Concerns about flow—assisted corrosion drove a successful conversion from a reducing toan oxygenated water treatment program at Kansas City Power & Light’s La Cygne Station.
For years, the 2500 psig drum unit on Unit 2 at Kansas City Power & Light Company’s La Cygne generating station operated on the classic program of all—volatile reducing [AVT(R)] feedwater treatment with phosphate treatment of the boiler water. Based on expert advice from EPRI, La Cygne switched to an all—volatile oxygenated [AVT(O)] feedwater treatment program in January 2003 to avoid concerns with flow—assisted corrosion, a potentially dangerous pipe wall—thinning phenomenon that can occur in units relying on reducing treatment programs. This article outlines the reasons for the change and the results seen so far.
Reducing Chemistry Cautions
When a steam generator is first placed into service, carbon steel feedwater piping and boiler tubes react with water to form a thin, relatively protective layer of magnetite (Fe3O4) that inhibits corrosion of the underlying base metal. Traditional chemistry taught that pH elevation through ammonia or amine feed and injection of an oxygen scavenger/metal passivator were necessary to maintain integrity of the protective magnetite layer.
While pH control is still very important, the issue of oxygen scavenger feedwater treatment has undergone some radical changes. The simplified basics of oxygen scavenging and metal (iron) passivation are illustrated in the following two equations, which show reactions of the most fundamental scavenger/passivator, hydrazine (N2H4), with dissolved oxygen and oxidized ferrous metal.
As these reactions indicate, hydrazine, and the organic substitutes that are sometimes used in place of hydrazine, are reducing agents. What EPRI personnel and other researchers discovered was that significant iron dissolution can occur in reducing environments. Particularly troublesome is the mechanism of flow—assisted—corrosion (FAC), where the combination of a reducing environment, temperature, and a flow disturbance can accelerate corrosion in localized areas (Power Engineering, September 2002, pp. 32—34).
Figure 1. Flow—assisted corrosion
For many years, researchers recognized that oxygen in condensate and feedwater would cause severe corrosion. Oxygen scavengers greatly reduced oxygen corrosion, which at the time appeared to be a “magic bullet.” However, in recent years wall thinning has been discovered at numerous plants that followed proper oxygen scavenger feed procedures. Magnetite, Fe3O4, contains both ferrous, Fe+2, and ferric, Fe+3, ions. During FAC, the ferrous ions continually migrate from the magnetite layer at the affected areas (high fluid impingement locations). The reducing effects of the oxygen scavenger continually regenerate ferrous ions, which perpetuates the corrosion mechanism. OT does not generate ferrous ions, and the protective layer that forms in a properly applied OT program is tighter and stronger than magnetite. Wall thinning caused by FAC is graphically illustrated in Figure 1. FAC is often at its worst in the feedwater and economizer circuits of the steam generator. The wall thinning generated by FAC can result in a sudden, catastrophic pipe or tube failure that can be very dangerous. Several power plant personnel have been killed by FAC—induced failures in the last decade or so.
State—of—the—Art FeedwaterChemistry Control
In once—through units with all—ferrous feedwater systems, the feedwater chemistry program that has become quite popular is oxygenated treatment (OT), in which oxygen is deliberately injected into the condensate and feedwater systems to maintain a 30 to 50 ppb residual. Controlled oxygen injection causes the magnetite film to become overlaid and interspersed with ferric oxide hydrate (FeOOH). Not only is FeOOH more tenacious than magnetite, but the oxidizing environment also eliminates one of the primary conditions for FAC.
Effective OT depends on high—purity feedwater (cation conductivity less than 0.15 microsiemens) with almost no possibility of contaminant ingress. Upsets in condensate quality can initiate severe oxygen corrosion. These strenuous condensate requirements virtually demand condensate polishing.
Figure 2. On—line dissolved oxygen instrumentation
But what about units that have no condensate polishers, where plant personnel are concerned about FAC? This was the case at La Cygne Unit 2. The feedwater treatment program in place when plant management initiated discussions with EPRI personnel relied on ammonia injection for pH control, with carbohydrazide (N4H6CO) as the oxygen scavenger. Carbohydrazide breaks down to hydrazine as temperatures rise in the feedwater system. EPRI pointed out that in units with tight condensers, such that dissolved oxygen levels in the condensate pump discharge remain at or below 10 ppb, it is possible to shut down the oxygen scavenger feed and let FeOOH develop naturally in the feedwater piping. This sounded ideal for Unit 2, as dissolved oxygen concentrations normally range from 5 to 10 ppb. Also, the condenser tubes are only a few years old, and tube leaks have been non—existent. Even minor raw water in—leakage and OT are not compatible.
In January 2003, La Cygne tested the idea. Via extensive sampling just prior to the test, the plant obtained good data on the economizer inlet iron concentrations in the reducing environment. The average iron concentration was 2.6 ppb. This data indicated that AVT(R) was performing fairly well, but plant management decided to switch to AVT(O) to reduce the potential for flow—assisted—corrosion. After initiating the natural OT program, follow—up testing showed an average iron concentration at the economizer inlet of 1.7 ppb. This provided convincing evidence that the plant could operate with AVT(O). A recent battery of tests on economizer inlet samples showed dissolved oxygen concentrations of less than 1 ppb. These results indicate that the condensate/feedwater system is completely treated and has a tight film of FeOOH.
OT a No—Nofor Copper—Alloy
La Cygne is fortunate in that the feedwater heaters in Unit 2, and Unit 1 for that matter, are comprised of all—ferrous material. OT cannot be used in systems with copper—alloy feedwater heaters because the combination of oxygen and ammonia would quickly attack and destroy the tubes. Yet, these units also suffer FAC. A developing technique for feedwater chemistry control in mixed—metallurgy systems is monitoring via measurement of oxidation—reduction potential (ORP).
Shortly after beginning the AVT(O) test on Unit 2, La Cygne commissioned a new Mettler—Toledo Thornton on—line dissolved oxygen instrument (Figure 2) to monitor condensate and economizer inlet conditions. When purchasing the meter, La Cygne specified that it have a second channel, which was set up to monitor ORP. AVT(O) gives ORP readings that stay relatively close to 0 millivolts (vs. Ag/AgCl, sat. KCl). For mixed—metallurgy systems, a standard rule—of—thumb is to feed the oxygen scavenger/metal passivator such that the ORP remains within a range of -350 to -300 millivolts. This range has been deemed safest to minimize copper corrosion, but not overdose the feedwater with oxygen scavenger and exacerbate FAC.
To test this principle, La Cygne started up the Unit 2 carbohydrazide feed system to see what the ORP would do. The ORP reached the -350 to -300 millivolt range within about two days, and stabilized there with a residual of 10 ppb or so hydrazine in the feedwater. After shutting down the carbohydrazide system, ORP readings recovered in a couple of days.
In November 2003, what turned out to be a sudden failure of a steam seal line to the turbine instantaneously began allowing excessive quantities of air into the condenser. The condenser cleanliness factor, which had been very high at 90% or greater, precipitously dropped to 40% overnight. This corresponded to a rise in turbine backpressure from 1.59″ (Hg) to 4.11″ (Hg). Dissolved oxygen concentrations in the condensate pump discharge also increased, from 6 ppb to 30 ppb. Of course, air that leaks into a condenser contains carbon dioxide, and CO2 is a condensate/feedwater contaminant that can upset OT chemistry. La Cygne suspended the natural OT program in Unit 2 until the steam seal line was repaired.
The Unit 2 experience at La Cygne indicates that AVT(O) is a viable option when forced OT is impossible but FAC prevention is of concern. A tight condenser from both an air and raw water in—leakage perspective is mandatory, however.
Author Brad Buecker is the chemistry supervisor at Kansas City Power & Light Company’s La Cygne, Kansas generating station. His responsibilities cover the areas of steam generation chemistry, makeup water treatment, flue gas desulfurization chemistry, cooling water treatment, condenser performance monitoring, and waste stream monitoring and treatment. Buecker is also a contributing editor for Power Engineering magazine.