On line tube cleaning systems for polyphosphate plants

Abstract

The feedwater to MSF (multi-stage flash) seawater evaporator plants must be pre-treated to prevent the formation of the so-called scales, namely calcium carbonate, CaCo S and magnesium hydroxide, Mg(OH)2. Untreated seawater contains calcium bicarbonate, Ca(HC0)2, which when heated above 50°C (122OF) decomposes to form CaCOS scale and carbon dioxide. The accompanying rise in pH can also cause the precipitation of Mg(OH)2. The deposition of these alkaline scales on the tubes of the evaporators and particularly in the brine heater, severly impairs the heat transfer in the evaporator. Many MSF seawater distillation plants, operating at maximum brine temperatures of 90°C (i35OF) and brine concentrations below 80,000 mg/l, use polyphosphate-based feed treatment chemicals to control scale deposition. Figure 1 shows a typical simplified process schematic of a MSF cycle. These polyphosphate compounds act to suppress the precipitation of the alkaline scales. The mechanism of the precipitation suppression generally depends upon chelation, whereby the precipitating scales are complexed or otherwise inhibited so that precipitation occurs during flashdown instead of during heating. Under these conditions of operation, however, an adherent soft sludge does tend to form on the evaporator tube surfaces. This sludge build-up must be minimized and peroditally removed in order to sustain plant operation at rated performance. It is known that the rate of sludge build-up and degree of adherence to the tube surfaces is influenced by temperature, velocity, seawater composition, brine concentration,