PURPOSE OF BOILER WATER TREATMENT

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PURPOSE OF BOILER WATER TREATMENT

• Failure Prevention
• Deposit Control
• Corrosion Minimisation
• Maximising Steam Purity

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• CORROSION Corrosion is the deterioration of metal
  by chemical or electrochemical reaction with its
  environment. Corrosion products that are formed
  in the boiler or transported to the boiler can
  deposit and impede heat transfer, causing tube
  metal overheating. They can also act as a
  concentrating mechanism for boiler water salts,
  yielding metal loss by corrosion.
• TYPES OF CORROSION
• Oxygen (refer to Oxygen Treatment)
• General corrosion of iron and copper
• Embrittlement
• The lowering of normal metal ductility due to
  physical or chemical change.
• Chemical (acid, caustic, chelant)
• Erosion (high velocity fluid flow, cavitation)

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• FACTORS INFLUENCING IRON AND COPPER SOLUBILITY
• pH
• Temperature
• Flow
• CORROSION REACTIONS OF IRON AND COPPER
• 3Fe0 4H2O ----------gt Fe3O4
  4H2
• IRON WATER OR STEAM
  MAGNETITE HYDROGEN GAS
• 4Fe0 H2O 3O2
  ----------gt 2Fe2O3 H2O
• IRON WATER OR STEAM OXYGEN
  HEMATITE WATER
• 8Cu O2 2H2O ----------gt
  4Cu2O 2H2
• COPPER DISSOLVED WATER
  CUPROUS HYDROGEN
• OXYGEN OXIDE
  GAS

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HIGH OR LOW BOILER WATER pH CORRODES BOILER STEEL
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RELATIVE CORROSION RATE OF COPPER ALLOYS AND
CARBON STEEL VS pH
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CAUSTIC EMBRITTLEMENT (STRESS CORROSION
CRACKING)

• A form of intergranular cracking originating at
  the surface of the metal.
• High stress level in steel
• Concentration mechanism for caustic in the water
• Leak
• Cracks
• Water must be embrittling
• Lack of natural inhibition (sodium nitrate)

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CRACKS at the ROLLED END OF A TUBE CAUSED BY
CAUSTIC EMBRITTLEMENT
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CAUSTIC CORROSION

• Results from the concentration of caustic soda
  beneath scale deposits or as a result of steam
  blanketing.
• Steam blanketing is a condition which permits
  stratified flow of steam and water, usually
  occurring in a high heat input zone such as a
  horizontal or inclined roof tubes
• Dissolves the protective magnetite layer.
• Iron restores the magnitite layer, only to be
  dissolved again.
• Typically characterised by irregular longitudinal
  patterns, often referred to as gouges.

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HYDROGEN EMBRITTLEMENT

• Usually at higher pressure (gt105 kg/cm²)
• Usually associated with low pH excursions
• Formation of nascent hydrogen (HO) at the boiler
  tube surface
• Hydrogen permeates the tube where it can react
  with
• Iron carbides to form methane gas
• Other hydrogen atoms to form hydrogen gas
• Gases collect at grain boundaries to increase
  pressure
• Microfissures occur within the material and
  weaken the tube
• Brittle failure - often blows out a window

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TYPICAL WINDOW FAILURE CAUSED BY HYDROGEN
EMPRITTLEMENT
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ACID CORROSION

• General thinning of all surfaces in contact
• Visually irregular surface appearance
• Smooth surface where flow has allowed the attack
  to intensify
• Typical Contaminants
• Process material or organic substances reverting
  to acids
• Cation regeneration acids
• Raw water depleting the alkalinity in the boiler
  and in high purity systems, producing acid
  conditions by magnesium salt hydrolysis and
  magnesium hydroxide precipitation

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ACID CORROSION IN A BOILER TUBE
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Phosphate/pH/Coordination PROGRAMME

• OPTISPERSE HP series products
• Usage of phosphates as buffer
• Usage of polymers to keep surfaces clean
• OPTISPERSE HTP series products

HTP - 2 polymer
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METAL CONDITIONING

• IRON
• Formation of dense magnetite inner oxide layer -
  inhibits ionic and charge transfer
• Formation of stable magnetite outer layer -
  corrosion products are released slowly
• Corrosion is self-limiting

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MODEL OF OXIDE LAYERS ON IRON /WATER INTERFACE
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Caustic Corrosion

• Chemistry involved
• 4 NaOH Fe3O4 2 NaFeO2
  Na2FeO2 H2O
• Fe 2 NaOH Na2FeO2
  H2
• 3 Fe 4 H2O Fe3O4
  4 H2
• Na2FeO2 2 NaFeO2 H2O 4
  NaOH Fe3O4

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Caustic Corrosion

• The concentration of NaOH in the boiler water is
  normally not high enough to create corrosion
• Concentration of NaOH
• Under deposits
• Places where departure from nucleate boiling
  occurs
• At grooves and cavities where steam escapes
• Places where water is injected to cool steam

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magnetite
Caustic Corrosion
steam
escapes
porous
deposit
Na
OH -
OH -
OH -
Na
Na
Na
OH -
Na
OH -
Boiler water in
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Under Deposit Wick Boiling
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pH/PO4 coordinationBoiler Internal Treatment
Programme

• Provides corrosion control in boilers with high
  purity water
• Make-up water quality is demineralised
• Both pH and phosphate concentrations are
  monitored and controlled
• The control ranges for pH and phosphate depend on
  the boiler pressure
• Optimum programme for high pressure boilers with
  demineralised make-up
• Industry standard
• Most boiler manufacturers recommendation
• ASME Consensus recommendation

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2 Na2 HPO4 NaOH Na3PO4
Na2HPO4 H2O
Na2 HPO4 NaOH
Na3PO4 H2O
Na2 HPO4 2 NaOH Na3PO4
NaOH H2O
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magnetite
Prevention
steam
escapes
porous
deposit
HPO42 -
Na
Na
HPO42 -
Na
HPO42 -
Na
HPO42 -
Na
HPO42 -
boiler water in
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Coordinated pH/PO4 Control Graph
1
2
3
4
5
6
7
8
10
20
30
40
50
10.6
10.6
10.4
10.4
Free Caustic
Region
10.2
10.2
10.0
10.0
9.8
9.8
9.6
9.6
9.4
9.4
9.2
9.2
9.0
9.0
8.8
8.8
8.6
8.6
Captive Alkalinity
Region
8.4
8.4
8.2
8.2
1
2
3
4
5
6
7
8
10
20
30
40
50
Boiler Water ppm of O-PO4
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Chemical Feed and Control Options
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Chemical OptionsHigh Pressure Boiler Chemistry
Internal Treatment

• Powders
• Powders/Dispersant
• Single Liquid
• Dual Up/Down Approach
• Optisperse

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What are our Options?Powders

• With powders, there is no choice but to utilise
  batch makedown facilities (day tank)
• Separate dispersant is usually fed based on
  feedwater/steam flow
• Difficult to maintain desired control unless
  feedwater quality is excellent or at least
  consistent

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Powdered PhosphatesThe Stone AgeKey
Application Problems

• Each phosphate has a different PO4 content and
  Na/PO4 ratio
• Dusting when handling
• Non-characterised - all phosphates look the same

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pH/PO4 Program Control
1. Manually measure PO4 and pH 2. Visually
compare the results to the control limits
3. Logically decide on the appropriate
changes for the system 4. Empirically determine
the magnitude of the change CRITERION FOR
SUCCESS OF TIME IN THE BOX
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Manual Control Algorithms
PO4 pH Recommended Action OK OK No changes OK
low Add NaOH OK high Increase BD, Substitute
mono-Na PO4 low OK Add di-Na PO4 high OK Increase
BD, Substitute di-Na PO4 low low Decrease BD, Add
tri-Na PO4 high high Increase BD, Substitute
mono-Na PO4 low high Add mono-Na PO4 high
low Add NaOH, Increase BD
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Manual Control Procedure

• Test the boiler water PO4 and pH

• Graph the results on the coordinated control chart

• If the PO4 and/or pH are not within the control
  limits, make a decision regarding the correct
  change
• Vary the daytank concentration of PO4
• Vary the type of PO4 (mono, di, tri)
• Vary the blowdown flowrate
• Vary the caustic feedrate

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Manual Control Procedure

• Empirically determine the magnitude of the change

• Repeat the process in an 8 or 12 hour time period

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Conclusion
The magnitudes of change in blowdown rate,
caustic feedrate, type of phosphate, sodium to
phosphate ratio, are empirically determined
frequent manual adjustments are required
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The Iron Age

• Single liquid product
• Fed from day tank
• Some of the same problems exist except we have
  better control of cycles and polymer
• Caustic feed an issue

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Why its difficult to stay In The Box

• Many independent variables
• day tank concentrations
• pump stroke adjustment
• steaming rate
• feedwater quality
• demineraliser sodium leakage
• condensate quality
• percent condensate return
• blowdown flowrate adjustment
• Two interdependent variables
• pH
• phosphate

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The Modern Age

• Dual liquid formulations
• Up/Down fed in day tank (good)
• Up/Down fed via seperate pumps (better)
• OPTISPERSE HP series
• Same phosphate level
• Same dispersant level
• Different Na/PO4 ratio

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Traditional Polymer Structures
CH
CH2
CH2
CH
CH
CH2
C O
C O
C O
O-
OH
NH2
X
X
POLYACRYLATE
ACRYLATE-ACRYLAMIDE COPOLYMER
CH3
CH2
CH
CH
CH
C
CH2
C
C
CO
O
O
O
SO-3
Y
X
O-
X
POLYMETHACRYLATE
SULFONATED STYRENE-MALEIC ANHYDRIDE COPOLYMER
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HTP-2 Polymer Structure
CH3
C
CH2

O
P
OH
O-
X
Poly (isopropenyl phosphonic acid) . . . PIPPA
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Iron Chemistry - Magnetite FormationTwo
Distinct Magnetite Layers

• Particulate Deposition
• Outer Layer
• Inner Layer
• Base Metal

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Magnetite FormationInner Layer

• 3 Fe 4 H2O Fe3O4
  4H2
• iron water
  magnetite hydrogen
• self limiting, tenacious, protective oxide
• does not impede heat transfer

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Magnetite Formation Outer Layer

• Formed from Fe2 diffusion through inner layer
• Porous, less tenacious, non-protective
• Contributes to deposit weight density (DWD)
• Does impede heat transfer

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MODEL OF OXIDE LAYERS ON IRON / WATER INTERFACE
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Deposition Layer

• Deposited particulate material
• Typically iron (in high purity systems)
• Porous, loose and non-protective
• Contributes to DWD
• Impedes heat transfer

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How Boiler Polymers Function on Existing
Deposition

• Boiler polymers are surface/particle active
• Some polymers show clean-up activity on particle
  deposition and the outer porous magnetite layer
• Polymers have no impact on the tenacious inner
  magnetite layer

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Characteristics of HTP-2

• A unique new phosphorylated boiler polymer
• Particularly effective on iron
• Demonstrated clean-up ability
• Designed for high purity/high cycles systems
• Suitable for use up to 125 Kg /cm²

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Characteristics of HTP-2

• Where can HTP - 2 be used
• High purity feed water systems / High cycles
• Iron is the main source of contamination
• running on a phosphate/pH/coordination programme
• High pressure system 63 - 105 kg/cm²(max. 125
  kg/cm²)
• The phosphate level limits the usage
• Lower pressure boilers lt 42 kg/cm²
• with demin water and known iron problems
• becomes expensive at lower cycles

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Characteristics of HTP-2

• Level of Phosphate
• The dephosphorylation depends on
• Pressure
• Cycles / Residence time
• Computer Simulation

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Characteristics of HTP-2

• Dephosphorilation releases phosphate for
  pH / Coordination
• Efffective dispersant eliminates deposits

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Can You Summarise for Me?

• Get away from powders now
• If feedwater quality dictates ,use single product
  and neat feed .
• Use Up/Down in all possible locations for maximum
  flexibility
• Use OPTISPERSE HTP series when there is an Iron
  problem

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Key High Pressure Concern
PHOSPHATE HIDEOUT
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Phosphate Hideout -What is it?

• Broadly defined any mass loss of phosphate from
  the boiler water
• Technically defined an interaction of phosphate
  with the boiler metal or metal oxide deposition
• Operationally defined a key problem knocking us
  out of the box at times when we don't have time
  or the ability to respond

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Phosphate HideoutWhat Types Are There?

• Related Type
• Dry-out
• Classic Types
• Incongruent hideout
• Congruent hideout

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Dry-out

• Not a true phosphate hideout problem but related
  in terms of PO4 mass loss and start-up/shut down
  chemistry
• A function of poor circulation due to over
  capacity, mechanical design or internal
  deposition problems
• Most significant impact is the increase in tube
  metal temperature due to deposition

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Dry-out

• Dry-out return is seen with a change in flow
• Typically the change in the boiler water is
  consistent with the amount solubilised and the
  mole ratio of the deposit at the time of dry-out

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Phosphate Hideout

• The classic phosphate hideout mechanism relates
  to the change in the solubility of phosphate
  during conditions of increased heat transfer
  (flux) and temperature
• PO4 solubility drops and reactions (adsorption)
  with tube metal magnetite and/or some deposition
  occur
• Hideout is boiler, temperature, pressure and
  system specific

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Phosphate HideoutWhat causes it ?

• Increase in load
• Start-up conditions
• Load swings/process demand
• Increase in tube metal temperatures/heat transfer
• Load increase or change
• Fuel change

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Classic Incongruent Phosphate Hideout - What
Happens?

• During initial hideout PO4 drops and pH
  increasesWhy?
• Less PO4 for a given NaOH inventory

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Classic Incongruent Phosphate Hideout -What
Happens?

• During reduced heat flux and load conditions or
  shutdown phosphate species come back into
  solution - PO4 increases and pH decreasesWhy?
• Depleted or same NaOH inventory and higher PO4

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Classic Incongruent Phosphate Hideout Reactions
Hideout Conditions
pH
PO4
PO4
Na
PO4
NaOH
-
Na
PO4
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Classic Incongruent Phosphate Hideout Reactions
Hideout Conditions
Hideout Return Conditions
pH
Na
PO4
PO4
PO4
Na
PO4
pH
PO4
Na
PO4
NaOH
-
NaOH
Na
PO4
PO4
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Incongruent PrecipitationWhat are the Risks?

• Caustic Corrosion
• Acid phosphate attack can occur accelerated if
  acidic phosphates are fed to counteract the high
  pH/low PO4 (i.e. hideout) conditions

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Acid Phosphate Corrosion How is it Identified?

• Deposit Chemistry/morphology
• Confirmation of the presence of maricite
• In some cases it has been confused with caustic
  corrosion

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Classic Congruent Phosphate Hideout - What
Happens?

• During initial hideout PO4 drops and pH does not
  change or gradually decreasesWhy?
• Congruent loss of PO4 from boiler water

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Classic Congruent Phosphate Hideout Return What
Happens?

• During reduced heat flux, load conditions or
  shutdown, phosphate species come back into
  solution - PO4 increases and pH IncreasesWhy ?
• Congruently precipitated phosphate comes back
  into solution. pH and PO4 increase consistent
  with molar ratio

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Classic Congruent Phosphate Hideout Reactions
Hideout Conditions
pH
PO4
PO4
Na
Sodium change consist with deposit mole ratio
change
PO4
Na
PO4
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Classic Congruent Phosphate Hideout Reactions
Hideout Conditions
Hideout Return Conditions
pH
Na
PO4
PO4
PO4
Na
PO4
pH
PO4
Sodium change consist with deposit mole ratio
change
Na
PO4
Sodium change consist with deposit mole ratio
change
Na
PO4
PO4
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Congruent HideoutWhat are the Risks?

• Deposition and tube metal temperature increase
• Not a significant threat re corrosion

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Phosphate Hideout -How Can We Cope?

• Keep the boiler clean - the more iron present the
  more phosphate uptake and degree of chemistry
  change
• Carry no more PO4 in the boiler water than needed
  to stay in congruent control - dictated by FW
  quality and chemical feed system capability

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Phosphate HideoutHow Can We Confirm?

• Accurate mass balances - chemical feed, feedwater
  and blowdown flow
• Without blowdown flow measurement use traced
  products or add ReckonCycle to batch
• Many operating artifacts can present hideout
  like symptoms

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Phosphate Hideout -How is Industry Responding?

• Reducing PO4 control levels to minimise the
  impact of hideout on boiler chemistry
• Equilibrium Phosphate Treatment (EPT) or
  Phosphate Treatment (PT)
• Living with the problem as a normal event

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Advancements in Phosphate Treatments
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Phosphate Treatments

• Coordinated Phosphate-pH Treatment
• Congruent Phosphate Treatment CPT
• Equilibrium Phosphate Treatment EPT
• Phosphate Treatment PT

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Coordinated Phosphate-pH Treatment

• Chemical Controls
• Hydroxide sodium to phosphate mole ratio
  between 2.2 to 1.0 and 3.0 to 1.0

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Coordinated Phosphate-pH Treatment

• Buffers the boiler water
• Acid or caustic corrosion possible
• Signs of trouble are phosphate hideout with pH
  excursions

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Congruent Phosphate Treatment CPT

• Chemical Controls
• Hydroxide to phosphate mole ratio between 2.2 to
  1.0 and 2.6 or 2.8 to 1.0

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Congruent Phosphate Treatment CPT

• Buffers boiler water
• Controls caustic corrosion
• Acid corrosion possible

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Congruent Phosphate Treatment CPT

• Signs of trouble
• Phosphate hideout and increasing pH with rising
  load
• Phosphate return and decreasing pH with falling
  load

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Equilibrium Phosphate EPT

• Chemical Controls
• Phosphate less than that soluble in the boiler
  water at maximum load
• minimum pH of 9.0
• Hydroxide to phosphate mole ratio a minimum of
  2.8 to 1.0
• Less than 1.0 ppm of sodium hydroxide

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FIGURE 3.7-4 CONTROL DIAGRAM FOR A pH/PHOSPHATE
TREATMENT
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Equilibrium Phosphate Treatment EPT

• Trisodium phosphate and caustic treatment only
• Adjust pH controls for the amine or ammonia
  present

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Equilibrium Phosphate Treatment EPT

• Controls phosphate hideout
• Controls acid phosphate corrosion
• Eliminates boiler blowdown?
• Is caustic corrosion possible?
• Any impact on steam turbines?

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Phosphate Treatment PT

• Chemical Controls
• Phosphate of 3 to 10 ppm
• Minimum hydroxide sodium to phosphate mole
  ratio of 2.8 to 1.0
• Less than 1.0 ppm of sodium hydroxide
• pH less than 10.0

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FIGURE 3.7-4 CONTROL DIAGRAM FOR A pH/PHOSPHATE
TREATMENT
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Phosphate Treatment PT

• Controls acid phosphate corrosion
• Protects against low level feedwater feedwater
  contamination
• Is caustic corrosion possible?