Boiler troubles are 1) Priming & foaming 2) Sludge & Scales3) Causticembrittlement4) Boiler corrosion
Sludge & Scale formation
In a boiler, water is continuously evaporated and converted into steam.
As a result the water becomes saturated due to increase in the concentration of dissolved impurities. Finally a stage is reached where the ionic products of these salts exceeds their solubility product and are
thrown out as precipitates on the inner walls of the boiler.
Sludge:
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 which have greater solubility in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc
Easily removed with wire brushDisadvantages of sludge formation
Sludges are poor conductors of heat, so they tend to waste a portion of 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.
Scales:
Hard deposits firmly sticking to the inner walls of the boiler.
Difficult to remove, even with the help of hammer & Chisel.
Formed due to:
i) Decomposition of Calcium bicarbonate:
Ca (HCO3)2 CaCO3↓ + H2O+CO2 Scale
(Soft, formed mainly in low pressure boilers)
In high pressure boilers, CaCO3 is soluble due to formation of Ca (OH)2 CaCO3 + H2O Ca (OH)2 +CO2↑
ii) Deposition of CaSO4:
Solubility of CaSO4 decreases with 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 high pressure boiler.
MgCl2 + 2H2O Mg (OH)2↓ + 2HCl
iv) Presence of silica:
Even very small amounts of silica leads to deposits of Ca/Mg silicates(CaSiO3 & MgSiO3), which adheres firmly to the inner boiler walls. It is difficult to remove.
Disadvantages of scale formation:
i) Wastage of fuel:
Rate of heat transfer is greatly reduced due to poor conductivity of scales Overheating is required for 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 & makes the boiler unsafe to bear the high pressure.
iii) Decrease in efficiency of boiler:
Deposition of scales in valves and condensers choke them partially& decreases the efficiency of boiler
iv) Danger of Explosion:
Due to uneven expansion the thick scales gets cracked, results in formation of large steam & develops high pressure. It may cause explosion of 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 operation (frequently removing precipitates)
ii) Deposition of CaSO4:
Solubility of CaSO4 decreases with 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 high pressure boiler. MgCl2 + 2H2O Mg (OH)2↓ + 2HCl
iv) Presence of silica:
Even very small amounts of silica leads to deposits of Ca/Mg silicates(CaSiO3 & MgSiO3), which adheres firmly to the inner boilerwalls. It is difficult to remove.
Disadvantages of scale formation:
i) Wastage of fuel:
Rate of heat transfer is greatly reduced due to poor conductivity of scales Overheating is required for 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 & makes the boiler unsafe to bear the high pressure.
iii) Decrease in efficiency of boiler:
Deposition of scales in valves and condensers choke them partially& decreases the efficiency of boiler
iv) Danger of Explosion:
Due to uneven expansion the thick scales gets cracked, results in formation of large steam & develops high pressure. It may cause explosion of 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 operation (frequently removing precipitates)
Tags
waterchemistry