Final Superheater Research Articles

Effect of steamside oxidation and fireside corrosion degradation processes on creep life of service exposed boiler tubes

In conventional steam plants, the efficiency mainly depends on the outlet steam temperature of boiler. The outlet steam temperature is maintained by subjecting different sections of boiler to high temperatures. In these harsh conditions boiler tubes undergo different degradation processes viz creep, fatigue, steamside oxidation, fireside corrosion and erosion. Therefore, life evaluation of boiler components is required at regular intervals of time for better functionality of power plant. Various life assessment techniques like microstructural evaluation, hardness measurements, Oxide scale measurements, Accelerated temperature tests are used for assessing the residual life of boiler tubes. In this paper, the creep life is estimated using destructive accelerated temperature tests on several T22 tubes from various sections of Platen Superheater, Final Superheater and Reheater of boiler. Accelerated temperature tests are carried out at higher temperatures under the nominal steam load and remaining life is estimated by extrapolating the results for the constant service temperature. But, the service temperature of tube never remains constant during a regular plant operation due to insulation effect of steamside oxidation and fireside corrosion. Further, during start up and shut down of plant, tubes undergo low cycle fatigue damage. Though, the life consumption of boiler tube due to fatigue is infinitesimal compared to creep because of thinner section of tubes. Therefore, the effect of cyclic operation is not considered, but the effect of steamside oxidation, fireside corrosion and erosion are incorporated in the accelerated temperature test results in estimating the residual life of boiler tubes. Further, these results are compared with the life obtained from Oxide scale thickness measurements carried out by Non-destructive Ultrasonic technique. In both the methods, metal temperature calculation using steady state conductive heat transfer equations play a crucial role in estimating the residual life. Mainly, these methods are based on thermal properties of oxide scale and short term stress rupture tests. Therefore, these methods can be applied for different grades of boiler materials such as T23, T24, T91 and T92 etc for residual life estimation.

Reduction of main steam temperature deviation in a tangentially coal-fired, two pass boiler

This paper describes the main steam temperature deviation and its reduction by changing the injection angles of over-fire air in a 800MW, tangentially coal-fired, two pass boiler. The CFD simulation showed that the simulated gas temperatures and velocities in the furnace are well matched with the steam temperatures along the steam pathways and the tube metal temperatures of the heaters. The changes in injection angles of over-fire air clearly affect the temperature and flow fields in front of the final superheater, in turn, the main steam temperatures at the final superheater outlet. With the findings of the CFD simulation and the relevant plant data, the field tests to reduce the deviation were performed by changing the yaw and tilt angles of Separate Over-Fire Air (SOFA) and Closed Coupled Over-Fire Air (CCOFA). The field tests showed that, under the optimized injection angles of SOFA and CCOFA, the main steam temperature deviation was reduced from 27°C to 9°C and the reheat steam temperature was increased to the desired value.

Application of an empirical model in CFD simulations to predict the local high temperature corrosion potential in biomass fired boilers

To gain reliable data for the development of an empirical model for the prediction of the local high temperature corrosion potential in biomass fired boilers, online corrosion probe measurements have been carried out. The measurements have been performed in a specially designed fixed bed/drop tube reactor in order to simulate a superheater boiler tube under well-controlled conditions. The investigated boiler steel 13CrMo4-5 is commonly used as steel for superheater tube bundles in biomass fired boilers. Within the test runs the flue gas temperature at the corrosion probe has been varied between 625 °C and 880 °C, while the steel temperature has been varied between 450 °C and 550 °C to simulate typical current and future live steam temperatures of biomass fired steam boilers. To investigate the dependence on the flue gas velocity, variations from 2 m·s−1 to 8 m·s−1 have been considered. The empirical model developed fits the measured data sufficiently well. Therefore, the model has been applied within a Computational Fluid Dynamics (CFD) simulation of flue gas flow and heat transfer to estimate the local corrosion potential of a wood chips fired 38 MW steam boiler. Additionally to the actual state analysis two further simulations have been carried out to investigate the influence of enhanced steam temperatures and a change of the flow direction of the final superheater tube bundle from parallel to counter-flow on the local corrosion potential.

Failure Investigation of a Final Super Heater Tube in a 140 MW Thermal Power Plant

This article describes the findings of a detailed investigation into the failure of a final super heater tube in a 140 MW thermal power plant. Preliminary macroscopic examinations along with visual examination, dimensional measurement, chemical analysis, evaluation of mechanical properties, and oxide scale thickness measurement were carried out to deduce the probable cause of failure. In addition, optical microscopy was a necessary supplement to understand the cause of failure. It was concluded that the tube had failed due to stress rupture. The tube metal had been exposed to a temperature above the safe operating limit for a long duration. As a result the tube metal had lost its strength, suffered creep deformation and failed to support the working stress resulting in final rupture. The adherent scale, particularly on internal surface formed due to exposure at high temperature. It had acted as an insulating blanket in view of its much lower thermal conductivity than that of the tube metal. The progressive build-up of scale had greatly affected heat transfer and raised the tube metal temperature above A1 for a short duration before bursting of the tube. The overheating in this case may be due to poor circulation over the localized area.

Investigation of overheating of the final super-heater in a 660 MW power plant

Overheating of the final super-heater in a 660MW power plant after service for 4700h was investigated. The scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) was employed to observe the surface morphology and cross-section of the tube’s steam side and analyze the ingredient of oxide scales. Results show that exfoliation of iron oxides was detected at the steam side of the tube, resulting in the over-heating of the final super-heater. Compressive stresses in the oxide scale generated during the unexpected shutdown of the power plant, which was responsible for the exfoliation of iron oxides. Finally, the growth and exfoliation mechanism of the oxide scale were put forward to conduct the related prevention and control measures.

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.

화력발전소 주증기배관에서 밸브 차단에 따른 수증기 충격 특성에 관한 연구

본 연구는 화력발전소 최종과열기에서 고압터빈 사이 배관과 고압터빈을 지난 곳에 있는 체크밸브와 1차 재열기 사이 배관을 포함한 수증기 배관시스템에서 터빈의 급작스런 사고로 인해 터빈으로 들어가는 수증기를 차단할 때 발생하는 수증기 충격이 배관시스템에 미치는 영향을 분석하는 연구이다. 이를 위해서 수격현상 해석에 많이 사용하는 Flowmaster 소프트웨어로 배관시스템을 모델링하고 시간 변화에 따라 배관 내부의 압력, 질량유량률의 특성을 파악하였다. 이러한 특성으로부터 수증기 충격이 주로 영향을 미치는 곡관에서 수증기 충격에 의한 힘을 도출하였다. 본 연구를 통해서 수증기 충격은 주증기 차단 밸브 직전의 곡관과 체크밸브 이후에 바이패스 배관과 연결되는 곡관에서 수증기 충격에 의한 힘이 가장 크게 나타남을 밝혀냈다. 본 연구에서는 이렇게 도출한 힘의 기본 자료를 이용하여 차후 연구에서 화력발전소 수증기 배관시스템의 수증기 충격 시 곡관과 지지대의 안전성을 진단하는 토대를 구축하였다. The present study has been carried out to analyze the effect of steam hammering on the steam piping system including the final superheater, the high pressure turbine, check valve and the first reheater by sudden stoping of main stop valve in a power plant. For the present steam hammering analysis, the well known Flowmaster software has been used to model the steam piping system and the time dependent characteristics of pressure and steam mass flow rate has been conducted. Using the result of the unsteady pressure and steam mass flow rate, the forces acting on the elbows in the piping system has been derived. From the present analysis, it has been elucidated that the elbow just before the main stop valve and the elbow near the connection pipe between bypass pipe and check valve had the largest force among the elbows in the steam piping system. The structural safety diagnostics study on the elbow and the supporting structures of the steam piping system of a power plant will be conducted in the future by the present results of the forces acting on the elbow.

The Exploration and Practice of Using Additive to Inhibit the Heating Surface Deposition in Biomass-Fired Boiler

In response to the deposition problem in biomass-fired boiler, inert addictive was tested to inhibit the possibility of coking. The deposition samples collected from final and primary superheater of biomass-fired CFB (circulating fluidized bed) boiler were grinded into fine powder. The inhibitive ability of additive was tested by mixing the deposition sample with different proportions of inhibitor under high temperature. By means of scanning electron microscope (SEM), X-ray diffraction (XRD) and other analytical procedures, the properties of the samples with inhibitor such as microstructure, composition were studied. The inhibitor was also introduced to the industrial biomass CFB boiler to verify its inhibition effect on superheater deposition. Unfortunately, the effect of adding inhibitor in industrial boiler is not certain, though the inhibitor added to the samples is obviously effective in weakening coking in the laboratory scope.

Research on Welding Repair for Tee Joint Crack of Final Superheater Header of 300MW Station Boiler

Through methods, such as microscope analysis, metallographic examination, the crack located nearby the fusion line between tee joint and outlet steam pipe of subcritical power station boiler final superheater header is reheat crack (the main crack occurred in the heat affected zone and extended along the fusion line and grain boundary in HAZ. There are many small cracks paralleling with main crack). The base metal material (12Cr1MoV) and excessive welding residual stress are the primary inducement of crack. In repair crack, the measures of reducing residual stress, such as lower welding heat input, reducing thickness of welding layer, hammer blowing, assigning reasonably heat treatment power ,lengthening holding time and so on, were taken. The welding repair succeeded greatly.

Numerical and experimental investigations on the gas temperature deviation in a large scale, advanced low NOx, tangentially fired pulverized coal boiler

This paper describes the numerical and experimental investigations of the gas temperature deviation in the crossover pass of a large scale, advanced low NOx, tangentially fired pulverized coal boiler. Major attention was paid to the region of the final super-heater where the tube ruptures occasionally had been occurred due to the tube overheating. Computational Fluid Dynamics (CFD) simulation and measurements of gas temperature just in front of the final super-heater showed the gas temperature distribution was biased to the left, lower region of that heater. It was found that this deviation is originated from the residual swirl flow in the upper furnace. To alleviate the gas temperature deviation, the changes in yaw angles of over-fire air have been suggested and the field tests carried out to identify their effects on that deviation. The test showed it is quite effective method to reduce that deviation.

Practical use of defect assessment procedures for industrial component integrity assessment

The practical use of defect assessment procedures for industrial component integrity assessment is described through two practical examples in this paper. In the first example, the procedure is used to perform low temperature fitness-for-service (FFS) analysis of a longitudinal seam welded vessel manufactured from a duplex stainless steel. The impact energy obtained from the Charpy impact test performed on the weld of the vessel was found lower than the value of the minimum impact energy criteria given by the British Standard BS 5500. Charpy impact energy and fracture toughness correlation and failure assessment diagram (FAD) methodology were used in this FFS analysis to determine the fracture resistance of the vessel. For the assumed defect size used in the assessment, the weld was found to meet the fracture resistance criteria and therefore would still be fit-for-purpose. The FAD analysis was, however, repeated using the J-values obtained from the CTOD test to gain better confidence as the Charpy impact test does not provide direct measurement of the fracture toughness. The FAD was later used to determine the critical surface crack length which was then represented as a function of crack depth. The Charpy impact energy correlation method was found to be more conservative than the method of evaluation using the CTOD J-values. The lack of side wall fusion (LOF) which is a typical defect in this type of vessel would usually be influenced by the size/diameter of the electrode wire used in the weld and the number of runs. In this case, the fracture resistance of the 15mm thick vessel with the longitudinal seam weld should be adequate if less than 5mm diameter of electrode wire is used.The second example illustrates defect assessment of a high temperature plant component. A defect was found in the high pressure final superheater header. A defect assessment incorporating FAD on the header showed that the defect was non-critical. This led to the need to perform creep and fatigue crack growth calculations and remaining life assessment in order to determine the mitigation plan for the engineers. The deterministic approach, which mainly considers the worst case scenario, suggested that the remaining life of the header was approximately 4.5 years. Probabilistic analysis showed that the component could still be fit for service up to 6 years. This will allow the engineers to mitigate a more efficient plan with a decision to either repair or replace the header and when. The use of probabilistic lifing methodologies and algorithms could consequently bring considerable financial benefits to the plant owners/operators e.g. by avoiding premature component repair or replacement. Nevertheless, it would be in the management’s interest to avoid a forced outage. The recommendation would be that the component could still be fit to use until their next minor outage (in 3 years). From then, the remedy option would be to grind down to a depth of suspected crack, excavate a small area/surrounding which might be affected, followed by correct and regular monitoring, or alternatively replace the component.

Failure Analysis on Tee Joint Crack of Subcritical Power Station Boiler Final Superheater Header

Through methods, such as microscope analysis, mechanical analysis, chemical analysis, metallographic examination, the causes resulting in tee joint crack of subcritical power station boiler final superheater header (12Cr1MoV) have been analyzed. The result indicates the tee joint crack is reheat crack, and irrational structure and excessive welding residual stress are the primary inducement of tube joint crack.

Failure analysis of a boiler tube in USC coal power plant

This paper presents failure analysis of final superheater tube in ultra-supercritical (USC) coal power plant. Visual inspection was performed to find out the characteristics of fracture of the as-received material. And the micro-structural changes such as grain growth and carbide coarsening was examined by scanning electron microscope. Detailed microscopic studies were made to find out the behavior of the scale exfoliation on the waterside of tubes. From those investigations, the creep rupture may be caused by the softened structure induced by carbide coarsening and accelerated by the metal temperature increase by the impediment of heat transfer due to voids.

Some aspects of metallurgical assessment of boiler tubes—Basic principles and case studies

Microstructural changes in boiler tubes during prolong operation at high temperature and pressure decrease load bearing capacity limiting their useful lives. When the load bearing capacity falls below a critical level depending on operating parameters and tube geometry, failure occurs. In order to avoid such failures mainly from the view point of economy and safety, this paper describes some basic principles behind remaining life assessment of service exposed components and also a few case studies related to failure of a reheater tube of 1.25Cr–0.5Mo steel, a carbon steel tube and final superheater tubes of 2.25Cr–1Mo steel and remaining creep life assessment of service exposed but unfailed platen superheater and reheater tubes of 2.25Cr–1Mo steel. Sticking of fly ash particles causing reduction in effective tube wall thickness is responsible for failure of reheater tubes. Decarburised metal containing intergranular cracks at the inner surface of the carbon steel tube exhibiting a brittle window fracture is an indicative of hydrogen embrittlement responsible for this failure. In contrast, final superheater tube showed that the failure took place due to short-term overheating. The influence of prolong service revealed that unfailed reheater tubes exhibit higher tensile properties than that of platen superheater tubes. In contrast both the tubes at 50 MPa meet the minimum creep rupture properties when compared with NRIM data. The remaining creep life of platen superheater tube as estimated at 50 MPa and 570 °C (1058 °F) is more than 10 years and that of reheater tube at 50 MPa and 580 °C (1076 °F) is 9 years.

Nature of fireside deposits in a bagasse and groundnut shell fired 20MW thermal boiler

The nature of deposit formation on the fireside surfaces of the boiler tubes in the various parts (water walls, platen superheater, final superheater, economizer, electrostatic precipitator etc.) of a commercial 20 MW stoker-fired boiler being fired with a mixture of 80% bagasse and 20% groundnut shell has been analyzed. The deposits in the various portions of the boiler were characterized by particle size analysis, chemical analysis, X-ray diffraction and scanning electron microscopy. The deposits were found to be mainly quartz, alkali and alkaline earth silicates and sulfates. From the phase constitution and other microscopic characteristics of the deposit, it can be inferred that the silicates in the deposit formed through inertial impaction and the sulfates formed by vapor phase deposition.

SEM Investigation of Superheater Deposits from Biomass-Fired Boilers

Straw is used as fuel in relatively small-scale combined heat and power producing (CHP) grate boilers in Denmark. The large content of potassium and chlorine in straw greatly increases the deposit formation and corrosion of the superheater coils, compared to boilers firing coal. In this study, mature superheater deposit samples were extracted from two straw-fired boilers, Masnedø and Ensted, with fuel inputs of 33 MWth and 100 MWth, respectively. SEM (scanning electron microscopy) images and EDX (energy dispersive X-ray) analyses were performed on the deposit samples. Different strategies are adopted to minimize deposit problems at the two boilers. At Masnedø the final superheater steam temperature is 520 °C, no soot blowing of the superheaters is applied and a relatively large superheater area is used. At Ensted, an external wood-fired superheater is used in order to obtain a final steam temperature of 542 °C, while the steam exit temperature of the straw-fired boiler is 470 °C. The mature Masnedø deposit had a thickness of 2 to 15 centimeters and consisted of three distinct main layers. The thick intermediate layer was depleted in chlorine but rich in Si, K, and Ca. This Masnedø intermediate layer was probably generated by in-situ reaction between KCl and Si-rich ash particles, which leads to release of chlorine-containing gases. The innermost layer contained many sublayers of mainly iron oxide, KCl, and K2SO4. The Ensted deposit had a maximum thickness of a few centimeters. The intermediate Ensted layer consisted of melted KCl with inclusions of Ca- and Si-rich particles, and the innermost layer was an iron oxide next to a potassium sulfate layer. Compared to deposits formed on a probe during short-time experiments, the mature superheater deposits contained larger dense inner sublayers of pure KCl and K2SO4. The present study indicates that the innermost layer of the superheater deposits expands by condensation of KCl, even when the deposit has a thickness of several centimeters.

Combustion and deposit formation behavior on the fireside surfaces of a pulverized fuel boiler fired with a blend of coal and petroleum coke

The thermochemistry of the combustion of a blend of coal and 5% petroleum coke was analyzed. Thermodynamic modeling and microscopic techniques were used to study the behavior of the inorganic constituents upon combustion of the blend of coal and petroleum coke. The chemical composition and phase constitution of the combustion products, as well as the deposits at several temperatures corresponding to those at the various parts of the boiler, were deduced by free-energy minimization. These results were compared with actual results obtained from a commercial pulverized fuel boiler fired with coal and petroleum coke blend. The deposits on the fireside surfaces of the boiler tubes in the various parts (water walls, platen superheater, final superheater, economizer, and electrostatic precipitator) of the commercial pulverized fuel boiler fired with coal and 5% petroleum coke were characterized by particle size analysis, chemical analysis, x-ray diffraction, optical microscopy, and scanning electron microscopy. The combustion gas composition was measured using a portable on-line gas analyzer for N 2 , O 2 , CO 2 , H 2 O, CO, NO, and SO 2 . The thermodynamically predicted compositions and phase constitutions for the gas phase as well as the condensed phases are in good agreement with the experimental results.

HGSSP?A Computer Program for Simulation of Once-through Boiler Start-up Behavior

In this study a computer program, HaGuo Start-up Simulation Program (HGSSP), for thermal-hydraulic analysis is developed for simulation of start-up behavior of a once-through boiler equipped with various integral separator start-up systems. With the simulation results, design principles for optimizing a start-up system for design and optimization of start-up procedures for operation, which will result in minimum start-up losses by reducing the start-up time as much as possible, can be obtained. On the basis of Dolezal's model, an improved analytical-numerical method is proposed to solve the coupled energy equations for flue gas and working fluid describing the transient heat transfer occurring in individual heating surfaces. In order to verify the program presented here, the cold start of a 600-MW supercritical once-through boiler located in China on July 6, 1993, is simulated and a comparison with measured data made. This comparison indicates reasonable agreement between the computational main steam pressure, water separator level, and drain flow, economizer inlet and outlet temperatures, economizer pressure, and final superheater inlet temperature and those measured. Finally, the standard cold and hot start-up processes of a Harbin Boiler Works supercritical once-through boiler are simulated with HGSSP to acquire the start-up curves and suitable drain valve dimensions corresponding to a predetermined pressure-raising curve.

99/01427 Ash fusion temperatures and the transformations of coal ash particles to slag : Gupta, S. K. et al. Fuel Process. Technol., 1998, 56, (1–2), 33–43

The nature of deposit formation on the fireside surfaces of the boiler tubes in the various parts (water walls, platen superheater, final superheater, economizer, electrostatic precipitator etc.) of a commercial 20 MW stoker-fired boiler being fired with a mixture of 80% bagasse and 20% groundnut shell has been analyzed. The deposits in the various portions of the boiler were characterized by particle size analysis, chemical analysis, X-ray diffraction and scanning electron microscopy. The deposits were found to be mainly quartz, alkali and alkaline earth silicates and sulfates. From the phase constitution and other microscopic characteristics of the deposit, it can be inferred that the silicates in the deposit formed through inertial impaction and the sulfates formed by vapor phase deposition.

Estimation of Unmeasurable Input and States by Applying Fuzzy Control. Fuzzy Control for Steam Temperature of a Boiler Heating Surface.

Previous papers proposed a fuzzy applied observer(FAO)as a solution to the nonlinear estimation problem with unmeasurable input. This paper deals with a steam-temperature control(STC)of a once-through boiler using FAO. The STC consists of a final superheater(FSH), a spray attemperator at the FSH inlet, the FAO and two compensators. One object of compensation is delay in the dynamic behavior of steam temperature and the other is the effect of imbalance between disturbances in heat absorption and steam flow. The former is compensated for by state variable feedback and the latter is compensated for using fuzzy control to deviations in the temperature and the ratio of these two disturbances. Simulations are performed to confirm the control. The results show better control performance due to the suitable fuzzy applied observer compared to the uncompensated case.