This is the first installment of a multi-part series by Brad Buecker, Buecker & Associates.

When I began my power industry career at a coal-fired plant in 1981, the common industry belief was that any dissolved oxygen (D.O.) in high-pressure boiler feedwater was deleterious and would cause extensive corrosion.  Foremost, the feedwater heaters in many of these fossil-fired units had copper alloy tubes, where the combination of D.O. and ammonia for pH control could cause extensive damage.  

Figure 1.  Cross-sectional view of tube wall thinning from FAC.  Photo courtesy of ChemTreat, Inc.

A corresponding assumption was that D.O. would also induce significant carbon steel corrosion, even though a decade before researchers and chemists at supercritical units in Europe had discovered that high-purity feedwater with a small concentration of oxygen could provide substantial carbon steel corrosion protection. 

Regardless, basically all U.S. power plant feedwater systems were equipped with a mechanical deaerator to reduce D.O. to approximately 7 parts-per-billion (ppb), with supplemental oxygen scavenger/reducing agent feed to further reduce D.O. concentrations.  The combination of ammonia (or alkalizing amine) use for pH conditioning and reducing agent injection for D.O. control came to be known as all-volatile treatment reducing (AVT(R)) chemistry.  Feedwater free of dissolved oxygen was the goal.

Figure 2.  Catastrophic FAC failure at a feedwater elbow.  Original source:  Electric Power Research Institute (EPRI), re-released in the Reference 3 seminar.

AVT(R) received a severe jolt in 1986, for “On December 9 of that year, an elbow in the condensate system ruptured at the Surry Nuclear Power Station [near Rushmere, Virginia.] The failure caused four fatalities and tens of millions of dollars in repair costs and lost revenues.” (1)  The culprit was single-phase flow-accelerated corrosion (FAC) of carbon steel; a phenomenon that has since been discovered at many plants, with additional fatalities from some FAC-induced failures. 

For high-pressure steam generators without copper alloys in the feedwater system, which includes virtually all combined cycle heat recovery steam generators (HRSGs), reducing agent/oxygen scavenger feed is not recommended.  However, the oxygen scavenger mindset persists at many combined cycle facilities, to the potential detriment of plant operation and employee safety.  This series will examine up-to-date knowledge regarding FAC control, with much of the information coming from the recent 40th Annual Electric Utility Workshop (2) and its predecessor (3).

FAC Influences

Figure 3.  Influence of temperature and pH on iron dissolution from carbon steel.  Source:  Sturla, P., Proc., Fifth National Feedwater Conference, 1973, Prague, Czechoslovakia.

As the name implies, flow-accelerated corrosion occurs at flow changes or disturbances, including elbows in feedwater piping and economizers; feedwater heater drains; locations downstream of valves and reducing fittings; attemperator piping; and, most notably for combined-cycle HRSGs, low-pressure evaporators, where the waterwall tubes, aka harps, have many short-radius elbows. 

FAC induces gradual metal loss, which leads to sudden failure when the pipe wall can no longer withstand the pressure.

Researchers learned that the reducing environment produced by oxygen scavengers is the prime ingredient for single-phase FAC of carbon steel.  However, several other factors influence FAC, with two of the most important being pH and temperature.  This is clearly shown in the classic illustration below, which has been known to power industry personnel for nearly five decades.

Two main features stand out in this diagram (right).

  1. Corrosion is maximized near 300o F, and diminishes on either side of this value.  That is why feedwater systems are usually most susceptible to FAC, as compared to other circuits.
  2. Carbon steel corrosion potential rapidly diminishes as pH increases from 8.75 to 9.6, especially at the temperatures that maximize FAC.

For traditional fossil units, the most FAC-prone locations are the feedwater circuit after the deaerator, the economizer, and attemperator lines.  Figure 4 below clearly outlines HRSG locations that are most susceptible.

Note that in this design, the low-pressure drum also appears as a location of concern.  This issue arises from the potential for two-phase FAC, a phenomenon that we will examine in the next part to this series.

Figure 4.  FAC areas of concern in the most common type of HRSG, the feed-forward, low-pressure (FFLP) design.  Yellow indicates moderate risk, and red indicates high risk.  Original source:  EPRI, re-released in the Reference 3 seminar.

Conclusion

My co-author, and world-class steam generation chemistry expert, for the seminar from Reference 2 included the following comments as key drivers for the FAC discussion, “Aggravating and puzzling [are] the continued large number of new HRSG specifications that call for reducing agent (oxygen scavenger) feed. . . .  Far too many people still do not understand the severity of this issue.” 

In the next installment to this series, we will dig more deeply into chemistry influences on FAC and the evolution of chemistry programs for FAC control.  Typical HRSG design lends itself well to modern treatment methods during normal operation, but high cycling frequency can present difficulties.

References

  1. Guidelines for Control of Flow-Accelerated Corrosion in Fossil and Combined Cycle Power Plants, EPRI Technical Report 3002011569, the Electric Power Research Institute, Palo Alto, California, 2017.  This document is available to the industry as a free report because FAC is such an important safety issue.
  2. Buecker, B., and S. Shulder, “Combined Cycle and Co-Generation Water/Steam Chemistry Control”; pre-workshop seminar for the 40th Annual Electric Utility Chemistry Workshop, June 6-8, 2022, Champaign, Illinois.
  3. Buecker, B., Shulder, S., and A. Sieben, “Fossil Plant Cycle Chemistry”; pre-workshop seminar for the 39th Annual Electric Utility Chemistry Workshop, June 4-6, 2019, Champaign, Illinois.


About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing.  Most recently he served as Senior Technical Publicist with ChemTreat, Inc.  He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station.  Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry.  He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control.  He may be reached at beakertoo@aol.com.

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