Flow-Accelerated Corrosion in HRSGs
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Jeff Anderson AEP Plant Engineering Programs August 10th, 2009 Flow-Accelerated Corrosion in HRSGs
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Transcript of Flow-Accelerated Corrosion in HRSGs
Jeff AndersonAEP Plant Engineering Programs
August 10th, 2009
Flow-Accelerated Corrosion in HRSGs
Helpful Articles
Flow-Accelerated Corrosion in Fossil and Combined Cycle/HRSG Plants by Barry Dooley, Power Plant
Chemistry, 2008
Assessments of HRSGs – Trends in Cycle Chemistry and Thermal Transient Performance By Barry Dooley and
Bob Anderson in Power Plant Chemistry, 2009
For article copies: [email protected]
Presenter
Presentation Notes
This presentation was taken from this article. If you would like a copy of the article, send me an email and I’ll send it to you.
The process whereby the normally protective oxide layer on carbon or low-alloy steel dissolves into a stream of flowing water or saturated steam. When the protective oxide layer on the carbon steel piping is dissolved into the fluid, a new protective oxide layer forms, and the cycle repeats until the pipe or vessel is thin enough to rupture.
Influencing factors: fluid chemistry, fluid temperature, flow turbulence, and metal composition.
FAC
FAC is the leading cause of tube failure in HRSGs. The FAC corrosion products from the LP areas of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This LP-HP corrosion link forms the main focus of the cycle chemistry assessments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Acting proactively will reduce the risk for both.
Corrosion Facts
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
5
FLOW
Simplified Mechanism of FAC
Reaction between dissolved oxygen and metal surface forms a protective oxide layer (rust).
Pipe OD
Pipe IDprotective oxide layer (rust)
Presenter
Presentation Notes
This oxide layer is a good thing, hence we refer to it as “the protective oxide layer”.
6
FLOW
Simplified Mechanism of FAC
The oxide layer is dissolved into the flow stream.
OD
ID
7
FLOW
Simplified Mechanism of FAC
The oxide layer is replenished as the base metal is converted into more oxide.
OD
ID
8
FLOW
The oxide layer is again dissolved into the flow stream. The base metal again
has to form a new
protective oxide layer. The cycle repeats, thinning the pipe wall until a rupture occurs.
OD
ID
Single-Phase FACOccurs when the fluid is in the liquid water phase. Damage is characterized by a bumpy “orange peel” surface.
Two-Phase FACOccurs when the fluid is in the saturated “wet” steam phase. Damage is characterized by a shiny black surface.
Two-phase FAC is more aggressive than single-phase FAC due to the hyper-turbulent nature of wet steam versus water.
Note: FAC does not occur in superheated “dry” steam environments.
Types of FAC
Examples of single-phase FAC.
Left: FAC failure in an economizer inlet header.
Right: FAC in a reducer which was attached to an HP FWH drain control valve.
Two views of the surface appearance of single-phase FAC.
Left: A close-up of the economizer inlet header tube above.
Right: Microscopic view of an HRSG LP evaporator tube.
In both cases the “horseshoes” point in the direction of flow.
Single-Phase Examples of FAC
Examples of FAC in HRSG LP evaporator tubing: A) single-phase FAC in a vertical tube. B) two-phase FAC in a vertical tube. C) two-phase FAC in a hairpin bend of a horizontal tube. D) microscopic view of the scalloped appearance that is
always present with FAC.
MagnetiteBlack or gray surface color. Porous, easily dissolved, susceptible to FAC. Forms when cycle chemistry operates at low oxygen levels (reducing agent is used).
HematiteRed surface color. Dense, not easily dissolved, resists FAC. Forms from magnetite, on top of it. Forms when cycle chemistry operates at higher oxygen levels (reducing agent is not used).
Types of Oxide Layers
Magnetite
Hematite with Magnetite
Two-phase FAC inside an LP feedwater heater.
Amos 3, #63 HP heater drain entry into the deaerator.
Presenter
Presentation Notes
Email from: William E Sayre/CH1/AEPIN 08/04/2008 08:54 AM During our recent U-3 outage due to #62 HP Heater drain piping to the Deaerator leaking due to a stress fracture caused by water hammer which caused internal damage to Deaerator and to the pipe hangers. While we were inside doing inspection and repairs we found FAC damage in the #63 HP heater drain piping. I have attached photo of this damage. During this outage we made pad weld repair to restore wall loss due to FAC to this piping. We will write work request to replace during our upcoming U-3 GBIR outage along with the #62 HP Heater drain piping where they enter Deaerator. Will this info/picture on damage/repairs be adequate for credit for our 2012 inspections/repairs?
Welsh 1, 6” superheat attemperator supply piping. Nominal wall = 0.864”. Remaining wall, low point = 0.152”. FAC occurred when cycle chemistry used a reducing agent. Red hematite formed after agent was removed, then FAC
stopped.
Presenter
Presentation Notes
Welsh Unit 1 Superheat Attemperator Supply Line 6” Straight Pipe, . The lip that can be seen in the picture is evidence of uniform thinning of the pipe on the downstream side of weld to the elbow, since this lip is essentially the ID of the elbow. The picture also shows three deeper pits just downstream from the weld. This pipe is originally 6 in. XXH, which has a nominal wall thickness of 0.864 in. The minimum wall thickness is 0.694 in. The remaining wall thickness for the pit at 12:00 in the picture is 0.152 in. and the pit at the 4:00 position has a remaining wall thickness of 0.224 in. All of the downstream piping that was replaced on the SH attemperator spray piping during the spring of 2008 had similar characteristics as those shown in the picture above.
We want to be proactive to protect against FAC rather than waiting for failures to occur.
We want to optimize cycle chemistry to prevent damage while performing inspections to find existing damage.
It must be recognized that chemistry alone cannot always eliminate FAC. An inspection program must be in place, combined with low-chrome alloy replacements.
AEP’s FAC Program Philosophy
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
1) Operate with an oxidizing chemistry (remove reducing agent) to control single-phase FAC.
2) Operate with an elevated pH (around 9.8) to control two- phase FAC.
3) Monitor the color of the LP and IP drums.
4) Monitor the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working (The “Rule of 2 and 5” is the goal: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum).
The Control of FAC
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
1. Economizer/preheater tubes at inlet headers. 2. Economizer/preheater tube bends where steaming takes place. 3. LP evaporator inlet headers which have a contortuous fluid entry
path and where orifices are installed. 4. Verticle LP evaporator tubes, especially in bends near outlet
headers. 5. LP evaporator transition headers. 6. LP riser tubes/pipes to the LP drum. 7. LP drum internals. 8. IP economizer inlet headers. 9. IP economizer outlet headers, especially in nearby bends which
have steaming. 10. IP riser tubes/pipes to the IP drum. 11. IP evaporator tubes on triple-pressure units operated at reduced
pressure.
HRSG Areas of Concern(single and two phase)
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
Waterford - 33 inspections, no damage found.
Lawrenceburg - ?
Comanche & Northeastern 1 - Extensive damage found:
-- Elbows around the LP economizer headers (at NE1 the inlet and outlet headers were replaced with P11 material because of the damage in the hard-to-reach bends).
-- The entire feedwater, attemperator supply, by-pass, and recirculation lines (the smaller diameter lines appear to be corroding faster than the FW piping due to the tighter radius bends).
--Since they have found so much FAC in these areas, they are systematically replacing the lines as money permits.
HRSG Areas of Concern
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
AEP’s FAC Program
Database
Scorecard
Data Storage
Support
Presenter
Presentation Notes
Currently, HRSG’s and Gas-Fired Boilers are not included in the program. But if anyone wants to voluntarily join the program, I’d be more than happy to accommodate them.
Location Legend:Required feedwater and heater drain inspection locations. Wall loss found; piping was repaired/replaced.Third phase locations (additional pts. chosen by the plant). Wall loss found; reinspection needed at a later date.
Y/N --> Submitted data does not conform to Circ. Letter.
Plant Unit LocationReq'd Loc
Date Inspected Comments
AM 1 Feedwater, 18", D/S of flow nozzle Y 05-04-99 Some apparent wall loss likely due to boring. Reinspect inAM 1 Feedwater, 18", D/S of flow nozzle Y 05-04-99AM 1 Feedwater, 18", D/S of check valve Y 05-04-99AM 1 RH Attemp., 6", D/S of flow nozzle Y 05-04-99 Some apparent wall loss likely due to boring. Reinspect inAM 1 RH Atemp., 4", 90 deg elbow Y 05-04-99AM 1 Heater #8, Drain, U/S of control valve, 10" AM 1 Heater #8, Drain, U/S of control valve, 8" AM 1 Heater #8, Drain, U/S of control valve, 4" Y 5/1/05 Data not submitted.AM 1 Heater #8, Drain, D/S of control valve, 4" Y 5/1/05 Data not submitted. Replaced deaerator drain elbows in 20AM 1 Heater #8, Drain, D/S of control valve, 8" Y 5/1/05 Data not submitted.AM 1 Heater #8, Drain, D/S of control valve, 12" AM 1 Heater #8, Alt Drain to Deaerator ElbowsAM 1 Heater #7, Drain, U/S of control valve, 12" AM 1 Heater #7, Drain, U/S of control valve, 8" AM 1 Heater #7, Drain, D/S of control valve, 8" YAM 1 Heater #7, Drain, D/S of control valve, 12" YAM 1 Heater #7, Alt Drain to Deaerator ElbowsAM 1 Heater #4, Drain, U/S of control valve, 6"
Comments Legend:
FAC Database
FAC Database
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
FAC ScorecardPhase 3
2008-2012
Attemp Supply
Misc. BFP
FWH Alt Drains
Conden -sate
FWH Shells
FWH Vents
Plant's Choice
Phase 3 Total
4 Req’d 4 Req’d 4 Req’d 4 Req’d 2 Req’d 4 Req’d 10 Req’d 32 Req’d
Presenter
Presentation Notes
We want to be proactive to protect against FAC rather than waiting for failures to occur. Root causes can be identified and eliminated before serious damage and failure. We want to optimize cycle chemistry to aviod FAC. Independent of the manufacturer or type of HRSG, there are common features of cycle chemistry. There is very little variation across the fleet worldwide. We can take corrective action in older units, and prevent damage in newer plants which haven’t experienced FAC failures. On HRSGs, it is never satisfactory to sit back complacently and wait for undetected damage to become failures. FAC is the leading cause of damage and failure in HRSGs. FAC can be single- or two-phase, occurs predominantly in low-pressure piping (economizer/preheater, evaporator tubes headers & risers), with an increasing number of incidents in intermediate pressure tubes and risers. All components with the temperature range of 200F-500F are suseptable. Tube Failure Prevention Program: The root cause of each failure must be determined so that the proper corrective actions can be taken to prevent additional failures. Failure sites should be removed and the failure mechanism identified through metallurical analysis. Root causes can range from “a bad weld” to chemistry imbalances, design features, or operating procedures. These root causes have the power to repeatedly inflict corrosion, corrosion fatigue, or thermal-mechanical fatigue damage. The Tube Failure Prevention Program should also keep track of damage locations as well as stock a small supply of replacement tubing. Regions of concern are: (underline=2 phase areas) Econ/preheater tubes at inlet headers 2. Econ/preheater tube bends where steaming takes place 3. LP evaporator inlet headers which have a contortuous fluid entry path and where orifices are installed 4. Verticle LP evaporator tubes, especially in bends near outlet headers 5. LP evaporator transition headers 6. LP riser tubes/pipes to the LP drum 7. LP drum internals 8. IP economizer inlet headers 9. IP economizer outlet headers, especially in nearby bends which have steaming 10. IP riser tubes/pipes to the IP drum 11. IP evaporator tubes on triple-pressure units operated at reduced pressure The corrosion products from the lower pressure parts of the HRSG are deposited in the HP evaporator tubing and form the basis of the under-deposit corrosion damage mechanisms (hydrogen damage, acid phosphate corrosion, and caustic gouging). This link forms the main focus of the cycle chemistry assesments in the plants, which if left unaddressed, will eventually lead to damage and failure by one or both mechanisms. Actiing proactively will remove the risk for both. The control of FAC usually takes a three-pronged approach: 1) To control single-phase FAC, operate with an oxidizing chemistry (all-volatile treatment (AVT(O)), or oxygenated treatment (OT); 2) To control two-phase FAC, operate with an elevated pH (at least 9.8); and 3) Monitoring the total iron concentration in the condensate, feedwater, and in each drum to evaluate how the first two approaches are working. Single-Phase FAC control: 1) Ensure that a reducing agent is not used in the cycle during any periods of operation or shutdown. Single-phase FAC is controlled by the oxidizing-reducing potential (ORP) of the condensate and feedwater. The potential should always be oxidizing. 2) Identify whether sufficient oxidizing power is available to passivate all the single-phase locations by a) Monitoring the total iron in the LP and IP drums. This is the main indicator of the extent of passivation (the “Rule of 2 and 5” is used: less than 2 ppb total iron in the condensate/feedwater and less than 5 ppb total iron in each drum); and b) Monitoring the color of the LP and IP drums (it should be an even red surface color, indicating that the corrosion-resistant hematite is in place. Black or gray magnetite is not FAC-resistant). The amount of oxidizing power is controlled by the oxygen level in the condensate and feedwater. The oxygen level may not be able to satisfactorily passivate all the single-phase flow locations in condensate and feedwater circuits and drums. The possibility of increasing the level of oxygen may need to be investigated to provide better single-phase protection while being cognizant of oxygen levels in other areas of the HRSG. In cases where the single-phase areas have been passivated by AVT(O) or OT but the total iron levels remain high, the only other chemical option is to try and increase the pH of the water to about 9.8. It must be recognized that chemistry alone cannot always eliminate FAC. This is why an inspection program must be in place and replacements with low-chrome materials be made.
AEP’s FAC Program
Data Storage
Support
Presenter
Presentation Notes
Currently, HRSG’s and Gas-Fired Boilers are not included in the program. But if anyone wants to voluntarily join the program, I’d be more than happy to accommodate them.
Questions?
Examples of two-phase FAC in deaerators. Example A is located adjacent to an HP cascading drain entry. Example B is directly in the path of flashing
steam from another drain entry. In both cases the two-phase FAC areas are easily seen by a black, shiny, extremely thin layer of magnetite. There may
also be pitting on the surface. The red areas indicate where single-phase water has provided a protective hematite oxide film.
Potential-pH Diagram for Iron
7
0
0 14
pH
Potential, V (ORP)
Oxidizing (+)
Reducing (-)
Corrosive
Passive
Immune
Magnetite
Hematite
1.6
-1.6
Iron-Chromium Oxide
Presenter
Presentation Notes
Potential-pH diagrams are a concise and compact way to help us understand corrosion of various metals. We can use these diagrams to develop strategies to control corrosion by looking at he regions of stability of a metal in various water environments. The Immune region is stable and will not corrode, the Corrosive or Active region is where the metal will quickly corrode, and the Passive region is an initially corrosive region which quickly forms a very thin protective oxide layer or film which then protects the metal from further corrosion. The horizontal axis identifies the pH, where acidic conditions are on the left side (0-7) and alkaline conditions are on the right (7-14). 7 is neutral. The vertical axis identifies the electrochemical potential in volts of the water. These values range from strongly reducing negative values at the bottom of the scale to strongly positive oxidizing values at the top. This diagram is configured for pure iron in pure water, so obviously there’s a bit of difference when we’re considering carbon steel, but carbon steel is about 97% iron, so we can approximate its behavior with the iron diagram. Define “electrochemical potential”. Go to previous lessons to find where electrical reactions are discussed. “Corrosion is an electro-chemical reaction....If conditions are right for corrosion, then at the atomic level metal atoms naturally want to react with water atoms and transfer electrons”. Each metal has a characteristic, inherent tendancy to react or corrode in a water environment. This reactivity is described as electrochemical potential. In power plants where we’re trying to keep the corrosion of carbon steel in the Passive region (because we apparently can’t create an evironment which is reducing enough to be in the Immune region), the corrosion of carbon steel in water always begins with the iron surface being converted into Magnetite. Magnetite always forms first, then the hematite forms from it, on top of it. If the water is reducing, the porous magnetite is the only oxide that will form and it will be suseptible to FAC. But if the water’s potential is at least zero or slightly positive, the oxide film will develop a second layer called hematite on top of the magnetite. This dual oxide is much less porous (or soluable) and protects us from FAC. “Possible passivation” - Not all oxides which form on the metal’s surface are created equally. Some are tougher and more corrosion resistant than others. The tougher the oxide the more it adheres to the metal’s surface, is nonporous, and resists iron transport into the surrounding water. Depsite all the research that has been done, there is still a lack of understanding in how these oxides grow and the associated electrochemical processes. The addition of chromium to carbon steel doesn’t make it immune to corrosion, but rather, a different type of passive oxide is formed. This chromium containing oxide is better at resisting corrosion. This is why when water chemistry is unable to stop FAC (such as with two-phase FAC) replacing carrbon steel with a low-chrome alloy is so effective.
