Boiler Water Wall Tubes Research Articles

Comprehensive Inspection Solution for Boiler Water Wall Tube Corrosion

A substantial number of domestic coal-fired boilers, most of which have been in operation for numerous years, are susceptible to corrosion and tube ruptures within their high-temperature environment, often resulting in unplanned shutdowns. Despite the annual planned maintenance shutdowns, which involve routine inspections of water wall and furnace tubes to detect potential corrosion and erosion, the current non-destructive testing methods are generally localized. This approach, particularly with the considerable height variation of the water wall tubes (ranging from approximately 10 to 25 meters), may overlook segments with severe corrosion during localized ultrasonic thickness gauging. Although the experience of inspection teams reduces the chance of defect oversight, the demand for a comprehensive and effective inspection technique remains a priority for boiler operators. This study utilizes the characteristic of guided waves to propagate over a certain distance along pipelines, initiating an exploration into a comprehensive inspection solution for water wall tubes. Finite element simulation results demonstrate the suitability of utilizing a 120 kHz frequency for shear horizontal mode (SH0) testing on the fire side of water wall tubes. This mode effectively screens for localized corrosion within a 3-meter range. In the field of on-site boiler inspection, electromagnetic coils are employed to generate guided waves within water wall tubes. The gathered signals from each electromagnetic guided wave testing (EMGWT) are consolidated into a distribution map, indicating suspicious signal locations. Pulse eddy current testing (PECT) is employed to identify potential issues with guided waves without requiring surface preparation. If PECT reveals anomalies, the affected surface is polished, and phased array ultrasonic testing (PAUT) is conducted to quantitatively measure the remaining wall thickness due to corrosion. The theoretical exploration and on-site inspection outcomes affirm the feasibility of this approach, aimed at enhancing operational safety for boiler water wall tubes and mitigating the risks of tube ruptures and leaks. This comprehensive inspection solution amalgamates three advanced detection technologies, each capitalizing on its unique inspection characteristics to facilitate corrosion screening, corrosion scanning, and wall thickness sizing. Through the integration of these technologies, the overall inspection strategy ensures the utmost level of reliability.

Numerical investigation of boiler waterwall corrosion by integrated tube temperature prediction and sulfur evolution model

High temperature corrosion has become one of the leading causes of boiler waterwall tube failures because of the wide application of air-staged combustion in coal-fired boilers. High temperature corrosion is strongly affected by the H2S concentration and wall tube temperature distributions in the furnace. Particularly, the corrosion rate increases exponentially with the increase of tube temperature. However, by far most of the numerical studies on high temperature corrosion only considered the effects of H2S concentration while the critical effects of tube temperature were overlooked. Thus, this paper presents a comprehensive corrosion model that incorporates the key effects of both tube temperature and H2S concentration. Specifically, the tube temperature is predicted by a coupled combustion and hydrodynamic model that integrates the critical effects of both the boiler gas flow and steam flow on waterwall tube temperature. The H2S concentration is predicted by a sulfur evolution model describing the devolatilization process of coal sulfur species and their subsequent gaseous reactions. The tube temperature prediction model and H2S evolution model provide all the key parameters for accurate prediction of boiler waterwall corrosion. The corrosion model is applied to a 350 MW supercritical wall-fired boiler. The results show that the tube temperature varies significantly over the boiler waterwall, and hence, has significant impacts on the distribution of wall tube corrosion. Particularly, local high tube temperature zones could form on the waterwall areas of relatively low H2S concentration leading to substantially higher tube corrosion rates than the adjacent wall areas. The results demonstrate the importance of incorporating both the wall tube temperature and H2S concentration in the prediction of boiler waterwall corrosion.

Degradation modelling and lifetime assessment for boiler waterwall with incomplete inspection data

Thermal fatigue cracking is a common problem for boiler waterwall tubes in power plants, resulting the risk of unscheduled plant shutdown. Developing an effective degradation model to predict the future condition of waterwall tubes is vital to assess boiler remaining life. Even though many numerical studies are available in the literature for understanding the physical mechanism (thermal fatigue cracking), the development of predictive models for future risks that can inform decision making are limited. One of the key impediments for developing predictive models is the lack of data. In the case of boiler critical subsystems, very few failures (if any) are recorded in its history. In addition, the number of inspections is limited due to access difficulties. Thus, in many practical cases, the condition data available for degradation modelling is sparse with both infrequent and (spatially) incomplete inspections. This paper presents the development of a stochastic degradation model prediction of thermal fatigue cracking severity in a boiler waterwall when the data is limited. The time evolution of the condition indicator for each waterwall tube is modelled using Markov Models and Bayesian identification algorithms are used to tractably address the large amount of missing data. The methodology presented in this paper employs a Markov Chain Monte Carlo (MCMC) scheme to account for the parameter uncertainty in the presence of missing data. Moreover, this paper also incorporates a novel grouping strategy in which the neighbouring components are grouped to tackle the problem of (spatial) data sparsity. The degradation modelling and prediction tools developed in this paper will support renewal decision making by using available onsite asset data and knowledge. A real-world case study of a boiler waterwall operating in an Australian power industry is also presented. It is found that the waterwall degradation appears to be slowing, but an additional inspection is recommended to confirm the trend.

Condition Assessment of Boiler Water Wall Tubes using Robotics

Boiler tube failures continues to be the primary cause for forced outages in boilers. To get the boiler back on line and reduce (eliminate) future forced outages due to tube failure, it is extremely important to determine and correct the failure root cause. Detecting and sizing flaws before they cause failures is of critical importance in boiler maintenance. Localized and general wall thinning due to corrosion & erosion in boiler water-wall tubing is a significant inspection concern for boiler operators. There are at least four (4) other methods used for the inspection of Boiler Water walls. These methods are Spot Check UT, A-Scan UT, EMAT, and Scanning Thermograpy. Spot Check UT only gives thickness readings and gets very minimal coverage of the total surface area of the furnace water walls; the chances of finding I.D. flaw mechanisms using Spot Check UT are minimal at best. If Boiler Water walls have been sandblasted, A-Scan UT may be used to inspected larger areas of the furnace walls; in these cases, a steady flow of water is most often used as the couplant. The EMAT technique requires that any Boiler Water wall surfaces be sandblasted. This paper will present a discussion on the deployment of robotic wall crawler using electromagnetic technique to inspect boiler water walls from outside of the tube together with the theoretical background of the technique which explains the quantitative nature of the inspection. Further, case studies will be presented for the technique that allows the extraction of tube wall thickness information from the inspection data.

Experimental and Theoretical Study on CHF of a Ultra-Supercritical Circulating Fluidized Bed Boiler Water-Wall Tube at Near-Critical Pressures

Experimental and Theoretical Study on CHF of a Ultra-Supercritical Circulating Fluidized Bed Boiler Water-Wall Tube at Near-Critical Pressures

A coupled combustion and hydrodynamic model for the prediction of waterwall tube overheating of supercritical boiler

Tube overheating is the leading cause of boiler tube failures that had led to tremendous replacement and maintenance costs. Boiler tube temperature is simultaneously affected by the heating effect of hot combustion gas in the furnace and the cooling effect of steam flow in boiler tubes. Thus, prediction of tube temperature needs to include the simulations of both the gas and steam flows of the boiler in the model. To compromise the significant scale difference between the boiler furnace and tubes, a coupled model that integrates a three-dimensional (3D) CFD model describing the gas flow and combustion processes in the furnace with a one-dimensional (1D) hydrodynamic model describing the steam flow in the boiler waterwall tubes was developed in this paper. The 3D CFD and 1D hydrodynamic models are coupled by iterative data exchange of boiler heat flux and steam temperature between the two models. In this way, this coupled model can incorporate all the key parameters that strongly affect the waterwall tube temperature, including the distributions of boiler heat flux, steam temperature and the convective heat transfer coefficient of steam flow in boiler tubes. In particular, the critical effects of steam maldistribution caused by both the structure of tube network system and uneven heat absorption of parallel tubes on tube temperature can be resolved by this model. This coupled model provides the capability to predict the detailed distribution of waterwall tube temperature and locate the areas of tube overheating under different boiler operating parameters. This coupled model was employed to predict the tube temperature distribution of the spiral waterwall of a 350 MW supercritical boiler. The predicted results show that the areas having higher risk of tube overheating are located near the outlet of spiral waterwall tubes where the steam temperature is the highest and on the wall areas where the local wall heat flux is the highest. The results strengthen the necessity of incorporating both the gas and steam sides of boiler into the model for accurate prediction of waterwall tube temperature.

Experimental Verification of a New Method for Determining the Thermal Load of the Combustion Chamber Waterwalls of Power Boilers

The aim of the furnace chamber calculations is to determine the heat flux absorbed by the walls along the furnace chamber height and the temperature of flue gases at the chamber outlet. This paper presents the boiler furnace chamber calculation methodology using the zone method. A new design is also presented of the thermometric insert intended for the determination of the heat flux absorbed by the walls of the boiler furnace chamber. The inserts are mounted at different levels of the chamber and operate in conditions similar to those of the boiler waterwall tubes. The results of the zone method-based calculations of the furnace chamber of a boiler with the steam output of 210 × 103 kg/h are compared with the results obtained from measurements performed with the use of thermometric inserts. Calculations are also carried out of the thermal load and of the flue gas temperature along the furnace chamber height for a supercritical boiler with the steam output of 2400 × 103 kg/h. The calculation results are compared with the measured values obtained from the boiler manufacturer.

Hydrogen Embrittlement of a Boiler Water Wall Tube in a District Heating System

A district heating system is an eco-friendly power generation facility with high energy efficiency. The boiler water wall tube used in the district heating system is exposed to extremely harsh conditions, and unexpected fractures often occur during operation. In this study, a corrosion failure analysis of the boiler water wall tube was performed to elucidate the failure mechanisms. The study revealed that overheating by flames was the cause of the failure of the boiler water wall tube. With an increase in temperature in a localized region the microstructure not only changed from ferrite/pearlite to martensite/bainite, which made it more susceptible to brittleness, but it also developed tensile residual stresses in the water-facing side by generating cavities or microcracks along the grain boundaries inside the tube. High-temperature hydrogen embrittlement combined with stress corrosion cracking initiated many microcracks inside the tube and created an intergranular fracture.

Steam Temperature Characteristics in Boiler Water Wall Tubes Based on Furnace CFD and Hydrodynamic Coupling Model

With the development of power plant units of higher capacity and with improved parameters, the proportion of high-capacity units for generating power has increased; this requires large capacity units to take responsibility for power-grid peak shaving. When the boiler operates at low loads, the working fluid in the boiler water wall tubes is subjected to high heat flux in the furnace, which can cause heat transfer deterioration, tube overheating or even leakage. Therefore, it is particularly important to study the reliability of the boiler hydrodynamic cycle during peak shaving (low load operation). This study takes a 1000 MW ultra-supercritical single-reheat Π type boiler with single furnace and double tangential firing as the research object. A furnace Computational Fluid Dynamics (CFD) and hydrodynamic coupling analysis model is established and verified according to the actual operating conditions on site. The calculation results show that the simulated value is in good agreement with the actual operating value. Therefore, the model established in this study can reflect the real situation of the on-site furnace to a certain extent, and has high reliability. Based on the fitting results, the causes of steam temperature deviation of the boiler water wall are analyzed, and measures to reduce the deviation are proposed to provide a necessary basis for power plant operation and boiler design.

Root cause failure investigation of a boiler waterwall tube employed in a 325 MW thermal power plant: Caustic corrosion phenomenon and its effects

This paper focuses on failure analysis of a boiler waterwall tube, ASTM A213 Grade T2 steel, used in a 325 MW fuel oil-fired thermal power plant. The influence of fire/waterside factors on the failure was identified through multiple characterization techniques. Preliminary macroscopic observations revealed uniform heavy deposition and repair welding indications on the fireside surface of the failed tube. Moreover, a longitudinally hemielliptical inner groove along with excessive weld metal penetration and discrete cavities were observed along the crown of the tube, leading to local wall thinning down to around 50% of its nominal thickness. No noticeable hardness and microstructural variations such as pearlite colonies decomposition caused by long term overheating and formation of decarburized layer were detected. According to the results of micro-zone analyses, considerable quantity of sodium vanadium oxide compounds accumulated throughout the fireside surface of the tube was due to poor fuel quality used. The authors postulated that evaporation at the waterline of the approximately filled tube resulted in water/steam stratification and localized caustic constituents concentration within the inner groove like NaOH and alkaline producing salts containing Na, Mg, Ca and P elements entered into the water/steam cycle through condenser copper-based tube leakages. This trend solubilized protective magnetite film locally and, consequently, triggered caustic corrosion of the bare steel.

Real-World Data-Driven Machine-Learning-Based Optimal Sensor Selection Approach for Equipment Fault Detection in a Thermal Power Plant

Due to growing electricity demand, developing an efficient fault-detection system in thermal power plants (TPPs) has become a demanding issue. The most probable reason for failure in TPPs is equipment (boiler and turbine) fault. Advance detection of equipment fault can help secure maintenance shutdowns and enhance the capacity utilization rates of the equipment. Recently, an intelligent fault diagnosis based on multivariate algorithms has been introduced in TPPs. In TPPs, a huge number of sensors are used for process maintenance. However, not all of these sensors are sensitive to fault detection. The previous studies just relied on the experts’ provided data for equipment fault detection in TPPs. However, the performance of multivariate algorithms for fault detection is heavily dependent on the number of input sensors. The redundant and irrelevant sensors may reduce the performance of these algorithms, thus creating a need to determine the optimal sensor arrangement for efficient fault detection in TPPs. Therefore, this study proposes a novel machine-learning-based optimal sensor selection approach to analyze the boiler and turbine faults. Finally, real-world power plant equipment fault scenarios (boiler water wall tube leakage and turbine electric motor failure) are employed to verify the performance of the proposed model. The computational results indicate that the proposed approach enhanced the computational efficiency of machine-learning models by reducing the number of sensors up to 44% in the water wall tube leakage case scenario and 55% in the turbine motor fault case scenario. Further, the machine-learning performance is improved up to 97.6% and 92.6% in the water wall tube leakage and turbine motor fault case scenarios, respectively.

An external through type RA-EMAT for steel pipe inspection

The online inspection system for pipe rolling production lines is essential for product quality assessment and cost control. The electromagnetic acoustic transducer (EMAT) is contactless and can achieve a rapid quantitative inspection, however, the effective detection range of common portable EMAT is limited. To increase the detection range of a single EMAT without increasing the number of the channels, we propose a different EMAT configuration—ring array EMAT (RA-EMAT). With only one solenoid coil and sufficient permanent magnets, the entire cross section of the steel pipe can be test when using this configuration. Through simulations and experiments, the proposed RA-EMAT is capable of generating ultrasonic waves in multiple positions of the same circumference of the steel pipe simultaneously. The comparison tests show that the RA-EMAT has better inspection ability and accuracy than the piezoelectric ultrasonic transducer. Verification tests were also carried out on the sample pipe with four different wall thicknesses and the boiler water-wall tube with simulated corrosion defects, among which the blind-hole artificial defect with the minimum depth of 0.75 mm is accurately measured.

Investigation on the welding-induced multiple failures in boiler water wall tube

In this study, by performing visual inspection, chemical composition analysis, mechanical performance testing, micro-structure and micro-morphology analyses, a failure investigation is presented on the surfacing weld induced bursting of 20G water wall tube in a thermal power plant. Besides, to identify the environmental medium and deposits, the Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) have been conducted. The tube has undergone the localized wall thinning by fly-ash erosion burst after being repaired for 4000 h. As revealed from the macroscopic examination on the tube, significant corrosive products have been produced on the water-side of the surfacing layer position of the tube. According to the metallographic analysis of the rupture region, the massive widmanstatten overheated structure has been found in the heat-affected zone (HAZ). Furthermore, in the failure parent metal, extensive intergranular cracks, severe decarburization and mechanical performance degradation (∼20 HV10) have been identified. The micro-morphology analysis has described the partial-melting process, spherical particles and the dendrite on the fracture surface. As revealed from the EDS and XRD results, the accumulation of Cu on the inner surface of the rupture region of the tube was demonstrated. The comprehensive study has suggested that the excessive welding heat-input leads to the generation of local over-heating and over-burning. Lastly, the electrochemical corrosion and the hydrogen corrosion cracking are generated in the inner wall of the welding region.

Intelligent Steam Power Plant Boiler Waterwall Tube Leakage Detection via Machine Learning-Based Optimal Sensor Selection.

Boiler waterwall tube leakage is the most probable cause of failure in steam power plants (SPPs). The development of an intelligent tube leak detection system can increase the efficiency and reliability of modern power plants. The idea of e-maintenance based on multivariate algorithms was recently introduced for intelligent fault detection and diagnosis in SPPs. However, these multivariate algorithms are highly dependent on the number of input process variables (sensors). Therefore, this work proposes a machine learning-based model integrated with an optimal sensor selection scheme to analyze boiler waterwall tube leakage. Finally, a real SPP test case is employed to validate the proposed model’s effectiveness. The results indicate that the proposed model can successfully detect waterwall tube leakage with improved accuracy vs. other comparable models.

Failure analysis of a water wall tube

The failure reasons of a boiler water wall tube used in coal-fired power plant were analyzed by the chemical composition, optical microscope and scanning electron microscope analysis. The results of metallographic analysis show that the microstructure is ferrite and pearlite, pearlite has no obvious spheroidization. The comprehensive analysis results show that the abnormal oxidation erosion leads to the leakage failure of the water wall tube at the fire-facing side.

Analiza vzrokov puščanja sten cevi izmenjevalnikov toplote v termoelektrarni

Analiza vzrokov puščanja sten cevi izmenjevalnikov toplote v termoelektrarni

Analysis of the causes of leakages and preventive strategies of boiler water-wall tubes in a thermal power plant

The reasons for the failures of a boiler water-wall tube used in the thermal power plant were investigated in this paper. The material of water-wall tube was made of medium-carbon steel, SA-210 Grade C, which is pearlitic heat-resistant steel with a working temperature lower than 500 °C and is widely used in boilers tubes. To investigate the causes of failure, various techniques including macroscopic inspection, chemical composition analysis, mechanical performance testing, optical microscopy observation and Energy Dispersive Spectroscopy (EDS) were carried out. Corrosive products were found on the inner wall of the tube’s bulge positions. The thickness of the tube wall covered by the corrosive products reduced due to the corrosion. The boiler water quality was analyzed and it was found that there was excessive Sodium Hydroxide (NaOH), which was the main corrosion-induced substance. Metallographic examination results showed that the microstructure in the corroded area has aged. Based on the analysis results, failure causes can be drawn: excessive NaOH in the boiler water was the main reason for the corrosion. The characters of alkaline corrosion products provided the condition that corrosion can proceed continuously which caused the thinning of the water-wall tube. As the process proceeds, it reduces performance of the tube and creates bulge which finally causes leakage.

Failure analysis of boiler leakage due to caustic corrosion on Cr-Mo steel

In the system of steam generation, condenser and boiler are the most prone to suffer corrosion attack. It usually occurs due to the interaction between the pipe surface that made from steel with the fluid of boiler. One of the dangerous types of failure is pitting corrosion. There are several causes of pitting corrosion, such as; the oxygen content in the fluid, the amount of contaminants that may easily form a deposit on the Cr-Mo steel surface, and the use of improper concentration of inhibitors. The electrochemical process takes place during the occurrence of pitting corrosion. Failure analysis of the boiler water-wall tube is presented in this work. In order to examine the causes of failure, various techniques including visual inspection, optical microscopy, and energy dispersive spectroscopy were carried out. Tube wall thickness measurements were performed on the failed tube. The water-facing side of the tube was observed to have experienced significantly localized wall thinning. The failure mechanism is suggested as oxidation due to caustic corrosion as a result of inappropriate usage of phosphate inhibitors.

Analysis of temperature drops on the wall thickness of a supercritical boiler water-wall tube

The paper presents selected results of numerical computations related to simulations of a supercritical power boiler evaporator operation. A detailed analysis is carried out of temperature drops on the thickness of the water-wall tube and the tube fin. A comparison is also made between the results obtained at the fin division into one and two control volumes. The tube cross-sections analysed along the tube length and including the fins are divided into 24 control volumes, for which 2-D transient heat conduction equations are formulated. The 1-D equations describing the principles of mass, momentum, and energy conservation are solved on the side of the working fluid. A convective condition, described by an empirical heat transfer coefficient, is set on the water-wall tube inner surface. The developed mathematical model is a distributed parameter model. The numerical computations are performed for a boiler operating in a power plant in Poland. The analysis takes account of the non-uniformity of the furnace chamber thermal load along its height.

Failure Analysis of Boiler Water Wall Tube for Power Generation in District Heating

A corrosion failure of materials, often exposed to the corrosive environment in the district heating facility, has a critical influence on structural integrity. Thus, it is very important to investigate the cause of the failure for the establishment of effective prevention methodology. In this study, the failure analysis was performed in the case that a water wall tube for a boiler in district heating was fractured along the axial direction of the tube that is SA-210 Grade A1, which is frequently used for the boiler. The feed water temperature was about 220°C and the operating pressure was about 10 MPa. For the visual inspection of the failed part, the wall thinning and failure were observed in the location where the heat was directly reached. Sodium and iron oxides were only found in the inner surface of the tube, as confirmed by x-ray diffraction, and it indicates that hydrogen is generated by the mechanism of the caustic corrosion, resulting in hydrogen embrittlement near the inner surface. The microstructural changes along the direction of radial and hoop were intensively examined. A number of microvoids were formed along the grain boundary, leading to some microcracks in the location inside the tube. As a result, it is concluded that an unstable magnetite film at elevated temperature under tensile residual stresses developed along the hoop direction in the interior of the tube undergoes the caustic corrosion, and microvoids or microcracks along the grain boundary formed due to hydrogen embrittlement cause a final fracture of the tube.