Fatigue Failure Analysis Of Water Wall Tubes In Utility Boilers

The utility boilers are one of the significant and substantially-used equipment in the field of thermal power generation,and the water wall tubes inside them are the most key parts for heat exchanging and transferring.However,owing to the complexity of the service conditions,e.g.,the interactions between different corrosion mechanisms,corrosion and scaling,corrosion and abrasion,as well as high temperature and high pressure,the water wall tubes are always encountered with premature failures.As a result,the utility boilers would be forced to stop operating,great economic losses would be suffered,and even safety accidents would occur.Thus,failure analysis of water wall tubes in utility boilers has important practical significance.In this paper,the causes and mechanisms of the premature failure of water wall tubes in utility boilers were analyzed by macroscopic analysis,microanalysis,and property testing.According to the results,pertinent prevention measures were proposed which could serve as a reference for detecting the premature failure of the water wall tubes that operate under similar service conditions.Detailed research contents and results are summarized as follows:(1)Through failure analysis of water wall tubes in a 320 MW supercritical boiler,the characteristics and mechanism of corrosion thermal fatigue for the water wall tubes by alternate thermal stress and high temperature corrosion caused by free sulfur were clarified.When the combustion flame repeatedly scrubs the water wall tubes,the water wall tubes would be subjected to a thermal stress that changes with temperature.The free sulfur from the pulverized coal corroded the water wall tubes under an oxidizing atmosphere.Thecombination of sulfur corrosion and thermal stress resulted in the initiation and propagation of the corrosion fatigue cracks of the water wall tubes.(2)Through failure analysis of water wall tubes in a 1000 MW supercritical boiler,the characteristics and mechanism of corrosion thermal fatigue of the water wall tubes by alternate thermal stress and high temperature corrosion caused by hydrogen sulfide gas were clarified.The water wall tubes in the area with the highest heat load would experience violent temperature fluctuations under different operating conditions,and would be subjected to changing thermal stress.A large amount of hydrogen sulfide gas generated during the combustion of coal in a strong reducing atmosphere corroded the water wall tubes in the high-temperature environment.The simultaneous action of thermal stress caused by temperature fluctuations during operation of water wall tubes and hydrogen sulfide gas corrosion leaded to the initiation and propagation of corrosion fatigue cracks.(3)Through the failure analysis of water wall tubes in a 1036 MW ultra-supercritical boiler,the characteristics and mechanism of fatigue cracking of welded area on water wall tubes caused by thermal expansion stress from suppressed expansion and shrinkage were clarified.Due to the particularity of position and structure,the relief tube and the adjacent straight tube in water wall tubes had different heating conditions,then their axial expansion and shrinkage were inconsistent.Inconsistent expansion and shrinkage caused the welded area to be subjected to thermal expansion stress.The stress value and direction of thermal expansion stress varied with the expansion and shrinkage of the water wall tubes.Therefore,fatigue cracking occurred at the welded area which lead to the leakage the adjacent straight tube in water wall tubes.In addition,presence of incomplete penetration inside welded area was also an important factor leading to fatigue leakage.