water technology-2

7/30/2019 water technology-2

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Boiler Troubles-Carry over:Priming and foaming: When a boiler is producing steam rapidly, some particles

of the liquid water are carried along with the steam.This process of wet steam formation is called priming.

Priming is due to

The presence of large amount of dissolved solids. High steam velocities. Sudden boiling. Improper boiler design Sudden increase in steam production rate.

.


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Foaming:

Foaming is the production of bubbles in boilers.

Which do not break easily.Foaming is due to presence of substances like oils.

Priming can be avoided by:

Maintaining low water levels in boilers.

Fitting mechanical steam purifiers.

Avoiding rapid change in Steaming rate.

Foaming can be avoided by:

Removing oil from boiler water by addingcompounds like Sodium Aluminate.


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Boiler Corrosion

.

Boiler corrosion is decay of boiler material by a chemicalor electro-chemical attack by its environment . Mainreasons for boil corrosions are

1) Dissolved oxygen:Water usually contains about 8ml of dissolved oxygenper litre at room temperature . Dissolved oxygen inwater reacts with iron at high temperature.

2Fe + 2H2O + O2 2Fe(OH)2

4Fe(OH)2 + O2 2 (Fe2O3.2H2O)Ferrous Hydroxide (Rust)


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Removel of dissolved oxygen:

Sodium sulphite or hydrazine or sodium sulphide onreaction with oxygen, thus

2Na2SO3 + O2 2Na2SO4N2H4 + O2 N2 + 2H2O

Na2S + 2O2 Na2SO4


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2. By mechanical-de-aeration: Water spraying in a perforated plate-fitted

tower, heated from sides and connected to

vacuum pump. High temperature, low pressure, and large

exposed surface (provided by perforated plates)

Reduces the dissolved oxygen in water.


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Dissolved-carbon dioxide:

CO2 + H2O H2CO3

Which has a slow corrosive effect on the boiler material.CO2 is also released inside the boiler if water used forsteam generation contains bi-carbonate

Mg(HCO3)2 MgCO3 + H2O + CO2

Removal of carbon dioxide:

Ammonia on reaction with carbon dioxide

2NH4OH + CO2 (NH4)2CO3 + H2O


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Acids from dissolved salts:

Magnesium salts undergoes hydrolysis to giveMagnesium Hydroxide and hydrochloric acid

MgCl2 + 2H2O Mg(OH)2 + 2Hcl

the liberated acid reacts with iron in chain likereactions producing Hcl again & again thus,

Fe + 2Hcl FeCl2 + H2

FeCl2 + 2H2O Fe(OH)2 + 2HclConsequently , presence of even a small amount ofMgcl2 will cause corrosion of iron to large extent.


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Scale and sludge formation in boilers:

In boilers, water evaporates continuously and theconcentration of dissolved salts increases progressively.

If the precipitation takes place in the form of loose and slimyprecipitate is called sludge.

If the precipitation takes place in the form of a hard ,adhering coating on the inner walls of the boilers is calledscale.

water hard coating

on inner wallBoiler of boiler

walls

loose ppt in water (scale)

(sludge)

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Sludge is a soft, loose, and slimy precipitate formed

with in the boiler. Sludge is formed by substances which have greater

solubilities in hot water than in cold water .

Ex: Mgco3,Mgcl2,Mgso4,Cacl2 etc.

Disadvantages of sludge formation: Sludges are poor conductor of heat, so they tend to

waste a portion of heat generated.

Excessive sludge formation disturbs the working of the

boiler.Prevention of sludge formation:

By using well softened water.

By frequent blow-down operation .


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Scales are hard deposits, which stick very firmly to the

inner surfaces of the boilers. Scales are difficult to remove, even with the help of

hammer . Scales are the main sources of boiler troubles. Formation

of scales may be due to

i) Decomposition of calcium bicarbonate:Ca(HCO3)2 CaCO3 + H2O + CO2

(scale)

This type of scale formation at low-pressure boilers.

But in high-pressure boilers, CaCO3 is solubleCaCO3 + H2O Ca(OH)2+CO2

(Soluble)


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ii) Hydrolysis of magnesium salts:MgCl2 + 2H2O Mg(OH)2 + 2HCl

(scale)iii) Presence of silica (SiO2):

CaSiO3, MgSiO3 these are stickly binding with inner surface ofthe boilers and are very difficult to remove.

Disadvantages of scale formation:

1)Wastage of fuel:The wastage of fuel depends upon the thickness and the nature

of scaleThick ness of scale (mm) 0.325 0.625 1.25 2.5 12

wastage of fuel 10% 15% 50% 80% 150%2) Lowering of boiler safety.3) Decrease in efficiency .4) Danger of explosion.


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Prevention of scales formation:1) Internal treatment .

2) External treatment.


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Caustic Embrittlement Is a type of boiler corrosion, caused by using highly alkaline

water in the boiler. In high pressure boilers, Na2CO3 decomposes to give NaOH

and CO2

Na2CO3 + H2O 2NaOH + CO2and this makes the boiler water caustic.

Caustic soda attacks the surrounding area, for example the

iron surrounded by dilute NaOH becomes the cathodic side The iron surrounded by conc. NaOH becomes Anodic Part,

Which is consequently corroded

At Anode : Fe Fe+2 + 2e-At Cathode : Fe+2 + 2OH- Fe(OH)2


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This causes embrittlement of boiler parts, particularly

stressed parts like bends, joints etc. , causing even failure ofthe boiler.

Caustic embrittlement can be avoided:

By using sodium phosphate as softening reagent, instead of

Na2CO3. By adding sodium sulphate to boiler water.

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water technology-2