Boilers

Boilers

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  1 SCALE & SLUDGE FORMATION PRIMINGAND FOAMING BOILER CORROSION CAUSTIC EMBRITTLEMENT BOILER PROBLEMS
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  Causes High steam velocity,+nce of large quantities of dissolved salts, Very high water level in boiler, Sudden boiling and improper design of boiler Prevention: by using soft water, fitting mechanical steam purifiers, controlling steam velocity, improving design of boiler, removing scale & sludge time to time 2 PRIMING Process of wet steam formation is called priming (When a boiler is producing steam rapidly, some droplets of liquid water are carried along with the steam)
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  FOAMING: Formation of persistentfoam or bubbles, at the surface of water in boiler which do not dissolve or break easily normal buble carry over buble Causes +nce of dissolved substances like oil, grease, and alkaline impurities Oil and alkalis react to form soap, lowers the S.T. & increases foaming Prevention: by using anti-foaming chemicals like castor oil By addition of coagulants such as sod. aluminate 3
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  DISADVANTAGES OF PRIMING& FOAMING  Wet steam carries over the dissolved salts of boiler water to turbine blades  B.P. of water is increased due to foaming- results in wastage of fuel  Reduces efficiency of boiler  Due to +nce of foam, actual water level can not be assessed in boiler 4
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  BOILER CORROSION Decay ofboiler material due to chemical or electrochemical reactions with its environment 1. Dissolved oxygen 2. Dissolved CO2 3. Mineral acids 5 CAUSES
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  1. Dissolved oxygen Watercontains 8 ppm of dissolved oxygen. On heating water in boilers, dissolved oxygen is set free. In +nce of water, this oxygen reacts with iron walls of boiler at high temperature to form rust 2 Fe + 2H2O + O2 → 2Fe (OH)2 4Fe (OH)2 + O2 → 2 [Fe2 O3. 2H2O] rust Hence to prevent this corrosion, it is essential to remove dissolved oxygen in boiler feed water. It can be done by adding hydrazine (N2H4), Sod. Sulphite (Na2SO3) or sod sulphide Na2S 6
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  2. Dissolved CO2 CO2forms carbonic acid which has a corrosive effect on boiler material CO2 + H2O → H2CO3 • CO2 is +nt in boiler water or it results from decomposition of bicarbonates +nt in water Ca (HCO3)2 ----> CaCO3 + H2O + CO2↑ 7 *CO2 can be removed by adding NH3 2NH3 + CO2 +H20  (NH4)2CO3
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  8 Boiler Tube: OxygenPitting Pipe: Low pH Corrosion
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  9 3. Mineral acids Hydrolysisof dissolved salts like MgCl2 present in boiler water results in formation of HCl MgCl2 + 2H2O  Mg(OH)2 + 2HCl HCl reacts with iron walls & form rust Fe + 2HCl  FeCl2 + H2 FeCl2 + 2H2O  Fe (OH)2 + 2HCl 4Fe (OH)2 + O2  2 [Fe2 O3. 2H2O] rust Small amount of HCl may cause corrosion to a large extent due to chain reaction
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  CAUSTIC EMBRITTLEMENT “The phenomenonin which the material of a boiler becomes brittle due to the accumulation of caustic substances” • As water evaporates in the boiler, conc. of sod. carbonate increases • Sod. Carbonate is also used in water softening by lime-soda process, due to this some sodium carbonate maybe left behind in the water • As the concentration of sod. carbonate increases, it undergoes hydrolysis to form sod. hydroxide. Na2CO3 + H2O → 2NaOH + CO2 10
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  11 • Presence ofNaOH makes the water alkaline. • NaOH attacks the surrounding material and dissolves the iron of the boiler as sodium ferrate (Na2FeO2). • It decomposes leading to rusting of boiler 3Na2FeO2 + 4H2O → 6NaOH + Fe3O4 + H2 • This causes embrittlement of boiler parts like rivets, bends and joints Na2CO3 + H2O → 2NaOH + CO2
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  Prevention • by usingsodium phosphate instead of sodium carbonate as softening reagents • by adding tannin or lignin to boiler water- blocks the hair- line cracks and prevents infiltration of caustic salts into these areas • Adding Na2SO4 to boiler water also blocks hair-line cracks 12 Tannic acidC76H52O46 from wood Lignin or lignen is a complex chemical compound most commonlyderived from wood, and an integral part of the secondary cell walls of plants and some algae. C9H10O2, C10H12O3, C11H14O4
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  BOILER A device togenerate steam for heating as well as to generate power BOILER FEED WATER Water used in boiler in this regard is termed as boiler feed water Boiler feed water = Return water + Makeup water 13
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  SPECIFICATION FOR BOILERFEED WATER It should be free from: Hardness (sludge & scale formation) Organic matter, oil, grease (foaming) Dissolved gases like CO2, H2S, O2 (to avoid corrosion) Alkalinity (caustic embrittlement) Turbidity and sediments 14
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  15 Following methods (a)External TreatmentMethods- (water treatment before being fed into boilers) • Lime Soda Process, • Zeolite Process • Ion Exchange Process (b) Internal Treatment Methods- (water treatment inside the boiler) Can be achieved by adding certain chemicals to water in the boiler • Colloidal conditioning • Carbonate conditioning • Phosphate conditioning • Calgon conditioning Treatment of boiler feed water
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  Internal Treatment Methodsof Boiler Feed Water 1. Colloidal conditioning • Scale formation can be reduced by adding colloidal substances like kerosene, agar-agar, starch etc to the boiler water • These substances surround the minute particles of scale forming salts & prevent their coagulation • Hence salts remain loose in the form of sludge and can be removed by blow-down operation. • This process is suitable for low pressure boilers. CnH2*n+2. Kerosene is a mixture of hydrocarbons of this formula, agar(C12H18O9)n , starch (C6H10O5)n 16
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  17 2. Carbonate Conditioning •Formation of CaSO4 scales can be avoided by addition of Na2CO3 • CaSO4 gets converted into CaCO3 CaSO4 + Na2CO3 → CaCO3 + Na2SO4 Loose sludge (Can be removed by blow down operation)
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  (3) Phosphate Conditioning Additionof sod. Phosphate 2 MgCl2 + 2 Na3PO4 → Mg3(PO4)2 + 6 NaCl Soft sludge 2 CaCl2 + 2 Na3PO4 → Ca3(PO4)2 + 6 NaCl Soft sludge 18
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  19 (4) Calgon Conditioning- Additionof Sod. Hexa-meta-phosphate (calgon) Calgon converts scale forming salts into soluble complexes Na2[Na4PO3)6] 2Na+ + [Na4 (PO3)6]2- Calgon CaSO4 + [Na4PO3)6]2- [Ca2 (PO3)6]2- + 2Na2SO4 Soluble complex (do not cause any hardness)