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