Boiler Tubes Research Articles

Development of miniature cruciform specimen and testing machine for multiaxial creep investigation

High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to extreme temperature conditions and their complex shapes. A multiaxial creep investigation is required for the safety of high temperature components in the design phase of the manufacture of such a component. However, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. A miniature cruciform specimen having a plane stress condition gauge section that is 5 mm square was designed using finite element (FE) analysis. Further, a biaxial testing machine capable of tensile loading was designed to conduct multiaxial creep tests. The testing machine had a 2 kN loading capacity and a 1 kW furnace. We also developed a non-contact displacement measuring method for the miniature specimen, which had an excessively small gauge section to permit the attachment of a mechanical extensometer. The proposed method uses a conventional optical camera to capture some images of the surface. We obtained the displacement value of the specimen, by tracking the trace of the target marks painted on the surface of the specimen. The measured strain value that was obtained from the non-contact displacement measuring method corresponds to the strain obtained by the mechanical method at room and high temperatures. By using the developed multiaxial creep testing machine and the non-contact observation system, we can investigate not only the deformation of the testing specimen, but also the surface conditions of materials during the creep test.

Measurement of Accumulated Height of Exfoliated Oxide Scales in Austenitic Boiler Tubes Using Pulsed Eddy Current Testing

Austenitic boiler tubes are key components in modern coal-fired power plants but often suffer from blockage or even rupture caused by the accumulation of steam-side oxide scales. Existing nondestructive testing methods are somewhat deficient to quantify the accumulated height of exfoliated oxide scales. Pulsed eddy current (PEC) technique has unique merits such as deep penetration depth and diverse signal features, offering a great possibility to address this issue. This work serves as an exploratory study for applying PEC technique to measure the scales accumulation. Finite element modeling and experimental work are conducted to validate the feasibility and effectiveness of PEC measurement under different circumstances. The PEC signal peak shows a linear relationship with the accumulated height and thereby is used as the signal feature for the quantitative measurement of accumulated height. A flexible sensor consisting of a replaceable flat cable with connectors at the ends is designed, following by the analyses of sensor offset and liftoff effects. To distinguish the unevenness of accumulation, the sensor-coil array scheme is proposed and the output array signals successfully characterize the profile of inner accumulation. Finally, a referencing strategy is explained to eliminate the interference due to magnetism transformation of the tube wall.

An experimental study of effects on tube to tube sheet expansion joint grip strength due to seal welding or blanking of adjacent bank tubes of bi-drum boiler

Industrial boilers used in processing industry and auxiliary units for start up steam supply of super critical boilers generate steam with the help of bi-drum arrangement. The tube connecting upper and lower drums are called bank tubes and situated in the flue gas path. Bank tubes are fitted in both the drum with the help of tube to tube sheet expansion joint. During prolonged operation this tubes got punctured due to flue gas erosion or becomes loose at its expansion joints due to improper operation, during drum repair and started leaking. Leaking tubes were repaired either by blanking the tubes or seal welding depends on the nature of failure. As drum is of higher thickness and tubes are of lower thickness, as a procedural drum surface required to be pre-heated before seal welding. This pre-heating and welding affects the joint strength of surrounding tubes. This study focuses on lose of grip strength of surrounding tubes due to pre-heating and welding of central tube during repair, by comparing its micro structure, hardness and pull out load strength. During micro structure analysis polygonal grains of ferrite and pearlite were observed. No significant change in microstructure dimensions between each expansion trail was observed visually. Hardness value falls around 20 Hv near 0° angle of HAZ and falls around 10 Hv at 180° angle of HAZ for 12% expansion. For wall thinning above 10% and ligament efficiency less than 0.4 grip strength falls drastically, and requires complete re expansion of surrounding tubes.

High-Temperature Erosive Wear Behavior of High-Velocity Oxy-Fuel Sprayed Cr3C225 (Ni20Cr) Coating on (AISI 316) Austenitic Steel

Abstract AISI 316 austenitic steel is extensively used in various components of power plants like boilers, boiler tubes and pipes, which suffer severe air jet erosion. Fly ash is a major erodent in that case. Present study is conducted to study the effect of Cr3C225 (Ni20Cr) coating on 316 substrates, when it is exposed to thermal power plant erosive conditions. High-velocity oxy-fuel deposition technique is used for coating. Major constituent of Indian fly ash is alumina, so alumina particles are taken as erodent. Erosion wear behavior is investigated for coated and uncoated conditions using an air jet erosion tester at 100 m/s impact velocity. Three impact angles, i.e., 30 deg, 60 deg, and 90 deg, and four working temperatures, room temperature, 200 °C, 400 °C, and 600 °C, were chosen to identify wear mechanism. Scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX) were utilized to characterize the coated, uncoated, and eroded surface. Erosion behavior is correlated with micro hardness, roughness, and microstructure. Results reveal that the coated surface offers better erosion resistance than the uncoated surface. Substrate exhibits ductile wear behavior as it shows higher wear rate at low angle and decreases on increasing the impact angle, while coating offers good wear characteristics at 30 deg and 90 deg impact angles. An increase in working temperature favors wear rate increment for both coated and uncoated samples. However, coated samples exhibit ductile erosive behavior at high temperatures. Uncoated surfaces have micro-cutting and deformation as major erosive mechanisms. Whereas for coated samples at high temperature, oxide layer formation takes place and erosion takes place due to spalling of the oxides from the coated surface.

Microstructural analysis and hot corrosion behavior of HVOF-sprayed Ni-22Cr-10Al-1Y and Ni-22Cr-10Al-1Y-SiC (N) coatings on ASTM-SA213-T22 steel

The present paper deals with the investigation of microstructure and high-temperature hot corrosion behavior of high-velocity oxy fuel (HVOF)-produced coatings. Two powder coating compositions, namely, Ni22Cr10Al1Y alloy powder and Ni22Cr10Al1Y (80wt%; microsized)-silicon carbide (SiC) (20wt%; nano (N)) powder, were deposited on a T-22 boiler tube steel. The hot corrosion behavior of bare and coated steels was tested at 900°C for 50 cycles in Na2SO4-60wt%V2O5 molten-salt environment. The kinetics of corrosion was established with weight change measurements after each cycle. The microporosity and microhardness of the as-coated samples have been reported. The X-ray diffraction, field emission-scanning electron microscopy/energy dispersive spectroscopy, and X-ray mapping characterization techniques have been utilized for structural analysis of the as-coated and hot-corroded samples. The results showed that both coatings were deposited with a porosity less than 2%. Both coated samples revealed the development of harder surfaces than the substrate. During hot corrosion testing, the bare T22 steel showed an accelerated corrosion in comparison with its coated counterparts. The HVOF-sprayed coatings were befitted effectively by maintaining their adherence during testing. The Ni22Cr10Al1Y-20wt%SiC (N) composite coating was more effective than the Ni-22Cr-10Al-1Y coating against corrosion in the high-temperature fluxing process.

Failure analysis of cracked 2.25Cr–1.0Mo steel tubes of an oil refinery boiler

After failure of a refinery boiler, some cracks were detected in its tubes made of 2.25Cr-1Mo steel. The tubes were in the boiler superheater section. To determine causes of the cracking, samples were taken from the tubes, and different microscopical and analytical techniques such as scanning electron microscopy (SEM) and mechanical tests were used. Furthermore, finite element analysis (FEA) was used to evaluate the stress conditions in one of the cracked tubes. The results of analysis showed that the tube material was in accordance with the ASTM A213 chemical composition requirement however based on the calculated J-factor, the material was susceptible to temper embrittlement. The FEA results showed tensile residual stress formed at welding zone due to non-alignment before tubes welding. Considering the intergranular propagation of the cracks, the hardness of HAZ and other findings, it was concluded that stress corrosion cracking was the major damage mechanism.

Oxidation resistance of Ni-Cr-TiO2 powder HVOF coating on carbon steel at elevated temperature

In this research a Chromium Carbide (Cr3C2)-based powder consisting of 80% Cr3C2 and 20% Titanium dioxide (TiO2) is sprayed to SA 210 Grade C boiler tube material for better performance of power plant boilers. Cr3C2-TiO2 coating is deposited by high velocity oxygen fuel (HVOF) process with powder particle of size 45 ± 15 μm. High temperature oxidation test under cyclic condition is performed in air environment up to 900 °C. The properties of specimen are characterized by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The results shown that the Cr3C2-TiO2 coating exhibit excellent corrosion resistance at 900 °C compared to bare SA 210 Grade C boiler tubes and the coating also reduces the weight gain of the steel by about 50%. High temperature oxidation resistance imparted by Cr3C2-TiO2 coating may be attributed to the formation of alloy phases during the powder processing and coating processes.

Study of circulation ratio for natural circulation in water-tube boiler at different operating conditions.

The basic objective of the paper is to identify the desired circulation ratio for the natural circulation of water tube boilers in different operating conditions. This requires the basic study of heat flux and the mode of the boiling heat transfer, and the phenomenon like departure from nucleate boiling and tube overheating. The parameters, which need to be studied are heat flux, pressure, dryness fraction, void fraction, liquid velocity and their impact on the required circulation ratio. For a natural circulation boiler, the circulation ratio is one of the most important design parameters as the other design parameter like critical heat flux and skin temperature are mainly derived from the circulation ratio. The required circulation ratio can vary with the boiler pressure, liquid velocity and maximum heat flux. This study is intended to provide input for the safe and optimum design of a natural circulation boiler.

Desired Circulation Ratio for Natural Circulation in Water-Tube Boiler

The basic objective of the paper is to identify the desired circulation ratio for the natural circulation of water tube boilers in different operating conditions. This requires the basic study of heat flux and the mode of the boiling heat transfer, and the phenomenon like a departure from nucleate boiling and tube overheating. The parameters, which need to be studied are heat flux, pressure, dryness fraction, void fraction, liquid velocity and their impact on the required circulation ratio. The present work is to develop a circulation analysis model for the natural circulation of the water tube boiler and to check the boiler design for different failure modes. For a natural circulation boiler, the circulation ratio is one of the most important design parameters as the other design parameter like critical heat flux and skin temperature are mainly derived from the circulation ratio. The required circulation ratio can vary with the boiler pressure, liquid velocity and maximum heat flux. This study is intended to provide input for the safe and optimum design of a natural circulation boiler.

English

English

The Effect of Fuel Emulsion on Fuel Saving in Fire Tube Boilers of Tartous Company for Cement and Construction Materials

In this work, the combustion of heavy oil and its emulsions with water, in the fire-tube boilers, was investigated in experiments on an industrial scale. The performance of the fire-tube boilers, used in Tartous company for cement and construction materials, was studied when they were employed to be operated with heavy fuel oil (HFO) and with the water phase of emulsified heavy oil containing 8 vol. % water and 92 vol. % heavy fuel oil (HFO). The emulsified water/heavy fuel oil (W/HFO) with 8 vol. % of water content showed no separation and contained the smallest and most homogeneous water-in-HFO (W/HFO) droplets after stability tests. Four boilers, each with 8 ton h-1 steam capacity, have been operated for 4 months with a regular heavy fuel oil HFO and W/HFO. It has been found that the micro-explosion, observed in W/HFO, improved the boiler efficiency and reduced the fuel consumption by 12.99%.

Coal-fired thermal power plant performance optimization using Markov and CFD analysis

In the present stimulated business environment, power sector is playing a major role in the economic growth of India. During the last 20 years, the country had been facing a poor supply of energy and this supply–demand gap is increasing continuously. Therefore, it is important for power plants to improve its power generation capacity drastically by reducing the failure rate. In the present paper, to analyze the causes of poor availability, thermal power plant has divided into six different systems and a system comprising waste gases heating system has been considered. With the help of transition diagram, mathematical equations have been used to find out the availability. After analyzing, it was found that the value of availability is very low and boiler tube failure is one of the most critical factors for this low availability of system. The power plants have low availability causing serious concern and need to identify the responsible factors for this low availability. Boiler tube failure is identified as a responsible factor for low availability, and economizer is the zone where maximum failure occurred. From the maintenance history sheet, it was identified that economizer is a critical zone where maximum tube failures occur and the main cause of economizer failure is due to high-velocity flue gas particle, i.e., erosion. To increase the availability and minimize the failures, erosion process must be reduced in economizer tubes which is mainly responsible for economizer failure and then CFD analysis has been done for this purpose. This results in a decrease in the shutdown period of the plant and an increase in the system availability as well as the power of the system.

Optical microscopy as a simple method for analysis of boiler tube failure

A severely damaged low carbon steel boiler tube was the object of this investigation. Detailed microstructural characterization was performed by optical microscopy, whereas scanning electron microscopy (SEM) was applied only in a few cases. Results show that a variety of microstructures was formed in the material of the damaged boiler tube during its exploitation. The failure of the tube is the result of very inhomogeneous overheating. The side of the boiler tube toward fire (F) was exposed to high overheating temperature, which in some locations was well above the A3 transformation temperature. The side toward boiler (BL) was subjected to lower temperatures, i.e. in the region mostly between A1 and A3 temperatures. Variations in temperatures and cooling rates, which resulted in microstructural inhomogeneity, are the main cause for the formation and multiplication of stresses leading to the rupture of the tube.

Studies on solid particle erosion behaviour of D-Gun sprayed WC-Co, Stellite 6 and Stellite 21 coatings on SAE213-T12 boiler steel at 400 °C temperature

Erosion wear of boiler tubes occurs from room temperature to high temperature and is accelerated by oxidation and hot corrosion above 400 °C temperature. The demand for hard coatings compatible in improving erosion, oxidation and hot corrosion resistance while maintaining the desired mechanical properties of the base material is increasing with the advancement of technology. In this work, WC-12Co coating, Stellite 6 coating and Stellite 21 coatings have been deposited on SAE213-T12 boiler steel using a Detonation gun (D-gun) spray method, and their erosion performances have been evaluated at 400 °C temperature using an air jet erosion tester. The erosion tests have been carried out using the alumina erodent particles at 30° and 90° impact angles. The surfaces of the as-sprayed coatings and the specimens eroded at 400 °C temperature were analysed using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscope (EDS). Stellite 21 coating has shown comparatively lower erosion rate than the Stellite 6 and WC-Co coatings at 400 °C temperature attributed to the formation of protective oxides of chromium and cobalt at 400 °C. The effects of microhardness and temperature on the erosion behaviours of the coatings at 400 °C temperature are discussed. The oxide layers of chromium and cobalt and accumulation of hard phases of nickel, molybdenum and carbon contributed in the protective behaviour of the deposited Stellite 6 and Stellite 21 coatings at 400 °C. The oxidation enhanced erosion wear is observed at 400 °C temperature for all the coatings.

Performance of different coating materials against high temperature oxidation in boiler tubes – A review

At presently, the failure of a boiler is big hazard to the effectiveness of the boiler in the power plants. In coal fired boilers, erosion and corrosion at high temperature is responsible for the downtimes, which directly affect the economic loss. In the present review report, the current situation of combating the high temperature erosion and corrosion to the boiler surface has been discussed. The thermal spray coating techniques used for protecting the boilers has been studied in regard to the performance of different coating materials deposited using different coating methods. The performance of the developed coatings under high temperature oxidation has been reported. This article may help to researchers to select the best coating combination and method to resist the failure of boiler.

A Novel Technique for Surface Modification of SA 210 Gr A1 Steel

Abstract Failure of boiler tubes due to High temperature corrosion is commonly reported cause of material degradation. So in order to surface modification boiler tube material (SA-210 Gr A1- low carbon steel) was Friction stir processed (FSPed) using WC tool with plunge depth of 01 mm, transverse speed as 20 mm/min and tool-spindle rotational speeds of 700 rpm, 1300 rpm and 1900 rpm respectively with 03 numbers of passes in each case. In order to characterize; XRD analysis has been carried after processing followed by Optical microscopy (OM) analysis to identify percentage of phases. Further Electron Back Scattered Diffraction (EBSD) analysis was done for calculation of the grain size of bare and FSPed steel: It has been found that coarse grain sizes of the base metal (25µm) has been reduced by FSP to 4.2 µm, 2.3 µm and 1.05 µm at 700 rpm, 1300 rpm and 1900 rpm respectively. The microhardness of the FSPed samples was enhanced by factor of 1.5, 2 and 2.5 in comparison to base steel respectively, this may be due to two dominating mechanisms firstly phase transformation from austenite to martensite and ferrite and secondly grain refinement.

Numerical studies on furnace water walls failure in water tube boiler applications

Boiler as one of the main equipment in steam power plant system that has important function as steam generation for rotating turbine. Failure of boiler result in operational cause lowering the power capacity of the steam power plant system on the whole. From the data observation, boiler failure frequencies because of leakage on boiler tubes. To analyze the boiler tube failure accurately, information of temperature operated on the outer surface tube and temperature distribution on the furnace wall are needed. The main objective of this study is to investigate several areas in which the failure of furnace water walls boiler exists. A combined analysis of thermal and strength of material has been performed, in order to calculate the temperature and velocity fields inside furnace of the water tube boiler. Due to the geometrical complexity of the model, GAMBIT applications was used to generate a computer-based model. All numerical calculations have been performed using the commercial CFD package FLUENT. The results obtained were numerically that failure on furnace water walls in water tube boiler because there is localized overheating.

Analysis of Hydrogen Corrosion of Water Wall Tube of a Power Plant Boiler

During the operation of a power station boiler with the rated evaporation of 1025 t/h, its water wall tube, which is made of SA-210C, bursts continuously. Based on the fracture analysis, material inspection, metallographic examination, energy spectrum analysis and mechanical property analysis of the tube sample, this paper confirms that the cause of the water wall tube failure is hydrogen corrosion, which is accelerated by the copper impurity.

Erosion behaviour of boiler component materials at room temperature and 400 °C temperature

Degradation of boiler tube components in Indian thermal power plants is mainly done through solid particle erosion due to exposure in erosive ash environment. Some important parameter such as hardness, erodent velocity, temperature, impact angle, shape and size are majorly affected the erosion wear. In this study, the erosion wear behaviour of SAE 208 grade steel, SAE213-T12 boiler steel, SAE213-T91 boiler steel and IS 2062 structural steel were investigated using an air jet erosion test rig. The erosion tests were carried out at the impact angles of 30° and 90°, impact velocity of 42.3 m/s at room temperature and 400 °C. The position of sample holder was adjusted in such a way that erosion wear scar occur at the central region of the samples. For each experiment, the erosion test was continued until the steady state erosion rate was achieved as per ASTM standard. The variation in erosion rate data has been plotted against cumulative mass of erodent for boiler and structural steel samples. The steady state erosion rates for different steel samples are calculated and compared. The surfaces of tested samples are analyzed using scanning electron microscopy to understand the erosion mechanism. The results showed that the ductile behaviour of erosion mechanism is dominated at both room temperature and 400 °C temperature. Mechanism involved at 90° impact angle is plastic deformation with formation of craters and at 30° impact angle is cutting and ploughing.

Oxidation Behaviour of HVOF Sprayed NiCrAlY and NiCrAlY-20SiC Coatings on T-91 Boiler Tube Steel

In present study, the high temperature oxidation behavior of HVOF sprayed NiCrAlY alloy coating and NiCrAlY–20SiC composite coating on T-91 boiler tube steel has been investigated at elevated temperature of 900°C for 50 cycles. The kinetics of oxidation has been established with weight change measurements. Both coatings and uncoated material obeyed the parabolic law during oxidation studies. XRD, FE-SEM along with EDS, X-ray Mapping characterization tools were utilized to check phase formation, elemental distribution and microstructure. The study resulted that both coatings performed better in oxidizing environment due to the formation of protective oxides in coated surface, however in case of uncoated steel unprotective Fe2O3 oxide was formed which led to extensive spallation. Moreover the composite NiCrAlY–20SiC coating provided more resistance to high temperature oxidation than NiCrAlY alloy coating.