The leakage failure of a drainage tank of reheat pipeline in ultra supercritical units will pose a great threat to personal and equipment safety. To find out the factors resulting in the drainage tank leakage

several tests such as non-destructive testing

component analysis

microscopic analysis and mechanical tests were carried out on defects position of inner wall of the drainage tank in hot section of reheat pipeline in a 1 000 MW unit. The factors such as the drainage logic of the drainage tank

the position of the drainage valve and the pipeline layout of the water level measuring were studied. It was found that the leakage of the tank was mainly caused by the thermal fatigue

which was resulted from the condensed water. Furthermore

the effect of turbulence invasion

the change of unit load

and the condensate return at the gauge nozzles made the liquid level fluctuate at the bottom of the tank

resulting in thermal fatigue cracks in a wide range. The reasons for the formation of condensate water included incomplete drainage

too far position of drainage valve

easy stream liquefaction in the water level measuring pipeline. In order to avoid re-leakage of the drainage tank of 1 000 MW unit

it is necessary to improve the drainage logic

the location of the drainage valve

the pipeline layout of the water level measuring and so on.