Water treatment is the process of making the raw water from any natural source, such as a river, well, or lake, suitable for feeding into a steam boiler. Water must be treated to prevent scale deposit and to control corrosion. If a steam boiler surface is covered with scale, heat normally absorbed by the water goes up the stack instead, the tube metal temperature of steam boiler rises to the point of failure because scale mostly calcium and magnesium, is an excellent insulator and slows the transfer of heat to the water.
The objective of water treatment, combining the external treatment and internal conditioning of a steam boiler, in one word is cleanliness—cleanliness of the wetted parts. This, in turn, facilitates the production of clean steam, which keeps the steam boiler, piping, and turbine protected. External water treatment is done before water is fed into the steam boiler and is differentiated for a better clarity from the internal water conditioning within the steam boiler island. It converts raw water to feedwater. Water conditioning is the dosing of appropriate chemicals at proper places to the treated water to prevent damage to the internal surfaces of a steam boiler and make the steam suitable for a turbine or process.
Water treatment before supplied to steam boiler consists of the following stages:
1. Clarification (sedimentation followed by filtration): to remove suspended solids
2. Softening or demineralization: to remove hardness and dissolved solids
3. Deaeration: Oxygen in the boiler causes corrosion in the form of deep pits. Deaerator work on the principle that hot water holds less dissolved gas than cold water. Heated water in a pot has bubbles that form under the surface long before the water boils. These are dissolved gases coming out of solution as the water warms up.
Deaerator performs three functions:
§ Removal of dissolved non condensable gases (air) such as oxygen, nitrogen, and carbon dioxide.
§ Heating of feedwater
§ Storage of feedwater