ORP
Essential Expertise for Water, Energy and Air SM
Monitoring and Control of Feedwater Corrosivity – A Case History Southwest Chemistry Workshop June 24, 2009
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1
Agenda • Measurements and Monitoring Tools • Room Temperature vs. At-Temperature ORP (AT ORPTM) • Tools of the Trade • Using the Tools • Key Findings • Conclusions
Online Analyzers – NCSM (top left), Particle Counter (lower left), Particle Monitor (bottom right)
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Measurements and Monitoring Tools • The ORP “Space” • ORP Measurements – Establishing a Reference • The Nalco Corrosion Stress Monitor (NCSM) and AtTemperature ORP • Particle Analysis • Particle Counter or Particle Monitor? • Using the Tools • Sample Locations
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The ORP “Space” • EPRI advocates use of ORP to monitor and control boiler system cycle corrosion chemistry • Current preboiler corrosion control and monitoring methods can be unreliable, unpredictable, impractical and often inadequately deployed
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The ORP “Space” • ORP analogous to temperature • Systems tend to not be in thermodynamic equilibrium • ORP is really an ever-changing kinetic indicator of the system at both the point of measurement and throughout the steam cycle • ORP is thus not a measure of a thermodynamic finality but rather a kinetic continuum • Corrosion stress events often occur, but traditional monitoring programs often fail to detect or react to such events
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The ORP “Space” • Many plants focus control of corrosion stress at a single location • Corrosion “space” changes, and single-point monitoring with slow response can miss short-lived changes in chemical demand or the corrosion environment • Traditional corrosion monitoring and control programs miss dynamic stresses • They assume that slow response is “good” and a “good” measurement in one location provides adequate protection in all locations • Ever-changing ORP “space” requires a rapidly responding, realtime monitoring and diagnostic measurement that reacts immediately to the changing corrosivity of the system • Need to give operators/controllers time to respond in a timely fashion to the cause of the variation (minimizing any of its negative impacts) Essential Expertise for Water, Energy and Air
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ORP Measurements – Establishing a Reference • ORP: the electrical potential of a liquid at a specific temperature • Net sum of redox potentials/reactions in the sample stream as measured on noble metals •
• •
Voltage • Solution
[DO] ORP [Scavenger] ORP
Noble Metal
Type Temp.
Reference Electrode EPBRE vs sat KCl
Difference in voltage between two electrodes – but which electrodes? Must quote ORP numbers with reference to the potential being measured and the reference electrode This paper reports the ORP value versus the reference electrode with temperature. For example, an ORP measurement of –0.6 V obtained with the SHE at 400 oF (204.4 oC) is reported as –0.6 V vs. SHE (400 oF).
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AT ORP and REDOX STRESS Controlling the REDOX Stress is essential to controlling CORROSION stress and maintaining system integrity
FW “REDOX Stress” Design
Metallurgy DO ingress Other chemical additions (e.g. pH) Ts and Ps Flow
Deaerator “REDOX Stress” DA design Temperature differentials Temperatures and pressures Mechanical effects (e.g. DA tray alignment) DA venting DA steam supply Seasonal variations T and the DO content of makeup and condensate return Condensate versus makeup ratio. Changes in FW flow / steam load. Scavenger used (kinetics / thermodynamics with [DO]) Catalyst Scavenger residence times and concentrations
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Room Temperature vs. At Temperature: Sensitivity Matters
• • • •
Sample Lag Time Reductant Activity Dissolved Oxygen Corrosion
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Sensitivity Matters: Sample Lag Time • Sample water may travel hundreds of feet from the sample take-off to the steam sample panel – Typical lag time 6-15 minutes – May be much longer (up to 60 minutes)
• Room temperature (sample after cooling and filtering) – Not representative of chemistry or corrosion potential – Additional corrosion reactions can occur during sample transit FW Line
Rough Cooler Fine Cooler Filter
77°F
?
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RT ORP Cell
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10
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Sensitivity Matters: Reductant Activity
• Reductants more active at temperature (especially passivating reductants like hydrazine, carbohydrazide) • Reductant impact on corrosion stressors enhanced at temperature, so control is more sensitive and realistic if ORP measured at temperature
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Sensitivity Matters: Reductant Activity
Effect of increasing temperature on ORP response to DO and reductant feed
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Sensitivity Matters: Dissolved Oxygen
• Excellent tool for plants that don‟t feed reductant
80
200 100
ORP (mV)
low DO concentrations)
o
ORP variations on adding DO at 400 F
60
0 -100
40
-200 -300
20
-400
% Stroke or DO (ppb)
• AT ORP responds more quickly with higher sensitivity (even at very
-500 0
-600 7 Hours o
ORP (400 F) vs EPBRE ORP "RT" vs sat. KCl//AgCl/Ag
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% Stroke Air Addition Dissolved Oxygen (ppb)
SM
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Sensitivity Matters: Dissolved Oxygen Dissolved Oxygen Variations with Deaerator Conditions
237
43 Minutes
4/25/2005 3:51:36
4/25/2005 1:51:36
4/24/2005 23:51:36
4/24/2005 21:51:36
4/24/2005 19:51:36
4/24/2005 17:51:36
A1 (DA P = 11.7 psig)
7
(DA P = 11.2 psig)
6
B2
234
5
231
4
228
B1 (DA P = 9.7 psig)
DO (ppb)
3
4/25/2005 4:00:00
4/25/2005 2:00:00
SM
2 4/25/2005 0:00:00
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4/24/2005 22:00:00
4/24/2005 20:00:00
225
4/24/2005 18:00:00
A2 (DA P = 9.5 psig)
4/24/2005 16:00:00
DA #5 Dome Section Temperature (F)
240
4/24/2005 15:51:36
DA #5 dome section temperatures and DO data recorded at the EI
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Sensitivity Matters: Dissolved Oxygen 6
-0.275 -0.285
5 -0.295
10
Log (DO (ppb))
@T ORP (220F) vs EPBRE (V)
Includes a 9 minute delay-time offset
-0.305 4
-0.315 -0.325 -0.335
3
-0.345 -0.355
2 Hours Essential Expertise for Water, Energy and Air
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Sensitivity Matters: Reductant and DO
o
ORP variations with CHZ additions: 250 F Tests RT ORP vs sat KCl//AgCl/Cl (V) ORP (250F) vs EPBRE - V
0 -0.1
ORP - V
-0.2 -0.3 -0.4 -0.5 -0.6 A B C D E A = No CHZ; No DO; 250F B = 1.5 ppm ELIMIN-OX; No DO; 250F C = 30 ppm ELIMIN-OX; No DO; 250F D = 1.5 ppm ELIMIN-OX; + 145 ppb O2; 250F E = 30 ppm ELIMIN-OX; + 145ppb O2; 250F Essential Expertise for Water, Energy and Air
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Sensitivity Matters: Corrosion All corrosion processes are electrochemical reactions
AT ORP
TM
o
RT ORP vs sat KCl//AgCl/Ag - mV
vs EPBRE (350 F) - V
• AT ORP responds AT and RT ORP Data: Feedwater Heater Outlet to any species -100 -0.1 within the water that affects the corrosion -200 -0.2 space -300 -0.3 • If corrosion occurs, -400 additional iron -0.4 and/or copper, -500 -0.5 soluble corrosion products are -600 -0.6 sensed by the AT -700 -0.7 ORP (but not by RT 1 Hour ORP) Essential Expertise for Water, Energy and Air
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Sensitivity Matters: Corrosion
Carbon Steel Corrosion Control within the AT ORP Control Space AT ORP (mV) vs EPBRE (T of DA) +300 +200
Dissolved Oxygen (ppb DO) >100 30-50
+100 0 -100 -200 -300 -400 -500
4-7
0
Carbon Steel Corrosion Increasing Severity for Localized Corrosion
Sulfite residual (ppb DO scavenging „Stay below this confidence zone‟ equivalence) 0.8 mpy
30 60
0.5 mpy 200 0.2 mpy 2500
-600 Essential Expertise for Water, Energy and Air
Lower General Corrosion
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Sensitivity Matters: Corrosion Copper corrosion control within the AT ORP control Space -20000
Copper Corrosion A and B: 3 ppb DO baseline C and D: 45 ppb DO baseline
-Imag Ohm
-16000 -12000 -8000
A
B
C
-4000
D 0 0
10000 20000 30000 40000 50000 Real Ohm
Parameter / Corrosion Test
A
B
C
D
ORP Setpoint vs EPBRE (400F) - mV Increase in mpy (%)
-650 21
-650 0
-100 2560
-100 1730
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Summary: Why AT ORP? • Corrosion is occurring AT – We want to see effects in this „space‟ • Might see an effect AT that cannot be seen at RT as equilibria vary with T - Need to capture the chemical soup AT, ideally • AT sampling is earlier in the sample stream so response will be quick • With scavengers its well known that they work better AT. The power of the reductant is enhanced and thus the control based on reductant residual is more sensitive and realistic • Quicker response – Probe kinetics and thermodynamics. RT probes are more sluggish and more likely to become polarized
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Tools of the Trade
• • • •
At Temperature ORP Particle Analysis Using the Tools Location, Location, Location – Where to Sample
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The Nalco Corrosion Stress Monitor (NCSM) and AtTemperature ORP
• NCSM (AT ORP) continuously monitors the corrosion stress in a hot water system (like boiler feedwater) • Provides a window into the process not available from the traditional room-temperature measurements (DO, ORP or reductant residuals) • Allows real-time analysis of the corrosion kinetic continuum
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The Nalco Corrosion Stress Monitor (NCSM) and AtTemperature ORP • • •
• •
•
AT ORP cell is a combination probe designed by Nalco Uses EPBRE Years of laboratory and field data demonstrate excellent reliability, longevity and stability Environments up to 500oF (260oC) and 3,000 psi (20.68 MPa) Signals can be used to control any species affecting the ORP space (typically reductant feed) and for advanced diagnostics Measures ORP in-situ in feedwater at the elevated temperatures where sensing electrodes are actually in contact with high-temperature water
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Particle Analysis • Traditional monitoring of metal transport and generation in the steam cycle relies primarily on periodic wet tests • Wet tests valuable, but leave significant holes in the data stream • Every thermal, chemical, or hydraulic event liberates or generates metal oxides in the steam cycle • These events occur often and cannot be scheduled - they occur as the plant operates
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Particle Analysis
• Time-based testing (iron sampling at a specific frequency) important, but it cannot detect the majority of these events • Particle analysis provides a window into metal liberation and transport as it occurs • Two different technologies that can be used - particle size analysis and particle counts
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Particle Counter or Particle Monitor? • PC reports counts in different size ranges – Requires 4 DCS inputs for each sample – Provides more data than PM, but requires more storage and data infrastructure
• PM provides only one reading – “index” – Only 1 DCS input/PI tag per sample – Index represents the total surface area of all particles passing through the sensor
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Particle Counter or Particle Monitor?
• •
•
Both may be useful Testing indicates that the majority of iron transport occurs as particles < 5 microns in size Most iron transport occurs as particles of similar and smaller size
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Particle Counter or Particle Monitor?
Condensate After Chemical Feed Particle Distribution Essential Expertise for Water, Energy and Air
LP Economizer Outlet Particle Distribution
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Particle Counter or Particle Monitor?
Good agreement between PC and PM
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Using the Tools • NCSM and particle analysis technology provides visibility to previously undetectable events. • The two technologies were used to correlate particle index to metal transport (iron) test results and operating environment (NCSM) • Combining particle counts with NCSM technology "closes the loop" on steam cycle metal transport. The combination offers two windows into the process – an extremely accurate measurement of the oxidizing environment (AT ORP technology) with control capabilities – a measurement of the impact of that operating environment (particle index and metal transport)
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Sample Locations
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Sample Locations
• Sample locations already exist at most plants • Conventional plant monitoring locations – – – –
Condensate pump discharge Deaerator inlet Deaerator outlet Economizer inlet
• Combined-cycle plant monitoring locations – Condensate pump discharge – Deaerator or LP drum outlet
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Sample Locations • Want AT ORP sampling as close as possible to the actual sample take-off point - provides quick and representative indications of the actual corrosion state – Steam cycle corrosion occurs at temperature and at pressure – Active species should not be quenched to low temperature – want to see any correlations that exist at actual operating temperatures and pressures – Minimizes lag time
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Sample Locations • Plants may lack sample points for some areas of particular concern, especially FAC has occurred. • Economizer outlet sample points not common in either coalfired or combined-cycle plants. – Significant corrosion and corrosion stresses do occur across economizers – Consider economizer outlet sampling if problems have occurred in the past or if particle analysis indicates significant corrosion product transport
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Key Findings • Reducing Agent Feed Has No Impact After LP Drum • Oxidation Potential Lowers Rapidly Even With No Reducing Agent Feed • Reducing Agent Feed “Quenches” Corrosion Reactions in LP Economizer and Minimizes Iron Transport • ATORP Control of Oxidizing Environment Results in Lower Corrosion Product Transport During Base-Load Operation • ATORP Control of Oxidizing Environment Results in Lower Corrosion Product Transport During Cycling Operation
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Reducing Agent Feed Has No Impact After LP Drum
•
•
•
•
No impact on LP Drum ATORP when reducing agent feed stops Water is already in a highly reduced state due to low oxygen levels (usually < 1 ppb) after the LP Drum Large surface area of the LP Drum effectively removes any remaining oxygen ATORP still provides valuable diagnostics
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Reducing agent off
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Oxidation Potential Lowers Rapidly Even With No Reducing Agent Feed • •
•
•
AVT(O) program minimizes the potential for FAC Oxidizing conditions exist only in the early portions of the condensate and FW Large transients in ATORP at LPEO are individual corrosion events Impact on redox potential is undetectable in the room temperature instruments, but significant impact on redox measured at temperature (the LPEO sample). Essential Expertise for Water, Energy and Air
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Reducing Agent Feed “Quenches” Corrosion Reactions in LP Economizer and Minimizes Iron Transport
•
•
•
Small amount of reducing agent feed lowers condensate ATORP by approximately 50 mV Condensate ATORP lowers because of the additional reducing agent. Corrosion redox stress has no discernible impact on the condensate ATORP Essential Expertise for Water, Energy and Air
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Reducing Agent Feed “Quenches” Corrosion Reactions in LP Economizer and Minimizes Iron Transport
• • •
•
LPEO at higher temp, corrosion redox stress has greater impact Greatest impact occurs after feed event Reducing agent “quenches” corrosion redox stress (perhaps through passivation) Reducing agent may provide benefit for systems with highly variable ATORP, but FAC risk increases
Avoid use of reducing agents in all-steel plants – but stable oxidizing environment is critical. May get better results with oxygen feed.
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ATORP Control Results in Lower Corrosion Product Transport During Base-Load Operation Avg PI = 81.45
Avg PI = 63.72
ATORP Correlation to Particle Index During Period of High ATORP Variability (No Control of Oxidizing Environment)
ATORP Correlation to Particle Index During Period of Low ATORP Variability (Reducing Agent Used to Control Oxidizing Environment)
• Consistency in the redox environment is as important as the actual ATORP value Essential Expertise for Water, Energy and Air
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ATORP Control Results in Lower Corrosion Product Transport During Base-Load Operation
• Corrosion product transport increases any time there‟s a significant change in ATORP • Use ATORP to detect and minimize variability, but reducing agent is only one option to obtain stability (better options may exist) – Oxygen feed – Mechanical/Operational diligence
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ATORP Control Results in Lower Corrosion Product Transport During Cycling Operation • Startup occurred after a brief shutdown typical of cycling operation – drums still have pressure
ATORP Correlation to Particle Index During Startup (No Control of Oxidizing Environment)
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ATORP Correlation to Particle Index During Startup (Reducing Agent Used to Control Oxidizing Environment)
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ATORP Control Results in Lower Corrosion Product Transport During Cycling Operation
• PI lowered to same value after 12 hours of operation in both cases, but was 38.5% higher when no attempt was made to control the oxidizing environment • Variability in the oxidizing environment had a greater impact on iron transport than the strength of the reducing environment • Cycling plants would benefit greatly from an increased emphasis on ATORP stability during both startup and normal operation • Reducing agent used in this study for convenience, but better results may come from reduced ATORP variability with other operational, mechanical, or chemical approaches
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Conclusions • Does reducing agent always increase corrosion product transport? – No – use of reducing agent to minimize variability resulted in lower corrosion product transport – Variability appears to have a greater impact than the actual ATORP value – Reducing agent increases risk of FAC. Consider oxygen feed to minimize variability
• There is a “sweet spot” above and below which corrosion product transport increases – But it‟s plant specific – Must study ATORP and corrosion product transport data for each plant
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News You Can Use • AT ORP responds quickly and with greater sensitivity to all corrosion stresses (much better than RT ORP) • Detects ORP changes caused by presence of oxidized species • Significant improvement in control of reductant and oxidizing environment (24/7) • Significant improvement in diagnostics – detected loss of product feed, pump changes, load changes, increased makeup water usage (nascent tube leak), other events • Reliability and robustness demonstrated through years of use and many field trials
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Laboratory Evaluations; Field Trials; Over 50 Reports; Documented and Published REDOX Stress Control Using AT ORP - Peter D. Hicks
PowerPlant Chemistry 2007, 9(5), pp 301-312
Feedwater Redox Stress Management: A Detailed AT ORP Study at a Coal-Fired Electrical Utility - Peter D. Hicks; David A. Grattan; Phil M. White; Kurt M. Bayburt
PowerPlant Chemistry 2007, 9(6), pp 324-336
Nalco Proprietary and Confidential
Boiler 3DT with AT ORP 2008
Questions?
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Monitoring and Control of Feedwater Corrosivity – A Case History Southwest Chemistry Workshop June 24, 2009
Essential Expertise for Water, Energy and Air
SM
1
Agenda • Measurements and Monitoring Tools • Room Temperature vs. At-Temperature ORP (AT ORPTM) • Tools of the Trade • Using the Tools • Key Findings • Conclusions
Online Analyzers – NCSM (top left), Particle Counter (lower left), Particle Monitor (bottom right)
Essential Expertise for Water, Energy and Air
SM
2
Proprietary and Confidential
Measurements and Monitoring Tools • The ORP “Space” • ORP Measurements – Establishing a Reference • The Nalco Corrosion Stress Monitor (NCSM) and AtTemperature ORP • Particle Analysis • Particle Counter or Particle Monitor? • Using the Tools • Sample Locations
Essential Expertise for Water, Energy and Air
SM
3
Proprietary and Confidential
The ORP “Space” • EPRI advocates use of ORP to monitor and control boiler system cycle corrosion chemistry • Current preboiler corrosion control and monitoring methods can be unreliable, unpredictable, impractical and often inadequately deployed
Essential Expertise for Water, Energy and Air
SM
4
Proprietary and Confidential
The ORP “Space” • ORP analogous to temperature • Systems tend to not be in thermodynamic equilibrium • ORP is really an ever-changing kinetic indicator of the system at both the point of measurement and throughout the steam cycle • ORP is thus not a measure of a thermodynamic finality but rather a kinetic continuum • Corrosion stress events often occur, but traditional monitoring programs often fail to detect or react to such events
Essential Expertise for Water, Energy and Air
SM
5
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The ORP “Space” • Many plants focus control of corrosion stress at a single location • Corrosion “space” changes, and single-point monitoring with slow response can miss short-lived changes in chemical demand or the corrosion environment • Traditional corrosion monitoring and control programs miss dynamic stresses • They assume that slow response is “good” and a “good” measurement in one location provides adequate protection in all locations • Ever-changing ORP “space” requires a rapidly responding, realtime monitoring and diagnostic measurement that reacts immediately to the changing corrosivity of the system • Need to give operators/controllers time to respond in a timely fashion to the cause of the variation (minimizing any of its negative impacts) Essential Expertise for Water, Energy and Air
SM
6
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ORP Measurements – Establishing a Reference • ORP: the electrical potential of a liquid at a specific temperature • Net sum of redox potentials/reactions in the sample stream as measured on noble metals •
• •
Voltage • Solution
[DO] ORP [Scavenger] ORP
Noble Metal
Type Temp.
Reference Electrode EPBRE vs sat KCl
Difference in voltage between two electrodes – but which electrodes? Must quote ORP numbers with reference to the potential being measured and the reference electrode This paper reports the ORP value versus the reference electrode with temperature. For example, an ORP measurement of –0.6 V obtained with the SHE at 400 oF (204.4 oC) is reported as –0.6 V vs. SHE (400 oF).
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7
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AT ORP and REDOX STRESS Controlling the REDOX Stress is essential to controlling CORROSION stress and maintaining system integrity
FW “REDOX Stress” Design
Metallurgy DO ingress Other chemical additions (e.g. pH) Ts and Ps Flow
Deaerator “REDOX Stress” DA design Temperature differentials Temperatures and pressures Mechanical effects (e.g. DA tray alignment) DA venting DA steam supply Seasonal variations T and the DO content of makeup and condensate return Condensate versus makeup ratio. Changes in FW flow / steam load. Scavenger used (kinetics / thermodynamics with [DO]) Catalyst Scavenger residence times and concentrations
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Room Temperature vs. At Temperature: Sensitivity Matters
• • • •
Sample Lag Time Reductant Activity Dissolved Oxygen Corrosion
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9
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Sensitivity Matters: Sample Lag Time • Sample water may travel hundreds of feet from the sample take-off to the steam sample panel – Typical lag time 6-15 minutes – May be much longer (up to 60 minutes)
• Room temperature (sample after cooling and filtering) – Not representative of chemistry or corrosion potential – Additional corrosion reactions can occur during sample transit FW Line
Rough Cooler Fine Cooler Filter
77°F
?
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RT ORP Cell
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10
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Sensitivity Matters: Reductant Activity
• Reductants more active at temperature (especially passivating reductants like hydrazine, carbohydrazide) • Reductant impact on corrosion stressors enhanced at temperature, so control is more sensitive and realistic if ORP measured at temperature
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11
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Sensitivity Matters: Reductant Activity
Effect of increasing temperature on ORP response to DO and reductant feed
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12
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Sensitivity Matters: Dissolved Oxygen
• Excellent tool for plants that don‟t feed reductant
80
200 100
ORP (mV)
low DO concentrations)
o
ORP variations on adding DO at 400 F
60
0 -100
40
-200 -300
20
-400
% Stroke or DO (ppb)
• AT ORP responds more quickly with higher sensitivity (even at very
-500 0
-600 7 Hours o
ORP (400 F) vs EPBRE ORP "RT" vs sat. KCl//AgCl/Ag
Essential Expertise for Water, Energy and Air
% Stroke Air Addition Dissolved Oxygen (ppb)
SM
13
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Sensitivity Matters: Dissolved Oxygen Dissolved Oxygen Variations with Deaerator Conditions
237
43 Minutes
4/25/2005 3:51:36
4/25/2005 1:51:36
4/24/2005 23:51:36
4/24/2005 21:51:36
4/24/2005 19:51:36
4/24/2005 17:51:36
A1 (DA P = 11.7 psig)
7
(DA P = 11.2 psig)
6
B2
234
5
231
4
228
B1 (DA P = 9.7 psig)
DO (ppb)
3
4/25/2005 4:00:00
4/25/2005 2:00:00
SM
2 4/25/2005 0:00:00
Essential Expertise for Water, Energy and Air
4/24/2005 22:00:00
4/24/2005 20:00:00
225
4/24/2005 18:00:00
A2 (DA P = 9.5 psig)
4/24/2005 16:00:00
DA #5 Dome Section Temperature (F)
240
4/24/2005 15:51:36
DA #5 dome section temperatures and DO data recorded at the EI
Proprietary and Confidential
14
Sensitivity Matters: Dissolved Oxygen 6
-0.275 -0.285
5 -0.295
10
Log (DO (ppb))
@T ORP (220F) vs EPBRE (V)
Includes a 9 minute delay-time offset
-0.305 4
-0.315 -0.325 -0.335
3
-0.345 -0.355
2 Hours Essential Expertise for Water, Energy and Air
SM
15
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Sensitivity Matters: Reductant and DO
o
ORP variations with CHZ additions: 250 F Tests RT ORP vs sat KCl//AgCl/Cl (V) ORP (250F) vs EPBRE - V
0 -0.1
ORP - V
-0.2 -0.3 -0.4 -0.5 -0.6 A B C D E A = No CHZ; No DO; 250F B = 1.5 ppm ELIMIN-OX; No DO; 250F C = 30 ppm ELIMIN-OX; No DO; 250F D = 1.5 ppm ELIMIN-OX; + 145 ppb O2; 250F E = 30 ppm ELIMIN-OX; + 145ppb O2; 250F Essential Expertise for Water, Energy and Air
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Sensitivity Matters: Corrosion All corrosion processes are electrochemical reactions
AT ORP
TM
o
RT ORP vs sat KCl//AgCl/Ag - mV
vs EPBRE (350 F) - V
• AT ORP responds AT and RT ORP Data: Feedwater Heater Outlet to any species -100 -0.1 within the water that affects the corrosion -200 -0.2 space -300 -0.3 • If corrosion occurs, -400 additional iron -0.4 and/or copper, -500 -0.5 soluble corrosion products are -600 -0.6 sensed by the AT -700 -0.7 ORP (but not by RT 1 Hour ORP) Essential Expertise for Water, Energy and Air
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Sensitivity Matters: Corrosion
Carbon Steel Corrosion Control within the AT ORP Control Space AT ORP (mV) vs EPBRE (T of DA) +300 +200
Dissolved Oxygen (ppb DO) >100 30-50
+100 0 -100 -200 -300 -400 -500
4-7
0
Carbon Steel Corrosion Increasing Severity for Localized Corrosion
Sulfite residual (ppb DO scavenging „Stay below this confidence zone‟ equivalence) 0.8 mpy
30 60
0.5 mpy 200 0.2 mpy 2500
-600 Essential Expertise for Water, Energy and Air
Lower General Corrosion
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Sensitivity Matters: Corrosion Copper corrosion control within the AT ORP control Space -20000
Copper Corrosion A and B: 3 ppb DO baseline C and D: 45 ppb DO baseline
-Imag Ohm
-16000 -12000 -8000
A
B
C
-4000
D 0 0
10000 20000 30000 40000 50000 Real Ohm
Parameter / Corrosion Test
A
B
C
D
ORP Setpoint vs EPBRE (400F) - mV Increase in mpy (%)
-650 21
-650 0
-100 2560
-100 1730
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Summary: Why AT ORP? • Corrosion is occurring AT – We want to see effects in this „space‟ • Might see an effect AT that cannot be seen at RT as equilibria vary with T - Need to capture the chemical soup AT, ideally • AT sampling is earlier in the sample stream so response will be quick • With scavengers its well known that they work better AT. The power of the reductant is enhanced and thus the control based on reductant residual is more sensitive and realistic • Quicker response – Probe kinetics and thermodynamics. RT probes are more sluggish and more likely to become polarized
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Tools of the Trade
• • • •
At Temperature ORP Particle Analysis Using the Tools Location, Location, Location – Where to Sample
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The Nalco Corrosion Stress Monitor (NCSM) and AtTemperature ORP
• NCSM (AT ORP) continuously monitors the corrosion stress in a hot water system (like boiler feedwater) • Provides a window into the process not available from the traditional room-temperature measurements (DO, ORP or reductant residuals) • Allows real-time analysis of the corrosion kinetic continuum
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The Nalco Corrosion Stress Monitor (NCSM) and AtTemperature ORP • • •
• •
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AT ORP cell is a combination probe designed by Nalco Uses EPBRE Years of laboratory and field data demonstrate excellent reliability, longevity and stability Environments up to 500oF (260oC) and 3,000 psi (20.68 MPa) Signals can be used to control any species affecting the ORP space (typically reductant feed) and for advanced diagnostics Measures ORP in-situ in feedwater at the elevated temperatures where sensing electrodes are actually in contact with high-temperature water
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Particle Analysis • Traditional monitoring of metal transport and generation in the steam cycle relies primarily on periodic wet tests • Wet tests valuable, but leave significant holes in the data stream • Every thermal, chemical, or hydraulic event liberates or generates metal oxides in the steam cycle • These events occur often and cannot be scheduled - they occur as the plant operates
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Particle Analysis
• Time-based testing (iron sampling at a specific frequency) important, but it cannot detect the majority of these events • Particle analysis provides a window into metal liberation and transport as it occurs • Two different technologies that can be used - particle size analysis and particle counts
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Particle Counter or Particle Monitor? • PC reports counts in different size ranges – Requires 4 DCS inputs for each sample – Provides more data than PM, but requires more storage and data infrastructure
• PM provides only one reading – “index” – Only 1 DCS input/PI tag per sample – Index represents the total surface area of all particles passing through the sensor
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Particle Counter or Particle Monitor?
• •
•
Both may be useful Testing indicates that the majority of iron transport occurs as particles < 5 microns in size Most iron transport occurs as particles of similar and smaller size
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Particle Counter or Particle Monitor?
Condensate After Chemical Feed Particle Distribution Essential Expertise for Water, Energy and Air
LP Economizer Outlet Particle Distribution
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Particle Counter or Particle Monitor?
Good agreement between PC and PM
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Using the Tools • NCSM and particle analysis technology provides visibility to previously undetectable events. • The two technologies were used to correlate particle index to metal transport (iron) test results and operating environment (NCSM) • Combining particle counts with NCSM technology "closes the loop" on steam cycle metal transport. The combination offers two windows into the process – an extremely accurate measurement of the oxidizing environment (AT ORP technology) with control capabilities – a measurement of the impact of that operating environment (particle index and metal transport)
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Sample Locations
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Sample Locations
• Sample locations already exist at most plants • Conventional plant monitoring locations – – – –
Condensate pump discharge Deaerator inlet Deaerator outlet Economizer inlet
• Combined-cycle plant monitoring locations – Condensate pump discharge – Deaerator or LP drum outlet
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Sample Locations • Want AT ORP sampling as close as possible to the actual sample take-off point - provides quick and representative indications of the actual corrosion state – Steam cycle corrosion occurs at temperature and at pressure – Active species should not be quenched to low temperature – want to see any correlations that exist at actual operating temperatures and pressures – Minimizes lag time
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Sample Locations • Plants may lack sample points for some areas of particular concern, especially FAC has occurred. • Economizer outlet sample points not common in either coalfired or combined-cycle plants. – Significant corrosion and corrosion stresses do occur across economizers – Consider economizer outlet sampling if problems have occurred in the past or if particle analysis indicates significant corrosion product transport
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Key Findings • Reducing Agent Feed Has No Impact After LP Drum • Oxidation Potential Lowers Rapidly Even With No Reducing Agent Feed • Reducing Agent Feed “Quenches” Corrosion Reactions in LP Economizer and Minimizes Iron Transport • ATORP Control of Oxidizing Environment Results in Lower Corrosion Product Transport During Base-Load Operation • ATORP Control of Oxidizing Environment Results in Lower Corrosion Product Transport During Cycling Operation
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Reducing Agent Feed Has No Impact After LP Drum
•
•
•
•
No impact on LP Drum ATORP when reducing agent feed stops Water is already in a highly reduced state due to low oxygen levels (usually < 1 ppb) after the LP Drum Large surface area of the LP Drum effectively removes any remaining oxygen ATORP still provides valuable diagnostics
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Reducing agent off
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Oxidation Potential Lowers Rapidly Even With No Reducing Agent Feed • •
•
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AVT(O) program minimizes the potential for FAC Oxidizing conditions exist only in the early portions of the condensate and FW Large transients in ATORP at LPEO are individual corrosion events Impact on redox potential is undetectable in the room temperature instruments, but significant impact on redox measured at temperature (the LPEO sample). Essential Expertise for Water, Energy and Air
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Reducing Agent Feed “Quenches” Corrosion Reactions in LP Economizer and Minimizes Iron Transport
•
•
•
Small amount of reducing agent feed lowers condensate ATORP by approximately 50 mV Condensate ATORP lowers because of the additional reducing agent. Corrosion redox stress has no discernible impact on the condensate ATORP Essential Expertise for Water, Energy and Air
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Reducing Agent Feed “Quenches” Corrosion Reactions in LP Economizer and Minimizes Iron Transport
• • •
•
LPEO at higher temp, corrosion redox stress has greater impact Greatest impact occurs after feed event Reducing agent “quenches” corrosion redox stress (perhaps through passivation) Reducing agent may provide benefit for systems with highly variable ATORP, but FAC risk increases
Avoid use of reducing agents in all-steel plants – but stable oxidizing environment is critical. May get better results with oxygen feed.
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ATORP Control Results in Lower Corrosion Product Transport During Base-Load Operation Avg PI = 81.45
Avg PI = 63.72
ATORP Correlation to Particle Index During Period of High ATORP Variability (No Control of Oxidizing Environment)
ATORP Correlation to Particle Index During Period of Low ATORP Variability (Reducing Agent Used to Control Oxidizing Environment)
• Consistency in the redox environment is as important as the actual ATORP value Essential Expertise for Water, Energy and Air
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ATORP Control Results in Lower Corrosion Product Transport During Base-Load Operation
• Corrosion product transport increases any time there‟s a significant change in ATORP • Use ATORP to detect and minimize variability, but reducing agent is only one option to obtain stability (better options may exist) – Oxygen feed – Mechanical/Operational diligence
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ATORP Control Results in Lower Corrosion Product Transport During Cycling Operation • Startup occurred after a brief shutdown typical of cycling operation – drums still have pressure
ATORP Correlation to Particle Index During Startup (No Control of Oxidizing Environment)
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ATORP Correlation to Particle Index During Startup (Reducing Agent Used to Control Oxidizing Environment)
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ATORP Control Results in Lower Corrosion Product Transport During Cycling Operation
• PI lowered to same value after 12 hours of operation in both cases, but was 38.5% higher when no attempt was made to control the oxidizing environment • Variability in the oxidizing environment had a greater impact on iron transport than the strength of the reducing environment • Cycling plants would benefit greatly from an increased emphasis on ATORP stability during both startup and normal operation • Reducing agent used in this study for convenience, but better results may come from reduced ATORP variability with other operational, mechanical, or chemical approaches
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Conclusions • Does reducing agent always increase corrosion product transport? – No – use of reducing agent to minimize variability resulted in lower corrosion product transport – Variability appears to have a greater impact than the actual ATORP value – Reducing agent increases risk of FAC. Consider oxygen feed to minimize variability
• There is a “sweet spot” above and below which corrosion product transport increases – But it‟s plant specific – Must study ATORP and corrosion product transport data for each plant
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News You Can Use • AT ORP responds quickly and with greater sensitivity to all corrosion stresses (much better than RT ORP) • Detects ORP changes caused by presence of oxidized species • Significant improvement in control of reductant and oxidizing environment (24/7) • Significant improvement in diagnostics – detected loss of product feed, pump changes, load changes, increased makeup water usage (nascent tube leak), other events • Reliability and robustness demonstrated through years of use and many field trials
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Laboratory Evaluations; Field Trials; Over 50 Reports; Documented and Published REDOX Stress Control Using AT ORP - Peter D. Hicks
PowerPlant Chemistry 2007, 9(5), pp 301-312
Feedwater Redox Stress Management: A Detailed AT ORP Study at a Coal-Fired Electrical Utility - Peter D. Hicks; David A. Grattan; Phil M. White; Kurt M. Bayburt
PowerPlant Chemistry 2007, 9(6), pp 324-336
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Boiler 3DT with AT ORP 2008
Questions?
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