All Volatile Treatment Research Articles

The Study of Multiaxial Loading and Damage to the Structure and Materials in the PWR Steam Generator of Nuclear Reactor

In Pressurized Water Reactor (PWR) Nuclear Power Plants (NPPs), the steam generator is crucial for transferring heat from the primary to secondary cooling systems, vital for steam production to drive turbines, and central to nuclear power safety. This study explores recent research on multi-axial loading, structural integrity, and material durability in PWR steam generators, shedding light on key factors affecting these systems. Common corrosion-related degradation in steam generators often arises from design, material, and water chemistry factors. However, the shift to All Volatile Treatment (AVT), the development of advanced material alloys, and enhanced water quality control in primary and secondary systems have significantly reduced instances of steam generator degradation. These findings promise to enhance the reliability and safety of steam generators in future nuclear applications.

Characterization of the Oxide Scale Formed on T12 Water Wall Tube After Long-term Service in Supercritical Power Plant

Oxide scale control is one of the critical maintenance issues in fossil fuel power plant. Hence, the water treatment of the feed water has been changed from all-volatile treatment (AVT) to oxygenated treatment (OT) by added some amount of oxygen in the system to form a stable oxide and minimize the scale growth rate. In this work, the oxide scales formed on T12 water wall tube after one cycle of chemical cleaning treatment under OT condition in a supercritical coal power plant were characterized by using various methods. It was found that the oxide scale formed on the inner surface of the tube had a multi-layer structure with a porous outer layer consisting of Fe3O4 and Cr-rich α-Fe2O3 oxides and a dense inner layer consisting of Cr-rich spinel oxide. A Cr- and Mn-enriched outermost layer was also observed. Cavities, pits, and exfoliation were observed in the oxide scales. The cavities and grain boundaries behaved as the short path of the diffusion of oxygen into the metal. Intergranular oxidation was observed between the inner layer of oxide scale and matrix. The distribution, morphology, and chemical compositional of these oxides were assessed and discussed from a point of view of the suitability of current chemical cleaning solution and procedure which has been used for AVT descaling.

Using of Pourbaix diagram for evaluation of water chemistry recommended for ultra-supercritical parameter of water coolant

An important indicator the reliable operation of the steam-water cycle at power plants is water chemistry. Special chemicals are used to correct water chemistry. At designing a new unit the selection of the chemicals is determined by the parameters of the water and steam and materials, used in different parts of the cycle. Oxygen and ammonia are used to correct water chemistry at units with supercritical parameters In the presence of these reagents the protective film is formed on the surface of the metal.The transition to ultra-supercritical parameters requires the using of new materials, used in the water-steam cycle. Austenitic steels are recommended to replace on inconel steels. Oxygenated treatment and all-volatile treatment under oxidizing conditions were recommended to apply at units of ultra-supercritical parameters. However to select the optimum water chemistry is necessary to have date on the conditions of formation the protective film on the inconel surfaces.On the basis of Pourbaix diagram is proposed to evaluate the influence of water chemistry on the forms of existence the oxides of nickel and chromium on the surface of inconel at high parameters of water coolant. The results of calculations are compared with the experimental data.

Removal of Iron Oxide Scale from Feed-water in Thermal Power Plant by Using Magnetic Separation

One of the factors of deterioration in thermal power generation efficiency is adhesion of the scale to inner wall in feed-water system. Though thermal power plants have employed All Volatile Treatment (AVT) or Oxygen Treatment (OT) to prevent scale formation, these treatments cannot prevent it completely. In order to remove iron oxide scale, we proposed magnetic separation system using solenoidal superconducting magnet. Magnetic separation efficiency is influenced by component and morphology of scale which changes their property depending on the type of water treatment and temperature. In this study, we estimated component and morphology of iron oxide scale at each equipment in the feed-water system by analyzing simulated scale generated in the pressure vessel at 320 K to 550 K. Based on the results, we considered installation sites of the magnetic separation system.

Corrosion Behavior and Oxide Film Formation of T91 Steel under Different Water Chemistry Operation Conditions

The corrosion behavior of a ferritic/martensitic steel T91 exposed to an aqueous solution containing chloride and sulfate ions is investigated depending on the stimulated all-volatile treatment (AVT) and under oxygenated treatment (OT) conditions. The corrosion of T91 steel under OT condition is severe, while the corrosion under AVT condition is not. The co-existence of chloride and sulfate ions has antagonistic effect on the corrosion of T91 steel in both AVT and OT conditions. Unlike to corrosion resistance in the aqueous solution, OT pretreatment provides T91 steel lower oxidation-resistance than VAT pretreatment. From scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis, the lower corrosion resistance in the aqueous solution by VAT conditions possibly is due to the formation of pits. In addition, the lower oxidation resistance of T91 steel pretreated by OT conditions is explained as follows: the cracks formed during the immersion under OT conditions accelerated peeling-off rate of the oxide film.

International water and steam quality standards on thermal power plants at all-volatile treatment

One of the methods for the improvement of reliability and efficiency of the equipment at heat power plants is the decrease in the rate of corrosion of structural materials and sedimentation in water/steam circuit. These processes can be reduced to minimum by using the water with low impurity content and coolant treatment. For many years, water and steam quality standards were developed in various countries (United States, Germany, Japan, etc.) for specific types of equipment. The International Association for the Properties of Water and Steam (IAPWS), which brings together specialists from 21 countries, developed the water and steam quality standards for various types of power equipment based on theoretical studies and long-term operating experience of power equipment. Recently, various water-chemistry conditions are applied on heatpower equipment including conventional boilers and HRSGs with combined cycle power plants (Combined Cycle Power Plants (CCPP)). In paper, the maintenance conditions of water chemistry with ammonia or volatile amine dosing are described: reducing AVT(R), oxidizing AVT(O), and oxygen OT. Each of them is provided by the water and steam quality standards and recommendations are given on their maintenance under various operation conditions. It is noted that the quality control of heat carrier must be carried out with a particular care on the HPPs with combined cycle gas turbine units, where frequent starts and halts are performed.

Feed Water Chemistry-Related Corrosion Failures in Subcritical 250 MW Coal-Fired Boiler

This paper emphasis on the study of typical premature failure of water wall tubes of two thermal power plant boiler of same capacity (250 MW) and same operational parameter but with different boiler water chemistry. The investigation concludes on the water-side corrosion in both of the cases. One boiler is running with coordinated phosphate treatment (CPT) and another with all volatile treatment (AVT). The causes of corrosion were discovered and proposed measures for their elimination were given. Visual examination, chemical analysis of deposits, oxide scale thickness measurement, and microstructural examination were carried to ascertain the probable cause/causes of failure. From the investigation, it was finally concluded that the combination of localized high tube metal temperature and wall thinning due to under deposit corrosion led to the premature tube failure in boiler running with CPT and localized pitting corrosion in boiler running with AVT. Based on the results and discussions, a possible way to combat the corrosion was proposed.

Deposition of Magnetite on 304 SS Under Phosphate & Caustic Treatment Regimes in Boiler Environments

Under the high pressure high temperature (HPHT) conditions in boilers, iron oxides, like magnetite and hematite, are the most common corrosion products. Apart from just degrading the base metal in the feedwater system, corrosion products can also be transported downstream, where they can potentially deposit on surfaces in the boiler. Deposition of magnetite on orifice plates in lower headers of forced circulation boilers leads to improper flow patterns, poor heat transfer and in some cases lead to tube failures. Field data suggests that the boiler operating conditions, like temperature, pressure and boiler water chemistry could influence the deposition of magnetite from the aqueous stream on to the metal surface. It has been experimentally shown previously that boiler water conditioned to pH25 ⁰C 9.3 using NH4OH, favors the deposition of suspended magnetite nanoparticles on 304 stainless steel surfaces at 300 ⁰C and 100 bar. As an extension, the work presented here, explores the effects of additional chemistries on the deposition process and behavior of suspended magnetite particles. This study looks at the influence of all-volatile treatment (AVT) regime with the addition of solid alkali to the boiler water. The aim is to understand the influence of the phosphate treatment-addition of sodium triphosphate-and caustic treatment-addition of sodium hydroxide- on the deposition process with respect to the conventional AVT regime. In addition, the study also investigates the influence of solid alkali addition on the suspended magnetite and hence the nature of the deposits formed on the stainless steel substrate. The boiler environment was simulated in an autoclave pressure vessel. The test solutions were prepared in de-aerated conditions to closely match the boiler feedwater in terms of chemistry. The concentrations of sodium triphosphate and sodium hydroxide used were decided based on the standard operating guidelines followed in the industry. A standard three electrode assembly was used to monitor the deposition process using Electrochemical Impedance Spectroscopy (EIS). The nature of the deposits were investigated using surface characterization techniques. The results showed a clear influence of the zeta potential and double layer compression on the behavior of the suspended particles and their affinity to deposit at 300 ⁰C and 100 bar. The results will add to the existing work on understanding magnetite deposition and suspended particle behavior in high pressure high temperature environments.

Stress Corrosion Cracking of SA-543 High-Strength Steel in All-Volatile Treatment Boiler Feed Water

Susceptibility of SA-543 steel, its welds (with and without stress relief treatment), and the heat-affected zone (HAZ) to stress corrosion cracking (SCC) was investigated in de-aerated and aerated boiler feed water subjected to the all-volatile treatment (AVT-BFW), and distilled water at 275 °C using the slow strain rate testing (SSRT) technique. The SSRT specimens were tested at three extension rates (3.50 × 10−6, 9.00 × 10−6, and 7.50 × 10−5 mm/s) using a novel SCC testing rig capable of testing at high temperatures and pressures. There are no significant differences in the time-to-failure among the four tested specimens. The elongation of the specimens at the time of failure is in the range of 10-23%. The reduction of the cross-sectional area of the failed specimens is large (45-77%) and the absence of any signs of intergranular propagation in fractured specimens, determined by scanning electron microscopy, indicates that the failure is due to mechanical load and not due to SCC. Dissolved oxygen does not affect the susceptibility of the specimens to SCC, which could be due to the inhibition effect of the test solution. SA-543 steel as the base metal, its welds (with and without stress relief treatment), and the HAZ are suitable for use in hot AVT-BFW and distilled water.

Removal of Iron Oxide With Superconducting Magnet High Gradient Magnetic Separation From Feed-Water in Thermal Plant

By the accident of Fukushima Daiichi Nuclear Plant, all the nuclear plants have stopped in Japan. As a result, the operation rate of thermal power plants has been increased. It caused growth in CO 2 emission, which requires some kind of countermeasure. We focused on the iron oxide scales deposited on the piping system and boiler which declines the heat exchange efficiency of thermal power plants. In this study we attempt to remove the iron scales from the piping system and the boiler to maintain the power generation efficiency. In the current thermal plant treated by All Volatile Treatment (AVT) the iron elutes to the feed water in the low-temperature part which changes into iron ion or the paramagnetic fine iron oxide particles. On the other hand at the high-temperature part the main component of the scales is large ferromagnetic particles of magnetite. Therefore the magnetic separation at the high-temperature part is the more effective to remove the scale than that at the low-temperature part. For the reason, the existing method using the electromagnetic filter placed in the low-temperature part is not effective to remove the scales. We studied the high gradient magnetic separation (HGMS) at high-temperature part to remove a large amount of the scale. In this study, we assumed to install the HGMS system using the superconducting magnet at the inlet of the boiler.

Studies of Oxide Layers Grown at 260°C on A106 B Carbon Steel in Aqueous Medium with Ethanolamine or Morpholine

The water chemistry of the secondary coolant in the majority of Nuclear Power Plants is controlled by AVT (All Volatile Treatment), wherein volatile amines are used to maintain the alkaline pH required for minimizing the corrosion of structural materials which one of them is Carbon Steel. In this treatment, ammonia, morpholine and ethanolamine are commonly used as conditioning reagents. In this context, experiments were carried out by exposing carbon steel A106 B samples in a simulated secondary coolant in order to study the nature of the oxide films. The tests were performed in a static autoclave at 260°C using two media: I) hydrazine + morpholine and II) hydrazine + ethanolamine during different exposure periods up to ≈1020 h. The oxide film characterization was mainly studied using Scanning Electron Microscopy and X-ray diffraction. A chemical descaling procedure was used to obtain the material weight loss (W) of samples, the adherent and released oxide. The XRD analyses, for all exposures studied, showed that magnetite was the corrosion product formed in the films grown in both media. The material weight loss, after descaling, could be fitted by a law of the type W = ktn, up to 1020 h of exposure tested, resulting in n = 0.42, k = 6.24 for films grown in medium I) and n = 0.39, k = 6.08 for films grown in medium II) respectively (W is in mg/dm2 and t in h). The higher corrosion product release measured in the medium with morpholine could be important in power plant operation.

Diagnosis for Water and Steam Quality Problem in Taishan Ultra-Supercritical Power Plant

Water and steam quality during operation must be strictly controlled for ultra-supercritical unit. Changes of indicators in water and steam are supervised, water and steam quality and working condition of boiler water, as well as supervision of water and steam quality are reviewed in this paper. Risks of corrosion and scaling in Taishan power plant is assessed. As a result, control effect of TOC is remarkable in Taishan power plant, content of Cl- and SO42- can satisfy the control standard, that is , risk of corrosion and scaling is low. Dissolved oxygen is significantly exceeded in the condensate polishing desalination device , and there has risk of iron deposit. To convert the feed water treatment from AVT(All Volatile Treatment) to OT(Oxygenated Treatment) can solve the problems above .

Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam Turbine Materials in the Simulated Boiler Water Added Chloride and Sulfate Ions

Using a rotor material, 3.5NiCrMoV steel, and a blade material, 13Cr steel, for low-pressure (LP) steam turbines of thermal power plants, electrochemical crevice corrosion tests were conducted in the simulated AVT (All Volatile Treatment) boiler water added chloride and sulfate ions. The crevice corrosion behaviors as well as the films formed on the specimen surfaces inside crevices were investigated. The 3.5NiCrMoV steel and the 13Cr steel in the test water showed crevice corrosions in a general type and a pitting type, respectively. For both the two steels, however, passive films formed on the specimen surfaces inside crevices. It was found that the passive film formed on 3.5NiCrMoV steel was composed mainly of Fe3O4, while that on 13Cr steel was composed mainly of Cr-oxides and partly of inward Fe-oxides. Especially, it was confirmed that CrOOH and CrO3 were concentrated in the outermost surface of the passive film formed on 13Cr steel. [doi:10.2320/matertrans.M2013202]

火力発電プラントボイラチューブ鋼の酸化還元反応挙動および形成皮膜特性に及ぼすボイラ水 pH 低下の影響

Concerning boiler tube materials of carbon-steel STB410, low-alloy steel STBA24 and stainless-steel SUS304, cyclic voltammetry measurements and immersion tests are conducted in simulated AVT (All Volatile Treatment) waters with the mixed formic acid (HCOOH) and chloride ions (Cl−) at pH9.5, pH8.0 and pH7.0. Characteristics of the oxide films formed on the specimens in immersion tests are evaluated by X-ray photoelectron spectroscopy (XPS). It is confirmed that the corrosions of STB410 and STBA24 steels are accelerated with the decreasing pH in the waters, and that SUS304 steel has the better corrosion resistance in all the waters tested. It is clarified that oxide films formed on STB410 steel in the simulated AVT waters are composed of Fe-oxides and become unstable with the decreasing pH. On the other hand, the films formed on STBA24 steel, even in the waters with lower pH values, show the oxide films with the concentrated Cr in them. Furthermore, the pH decrease in the water results in no influences on the corrosion behaviors of SUS304 steel due to the formation of oxide films with Cr.

Experimental investigation and optimization of carbohydrazide application using different alkalization agents on boilers All-Volatile treatment

Abstract This paper presents an experimental investigation of a carbohydrazide application on three Units with mixed metallurgy boilers water–steam cycles in the Power Plant of Ptolemais (Greece). The tests were performed using carbohydrazide as an oxygen scavenger and metal passivation agent, with the parallel application of different alkalization agents on All-Volatile regime, in order to find the most appropriate. The optimization of this carbohydrazide application concerns the adaptation of the final products dosage for the Units needs. The tests that were performed, the measurements, the analyses and other relevant data, the results evaluation and the conclusions, are presented. The results were obtained under real industrial conditions and may provide a general tool for developing the know-how of this kind of treatment.

塩化物イオン混入模擬 AVT 水中におけるボイラチューブ鋼の腐食挙動に及ぼす有機酸の影響

The influence of formic and acetic acids on the corrosion behavior in the simulated all volatile treatment (AVT) waters at pH9.5 is investigated by electrochemical and immersion corrosion tests for boiler tube steels of STB410 carbon steel, STBA24 low- alloy steel and SUS304 stainless steel used in thermal power plants. For STB410 and STBA24 steels, the pitting corrosion susceptibilities are lowered by adding formic acid (HCOOH) and acetic acid (CH3COOH) into the simulated AVT waters added also with chloride ions (Cl−). It is suggested that both these organic acids, coexisted with Cl− ions in the water, will act as inhibitors for corrosion protection of the carbon and low-alloy steels. On the other hand, almost no inhibiting effect of both these organic acids on the corrosion behavior of SUS304 steel is observed, due to the formation of thin and tight passive films containing chromium.

発電プラント低圧蒸気タービン材料の孔食感受性に対する Cl− および SO42− の影響

Pitting corrosions of 3.5NiCrMoV steel and 13Cr steel, the materials used respectively for rotors and blades of low-pressure steam turbines in power plants, are evaluated by anodic polarization tests in the simulated AVT (All Volatile Treatment) water, and also in the waters added with chloride ions (Cl−) and sulfate ions (SO4 2−), at 308 K. It is clarified that the pitting corrosions of 3.5NiCrMoV steel, in addition to the uniform corrosions, are promoted by adding Cl− and SO4 2− ions into the test waters. As for 13Cr steel, the high pitting corrosion resistance is shown in the test water with only SO4 2− ions due to the stable passive film formed on the specimen while the pitting corrosion is remarkably promoted by adding Cl− ions into the water. It is clear that the coexistence of SO4 2− ions with Cl− ions in the test water exhibits an inhibiting effect on the initiation of corrosion pits but also an accelerating effect on the pit growth, for the high-alloyed 13Cr steel.

Experimental Simulation of Iron Oxide Formation on Low Alloy Steel Evaporator Tubes for Power Plant in the Presence of Iron Ions

Presented are the formation of iron oxide layers on evaporator tubes in an actual fossil power plant operated under all volatile treatment (AVT) condition and an experimental simulation of iron oxide formation in the presence of ferrous and ferric ions. After actual operations for 12781 and 36326 hr in the power plant, two iron oxide layers of magnetite on the evaporator tubes were found: a continuous inner layer and a porous outer layer. The experimental simulation (i.e., artificial corrosion in the presence of ferrous and ferric ions at 100 ppm level for 100 hr) reveals that ferrous ions turn the continuous inner oxide layer on tube metal to cracks and pores, while ferric ions facilitate the production of porous outer oxide layer consisting of large crystallites. Based on a comparison of the oxide layers produced in the experimental simulation with those observed on the actually used tubes, we propose possible routes for oxid layer formation schematically. In addition, the limits of the proposed corrosion routes are discussed in detail.

E302 EVALUATION ON THE FILMS FORMED ON BOILER TUBE STEELS IN SIMULATED AVT WATERS(Corrosion)

The films formed on boiler tube steels are investigated and the corrosion resistances of the films formed are evaluated in this work. Corrosion potential measurements are conducted in simulated AVT (All Volatile Treatment) waters at 308K for 100h on STB410 carbon steel, STBA24 low-alloy steel and SUS304 stainless steel, which are used as the boiler tube steels in thermal power plants. Four kinds of the simulated AVT waters, i.e., the base water and the waters added with chloride and sulfate ions, are used in the tests. After corrosion potential measurements the films formed on the steels are observed with SEM (Scanning Electronic Microscope) and are analyzed with XPS (X-ray Photoelectron Spectroscopy). Anodic polarization behaviors of the materials with the films formed are also investigated in the same test waters. In all of the test waters, STB410 steel exhibits base corrosion potentials, while STBA24 and SUS304 steels show nobler. It is found that the films formed on STB410 steel consist of Fe-oxides and the films formed on STBA24 and SUS304 steels contain stable Cr-oxides. In this work, it is confirmed that the STBA24 and SUS304 steels show better corrosion resistances to the anions such as chloride and sulfate ions in the simulated AVT waters at 308K by virtue of the passive films.

A decade's experience in chemistry maintenance of once through steam generator of fast breeder test reactor

The steam generator of fast breeder test reactor (FBTR) at Kalpakkam (India) is a once through Steam Generator (OTSG) which requires the feed water at high purity level. Therefore, for maintaining feed water chemistry, all volatile treatment (AVT) is adopted along with a full-flow deep bed Condensate Polishing Unit (CPU) in the steam water system. Operational difficulties such as premature termination of operation cycle of the CPU, enhanced impurity pickup resulting in increased load for CPU, early silica breakthrough, etc. were observed on occasions. This paper describes the modifications carried out in the steam water circuit to overcome these problems. A decade's experience in operating the CPU and maintaining the feed water quality is also discussed.