Paper No.
5574
FLOW ACCELERATED CORROSION – DETECTION AND MITIGATION
Aaron D. Kelley
Altran
846 East Algonquin Road, Suite 102
Schaumburg, IL 60173 USA
ABSTRACT
Flow Accelerated Corrosion (FAC) has plagued the nuclear and fossil power industry with piping and
components failures for more than 40 years. Two-phase FAC has been recognized since about 1970,
while single phase FAC has been acknowledged since the mid-1980s. U.S. and international utilities
recognize FAC as a major contributor to piping and component degradation.
Keywords: Flow Accelerated Corrosion, FAC, GL89-08, detection, mitigation, elements.
INTRODUCTION
Since the catastrophic rupture which occurred at the Surry Nuclear Power Plant in December 1986, the
nuclear industry has spent considerable time and resources in efforts to detect and mitigate Flow
Accelerated Corrosion (FAC) before a catastrophic rupture or leak occurs. The U.S. Nuclear Regulatory
Commission (NRC)(1) became engaged in the issue by ensuring that all nuclear plants were fully aware
of the implications of the catastrophic rupture at the Surry Nuclear plant. The primary focus of the
industry FAC Programs has been on plant personnel and power plant safety, helping to ensure that our
co-workers go home as safely as they were when they came to work.
BACKGROUND
On December 9, 1986, a high pressure condensate line in Virginia Power’s Surry Nuclear Power Plant
catastrophically ruptured and killed four maintenance personnel and severely scalded four additional
personnel (Figure 1). This failure caught the industry off-guard due to the fact FAC was well known in
two-phase flow lines (water and steam mixture), but this line was a single phase line. Investigations
immediately following this rupture revealed a similar rupture at a fossil station; however, it was not
reported since no one was injured. Following the rupture at Surry, the NRC became concerned
because it was apparent that safety related systems can be damaged by failures in non-safety related
systems.
1
U.S. Nuclear Regulatory Commission (NRC), 11555 Rockville Pike, Rockville, MD 20852
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
In response to the Surry accident, the Nuclear Management and Resources Council (NUMARC)(2)
formed an industry working group to address FAC. This working group published a set of guidelines
with their major recommendations focused on single phase lines due to the fact they felt they had
adequate controls in place to address two-phase FAC. The recommendations focused on developing a
consistent approach to perform a susceptibility review of the plant systems to FAC, risk rank the lines,
and then perform a limited number of inspections.
The NRC agreed in general principle to the NUMARC guidelines and recommended that the industry
implement these guidelines. In 1989, Generic Letter (GL) 89-08 was issued which required the U.S.
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 tightly regulated, although numerous failures and fatalities
are increasing industry awareness, resulting in increased attention to FAC.
The industry programs are largely geared toward mitigating the effects of FAC on plant piping and
components such as feedwater heaters, tanks, and equipment nozzles. This helps to ensure personnel
and plant safety, as well as plant availability.
Figure 1: 18” Condensate elbow rupture at Surry Nuclear Plant.
Since the majority of nuclear power and fossil plants were already designed and constructed prior to
the Surry failure, the industry was not aware of this phenomenon or how it would affect personnel and
plant safety, as well as safe plant operations. However, over the past 30 years the industry has learned
an extensive amount in the causes of, and ways to mitigate FAC. These lessons learned must be
incorporated into new plant construction for conventional nuclear power plants, Small Modular Reactors
(SMRs), and fossil and combined cycle plants.
For the first time in decades, the United States is constructing new nuclear power plants in South
Carolina and Georgia. The utilities now have the opportunity to incorporate what we have learned about
FAC to ensure personnel and plant safety. Incorporating FAC mitigation strategies, coupled with the
safe efficient operation of the power plants, will help to reduce the costs associated with inspections
and repair/replacements due to FAC wear. It is essential that the utilities responsible for the
construction of these plants take into account measures to reduce the potential for FAC.
2
Nuclear Management and Resources Council (NUMARC), now part of the Nuclear Energy Institute (NEI), 1201 F St., NW,
Suite 1100, Washington, DC 20004-1218.
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
DEFINITION OF FAC
Corrosion can be defined in numerous ways, but one definition is as simple as the degradation of a
metal with its reaction to the environment. Corrosion can be categorized as two types: general, or
localized. General corrosion results in a uniform degradation of a metal’s surface; localized corrosion
includes pitting, crevice corrosion, etc. FAC is a chemical dissolution process whereby the normally
protective oxide layer on carbon and low alloy steel dissolves into the flow of water and wet steam
(Figure 2).1 As the oxide layer thins, it becomes less and less protective and the corrosion rate
increases. 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 local wear such as pitting or cracking.1 This process is why FAC is deemed to be a relatively linear
wear mechanism in steady state conditions which allows the corrosion rate to be readily predicted and
evaluated and the life of the piping and components accurately determined.
Although FAC may occur over a wide area in a component, it is often localized to a limited area, largely
due to high turbulence such as immediately downstream of control valves, flow venturies, flow orifices,
and other reducing or flow straightening fittings. It should be noted that, if one component is worn,
others within the same line, as well as those located in sister trains which operate under similar
conditions, may have similar wall loss. The investigation results from the rupture at Surry revealed an
additional 190 components which needed to be replaced due to wall thinning at Surry Unit 1 and Unit 2.
Figure 2: Representation of the FAC process.1
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
UNDERSTANDING FAC
The phenomenon of FAC is well understood. There are a number of factors which contribute to the
likelihood of FAC wear in plant piping and components:
•
Hydrodynamic such as velocity, pipe roughness, and geometry. FAC wear rates increase 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. FAC typically occurs
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 rates
Large-bore components >2.5” NPS (63.5 mm) can fail catastrophically due to overstress from the
operating pressure, fluid transients, start-up loading, etc. Typically there is no warning prior to a sudden
rupture, which releases hot water and flashing steam into the area. Small-bore components < 2” NPS
(5.08 mm) will typically leak before rupture.
Understanding the parameters which influence FAC will help to identify the piping and components
which are susceptible to FAC. This, in turn, allows us to better predict the corrosion rates utilizing
computer analytic models. All power plants, both nuclear and fossil, have a vast amount of piping
susceptible to FAC. Nuclear and fossil plants must be able to identify the most susceptible components,
measure the wall thicknesses, and utilize this information to help determine the life of the remaining
uninspected components. The thickness information is fed into the analytical model in order to
recalibrate initial predictions.
AN EFFECTIVE FAC PROGRAM
Currently the nuclear industry utilizes common guidelines developed with Electric Power Research
Institute (EPRI)(3) to determine susceptibly to FAC. These include standard methods of inspecting,
evaluating, and calculating continued service of plant piping and components. The principles behind a
highly effective FAC program are based on the following six key elements (Figure 3).2
1. Corporate Commitment – required to ensure proper resources to implement and maintain the
program, as well as responsibilities are clearly defined,
2. Analysis – the objective is to identify the systems most susceptible to FAC and to ensure
inspections are performed. The analysis is continually updated to include changes to operating
conditions,
3. Operating Experience – it is key to be aware of operating experience to share information
across the industry,
4. Inspections – accurate inspections are key to determine the presence of FAC and that they are
used to verify the accuracy of the analytical model,
5. Training and Engineering Judgment – managing a successful FAC program requires expertise
in various areas as well as the ability to exercise sound judgment in many of the required tasks,
6. Long Term Planning – it is necessary to reduce the rates of FAC to ensure the safety of the
plant and personnel which, in turn, helps to ensure operating efficiency and reduce costs.
3
Electric Power Research Institute (EPRI), 1300 West WT Harris Blvd, Charlotte, NC 28262
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
Figure 3: 6 Elements of an effective FAC program.2
FAC DETECTION
Detection of FAC can be a rigorous task. Since the FAC process employs several parameters, such as
chemistry, material composition, and flow characteristics, it is critical that the plant engineer utilizes all
of the tools available to ensure that targeted inspections are carried out to detect FAC.
Once the plant systems susceptible to FAC have been identified, a detailed analysis should be
performed using a predictive analytical model. The purpose of the model is to predict FAC wear rates
based on known operating conditions, chemistry, and material parameters. This will assist the engineer
in determining the service life of the plant piping and components.
Inspections locations are then selected using the results from the analytical model, previous inspection
results (trending), plant and industry experience, and engineering judgment.
New conventional nuclear plants, SMRs, and fossil (including combined cycle) plants have a distinct
advantage over existing plants in that they are able to apply the best practices learned over the last 26
years of FAC Program implementation.
FAC MITIGATION
Two of the major parameters that influence FAC wear rates are fluid chemistry and chromium content
of the material. FAC wear rates are strongly dictated by the amount of dissolved oxygen (DO) and the
pH (Figure 4). Plant chemistries have changed in order to reduce Inner-Granular Stress Corrosion
Cracking (IGSCC). FAC damage is highly dependent on the oxidizing reducing potential (ORP) and the
additives—such as hydrogen or hydrazine—used to reduce IGSCC. These two additives have proven
to lower the DO or raise the pH of the system.
In general, the higher the pH and the higher DO in a system, the lower the FAC wear rates due to the
magnetite layer will be more stable and offer a greater degree of corrosion protection. Oxygen
promotes the establishment of magnetite, which is the normally protective layer of corrosion. Higher
levels of oxygen will further promote the formation of hematite, which is even less soluble than the
magnetite. A more acidic environment enhances the dissolution of the magnetite layer, preventing the
formation of hematite. Understanding these chemistry parameters will assist in the safe operation and
reduction of FAC wear rates in future conventional nuclear, SMRs, and fossil including combined cycle
plants.
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
Figure 4: Effects of pH on FAC wear rates in single phase lines.2
It is well known that chromium content in plant piping and components plays a large part of reducing
the effects of FAC. Original documentation provided to the utilities in the early 1990s from EPRI stated
that plant piping systems constructed of 1¼% chromium could be excluded from evaluation of FAC
based on the very low susceptibility. Since that that time, the industry has completed a substantial
amount of research on the effects of chromium on FAC wear rates (Figure 5). We have learned that
piping and components with as little 0.04% chromium have reduced FAC wear rates nearly to the point
of not being able to accurately measure and calculate the wear rate. In fact, when a component is
inspected with no measureable FAC wear detected, and the chromium content is measured to be
0.10% or greater, this component can be excluded from further evaluation.
Figure 5: Effects of chromium on predicted FAC wear rates.6
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.
Much of what the industry has learned over the past 26 years can be incorporated into the design of
conventional nuclear plants, SMRs, and fossil and combined cycle plants. We fully understand how the
construction costs play an integral role in the selection of materials for piping systems. Plants will have
miles of piping installed; some of it will be susceptible to FAC by nature of the operating conditions. As
the plants ages, implementing and maintaining an Aging Management Plan (AMP) becomes even more
costly. By performing a cost-benefit analysis, utilities can determine if investing in the installation costs
of chromium material during construction can lead to overall operating and maintenance (O&M) cost
reduction over the life of the plant. It is generally enough to simply design the system using material
constructed of 1¼% chromium or greater.
Another available option is, during the design process, to mandate that the piping vendor supplies a
minimum amount of chromium in the material being used for systems which are normally susceptible to
FAC. For example, the maximum chromium content for ASTM A106 A/B/C is 0.40%. Designers and
utilities can mandate the material to meet a maximum chromium content of 0.40%, thus reducing the
amount of systems which will be affected by FAC. A few utilities are implementing this practice during
the replacement of existing piping systems due to FAC damage.
CONCLUSION
By implementing the lessons learned over the past 26 years in the design, construction, and operation
of new conventional nuclear plants, SMRs, and fossil (including combined cycle) plants, FAC can be
greatly reduced, or eliminated, to ensure personnel and plant safety. This, in turn, will help with the
safe, efficient operation of the plants, reducing O&M costs while helping to ensure plant availability. We
know that if a power plant is not producing electricity, the utility is not making money. Reducing O&M
costs is high priority for all utilities due to the effects of increased O&M costs on profitability. Therefore,
it is critical utilities considering new conventional nuclear plants, SMRs, and fossil plants to take the
measures to reduce FAC as part of design.
REFERENCES
1. B. Chexal, et. al., Flow-Accelerated Corrosion in Power Plants, TR-106611-R1, (Pleasant Hill, CA:
Electric Power Research Institute, 1998), pp. 1-1 through 1-13.
2. EPRI Final Report 1011838 “Recommendations for an Effective Flow-Accelerated Corrosion
Program (NSAC-202L-R3)”, Munson & Associates, and Horowitz, J., 2006.
3. EPRI Final Report 1008082. “Guidelines for Controlling Flow-Accelerated Corrosion in Fossil and
Combined Cycle Plants”, EPRI, 2005.
4. EPRI Final Report 1008047 “Flow Accelerated Corrosion Investigations of Trace Chromium”, EPRI,
2003.
5. J. Mathews, “Importance of Cycle Chemistry Flow-Accelerated Corrosion in Conventional Fossil
Plants and HRSGs” 2009 Fossil FAC Workshop (Charlotte, NC: Electric Power Research Institute:
2009)
6. EPRI Final Report Number NSAC 145 Revision 1 “CHECMATE Computer Program User’s Guide”
(Palo Alto, CA: EPRI, 1991)
©2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to
NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in
this paper are solely those of the author(s) and are not necessarily endorsed by the Association.