric acid in the geothermal steam flow. Corrosion and dust particles are often present in the flow path turbine components affecting the performance and maintenance. In addition, the paper includes topics for on-site rehabilitation which include complex activities and challenges that must be taken into account for a successful rehabilitation process. The mapping process for horizontal joint casing, the processes used for recovery of planarity, the post weld heat treatments used for these components, and the machining process used for recovery inner diameters of casings.

This paper presents the thermal and fluid dynamic analysis of the gas turbine transition piece, applying the Finite Volume Method (FVM) through the Computational Fluid Dynamics (CFD). The study is carried out to examine the flow field and distribution of temperatures of the combustion gases along the transition piece and exit mouth, getting profiles and contours of velocity and temperature. This study is important to know the paths of flow and distribution of temperatures of the hot streaks through the transition piece, which impact on cooling system in stator and rotor. Also, these flow field and distribution of temperature have an effect in performance and life of the vanes and blades in the first stage of the turbine, principally by the difference of heat load. The study was carried out in a steady state three-dimensional model to avoid the geometric simplifications, using code FLUENT® version 6.3.26 where the k-ε turbulence model was applied and different boundary conditions in the inlet of the transition piece were considered. To obtain the results, a structured grid about 5.1 millions of cells with second-order upwind scheme and coupled solver was applied. The results show the effect of the velocity and temperature along the transition piece and exit mouth due to the change of the curved section. In the exit mouth of the transition piece is identified a dimensionless peak temperature for about 1.019 in a point near to 68% of the radial edge, while in the circumferential direction the peak temperature is about 1.027 in a point near to 50% of the circumferential edge with symmetry profiles.

This paper provides an overview of a steam turbine low pressure blades failures induced by flow excitation and blades torsional vibrations due to sudden changes on the grid. The analysis include L-0 and L-1 blades failures of the 110 MW [1], 28 MW [2] and 35 MW [3] geothermal units induced by unstable flow due to operation at low load low vacuum, and L-0 blades failure of the 660 MW [4] nuclear unit due combined effect of a transient phenomenon provoked by sudden load changes on the grid and some low vacuum operation period. The failure initiation was registered at different zones of the blades depending on the case, and were localized at the blades cover segments, blade root and blade airfoil close to the platform. Laboratory evaluation of the blades fracture surface indicates the failure mechanism to be high cycle fatigue (HCF).

The unsteady aerodynamic and aero-thermal performance of a first stage gas turbine bucket with thermal barrier coating (TBC) and internal cooling configuration were investigated by application of a three dimensional Navier-Stokes commercial turbomachinery oriented CFD-code. Convection and conduction were modeled for a super alloy blade with TBC. The CFD simulations were configured with a mesh domain including the nozzle and bucket inter-stage in order to accurately predict the fluid parameters at inlet and outlet of bucket. Comparisons to the gas turbine manufacturer data have permitted to validate the flow conditions at the inlet of the rotor. The effects of blade TBC surface temperature changes during a start-up cycle were simulated by means of an unsteady simulation, with unsteady inlet/outlet boundary conditions specified according to test data. The calculations include not only the fluid but also the solving of conduction within the blade, allowing for a correct modeling of the large difference of thermal inertia between the fluid and solid. The role of thermal barrier coatings (TBC) is, as their name suggests, providing thermal insulation of the blade. A coating of about 100–400 μm can reduce the temperature by up to 200°C. A TBC can be used either to reduce the need for blade cooling (by about 36%) increasing the turbine efficiency, while maintaining identical creep life of the substrate; or to increase considerably the creep life of the blade while maintaining level of blade cooling (and therefore allowing the blade to operate at lower temperature for an identical turbine inlet temperature).

The unsteady aerodynamics and aerothermics of a first stage gas turbine bucket with thermal barrier coating (TBC) and internal cooling configuration were investigated by application of a three dimensional Navier-Stoke commercial turbomachinery oriented CFD-code. Convection and conduction were modeled for a super alloy blade with TBC. This work is the second part of the paper “Unsteady 3-d conjugated heat transfer simulation of a thermal barrier coated gas turbine bucket”, and includes the simulation of shut down cycle. The CFD simulations were configured with a mesh domain of nozzle and bucket inter-stage using real turbine parameter data as boundary condition (BC) at nozzle inlet. The BC’s were adjusted in accordance with standard start-up and shut-down diagram for a gas turbine from Takahashi work [3]. Additionally a parabolic turbine inlet temperature was set for main gas flow. The problem was launched in a minicluster of 8 HP Workstation. Reasonably good comparisons in the main flow parameters with the manufacturer data were obtained. The effects of blade TBC surface temperature changes during a start-up and shut-down cycle were simulated using the Spalart-Allmaras turbulence model. A TBC can be used either to reduce the need for blade cooling (by about 36%) increasing the turbine efficiency, while maintaining identical creep life of the substrate; and increase considerably the creep life of the blade while maintaining level of blade cooling (and therefore allowing the blade to operate at lower temperature for an identical turbine entry temperature).

As a gas turbine entry temperature (TET) increases, thermal loading on first stage blades increases too and therefore, a variety of cooling techniques and thermal barrier coatings (TBCs) are used to maintain the blade temperature within the acceptable limits. In this work a multi-block three dimensional Navier-Stokes commercial turbomachinery oriented CFD-code has been used to compute steady state conjugated heat transfer (CHT) on the blade suction and pressure coated sides of a rotating first inter-stage (nozzle and bucket) with cooling holes of a 60 MW Gas turbine. A Spallart Allmaras model was used for modeling the turbulence. Convection and radiation were modeled for a super alloy blade with and without TBC. The CFD simulations were configured with a mesh domain of nozzle and bucket inter-stage in order to predict the fluid parameters at inlet and outlet of bucket for validate with turbine inter-stage parameter data test of gas turbine manufacturer. The effects of blade surface temperature changes were simulated with both configurations coated and uncoated blades.

A last stage turbine blades failure was experienced in two units of 660 MW. These units have one high-pressure turbine and two tandem-compound low-pressure turbines with 44-inch last-stage blades. The blades that failed were in a low pressure (LP) turbine connected to the high pressure (HP) turbine (LP1) and in LP turbine connected to the generator (LP2). The failed blades had cracks in their roots initiating at the trailing edge, concave side of the steeple outermost fillet radius. Laboratory evaluation of the cracking indicates the failure mechanism to be high cycle fatigue (HCF). The last-stage blades failure evaluation was carried out. The investigation included a metallographic analysis of the cracked blades, natural frequency test and analysis, blade stress analysis, unit’s operation parameters and history of events analysis, fracture mechanics and crack propagation analysis. This paper provides an overview of this failure investigation, which led to the identification of the blades torsional vibrations near 120 Hz and some operation periods with low load low vacuum as the primary contribution to the observed failure.

Excessive erosion of the low-pressure shaft end gland seal of a 25 MW geothermal turbine has been investigated. Due to excessive erosion of the gland seal rotor surface the turbine vacuum was partially destroyed and the efficiency of the cycle degraded. This study uses computational fluid dynamics (CFD) to identify the causes of erosion and the optimal steam seal system flow conditions for reducing the erosion problem. The predictions were based upon a numerical calculation using a CFD model of the rotor end gland seal with a steam flow containing hard solid particles and solved with the commercial CFD code: Adapco STAR-CD. The results confirmed the existence of flow conditions that play a major role in the rotor gland seal erosion. Afterwards, the flow was simulated changing seal steam flow conditions (flow pressure). It was confirmed that there exists threshold seal flow conditions below which erosion does not occur. The recommendations for adjusting shaft end gland seal system are provided to avoid erosion problem.

The assessment of service induced degradation of nickel-base alloy Inconel 738LC of the gas turbine blade airfoil hot section, after 24000 hours of operation at high temperature is presented. The assessment include the blade coating degradation, changes of the gamma prime (γ′) phase (aging and coarsening), carbides and brittle phases precipitation, grain type and size characterization, and evaluation of interface coating/base alloy and cracks. The results of blade hot section (airfoil) microstructural assessment are compared to the cold reference zone (bucket root) to show the degradation degree of the alloy and its lifetime consumed. Finally, conclusions are presented indicating the alloy main microstructure elements which influence of blade behavior and the factors that can be used to determine the grade of material deterioration and lifetime consumed.

Steady-state analysis of heat transfer in a base-load power generation gas turbine was conducted by thermal conjugation inside and outside of the first stage nozzle, which consists of thermal convection and conduction by coupling of fluid flow and solid body. A computer CFD code was used to solve the problem. The principal issues of the work were concerned with three-dimensional behaviors of the temperature distribution of the nozzle vane, which are influenced by inlet gas flow conditions, internal cooling conditions and film cooling conditions. The numerical results of the effects of cooling flow rate and temperature on heat transfer rates in the nozzle are also presented. The paper focuses on the estimation of the temperature distribution on the nozzle vane by prediction of the thermal environment around the nozzle vane and heat conduction in the nozzle which is necessary to carry out the nozzle thermal load analysis and finally life assessment. Also, the evaluation of service induced degradation of cobalt base alloy FSX-414 of the nozzle, after 24000 hours of operation at high temperature is presented. The assessment includes the nozzle carbides precipitation and grain type and size characterization.

In-site machining of a high-pressure casing of a 75 MW steam turbine, which experienced severe deformation after a long period of service is fully described. The casing deformation was measured using an optical instrument (transit) which revealed a bottom casing concavity of about of 4.4 mm and top casing convexity about of 2.8 mm. This deformation caused rotor-stator rubbing, an increase in steam path radial clearances, turbine efficiency deterioration and maintenance problems. To restore the casing horizontal joint surface flatness in the top and bottom casing halves, a special portable milling machine tool was used, which was supported directly on the casing. Also due to its deformation, the casing central guide bores of the diaphragms carriers needed to be re-machined. This was made by using a special rotary arm supporting an electrical angle grinder which was centred individually for each guide bore. The casing restoration was made without any needs for welding. Due to the machining of the horizontal joint, the casing joint surface was displaced approximately 6 mm in the vertical direction without needing to change the turbine rotor axis position. After the re-machining, the casing was put back into service reaching the design operational conditions without any problems.

A last stage (L-0) turbine blades failure was experienced in a 110 MW geothermal unit after one year of operation period. This unit has two tandem-compound intermediate/low-pressure turbines (turbine A and turbine B) with 23-inch/3600 rpm 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 Hz to 588 Hz induced due to unstable flow excitation (stall flutter) as the primary contribution to the observed failure.

Solid particle erosion in a turbine nozzle (turbine control stage) has been investigated by means of CFD. Literature attempt to couple fluid mechanics and erosion modeling and improvements in the hydrodynamics models together with improvements in the erosion models are reviewed. The solid particle bearing steam flow through the nozzle was investigated using a 3D numerical model and the finite volume code Fluent V6.0.12, looking for a reduction of the erosion process. The flow simulation was carried out for the original and modified (nozzle) designs with changes of the angle of particle impact on the nozzle surface. Numerical predictions have been carried out using the Renormalization Group ( RNG ) k -ε turbulence mode. To account for the influence of turbulent fluid fluctuations on particle motion, the stochastic tracking Discrete Random Walk model is used, which includes the effect of instantaneous turbulent velocity fluctuations on the particle trajectories. The removal of wall material due to erosion is calculated using the Finnie model developed for ductile materials. The numerical predictions showed a 50 percent reduction of the erosion rate for the modified (nozzle) design due to changes of the particles trajectories and impingement angle (angle of particle impact). The results obtained show that numerical simulation can be used in a predictive manner to solve a real practical design problem.