An Investigation On The Reasons And Improvement Of The Tube Explosion In Back Pass Cage

The thesis theoretically analyzed the causes of the accident of tube explosion occurred in the side wall of back pass cage of the 660MWe pulverized coal fired boiler at Dezhou Power Plant, which strongly affected the reliability of the facility. Numerical simulations of the fluid dynamics inside the associated tubes were conducted. Based on the theoretical analysis and the numerical simulation, the causes of the accident were identified and corresponding treatments were proposed. Following the theoretical analysis, numerical simulations were conducted by using standard code in combination with the discrete model. Detailed information of the steam pressure resistance, flow rate difference, and the static pressure distribution in outlet header of the second pass cage system were obtained. The results showed that the key reason for the tube explosion was the inappropriate heat surface structure, which caused the uneven distributions of static pressure and the mass flow rate in header of the tubes. Thus, some tubes were not sufficiently cooled down to the safety temperature.Based on the analysis, four retrofit approaches were suggested as follow: improvement of the pipe system, injection of saturated steam directly to inlet header of the cage wall, alteration of the tube diameter and addition the throttle orifices in every tubes. Through the theoretical calculation and comparison of the proposal approaches, the approach of the throttle orifices addition was adopted and put into practice. The operation experiences showed that, for the same load, after the treatment, the tube wall temperature distribution of the side wall tubes became more uniform and the maximum temperature of the tube became much lower. As a result, the boiler has being run for a long period without a cage tube explosion, confirming the success of the treatment.