Fireside Surface Research Articles

Root cause failure investigation of a boiler waterwall tube employed in a 325 MW thermal power plant: Caustic corrosion phenomenon and its effects

This paper focuses on failure analysis of a boiler waterwall tube, ASTM A213 Grade T2 steel, used in a 325 MW fuel oil-fired thermal power plant. The influence of fire/waterside factors on the failure was identified through multiple characterization techniques. Preliminary macroscopic observations revealed uniform heavy deposition and repair welding indications on the fireside surface of the failed tube. Moreover, a longitudinally hemielliptical inner groove along with excessive weld metal penetration and discrete cavities were observed along the crown of the tube, leading to local wall thinning down to around 50% of its nominal thickness. No noticeable hardness and microstructural variations such as pearlite colonies decomposition caused by long term overheating and formation of decarburized layer were detected. According to the results of micro-zone analyses, considerable quantity of sodium vanadium oxide compounds accumulated throughout the fireside surface of the tube was due to poor fuel quality used. The authors postulated that evaporation at the waterline of the approximately filled tube resulted in water/steam stratification and localized caustic constituents concentration within the inner groove like NaOH and alkaline producing salts containing Na, Mg, Ca and P elements entered into the water/steam cycle through condenser copper-based tube leakages. This trend solubilized protective magnetite film locally and, consequently, triggered caustic corrosion of the bare steel.

Evaluation of near-drum thinning data in recovery boiler generating bank tubes

Near-drum thinning affects the fireside surface of recovery boiler generating bank tubes near the surface of the mud drum. Although sophisticated thickness scanning equipment has been developed to rapidly make tens of thousands of thickness measurements in the portion of a tube that is vulnerable to near-drum thinning, methods for using these data to evaluate fitness-for-service have not shown similar advances. Non-destructive testing companies typically use a technician’s subjective judgment to identify the “thinnest reliable" thickness measurement on each tube. Some mills decide whether tubes can continue in operation or should be plugged or replaced based on this single thickness measurement. However, finite element analysis of the remaining strength of individual tubes thinned in the near-drum area suggests that it is essentially impossible to identify the weakest tubes from simple empirical rules. In the absence of an industry standard for evaluating these data, different mills could reach different conclusions about the fitness-for-service of a tube from the same data set. This paper reviews the technology for scanning the thickness of generating bank tubes and discusses approaches that have been used to identify the tubes most weakened by near-drum thinning and to evaluate the fitness-for-service of individual tubes.

Numerical simulation of porosity on thermal properties and fire resistance of foamed concrete

The relationship between thermal insulation properties and porosity of fly ash based foam concrete was built, in which effective density, effective heat conductivity, and effective specific heat of fly ash based foam concrete were deduced as functions of porosity. Using the model, the effective heat conductivity of density of 580 kg/m3 fly ash based foam concrete was the theoretically calculated as 0.145 W/(m K) while the experimental measured value was 0.142 W/(m K). The relative error of heat conductivity was very low at 2.1%. The effective specific heat within the model was 967.05 J/kg K and the experimental value was 920 J/kg K with a relative error of 5.1%. Then, the effective heat conductivity and specific heat models were incorporated into heat transferring model to calculate the temperature field of fly ash based foam concrete wall during a fire incident. Finally, the temperature field of fly ash based foam concrete wall and traditional dense concrete wall during fire incident were calculated and compared. Comparing the temperature field of the fly ash based foam concrete wall with the traditional concrete wall, it was found that at close to fire-side surface, the temperature in the fly ash based foam concrete wall could reach 1039 °C, while the lowest temperature in the fly ash based foam concrete wall remained at 20 °C for a thickness of 7 mm. In contrast, at close to fire side of surface, the temperature of traditional concrete wall was 987.2 °C at 360 s and the lowest temperature in the traditional wall was 102.9 °C at the opposite side-wall surface far away from the fire direction. As expected, the data demonstrated that the use of fly ash based foam concrete in wall construction adds greatly to the time for people to leave in safety.

Optimised selection of new protective coatings for biofuel boiler applications

AbstractUsing biofuels in power and CHP boilers can pose a challenge for materials performance. Formation of deposits containing e.g. potassium, sulphur, calcium, sodium, and chlorine can result in severe corrosion of conventional steels and alloys at relatively modest temperatures. Given suitable component design and fabrication facilities, coatings may be considered to protect the fireside surfaces. This paper aims to present a systematic approach to the design and selection criteria for protective coatings of boilers. The approach includes modelling of the process and surface conditions, optimisation of the coating process and structure, and performance validation in the laboratory and plant scales. The applied examples have included iron and nickel based HVOF and arc sprayed coatings subjected to verification field testing in boiler testing under aggressive biofuel conditions. The coatings have shown good corrosion resistance in both laboratory tests and long‐term harsh field tests. The paper discusses the used approach for finding a suitable and cost effective coating for biofuel boiler applications. The paper gives test results from microstructural, corrosion resistance and field testing experience for the selected coatings.

Examination of cracked primary air ports fabricated from modified Alloy 825-based composite tubes

Twenty primary air ports fabricated from modified Alloy 825-based composite tubes underwent a metallurgical examination to document the mode and extent of cracking on the external fireside surface of a kraft recovery boiler. Collectively, the crack features found are most consistent with thermal fatigue, but corrosion fatigue cannot be ruled out. Regardless of the true cracking mechanism, temperature cycling is implicated as a critical factor for crack propagation. On the basis of the relative crack lengths observed, membrane welds and tube weld repairs, and their adjacent heat-affected zones, appear to be more susceptible to cracking than the cladding itself.

Failure of evaporator tubes initiated by lamellar tearing during the commissioning of a waste heat recovery boiler

Successive failures of several new evaporator tubes during commissioning and trial run of a waste heat recovery boiler has been analyzed. The evaporator tubes are cold bent into U-shape and subsequently TIG welded to a 24-mm thick wear plate on either sides of the tube immediately adjacent to the bend. The failed samples showed stepped longitudinal cracks along the rolling direction and parallel to the weld fusion boundary at the fireside surface of the inner bend of the tube, prominently at the center of the bend. It was found that the failure of the evaporator tubes at the tube bends was initiated by lamellar tearing because of inherent defects in the material (segregation leading to banded structure), improper processing of material (inclusions with high aspect ratios along the rolling direction) and accelerated by high transverse shrinkage stresses over the entire bend portion of the tube introduced by the weld. The pre-existing fine cracks in the welded evaporator tube initiated by lamellar tearing have subsequently opened up during service when the total strain in the region increased because of steam pressure resulting in catastrophic failure of the tubes. Contrary to expectation, many of the elongated inclusions were found to be iron oxide scales and only few were manganese sulfide stringers enveloped by iron oxide scales. The iron oxide inclusions were resolved from the manganese sulfide by scanning electron microscopy and EDS spot analysis of the inclusions.

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.

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.

Phosphate induced stress corrosion cracking in a waterwall tube from a coal fired boiler

The failure of a waterwall tube in a coal-fired boiler from a thermal power station was analyzed. Mild whitish deposit formation in the water-side of the tube with multiple fine cracks on the internal surface (water-side) of the tube was observed in the failure zone. This was associated with localized wall thinning at the failed section. Swelling with a visible thin-lipped crack at the fireside surface of the failure zone was also observed. The failure zone was studied using stereo and optical microscopy, micro-hardness measurements and scanning electron microscopy in the secondary electron imaging and back scattered electron imaging mode. Further, the deposit was analyzed by X-ray diffraction and chemical analysis and the composition at the fracture surface was determined by EDAX. It was observed that the failure occurred because of localized caustic corrosion in the internal surface of the water wall tube followed by inter-granular stress corrosion cracking. The corrosion products at the grain boundaries were found to be mainly iron and sodium phosphates.

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.

The Corrosion Contribution to Circumferential Cracking in Waterwall Boiler Tubes

Abstract Circumferential cracking of waterwall boiler tubes is an important problem facing modern electric utilities that operate coal-fired boilers. Analysis of the problem indicated that the crack mechanism had two components, thermal stress and corrosion. The corrosion component was studied and found to be localized sulfidation corrosion. The sulfidation mechanism occurs beneath a previously existing oxide layer and attacks the fireside surface grain boundaries. These corroded grain boundaries act as surface notches, which concentrate the thermal stresses at the grain boundaries, leading to crack initiation. The corrosion mechanism also contributes to circumferential crack growth by attacking the tube steel in front of the crack tip.

A Laboratory Study of High-Temperature Corrosion on Fireside Surfaces of Coal-Fired Steam Generators

Three experimental approaches gave significant information on high-temperature fireside corrosion: 1 Sodium and potassium chlorides, injected through a gas flame, were deposited as such and then converted to sulfates on an iron surface. 2 Samples of mixed alkali and iron sulfates first showed decomposition and then formed melts at temperatures as low as 1070 deg F. This temperature is within the range that might be expected for fireside surface temperatures on superheater and reheater tubes of large steam generators. 3 Alkali metal chlorides and sulfates, in partial coverage of metal disks and in a flue gas atmosphere, formed fused deposits at temperatures as low as 900 deg F. Complex alkali iron sulfates may cause corrosion either by direct electrolytic corrosion or by altering the normally protective iron oxide scale so that the metal is exposed to direct oxidative corrosion.