C. Boiler Internal Cycle Chemistry & Control

8/10/2019 C. Boiler Internal Cycle Chemistry & Control

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Boiler Internal Cycle Chemistry

Indonesia Customer Seminar

June 13 & 14 2012

Jakarta Indonesia


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High Pressure Boiler Definition

Terminology:

Boilers in which steamproduced at or above 70 bar

(1000 psi) Sub-critical, below critical pressure(225 kg/cm2). Drum type boilers.

Supercritical, mostly operate at 245

kg/cm2. Highest is 350 kg/cm2.Once-through boilers


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Factors Affecting Chemistry in HP Boiler

Smaller densitydifference and greatervolatility

High Cycle ofConcentration

ConcentratingMechanisms


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5

Higher pressure =greater volatility ofdissolved solids

Higher pressure =smaller difference ofliquid and vapor

density

SiO2NaOH

NH4Cl

(NH4)2SO4

CuO NaCl

Na3PO4

Volatile Mechanical

Fe3O4

#1 Boiler Pressure and Carry Over


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#2 High Cycle of Concentration

Make-up water cost for HPboiler is high

Energy loss from blow down inHP boiler is also high

HP boiler running with highcycle compare (50- >100 cycles

or 2-1% BD)to save the costfrom blow down and make-up


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Higher solids accumulation inthe boiler higher potentialfor deposition problem

Longer holding timeLimiting effectiveness of chemicaldispersant that commonly use tominimize deposition

#2 High Cycle of Concentration


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Iron Transport and Boiler Deposition

The majority of iron entering the boiler remains inthe boiler (even when dispersants are used)

Cycle % Fe Transport

(with dispersant)

25-50 20-30

75-100 5


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Pressure, Temperature and Deposits

Deposit will retard the heattransfer and increase the tubemetal temperature

High tube metal temperature inHP boiler makes its tolerancefor water deposit is reduce


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#3 Concentrating Mechanisms

The concentration of TDSin the film is higher thanthe bulk

The higher thetemperature difference, thegreater TDS concentrationin film (tube surfaces)

High concentration ofagressive ions could leadto localized corrosion


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#3 Concentrating Mechanisms

Concentrating mechanismslead to localized corrosionstype that only found in HPBoiler

Critical Factors: DNB,Deposition & Evaporation inwater line

Deposit will enhanceconcentrating mechanisms andincrease potential for underdeposit corrosion


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Chemical Cleaning Guide Line


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ObjectivesHP Boiler Internal Cycle Chemistry

Prevent Internal

Corrosion

Prevent Deposition

Promote Production of

Pure Steam


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Achieinge non-corrosive boiler water pH levels

Providing adequatebuffer to deal with BFWcontamination

Providing minimumsolids contribution to theboiler water

15

Boiler Internal Treatment Chemistry


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Why pH Control is Important ?


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Caustic Corrosion

Causes: DepositionFree NaOH > 1 ppm

- Na contamination- Caustic overfeed

Solution

Minimize NaOHconcentrationNa:PO 4 = 2.3-2.6(Congruent PO 4)

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Acid Phosphate Corrosion18

CausesDepositionPhosphate Hide Out

Control Na:PO 4 < 3:1Solution

Control PO 4 residual atlower rangeControl Na:PO 4 > 3:1Equilibrium PO 4 (EPT)

HideoutLoad

PO4

pH

Hideout Return


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Hydrogen Damage

Causes: Deposition Acidic condition

- Condenser in-leakage- Pretreatment upsets

- Un-proper cleaningInadequate buffer

SolutionBetter BFW quality

Control Na:PO 4 3:1 to 1 ppm NaOHContinuum PO 4

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SolidTreatment

Phosphate Continuum(PC)

Caustic Treatment (CT)

All VolatileTreatment

All Volatile (AVT)- Oxidizing- Reducing

Oxygenated Treatment(OT)

ContinuumEPRI New Cycle Chemistry Guide Line 2004


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Phosphate Continuum (PC)

Control at Na:PO4 = 3: 0 to 1 ppm NaOHTSP and NaOH only no Phosphate with ratio < 3: 1

Tight boiler water pH control required

Transition from PC (Low) to PC (High) at approximately 3ppm PO 4

Advantages:Better pH control and bufferingGreater tolerance of FW contaminationReduced potential for acid phosphate corrosion if hideout occursEasier to stay in specification


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Phosphate Continuum Control Chart

8.6

8.8

9.0

9.2

9.4

9.6

9.8

10.0

10.2

0 1 2 3 4 5 6 7 8 9 10

mg/L PO4

p H

a t 2 5 d e g

C

Na/PO4 = 3.0

TSP + 1 PPM NaOH

TSP + 2 PPM NaOH

PC

Na/PO4 = 2.6

Note:Minimum PO4 > 0.2 mg/L

Na/PO4 = 2.8

Na/PO4 = 4.0

PC (L) PC (H)

Na/PO4 = 3.4


NaOHTSP

BD

23


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PC(L) CC BFW : < 0.15 S/cm Phosphate : 0.3 3 ppm pH : 9.0 9.7

PC(H)

CC BFW : < 0.20 mS/cm Phosphate : 3 10 ppm pH : 9.5 10.1



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8.6

8.8

9.0

9.2

9.4

9.6

9.8

10.0

10.2

0 1 2 3 4 5 6 7 8 9 10

mg/L PO4

p H

a t 2 5 d e g

C

Na/PO4 = 3.0

TSP + 1 PPM NaOH

TSP + 2 PPM NaOH

PC

Na/PO4 = 2.6

Note:Minimum PO4 > 0.2 mg/L

Na/PO4 = 2.8

Na/PO4 = 4.0

PC (L) PC (H)

Na/PO4 = 3.4

Phosphate Continuum Product Options

Na:PO4 = 4:1

Na:PO4 = 3.4 :1

Na:PO4 = 3 :1


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Chemical Product Options

Dry TSP (Nalco 444) plus Caustic

Liquid TSP (BT 3000) plus diluteCaustic

Blended Na:PO4 ratio productsNalco BT-3400 (3.4:1)

Nalco BT-4000 (4.0:1)


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Caustic Treatment

Candidates are boilers with severe phosphate hideout, condenserleaks, and saline cooling water.Would not be able to use AVT under these conditions.

Maintain to EPRI specifications

Cl- allowed varies by pressure (0.3 ppm @ 2500 psi)

NaOH = 2.5 x Cl -

SO 4 < 2 x Cl -

Provides more buffering capacity for SO 4 or Cl contaminant ingress

than PC(L), less than PC(H).


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Boiler Chemistry Guidelines

Parameter Units psi 900 1500 1900 2300 2700 < 136 bar > 136 bar MPa 6.2 10.3 13.1 15.8 18.6 < 13.6 > 13.6

Na mg/L 3.50 2.70 2.10 1.60 1.30 PO4 mg/L 5.00 3.50 2.90 2.30 1.70 < 6 pH 9.68 9.53 9.45 9.35 9.23 9.8 - 10.2 Cl mg/L 1.70 1.20 0.90 0.70 0.52 SO4 mg/L 3.40 2.40 1.80 1.40 1.10 SiO2 mg/L 2.80 0.90 0.45 0.25 0.12 pressure Conductivity uS/cm 40 28 21 17 14 Cation Cond uS/cm 75 48 37 28 22 < 50

Na mg/L 2.30 1.80 1.50 1.20 0.95 PO4 mg/L 3.00 2.40 2.00 1.60 1.20 < 3 pH 9.52 9.38 9.30 9.20 9.09 9.3 - 9.7 Cl mg/L 0.90 0.45 0.30 0.19 0.12 SO4 mg/L 1.80 0.90 0.60 0.37 0.24 SiO2 mg/L 2.80 0.90 0.45 0.25 0.12 pressure Conductivity uS/cm 24 15 12 9 7 Cation Cond uS/cm 38 25 17 12 8 < 50

Na mg/L 2.20 1.60 1.20 0.90 0.70 NaOH mg/L 2.60 1.80 1.40 1.10 0.90 pH 9.80 9.65 9.55 9.45 9.35 9.3 - 9.7 Cl mg/L 0.90 0.58 0.44 0.34 0.27 SO4 mg/L 1.80 1.20 0.90 0.68 0.55 SiO2 mg/L 2.80 0.90 0.45 0.25 0.12 pressure Conductivity uS/cm 27 18 13 10 8 Cation Cond uS/cm 27 18 13 10 8 < 50

Phosphate Continuum - Low

Caustic Treatment

EPRI Guideline VGB Std Drum Pressure

Phosphate Continuum - High


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All Volatile Treatment

Only use volatile chemicals. No solid chemicals.

Excellent program:For systems with condensate polishers and/or tight condensers

For once through boilers

Default program during steam blows and commissioning

Requires very high purity feedwater at all times!

< 0.2 uS/cm Cation ConductivityDissolved O 2 at CPD < 10 ppb


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Source: Albert Busik, Drum Boiler on All Volatile Treatment - The pH Pitfall, Power Plant Chemistry, 2004

AVT Feed Water Purity is EssentialBoiler Water pH @Temperature with Contaminants


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Source: Albert Busik, Drum Boiler on All Volatile Treatment - The pH Pitfall, Power Plant Chemistry, 2004

AVT Feed Water Purity is EssentialBoiler Water pH (9.5) @Temperature with Chloride 10-50 ppb


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Oxygenated Treatment (OT)

Most preferred programfor Once Through Boiler

All steel metallurgy

Very high purity boilerBFW

CC < 0.15 mS/cmDO @CPD < 10 ppb

Full flow polisher is a mustNo oxygen scavengers


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Disadvantages of OT

Very high purity ofBFW is required

Upset of cationconductivity will leadto serious corrosionproblem

Source:, EPRI, Cycle Chemistry Guidelines forFossil Plants: Oxygenated Treatment, 1994


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Continuum Program Selection


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Feedwater and Steam Quality Limits

0

0.5

1

1.5

2

2.5

3

3.5

4

OT AVT PC(L) PC(H) CT

p p b

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

u S / c m

Na, Cl, SO4 Cat Cond

Continuum: Feed Water Quality


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Boiler Water Quality Limits @ 2500 psi

0

200

400

600

800

1000

1200

1400

OT AVT PC(L) PC(H) CT

p p b

0

4

8

12

16

20

24

28

u S / c m

Cl ppb SO4 ppb Cat Cond uS/cm

Continuum: Boiler Water Quality


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Action Levels

EPRI developed Action Levels to specify how long and howfar chemistry could be safely out of the normal controlspecification.

Action Level 1:


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Drum Normal Level 1 Level 2 Level 3Parameter Min Max Min Max Min Max Min MaxpH Consistent with selected PC programS. Cond (uS/cm) 28 > 28 < 56 > 56 < 112 > 112Cat. Cond (uS/cm) 48 > 48 < 96 > 96 < 192 > 192

Silica (mg/L) 0.9 > 0.9 < 1.8 > 1.8 < 3.6 > 3.6PO4 (mg/L) Consistent with selected PC programNa:PO4 Ratio Consistent with selected PC programSodium (mg/L) 2.7 > 2.7 < 5.4 > 5.4 < 10.8 > 10.8Chloride (mg/L) 1.2 > 1.2 < 2.4 > 2.4 < 4.8 > 4.8

Sulfate (mg/L) 2.4 > 2.4 < 4.8 > 4.8 < 9.6 > 9.6NaOH (mg/L) 1.0 > 1.0 < 2.0 > 2.0Note: Recommend immediate shutdown if pH < 8.0 or > 10.5

Action Levels: PC Program


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Why Boiler Lay Up?

Oxygen corrosion found in a boileris generally occurred during theboiler in the idle condition andresult of improper lay up procedure

Factors that need to beconsidered:

What is the duration of downtime Need to provide backup steam capacity ?

Superheater? Drainable ? Can the boiler drained and left drywithout affecting plant operation ?

Can the boiler be sealed ? Do the valveshold ?

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Boiler Lay Up

Wet Lay Up

Idle < 3 weeks

pH & O2 Scavenger atoptimum level

Pressurized blanket of inertgas to prevent air ingress

Method:

Short Term (Bottle Up) 72 hrs

Dry Lay Up

Best lay up method

Idle > 3 weeks

Method Nitrogen Blanketing Dessicant Vapor Corrosion Inhibitor


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Boiler Lay-up Decision Tree


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Short Term Wet Lay UpWater Chemistry Control Range

Lay-up Method Short-term Wet Lay-up

Time Period (After Shutdown) < 72 Hours (as long asretains heat and pressure)

pH Target Range (All-steel) 9.5-9.6 (High in normalrange)

pH Target Range (Cu Alloys) 9.2-9.3 (High in normal

range)Dissolved Oxygen (ug/L)


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Wet Lay Up with N 2 Blanketing


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Wet Lay Up with Steam Sparging


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Lay-up Method Long-term Wet Lay-upTime Period (After Shutdown) > 72 Hours or when unit cools to

ambient pressure and temperaturepH Target Range (All-steel) 10.0-10.5pH Target Range (Cu Alloys) 10.0-10.5Dissolved Oxygen (ug/L)


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Thank You !

Documents

C. Boiler Internal Cycle Chemistry & Control