Boiler Water Chemistry

Pre-Treatment

BoilerProcess

Process

Process Low pressure steam Blowdown flash tank Flash tank Condensate Receiver

Nalco Chemical Co.

Major Problems

Corrosion

Scale

SolubilitiesCompound Calcium Bicarbonate Carbonate Sulfate Magnesium Bicarbonate Carbonate Sulfate Sodium Bicarbonate Carbonate Chloride Hydroxide Sulfate ppm as CaCO3 o o o o 32 F (0 C) 212 F (100 C) 1620 15 1290 decomposes 13 1250

37,100 101 170,000

decomposes 75 356,000

38,700 61,400 225,000 370,000 33,600

decomposes 290,000 243,000 970,000 210,000

Scale Formation Mechanisms

Precipitation of insoluble hardness Ca(HCO3 )2 + HEAT ---> CaCO3 + H2O + CO2

Mg+2 + OH- ---> MgOH+H2SiO3 ---> H+ + HSiO3MgOH+ + HSiO3- ---> MgSiO3 + H2O

Exceeding saturation through evaporation, resulting in crystallization eg. CaSO4, SiO2

Found in Boiler DepositsName Acmite Analcite Anhydrite Aragonite Basic magnesium phosphate Brucite Calcium hydroxide Calcite Copper Cuprite Ferrous oxide Goetnite Gypsum Hematite Hydroxyapatite Magnetite Serpentine (magnesium silicate) Sodium ferrous phosphate Tenorite Thenardite Xonotlite Formula Na2OFe2O34SiO2 Na2OAl2O34SiO22H2O CaSO4 CaCO3 (gamma form) Mg3(PO4)2Mg(OH)2 Mg(OH)2 Ca(OH)2 CaCO3 (beta form) Cu Cu2O FeO Fe2O3 H2O (alpha form) CaSO42 H2O Fe2O3 Ca10(PO4)6(OH)2 Fe3O4 3MgOSiO22 H2O NaFePO4 CuO Na2SO4 5 CaO5 SiO2 H2O

Silica

Forms deposits in boilersOccurs as magnesium silicate or silicic acid

Selective silica carryover Cannot be controlled mechanically by steam separators Not usually a problem with boilers with less than 600 psig pressure

Selective Silica Carryover

Silica is selectively dissolved into the steam Controlled by limiting the silica concentration in the boiler water Controlled by limiting boiler pressure Controlled by maintaining high pH

Forms of Iron ScaleIron is usually found in a boiler as one or more of the following:

A complex with calcium A complex with phosphate

HematiteMagnetite

Fe2O3Fe3O4

Effect of Scale on Heat Transfer

Effect of Scale on Tube Temperature

Effect of Scale - Example

Effect of Scale - Example

Temp Drop Across Water Film = 76o F. Temp Drop Across Internal Scale = 362oF. Temp Drop Across Tube Wall = 66o F.

Max. Tube Temp (1004o F.) is above allowable Oxidation Temp limit of SA-210 Carbon Steel

Assume CaSO4 scale (0.024)Thermal Conductivity = 10 BTU/ft2-hr-o F/in

Scale Problems

Boiler tube failure Caused by reduced heat transfer and tube overheating

Under-deposit corrosion Caused by high concentration of corrosive agents (usually NaOH)

Scale Prevention

Precipitation of hardness in the boiler Reduce amount of hardness entering boiler

Keep the hardness soluble

Coagulation TreatmentsPrinciples

Calcium hardness is precipitated as Calcium CarbonateMagnesium hardness is precipitated as Magnesium Hydroxide or Magnesium Silicate

Coagulation Treatment ApplicationsA Coagulation Program Can Be Used When:

Boiler pressure < 350 psig

Feedwater hardness > 60 ppmBoiler alkalinity < 500 ppm

Coagulation TreatmentsAdvantage

Can treat very high feedwater hardness

Disadvantages

Low cycles

High boiler TDSHigh blowdown - wastes heat Scale formation

Treatment With Makeup SofteningRequirements for Success

Proper Operation and Maintenance of Make-up Equipment

Chemical Conditioning for Residual Hardness

Current Treatment TechnologiesThere are three technologies in common use today:

Phosphate residual Chelants All-polymer treatment

Phosphate ResidualChemistry

Precipitates calcium as calcium phosphate Precipitates magnesium as magnesium hydroxide

Phosphate TechnologyTypes of PhosphatesOrtho phosphates

Mono-, di-, tri- sodium phosphates

Poly Phosphates

Sodium hexa meta phosphate Sodium hepta meta phosphate

Sodium tripoly phosphateTetra sodium pyro-phosphate

Phosphate TechnologyFeedpoints

Ortho phosphate Feed to boiler drum directly

Poly phosphateFeed to feedwater line

If feedwater hardness is > 3 ppm, feed both to steam drum

Phosphate TechnologyDetailed Chemistry

(poly)(ortho)

Na5P3O10 + 4NaOH -> 3Na3PO4 + 2H2ONa2HPO4 + NaOH -> Na3PO4 + H2O

3CaCO3 + 2Na3PO4 -> Ca3(PO4)2 + 3Na2CO3

3CaSO4 + 2Na3PO4 -> Ca3(PO4)2 +3Na2SO4Mg(HCO3)2 + 4NaOH -> Mg(OH)2 + 2Na2CO3 + 2H2O MgCl2 + 2NaOH -> Mg(OH)2 + 2NaCl

Phosphate TechnologyAdvantages

Easy to monitor and controlDoes not require high purity makeup Can be used at high pressure

Well understoodCan be FDA and/or USDA approved Can handle feedwater hardness fluctuations

Residual PO4 is non corrosiveLarge PO4 residual - buffer for excursions Relatively low cost ingredients

Phosphate TechnologyDisadvantages

Produces precipitates in boiler water Excess alkalinity can produce corrosion May require more blowdown

If so, more heat lost, more chemical usedPossibility of scaling Normally used with sludge dispersant

Sludge Conditioning

Insoluble Calcium Phosphate and Magnesium Hydroxide solids formed (Sludge) Particulate Iron Oxide returned in condensate Solids settle on hot boiler surfaces Heat transfer impaired, tube failure risk

Types of Sludge Conditioners

Synthetic polymers Tannins Lignins Starches

Use of Sludge ConditionersStarch Organic Products

When Mg:SiO2 ratio < 2

When oil contaminates the boilerIn food processing plants

Lignin Organic ProductsTo condition Calcium Phosphate & Iron Oxide

Feed & Control of Sludge ConditionersFeed to feedwater as far ahead of boiler as possible. preferred addition points are:

Deaerator storage Boiler feedwater line Direct to steam drum

Phosphate-Polymer Programs

Affects Calcium & Magnesium precipitation Same precipitation chemistry as other phosphate programs but different dosage requirements for sludge conditioner & phosphate Provides cleaner boilers

Application of Phosphate-Polymer Programs

Feedwater hardness less than 3 ppm

Softeners or naturally low hardness

Phosphate-Polymer ProgramsAdvantage

Can provide much cleaner boilers than other conventional Phosphate programs

Disadvantage

Requires much stricter control of feedwater hardness and chemical program

Chelants

Act on dissolved metal ions Create very soluble complexes

Competing ions (PO4, SiO2, OH) reduce effectiveness

Common ChelantsEDTA (Ethylene diamine tetracetic acid)

Has 6 metal complexing sites which include nitrogen and oxygen atoms NTA (Nitrilo triacetic acid)

Has 4 metal complexing sites

Comparison of Chelants

NTA is more thermally stable 900 psig max. for NTA, 600 psig max. for EDTA

NTA has lower cost than EDTAEDTA chelates Magnesium better than NTA

EDTA chelates ferrous iron better than NTAEDTA has full FDA approval

Application of Chelants

Must be fed continuously to feedwater using a stainless steel injection quill & piping Oxygen must be absent

Residual concentration must be kept below 10 ppm as CaCO3 in boiler water to minimize corrosion Accurate feed control is required

Chelant Control RangesBoiler Pressure psig (Bar) 400 401 - 600 601 - 1000 (30) (30 - 40) (40 - 70) Chelant Residual ppm as CaCO3 4-8 3-6 3-5

Chelant Advantages

No precipitates formed Heat transfer surfaces cleaner Less frequent acid cleaning Can sometimes reduce blowdown

Chelant Disadvantages

Cost more than phosphates Require stricter control of feedwater quality More difficult control test Excessive residuals are corrosive Competing ions can form deposits

All-Organic Polymer Programs

All-polymer program, polymeric blend Contains no chelants or phosphates, does not require supplemental dispersants

Functions by solubilization for Calcium and Magnesium and by dispersancy for iron and other particulatesNon-aggressive to boiler metals

All-Organic Polymer Programs

Feed to deaerator storage for boilers at < 600 psig and using softened water Use other feed points for high pressure boilers using high purity (e.g. demineralized) makeup

Feed program based on statistical upper control limit for hardness and iron, not average valuesUnder dosing ( Fe3O4 + H2 + 2H2O

Rate of Magnetite Formation Is:

Temperature dependent Spontaneous above 180 oC

Effect of pH on Boiler Corrosion

Types of Caustic Damage in BoilersThere are two forms of damage caused by caustic soda

to high pressure boilers, namely:

Caustic corrosion Caustic embrittlement

Caustic Corrosion

Usually found only in high pressure boilersProblem usually due to deposits

Localized in boilerAlso called crater attack or caustic gouging

No embrittlement of metal

Requirements for Caustic CorrosionTwo conditions are necessary for caustic corrosion to occur:

The presence of a corrosive material in the boiler water (caustic soda) A mechanism for concentrating this material

Concentrating MechanismsThe following conditions can result in dangerously high localized caustic soda concentrations

Porous metal oxide deposits

Metal oxide depositsOperation above rated capacity Excessive rate of load increase

Excessive localized heat inputLocalized pressure differentials Restrictions in generating tube(s)

Prevention of Caustic CorrosionPrevention of caustic corrosion is achieved by minimizing

or eliminating the presence of free caustic soda in theboiler water.

Coordinated phosphate Congruent sodium phosphate Phosphate-low hydroxide (tri-ad) Equilibrium phosphate control

All-volatile treatment

Coordinated Phosphate

Control of pH comes from hydrolysis of trisodium phosphate in water Na3PO4 + H2O -> Na2HPO4 + NaOH

Molar ratio of sodium : phosphate is 3 : 1 in water Feedwater contamination usually dictates caustic-consuming chemicals, such as disodium and trisodium phosphate Does not ensure absence of caustic under concentrating conditions

Coordinated pH/ Phosphate Control Limits

Nalco Chemical Co.

Congruent Control

This program was developed to prevent free caustic in boiler water during concentrating conditions At sodium:phosphate ratio of 2.85 in boiler water, precipitated solids have same concentration Safe range is between ratio 2.3 - 2.6 Control is based on pH and PO4 values

Tri-Ad Programs

Boiler water contains low level of caustic soda

Also called PRECISION CONTROLUseful when traces of hardness in feedwater

Greater risk of caustic corrosionNot for high heat flux boilers

Not for pressure greater than 1800 psig

Equilibrium Treatment

Coordinated and congruent treatments can be difficult to control Phosphate hideout interferes PO4 levels kept between 1 - 5 ppm Controlled by pH, OH and PO4 in boiler water

Phosphate Hideout

Shows as drop in boiler water phosphate under high loadUnder concentrating condition, phosphate precipitates from boiler water Further addition of chemical to compensate can cause increased deposition

Can cause localized corrosionPhenomenon reverses when heat load drops

All-Volatile Treatment

Creates a non-corrosive pH without adding dissolved solids

Can be used at pressures above 600 psigUseful in boilers with severe phosphate hideout

Has no buffering for feedwater contaminationUseful when ultrapure steam is needed

Caustic Embrittlement

Should not be confused with caustic corrosionEmbrittlement is a special form of stress corrosion cracking

Three conditions must be present Concentrating mechanism present Metal under high stress Must contain silicaInhibited by improved fabrication techniques and by organic and nitrate-based inhibitors

Other Causes of Boiler Corrosion

Feedwater acid contamination Surface condenser leaks Acid leaks from demineralizer Organic materials Chelant corrosion