Simulation Analysis Of Fluid Scouring Inside Tube Sockets Of Thermometers For High-pressure Steam

Inserted thermocouple thermometer is one of the commonly used equipment to detect the temperature of main steam in power plant.It is usually inserted into the pipelines by a tube socket and provides important monitoring parameters for the operation of power plant.During the operation of a 300 MW unit,a perforation leakage occurred in a tube socket of main steam thermometer and lead to the abnormal shutdown of the unit.Therefore,it is necessary to explore the causes of leakage and propose corresponding solutions.The tube sockets of the main steam thermometers belonging to the unit were selected to the main research object.The physical model of the forky main steam pipelines in front of the main valves was established.The three-dimensional flow characteristics inside the tube sockets were numerically simulated and analyzed using the Fluent software.Besides,the rationality of the established physical model and the simulation method were verified.On this basis,the characterization method of the characteristic parameters of spiral flow was defined.The influence of the pyramid setting angle,low-speed wake,secondary flow inside elbow and unit load on spiral flow inside the tube sockets were quantitatively analyzed.Two different vortex structures inside the tube sockets were explored,and the distribution law of local vortex scour frequency was obtained.The results show that: 1)the physical model and simulation method are reasonable;2)the main steam flow inside a tube socket is dominated by spiral flow which is around the cylindrical protective casing,and the defined spiral flow characteristic parameters can characterize the shape and velocity changes of spiral flow.3)when the pyramid setting angle is 60°,the high-speed main steam flow entering from the gap entrance drives the fluid inside the tube socket to form a spiral flow and the spiral angle decays along the axial direction of tube socket.The circumferential velocity of the near-wall area inside the upper section of tube socket can reach up to 4.5 m/s,which is about twice that of the pyramid setting angle of 0°.A low-speed wake is formed behind the pyramid when the main steam flows around the pyramid.The existence of wake makes the velocity level of spiral flow inside the downstream tube socket decrease,and its influence on the pattern of spiral flow is related to the distance between the thermometers.5)the average total pressure inside the tube socket of NO.3 thermometer,which is located at curved pipeline,is 2900 Pa higher than that,which is located at straight pipeline.This phenomenon indicates that the secondary flow inside curved pipeline can provide more energy for the spiral flow inside the tube socket.6)the velocity level of spiral flow inside the tube socket increases with the increase of unit load: when the pyramid setting angle is 60°,the circumferential velocity within the near-wall region of upper section increases by more than 30% as the unit load increases from 80% to 100%.7)the vortex with high energy and high frequency is easy to form within the region with large spiral angle,which may lead to the increase of the local erosion intensity.Paired circumferential vortices are easy to form within the region with low spiral angle,which induces the secondary flow inside the tube socket.On this basis,the structural improvement scheme of adding straight fins inside the tube socket was proposed.The simulation results show that the spiral flow pattern inside the tube socket can be effectively changed after the installation of straight fins,and the circumferential velocity within the near-wall region of tube socket decreases by more than 90%,which is beneficial to greatly weaken the cutting wear of the inner wall caused by the main steam flow and oxide particles.