Q. Explain scale formation and sludge formation in boilers. How are they formed?
Boilers are used for steam generation. When hard water is evaporated, progressively the concentration of dissolved salt is increased. When their saturation points are reached, the dissolved salts of calcium and magnesium along with other soluble impurities are precipitates on the inner walls of boilers and in due course of time adhere to the metal surface in the form of scales and sledges. The sludge are generally form by the substance, which have greater solubility in hot water than cold water. So, sludge is formed in colder parts of boiler.
Theories of scale formation
1. Decrease in solubility: Certain impurities in water decreases solubility with increase of temperature, so such impurities separates out from water at a higher temperature e.g. In CaSO4solubility decreases with increase of temperature.
2. Hydrolysis of salts: The soluble magnesium salts are hydrolysed under high temperature conditions to give soft scale.
Mg (Cl2) +2H2O = Mg (OH)2
3. Chemical decomposition: The decomposition of bicarbonates of calcium and magnesium takes place to give rise two carbonates CaCO3 and MgCO3 respectively.
Ca (HCO3)2 = CaCO3 + H2O + CO2
i. Mg (HCO3)2 = MgCO3 + H2O + CO2
MgCO3 may further decomposes near boiling point of water to give Mg (OH)2
MgCO3 + H2O = Mg (OH)2 + CO2
These salts precipitate and deposited inside the boiler as scale and sludge.
4. Formation of silicates: If small amount of SiO2 is present in boiler feed water it gives rise to formation