Acid cleaning operation for a natural circulation high pressure drum boiler unit of a thermal power station

Two-stage acid cleaning (TSAC) operation has been employed and investigated to clean up undesired impurities (deposit) in boiler water wall tubes (BWW T), boiler drum, super heater, economizer, feed water lines, live steam lin es, hot reheat lines, cold reheat lines, dearator, collectors, headers for a natural circulation high pressure drum boiler unit, TGME - 206-COB, Russi a (158 kG(f) cm(-2), capacity steam generation 670 ton h(-1), 210 MW unit) of a thermal power station in Bangladesh. The deposit and loose impurities consist of Fe2O3, CaO, MgO and SiO2. On area basis the maximum quantity (98 g (-2)) of deposit was measured at boiler WWTs. During cleaning operation t he loose impurities were withdrawn at first by flashing of the water. TSAC operation was carried out at two stages by re-circulation of hot 105 (+/-5 degrees C) cleaning solution H2SO4+N2H4.H2O (250 +/- 25 mg L-1) at pH 2.0 ( +/-0.5) for 24 h. The TSAC operation revealed that not less than 95% of imp urities were removed. The cleaning mechanism and passivation on clean surfa ce of tubes by circulation of 500 mg L-1 N2H4 and 0.4% NH4OH with pH 10.8 ( +/- 2) at 60 degrees C were found to be satisfactory.