WATER CHEMISTRY ASPECTS FOR

SUPERCRITICAL UNIT

V ChandrasekharanSr. ManagerNTPC SIPAT

DETAILS

WATER TREATMENT OT – INTRODUCTION BOILER ACID CLEANING BOILER WATER PARAMETERS LP & OXYGEN DOSING SYSTEM ON LINE MONITORINGCHANGE OVER FROM ALKALINE TREATMENT TO OT

WATER TREATMENT - EXTERNAL

Depending on the source of water

1. Clarification

2. Filtration

3. Demineralization

4. DM With RO

Make up water quality at DM Outlet

Conductivity of > 0. 1 ms/cm

Silica - <10 ppb

FEED WATER TREATMENT

Type - A: AVT(R) ;using Ammonia and a Reducing Agent (such as Hydrazine)

Type – B: AVT (O); Similar to Type – A minus reducing agent

Type – C : (OT); Oxygenated Treatment using only ammonia and oxygen

SUPERCRITICAL DRUMLESS (ONCE THROUGH)BOILER WITH OXYGENATED TREATMENT

THE KEY WORDS ARE

SUPERCRITICAL+DRUMLESS BOILER +OXYGENATED TREATMENT

OT AN ALTERNATIVEPROVIDES LONG TERM PROTECTION OF PRE

BOILER SYSTEM BY FORMING HAMETITE LAYER.CPU OPERATING PERIOD WILL BE MORE DUE TO

LOW CONDENSATE AMMONIA CONTENTIRON TRANSPORT WILL BE REDUCED BY 90% LESS GENERATION OF CRUDREDUCTION IN CHEMICAL CLEANING

FREQUENCYREDUCTION IN OUTAGE TIME AND FASTER

START UPFAC WILL BE MINIMIZEDALL THESE LEADS TO VERY LESS BTF

SCHEMATIC OF OXIDE GROWTH AND MORPHOLOGY UNDER 0XIDISING AVT AND OT

2 Fe3O4 + 0.5 02 = 3 Fe2O3

CHEMICAL CLEANING OF SUPER CRITICAL BOILER BY FORMIC ACID

EconomiserBottom ring HeaderVertical waterwallSpiral WaterwallSeparatersSeparator storage tank

AREAS UNDER CLEANING

STEPWISE CHEMICAL CLEANING PROCESS

Inhibitor efficiency testHydro test of the system SH filling & DM Water Flushing of Boiler Chemical Cleaning Of Boiler Rinsing SH Back Flushing Pre Neutralization Neutralization Passivation preservation

Inhibitor efficiency test

Acceptable limits

corrosion rate should be less than 0.1 Mg/cm2/hr

Formic Acid 10.0 %

Inhibitor Coronil-200 S

0.30 %

Sorbic Acid 0.15 %

Citric Acid 0.10 %

Wetting agent 0.05 %

Temperature 85oC

Duration 6 hrs.

Materials Used SA-106 / SA-210C / SA213-T-12 / SA213 T-22 / SAT-23 / SA-335 P-91.

Temporary arrangements for chemical cleaning of boiler

CHEMICAL CLEANING OF SIPAT *Total volume of boiler including temporary pipelines-370 M3

No Chemical Purity (%)

WorkingConc. (%)

Q'ty (Kg) Remarks

1 Hydrazine 50 0.001 Pre-cleaning, Hydrotest

2 Sorbic Acid 98.0 0.15 540

SimultaneousDegreasing andDescaling

Formic Acid 85.0 10.0 36000

INHIBITOR (CORRONIL 200S)

100.0 0.3% 1070

Citric Acid 98.0 0.1 360

Non Ionic Detergent 100 0.05

3 Hydrazine 50 0.001 750 Rinsing

4 Citric Acid 98.0 0.1 360 Pre-NeutralizationNeutralizationPassivation

Ammonia 25 0.075 1050

Hydrazine 50 0.050 350

Function of Chemicals

Sorbic Acid: acts as a reducing agent convert Fe3+ to Fe2+

Formic Acid: (HCOOH) Removes mill scale, weld slag

Citric Acid: which dissolves the rust. The pH is raised to an alkaline value

using ammonia, and the sequestered iron remains in solution

Function of Chemicals

Inhibitor: Makes a passivation layer to save

the parent metal

Detergent: Removes oil & grease

COMPLETION CRITERIA FOR ACID CLEANING

Check point: Fe++, Fe+++ ,pH, TOC

Requirements: Fe++, pH, TOC Should be stable

Coupons to be placed in supply & return line prior to chemical addition to circulation tank made of exactly same composition as of boiler tubes of different locations.

COUPON PLACEMENT

PASSIVATED COUPON

QUALITY OF MAGNETITE LAYER FORMATION AFTER PASSIVATION AS VISUAL INSPECTION.

FEED WATER SPECIFICATIONS.NO Parameter Units Normal Operation During

Start up

Alkaline water Treatment

Combined water Treatment

1 pH Min 9.0 8-8.5 Min 9.0

2 Cation Conductivity, ms/cm

ms/cm

Max 0.2 <0.15 Max 0.5

3 Dissolved Oxygen ppb < 5 30-150 Max 100

4 Iron ppb < 2 < 2 < 20

5 Sodium ppb < 2 < 2 < 10

6 Silica ppb <10 < 10 < 30

7 Turbidity NTU <2 < 2 <5

FEED WATER RECIRCULATION VIA BOILER TO ACHIEVE FEED WATER QUALITY.

LP DOSING AND OXYGENATED TREATMENT SYSTEM AT SIPAT – AMMONIA DOSINGCycle pH is controlled by feeding an ammonia

from a pump having a automatic stroke adjustment.

The adjustment is automatically adjusted by CEP outlet pH and Feed water flow.

The dosing system includes two positive displacement pumps with auto stroke adjustment

Automatic switching of pumps upon pump failure.

LP DOSING AND OXYGENATED TREATMENT SYSTEM AT SIPAT – HYDRAZINE DOSING

Cycle oxygen is controlled by feeding an hydrazine from a pump having a automatic stroke adjustment.

The adjustment is automatically adjusted by Deaerator outlet Dissolved Oxygen and Feedwater flow.

LP DOSING AND OXYGENATED TREATMENT SYSTEM AT SIPAT – OXYGEN DOSING

The cycle oxygen is controlled by flow control valve having a automatic controller.

The injection control is automatically adjusted by Feed water flow and Condensate extraction pump outlet dissolved oxygen / Deaerator outlet dissolved oxygen.

The dosing system includes four(4) flow control valves with auto controller.

STEAM WATER ANALYSIS SYSTEM (SWAS)

FOLLOWING IS THE PROCESS MONITORING FOR CHEMICAL CONTROL OF STEAM AND WATER USED FOR BOILER AND STEAM TURBINE

S.NO SYSTEM TYPE OF MEASUREMENT

1 MAKE UP DM WATER SP.COND., CATION CONDUCTIVITY (ACC)

2 CEP DISCHARGE pH, ACC, Na, DO, SP.COND.,

3 CONDENSATE POLISHER O/L pH, ACC, Na, SILICA, SP.COND.,

4 DEAERATOR OUTLET DO

5 FEED WATER AT ECONOMIZER INLET

pH, ACC, COND.,HYDRAZINE,SILICA, TURBIDITY

STEAM WATER ANALYSIS SYSTEM (SWAS)

FOLLOWING IS THE PROCESS MONITORING FOR CHEMICAL CONTROL OF STEAM AND WATER USED FOR BOILER AND STEAM TURBINE

S.NO SYSTEM TYPE OF MEASUREMENT

6 VENT HEADER OF BOILER (SEPARATOR OUTLET STEAM)

ACC, SP.COND., HYDRAZINE, SILICA

7 MAIN STEAM pH, ACC, Na, SILICA, SP.COND.,

8 WATER SEPARATION STORAGE TANK OF BOILER

CATION CONDUCTIVITY(ACC)

9 REHEATED STEAM CATION CONDUCTIVITY(ACC)

10 TG ECW COOLING WATER pH

PROCEDURE TO CHANGE OVER TO OXYGENATED TREATMENT

CPU TO BE IN SERVICE WITH FULL CONDENSATE FLOW.

UNIT IN FULL LOAD & STABLE CONDITION. THERE SHALL BE 4 CPU VESSELS (3 IN

SERVICE OF 33% CAPACITY EACH)REQUIRED WATER QUALITY:

FEED WATER CATION CONDUCTIVITY : < 0.15 us/cm

FEED pH value : 9 ~ 9.5D/A O/LOxygen : ≤ 7 ppb

DURING SHUTDOWN

Stop oxygen feed at least one hour before shutdown

Increase ammonia feed rate to achieve a pH > 9.0

Open Deaerator vents

LP & HP heaters should be vented