This article aims to serve as a reminder of issues related to flow-accelerated corrosion and that dissolved oxygen monitoring is an important tool for any chemistry program.
By Brad Buecker, Buecker & Associates Analyzer Oxygen
By Denton Slovacek and Jean Holz, Hach
Frequently, author Buecker sees a post on LinkedIn offering a blanket recommendation for feedwater deaeration in high-pressure utility steam generators. The various authors of these commentaries seem to be unaware of the issue of flow-accelerated corrosion (FAC). Given the potentially large distribution of LinkedIn posts, many people may be exposed to critical misinformation in this regard. I wrote about FAC issues in a Power Engineering series last autumn,1 followed by a recent article on trace metal analysis for feedwater corrosion monitoring.2 The present article provides an overview of the details, with additional discussion from Hach on the importance of accurate dissolved oxygen (D.O.) monitoring for feedwater chemistry control.
The following bullets provide a condensed, mostly chronological review of high-pressure boiler feedwater chemistry evolution from the middle of last century to present times.
· The common material of construction for condensate/feedwater piping and boiler tubes has always been mild carbon steel. It provides good strength at low cost.
· Steam generator pressure and temperature were steadily increased from the 1930s into the middle of the century and beyond to improve boiler efficiency. Adoption of regenerative feedwater heating represented a major improvement to recover some energy that would otherwise be lost in the condenser. Copper alloys became a common choice for heater tube material, per copper’s decent strength and excellent heat transfer properties. Feedwater networks with carbon steel piping and copper alloys in the feedwater heaters are known as mixed-metallurgy systems.
Figure 1. Basic schematic of a large coal-fired power unit. Note the multiple feedwater heaters, including the deaerator.3
· Iron and copper exhibit minimal general corrosion at a mildly alkaline pH, with the optimal value for iron shown below in the well-known Sturla diagram.
Figure 2. Feedwater carbon steel dissolution as a function of pH and temperature. Note: The pH analyses are at 25o C.4
As is evident, general corrosion greatly diminishes as pH rises into a mid- to upper-9 range.
However, a lower range in the mid-8s is better for the protective oxide that forms on copper.5 For mixed metallurgy systems, a common guideline for years was 8.8-9.1 to balance corrosion control between the two metals, but modern guidelines now suggest 9.1-9.3.6 Ammonia or in some cases a neutralizing amine (the new term is alkalizing amine) was, and still is, the treatment chemical to establish the proper pH range. Alkalizing amines offer potential benefits and drawbacks, and must be carefully evaluated.7
· As power boilers grew in size and sophistication in the last century, researchers became convinced that even trace amounts of dissolved oxygen during operation would cause serious metal corrosion, which is true for copper alloys in ammoniated water. Virtually all units were equipped with a mechanical deaerator. The common DA effluent guarantee is 7 ppb D.O.
· Even 7 ppb was considered excessive, so chemical oxygen scavenging became standard. Originally, hydrazine was the oxygen scavenger/reducing agent of choice, but health concerns from handling the chemical led to hydrazine replacement with such compounds as carbohydrazide, diethylhydroxylamine (DEHA) and others.
· The combination of ammonia or an amine for pH control and oxygen scavenger feed became known as all-volatile treatment reducing (AVT(R)). The reducing chemistry generates the familiar gray-black iron oxide layer magnetite (Fe3O4) on carbon steel, and it maintains the reduced copper oxide layer, cuprous oxide (Cu2O)), on copper alloys.
· 1986, “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.” 8 This phenomenon is known as single-phase FAC. (Two-phase FAC, which can affect deaerators, feedwater heater drains, and low-pressure HRSG evaporators is discussed in greater detail in references 8 and 9.) Other single-phase FAC-induced failures over the last three decades have caused additional fatalities and much damage at several power plants. FAC has been observed in many HRSGs around the globe. Gradual metal loss occurs at FAC locations, until the affected area can no longer resist the fluid pressure.
Figure 3a. Photo of tube-wall thinning caused by single-phase FAC.3
Figure 3b. Surface view of single-phase FAC. Note the orange peel texture.3
Figure 3c. Catastrophic failures induced by FAC.9
· In the late 1960s and early 1970s, chemists at supercritical units in Russia and western Europe discovered that with high-purity makeup water (conductivity after cation exchange (CACE) <0.15 mS/cm), direct injection of oxygen induced formation of ferric oxide hydrate (FeOOH) on carbon steel surfaces. (OT cannot be employed in systems with copper alloy feedwater heater tubes.) This oxide layer, a rather deep red in color, is denser and stronger than magnetite. After some evolution, this chemistry regime became known as oxygenated treatment (OT). Current guidelines from the International Association of the Properties of Water and Steam (IAPWS) call for a feedwater D.O. range of 30-150 parts-per-billion.6 OT has been adapted at most supercritical units around the world that have all-ferrous feedwater systems. With proper control and monitoring, total feedwater iron concentrations should remain at or below 1 ppb.
· While OT can be employed for feedwater treatment in drum units, personnel from the Electric Power Research Institute (EPRI) developed all-volatile treatment oxidizing (AVT(O)) for high-pressure drum boiler feedwater. The primary source for oxygen is the small amount (usually) of oxygen that enters the condenser through small air leaks at condenser shell penetrations, turbine/condenser expansion joints, etc. Original AVT(O) guidelines recommended <20 ppb D.O. in the condensate with a 5-10 ppb residual at the economizer. EPRI has since expanded the latter range to 5-30 ppb.10 The key point is that with OT or AVT(O), all surfaces in the feedwater system and economizer should have the deep red color mentioned above. Patches of gray-black magnetite indicate insufficient protection. In some cases, and most notably for feed forward low-pressure HRSGs, direct oxygen injection (similar to OT applications but lower feed rates) may be needed to protect intermediate- and high-pressure economizer circuits.1, 8
As the discussion above indicates, each of the feedwater treatment programs has a well-defined D.O. range. Thus, along with analytical measurements for trace metal concentrations and, for mixed-metallurgy systems, oxidation-reduction potential (ORP), continuous on-line D.O. monitoring is of major importance. Like other technologies, D.O. measurement has evolved and become more precise. For many years, amperometric methods were de rigueur for oxygen analyses. This is an electrochemical technique that can be quite accurate. However, amperometric instruments are labor intensive with frequent calibrations and sensor maintenance, the latter of which often requires replacement of a fragile membrane, especially if flow is discontinued and the membrane dries. These difficulties have only been exacerbated by the now common load cycling of most combined cycle (and even many remaining traditional) power plants. The figure below shows the response time of an amperometric sensor vs the luminescent dissolved oxygen (LDO) technology (in this case an Orbisphere K1100 instrument) that continues to increase in popularity.
Figure 4. Response time of an amperometric sensor vs. LDO.
This graph outlines the analysis of a sample with D.O. concentrations that are common for OT applications, but “Since 2009, accurate measurement at levels below 1 ppb has now been made possible.” 11 The technology is a practical example of quantum mechanics. In short, the instrument uses shorter-wavelength blue light to excite electrons in the atoms of the measuring device. The electrons release longer-wavelength red light as they return to an unexcited state. Oxygen molecules capture this released energy and lower the amount of red light to the sensor. O2 also reduces the duration that the electrons exist in the excited state. Measurement of these two parameters allows very accurate calculation of D.O. concentrations, where “A constant alignment of the sensor occurs with the help of [a] red LED fitted in the probe. Before each measurement, this [LED] sends out a light beam of a known radiation characteristic. Changes in the measurement system are hence detected without any time delay.” 12
Figure 5. Basic representation of a luminescent dissolved oxygen measurement system.12
Beyond the above-mentioned technical capabilities, the LDO instrument typically only requires one, 30-minute calibration per year. And, the analyzer is not affected if sample flow is discontinued. Startup is immediate. Most combined-cycle plants operate with minimal personnel, who often have limited chemistry training. Yet, proper operation of on-line water/steam chemistry analytical instruments is critical to prevent major upsets that can severely damage equipment and jeopardize employee safety. Maintenance un-intensive instruments like LDO can be of great benefit in that regard.
Note: The choice of sample tubing for low-range dissolved oxygen analyzers is very important. Outside air can penetrate polyethylene tubing and significantly increase the oxygen concentration of the sample, making readings meaningless. Alternatives include stainless steel and specially-fabricated nylon.
Water cooled stator coils for turbine generators are typically of copper-alloy construction. They are normally designed to operate with a either a very low dissolved oxygen concentration (<10 ppb) or in a several parts-per-million (ppm) range. The middle ground between these two ranges can lead to severe corrosion. D.O. measurements are valuable for monitoring stator chemistry.
Increasingly, and especially in locations where water conservation is of concern, new power plants have air-cooled condensers (ACC) rather than water-cooled condensers. ACCs are enormously larger due to the much lower density of air than water. The extensive piping in an ACC offers many locations for air in-leakage. Condensate D.O. monitoring can help plant technical personnel to select and adjust feedwater treatment chemistry. Because of high air in-leakage and subsequent carbon steel corrosion, often recommended is a particulate filter on the condensate discharge to remove iron oxide particulates and prevent transport to the steam generator.
The complexity of modern steam-based power generation requires up-to-date information sharing and analytical technology to maintain water/steam chemistry within acceptable parameters. This article hopefully serves as an additional reminder of issues related to flow-accelerated corrosion and that dissolved oxygen monitoring is an important tool for any chemistry program.
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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