corrosion, fouling & scale formation in … 2013 presentations/day-2 at pmi...biggest sea water...

CORROSION, FOULING & SCALE

FORMATION IN SEA WATER COOLING

SYSTEM - A CASE STUDY WITH

REFERENCE TO SCALING PROBLEM OF

NTPC-SIMHADRI UNIT#III

BY

P GHOSH

AGM CHEMISTRY

Presentation Elutes..,

SEA WATER CONDENSER COOLING

SEA WATER RELATED PROBLEMS

RECENT PROBLEMS FACED ON SCALING

DATA COLLECTION

RESEARCH AND ANALYSIS OF THE PROBLEM

CORRECTIVE ACTIONS TAKEN

UNIT 3 CONDENSER CHEMICAL CLEANING

OBSERVATIONS AFTER CHEMICAL CLEANING

PROPOSED DESIGN FOR PROCESS IMPROVEMENT

CONCLUSION

UNIQUE FEATURES OF SIMHADRI

First Coastal Coal fired

Thermal Power Project of

NTPC

Simhadri has the First Sea

Water Cooled Condenser in

NTPC

Biggest Sea Water Intake well

constructed in Bay of Bengal

Biggest Natural Draft Cooling

Tower of Asia (165 M)

CW fore-bay

CW pumps

CONDENSER

Cooling towers

1 2

Sampling points for ReCl2

chlorine dozing

Gas chlorination For CW system

Electro-chlorinator at

Sea water intake Sampling point for ReCL2

SCHEME AT SIMHADRI

1.5 COC

DESIGN OF SIMHADRI COOLING WATER SYSTEM

Sl. No. Parameter UM Design value

1 Design CW flow M3/hr 54,000

2 Temp. rise of CW deg C 10.4

4 Design CW inlet temp. deg C 33

5 CW side pressure drop mwc 4.0

6 No. of condenser passes No. 2

7 Total no. of tubes No. 24,398

8

Tube material :

a) Condensating zone

b) A/c zone

--- Welded Titanium SB-338 Gr II

9 Water box design pressure kg/cm2 5.0

CORROSION

SCALING

FOULING

SALT DEPOSITION

NEAR BY

INSULATORS

SEA WATER

RELATED

PROBLEMS

MARINE FOULING

ALGAE FUNGI ,

BACTERIA

MUSSELS AND

VERNACLES

GENERAL FOULING

POROUS DEPOSITS

DIRT,SILTS

HARD CRYSTALINE

DEPOSIT OF SALTS

OF Ca & Ma, Sio2

ELECTROCHEMICAL

REACTIONS

INVOLVING O2

Nearly saturated

with salt

And large no of

marine species

PROCESS

MANAGEMENT

MAINTENANCE

PRACTICES

ENGINEERING

SW LARGER PIPELINES-PU COATED

SW SMALLER PIPE LINES - HDPE OR

SS 316L

CONDENSER TUBES-TITANIUM

WATER BOX-FRE COATED

ELECTROCHLORINATION

GAS CHLORINATION AT CW

FOREBAY

CONTINUOUS CHLORINATION

ONLINE TUBE CLEANING

THOROUGH INSPECTION AND

RECOATING OF DEFECTIVE LAYERS

CLEANING OF CT SUMP AND

NOZZLES

CONDENSER HPW JET CLEANING

CARRYING OUT CORROSION TEST

WITH CECRI

FEATURES TO COMBAT SEA WATER

RELATED PROBLEMS

Huge colonial growth of marine species (vernacles) inside and

outside submerged surfaces of the AW overflow lagoon and

pipelines of the AW recirculation sump.

The growth inside the pipe line caused huge flow restriction

Lead to stopping of AW recirculation pumps.

The entire system has been taken under shutdown for manual

cleaning.

PROBLEMS FACED BY THE STATION

FOULING


Fouling in Ash pond at NTPC Simhadri


CORROSION

cooling towers, condenser water box, ARCW, Sea

water pp house area - some of the vulnerable places

for corrosion

station has faced corrosion problem in its cooling

tower racer columns

For the stage I cooling towers lot of study has been

carried out through CBRI. Accordingly maintenance

of the columns has been done by the site

Study regarding suitability of the paint for marine

environment -through CECRI

A coating of thick hard scale had developed in the UIII of stage II

500MW condenser tubes(more in top half ), the water boxes , the

complete hot water duct and its CT nozzles fill material

Scale is of almost 0.7 to 1.2 mm thickness ,primarily composed of

Calcium Carbonate (~92%)

The scale formed was too hard to remove with high pr water jet and

CONCO bullet cleaning

Ti tube condenser scaling and its chemical cleaning was unique in

nature and was never done anywhere in India

The problem jointly studied by site, NETRA and OS

RECENT PROBLEM FACED BY THE STATION

SCALING

Condenser-II Condenser-III

With scale No scale

SCALING ON TITANIUM CONDENSER TUBES

SCALING ON FRE COATED WATER BOX

SCALING ON PU COATED SW DUCT

SCALING ON COOLING TOWER NOZZLES

DATA COLLECTION AND

ANALYSIS OF THE PROBLEM

To ETP 4200

All values in m3/hr SEA WATER BALANCE

ST I 2x500 MW

SYSTEM

To Ash Handling Plant 934

Evaporation Loss 1795

Debris Filter 5

From Sea 6934

Blow down required to maintain <1.5 COC is ~2000 M3/unit

Actual blow down in ST-I was 2600/unit with 1.3 COC

To ETP NIL (blow down

system not available)

All values in m3/hr SEA WATER BALANCE FOR

3x500MW during initial period

SYSTEM

To Ash Handling Plant 1000

Evaporation Loss 900

Debris Filter 5

From Sea

9000

To ST-I 6930

To U#III 2070

Leads to actual COC around 2.0 against design COC 1.5

ST II was under higher COC for about 06 to 07 months

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

2.00

2.10

2.20

0 1 2 3 4 5 6 7 8 9 10

COC LIMIT

---- ST II

---- ST I

JANUARY-OCTOBER

CO

C

PROBLEM IS KNOWN

Standard practice of scale removal a) HP water jetting b)

CONCO Cleaning c) Chemical cleaning followed by HP and

CONCO cleaning

HP and CONCO was not at all able to remove the scale

Chemical cleaning

Needs careful study as the

Tubes are Ti and the water box tube plate

is FRE coated and the ducts are PU coated

Calcium Carbonate Super saturation

Supersaturated solutions of calcium carbonate can be formed from

saturated or under saturated solutions when the calcium hardness, pH

or alkalinity are increased

The degree of calcium carbonate saturation (S) is given by the ratio

of the actual ion activity product (IAP) and the

thermodynamic solubility product constant at infinite dilution (KS):

S = IAP/KS = {Ca2+}{CO32–}/KS

= [Ca2+] γCa+2 [CO3 2–] γCO2 3–/KS

S = [Ca2+][CO32–]/Cks

S values of <1, 1, and >1 represent under saturation, saturation,

and oversaturation, respectively

Conditions effecting scale formation Physical :

Thermodynamic: The solubility limit must be exceeded. .i.e.

there must be super saturation

Kinetic: The deposition rate must be sufficiently high

Increase in temperature decrease solubility of CaCO3

Increase in Pressure increase solubility of CaCO3

Conditions effecting scale formation

Chemical:

Increase of pH favours formation of CaCO3 scale

With increase in alkalinity scale formation tendency increases

The solubility of CaCO3 in saline water is more than that in

natural sweet water

Both Calcite and Aragonite are more soluble

at higher salinity because of the

thermodynamics of the system. Thus, the

saturation concentration of calcite and

aragonite will also depends on salinity

As pH increases, calcium carbonate solubility

exponentially decreases. Therefore, in case of

increasing CO2 input into the ocean ,which

reduces pH, calcium carbonate will more

susceptible to dissolution.

Useful indices for determining corrosion and scale

tendencies of makeup water

Ryzner (or Stability) Index (RSI) = RSI = 2pHs – pH

Langelier (or Saturation) Index (LSI) = LSI = pH – pHs

Puckorious (or modified stability) index=PSI = 2pHs – pHe,

(pHequilibrium = 1.465log (total alkalinity))

8.70

8.80

8.90

9.00

9.10

9.20

9.30

9.40

9.50

9.60

9.70

9.80

Jan Feb Mar APR May Jun

PSI

4.00

4.10

4.20

4.30

4.40

4.50

4.60

RSI

1.70

1.75

1.80

1.85

1.90

1.95

2.00

Jan Feb Mar APR May Jun

LSI

OBSERVATIONS FROM INDICES

RSI and PSI values indicate the increased tendency towards scaling

The literature survey does not clearly reflect about the applicability

of index calculation to sea water

Actual experiment is done with Marble test

pH pHs CaH M Alk LSI by

calculati

on

LSI by

Marble

test

RSI by

calcula

tion

RSI by

marble

test

PSI by

calculati

on

PSI by

marble

test

Ionic

product

Sol. Prod

Const5

SW 8.41 8.13 930 115 1.2-1.4 +0.28 5.7-6.5 7.85 10.5-11 12.9 1.07x10 -5 3.7x10-9

CWI

I

8.39 7.96 1395 150 1.79-

1.94

+0.43 4.5-4.6 7.53 9.6-9.9 13.07 2.1x10-5 3.7x10-9

RW 8.96 9.27 40.45 99 0.5-0.9 -0.31 10.5-

11.5

9.58 12.8-

13.4

15.61 0.5x10-5 3.7x10-9

NOTE: LSI -ve unsaturated .i.e. corrosive tendency

LSI +ve super saturated .i.e. scaling tendency

For sea water: By calc-severe scaling and by test -slight scaling (LSI taken for reference)

For Circulating water: By calc-severe scaling and by test-moderate scaling(LSI taken for reference)

It is apparent that in case of Sea water, at and more than design

COC of 1.5 the value of Ca hardness (1395 as CaCO3) is much

more than 5 times than that of 5 COC wrt Sweat water (202 as

CaCO3)

The crystal of the scale deposit as observed in Simhadri was of

glassy in appearance and extremely hard and non porous

Sweet water Scale at KhSTPP Sea water scale at SIMHADRI

Effect of the problem

0

5

10

15

20

25

30

Feb-12 Mar-12 Apr-12 May-12 Jun-12 Jul-12 Aug-12

Avg vacuum loss

S

H

U

T

D

O

W

N

mm

Hg

UIII 500MW Scale Deposit analysis report

PARAMETER CONDENSER CONDENSER

BACK SIDE

% Moisture 2.9% 7.2%

Loss on

ignition (at

400oC)

8.5% 5.69%

% OF Ca as

CaCO3

90.12% 91.91%

% of sulphate 2.16% 2.52%

% of Iron 0.1% 0.095%

Acid insoluble 4.12% 4.56%

Sea Water & CW Analysis Report during the 06months period

Parameter Unit Sea water Stage - I CW Stage - II CW

pH 8.2 8.4 8.42

TSS ppm 22 26 28

TDS ppm 37355 56800 58900

Turbidity NTU 1.58 3.2 7.81

Conductivity µS/cm 45200 67300 70300

Residual Chlorine ppm 1 0.6 0.6

Total Hardness ppm as CaCO3 5600 7600 10400

Ca hardness ppm as CaCO3 800 1000 1600

Mg Hardness ppm as CaCO3 4800 6600 8800

Chloride ppm as CaCO3 26790 33840 47940

P- Alkalinity ppm as CaCO3 6 18 20

M- Alkalinity ppm as CaCO3 102 108 146

Silica ppm as SiO2 2.1 2.2 2.53

Sodium + Potassium ppm 21000 29200 32000

Iron ppm 0.117 0.29 0.4

COMPATIBILITY STUDY

Before adopting a cleaning method analysis has been done

jointly by SITE and NETRA and for :-

The solubility of the scale (.i.e. 92% CaCO3) in the acid

Compatibility of Titanium metal in the acid used to dissolve

the scale by measuring the corrosion rate

Effect of chemicals, temperature and process on the FRE and

the PU coating

Acid Solubility of scale Compatibility of titanium

tube

*3-10% phosphoric

acid

Negligible solubility compatible

* 3.5 % Hydrochloric

acid

Completely soluble

within 15 minutes

Compatible with 0.2% FeCl3

inhibitor

* 3% citric acid +

Ammonia

Negligible solubility compatible

* 3.5% Sulphamic

acid

Completely soluble

within 30 minutes

Compatible with 0.2% FeCl3

inhibitor

* Chromic acid Negligible solubility compatible

* From literature

* From site study

S.

N

O

Sample description pH Initial

Weight

Final

weight

Weight

loss in g

Density

in g/cc

Total

Surface

area

mm2

Time

in Hrs

Corrosion

rate in

mpy

1 3.5% Sulphamic acid +

0.1% FeCl3 at 62deg C

1.75 9.682 9.679 0.0023 5.407 6716 6 3.699



1.67 10.0525 10.051 0.0011 5.407 6817 6 1.743



1.61 11.2629 11.26 0.0025 5.407 7825 6 3.451

4 3.5% HCl + 0.2%

FeCl3 at 62deg C

1 9.9562 9.9535 0.0027 5.407 6817 6 4.278

From all the above tests and analysis, the suitable methodology

has been proposed for scale removal .i.e. by using 3.5%

sulphamic acid with 0.2% FeCl3 as inhibitor at 62 deg C

CaCO3 + 2(NH2.SO3H) Ca(NH2 SO3)2 +H2O + CO2

CORRECTIVE ACTIONS TAKEN

To ensure that further scale formation does not take

place a contingency blow down arrangement has been

made maintaining COC less than 1.5

CW chemical treatment started for the control of

further scaling (till the CW blow down system for

Stage II is commissioned .i.e. about 4 months)

UNIT 3 CONDENSER CHEMICAL CLEANING

Chemical cleaning Procedure is by Inhibited Sulphamic acid

(along with ferric chloride) cleaning followed by high pr water

jetting and CONCO cleaning

Prior to start of chemical a 150mm wide FRE coating was

removed from the CW line for fixing of 16mm MS dummy

plates for isolation of CW inlet and outlet ducts

CHEMICAL CLEANING SET UP AREA

0.2% of technical grade ferric chloride inhibitor is added to the

water and when the ferric chloride is observed in the outlet,

inhibited Sulphamic acid (3.0 to 3.5 % w/w) is introduced into the

system monitoring the acidity level

Temperature oC Acid strength % Total Hardness ppm

as CaCO3

Ca Hardness ppm as

CaCO3

pH

Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

65 60 0.77 0.194 - - - 2.3 7.0

67 61 0.582 0.194 - 6900 - 6250 - -

67 62 3.10 0.194 - 10250 - 9500 2.1 4.69

68 63 2.52 0.194 - 13750 - 13000 - -

68 64 2.52 0.194 - 17500 - 16750 2 3.58

66 64 3.52 0.58 - 22000 - 21500 - -

63 61 1.35 0.58 - 32000 - 30000 - -

65 59 3.10 0.58 - 34000 - 32500 1.82 2.8

65 59 4.65 1.64 - 45000 - 44000 - -

65 59 3.29 1.9 - 47000 - 45500 1.7 2.2

65 60 3.49 2.13 50500 51363 48000 48500 1.75 1.9

65 64 2.91 2.32 54000 53000 52000 51500 - -

65 64 1.94 1.94 54500 54500 53000 53000 1.8 1.8

Results of analysis for condenser chemical cleaning of Pass - A

Results of analysis for condenser chemical cleaning of Pass – B

Temperature oC Acid strength % Total Hardness ppm as

CaCO3

Ca Hardness ppm as

CaCO3

pH

Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

61 53 2.13 - - - - - 2.4 7.0

64 54 1.34 0.194 - 17500 - - 2.2 6.0

61 57 5.62 0.38 - 28500 - 16250 2.0 5.7

62 58 5.23 0.58 - 30000 - 29500 - -

65 60 2.71 0.97 - 38500 - 1.94 3.2

65 64 4.46 0.97 - 50000 - 49000 - -

65 64 3.49 1.16 - 53500 - 52500 1.92 2.74

66 64 3.64 1.16 - 57000 - 56500 - -

58 55 1.9 1.26 - 54500 - 54000 1.7 2.3

58 54 4.85 1.16 36750 54500 49500 54000 - -

60 58 3.49 1.74 55000 57000 52000 55500 1.9 1.76

62 59 3.49 2.32 55500 57000 52500 55500 1.85 1.7

63 62 1.74 1.74 56500 57000 55000 54500 1.8 1.7

OBSERVATIONS AFTER CHEMICAL CLEANING

About 28MT of scale was removed apart from huge quantity of

loose debris accumulated on the partition and the bottom plates

After chemical cleaning and water jetting at 320 bar pr also a thin

but adherent layer remained on the inside of the tube surfaces more

so on the upper 30-40% tubes of the top half of the condenser

With CONCO bullet cleaning about 1000kgs of loose and broken

hard scale was removed from each pass.

Still a layer of thin scale is observed on the upper half of the top

half of the condenser

AFTER CHEMICAL CLEANING Observations Continued…

After CONCO cleaning of

pass A, the top rows rear view

still having scale deposit

After CONCO cleaning of pass B,

the top rows front view still having

scale deposit

Observations Continued…

ACTUAL SCHEME FOR chemical cleaning

P/P

Titanium Tubes

Rear water box Front water box

CW INLET DUCT

CW O/L

DUCT

Man

hole

M

an

hole

Man

hole

ACID I/L

Recirculation tank

vent

ACID O/L

vent

PROPOSED SCHEME FOR IMPROVEMENT

Proposed design by site for process

improvement

The process design proposed is mainly to ensure the following

To ensure the upper rows remain in acid contact

Rear side and front side venting to remain always open

P/P

Titanium Tubes

Rear water box Front water box

CW INLET DUCT

CW O/L

DUCT

Man

hole

M

an

hole

Man

hole

ACID I/L

Recirculation tank

vent

ACID O/L

vent

CONCLUSION

Sea water application is corrosive but it is equally

dangerous from fouling and scaling point of view

The nature of the scale developed due to Sea water

is extremely hard with Ca content ~ 92%

compare to sweet water scale with Ca content 70-80%

In Simhadri the scale developed is so hard and non

porous though it dissolves in acid, it does not get

softened

0

20

40

60

80

100

120

31.10.1

1

21.01.1

2

08.02.1

2

28.02.1

2

05.03.1

2

10.03.1

2

23.04.1

2

07.05.1

2

20.07.1

2

08.08.1

2

29.12.1

2

-5

0

5

10

15

20

25

30

TREND OF VACUUM

Vacuum

(mmhg)

Loss due to

Dirty tubes

(mmhg)

CW I/L

Temp

(Deg.C)

TTD

CW dT

104.5

64.04

5.15

23.92

Area Actions Heat Rate Recovery

(Kcal/Kwh)

Condenser Acid cleaning carried out.

( Vacuum Improved by 18.77 mm hg) 21.77

CT

Internal inspection, Nozzle cleaning &

replacement done (Performance

Improvement factor Improved by 2.73

Deg.C)

10.96

Total Improvements 32.73

Improvements Achieved

As the bottom half of the condenser tubes are

completely cleaned it was found not advisable to go

for another immediate chemical cleaning

The top rows need more time of contact with acid

Proper release of CO2 evolved during the process

needs to be appropriately taken care

The problem faced by the station gives an

opportunity for in depth study and come out with

solution

Contd…

REFERENCES

Journal of the Swimming Pool and Spa Industry Volume 2,

Number 2, pages 23–29 Copyright © 2001by JSPSI

Calcium carbonate saturation index and alkalinity interpretations

by T.E.Larson AND A.M.Buswell

Saturation, stability, and scaling indices by Robert R. Cavano –

Scranton Associates, Incorporated

Factors affecting calcium carbonate equilibrium, Wikipedia

Solubility product constants, Wikipedia

Industrial Chemical cleaning by James McCoy

ACKNOWLEDGEMENTS

Corporate-OS

NETRA

corrosion, fouling & scale formation in … 2013 presentations/day-2 at pmi...biggest sea water...

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