water hardness and associated problems ppt.pptx

water hardness, and associated problems

1. Unit – III WATER Introduction and specifications of water , Hardness and its determination by EDTA method (Numerical), Alkalinity and its determination (Numerical), Reverse Osmosis, Electro-dialysis, Disinfection by break-point chlorination. Boiler feed water, boiler problems– Scale, sludge, priming & foaming: causes prevention

2. caustic embrittlement & corrosion: causes & prevention, Water Softening by Internal Treatment: carbonate & phosphate conditioning, colloidal conditioning & calgon treatment Water softening by External Treatment: Lime-Soda Process (Numerical) Zeolite method Ion-Exchange process.

3. Water & Water Treatment Hardness The soap consuming capacity of water is known as the hardness of water. Or Water samples that do not readily produce lather with soap, or deposit scale on the walls of the container when there is appreciable change in temperature, are called hard water.

4. Hardness is generally caused by calcium & magnesium ions present in water. Other divalent metallic cations that cause hardness are Fe +2 , Mn +2 and Sr +2 but these are usually present in small amounts. The cations react with soap (Sodium or Potassium salts of fatty acids ) to form precipitates: 2C 17 H 15 COONa + M +2 Sodium Stearate(Soap) (C 17 H 35 COO) 2 M + 2Na + Ppt..

5. Types of hardness Temporary hardness – Due to bi carbonates and carbonates of Ca +2 and Mg +2 ions and can be removed by boiling. Permanent Hardness – Due to sulphates , chlorides and nitrates of Ca +2 and Mg +2 ions. The hardness in water can be determined by EDTA method.

6. Units of Hardness – mg/l Ppm Cl Fr 1 mg/l = 1 ppm = .07 Cl = 10 Fr Numerical – 1. 100 ml of water sample has a hardness eq of 12.5 ml of .08 N MgSO 4 . What is the hardness in ppm ? 500 mg/l

7. 2. A water sample contains 408 mg CaSO 4 /lit. Calculate hardness in CaCO 3 eq. 50 ml sample water consumed 15 ml of .01 M EDTA before boiling & 5 ml of the same after boiling. Calculate total hardness.

8. Boiler Feed Water – Water is mainly used in boilers for the generation of steam. It should correspond with the following composition- Its hardness should be below .2 ppm. Its caustic alkalinity(OH ions) should lie b/w .15 and .45 ppm. Its soda alkalinity (due to sodium carbonate) should be .45 to 1 ppm. Excess impurities in water results in the following problems-

9. Boiler Problems-

10. BOILER TROUBLES : In treatment of water complete elimination of all the impurities is not possible. The impurity that gives rise to certain troubles will be removed to certain extent. In modern pressure boilers and laboratories, water purer than the distilled water is required. Some of the boiler troubles caused by the use of unsuitable water are.. 1. Carryover – Priming and Foaming 2. Scale formation 3. Boiler corrosion and 4. Caustic embrittlement

11. Priming – It may defined as the violent or rapid boiling of water occurring in the boiler which results in carrying out of water with steam in in the form of a spray. When a boiler is producing steam rapidly, some particles of liquid water are carried along with the steam. This process of wet steam formation is called priming. Priming mainly caused due to very high water level. The presence of large amount of dissolved solids, high steam velocities, sudden increase in steam production rate.

12. Sudden steam demand which leads to sudden boiling, presence of excessive foam filling the foam spare, and due to faulty boiler design. priming can be controlled by proper boiler design, fitting mechanical steam purifier, avoiding rapid change in steam-rate, proper evaporation & adequate heating surfaces, uniform distribution of fuel and providing anti priming pipes, keeping the water level low & avoid sudden steam demands, efficient softening & filtration of the boiler feed water.

13. Foaming – Foaming is the formation of small but stable bubbles above the surface. The main reason for foaming is being presence of fatty acids and other impurities. Foaming can be controlled by using anti-foaming chemicals, removal of concentrated boiler water and replacing it by fresh feed water. Removing oil from boiler water by adding compounds like sodium aluminate

14. SLUDGE – Sludge is a soft, loose and slimy precipitate formed within the boiler. It is formed at comparatively colder portions of the boiler and collects in the area where flow rate is slow. These are formed by substances which have greater solubility in hot water than in cold-water. Eg . MgCO 3 , MgCl 2 , CaCl 2 , MgSO 4 .

15. SCALES – Scales are hard, adhering precipitates formed on the inner walls of the boilers. They stick very firmly on to the inner wall surface and are difficult to remove with chisel and hammer. C auses of scale formation: a. decomposition of calcium bicarbonate: Ca(HCO 3 ) 2 ------- CaCO 3 + H 2 O + CO 2 In low pressure boilers, CaCO3 causes scale formation. In High pressure boilers, CaCO 3 becomes soluble. CaCO 3 + H 2 O Ca(OH) 2 + CO 2

16. b. Decomposition of calcium sulphate : The solubility of CaSO 4 in water decreases with rise of Temperature. In super heated water CaSO 4 is insoluble. This is the main cause in high-pressure boilers. c . Hydrolysis of Magnesium salts: Dissolved Magnesium salts undergo hydrolysis forming Mg(OH)2 precipitate. MgCl 2 + 2H2O Mg(OH) 2 + 2 HCl Mg(OH) 2 so found by hydrolysis of magnesium salts is a soft type of scale.

17. d. Presence of Silica: Silica present in small quantities deposits as silicates like CaSiO 3 and MgSiO 3 . These are very difficult to remove. Disadvantages: a. Danger of Explosion: The hot scale cracks because of expansion and water suddenly comes in contact with overheated Iron plates. This causes in formation of large amount of steam suddenly. This results high pressure causing boiler to burst.

18. b. wastage of fuel: The scale formation causes decreases of heat transfer. As a result over heating is required this causes consumption of fuel. Boiler Corrosion: The chemical or electro chemical eating away of metal by its environment in a boiler is known as boiler corrosion. The main reason for this problem is the presence of excess of oxygen in water. It can be prevented by mechanical deaerator, pre-heating and chemical treatment.

19. a. Dissolved oxygen: Water usually contains about 8 ml of dissolved oxygen per liter at room temperature. Dissolved oxygen in water, in presence of prevailing high temperature, attacks boiler material: 2Fe + 2H2O + O2 2Fe(OH)2 4Fe(OH)2 + O2 2[ Fe2O3.2H2O]

20. Removal of dissolved oxygen : 1. By adding calculated quantity of sodium sulphite or hydrazine or sodium sulphide . 2Na2SO3 + O2 2Na2SO4 N2H4 N2 + 2H2O Na2S + O2 Na2SO4 Hydrazine is an ideal internal treatment chemical for the removal of dissolved oxygen. It reacts with oxygen, forming nitrogen and water. Nitrogen is harmless. Recently Azamina 8001-RD a poly organic compound, has been employed for degassing of water.

21. b. Dissolved carbon dioxide: carbon dioxide is carbonic acid, CO2 + H2O H2CO3 This has a slow corrosive effect on the boiler material. Carbon dioxide is also released inside the boiler, if water used for steam generation contains bicarbonate, Mg(HCO3)2 Heat MgCO3 + H2O + CO2 Removal of carbon dioxide: 1. By adding calculated quantity of ammonia, 2NH4OH + CO2 (NH4)2CO3 + H2O 2. By mechanical – aeration process along with oxygen.

22. c. Acids from dissolved salts: Water containing dissolved magnesium salts liberates acids on hydrolysis. MgCl2 + 2H2O Mg(OH)2 + 2HCl The liberated acid reacts with iron in chain – like reactions producing HCl again and again. Fe + 2HCl FeCl2 + H2 FeCl2 + 2H2O Fe(OH)2 + 2HCl Consequently, presence of even a small amount of MgCl2 will cause corrosion of iron to a large extent.

23. Caustic Embrittlement : The formation of brittle and in crystalline cracks in the boiler shell is called caustic embrittlement . It is a type of boiler corrosion and the main reason for this, is the presence of alkali-metal carbonates and bicarbonates in feed water and also the presence of sodium sulphate . In lime-soda process, it is likely that, some residual Na 2 CO 3 is still present in the softened water. This Na 2 CO 3 decomposes to give NaOH and CO 2 , due to which the boiler water becomes “Caustic”. Na 2 CO 3 + H 2 O NaOH + CO 2

24. This caustic water flows inside the boiler and causes some minutes hair-cracks, by capillary action. On evaporation of water, the dissolved caustic soda increases its concentration which attacks the surrounding area, thereby dissolving Iron of boiler as Sodium ferroate . This causes embrittlement of boiler parts such as bends, joints, reverts etc, due to which the boiler gets fail. Therefore, caustic embrittlement can be prevented. 1. By maintaining the pH value of water and neutralization of alkali. 2. By using Sodium Phosphate as softening reagents, in the external treatment of boilers.

25. 3. Caustic embrittlement can also be prevented by adding Tannin or Lignin or Sodium sulphate which prevents the infiltration of caustic-soda solution blocking the hair-cracks.