State harmful effects of Scale and Sludge formation in boilers.
Answer: 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 precipitated on the inner walls of boilers and in due course of time adhere to the metal surface in the form of scales and sludges.
When the precipitate formed is soft, slimy, and loose, known as sludge, and is hard and firmly adhering to the inner walls of boilers known as scale. The sludges are generally formed by the substance, which has a greater solubility in hot water than in cold water. So, sludges are formed in colder parts of boilers.
Scale Formation
Scales are hard, adherent crusts or coating formed on the inner walls of the boiler during steam generation. They are difficult to remove, even with the help of a hammer and chisel, and are the main source of boiler trouble
Causes
Scales are formed mainly by Ca (HCO3)2 and MgCl2 in low-pressure boilers, whereas they are formed mainly by CaSO4 and silica in high-pressure boilers.
(a) Hydrolysis of Magnesium Salts: The soluble MgCl2 salts are hydrolyzed under high-temperature conditions, to give a soft Mg(OH)2 scale
MgCl2 + 2H2O --> Mg(OH)2 + 2HCI
Scale
(b) Decomposition of Bicarbonates
Ca(HCO3)2 -> CaCO3+ + H2O + CO2
Mg (HCO3)2 -> MgCO3 + H2O + CO2
MgCO2 may further decompose near the boiling point of water to give Mg (OH)2.
MgCO3 + H2O -> Mg(OH)2 + + CO2
CaCO3 is soft and is the main cause of scale formation in low-pressure boilers. But in high-pressure boilers, Caco3 is soluble.
CaCO3 + H2O ----> Ca(OH)2 + CO2
(c) Deposition of CaSO4: The solubility of CaSO4 decreases with an increase in temperature. The CaSO4 gets precipitated as a hard scale on the hotter parts of the boiler. This type of scale causes trouble mainly in high-pressure boilers. CaSO4 scale is quite adherent and difficult to remove, even with the help of a hammer and chisel.
(d) Formation of Silicates: Formation of silicates of Ca and Mg may take place as CaSiO3, and MgSiO3 respectively even if a small amount of SiO2 is present in boiler feed water. These scales once deposited, adhere very firmly to the inner walls of the boiler which are difficult to remove. One important source of silica in water is the sand filter.
Sludge Formation
(a) Decrease in Solubility: Certain impurities in water decreases solubility with an increase in temperature, so such impurities separate out from water at a higher temperature e.g. In CaSO4 solubility decreases with an increase in temperature.
(b) Hydrolysis of Salts: The soluble magnesium salts are hydrolyzed under high-temperature conditions to give a soft Mg(OH)2 scale.
MgCl2 + 2H2O -> Mg(OH)2 + 2HCI
(c) Chemical Decomposition: The decomposition of bicarbonates of calcium and magnesium takes place to give rise to two carbonates CaCO3 and MgCO3 respectively.
Ca(HCO3)2 -> CaCO3 + +H2O + CO2
Mg(HCO3)2 -> MgCO3 + H2O + CO2
MgCO3 may further decompose near the boiling point of water to give Mg(OH)2
MgCO3 + H2O + Mg(OH)2 + +CO2 These salts precipitate and deposit inside the boiler as scale and sludge.
(d) Formation of Silicates: If a small amount of SiO2 is present in boiler feed water it gives rise to the formation of Casio3 and MgSiO3 adhere very firmly to the inner walls of the boiler
Explain disadvantages of scale & sludge formation in Boilers
Disadvantages of Scale & Sludge Formation
1. Decrease in Efficiency of Boiler: The excessive scale formation may cause clogging of boiler tubes which decreases the efficiency of the boiler.
2. Wastage of Fuel: Scale is a poor conductor of heat. This results in a reduced rate of heat transfer, and thus the evaporative capacity of the boiler will be reduced. Thus scale formation also decreases the efficiency of the boiler and causes wastage of fuel.
3. Danger of Explosion: The scale formation also leads to the uneven expansion of the boiler metal. Due to uneven expansion, thick scales crack and water comes suddenly in contact with overheated iron plates. This results in the formation of a large amount of steam suddenly which in turn develops a sudden high pressure. The sudden high pressure thus developed may lead to the explosion of the boiler.
4. Boiler Becomes Unsafe: The overheating of boilers occurs due to scale formation and metal becomes soft and weak. This makes the use of boilers unsafe, particularly at
Removal of Scales and Sludges
1. If they are loosely bound to the inner surface of the boiler, can be removed with the help of a scrape or wire brush or wood.
2. If they strongly adhere to walls and are hard, can be removed by dissolving them by adding certain chemicals like
(a) 5-10% HCl solution.
(b) EDTA solution.
3. Blow down operations: In continuous blowdown, a definite concentration of a substance in boiler water is maintained. Intermittent blowdown is aimed at removing sludge from boiler water. Thus the blow-down operation is attempted to keep salt concentration at a specific level and to control sludge in the boiler.