THE MECHANISM OF SCALING & CORROSION.pptx
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Gives good concept on scaling and corrosion with respect to ph of water
THE MECHANISM OF SCALING & CORROSION.pptx
- 1. D.M.PLANT AND IT’S IMPORTANT ROLE OF POWER PLANT
- 3. D.M.PLANT
- 4. To prevent scaling of pressure vessels due to dissolved and suspended impurities. To prevent corrosion of metallic parts of the boiler, with which water steam comes in direct contact. To establish protective coating over metallic surfaces to prevent corrosion attack. To avoid salt deposits over turbine blades. To ensure better utilisation of heat energy and to improve on efficiency. NEED FOR D.M.WATER TREATMENT
- 6. CLARIFIER Before water enters into Demineralised plant, it should be free from suspended, colloidal and organic impurities. Presence of such suspended impurities adversely affects the deionization properties of the resins, which will affect the end quality of demineralised water. Suspended and colloidal particles are removed by clarifying the water in a clarifloculator added by suitable coagulating agents. PRE- TREATMENT OF RAW WATER
- 7. Turbidity in the water is suspended insoluble matter including coarse particles (mud, sediment, sand etc.) that rapidly on standing the turbidity of feed water should not exceed 5 NTU. These materials can be removed by settling, coagulation and filtration. Their presence is undesirable because evaporation on heating produces hard stony scale deposits on the heating surface. Both are objectionable as they cause damage to the boiler system. TURBIDITY AND SEDIMENT
- 8. CALCIUM AND MAGNESIUM SALTS The Ca and Mg salts present in the water in the form of carbonates, bicarbonates, sulphates and chlorides. The presence of these salts is recognized by the hardness of the water. The hardness of the water is classified as temporary hardness and permanent hardness. DISSOLVED SALTS & MINERALS
- 9. Presence of certain salts of Calcium, Magnesium and other metals TEMPORARY HARDNESS Dissolved bi carbonate of Calcium, Magnesium carbonates of Iron and other heavy metals Removed by boiling the water PERMANENT HARDNESS • Chlorides and Sulphates of Calcium, Magnesium, Iron and heavy other metals • Not destroyed easily like Temporary Hardness HARDNESS OF WATER
- 10. Oxygen Its presence is highly objectionable as it is corrosive to iron, zinc, brass and other metals. It causes corrosion and pitting of water lines, boiler exchanges. Its effect is further accelerated at high temperatures. Carbon dioxide It also causes the corrosion of steam water and condensate lines. Also helps to accelerate the corrosive action of oxygen. The other gases are H2S, CH4, N2 and many others but their percentage are negligible. DISSOLVED GASES
- 11. Scale and sludge formation Corrosion Priming & Foaming Caustic Embrittlement DISADVANTAGES OF HARDNESS
- 12. DUEL MEDIA FILTER Duel Media filters have different sizes of gravels. The function of the DMF is to decrease the turbidity further. It decreases turbidity < 2NTU. Hypochlorite dosing is given to the water when it enters into the DMF due to kill the algae & fungi. Here residual chlorine will not exceed 0.5 ppm of inlet water. After exhaust DMF is Backwashed.
- 13. ACTIVATED CARBON FILTER • This filter contains charcoal particles. The main function of the ACF is to remove the bad odor of the water. • It also removes the excess chlorine in water. From ACF water goes to Strong Acid Cation. After exhaust ACF is backwashed.
- 14. ION EXCHANGE PROCESS Ion Exchange resins are insoluble, cross linked, long chain organic polymers with a micro porous structure and the functional groups attached to the chains are responsible for Ion Exchange properties. Resin containing Acidic functional groups (-COOH – SO3H) are capable of exchanging their H+ Ions with other cations which comes in their contact where as containing basic functional groups (-NH2 +/- NH) are capable of exchanging their Anions with other anions. Ion exchange resins are two types: 1) Cation Exchange Resins 2) Anion Exchange Resins.
- 15. Cation & Anion Resins
- 16. Cation exchange resins(RH+ ) These resins are capable to exchange their Hydrogen Ions with cations in the water. After exhausted of the Bed these Resins are treated with HCl or H2SO4. Anion Exchange Resins:(OH- ) These resins are capable to exchange their OH- Ions with anions in the water. The exhausted Bed treated with caustic soda(NaOH).
- 18. STRONG ACID CATION(SAC) From ACF water enters into the Cation bed. When hard water enters through cation exchange column which removes all the cations like Ca2+ , Mg2+ from it and equivalent amount of H+ ions are released from this column to water. 2RH+ + Ca2+ -- R2Ca2+ + 2H+ 2RH+ + Mg2+ - R2Mg2+ + 2H+ When the exchanging capacity of H+ Ions completed the bed is declared as exhausted. Again the bed is re-activated by treating with Hydrochloric Acid and after regeneration the bed has a capacity to exchange H+ ions. R2Ca2+ + 2H+ ---- 2RH+ + Ca2+ (washing)
- 19. After SAC water goes to the degassed tower in which gases like CO2, CO3 removes. In degassed tower pal rings are present, water sprinkles from top to bottom. Air is blow into the chamber through air blower, so by this process gasses are removed from the water. After that this water collected in a degassed water tank.
- 20. STRONG BASE ANION(S.B.A) From degasified water tank water comes into the SBA with degassed water pump. When water passes through Anion Exchanger which removes all anions like SO4 2, Cl- , SiO2 etc. from it and equivalent amount of OH- ions are released from this column to water. R-OH- + Cl RCl- + OH- R-OH- + SO4 2 R2SO4 2 + 2OH- After exhausted SBA is treated with Caustic Soda (NaOH) and the bed is capable to exchange OH- ions. R2SO4 2 + 2OH- 2ROH- + SO4 2 (washing) H+ and OH- ions releases from SAC and SBA combines and from water molecule H2O.
- 21. MIXED BED In this bed both types of resigns SAC and SBA will be present. After SBA water enters in to the mixed bed and the water further purifies, this water collects into the DM Water storage tank (DMWST).
- 22. DW WATER STORAGE TANK
- 23. THE MECHANISM OF SCALING & CORROSION
- 24. SCALING
- 25. A hard coating deposited on the surface of plates and tubes in steam boilers is called boiler scales. The formation of scale and sludge can cause tube failures Scaling reduces boiler heat transfer efficiency The accumulation of material on boiler surfaces can cause overheating and corrosion. Most boiler systems have demineralised make up water. SCALING
- 26. Deposition is a major problem in the operation of steam generating equipment Most deposits can be classified as one of two types: • Scale that crystallized directly on to tube surfaces. • Sludge deposits that precipitated elsewhere and were transported to the metal surface by the flowing water DEPOSITS
- 28. Types of corrosion • Galvanic corrosion • Caustic corrosion • Acidic corrosion • Oxygen attack • Steam side corrosion CORROSION
- 29. Corrosion is one of the main cause of reduced reliability in steam generating systems. Many corrosion problems occur in the hottest areas of the boiler, the water wall, screen, and super heater tubes. Other common problem areas include deaerator, feed water heaters and economizers. The most common causes of corrosion are dissolved gases(Primarily oxygen and carbon dioxide) CORROSION
- 30. Maintenance of proper pH and alkalinity levels Control of oxygen and boiler feed water contamination Reduction of mechanical stresses Operation with design specifications, especially for temperature and pressure. Proper precautions during start-up and shutdown Effective monitoring and control. CORROSION CONTROL MEASURES
- 31. DEAERATOR
- 32. Deaerators are used to heat feed water and reduce oxygen and other dissolved gases to acceptable levels. Corrosion fatigue at or near welds is a major problems in deaerators. Most corrosion fatigue cracking has been reported to be the result of mechanical factors, such as manufacturing procedures, poor welds, and lack of stress-relieved welds. DEAERATORS
- 33. ECONOMISERS
- 34. Economiser helps to improve boiler efficiency but extracting heat from flue gasses discharged from the fire side of a boiler. Economisers can be classified as steaming and non steaming. Steaming economizers are particularly sensitive to deposition from feed water contaminants and resultant under-deposit corrosion. Corrosion at tube bends is also a problem in steaming economisers. Whenever possible, tubes in this area should be inspected closely for evidence of corrosion. ECONOMIZERS
- 35. SUPERHEATERS
- 36. Super heaters corrosion problems are caused by a number of mechanical and chemical conditions. One major problem is the oxidation of super heater metal due to high gas temperatures, usually occurring during transition periods, such a start-up and shutdown. Deposits due to carryover can contribute to the problem. SUPERHEATERS
- 38. Anything that results in a difference in electrical potential at discrete surface locations can cause a galvanic reaction. Causes include • Scratches in a metal surface • Differential stresses in a metal • Differences in temperatures • Conductive deposits GALVANIC CORROSION
- 40. By the reaction of Iron contact with the boiler water, forms magnetite and the protective layer is continuously restored. Caustic corrosion occurs when caustic is concentrated and dissolves the protective magnetite layer. However, as long as a high caustic concentration exists, the magnetite is constantly dissolved, causing a loss of base metal and eventual failure. Disodium phosphate neutralizes caustic by the following reactions: Na2HPO4 + NaOH ↔ Na3PO4 + H2O CAUSTIC CORROSION
- 41. ACIDIC CORROSION
- 42. Low make up or feed water pH can cause serious acid attack on metal surfaces in the pre boiler and boiler system. Common causes include the following: • Improper operation or control of demineralizer cation units • Cooling water contamination from condensers ACIDIC CORROSION
- 43. CAUSE As water evaporates in the boiler, the concentration of sodium carbonate increases in the boiler. Sodium carbonate is used in softening of water by lime soda process, due to this some sodium carbonate may be left behind in the water. As the concentration of sodium carbonate increases, it undergoes hydrolysis to form sodium hydroxide. Na2CO3 + H2O → 2NaOH + CO2 The presence of sodium hydroxide makes the water alkaline. This alkaline water enters small cracks present in the inner walls of the boiler by capillary action. Inside the cracks, the water evaporates and amount of hydroxide keeps increasing progressively. This sodium hydroxide attacks the surrounding material and the dissolves the iron of the boiler as sodium ferrate. This causes embrittlement of boiler parts like rivets, bends and joints, which are under stress. CAUSTIC EMBRITTLEMENT
- 45. Oxygen is highly corrosive when present in hot water. Even small concentration can cause serious problems. Because pits can penetrate deep into the metal, oxygen corrosion can result in rapid failure of feed water lines, economisers, boiler tubes and condensate lines. Additionally iron oxide generated by the corrosion can produce iron deposits in the boiler. OXYGEN ATTACK
- 46. OXYGEN ATTACK
- 47. Carryover in low pressure boiler steam generated in boilers essentially includes no dissolved solids, however, the dissolved solids in boiler water sometimes transfer to the steam due to various causes. This phenomenon is called “Carryover”. The fallowing factors are the main causes of carryover, and if the “Priming” or “Foaming” occurs, it accelerates the carryover. CARRYOVER
- 48. CARRYOVER
- 49. PRIMING It is the generation of foam In boiler due to high concentration of solids, organic matter, bubbles are build up on the surface of boiler water and passed out with the steam. FOMING It Is the carrying amount of droplet water in the steam, witch leads to carryover of Salt crystals on the super heater and turbines. PRIMING AND FOAMING