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Pradeep Jain 3.1- 1/17

HOW TO INCREASE HEAT TRANSFER AND REDUCE WATERWALL

TUBE FAILURE IN HIGH PRESSURE BOILERS

Dr. Pradeep Jain -DGM, NETRA- R&D, NTPC Limited,

ABSTRACT:In spite of maintaining the very good water chemistry in water / steam cycle of thermal power plant,

corrosion phenomena takes place in the boilers. The product of corrosion in the feed water system

transported into the boiler and gets deposited on the internal surface of waterwall tubes. It leads to

overheating and On-load corrosion and ultimately tube failure. To achieve the almost zero tube failure

in waterwall of high pressure boilers, post-operational chemical cleaning is essential at the frequent

intervals in the life span of power plant which will improve heat transfer and reduce under deposit on-

load corrosion. This paper will discuss the phenomena of localised corrosion, its remedial measures to

achieve near zero tube failure.

INTRODUCTION

The NTPC Limited has more than 125 numbers of fossils fired, sub critical, high pressure boilers ranging

from 100 to 190 kg/cm2. The saturation temperature of boiler water varies from 250 370 C depending

upon the boiler pressure. The capacity of these boilers varies from 60 MW to 500 MW, whereas the

heat flux varies from 200-300 KW/m2. These boilers are generally drum type and a few once through

types. Waterwall tubes of these boilers are made of carbon steel or low alloy steel. Dematerializedwater is used in these boilers with proper boiler water treatment generally recommended by Original

Equipment Manufacturer (OEM). Generally Non volatile treatment (NVT) i.e. TSP is used in most of the

boiler to maintain the pH except a few boilers where, all volatile treatment (AVT) chemicals is used. Due

to On-load corrosion in low pressure parts, condenser and heat exchangers of boilers, deposition of

corrosion products and salt concentration takes place on the internal surface of waterwall tubes. Ingress

of raw water due to condenser leaks into the feed water also increases the salt deposition and oxygen

concentration on internal surface due to boiling. The condenser of 500 MW generators can contain

300 miles of tubing and approx. 50000 tube-to-tube-plate joints, so some in-leakage of the external

cooling water is inevitable from time to time. Thermal conductivity of this deposit is very low (about 2

W/m/C) in comparison of carbon steel (about 50 W/mC), thus significantly reduces the heat transfer

and increases the outer metal temperature. Frequent tube failure has been observed in old boilers

ran more than 100,000 hours due to internal localised corrosion. Tube failure investigation indicated

that the main reason of waterwall tube failure is either due to hydrogen damage or caustic corrosion or

overheating or the effect of all. Some of the case histories experienced in different capacities boilers

of State Electricity Boards as well as NTPC Ltd. along with their remedial measures to attain zero tube

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failure have been discussed in this paper.

How mild steel corrodes in boiler water

Waterwall tubes in most of the fossil-fired boiler are made of carbon steel. In pure DM water, or in very

dilute acid or alkaline solutions at boiler temperature, it normally corrodes very slowly to form the black

iron oxide known as magnetite (Fe3O

4). The overall reaction is:

3 Fe + 4 H2O ------- Fe

3O

4+ 4 H

2

The corrosion rate is dependent by the rate at which the reactant (water) can reach the metal surface

and the reaction product can leave the surface. In nearly neutral solutions, magnetite is very slightly

soluble and it deposits as coherent and tenacious surface film, which greatly impedes this two-waychemical traffic. The transport processes are dominated by slow state diffusion through the oxide

layer and the corrosion rate is virtually independent of solution composition. The corrosion rate is also

diminishes with time as the oxide thickness grows. Even after years of exposure, the layer is no more

than a few microns thick.

In more alkaline or more acid solutions, magnetite becomes increasingly soluble and precipitates in

a different physical form. Instead of yielding a strongly coherent film, it has a more porous structure.

Soluble species can now diffuse relatively rapidly through the film and the corrosion rate is much

faster, although it still falls off the time as the oxide accumulated. The deposition of salts and corrosion

products observed in different waterwall tubes has shown in the photograph no.1-4.

Photograph no.1 & 2 showing waterwall tubes of 60 MW boilers having a very thick deposit ranging

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thickness 1.0-1.5 mm.

Photograph no. 3 & 4 showing a waterwall tube of 200 & 110 MW boilers having a thick iron oxide as

well as hardness salt deposition which is non uniform in nature.

Regions of Heat Transfer in Boi lers

Both recirculating drum type boilers and once-through boilers are in use in NTPC. Drum type boiler

consists of three separate components: an evaporator (waterwall area), where water is converted to a

steam-water mixture: a drum, in which two phases are separated and a super heater. A once-throughboiler, on the other hand, is a single unit with sub-cooled (below the boiling point at that pressure) water

entering at one end and superheated steam leaving at the other end. Because of its lower capital cost,

the once-through type is becoming more popular and now NTPC is going for 660 MW super critical

once-through boilers.

The various regions of heat transfer in a once-through boiler tube, with uniform heat flux, are shown in

Figure-1.

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Figure-1Showing water-steam formation in waterwall tube.

Of the six different heat transfer regions, five are usually present in a recirculation boiler (the exception

is the dryout / post-dryout regions. dryout should not occur in recirculation boilers, but is possible if the

flow in a tube is reduced for any reason, or if one or more adjacent tubes are plugged).

At the inlet, both wall and liquid are below saturation temperature and heating is by convection. At some

point along the tube, the wall temperature exceeds the saturation temperature and bubbles are formed

on the wall, although the bulk liquid temperature is below saturation. This is the region of sub-cooled

nucleate boiling. (Continuing along the tube, the region of saturated nucleate boiling, where the bulk

liquid temperature reaches saturation, is then enters).

As more steam is formed, small bubbles coalesce to form large bubbles (slug flow). Eventually, these

combine to give a central core of steam and leave an annular flow of water along the wall. Further

along the tube, the liquid film on the wall becomes sufficiently thin for convective heat transfer to the

liquid surface coupled with direct evaporation to become the only means of heat transfer, because

this method of heat transfer is highly efficient, nucleation of bubbles at the wall ceases through lack of

sufficient superheat. At the same time, increasing steam velocity results in entrainment of liquid in the

form of droplets. Depletion of the liquid film by evaporation and droplet formation eventually results in

complete dryout of the tube wall.

A dramatic increase in wall temperature occurs at the dryout point. In the post-dryout region heat

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transfer is by convection in the steam phase, the droplets gradually disappearing by evaporation. At

low heat fluxes direct impingement of droplets on to the wall may also occur. Evaporation of all droplets

finally produces single phase superheated conditions.

WATER CHEMISTRY

Impurities and additives present in water fed to high pressure boilers are controlled within closely

defined limits for two very good reasons.

(1) To reduce corrosion in the water-steam circuits to a minimum under normal and fault

conditions

(2) To reduce transport of iron oxide, hardness salts, silica etc. into the boiler and turbine. These

substances deposit out on the heat transfer surfaces and turbine blades.

Accelerated corrosion in many regions of high pressure boilers is known to be brought about byconcentration of corrosive substances (chlorides, sulphates, hydroxides and phosphates) by boiling.

The main water chemistry regimes used to reduce corrosion and transport of iron oxide etc. to a minimum

are given in Table below together with the advantages and disadvantages of each regime.

Regime Dissolved

Oxygen level

Protective

oxide

on tube

surface

Advantages Disadvantages

All volatile

treatment

(0.5 ppm

NH3)

Low

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in Table, very strict limits are specified on the concentrations of many other substances. For example, in

the case of a typical once-through boiler using all volatile treatment (AVT) water chemistry, upper limits

for various substances under steady load conditions are as follows:

Sodium 5 ppb Sulphate 2 ppb

Chloride 2 ppb Silica 20 ppb

N2H

4 1.5 X O

2(Min 10 ppb) Iron 5 ppb

Cu + Ti 2 ppb

CORROSION AND DEPOSITION IN VARIOUS HEAT TRANSFER REGIONS:

Convective Heating region:

In this region, at the inlet to a once through boiler, there have been no reported instances of tube failure

owing to heat flux conditions. In the absence of boiling, the increase in corrosion rate should be small

and related to the higher metal temperature and activation energy of the corrosion process in pure

water. Studies on surfaces which have been oxidized under good water chemistry conditions show that

even at high heat fluxes as high as 800 kW/m2, negligible amounts of sodium and chlorine are taken

up from NaCl, NaOH, NaHSO4, Na

2SO

4and Na

3PO

4solutions. Some iron is deposited from Fe (OH)

2,

which is likely to be the initial form of iron produced by protective or aggressive corrosion under alkaline

conditions.

Nucleate boiling regions:

Three types of situation are possible. Two of these are concerned with the formation of oxide deposits

with different porosities on a tube surface and the effect of this on corrosion. In the first instance, all

oxide is assumed to deposit from iron dissolved in solution and to form an oxide layer of low porosity

(50%). The situation in a real boiler may lie anywhere between these two

extremes. The third situation is concerned with boiling at defects on a tube surface.

Oxide deposit of high porosi ty:

Oxide of high porosity (>50%) is found to deposit in drum as well as once-through boilers under both

low and high oxygen water chemistry conditions. The deposition rate is approx. proportional to the

concentration of particulate iron oxide and the square of the heat flux. The best approximation to the

real situation is given by

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D = k q2c t

Where, D = amount of magnetite deposited (kg/m2)

q = the heat flux (W/m2)

c = concentration of iron in water (kg/m3)

t = time (hour)

k = constant ( approx. 5 X 10-13 / W2m2/s

In a wick boiling mechanism, salts dissolved in the boiler water can be concentrated by factors > 104

as shown in the figure-2.

Generally, the protective magnetite scale thickness is 10-15 microns in the waterwall tube. When the

corrosion rate increases due to upset of water chemistry parameters in boiler, (due to salt ingress and

concentration), the deposit formation also increases due to corrosion of metal and precipitation of

contaminants whose water solubility decreases at higher temperature on the evaporator tube surface.

To maintain the pH in boiler water, in case of reduction of pH due to salt ingress, addition of more

Tri Sodium Phosphate (TSP) is required. In this process, at some places on the internal surface of

waterwall tubes, deposit thickness increases and the protective iron oxide scale becomes non protective

and porous in nature. Porous, insulating types of deposits allow boiler water to diffuse into the deposit

where the water becomes trapped and boils.

The boiling of deposit in entrapped water produces relatively pure steam which tends to diffuse out of

the deposit, leaving behind super heated non-boiling equilibrium solution of caustic, which is responsible

for caustic corrosion or acidic solution, which is responsible of hydrogen damage in waterwall tubes as

discussed below.

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CAUSTIC CORROSION

Figure-2showing salt concentration under the deposit by Wick boiling phenomena

If the salt concentrated under the deposit is having high pH due to concentration of caustic from TSP

dosing, it start dissolution of protective magnetite (Fe3O

4) layer on the evaporator tube wall inner surface

and form sodium ferrite (NaFeO2)and sodium ferroate (Na

2FeO

2) as shown in the equation.

Fe3O

4+ 4 NaOH ------- NaFeO

2+ Na

2FeO

2+ 2 H

2O

Caustic corrosion has been shown in the photographs no.5 & 6. in waterwall tube of 200MW boilers.

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ACIDIC CORROSION

Solution of low pH is generated in high pressure boilers in two different ways:

1. pH of the entire boiler water is reduced when contaminants which are acidic or becomes acidic

when heated in to the boiler.

2. The bulk boiler water remains alkaline but acidic solutions are generated within corrosion pits

by the action of dissolved oxygen and chloride. The most common acid forming contaminant

is sea water or a river water which is low in carbonate and sulphate. In the boiler, the acidity is

increased locally to corrosive concentrations by boiling.

In the acidic or highly alkaline conditions, iron reacts and hydrogen is liberated.

Fe + 2 NaOH = Na2FeO

2+ H

2

Fe + 2 HCl = FeCl2 + H

2

If the hydrogen is liberated in an atomic form, it is capable of diffusing into the steel. Some of this

diffused, atomic hydrogen will combine at metal grain boundaries or inclusions to produce molecular

hydrogen, or it will react with iron carbides in the metal to produce methane.

Fe3C + 4 H = CH

4+ 3 Fe

Because neither molecular hydrogen nor methane is capable of diffusing through the steel, these gases

accumulate, primarily at grain boundaries. Eventually, the gas pressure created will cause separation

of the metal at its grain boundaries, forming discontinuous, intergranular micro cracks as shown in the

micrograph-1.

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Micrograph-1showing the fissures and cracks inside the metal due to hydrogen damage.

As these micro cracks accumulate, tube strength diminishes until stresses imposed by the internal

pressure exceed the tensile strength of the remaining, intact metal. At this point a thick-walled, longitudinal

burst may occur depending on the extent of hydrogen damage as shown in photographs no. 7 & 8.

Photographsno. 7 & 8, Waterwall tubes show the failure due to hydrogen damage due to localised

acidic condition.

EXPERIMENTAL PROCEDURE FOR DEPOSIT ASSESSMENT IN WATERWALL

TUBE

Boiler tube sampling:

Four waterwall tubes are cut from the four corner / sides of the high heat flux zone of the boilers, i.e.

mainly from the uppermost portion of (2-3 meters above) burner zone. The tube sample are preparedas per ASTM D-3483, machined and cut longitudinally in two parts, one being the hot side (internal

surfaces facing fire side) and the other, the cold side (internal surfaces facing remote side). Internal

deposition from the samples is removed for chemical analysis mechanically by pressing the machined

sample in a vice. The average quantity of internal deposits is calculated separately for both the sides

from the difference between sample tube weights, measured before and after the deposits are removed

chemically.

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Deposit quantity assessment:

Waterwall tube samples are collected from site and machined on the outer surface. The outer machined

surface of the tube is painted with corrosion resistant lacquer. The internal surface area (A, in cm2)

is measured and the initial weight (W1

, in mg) of the tube samples is measured. The waterwall tube

sample is cleaned in 5% inhibited hydrochloric acid solvent at 65 C and put on a magnetic stirrer till the

deposit is removed completely from the internal surfaces. Then it is washed with demineralised water

and an alkaline solution and dipped in a surface passivation solution for 5-10 minutes. The final weight

of the sample (W2, in mg) is measured. The quantity of internal deposit (DQ, in mg.cm2) is calculated

by the following formula.

W1 W

2

DQ = ---------------

A

CRITARIA FOR CHEMICAL CLEANING

Chemical cleaning of a boiler is suggested on the basis of the quantity of intenal deposit present in the

waterwall tubes of the boiler. When the quantity of deposit exceeds 40 mg/cm2, the tube surfaces are

considered to be very dirty surfaces as per the Indian Standard 10391-1998 and the chemical cleaning

is suggested to improve the heat transfer and reduce the overheating. The guidelines are given in

Table-1.

Table-1showing limits of deposit quantity allowed in waterwall tubes at different pressure.

The deposit quantity measured in high pressure boilers of different site are given here along with the

trend chart and recommendations for chemical cleaning.

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A List of NTPC boilers in which chemical cleaning carr ied out under the

supervision of NTPC (R&D)

1. Ramagundam, Boiler # 1, 200 MW Oct. 19982. Ramagundam, Boiler # 2, 200 MW Sept. 1999

3. Ramagundam, Boiler # 3, 200 MW July 2000

4. Tanda, Boiler # 1, 110 MW June 2000

5. Tanda, Boiler # 2, 110 MW Feb. 2002

6. Tanda, Boiler # 3, 110 MW Jan. 2001

7. Tanda, Boiler # 4, 110 MW Aug. 2002

8. Talcher Thermal, Boiler # 1, 60 MW Dec. 1995

9. Talcher Thermal, Boiler # 2, 60 MW Feb. 1996

10. Talcher Thermal, Boiler # 3, 60 MW June 200211. Talcher Thermal, Boiler # 4, 60 MW May 2000

12. Badarpur, Boiler, #1 95 MW Dec. 2006

13. Badarpur, Boiler # 3 95 MW Oct. 2004

14. Badarpur, Boiler # 2 95 MW Nov. 2004

15. Badarpur, Boiler #4 210 MW Aug. 2006

16. Korba, Boiler #1 200 MW Oct. 2007

17. Singrauli, Boiler #3 200 MW April 2008

18 . Tanda, Boiler # 3, 110 MW May 2008

19. Vindhyachal, Boiler # 1, 210 MW May 2008

20. Farakka, Boiler #3 200 MW July 2008

21. Ramagundam, Boiler #3 200 MW Aug. 2008

22. Unchhahar, Boiler #1 210 MW Aug. 2008

23. Rihand, Boiler #1 500 MW Sept. 2008

24. Ramagundam, Boiler #1 200 MW Sept. 2008

25. Vindhyachal, Boiler # 2 210 MW Oct. 2008

26. Vindhyachal, Boiler # 4 210 MW May 2009

27. Kahalgaon, Boiler # 4 210 MW May 2009

28. Farakka, Boiler # 2 200 MW May 2009

CONCLUSIONS

Failure investigation studies of waterwall tubes of different capacity boilers indicated the reasons of

tube failure are:

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4. David E. Hendrik, Hydrogen attack on waterwall tubes in a high pressure boiler, Material

Performance, Aug. 1995, pp-46-51.

5. C. Syrett, Corrosion in fossils fuel power plant, EPRI, USA

6. L. Tomlinson and A.M. Pritchard, Effects of heat flux on corrosion of high pressure boilers, Br.

corrosion J., 1985, vol.20, No.4, pp-187-195.

7. R.D. Port, Identification of corrosion damage in boilers, Material Performance, Dec. 1994, pp

45-51.

8. G.M.W. Mann, History and causes of On-load water side corrosion in power plants, Br.Corrosion J., 1977, vol. 12 No.1, pp 7-14.

ABOUT THE AUTHOR

Dr. Pradeep Jain, M.Sc., MBA, Ph.D. (Chemistry)

4 years experience in O&M, Water Chemistry at Thermal Power plants, Satpura and Korba

(MPEB) from 1982-86.

Joined NTPC (R&D) in 1986 and having 23 years of work experience at in thefield of water

chemistry and corrosion studies in High pressure Boilers, Turbines and Generators of power

plants.

Specialized in the field of Post-operational chemical cleaning of high-pressure boilers and

condensers.

Carried out solvent selection studies and supervised post-operational chemical cleaning of 35

nos. of NTPC and 06 numbers of State Electricity Boilers and improve the heat transfer efficiency

and heat rate in the range of 20-30 Kcal/kwh.

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Recently, he has visited to Fujairah water and power plant, UAE to find out the Root cause

Failure in evaporator tubes in HRSGs. It is an international consultancy work through NTPC

- International consultancy group.

Working in the field of void fraction measurement and thermal monitoring in waterwall tubes to

measure the heat flux, internal deposit and steam water ratio.

Published research papers in national and international journals andfile two patents for void

fraction measurement techniques.

Presently working as Deputy General Manager, NETRA-R&D

Supporting Tables:

Table-2, Badarpur uni t no. 1 was chemically c leaned once in Dec. 2006

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Table-3, Badarpur unit no. 2 was chemically cleaned once in Nov. 2004

Table-4, Badarpur unit no. 3 was chemically cleaned once in Oct.. 2004

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Table-5, Badarpur unit no. 4 was chemically cleaned once in Aug. 2006

Table-6, Badarpur unit no. 5, two stage chemical c leaning recommended in Dec. 2005

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