| The main steam valve is an important component of the steam turbine unit for regulating and controlling high temperature and high pressure steam from the boiler.Compared with the fatigue problem of the main steam valve in the cold start with sliding parameter,the cold start process with rated parameter studied in this paper will significantly affect the fatigue life of the valve structure,mainly because the steam parameters have reached the rated value during the start-up process.The value causes the valve to produce greater alternating thermal stress.In order to reduce the impact of thermal stress on the life of the valve during the process,the power plant often uses warm up the mechine to increase the temperature of the valve body before starting to reduce the thermal shock.The preheating process consumes a certain amount of steam,and the increase in the preheating temperature not only increases the operating cost,but also brings fatigue damage to the main steam valve structure.In summary,the fatigue life prediction of the main steam valve during the cold start process and the selection of the preheating temperature are of great significance to the structural strength design of the main steam valve.In this paper,the main steam valve of a 50 MW mother-controlled back pressure steam turbine unit(the inlet steam parameter is 10MPa/565°C)is taken as the research object,and the high temperature strength and fatigue life of the the main steam valve under the cold start condition of the rated parameters is numerically studied.An optimization scheme for prolonging the fatigue life of the valve and the influence of different preheating temperatures on the fatigue life of the main steam valve were analyzed.The main research contents of this paper are as follows:Firstly,the temperature field,stress field and strain field of the main steam valve are calculated numerically for the cold start condition of the parameters under service conditions.Among them,the credibility of the heat transfer calculation method is verified by comparing the calculated and measured temperature values of the measured point.The law of the stress and strain field of the main steam valve changing with the opening degree of the valve during the starting process is analyzed in detail.The results show that the change in the opening of the main valve during the start-up process causes a large fluctuation in the stress of the valve seat.Secondly,according to the numerical analysis of stress and strain field results,the ASME specification is used to calculate and analyze the fatigue damage of the key position in the main steam valve,and then the main steam valve opening degree is changed to reduce the structural damage during the starting process.The results show that the fatigue life loss of the valve seat is more serious than that of the valve casing during the starting process.By optimizing the opening mode of the main steam valve,the predicted valve seat fatigue life can be extended by 167%.Finally,for the starting process of the main steam valve under different preheating temperatures,the variation of the fatigue life of the main steam valve with the preheating temperature is numerically analyzed.The results show that when the preheating temperature changes within the temperature range of 100 ° C to 250 ° C,the higher the preheating temperature,the longer the fatigue life of the main steam valve.Considering the safety and economy of the unit operation,the preheating steam temperature is increased from about 100 °C under current service conditions to about 200 °C,which can greatly improve the fatigue life of the main steam valve. |
