Download - U#3 BOILER CHEMICAL CLEANING
POST OPERATIONAL CHEMICAL CLEANING OF BOILER
BY DILIP KUMAR ,CHEMISTRY NTPC KAHALGAON
BY
CHEMCLEAN SERVICES
A-6, Sagar Co-op Hsg. Society, Near
Sushrut Hospital,
Chendhare, Alibag - 402201
DISTT. –RAIGAD (MAHARASHTRA)
(INDIA)
Post Operational Chemical Cleaning
of 210 MW Boiler Unit No.3 at NTPC Kahalgaon SEPT-2016
INTRODUCTION
Water-side scale build-up in boilers is
a progressive, inevitable process.
Even with stringent control of feed
water and condensate chemistry,
scale and deposition will occur.
The main problems caused by boiler
scales are:
1. Increase in tube wall temperature,
hence, boiler tube ruptures.
2. Decrease in overall boiler
PROBLEMS BY BOILER DEPOSITS
The increase in tube wall temperature is a
result of the low thermal conductivity of
scales as compared to metal. The
reduction in heat-transfer can lead to the
design temperature of the tube wall being
exceeded, which in turn may lead to failure
of the tube by creep rupture.
BOILER TUBESFINS
INSULATION MATERIAL
FURNACE FLAME
PROBLEMS BY BOILER DEPOSITS
Overall efficiency can be
defined as the ratio of
steam output to the fuel
consumption ratio.
Again, since scaling
impedes heat transfer,
more fuel is required to
produce a given amount
of steam, thus reducing
overall efficiency and
loss of energy.
REQUIREMENT OF CHEMICAL CLEANING
Removal of scale from the boiler becomes
essential if damage to the boiler is to be
prevented. One way of removing scale is to
chemically clean the boiler. Chemical
cleaning is a multiple stage process that
seeks to remove all the existing scale from
the boiler internals, leaving a clean,
passivated waterside system.
One step in the process involves the use
of inhibited acid to dissolve the scales.
This acid stage is potentially damaging to
THE CHEMISTRY OF BOILER SCALES
The primary constituent of boiler
scales is magnetite (Fe3O4), which is
formed as a result of the reaction of
metallic iron with high-temperature
steam.Copper is present due to corrosion of
the copper alloy, aluminium bronze
feed water condensers and pre-
heaters, often because of oxygen
ingress into these systems. Copper is
transported through the steam cycle
where it forms on the boiler internals.
THE CHEMISTRY OF BOILER SCALES
Other crystalline materials, some shown in Table
Table 1: Compounds Found in Boiler Scales
Compound
FormulaAnhydrite CaSO4
Aragonite CaCO3
Brucite Mg(OH)2
Copper Cu
Calcite CaCO3
Hematite Fe2O3
Hydroxyapetite
Ca10(OH)2(PO4)6
Magnetite Fe3O4
Quartz SiO2
Thenardite Na2SO4
Wollastonite CaSiO3
CHEMICAL CLEANING PROPOSAL
Chemical cleaning proposals must
take into consideration the
different compounds present in the
scale in order to formulate the
optimum cleaning solutions. This
recommendation should effectively
remove scale without damaging
the underlying metal.
BOILER TUBE SAMPLES
Tube samples should be removed
from locations where heavy scaling
is suspected. The tube sample
should be at least three feet long
so the method of removal (cut-off
wheels and cutting torches) does
not contaminate the scale at the
centre of the sample with slag or
filings.
ANALYSIS OF SCALES
The scale density is determined
gravimetrically after dissolution of
scale in inhibited HCl. Loss of
weight on firing in a furnace will
measure the percentage of
hydrocarbons present, which then
determines the need for alkaline
degreasing. The need to clean is
based on the scale density.
SCALE DENSITY Vs ACTION PLAN
Scale Density Ranges and Required Action
Scale Density g/ft2 or (mg/cm2) Recommended Action
< 23 (25) No action required23 – 70 (25 – 75) Chemically clean within one year70 – 93 (75 – 100) Chemically clean within three months> 93 (100) Chemically clean before further operation
NTPC- NETRA
ANALYSIS OF SCALES DONE AT
NETRA( NTPC Energy Technology
Research Alliance) GREATER
NOIDA AND RECOMMENDED
TWO SAGE BOILER CHEMICAL
CLEANING OF KhSTPP U#3
CHEMICAL CLEANING RECOMMENDATIONS
BY NETRA
Internal deposit 18.6-
52.5mg/cm2 at hot side of
surfaces and 15.6-48.6
mg/cm2 at cold side of
surfaces which is more than
40 mg/cm2 as per the limit if
IS-10391 and copper in
deposit is > 20%. On the
basis of internal deposit
quantity and deposit
analysis, two stage post –
operational chemical
cleaning was recommended.
Deposits and scale may consist of silicates, sulphates, sulphites, carbonates, calcium, organic growths, etc. and all have the potential to cause tube wall damage and/or decrease efficiency.
THE STEPS TO BOILER CHEMICAL CLEANING
AND TREATMENT SELECTION
Cleaning a boiler usually consists
of a combination of the following
stages:
o Mechanical cleaning
o Water flushing
o Alkaline treatment
o Solvent cleaning
o Neutralization and Passivation
THE STEPS TO BOILER CHEMICAL CLEANING
MECHANICAL CLEANING: Mechanical cleaning and
water flushing can remove loose scale and other debris
from the boiler.
ALKALINE TREATMENT: Alkaline treatment removes oils
and hydrocarbons that might interfere with the
dissolution of the scale by acid solvents.
SOLVENT CLEANING: The solvent cleaning stage is the
process in which inhibited acid is used to remove scale
form the boiler.
Once the scale is removed in the solvent stage, fresh
active metal is exposed.
NEUTRALIZATION AND PASSIVATION: The neutralization
HOT ALKALINE TREATMENT SELECTION
If oil, grease, carbon or other
organic compounds are present,
they must be removed during
chemical cleaning. Selection
depends on the degree of
contamination. Use hot alkaline
treatment only when organic
deposits interfere with solvent
cleaning. If solubility of deposits is
greater than 70 percent in solvent
PROPOSED SCHEME FOR CHEMICAL
CLEANING
CHEMICAL CLEANING SITE
CHEMICAL CLEANING SITE
(TOP VIEW)
CHEMICAL RECIRCULATION PUMPS
CHEMICAL CLEANING TECHNIQUE
Chemical cleaning shall be carried out
by circulation method using adequate
capacity and head pumps for achieving
desired velocity in each down comer and
riser tubes as well as the other parts of
the boiler. In general, circulating water
pumps will take suction from chemical
mixing tank and discharge is connected
to feed water line inlet to economizer.
The return from boiler is taken from
lower water wall headers to the chemical
SEQUENCE OF CLEANING OPERATION
The following sequence of operations shall be
carried out for complete removal of various
post-operational deposits such as Copper
Oxide, Iron oxide and other water side
impurities:
1. Preservation of super heater.
2. Water Rinse (Cold & Hot)
3. Copper Removal 1st Stage.
4. Acid Pickling / Iron & Copper
Removal
5. Citric Acid Rinsing
6. Neutralization
PRESERVATION OF SUPER HEATER
The Super Heater Tubes Outlet is to be
plugged from inside of drum first and
Super Heater Coils are preserved by
filling of Ammoniated Hydrazine Solution
through Super Heater Drain Pipe Line.
Hydrazine concentration is to be
maintained at 200ppm and pH>10.This
system is to be kept under Constant
pressure by pumping the preservation
solution (The Super Heater System is
TEST SAMPLES
Water wall tube pieces
of about 4”-6” long
should be cut from
boiler and hanged
inside the boiler drum
by steel wire to see
the efficacy of
chemical cleaning just
after the completion
of procedure.
BOILER TUBES SAMPLE BEFORE CHEMICAL CLEANING
1 2 3 4 5
COLD WATER RINSE
Cold D.M. Water flushing of the
system are carried out to
remove the loose deposit and
water soluble impurities from
the system. Turbidity should
be observed visually.
The process is to be
terminated once the water is
clear.
ACID R/C TANK
PUMP
TEMPORARY HEADER
ECO
NO
MISER
DO
W N
C O
M E R
RI S E R TUBE
DRUM
DR
UM
VEN
T
BOTTOM RING HEADER
TO ECONOMISER INLET
W /
W T
UB
ES
Eff Pit
DM WATER
HOT WATER RINSE
Hot Water Flushing shall be
done at 60-65°C using external
steam for heating of the system
to removed the loose deposit
and water soluble impurities.
Turbidity should be observe
visually. The process is to be
terminated once the water is
1st STAGE COPPER REMOVAL
Copper removal shall be carried out by using the
following chemicals quantity as in % with D.M.
Water and temperature maintained 65-70°C.
a] Add Liquid Ammonia 1% (approx.) to raise pH
9.5 – 10.0
b] Sodium Nitrite (0.75%)
c] Ammonia Bi Carbonate (0.25%)
Add chemicals in following sequence after get
desired temp.
1. Add Liquid Ammonia 1%w/v.
2. Add 0.75% w/v Sodium Nitrite slowly, which acts
as an oxidizing agent.
3. Then Add Ammonium bi-Carbonate (0.25).
ACID R/C TANK
PUMP
TEMPORARY HEADER
ECO
NO
MISER
DO
W N
C O
M E R
RI S E R TUBE
DRUM
DR
UM
VEN
T
BOTTOM RING HEADER
TO ECONOMISER INLET
AMMONIA,
SODIUM NITRITE’
AMMONIUM BICARBONATE
W /
W T
UB
ES
To Effluent Pit
STEAM
CHEMICAL REACTIONS IN COPPER REMOVAL
D.M. WATER FLUSHING
D.M. Water filled and re-
circulation establish along with
temp.60°C raising and drained
off the entire system and
analyzed the alkalinity nil traces
and pH normal.
Terminated the process at get
pH normal i.e., 7± 0.2 and
IRON AND COPPER REMOVAL
Iron & Copper removal shall be carried
out by the using of following
Chemicals quantity as in
% with D.M. water.
1. Acid Inhibitor .(0.25%) Rodine 213Spl
2. Hydrochloric Acid (5%)
3. Ammonium Bi-fluoride (0.25%)
4. Thio-Urea (1.0%)
Inhibited hydrochloric acid is a most
widely used solvent since it produces
good solubility with a wide variety of
scales, is economical and is easy to
handle. It shows good corrosion
characteristics when adequately
inhibited and the process is controlled
within the accepted limits.
The process is flexible and can be
tailored to complex copper by the
addition of thio-urea to enhance silica
IRON AND COPPER REMOVAL
IRON AND COPPER REMOVAL PROCEDURE
1.Fill the boiler system by DM water and establish
recirculation.
2. Raise the temperature of circulating water about
65-700C in return line.
3. Cut off steam and add a small quantity of HCl and
immediately followed by required quantity of acid
inhibitor. Acid concentration not exceeded by 6%.
4. Add required quantity of thio-urea into the
circulation tank and circulate the system for complete
mixing of chemicals.
5. Add calculated quantity of Ammonium bi-fluoride in
the mixing tank slowly and circulate the system for
complete mixing of chemical.
6. The circulation will be terminated once steady
IRON AND COPPER REMOVAL REACTIONS
HYDROCHLORIC ACID
THIO-UREA
AMMONIUM BI-FLUORIDE
CHEMICALS
IRON AND COPPER REMOVAL REACTIONS
DM WATER RINSE
Fill the system with the D.M. Water
and raise the temp up to 60°C.
charging steam to the circulating
water. Analyze the samples from
inlet sampling point and a system
return line for acid, copper and iron
concentration. Terminated the
process after circulate the system
for half an hour. And drain the
system completely under Nitrogen
CITRIC ACID RINSE
Fill the system with the D.M.water.
Charges steam in the Mixing tank and
raise the temp. Of circulating water to
60-65°C. Add. 0.2% w/v Citric Acid and
Liquid Ammonia to raise the pH to 3.5-
4.0
Terminated the process after circulate
the solution for 2 hrs. Analyzed the
sample for pH, copper and Iron
CITRIC ACID RINSE
The normal reasons for Citric Acid
selection are:
oPresence of austenitic materials of
construction.
oExtremely effective copper removal
from high copper- content scales.
oReduces cleaning time by eliminating
the need to drain, flush and refill the
boiler between stages because iron
CITRIC ACID RINSE CHEMICAL REACTIONS
Citric acid is weak and tri-basic organic acid and forms complex compound with Fe2+ and Fe3+ which are stable in solution.
D.M. WATER RINSE
Fill the water with D.M.water by
circulating pump. Rise the
temperature up to 600C.
Terminated the process on
getting Iron Concentration < 25
ppm, copper concentration <1
ppm and Acidity Nil. Drain the
total system under Nitrogen
Capping.
NEUTRALISATION
Fill the system with DM water and
establish circulation. Measure the iron
content in the water. When the iron
content is less than 25 ppm, Add 0.5% of
soda ash in the circulating tank to
neutralize the system. Raise the
temperature of circulating water to 80-85
ºC by addition of steam and circulate for
6 hours. Measure the pH in the inlet and
outlet of samples. Drain the system
without nitrogen capping.
The purpose of neutralization is to end acidic nature of solution.
PASSIVATION STAGE-1
Purpose of this activity is to
create a uniform layer of
protective iron oxide on the
freshly treated surface. Use the
same system as used in acid
cleaning and carry out first
passivation immediately after
acid cleaning.
PASSIVATION STAGE-1
Fill the system with D.M. water. Raise Temp.
of circulating water to 80-90°C by charging
steam. Add required quantity of Hydrazine
hydrate to maintain 200ppm and ammonia
added to maintain the pH above
9.0.circulate for 20 hrs. Measure the
Hydrazine concentration and Ph of
circulating water periodically. Add the
hydrazine into the solution when the
concentration goes below 200 ppm. Drain
the system in hot condition. Open vent,
drum manholes and cut bottom ring header
Inspect the drum (after cooling) for magnetite coating and remove debris if available.
PASSIVATION STAGE-1
INSIDE VIEW OF BOILER DRUM AFTER STAGE-1 PASSIVATION
PASSIVATION STAGE-1
PASSIVATION STAGE-1
BOILER TUBE SAMPLES BEFORE CLEANINGBOILER TUBE SAMPLES AFTER CLEANING
PASSIVATION STAGE-1
• The magnetite layer of thickness
50-100A thickness is temporary
during this process so second
stage passivation is required.
• After draining the system hot cut
bottom ring header stubs and
cool down the system under
aeriated condition.
• After the chemical cleaning the drum, internals are restored and the boiler is prepared for regular operation.
• Fill up the boiler with DM water having N2H4 200ppm and for pH>10.
• Light up the boiler at 40kg press and maintain the press for 24 hrs during the process maintain N2H4 at 200ppm now the boiler can be
PASSIVATION STAGE-2
EFFLUENT NEUTRALISATION & DISPOSAL
Remnant Hydrochloric Acid can be
neutralized by adding Soda Ash in
Neutralizing pit.
• Make arrangement to dispose the effluents
into the ash dyke. confirm regulation of
pollution control board norms. The large
ash pond with sufficient retention time can
help non-complex copper, Iron to precipitate
as their hydroxide.
• The effluent after first stage passivation
will be treated with bleaching power.
EFFLUENT NEUTRALISATION & DISPOSAL
Effluent
Neutralization
System contains
Effluent pit,
Effluent recirculation
pump,
Effluent disposal
pump, and
Air compressor for
effluent mixing
EFFLUENT NEUTRALISATION AND DISPOSAL SYSTEM
Chemical Cleaning Evaluation
Inspection of the boiler after the chemical
cleaning is crucial to determine if the
procedure has been successfully
completed. Visual and video boroscope
inspections determine the effectiveness of
the cleaning. There should be no visible
traces of water and loose or adherent scale
inside the boiler drums and tubes. Remove
the corrosion coupons and the polarization
probes, visually examine them, determine
their weight loss and calculate the loss of
Chemical Cleaning Evaluation
Scale Density after cleaning - Cut a tube
sample after chemical cleaning and determine
the density of any deposit.
CONCLUSION
It is essential to clean a boiler periodically for
efficient operation, corrosion control, reliability
and prevention of tube failures. Cleaning is
accomplished by a combination of steps. For
some boilers it may not be necessary to use all
the cleaning steps, since the degree of
contamination will vary from one boiler to
another. The exact procedure to be used
depends upon the scale density and its analysis,
tube bulging or failure, water treatment analysis,
inspection and history or the unit itself. The
formulations recommended do not override the
boiler manufacturer's chemical cleaning
THANK YOUSAVE NATURE