Research On Stress And Fatigue Life For Supercharged Boiler Drum

During the typical operation processes,such as startup,shut down,and load alteration,of marine supercharged boiler,the temperature and pressure of the working substance in the boiler drum fluctuate frequently in a short time,resulting in the boiler drum subject to high level and cyclic mechanical and thermal stresses,which cause the potential safety hazard of plastic fracture and fatigue failure that affect the safety of personnel and equipment.Thus,conducting the research on stress and fatigue life of boiler drum and developing the on-line fatigue life monitoring system have practical importance for safety operation of marine supercharged boiler unit.Comparing to the utility boiler,the studies on stress and fatigue life of marine supercharged boiler drum is still in early stage and has the disadvantages of limited research approaches,inadequate research contents and inaccurate research results.Also,the research results on the safety issues of utility boiler drum cannot be directly applied to supercharged boiler since the distinct differences exist in the aspects of structure and heat exchange pattern between two types of boilers.Therefore,this work conducts the research on the stress and fatigue life of supercharged boiler drum in the typical operation processes.The experimental research on the temperature field of the supercharged boiler drum was carried out in the first place.In view of the current research status that be lacking of the temperature change regularities of drum walls,the drum pressure and wall temperature of four typical operation processes were collected using the modified temperature data acquisition system.The regularities of the wall temperature versus time and space were obtained;also the correlations between the wall temperature and the drum pressure were analyzed.The research results show that the basic curves of temperature versus time of four typical operation processes can be determined based on the attributes of shapes and inflection points of the temperature curves.On the above basis,the approximate temperature curves on the different areas of the drum can be obtained with the numerical ranges of temperature differences along the circumferential and radial directions of the drum.Also,basing the correlations between the wall temperature and the drum pressure,the approximate temperature curves can be obtained with the drum pressure data in the processes of quick pressure relief and steam charging.Secondly,the numerical simulation on the stress field of the supercharged boiler drum was conducted.The determination methods of boundary conditions for stress calculation of static,cold startup,and steam charging processes were obtained on the basis of wall temperature regularities and heat transfer theory.The distributions,values,influencing factors,and curves of stress versus time,as well as the relationships among the mechanical stress,thermal stress,and total stress were obtained,by conducting the numerical simulation.Also the safety assessments were carried out for the dangerous points of the drum in different operation processes.The research results show that high level stress exist in the local areas of drum body,tube sheet,downcomer,water wall,and convection tube bank during the static and cold startup processes.Although the strength assessment of the structure shows a satisfactory result,there is a low safety margin for convection tube bank since its value of stress is very close to the allowable limit in the process of cold startup.The combination of cold startup and shut down processes will cause fatigue damage to the drum because of the cyclic stress while the steam charging process cannot affect the safety of the drum.Based on the previous studies,the on-line fatigue life monitoring system for supercharged boiler drum was developed.In order to meet the need of on-line monitoring the temperature,stress,and fatigue life of dangerous points on the drum,the real time calculation methods of the parameters were investigated and compiled to code.The first on-line fatigue life monitoring system which can be applied to supercharged boiler drum was established basing on the software program and hardware devices.The research results show that the accuracy of the real time calculation program meets the requirement of practical application by testing with the measured data and numerical results of the processes of cold startup,steam charging,and shut down.The relative error of the result of temperature calculation program is large in some individual areas of the drum for a short time,while the maximum relative errors for inverse method and direct method are-8%and 20%respectively.Besides the above-mentioned values,the errors are within a range from-5%to 5%for most of the time in the operation processes.Comparing the result of stress calculation program against the result of ANSYS software,the two curves of stress versus time show a high goodness of fit in the areas of air layers and insulating layer on the drum while a large deviation exists in the area of tubes.The system has the functions of data collection,calculation,storage,real time display and history querying.