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Inside a boiler surface, scales are formed due to the presence of ____ in the boiler feedwater.


1. insoluble compounds
2. Ca and Mg salts
3. oils
4. dissolved gases

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Correct Answer - Option 2 : Ca and Mg salts

Explanation:

Scale and Sludge formation

In a boiler, water is continuously converted into steam which in turn increases the salt concentration.

A stage is reached when these change into a loose and slimy precipitate called sludge and if there are hard adhering coatings called scales.

Thus, scales are hard deposits, which stick very firmly to the inner surface of the boiler.

On the basis of chemical composition, scales formed in the boiler surfaces of the surface.

  • Alkaline earth metal scales
    • These consist of compounds such as CaCO3, CaSO4, CaSiO3.(PO4)2, Mg(OH)2, Mg3(PO4)2, Mg3(PO4)2, 3MgO.2SiO2.H2O.
    • These substances are precipitated from the solution and get deposited on a hot or cooled surface.
  • Iron-oxide scales
    • These consist of iron oxides, iron silicates, and iron phosphates.
    • These are formed due to the presence of iron impurities in feed water.
  • Copper scales
    • These contain 20-30% of copper, iron oxides, and calcium and magnesium compounds.
  • Silicates scales
    • Silica present in feed water forms scales with calcium forming calcium silicate and with aluminum forming sodium aluminum silicate.

Effect of scale or sludge formation

  • They are poor conductors of heat so they hinder the flow of heat from the source to water.
  • Decreases the efficiency of the boiler by clogging the boiler tubes
  • Due to scale, boilerplates are overheated which reduces the life of the plates.
  • Scales of magnesium chloride corrode the plates.
  • Boilers sometimes crack with the deposits on of scales. The water enters through these cracks to the very hot metal and gets changed into steam. This sudden change sometimes burst the boiler.

Prevention of scale

  • Scale or sludge formation is prevented by introducing kerosenetannins, and other colloidal substances. These colloids get coated over the scale forming particles and suspended in water.
  • Carbonate conditioning process where sodium carbonate is added to boiler water, scales react with it and form insoluble CaCO3.

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