BOILER WATER TREATMENT
Waters used as boiler feedwater, such as tap water, industrial water, underground water and river water, usually contain various substances such as suspended solids, dissolved solids and gases. The amounts of these substances vary largely depending on the sources of raw waters.
The use of such raw water without the pretreatment may result in problems, such as scale, corrosion and carryover, in boilers and the auxiliary equipments.
Most of low pressure boilers* use raw water or softened water as the feedwater and usually no deaerator is employed. Therefore, those boilers are subjected to troubles such as hardness and silica scale adhesion, corrosion due to dissolved oxygen and corrosion by carbon dioxide in the condensate line. Medium or high pressure boilers* are generally supplied deaerated and demineralized water as the feedwater. However, since they are operated at the high temperature and high pressure, the presence of a small amount of impurities causes problems, such as metal oxide deposits on the heating surface of the boiler, the corrosion of the auxiliary equipments, and scale adhesion in the superheaters or on turbine blades.
In order to prevent these problems and to operate the boilers safely and efficiently, the application of suitable water treatment is required for each boiler.
The water treatment for boilers is divided into the external (mechanical) and internal (chemical) treatment. The mechanical treatment is to remove the impurities in water by applying coagulation, sedimentation, filtration, ion exchange, deaeration
treatments, etc. The chemical treatment is divided into the treatment for the feedwater and condensate lines, and for the boiler itself.
The chemical treatment for the feedwater and condensate lines aims to control corrosion by adding oxygen scavengers and corrosion inhibitors to the lines, and to supply water containing as little impurities as possible into the boiler. Boiler compounds, oxygen scavengers, sludge dispersants and so on are used for the chemical treatment of boilers. Those chemicals prevent corrosion and make the scale forming components the water insoluble and dispersed particles to discharge them from the boiler with the blowdown water.