boiler and scales in hardness of water treatment.pptx

boiler scales issue and solutions

1. BOILER FEED WATER In industry one of chief uses of water is generation of steam by boilers. Boiler feed water – the water fed into the boiler for the production of steam.

2. Requirements of boiler feed water It should have zero hardness. It must be free from dissolved gases like O 2 , CO 2 , etc. It should be free from suspended impurities. It should be free from dissolved salts and alkalinity. It should be free from turbidity and oil. It should be free from hardness causing and scale forming constituents like Ca and Mg salts.

3. Boiler troubles (or) Disadvantages of using hard water in boilers Formation of deposits (scales and sludges ). Priming and foaming (carry over). Caustic embrittlement . Boiler corrosion.

4. Formation of deposits in boilers and heat exchangers On continuous evaporation of water in boiler the concentration of soluble matters increases progressively which leads to the deposition of salts.

5. SLUDGE – the precipitate is loose and slimy formed by the substances like MgCl 2 , MgCO 3 , MgSO 4 and CaCl 2 . have greater solubility in hot water than cold water.

6. SCALE - precipitate forms a hard and adherent coating on the inner walls. - formed by the substances like Ca(HCO 3 ) 2 , CaSO 4 and Mg(OH) 2 .

7. Comparison of scales and sludges S. No. Sludge Scale 1. A loose, slimy and non-adherent precipitate S hard and adherent coating 2. MgCO 3 , MgCl 2 , MgSO 4 and CaCl 2 Ca(HCO 3 ) 2 , CaSO 4 , Mg(OH) 2 3. Poor conductors of heat and decreases the efficiency of boiler Decreases the efficiency of boiler, crack developed leads to explosion 4. Prevention: Using softened water Using HCl , H 2 SO 4 By blow-down operation Internal and external treatment, by applying thermal shocks, scrapers, wire brush, etc.

8. 1. Wastage of fuels The heat transfer from boiler to inside water in not efficient because of the low thermal conductivity of scales. Overheating is done to provide steady supply of heat to water which causes wastage of fuel. The wastage of fuel depends on the thickness and nature of the scale, which is shown in the table. Disadvantages of scales & sludges

9. 2. Decrease in efficiency – due to the deposition of scales in the valves and condensers of the boiler and choke. 3. Boiler explosion Sometimes due to over heating the thick scales may crack and causes sudden contact of high heated boiler material with water. This causes formation of a large amount of steam and high pressure is developed which may lead to explosion.

10. Prevention of scales Scales can be removed using scraper, wire brush etc. at the initial stage. If scales are brittle, they can be removed by thermal shocks. By using suitable chemicals like dil. acids (for CaCO 3 scale), EDTA (for CaSO 4 scale) with which they form suitable complexes. If the scales are loosely adhering, they can be removed by frequent blow down operation.

11. Scales are hard, adhering precipitates formed on the inner walls of the boilers . Scales are stick very firmly on to the inner walls of the boiler. It is removed with chisel and hammer. Scales are formed by decomposition of calcium bicarbonate in low pressure boilers . Scales

12. 1. Decomposition of calcium bicarbonate: The calcium bicarbonate at high temperature decomposes to calcium carbonate which is insoluble salt, forms scale in low pressure boilers. Ca(HCO3)2 → CaCO3 + H2O + CO2 2. Hydrolysis of Magnesium salts: Magnesium salts gets hydrolyzed at high temperature forming Mg(OH)2 precipitation which forms salt type scale. MgCl2 + H2O → Mg(OH)2↓ + 2HCl 3. Decomposition of calcium sulphate: The solubility of CaSO4 in water decreases with the increase in temperature and forms precipitation on the surface of the boiler further which forms hard scale. This type of scales is formed in high-pressure boilers.

13. 4. Presence of silica: Sio2 present even in small quantities, deposits as Calcium silicates (CaSiO3) or Magnesium silicates (MgSiO3). The deposits form hard scale and are very difficult to remove. Disadvantages of Scales: 1. Wastage of heat and fuels: Scales poor thermal conductivity so that rate of heat transformation is reduced. 2. Lowering of boiler safety is due to overheating of the boiler material becomes softer and weaker, which causes distortion of boiler. 3. If scale is very hard that is formed by CaCO3 can be removed by washing with 5-10% HCl and CaSO4 can be removed with EDTA solution.