Frequent leakages occurred unusually at outlet of vertical section of the low-temperature reheater(LTR) in an ultra-supercritical 2×1 000 MW unit which had been put into service for just two years. To find out the main leakage causes

macroscopic

microstructural inspections and mechanical tests were carried out on the failed LTR tubes. The actual working condition curves

regarding LTR wall and circumjacent flue-gas temperatures were analyzed and discussed. Finally

long-time overheat of the vertical section was considered as the representative reason. The installation positions of the wall temperature measuring points were unreasonable

the measuring points number was too small and the alarm temperature was too high

which caused the over temperature can not be alarmed in time. The essential cause was that the heat exchange area of the upstream tubes was too small

so the heat absorption was insufficient

resulting in over temperature of the flue gas at inlet of the vertical section of downstream LTR

which severely deteriorated the heat exchange when the units operated at(ultra) low load. Thus

the temperature measuring points positions were changed

the measuring points number was increased

the heat insulation tile was installed properly and the alarm temperature was set lower. The unit has ran for more than one year after these transformation

and the results show that the tube burst caused by over temperature of LTR is effectively inhibited