Investigation of Fatigue Failures of Titanium Alloy Blades Used in Compressor Modules of Aeroderivative Industrial Gas Turbines

Gas turbine axial compressor blades are designed to withstand cyclic stress loading resulting of modal vibrations. However, when surface suffers from foreign object damage, a fatigue crack is likely to initiate. In this paper two failure case-studies of titanium alloy used for aeroderivative gas turbine compressor blades are presented. Both are related to a failure of first stage compressor blades coming from different turbine power ranges. Observed cracks were found at blade spans corresponding to modal vibrations.     

A New Approach to Characterization of Gas Turbine Components Affected by Pitting Corrosion

Among gas turbine components such as compressor blades made of precipitation-hardened martensitic stainless steel, a phenomenon known as pitting corrosion can occur on the surfaces of components during exposure to a marine environment. This can lead to surface degradation followed by crack initiation and further propagation with high-cycle fatigue as the main mode of damage. In order to obtain detailed statistical characterization of pitting severity, which may enable obtaining preventive models of the phenomena, a new study on pit size analysis and pitting inspection technique is proposed. In this investigation, pit depth values are considered as the main factor behind this type of failures. This article presents the results of pit depth assessment and other pitting characteristics with the use of confocal microscopy as a non-destructive technique. The results obtained show that confocal microscopy can be successfully used as a non-destructive tool for the evaluation of pitting severity.     

Failure of the J79 Engine Compressor Blade Due to Stall

The fractured and damaged compressor rotor blades of a J79 engine have been examined. Optical, stereoscopic, microhardness testing, and SEM examinations were carried out to find out the causes of the fracture. The material of the blades was STS403 and blades were used without coating. From the 15th through the 17th compressor stages the rotor blades, stator vanes, combustion, and turbine sections were damaged. The fractured surface of the 17th blade show multiple origins, secondary cracking in the blade’s convex surface, and extensive propagation before separation and rougher surfaces. The metallographic analysis of the microstructure suggested work hardening. Based on the results, the cause of the fractured blade was high-amplitude fatigue, due to severe stall. After normal engine usage of 5 months, the blade fractured and blade fragments hit the other compressor blades, stators, and casing, and caused damage in both combustion and turbine sections.     

Vibration based damage detection of rotor blades in a gas turbine engine

This paper describes the problems concerning turbine rotor blade vibration that seriously impact the structural integrity of a developmental aero gas turbine. Experimental determination of vibration characteristics of rotor blades in an engine is very important from fatigue failure considerations. The blades under investigation are fabricated from nickel base super alloy through directionally solidified investment casting process. The blade surfaces are coated with platinum aluminide for oxidation protection. A three dimensional finite element modal analysis on a bladed disk was performed to know the likely blade resonances for a particular design in the speed range of operation. Experiments were conducted to assess vibration characteristics of bladed disk rotor during engine tests. Rotor blade vibrations were measured using non-intrusive stress measurement system, an indirect method of blade vibration measurement utilizing blade tip timing technique. Abnormalities observed in the vibration characteristics of the blade tip timing data measured during engine tests were used to detect the blade damage. Upon disassembly of the engine and subsequent fluorescent penetrant inspection, it was observed that three blades of the rotor assembly were identified to have damaged. These are the blades that exhibited vibration abnormalities as a result of large resonant vibration response while engine tests. Further, fractographic analysis performed on the blades revealed the mechanism of blade failures as fatigue related. The root cause of blade failure is established to be high cycle fatigue from the engine run data history although the blades were put into service for just 6 h of engine operation.     

Failure Prevention of Gas Turbine Blades Through Pack Aluminization

Pack aluminization of low-pressure turbine blade of an aero gas turbine engine has been carried out following a well-defined systematic procedure. The process parameters are first optimized on dummy blades, and optimized process is followed for the actual blades for evaluation and testing. Visual and binocular examination followed by metallurgical evaluation has been carried out to validate the process and to establish the adequacy and correctness of the coating. The coated blades are then evaluated through component-level test and engine-level test followed by field evaluation trials for performance and durability. The results of engine-level tests and inspection post-accelerated mission test cycles ensure that the blades with aluminide coating can withstand severe engine operating cycles without any damage or failure which otherwise would have experienced. The failure-free operation for an equivalent TBO life and post-AMT condition of blades are an indication of enhanced life of aluminide blades and prevention of failure of the turbine blades through pack aluminization.     

Possibility of increasing the service life of damaged blades

Results are given of an investigation of the effectiveness of a method of strengthening the working titanium alloy blades of a gas turbine engine compressor. It is established that the proposed strengthening method can lengthen to more than double the service life of damaged blades. In addition it is shown that in these blades, in comparison with nonstrengthened blades, cracks develop in a narrower zone and predominantly on the side of the inlet edge.Translated from Problemy Prochnosti, No. 4, pp. 45–49, April, 1992.     

重型F级燃气轮机IGV开度对压气机效率的影响

压气机是燃气轮机的重要设备,其性能的高低直接影响燃机最大输出能力及性能,而压气机进口导叶（inlet guide vane,IGV）开度的变化,对压气机的运行效率及性能实现具有重要影响。研究实际运行中燃机IGV开度的变化与压气机效率之间的关系,对压气机运行特性分析,机组运行效率及安全具有重要意义。对三菱M701F4燃机的压气机运行数据进行收集分析,通过实际运行中的数据分析发现,在外界环境压力及温度变化较小的情况下,IGV开度对压气机效率的影响存在一定的动态变化关系。当IGV开度达到某一个值之前,IGV开度对压气机的效率影响较大,而对压气机出口压力及温度的影响相对较小;随着IGV开度的增加,其对压气机出口压力及温度影响逐渐加强,直至IGV全开,压气机出口压力及温度达到稳定。     

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.     

Investigation of Efficiency Deterioration Causes in Process Centrifugal Compressor Operation

The high operating efficiency of centrifugal compressor is a basic requirement to maintain the gas productivity and machine availability. However, there are several factors influencing the stage efficiency in the operating environment leading to a deterioration in the compressor performance in addition to its impact on the mechanical integrity of the internal components. These variables can be classified into three main groups which are suction parameters variation, flow profile distortion, and compressor component damage. The determination of the root cause will help for a proper maintenance plan and to reduce the machine downtime. Hence, this paper will introduce a new approach to identify the inefficient compressor operation causes based on the available operation data. The investigated case is a three-stage gas transport centrifugal compressor driven by 2.9 MW two-shaft gas turbine. The outcomes from the conducted optimisation are compared with the measured discharge parameters and the findings from the internal inspection observation to emphasize the outcomes from the derived approach.     

Abrasion predictions for Francis turbines based on liquid–solid two-phase fluid simulations

The runner blades and guide vanes of Francis turbines are worn by sediment in the flow. However, there are few studies about abrasion of the runner blade and guide vane for normal turbine operating conditions. This study investigated the relation between the wear rates on the surfaces of the runner blade and guide vane and the sediment concentration, and analyzed the distribution of the wear rates for normal turbine operating condition. An Eulerian–Lagrangian Computational Fluid Dynamics (CFD) procedure was used to simulate steady liquid–solid two-phase flow for various operating conditions. The Finnie model was then used to predict the abrasion. The conditions leading to abrasion in the inner flow passage components of a Francis turbine are clarified through analysis of the abrasion conditions for the runner blades and guide vanes. Field tests and simulations show that the relative wear rate on the runner blades and guide vanes increases with increasing sediment concentration, and the maximum wear on the runner blades occurs in a small opening region with the maximum increasing as the head increases. The maximum wear on the guide vanes occurs at the maximum output and the relative wear rate on the runner blades is much greater than that on the guide vanes. There is no good data, so the relative wear rates on the runner blades and the guide vanes can only be obtained numerically. Thus actual wear rates cannot be given and are beyond the scope of this paper. This paper shows the abrasion characteristics on the runner blades and guide vanes with sediment flow and provides reference data for predicting the abrasion conditions in the flow passage components of a Francis turbine.     

Failure analysis of a set of compressor blades

This paper analyses the root causes of the failure of a set of blades belonging to the high pressure compressor of an aircraft engine. All these blades were manufactured using a 718 nickel base alloy. The performed study consisted in a fractographic analysis by scanning electron microscopy and a microstructural study using both scanning and optical microscopy. Phases which were present in the fracture surfaces were identified by means of X-ray energy dispersive spectrometry. As a result of this labour the failure was attributed to the impact of sand and stones; that is the so-called foreign object damage mechanism.     

INFLUENCE OF ADIABATIC SHEAR BANDS ON THE FATIGUE STRENGTH OF A TITANIUM ALLOY

An experimental investigation is reported of the possible effects of adiabatic shear bands, caused by projectile impact, on the fatigue strength of a titanium alloy (Ti-6%A1-4%V). No significant reduction in fatigue strength due to the presence of adiabatic shear bands was found, nor did the fatigue cracks initiate from the bands. The results are relevant to the problem of foreign object impact damage to compressor blades in gas turbine aeroengines.     

Second Stage Gas Turbine Blade Premature Replacement Investigation

This article investigates the root causes of the premature failure and replacement of a set of second-stage turbine blades from a heavy industrial gas turbine engine. The investigations included dye-penetrant testing, optical microscopy, X-ray diffractometry (XRD), Environmental Scanning Electron Microscopy (ESEM), and energy dispersive X-ray spectroscopy (EDS) techniques. Moreover, the effect of heat treatment process on restoring the blade microstructure so that the properties were suitable for service was also explored. As a result of the investigation, the second-stage turbine blades premature failure was attributed to the grain boundary secondary phase precipitates. These precipitates were present in the “as-found” condition of the investigated blades.     

A study on thermo mechanical fatigue life prediction of Ni-base superalloy

Gas turbine blades are exposed to high-temperature degradation environments due to flames and mechanical loads as a results of high-speed rotation during operation. In addition, blades are exposed to thermo-mechanical fatigue due to frequent start and shutdown. Therefore, it is necessary to evaluate the lifetime of blade materials.In this study, the TMF life of a Ni-base superalloy applied to gas turbine blade was predicted based on LCF and TMF test results. The LCF tests were conducted under various strain ranges based on gas turbine operating conditions. In addition, IP (in-phase) and OP (out of-phase) TMF tests were conducted under various strain ranges.Finally, a fatigue life prediction model was drawn from the LCF and TMF test results. The correlation between the LCF and TMF test results was also evaluated with respect to fatigue life.     

Phase reactions on surface of nickel-base alloys during tests and service

Abstract

X-ray diffraction analysis has been used to study the oxide phase composition of the surface of nickel-base alloys, which have different alloying levels under air conditions, and in tests with an ash film environment. Several stages of phase reaction and the directivity of phase reactions during oxidation have been revealed. The lattice parameter of the spinel oxide depends on the alloy content (mainly Cr/Al ratio). The surface structure of gas turbine blades after service in power plants and gas compressor stations has been examined. The different types of sulphate, oxide and sulphide compounds formed on the uncoated blade surface have been established. This suggested the development of sulphide-oxide corrosion process. The surface study of MCrAlY-coated blades revealed the formation of different types oxides during operation.     

Identification of failure mechanisms in nickel base superalloy turbine blades through microstructural study

Failures in gas turbine blades can occur by various mechanisms that are operative at high temperatures. Most frequent ones are the creep and stress rupture. These mechanisms are strongly dependent on the microstructure of the blades and often, they are facilitated due to microstructural degradation that occurs during service exploitation. The study of these failures, therefore, requires detailed microstructural examination. This paper reports the basic microstructural features in cast nickel base superalloys that control the failure mechanisms in gas turbine blades followed by analysis of two service failures. A methodology has been suggested for effective microstructural analysis of turbine blades after service exposure. It has been shown how systematic microstructural study on the turbine blades can be helpful in establishing engine operating conditions.     

Field Experience of Platinum Aluminide Coated Turbine Blades

This paper describes the results obtained from an evaluation of several platinum aluminide coated first stage turbine blades returned from the field. The IN-738LC blades had accumulated from 3,900 to 27,500 service hours in Centaur (1) 50 industrial gas turbine engines, operating in a high temperature oxidizing environment. The coating performance and condition were assessed using optical and electron microscopy. The condition of the coating was correlated to blade operating temperatures, which were estimated using the gamma prime coarsening technique. The degradation mechanism of the coating, remaining coating life, and blade repairability were also addressed.     

GE9FA燃气轮机进气加热系统的控制

当9FA燃气轮机压气机进口温度低时,为防止压气机进口结冰和超过运行极限,9FA燃气轮机的运行范围将受到限制。为了降低排放和防止压气机进口结冰,根据压气机进口可转导叶(IGV)来控制进气加热。压气机进气加热是将部分压气机的排气循环到压气机进口。该系统有三个作用:1)防止压气机进口结冰;2)通过降低IGV角度来满足低负荷时的排放要求,降低压气机压比;3)保证压气机有足够的安全裕度。进气加热系统保证了压气机的全范围运行。     

Cyclic strength of turbine blades made of cast nickel-based alloys

We study the cyclic strength and durability of service-exposed turbine blades made of IN-738, ZMI-3U, and éP539LM nickel alloys, some of them with a protective coating, upon a long operating time in gas turbine compressor sets. The blades of IN-738 alloy are found to have the highest cyclic strength. The cyclic strength of uncoated IN-738 blades is about 10% higher than that of coated blades. __________ Translated from Problemy Prochnosti, No. 2, pp. 5–14, March–April, 2007.     

Erosion failure mechanisms in turbine stage with twisted rotor blade

Blades of steam turbine are very important elements in power plant turbo aggregates. If blades of turbine fail, this will provoke further failures and high economical losses. Therefore, it is crucial to perform detailed research on reasons for failure of turbine blades to increase the reliability of turbine systems. The present paper deals with numerical simulation and research of erosion over blades in a low pressure stage of K-1000-6/1500 steam turbine working in a Nuclear Power Plant. Attention is paid to the effect of the amount of moisture in the stage; to the impact of droplets' diameter, their mass flow rate and forces acting on blade surfaces, to their aerodynamic behavior and influence on the energy conversion efficiency.Specific trajectories of water particles, reasons for the occurrence of erosion wear and erosion of certain parts of the streamlined surfaces are established and discussed. An approach to obtain incidence time to erosion effects appearance is formulated and implemented in the code. Research methodology and obtained results are applicable to determine specifics of erosion effects over streamed complex surfaces; replace expensive measurements campaigns; introduce approaches to decrease wetness in the last stages of condensation turbines and prolong the reliability of blades operated in wet steam conditions.