Abnormal noise occurred in operation of 300 MW steam turbine equipped for unit no.4 in one Power Plant.In disassembled major repair

the plug-in pipe for HP steam incoming has been discovered to be ruptured

and being seriously damaged.Test and analysis for the structure of said plug-in pipe show that the structure of said plug-in pipe was originally designed with two sealing rings

and later being retrofitted into bell-shaped seal

but during the time of drilling screw hole

a partial of screw threads of the pin hole have been damaged

making the fastening screw to be drop-off

and the special pin to be out from its position

and the plug-in pipe to lose movement allowance in the horizontal direction

finally the plug-in pipe is ruptured due to bearing large moment of force under the effect of thermal stress.For this

the ruptured plug-in pipe and that with defects have been replaced

reinforcing the fastening screw

and further preventive measures being put forward.