In start-up process after more than one month of acid cleaning for once-through boiler of 600 MW unit in one power plant

over-temperature tube-burst due to short-term superheating has occurred at many places in the transition zone of water-steam conversion on the vertical water-wall.It is believed through inspection by using an endoscope that the direct cause resulting in tube-burst is large amount of mixed solid deposits accumulated on corresponding positions of the inner surface.For this

it is affirmed through analysis that the deposits are mainly the residual impurities of chemical cleaning before starting-up

and being not flushed away during the starting-up.After start-up

these residual impurities moved along with steam-water cycle to the transition zone of water-steam conversion

and strengthened deposition in the turbulent positions may exist.