Prepared by
P T. Abdul Sakkur
B. Tech. Mechanical Engineering
MBA
Utility
Qatar Steel Company Ltd
2
Useful Guideline on Water
Preface
My career with the process cooling system in QASCO inspired me to begin
with a concentrated study on water. The study notes when put altogether in an Ms.
Word file ended with a useful guide to both the beginners and the professionals.
Here are discussed many factors related to water, such as water physics,
water chemistry, water treatment, R O System, water supply engineering, System
Designing and analytical method to maintain the Quality. Some relevant data are put
in tables. So a collection of knowledge on a single topic is available at a place. A
number of References of famous authors on Mechanical and Chemical Engineering
are utilized to prepare this Guide. The authors’ efforts are gratefully acknowledged.
You may find some unfamiliar notations in some of the equations, like those in
the blow down quantity of water. In fact, such equations are derived by myself as I
could not find any materials on this regard when I was essential such relations.
I am grateful to Mr. C. Imthiyas, M. Sc. Chemistry, PhD (Undergoing) on
advising and clarifying the matters related to water chemistry. QASCO is blessed
with his presence as a chemist in its QA/QC laboratory. As he has exposure on
industrial, teaching and research, his advice, no doubt is worth.
Further suggestions will be greatly appreciated.
With Regards and Thanks
P T Abdul Sakkur
B Tech. Mechanical Engineering
MBA
QASCO, Utility
Date: 23.11.2006
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Useful Guideline on Water
WATER
Planetary significance
Water constitutes one third part of the earth. In terms of number and variety of
organisms, water is the first to mention. Water is one of the most important
substances in human life. It makes up over 65% of the human body. It is said that
the human body is the replica of the earth where blood and the body fluids take the
part of water. It is no surprising if one accepts the premise that life emerged from
water. All forms of life, even in the desert, require a reliable intake of water and most
of the nutritional and excretory functions are water based. Some organism such as
jellyfish is about 98% water, while trees may be only about 50% water. It is the most
abundant and universal solvent.
Water as oxygen provider
For plants, water is required along with carbon dioxide in photosynthesis.
Oxygen is liberated during this process. The process can be approximately
represented by the equation:
CO2 + 2H2O*
h
1/n (CH2O) n + H2O + O2*
The Oxygen molecule (O2) in the product side comes from water, not from
carbon dioxide. It is experimentally proved through radio active labeling (*) studies.
Another way for the natural production of oxygen is photodecomposition of
water in the atmosphere under the influence of ultraviolet radiation from the sun,
according to the formula;
2H2O
h
2H2 + 2O
2H2 + O2
Water as steadying media of climate:
Water has capacity to absorb large quantities of heat and the reverse, with
small change in temperature owing to the high heat capacity and high latent heat of
fusion and vaporization. Water has the highest heat capacity of all liquids and solids,
except NH3.
Water as both acid and base
Water can function as both acids and bases. Such compounds are called
Amphoteric or ampheprotic compounds. See the reactions:
HCl (aq) + H2O (l)  H3O+(aq) + Cl-(aq)
acid
base
acid
base
NH3 (aq) + H2O(l)  NH4+(aq) + OH-(aq)
base
acid
acid
base
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Useful Guideline on Water
Water as Energy Source
The energy of flowing water can be converted to electric energy by means of
Turbines, which is commonly termed as ‘hydro’ power. The imparted energy to the
impeller rotates the shaft connected with it kept in between two opposite poles of
magnetic field causes to generate an electric flux. It can be taken out as useful
energy.
Moving water
Hydro Turbine
Work Output
Shaft
Electric energy
Generator
WATER PHYSICS
Water is colorless, it freely occurs in all the three phases of matter. The
melting point of water is 273K and boiling point is 373K. (1oC =273K) It means that it
has a convenient liquid range at these two temperature ranges. Water contains two
hydrogen atoms and an oxygen atom (H2O). Due to higher electro negativity, oxygen
atom attracts the bonding electrons towards itself. So it acquires a partial negative
charge and, the hydrogen atom a partial positive charge. On account of this, water
molecule becomes polar.
The positively charged hydrogen atom attracts the negatively charged oxygen
atom of a second molecule by electrostatic force of attraction. The attractive force
which binds hydrogen of one molecule with a highly electronegative atom of another
or different part of the same substance is known as hydrogen bonding. A number
of water molecules associate together to form a larger cluster of molecules (H2O) n.
As it is clear in the picture, in water the hydrogen atom is attached to two oxygen
atoms, one through covalent bond and the other through the hydrogen bond.
A number of water properties are explained on the basis of hydrogen bonding.
Example is the lighter density of ice. In ice, each oxygen atom is surrounded by four
other oxygen atoms located at the corners of a regular tetrahedron. There is one
hydrogen atom between any two oxygen atoms; vacant spaces are created and as
a result the volume of ice gets increased than water, and so the density gets
decreased.
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Useful Guideline on Water
When ice melts, the hydrogen bonds start to break and the tetrahedral
structure starts to destroy. The molecules are packed closely together to cause a
greater density at its melting point (273K). As the temperature rises above 273K, a
gradual increase in density occurs since the increase in kinetic energy of the
molecules breaks down more hydrogen bonds. Upto 277K, this effect is more
significant, and so the maximum density of water is at 4 0C. Further, the increasing
vibration of molecules becomes significant and overcomes the compacting effect due
to breaking of hydrogen bonds.
Physical Property of water
specific heat capacity of water (20oC)
surface tension (20oC)
Heat of fusion (0oC)
Heat of vaporization
Heat conductivity (at 20oC)
4.18JK-1g-1
72.8 x 10-3 Jm-2
0.333kJg-1
2.257kJg-1
59.8Js-1m-2
Pascal’s Law
When a certain pressure is applied at any point on a fluid in a closed vessel at
rest, the pressure is equally transmitted in all the directions and to every other point
in the fluid.
Principle of flotation
The weight of a body floating in a fluid is equal to the buoyant force which in
turn is equal to the weight of the fluid displaced by the body.
Capillarity of water
Weight of the water column in the tube above the water surface
acting downwards
= hd2
4
Vertical component of the force of surface tension
= dcos
Equating these two equations
dcos = hd2
4
The capillarity rise, h = 4cos
d
specific weight a = angle of contact of water surface
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Useful Guideline on Water
Bernoulli’s Theorm
If a fluid flows at a constant rate through a pipe, it pressure will be constant unless
the diameter changes. When the diameter reduces, fluid velocity increases, the fluid
pressure reduces. The height to which the fluid rises in each vertical pipe is
proportional to the fluid pressure in the main pipe. The figure illustrates the theorm
neglecting the frictional loss.
CHEMISTRY OF WATER
The basic equation of formation of water, as all know, is given below.
H2 + O2
H2O
On studying the stoichiometry of the reaction, since an atom of oxygen
disappears, a balanced equation is formed.
2H2 + O2
2H2O
It means that, for every mole of molecular water to form one mole of hydrogen
undergoes reaction with half a mole of oxygen. (H2 + ½O2
H2O)
The atomic number of central oxygen is 8 and its electronic configuration is
1s2, 2s2, 2px2, 2py1, 2pz1. The oxygen in water is sp3 hybridized. Out of the four
hybridized orbitals, two form bonds with 1s orbitals of hydrogen atoms. The
electron present in the other two hybrid orbitals are not involved in the bond
formation. They constitute two lone pairs.
The structure of H2O molecule is a bent triatomic, with the H-O-H angle of
and the two interatomic O-H distances the same, 0.096 nm. The bond energy
for these covalent O-H bonds is 463kJ
104.5o,
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Useful Guideline on Water
CHARACTERISTICS OF WATER COMPOSITION
Common Impurities in Water are as follows
1) Suspended impurities
2) Dissolved impurities
3) Colloidal impurities
According to O.A. Alekin, the chemical composition of natural waters, which is
understood as a complex of mineral and organic substances present in various
forms of ionic, molecular or colloidal state, may be thought to consist of five principal
groups:
1. Main ions which are present in appreciable concentrations (Na+, K+, Ca2+,
Mg2+, SO42-, CO32-, Cl-, HCO3)
2. Dissolved gases (N2, O2, CO2, H2S)
3. Biogenic elements (compounds of phosphorus, nitrogen and silicon)
4. Microelements (compounds of all other chemical elements)
5. Organic substances.
Suspended matter in water may contain particles of different size; from colloidal
to coarse disperse as may be seen in the following table.
Suspended matter in water
Suspended matter
Colloidal particles
Grain size, mm
2x10-4 -- 1x10-6
Fall velocity mm/s
7x10-6
Time of settling to
depth of 1m
4years
Fine clay
1x10-3 -- 5x10-4
7x10-4 -- 17x10-5
0.5 – 2 months
Clay
27x10-4
5x10-3
2 days
Fine sludge
1x10-2 -- 5x10-3
7x10-2 -- 17x10-3
4-18 h
Sludge
5x10-2 -- 27x10-3
1.7—0.5
10-30 min
Fine sand
0.1
7
2.5 min
Medium sand
0.5
50
20 s
Coarse sand
1.0
100
10 s
L.A.kulsky has proposed to classify water impurities by their phase state and
dispersity. Accordingly all water impurities can be divided into four groups. The
property and the quality of water is determined by the concentrations of particular
impurities the quality of water can be determined in terms of physical indices
(temperature, suspended matter, coloration, odour, flavor), chemical (hardness,
alkalinity, oxidation susceptibility, dry residue), biological (hydrobionts), and
bacteriological (total concentration of bacteria, Coli index)
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Useful Guideline on Water
pH value
pH is a term used to express the intensity of the acid or alkaline condition of a
solution. Even if no acid or base is added, there is some hydronium (H3O+) and
hydroxyl (OH-) present in pure water. Some of the water molecules react with each
other, one molecule of water acting as an acid, another as a base, to produce the
ions.
H2O(l) + H2O(l)  H3O+(aq) + (OH-)(aq)
ACID
BASE
ACID
BASE
The equilibrium constant for this reaction is,
With our usual convension,

K = [H3O+][OH-]
[H2O] 2
[H2O] = 1
K[H2O]2 = KW = [H3O+][OH-]
The ionic product of water at 298K is given by, kW = 1.0x10-14mol2/l2
For pure water,
[H3O+]
= [OH-]
= 1x10-7 mol/l
pH value of a liquid is the negative logarithm of the concentration of hydronium
(H3O+) ions.
pH value = -log [H3O+]
]
From the above definition, it is clear that the pH value of the neutral water = 7
There fore, if the pH value > 7, water has alkaline quality. Since acidic water has
more tendency to react with metals, it is recommended to keep the pH value of
circulating water more than 7.
pH meter
pH value can be directly measured using pH meter. pH meter comprised with
a glass electrode with a saturated potassium chloride-calomel reference electrode. A
glass electrode in contact with H+ in solution measures the pH potentiometrically
against calomel electrode as reference. The potential difference between glass
electrode and calomel electrode is expressed as pH.
Total Dissolved Solids (TDS)
It denotes the various types of minerals present in water in the dissolved form.
In natural waters, dissolved solids are composed of mainly carbonates,
bicarbonates, chlorides, sulphate, phosphate, silica, calcium, magnesium, sodium,
and potassium.
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Useful Guideline on Water
Hardness
Hardness is caused by the presence of the salts of calcium and magnesium.
Mg2+). Hard water possesses problems when used in industrial cooling
purposes because it causes chocking and clogging troubles of plumbing due to
precipitation of salts causing hardness, and formation of scales on the hot water
systems which affects the heat transfer efficiency.
(Ca2+,
Classification of hardness
Classification
1. Soft
2. Moderately hard
3. Hard
4. Very Hard
Total hardness as mg/l of CaCO3
50
50-150
150-300
300
Hardness is usually expressed in terms of the dissolved calcium and
magnesium salts calculated as calcium carbonate equivalent. It can be divided into
two classes temporary and permanent. The temporary hardness is caused by the
presence of dissolved bicarbonates. It can be reduced by heating or by the addition
of a calculated amount of lime whereupon Mg/Ca carbonate is precipitated.
Reaction on heating
Heating
Ca (HCO3)2 →
Mg (HCO3)2 →
Bicarbonate
hardness
CaCO3 + CO2 + H2O
MgCO3 + CO2 + H2O
Ca/Mg carbonate get
precipitated
Reaction on addition of lime
Ca (HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
Mg (HCO3)2 + Ca(OH)2 → MgCO3 + CaCO3 + 2H2O
Bicarbonate
hardness
Ca/Mg carbonate get
precipitated
The Ca/Mg Carbonate so formed can be removed in the sedimentation tanks
since these are insoluble in water.
Hardness Compound
Causing temporary hardness
1.Calcium bicarbonate Ca(HCO3)2
2. magnesium bicarbonate Mg(HCO3)2
Causing Permanent hardness
1. Calcium Sulfate (CaSO4)
2. Magnesium Sulfate(MgSO4)
3. Calcium Chloride (CaCl2)
4. Magnesium Chloride( MgCl2)
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Useful Guideline on Water
Removal of Permanent hardness
The permanent hardness is due to the presence of sulfates and chlorides of
calcium and magnesium. It can be removed by one of the following methods.
1. Lime-soda process
2. Zeolite process
3. Demineralization or de-ionization process.
1. Lime-soda process
This process is commonly known as Clark’s’ system. The addition of lime
[Ca(OH) 2] and soda ash (Na2CO3) into the raw water is known as lime soda
process. Both the temporary and permanent hardness can be removed by this
method. The process involves the thorough mixing of the chemicals with water,
followed by slow agitation for 30 to 60 min. to allow completion of chemical reaction.
Precipitated cahemicals are removed by sedimentation or filtration or both. In the
following process  indicates the precipitation.
CO2 + Ca(OH)2
 CaCO3 + H2O
Ca(HCO3)2
+ Ca(OH)2  2CaCO3 + 2H2O
Mg(HCO3)2
+ Ca(OH)2  CaCO3 + MgCO3 2H2O
MgCO3+ Ca(OH)2  Mg(OH)2 + CaCO3
MgSO4 + Ca(OH)2  Mg(OH)2 + CaSO4
CaSO4 + Na2CO3  CaCO3 + Na2SO4
CaCl2 + Na2CO3  CaCO3 + 2NaCl
MgCl2 + Na2CO3  Mg(OH)2 + CaCl2
Summary of Chemical used
Type of hardness
Chemical required
CaCO3 alkalinity (as bicarbonate)
Lime
Ca(HCO3)2
Lime
Ma(HCO3)2
Lime (2 units)
MgSO4
(i) Lime, (ii) Soda ash
CaSO4
Soda ash
CaCl2
Soda ash
MgCl2
(i) Lime (ii) Soda ash
This method is essentially a precipitation method of water softening. A lot of
arrangements are required to get out of the product water. So, in industrial cooling
purposes it is not used.
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Useful Guideline on Water
2. Zeolite process – water softening
Removal of dissolved salts of calcium and magnesium is called water softening.
Water softeners containing zeolite are used for the industrial purposes. The zeolites
exchange the positively charged sodium cations (Na+) for the cations of calcium
(Ca+) and magnesium (Mg+). So softening is essentially a cation-exchange process
and the process is also known as Base Exchange or ion-exchange process. It is
most commonly used for the softening of water for industrial purposes.
The zeolites are complex compound of aluminium, silica, and soda. Some
forms of which are synthetic and others are naturally occurring. Natural zeolites are
mainly processed from Green Sand. (Glauconite). It is green in colour and has an
exchange value of 6500 to 9000gm of hardness per m 3.
The common artificial zeolite is permutit. It has larger grains and has high
exchange value of 35000 to 40000gm of hardness per m 3. Its chemical formula is
SiO2Al2O3Na2O. Since it absorbs moisture from atmosphere it should be stored in a
dry place.
When hard water passes through a bed of permutit, the following reactions take
place:
2SiO2Al2O3Na2O + Ca(HCO3)2  2SiO2Al2O3CaO + 2NaHCO3
2SiO2Al2O3Na2O + CaSO4
→ 2SiO2Al2O3CaO + Na2SO4
2SiO2Al2O3Na2O + CaCl2 → 2SiO2Al2O3CaO + 2 NaCl
Considering the permutit resin as an activated ion, ‘A’, the above process can be
rewritten as follows:
CaCl2
MgCl2
+
+
Non-Carbonate
hardness
→
→
+
+
Na2 . A.
Na2 . A.
Ca . A
Mg . A
+
+
2 NaCl
2 NaCl
Sodium hydrogen
carbonate (remains
in soft water)
Exhausted ion
exchange resin
Active ion
exchange resin
Carbonate
hardness
Ca SO4
Mg SO4
Na2. A
Na2 . A
→
→
Active ion
exchange resin
Ca . A
Mg . A
+
+
Na2SO4
Na2SO4
Sodium sulphite
carbonate (remains
in soft water)
Exhausted ion
exchange resin
Where, ‘A’ stands in place of SiO2Al2O3Na2O
Regeneration of softeners
Due to continuous use of zeolite, the sodium present in it is exhausted. At this
stage, zeolite is regenerated by passing brine solution (Aqueous NaCl) through it.
The sodium in the brine replaces the calcium and magnesium in the exhausted
zeolite. The following reactions take place during regeneration:
2SiO2Al2O3CaO + 2NaCl → 2SiO2Al2O3Na2O + CaCl2
2SiO2Al2O3MgO + 2NaCl → 2SiO2Al2O3Na2O + MgCl2
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Useful Guideline on Water
Steps in regeneration
1. Backwashing: It loosens the resin bed and removes the debris and suspended
solids
2. Brine injection: This is the actual regeneration stage where the ion replacement
as takes place.
3. Slow/rapid rinse: It rinses away the residual brine and calcium/magnesium salts
removed from the resin.
Notes:
1. The process is unsuitable for acidic water which irreversibly substitutes hydrogen
for sodium in the zeolite. The acidic water may aggressively attack the zeolite by the
dissolving alumina or silica from it.
2. The process is not suitable for water containing iron and manganese. Iron or
manganese bearing water either deposit hydroxides on the surface of zeolite or
irreversibly react with the zeolite.
3. There is likelihood of growth of bacteria on the bed of zeolite. It should therefore
be flushed annually with chlorinated water.
3. Demineralization
or de-ionization process.
In this process both anionic and cationic exchanges are taking place. A typical
ion exchange plant comprises two groups of ion exchange softeners one of them
operates by the cycle of H-cations from water and the other removes the anions.
The regeneration is made by H2SO4 (concentration 2-4%) of HCl (conc. 3-5%)
on the cationic exchanger and NaOH (conc. 2-4%) on the anionic exchanger)
Cations 
Anions ()
Na+, K+, Ca2+, Fe3+
SO42-, Cl-,
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Useful Guideline on Water
1. Cationic Exchanger:
The cationic resin takes 4 calcium ions (Ca++) and gives out 4 hydrogen ions
(H+). Pictorially it can be shown as given below:
Before
Normal usage
After
Cationic Resin
Cationic Resin
H+
ClClClCl-
Ca++
Ca++
Ca++
Ca++
H+
semi demi water
Ca++
H+
Ca++
H+
H+
H+
H+
H+
Ca++
Ca++
ClClClCl-
Regeneration of Cationic exchanger is shown below:
HCl  H+ + ClH2SO4  2H+ + SO42-
 cationic resin
 cationic resin
H+
Ca++
Ca++
Ca++
+ HCl 
H+
H+
+ Cacl2
H+
Ca++
2. Anionic Exchanger:
The cationic resin takes 4 chlorine ions (Cl-) and gives out 4 hydroxyl ions (OH-).
Pictorially it can be shown as given below:
Before
Normal usage
H+
H+
H+
H+
ClClClCl-
After
anionic resin
anionic Resin
OH-
ClOH-
OHOH-
Cl-
ClCl-
semi demi water
H+
H+
H+
H+
OHOHOHOH-
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Useful Guideline on Water
Regeneration of anionic exchanger is shown below:
NaOH  Na+ +OH-
( - ) anionic resin
OH-
ClCl-
Cl-
+ NaOH 
OH-
OH-
+ NaCl
OH-
ClElectrical Conductivity (E C)
It is a term in electrochemistry. When a voltage is applied to electrodes
immersed into an electrolyte solution, ions of electrolyte move and the electric
current thus flows through the electrolytic solution obeying the Ohm’s law. The
reciprocal of resistance is called conductance and its unit is siemens (ie.1S = -1).
Conductivity is the reciprocal of resistivity (). The usual symbol of conductivity is 
(kappa) and its unit is -1cm-1 or Scm-1. Very pure water has a conductivity of 6.0 x
10-5 Scm-1.
The conductivity of water increases with temperature. The temperature
dependence of conductivity reflects the speed of the ions as well as degree of
dissociation and an abnormal transport mechanism. It can be measured directly by
conductivity meter.
An empirical relation exists between Specific Conductance and TDS. The
conversion factor is given in the table:
Multiplication Factors between TDS and Electric Conductance
1
2
3
4
TDS
0 - 700
Conductance
0.55
700 - 1200
1200 – 2000
Above 2000
0.6
0.65
0.70
Alkalinity
Alkalinity is determined as the sum of the hydroxyl ions and anions of weak
acids. It results from the presence of the hydroxides [OH-], carbonates [CO32+] and
bicarbonates [HCO3-] of elements such as calcium, magnesium, sodium, potassium
and ammonia. Of these, calcium and magnesium bicarbonates are most common.
Borates, silicates, phosphates, and similar compounds can also contribute to the
alkalinity. It is distinguished between bicarbonates, carbonate and hydrate alkalinity.
Their values can be found by reference to nomograms. Their sum makes what is
called the total Alkalinity.
Alkalinity is the capacity of a system to neutralize acid. The alkalinity in water
helps to resist changes in pH. The components of Alkalinity that can form during
water treatment are given below
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Useful Guideline on Water
Method of water treatment
Components in total alkalinity
Lime treatment
Lime treatment followed by Na-cation
exchange softening
Na-cation exchange softening
CaCO3, Ca(OH)2, Ca(HCO3)2
Na2CO3, CaCO3, NaOH, Ca(OH)2
NaHCO3
The general method for measuring alkalinity is the potentiometric titration
technique. This method involves continuously adding volumes of acid with a certain
concentration to a water sample until the pH of the water reaches a specified
endpoint. "Potentiometric" refers to the use of a pH meter to identify when the
desired pH has been reached. The amount of acid added is converted to equivalent
mg CaCO3/L and reported along with the titrated pH endpoint.
CORROSION
Corrosion is the deterioration of a material as a result of an unintentional
chemical or electrochemical reaction that occurs between the environment and the
surface of the material in question. The basic cause of corrosion of metals is their
inherent instability in their refined forms. Thus corrosion can be regarded as the
tendency of metals to go back to their more stable chemical forms such as oxides
and sulphides in which they occur in nature. In water pipelines, the direct corrosion is
taking place. It is a chemical attack as a result of chemical reaction between the pipe
inner surface and the water of acidic nature.
The chemical attack will be uniform and continuous at an almost constant rate
that can be measured in standard units, e.g., milligrams per square decimeter per
day (mdd) and mills (1/1000”) per year (mpy). One means of controlling the direct
corrosion is the addition of certain inhibiting chemicals into corrosive solution. They
act as barriers between the metal involved and the environment.
Mechanism of Corrosion
A metal in contact with an aqueous solution functions as an electrode. The
potential of this electrode is a measure of the tendency of the metal to act as an
anode and become oxidized when coupled with some other material or with localized
portions of the same metal which can act as cathodes. The anodic or corrosive half
reaction can be represented as
Ml → Ml2++ 2e
Where, Ml is metal corroded. The e.m.f of the galvanic corrosion cell formed is given
by;
Ecell = cathode-anode
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Useful Guideline on Water
This gives a driving force behind the process under equilibrium conditions.
The values of  in the above expression for both anode and cathode are considered
as reduction potentials which explain the negative sign for anode.
Cooling water systems provide an ideal environment for corrosion of the
material of supply pipe and equipment, by which the metal returns to its natural state.
Water acts as a galvanic corrosion cell unit in which one part of the system acts as
an anode and becomes corroded, while another part of the system functions as a
cathode. The rate of corrosion is proportional to the current density and this in turn is
proportional to the cell potential. Metal ions dissolve into water (electrolyte) at anode
and electrically charged particles are left behind. These electrons flow through the
metal to other points (cathode) where electron consuming reactions occur. As a
result of this, metal losses and often deposit forms.
The half reactions of the unit are partially shown below:
Anodic reaction:
Feo
Fe2+ + 2e-
Cathodic reaction
2e- + ½O2 + H2O
2(OH-)
Method to avoid corrosion
1. Cathodic Protection, 2. Proper selection of Pipe material, 3. Protective linings and
coatings, 4. Treatment of water
The treatment of water includes (i) pH adjustment (ii) Control of Calcium
carbonate (iii) Removal of dissolved oxygen (iv) Removal of CO2 (v) Addition of
corrosion inhibitors like Sodium Silicate, sodium molybdate and sodium phosphate.
A calculated amount of Chemical inhibitors are dosed in the supply water in
order to reduce or stop corrosion by interfering with the corrosion mechanism.
Usually it affects either anode or cathode such that it creates a protective film in
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Useful Guideline on Water
between the metal and water. Caution should be taken when the inhibitors are being
dosed. For example, if inhibitors dosed less than required, the entire corrosion
potential occurs at the unprotected sites. This causes severe localized (pitting)
attack. Pitting is the most serious form of corrosion because the action is
concentrated in a small area; it may perforate the metal in a short time. At the same
time, if inhibitors dosed more than the required, film produced will be more and it
may cause to reduce the heat exchange, or the cooling efficiency will be affected.
OSMOSIS
Osmosis occurs when pure water flows from a dilute saline solution through a
membrane into a high concentrated saline solution. The phenomenon is illustrated in
the figure. A semi permeable membrane is placed between two compartments.
Semi-Permeable means that the membrane is permeable to some species and not
permeable to others.
Assume that the membrane is permeable to water
and not to salt. Place a salt solution in one of the
compartments and pure water in other. The membrane
will allow water to permeate through to either side, but
salt cannot pass through.
As a fundamental rule of the nature, the system
will try to reach equilibrium, i.e., it will try to reach the
same concentration on both sides of the membrane.
More specifically, the vapour pressures of liquids at both
the sides will play on the liquids to reach the
concentrations the same. The only possible way to reach the same concentration for
water is to pass pure water compartment to salt containing compartment, to dilute
the salt solution, consequently the level of the salt solution increases as shown in
figure. The level will increase until the pressure of the
column of water (salt solution) is so high that the weight of
this water column stops the water flow. The equilibrium point
of this water column height in terms of water pressure
against the membrane is called Osmotic Pressure.
Reverse Osmosis: As the term indicates, it is the reverse of
osmosis process, externally made by applying pressure.
When the applied pressure exceeds the osmotic pressure,
the flow water occurs in reverse direction as shown in the
figure. The process can produce pure water from salt water
since the membrane is permeable to salt.
Seawater
Brackish water
Osmotic Pressure
23.8 ~27.7 bar (350 ~400psi)
0.7 ~ 3.4bar (10 ~ 50psi)
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Useful Guideline on Water
Reverse Osmosis Plant
Shown is an R.O Plant Schematically. Here the feed water is taken from a beach
well. The Water is pumped from the Beach well is first filtered through a dual media
sand filter and passed through a cartridge filter by means of a pump. Using a High
Pressure Pump (HPP) it is allowed to pass through the Seawater membrane
(Polyamide). The product water from the membrane is collected into an intermediate
suck tank. The remaining water is thrown out through an Energy Recovery Turbine
(ERT), the work output of the turbine is used as an input energy to the HPP as
shown in the figure.
Water from the Intermediate Suck Tank is passed through the brackish
water membrane by means of a booster pump. The product water is collected in the
surge tank, among with the overflow of the Intermediate tank, and is pumped into the
Over Head Tank, from where; it is supplied to the necessary areas. Necessary
chemical treatments are given at different points as it is shown in the figure.
Chloride Content in water
Chlorides are mineral salts. The solubility of common salt is 360g/l and that of
magnesium chloride is 546g/l. when present in substantial concentration, chloride
makes the water aggressive relative to concrete. The chlorides of calcium lead to the
formation of non-carbonate hardness of water. The traditional method of reducing
chloride ion concentration from a water system is to blow down. The exact amount of
blow down water could be calculated.
The chloride concentration can be found out by titrating the sample of water
with a standard silver nitrate solution, using potassium chromate as indicator. Such
method is called Mohr’s method. Normally a sheet of calculated value is kept in
laboratories.
The chloride ions react with standard silver nitrate solution to form the white
silver chloride precipitate:
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Useful Guideline on Water
ACl + AgNO3  AgCl + ANO3 (where A can be any cations)
As all the Cl- ions react with AgNO3, the silver ions will react with Chromate
ions present in the indicator to give an orange red coluor. The red colour is an
indication of absence of chloride ions other than that of silver, and it is taken as the
endpoint. The reaction of AgNO3 is given below.
2AgNO3 + K2CrO4 Ag2CrO4+ 2KNO3
Seawater and brine water
The chloride concentration of seawater is of the order of 200, 000mg/l
whereas brine may contain as much as 10 times that amount.
Chlorination of water
According to ‘Enzymatic Hypothesis’, chlorine compounds formed when
chlorine is added to water, interfere with certain enzymes in the bacterial cells which
are vital for the support of life. Chlorine water is an effective bleaching and sterilizing
agent. That ability is used in the treatment of water supplies to reduce bacterial
contamination.
When chlorine in the form of Cl2 gas is added to water, two reactions take
place; hydrolysis and ionization. Hydrolysis may be defined as the reaction in which
chlorine gas combines with water to form hypochlorous acid (HOCl)
Cl2 + H2O  HOCl + H+ + Cl- (hydrolysis)
The equilibrium constatnt KH for this reaction is
KH = [HOCl] [H+][Cl-] = 4.5 x 10-4 (mole/L)2 at 25oC
[Cl2]
Because of the magnitude of the equilibrium constant, large quantities of
chlorine can be dissolved in water. (2.2 liters in 1 liter of water at 20 0 C)
Ionization of hypochlorous acid to hypochlorite ion (OCl-) may be defined as
HOCl  H+ + OCl- (ionization)
The ionization constant Ki for this reaction is
Ki = [H+][OCl-] = 3 x 10-8 mole/L at 25oC
[HOCl]
The total quantity of HOCl and OCl- present in water is called ‘free available
chlorine’. It is hypochlorous acid and hypchlorite ions which accomplish disinfection.
The killing efficiency of HOCl is 40 to 80 times that of OCl-.
When chlorine is added to water, all the three (HCl, HOCl, OCl-) are formed
and they remain the equilibrium at different concentrations depending upon the pH of
water which controls the amount of dissociation.
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Useful Guideline on Water
Hypochlorite reaction with water
The hypochlorite salts ionize in water and yield hypochlorite ions, which
establish equilibrium with hydrogen ions:
Ca(OCl)2 + 2H2O  2HOCl + Ca(OH)2
NaOCl + H2O   HOCl + NaOH
The process of chlorination with hypochlorites is known as hypochlorinaiton.
Factors affecting bactericidal efficiency of chlorine
Turbidity, presence of metallic compounds, ammonia compounds, pH,
temperature, time of contact, number of concentration of bacteria are those factors
affecting the bactericidal efficiency of chlorine.
1. Turbidity: it makes difficulty to obtain free residual chlorine, the penetration of
chlorine so the destruction of bacteria in particles of suspended matter of turbid
water is uncertain.
2. Presence of metallic particles: metallic particles utilize chlorine to convert into
their higher stages of oxidation which are insoluble in water. Since the shop return
water of a steel industry will be both turbid and full of iron particles, chlorination is
preferred after doing all kind of physical treatment including filtration.
3. Ammonia compounds: it may form combined available chlorine, which is not so
effective a bactericide as free available chlorine. If similar doses of free and
combined chlorines are used, then the combined chlorine will take 100 times as long
as the free chlorine to achieve the same degree of kill.
4. pH of water: Increasing the pH reduces the effectiveness of chlorine. The
effective sterlising compound, hypochlorous acid, is formed in greater quatities at low
pH than at high pH values as is clear from table below:
pH Value
Amount of hypochlorous acid
Upto 6.7
95% of total free chlorine
7.0
80% of total free chlorine
8.0
30% of total free chlorine
9.0
5% of total free chlorine
5. Temperature of water: As temperature increases the killing power of chlorine
increases. The relationship between time and temperature, to effect a given
percentage of kill can be expressed as:
Log10 t1 = . E (T2-T1) .
t2
2.303RT1T2
t1, t2 = time for given % of kill at temperature T1, T2 (K) respectively
E = activation energy, J/mol K,
R = gas constant, = 8.314 J/mol K
= 1.99 Cal/mol K
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Useful Guideline on Water
Electrolysis of salt solution
In the solution of NaCl there will be Na+, Cl-, H3O+, OH- ions present. When
circuit is closed, chlorine appears as a gas at the anode and hydrogen at the
cathode, whereas NaOH is formed in the solution. This is due to the secondary
reaction taking place at the cathodes.
Following reactions take place at the electrodes:
At anode,
2Cl-(aq)  Cl2(g) + 2e- (oxidation)
At cathode, 2H2O + 2e-  2OH-(aq) + H2 (g) (reduction)
Net reaction 2Cl-(aq) + 2H2O  2OH-(aq) + H2(g) + Cl2(g)
The OH- ions migrate from the cathode area and react with Na+ to produce NaOH.
Na+ + OH-  NaOH
The liberated chlorine at anode is dissolved in water and reacts with NaOH to
form NaOCl which acts as the bleaching agent
ie., 2NaOH + Cl2  NaOCl + H2O + NaCl
ELECTRO-CHLORINATOR
Electro-chlorinator is an equipment to produce nascent chlorine in the form of NaOCl
from seawater. It works on the same theory as explained above. Since seawater
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Useful Guideline on Water
contains the ions of Mg and Ca, there will be formation of their hydroxides and
carbonates, thus the efficiency gets reduced. So after a long operation the
chlorinator should be cleaned by diluted HCl (3 – 5% concentration). Hydrogen is
highly flammable with an ignition temperature of 530 oC. A spark will ignite the
explosion if the air/hydrogen is in explosive range. So, care must be taken,
especially during the maintenance activities like arc welding inside the plant.
Reaction of Chlorine with Hydrogen
Hydrogen gas readily reacts with chlorine gas very aggressively to give
Hydrogen Chloride.
Cl2 + H2  2HCl
But, it is purely a photochemical reaction. In the dark, no reaction occurs. So a
degassing tank should be provided with proper design in order to keep it away from
photons (sunlight).
Calculations of Electrolysis
The amount of chemical change which occurs at any electrode or the amount
of substance liberated or deposited on the electrode is directly proportional to the
quantity of the electricity passed.
Suppose, W grams of a substance is deposited at an electrode and Q
coulombs of electricity are passed,
Then, W  Q.
If a current of I ampere is passed for t seconds, then the quantity of electricity
is give current strength x time, i.e., ct coulombs.
W  It = ZIt.
Where, Z = electrochemical equivalent.
The quantity of electricity which can liberate or deposit 1g equivalent of any
substance is called 1 Faraday, F and its value is 96500 coulombs/eqv.
Sedimentation
Plain Sedimentation and coagulation:
When the impurities are separated by the action of gravity force only, it is called the
plain sedimentation
When it is with the addition of chemicals or other substances, it called sedimentation
with coagulation or simply clarification.
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Useful Guideline on Water
Types of Sedimentation
1. Discrete Settling (1) (free settling): Corresponds to the sedimentation of
discrete particles in a suspension of low solids concentration.
2. Type (2) Hindered settling: refers to rather dilute suspension of particles that
coalesce or flocculate during sedimentation process.
3. Types (3) Zone Settling: Refers to flocculent suspension of intermediate
concentration.
4. Types (4) Compression settling: Refers to flocculent suspension of high
concentration that particles actually come in contact with each other resulting in the
formation of a structure.
Settling of discrete particles
Settlement Velocity (Vs)
The settling velocity of particles in water can be calculated by the formula:
Vs =
4 g (Ss – 1) d
3 CD
Vs = Settling velocity (mm/sec)
Ss = specific gravity of particle
d = Dia. Of particle, in mm
CD = Drag Coefficient which is related to Reynolds’s number Re.
1. For Re between 0.5 to 104,
2. For Re > 103 to 104,
3. For Re < 0.5,
CD = 24/Re + 3/Re + 0.34
CD = 0.4
CD = 24/R
Sedimentation with coagulation (clarification)
The chemically assisted sedimentation is called the clarification or
sedimentation with coagulation. The flocculent should be dosed at a point of high
turbulence. Flocculation refers to the building up of the particles of floc to a larger
size which can be removed by sedimentation. When coagulant is dissolved in water
and thoroughly mixed in it, a thick gelatinous precipitate, known as floc is formed.
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Useful Guideline on Water
Common coagulants
Depending upon the impurities the coagulant differs. In steel industries the
return water will contain positively charged ferric ions, and so they attract the
negatively charged particles. Anionic flocculent is advised in such places. The
Cationic flocculent is recommended for using in oily waste water clarification
Aluminium sulphate or Alum, Chlorinated copperas, ferrous sulphate and lime,
Magnesium carbonate, Polyelectrolyte and Sodium aluminate are the common
coagulants
Blow down quantity (self derived)
Suppose that, Q = Total Quantity of system water and, P1 = Cl- amount in
ppm (mg/l) for the system water, P2 = Cl- (say)a mount in ppm for make up water,
P3 = required ppm of the system water. Take 1 liter of supply water and remove x
liters out and add same quantity of make up
Ie, P3 = (1-x) P1 + xP2
Solve for x
P3 = P1 – xP1 + xP2
(P3 – P1) = x (P2 – P1)
X = (P2 – P1)
(P3 – P1)
Total amount of water to be blow down q, to normalize the Cl- is
q = xQ
Note: P3 is available from the manual, and P1, P2 are getting on analysis.
Water Supply Engineering
Pump
Centrifugal pumps are the common
supply pumps. The pump will be primed
before starting because the pressure
generated in a centrifugal pump impeller
is directly proportional to the density of
the fluid that is in contact with it. Hence, if
an impeller is made to rotate in the
presence of air, only a negligible pressure
would be produced with the result no
liquid will be lifted up by the pump. To
produce pressure centrifugal pumps
depend on the basic law of inertia, the
Newton’s first law of motion.
Negative
pressure
Atmospheric
pressure
Water to
impeller
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Useful Guideline on Water
The increase of pressure at any point is proportional to the square of the
angular velocity and the distance of the point from the axis of rotation. As the
delivery valve is opened, the flow causes creating a partial vacuum at the eye of the
impeller, to which water is rushed up due to the pressure exerted by the atmosphere
on the liquid surface. Since the atmospheric pressure is the real cause to lift up the
water in a centrifugal pump, the suction line should not be exceeded more than the
atmospheric pressure in terms of water head. Practically it is 7.5m.
Pump Symbols
Power required for pumping
Power in kW =
HQ
3.67 x 105
is the mass density kg/m3, H = Head of pump in terms of
metes of water, Q = flow in m3/sec
Power in HP =
HQ
3960
specific weight of water
Performance Curve of a Centrifugal Pump
Vibration Check Points of the pump
1. Left bearing
3. Right Bearing (inboard bearing)
5. Motor bearing
7. Motor base bolt
2. Pump case
4. Base bolt (pump side)
6. Motor Middle
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Useful Guideline on Water
Method to check vibration
Keep the vibrometer tip on the points mention as
follows (see the figure)
1. vertical
2. horizontal
3. longitudinal
Practical Significance of vibration
Comparison of vibrations of same pump during foreign materials inside
impeller and after removal
Vibr. While material inside
Vibr. After Removal materials
Points
1
2
3
4
5
6
7
1
2
3
4
5
6
7
vertical
25 15 20 16 6
6
8
2
5
10 3
7
3
4
horizontal
14 16 21 10 6
7
7
7
9
17 2
6
5
3
longitudinal 70 35 20 16 9
8
6
12 8
5
2
6
4
2
Delivery pressure of Pump
2.0kg/cm2
Delivery pressure 2.8Kg/cm2
Motor Current of pump
12.5A
Motor Current
13.5A
Experiment date: 10.11.2006, QASCO, Water Treatment plant for CC # 3
Minimum starting speed:
[(N/60)2 (D12 –D2)] = (2gHm)
Where: N = rpm, D1= outer diameter of the impeller, D = inner diameter, Hm =
the manometric Head.
Variable Speed
Q2 = Q1 (N2/N1), or H2 = H1 (N2/N1)2
Specific Speed (Ns)
It is the speed in rpm of a geometrically similar pump of such a size that under
corresponding conditions it would deliver 1 litre of liquid per secd against a head of 1
metre.
Ns = NQ
Hm3/4
NPSH: Net Positive Suction Head
NPSH is defined as the absolute pressure head at the inlet to the pump minus
vapour pressure head (in the absolute units) corresponding to the temperature of the
liquid pumped, plus the velocity head at this point.
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Useful Guideline on Water
NPSH = (pa/w + ps/w) – pv/w + Vs2 /2g
= pa/w - pv/w – hs - hfs
pa = ambient pressure, pv = vapour pressure, ps = pressure at inlet,
vs = suction velocity, hs = static suction lift, hfs = frictional loss in suction pipe
Criterion for NPSH in installation of pumps:
NPSH available should be greater than the
required.
Or, (NPSH)a > (NPSH)req
Cavitation
When a liquid flows into a region where its pressure is reduced to vapour
pressure, it stars vaporizing and vapour pockets or bubbles are formed in the liquid.
These vapour bubbles are carried along with the flowing liquid until a region of higher
pressure is reached, where they suddenly collapse as the vapor condenses to liquid
again. When a vapour bubble collapses a cavity is formed and the surrounding liquid
rushed in to fill it. This process of formation of bubbles and their collapsing is called
cavitation.
This becomes problem in water flow in certain cases. 1. Pump suction: the
centrifugal pumps are creating negative pressure to raise the water. if the fluid
pressure inside the impeller equals the then vapour pressure of the fluid, vapour
pockets will loss the priming. 2. the heat generated by the rotation of impeller is
carried away by the supply water. so if the delivery valve is throttled more than a
certain limit (40%), its temperature increases, resulting to generate the cavities to
loss the prime. 3. When the velocity of flow is high, the pressure will be less at that
region. if the vapour pressure and fluid pressure are same at one region and differ at
another, cavitation occurs. The pressure waves propagate to the inner surfaces of
the pipes resulting in erosion, which is known as pitting. Cavitatiion reduces the
efficiency of the system.
Cavitation in pumps
Thomas cavitation factor for pumps  [(pa – pv)/w] – (hs + hfs)
Hm
It can be proved that [(pa – pv)/w] – (hs + hfs) = NPSH
(1)
NPSH
Hm
Cavitation will occur if the value of is less than the critical value c at which
the cavitation just begins. The c can be found out from the formula:
c = 0.103(Ns/1000)4/3
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Useful Guideline on Water
The relation [(pa – pv)/w] – (hs + hfs) = NPSH indicates that the value of can
be increased by reducing the suction lift. In some cases especially hot liquids are to
be handled, the pump my to be installed either at the liquid surface or even below
the liquid surface.
Vapour pressure of water at operating temperature of water
(2)
Possible Operating temperature of water
Temperature
o
C
Vap.
pressure
kg/cm2
15
20
25
30
35
40
45
50
55
1.734
2.549
3.263
4.385
5.812
7.648
9.789
12.644
16.112
Water hammer
When the water flow in a long pipe is suddenly brought to rest, a sudden rise
in pressure will occur due to the momentum of the moving water being destroyed.
This causes a wave of high pressure to be transmitted along the pipe, the
phenomenon of sudden rise in pressure is known as water hammer or water blow. It
creates a noise known as knocking and even the pipe may burst on water hammer.
So care should be taken while closing a valve on the supply line.
Water hammer pressure pi =
V
a
(g/w (1/K + D/(TE)(1-1/(2m)))
V = velocity of water, E = Young’s modulus of the material of the pipe, K = bulk
modulus of water, T = wall thickness of pipe, 1/m = Poisson’s ratio, D = diameter of
pipe.
Valves
Needle valve
owing to the pointed stem needle valve can control accurately the rate of fluid
flow. It is used both as throttle and shut off valve.
Globe valve
Since the flow area of a globe valve is larger, it has larger flow capacity at low
pressure drop than a needle valve of the same size. Globe valves are not suitable for
throttling service.
Gate valve
Mostly used as either shut off valve or open the line to full flow. It gives large
opening with minimum pressure drop.
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Useful Guideline on Water
Pipes and Network components
Pipe Network Diagram
A group of interconnected pipes
forming several loops or circuits is called
a network. According to the principle of
continuity, the flow into each junction
must be equal to the flow out of the
junction. The Darcy – Weisbach equation
must be satisfied for flow in each pipe.
According to it, the loss of head hf through
any pipe discharging at the rate of Q is
hf = rQn
Where, r = proportionality factor, which can be determined by means of the
below equation for each pipe knowing the friction factors f, the length L and the
diameter D of the pipe.
r=
fL
.=
fL . ; n is a an exponent having a numerical value
2
5
2gH(/4) D
12.D5
Ranging form 1.2 to 2.00
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Useful Guideline on Water
Water System Designing
When designing a water system, two things are considered, 1. Physical
characteristics and, 2. chemical characteristics. Physical characteristics include
Pressure, flow, temperature etc. Chemical characteristics include corrosion etc. The
battery point is the first one to be considered. As per it all the other parameters are
decided. The technology suppliers provide all the necessary utility requirements and
the project engineers need to decide as per that only.
Pump Selection
From the battery point requirements, Find out the T.D.H, (total developed
head). From the performance curves of pump manufactures select the required
pumps that satisfy the battery point requirements. Analyze the isometric drawing of
the pipe lines to study the possible pressure drop of supply water from the supply
station.
Lubricants
A number of lubricants are used in various parts of the equipment. SAE
(Society of Automotive Engineers) relates viscosity range to a number. SAE20 oil
has a viscosity of 120 to 185 SSU at 130 oF, SAE30 oil has a viscosity of 185 to 255
SSU at the same temperature.
SSU refers to Seconds Saybolt Universal. The saybolt viscometer/viscous
meter measures the number of seconds it takes for a fixed quantity of liquid (6)cm3)
to flow through a small orifice of standard length and diameter at a specified
temperature. The time taken to fill the container is considered as a measure of the
viscosity and represents with SSU.
Viscosity Index (VI)
This value shows how temperature affects the viscosity of an oil. Parrafinic
(Pennsylvania) oil vary comparatively little in viscosity. While naphathenic (Gulf
cost) vary considerably.
The change in viscosity of Pennsylvania crudes between 100 and 210F is
rated as 100 and of Gulf cost is rated as 0. Other oils are then assigned a viscosity
index in terms of the degree to which this viscosity changes over the range as
compared with these standard oils. So, the lower the V.I the greater the variations in
viscosity with changes in temperarutre. V.I figures may range above 100 or below 0
if the viscosity of the oils being measured varies less or more than the standard oils.
IMPORTANT TERMS AND DEFINITIONS
ppm
The quantity of constitutes of water is measured in ppm. The treatment
chemical agent is dosed in water at a rate of ppm. It means the parts of chemical
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Useful Guideline on Water
agent per million of water. In pure water it is equivalent to 1mg/l, or 10 -6/l. This
relation may not be correct in the case of contaminated water. Since the supply
water is free from contaminations, its quality could be approximated to that of pure
water.
The amount of constituents in pure water can simply calculated in ppm as
follows. If ‘p’ is the amount of chemical recommended to be added in ppm, , ‘q’ the
volume of water added in the water basin, then the amount of chemical ‘w’ in kg is
given by,
w in kg = p . q /1000
The same formula can be used for the purpose of calculations in the case of
cooling supply water also.
Specific gravity
The specific gravity is the ratio of specific weight of a fluid to the specific
weight of pure water at 40C. on the basis of this definition we can derive a relation for
the weight of chemical delivered as follows.
Weight of chemical delivered = Volume in liter of the chemical
Specific gravity of the chemical
Chlorine demand
It is the difference between the amount of chlorine added to water and the
quantity of free available chlorine remaining at the end of a specified contact period.
Electrochemical equivalent
It is the amount of substance (in gram) liberated or deposited on the
electrode, on passing a current of 1 ampere for 1 sec, or on passing 1 coulomb of
electricity.
Electrolytic conduction
The electrolytes are termed as ionic conductors. The acids, bases or salts in
their fused state or in aquies solution will function as electrolytes. It consists of the
movement of ions in the electrolyte. The resistance of the flow decreases with
increase in temperature.
Conductivity of solution
According to ohm’s law, the current 1A flowing through a conductor of
resistance R ohms is related to the potential difference E volts, through the equation,
I = E/R. The reciprocal of R is called the conductance C.
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Useful Guideline on Water
Specific resistance
The resistance R of any uniform conductor is directly proportional to its length,
l and is inversely proportional to its area of cross section a.
Mathematically, R  l/a = xl/a
An electrolyte specific resistance () may be defined as the resistance of a
column of a solution of unit length and unit area of cross section , i.e., it is the
resistance of a unit cube of the solution.
Specific conductance
From the above equation, 1/R =C= 1/x a/l = K a/l, where K = 1/ the
reciprocal of specific resistance is called specific conductance, K and is expressed in
m. thus K = C x l/a. the specific conductance is defined as the conductivity of the
electrolytic solution, a unit cube of which offers to the passage of electricity.
Variation of specific conductance with dilution
The specific conductance will reduce with the decreasing concentration of the
current conducting particles.
Bond energy: Average bond dissociation energy of the entire bonds is called as
bond energy
Cell constant
It is the ratio between length of the cell to the cross sectional area
Cell constant = l/a = Resistance x specific conductance.
Detention time:
It is the theoretical time taken by a particle of water to pass between entry
and exit of a settling basin.
Dielectric constant:
It is a measure of the ability of a medium to reduce the force of attraction
between oppositely charged particles. OR It is the ability to break a molecule into its
constituent ions.
Electro negativity:
It the tendency of an atom to attract the bond pair electron in a covalent bond
Hybridization:
The mixing up of various atomic orbitals to give equal number of new orbitals
having equality in all respects.
Recycle rate: It is the ratio of gross water use to intake.
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Useful Guideline on Water
Stoichiometry
The study of chemical reactions and the mass relationships implied by them is
called stoichiometry
Pipe & Tube
Tube is specified by outside dia and wall thickness expressed either BWG
(Birmingham Wire Gauge) or thousands of an inch. Pipe is identified by norminal
size with wall thickness defined by schedule number.
bond: It is a bond formed by axial overlapping of atomic orbitals.
EDTA: ethylene di-amine tetra acetic Acid
Instrumentation
Pressure, temperature and flow are the most important parameters in a
process flow. Calibrated instruments are available to measure these parameters.
The basic theory of the instruments is given in short.
Pressure Gauges
Mostly the elastic property of material is utilized to measure the pressure. The
pressure above or below the atmospheric can be measured by a Bourdon’s Tube
pressure gauge as well as the diaphragm pressure gauge. Since the tube of a
Bourdon’s pressure gauge is encased in a circular cover, it tends to become circular
instead of straight, and the calibrated scale gives the pressure. The diaphragm
gauges utilizes the expansion of a diaphragm for measuring pressure.
Temperature measurement
The temperature at which a sample is collected is important for data
correlation and interpretation purposes, like in tests on pH, specific conductance,,
salinity and forms of alkalinity. Temperature affects the stability of salts. It influences
the bioproductivity in the aquatic environment determines the degree of dissociation
of dissolved salts, and controls to some extent the rate of oxidation of the organic
matter.
Thermometric property of materials such as, the thermal expansion and the
change in resistance is utilized for the measurement of temperature like in Mercury
thermometer and thermocouple.
The zeroth law of thermodynamics is applied in any kind of thermal gauges.
The zeroth law of thermodynamics states that, when a body A is in thermal
equilibrium with a body B, and also separately with a body C, then B and C will be in
thermal equilibrium with each other.
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Useful Guideline on Water
Flow of water
The basic equation of flow measurement is the velocity of the fluid x cross
sectional area of the pipe
Q = AV
For a circular pipe, A = r2,
Where, r = the radius of the cross section.
The cross sectional area of other shapes can be found out the relevant
equations.
And the velocity of moving water is found out by the relation
V = (2gH)
g = acceleration due to gravity,
H = head in meters of water
Instrument such as Orifice meters are used for the measurement of flow. The
differential pressures between the inlet and the throat is utilized for deriving an
equation, from the basic equation of flow.
Q = C a0a1(2gH)
(a12 – a02)
A variety of methods are available to get the direct measurement of flow either
by calibrating or using transducers.
Thermal Calculation
Thermal energy is carried away by the cooling water. It is further dissipated
by the secondary cooling media. the amount of heat carried away is calculated using
the following formula:
Q = mCpΔT
Where, m = quantity of water flow in m3/sec,
Cp = the specific heat at constant pressure,
ΔT = the differential temperature.
Water balance
Suppose Q amount of water supplies to 3 utility points as shown in figure from
water storage. Suppose A, B, C are the amount of water supplied to various utility
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Useful Guideline on Water
points respectively, and an amount of x, y, z water evaporates from each utility
points. Suppose E is the total amount of water evaporated,
Then, supply water
Return water
Where

Q = A + B + C.
R=Q–E
E = x + y + z, the total evaporation loss
Make up water, M = E
Water Equivalent
Water equivalent of a substance may be defined as the quantity of water
which requires the same quantity of heat as the substance to raise its temperature
through one degree. Mathematically,
Water Equivalent of substance = mc
m = mass of substance in kg,
c = specific heat of the substance in kJ/kg.K
Note:
The numerical value of the thermal capacity and the water equivalent of the
substance are the same, but they are expressed in different units.
Heat Exchanger Calculation for conduction (Fourier’s Law)
Q = kA (T0 – TL) = 1 (To –TL)
L
L/KA
K = thermal conductivity (W/m0C)
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Useful Guideline on Water
Reynolds number and Turbulent flow of water
Reynold’s Number = Inertia force/viscous force
Re = Fi/Fv
On substituting, Re = VL/
 = mass density, V = velocity of flow, L = characteristic linear dimension
 = viscosity of water.
In circular pipes, the flow will be laminar if Re is less than 2000 and turbulent
if it is greater than 4000. Between Reynold’s numbers 2000 and 4000 the transition
region occurs.
W H O Std. Of Drinking water
Physical And Chemical Standards of Drinking Water (WHO)
No Characteristic
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Highest
Desirable level
Max.
Permissible
Turbidity on Jackson Scale
Colour on Platinum Cobalt Scale
Taste and colour
pH
Total Solids
5
5
Unobjectionable
7- 8.5
500
10
50
Unobjectionable
6.5 -9.2
600
Chloride
Sulphates
Total Hardness (as CaCO3)
Nitrates
Flurides
200
200
200
45
1.0
400
600
600
45
1.5
Iron
Manganese
Copper
Zinc
Calcium
Magnesium
Phenol
Anionic Detergent
Arsenic
Cadmium
Hexavalent Chromium
Cynades
Lead
Selenium
Mercury
Gross alpha activity (pc/L)
Gross beta activity (pc/L)
0.1
0.05
0.05
5
75
30
0.001
0.02
0.05
0.01
0.05
0.05
0.1
0.01
0.001
3
30
1.0
0.5
1.5
15
200
150
0.002
1.0
0.05
0.01
0.05
0.05
0.1
0.01
0.001
3
30
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Useful Guideline on Water
Quantitative analysis for Quality Control of Water
Total Hardness
Total Hardness is estimated by complexometric titration with standard EDTA solution
using Dotite BT as indicator
Procedure
Pipette out 50ml water sample into a conical flask. Add 10ml of buffer solution and
wait for 5 minutes. Titrate with standard M/100 EDTA solution taken in the burette
using Dotite BT as Indicator. End point is sharp color change from Wine red to blue.
Total Hardness is calculated as follows.
Total Harness = V x 1000/50 mg/litre
Ca Hardness
Ca Hardness is estimated by complexometric titration with standard EDTA solution
using Dotite NN as indicator.
Procedure
Pipette out 50ml water sample into a conical flask. Add 5ml of 10% KOH solution
and wait for 5 minutes. Add 0.5ml NH2OH.HCl. Titrate with standard solution of
M/100 EDTA taken in the burette using Dotite N’N’ Indicator. End point is sharp color
change from Wine red to blue. Ca Hardness is calculated as follows.
Ca2+ = V x 0.4008 x 1000/50
In terms of CaCO3 = Ca2+ x 2.495 mg/litre
Chloride in Water
Chloride content in water is estimated by Argentometric titration with standard
AgNO3 using K2CrO4 as indicator.
Procedure
Pipette out 50 ml water sample into a clean conical flask. Add I ml of K2CrO4
indicator solution into to the sample. Titrate with standard 0.1N AgNO3 solution.
Endpoint is sharp color change to Orange red. Chloride ion is estimated as
Chloride (mg/l) = A x N x 35.45 x 1000
Vol. of sample
mg/litre
Where, A = Vol. of AgNO3 required for sample
N = Normality of AgNO3
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Useful Guideline on Water
Total Alkalinity
Total alkalinity in water sample can be determined by titrating a known volume of
sample against standard sulphuric acid solution using methyl red indicator.
Procedure
Pipette out 50 ml water sample into a clean conical flask. Add 2-3 drops of methyl
red indicator. Titrate with standard N/50 H2SO4 solution taken in the burette.
Endpoint is sharp color change from blue to white. Alkalinity is calculated as
Total alkalinity
=
VX 1000/50 ppm
Total Dissolved Solids (TDS)
At present Conductivity/TDS meter is used to find the both parameters. The
following chemical analysis can also be used.
1. Take 100ml of water into a pre-weighed clean, dry porcelain dish.
2. Evaporate the water in the basin to dryness over a water bath
3. Clean the outside of the dish, dry it in an air oven at 105oC for an hour to
remove moisture, cool and weigh.
4. the difference between the weights is the weight of total soluble salts.
Calculation
Volume of water taken
= 100ml
Weight of empty dish
= Agrams
Weight of dish + residue
= B grams
Weight of TDS in 100ml sample = (B – A) grams
TDS (mg/l)
= (B – A) x 106
100
Total Suspended Solids
TSS in water sample is determined by filtering the sample through filters with
nominal pore sizes varying from 0.45m to about 2.0m. TSS test is somewhat
arbitrary because it depends on the pore size of the filter paper.
Procedure
Accurately weigh a GS 25 filter paper. Shake the given water sample vigorously and
Filter 500ml sample through the filter paper using a filtration a pump. Dry at 100 oC
for 2 hr. Cool and weight is noted .TSS is calculated as
TSS = (W2-W1) /Vx 106 mg/litre
W1= Weight of filter paper
W2 = Weight of filter paper with sample
V= volume of sample taken
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Useful Guideline on Water
Total Phosphate in Circulating water
The estimation is based on the reaction of the phosphate ions with acidified
molybdate reagent to give phosphor-molybdate complex which is then reduced to a
blue compound by SnCl2 or by suitable reducing agent. The absorbance of the
compound is measured by spectrophotometry.
Procedure
Pipette out 10 ml water sample into a clean graduated test tube. Add 1ml H2SO4 and
10 ml potassium per sulphate solution. Cover with aluminum foil, autoclave for 1 hr.
Cool, add 2-3 drops phenolphthalein and add NaOH drop wise to pink colour. Add
5ml ammonium Molybdate solution. Mix well and add 0.25ml SnCl2. Make up to
50ml. Mix well and allow t stand 15mts. Measure absorption rate at 700nm using
spectrophometer. Prepare a blank solution with distilled water and follow same
procedure.
HCl Concentration (for acid washing of Chlorinator)
Before circulating the HCl in the hypo-chlorinator, its concentration will be analyzed.
Concentration of HCl can be estimated by potentiometric titration using standard
NaOH solution.
Procedure
Pipette out 10ml acid in to a 100ml beaker. Add 50ml Distilled Water and keep a
magnetic stirrer in it. Dip pH meter electrode and slowly add standard NaOH (1N)
from burette with stirring. When pH increases to 3 minimize addition of NaOH.
Continue addition slowly with stirring till pH increases to 8. Note volume of NaOH.
Then HCl percentage is calculated as
% HCl = (V1 x N1 x 0.3646)/sp. Gravity of HCl
Where V1 = Volume of NaOH
N1 = Normality of NaOH
Sp. Gravity is measured using a hydrometer
T Fe of water (Soluble Fe)
Total Iron in circulation water is estimated by absorption photometric method.
Procedure
Pipette out 50ml sample in to a conical flask. Add 5ml 3N HCl. Boil and reduce the
volume to 25%. Cool and transfer in to a 50ml standard flask. Add 1ml NH2OH.HCl
and 2.5ml O-phenanthrolein hydrochloride. Put a small piece of Congo red paper.
Add 6N NH4OH drop by drop till color turns red. Add 2.5ml buffer solution and make
up to volume 50ml. Wait for 30mts and take spectrophotometer reading at  =
510nm
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Useful Guideline on Water
HISTORICAL DEVELOPMENT OF WATER TREATMENT
Ancient civilizations established themselves around water sources. It took
thousands of years for people to recognize that their senses alone were not accurate
judges of water quality. Water treatment originally focused on improving the
aesthetic qualities of drinking water. Methods to improve the taste and odor of
drinking water were recorded as early as 4000 B.C. Turbidity was the main driving
force behind the earliest water treatments. Ancient Sanskrit and Greek writings
recommended methods such as filtering through charcoal, exposing to sunlight,
boiling, and straining. To clarify water, the Egyptians used the chemical alum as
early as 1500 B.C. to settle down the suspended particles.
By the early 1800s, slow sand filtration began to use in Europe. During 1800s,
scientists understood the sources and effects of drinking water contaminants. In
1855, Dr. John Snow proved that cholera was a waterborne disease. In the late
1880s, Louis Pasteur demonstrated the “germ theory” of disease, which explained
how microbes could transmit disease through water. During the late nineteenth and
early twentieth centuries, concerns regarding drinking water quality continued to
focus on disease-causing microbes (pathogens) in public water supplies.
It was disinfectants like chlorine that played the largest role in reducing the
number of waterborne disease outbreaks in the early 1900s. In 1908, chlorine was
used for the first time as a primary disinfectant of drinking water in Jersey Citty. The
use of other disinfectants such as ozone also began in Europe around this time.
By the late 1960s it became apparent that industrial and agricultural
advances and the creation of new man-made chemicals also had negative impacts
on the environment and public health. Many of these new chemicals were finding
their way into water supplies. Treatment techniques such as aeration, flocculation,
and granular activated carbon adsorption at this time were ineffective at removing
some new contaminants. A study in 1972 found 36 chemicals in treated water taken
from treatment plants that drew water from the Mississippi River in Louisiana.
Scientist continued their effort on the quality of water.
Ancient treatment techniques are still in use. However, techniques like
reverse osmosis and granular activated carbon are newer. Since water is a raw
material in a variety of industries, and it being the best and cheap cooling medium, a
variety of treatment program was designed to make it suitable for industrial purposes
and eco-friendly manner.
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Useful Guideline on Water
Foot Notes
1. Hydraulics and fluid mechanics, P.N.Modi P: 1168
2. Hydraulics and Fluid Mechanics, P N Modi - Page: 35
References
1.
George T. Austin, Shreve’s Chemical Process Industries.
2.
David R Sherwood, The piping Guide; For Design & Industrial Piping systems
3.
P N Modi, S M Seth, Hydraulics and Fluid Mechanics
4.
John J Pippenger, Industrial Hydraulilcs
5.
R S Khurmi, Hydraulics
6.
B C Punmia, Ashok Jain, et-al Water Supply Engineering
7.
P K Nag, Engineering Thermodynamics
8.
Breck, Brown, McCowan, Chemistry For science and Engineering
9.
MetCalf & Reddy, Waste water Engineering; Treatment &
Reuses
10. B C Punmia, Ashok Jain, Wastewater Engineering
11. A.O Thomas, Modern chemistry vol - I
12. R S Khurmi, J K Gupta, Thermal Engineering
13. G. Nikolandze, D. Mints et-al, Water treatment for Public & Industrial supply
14. A O Thomas, Practical Chemistry (BSc Main)
15. R Ramesh & M Anbu, Chemical Methods for Environmental Analysis: Water &
Sediment
16. Hoffman, Heat and Mass Transfer
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