4. Insoluble impuritiesInsoluble impurities are very largely
metal oxidesmetal oxides derived from corrosion of
the plant itself, particularly the materials
of construction of the condenser and feed
systems. Such corrosion is minimised by
maintaining the feed water at an alkaline
pH and by removing dissolved oxygen
down to very low concentrations.
5. Soluble impuritiesSoluble impurities in feed water most frequently
arise from in leakage of cooling water from
condensers and other coolers, but can also results
from poor quality of make-up water.
6.
7. Drum Pr.
Kg / sq.cm
61- 100 101-165 166 - 205 Once-thro
ugh units
Hardness, ppm, max nil nil nil nil
pH at 25o
C 8.8 - 9.2 8.8 – 9.2 8.8 – 9.2 8.8 – 9.2
Diss.Oxygen,ppm,max 0.007 0.007 0.007 0.007
Cond. ( H+
), µS/cm, max 0.5 0.3 0.3 0.2
Silica, ppm, max 0.02 0.02 0.01 0.01
Iron , ppm , max 0.01 0.01 0.005 0.005
Copper , ppm , max 0.01 0.005 0.003 0.003
Hydrazine , ppm , residual 0.01– 0.02 0.01-0.02 0.01-0.02 0.01-0.02
8. o Feed water pH- the use of volatile alkalis
(NH3
)
o By making the pH of feed water slightly
alkaline (9.0-9.2) the corrosion rate of ferrous
alloys can be minimised. Where the system is
also contain copper based alloys similar
benefits can be obtained
o Oxygen Control
o Physical de-aeration
o Chemical de-aeration
11. There is no excessive accumulation of alkali in the
boiler water, the volatile alkali passing out the boiler
with the steam.
As condensate are formed in the lower pressure
section of the turbine, in the condensers and on the
steam side of the feed heaters, the alkali immediately
available in these parts of the steam/water system,
raising the ph of the condensates and hence
suppressing corrosion.
Ammonia (NH3) is the most widely used volatile
alkali because of its low cost, ready available and
its stability at high temperatures
12.
13. The objectives of any chemical treatment
for a modern, safe and efficient thermal
power plant are:
To reduce corrosion of metals and
equipments
To avoid scale formation
14. No turbine deposits/corrosion
No BTF due to cycle chemistry problem
Optimize feed water treatment / no FAC
Eliminate the need to chemical cleaning
Optimize procedures for shutdown/lit up
Bench mark in the world class
15. Corrosion is a natural process and is a result of the
inherent tendency of metals to revert to their more stable
compounds, usually oxides. Most metals are found in
nature in the form of various chemical compounds called
ores. In the refining process, energy is added to the ore,
to produce the metal. It is this same energy that provides
the driving force causing the metal to revert back to the
more stable compound.
IRON OXIDE REFINING MILLING
STEEL
CORROSION IRON OXIDE
23. Economizer tube severely
damaged by oxygen
Deposit accumulation restricted
heat transfer, leading to long-term
overheating
NN22HH44 + O+ O22 NN22 + H+ H22OO
Physical de-aeration by De-aerator
Reduce Flow Accelerated Corrosion
by Feed Water Treatment
24. Thin-lipped burst caused by
rapid overheating
Caustic gauging
Caustic reacts with the magnetite and
forms sodium ferrite (NaFeO2) and
sodium Ferroate (Na2FeO2).
Maintain boiler water Phosphate
to avoid caustic gauging
25. Boiler system failure
Boiler tube shows effect of acid attack
Violent rupture caused by hydrogen
embrittlement
Acidic condition generated on tube surface
beneath the deposits. Presence of chloride
was confirmed
26. Boiler system failure
Liquid metal embrittlement of boiler tube
caused by copper deposits and high
temperature (greater than 1600 °F).
Steam blanketing caused metal
wastage on top of sloped tube
27. Steam purity
. Overheating of this superheater
tube was caused by deposits that
resulted from boiler carryover into
the steam
28. Chemical de-aeration
The most widely used agent is Hydrazine (N2H4).
It is steam volatile, slightly basic and reacting
readily to remove even traces of dissolve
oxygen at temperature above 1500
C producing
only volatile or gaseous products, i.e.,
NN22HH44 + O+ O22 NN22 + H+ H22OO
At higher temperature hydrazine itself decomposes to
produce ammonia and nitrogen, i.e.,
NN22HH44 4NH4NH33 + N+ N22
29. To maintain boilers and turbines at a high level
of availability and efficiency, the chemical
control of water and steam purity is aimed at the
prevention of:
1.Corrosion of feed, boiler and steam systems.
2.Scale and deposit formation on heat transfer
surfaces.
3.Deposition and corrosion of turbines.
30. Drum Pr.
Kg / sq.cm
61-125 126-165 166-205 166-205
Treatment
Type
phosphate phosphate phosphate AVT
pH at 250
C 9.1-10.1 9.1-9.8 9.1-9.6 > 8.5
Silica, ppm , max 7.0 – 0.9 0.9 – 0.2 0.10 0.10
PO4 residual , ppm 5 - 20 5 - 10 2 - 6 ---
TDS , ppm , max 100 50 15 ---
Cond. µS / cm , max 200 100 30 ---
Cond. ( H+
), µS/cm,
max
--- --- --- 0.2
31. Trisodium phosphate (Na3PO4) and disodium
phosphate (Na2HPO4) can both effectively produce
alkalinity by hydrolysis in water:
Na3PO4 + H2O Na2HPO4 + NaOH
Na2HPO4 + 2 H2O NaH2PO4 +
NaOH
The important difference between this source of
alkalinity and that produced by free caustic soda lies
in the reversibility of the above reactions. If acid
species appear in the boiler water they will be
neutralised by the sodium hydroxide:
HCl + NaOH NaCl + H2O
32. Recommended parameters of Super heated Steam
Purity Guidelines
Parameters 210 MW 500 MW
pH at 25°C 8.8-9.0 9.0-9.2
ACC Cond. µm/cm, 0.2 0.2
Silica, ppb, (max) 20 10
Sodium, ppb, (max) 5.0 5.0
NH3, ppm, (max) 0.5 1.0
Total Fe, ppb, (max.) 10 10
Total Cu, ppb, (max.) 5 3
Ref: Code of Practice on Power Plant chemistry, by (OS) COS-ISO-00-OGN-OPS-
CHEM/015, Oct.2003
33. o Solubility of impurities in steam decreases as
steam expands in turbine
o NaCl and NaOH most corrosive chemicals
o Solubility of caustic exceeds 100 ppb at HP
turbine pressure and temp.
o In LP turbine, caustic solubility in steam
decreases and deposit concentration goes up
to 90%.
35. A potential major source of ingress of impurity
into the boiler water system is from leakage of
cooling water into the main condenser steam
space (because, steam space is maintained by
vacuum) which is called condenser leakage
36.
37. Left side hot well conductivity increased more rapidly than
right side indicates condenser tube leak in left pass of
condenser
38.
39. Boiler and condensate silica increased and cannot be controlled
without CBD and/or CPU.
40. Boiler and condensate silica increased and cannot be controlled
without CBD and/or CPU.
41.
42. o Online sodium increases (normal <2ppb).
o After Cation Conductivity increases ( normal
value <0.2 µS/cm for 500MW plant)
o Total hardness of condensate will be high.
o Boiler and condensate silica will go on
increasing and cannot be controlled without
CBD and/or CPU.
o Hot well makeup will be low as well as level
will be high.
o De-aerator level will go high.
43. 1. Open CBD 100%, makeup will go high. (Heat
as well as DM water loss)
2. Increase the concentration of phosphate and
free alkalinity in boiler.
3. Limit as for as possible the boiler de-
superheater spray water to prevent
contamination of the system.
4. Isolate one path of condenser, and see the
results and vice versa.
5. Put CPU into service if available.
44. If the leak is in minor in nature:If the leak is in minor in nature:
o In running units, isolate one half the
condenser at a time and minor the chemical
parameters and find out which half is leaking.
o Isolate the leaky half portion.
o Plug or repair the leak.
o Normalise and comeback to full load.
45. First is path detection
Flood test
Candle Test
Dye test
Foam Test
Bubbler Method
46. Condensate polishing is employed to purify the
return steam condensate in order to meet the
quality requirements of high pressure thermal
cycles and minimize consumption of make-up
water.
47. Improvement in the quality of condensate
and "cycle" clean up
Reduced blow down & make up requirements
Improvement in boiler water quality for drum
type boilers
Quick start up and as a result, full load
conditions are reached early giving economics
48. S.NO. PARAMETER CONTROL LIMITS FREQUENCY
OF TESTING
WITHOUT
ALKALIZER
WITH
ALKALIZER
1. Conductivity at
25oC, µS/cm
<1.5
(preferably
below 0.5)
<2.2 Continuous
2. Dissolved
oxygen, µg/l
<100 <10 Continuous
3. Total copper,
µg/l
<20 <20 Once in 2-
months
4. Total iron, µg/l <20 <20 Once in 2-
months
5. pH at 25oC 6.0-8.0 8.5-9.0 Once in 2-
months
49. Conductivity is the basic criteria of quality for the
stator cooling water and a low level must be
maintained to eliminate the possibility of flashover.
Dissolved oxygen in stator water is the main reason
for corrosion.
Dissolved oxygen in stator cooling water is generally
removed by purging with nitrogen gas of high purity
(99.99%, min.). Removal of dissolved oxygen
depends on the nitrogen pressure which is normally
maintained at 0.2 kg/cm2
with alarm for nitrogen
pressure in primary water tank set at 0.4 kg/cm2
for
better efficiency.
50.
51. A bypass ion exchange polishing plant is
commonly installed to control the conductivity
of the stator water and can also trap some of the
suspended matter in water.
Polishing plant is generally designed to treat
about 2-5% of total stator water flow. The
polisher contains a bed of mixed resins
consisting of strong cation resin in h-form and
strong anion resin in oh-form. Analysis of the
exhausted resin can be used to know how much
corrosion products are removed by the mixed
bed.
