Superheater Tubes In Boilers Research Articles

Investigation into Cause of High Temperature Failure of Boiler Superheater Tube

AbstractThe failure of the boiler tubes occur due to various reasons like creep, fatigue, corrosion and erosion. This paper highlights a case study of typical premature failure of a final superheater tube of 210 MW thermal power plant boiler. Visual examination, dimensional measurement, chemical analysis, oxide scale thickness measurement, microstructural examination are conducted as part of the investigations. Apart from these investigations, sulfur print, Energy Dispersive spectroscopy (EDS) and X ray diffraction analysis (XRD) are also conducted to ascertain the probable cause of failure of final super heater tube. Finally it has been concluded that the premature failure of the super heater tube can be attributed to the combination of localized high tube metal temperature and loss of metal from the outer surface due to high temperature corrosion. The corrective actions have also been suggested to avoid this type of failure in near future.

Long-term Overheating Cracking Analysis of 12Cr1MoV Boiler Superheater Tube

After running 39 840 h, a boiler superheater tube made by 12Cr1MoV steel in a plant cracked. The tube is blocked and the thick-lip-shape fracture is covered with oxides. By utilizing optical microscope(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), transmission electron microscopy(TEM) and other methods to detect the tube, it is found that the compositions of blocking substances are Na3PO4, Fe2O3 and NaFeO2. In the unblocking position of the inner wall oxide is Fe3O4, however, in the blocking position, it is Fe2O3. In terms of the shape of the fracture morphology, it is inter granular, along with secondary cracks and pores in the grain boundary. The research results illustrate that the metallographic microstructures in samples of near crack and away from the crack are the same. With the lamellar pearlite of the initial state dissolves, the carbides gather and grow at the grain boundaries. The organization spherifies seriously, and the carbide is M23C6. According to the electric power industry standard, the spherical degree is identified as level 5. By utilizing spheroidization level and run-time to estimate the wall temperature, it can be know the tube run in over-temperature environment. To be specific, the hardness in all the samples is about 135 HV, which is relatively low. In all, the failure mode of superheater tube is high temperature creep cracking which is caused by long-term overheating resulted from the oxide layer and the blocking substances.

Investigations on the behavior of laser cladding Ni–Cr–Mo alloy coating on TP347H stainless steel tube in HCl rich environment

Combustion of biomass with a high content of alkali and chlorine can result in operational problems including deposition and rapid corrosion of critical components such as superheater tubes. Alloy modification is an effective way to reduce corrosion problems. Considering the excellent corrosion resistance but high cost of Ni–Cr–Mo alloy bar, the microstructure and high-temperature corrosion behaviors of the laser cladding Ni–Cr–Mo alloy coating were investigated and compared with the Ni–Cr–Mo alloy bar and the biomass boiler superheater tube material TP347H stainless steel in this study. Corrosion exposures were carried out in a tube furnace under simulated combustion gases containing 12%CO2–5%O2–0.05%HCl–N2 for 168h at 650°C and 700°C. The performance of three materials was evaluated by means of several analysis methods including: kinetic analysis (corrosion rate), SEM (microcosmic morphology of surface and cross-section),EDS (distribution of elements of cross-section) and XRD (phases formed on the surface layer). Compared to the other two materials, the overall performance was better for the laser cladding Ni–Cr–Mo alloy coating which on one hand had optimal ratio of corrosion resistance elements and on the other hand had grain strengthening effect by laser cladding technique.

Probabilistic creep rupture life evaluation of T91 alloy boiler superheater tubes influenced by steam-side oxidation

A systematic probabilistic evaluation method for the long‐term creep rupture life (LCL) of T91 alloy superheater tubes is proposed in this work. The effects of steam‐side oxidation on the effective load‐bearing thickness and temperature of tube wall are taken into account, and the latest assessment results of the creep rupture strength of T91 alloy are utilized. Since the uncertain factors including not only the geometry dimensions but also the operational conditions may hinder a reliable analysis, instead of usual deterministic methods, probabilistic life evaluation employing Monte Carlo simulation method is performed. The effects of initial inner tube diameter, steam temperature, and steam flow on the LCL of superheater tubes are analyzed, and simple fitting functions formulating the analysis results are obtained. The survival probability of the tubes, which reflects the possibility for the LCL to be longer than a given time, is calculated. It is found that when the initial inner tube diameter or the steam temperature increases, the survival probability of the tubes exhibits a decrease trend. However, the steam flow has an opposite effect. Based on the conclusions, feasible measures that can be adopted to improve the survival probability of high temperature steam generation components in coal‐fired power plants are introduced.

Numerical Analysis of Stress and Temperature Coupled Field for Boiler Superheater in Thermal Power Plant

Boiler failure is the main power plant boiler operation fault. And boiler superheater tube is one of the important reasons for boiler failure, which affects the safe and stable operation of power generation equipment. Structure design and calculation of boiler superheater has important influence on performance and life of boiler, so three-dimensional structural model of boiler superheater is built in this paper. Based on the finite element method boiler superheater in temperature and stress coupled field are analyzed. The highest temperature, maximum stress, and distribution characteristics of stress and temperature couple field of boiler superheater including straight and elbow section are computed. The date on the actual operation of a power plant was used in the process of analysis. And thermal physics and mechanical properties of the boiler superheater tube were taken full consideration. It provides a reliable basis for reasonable design and safety evaluation of the boiler superheater.

Oxide scale exfoliation and regrowth in TP347H superheater tubes

AbstractThis paper provides an introduction to a comprehensive model being developed to predict and control oxide scale exfoliation from the steam‐side of superheater and reheater tubes in steam boilers. The model deals with the main phenomena involved in scale growth and failure in steam, and incorporates major variables related to boiler design and operation. The considerations used to calculate oxide growth under the specific constrains of small diameter tubes carrying high‐pressure steam and operating with large temperature gradients under temperature and pressure cycling conditions, as well as the evolution of stresses and strains in the scales, are indicated but only a cursory description is given of the details of the analytical treatments. An example is presented of calculations made with the model to predict the extent of blockage expected in a single superheater loop as a function of time and outlet steam temperature under several realistic service conditions. The results suggest that problems due to scale exfoliation would be expected early in the operating life of superheater tubes made from austenitic steel TP347H.

X20CrMoV12.1강의 열화평가에 관한 연구

Power plant boiler is one of the most important utilities providing steam to turbine in thermal power plant. It is composed of thousands of boiler tubes for high efficient heat transfer. Boiler tube material is used in such high temperature and pressure as <TEX>$540^{\circ}C$</TEX>, <TEX>$170kg/mm^2$</TEX>. The boiler tube material is needed to resist corrosion damage, creep damage and fatigue damage. 2.25%Cr-1Mo steel is used for conventional boiler tubes. In these days steam temperature and pressure of the power plant became higher for high plant efficiency. So, the material property of boiler tube must be upgraded to meet the plant property. Several boiler tube material was developed to meet such condition. X20CrMoV12.1 steel is also developed in early 1980's and used for superheater and reheater tubes in supercritical boilers. The material has martensitic structure, which is difficult to evaluate the material degradation. Boiler tube material at severe condition was tested to evaluate long term and short term degradation and creep. Through long term and high temperature degradation test, lath structure was decreased and recrystallization has been proceeded by sub-crystal. And in this research the effect of temperature and stress on boiler tube characteristic,for example, deformation by creep was changed rapidly at relatively high temperature and stress because creep was affected easily by temperature and stress.

The Application of Radiation Shields for Thermal Control of Superheater Tubes in Boiler

Superheater tubes temperature control is a necessity for long lifetime, high efficiency and high load following capability in boiler. This study reports a new approach for the control strategy design of boilers with special shields. The presented control strategy is developed based on radiation thermal shields with low emissivity coefficient and high reflectivity or scattering coefficient. In order to simulate the combustion event in boiler and heat transfer to superheater tubes, an effective set of computational fluid dynamic (CFD) codes is used. Results indicate a successful identification of over- heated zones on platen superheater tubes and effect of radiation shields for solving this problem.

Fatigue Failure of 321 Stainless Steel Superheater Tubes from a Thermal Incinerator

Several boiler superheater tubes showed circumferential cracking at weld seams after 2 years in noncontinuous service (several shutdowns). On-site inspection revealed that several tubes were cracked and leaked; while many others were cracked, however, the severity was less pronounced. Two types of superheater tubes samples were collected: one with butt-welded tubes and the other with fillet-welded sleeve. The latter was found to be out of the boiler fireplace, and the sleeve was used as tubing support to the boiler shell. Detailed investigation showed that the butt-welded tubes contained circumferential fatigue cracks that initiated from the internal surface. The cracks initiated in the heat-affected zone and propagated as a result of tube vibration. The variations in the tube internal diameter and tube wall thickness are expected to play a role in tube fatigue failure. On the other hand, tubes with fillet-weld sleeve showed circumferential cracking as a result of fatigue crack initiation from weld defects on the tube external surface. The high vibration during several unscheduled shutdowns in addition to several other factors such as variations in tube inside diameter, wall thickness, and weld defects resulted in the initiation and propagation of fatigue cracks and premature failure. White deposits, similar to those observed when boiler tubes failed by caustic exposure, were seen in the vicinity of the tube cracks. Therefore, it was difficult to confirm whether the boiler tubes failed because of the fatigue cracks or because of the caustic salts (pH control chemical).

Características de los recubrimientos WC-NiCrBSiFe en función del sistema de proyección térmica

WC-NiCrBSiFe coatings are normally used as protection against heat, corrosion and erosion actions encountered in superheater and reheater tubes in power plant boilers. This coating was thermal-spray projected using different procedures (plasma, HVOF and HFPD) onto stainless steel specimens. The microstructures, porosities, oxide contents and microhardnesses of the coatings were determined. Thermal fatigue tests under simulative power plant service conditions were conducted in an experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer was evaluated by means of tensile tests.The obtained results are discussed, and special attention was paid to the specific characteristics of the different spraying procedures.

Influence of thermal-spray procedure on the properties of Cr&lt;sub&gt;3&lt;/sub&gt;C&lt;sub&gt;2&lt;/sub&gt;-NiCr coating

A Cr 3 C 2 -NiCr coating ( 5 y 106 pm.), was thermal-spray projected using different procedures (plasma, HVOF and HFPD) onto stainless steel specimens. This type of coating is normally used as protection against heat, corrosion and erosion actions encountered in superheater and reheater tubes in power plant boilers. The microstructures, porosities, oxide contents and microhardnesses of the coatings were determined. Thermal fatigue tests under simulative power plant service conditions were conducted in an experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer was evaluated by means of tensile tests. The obtained results are discussed, and special attention was paid to the specific characteristics of the different spraying procedures.

Monitoring of oxidation of metals in gas environments for energy production

Monitoring of oxidation in gas environments typical of energy production is essential for determination of lifetime and condition of various components operating at high temperatures. One of the most important phenomena to be monitored is the oxide growth kinetics and its changes during the exposure. Three different monitoring methods have been presented in this paper. First, a conventional method called thermogravimetry based on the weight change measurement during oxidation is reviewed. Second, a novel monitoring technique, HT-CER technique, where contact electric resistance is measured as a function of time and distance of the measuring tip is presented. The third approach described involves a method under development, based on the change of capacitance over two growing oxide surfaces. Simple oxidation in air at a typical temperature for superheater tubes in boilers, 600 °C, has been selected as the reference condition for all of the monitoring methods.

Influence of the thermal-spray procedure on the properties of a nickel-chromium coating

A modified NiCr coating was thermal-spray projected using different procedures (flame, plasma, HVOF and HFPD) onto stainless steel specimens. This type of coating is normally used as protection against heat, corrosion and erosion actions encountered in superheater and reheater tubes in power plant boilers. The microstructures, porosities, oxide contents and microhardnesses of the coatings were determined. Thermal fatigue tests under an atmosphere similar to power plant service conditions were conducted in an experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer was evaluated by means of tensile tests. The results obtained are discussed, with special attention being paid to the specific characteristics of the different spraying procedures.

Effects of fuel impurities on the fireside corrosion of boiler tubes in advanced power generating systems—a thermodynamic calculation of deposit chemistry

Thermodynamic equilibrium calculation relating fuel chemistry with flue-gas composition and volatile condensate deposits on tube metals upon combustion of various “dirty” fuel was conducted for a better understanding of the deposit chemistry of superheater tubes in steam-generating boilers. Corrosive impurities such as sodium, potassium, chlorine, and sulfur, inevitably involved in fuel, were considered in the calculation. Possible influence of flue-gas temperature on deposit chemistry was investigated as well. Based on the flue-gas composition and the deposit chemistry, corrosion environments of steam-generating boilers firing various “dirty” fuel were discussed.

Modelling Of Particle Deposition On Superheater Tubes In Kraft Recovery Boilers

A numerical model ia developed for predicting the rate of the deposition of carryover materials on superheater tubes in kraft recovery boilers. The rate is calculated by predicting the probability for a given carryover particle, exposed to turbulent fluctuations, to reach the superheater tube bank. Results indicate that large particles tend to deposit at the leading edge of the tube bank while smaller particles are collected further downstream. The predicted deposit growth rates are consistait with observations in recovery boilers.

A study on the evaluation for material degradation of 0.0Cr-0.5Mo steel by a electrochemical polarization method

The contents of this paper include a non-destructive technique for evaluating the degradation of the boiler superheater tube in a fossil power plant through an electrochemical polarization test. Correlation between Ip of polarization parameter and SP-DBTT for the superheater tubes in long-term use was obtained. 1.0Cr-0.5Mo steel was degraded by softening, and the degree of degradation was dependent upon carbides with Cr and Mo elements. Since brittle fracture at low temperature and ductile fracture mode at high temperature were shown, similarity between standard Charpy and small punch tests could be found. In addition, SP-DBTT showing the degree of degradation was higher, as the time-in use of the materials got longer. Electrolyte including picric acid of 1.3 g in distilled water of 100ml at 25.deg. C temperature and sodium tridecylbenzene sulfonate with 1g could be applied to evaluate the degradation of 1.0Cr-0.5Mo steel by means of the electrochemical polarization test. Ip and Ipa values measured through the electrochemical test are the appropriate parameters for representing the degradation of the superheater tube(1.0Cr-0.5Mo steel) for the fossil power plant. It is poassible to evaluate the degradation of materials with different time histories electrochemically, by Ip value only, at field test.

95/06279 A creep damage estimation method for in-service fossil fuel boiler superheater tubes

95/06279 A creep damage estimation method for in-service fossil fuel boiler superheater tubes

Errata: “A New Approach to Stress Analysis of Various Wound Vessels” (Journal of Pressure Vessel Technology, 1994, 116, pp. 359–364); “A Creep Damage Estimation Method for In-Service Fossil Fuel Boiler Superheater Tubes” (Journal of Pressure Vessel Technology, 1995, 117, pp. 14–18); “Life Assessment of 1 1/4 Cr-1/2 Mo Steel Catalytic Reformer Furnace Tubes Using the MPC Omega Method” (Journal of

Errata: “A New Approach to Stress Analysis of Various Wound Vessels” (Journal of Pressure Vessel Technology, 1994, 116, pp. 359–364); “A Creep Damage Estimation Method for In-Service Fossil Fuel Boiler Superheater Tubes” (Journal of Pressure Vessel Technology, 1995, 117, pp. 14–18); “Life Assessment of 1 1/4 Cr-1/2 Mo Steel Catalytic Reformer Furnace Tubes Using the MPC Omega Method” (Journal of

Experience of high chromium ferritic steel tubes in power plant

A new steel low C-9Cr-2Mo steel has been developed for use as superheater and reheater tubes in power boilers. The steel has higher strength than 21/4Cr-1Mo or 9Cr-1Mo steel and fills the gap in elevated temperature strength between these ferritic grades and the austenitic stainless grades. Because of the steel’s strength and its good oxidation resistance up to 1202 °F (650 °C), it may provide an economic alternative for austenitic stainless tubes. The steel has been installed in a number of power boilers. This paper reports on the examination of 9Cr-2Mo reheater and superheater tubes removed from the oil and blast furnace gas fired 156 MW power boiler.

Corrosion of austenitic steels in molten sulphate deposits

Thermal analysis has been used to determine the melting behaviour of alkali metal sulphate deposits typical of those found on superheater tubes in coal-fired power station boilers. Additions of 5–30 m/o Fe 2(SO 4) 2 to Na 2SO 4K 2SO 4 mixtures under a simulated flue gas (N 2 + 15 v/o CO 2 + 1 v/o O 2 + 0.3 v/o SO 2) are shown to reduce the melting point from 820°C to below 550°C. Alkali-iron trisulphates are formed which re-solidify on heating above 720°C by decomposition of Fe 2(SO 4) 3 at low thermodynamic activity. It is suggested that the effect of heat flux and SO 3 potential gradient on the melting behaviour of a superheater deposit could account for the observed ‘bell-shaped’ temperature dependence of corrosion rate in the range 550–750°C. Results are presented of laboratory corrosion tests on three commercial austenitic stainless steels using a melt of composition 74 m/o Na 2SO 4 + 20 m/o K 2SO 4 + 6 m/o Fe 2 (SO 4) 3. A model is proposed for corrosion by an acid fluxing mechanism involving refractory metal alloy elements in the steels. The model offers a quantitative basis for prediction of fireside corrosion resistance in coal-fired boilers, and a comparison of relative corrosion rates of a range of steels in laboratory tests, probe trials and service performance confirms the validity of the present model.