Design And Reliability Analysis Of Molten Salt Pump At High Temperature

Since the molten salt has good heat storage capabilities, great thermal conductivity and stable chemical properties, it is considered to have great potential in future advanced nuclear plants and solar thermal power plants. Molten salt pumps are the key equipment for transporting high temperature molten salt. The operating temperature of the molten salt pump can be as high as 700~1000℃, which has posed tremendous challenges to the reliability for pump running, not to mention the molten salt is high in density and viscosity. Therefore, it is extremely important and necessary to develop a high-performance molten salt pump which is highly reliable as well. In this paper, a vertical high-temperature molten salt pump was investigated by both theoretical and numerical methods. The hydraulic components were designed and the operating reliability of the pumps were analyzed. And the predicting results were verified by the tests of the prototype pump. The main contents and conclusions of this study are shown below:(1) An overview of the applications of molten salt pumps in varied fields were stated. In addition, by exploring the research progress of numerical simulations and fluid-thermal-structure coupling, as well as their theoretic basis in centrifugal pumps, the main challenges the pumps have to deal with were presented.(2) Based on the conditions where the pump operates, the structure of the pump was developed. The impeller, guide vane and annular casing of the pump were hydraulically designed as well. The methods and process for determining the main parameters of the components were given. And the hydraulic performance of the designs was verified by prototype pump tests. The results shows that the designs are suitable and the design requirements have also been met.(3) Based on the hydraulic designs, steady simulations for the whole-field of the pumps were performed. The hydraulic performances of the pump when transporting both the water and the molten salt were predicted, and the prediction accuracies were verified by the test results. Furthermore, the internal flow characteristics of the pump were analyzed and the results indicate: The efficiency of the pump is slightly higher by 0.96% when transporting the molten salt under1.0Qd. The pressure of molten salt is significantly higher than water. Under 1.0Qd, the maximum pressure of water is 0.35 MPa, which is only 53.8% of that for molten salt. Under most operating conditions, swirls and reflux exist inside the guidevane, and this phenomenon becomes more evident at low flow rate, which indicates the guidevane could be further optimized. The positions of the outlet pipe for the pump exert significant influence on the internal pressure: The pressure is lower near the outlet pipe area compared to other areas inside the pump. Large vortex will exist inside the impeller passage at low flow rate. With the flow rate increasing, the vortex will gradually disappear and the flow conditions get better as well.(4) The temperature distributions of the pump during the “Temperature Rising Process(TRP)” was calculated. The radial plane from the middle of the impeller and the axial plane from the middle of outlet pipe were extracted for analysis. The results shows that: During the TRP, the temperature differences on the axial plane are higher than those on the radial plane. The temperature differences remain essentially stable at different moments of the TRP process. With the increase of temperature rising rate, the temperature differences inside the pump will increase. With the decrease of the flow rate, the internal temperature differences will rise significantly. When the flow rate drops from 1.2Qd to 0.3Qd, the temperature differences on the radial plane climb from 5℃ to 20℃, while those on axial plane rise from 11℃ to 53℃. So when designing the molten salt pump, the axial space should be designed to be relatively larger so as to make enough room for thermal deformations. Large flow rate are recommended when conducting the TRP process, which can be helpful for reducing temperature differences inside the pump.(5) The impeller stress under different TRP conditions were calculated and analyzed, the results shows: Under 1.0Qd, the thermal stress resulted from the temperature differences is 5.08 MPa, which is only 37.6% of the mechanical stress. The mechanical stress generated by the fluid-structure coupling is the main load when the pump operates. The mechanical and thermal stresses are quite stable when the temperature rises, where they fluctuates by only 0.6% and 1.4% respectively. During the TRP process, when the flow rate rises, the thermal and mechanical stresses will be reduced by 38.31% and 80.19% respectively, of which the thermal stress enjoys a sharper drop. The climbing of the temperature rising rate will lead to a significantly increase of the thermal stress, while the mechanical stress changes little. According to the ASME standards, the structural strength of the impeller under 1.0Qd were checked when the temperature rising rate is at 10℃/s. And the results show that the impeller could withstand a rate of 10℃/s for TRP.(6) Multi-phase modal analysis of the pump at high temperature was conducted and the results show that: The first six natural frequencies of the rotational and stationary parts are away from the main flow-induced excitation frequency. Under different phases, natural frequencies and deformations of the molten salt pump change little, where the maximum variations of frequency and deformation for the rotating parts are 0.5% and 0.03%, while those for the stationary parts are 0.7% and 1.3% respectively. The natural frequencies calculated from pre-stressed analysis are higher compared to the none-prestress analysis, where the natural frequency for the rotational parts enjoys a higher climb. The maximum climbs of frequencies for rotating and stationary parts are 1.35% and 0.74% separately, while the maximum increases of deformations for rotating and stationary parts are 0.6% and 2.6% respectively.(7) The critical speeds of the rotor system were calculated based on the dynamics analysis module in Workbench. Then the recommended axial and radial supporting stiffness were calculated according to the critical speeds.