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51315-5574-Flow Accelerated Corrosion—Causes, Detection, and Mitigation

Product Number: 51315-5574-SG
ISBN: 5574 2015 CP
Author: Aaron Kelley
Publication Date: 2015
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$20.00
$20.00
Flow Accelerated Corrosion (FAC) has plagued the nuclear and fossil power industry with piping and components for over 30 years. Since the catastrophic rupture which occurred at the Surry Nuclear Power Plant in 1987 the nuclear industry has spent considerable time and resources in efforts to detect and mitigate FAC before a catastrophic rupture or leak occur. The primary focus has been on plant personnel and power plant safety helping to ensure our co-workers go home as safe as they were when they came to work. Additionally our programs are geared towards mitigating the effects of FAC on plant piping and components such as feedwater heaters tanks and equipment nozzles.Flow Accelerated Corrosion (FAC) is a chemical dissolution process where the normally protective oxide layer on carbon and low alloy steel dissolves into the flow of water and wet steam. As the oxide layer thins it becomes less and less protective and the corrosion rates increase. The rate of corrosion will eventually reach a steady state whereby the corrosion and dissolution rates equal and a stable corrosion rate is maintained.FAC damage is characterized as general reduction of the piping and component wall thickness rather than a local wear such as pitting or cracking. Although FAC will occur over a wide area in component it is localized based on it frequently occurs over a limited area of piping systems largely due to turbulence. It should be noted that if one component is worn others with have similar wall loss. This process is why FAC is deemed to be a linear wear mechanism which allows the corrosion rate to be readily predicted evaluated and the life of the piping and components accurately determined.The phenomenon of FAC is well understood. Two-phase FAC has been recognized since about 1970 while single phase FAC has been acknowledged since the mid-1980s. There are a number of factors which contribute to the likelihood of FAC wear in plant piping and components and they are:•Hydrodynamic such as velocity pipe roughness and geometry. FAC wear rates increase in with velocity and turbulent conditions or with increased pipe roughness•Environmental factors such as steam quality temperature pH and oxygen concentration. The solubility of ferrous hydroxide increases with increasing temperature with FAC typically occurring between 200° F and 400° F (100° C to 280° C). Low pH increases the solubility of the oxide layer whereas higher levels of Dissolved Oxygen (DO) promote the growth of hematite which is much less soluble in water than magnetite•Material Composition – the use of alloying elements namely chromium helps to reduce FAC wear ratesLarge-bore components >2.5” NPS (63.5 mm) will fail due to overstress from the operating pressure or abrupt changes such as water hammer start-up loading etc. Typically there is no warning due to the sudden rupture which will release hot water and flashing steam into the area. Small-bore components < 2” NPS (5.08 mm) will typically leak before rupture.Understanding these parameters in operating systems helps to identify the piping and components which are susceptible to FAC which allows us to better predict the wear rates utilizing computer analytic models. All power plants both nuclear and fossil have a vast amount of piping which is susceptible to FAC. Nuclear and fossil plants need to be able to selectively measure the wall thicknesses on the highly susceptible components and utilize this information to help determine the life of the remaining uninspected components. The thickness information is feed into the analytical model in order to recalibrate our initial predictions.Following the catastrophic rupture at the Surry Nuclear Power Plant the NRC issued Generic Letter (GL) 89-08 which required nuclear utilities to implement a long-term program for protecting piping from single and two-phase FAC. Fossil and industrial steam plants are not as well documented and enforced mainly due to the fact they are not as tightly regulated although due to numerous failures and fatalities at fossil plants FAC is getting higher attention.Currently the nuclear industry utilizes common guidelines developed with Electric Power Research Institute (EPRI) to determine susceptibly to FAC which includes standards methods of inspecting evaluating and calculating continued service of plant piping and components. The principles behind the programs are focused on six (6) key elements of a highly effective FAC program and are:1.Corporate Commitment – required to ensure proper resources to implement and maintain the program as well as responsibilities are clearly defined2.Analysis – objective is to identify the most susceptible systems to identify systems susceptible to FAC and ensuring inspections are performed. The analysis is continually updated to include changes to operating conditions3.Operating Experience – it is key to be aware of operating experience so that we learn from everyone4.Inspections – accurate inspections are key to determine presence of FAC and use to verify the accuracy of the analytical model5.Training and Engineering Judgment – running a FAC program requires expertise in various areas. Judgment is used in many of the tasks that are performed6.Long Term Planning – it is necessary to reduce the rates of FAC to ensure the safety of the plant and personnel which helps to ensure operating efficiency and reduce costs.This presentation will focus on the elements in implementing an effective program to detect and mitigate FAC. Although this presentation will be largely geared towards nuclear power plant issues many of the same issue are common in our fossil and renewable industry.
Flow Accelerated Corrosion (FAC) has plagued the nuclear and fossil power industry with piping and components for over 30 years. Since the catastrophic rupture which occurred at the Surry Nuclear Power Plant in 1987 the nuclear industry has spent considerable time and resources in efforts to detect and mitigate FAC before a catastrophic rupture or leak occur. The primary focus has been on plant personnel and power plant safety helping to ensure our co-workers go home as safe as they were when they came to work. Additionally our programs are geared towards mitigating the effects of FAC on plant piping and components such as feedwater heaters tanks and equipment nozzles.Flow Accelerated Corrosion (FAC) is a chemical dissolution process where the normally protective oxide layer on carbon and low alloy steel dissolves into the flow of water and wet steam. As the oxide layer thins it becomes less and less protective and the corrosion rates increase. The rate of corrosion will eventually reach a steady state whereby the corrosion and dissolution rates equal and a stable corrosion rate is maintained.FAC damage is characterized as general reduction of the piping and component wall thickness rather than a local wear such as pitting or cracking. Although FAC will occur over a wide area in component it is localized based on it frequently occurs over a limited area of piping systems largely due to turbulence. It should be noted that if one component is worn others with have similar wall loss. This process is why FAC is deemed to be a linear wear mechanism which allows the corrosion rate to be readily predicted evaluated and the life of the piping and components accurately determined.The phenomenon of FAC is well understood. Two-phase FAC has been recognized since about 1970 while single phase FAC has been acknowledged since the mid-1980s. There are a number of factors which contribute to the likelihood of FAC wear in plant piping and components and they are:•Hydrodynamic such as velocity pipe roughness and geometry. FAC wear rates increase in with velocity and turbulent conditions or with increased pipe roughness•Environmental factors such as steam quality temperature pH and oxygen concentration. The solubility of ferrous hydroxide increases with increasing temperature with FAC typically occurring between 200° F and 400° F (100° C to 280° C). Low pH increases the solubility of the oxide layer whereas higher levels of Dissolved Oxygen (DO) promote the growth of hematite which is much less soluble in water than magnetite•Material Composition – the use of alloying elements namely chromium helps to reduce FAC wear ratesLarge-bore components >2.5” NPS (63.5 mm) will fail due to overstress from the operating pressure or abrupt changes such as water hammer start-up loading etc. Typically there is no warning due to the sudden rupture which will release hot water and flashing steam into the area. Small-bore components < 2” NPS (5.08 mm) will typically leak before rupture.Understanding these parameters in operating systems helps to identify the piping and components which are susceptible to FAC which allows us to better predict the wear rates utilizing computer analytic models. All power plants both nuclear and fossil have a vast amount of piping which is susceptible to FAC. Nuclear and fossil plants need to be able to selectively measure the wall thicknesses on the highly susceptible components and utilize this information to help determine the life of the remaining uninspected components. The thickness information is feed into the analytical model in order to recalibrate our initial predictions.Following the catastrophic rupture at the Surry Nuclear Power Plant the NRC issued Generic Letter (GL) 89-08 which required nuclear utilities to implement a long-term program for protecting piping from single and two-phase FAC. Fossil and industrial steam plants are not as well documented and enforced mainly due to the fact they are not as tightly regulated although due to numerous failures and fatalities at fossil plants FAC is getting higher attention.Currently the nuclear industry utilizes common guidelines developed with Electric Power Research Institute (EPRI) to determine susceptibly to FAC which includes standards methods of inspecting evaluating and calculating continued service of plant piping and components. The principles behind the programs are focused on six (6) key elements of a highly effective FAC program and are:1.Corporate Commitment – required to ensure proper resources to implement and maintain the program as well as responsibilities are clearly defined2.Analysis – objective is to identify the most susceptible systems to identify systems susceptible to FAC and ensuring inspections are performed. The analysis is continually updated to include changes to operating conditions3.Operating Experience – it is key to be aware of operating experience so that we learn from everyone4.Inspections – accurate inspections are key to determine presence of FAC and use to verify the accuracy of the analytical model5.Training and Engineering Judgment – running a FAC program requires expertise in various areas. Judgment is used in many of the tasks that are performed6.Long Term Planning – it is necessary to reduce the rates of FAC to ensure the safety of the plant and personnel which helps to ensure operating efficiency and reduce costs.This presentation will focus on the elements in implementing an effective program to detect and mitigate FAC. Although this presentation will be largely geared towards nuclear power plant issues many of the same issue are common in our fossil and renewable industry.
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