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7/31/2019 Chemical Cleaning NPTI
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CHEMICAL CLEANING
OF BOILERS
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Need For Chemical cleaning ?
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Different types of Chemical
cleaning
Alkali Boil out
Acid 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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SULFATE: CAUSES ADDS TO SOLIDS
SOLUTION: DM PLANT
CHLORIDE: ADDS TO SOLIDS
AND
ALSO CORROSION
SOLUTION: DM PLANT
NITRATE: ADDS TO SOLIDSAND
ALSO CORROSION
SOLUTION: DM PLANT
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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 OUTFlushing 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.