What are Boiler Troubles? Explain Scales and Sludge Formation

Boiler troubles are -

1) Priming & foaming 

2) Sludge & Scales

3) Caustic embrittlement 

4) Boiler corrosion

The most common reason is the presence of dissolved oxygen and carbon dioxide, which eat away at areas of the boiler causing system stress and cracking. The higher the boiler pressures, the more severe the corrosion. When chlorides are present at an unacceptable level, chloride stress cracking can also occur.

Sludge & Scale formation

In a boiler, water is continuously evaporated and converted into steam. As a result, the water becomes saturated due to an increase in the concentration of dissolved impurities. Finally, a stage is reached where the ionic products of these salts exceed their solubility product and are thrown out as precipitates on the inner walls of the boiler Sludge:

  • The soft, loose, and slimy precipitate formed within the boiler.
  • Formed at comparatively colder portions of the boiler and are collected at the bends.
  • Formed by substances that have greater solubility in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc
  • Easily removed with a wire brush

Disadvantages of Sludge Formation

  • Sludges are poor conductors of heat, so they tend to waste a portion of the heat generated.
  • Excessive sludge formation disturbs the working of the boiler.
  • Sludge can be removed by using

1) Softened water

2) by blow down

operation i.e. drawing off a portion of the concentrated water.


  • Hard deposits firmly stick to the inner walls of the boiler.
  • Difficult to remove, even with the help of a hammer & Chisel.

Formed due to:

i) Decomposition of Calcium bicarbonate:
Ca (HCO3)2 CaCO3↓ + H2O+CO2

 (Soft, formed mainly in low-pressure boilers)
In high-pressure boilers, CaCO3 is soluble due to the formation of Ca (OH)2

CaCO3 + H2O Ca (OH)2 + CO2↑

ii) Deposition of CaSO4:

Solubility of CaSO4 decreases with an increase in temperature. It is completely insoluble in super heated water. Hard scale formation takes place in high-pressure boilers

iii) Hydrolysis of Magnesium salts:

Soft scale formation due to hydrolysis of Mg salts in the high-pressure boiler.

 MgCl2 + 2H2O Mg (OH)2↓ + 2HCl

iv) Presence of silica:

Even very small amounts of silica lead to deposits of Ca/Mg silicates(CaSiO3 & MgSiO3), which adhere firmly to the inner boiler walls. It is difficult to remove.

Disadvantages of scale formation:

i) Wastage of fuel:

The rate of heat transfer is greatly reduced due to the poor conductivity of the scales Overheating is required for a steady supply of heat hence fuel consumption increases.

ii) Lowering of boiler safety:

To supply steady heat overheating is required, which makes the boiler material weak &soft. Results in distortion of boiler tube & make the boiler unsafe to bear the high pressure.

iii) Decrease in the efficiency of boiler:

Deposition of scales in valves and condensers chokes them partially & decreases the efficiency of the boiler

iv) Danger of Explosion:

Due to uneven expansion, the thick scales get cracked, resulting in the formation of large steam & develop high pressure. It may cause an explosion of a boiler.

Removal of scales:

  • Mechanical/chemical method
  • Loosely adhering scales are removed with the help Scraper/wire brush
  • Brittle scales are removed by giving Thermal shocks
  • Loosely adhering scales are removed by frequent blow-down operations (frequently removing precipitates)
  • Adherent & hard scales are removed by dissolving them by adding chemicals

     CaCO3 scales - 5-10% HCl
     CaSO4 scales – EDTA

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