Boilers

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SCALE & SLUDGE FORMATION

PRIMING AND FOAMING

BOILER CORROSION

CAUSTIC EMBRITTLEMENT

BOILER PROBLEMS

CausesHigh steam velocity, +nce of large quantities of dissolved salts, Veryhigh water level in boiler, Sudden boiling and improper design ofboilerPrevention: by using soft water, fitting mechanical steam purifiers, controllingsteam velocity, improving design of boiler, removing scale & sludgetime to time

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PRIMINGProcess of wet steam formation is called priming

(When a boiler is producing steam rapidly, some droplets of liquid water are carriedalong with the steam)

FOAMING:Formation of persistent foam 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 impuritiesOil and alkalis react to form soap, lowers the S.T. & increasesfoamingPrevention: by using anti-foaming chemicals like castor oilBy addition of coagulants such as sod. aluminate

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DISADVANTAGES OF PRIMING & FOAMING

Wet steam carries over the dissolved salts of boiler water toturbine blades

B.P. of water is increased due to foaming- results in wastageof fuel

Reduces efficiency of boiler

Due to +nce of foam, actual water level can not be assessedin boiler

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BOILER CORROSION

Decay of boiler material due to chemical or electrochemical reactions with its environment

1. Dissolved oxygen

2. Dissolved CO2

3. Mineral acids

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CAUSES

1. Dissolved oxygenWater contains 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 athigh 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 removedissolved oxygen in boiler feed water. It can be done by addinghydrazine (N2H4), Sod. Sulphite (Na2SO3) or sod sulphide Na2S

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2. Dissolved CO2

CO2 forms carbonic acid which has a corrosive effecton boiler material

CO2 + H2O → H2CO3

• CO2 is +nt in boiler water or it results fromdecomposition of bicarbonates +nt in water

Ca (HCO3)2 ----> CaCO3 + H2O + CO2↑

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*CO2 can be removed by adding NH3

2NH3 + CO2 +H20 (NH4)2CO3

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Boiler Tube: Oxygen Pitting Pipe: Low pH Corrosion

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3. Mineral acids

Hydrolysis of dissolved salts like MgCl2 present in boiler waterresults 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 dueto chain reaction

CAUSTIC EMBRITTLEMENT“The phenomenon in 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, itundergoes hydrolysis to form sod. hydroxide.

Na2CO3 + H2O → 2NaOH + CO2

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• Presence of NaOH makes the water alkaline.

• NaOH attacks the surrounding material and dissolves the ironof 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 andjoints

Na2CO3 + H2O → 2NaOH + CO2

Prevention

• by using sodium 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

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Tannic acidC76H52O46 from woodLignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of

the secondary cell walls of plants and some algae. C9H10O2, C10H12O3, C11H14O4

BOILERA device to generate steam for heating as well as togenerate power

BOILER FEED WATERWater used in boiler in this regard is termed as boilerfeed waterBoiler feed water = Return water + Makeup water

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SPECIFICATION FOR BOILER FEED 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

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Following methods

(a)External Treatment Methods- (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

Internal Treatment Methods of Boiler Feed Water

1. Colloidal conditioning

• Scale formation can be reduced by adding colloidalsubstances like kerosene, agar-agar, starch etc to the boilerwater

• These substances surround the minute particles of scaleforming salts & prevent their coagulation

• Hence salts remain loose in the form of sludge and can beremoved 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

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2. Carbonate Conditioning

• Formation of CaSO4 scales can be avoided by addition ofNa2CO3

• CaSO4 gets converted into CaCO3

CaSO4 + Na2CO3 → CaCO3 + Na2SO4

Loose sludge(Can be removed by blow down operation)

(3) Phosphate Conditioning

Addition of sod. Phosphate

2 MgCl2 + 2 Na3PO4 → Mg3(PO4)2 + 6 NaClSoft sludge

2 CaCl2 + 2 Na3PO4 → Ca3(PO4)2 + 6 NaClSoft sludge

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(4) Calgon Conditioning-

Addition of 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- + 2 Na2SO4

Soluble complex (do not cause any hardness)