Water Wall Tubes Research Articles

석탄화력발전용 초초임계압(USC) 보일러 수냉벽 튜브 용접 신뢰성 향상에 대한 연구

The low alloy-steel material(1.0Cr-0.5Mo, SA213T12), which has widely been used for the waterwall tube in the conventional power plant, do not have enough creep rupture strength for waterwall tubes of the Ultra-supercritical(USC) boilers. According to this reason, the high-strength low alloy-steel(2.25Cr-1.0Mo, SA213T22) has newly been adopted for the waterwall tube in the USC boilers. This paper presents failure analysis on weld-joint of the waterwall tubes in USC boilers. Visual inspections were performed to find out the characteristics of the fracture. Additionally both microscopic characteristics and hardness test were carried out on failed tube samples. Failures seem to happen mainly because the welding process has not been conducted strictly.(preheating, P.W.H.T and so forth). Thus, this paper has the purpose to describe the main cause of the poor welding process and to explain how to prevent similar failures in those weld-joints.

Hydrogen damage in a rear riser water wall tube of a power plant

Hydrogen damage in a rear riser water wall tube of a power plant

Effect of high-temperature corrosion on the gas-abrasive resistance of electric-arc coatings

We study the high-temperature gas-abrasive wear resistance of electric-arc coatings sprayed from powder wires. It is shown that the wear resistance of these coatings increases with temperature. This is explained by the effect of strengthening of lamellae in the coating, segregation of the dispersed phases, and the appearance of aluminum and chromium oxides (characterized by high shock resistance) both on the surface and between the lamellae. These coatings can be used for the restoration and protection of the heat-exchange surfaces of boilers of thermal power plants, including the large-size waterwall tubes and the tubes of economizers.

Weld Heat Tented Area Induced Failure of Boiler Water Wall Tubes in Power Generation Plant

The subject boiler is a vertical type with a capacity of 2063 ton/hr steam. Its operating and design pressure are 2608psi (18MPa) and 2928psi (20.2MPa) respectively while maximum outlet operating temperature is 1008oF (542.2oC). Water wall tubes are seamless tubes made of SA 210 and SA 213 T2 steels with nominal outer diameter of 2.5-3.0(63.5-76.2mm) and wall thickness of 0.25-0.313 (6.35-7.95mm). After 77360 operating hours, some water wall tubes have experienced leakage. Leaked tubes were subjected to different non-destructive and destructive examinations to clarify cause of failure. Visual investigation showed through-thickness hole in base metal, close to weld region of twoleaked tubes. Optical and scanning electron microscopic examinations of leaked zones disclosed complete decomposition of pearlite into ferrite and spheroidal carbides, creep voids, and grain boundary micro-cracking. It is concluded that failure of the subject boiler tubes is attributed mainly to long term localized overheating. This could be related to heat tinted area (oxidation) formed on tube outer surface by heating (arc strike) during welding. During operation, oxidized or scale layerformed on outer surface of heat-tinted area will be exfoliated due to thermal stresses. Repeated cycles of thermal oxidation and exfoliation of scale layer will result in localized thinning and subsequently, localized overheating leading to acceleration of microstructure degradation. Thinning of water wall-tubes together with its microstructure degradation willresult in increase in boiler pressure above the maximum safe working pressure leading to complete failure or leakage of tubes. It was recommended that boiler water wall tubes to be checked using different nondestructive methods during the nearest shut down and all thinned zones of tubes to be replaced. In order to decrease the possibility of such failure in future, proper welding procedure should be applied so that heat-tinted areas due to heating or arc strike on tube outersurface to be avoided.

Corrosion of Carbon Steel Tubes in the Mid-Furnance of Kraft Recovery Boilers--Environmental Characterization

Carbon steel tubes in the mid-furnace of kraft recovery boilers have experienced accelerated corro-sion in recent years. Gaseous environments in the mid-furnace of kraft recovery boilers were generally considered to be oxidizing and not very corrosive to the carbon steel waterwall tubes. However, mid-furnace corrosion has occurred above the cut line where composite tubes that have a stainless steel outer layer used in the lower furnace are welded to the carbon steel waterwall section. This study characterized the local environments in three boilers to find the reasons for accelerated corrosion of waterwalls in the mid-furnace. Gas sampling ports were installed in the mid-furnace with high and low corrosion rates of carbon steel waterwall tubes. During boiler operation, gases were sampled and analyzed using a specially designed gas chromatograph. In each boiler, the areas with higher corrosion rates had higher concentrations of reducing gases and sulfur-containing gases at the waterwall surface than in the low-corrosion areas. Results from this study indicate that accelerated mid-furnace corrosion likely results from changes in the boiler operation, such as decreases in the air supply at the lower air port levels of the furnace. These changes also might result in higher local gas temperatures and local thermal cycling in the mid-furnace of boilers.

An investigation of the heat transfer behavior of longitudinal finned membrane water wall tubes in circulating fluidized bed boilers

Experiments were conducted in a cold model circulating fluidized bed (CFB) having riser cross sectional area of 100 mm × 100 mm, height of 4.8 m, bed temperature of 75°C and superficial velocity of 8 ms − 1 . Local sand having average diameter of 231 μm, particle density of 2774 kg m − 3 and bulk density of 1515 kg m − 3 was used as bed material. The experiments were conducted for three tube configurations: a membrane tube, a membrane tube with a longitudinal fin at the tube crest and a membrane tube with two longitudinal fins at 45° on both sides of the tube crest. Heat transfer, heat transfer coefficient ratio and heat transfer ratio of the three tubes are presented as a function of cross sectional average suspension density. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes although they have lower heat transfer coefficient. In addition, numerical simulations were conducted to investigate the heat transfer behavior of the three tubes under the normal operating conditions of CFB boilers. It was found that the membrane tube with two longitudinal fins at 45° on both sides of the tube crest had the flattest inner wall tube heat flux profile. Moreover, in terms of temperature distribution in the tube material, it has the lowest profile.

Failure investigation on rear water wall tube of boiler

This paper presents failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements and microscopic examination. The rear water wall tube has failed with wide open burst and was situated at the boiler nose lower bent region. On-site wall thickness measurements were performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes were observed to have experienced significant wall thinning. Microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted in order to support in determining the failure mechanism and failure root cause. Failure mechanisms are discussed and the findings obtained from the site inspection, wall thickness measurements, microscopic examinations and creep analyses may finally reveal the failure mechanism and main root cause of the failure. The failure mechanism is identified as a result of the combination of the significant localized wall thinning of the rear water wall tube due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures.

Corrosion Characteristics and Mechanical Properties of Inconel 622 Weld Overlay of Waterwall Tubes in Coal Fired Boilers

Recently in Japan, boiler waterwall tube damage such as fireside corrosion and circumferential cracking in a low NOx environment has become a serious issue, despite the fact that relatively lower sulfur content coal is typically being used than in the US. Thermal spray coating is the most popular method for tube protection in Japan, and thermal spray coating tubes are used for this purpose.However, extensive damage to thermal spray coating tubes from cracking and exfoliation has recently been experienced. It is reported that the thermal fluctuations that occur as a result of operating changes create alternating stress, leading to cracking and exfoliation of the thermal sprayed thin coating. Corrosion-resistant weld overlays, such as stainless steel type 309, alloy 625 and alloy 622, are now commonly used to protect corrosion of boiler tubes in low NOx coal fired boilers in the US. In order to develop a fundamental understanding of the high temperature corrosive behavior of alloy 622 weld overlay, gaseous corrosion testing and certain mechanical tests for consideration of long-term aging were undertaken. After one year of service in the low NOx combustion environment of a coal fired supercritical boiler, field tests on alloy 622 weld overlay panels are continuing.This report describes the behavior of alloy 622 overlay panels in a field test installed in a Japanese supercritical boiler, the laboratory results of weight loss corrosion testing, and mechanical tests related to aging.

Origins of tensile stress-induced circumferential cracking of waterwall tubes in boilers

The leakage and rupture of boiler tubes in power plants is a serious problem that can lead to unscheduled and costly outages. The predominant failure location of current concern is circumferential cracking on the fireside of waterwall tubes in the furnace waterwall section of a boiler. Although there is basic agreement that cracking results from a combination of thermal fatigue and corrosion, a complete explanation of the basic phenomena needed to establish the root causes of this problem is lacking. The purpose of the present study was to analyse the sources of the tensile stress responsible for initiating circumferential cracking and to identify the key parameters controlling this tensile stress. The results of analytical modelling suggested that a combination of increasing tube wall temperature with increasing thickness of internal oxide layers, and temperature spiking due to deslagging events eventually may result in tensile stresses sufficient to crack the fireside oxide and initiate the development of circumferential cracks. This scenario also led to suggestions for reducing tensile stresses in waterwall tubes which, in turn, would be expected to delay/avoid circumferential cracking and improve the reliability of waterwall tubes.

A new method for identification of thermal boundary conditions in water-wall tubes of boiler furnaces

In this paper, a method for determining fireside heat flux, heat transfer coefficient on the inner surface and temperature of water-steam mixture in water-wall tubes is developed. The unknown parameters are estimated based on the temperature measurements at a few internal locations from the solution of the inverse heat conduction problem. The non-linear least squares problem is solved numerically using the Levenberg–Marquardt method. The diameter of the measuring tube can be larger than the water-wall tube diameter. The view factor defining the distribution of the heat flux on the measuring tube circumference was determined using exact analytical formulas and numerically using ANSYS software. The method developed can also be used for an assessment of scale deposition on the inner surfaces of the water-wall tubes or slagging on the fire side.

An experimental investigation of the effect of longitudinal fin orientation on heat transfer in membrane water wall tubes in a circulating fluidized bed

Experiments were conducted in a cold model circulating fluidized bed having riser cross-sectional area of 100 mm × 100 mm, height of 4.8 m, bed temperature of 75 °C and superficial velocity of 8 m s −1. Local sand having average diameter of 231 μm was used as bed material. The experiments were conducted for three tube configurations: membrane tube, membrane tube with a longitudinal fin at the tube crest and membrane tube with two longitudinal fins at 45° on both sides of the tube crest. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes and the heat is mainly transferred in the combination portion of tube and membrane fins. In addition, the membrane tube has the highest heat transfer coefficient.

Reliability of furnace waterwalls of hot-water boilers

It is shown that scale formation plays a deciding role among the factors that cause the wall temperature and axial thermal stresses to rise to limiting values at which waterwall tubes lose their strength. Conditions under which hydraulic nonuniformity of waterwall tube panels, a phenomenon stimulating the growth of scale in misadjusted tubes, occurs are considered. Ways for achieving better operational reliability of waterwall tubes are discussed.

A PDE model of a waterwalls steam generation process

This paper describes a model of a forced circulation waterwalls steam generator, derived from first principles. The distributed parameter criteria were applied to the heat transfer process and to the steam production inside the waterwalls. The model is capable of representing swell and shrink effects as well as the condensation–vaporization phenomena that take place inside the waterwall tubes, when large drum steam pressure variations are introduced. The swell and shrink effects are responsible for water displacement from the waterwalls to the drum and from the drum to the waterwalls. Open loop simulated test were produced with the steam pressure disturbance. Closed loop tests, including the models of the drum level and the combustion system and their control systems are presented.

Scale formation in waterwall tubes of boilers

It is shown that the amount of scale formed in waterwall tubes of boilers, as a function of the heat flux density, should be described by a second-power polynomial. At values of the heat flux density typical for modern boilers, this function can be replaced by a linear one. Comparison with experimental data was made.

A failure analysis and remnant life assessment of boiler evaporator tubes in two 250 MW boilers

It has been established from (a) the tube thermal flux calculations, (b) the failure surface fractographic features, (c) the data sheets on evaporator tube temperatures from both Mitsubishi and Seimens, (d) the reported data from the literature and (e) to a certain extent, the tube strain gauge data that the recent waterwall tube failures in two 250 MW boilers were the result of high stresses being active at certain boiler locations. At start-up these stresses were estimated from fractographic features to be around 370–420 MPa in magnitude, which resided between the tube materials yield and ultimate tensile strengths. The defect extension process in the present waterwall tube failures was shown to be fatigue crack growth, which is the most common failure mode in waterwall boiler tubes. By adopting two separate approaches, viz., an “upper bound” ASME XI design line and an influence diagram model approach which was designed specifically for waterwall tubes, detailed remnant life values could be estimated for a number of different situations. In the case of new replacement tubes, both approaches estimated a remnant life of about 9 years. Note that the failure probability of the upper bound ASME XI predictions was demonstrated to be around 10 −5. Also the influence diagram model approach was extremely helpful in identifying the probable root cause of corrosion fatigue failures in waterwall boiler tubes. With respect to the remnant life estimates two separate cases were considered, viz., (1) tubes with defect populations as recorded during the recent NDT study and (2) new replacement evaporator tubes which contained zero defects.

Erosion Wastage at High Temperature for Arc Spraying Coatings

A new novel cored wire for producing FeCrAl/WC intermetallic coating by arc spraying was developed. The erosion wastage at high temperature of the coating was researched with different impingement angles and temperatures. The results indicated that impingement angle and temperature greatly affected erosion wastage of the coating. At the same time, when impingement angle was 90°, different temperature had a little effect on the erosion wastage. And when temperature was 650°C, different impingement angle had a little effect on the erosion wastage too. As compared with water wall tube material (G20 steel) of power station boiler, the erosion resistance at high temperature of the coating was higher. The erosion resistance rate of the coating was 1.94~2.89 times of that of G20 steel when temperature was 650°C and particle was polygonal silica. The results indicated that erosive condition of high temperature flue gas and fly ash mixture in boiler tube of pulverized coal fired power station could be solved with the FeCrAl/WC coating.

A thermodynamic approach on vapor-condensation of corrosive salts from flue gas on boiler tubes in waste incinerators

Thermodynamic equilibrium calculation was conducted to understand the effects of tube wall temperature, flue gas temperature, and waste chemistry on the type and amount of vapor-condensed “corrosive” salts from flue gas on superheater and waterwall tubes in waste incinerators. The amount of vapor-condensed compounds from flue gases at 650–950 °C on tube walls at 350–850 °C was calculated, upon combustion of 100 g waste with 1.6 stoichiometry (in terms of the air–fuel ratio). Flue gas temperature, rather than tube wall temperature, influenced the deposit chemistry of boiler tubes significantly. Chlorine, sulfur, sodium, potassium, and calcium contents in waste affected it as well.

Computer system for monitoring power boiler operation

The computer-based boiler performance monitoring system was developed to perform thermal-hydraulic computations of the boiler working parameters in an on-line mode. Measurements of temperatures, heat flux, pressures, mass flowrates, and gas analysis data were used to perform the heat transfer analysis in the evaporator, furnace, and convection pass. A new construction technique of heat flux tubes for determining heat flux absorbed by membrane water-walls is also presented. Flux tubes mounted at different levels in the boiler work at similar conditions as water-walls tubes. The current paper presents the results of heat flux measurement in coal-fired steam boilers. During changes of the boiler load, the necessary natural water circulation cannot be exceeded. A rapid increase of pressure may cause fading of the boiling process in water-wall tubes, whereas a rapid decrease of pressure leads to water boiling in all elements of the boiler's evaporator — water-wall tubes and downcomers. Both cases can cause flow stagnation in the water circulation leading to pipe cracking. Two flowmeters were assembled on central downcomers, and an investigation of natural water circulation in an OP-210 boiler (with steam capacity of 210 × 103 kg/h) was carried out. On the basis of these measurements, the maximum rates of pressure change in the boiler evaporator were determined. The on-line computation of the conditions in the combustion chamber allows for real-time determination of the heat flowrate transferred to the power boiler evaporator. Furthermore, with a quantitative indication of surface cleanliness, selective sootblowing can be directed at specific problem areas. A boiler monitoring system is also incorporated to provide details of changes in boiler efficiency and operating conditions following sootblowing, so that the effects of a particular sootblowing sequence can be analysed and optimized at a later stage. The current paper also presents an analysis of stresses occurring in the boiler drum and the downcomer junction during-start up and shut-down of the boiler.

An analysis and assessment of factors causing damage to the waterwall tubes of high-pressure drum boilers

Possible factors causing damage to the waterwall tubes of high-pressure drum boilers are analyzed and assessed on the basis of data from the Ust’-Ilimsk cogeneration station and papers published over the last 10–12 years. The question of diagnosing the period between washing operations is considered.

Wastage of Water Wall Tubes in a Circulating Fluidized Bed Combustor with Two Gas Exits

The tube wastage profiles of water walls are discussed in a commercial CFB furnace with two gas exits. The lateral profile of wear rate confirms that tube wastage is influenced mainly by secondary flow, stronger toward the center than in the corners, and somewhat by the wing walls and gas exits. The effect of two superposed secondary flows of the second kind, one caused by the shape of the entire freeboard cross-section and the other by sections between the wing walls, could also be confirmed in lateral profiles of tube erosion rate on the wall connected with wing walls. Appreciable tube erosion rate was confirmed near the center of the rear wall at lower heights and near the center of both sidewalls at heights above the gas exit level. Wastage of wing walls increased with distance into the interior of the furnace.