Coal, Emissions

Plant Management: Don’t Lose Focus on FAC Issues

Issue 7 and Volume 117.

Single-phase FAC. Note the orange peel texture. Photo courtesy of Dave Johnson, ChemTreat.
Single-phase FAC. Note the orange peel texture. Photo courtesy of Dave Johnson, ChemTreat.

By Brad Buecker, Contributing Editor

A main topic at the spring 2013 meeting of the ASME Research Committee on Power Plant & Environmental Chemistry was once again flow-accelerated corrosion (FAC) and its prevention. The issue has not lost any importance since four workers were killed by an FAC-induced failure in 1986, [1] with a number of fatalities since. In fact, FAC is the top corrosion mechanism in heat recovery steam generators (HRSG), so the issue has, if anything, become more important.

However, as was pointed out by several of the meeting attendees (and most notably Dr. Barry Dooley of Structural Integrity Associates and formerly of EPRI), concern over FAC seems to be fading away in the minds of plant management at many facilities. A contributing factor undoubtedly is the many retirements in the power industry, where new personnel simply do not understand the importance of FAC control. Yet, FAC continues to generate international conferences, the most recent of which was held in March in Washington, D.C. In a presentation to the PPEC meeting attendees, Kevin Shields, one of Dr. Dooley’s colleagues, provided the following bullet items in an introductory slide. [2]

  • FAC occurs in >70% of fossil plants, and represents >40% of all tube failures in HRSGs despite R&D since the 1960s, and
  • Many 100s of plant assessments worldwide, and
  • Numerous fatalities and serious failures, and
  • Much application and development

This article focuses upon FAC and methods to prevent it, and will hopefully serve as a warning document for plant management at the many hundreds of facilities that continue to be constructed and brought on-line not only in the U.S. but worldwide.

When I began my utility career in 1981, conventional wisdom said that any dissolved oxygen which entered the condensate/feedwater system of utility boilers was harmful. At that time, more than 50 percent of the power produced in the U.S. came from coal. Coal-fired units typically have complex condensate/feedwater networks with numerous feedwater heaters. The prevalent thinking was that any trace of dissolved oxygen (D.O.) would cause corrosion, and indeed oxygen corrosion can be very problematic in uncontrolled situations. Therefore, virtually all feedwater systems for high-pressure steam generators were equipped with a deaerator for dissolved gas removal. A properly operating deaerator can lower D.O. concentrations to 7 parts-per-billion (ppb).

However, any residual D.O. concentration was still considered harmful, so chemical deaeration was a standard process at most plants. The workhorse for many years was hydrazine (N2H4), a reducing agent which reacts with oxygen as follows:

N2H4 + O2 → 2H2O + N2↑ Eq. 1

Photo of tube-wall thinning caused by single-phase FAC. Photo courtesy of Dave Johnson, ChemTreat.
Photo of tube-wall thinning caused by single-phase FAC. Photo courtesy of Dave Johnson, ChemTreat.

Also, a primary, and arguably the primary, benefit of hydrazine is that it will passivate oxidized areas of piping and tube materials as follows:

N2H4 + 6Fe2O3 → 4Fe3O4 + N2↑ + 2H2O Eq. 2

N2H4 + 4CuO → 2Cu2O + N2↑ + 2H2O Eq. 3

Fe3O4, magnetite, is the protective layer that forms on carbon steel when it is placed into service. Cu2O forms on copper alloys, although we will not discuss this chemistry in great depth, as the use of copper alloys in condensate/feedwater systems has greatly diminished in large part due to the potential for copper carryover to steam in high-pressure utility boilers.

Hydrazine residuals were typically maintained at relatively low levels of perhaps 20 to 100 parts-per-billion (ppb). Oxygen scavenger treatment was coupled with feed of ammonia or an amine to maintain feedwater pH within a mildly alkaline range, now 9.1 to 9.3 for mixed-metallurgy feedwater systems and a bit higher for all-ferrous systems.

NH3 + H2O ⇔ NH4+ + OH- Eq. 4

This program became known as all-volatile treatment reducing [AVT(R)].

Due to the suspected carcinogenic nature of hydrazine, alternative chemicals such as carbohydrazide, methyl ethyl ketoxime, and others gained popularity. Regardless, all still had the same purpose, to establish a reducing environment in the feedwater circuit, thus inhibiting oxidation of metal. The technique became a standard in the industry.

“This changed in 1986. On December 9 of that year, an elbow in the condensate system ruptured at the Surry Nuclear Power Station [near Rushmere, Va.] The failure caused four fatalities and tens of millions of dollars in repair costs and lost revenues.” [1] Researchers learned from this accident and others that the reducing environment produced by oxygen scavenger feed results in single-phase flow-accelerated corrosion (FAC).

The attack occurs at flow disturbances such as elbows in feedwater piping and economizers, feedwater heater drains, locations downstream of valves and reducing fittings, attemperator piping, and, most notably for the combined-cycle industry, in low-pressure evaporators. The effect of single-phase FAC is outlined in the next illustration.

carbon steel

Metal loss occurs gradually until the remaining material at the affected location can no longer withstand the process pressure, whereupon catastrophic failure occurs. The thinning is due to the combination of a reducing environment and localized fluid flow disturbances, which cause dissolution of ferrous ions (Fe+2) from the metal and metal oxide matrix.

Results from EPRI showed that iron dissolution is greatly influenced by not only reducing conditions but also by solution pH and temperature.

As Figure 1 illustrates, corrosion reaches a maximum at 300° F. Thus, feedwater systems and HRSG low-pressure evaporators are particularly susceptible locations. Also note the influence of pH, as reflected by ammonia concentration, on the corrosion characteristics. As we shall see, this factor is quite important with regard to control of FAC.

The quest to maintain a non-detectable oxygen residual in feedwater systems led to FAC at many coal-fired power plants. I observed this first hand at one of two utilities in which I was employed in the past. At this plant, a feedwater heater drain line failed due to FAC, shutting down an 800 MW supercritical unit. Infinitely more serious was FAC-induced failure of an attemperator line in 2007 at another of the utility’s stations, which killed two workers and seriously injured a third.

In large measure, coal plant personnel have recognized the problem of single-phase FAC, and have adopted alternative feedwater treatment methods to mitigate the issue. However, I regularly review combined-cycle proposals in which the developer specifies an oxygen scavenger feed system for HRSG chemistry control. It is obvious that this mindset clearly has not been expunged at many locations.

Solutions to Single-Phase FAC and Changing the Reducing-Agent Mindset

HRSGs by their very nature typically have many waterwall tubes with short-radius elbows. Thus, the HRSG contains numerous spots susceptible to single-phase FAC. A primary method to mitigate this attack is selection of proper feedwater treatment, which we will now examine.

More than 40 years ago, researchers in Germany and then Russia began using a program known as oxygenated treatment (OT) to minimize carbon steel corrosion and iron dissolution in supercritical steam generators. The key component of the program was, and still is, deliberate injection of pure oxygen into the condensate/feedwater network to establish oxygen residuals of up to 300 ppb. What chemists discovered is that in very pure feedwater (cation conductivity, ≤ 0.15 µS/cm), the magnetite becomes overlayed with a tenacious and very insoluble film of ferric oxide hydrate (FeOOH). Results quickly showed that OT can lower feedwater iron concentrations to 1 ppb or less, and greatly minimize single-phase FAC. Now, OT is the preferred feedwater treatment for once-through utility steam generators around the world. Common in the United States is an oxygen residual range of 30 to 150 ppb, with a recommended pH range of 8.0 to 8.5. OT has been applied to a few drum units, where EPRI guidelines call for a feedwater pH range of 9.0 to 9.4 with a dissolved oxygen concentration of 30 to 50 ppb.

Although OT has been successfully applied to drum boilers, another program has evolved that is very popular for condensate/feedwater in these steam generators. To this point in time with AVT(O), oxygen has not been deliberately injected into the condensate, but rather the amount that enters from condenser air in-leakage (per “normal” conditions, where we will examine “normal” shortly) is allowed to remain without any oxygen scavenger/metal passivator treatment. It should be noted at this point that OT or AVT(O) are not permissible for feedwater systems containing copper alloys, as the oxygen would simply be too corrosive to the metal. The following text therefore focuses upon AVT(O) for all-ferrous systems.

When researchers developed AVT(O), they took into account the pH effect on carbon steel dissolution, as previously illustrated in Figure 3. AVT(O) guidelines evolved to the following parameters.

  • Recommended pH range, 9.2-9.6
  • Feedwater D.O. concentration, 1-10 ppb

As with OT, the condensate in an AVT(O) programs must be quite pure to allow oxygen to generate the FeOOH protective layer rather than cause pitting. However, the cation conductivity upper limit with AVT(O) is a bit more relaxed at ≤ 0.2 µS/cm.

AVT(O) chemistry is evolving. Chemists have discovered that even 10 ppb D.O. in the feedwater may not be sufficient to generate a complete FeOOH passivation layer. It is possible that deliberate injection of oxygen, similar to OT programs, may in the future be recommended where needed.

The amount of air in-leakage that establishes the “normal” condition of 10 ppb or less dissolved oxygen in the condensate is not a hard and fast value. The old rule of thumb for proper condenser conditions is a limit of 1 scfm (standard cubic feet per minute) of air in-leakage per 100 MW of capacity. However, I have worked with units in which the air in-leakage ratio was significantly higher, but where the condenser vacuum pumps had sufficient capacity to remove the gases. Quite often, a failure at the condenser shell or within auxiliary equipment may cause a sudden spike in dissolved oxygen concentration. As contrasted to pure oxygen feed such as with OT, air in-leakage also allows carbon dioxide to be drawn into the condensate, which raises the conductivity. In such cases, plant personnel need to search for the leak or leaks and repair them promptly. Much more problematic is a condenser tube leak, which not only raises the condensate dissolved solids concentration, but introduces impurities to the steam generator. These effects can be quite dramatic.

Additional Chemical and Material Selection Techniques to Minimize FAC

Elevated pH also has a beneficial effect in mitigating FAC. Thus, the guidelines for feedwater pH now recommend a range of 9.2 to 9.6. With EPRI’s phosphate continuum program or with caustic treatment alone, the drum pH can be controlled within a range of 9 to 10 quite readily. However, a complication sometimes arises due to HRSG design. Most HRSGs are of the multi-pressure, drum, vertical tube style. In some cases, the feedwater circuit is designed such that feedwater separately enters each pressure circuit separately. In many others, however, the entire feedwater stream is routed to the low-pressure (LP) evaporator for heating before being distributed to the intermediate-pressure (IP) and high-pressure (HP) steam generators. For this configuration, phosphate or caustic feed to the LP circuit is not permissible due to the downstream effects on attemperator chemistry, and IP and HP economizers. In these situations, LP pH control is dependent upon the ammonia injected into the feedwater. If the condenser is tubed with ferrous materials, the pH may be taken higher than the 9.2 to 9.6 range listed above without ill effects. However, copper-alloy tubes would suffer corrosion at higher ammonia concentrations.

For new HRSGs, single-phase FAC control can also be addressed by materials selection. The addition of a small amount of chromium in the material at FAC-susceptible locations virtually eliminates the corrosion. A primary example is LP waterwall elbows. Fabrication of the elbows from 1¼ or 2¼ chrome alloy can provide great benefit. While this alloy addition adds some cost to the project, the materials are quite resistant to FAC.

Two-Phase FAC

Many steam generators, regardless of type, are susceptible to two-phase FAC. As the name implies, this corrosion mechanism occurs where water flashes to steam, resulting in a mixed-phase fluid.

Two-phase FAC in a deaerator. Photo courtesy of Tom Gilchrist, Tri-State G&T (ret.)
Two-phase FAC in a deaerator. Photo courtesy of Tom Gilchrist, Tri-State G&T (ret.)

For conventional units, feedwater heater shells and heater drains are common locations for two-phase FAC, but this equipment is not common for HRSGs.

However, deaerators also experience two-phase fluid flow. As fluid flashes upon entering a deaerator, oxygen departs with the steam. Thus, the water that impinges upon metal surfaces does not maintain an oxidizing environment.

Also, the pH of entrained water droplets within the steam is usually lower than the bulk water pH. The combination of these factors often initiates FAC.

As has been noted, elevated pH will help to mitigate FAC, but the HRSG configuration dictates how the maximum treatment allowed.

If the LP system is utilized for heating of feedwater to the IP and HP circuits, solid alkali treatment (tri-sodium phosphate or caustic) of the LP circuit is not permissible. Control of pH can only be accomplished by ammonia, but it should be noted that ammonia hydrolysis, as previously outlined in Equation 4, decreases with increasing temperature.

As with single-phase FAC, a method to combat two-phase FAC is fabrication of susceptible locations with chromium-containing steel. Again, this adds cost to the project.

Conclusion

FAC is an issue to be taken very seriously.

I continue to see a large number of power plant proposals that still call for an oxygen scavenger feed system, and this is quite frustrating.

In addition to the references I have included in this article, I also encourage readers to access the web site of the International Association for the Properties of Water and Steam (http://www.IAPWS.org“>www.IAPWS.org).

This group, in which Dr. Dooley is one of the directors, offers free downloadable and cutting-edge technical information regarding power plant water/steam chemistry.

References

Guidelines for Controlling Flow-Accelerated Corrosion in Fossil and Combined Cycle Plants, EPRI, Palo Alto, CA: 2005. 1008082.

K. Shields, [A report on the] “International Conference on Flow-accelerated Corrosion in Fossil, Combined-Cycle/HRSG and Renewable Energy Plants” at the 2013 spring meeting of the ASME Research Committee on Power Plant & Environmental Chemistry, April 15-17, Houston, Texas.

Author

Brad Buecker is a contributing editor for Power Engineering and also serves as a process specialist with Kiewit Power Engineers in Lenexa, Kan.

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