In order to explore the reason for perforation leakage occurred in inner wall of tube sockets resulting from wear

the tube sockets for thermometers of main steam in a 300 MW unit are taken as the research object to carry out simulation study on the internal three-dimensional flow characteristics. The results show that

large scale axial spiral flow is formed after the main steam enters a tube socket and continuously scours the inner wall with high frequency(7～40 Hz). When the installation angle of protective casing pyramid is 60°

the tangential scouring velocity within the near-wall region of the tube sockets is up to 8 m/s

which is twice about that of the installation angle of 0°. The difference of total pressure between the tube sockets for No.3 thermometer

which locates at curved and straight pipelines respectively

is up to about 6 500 Pa

indicating that the axial spiral flow inside the tube sockets has obtained stronger energy from the secondary flow in curved pipeline. The axial vortex trends to form in the region with high spiral angles and is able to accelerate local fluid scouring frequency. Thereby

a structural improvement scheme of adding straight fins in the tube sockets was proposed and the simulated results show that the installation of straight fins effectively change the original spiral motion mode of fluid inside the tube sockets

and the tangential scouring velocity is reduced by more than 90%

which should be beneficial to greatly alleviating the inner wall damage caused by the flow of high-pressure steam.