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96534 Sulphur Containing Sediments Influenced Wear of Superheater Tubes

Picture for 96534 Sulphur Containing Sediments Influenced
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Product Number: 51300-96534-SG
ISBN: 96534 1996 CP
Author: Pavel Yoffe
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The failed superheater tube was investigated at the point of failure and at a distance of 25 cm. At both of the cross sections two layers of sediment were detected The external one contained Na2SO4 and Na3VO3 in concentration relation 6.4:1 approximately. The “intend” layer that situated on the tube surface consisted of V2O3 mainly (about 70% ) and Sodium pyrosulphates Na2SO3.xSO3. The tube surface was covered with usual “passive” oxides such as Cr2O3, Fe2CrO4, and magnetite. In the faded section the last layer contained iron and chromium sulfides. Thinning of the wall was detected in both sections, but it was more evident in the failed section. Thermodynamic and kinetic estimations of the results were carried out. The following failure mechanism was proposed: 1. At 460°C the pyrosulphate dissociates with SO3 emanation. During SO3 interaction with steel both passive oxides and iron sulfide are produced simultaneously. Sulfides damage the passive (protective ) film. 2. The vicinity of partition the temperature increases and the structure of steel is changed SO3 penetration along grain boundaries is possible, and it leads to embrittlement of the steel through sulphidization. 3. The embrittled layer is stripped off by falling ash deposit and collapsed passive film. Thinning of the tube is accelerated at such a region. So, sulfure containing sediment accelerates the wear of the tube. This accelemtion is strong especially in the vicinity of the boiler walls. Keywords: superheater tubes, sulfur containing sediment, sulfidization, high temperature corrosion, boiler tube failure.
The failed superheater tube was investigated at the point of failure and at a distance of 25 cm. At both of the cross sections two layers of sediment were detected The external one contained Na2SO4 and Na3VO3 in concentration relation 6.4:1 approximately. The “intend” layer that situated on the tube surface consisted of V2O3 mainly (about 70% ) and Sodium pyrosulphates Na2SO3.xSO3. The tube surface was covered with usual “passive” oxides such as Cr2O3, Fe2CrO4, and magnetite. In the faded section the last layer contained iron and chromium sulfides. Thinning of the wall was detected in both sections, but it was more evident in the failed section. Thermodynamic and kinetic estimations of the results were carried out. The following failure mechanism was proposed: 1. At 460°C the pyrosulphate dissociates with SO3 emanation. During SO3 interaction with steel both passive oxides and iron sulfide are produced simultaneously. Sulfides damage the passive (protective ) film. 2. The vicinity of partition the temperature increases and the structure of steel is changed SO3 penetration along grain boundaries is possible, and it leads to embrittlement of the steel through sulphidization. 3. The embrittled layer is stripped off by falling ash deposit and collapsed passive film. Thinning of the tube is accelerated at such a region. So, sulfure containing sediment accelerates the wear of the tube. This accelemtion is strong especially in the vicinity of the boiler walls. Keywords: superheater tubes, sulfur containing sediment, sulfidization, high temperature corrosion, boiler tube failure.
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