Special Features of Phosphatizing in Stage Evaporation

Abstract

High-duty drum boilers (15.5 MPa in the drum) in Russia often function with double-stage evaporation, the first stage occurring in the drum and the second stage occurring in remote cyclones or end compartments of the drum connected to an individual circulation center. This process is accompanied by enhanced introduction of PO₄ phosphates that contaminate the heating surfaces, i.e., waterwall pipes of the boiler, and cause their corrosion under scale, which requires frequent acid cleanings (3 – 4 times a year). The experience of the Novosibirsk TÉTs-5 cogeneration plant employing single-stage evaporation and periodic introduction of phosphates at the expense of higher hardness of the feed and boiler water (as a result of worsening of the feed water) has shown that it is possible to increase the period between boiler cleanings to over ten years with periodic control of the sediment on the pipes by cutting specimens. The annual amount of introduced phosphates is 0.05 – 0.08 mg/dm³. The growth of sediment has virtually stopped in the last three years. The pipes are not damaged. Under conventional water chemistry and transfer of boilers with double-stage evaporation to a single-stage evaporation mode (by elimination of partitions in intradrum compartments of the second stage at the drum ends) the concentration of phosphates in the bleed water has decreased by more than a factor of 4.