TRAINING PROGRAMME ON WATER CHEMISTRY

Water Chemistry
25.Jan.2021
Akash Shah
L&T-Sargent & Lundy Limited
Knowledge City Vadodara

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CONTENT
• What is Role of Water in Power Plant?
• Source of Raw water
• How water get contaminated?
• Typical water analysis
• Effect of contaminants on Treatment scheme
• Power Cycle water and steam Chemistry control parameters
• Impact of Improper cycle chemistry control
• Difference between subcritical and supercritical water
• Feed cycle chemical treatment regime
• Condensate polishing plant
• Chemical feed system
• SWAS parameters and action levels
• System configurations of Malwa-II

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What Is Water’s Role ?

• Make up for boiler feed.
• Steam condensing media.
• Water requirement in HVAC System
• Coolant for heat exchange equipment.
• Fire fighting.
• Potable and service/sanitary demands.
• Water requirements in Coal handling plant
• Water requirements in Ash handling plant

Sources of Raw Water

• Surface water from rivers, lakes and sea.
• Sub surface water from underground wells.
• Treated water from Municipal waste
• Combination of River water & bore well water

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How Water Gets Contaminated ?

Global Water Cycle

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Typical Water Analysis

Parameters As River Water Value Bore Well Water Value Sea Water Value

Cations
Calcium ppm as CaCO3 84 268.4 1100
Magnesium ppm as CaCO3 33 158.4 5050
Sodium ppm as CaCO3 86 119.6 22465
Potassium ppm as CaCO3 3 11.3 845
Total cation ppm as CaCO3 206 557.7 29460
Anions
Bicarbonate ppm as CaCO3 141 425.5 98
Sulphate ppm as CaCO3 9 77.8 2712
Chloride ppm as CaCO3 40 29.2 26650
Phosphate ppm as CaCO3 1 0.2 -
Nitrate ppm as CaCO3 14 24.2 -
Fluoride ppm as CaCO3 1 0.8 -
Total Anion ppm as CaCO3 206 557.7 29460
Iron ppm as Fe 0.2 0.04 1
Colloidal Silica ppm as SiO2 2 0 0.5
Reactive Silica ppm as SiO2 10 23 3.3
Colour Pt-Co-Units -- 4 --
pH at 25 ºC 8.7 6.9 7.6
Turbidity NTU 275 Less than 1 68
Chemical Oxygen demand mg/l 20 10 --
Bio-Chemical Oxygen demand
mg/l 3 3.3 9
(for 5 days at 200 °C)
Total suspended solids mg/l 850 4 96
Conductivity at 25°C μS/cm -- 950 47500
Total dissolved solids mg/l 250 595 34000
Total solids mg/lit 1100 599

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Particle Size Scale

NF MF
RO UF Particle Filtration
0.0001 0.001 0.01 0.1 1.0 10 100
MICRON MICRON MICRON MICRON MICRON MICRON MICRON
Atom Molecules Viruses Bacteria Bacteria Pollen Sand
Invisible Visible
Dissolved Suspended
Colloids Settleable Solids

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Design Water Analysis

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Effect of Contaminants on Treatment Scheme

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Power Cycle Water and Steam Chemistry Control Parameters

Control Parameter Units Limiting Conditions
for Feed Water At Economiser Inlet At DM Plant
Outlet
AVT CWT
pH 9.3-9.6 8.5-9.3
Conductivity µS/cm ≤0.25 <0.2 (Target < 0.1) 0.2
Hydrazine mg/l ≥0.01
Dissolved Oxygen mg/l ≤0.007 0.02-0.2
(Set point < 0.05)

Iron mg/l ≤0.01 ≤0.002

Copper mg/l ≤0.002 ≤0.002
Silica mg/l ≤0.02 <0.02 <0.01
Total Organic Carbon ppb ≤300 <300
Sodium mg/l ≤0.005 <0.005

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Power Cycle Water and Steam Chemistry Control Parameters

Control Parameter for Normal Operation Limiting Condition
Steam Two Weeks 24 Hours
Conductivity (µS/cm) <0.3 0.3-0.5 0.5-1.0
Dissolved Oxygen (mg/l or <10 10-30 30-100
ppb) -AVT
Dissolved Oxygen (mg/l or 20-200 Max.200 -----
ppb) – CWT
Sodium (mg/l) <2 2-5 5-10
Chlorides (mg/l) <5 5-10 10-20
Silica (mg/l) <10 10-20 20-50
Copper (mg/l)* <1 Max. 3 -----
Iron (mg/l)* <5 Max. 20 -----
Sulfites and Sulphates* Less than BDL
* Should be analyzed once a week; Rest shall be analyzed continuously.

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Major Contaminants Present in Water & its effects

 Calcium and Magnesium
It increases the scale formation on the surfaces. Deposition on heated surfaces
leads to uneconomic operation and may lead to material over-heating and to
corrosion underneath the scale due to concentration of dissolved solids.
 Sodium and chloride
Carry-over of sodium compounds to the steam can result in super heater and
turbine deposits possibly also in corrosion.
 Silica
The solubility of silica in steam increases with pressure. As steam is cooled by
expansion through the turbine, silica solubility is reduced and deposits are formed
on turbine blades
 Iron and copper
Oxygen corrosion can result in rapid failure of feedwater lines, economizers,
boiler tubes, and condensate lines. Additionally, iron oxide generated by the
corrosion can produce iron deposits in the boiler.

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Major Contaminants Present in Water & its effects

 Carbon dioxide
Carbon dioxide is a common contaminant in the steam water cycle that leads to
an increase of acid conductivity which enhances the corrosive effect of oxygen.
 Sulphates and Carbonates
Sulphates and Carbonates have the potential to form insoluble, adherent,
insulating “hard water” scale deposits on heat exchanger surfaces.

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Impact on Improper Cycle chemistry control

Orifice fouling Deposition in boiler tubes

Boiler tube failure by Boiler tube failure by overheating

thermal fatigue mechanism.
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Impact on Improper Cycle

chemistry control
Turbine blade fouling

Deposition of
corrosion products on
BFP rotor.
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Know the Terms

• Specific Conductivity
• Cation Conductivity
• Magnetite (Fe3O4)
• Hematite (Fe2O3)
• Flow Accelerated Corrosion (FAC)

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Difference between Subcritical and Supercritical Water

 The quality of feed water required for a Super critical plant is

much higher than the sub critical plant.

 No Boiler Drum is Required in Super critical Plant while in sub

critical plant boiler drum is required and some water blowdown
can be done thru that.

 Water Cycle Chemistry control is done using following:

o Ensuring make-up water purity by DM/UF Plant
o Condensate Polishing Plant
o Chemical Feed/Dosing system
o SWAS
o Condenser & (Deaerator?)
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Difference between Subcritical and Supercritical Water

Feed Water Units Supercritical Subcritical
Quality (AVT) (AVT)
pH 8.9-9.2 >9.5
Acid Conductivity µS/cm <0.10 <5
Oxygen µg/kg 5-10 5-10
Silica µg/kg <5 <10
Iron µg/kg <5 <10
Sodium µg/kg <2
Copper µg/kg <1 <1
Organics mg/l <0.1 <0.2

Ref. VGB-R 450 Le

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Condensate Polishing Unit

Condensate Polishing Unit is used in a Power plant to remove the crud & dissolved
solids from the return condensate to make a boiler feed water of desired quality. For
this purpose an extremely highly regenerated resin is required. So resin after
exhaustion is transferred to a separation tank from the service vessel and after
separation, the Cations & Anions resins are regenerated in two separate vessels and
finally they are mixed up before sending in another CPU service vessel.
Each condensate polishing unit consists of Three - service vessel ( 2 W + 1 S i.e. 3 X
50%) with condensate flow 655 m³/hr

Types of Condensate Polishers

 Pre-coat Candle Polishers

 Back-flushable Cartridge Filters

 Deep Bed Polishers

 Magnetic Filters - Polishers

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Condensate Polishing Unit

Typical Process Flow Diagram

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Advantages of using Polishers

 Reduce Cost
• Reduce start-up time after outages
• Reduce heat loss due to fouling of heat transfer surfaces by
preventing transport of iron and/or copper corrosion products
• Extend the life of the power plant
 Improve Boiler and steam chemistry
 Enable power plant to stay on line with a small to moderate size
condenser leak
 Provide time to arrange for orderly shutdown in the event of a large
condenser leak

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Ion Exchange with Cation Resin

R - SO-3 H+ + NaCl R - SO-3 Na+ + HCl

2 R - SO-3 H+ + Ca(HCO3)2 (R - SO-3)2 Ca++ + H2CO3

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Ion Exchange with Anion Resin

Neutral salt
R - N+R’3 OH- + NaCl R - N+R’3 Cl- + NaOH

Strong acid
R - N+R’3 OH- + HCl R - N+R’3 Cl- + H2O

CO2 = weak acid

R - N+R’3 OH- + CO2 R - N+R’3 HCO-3

SiO2 = very weak acid

R - N+R’3 OH- + SiO2 R - N+R’3 HSiO-3

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Control Philosophy For CPU

CPU Service, Standby and OFF mode:
CPU system being a high pressure system, the service vessel cannot be taken into service immediately. Hence,
the stages of CPU off, CPU standby and CPU service are required.
When CPU vessel remain in un-pressurized condition its called as CPU off mode. To take CPU vessel into a
service mode, it is required to pressurize the service vessel and it is done by opening the solenoid valve provided
across the CPU auto inlet valve. When the differential pressure across the auto inlet valve comes below 2.0
kg/cm² then inlet auto valve gets open and the pressure inside the service vessel become equal to inlet
condensate line pressure and this condition is called CPU standby mode.

When auto outlet valve of CPU gets opened the mode of operation changes from stand by to service and CPU
starts performing to give desired quantity and quality of water.
Again when it is required to isolate the CPU service vessel from service mode, first it required to put the service
vessel to standby mode by closing the service vessel auto outlet valve then the vessel will be depressurized &
comes from standby to off mode.

Condensate Bypass Control :

Irrespective of any mode of operation, Bypass control Valve will play the role of Condensate flow diversion.
When no single vessel is in operation, total condensate inlet will get bypassed.
When one vessel is in service, 50% of condensate inlet flow will get bypassed and also this Control valve will
control the condensate differential pressure.
And when two service vessels are in service, ideally control valve will not bypassed condensate flow but it may
control the condensate inlet outlet differential pressure only.

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Control Philosophy For CPU

The following parameter will check before CPU taken in service:
Specific Inlet conductivity < 2.0 μS/cm
Inlet condensate temperature < 60 ºC
Bypass valve will open during high temperature of condensate

RESIN REGENERATION:
Conductivity at the outlet of CPU under rinse will check with respective Cation conductivity
meter. When the conductivity goes above 0.1 μS/cm the regenration of resin will start andrinse
will continue till the conductivity goes below 0.10 μS/cm. Once conductivity of CPU outlet
condensate (under rinse) below 0.10 μS/cm then it will come to standby mode.

EXHAUSTED RESIN TRANSFER FROM CPU TO SPT:

This stage describes exhausted resin (service end) need to be regenerated externally.
Hence, it is to be transferred from CPU vessels to resin separation tank SPT.

TRANSFER OF THE REGENERATED MIXED RESIN FROM CRT TO CPU:

One fresh charge of regenerated mixed resin is always stored in Cation regeneration tank cum
mixed resin storage tank. Once the resin transfer operation from service vessel to SPT is over.
Now the regenerated mixed (from previous batch) will be transfer to that empty service vessel.

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External Regeneration System

Resin Transfer is required for physical cleaning and chemical regeneration of exhausted
resins. The sequence, “transfer of exhausted resin from CPU service vessel to SPT in
regeneration area and transfer of fresh resin from CRT in regeneration area to empty
CPU service vessel” will be done.

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Resin Regeneration Process Flow Path

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Typical Installations

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Typical Installations

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Typical Installations

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Feed Cycle Chemical Treatment Regime

Why Chemical Treatment is Required ?

 To ensure availability and reliability of Power plant
By means of:
 Reduction in Corrosion potential
 Reduction of Deposition products

Types of Chemical Treatment

 All Volatile Treatment (AVT)
 Oxygenated Treatment (OT)
 Caustic Treatment (CT)
 Phosphate Treatment (PT)

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Comparison of OT and AVT

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Comparison of OT and AVT

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All Volatile Treatment (AVT)

The optimum pH level should be there in the condensate/feed water train upstream of the
low pressure heaters to ensure maximum corrosion protection. Where a condensate
polishing plant is installed, ammonia must therefore be dosed upstream of the low
pressure heaters.
Hydrazine dosing is done either at Deaerator outlet or CPU discharge.

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Schematic Representation of Oxide Formed on Ferrous Feed

water Surfaces During Operation with AVT(R)

(Outer layer)

(Inner layer)

• Fe = Fe2+ + 2e-
2H2O + 2e- = 2OH- + H2
• Fe2++ OH- = Fe(OH)+
2Fe(OH)+ + 2H2O = 2Fe(OH)2+ + H2
• Fe(OH)+ + 2Fe(OH)2+ + 3OH- = Fe3O4 + 4H2O
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Oxygenated Treatment

The concentration of iron in the feed water is reduced by adding both gaseous oxygen
and ammonia. The alkalizing agent promotes the oxidation process of Fe (II) to Fe (III)
in water containing oxygen and protects against the harmful effect of anions on the
protective layer.
Usually oxygen dosing is done at CPU outlet. However, a connection is provided at
Deaerator outlet.

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Schematic Representation of Oxide Formed on Iron-Based

Feedwater Surfaces During Operation with AVT(O) and OT

◊ 3Fe2+ + ½ O2 + 3H2O = Fe3O4 + 6H+

◊ 2Fe3O4 + H2O = 3Fe2O3 + 2H+ + 2e-
◊ Fe3O4 + 2H2O = 3FeOOH + H+ + 2e-
◊ Fe3+ + 3OH- => Fe(OH)3 => Fe2O3-nH2O => FeOOH=> Fe2O3
(AGING) (DEHYDRATION)
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Caustic and Phosphate Treatment

The pH of the boiler water must be maintained within a specific range in order to
minimize the solubility of the magnetite and to counteract the effect of contaminants,
which reduce the pH and can be carried over with the feed water and concentrated in
the boiler.
The alkalizing agents maintain a pH above 9 in the condensate/feed water area to
achieve satisfactory boiler water alkalinity and pH.
The alkalizing agent are usually sodium hydroxide or tri-sodium phosphate which are
added to the feed water downstream of the spray water tapping point.
Alternatively, the solid alkalizing agents can be dosed into down comers or to
the boiler drum

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Chemical dosing system

• Ammonia dosing system

• Hydrazine dosing system
• Oxygen dosing system

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Steam and Water Analysis System

 SWAS provide continuous online Analysis of key Parameters
of Water and Steam.
 It provides signals to control the feed rate of Chemical dosing
systems.
 It alert operators to Potential harmful deviations from desired
water quality.

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What conditions SWAS can detect ?

 Healthy condition of power cycle

 Condenser tube leakage
 Improper functioning of Chemical dosing system
 Air leakages in low pressure side of feed cycle
 End of Service run of CPU
 Quality of make-up water
 Acceptability of feed water for operation in closed loop

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Major Components
 Primary Cooler
 Secondary Cooler
 Chiller
 Pressure Reducing Station
 Back Pressure Regulator / Relief Valve

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System Scheme-SWAS

GSC

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CEP Discharge (Before CPU)

Parameter

Condenser
Sodium, ppb

Dissolved Oxygen, ppb

pH

Specific Conductivity, μS/cm

CEP
Cation Conductivity, μS/cm

GSC

CPU
LP Heaters

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Makeup Water Inlet

Parameter Makeup
Treatment
Cation Conductivity, μS/cm System

Specific Conductivity, μS/cm

CST
Condenser

Condenser Hotwell

Parameter

Specific Conductivity, μS/cm

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CEP Discharge (After CPU)

Parameter

Condenser
Sodium, ppb

Dissolved Oxygen, ppb

pH

Specific Conductivity, μS/cm

Cation Conductivity, μS/cm CEP

GSC

CPU
LP Heaters

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Parameter

Dissolved Oxygen,
ppb

Specific Conductivity,
μS/cm
Boiler
pH

Cation Conductivity,
μS/cm

Hydrazine, ppb

Silica, ppb

HP Heaters

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HP Steam (Before HP Bypass)

Parameter

HP Specific Conductivity
Turbine
Cation Conductivity

Chloride
Silica
Dissolved oxygen
RH

SH

Sodium

Steam Water Separator (Steam Side)

ws ws
WSDT

Parameter

Cation Conductivity, μS/cm

Boiler
To Condenser

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HP IP LP Turbine BFPT
Turbine Turbine
RH

SH

ws ws
WSDT

Reheat Steam (Before LP Bypass)

Boiler Parameter
To Condenser

Cation Conductivity, μS/cm

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Sampling and Analysis P&ID

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SWAS P&ID

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References

• EPRI’s Cycle Chemistry Guidelines for Fossil Plants: All-

Volatile Treatment

• EPRI’s Cycle Chemistry Guidelines for Fossil Plants:

Oxygenated Treatment of EPRI

• VGB’s Guidelines for Feed Water, Boiler Water and Steam

Quality for Power Plants / Industrial Plants

• Modern Power Station Practice of British Electricity

International (BEI)

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