pp 1 water tech
TRANSCRIPT
-
7/27/2019 PP 1 Water Tech
1/70
-
7/27/2019 PP 1 Water Tech
2/70
WATER TECHNOLOGYfor
HIGH PRESSURE BOILERS
-
7/27/2019 PP 1 Water Tech
3/70
-
7/27/2019 PP 1 Water Tech
4/70
-
7/27/2019 PP 1 Water Tech
5/70
-
7/27/2019 PP 1 Water Tech
6/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
water technology
Waterwall deposit analysisCHEMICAL COMPOSITION
0.10.20.10.2Sodium oxide,Na2O
0.40.13.01.8Magnesium oxide,MgO
1.10.529.819.1Calcium oxide,CaO
7.73.21.84.9Copper , Cu88.695.155.058.6
Iron oxide, Fe3O
4
0.80.58.714.7Silica , SiO2
% by weight
Sample 1 Sample 2 Sample 3 Sample 4
Parameter
-
7/27/2019 PP 1 Water Tech
7/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
water technology
QUANTITY OF DEPOSIT (mg / sq.cm.)
38.525.5---38.5RHS waterwall
80.928.1---64.4LHS waterwall
54.822.158.647.4Rear waterwall
80.521.955.053.7Front waterwall
Sample 4Sample 3Sample 2Sample 1
-
7/27/2019 PP 1 Water Tech
8/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
water technology
Quantity of deposit and unit cleanliness
Quantity of deposit Surface cleanliness
Less than 15 mg /sq.cm. Clean surface
15 to 40 mg / sq.cm. Moderately dirty
more than 40 mg/sq.cm. Dirty
Chemical cleaning should be done whenever depositsare more than 40 mg / sq.cm .
once in 4 years as a mandatory maintenance practice
( guidelines only / not a rule or code ) BIS : 10391
-
7/27/2019 PP 1 Water Tech
9/70
1-2 .. TUBE METAL RESISTANCE
( ~ 30O C FOR 2.5mm WALL )
2-3 .. INTERNAL DEPOSITRESISTANCE
3-4 .. WATER STEAM FILM
RESISTANCE
HEAT TRANSFER GRADIENTS
AND EFFECT OF DEPOSITS
1
2
3
4
FIRESIDE
Tube metal and water film resistance
*** more or less constant
the temperature of the inner & outer wallrises due to changes in the deposit
thickness & nature ( thermal conductivity
of the deposit ) Silica , Ca , Mg salts very poor thermal conductivity
-
7/27/2019 PP 1 Water Tech
10/70
285OC285OC
-
7/27/2019 PP 1 Water Tech
11/70
1
2
3
4
SCALE THICKNESS
250 m 500 m 750 m
410OC 478OC 550OC
360OC 428OC 500OC
297OC 297OC 297OC
285OC 285OC 285OC
1-2..TUBE METAL RESISTANCE
2-3.. INTERNAL SCALE & DEPOSIT
RESISTANCE
3-4.. WATER / STEAM FILM RESISTANCE
285OC
EFFECT OF DEPOSIT ON HEAT TRANSFER
250 m thickness
nail
-
7/27/2019 PP 1 Water Tech
12/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
water technology
Method of tube sampling location of tube samples very important
( since deposits may vary appreciably in different parts
of a boiler ) tube samples to be taken from locations representing
heaviest deposit (corrosion product ) formation
Band starting the central line of the uppermost burnerand extending up the furnace by about 2 to 3 metres
Operating experience in specific units
- problem areas : arch tubes in natural circulation units
or other horizontal or low-sloped circuits
Three adjacent tubes to be removed for comparison ;samples should be 600 mm in length and ends sealed
-
7/27/2019 PP 1 Water Tech
13/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Objectives of water treatment
To reduce corrosion of metals
To prevent formation of deposits
To produce good quality steam
-
7/27/2019 PP 1 Water Tech
14/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
New units Internal surfaces of heat exchanger tubes to
be clean --- before put into service
Oil, grease , sand etc removed by alkali boilout
Chemical cleaning ( acid pickling ) for removal
of rust and millscale Thin , dense , uniform layer of magnetite to be
formed on the internal surfaces of the tubes to
protect from corrosion ( passivation )
-
7/27/2019 PP 1 Water Tech
15/70
Saponification of oilO||
H2C O CCH2 CH3
O||
H C O CCH2 CH3 oilmolecule
O||
H2
C O CCH2
CH3
O||
H2C O H NaOCCH2 CH3
O||
H C O H NaO CCH2 CH 3
O||
H2 C O H NaO CCH2 CH3
-
7/27/2019 PP 1 Water Tech
16/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Why passivation ?Magnetite , Fe3O4 Ferric oxide , Fe2O3
colour black brownish red binding nature tightly binds to flakes off easilybase metal from base metal
Significance protects the does not protectw.r.to corrosion base metal the base metal
-
7/27/2019 PP 1 Water Tech
17/70
EFFECT OF IMPROPER WATER
CHEMISTRY FOR FEED WATER SYSTEMS Storage tanks :
** general corrosion , pitting
Condensers and feed water heaters :
** pitting , erosion , corrosion , exfoliation,
galvanic corrosion
Dearators :
** corrosion , pittingEconomizer :
** pitting , corrosion , hydrogen damage
-
7/27/2019 PP 1 Water Tech
18/70
EFFECT OF IMPROPER WATER
CHEMISTRY FOR BOILER SYSTEMS** general corrosion , pitting , caustic gouging ,
hydrogen damage , deposit induced overheating
EFFECT OF IMPROPER WATERCHEMISTRY FOR STEAM SYSTEMS
Reheaters and Superheaters
** pitting , corrosion , overheating (SHS), exfoliation (RH)
Turbine :
** corrosion , stress corrosion cracking , pitting , galvanic
corrosion , crevice corrosion , deposition & fouling
-
7/27/2019 PP 1 Water Tech
19/70
.
*** GUIDELINES---- normal operation
---- critical periods
~~ start up~~ condenser leakage
~~ shutdown ( lay-up)
*** MONITORING
*** MEASUREMENTPrevention is better than cure stitch-in-time
-
7/27/2019 PP 1 Water Tech
20/70
Blowdown
HP Heaters
Deaerator
Boiler
HP
Turbine
IP Turbine LP Turbine
LP Heaters
BFP
Condenser
Condensate
storage tank
Makeup
treatment
system
Condensatepolisher
Attemperation
Steam-Water cycle diagram for reheat type boilers
-
7/27/2019 PP 1 Water Tech
21/70
-
7/27/2019 PP 1 Water Tech
22/70
Bharat Heavy Electricals Limited, Tiruchirapalli
Research & Development
CRITERIA boiler design
pressure of operation
thermodynamic & engineering principles
type of water treatment ( boiler )
*** phosphate , AVT , etc
objectives
To achieve the expected steam purity
To minimize corrosion
To reduce deposition
-
7/27/2019 PP 1 Water Tech
23/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Steam purity recommendations
100100100TOC
333Sulphate
333Chloride
101010Silica
0.150.150.3Cation cond.
S/cm
335Sodium
Once-throughtreatment, ppb
All-volatiletreatment, ppb
Phosphatetreatment ,ppb
substance
-
7/27/2019 PP 1 Water Tech
24/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Feedwater guidelines
0.01-0.020.01-0.020.01-0.020.01 0.02Hydrazine , ppm , residual
0.0030.0030.0050.01Copper , ppm , max
0.0050.0050.010.01Iron , ppm , max
0.010.010.020.02Silica, ppm, max
0.20.30.30.5Cond. ( H+ ), S/cm, max
0.0070.0070.0070.007Diss.Oxygen,ppm,max
8.8 9.28.8 9.28.8 9.28.8 - 9.2pH at 25oC
nilnilnilnilHardness, ppm, max
Once-thro
ugh units
166 - 205101-16561- 100Drum Pr.
Kg / sq.cm
-
7/27/2019 PP 1 Water Tech
25/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Corrosion of boiler steel
Factors responsible for corrosion
pH
Dissolved oxygen
-
7/27/2019 PP 1 Water Tech
26/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Corrosion of steel vs boiler water pH
safe range
8.5 11.0
pH
Corrosion
rate
acidic alkaline
4 8 10 126 14
-
7/27/2019 PP 1 Water Tech
27/70
-
7/27/2019 PP 1 Water Tech
28/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology 8
An Electrochemical reaction
Electron flow
Fe2+O2 OH--
water
ANODE REACTION
Fe0 === Fe2+ + 2 e--
CATHODE REACTION
O2 + H2O + 2 e-- == 2 OH--
wwTubeFe
-
7/27/2019 PP 1 Water Tech
29/70
Cation conductivitysalient features
enhances sensitivity to mineral salt concentration
when sample passes through cation exchangerNH4OH + H
+ Resin NH4+ resin + H2O
NaCl + H+ Resin Na+ resin + HCl
Effect of ammonia nullified Effect of contaminant minerals increased to
nearly 3 times and
sensitivity of measurement increased and
trace amounts of contaminants identified
-
7/27/2019 PP 1 Water Tech
30/70
REFERENCE
1 Power Plant Water Chemistry -- a practicalguide by Brad Buecker ( Pennwell )
2 Betz Handbook of Industrial Water Conditioning BetzDearborn, Inc., ( Pennsylvania )
3 Drew Principles of Industrial Water Treatment
Cantafio. A. R., ed., ( New Jersey)
4 The Nalco Guide to Boiler Failure Analysis by Herro,H.M and R.D.Port ( McGraw-Hill )
5 Fundamentals of Steam Generation Chemistry by Brad Buecker ( Pennwell )
Bh t H El t i l Li it d
-
7/27/2019 PP 1 Water Tech
31/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Feed water chemistry
Condensate plus make-up water
Virtually all impurities are carried into the boiler in thefeedwater
Pre-boiler (condenser, feedwater heaters , dearators)corrosion --- most prevalent form of corrosion inmodern h-p boilers
Corrosion products formed in the pre-boiler sectiontransported to the boiler waterwall tubes
get deposited preferably on high heat transfer zones
results in overheating and subsequent tube failure Deposits contain
oxides of iron , copper and its oxides , traces of Zn,Ni ,etc.
Bh t H El t i l Li it d
-
7/27/2019 PP 1 Water Tech
32/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Condensate chemistry -- dissolved oxygen
Magnetite , Fe3O4 , protective film for iron
Cuprous oxide , Cu2O , protective film for copper
Oxygen , if excess , in feedwater oxidizes both theprotective layers
2 Fe3O4 + O2 3 Fe2O3 Cu2O + O2 2 CuO
Nodules of corrosion products & pits form at thecorrosion site
Corrosion products enter the solution , transportedto the boiler higher heat loads cause deposition
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
33/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Pre boiler corrosion Dissolved oxygen attacks copper in the presence of ammoniDissolved oxygen attacks copper in the presence of ammoniaa
more severely as follows :more severely as follows :
2 Cu2 Cu22O + OO + O22 44 CuOCuOCuOCuO + 4 NH+ 4 NH44OHOH Cu (NHCu (NH33))44 (OH)(OH)22 + 3 H+ 3 H22OO
(insoluble) ( soluble )
The corrosionThe corrosion product is transported more easily in a solubleproduct is transported more easily in a solubleform into the boilerform into the boiler
The copperThe copper--ammonia complex decomposes inside the boiler atammonia complex decomposes inside the boiler atelevated temperatures ( >elevated temperatures ( > 140140ooC )C )
The liberatedThe liberated free copperfree copper gets deposited on the heatgets deposited on the heat
transfer surfacestransfer surfaces
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
34/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Copper & Iron --- during startup
< 10< 536
352028503024
754020
1207016
1801608
2102304
2605000
Iron , ppbCopper , ppbTime ( hour ) afterstartup
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
35/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Water technology
Pre boiler corrosion salient features
Most-affected locations --- Economisers & feedwater heaters
High levels of oxygen can be expected in feed water more
during start-ups and low-load operations
Pitting severe during idle periods -- free entry of oxygen ---moisture can be collected in bends , sags , etc leading to
enhanced corrosion
Appearance : perforations or pits on the tubewalls throughlocalized corrosion
if severe , during very high levels of dissolved oxygen ,crater like sites develop having red,rusty appearance
Op ti n f D m t p B il s 165 t 180 k / 2
-
7/27/2019 PP 1 Water Tech
36/70
Operation of Drum-type Boilers 165 to 180 kg/cm2
boiler water control for [ 0.25 1.0 ppm (abnormal) ] condition :
o If pH < 8.0 & Total solids < 50 ppm do not operate the unit
p increase blowdown to limit TDS concentration
q avoid use of desuperheating spraywater by (a) reducing load
and (b) permitting reheater temperature to fall - contd
n chemical injectionto maintain excess
PO4 & pH conditions
TDS < 50 ppm
pH 9.1 10.1
PO4 5 20 ppm
Limited operations;
Inspection and
repair of condenser
Total solids
(abnormal)
0.25 1.0 ppm
If on volatile
treatment , changeto PO4 treatment
TDS < 25 ppm
pH 9.1 9.8
PO4 5 10 ppm
Volatile treatment
not suitable ;
Back to guideline
values soon
Total solids
(acceptable)
< 0.25 ppm
normalTDS < 25 ppm
pH 9.1 9.8
PO4 5 10 ppm
noneTotal solids
(recommended)
< 0.05 ppm
Boiler water controlControl limitsOperational
limitations
Hotwell
conditions
-
7/27/2019 PP 1 Water Tech
37/70
Operation of Drum-type Boilers 165 to 180 kg/cm2
Same as forabnormal hotwell
conditions of
0.25 to 1 ppm
plus
n prepare forwet lay-up of the
unito condenserrepair
TDS < 50 ppmpH 9.1 10.1
PO4 5 20 ppm
Emergencyoperation :
immediately
reduce load
orderly shut-
down of the unit
if hotwell
concentration
cannot bereduced below
1ppm
Excessive> 1.0 ppm
Boiler watercontrol
Control limitsOperationallimitations
Hotwellconditions
Total solids
C t d t i t d d t
-
7/27/2019 PP 1 Water Tech
38/70
Case study --- contaminated condensate Boiler parameters : 80 MW unit ; 90 kg / cm2 ; PO
4treatment
Problem : ~ 0.75 ppm of TDS in condensate detected
Situation : chemists request to stop the unit denied ; load demand
high ; unit ran for about 15 daysAction : blowdown increased
pH and PO4 controlled in boiler water
monitoring frequency increased
Consequence :
After about 2 months , water wall tubes started failing
frequently
unit stopped on numerous occasions
Remedy :
tubes replaced at a cost of US $ 2,000,000 /-
-
7/27/2019 PP 1 Water Tech
39/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
40/70
y
Water technology
Corrective actionsCorrective actions Control of dissolved oxygen and ammonia in feed water
Effects of pre-boiler corrosion severe during start-ups whenchemical control is difficult
Proper care and attention during condenser leakage Condensate purification plants : to minimize soluble and
insoluble materials
` Alternate materials : Titanium , stainless steel` Air-inleakage control : Expansion joints between turbine and
condenser ; turbine seals, diaphragms ; condensate pump seals
Dearator performance
Steam temperature and pressure and quantity
Tray alignment etc
Monitoring of dissolved oxygen . On-line
Relative priority of
-
7/27/2019 PP 1 Water Tech
41/70
Relative priority ofWater Chemistry Instrumentation
Dissolved oxygen
Cation conductivity / degassed conductivitysodium
silica
chloride
pH
specific conductivity
phosphate
N b f li lN b f li l
-
7/27/2019 PP 1 Water Tech
42/70
Number of online analyzersNumber of online analyzers
0102Ammonia
2222pH
5757Sodium
2525Silica
2323Dissolved oxygen
1111Conductivity/
degassed cation
5544Conductivity/cation
0303Chloride
Volatile treatment
max min
Phosphate treatment
Max min
Parameter
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
43/70
Water technology
Solubility of oxygen vs temperature
0.121099
1.519088
2.417077
3.1150663.813055
4.411044
5.19033
6.27022
Oxygen ( cc per litre )Temperature ( 0F )Temperature ( 0C )
Case study --- organics
-
7/27/2019 PP 1 Water Tech
44/70
Case study organics Boiler parameters : 50 tph unit ; 40 kg / cm2 ; amine treatment boilers generating steam for phenol production ; 80% condensate return
Problem : frequent failure of super heater tubes
Situation : deposits found in SH tubes having iron oxide and silica
Reason :boiler water indicated indicated organics
observed value : 20 to 200 ppm
recommended value : 0.5 ppm
Consequence :foaming inside boiler drum resulting in carryover
Remedy :
Upgrade analytical system to detect organics in feed
water , and boiler water
organics to be removed condensate polisher
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
45/70
Water technology
Boiler water guidelines
0.2---------Cond. ( H+ ), S/cm,
max
---30100200Cond. S / cm , max
---1550100TDS , ppm , max
---2 - 65 - 105 - 20PO4 residual , ppm
0.100.100.9 0.27.0 0.9Silica, ppm , max
>8.59.1-9.69.1-9.89.1-10.1pH at 250C
AVTphosphatephosphatephosphateTreatment
Type
166-205166-205126-16561-125Drum Pr.
Kg / sq.cm
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
B il id li
-
7/27/2019 PP 1 Water Tech
46/70
Water technology
Boiler water guidelines
1.50.4---0.1190
1.60.9---0.14175
1.71.50.40.2160
1.91.80.540.271452.22.00.80.4
135
2.42.41.10.55120
2.62.61.60.8105
3.02.82.61.390
SulphateSO4 , ppm,max
ChlorideCl, ppm, max
SiO2 , ppm,max(0.02 ppm steam)
SiO2 , ppm ,(0.01 ppm steam )
Operatingpressure
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
-
7/27/2019 PP 1 Water Tech
47/70
Boiler water treatmentTrisodium phosphate provides the needed alkalinity inboiler systems as follows :
o Na3PO4 + H2O === NaOH + Na2HPO4
Absorption of contaminants :
o 10Ca2+ + 6PO43-- + 2OH-- 3Ca3(PO4).Ca(OH)2
calcium hydroxyapetite
o 3 Mg2+ + 2SiO32- + 2OH-- + H2O 3MgO.2SiO2.2H2O
serpentine
Calcium hydroxyapetite and serpentine exist as soft sludgesand much easier to remove ; typically settle in the drum
and removed by blowdown
H V PO4 & N OH
-
7/27/2019 PP 1 Water Tech
48/70
pH Vs PO4 & NaOH
8.5
9
9.5
10
10.5
11
11.5
0 5 10 15 20 25 30 35 40
ppm
pH
PO4 NaOH
EFFECT OF PO4 / N OH H
-
7/27/2019 PP 1 Water Tech
49/70
EFFECT OF PO4 / NaOH on pH
8.8
9
9.2
9.49.6
9.8
10
10.2
10.4
10.6
0 2 4 6 8 10 12
ppm
pH
PO4 NaOH
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
C ti i
-
7/27/2019 PP 1 Water Tech
50/70
Water technology
Caustic corrosion
Irregular thinning of waterwall tube due to the corrosiveaction of sufficiently concentrated NaOH
Under porous deposits , NaOH builds up from safe
level to corrosive level Corrosion occurs by concentrations at the local tube wall
& not by concentrations existing in the bulk boiler water
Concentrated NaOH first attacks the protective magnetite
Fe3O4 + 4 NaOH === 2 NaFeO2 + Na2FeO2 + 2H2O
Further attack of NaOH on boiler metal
Fe + 2 NaOH === Na2FeO2 + H2
-
7/27/2019 PP 1 Water Tech
51/70
CAUSTIC DAMAGE
-
7/27/2019 PP 1 Water Tech
52/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
C i i
-
7/27/2019 PP 1 Water Tech
53/70
Water technology
Corrective actions
control of boiler water chemistry ( PO4 programmes )
control of water-side deposition
Periodic ( usually yearly basis ) assessment of tubecleanliness & chemical cleaning to be carried out , ifneeded based on deposit analysis
minimizing ingress of deposit forming materials ( pre-boilercorrosion and condenser leakage )
Wall thinning , if severe , tubes to be replaced
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Hydrogen damage
-
7/27/2019 PP 1 Water Tech
54/70
Water technology
Hydrogen damage
Hydrogen damage occurs in boilers operated with low pHHydrogen damage occurs in boilers operated with low pHwater chemistrywater chemistry
by aggressive anions like chloridesby aggressive anions like chlorides
concentration of acidic species under depositsconcentration of acidic species under deposits
During periods like condenser leakage , specially in sDuring periods like condenser leakage , specially in seaeacooled power plants , lots of acidic species are introduccooled power plants , lots of acidic species are introduceded
MgClMgCl22 + 2 H+ 2 H22OO Mg(OH)Mg(OH)22 + 2+ 2 HClHCl Atomic hydrogen , so formed , can diffuse into steel and react
with iron carbide in steel
Fe3C + 4 H 3 Fe + CH3 Fe + CH44 Methane , being a bigger molecule, can not diffuse easily
Grain boundaries affected ; internal structure of the tubedamaged , making it brittle
-
7/27/2019 PP 1 Water Tech
55/70
HYDROGEN DAMAGE
-
7/27/2019 PP 1 Water Tech
56/70
HYDROGEN DAMAGE
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
C ti ti
-
7/27/2019 PP 1 Water Tech
57/70
Water technology
Corrective actions
Restoration water chemistry
condenser leakage
Removal of unit from service , if pHfalls below 8
Inadvertent release of water treatmentregeneration chemicals
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Deposition
-
7/27/2019 PP 1 Water Tech
58/70
Water technology
Deposition Even a relatively thin deposit layer significantly reduce
heat transfer . Leading to overheating of the tubes
In cases of severe deposition, overheating may occur veryrapidly resulting in catastrophic tube failure
deposits are also the precursor to under deposit corrosion water enters a porous deposit through some opening
and boils off through other channels , leaving the solidsbehind
the concentrated species may be an alkali (eg.NaOH) oran acidic species ( eg, Chlorides during condenser leak)
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
UNDER DEPOSIT CORROSION
-
7/27/2019 PP 1 Water Tech
59/70
UNDER DEPOSIT CORROSION
Water wall tube
without deposit
Water wall tube
with deposit
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
UNDER DEPOSIT CORROSION
-
7/27/2019 PP 1 Water Tech
60/70
UNDER DEPOSIT CORROSION
Na3PO4 + H2O
NaOH + Na2HPO4
Boiler water with
Na3PO4 , Na2HPO4 , NaOH
enter through the pores
of the deposit and wateronly comes out as steam
leaving the solids to
concentrate
HEAT
WATER WALL TUBE FACING
THE FURNACE ( HOT ) SIDE
-
7/27/2019 PP 1 Water Tech
61/70
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Lay-up and offline corrosion
-
7/27/2019 PP 1 Water Tech
62/70
Water technology
Lay-up and offline corrosion
More corrosion can occur during an outage than any
other time
This is due air intrusion and subsequent corrosion by
oxygen Lay up guidelines : boiler filled with water containing
200 ppm hydrazine and ammonia to pH 10
Nitrogen blanket ( 0.5 kg/sq.cm) to the drum and superheaterthrough the vent lines
The boiler lay up solution should be circulated periodicallyto avoid stagnant zones, analyzed and replenished if needed
Before the boiler is started up , the lay up chemicals needto be removed
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Steam Chemistry
-
7/27/2019 PP 1 Water Tech
63/70
Water technology
Steam Chemistry
Steam purity affected by carryover --- the process by whichsolids are transported to steam
Carryover influenced by
solids become more soluble at high pressures
some like silica carry over as vapour drum level , drum design (internals) , foaming
contaminants can also enter via attemperator systems ;greatly exacerbated during upset conditions such as a
condenser leak
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
CRITICAL FACTORS IN WATER CHEMISTRY
-
7/27/2019 PP 1 Water Tech
64/70
Water technology
CRITICAL FACTORS IN WATER CHEMISTRY
1 Recommended guidelines in entire water steam cycleto be followed always !
2 Special care to be taken in controlling and monitoringdissolved oxygen , silica , & cation conductivity
3 Critical periods of water chemistryg Start ups gCondenser leakage g lay- up
4 Periodic chemical cleaning --- a routine maintenance step tokeep heat exchanger tubes clean
5 Management support : on-line and laboratory measurement facilities updating chemical technology knowledge base
-
7/27/2019 PP 1 Water Tech
65/70
THANK YOU
Bharat Heavy Electricals Limited TIRUCHIRAPALLI
Chemical Cleaning
-
7/27/2019 PP 1 Water Tech
66/70
Chemical Cleaning
Post operational chemical cleaning
To be based on waterwall tube deposit analysis
single stage or multi stage --- based on laboratory trials
and copper content
sometimes chemical cleaning may not remove the depositscompletely , if the deposition is very heavy and/ or having
hardness and silica , necessitating tube replacement
To be carried out in a planned manner and entrusted to
experienced and reliable contractors
-
7/27/2019 PP 1 Water Tech
67/70
To return line
-
7/27/2019 PP 1 Water Tech
68/70
CHEMICAL TREATMENT PROGRAMSCHARACTERISTICS
-
7/27/2019 PP 1 Water Tech
69/70
Can cause rapid corrosion whenconcentrated (specially under
deposit ) ; vaporous carryover in
steam at high pressure ; dosingcontrol very essential
Acid neutralization ;No phosphate hide-out
Sodiumhydroxide
Controlling molar ratio of Na
and PO4 ;Hide-out
Caustic corrosion mostly
eliminated ; deposit form ..Easy for removal ; acids
neutralized ; surface
passivation by phosphate
Congruent
Phosphate
Na:PO4 ..2.8
Possible under-deposit corrosion
by concentrated NaOH ;
Hide-out
Caustic corrosion may beeliminated; deposit form ..
easy for removal ; acidsneutralized; surface
passivation by Phosphate
Coordinated
phosphate
unfavourablefavourableprogram
CHEMICAL TREATMENT PROGRAMSCHARACTERISTICS
-
7/27/2019 PP 1 Water Tech
70/70
Can tolerate very low
concentration of impurities ; no
corrosion protection in case of
upset ; copper alloys should notbe used in the system ; requiresexcellent purity feed water ;
precise chemical control required
Low corrosion rates of
ferritic steels and conden-
ser tubes ; better oxide
coating , hence frequencyof chemical cleaningincreased
oxygenated
treatment
Feed water contamination mayexceed inhibiting ability of
volatile feed , leading to boilercorrosion ; marginal acid
neutralization ; no protection
during mild hardness ingress
deposition of salts can beeliminated ; high purity
steam under ideal feedwater conditions ; no carry
over of solids
All volatile
unfavourablefavourableprogram