HOW TO INCREASE HEAT TRANSFER AND REDUCE WATERWALL ABSTRACT:

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 onload 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 ºC 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. Dematerialized
water 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/ºC) in comparison of carbon steel (about 50 W/mºC), 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 (Fe3O4). The overall reaction is:
3 Fe + 4 H2O -------à Fe3O4 + 4 H2
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-way
chemical 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 Boilers
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-through
boiler, 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-1 Showing 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 by
concentration 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
All volatile
treatment
(0.5 ppm
NH3)
Low <5 ppb)
Protective
oxide
on tube
surface
Magnetite,
Fe3O4
Advantages
Disadvantages
Presence of ammonia No neutralisation action at
reduces transport of
high temperature.
iron from feed train
Ammonia can attack copper
into boiler
containing alloys in condenser
Ammonia is not
and feed heaters in presence
corrosive at high
of oxygen
temperature
Na3PO4
Low <5 ppb)
Magnetite, Neutralizes acid
Na3PO4 or NaOH can be
or NaOH
Fe3O4
chloride and acid
concentrated by boiling,
dosed
sulphates
forming corrosive solutions.
Reduced rate of
No neutralising action
High
High >200 ppb) Hematite
transport of iron into
Ingress of chloride results in
Oxygen
(Alpha
boiler compared with rapid corrosion.
Fe2O3)
low oxygen.
Besides controlling the levels of dissolved oxygen and pH by means of chemical additives, as indicated
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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
Chloride
N 2H 4
Cu + Ti
5 ppb
2 ppb
1.5 X O2 (Min 10 ppb)
2 ppb
Sulphate
Silica
Iron
2 ppb
20 ppb
5 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, Na2SO4 and Na3PO4 solutions. 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
(<10%). In the second situation, all oxide is considered to deposit from particles suspended in the liquid
and to have a high 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 porosity:
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 q2 c 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 / W2 m2/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-2 showing 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 (Fe3O4) layer on the evaporator tube wall inner surface
and form sodium ferrite (NaFeO2) and sodium ferroate (Na2FeO2) as shown in the equation.
Fe3O4 + 4 NaOH
-------à
NaFeO2 + Na2FeO2 + 2 H2O
Caustic corrosion has been shown in the photographs no.5 & 6. in waterwall tube of 200
MW 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 =
Na2FeO2 + H2
Fe + 2 HCl
FeCl2 + H2
=
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
=
CH4 + 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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3.1- 10/17
Micrograph-1 showing 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.
Photographs no. 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 prepared
as 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 ºC 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 – W2
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-1 showing 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 carried out under the
supervision of NTPC (R&D)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18 .
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
Ramagundam, Boiler # 1,
Ramagundam, Boiler # 2,
Ramagundam, Boiler # 3,
Tanda, Boiler # 1,
Tanda, Boiler # 2,
Tanda, Boiler # 3,
Tanda, Boiler # 4,
Talcher Thermal, Boiler # 1,
Talcher Thermal, Boiler # 2,
Talcher Thermal, Boiler # 3,
Talcher Thermal, Boiler # 4,
Badarpur, Boiler, #1
Badarpur, Boiler # 3
Badarpur, Boiler # 2
Badarpur, Boiler #4
Korba, Boiler #1
Singrauli, Boiler #3
Tanda, Boiler # 3,
Vindhyachal, Boiler # 1,
Farakka, Boiler #3
Ramagundam, Boiler #3
Unchhahar, Boiler #1
Rihand, Boiler #1
Ramagundam, Boiler #1
Vindhyachal, Boiler # 2
Vindhyachal, Boiler # 4
Kahalgaon, Boiler # 4
Farakka, Boiler # 2
200 MW
200 MW
200 MW
110 MW
110 MW
110 MW
110 MW
60 MW
60 MW
60 MW
60 MW
95 MW
95 MW
95 MW
210 MW
200 MW
200 MW
110 MW
210 MW
200 MW
200 MW
210 MW
500 MW
200 MW
210 MW
210 MW
210 MW
200 MW
Oct. 1998
Sept. 1999
July 2000
June 2000
Feb. 2002
Jan. 2001
Aug. 2002
Dec. 1995
Feb. 1996
June 2002
May 2000
Dec. 2006
Oct. 2004
Nov. 2004
Aug. 2006
Oct. 2007
April 2008
May 2008
May 2008
July 2008
Aug. 2008
Aug. 2008
Sept. 2008
Sept. 2008
Oct. 2008
May 2009
May 2009
May 2009
CONCLUSIONS
Failure investigation studies of waterwall tubes of different capacity boilers indicated the reasons of
tube failure are:
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Formation of localised alkaline / acidic conditions is as per the mechanism of wick boiling phenomena
under the deposit due to ingress of raw water into the condensate water. Acidic localised condition is
responsible for hydrogen damage as observed in many old boilers where the deposit is very high and
adherent type. Alkaline condition is responsible for caustic corrosion where the deposit is dense and
porous in nature.
Overheating of tubes due to deposition of salts on the internal surface of tube.
REMEDIAL MEASURES
It is suggested, to control the ingress of cooling water from the condenser tube leakage which will
reduce the phenomena of salt concentration on the internal surface of boiler tubes.
If the internal oxide growth increases from the limit as per the Indian Standards of IS-10391-98. Post
– operational chemical cleaning of boilers should be carried out to remove the existing porous deposit
and to form a new adherent magnetite layer. Magnetite layer will work as a protective layer and reduce
the possibilities of on-load corrosion.
ACKNOWLEDGEMENT
Authors are thankful to Shri Sharad Anand, E.D., NETRA, Shri A.K.Mohindru G.M. (NETRA-R&D)
and Shri J. Rajendran AGM (NETRA-R&D) for his continued interest, valuable guidance and constant
encouragement for chemical cleaning of boilers. The contributions of our colleagues of NTPC sites
and NETRA, who have helped directly or indirectly for carryout the chemical cleaning, are greatly
acknowledged.
REFERENCES
1.
Test methods for accumulated deposition in a steam generator tube, 2005, ASTM International,
West Conshohocken, PA, USA, ASTM Standard D-3483-05.
2.
Code of practice for chemical cleaning of boilers, 1998, Bureau of Indian Standards, New
Delhi, IS-10391-98.
3.
Sugimoto A, Ueki, H. Sakuma, S., Proc. American Power Conference, 1972. Illinois institute of
Technology, Chicago, IL, USA, 34, 764
Pradeep Jain
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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 the field 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 HRSG’s. 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 and file two patents for void
fraction measurement techniques.
Presently working as Deputy General Manager, NETRA-R&D
Supporting Tables:
Table-2, Badarpur unit no. 1 was chemically cleaned 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 cleaning recommended in Dec. 2005