PGESCO TRAINING APRIL 2008

BOLIER CHEMISTRY

KUMAR SINHAPRINCIPAL ENGINEER

BECHTEL POWERFREDERICK, MD

BASIC CHEMISTRY

• DEFINITIONS OF pH, CONDUCTIVITY (SPECIFIC AND CATIONIC), PO4, CAUSTIC, ACID, MOLAR RATIO, DISSOLVED OXYGEN, DISSOLVED IRON AND TOTAL IRON,

• HARDNESS IN WATER– TEMPORARY HARDNESS– PERMANENT HARDNESS

• CATIONS AND ANIONS

POWER PLANT CHEMISTRY• STEAM, BOILER WATER, FEEDWATER, &

CONDENSATE• IMPURITIES IN STEAM

– SILICA– CHLORIDES– SULFATES– SODIUM– TOC– CATIONIC CONDUCTIVITY

• CARRYOVER-MECH AND VAPOR• CYCLE MAKEUP TREATMENT• CONDENSATE POLISHING

HISTORY OF BOILER WT• COORDINATED PO4 TREATMENT (TRI SODIUM PHOSPHATE

OR TSP INTRODUCED IN 1942) TO PREVENT CAUSTIC EMBRITTLEMENT AND HRDNESS CONTAMINANTS FROM CONDENSER IN LEAKAGE---------STILL LEAD TO TUBE FAILURES

• CONGRUENT PO4 TREATMENT (TSP AND DSP IN SODIUM TO PO4 MOLAR RATIO OF 2.6-2.8) WAS RECOMMENDED BY EPRI MUCH LATER----STILL DID NOT PREVENT HIDEOUTS (AN OPERATING NUISANCE)-------ACID PHOSPHATE COROSION WAS DISCOVERED IN WATR WALL TUBES

• EQUILIBRIUM PO4 TREATMENT(TSP + 1 PPM OF NaOH)• HIDEOUT IS DECREASE OF Na/PO4 MOLE RATIO WITH pH

DEPRESSION AND PO4 INCREASE-----CAUSE LOAD CHANGES• HIDEOUT IS BOILER, TEMP, PRESSURE, DESIGN AND SITE

SPECIFIC

CHEMICAL INJECTION POINTS

• CONDENSATE PUMP DISCHARGE (WHEN NO CONDENSATE POLISHERS)

• DOWNSTREAM OF COND. POL • DOWNSTREAM OF DA• PO4 AND NaOH DIRECTLY IN BOILER DRUM

(IF DRUM BOILER)• CHEMICALS ADDED ARE:

– OXYGEN SCAVENGERS (AVT)– AMMONIUM HYDROXIDE FOR pH control– O2 (OXYGENATED TREATMENT)

COORDINATED PO4

CONGRUENT PO4

EQUILIBRIUM PO4

BOILER WATER PHOSPHATE

WHY ADD PO4 OR NaOH?• Phosphate and caustic form a buffer in the boiler

drum• React to neutralize small amounts of acid or caustic

that may concentrate up in the drum• Can complex impurities so they can be blown down

& out of drum• Control of both the pH and phosphate

concentrations are critical to prevent damage to boiler

• Unless unusual circumstance phosphates need not be fed until after ST roll off

DRUM BOILERS

• Four possible choices for drum boiler water treatment– AVT (ALL VOLATILE TREATMENT– OT (OXYGENATED TREATMENT– PC (PHOSPHATE CONTINIUM)– CT (CONGRUENT TREATMENT)

• Three possible feedwater treatments– AVT (R)– AVT (O)– OT

ONCE THROUGH BOLIERS

• Three possible feedwater treatments– AVT (R)

» USES AMMONIA AND AREDUCING AGENT» ORP POTENTIAL (-300 TO -350 Mv)

– AVT (O)» REDUCING AGENT HAS BEEN ELIMINATED» ORP POTENTIAL (0 Mv)

– OT» OXYGEN AND AMMONIA ARE ADDED» ORP POTENTIAL (+100 TO +150 Mv)

AVT(R) VS AVT(O)PARAMETER AVT (R) AVT (O)

pH 9.2-9.6 9.2-9.6

CATION COND (US/CM)

<0.2 <0.2

Fe, PPB at EI <2 <2 (<1)

Cu, PPB at EI <2 <2

OXYGEN, PPB at EI <5 (<2) <10

OXYGEN, PPB at CPD <10 <10

REDUCING AGENT YES NO

ORP (Mv) at DAI -300 TO -350 0

AVT FEEDWATER CHEMISTRY ECONOMOZER INLET

PARAMETERS FW CYCLE MATERIALS

NORMAL TRAGET VALUES

CATION CONDUCTIVITY

(us/cm)

ALL FERROUS AND MIXED Fe-Cu

METALLURGY

LESS THAN OR EQUAL TO 0.2

pH ALL FERROUS METALLURGY

MIXED METALLURGY

9.2-9.6

9.0-9.3

OXYGEN, ppb (AT CONDENSATE PUMP)

ALL FERROUS METALLURGY

MIXED METALLURGY

LESS THAN OR EQUAL TO 10

LESS THAN OR EQUAL TO 5

OT FEED WATER CHEMISTRY ECONOMIZER INLET-ONCE THROUGH

BOILERSPARAMETER NORMAL TARGET

VALUE 1TARGET VALUE 2

TARGET VALUE 3

pH 8-8.5

AMMONIA, PPM 0.02-0.07

SPECIFIC COND (US/CM)

0.25-1.0

CATION COND (US/CM)

LESS THAN OR EQUAL TO 0.15

LESS THAN OR EQUAL TO 0.3

LESS THAN OR EQUAL TO 0.6

GREATER THAN 0.6

IRON, PPB LESS THAN OR EQUAL TO 2

>2

OXYGEN, PPB 30-150

OT FEED WATER CHEMISTRY ECONOMIZER INLET-DRUM BOILERS

PARAMETER NORMAL TARGET VALUE 1

TARGET VALUE 2

TARGET VALUE 3

pH 9-9.4

AMMONIA, PPM

0.28-1.1

SPECIFIC COND (US/CM)

2.7-6.7

CATION COND (US/CM)

LESS THAN OR EQUAL TO

0.15

LESS THAN OR EQUAL TO

0.3

LESS THAN OR EQUAL TO

0.6

GREATER THAN 0.6

IRON, PPB LESS THAN OR EQUAL TO

2

>2

OXYGEN, PPB 30-50

STARTUP CHEMISTRY CONTROL

• Major Areas of Emphasis– Develop with the Project a Site Specific

“Startup Chemistry Operating Procedure”• Specific Monitoring & Alarm Response

– Offer On-Site Chemistry Training• For Startup Team & Operations

– Approve Site Specific Procedures for Flushing, Chemical Cleaning & Lay-up

– Conduct On-Site Chemistry Assessments

CHEMISTRY CONTROL

• Site Specific– Specific to equipment at plant (e.g.

Condensate Polishers, ACC, seawater cooled condenser)

– With the chemistry program of choice:• Ammonia & Hydrazine with PO4 in Drum

– Specific to on-line analyzers & alarms on unit

CORROSION IN BOILERS• Iron based metals corrode if given a chance• Obvious things that speed up corrosion – acids, too

much caustic, and corrosive impurities (chloride & sulfate)

• Not so obvious things – heavy deposits of corrosion products, temperature, time

• Dissolved oxygen speeds iron corrosion in most cases– Reacts with magnetite to form hematite (black rust to

red rust)– Must be controlled by de-aeration and scavenger

FLOW ACCELERATED CORROSION

• FAC is real and prevalent in HRSGs particularly in horizontal tube arrangements in LP

• Conditions causing FAC that are common in HRSGs are: – tight radius bends – and high temperature – turbulent water flow– no magnetite layer being formed

• Overfeed of oxygen scavengers speeds process – by creating a reducing environment

2500 PSI AND HIGHER PRESSURE DRUM BOILERS

ASSUMES 0.18% MECHANICAL CARRYOVER

OT AVT PC (L)(Na/PO4 MOLAR

RATIO =3)

PC (H)(Na/PO4 MOLAR

RATIO =3)

CT

CATION COND

(US/CM)

1.5 1.5 10 25 9

Cl, PPB 50 50 150 600 300

SO4, PPB 100 100 300 1200 600

SiO2, PPB 180 180 180 180 180

pH CONTROL• It is important to maintain an alkaline pH - >9 at

all times • Alkaline pH is important to lower iron corrosion

and to maintain passivation on boiler surfaces• Unless the boiler is fired directly (by GTs, Duct

Burners, etc.) and steam is bypassed to the condenser, pH above 9.6 can be tolerated

• Once the unit is bypassing steam to the condenser, pH should be lowered to normal operating levels (typically 9.2 – 9.6)

OXYGEN CONTROL• DM water exposed to atmosphere becomes saturated with

Dissolved Oxygen • Oxygen scavengers require heat to react

– Hydrazine reacts @450 F and Carbohydrazide reacts @250 F– Other oxygen scavengers react at lower temps but can significantly

add to steam cation conductivity and should be avoided• Whenever oxygen scavengers are added to the water they

need to be monitored– to show DO is low and a residual of the scavenger

exists in the condensate – Overfeed of oxygen scavengers can lead to FAC (more

so in HRSGs)

Boiler Water Chemistry-PHOSPHATE CONTINIUM

Rankine

Combined Cycle