WATER CHEMISTRY ASPECTS FOR SUPERCRITICAL UNIT V Chandrasekharan Sr. Manager NTPC SIPAT

DETAILS WATER TREATMENT OT – INTRODUCTION BOILER ACID CLEANING BOILER WATER PARAMETERS LP & OXYGEN DOSING SYSTEM ON LINE MONITORING CHANGE 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 ALTERNATIVE PROVIDES LONG TERM PROTECTION OF PRE BOILER SYSTEM BY FORMING HAMETITE LAYER. CPU OPERATING PERIOD WILL BE MORE DUE TO LOW CONDENSATE AMMONIA CONTENT IRON TRANSPORT WILL BE REDUCED BY 90% LESS GENERATION OF CRUD REDUCTION IN CHEMICAL CLEANING FREQUENCY REDUCTION IN OUTAGE TIME AND FASTER START UP FAC WILL BE MINIMIZED ALL 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

AREAS UNDER CLEANING Economiser Bottom ring Header Vertical waterwall Spiral Waterwall Separaters Separator storage tank

STEPWISE CHEMICAL CLEANING PROCESS Inhibitor efficiency test Hydro 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 CHEMICAL CLEANING OF SIPAT *Total volume of boiler including temporary pipelines-370 M3 No Chemical Purity (%) Working Conc. (%) Q'ty (Kg) Remarks 1 Hydrazine 50 0.001 Pre-cleaning, Hydrotest 2 Sorbic Acid 98.0 0.15 540 Simultaneous Degreasing and Descaling Formic Acid 85.0 10.0 36000 INHIBITOR (CORRONIL 200S) 100.0 0.3% 1070 Citric Acid 0.1 360 Non Ionic Detergent 100 0.05 3 750 Rinsing 4 Pre-Neutralization Neutralization Passivation Ammonia 25 0.075 1050 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

COUPON PLACEMENT 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.

PASSIVATED COUPON QUALITY OF MAGNETITE LAYER FORMATION AFTER PASSIVATION AS VISUAL INSPECTION.

FEED WATER SPECIFICATION S.NO Parameter Units Normal Operation During Start up Alkaline water Treatment Combined water Treatment 1 pH Min 9.0 8-8.5 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 < 2 < 20 5 Sodium < 10 6 Silica <10 < 30 7 Turbidity NTU <2 <5 FEED WATER RECIRCULATION VIA BOILER TO ACHIEVE FEED WATER QUALITY.

LP DOSING AND OXYGENATED TREATMENT SYSTEM AT SIPAT – AMMONIA DOSING Cycle 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 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 ~ 9.5 D/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

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