3 water treatment plant

8/18/2019 3 Water Treatment Plant

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Water Treatment Plant at JPL comprises of following three sections –

1) Pre Treatment Plant.

2) DM Plant.

3) CW Treatment Plant

IMPURITIES in Water :

i. Suspended Solids : Muds, Silts, Sand , organics etc.

ii. Dissolved Solids : Salts of Calcium, Sodium and Magnesium, Iron, Manganese,

Fluoride salts and Gases.

Raw Water is taken from raw water reservoir through pumps

having capacity 2200 m3/hr each (2W+2S) into the cascade aerator for deaeration for removal of

Oxidisable impurities like iron etc. then to two stilling chambers where turbulence of raw water

will be broken. Water then flows through individuals 3 nos. of venturi flumes (for flow

measurement) to (3X100%) flash mixer units where water will be then fed to three

clalriflocculators each of 1750-m3/hr. capacities. Chemical such as Alum, Lime &

polyelectrolyte are added at the downstream of each venturi. The clarified Water thus producedis stored in two compartments reservoir separated by partition wall along with necessary

isolation gauge. The under Sludge from each clarifiers is collected to a common sludge pit from

where sludge is pump out by means of vertical centrifugal pumps. Chlorine is dosed in stillingchambers by means of 3 vacuums feed chlorinators (cap. -11Kg/hr. each).

– DM Plant primarily meets heat cycle make up water requirement of 4X250MW

units. It consists (3) DM chains having capacity 100m3/hr.each. (2W+1Stand by for regeneration

& maintenance purpose).

Each chains comprises of following units –

DM Feed Pump - DMF-ACF-LBC-DEGASSER TOWER-DEGASSED WATER STORAGE

TANK-Degassed Water Transfer Pump-WBA-SBA-MB-UF Break Tank-UF feed pump-UF

SKID-DM STORAGE TANKS.


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DMF (Dual Media Filter)– It consists of vessel filled with filter media comprising of sand,

supported on graded gravel for filtering of suspended solids carried over from clarifiers. On inlet

design turbidity of 25 NTU & Outlet Turbidity of 2 NTU (Nephelometric Turbidity Unit).

ACF (Activated Carbon Filter)- Water from DMF goes down stream into ACF, which consistof vessel filled with activated carbon in granular form for removal of free chlorine dosed in

clarifier.

LBC (Layered Bed Cation)- Water from ACF goes down stream into LBC, which consist ofvessel filled with Cationic Resin in bead form for removal of cations present in water after it’s

service cycle is over, it is again regenerated with 5% HCL.

Degasser Tower- It is mounted on a plat form over a degassed water storage tanks wheredegassed water is stored. Water from LBC goes down stream into Degasser Towers (3 nos.) each

of rubber lined MS construction to ensure removal of CO2 gas based on the theory of Partial

Pressure Mechanism. A counter current of air to water is provided for effective removal of CO 2

by 4 nos. of Air Blowers.

Degassed Water Transfer pumps – four horizontal centrifugal type pumps which are capable

are taking suction from Degassed water storage tanks & pumped it to WBA & SBA units.

WBA (Weak Base Anion)- Water from Degassed Water Storage Tanks goes down stream into

WBA, which consist of vessel filled with weakly basic type-high capacity Resin for removal of

anions present in water.


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SBA (Strong Base Anion)- Water from WBA goes down stream into SBA which consist of

vessel filled with Strongly Basic type high capacity Resin for removal of anions specificallySilica present in water.

Regeneration of SBA & WBA: Alkali regeneration system consists of solution tanks ejector

etc. design such that during regeneration Anion resin Bed in SBA is regenerated in 5% of NaOH

& the spent lye is used to regenerate WBA.

MB (Mixed Bed)- Water from SBA goes down stream into MB which consist of vessel filled

with Strong Acid Cation & strong Basic type high capacity Resin for Polishing any slippage of

cations or anion from their respective removal units & water collected at UF break tanks.

Regeneration of MB units: Concurrent regeneration is adopted in MB to regenerate CationicResin with HCl & Anion Resin with NaOH.

Ultra Filtration System – Water from UF break tank is pumped by UF feed pumps to three UF

Skid systems having capacity of 100m3/hr. each. Suitable no. of membrane units for each units

are provided for removal of Colloidal Silica. Membrane pore is 10000MWC (Molecular Weight

Cut off). Membrane Material is poly Sulphone/Poly amide & is able to with stand a load of about250mg/1maximum at a pH range of 1.5 to 13.

DM Storage Tanks- Dematerialized water is collected in two DM Storage tanks having Water

holding Capacity 1000 m3 each by which water is transfer to the heat cycle make up water

requirement of 4X250MW units with the help of Six nos. of DM transfer pumps out of whichthree pumps (2W+1S) for Phase-I units (i.e. units-1 &2) & Three pumps (2W+1S) for Phase-II

units (i.e. units-3&4).

DESCRIPTION OF CW SYSTEM: -

CW System comprises of following three sections:-

1. Chlorination System

2. Acid Dosing & Chemical Dosing System

3. Side Stream Filtration System

Chlorination System - In CW Chlorination System Cl2 is shock dosed for an average period of

about one hour daily. In the CW bay to prevent microbiological growth. There are three nos. ofVacuum feed chlorinator each of capacity 200kg/hr. for each set of 2X250MW units. Two

Chlorinators will be in operations & other will be kept on common stand by for each 2 units. The

chlorination system consists of Chlorine Ton container, Booster Pumps, Strainers, piping &

Diffuser systems up to dosing points. The Water to the Booster Pumps is supplied from CWpump discharge header at a pressure of 2 kg/cm2.

Acid Dosing & Chemical Dosing System – Sulphuric acid dosing system comprises of two nos.

conc. Sulphuric acid storage tanks two sulphuric acid day tanks & three sulphuric acid dosing


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pumps. For addition safe guard against corrosion, scaling & microorganism, corrosion as well as

scale inhibitor pumps has been provided along with chemical dosing tanks.

Side Stream Filtration System- To reduce the turbidity in Cooling Tower, 2% of therecalculating Water is passed through Vertical Automatic valve less gravity Filters (AVGF).

Total 10 in numbers AVGF for each unit. Each having capacity of 175 m3 /hr. The Filters are

getting supply of circulating Water from CW Pump discharge header & after filtration, filtered

water is returned to CW basin.

Note: It is to note that the treatment of Circulating Water at Phase-I & Phase-II is an outsource

activity & carried out by Contractor hence they are accountable to ensure end result/outcome ofguaranteed parameters are under specify limits for Corrosion rates and deposition index. The

monitoring of parameters at CW Phase-I & Phase-II is determined by testing of water for someparameters at their respective site laboratory at CW treatment Plant Phase-I and is also verified

and recorded at JPL Chemical Laboratory for to make sure the legitimacy of report are under

specify limits & smooth operation of the system could be done

CHEMICAL TERMINOLOGY AND DEFINITIONS

Coagulation: Coagulation is simply the accumulation of the smaller particles into larger ones,

which are big enough to settle down .The addition of a coagulant (Alum) neutralizes Charges,

collapsing the "cloud" surrounding the colloids so they can Agglomerate and consequently

become heavy to settle down. This neutralization of surface charge by addition of Co-agulant isknown as Co-agulation.

Flocculation: The floe formed by the agglomeration of several colloids may not be large enough

to settle or dewater at the desired rate. A flocculants gathers together floe particles in a net,

bridging from one surface to another and binding the individual particles into large agglomerates,

this agglomeration of particles is called Flocculation.

Disinfections: Killing of Bacteria or microbiological organism in water by help of Chlorination

or addition of chlorine is known as disinfections.

Turbidity: Turbidity is the optical property of water, which renders objects viewed through it

indistinct in outline. The presence of suspended matter such as silt, clay, course particles of

organic matter, plankton & other microscopic organism caused turbidity.

Resin: Ion Exchange resins are porous materials, which contain an inert base attached to which,

are free ions. These ions are Free to move about within the resin structure and can be replaced by

other ions of the same type from a surrounding solution.

Suspended matter: Such matters, which can be filtered, out physically or can be observed by

naked eye in water, are termed as Suspended matter.


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Dematerialization: Demineralization is a process in which mineral salts dissolved in water are

completely removed. Removal of dissolve solids are accomplished with the help ofchemicals/Resins through ion exchange process.

Regeneration: It is a process by which exhausted ion exchanging chemicals are charged back to

their ion exchange replacing forms.

CV: Calorific Value is the amount of heat liberated on combustion of 1 g of any combustible

matter.

HGI: Hard Groove Grindibility Index of Coal

HP & LP Dosing: Chemical (Tri-Sodium Phosphate) Dosing by the aid of High-pressure pumpto Boiler Drum. Chemical Dosing (Hydrazine & Ammonia) at De-aerator by the help of Low

Pressure Pump.

Viscosity -- measurement of a fluid's resistance to flow. The common metric unit of absolute

viscosity is the poise, which is defined as the force in dynes required moving a surface one

square centimeter in area past a parallel surface at a speed of one centimeter per second, with the

surfaces separated by a fluid film one centimeter thick. In addition to kinematics viscosity, there

are other methods for determining viscosity, including Saybolt Universal Viscosity (SUV),

Saybolt Furol viscosity, Engier viscosity, and Redwood viscosity. Since viscosity varies in

inversely with temperature, its value is meaningless until the temperature at which it isdetermined is reported

Mechanical Impurities: Presence of Foreign particles or eroded particles due to wear & tear of

mechanical movements in Lubrication oil which can be separated by the help of Micron pore

size filter paper.

De-aeration: Removal of oxygen to reduce corrosion and fouling in Boiler Water tubes.

Organic Fouling: Obstruction in flow of water due to growth of microbiological organism is

known as Organic Fouling.

Scale - is a very hard substance that adheres directly to heating surfaces forming a layer of

insulation. This layer of insulation will decrease heat transfer efficiency. Scale also results in

metal fatigue/failure from overheating, energy waste, high maintenance costs and unnecessary

safety risks. A one-sixteenth inch thickness of scale in a fire tube boiler can result in a 12.5%

increase in fuel consumption.

Corrosion - is defined as the destruction of a metal by chemical or electromechanical reaction

with its environment. The metal is eaten away in much the same manner as fender rusts on a car.

Corrosion dramatically increases maintenance costs and can cause unnecessary safety risks. It


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will occur when levels of oxygen or carbon dioxide are high, where pH values are low, where

contact occurs between dissimilar metals and in damp environment or corrosive atmospheres.

Corrosion is an electrochemical process in which electricity flows through a solution of ions

between areas of metal. Deterioration occurs when the current leaves the negatively charged

metal or anode and travels through the solution to the positively charged metal or cathode,

completing an electrical circuit in much the same manner as a battery cell. The anode and the

cathode can be different metals or areas of the same metal. Corrosion occurs when there is a

difference in the electrical potential between them.

Fouling - occurs when a restriction develops in piping and equipment passageways and results in

inefficient water flow. The fouling of boiler room equipment directly impacts energy efficienciesand cost of operations.

Foaming - is a condition in which concentrations of soluble salts, aggravated by grease,

suspended solids or organic matter, create frothy bubbles or foam in the steam space of a boiler.

When these bubbles collapse it creates a liquid that is carried over into the steam system.

Foaming degrades steam quality and in some cases can create a water slug that is discharged into

the steam lines.

Alkalinity: The capacity of water for neutralizing an acid solution. Alkalinity of natural waters is

due primarily to the presence of hydroxides, bicarbonates, carbonates and occasionally borates,

silicates and phosphates. It is expressed in units of milligrams per liter (mg/l) of CaCO 3 (calcium

carbonate) or as micro equivalents per liter (ueq/l) 20 ueq/l = 1 mg/l of CaCO3. A solution

having a pH below 4.5 contains no alkalinity. Low alkalinity is the main indicator of

susceptibility to acid rain. Increasing alkalinity is often related to increased algal productivity.

Lakes with watersheds that have sedimentary carbonate rocks are high in dissolved carbonates

(hard-water lakes). Whereas lakes in granite or igneous rocks are low in dissolved carbonates

(soft water lakes).

BOILER WATER TREATMENT:

High pressure boiler (operating above 60 kg/cm2) design needs a close look at water & steam

quality to avoid (1) corrosion control and (2) deposition on turbine blades. The cost of corrosion

and deposition is very high due to repairs and loss of production (shutdowns).Thus the successful

operation of high pressure boilers and turbine units require a strict vigil on the Water Treatment

practices and controls, particularly for high pressure drum type and once through boilers.

a. MAKE - UP WATER TREATMENT


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Total solids and silica are the main constituents for carry-over and deposition. These constituents

are to be controlled and maintained at low levels in makeup water. Silica in particular, isCARRIEDOVER in the form of VAPOUR at high pressures. Contaminated Feed water, used for

de-superheating spray, directly enters the superheated steam and may cause boiler tube corrosionand deposition on turbine blades. More blowdown, to control contamination, is uneconomical.

Hence feed water is required to be very pure for high pressure boilers. Modern demineralisation

plants are capable of producing the required quality of makeup water with specific electrical

conductivity less than 0.2 micro mhos/cm and silica 0.02/0.01 ppm.

b. INTERNAL CORROSION

Corrosion is a common phenomenon in high pressure boilers. Corrosion in boiler circuits as

well as in pre-boiler circuits can cause tube failures followed by force shut down of boilers.The causes of corrosion are :

1. pH ( acidity or high alkalinity ) value.

2. Oxygen

3. Excessive ammonia (on copper base alloys )

4. Concentration of alkalising agents due to localised over heating

5. Poor quality of passivating layer or breaking of passivating layer due to thermal shocks6. Decomposition of organics into corrosive products.

c. EFFECT OF pH

The reaction of feed water on steel is spontaneous and rapid at high temperatures. For protecting

this reaction a passivated layer of magnetite (Fe3O4) / hydrated iron oxide (FeOOH) is created on

the steel surface. Maintaining alkalinity in boiler water protects this oxide film.

It was found that the protective layer is getting dissolved at pH values below 5.0 and above 13.0

Minimum corrosion is indicated at pH of 9.0 to 11.0 An optimum pH of 8.8 to 9.2 is

recommended for feed water. Any excess presence of ammonia (indicated by higher pH values)

will cause copper corrosion in the pre-boiler system.

Another parameter of different temperature / pressure ranges in boiler water different pH values

are to be maintained to minimize corrosion.

Accordingly boiler water pH requirements are higher than the feed water limits and different for

different pressure ranges. Boiler water pH is elevated as per recommended levels using

Trisodium phosphate (Na3PO4). The use of caustic soda is ‘not recommended for this purpose as

it has the danger of concentration and destruction of protective oxide film to cause corrosion.

d. EFFECT OF OXYGEN

Oxygen free feed water is essential to avoid corrosion. Small quantities of dissolved oxygen arecapable of causing severe corrosion pitting in boiler tubes. A combination of poor oxygen


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control and chlorides in boiler water can result in serious hydrogen damage type corrosion of

water wall tubes. Continuous monitoring of oxygen is required in high pressure system.

Poor start-up procedures are also responsible for oxygen ingress. Feed water at a temperatureless than 100 0C contains excessive quantities of dissolved oxygen. Deaerator is the main

equipment to control oxygen within 0.01 - 0.02 ppm. This problem can be reduced by

pressurising the deaerator with steam at about 0.5 kg/cm2 .

With the main oxygen removal by deaeration, residual oxygen in small quantities can be reduced

further by reducing agents such as hydrazine N2H4). Hydrazine being a volatile chemical, should

only be used for high pressure boilers. Hydrazine reacts with oxygen to form nitrogen and water

( N2H4 + O2 → 2H2O + N2 ↑ ). It is advantageous to add hydrazine to the cycle at the outlet of

deaerator.

e. BOILER WATER TREATMENT

The principal objective of boiler water treatment is to prevent:

1. Scaling.2. Corrosion.

3. Steam contamination

SCALING :

Water contains many impurities like dissolved salts and / or suspended matter. Thesuspended impurities such as biological growth, mud, and bacterial growth can be removed

easily as compared to dissolved solids. These dissolved impurities are insoluble at an

elevated temperature. When temperature rises, solubility of these dissolved salts decreases

and some precipitation occurs locally. These precipitations are sticky in nature and form

coating on the metallic surface. This is called scaling. It can also be described as continuous,

adherent layer of foreign material formed on the waterside of a surface through which heat is

exchanged. Scales are objectionable because of their heat insulating effect.

It is sometimes said that a thin layer of CaCO3 ( Calcium Carbonate) should be maintained to

protect the surface from corrosion. But it is impossible to lay down uniform thickness ofscale, because the scale thickness depends upon the amount of heat being transferred, which

is not same in all sections of boiler.

Any scale in the boiler, however, is absolutely undesirable. Scales and deposits are formed

because the compounds, of which they are composed, are insoluble under high temperatures

prevailing in the boiler. Certain dry calcium salts especially sulphate, decrease insolubility as

temperature and pressure increase. Similarly, solubility of CaSO4 decreases rapidly with

increasing temperature, producing extremely hard adhering coating on boiler tubes,

especially in location where heat flux is high. Accumulation in boiler drums are most often in


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the form of mud or sludge. When oil is present as a contamination in boiler water, loose

scales may form particularly in water tubes. Oil serves as a nucleus and of scaling at hotspots. The ‘oil balls’ found in steam drum and water wall headers.

PREVENTION OF SCALING :

The most effective method of prevention of scaling is to eliminate scale-forming elements

from the feed water, or to transform them by some means into a safe form. That is the reason

why deminerlised water is used in the system. Demineralisers can produce water quality

with nearly zero hardness. All ionized salts are removed in these processes, which greatly

minimize the potential for boiler deposits, corrosion and turbine fouling. On line removal of

scales forming salts are done by phosphate treatment. These salts are inevitably present inthe boiler water in the form of residual hardness even after demineralization.

The tri-sodium phosphate, which is used, as phosphate treatment tends to increase the pH

value while di-sodium phosphate formed as a byproduct, is a neutral salt. Tri-sodium

phosphate reacts with salts of Magnesium and Calcium to form sludge ( Calcium &

Magnesium phosphate) . The reason is as follows :

2Na3PO4 + CaSO4 → Ca3(PO4)3 + 3Na2SO4

Na3PO4 + H2O → NaOH + Na2HPO4

Na2HPO4 + H2O → NaOH + NaH2PO4

The advantages of Phosphate treatment are:

Adequate alkalinity can be maintained in the boiler water.

Na2HPO4 is additionally available to form phosphate sludges.

Self-containing hydrolysis, hence proper control over pH ( above pH = 10.2 reactions

reverts to left) can observed. No problem of caustic corrosion.

The only disadvantage of phosphate treatment is phosphate hideout.

PHOSPHATE HIDEOUT:

Sometimes, objections are raised against co-ordinated phosphate treatment because of the

phenomenon of the phosphate hideout. At higher loads phosphate comes out of the solution

because of its low solubility at raised temperatures. But when the load is reduced it goes back

into the solution adding to the total phosphate content of water. This may lead to excessive total

dissolved solids content of boiler water in drum. However, there is no danger in phosphate

hideout. It is nothing more than a nuisance to the operators for control of boiler water chemistry


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regarding (PO4) – level control. Any phosphate salts, hidden out, are available to take care of any

hardness in water it is never advisable to resort to any boiler water treatment with free caustic,even in small amounts. It does much more damage to the boiler tubes than any ‘phosphate

hideout’.

The recommended boiler water limits for phosphate & drum water pH are as follows:

Phosphate should be , 3- 10 ppm. & pH = 9.4 – 9.7.

CORROSION :

Scattered pitting in the presence of oxygen is sometimes observed in the water line in the

steam drum and in the down-comer tubes of boilers. Economizer, on account of their hightemperature, is susceptible to corrosion by oxygen. The mechanism of pitting in a metallic

surface produced by a bubble of air is shown in the figure below;

4e¯ + O2 + H2O = 4(OH) ̄

4(OH) ¯ + 2Fe = 2Fe(OH)2 + 4e¯

It ultimately forms a cell and is continuous in nature unless surface of oxygen is removed.

Corrosion of iron and copper in condensate system leads to formation or porous deposits

under which salts in boiler water concentrate and damage the underlying surface of boiler

steel. Then, even in absence of deposits, caustic gauging can occur owing to the

concentration of sodium hydroxide particularly in places where the rate of heat transfer is

unusually high. Other possibilities are the corrosion of stressed metal. The severity of these

effects can be controlled of some extent by reducing the concentration of oxygen and free

alkali, and by eliminating products introduced from pre-boiler system.

Corrosion is the oxidation of metal by some oxidizing agent in the environment. The area

over which the metal is oxidized is called the anode and at which the oxidizing agent is

reduced is called cathode. These areas are necessarily separated but usually are not far apart.

As corrosion products, electrons flow between these areas through the metal while ions

migrate through the solution. This system constitutes an electro-mechanical cell. In boiler the

oxidation of iron is accompanied by the reduction of hydrogen ions supplied by the hot

water.

3Fe + 4H2O = Fe3O4 + 4H2 ……….(A)

In case of acidic water :

2H+ + Fe = Fe++ + H2 …………(B)


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The reaction (A) is self-limiting on account of the barrier of Fe3O4 that forms on the surface

of the metal. The reaction (B) on the contrary, continues until the supply of hydrogen ion isdepleted in boilers. Both reactions are opposed by an irreversible potential called the

hydrogen over-voltage, which is affected, by the condition of the surface of the metal.

Prevention of Corrosion:

I. Removal of Oxygen:Oxygen is introduced into the boilers dissolved in feed water. When this water enters the

steam drum, most of the oxygen flashes into steam space, producing characteristics

pitting at the water wall lines and in the vicinity of the discharge of the feed line.

In addition, several local corrosion, pinhole failures and pitting in the rear furnace wall

tubes in high-pressure boiler, are attributable to attack by the dissolved oxygen.

The concentration of dissolved oxygen in feed water should be less than 0.03 ppm and

preferably less than 0.005 ppm in water for high-pressure boilers. Cold water saturated

with air contains about 10 ppm of oxygen . This can be reduced to 0.3 to 0.7 ppm in an

open heater and about 0.01 ppm in a spray type deaerator normally used in a power

plant. The greater part of corrosive gases, and carbon dioxide and oxygen that are

dissolved in water can be removed by deaeration. Open heaters are suitable for low

pressure but spray type deaerating heaters are commonly used. In these units steam heats

the feed water in primary heater and also scrubs the heated water. Hot spray flows down

through a baffle arrangement against a rising flow of steam that sweeps the liberated

gases out through a vent at the top of the vessels, while deaerated water collects in a

storage section at the bottom. The vent is equipped with a condenser through which cold

feed water flows to prevent excessive wastage of steam. The oxygen concentration of less

than 0.007 ppm can be obtained through this method. Because of volatilization of CO2

and thermal decomposition of bicarbonate the pH of deaerated water is normally between

8.5 & 9.5.

2 HCO3 ¯ → CO3¯ + CO2 + H2O

H+ + HCO3¯ → CO2 + H2O

So far the mechanical deaeration has been discussed. Now it may be seen how effectively

the corrosive oxygen can be removed with the help of chemicals. There are two

chemicals, which are primarily used to remove oxygen.

a. Sodium Sulphite.

b. Hydrazine.


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Sodium Sulphite is commonly used in boilers operating at less than 60 kg/cm2. While

hydrazine is the reducing agent at higher pressures. The reaction of hydrazine is asfollows:

N2H4 + O2 = N2 + 2H2O.

3N2H4 = 4NH3 + N2 ( at 2000C )

2NH3 + CO2 ⇆ (NH4)2CO3 ( at 270 0C)

Effect of time and temperature on reactions between hydrazine and dissolved oxygen.

Nitrogen being inert gas liberated and is removed as non-condensable gas.

Advantages of hydrazine treatment are ;

• Low equivalent weight.

• Does not increase dissolved solid content of drum water.

Disadvantages of hydrazine treatment are :

• Vapour toxic nature.

• Excess of hydrazine at high temperature disintegrates into ammonia.

• Concentrated solution of hydrazine is flammable.

At the economizer inlet the concentration of hydrazine is to be limited to 0.050 ppm.

The presence of porous deposits on the water side of boiler tubes lead to serious

corrosion, especially when there is free alkali in the water. Various conditioners are

added to disperse insoluble materials and prevent the accumulation of sludge on surface

where the heat is transferred. Recently polyacrylates and other synthetic polymers have

come into use.

Small amount of particulate matter comprising finely divided oxides of copper often

contaminate condensate. These oxides, besides to causing foaming deposit on boiler

tubes at a rate proportional to the heat flux. The rate of deposition increases rapidly above

55 kg/cm2. The presence of these deposits causes over heating of the tubes and

sometimes-ductile hanging. Direct reaction of steel with particles of ferric oxide is also

possible.

4Fe2O3 + Fe+ = 3Fe3O4

II. Monitoring Feed / Condensate water pH :


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The metallic surface on the water side of a boiler tube is naturally protected by a thin film

of magnetite formed by the action of hot water on steel.

3Fe + 4H2O = Fe3O4 + 4H2

Ideally there is no further oxidation of metal after the protective layer is formed. The

minimum rate of corrosion is realized at pH value 11 to 12. At lower pH values hydrogen

ions are discharged where as the values greater than 12, the magnetite layer thickness

peptizes (digest) to some extent and is made porous by diffusion of ions from underlying

metal. Above pH value of 13, the magnetic layer is completely destroyed. With the

pressure above 40 kg/cm2 hydrogen may diffuse into metal blistering and weakening it

severely. Hydrogen atom reacts with carbon in steel to form methane. The pressuregenerated in this may cause fissures along the grain boundaries, so ideally the pH of 8.5

to 9.5 should be maintained. The recirculation of small amount of alkaline boiler water

through BFP has been recommended but this can lead to plugging of feed lines in

economizer, feed water heaters by insoluble phosphate.

At lower pH values than 8.5 in the drum , the removable sludge formed by phosphate

treatment of scale forming salts becomes very sticky itself. Also at lower pH values the

silica carry over (Distribution Ratio x1/pH) increases very rapidly, not to say rapidly

increasing rate of corrosion due to pH values lower than those recommended.

STEAM CONTAMINATION :

Carryover of salts in steam occurs either due to mechanical or vapour carryover. Efficient

drum internal scan only reduce mechanical carryover. Silica is always carried over in

vapourous form. The vapourous carryover of remaining salts mainly sodium salts is

significant only at pressure above 180 kg/cm2. The carryover may occur in the following

four types:

i. Leakage carryover.

ii. Spray or mist carryover.

iii.

Priming.iv. Foaming.

i. Leakage Carryover :

Leakage of water droplets through seals or gasketed joints of steam purifying equipment

cause this type of carryover. It is usually highly localized and may not be detected in

steam purity test unless the sampling points are very near to the leakage point. This will

be revealed that steam purity will deteriorate with increase in load. The change water

level may not alter the degree of contamination.


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At this type of carryover is most frequently localized. This causes for localized super

heater failure. It is suspected then new gasketing or seal welding may be required toeliminate this problem.

ii. Spray or mist carryover:

In this case, atomized droplets of water will be carried with the steam. This is common in

all boilers to some extent. Spray or mist carryover can be avoided by installing steam

purification equipments. If the steam purification equipment is under designed, this will

be present even after the installation of these equipments.

iii. Priming :

It is relatively unimportant in present power plant boilers and is rarely encountered.

Carryover of this type is characterized by a sudden carryover of gross quantities of boiler

water, which would show up a drastic deterioration of steam purity. It can be caused by

the variation in pressure such as large pressure drop. In this case the water in the boiler

would swell due to expansion of steam and formation of additional steam. This action is

similar to the ‘bumping’ experienced when water is boiled in an open beaker. It is more

violent spasmodic action, resulting in the throwing slugs of boiler water with steam flow.

iv. Foaming :

The important factors that affect the carryover in steam are:

1. Drum and its internals.

2. Water level in the drum.

3. Boiler water concentration.

4. Foaming and vapourous and carryover.

To achieve purity of steam for power or industrial units, mechanical contrivances are

provided inside the boiler drum. These are known as drum internals. They distribute and

mix feed and chemicals added to boiler water while removing entrained moisture from

steam as it leaves the drum.

The three basic effects by the internal arrangements are:

Centrifugal action to produce separation force, which is many times greater than

gravity.

To direct the steam water mixture so that the upward velocity vector is zero.

Provision of drainable wetted surface in which fine spray can coalesced.


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Foaming is the condition resulting from the formation of bubbles on the surface of boiler

water. The foam produced may entirely fill the steam space of the boiler or may berelatively minor depth. In either case this foaming condition causes appreciable

entrainments of boiler water with steam. Generally presence of organic matter and/or oil

will promote foaming.

Silica Carryover :

Certain dissolved solids in the boiler water are carried away with the steam as vapour and

the internals have no control over such vapourous carryover. One of the detrimental

constituents is silica. In order to limit silica carryover, the concentration of silica in the

drum water must be limited to a specified value for a given operating pressure range.

In order to control the silica in boiler water, the most effective method used is blow-

down.

Blow-down :

As steam leaves the boiler, solids introduced in feed water are concentrated in the water

left behind in the drum. If this concentration were allowed to continue, the less soluble

components in the water eventually crystallize on the internal surface and in addition, the

steam would contaminate .In ideal operation, the concentration of solids is allowed to

reach the limit after which the concentrated boiler water is bled off at such a rate that theamount of solids entering in feed water is exactly balanced by that method in bleed

steam. This process is called continuous blow-down.

Suspended solids in the presence of iron tend to collect as sludge in the lower parts of

boiler i.e. down-comers or ring header. Opening the intermittent blow-down can blow out

the concentrated sludge. The turbulence caused by opening the valve disperses the

sludge. So there is no point in leaving the valve open longer than 15 secs. It is usual

boiler practice that water wall headers should never been blown down because the

circulation of water through them is usually critical when the boiler is on load.

The outlet of CBD should be below the level where the riser tubes enter the steam drum

because this is where the dissolved solids in recirculating water are most concentrated.

Chemicals for internal treatment (phosphate, sulphite etc.) should be introduced above

the down-comer tubes to prevent sludging on the hot risers, and also to promote mixing

and reaction with salines in the entering feed water. If the chemicals are injected near the

blow-down outlet, short circulating will occur.


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Continuous blow-down is the most effective way for controlling the amount of solids in

boiler water after the rate of withdrawal has been adjusted properly. If the rate of blow-down is too high, heat and water are wasted, if too low the permissible limits will be

exceeded.

f. CONDENSER LEAKAGE

Condenser leakage is a major source for corrosion. The type of cooling water and its interaction

with boiler water determines whether boiler water pH will become more acidic or alkaline duringa period of condenser leakage. It is very important to prevent condenser leakage of sea water as it

results in acidic boiler water. The hardness chloride salts present abundantly in sea water

generate hydrochloric acids at boiler water temperatures.

Uncontrolled large leakages of sea water can cause within hours extensive corrosion (hydrogen

damage) of water wall tubes. There should be no hesitation to shutdown and save the unit if

boiler water specifications, as recommended cannot be maintained during the condenser leakage.Any unit should have an on-line instrument with a cation column at the outlet of condenser to

monitor conductivity continuously and detect immediately any condenser leakage.

AUTOMATIC VALVELESS GARVITY FILTER (AVGF)

Automatic valve-less gravity filter (AVGF) operates automatically, on the loss of head principle.

Around 3 – 5% of CW Pump discharge water is taken into this AVGF system for filtration. It isused as Side Stream Filtration. Unfiltered water falls from a Raw Water Chamber and passes

downward through a filter. After filtration the filter water level increases and enters into

Backwash storage tank and another part of filter water passes through filter water outlet line.

During the process of filtration the filter is gradually chocked by silts, mud, etc. and unfiltered

water level above the filter bed starts increasing. As a result water level increases inside the

backwash pipe line. When the water reaches to its optimum level, backwash starts due to

symphonic action. When the water level inside backwash water chamber falls below the siphonbreaker inlet line backwashing stops and again normal filtration starts.


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When more than one filter is used, the flow is divided equally among all the filters by means of aflow splitting box. In addition, an interlock between filters is provided to prevent more than one

unit backwashing at any time.

3 water treatment plant

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