Effects On Metal Wall Temperature By Hydrodynamic Characteristics Of Vertical Water-wall In Ultra-supercritical Pressure Boilers

With the increase of the capacity of ultra-supercritical units,more and more problems are exposed during the operation of the boiler.The phenomenon of over-temperature of the water-wall caused by poor hydrodynamic characteristics occurs from time to time,and long-term wall temperature fluctuation will cause horizontal cracks in the water-wall,leading to water-wall tube leaking,which will seriously endanger the safe and efficient operation of the boiler.Hydrodynamic characteristics of the evaporation area of the boiler have an important influence on the safety of the water-wall.Referring to horizontal cracks of vertical water-wall in ultra-supercritical boilers,this paper focuses on the effects on metal wall temperature by hydrodynamic characteristics.In this study,a 660 MW ultra-supercritical once-through boiler is selected as the object.Firstly,the water-wall flow network is divided according to the structural characteristics of the water-wall system.At the same time,the appropriate hydrodynamic calculation method is determined.The calculation models of the single-phase fluid resistance under the supercritical pressure,the furnace heat load,and the metal wall temperature are established.A complete calculation program is compiled.Among them,the heat load model is extracted by calculating the flow distribution and heat distribution of each tube according to the measured data of the wall temperature.Then,the operating characteristics of the lower furnace water-wall of the boiler under BMCR operating condition are analyzed based on the verification of the correctness of the heat load model,including the flow distribution in the water-wall tube,the heat load distribution in the furnace,the outlet temperature distribution of the water-wall tube and the height distribution of the phase transition point of the working fluid.Also,the relationship between these distribution and the size of throttle orifice aperture is analyzed.Additionally,the temperature distribution of the metal wall,the temperature change of the metal wall,the height distribution of the phase transition point and the height change of the phase transition point are calculated under the conditions of different operating conditions,inlet flow,inlet temperature and inlet pressure.The effects of these changes on the outlet temperature of the metal wall tube and the height of the phase transition point of the working fluid are determined.And then,further analysis of the possible effects of these changes on horizontal cracks is conducted.The results show that the phase transition of the working fluid occurrs between the elevation of 36 m and 51 m.Between No.40 tube and No.190 tube of each wall,the metal wall temperature is high and varies widely,horizontal cracks are prone to occur,which is basically consistent with the location of horizontal cracks in the field.Finally,a numerical model is established for the irregular areas near the middle header of the water-wall.The local metal wall temperature distribution is calculated,and the effects of variable working conditions on the metal wall temperature distribution are analyzed.Moreover,the position with the highest temperature and the position with the largest temperature gradient of the irregular areas are determined,which provides an important basis for subsequent analysis of horizontal cracks.The results of this paper can be used to predict the location of horizontal cracks in the water-wall,and also provide a theoretical basis for the alteration of the throttle orifice.It is of great significance to improve the safety of the water-wall operation in the furnace.