Failure analysis of gas turbine transition pieces,leading to a solution for prevention

In this study the failure analysis of transition pieces of a gas turbine is investigated. Transition piece connects combustion chamber to the turbine and acts as a nozzle which leads hot gases to stationary blades of turbine. The problem of this transition piece in this gas turbine, which was common in other similar units, was cracks developing on the lower wall near the connection to the turbine. Study of gas turbine operation history, cracks apparent form microstructure analysis and fracture surface, revealed that the thermal fatigue was the main reason for the failure and also oxidation facilitated the crack propagation. In order to prevent the failure of transition pieces, it was proposed to create a row of holes, at 1 cm above the impaired region for local cooling and stopping probable cracks. Due to the implemented solutions, the failure of transition pieces and their annual repair are prevented.     

Investigation on Failure in Thermal Barrier Coatings on Gas Turbine First-Stage Rotor Blade

Failure in turbine blades can affect the safety and performance of the gas turbine engine. Results of coating decohesion, erosion and cracking at the first-stage high-pressure (HPT) blade working in gas turbine engine are being reported in this paper. This investigation was carried out for the possibility of various failure mechanisms in the thermal barrier coating exposed to high operating temperature. The blade was made of nickel-based superalloy, having directionally solidified grain structure coated with thermal barrier coatings of yttria-stabilized zirconia with EB-PVD process and platinum-modified aluminum (Pt–Al) bond coat with electro-deposition. The starting point of analysis was apparent coating decohesion close to the leading edge on the suction side of blade. The coating decohesion was found to be widening of interdiffusion zone toward the bond coat at higher operating temperature which could change the composition and induce thermal stresses in the bond coat. The erosion, cracking and decohesion of the coating on the pressure side was also observed during failure investigation. The erosion of the coating was coupled by two factors: one by increase in temperature as demonstrated by change in microstructure of the substrate and second by increase in coating inclination toward the trailing side. As a result of high operating temperature, swelling and thickening of TGO was observed due to outward diffusion of aluminum from the bond coat to form alumina (non-protective oxide) which causes internal stresses that leads to top coat decohesion and cracking. The possibility of hot corrosion was also investigated, and it was found that top coat decohesion did not involve this failure mechanism. Visual inspection, optical microscopy, scanning electron microscopy and energy-dispersive spectroscopy have been used as characterization tools.     

Failure Analysis and Remedy Procedure for Cracking of Fuel Nozzles in a Gas Turbine

A common failure in a certain type of gas turbine, observed during the first periodic inspection, is radial cracks in the tip plate of gas fuel nozzles. Here, each gas turbine has 18 nozzles. In all nozzles and in all similar units, these cracks of lengths ranging from 1 mm to a maximum of 14.5 mm are observed. As prescribed by the manufacturer, the defective part must be removed and replaced by welding and machining of a new one. But this problem is repeated and observed in the next periodic visits, and in all units. Depending on the number of nozzles in each gas turbine unit and the number of units in total, these repairs are very expensive and time-consuming. In this paper, the failure is analyzed and the causes of the cracks in the nozzles are investigated. Studies show that the main causes of nozzle failure are residual stresses caused by welding and thermal stresses caused by the start-up and shutdown processes. According to results, a solution has been proposed to release these residual and thermal stresses. After the implementation of this method in 1998, no more failure has been reported by the repair team, which proves the effectiveness of this solution. Since this paper has been prepared based on technical reports from the years between 1996 and 1998, the cited references of this paper are these technical reports.     

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 an un-cooled turbine blade in an aero gas turbine engine

Failure of an un-cooled turbine blade in an aero gas turbine engine is analyzed to determine its root cause. The operational condition of the engine was studied and metallurgical investigations are carried out on the fractured blade. The failure has originated from the leading edge and has propagated towards the trailing edge. Thermal cracks due to surface oxidation leading to fatigue were found to be the cause of the blade failure. Operation at elevated temperatures due to malfunction of sensors in the engine control system was found responsible for initiating the thermal cracks.     

Hot Corrosion of Protective Coatings

Improvement in efficiencies of gas turbine engines requires a significant increase of gas inlet temperatures. This results in an increased service temperature for blade materials and consequently in enhanced oxidation and hot corrosion attack of the blade coatings, which are usually of MCrAlY type where M is Ni, Co or NiCo. This type of coating can provide protection against oxidation and hot corrosion and act as a bond coat for thermal barrier coating systems. In both cases slow growth rates and optimum adherence of the alumina scales forming on the MCrAlY coatings during high temperature exposure are significant for component life. The above mentioned properties for the alumina scales strongly depend on the coating base composition as well as on the presence of minor alloying elements. In the present paper the performance of existing superalloys during hot corrosion is briefly described followed by the results obtained on hot corrosion of MCRAlY type coatings explaining the effect of trace elements on the life of coatings in the presence of NaCl and vanadium containing environments. Optimum thickness to improve the life of superalloys with NiCoCrAlY as a bond coat and yttria stabilized zirconia thermal barrier coatings has been identified. Based on the results, an electrochemical mechanism is proposed and shows that hot corrosion of protective coatings is an electrochemical phenomenon. Hence electrochemical techniques appear to be quite useful in evaluating the coatings for hot corrosion resistance.     

Effects of welding defects on reducing lifetimes of first stage nozzles of a 123 MW gas turbine made of FSX-414 alloy

In this paper, premature failure of one set of first stage nozzles (stationary blades) in a 123 MW gas turbine has been analyzed. Metallurgical and mechanical experiments showed that the main mechanism for such unexpected cracks is thermal fatigue phenomenon accelerated by presence of welding defects in nozzles. Based on the results, it is recommended that repaired nozzles be thoroughly checked before being re-installed in turbines. Moreover, it is recommended; based on the results; to coat nozzles by corrosion/oxidation resistant coatings after their welding by low chromium filler metals such as L-605.     

Resistance of thermal barrier ceramic coatings to hot salt corrosion

Stabilized zirconia ceramics appear to have considerable potential as thermal barriers or corrosion-resistant coatings for application to gas turbine components. Salt corrosion tests have indicated however that under some conditions vanadium impurities in the fuel may cause accelerated failure of these coatings as a result of destabilization of the zirconia phase. The corrosion resistance, thermal stability and structure of the coatings depend to a large extent on the chemical composition of the ceramic phase. The results of a study of the behavior of plasma-sprayed Y₂O₃ZrO₂ coatings towards combustion gases and the typical salt deposits likely to be encountered in gas turbines which burn fuels containing sodium, sulfur, lead and vanadium impurities are described.     

Failure analysis in after shell section of gas turbine combustion liner under base-load operation

The present study investigates the failure analysis and the lifetime prediction in the after shell section of gas turbine combustion liner with internal cooling passages called C-channel. To calculate distributions of temperature and stresses, 3D-numerical simulations using FVM and FEM commercial codes are performed. As a result, the discrepancy in thermal expansion between hot and coolant side walls induces high thermal stresses in the welding region and above the divider of the C-channel. Thus, these two regions are much weaker than the other regions. The locations match well to those of thermal cracks in actual gas turbine combustors in service.     

Corrosion Failures in Gas Turbine Hot Components

At elevated temperatures hot corrosion often takes place in gas turbines and other combustion turbomachinery. The main corrosion issues are high-temperature environmental attack in the form of high-temperature oxidation and hot corrosion. Two case histories of gas turbine hot components are presented. The mechanisms for different types of hot corrosion are discussed. The influence of temperature, composition, and microstructure of materials on hot corrosion is described. A variety of practical approaches to minimize hot corrosion are considered.     

Hot corrosion in gas turbine components

The macroscopic and microscopic characteristics as well as the proposed mechanisms of Type I (high-temperature) and Type II (low-temperature) hot corrosion are reviewed. Two case histories of gas turbine blade failures are presented. Different practical approaches to minimize hot corrosion are described.     

Fouling and corrosion of bed nozzles in a bubbling fluidized-bed power boiler

An inspection of the floor in a bubbling fluidized-bed power boiler during a planned shutdown revealed significant corrosion of the type 310H stainless steel (UNS S31009) bed nozzles. One severely corroded bed nozzle was removed, along with samples of the deposit encasing the surface of the bed nozzle, for a subsequent failure analysis. The deposit covering the external (fireside) surface of the bed nozzle was almost entirely sodium and potassium chlorides, which covered a thick layer of corrosion product on the bed nozzle surface. The corrosion product was predominantly oxides of chromium, nickel, and iron. Chlorides were also found interspersed through the layer of corrosion product. It is concluded that the fireside corrosion was predominantly caused by chlorine-containing compounds (active oxidation). Overheating induced by the fouling, particularly within the air holes, is believed to be a major factor influencing corrosion.     

浅析音速喷嘴装置中喷嘴间的相互影响

目前采用音速喷嘴作为传递标准的气体流量标准装置被国内外检测机构广泛使用。在使用时,将根据需要的流量使用单个或多个喷嘴组合。与国外音速喷嘴的实际应用相比较,国内装置在喷嘴的安装方式、结构排列和上游设计方面多有不同。在对装置进行不确定度评定时,需要考虑喷嘴间相互影响(包括数量和位置等)对流量不确定度的影响。本文基于国内两套音速喷嘴法气体流量标准装置,利用单个、多个喷嘴的组合,测得两组相近流量点下涡轮流量计仪表系数,计算出不同组合的仪表系数偏差。本文考虑了实验时各组实际体积流量差值的影响,利用曲线拟合的方法,拟合出涡轮流量计仪表系数特性曲线,给出拟合方程式。将流量不同带来的影响剔除,最终给出喷嘴间相互影响带来的偏差,两套装置得出由此影响带来的不确定度为0.02%和0.07%。因此,在对音速喷嘴装置进行不确定度分析时应考虑这一分量。此外,通过相近流量不同喷嘴的组合对流量计进行检测,可以发现和找出喷嘴的问题,也是一种装置期间核查的好办法。     

Corrosion in coal-fired gas turbines

The purpose of this paper is to review corrosion processes and experience related to coal-fired gas turbines. For over 40 years there have been major programmes of research directed towards burning coal in a gas turbine either directly or as a coal-derived fuel. This history is briefly reviewed, demonstrating the importance which corrosion of the hot section has had in limiting the achievements and defining the systems. The probable corrosion mechanisms are identified, and because of their synergistic interaction with corrosion, erosion and deposition are also considered. The discussion identifies pressurized fluidized bed combustion (PFBC) as the direct combustion technique most likely to be able to avoid serious corrosion problems in the immediate future. Recent results from PFBC pilot plant investigations related to turbine materials are compared, and thereby future directions for overcoming corrosion problems are proposed. MST/449     

6541B燃汽轮机叶片氧化腐蚀原因分析

利用扫描电镜、X-ray衍射仪及金相显微镜等分析方法,分析、研究了6541B燃气轮机在使用EC-2抑钒剂和镁基抑钒剂后,分别在叶片表面形成氧化产物的成分、结构分子式及其微观组织,得出该汽轮机叶片氧化腐蚀是由于重油中的钒形成了V2O5,致使叶片腐蚀.     

Lifing issues in hot gas path components of heavy-duty gas turbines

Abstract

Gas turbine hot-gas-path components, which include combustion liners, transition pieces, turbine nozzles and turbine buckets, are exposed to hot gases discharged from combustion systems and suffer from severe materials degradation and damage even in the early stage of operation. The severity of the damage and degradation increases with increasing inlet temperature and size of the gas turbines, which also increase the maintenance cost. ‘Lifing’ of components is, therefore, becoming a very critical issue. This paper describes several kinds of component damage and material degradation occurring in the 1,100°C- and 1,300°C-class heavy-duty gas turbines and then shows how we revised those component lives from the original design ones. Analytical-based assessment methods associated with condition-based assessment ones, some examples of assessment results, and component life extension technologies are also described.     

Verhalten von Gasturbinenschaufelwerkstoffen bei mechanischer Langzeitbeanspruchung unter Heißgaskorrosion – Teil I. Mechanisches Langzeitverhalten

Behaviour of Gas Turbine Blade Materials under Mechanical Long-Term Loading and Hot Gas Corrosion – Part 1. Mechanical Long-Term Behaviour The mechanical behaviour of several uncoated and coated nickel base superalloys for gas turbine blades was determined under typical hot gas corrosion conditions at relatively high temperatures. In case of severe corrosion, partly a decrease of creep rupture strength occurred which was no more explainable by the corrosion induced loss of cross sectional area. Under weaker corrosion conditions corresponding to usual gas turbine conditions this was not observed and coatings did not show premature failure. The LCF behaviour followed the creep rupture behaviour with an increased corrosion attack, however.     

Failure Analysis of High-Pressure Compressor Blade in an Aero Gas Turbine Engine

Failure of high-pressure compressor rotor blade in an aero gas turbine engine is analyzed to determine its root cause. Forensic and metallurgical investigations are carried out on the blade and failed parts. The failure of the platform ladder is found to the first in the chain of events that led to the compressor blade failure. The mode of failure in the blade is found to be fatigue and has originated from the damaged region on the leading edge caused by dislodgement of platform ladder. The failure has caused extensive damages in high-pressure compressor module and also in downstream turbine blades as a secondary effect.     

Hot corrosion: a modification of reactants causing degradation

Abstract

It has been conjectured that if sulfur in the fuel is removed, engine materials will cease to experience attack from hot corrosion since the fuel sulfur has been viewed as the primary source that has caused hot corrosion and sulfidation. Hot corrosion has been defined as an accelerated degradation process that is generally considered to involve deposition of corrosive species (e.g. sulfates) from the surrounding environment (e.g. combustion gas) to the surface of hot components, followed by subsequent destruction of the protective oxide scale. Most papers in the literature since the 1970s consider sodium sulfate as the single salt causing hot corrosion. There has been a push to remove fuel sulfur content to less than 15 ppm. However, sulfur species may still enter the combustion chamber via air intake or with seawater entrained in the air through the air intake of the ship. Seawater contains, in addition to sodium sulfate, magnesium, calcium, and potassium salts. Additionally, sulfate speciation and the content of atmospheric particulate matter (PM) varies considerably around the world and in some places, such as China and India, high PM seems to cause an increased risk of deposit-induced hot corrosion due to atmospheric pollutants rather than a combustion process (i.e. sulfur impurity in the fuel). The increasing operating temperatures of turbine engines and activities within regions of the world that have relatively high pollutant (PM, SO₂, C, Ca, etc.) levels are working conjointly to cause previously unobserved forms of high-temperature corrosion. This paper will cover some of our revised understanding of hot corrosion and consider other possible contaminants that could further complicate our understanding of hot corrosion.     

包装印刷设备的烘干风嘴分布参数规律研究

目的为了提升包装印刷设备烘干性能,针对风嘴分布进行研究。方法依据实际包装印刷设备风嘴为参考,建立风嘴分布模型并进行非结构网格划分,根据实际情况建立适用于其工况的边界条件和控制方程;控制风嘴间距和承印物距离这2个关键参数,建立风嘴分布参数与热风场性能之间的数学模型;依据所建立数学模型估算不同参数模型的烘干效果。结果根据多组仿真实验得到了风嘴间距和承印物距离等参数对热风场分布的影响,相应经验公式为v (=-0.007×L+2.568)x~(4.311/L+0.056)。风嘴间距对于烘干风嘴影响效果明显,增加风嘴间距离可以提升烘干风嘴风速和热风场的均匀性;同时发现随着承印物不断前进和风嘴作用效果的叠加,其表面风速将持续呈现上升。结论经过研究分析,掌握了不同参数下的热风场分布规律,这对指导实际生产对于烘干风嘴设计具有重要参考价值。