Boiler Tubes Research Articles

Low-temperature formation and steam oxidation of β-FeAl coating on P92 steel

The poor steam oxidation resistance of P92 steel limits its long term application in 620 °C double reheat ultra-supercritical steam boilers. Aluminizing has been considered as one cost-effective solution to improve the oxidation resistance of substrate alloys without significantly worsening their mechanical performance. A modified aluminising was developed to form a thin β-FeAl layer on heat-resistant P92 steel under relatively lower temperatures. Oxidation behaviour of the layer and the P92 steel was probed in flowing pure steam vapour up to 500 h at 650 °C. An unstable Fe-rich oxide layer of over 100 μm in thickness grew on the steel, whereas only an extremely thin (<0.2 μm) and stable α-Al2O3 scale grew on the β-FeAl layer that ultimately evolved to FeCrAl solid solution phase during oxidation, implying great potentials of applying the β-FeAl coating for protecting practical boiler tubing systems against oxidation.

Dissimilar Cladding of Ni–Cr–Mo Superalloy over 316L Austenitic Stainless Steel: Morphologies and Mechanical Properties

The solid solution strengthened Inconel 625, a Ni-based alloy is known for its excellent strength and good corrosion resistance at extreme environments used in thermal plants, boiler tubes, petrochemical industry and power plant. The presence of Cr content (~ 20 wt%) along with Mo-rich, Nb and Fe makes Ni–Cr–Mo–Nb austenitic alloy called as Inconel 625 to achieve excellent corrosion resistance property. Using cold metal arc transfer (CMT) cladding, the metallurgical, mechanical and corrosion properties of Inconel 625 on 316L is evaluated. The process parameters selected includes welding current, torch angle and travel speed with a constant voltage. From the results of microstructural and EDS inferences, the formation of cellular dendritic structure with secondary phases like Laves phase, complex nitrides along with the interdendritic segregation of Mo and Nb as well as microsegregation of Cr, Ni and Fe. In case of Ni–Cr–Mo alloy, Ni and Cr contribute to resistance to corrosion in NaCl environments. The formation of Cr2O3 and the passivation action of the clad zone is due to the presence of Cr. The solid solution effect in Ni–Cr matrix is contributed by the presence of Nb and Mo. Apart from that the strengthening action happens due to the precipitation of Ni3 (Al, Ti, Nb) commonly known as γ′, γ″ and MC carbides confirmed through XRD. Uni-axial tensile tests and Vickers-micro hardness indentation tests were performed on Inconel 625 cladded over 316L. Based on the fractographic results fatigue striations, tear rigdes with river markings, dimples with fibrous structure and cleavages are observed. Unlike other studies, unique type of cuboidal precipitates are seen, which is due to the presence of Ti, which form carbonitrides containing Ti, which are further characterised as NbC. The potentiodynamic polarisation tests is performed on 3.5% NaCl solution. The results suggest that Ni–Cr–Mo alloy protects the substrate from corrosion.

Application of bending creep for examining effect of service conditions on creep response of steel

We demonstrate that creep testing of cantilevers in bending is suitable for evaluating steady-state creep parameters, including the stress exponent. In particular, a low alloy bainitic boiler tube steel, namely T11, in as-fabricated condition and after 27 years of service in a coal-fired power-plant were tested in bending at 600 °C at effective steady-state stresses of 100–200 MPa. Steady-state strain rates obtained from the bending tests matched reasonably well with those obtained from uniaxial compression tests performed at equivalent stresses, thereby unambiguously validating the bending test methodology for the entire range of stress. All types of samples showed a stress exponent of 6; however, the steady-state strain rates of the in-service samples were ~7 times larger than those of as-fabricated samples. Although this suggests a substantial loss in the creep resistance of the tubes at the end of the design-life, the “to-be-discarded” tube may still have considerable residual life. Microstructural evaluation of the samples revealed coarsening of carbides in bainite and metal enrichment of carbides, with excess of M6C, with exposure to the service conditions. A structure-property correlation is established to explain the experimental observations, and results are discussed in light of existing literature.

Investigation of potassium vapor time-resolved adsorption and potassium-sodium competitive adsorption by modified kaolinite

Frequent corrosion of heat transfer tubes in boilers, which was closely related to the massive corrosion sources, such as KCl and NaCl vapors in flue gas, restricted the safe and economic development of MSW incineration technology. This study attempted to alleviate corrosion by using the method of modified kaolinite adsorbing alkali vapors in furnace. Firstly, an ingenious alkali vapor adsorption platform with the advantage of simple operation and precise control was established. Based on this, the time-resolved adsorption of K vapors and the competitive adsorption of K/Na vapors by kaolinite and modified kaolinite were investigated. Four classical adsorption models were applied to mathematically describe the adsorption process. The pseudo-first order model was suitable for kaolinite and pseudo-second order model can be used to fit modified kaolinite. The corresponding rate-controlling steps were external mass transfer and chemisorption step. Accordingly, the equilibrium adsorption capacity of kaolinite and modified kaolinite was estimated as 17.41 and 32.30 mg/g, respectively. During the competitive adsorption of K/Na vapors, it was obvious that more K was captured by kaolinite than Na. It might be caused by the higher alkalinity of potassium, which had a higher tendency to react with acidic kaolinite. Maximum K-loading capacity of kaolinite occurred at the K/Na atomic ratio of approximately 1:1. After the structural calculation of alkali feldspar based on DFT, the most stable state of alkali feldspar occurred at the K/Na atomic ratio of 1:1, which might explain the maximum loading capacity at the K/Na ratio of 1:1.

Corrosion behavior of wire arc sprayed Ni-based coatings in extreme environment

The current study represents the corrosion behavior of wire arc sprayed Ni-based (Ni-5Al and Ni-20Cr) coatings on commonly used boiler tube steels (T11 and Gr A1). A cyclic hot corrosion behavior of the samples has been carried out at 900 °C for 50 cycles with the application of Na2SO4-60%V2O5 salt. The coatings offered excellent corrosion resistance in comparison to bare steel. The surface and cross sectional analysis of coated samples before and after corrosion has been carried out using SEM/EDS and XRD analysis. It was observed that Ni-20Cr coating exhibited better corrosion resistance for both T11 and Gr A1 steel than a Ni-5Al coating. The Ni-20Cr coating has shown 87% and 75% better corrosion resistance in terms of weight change in comparison to bare T11 and Gr A1 boiler steel, while Ni-5Al offered 86% and 73% better corrosion resistance to bare steels respectively. This may be due to the formation of protective Cr2O3 oxide in the top oxide layer, lower porosity of coatings and high micro hardness.

An Investigation on Hot Corrosion Behaviour of Cermet Coatings in Simulated Boiler Environment

In this research work, hot corrosion behaviour of different Cr3C2–NiCr coatings has been investigated. The coatings were deposited on commercially available boiler tube steel with high velocity oxy fuel thermal spraying technique. The uncoated and coated specimens were exposed to elevated temperature in a silicon tube furnace at 700 °C in molten salt environment. The thermogravimetric technique was used to compare the corrosion resistance of different coatings for 50 cycles at high temperature. The microstructures of exposed specimens were evaluated with X-ray diffraction and scanning electron microscopy with elemental compositional analysis. 10Cr3C2–90NiCr and 20Cr3C2–80NiCr coatings provided the higher resistance to corrosion as compared to 100NiCr and 35Cr3C2–65NiCr. The formation of chromium carbide layer on the coated surface and this layer protect them from corrosion.

Failure analysis of boiler tube at a petrochemical plant

This research aims to assess the failure analysis of a boiler tube of a steam generation system at a petrochemical plant. To achieve this goal, firstly visual observation and thickness measurements were made to detect different features of failure. Secondly optical microscope and scanning electron microscope (SEM) were used observing the microstructures and also hardness measurements were carried out for metallurgical evaluation. Additionally, phase composition of deposits was studied by using X-ray diffraction (XRD). Also, the on-site water composition of the boiler was taken into consideration. Finally, Finite Element Analysis (FEA) was performed to model the condition of the tube before failure. Based on the results, “short-term overheating” was recognized as the root cause of the boiler tube failure.

Mathematical Modeling of the Working Conditions of the Ship’s Utilization Boiler in Order to Evaluate Its Performance

The paper presents a mathematical model allowing the determination of the amount of saturated steam produced in marine water tube boilers and smoke tube boilers. The mathematical model includes the impact of the exhaust gas temperature and its amount, ambient temperature, engine power load, and location of boiler tubes. In addition to the amount of steam generated in a boiler, it is also feasible to establish flow resistance of the exhaust gas in the boiler determined by the boiler tubes’ arrangement and the thickness of scale deposits and the exhaust gas temperature after the exhaust gas boiler. Due to the model universality, it may be applied not only to make calculations for existing boilers, but also to perform numerical experiments in order to determine the amount of steam produced by the entire range of boilers used in the waste heat recovery systems in power marine systems and the adopted limit values, such as exhaust gas flow resistance and their temperature, after the boiler. The reliability of the obtained results has been revised by comparing them with the outcomes of the experiments performed on the ships.

Failure Analysis of Superheater Boiler Tube SA 213 T12

Superheater tubes SA213 T12 (OD = 57 mm, WT = 6 mm) has been in service for 11 years in a supercritical coal fired boiler (315 MW). Tubes operating temperature are 430°C and pressure is 11 MPa. Failure occurred on tube no. 20, row 2. Three tubes specimens from the same row (failed tube, tube no. 3, and no. 4), were drawn for laboratory examinations. Vickers microhardnesses of failed and no. 3 tubes are the same (140 VHN), while tube no. 4 hardness is 170 VHN. These hardness values are much lower compared to the new tube (192 VHN). The lower hardness value of failed tube is due to complete decomposition of pearlite colonies in inner and outer side of failed tube to ferrite and spheroidized cementite. Failure of tube is characterized by thin lip rupture which indicated that the tube circumferential deformation occur gradually following spheroidization process. Failure by intergranular crack occurred along grain bondaries when the stress on the tube wall (hoop stress) exceeding material strength at service temperature. Tube no. 3 and 4 are partially spheroidized, mostly on the innerside. The severity of microstructure damage is related to the flow direction of flue gas, from tube no. 1 to no.2, no.3 and so on. Tube no. 1 was survived because it was protected by a front cover that will limit heat transfer from the flue gas. The previous failure analysis of similar boiler tube revealed that the hardness of the bulging tube is about 140 VHN which is the same as the hardness of the failed tube (no. 2) and no.3. Tube no. 3 is in the early stage of damage and shall be replaced because the hardness is much lower than 170 VHN. The effect of flue gas flow direction not only affects the hardness and microstructure but also deposit thickness. Deposit thickness in tube no. 2, 3, and 4 are 149, 123, and 100 µm respectively. Thicker deposit in tube no. 2 is also contributed to premature failure of this tube compared to the others. The local temperature of tube wall will be higher because thick deposit will reduce heat transfer between flue gas and steam inside the tube.

MAXIMUM ALLOWABLE THERMAL STRESSES CALCULATION OF WATER TUBE BOILER DURING OPERATION

In steam boiler industrial sector, pressure and temperature of the water tube are the two main factors that affecting the safety and efficiency of a steam boiler. Explosions may be occurring because of a sudden drop in pressure without a corresponding drop in temperature. Therefore, understanding the temperature distribution of the water tube boiler is essential control the failure and explosion of the boiler. Once the temperature distribution is known then the limiting factors that affect the water tube life such as maximum allowable thermal stresses can be determined. ANSYS software will be used to determine the temperature distribution in the water tube of a utility boiler during operation at elevated inlet water and furnace temperature. The theory of axisymmetric has been utilized since water- tube is cylindrical in shape. In axisymmetric theory, a three-dimensional cylindrical problem like water tube can be reduced to two dimensional by ignoring the circumferential Ө, while r-axis and z-axis became x-axis and y-axis or Cartesian coordinate. Then two-dimensional rectangular elements meshing for the profile cross-section along the water tube in r and z axes were implemented in a computerize simulation using ANSYS 10 to find out the steady state temperature distribution of the water tube.

On Line Dirt Based Soot Blowing System using Fuzzy Logic for Utility Boilers

Soot is a byproduct of burning coal in thermal power plants. The soot accumulates in the boilers tubes. It gradually spoils the heating surface which reduces the heating efficiency. Hence it has to be cleaned. This process is called soot blowing. The existing soot blower control system in the old boiler is composed of relays, timers and contactors which are not efficient. In order to improve the efficiency, an intelligent system is proposed which estimates the dirt in the boiler tube periodically and initiates soot blowing process to improve the thermal efficiency. Simulation results demonstrate the superiority of the proposed scheme when compared to relay logics.

Hot Corrosion Behavior and Mechanism of High-Velocity Arc-Sprayed Ni-Cr Alloy Coatings

High-temperature corrosion affects many components used in coal-fired power plants. A possible solution is the use of protective coatings. In this study, Ni-Cr alloy coatings with different Cr contents (30 at.%, 45 at.%, and 50 at.%) and Ni-Cr-Ti coatings were deposited on 20G boiler steels by high-velocity arc spraying. The hot corrosion behavior of the coatings was characterized in the aggressive environment of Na2SO4 + 30% K2SO4 molten salt under cyclic conditions at 750°C. x-Ray diffraction analysis, optical microscopy, and scanning electron microscopy were used to analyze the phase composition, microstructure, and corrosion products of the coatings. The corrosion kinetic curves of the coatings, established by thermogravimetry, conformed to the classic parabolic law, revealing that the Ni-Cr-Ti coatings exhibited better hot corrosion resistance than the Ni-45Cr or Ni-30Cr coatings. However, among the four tested coatings, the Ni-50Cr coating was found to offer the best protection with the lowest values of mass gain per area (2.9 mg/cm2) and parabolic rate constant (kp, 0.215 × 10−11 g2 cm−4 s−1). This can be explained by the formation of Cr2O3 and NiCr2O4. Therefore, such high-velocity arc-sprayed Ni-50Cr coatings could be used as protective layers for hot components in boiler tubes.

Analysis of the power boiler superheater strain-and-stress state under creep conditions

ABSTRACTThe paper presents a procedure for assessing the condition of the current structure of boiler tubes. First, a modified version of the Garofalo equation is proposed to model the strain-and-stress state under creep conditions in Super304H steel. In parallel with the creep model development, the microstructure is examined using SEM and TEM techniques. The Super 304H steel structure is tested for a new material and for the superheater material after 100,000 h of operation. According to the presented description of the tested steel degradation, it is possible to operate the material for another 100,000 h if the design conditions are met. The results show that the conventional approach based on European standards does not exclude the superheater safe operation. However, a more detailed analysis, taking account of additional loads from the pipeline and using the proposed creep equation, proves that the conditions for the superheater safety are not satisfied.

Key issues and practical design for cooling wall of supercritical carbon dioxide coal-fired boiler

When supercritical carbon dioxide (S–CO2) Brayton cycle is used for coal-fired power plants, the significantly increased mass flow rate and endothermic temperature of S–CO2 boiler leads to the over-temperature and high pressure drop on the cooling wall (CW). This paper presents a mathematic flow and heat transfer model of S–CO2 in combustion chamber CW. The results show that reducing the tube diameter can effectively reduce the temperature difference in cooling wall and reduce the effect of high heat flux on the temperature difference. The pressure drop decreases exponentially with the increase of tube diameter and boiler expansion factor. Increasing partial flow numbers will decrease the boiler pressure drop and improve system cycle efficiency in spite of increasing the temperature difference of CW. The change of flow direction has no obvious effect on temperature distribution of CW and has opposite effect on gravity and buoyancy force. Three boiler layout strategies, including partial flow strategy, flow symmetry strategy and boiler local expansion strategy are proposed in order to coordinate the pressure drop and uneven temperature distribution, which can not only decrease boiler heat exchange surface temperature difference but also reduce the boiler pressure drop to prevent the over-temperature and improve system performance.

Creep constitutive equations for as-received 9Cr1Mo steel

9Cr1Mo has a wide application in petrochemical and power generation plants, prediction of its creep behavior is a major concern in these industries. In this paper, the creep behavior and creep lifetime of test samples machined from 15 years of service exposed boiler tubes in an oil refinery plant, are studied using uni-axial creep tests. Tests are carried out according to ASTM E139 at different stress and temperature levels (from 80 MPa to 190 MPa and at 550 °C, 600 °C and 650 °C). Based on the experimental data, the creep constitutive coefficients for three types of creep laws: Norton, Plrandtl and Garofalo are obtained together with Monkman-Grant and Larson Miller parameters. It has been shown that the constitutive equations in the form of Garofalo law predict the creep behavior of the material more accurately. The results show that the creep life relationships obtained based on both Garofalo and Norton power law give acceptable estimation. Also, the results are implemented in reference stress method to produce a set of loci for the optimal geometry of the tubes under combined loadings with maximum creep lifetime.

Study on reduction mechanism of Fe2O3 by NH3 under SNCR condition

Amino compounds were injected into a boiler at high-temperature area usually to realized De-NOx. However, amino compounds were highly corrosive. The metal tube skin of stage III superheater in a medium temperature separation circulating fluidized bed boiler (MTS-CFBB) was investigated to verify the corrosion effect of amino compounds on the boiler tube wall. The sodium salt which was found in the inner layer of the metal tube skin indicated that large area cracking occurred at the metal tube during the operation process; and further indicated that the corrosion rate of the tube wall was speedy. Meanwhile, the tube furnace experimental system and the reaction equations module in HSC Chemistry 6.0 were used to obtain the reduction properties of NH3 on Fe2O3 at high temperature. The weight loss rate of Fe2O3 under N2-NH3 atmosphere increased from 3.3 wt% to 5.5 wt%, when the temperature rose from 600 °C to 900 °C, and the weight loss rate of samples did not increase any more with the temperature rising to 1000 °C. The injected NH3 reduced the Fe3O4 to form FeO quickly at 810 °C. With the thermal stress caused by the boiler load adjustment and start up or shut down, the looser FeO formed at the interior of tubes, and then, the different expansion coefficient of Fe based compounds led to the extensive cracking. In summary, NH3 generated by SNCR was the main reason for corrosion happened on the tubes at high-temperature.

High‐temperature corrosion behavior of some post‐plasma‐spraying‐gas‐nitrided metallic coatings on a Fe‐based superalloy

AbstractThe objective of the present study is to propose a cost‐effective process for modifying commercially available coatings by gas nitriding using commonly available equipment and starting materials. Al–Cr and Ti–Al metallic coatings were deposited on Superfer 800H (Fe‐based superalloy) using a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma‐sprayed‐coated specimens. Characterization and high‐temperature corrosion behavior of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDX, and X‐ray mapping analysis were used for the characterization of the coatings and analysis of the oxide scale. Both the coatings successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation (Type‐I hot corrosion) at 900°C for 50 cycles in air and molten salt (a salt mixture of Na2SO4–60%V2O5 dissolved in distilled water). Based on the findings of the present study, the coatings under study are recommended for tapplications to super‐heater and reheater tubes of boilers and all those surfaces that face fireside corrosion, such as fluidized beds, industrial waste incinerators, internal combustion engines, gas turbines or steam turbines, to provide protection against degradation in these environments. The cost of the product/process is approximately Rs. 0.62 per mm2 in case of Al–Cr coating and Rs. 1.86 per mm2 in case of Ti–Al coating.

Study of particle residence time in a pressurized fluidized bed with in-bed heat exchanger tubes

Anthropogenic climate change is amongst the greatest of challenges to human civilization. A key area that will play a large role in mitigating its effects are clean fossil fuel applications. Clean coal combustion can be achieved with an oxygen-fired pressurized fluidized bed combustor incorporating carbon capture and storage. In relation to pressurized fluidization processes, understanding the influence of pressure on fluidized bed hydrodynamics and, in turn, their effect on parameters including fuel residence time is essential. For the combustor under consideration here, a fraction of the heat exchanger boiler tubes are submerged in the fluidized bed such that the effect of the horizontal tube bundle on the fuel residence time is of great importance. The main focus of the present work was to evaluate the impact of gas velocity, pressure, presence or absence of a horizontal tube bundle and fuel feed rate on the average fuel residence time in a dense gas-solid fluidized bed. Experiments were conducted under cold flow conditions in a pressurized fluidized bed with an inner diameter of 0.15 m. The fluidization material was large glass beads (1.0 mm in diameter) while fuel particles were simulated with smaller glass beads (64 and 83 μm in Sauter mean diameter) that were susceptible to entrainment. Operating pressures and superficial gas velocities were maintained between 101.3 and 1200 kPa and 1.5 and 3.2 Umf, respectively. To simulate continuous fuel injection, experiments were conducted with the fuel surrogate particles being continuously fed to the fluidized bed of large particles over a desired period of time. Downstream, entrained particles were captured to determine the average entrainment rate and average mass of fuel particles inside the fluidized bed at steady state, which yielded the average fuel residence time. The combination of elevated pressure with the tube bundle present was found to have the most influential impact when compared to base conditions of atmospheric pressure and with no tube bundle present. It was found to enhance gas bubble break up and reduce the average gas bubble size substantially. In turn, this increased the average residence time of 83 μm particles by nearly three-fold in comparison to the case of atmospheric pressure with no tube bundle present. The effect of gas velocity on particle residence time was not found to be statistically significant under the range tested. Similarly, the effect of increasing fuel feed rate by 50% had no statistically significant impact.

Improving the hot corrosion resistance of boiler tube steels by detonation gun sprayed coatings in actual boiler of thermal power plant

PurposeThis paper aims to investigate the hot corrosion behavior of Ni-Cr and Cr3C2-NiCr coatings, deposited on T11, P91 boiler steels by detonation gun spray coating (D-Gun) process to enhance high temperature corrosion resistance.Design/methodology/approachHot corrosion studies were conducted in secondary super heater zone of boiler at 900 °C for 10 cycles on bare and D-Gun coated steel specimens. The microhardness and porosity values of as-sprayed coatings were measured before exposing the specimens in the boiler environment. Each cycle consisted 100 h of heating in the boiler environment followed by 20 min of cooling in air. The weight change measurements were performed after each cycle to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction, SEM techniques were used to analyze the corroded specimens.FindingsUncoated boiler steel experienced higher weight loss. The Cr3C2-NiCr coating was found to be more protective than Ni-Cr coating. The phases revealed the formation of oxide scale on coated specimens, mainly consist of nickel and chromium, which are reported to be protective against the hot corrosion.Originality/valueThere is very limited reported literature on hot corrosion behavior of Ni-Cr and Cr3C2-NiCr coatings deposited on the T11 and P91 substrates by detonation gun (D-gun) spray technique. T11 and P91 alloy steels have been chosen for this study because these two alloys are used to manufacture boiler tubes used in Indian thermal power plants.

Experimental Evaluation of High Temperature Corrosion Performance of 75Ni25Cr Coated and Bare 347H SS in Air and Simulated Husk Fired Boiler Environment

Corrosion occurring at elevated temperatures is a serious problem in areas such as gas turbine applications and power plants that use high temperature boilers. In this paper, we carry out experimental investigation for reducing corrosion rate so as to enhance the corrosion and wear characteristics of 347HSS and quantify the effect of 75Ni25Cr on 347HSS in terms of improved corrosion resistance. A coating of 75Ni25Cr was deposited on 347HSS boiler tube by the process of Detonation Gun. Corrosion studies were conducted on bare as well as D-Gun coated 347H stainless steel specimens in air and simulated husk fired boiler atmosphere at 800°C under cyclic conditions. Each cycle consisted one hour of heating in the tubular furnace and 20 min of cooling at room temperature. Fifty such cycles were completed to observe performance of these specimens. The weight change measurements were performed after each cycle to establish the kinetics of corrosion using weighing balance. Scanning Electron Microscopy, X-ray diffraction and X-ray mapping analysis techniques were used to analyse the corrosion products formed on the surface of these substrates.