Flow-accelerated Corrosion Wear Research Articles

Investigation into Factors Causing Damage to Low-Pressure Loop Evaporating Tubes of Large-Capacity Heat-Recovery Steam Generators

The article addresses factors causing damage—in particular, flow accelerated corrosion wear (FAC) of tube walls—to the evaporator operating in the low-pressure loop of the Ep-258/310/35-15.0/3.14/0.44-540/535/263 (P-132) heat-recovery steam generator used as part of a 795-MW combined-cycle plant. Factors influencing the FAC, namely, improper water chemistry, availability of gas shunts in the steam generator, and high mixture motion velocity in the tubes in combination with the flow path configuration, are described. The damages inflicted to the tubes and their locations in the heat-recovery steam generator low-pressure loop are described. It is shown that gas shunts in the heat-recovery steam generator result in an increased heat absorption and higher velocity of medium in the boundary tubes of low-pressure evaporator tube banks, which intensifies the tube’s wear process. It has been found that the water chemistry used for the Kirishi District Power Plant (DPP) heat-recovery steam generator was not the factor that caused damage to the low-pressure loop tubes. The Kirishi DPP steam generator was compared with other similar large-capacity heat-recovery steam generators. For analyzing the low-pressure evaporator performance indicators, the loop and the processes occurring in it were modeled using the Boiler Designer software. An analysis of the results from comparison of the steam generator design and performance characteristics has shown that it is necessary to change the two-phase mixture motion velocity in the Kirishi DPP steam generator low-pressure loop by modifying the design or operating parameters. The wear of the low-pressure evaporator tube walls was mathematically modeled, the results of which confirm that erosion wear is one of the main factors causing damage to the tubes. The erosion is caused by the intense dynamic effect of two-phase, high-velocity flow jets on the tube walls. General recommendations for decreasing the wear of heat-recovery steam generator tubes are given. It has been determined that the increased wear of tubes in the low-pressure loop of the P-132 steam generator at the Kirishi DPP is caused by a combination of a few factors, such as high velocity of steam–water mixture, availability of bends, and unsatisfactory quality of aligning the tubes at their welding places.

Методические особенности конечно-элементного исследования прочности трубопроводов водо-водяных энергетических реакторов с учётом локальных утонений от воздействия эрозионно-коррозионных процессов

The methodological features pertinent to strength analysis of the pipelines used in the secondary coolant circuit of nuclear power plants (NPPs) with taking into account local thinning caused by flow accelerated corrosion processes are addressed. The geometric features of pipelines are essential not only for assessing the flow accelerated corrosion intensity in metal, but also for the final strength assessment, in particular, in analyzing the strength of pipelines with walls containing local thinning spots. The pipeline structure lifetime prediction accuracy depends directly on the strength analysis of pipeline elements in which local thinning spots have occurred, especially if these elements themselves have a complex shape. Elbows, sockets, adapters (transitions), and shell rings can be referred to such elements. All of them are thin-walled structures having a complex shape. Software systems like ERCO and PELBOW are best suited for numerically studying them. These computer programs are based on the shell finite element method, using which the thickness at each finite element node can be specified either in the form of certain dependencies or (in some cases) manually. In accordance with the 3D finite element theory of thick-walled shells, a pipeline section can be approximated by a mesh of finite isoparametric 3D elements of a thick shell. The flow accelerated corrosion wear (FACW) depends essentially on the chemical composition of steels. Domestically produced Grade 10Kh9MFB steel is compared with foreign Grade P91 steel (that is close to the former in chemical composition and in performance characteristics), which has positively proven itself and widely applied for making NPP pipelines, with a view to recommend Grade 10Kh9MFB steel for use.

Local Flow-Accelerated Corrosion of Welds in NPP Power Unit Pipelines: Phenomenon Mechanism Features and Measures to Prevent Damage

Flow-accelerated corrosion of metal caused by single- or two-phase flow of working fluid leads to thinning and destruction of welds in NPP power unit pipelines. Field experience gained from the operation of Russian and foreign nuclear power plants testifies that there have been many cases of damages inflicted to welds due to local flow-accelerated corrosion, the number of which tends to grow with the time for which the power units have been in operation. The article gives examples of the most typical places and characteristic features pertinent to the way in which the flow-accelerated corrosion of weld elements' metal takes place. The role and influence of the chemical composition of the weld metal and heat-affected zone metal on the location and rate of local flow-accelerated corrosion thinning in welds are determined. The flow hydrodynamic characteristics having an essential effect on the rate of flow-accelerated corrosion processes and the local thinning occurrence places are determined. Results from numerical modeling of the flow-accelerated corrosion weld thinning development process for different contents of chromium in the weld and heat-affected zone metal are given. The possibility of increasing the flow-accelerated corrosion rate in pipelines downstream of a weld with a preceding extended straight pipeline segment made of metal not prone to flow-accelerated corrosion is analyzed. Methods aimed at preventing the occurrence of damage to pipelines of NPP power units due to flow-accelerated corrosion are discussed. The article gives information on the results of activities for modifying the standard programs for in-service inspection of pipelines at VVER-based NPPs and also on the use, at Russian NPPs, of software systems providing support to the personnel on optimizing the arrangement and scheduling of in-service monitoring of flow-accelerated corrosion wear of heat-affected zones in welds of the pipelines used at NPP power units equipped with VVER-440, VVER-1000, and BN-600 reactors. It can be expected that the application of regularly updated software systems for supporting the personnel will make it possible to timely prevent inadmissible thinning and abrupt flow-accelerated corrosion-induced failures of pipeline welds in the power units of Russian nuclear power plants.

Flow-Accelerated Corrosion Wear of Power-Generating Equipment: Investigations, Prediction and Prevention: Part 3. Managing the Flow-Accelerated Corrosion of Pipelines and Equipment

The third part of this review1 discusses matters concerned with managing the flow-accelerated corrosion of power-generating equipment’s and pipelines' components. The importance of these matters tends to increase with increasing the capacity of power plant units and the time for which they have been in operation. The article considers the capabilities of object-oriented software packages aimed at providing information support to the personnel (PSSPs) in regard to scheduling in-service inspections for timely revealing cases of close-to-inadmissible flow-accelerated corrosion-induced thinning. The prospects of using PSSPs for minimizing the carryover of iron-containing products of general flow-accelerated corrosion into the process circuit of NPP units are shown. The list of parameters and characteristics necessary for numerically estimating the thinning rates and residual life of pipelines and equipment susceptible to flow-accelerated corrosion is determined. The possibility of using some PSSPs to predict the thinning of process circuit component walls taking into account the effect of cavitation and droplet impingement erosion on the metal is pointed out. The need to optimize the scheduling of in-service inspections of near-seam zones of welded connections used in the pipelines of the condensate–feedwater and wet steam paths of power units is substantiated. It is pointed out that work is underway for fitting the Russian NPP units with PSSPs able to define— based on calculation results, data of in-service inspections, and other information—a priority list of components to be subjected to first-priority or next scheduled in-service inspection and to schedule repair (or replacement) of components susceptible to intense thinning. It is shown that the use of stainless steels for making the components of pipelines and equipment operating in the secondary circuit of NPP power units is often an excessive measure for preventing the occurrence of inadmissible local flow-accelerated corrosioninduced thinning and for minimizing the concentration of iron in feed water. The prospects for comprehensively solving the flow-accelerated corrosion problems by implementing appropriate measures at the designing, construction, and operation stages are considered. The main topical practical objectives for coping with the problem of flow-accelerated corrosion in power engineering are formulated.

Flow-Accelerated Corrosion Wear of Power-Generating Equipment: Investigations, Prediction, and Prevention: 2. Prediction and Prevention of General and Local Flow-Accelerated Corrosion

The second part of this review considers physicochemical models and computer codes used for predicting flow-accelerated corrosion wear of power generating equipment. Approaches used to prevent the occurrence of general and local flow-accelerated corrosion that are based on selecting metals resistant to flow-accelerated corrosion and adjusting the water chemistry of power units are also discussed. The existing computer codes use physicochemical models of flow-accelerated corrosion and statistical data on damages inflicted to power units due to flow-accelerated corrosion processes. Advantages and drawbacks of different analytical physicochemical models describing the flow-accelerated corrosion process are pointed out together with the specific features of using them in elaborating flow-accelerated corrosion computing codes. It is shown that the processes lying at the heart of the flow-accelerated corrosion mechanism include, on the one hand, the occurrence of a protective oxide layer on the metal surface and, on the other hand, the dissolution of this layer and carryover of dissolution products in the flow. Differences between the processes through which metal undergoes flow-accelerated corrosion in a single-phase water flow and in a two-phase wet steam flow are analyzed. Thus, the redistribution of admixtures and gases between the phases that takes place in two-phase media may cause a change in the pH values, thereby significantly influencing the flow-accelerated corrosion rate. In addition, the rate with which flow-accelerated corrosion products are carried over into a two-phase stream depends on the liquid film flow mode on the streamlined surface. The flow-accelerated corrosion rate computing codes most widely known around the world, including the COMSY code (Germany), CHECWORKS SFA code (United States), BRT-CICEROTM code (France), and RAMEK code (Russia) are considered. Their specific features and application limits are pointed out. Information on the effect the content of chromium, molybdenum, and copper has on the flow-accelerated corrosion rate is given. It is shown that the choice of metals resistant to flow-accelerated corrosion is a combined technical and economic problem, and the way in which it is solved has an effect on the safety and reliability of power unit operation. It is pointed out that the liquid phase pH value is essentially affected by the steam wetness degree if the latter exceeds 20%.

Flow-Accelerated Corrosion Wear of Power-Generating Equipment: Investigations, Prediction, and Prevention: 1. Flow-Accelerated Corrosion Processes and Regularities

The first part of this review considers the prerequisites and scales of damage inflicted to the equipment and pipelines of power plant units as a consequence of flow-accelerated corrosion, and examples of accidents involving lethal outcomes that occurred at nuclear power plants as a result of pipeline ruptures caused by flow-accelerated corrosion are given. It is shown that the scope of items susceptible to flow-accelerated corrosion includes components of condensate–feedwater and wet steam path equipment and pipelines made of carbon and low-alloy steels. The main negative consequences caused by flow-accelerated corrosion are local metal thinning spots that can lead to abrupt failures and depressurization of the process circuit and contamination of working fluid with iron containing products of flow-accelerated corrosion. Sedimentation of these products in a steam generator is one of factors causing damage to and failure of its heat transfer tubes. It is proposed to draw a distinction between general and local flow-accelerated corrosion. General flow-accelerated corrosion causes ingress of iron-containing compounds into the working fluid and is characterized by a moderate metal thinning rate that does not lead to destructions or occurrence of wormholes involving loss of process circuit leak tightness. The effects of local flow-accelerated corrosion manifest themselves in small parts of intricately shaped channels and are characterized by a significant metal thinning rate with possible occurrence of wormholes or abrupt destruction of the pipeline and power-generating equipment components. Typical cases of and statistical data on damages inflicted to nuclear power plant components due to flow-accelerated corrosion are presented. The physicochemical fundamentals and regularities relating to flow-accelerated corrosion of metal occurring in one- and two-phase flows are considered. It is pointed out that the hydrodynamic factor plays the determining role in the occurrence of zones and rate of local thinning caused by flow-accelerated corrosion. The key hydrodynamic characteristics influencing the local flow-accelerated corrosion rate are determined. Fundamental differences between the mechanisms governing flow-accelerated corrosion in two-phase medium and single-phase water flow are shown. These differences stem from the specific features of hydrodynamics pertinent to the motion of liquid film and interphase redistribution of admixtures and gases that gives rise to a change in the pH value of liquid. The results from experiments aimed at studying the effect of temperature on the flow-accelerated corrosion rate of different metals in a two-phase wet steam flow and the effect of steam wetness degree on the liquid film flow mode are presented.

Flow-accelerated corrosion 2016 international conference

The paper discusses materials and results of the most representative world forum on the problems of flow-accelerated metal corrosion in power engineering—Flow-Accelerated Corrosion (FAC) 2016, the international conference, which was held in Lille (France) from May 23 through May 27, 2016, sponsored by EdF-DTG with the support of the International Atomic Energy Agency (IAEA) and the World Association of Nuclear Operators (WANO). The information on major themes of reports and materials of the exhibition arranged within the framework of the congress is presented. The statistics on operation time and intensity of FAC wall thinning of NPP pipelines and equipment in the world is set out. The paper describes typical examples of flow-accelerated corrosion damage of condensate-feed and wet-steam pipeline components of nuclear and thermal power plants that caused forced shutdowns or accidents. The importance of research projects on the problem of flow-accelerated metal corrosion of nuclear power units coordinated by the IAEA with the participation of leading experts in this field from around the world is considered. The reports presented at the conference considered issues of implementation of an FAC mechanism in single- and two-phase flows, the impact of hydrodynamic and water-chemical factors, the chemical composition of the metal, and other parameters on the intensity and location of FAC wall thinning localized areas in pipeline components and power equipment. Features and patterns of local and general FAC leading to local metal thinning and contamination of the working environment with ferriferous compounds are considered. Main trends of modern practices preventing FAC wear of NPP pipelines and equipment are defined. An increasing role of computer codes for the assessment and prediction of FAC rate, as well as software systems of support of the NPP personnel for the inspection planning and prevention of FAC wall thinning of equipment operating in singleand two-phase flows, is accepted. Different lines of attack on the problem of FAC of pipelines and equipment components of existing and future nuclear power units are reviewed. Promising methods of nondestructive inspection of pipelines and equipment are presented.

A Numerical Study on Flow-Accelerated Corrosion in Two Adjacent Elbows

Flow-Accelerated Corrosion (FAC) is a well-known degradation mechanism that attacks the secondary piping in nuclear power plants. Since the Surry Unit 2 event in 1986, most nuclear power plants have implemented management programs to deal with damages in carbon and low-alloy steel piping. Despite the utmost efforts, damage induced by FAC still occurs in power plants around the world. In order to predict FAC wear, some computer programs were developed such as CHECWORKS, CICERO, and COMSY. Various data need to be input to these programs; the chemical composition of secondary piping, flow operating conditions and piping geometries. CHECWORKS, developed by the Electric Power Research Institute (EPRI), uses a geometry code to calculate geometry effects. Such a relatively simple geometry code is limited in acquiring the accuracy of FAC prediction. Recently, EPRI revisited the geometry code with the intention of updating it. In this study, numerical simulations were performed for two adjacent elbows and the results were analysed in terms of the proximity effect between the two adjacent elbows.

Use of software tools for calculating flow accelerated corrosion of nuclear power plant equipment and pipelines

The results obtained from calculations of flow accelerated corrosion of equipment and pipelines operating at nuclear power plants constructed on the basis of PWR, VVER, and RBMK reactors carried out using the EKI-02 and EKI-03 software tools are presented. It is shown that the calculation error does not exceed its value indicated in the qualification certificates for these software tools. It is pointed out that calculations aimed at predicting the service life of pipelines and efficient surveillance of flow accelerated corrosion wear are hardly possible without using the above-mentioned software tools.

Experimental investigation of flow accelerated corrosion under two-phase flow conditions

The main objective of this paper is to experimentally study the effect of two-phase flow on flow-accelerated corrosion (FAC) downstream an orifice. FAC is a major safety and reliability issue affecting carbon-steel piping in nuclear and fossil power plants. This is because of its pipe wall wearing and thinning effects that could lead to sudden and sometimes catastrophic failures, as well as a huge economic loss. In the present study, FAC wear of carbon-steel piping was simulated experimentally by circulating air–water mixtures through hydrocal (CaSO4·1/2H2O) test sections at liquid superficial Reynolds number, Re=20,000, and different air mass flow rates. Experiments were performed for a test section with different orifice to pipe diameter ratios (do/D=0.25, 0.5 and 0.74). The observed flow patterns were compared with the available flow pattern maps. Surface wear patterns downstream the orifices were also analyzed. The maximum FAC wear was found to occur at approximately 2–5 pipe diameters downstream of the orifice. The obtained results were found to be consistent with those from a single-phase flow study reported earlier. Moreover, FAC was found to depend on the relative values of the mixture mass quality and the volumetric void fraction. Lower values of FAC wear rate were obtained for higher values of mass quality. A modified correlation is developed in order to predict FAC wear rate downstream of the pipe-restricting orifice with an average RMS accuracy of ±10%. However, the location of maximum wear rate is well predicted. The current study is considered as an integrated effort to develop guidelines to FAC engineers in power plants in order to prepare more reliable plant inspection scope.

Flow and mass transfer downstream of an orifice under flow accelerated corrosion conditions

Local flow parameters play an important role in characterizing flow accelerated corrosion (FAC) downstream of sudden area change in power plant piping systems. Accurate prediction of the highest FAC wear rate locations enables the mitigation of sudden and catastrophic failures, and the improvement of the plant capacity factor. The objective of the present study is to evaluate the effect of the local flow and mass transfer parameters on flow accelerated corrosion downstream of an orifice. In the present study, orifice to pipe diameter ratios of 0.25, 0.5 and 0.74 were investigated numerically by solving the continuity and momentum equations at Reynolds number of Re=20,000. Laboratory experiments, using test sections made of hydrocal (CaSO4·½H2O) were carried out in order to determine the surface wear pattern and validate the present numerical results. The numerical results were compared to the plants data as well as to the present experiments. The maximum mass transfer coefficient found to occur at approximately 2–3 pipe diameters downstream of the orifice. This location was also found to correspond to the location of elevated turbulent kinetic energy generated within the flow separation vortices downstream of the orifice. The FAC wear rates were correlated with the turbulence kinetic energy and wall mass transfer in terms of Sherwood number. The current study found to offer very useful information for FAC engineers for better preparation of plant inspection scope.

Methodology for predicting flow-accelerated corrosion wear using unreferenced multiple inspection data

The lack of fixed spatial referencing in inspection data is a confounding factor in the assessment of wall thinning due to flow-accelerated corrosion (FAC). Because of the loss of wall thickness over time, it is impossible to match any two unreferenced points precisely between inspection outages, making the use of point-to-point comparison methods highly uncertain. In this study, we develop a new methodology for estimating the FAC rate and component lifetime from spatially unreferenced inspection data. The method is based on a small circular patch that represents the minimum wall thickness as a random variable. This allows the predictions to be performed in a probabilistic setting, hence providing valuable risk information for long-term replacement planning and life-cycle management. The uncertainty and errors introduced by inspection and grid coverage are minimized in the analysis by fitting a continuous surface through the inspection data using the method of kriging. The developed methodology is applied to the assessment of carbon steel feeder piping in CANDU reactors, where the economic and safety consequences of FAC can be highly significant. Based on the analysis of a large industry dataset, FAC in feeder piping appears to be a relatively uniform and stationary process. The results of the study demonstrate the key advantages of the developed methodology and how it can be used to support risk-informed decision making with respect to FAC at power plants.

Evaluation of the proximity effect on flow-accelerated corrosion

Flow-accelerated corrosion (FAC) is a degradation mechanism that affects carbon steel piping in power plants. The failures and degradation due to FAC have necessitated numerous replacements in many power plants. Several computer codes around the world were developed as part of a systematic program or process to control FAC in power plant utilities. The typical plant model requires the input of the flow parameters, piping configuration and the plant water chemistry. The results on FAC rate are considered the key to proper selection of components for inspection. The lack of information on the effect of the upstream components located in the proximity limited the accuracy of the FAC prediction tools and hence will affect the accuracy in identifying potential inspection locations. In the present study 211 inspection data for 90° carbon steel elbows from several nuclear power plants were used to determine the effect of the proximity between two components on the FAC wear rate. The effect of the velocity as well as the distance between the elbows and the upstream components is discussed in the present analysis. Based on the analyzed trends obtained from the inspection data, significant increase in the wear rate of approximately 70% on average is identified to be due to the proximity.

A two-phase methodology to predict FAC wear sites in the piping system of a BWR

Flow accelerated corrosion (FAC) wear is a serious degradation problem especially for nuclear power plants since it may result in the plant damage as well as risk to personnel. In this paper, a methodology which includes the two-phase hydrodynamic CFD models and FAC models, is proposed to predict severe FAC wear sites. Based on hydrodynamic simulation results, the present CFD models can precisely capture the two-phase characteristics within the piping system, which include the centrifugal effect, the gravitation effect and the imbalance of phase and mass separation in a T-junction, etc. Coupled with these flow characteristics, the appropriate FAC indicators can predict the possible locations of severe FAC wear. This methodology was validated against the measured results of wear site distributions for the piping system in a boiling water reactor (BWR) power plant. Good agreement between measurements and predictions at severe wear sites indicate that the present models can capture the characteristics of severe FAC wear and can help assist in the pipe wall-monitoring program for a nuclear power plant.