Paper 3 Acid Dew Point Corrosion in HRSGs
Transcript of Paper 3 Acid Dew Point Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
1/83
NETR
A Maharatna Company
14th Feb. 2012
ASHWINI K. SINHA
AGM (NETRA)[email protected]
NTPC Energy Technology Research Alliance NETRA)
NTPC LIMITED.
E 3, Ecotech II, Udyog Vihar, Greater Noida 201308 (UP)FAX 0120-2350469
1
Cases of Acid Dew Point and Flow
Accelerated Corrosion in HRSGs and their
Remedial Measures
mailto:[email protected]:[email protected]:[email protected]:[email protected]
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
2/83
NETR
A Maharatna CompanyOverview
1. Cold End (Acid Dew Point)
Corrosion of HRSGs
2. Flow Accelerated Corrosion ofHRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
3/83
NETR
A Maharatna Company
3
3
NETRA: Focus Areas
Efficiency & Availability Improvement and Cost Reduction:
Waste Heat Recovery, VFD Retrofit, Health Assessment, ANN Modeling, CFD analysis, CHEM Analyzer, MALAE
Cycle, Combustion Optimization, etc
Rankin
Corresponding NH3/H2O Absorption Cycle
Entropy
T e m p e r a t u r e
2
3
Higher Work ThanRankin Cycle
1
4
5
6
7
8
Rankin
Corresponding NH3/H2O Absorption Cycle
Entropy
T e m p e r a t u r e
2
3
Higher Work ThanRankin Cycle
1
4
5
6
7
8
Renewable and Alternate Energy: Solar Thermal Platform,
Solar PV, Integrated Biodiesel Systems, Energy fromMunicipality Waste, etc
Climate Change and Environment: CO2 Capture & Utilization
Technologies, Fly ash Mineralization by flue gas, Waste Water
Recycling, Emission Reduction, etc
Support to Stations (NTPC & Other Utilities): Condition
Monitoring of Transformers, Failure Investigations, Corrosion
Control, Boiler & Condenser Cleanings, Vibration Analysis,
Water & Waste Water Treatment, Robotic Devices, etc
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
4/83
NETR
A Maharatna CompanyCorrosion Activities at NETRA
CathodicProtection
ChemicalDevelopmentfor CW System
CorrosionMonitoring &
Audit
WaterManagement
Selection ofAnticorrosive
CoatingsHeat TransferImprovementfor Boilers &
HE
Acid Dew PointCorrosion of
HRSGs
FailureInvestigations
HealthAssessment ofBoiler Tubes
Corrosion of
Turbines &Other
Equipment
Corrosion
Analysis,
Monitoring
& Control
Laboratory
4
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
5/83
NETR
A Maharatna Company
Corrosion Analysis & Control
Objective: Preventing corrosion, scaling,fouling in Power plant
components
1. Corrosion Assessment
2. Development of Chemical treatment for CW
3. Design of cathodic protection systems
(Condenser water boxes & underground
pipes)
4. Failure analysis (PA Fan blade, condensertubes)
5. Energy efficient coatings (Pumps, Ducts)
6. Control of corrosion of RCC structures
(cathodic protection of RCC structures)
7. Chemical cleaning of condensers & HRSGs
8. Corrosion audit (CW systems, Structures)9. Development of water & waste water
treatment programs
10. Evaluation of Anti-Corrosive Coatings
Benefits: Improving Availability,
Reliability & life of
Stations
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
6/83
NETR
A Maharatna CompanyOverview
1. Cold End (Acid Dew Point)
Corrosion of HRSGs
2. Flow Accelerated Corrosion ofHRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
7/83
NETR
A Maharatna Company
Overview
L. P. Drum
EXHAUST GAS
DEAERATOR
Exhaust
FUEL
(GAS / NAPTHA / HSD / NGL
COMBUSTION
CHAMBER
(SILO / CAN TYPE)
AIR
FLUE GAS
W.H.R.B.
H.P.T.
CONDENSER.
L.P.T.
GENERATOR
CONDENSATE PUMP
GENERATOR
GAS TURBINECOMPRESSOR
COMBINED CYCLE GAS POWER PLANT
H. P. Drum
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
8/83
NETR
A Maharatna Company
8
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
9/83
NETR
A Maharatna Company
9
―Whenever tube wall surfaces in boiler air heater or
economizer fall below acid dew point temperatures of vaporssuch as hydrochloric acid,nitric acid,sulfuric acid or even water
vapor,condensation of these vapors can occur on these
surfaces,leading to corrosion and tube failures.Of course,one
could use teflon coated tubes as in condensing
exchangers,but the cost may be significant. A simple solutionis to ensure that the lowest tube wall or surface temperature is
above the acid dew point‖.
Acid Dew Point:
The acid dewpoint (also acid dew point) of a flue gas (i.e., acombustion product gas) is the temperature, at a
given pressure, at which any gaseous acid in the flue gas will
start to condense into liquid acid
Acid Dew Point Corrosion of HRSG
http://chemengineering.wikispaces.com/flue+gashttp://chemengineering.wikispaces.com/pressurehttp://chemengineering.wikispaces.com/pressurehttp://chemengineering.wikispaces.com/flue+gashttp://chemengineering.wikispaces.com/flue+gashttp://chemengineering.wikispaces.com/flue+gas
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
10/83
NETR
A Maharatna Company
10
Cold-end corrosion can occur on surfaces that are lower in temperature
than the dew point of the flue gas to which they are exposed.
Air heaters and economizers are particularly susceptible to corrosive attack.
Other cold-end components, such as the induced draft fan, breeching, and stack,
are less frequently problem areas. HRSGs are also susceptible to acid dew point
corrosion at the flue gas exit points. The accumulation of corrosion products often
results in a loss of boiler efficiency and, occasionally, reduced capacity due toflow restriction caused by excessive deposits on heat transfer equipment.
Acidic particle emission, commonly termed "acid smut" or "acid fallout," is another
cold-end problem. It is caused by the production of large particulates (generally
greater than 100 mesh) that issue from the stack and, due to their relatively large
size, settle close to the stack. Usually, these particulates have a high
concentration of condensed acid; therefore, they cause corrosion if they settle on
metal surfaces.
The most common cause of cold-end problems is the condensation of sulfuric
acid. Sulfur in the fuel is oxidized to sulfur dioxide:
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
11/83
NETR
A Maharatna Company
11
The most common cause of cold-end problems is the condensation of
sulfuric acid. Sulfur in the fuel is oxidized to sulfur dioxide:
S + O2 = SO2Sulfur oxygen sulfur dioxide
A fraction of the sulfur dioxide, sometimes as high as 10%, is oxidized to sulfur
trioxide. Sulfur trioxide combines with water to form sulfuric acid at temperatures
at or below the dew point of the flue gas. In a boiler, most of the sulfur trioxide
reaching the cold end is formed according to the following equation:
SO2 + 1/2 O2 = SO3sulfur dioxide oxygen sulfur trioxide
The amount of sulfur trioxide produced in any given situation is influenced by
many variables, including excess air level, concentration of sulfur dioxide,
temperature, gas residence time, and the presence of catalysts. Vanadium
pentoxide (V2O5) and ferric oxide (Fe2O3), which are commonly found on the
surfaces of oil-fired boilers, are effective catalysts for the heterogeneous
oxidation of sulfur dioxide. Catalytic effects are influenced by the amount of
surface area of catalyst exposed to the flue gas. Therefore, boiler cleanliness, a
reflection of the amount of catalyst present, affects the amount of sulfur trioxide
formed.
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
12/83
NETR
A Maharatna Company
12
Acid Dew Point Corrosion of HRSG
Typical Acid Dew Point Corrosion
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
13/83
NETR
A Maharatna Company
13
Acid Dew Point Corrosion of HRSG
Typical Acid Dew Point Corrosion
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
14/83
NETR
A Maharatna Company
14
Acid Dew Point Corrosion of HRSG
Typical Acid Dew Point Corrosion
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
15/83
NETR
A Maharatna Company
15
Acid Dew Point Corrosion of HRSG
Typical Acid Dew Point Corrosion
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
16/83
NETR
A Maharatna Company
16
Acid Dew Point Corrosion of HRSG
Typical Stack Liner Corrosion
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
17/83
NETR
A Maharatna Company
17
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
18/83
NETR
A Maharatna Company
18
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
19/83
NETR
A Maharatna Company
19
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
20/83
NETR
A Maharatna Company
20
Acid Dew Point Corrosion of HRSG
Loss on ignition (%)
Temperature 105 0C 400 0C 815 0CLoss on
ignition 1.13 6.5 3.94
Chemical Analysis of deposit
% Fe as Fe2O3 % Ca/Mg as
CaO/MgO % Acid Insolubles84 4.5 11.5
Chemical Analysis of 1% water extract of Deposit
pH
Cond Chloride Sulphate
Nitrate
Sodium
Potassiu
m
µs/cm ppm ppm ppm ppm ppm
3.4 240 10 57.2 4 0.2 0.1
X-Ray Diffraction
Phases Identified FeO (OH), Fe2O3 (Sample amorphous in nature)
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
21/83
NETR
A Maharatna Company
21
Acid Dew Point Corrosion of HRSG
S
No.
PARAMETER UNIT SAMPLE NO.
697/C-2084
HP EVA & ECO
Dust (1.0 %)extract
SAMPLE NO.
697/C-2085
CPH Area
Dust (1.0 %)extract
1 Temperature Deg C 25 25
2 pH 2.86 2.73
3 Conductivity S 2297 3137
4 Sulphate As SO42- ppm 1040 2400
5 Sodium As Na+ ppm 2.9 4.2
6 Potassium As K+ ppm 0.3 2.3
7 Nitrate As NO3- ppm 17.2 22.5
8 Water Soluble % 12.00 31.6
9 Acid Insoluble % 14.3 13.2
Sample
No.
Description Fe (%) as
Fe2O3
Na (%) as
Na2O
Si (%) as
SiO2
Cu (%) as
CuO
C- 2084 HP EVA &
ECO Area
Dust
54.2 0.9 7.6 0.1
C- 2085 CPH Area Dust 40.0 0.5 7.7 0.1
Chemical analysis of Deposit Extract
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
22/83
NETR
A Maharatna Company
22
Acid Dew Point Corrosion of HRSG
S. No. Sample No. Description Phase identified
1. C- 2084 HP EVA & ECO Area
Dust
Fe2O3, Fe+3(OH)SO4.2H2O,
FeO(OH)2. C- 2085 CPH Area Dust Fe2O3, Fe2S2O9.5H2O
Sample Fluoride
(ppm)
Chloride
(ppm)
Nitrate
(ppm)
Bromide
(ppm)
Phosphate
(ppm)
Sulphate
(ppm)
1 Nil 3.17 7.00 Nil Nil 43.67
2 Nil 1.89 0.812 Nil Nil 2518.6
3 1.64 1.49 14.46 7.6 Nil 60.14
4 Nil 3.08 16.57 Nil Nil 1190.8
Ion Chromatographic analysis of Deposit Extract
X-Ray Diffraction analysis of Deposit
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
23/83
NETR
A Maharatna Company
23
Acid Dew Point Corrosion of HRSG
Sl No Data Required by NETRA Data given by Site
1 Flue gas composition of each HRSG at
inlet to CPH, outlet to CPH and Stack.
A typical composition of flue gas (dry) is
as follows and these values remain more
or less the same throughout the stackpath as long as there is no air ingress in
to the flue gas duct:
1. Oxygen content = 15.4%
2. Oxides of Nitrogen (NOx) = 95 PPM
3. CO2 = 3.0 %
4. Carbon Monoxide = BDL (< 1 PPM)5. Oxides of = 8 - 10 PPM (Online value)
6. Temperature = 118 Deg C
7. The average sulphur = 0.010 %
2 Surface area of CPH structures/inside
walls & stack (steel chimney)
Stack ID= 6m. Height = 70 m .
The surface area is approx: 1320 Sqm.
Area of MS duct & structures in CPH area
approx.: 350 Sqm3 Mass flow rate of flue gas/velocity profile
in each HRSG
Aprox. 380 Kg/s
No data available on Velocity
4 Any repairs carried out at the flue gas
ducts/stack?
No repair has been carried out.
5 Any other information relevant to this. The chimney is of MS construction. Other
than the area between CPH and stack theduct internal surface is SS cladded.
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
24/83
NETR
A Maharatna Company
24
Acid Dew Point Corrosion of HRSG
S.No. Unit No. Reason Date Outage
hours From To
1 I Planned Outage 21.09.09 26.09.09 116.352 I Planned Outage 12.07.10 24.07.10 290.37
3 I No demand 24.05.09 29.05.09 105.31
4 I No demand 03.09.09 14.09.09 266.19
5 I No demand 27.09.09 10.10.09 325.34
6 I No demand 01.07.10 11.07.10 240.13
7 I No demand 24.07.10 19.08.10 635.158 I No demand 14.10.10 01.11.10 424.56
9 II Planned Outage 30.06.09 07.07.09 167.58
10 II Planned Outage 25.04.10 02.05.10 156.13
11 II Planned Outage 03.02.11 07.03.11 761.11
12 II No demand 11.11.09 16.11.09 110.15
13 II No demand 20.08.10 14.10.10 1311.45
14 II No demand 10.12.10 20.12.10 231.14
Average Relative Humidity during the year: 79.4% (Min. 22.4%, Max. 96.9%)
Average Temperature during the year: 27.4 oC (Min. 16.4 oC, 35.8 oC)
C f SG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
25/83
NETR
A Maharatna Company
25
Acid Dew Point Corrosion of HRSG
Method Advantages Disadvantages
Nitrogen - Effective
- No foreign Chemicals introduced
- Low oxygen environment may be
hazardous to personnel
- Difficult to confirm that all spaces are filledwith nitrogen (not air) unless cap is installed
as pressure decays.
- Large volume of inert gas required
- Does not remove standing water
Desiccant
Trays
- Proven traditional method
- Easy to source material (silica gel,
quick lime, activated alumina); rule of
thumb is 5 lb silica gel/100 cft ofvolume
- Need to handle chemicals
- Damp chemical is corrosive if spilled in
drum.
- Air circulation through HRSG is notaccomplished naturally
- Requires frequent checking
Dehumidified
Air
- Successful in humid climates
- Clears small pockets of water within
hours
- Simple and effective
- No foreign chemicals introduced
- Equipment intensive; requires blowers,
flexible ducting
- Seal must be maintained with relative
humidity of < 30% re-established
- Constant use of blowersVapour Phase
Corrosion
Inhibitor
- Simple to add
- Chemicals are water soluble
- Require flush and refill
- Personnel should not enter drums until after
a flush, refill and startup
- Handling and introduction of foreign
chemicals
- Do not clear residual water
- Difficult to confirm dispersion throughout
HRSG
A id D P i t C i f HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
26/83
NETR
A Maharatna Company
26
Acid Dew Point Corrosion of HRSG
Gas-side layup
Gas-side corrosion can be problematic for HRSGs in cycling service. Layup ofthe gas side historically has been given less consideration than it has for the
water side, but that may be changing.
As ambient temperature increases during the daylight hours, the cooler HRSG
components, with their considerable thermal inertia, lag behind, and moisture
condenses on metal surfaces. Condensation typically occurs when the relativehumidity is more than 35%.
Also, when HRSG internal surfaces are cooler than ambient temperature,
reverse draft through the stack occurs. Air entering through the stack exits via
the gas turbine, open gas-side manways, and other leakage points.
Dewpoint corrosion of tubes, fins, headers, and casing can cause many
problems including particulate emissions at restart, piping and hanger corrosion,
increased gas-turbine backpressure, and reduced heat transfer in the HRSG.
A id D P i t C i f HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
27/83
NETR
A Maharatna Company
27
Acid Dew Point Corrosion of HRSG
Corrosion can be minimized either by removing oxygen or moisture from
ambient air; the latter usually is easier. In either case, it is important to minimize
the amount of air that must be handled and conditioned. This requires blockingair flow through the stack with a damper or balloon.
Options for minimizing dewpoint corrosion include adding heat (1) by injecting
sparging steam on the water side, and (2) installing portable heating coils or
radiant heaters on the gas side. Another practical option is dehumidification. In
many cases, a combination approach may be required.
Finally, some plants that clean tube panels early in an outage see residual
deposits ―growing‖ as they absorb moisture. A good strategy for a long outage
may be to inspect the HRSG during the first five days of the outage, engage
heating or dehumidification, clean as close to restart as possible, and return to
the heating or dehumidification plan if startup is delayed.
A id D P i t C i f HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
28/83
NETR
A Maharatna Company
28
Acid Dew Point Corrosion of HRSG
Relationship between corrosion rate and the moisture content of air shows the
importance of maintaining relative humidity below about 40%.
A id D P i t C i f HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
29/83
NETR
A Maharatna Company
29
Acid Dew Point Corrosion of HRSG
The water vapour pressures from the water vapour table. A gas with 6.5 v% H2O has a
vapour pressure of 49.7 mm Hg (100 v% water has a vapour pressure of 758 mm Hg) and
a dewpoint of 38 °C.
A id D P i t C i f HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
30/83
NETR
A Maharatna Company
30
Acid Dew Point Corrosion of HRSG
A: Dewpoint equation of SO3 according to Verhoff:
T d=1000/{2.276 - 0.0294ln(PH2O) - 0.0858*ln(PSO3) + 0.0062*ln(PH2O*PSO3)}
B: Dewpoint equation of SO2 according to Kiang:
Td=1000/{3.9526 - 0.1863*ln(PH2O) + 0.000867*ln(PSO2) - 0.00091*ln(PH2O*PSO2)}
C: Dewpoint equation of HCl according to Kiang:
Td=1000/{3.7368 - 0.1591*ln(PH2O) - 0.0326*ln(PHCl) + 0.00269*ln(PH2O*PHCl)}
D: Dewpoint equation of NO2 according to Perry:
Td NO2 = 1000/(3.664 - 0.1446*ln(v%H2O/100*760) - 0.0827*ln(vppmNO2/1000000*760)+
0.00756*ln(v%H2O/100*760)*ln(vppmNO2/1000000*760)) - 273
Pressures (P) in the equations B, C and D are given in mm Hg; in equation A in
atmosphere.
A id D P i t C i f HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
31/83
NETR
A Maharatna Company
31
Acid Dew Point Corrosion of HRSG
Dew points of SO3 at various water contents of the gas, calculated from the formula
of Verhoff.
Acid De Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
32/83
NETR
A Maharatna Company
32
Acid Dew Point Corrosion of HRSG
Dew p oints of SO 2 at var iou s water contents of th e gas, calculated from the formula
of Kiang . The SO2 dew poin ts for al l gasses are low er than the water dew poin t of
the gasses .
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
33/83
NETR
A Maharatna Company
33
Acid Dew Point Corrosion of HRSG
Dew points of HCl at various water contents of the gas, calculated from the formula
of Kiang and the water vapour table.
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
34/83
NETR
A Maharatna Company
34
Acid Dew Point Corrosion of HRSG
Dew p oints of NO 2 at var ious water con tents of the gas, calculated from the formula
of Perry and the water vapou r table
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
35/83
NETR
A Maharatna Company
35
Acid Dew Point Corrosion of HRSG
Acid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
36/83
NETR
A Maharatna Company
36
Acid Dew Point Corrosion of HRSG
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
37/83
A Maharatna Company
37
Acid Dew Point Corrosion of HRSG
Condensate Pre-heater (CPH), HP Evaporator and Stack liner of HRSGs are getting
affected by Corrosion by Condensed gases (SO2, H2O, NO2).
Corrosion products consists of iron oxides, sulphate , nitrate, and acidinsolubles and the products are acidic in nature.
Naptha contains around 0.01% sulphur and at around 6.5% moisture in flue gas,
the expected acid dew point is around 95 oC.
The flue gas temperature at CPH outlet is around 125 oC (rated 120 oC). This
suggests that flue gases are above acid dew point temperature during normal operatingperiod. However; the exit gas temperature is higher than the rated temperature,
suggesting that there is lesser heat transfer than the design in CPH region perhaps due
to fouling of tubes.
The deposit analysis indicates presence of sufficient quantity of sulphates (ranging
from 1000 -2500 ppm on boiler tubes & 58 ppm on stack liner), nitrates are
ranging from 4 ppm on stack to 22 ppm on boiler tubes and pH of 1% solution ofthe deposit in water is ranging between 2.7 to 3.4.
The acid dew point of SO2 under the present conditions of operation is around 95
oC and dew point of NO2 is around 38 oC. These conditions can occur only when
the units are shutdown and the equipment are exposed to relative humidity of >
40% and ambient temperatures leading to corrosion from condensation of flue
gases.
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
38/83
A Maharatna Company
38
Acid Dew Point Corrosion of HRSG
Application of Novolac Vinyl Ester Glass Flake coating 1000 – 1200 microns
DFT on Structures of CPH and Stack Liners to improve life of the
structures.
To improve the performance of the HRSGs, there is a need to remove the
deposited corrosion/flue gas condensation products from the boilers. Some
methods of cleaning are indicated further.
Proper preservation of water-side and gas-side portions of HRSG during shutdown of the unit.
Prevent ingress of humidity & rainwater into the HRSG systems. One possible
method of keeping the gas side system dry is to install duct balloons at the
entrance of HRSG from gas turbine and in the stack.
It might be worthwhile to install online corrosion monitoring system to keep a
check on the corrosion initiation, progress and control.
There is a need to revisit the lay up strategy for the HRSGs (Gas Side) so that
ingress of atmospheric moisture can be prevented.
Control Measures
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
39/83
A Maharatna Company
39
Acid Dew Point Corrosion of HRSG
Cleaning
Method
Pros Cons
Water
Washing
1. Low Cost
2. Can be performed by plant O & M
1. Water reacts with ammonia salts
to form sulphuric acid
2. Water waste must be removed
and treated
3. Water can leak into the internal
insulation
Grit Blasting: 1. Low Cost
2. Can be performed by plant O & M
1. A small portion of metal is
removed along with the coating2. High amount of waste has to be
vacuumed
CO2 Blasting 1. Cleaning process causes no tube
or fin damage?
2. No cleanup except for what was on
the tubes?
1. Higher Costs?
2. Must be subcontracted
3. Environmentally friendly
MagnesiumHydroxide
Washing
(NETRA)
1. Neutralizes the acidic materials.2. Forms a passivating layer on the
boiler surfaces which gets
removed after firing of boiler.
3. Being in a slurry form can move
along the boiler surfaces and
remove the acidic deposit
1. Waste water needs to be removed2. Water can leak into internal
insulation (may need to place
polyethylene sheets on the joints
to prevent water ingressing into
insulations)
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
40/83
A Maharatna Company
40
Acid Dew Point Corrosion of HRSG
Advantage / Gain of WHRB-4 washing
Date of parameters :06/09/02
Time :10:00-10:30
Parameters WHRB-3 WHRB-4
(Without
washing)
(After
washing)
Fuel Unit Gas Gas
GT load MW 124.744 122.811 Frequency Hz 50.21 50.21
Power Gain by Washing by WHRB-4
WHRB outlet temp.(measured) deg. C 123.500 100.300
GT mass flow rated 471.59 471.59
Rated flue gas temp. at WHRB outletduring gas firing
deg. C 102 102
Rated flue gas temp. at WHRB outlet
during HSD firing
deg. C 150 150
Power loss When CHP is bypass totally
(from HBD)
MW 4.725 4.725
Loss / Gain due to CPH MW -2.11640625 0.16734375 Net Effect of boiler washing MW 2.28375
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
41/83
A Maharatna Company
41
Acid Dew Point Corrosion of HRSG
Advantage / Gain of WHRB-4 washing
Financial Gain
Power saving MW 2.28375
Total Energy saving in day KWHr 54810
Total saving of Gas with 80 % loading
per year
KWHr 16004520
Per unit cost with Gas Rs. 1.4
Net saving per annume Rs. 22406328 ( say Rs, 2.241crores)
Saving in Petroleum product
Sp. Gas Consumptions sm3/kwhr 0.215
Total Energy saving per year Kwhr 16004520
Total Natural Gas saving per year sm3 3440971.8
Cost of Gas per 1000 sm3 Rs. 4307.78
Saving due to Natural Gas saving Rs. 14822949.5
(Rs. One Corer Forty Eight Lakhs Twenty Two Thousand and Nine Hundred
Fifty Only)
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
42/83
A Maharatna Company
42
Acid Dew Point Corrosion of HRSG
HRSGHRSG manhole
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
43/83
A Maharatna Company
43
Acid Dew Point Corrosion of HRSG
Hanger RodInstallation of Duct Balloon
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
44/83
A Maharatna Company
44
Acid Dew Point Corrosion of HRSG
Deflated Duct Balloon Blower for inflating Duct Balloon
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
45/83
A Maharatna Company
45
Acid Dew Point Corrosion of HRSG
Inflated Duct balloon inside stack
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
46/83
A Maharatna Company
46
Acid Dew Point Corrosion of HRSG
Duct Balloons for isolating the gas path from atmosphere & humidity
NETRAcid Dew Point Corrosion of HRSG
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
47/83
A Maharatna Company
47
Acid Dew Point Corrosion of HRSG
Installation of dehumidifier in HRSG
Corrosion Monitoring
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
48/83
NETR
A Maharatna Company
V = I*R
R = ρ*l/A
Electricalresistance
probe
Corrosion Monitoring
Corrosion Monitoring
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
49/83
NETR
A Maharatna Company
Corrosion Monitoring
Online Corrosion Monitoring of HRSGs
Corrosion Monitoring
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
50/83
NETR
A Maharatna Company
Corrosion Monitoring
Electrical Resistance (ER) Monitoring
The ER technique measures the change in Ohmic resistance of a corroding metal
element exposed to the process stream. The action of corrosion on the surface ofthe element produces a decrease in its cross-sectional area with a corresponding
increase in its electrical resistance. The increase in resistance can be related
directly to metal loss and the metal loss as a function of time is by definition the
corrosion rate.
Although still a time averaged technique, the response time for ER monitoring isfar shorter than that for weight loss coupons. The graph below shows typical
response times.
Corrosion Monitoring
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
51/83
NETR
A Maharatna Company
Corrosion Monitoring
ER probes have all the advantages of coupons, plus:
• Direct corrosion rates can be obtained.
• Probe remains installed in-line until operational life has been exhausted.
• They respond quickly to corrosion upsets and can be used to trigger an alarm.
ER probes are available in a variety of element geometries, metallurgies and
sensitivities and can be configured for flush mounting such that pigging operationscan take place without the necessity to remove probes. The range of sensitivities
allows the operator to select the most dynamic response consistent with process
requirements.
NETR
Water Extraction from Flue Gas
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
52/83
A Maharatna CompanyWater Extraction from Flue Gas
Pilot Test Heat Exchanger installed for Studies
NETR
Pilot Heat Exchanger Instal led
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
53/83
A Maharatna Company
53
Pilot Heat Exchanger Instal led
Coal power station
Parameters unit Value
pH - 2.55
Conductivity µS/cm 2890
TotalHardness
ppm asCaCO3
Nil
Cl ppm as Cl- Nil
M-alk ppm as Cl- Nil
EMA - 1500 Acidity - 450
Quality of Water condensed from flue gas
Gas power Station
PARAMETERS Unit Value
pH - 4.3
K µS/cm 213
TDS ppm 107
Salinity % 0.1
Sodium
ppm as Na
1
Potassium ppm as K 0.7
Total Hardness ppm as CaCO3 Nil
Ca Hardness ppm as CaCO3 Nil
p-Alkalnity ppm as CaCO3 Nil
m-Alkalnity ppm as Cl- Nil
Chloride
ppm as Cl- 1 Sulphate ppm as SO4
2- 58
Nitrate ppm as NO3- 6
NETR
Overview
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
54/83
A Maharatna CompanyOverview
1. Cold End (Acid Dew Point)Corrosion of HRSGs
2. Flow Accelerated Corrosion ofHRSGs
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
55/83
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
Piping Rupture Caused by Flow Accelerated Corrosion
(FAC):
A piping rupture likely caused by flow accelerated corrosion and/or cavitation-erosion occurred at Mihama-3 at 3:28pm on
August 9, 2004, killing four and injuring seven. One of the
injured men later died, bringing the total to five fatalities.
The rupture was in the condensate system, upstream of the
feedwater pumps, similar to the Surry and Loviisa locations.
The AP reports that sections of the failed line were examined
in 1996, recommended for additional inspections in 2003, and
scheduled for inspection August 14 (five days after the
rupture). This story was published Wednesday, August 11th,
2004 By James Brooke, New York Times News Service
On Monday, four days before the scheduled shutdown,
superheated steam blew a 2-foot-wide hole in the pipe, fatallyscalding four workmen and injuring five others seriously. The
steam that escaped had not been in contact with the nuclear
reactor, and no nuclear contamination has been reported.
The rupture was 560 mm in size. The pipe wall at the
rupture location had thinned from 10mm (394 mils) to
1.5mm.
NETR
Flow Accelerated Corrosion in HRSGs
http://corrosion-doctors.org/Forms-Erosion/erosion.htmhttp://corrosion-doctors.org/Forms-cavitation/cavitation.htmhttp://corrosion-doctors.org/Forms-cavitation/cavitation.htmhttp://corrosion-doctors.org/Forms-cavitation/cavitation.htmhttp://corrosion-doctors.org/Forms-cavitation/cavitation.htmhttp://corrosion-doctors.org/Forms-Erosion/erosion.htmhttp://corrosion-doctors.org/Forms-Erosion/erosion.htmhttp://corrosion-doctors.org/Forms-Erosion/erosion.htmhttp://corrosion-doctors.org/Forms-Erosion/erosion.htmhttp://corrosion-doctors.org/Forms-Erosion/erosion.htm
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
56/83
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
OSHA Safety Hazard Information Bulletin - Potential for Feed Water Pipes in Electrical
Power Generation Facilities to Rupture Causing Hazardous Release of Steam and Hot
Water (Excerpts from OSHA Bulletin – 19961031)
October 31, 1996
MEMORANDUM FOR:REGIONAL ADMINISTRATORSFROM:STEPHEN J. MALLINGER
Acting Director Directorate of Technical SupportSUBJECT:Hazard Information
Bulletin(1): Potential for Feed Water Pipes in Electrical Power Generation Facilities to Rupture
Causing Hazardous Release of Steam and Hot Water. The Directorate of Technical Support
issues Hazard Information Bulletins (HIBs) in accordance with OSHA Instruction CPL 2.65 to
provide relevant information regarding unrecognized or misunderstood health hazards,
inadequacies of materials, devices, techniques, and safety engineering controls. HIBs are
initiated based on information provided by the field staff, studies, reports, and concerns
expressed by safety and health professionals, employers, and the public. Bulletins are
developed based on a thorough evaluation of available facts in coordination with appropriate
parties
The Chicago Regional Office has brought to our attention the potential for feed water pipes in
electrical power generation facilities to rupture causing hazardous release of steam and
hot water. During an investigation of a multiple fatality accident at an electrical power
generation facility in an industrial plant, the Appleton Area Office uncovered at least three
other feed water pipe failure incidents in other power plants. In two of the three incidents,
six additional fatalities had occurred. In all cases, the feed water pipe failures were attributed to
wall thinning as a result of single-phase erosion/corrosion, leading to rupture of the pipes underhigh working pressures
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
57/83
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
The rupture of feed water pipes due to wall thinning creates the potential for serious burns,
massive property damage, and power outages in electrical power generation plants. These feed
water pipe failures could not be linked to any specific aspect of system designs, materials, or
operating histories to support a conclusion that single-phase erosion/corrosion was distinctive
to these particular power plants. This suggests that these may not be isolated incidents but a
problem that may be widespread in the industry.
Several factors affect the rate of erosion/corrosion in piping. These factors include material
composition of carbon steel piping, temperature, low water pH, low dissolved oxygen content,
pipe geometry, and fluid velocity. The flow path through elbows, bends, tees, orifices, welds,
valves, and backing rings creates turbulence in flow which, with fluid velocity, has the potential
to react with the protective oxide layer of carbon steel piping, contributing to theerosion/corrosion process.
Feed water pipes are addressed in the standard boiler inspection. Generally only a visual
inspection with the pipe insulation in place is done or required. Since this will not reveal pipe
thinning, employers may not have actual knowledge of the pipe wall thinning that could be
occurring.
To minimize the potential for personal injury or loss of life, property damage, and power
interruptions resulting from feed water pipe failure, it is recommended that employers ofelectrical power generation facilities establish a flow-assisted corrosion (FAC) program:
to identify the most susceptible piping components/areas and establish a sampling protocol
consistent with engineering principles and practices;
• use appropriate nondestructive testing (usually ultrasound) to determine the extent of pipe
thinning (if any); and,
• where thinning is identified, establish a preventative maintenance program and replace pipingin accordance with ASME recommendations.
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
58/83
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
Flow Accelerated Corrosion:
Flow-accelerated corros ion (FAC) is a well-kn ow n damage mechanism
that affects carbon steel components carryin g water or two-phase f low.
Caused by the mechanical ly-assisted chemical dissolut ion of the
pro tect ive oxid e and base metal, i t has lead to fai lures or severe wal l
th inn ing in :
• Main Feed water Piping
• HRSG LP & IP Evaporator Tubes
• HRSG Economizer Tube and Piping
• LP and IP Drum Internals
• Feed water Heaters
•
Blowdown Lines
Frequent startups and low load operation results in substantial transients in
boiler water chemistry, therefore HRSGs in cycling operation can increase the
risk for FAC. In combined-cycle (CC) plants, thinning of pipe and damage to
system components made of carbon and low-alloy steel typically occur in the
feed water and wet-steam sections of the cycle.
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
59/83
A Maharatna Companyo cce e ated Co os o SGs
Difference between Erosion, FAC, Erosion-Corrosion, and Cavitation
Erosion?
Erosion - is defined as the damage result ing from water, steam, part icles, or thecom binat ion thereof on the mater ial at hand . It can be seen as etching, defined lines, or
the wallowing out of a certain area. Often this can be misdiagnosed as Flow Accelerated
Corrosion. Chemistry as well as velocity can be a factor.
Flow Acc elerated Corr osio n (FAC) - EPRI defines FAC, Flow Accelerated (or Assisted)
Corrosion, as “A proc ess whereby the normal ly pro tect ive oxide layer on carbo n or low -
al loy steel diss olves into a stream o f f lowin g water or a water-steam m ixture.” It canoccur in single phase and in two phase regions. EPRI has stated that the cause of FAC is
water chemistry. Two phase FAC can be differentiated between Cavitation by the evidence of
“tiger stripes” or “chevrons” . FAC has often been classified as Erosion-Corrosion. FAC is a
term originating with EPRI for a condition that the industry has previous labeled with the more
generic term Erosion-Corrosion.
Erosion -Corrosio n (EC) - EPRI defines this as “Degradation of mater ial caused by bothmechanical and chem ical proc esses . FAC is often mislabeled as Erosion-Corrosion, even
though FAC is caused by chemical and mass transfer effects” . The term Erosion-Corrosion
includes many erosion and corrosion mechanisms while FAC is very specific. It is not incorrect
to call FAC, erosion corrosion however; FAC refers to a specific set of erosion corrosion
conditions. FAC is a term originating with EPRI for a condition that the industry has previous
labeled with the more generic term Erosion-Corrosion. Although there is industry practice incalling FAC erosion corrosion, there are no mechanical processes associated with FAC.
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
60/83
A Maharatna Company
Difference between Erosion, FAC, Erosion-Corrosion, and Cavitation
Erosion?
Cavitat ion Erosion (CE) - Occurs downstream of a direct ional change or in the
presence of an eddy . Evidence can be seen by round pits and is often misdiagnosed as
FAC. Like Erosion, CE involves fluids accelerating over the surface of a material; however,
unlike erosion, the actual fluid is not doing the damage. Rather, cavi tat ion resul ts from
smal l bu bbles in a l iquid str ik ing a surface. Such bubbles form when the pressure of a
fluid drops below the vapor pressure, the pressure at which a liquid becomes a gas. Whenthese bubbles strike the surface, they collapse, or implode. Although a single bubble
imploding does not carry much force, over time, the small damage caused by each bubble
accumulates. The repeated impact of these implosions results in the formation of pits. Also,
like erosion, the presence of chemical corrosion enhances the damage and rate of material
removal. Cavitation is not a property of the material, but a property of the system itself. The
fluid pressure is determined by the size and shape of the vessel, not the material. While a
stronger material can be highly resistant to cavitation, no metal is immune.
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
61/83
A Maharatna Company
Difference between Erosion, FAC, Erosion-Corrosion, and Cavitation
Erosion?
Flow-accelerated corrosion (FAC) and erosion corrosion (EC) are often used interchangeablyto describe similar material degradation processes. As a result, confusion exists regarding the
identification of FAC and the differences between FAC and EC. Both types of damage
involve destruction of a protective oxide film on the surface of a material (usually a
metal or metal alloy). The elimination or removal of the oxide film is generally referred to
as the "erosion" process. This is followed by electrochemical oxidation, or corrosive
attack of the underlying metal. Both processes involve a fluid that flows across or impingeson a metal surface. The differences between FAC and EC involve the mechanism by which
the protective film is removed from the metal surface. In the EC process, the oxide film is
mechanically removed from a metallic substrate. This most often occurs under
conditions of two-phase flow (i.e., water droplets in steam, solid particles in water, or
steam bubbles in water). It is also possible, but less likely, for erosion to occur under single
phase flow conditions. For this to happen, the fluid velocity must increase the surface shear
stress to a level that causes the oxide film to breakdown. In addition to shear stress, theremust also be variations in the fluid velocity
In the FAC process, the protective oxide film is not mechanically removed. Rather, the
oxide is dissolved or prevented from forming, allowing corrosion of the unprotected
surface. Thus, flow-accelerated corrosion may be defined as corrosion, enhanced by mass
transfer, between a dissolving oxide film and a flowing fluid that is unsaturated in the
dissolving species.
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
62/83
A Maharatna Company
Failed HP Economizer Drain Tube
NETR
Flow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
63/83
A Maharatna Company
Failed LP Economizer Tube
NETR
A M h CFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
64/83
A Maharatna Company
Failed LP Feed Line ―T‖
NETR
A M h t CFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
65/83
A Maharatna Company
LP Feed Pipe
NETR
A M h t CFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
66/83
A Maharatna Company
LP Feed Pipe
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
67/83
A Maharatna Company
Failed LP Feed Line
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
68/83
A Maharatna Company
Failed LP Feed Line
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
69/83
A Maharatna Company
Failed LP Feed Line
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
70/83
A Maharatna Company
Thickness reduction along the length of the pipe
Single phase FAC Two phase FAC
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
71/83
A Maharatna Company
In combined-cycle (CC) plants,
thinning of pipe and damage tosystem components made of carbon
and low-alloy steel typically occur in
the feed water and wet-steam
sections of the cycle.
FAC is a mass-transfer process inwhich the protective oxide (mostly
magnetite) is removed from the steel
surface by flowing water. Material
wear rate depends on (1) steel
composition, temperature, flow
velocity and turbulence, (2) waterand water-droplet pH, and (3) the
concentrations of both oxygen and
oxygen scavenger.
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
72/83
A Maharatna Company
The FAC problem is most pronounced in carbon steels. In these materials, even small
concentrations of chromium, molybdenum, and copper can improve FAC resistance. Where
FAC problems cannot be resolved by changing water chemistry, carbon steels often are
replaced by low-alloy steels, such as P11 and P22 FAC is a mass-transfer process in which the protective oxide (mostly magnetite) is
removed from the steel surface by flowing water . Material wear rate depends on (1) steel
composition, temperature, flow velocity and turbulence, (2) water and water-droplet pH, and
(3) the concentrations of both oxygen and oxygen scavenger.
Temperature has a pronounced effect on the FAC wear rate and when a system is
inspected, components in the 250-400F range get a priority. Flow velocity has a strongeffect, which makes wet steam systems very susceptible to FAC. Reason is that the velocity
of the steam usually is much higher than that of the water.
Water chemistry effects on FAC often are not well interpreted. The pH of feedwater and
steam droplets must be kept above a certain threshold, which depends on the pH agent
used and on temperature. For ammonia and amines, their effect diminishes withtemperature. For feedwater treatment with ammonia, a room-temperature pH above 9.5
is desirable.
Oxygen actually is good for preventing FAC. Experience indicates that 5 ppb of oxygen
in feedwater can practically stop FAC, while excessive concentration of oxygen scavengers
accelerates it. In most CC units that do not have copper-alloy tubing, oxygen concentrations
can be as high as 20 ppb without causing any problem.
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
73/83
A Maharatna Company
Any carbon- or low-alloy-steel component or piping system at a CC plant is a
candidate for FAC. These include:
■ Single-phase systems—HRSG economizers, headers, drum liners, boiler tubes,and feedwater pipes in drums; condensate/feedwater; auxiliary feedwater, heater,
and other drains; pump glands and recirculation lines.
■ Two-phase systems—low-pressure (l-p) turbine wet-steam extraction sections
and pipes, glands, blade rings, casing, rotors, and disks; flashing lines to the
condenser (miscellaneous drains); feedwater-heater vents, shells, and support
plates; feedwater heaters; HRSG moisture separators; condenser shell and
structure.
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
74/83
A Maharatna Company
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
75/83
p y
1. Flowing water increases material loss rate exponentially with flow velocity. Data are for
neutral 580-psig/356F water with an oxygen content of less than 5 μg/kg. Exposure time is
200 hr
2. Decreasing pH increases material wear, particularly below 9.2
3. Oxygen content above 100 μg/kg gives maximum steel protection in neutral water
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
76/83
p y
Typical Locations for FAC in HRSGs
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
77/83
p y
FACTORS AFFECTING FAC:
When carbon steel is exposed to oxygen-free water, the following reaction occurs:
Fe + 2H2O Fe2+ + 2OH- +H2 Fe(OH)2 + H2 (1)
This reaction is then followed by the Schikorr reaction where precipitated ferrous
hydroxide is converted into magnetite:
3Fe(OH)2 Fe3O4 + 2H2O + H2 (2)
Magnetite (Fe3O4) forms a protective surface layer which inhibits further
oxidation of the steel. However, magnetite is slightly soluble in demineralized,
neutral or slightly alkaline water (pH in the range of 7.0 to 9.2) and low dissolved
oxygen concentration (
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
78/83
Control of FAC:
An effective FAC control program should include the assessment of the propensity of
different plant systems and components to FAC, the use of available software withwater and steam chemistry corrections and periodic inspections. Monitoring of iron
concentration around the steam cycle is also useful; elevated concentrations may
indicate ongoing damage in a specific subsystem. FAC and cavitation evaluation
procedures used include the combined effects of:
•
Component geometry
• Flow velocity
• Water and steam parameters
• Material composition
• Water chemistry (pH, oxygen, oxygen scavenger, CO2, organics)
• Operating experience.
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
79/83
Notes: EI - economizer inlet, CPD - condensate pump discharge, DAI - deaerator inlet,
D - drum unit, O - once-through unit
* - Copper alloys may be present in condenser.
+ - These ORP values are meant to be indicative of a reducing treatment where a reducing agent
is added to the feedwater, after the CPD, and oxygen levels are less than 10 ppb at the CPD.However, ORP is a sensitive function of many variables and may under these conditions be as
high as –80 mV.
For HRSG plants with all-ferrous feedwater systems the feedwater chemistry should be AVT(O)
to avoid single-phase FAC in the feedwater and LP evaporator circuit.
For both fossil and HRSG plants, the basic idea of AVT is to minimize corrosion and FAC by
using deaerated high purity water with elevated pH. The pH elevation should be achieved by the
addition of ammonia. The actual pH range depends on the cycle metallurgy. The use and application of AVT(R)
in either type of plant with all-ferrous feedwater systems can result in FAC
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
80/83
Effect of Temperature and Ammonia
on iron dissolution
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
81/83
Effect of pH on FAC
NETR
A Maharatna CompanyFlow Accelerated Corrosion in HRSGs
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
82/83
Recommendations:
1. AVT (O) water treatment should be continued with tighter control on water
chemistry parameters.
2. Turbulences should be minimized by proper design.
3. For new replacement and for new units material of construction may bechanged to P11 or P22.
4. NETRA has developed CHEMAnalyzer, implementation of the same (after
suitable modifications to meet HRSGs requirement) should be considered.
For this necessary instruments need to be procured & installed.
5. Regular inspection of susceptible components by ultrasonic (UT)
examination needs to be undertaken to prevent any catastrophic failure.
NETR
A Maharatna Company
TRANSFORMING LIVES
-
8/20/2019 Paper 3 Acid Dew Point Corrosion in HRSGs
83/83
83