boiler feed water.pptx

Boiler Feed Water

•

In industries, water is mainly required for cooling and

steam generation purposes.

•

Water as coolant need not to be of high quality.

Boiler Feed Water

•

Boiler feed water requires to be treated first.

•

If used untreated then it may lead to following

problems :

1. Scale and sludge formation.

2. Boiler corrosion

Scale and sludge formation

•

In boilers, water is converted into steam. When water containing impurities is used for generating

steam, the concentration of the dissolved salts increases.

•

If dissolved salts takes place in the form of loose, non adherent, slimy precipitates, it is called

sludge

.

•

If the precipitates form hard, adhering coating on the inner walls of the boilers, it is called

scales

.

•

Salts responsible for the formation of scales and sludges are CaCO

, CaSO

, Mg (OH)

MgCl

, and SiO

etc.

Sludge formation : Sludges are formed by the substances having greater solubilities in hot water than in the cold water e.g. MgCO₃, MgCl₂, CaCl₂, MgSO₄ etc.

Disadvantages of Sludge Formation

(i) Poor conductor of heat so it tends to waste lot of amount of heat. So boilers require more heat.

(ii) Decrease the efficiency of the boilers.

(iii) Slows down the water circulation as it cause choking of pipes.

Prevention of Sludge Formation

(iv) By using soft water.

(v) By frequently carrying out the cleaning operations.

Causes of Scale formation

(i) Decomposition of bicarbonates: When water containing bicarbonates is heated in the boiler, the bicarbonates

present are converted into insoluble CaCO₃. Ca (HCO₃)₂  CaCO₃ + CO₂ + H₂O

(ii) Hydrolysis of Mg Salts: Dissolved magnesium salts undergo hydrolysis forming precipitates of Mg(OH)₂.

MgCl₂ + 2H₂O  Mg (OH)₂ + 2HCl

(iii) CaSO₄ is less soluble in hot water so it forms hard scale on the heated portions of the boiler. CaSO₄ is the main cause of scale formation in high pressure boilers.

Disadvantages of Scale Formation

(i)

Wastage of Fuel

:

Because of low thermal conductivity of the scales, more heat is supplied to the boiler to

maintain the supply of steam leading to wastage of fuels.

(ii)

Bagging

:

The distortion of boiler material is known as bagging. The superheating of the boiler leads to

the distortion of the boiler material.

Due to overheating, it causes thinning of the boiler material.

Removal of Scales

(i)

Soft scales are removed with the help of scrapper or wire brush.

(ii)

Brittle or hard scales can be removed by giving thermal shocks i.e. heating the boiler and suddenly cooling

with cold water.

(iii)

Scales can also be dissolved in certain chemicals and hence can be removed along with water.

Prevention of Scale Formation

The scale formation can be minimized by subjecting the boiler feed water to

certain treatments which may be

1.

External treatment:

which includes the, water softening techniques.

Water Softening techniques for boiler feed water

The methods used for internal treatment of water are :

1.

Colloidal Conditioning

2.

Carbonate Conditioning

3.

Phosphate Conditioning

1.

Colloidal Conditioning

•

Scale formation can be reduced by introducing colloidal substances like

kerosene, tannin, glue, agar etc. to the boiler water.

•

These substances act as protective coatings.

•

They surround the minute particles of scale forming salts and prevent

their coalescence and coagulation.

•

Hence, the salt remains as loose, non sticky deposits in the form of

sludges which can be removed by simple 'blow down operation.

2. Carbonate Conditioning

•

This method is useful in low pressure boilers.

•

Any scale forming salts like CaSO

4

present in water is

precipitated in the form of insoluble CaCO

by the addition of

sodium carbonate.

CaSO

+ Na

CO



CaCO

+ Na

SO

3.

Phosphate Conditioning

•

This technique is useful for high pressure boilers.

•

Hardness causing salts are removed by converting them into insoluble soft

and non adherent, phosphates by treating them with sodium phosphates.

3MCl

+ 2Na

PO



M

(PO

)

+ 6NaCl

where M = Ca

and Mg

•

Calcium can be properly precipitated at a pH of 9.5 or above. Therefore,

the exact selection of phosphate salt depends upon the alkalinity of

water.

3.

Phosphate Conditioning(contd.)

•

The three different phosphates are as follows.

(a) Mono sodium phosphate

(NaH

PO

): It is highly acidic and used

for highly alkaline water. In other words it reduces the pH of the

highly alkaline water and brings it near the optimum pH.

(b) Disodium phosphate

(Na

HPO

): Its acidity is moderate. It is used

for moderate alkaline water.

(c) Trisoidum phosphate

(Na

PO

): It is highly alkaline and used for

low pH (acidic) water to increase the pH so that Ca

or Mg

may

4. Calgon Conditioning

• Calgon is the commercial name of sodium hexametaphosphate.

• It forms the complexes with the scale forming salt in which Ca2+ _{is trapped by}

the phosphate, even if present in very small amounts.

• _{The complex which is formed, is soluble in water and hence remains in the}

dissolved form without causing any harm to the boiler.

Na₂[Na₄ (PO₃)₆]  2Na+ _{+ [Na}

4P6O18] 2

-2CaSO₄ + [Na₄ P₆O₁₈]2-  _[Ca

2P6O18]

2- _{+ 2Na} 2SO4

• _{However, this treatment is not capable of preventing the formation of iron}

7.

Treatment with sodium aluminate

(NaAlO

)

:

•

When NaAlO

2

is added to the boiler water, it gets hydrolyzed.

•

NaAlO

2

+ 2H



2NaOH + Al(OH)

(Gelatinous ppt.)

•

The NaOH thus formed precipitates any Mg

salts.

MgCl

+ 2NaOH



Mg (OH)

+ 2NaCl

•

Both Al(OH)

3

and Mg(OH)

are flocculant precipitates. They can help in

Priming : Due to the extremely rapid boiling of water in the boiler, the formed steam carries away the water droplets. The phenomenon of formation of this wet steam is called priming.

If steam contains 1.5% moisture, its steam quality will be reported as 98.5%.

Priming is caused by

(i) The high steam velocities.

(ii) Very high water level in the boiler.

(iii) Sudden steam demands leading to sudden drop of pressure in the steam line

(iv) Faulty boiler design.

Foaming : Foaming is the formation of small persistent bubbles at the surface of water in the boiler.

• Bubbles are formed due to the difference in concentration of solute and the bulk of the liquid.

• Any material which lowers the surface tension of the water will collect at the interface and thus increase the foaming.

Disadvantages of Priming and Foaming

• Priming and foaming are often used synonymously.

• These are highly objectionable because water is contaminated by them.

• Impurities with the steam goes to super heaters or turbine blades, where they get deposited as the water evaporates.

• These deposits reduce their efficiency by hindering the flow of steam. (i) Dried salts may be deposited on the engine valves decreasing their life.

(ii) Water in the steam may lead to corrosion in the super heaters.

Prevention of Priming: Priming can be avoided by (i) Keeping the water level lower.

(ii) Reducing steam velocities also decreases the moisture content of steam

(iii) Efficient softening and filtration of boiler feed waters.

(iv) Good boiler design fitted with mechanical steam purifiers.

Prevention of Foaming : Foaming can be avoided by

(v) adding antifoaming chemicals. For example polyamide antifoamers reduces the surface tension. Castor oil spreads on the surface of water and prevents foaming.

(vi) removing oil from boiler water also prevents foaming.

Boiler Corrosion

•

D

ecay of boiler material due to the attack of certain chemicals on its surface.

•

The material of the boiler thus gets dissolved and rusted, thereby reducing the life of the boiler.

•

In general, corrosion occurs, if Fe of the boiler material combines with the water molecules to form

ferrous hydroxide, which reacts with oxygen to form Fe(OH)

, or rust.

Fe

+ 2H

2

O  Fe(OH)

+ 2H

4Fe(OH)

+ O

 2 [Fe

O

. 2H

O]

[ Rust ]

Main reasons for boiler corrosions are :

(i) Presence of dissolved oxygen.

(i) Presence of Dissolved Oxygen

• Dissolved oxygen is the main cause of boiler corrosion.

• When water containing dissolved oxygen is heated in the boiler, evolved free gas with high temperature of boiler attacks the boiler material.

2 Fe + 2H₂O + O₂  2Fe (OH)₂ 4Fe (OH)₂ + O₂ + 2H₂O  2 [Fe₂O₃. 2H₂O]

Removal of dissolved Oxygen: Oxygen can be removed by chemical or mechanical means.

• _{Chemical Means: Oxygen can be removed by adding calculated amounts of certain chemicals such as Na2SO3,}

Na₂S and N₂H₄.

• Hydrazine is extensively used to remove dissolved oxygen in high pressure boilers.

(ii) Presence of dissolved CO2

• _{Carbon dioxide is formed by the decomposition of dissolved bicarbonates under high temp of boiler.}

Mg (HCO₃)₂  MgCO₃ + CO₂ + H₂O.

• _{Carbon dioxide forms carbonic acid in presence of water which has slow corrosive effect on boiler} material.

CO₂ + H₂O  H₂CO₃ Carbonic acid

Removal of CO₂: Removal of CO₂ also involves chemical or mechanical means.

• _{Chemical means: By addition of calculated amount of NH4OH.}

• _{Mechanical means: Similar process as discussed in removal of oxygen.}

(iii) Presence of Acids

Free mineral acids, present in boiler water, reacts with iron

material in the following way

Fe + 2HCl



FeCl

+ H

FeCl

+ 2H

O  Fe (OH)

+ 2HCl

Fe(OH)

+ O



2[Fe

O

.2H

O]

•

Thus a small amount of HCl may lead to corrosion to a great extent.

Caustic Embrittlement

•

Boiler corrosion due to high alkaline water in the boiler is known as caustic embrittlement.

•

The material of the boiler becomes brittle due to accumulation of caustic substance.

•

During the softening of water by lime-soda process a slight excess of Na

CO

is used for the complete

removal of calcium and magnesium salts.

•

If low pressure boilers, the treatment with Na

CO

is quite satisfactory but in high pressure boilers the

free Na

CO

present undergoes hydrolysis to give NaOH as

Na

CO

+ H

O→ 2NaOH + CO

•

NaOH so formed in water makes the boiler water caustic.

•

The NaOH attacks the minute hair cracks, riveted seams, bends and joints

•

Thereby corroding the boiler material or dissolving the iron of the boiler. This causes embrittlement of

the boiler parts, particularly the stressed parts.

Prevention of Caustic Embrittlement:

The caustic embrittlement can be prevented by

(i) Using sodium phosphate as softening reagent instead of sodium carbonate.

(ii) By adding liquids or tannin to boiler water. These block up the hair cracks thus prevent the infiltration of sodium hydroxide.

(iii) It is observed that boiler waters containing sodium sulphate or sodium phosphate inhibit the caustic embrittlement by blocking the capillaries, thereby preventing the infiltration of caustic soda solution to these.

The concentration of Na₂SO₄ : NaOH are maintained at 1 : 1, 2 : 1 or 3 : 1 for operating pressures of 10, 20 and > 20 atmospheres respectively to check the caustic embrittlement.

Specification of Drinking Water

(a)

Primary Standards:

Primary standards specify the maximum contaminant levels of

various dissolved minerals based on their effect on human health.

(b)

Secondary Standards:

vary from place to place depending upon the taste, odour,

colour and hardness of water etc and do not have

Requisites of Drinking Water

•

Free from objectionable minerals.

•

pH value ranging from 6.5 to 8.5.

•

Total dissolved solids : Less than 500ppm.

•

Free from Pathogenic bacteria.

•

Coliform bacteria used as indicator. Water should contain one coliform per 100ml.

•

Free of Fe & Mg, permissible limit being 0.3 mg/L and 0.05mg/L resp.

•

Non corrosive

•

Free from objectionable dissolved gases like H

S.

CONTAMINANT PERM LIMIT(MG/L) HEALTH EFFECT

Arsenic 0.050 Dermal and nervous system toxicity effects. Barium 1.000 Circulatory system effects.

Cadmium 0.010 Kidney is effected.

Chromium 0.050 Liver and kidney are effected Fluoride 1.000 Fluorosis.

Lead 0.050 Central and peripheral nervous system damage, kidney effects, highly toxic to infants.

Mercury 0.001 Central nervous system disorders.

Nitrite & Nitrates 45.000 Methaemoglobinemia (blue baby syndrome) Selenium 0.010 Gastrointestinal effects.

Silver 0.050 Skin discolouration.

Sulphate 200.000 Gastrointestinal irritation.

Water treatment for Domestic/Municipal Purpose

For removal of Pathogenic bacteria

For removal of Colloidal matter &

Microorganisms

Domestic water treatment

Screening :

•

Water is made to pass through large screens having number of holes.

•

Floating material like wood pieces, leaves etc. are removed.

Sedimentation :

•

Process of allowing water to stand undisturbed in big tanks for 2-8

Domestic water treatment

Filteration :

•

process of removing insoluble, colloidal and bacterial impurities from

water by means of filters.

•

most widely used filtration units is the rapid-sand filter (gravity/press).

Concrete Tank Filter 3 ft. Fine Sand 1 ft. Coarse Sand 8 ft. Gravel Sand

Under drain channel Sedimented Water

Filtered Water

Domestic water treatment

Coagulation :

•

Fine particles of clay & silica doesn’t settle down by sedimentation.

•

A chemical called coagulant is added then it entraps the fine particles

of clay & silica making bigger particles which settle down easily.

•

Most commonly used coagulants in water treatment plants : Alum

[K

(SO

)

. Al

(SO

)

. 24H

O] , sodium aluminate[NaAlO

] and ferrous

sulphate[Fe SO

7 H

O].

Disinfection :

•

process of destroying pathogenic bacteria and microorganisms is

known as sterilization or disinfection.

Methods of disinfection :

1.

Disinfection by Chlorination

•

Break point chlorination is defined as the chlorination to such an extent

that micro organisms as well as bad taste and odour present in water

are also destroyed.

•

Chlorine dosage added to water should be slightly more than the break

point, for long term germicidal effect.

•

Chlorine is added to water in following forms: Chlorine gas/liquid,

bleaching powder/ hypochlorites, tablets, chloramines, ClO

gas.

•

Over chlorination(>0.2ppm) produces bad odour & taste in water.

Domestic water treatment

2. Disinfection by ozone

•

Ozone can be produced by passing high voltage electric current

through a stream of air in a closed chamber.

3O



2O

(unstable) it decomposes to nascent oxygen.

O



O

+ [O]

•

Nascent oxygen is a powerful oxidizing agent which removes the

organic matter & kills the bacteria.

•

Ozone removes colour, odour & taste from water.

Raw Hard Water

O 3 inlet