110844165 Chemical Cleaning NPTI

8/10/2019 110844165 Chemical Cleaning NPTI

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CHEMICAL CLEANING

OF BOILERS


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Need For Chemical cleaning ?


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TYPES OF CHEMICAL

CLEANING

PRE-COMMISSIONING / PRE-

START CLEANINGPLANNED / SCHEDULED

CLEANING


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PLACE OF IMPURITIESWelded joints in pressure

parts.Accumulation offlux,silicates in the electrodes.

Corrosion products.

Lubricants,grease,oil,mill

scales and other impurities.


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PURPOSE OF CHEMICAL CLEANINGThe purpose of chemical cleaning of

the internal surfaces of a new boiler

which come into contact with waterand steam is to keep them clean for

ensuring efficient heat transfer, for

maintaining steam purity and to avoidfailures of pressure parts.


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Corrosion:

Corrosion is nothing but when the

pure metal goes back into the

original oxide form for example

Iron metal into Iron oxide, Copper

into Copper oxide.

Because a pure metal cannot stayas such when it is exposed to

moisture, air, temperature etc. it

will try to convert into oxide form.

Like human being cannot stay in

isolation in the society.


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These may be classified into

two major categories. Pitting

corrosion and Galvaniccorrosion. The pitting

corrosion is due to the Oxygen

present in the water.


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The Galvanic corrosionis due to two different

metals present in the

material of construction.

I would like to give an

example for this.


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The Statue of Liberty is supported

by Bronze and iron rods during theerection stage. After prolonged

period of time it was found that

iron rods are getting corroded

because of galvanic corrosion.Then the supports are replaced

with Stain steel rods.


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UNDER GROUNDWATER:

1. WELLWATER

2. SPRING WATER


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COMMON IMPURITIES:

TURBIDITY: CAUSES DEPOSITS

SOLUTION: COAGULATION

COLOUR: CAUSES FOAMING

SOLUTION: CHLORINATION

HARDNESS: CAUSES SCALE

SOLUTION: SOFTENING


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ALKALINITY: CAUSES CARRY OVER

SOLUTION: LIME SODA/DM PLANT

FREE MINERAL ACIDITY: CORROSION

SOLUTION: NEUTRALISATION

CARBON DIOXIDE: CAUSES CORROSION

SOLUTION: AERATION

PH: 0.0 ACIDIC

7.0 NEUTRAL

14.0 BASIC


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FLOURIDE: CAUSES MOTTLED

ENAMEL IN TEETH

SOLUTION: DM PLANT

SILICA: CAUSES SCALESOLUTION: DM PLANT

WITH STRONG BASE ANION

IRON: CAUSES DISCOLOUR

SOLUTION: AERATION OR DM PLANT


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MANGANESE: CAUSES DISCOLOUR

SOLUTION: AERATION OR DM PLANT

OIL: CAUSES SCALE, SLUDGE, FOAMING

SOLUTION: STRAINERS /COAGULATION

OXYGEN: CAUSES CORROSION

SOLUTION: DEARATION / Na2SO4


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HYDROGEN SULFIDE: CAUSES

CORROSION

SOLUTION: AERATION /CHLORINATION

AMMONIA: CAUSES CORROSION

SOLUTION: CHLORINATION /

AERATION

CONDUCTIVITY: CAUSES CORROSION

SOLUTION: DM PLANT /

REVERSE OSMOSIS


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DISSOLVED SOLIDS: CAUSES FOAMING

SOLUTION: DM PLANT

SUSPENDED SOLIDS: CAUSES DEPOSITS

SOLUTION: FILTRATION


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SCOPE

Economizer, evaporator,upriser,

Down comer, SH, DESH andMS lines up to boiler stop valve,

interconnecting pipes and drains.


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REASONS FOR CLEANING

To enable the plant to be commissionedand operated without incurring any

physical or corrosive damage.

To eliminate deposits which affects the

heat transfer and efficient operation.

To ensure quality of the feed water and

steam.

To remove oil, grease and protectivecoating.

To remove construction scales so as to

avoid any blockage in lines, valves etc.,

Mech.damages may also be caused due to


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METHODS OF CHEMICAL

CLEANING

SOAKING METHOD

CIRCULATION METHOD


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1.SOAKING METHOD

4 TO 8 HOURS

TO ENSURE COMPLETE

REMOVAL OF DEPOSITSACID STRENGTH OF THE

SOLVENT SHOULD BE

GREATER. 5 TO 6% END RESULT IS NOT

SATISFACTORY


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2. CIRCULATION METHOD

4 TO 6 HOURS

LOWER SOLVENT

STRENGTH 3 TO 5 %

RESULT IS SATISFACTORY


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SOLVENTS

SELECTION:

EFFICIENCY AND SPEED OF

DISSOLUTION OF METALOXIDES

AVAILABILITY

COST

DEGREE OF SAFETY WITH

RESPECT TO PLANT AND

PERSONAL.


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SOLVENTS USED

1. MINERAL ACIDS: HCL, H2SO4,

HF

a) HCL

WIDELY USEDEFFECTIVE IN REMOVING

METAL OXIDES (Fe,Ni,Zn,Ca,Mg)

b) H2SO4

STRONG ACID-LIBERATE HEAT WHEN MIX

WITH WATE

c) HF

Effective at lower temp.


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SOLVENTS USED

2) ORGANIC ACIDS

3) COPPER REMOVAL

SOLUTIONS4) ALKALINE SOLUTIONS


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INHIBITORS

TO PREVENT /MINIMISE THE

ATTACK OF ACID ON THEBOILER PARENT METAL

IT PROVIDES BLANKET

OVER PARENT METALSURFACE.


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INHIBITORS

ARSENIC COMPOUNDS

BARIUM SALTS

QUINOLINE

PYRIDINE

COMMERCIAL INHIBITOR IS

RHODINE


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PROCESS OF CHEMICAL

CLEANING1.Cold water flushing2.Hot water flushing

3.Alkali boil out

4.RINSING AFTER ALKALI BOILOUT

5.ACID CLEANING

6.Citric rinsing7.DM water rinsing

8.Neutralising Boil out

9.Passivation


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1) Cold water flushing

up to boiler stop valve

Flushing with DM water

to remove loose debris and

silica bearing deposits.


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2) Hot water flushing

Hot DM water flushing at 80-

85 deg and draining.


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3) Alkali boil outActual firing of boiler with tri

sodium phosphate (0.1%) &Hydrazene hydrate (0.02%).

40 ksc-16 to 20hrs.-To

completely remove grease oroil.


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4) RINSING AFTER ALKALI

BOIL OUT

Flushing up to boiler stop

valve & SH backwashing

and rinsing till PO4 is nil.

Fill SH with hydrazene

dozed water and plug it.


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5) ACID CLEANINGa) Acid cleaning by methods ofsoaking and auto circulation inevaporator- To remove scales anddeposits.

b) raise DMT=80deg.

c) Chemicals: Hydro chloric

acid & ammonium biflouride.d) Once acid and iron

concentration equals acid cleaning isdeemed to be complete.


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6) Citric rinsing

To prevent iron oxide sludge

formation by contact of

freshly cleaned surface withDM water, 0.2% citric acid

ammoniated with DM water is

circulated.


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7) DM water rinsing

The system is flushed with

DM water till dissolvediron expelled from it.


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8) Neutralising Boil out

DM water with Disodium

phosphate and Trisodium

phosphate filled & lighted

up. Pr.10 ksc. Alkalinity

will decrease and stabilise.


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9) Passivation

Actual layer of magnetite

coating will be formed.

Drum pr.40 to 42 ksc

Hydrazine 200 ppm at

drum for 24 hours.


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Because the boiler water

circulates 7 times in theBoiler and getting

concentrated, like the COC

in circulating water

system, therefore may

cause harm to the boilerpressure parts if not

treated with much care.

110844165 Chemical Cleaning NPTI

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