Vibration analysis for failure detection in low pressure steam turbine blades in nuclear power plant

This paper presents an investigation of the failure of a low-pressure steam turbine blade in a pressurized water reactor (PWR) nuclear power plant. The dynamical behaviour of the blade is analyzed theoretically and experimentally. A three-dimensional finite element model is used to predict the blade resonances in the operational speed range. Natural frequencies and mode shapes of the blade at static condition are obtained, then natural frequencies of the blade at different rotational speeds are calculated with consideration of centrifugal force and steam flow forces. A Campbell diagram is plotted to predict the likely operational conditions that may cause resonant vibration of the blade. Vibration tests are conducted to determine the vibration characteristic of the blade. It is found that the 2nd natural frequency of the blade is very close to the 9th rotor speed harmonic. The experimental natural frequencies are in good agreement with the finite element predicted values. Fretting wear is observed at the concave root surfaces of the blade trailing edge caused by resonant vibration. The fracture surface of the cracked blade shows typical fatigue patterns. The fretting wear characteristics in the crack initiation regions are observed.Stress distribution of the blade at the 9th harmonic frequency is analyzed using an elastic-plastic finite element model. Fretting fatigue experiments indicate that the fatigue life of the blade is greatly reduced due to fretting wear. The results of the investigation show that the failure of the blade is attributed to a combination of high cycle fatigue (HCF) and fretting wear.     

Failure analysis of the final stage blade in steam turbine

A failure case of the low pressure blades of steam turbine is presented in this paper. The suction side of blades has been quenched to improve the erosion resistance. Cracks with different lengths were found in the quenched region of final stage blades after about 13,200 h service. The failure analysis of blades was performed in terms of composition analysis, microstructure and mechanical tests, etc. The yield strength and tensile strength conform to the corresponding standard, whereas the elongation, area reduction and impact toughness are lower than the criteria. From the crack morphology, fractography and composition analysis on the fracture surface, it was found that the failure mechanism of blades is the environment-assisted fatigue fracture. The location of fatigue crack initiation is related with the salient of blades due to the stress concentration. In order to decrease the blade cracking susceptibility, the increment of tempered temperature in both modified treatment and high-frequency hardening was recommended.     

Failure analysis of steam turbine last stage blade tenon and shroud

This paper presents an analysis of the cause of steam turbine blade fractures. Recently, several L-0 blades 28.5 (725 mm) long of a steam turbine fractured 5 in. (125 mm) from the blade root platform, causing the forced outage of the turbine. A finite-element analysis (FEA) of the blade was carried out in the beginning of the last decade to calculate the natural frequencies and de vibratory stresses on the blade. A telemetry test was also conducted. The current investigation analyzed the operational data during the last two years, reviewed the results of previous studies, conducted metallurgical investigations, and identified the mechanical and metallurgical modes of the failure. The results of the investigations showed that improper welding of the shroud to the blade was the principal cause of blade fracture.     

Erosion-fatigue of steam turbine blades

The premature failure of steam turbine rotor blades, manufactured in forged 12% Cr–NiMoV martensitic stainless steel, was investigated using visual inspection, non-destructive testing, macro and microfractography, microstructural characterization, EDS microanalysis, chemical analysis, micro hardness and tensile testing. The blades belonged to the last stage of a thermoelectric plant steam turbine generator (140 MV A). The results indicated that the failure of the blades was promoted by foreign-particle erosion, which attacked preferentially the low-pressure side of the lower trailing edge of the blades. The resulting wear grooves acted as stress raisers and promoted the nucleation of fatigue cracks, which probably grew during the transition events of the steam turbine operation. Finally, water drop erosion was observed on the blade upper leading edge (low-pressure side).     

Failure assessment of the first stage high‐pressure turbine blades in an aero‐engine turbine

In this paper, microscopic analysis and fatigue experiment were conducted to detect the damage degree of turbine blades after a 600 h service and determine the reliable service time. First, several service blades were cut into slices on different cross sections and microscopically examined. It is found that the γ′ phase particles are slightly coarsened and the γ′ phase parameters change with temperature and stress distribution. Then, fatigue test was conducted on service blades simulating the real working condition. The test result shows that the blades after a 600 h service would serve for 3596 h more until failure during actual flight. The ultimate fracture is mainly caused by the interaction of fatigue, creep and oxidation. Besides, the γ′ phase parameters change obviously compared with service blade without fatigue test. It indicates that the γ′ phase parameters could be used to evaluate the microdamage of service blades, which has great significance for service reliability of the turbine machinery.     

Failure probability estimation of steam turbine blades by enhanced Monte Carlo Method

Steam turbines are designed to work in stable operating conditions, including speed and load, to avoid mechanical stress variations. However, sometimes failures occur in the turbine components. The components having major breakdowns for fracture, an average of 75%, are the blades of the Low Pressure (LP) stage steam turbine. These blades produce around 10% of the output power turbines and 15% in some applications of combined cycle; generally longs, with a relatively low stiffness and such blades may present problems of high stress due to centrifugal forces. In this work probabilistic design procedure was applied to the group of ten blades of the LP stage steam turbine of 110 MW, in order to compute the stress changes and reliability due to variations in: damping, natural frequencies, vibration magnitude and density. The computed vibration stresses were analyzed by applying probability distributions and statistical parameters of input and output to compute the useful life. Monte Carlo technique and stochastic finite element method (SFEM) were applied. The results show that the Monte Carlo technique and SFEM are a good approach to estimate the useful life and reliability design of those blades.     

Axis position measurement for steam and gas turbine blades

Translated from Izmeritel'naya Tekhnika, No. 2, pp. 25–26, February, 1990.     

Failure analysis of gas turbine blades in a thermal power plant

A case study of failure analysis of a 40 MW gas turbine blade made of Udimet 500 is presented. The cause of failure is found to be intergranular cracks which started during exposure to high temperature. The cracks initiated from the grain boundaries and propagated to the critical length to result in catastrophic fracture. In many locations M₆C type secondary carbides were found agglomerated on grain boundaries. Also micro-cavities were found on fracture surfaces which served as an origin of creeping failure mechanism.     

Performance test and analysis after high pressure turbine retrofit for Maanshan nuclear power plant Unit 1

The purpose of the performance test for Unit 1 of Maanshan nuclear power plant was to determine the electrical output and heat rate after the retrofit of the high-pressure turbine during the refueling in 2012. The performance test was conducted in order to verify that the actual improvement in electrical output resulting from the replacement of the high-pressure turbine meets the vendor’s guarantee. A total of two performance test runs was conducted in accordance with the American Society of Mechanical Engineers performance test code (PTC) 6. The measured electrical powers for the two test runs were 977.4 and 975.0 MWe, respectively, and the average value was 976.2 MWe. After correcting the electrical power to the rated conditions specified in the performance test procedure, the gross electric output was 983.2 MWe. The corrected heat rate for the two performance tests were 10365 and 10353 kJ/kWh, respectively. The deviation between two corrected heat rates was 0.11%. Since the acceptable deviation between two test runs required by PTC 6 is no more than 0.25%, the quality of test results is acceptable. Moreover, the performance test results also demonstrated that the improvement in gross electrical output was 17.6 MWe, which was higher than the contract guarantee of 10.0 MWe.     

Failure of a low pressure turbine rotor blade of an aeroengine

During a test run of an aeroengine, a low-pressure turbine rotor blade had failed. The turbine blades were made of Ni-base superalloy of CM 247 LC grade and fabricated by DS investment casting. The blades were coated with platinum aluminide. Investigation revealed that the blade had failed by fatigue. It was concluded that the coating on the blade had developed cracks due to excessive bending/vibration, which in turn propagated by fatigue leading to the failure.     

T-root blades in a steam turbine rotor: A case study

The present work illustrates, 3D finite element analysis (FEA) of low-pressure (LP) steam turbine bladed disk assembly are carried out at a constant speed loading condition. The prime objective is to study structural integrity of bladed disk root with aid of design considerations at design stage. Secondly, design rules are developed for structural integrity of blades and disk considering a factor of safety for material, manufacturing and temperature uncertainties. These design rules are in turn used as design checks with aid of finite element analysis results.Investigations are performed based on Neuber formulae for solving a highly non-linear problem employing linear analysis tool ANSYS 12.0. Local peak stresses at blade and disk root fillet of linear analysis is used to identify the equivalent non-linear stress value by strain energy distribution method for estimating the minimum number of cycles required for crack initiation for low cycle fatigue (LCF) calculations.Design methodology is developed to address the structural integrity of blades at design point and for off-design conditions.     

Failure of last-stage turbine blades in a geothermal power plant

This paper presents an investigation into causes of failure of geothermal steam turbine blades. Several L-0 blades of geothermal steam turbines of 110 MW capacity suffered failures, causing forced outages of the turbines. To assess the causes of failure, the natural frequencies of the blades installed on the rotor were measured in the laboratory. The measured frequencies were compared with the natural frequencies calculated through a finite-element analysis (FEA) of the blade. The FEA was also used to calculate the vibratory stresses on the blade numerically. Also, the investigation analyzed the operational data and the history of the blade failures on several rotors of different units from the same system. The results of previous repairs were reviewed, and metallurgical investigations were conducted to identify the mechanical and metallurgical modes of failure. The results of the investigation showed that the fracture of two blades was attributed to installation and manufacturing errors and aggravated by general deterioration of the blades. The deterioration was caused by the erosion and corrosion process that resulted from moisture condensation in the steam.     

Failure analysis of a second stage blade in a gas turbine engine

The failure of a second stage blade in a gas turbine was investigated by metallurgical and mechanical examinations of the failed blade. The blade was made of a nickel-base alloy Inconel 738LC. The turbine engine has been in service for about 73,500 h before the blade failure at 5:50 PM on 14 August 2004. Due to the blade failure, the turbine engine was damaged severely. The investigation was started with a thorough visual inspection of the turbine and the blades surfaces, followed by the fractography of the fracture surfaces, microstructural investigations, chemical analysis and hardness measurement.

The observation showed that a serious pitting was occurred on the blade surfaces and there were evidences of fatigue marks in the fracture surface. The microstructural changes were not critical. It was found that the crack initiated by the hot corrosion from the leading edge and propagated by fatigue and finally, as a result of the reduction in cross-section area, fracture was completed.

An analytical calculation parallel to the finite element method was utilized to determine the static stresses due to huge centrifugal force. The dynamic characteristics of the turbine blade were evaluated by the finite element modal and harmonic analyses. Finally according to the log sheet records and by using a Campbell diagram there was a good agreement between the failure signs and FEM results which showed the broken blade has been resonated by the third vibrational mode occasionally before the failure occurred.     

Failure analysis of Ti6Al4V gas turbine compressor blades

The failure mechanism of Ti6Al4V compressor blades of an industrial gas turbine was analysed by means of both experimental characterisations and numerical simulation techniques. Several premature failures were occurred in the high pressure section of the compressor due to the fracture of the blade roots. Metallurgical and mechanical properties of the blade alloy were evaluated. A 2D finite element model of the blade root was constructed and used to provide accurate estimates of stress field in the dovetail blade root and to determine the crack initiation in the dovetail.

The results showed no metallurgical and mechanical deviations for the blade materials from standards. SEM fractography showed different aspects of fretting fatigue including multiple crack initiation sites, fatigue beach marks, debris particles, and a high surface roughness in the edge of contact (EOC). The numerical model clearly showed the region of highest stress concentration at the front EOC of the blade root in the dovetail region, correlated closely with the experimentally characterised fatigue crack region. It was concluded that this failure has occurred due to the tight contact between the blade root and the disk in the dovetail region as well as low wear resistance of the blade root.     

Stress concentration around the shroud hole of steam turbine blades

Results of study of the stress-strain state around shroud holes of steam-turbine rotor blades are presented. Methods of volumetric photoeleasticity with freezing were used for experiments on models of blade elements in the form of plates with oblique holes passing through the plates which are diamond-shaped in the plan of their thickening. The angle between the hole axis and thickening face plane was varied while that between this axis and plane of the plate was fixed. Dependences between thickening parameters and maximum stress values are obtained. These facilitate selection of an optimum version of a structure during design.Translated from Problemy Prochnosti, No. 8, pp. 34–37, August, 1990.     

Causes of damages to fan blades of the generator of a steam turbine

It is shown that the fracture of blades of the fan of a TGV-200 generator is induced by corrosion fatigue intensified by residual hydrogen accumulated in steel in the process of electroplating of a cadmium coating on the surface of blades. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 109–112, May–June, 1997.     

Failure investigation of an auxiliary steam turbine

This paper presents the results of failure investigation of an auxiliary steam turbine in a power plant. Fractures were occurred at the lacing wires in the L1 blade cascade. The failure was occurred in repaired stages of blades after 47 days of an overhaul operation period. Visual inspection showed some regular fractures in the improper brazed joints and dimensional analysis showed that the lacing wire holes in the blades of the L1 stage are smaller than the originals. Fractographic investigation of fractured surface showed that the lacing wires had been exposed to a fatigue stress phenomenon. Finite element analysis showed that there is a high stress critical point near the brazing regions in comparison with original elements. Vibration analysis was performed experimentally and computationally to find the probable intersection points between the excitation harmonics and natural frequencies of blade cascade. Experimental test results verified the FEM analyses with good agreements. Obtained results from harmonic response analysis showed an approximate resonant condition of L1 blades during the operation of boiler feed pump turbine.     

汽轮机叶片断裂失效分析

利用电子显微镜对汽轮机叶片断裂原因进行分析,结果是疲劳断裂。