Boiler Water Chemistry Control: Preventive vs. Reactive Maintenance  

Brad Buecker, President, Buecker & Associates, LLC

Posted 4/29/2024

Introduction – Boiler Water Treatment Concepts

In Part 1 and Part 2 of this series, we examined the importance of good water quality in makeup and condensate return to steam generators.  Any number of impurities can cause scaling and corrosion in boilers unless properly controlled.  But even with properly-treated makeup, boilers require internal treatment to minimize corrosion and other problems.  The high-temperatures and pressures in boilers magnify chemical reactions and corrosion mechanisms.  This installment examines important boiler water treatment concepts.

Some General Steam Generation Fundamentals

Steam is the ideal medium for process heating at many thousands of plants.  For decades, fossil fuel-fired, drum-type boilers have represented the most common design for industrial steam generation.

boiler water treatment O type package boiler
Figure 1.  Basic configuration of an “O” type package boiler.  Other varieties include “A” and “D” type boilers. (3)

Many boiler configurations are possible, ranging from the package type shown above to the relatively complex multi-pressure heat recovery steam generators (HRSGs) of modern combined cycle power units to the remaining coal-fired units (with extensive feedwater heating systems) erected in the previous century.  Some facilities may have specialty waste heat boilers that present unique challenges.  A good example is the Transfer Line Exchangers (TLE) at refineries and petrochemical plants, whose heat source is effluent gas from thermal and catalytic crackers. (4) These units somewhat represent fire-tube boilers in that the hot gas flows internally through the tubes with the water on the outside.

When steam boilers were first developed for power generation, makeup treatment system operation was often unreliable.  Poor performance allowed impurities including hardness to enter boilers and form insulating deposits.  Tube overheating and failure were frequent outcomes.

Figure 2.  A modern example of a boiler tube with blisters and bulges from overheating induced by internal deposits. (3)  

The insulating effect of deposits is clearly shown in Figure 3 below.

boiler water treatment metal with and without deposits
Figure 3.  The influence of internal tube deposits on external wall temperatures. (3)  Higher firing rates are necessary to maintain required boiler water temperatures, which raises the tube wall temperature.

Although makeup treatment technology improved considerably over the years, boilers were still subject to contaminant ingress from numerous sources as we explored in Part 1 and Part 2.  Boiler water treatment programs evolved to protect units against impurity ingress.

A primary issue, then and now, is to maintain a moderately basic boiler water pH to minimize corrosion.  Figure 4 shows the amphoteric nature of iron, in which corrosion rates increase at both low and high pH.

Figure 4. General corrosion rate of iron as a function of pH. (3)  The “sweet spot” for this example, representative of mild temperatures, is between a pH of 10 to 12.  At the other extreme, in the harsh conditions of utility boilers with very high temperatures, the zone shifts to the left and contracts, as outlined below.

In the 1930s, tri-sodium phosphate (Na3PO4) treatment emerged to generate alkaline conditions and minimize general corrosion per the figure above.

Na3PO4 + H2O ⇌ Na2HPO4 + NaOH             Eq. 1

For high-pressure utility steam generators, the optimal pH range typically resides between 9.2 and 10.0, with adjustments within this range dependent on boiler configuration.  Multi-pressure HRSGs often have slightly different ranges for each circuit.  If the HRSG is of the feed-forward low-pressure (FFLP) type, a solid alkali, e.g., phosphate or caustic, cannot be used in the low-pressure evaporator because of the potential for direct transport of these solid alkalis to the steam system via attemperator sprays. (3)  The pH range for lower-pressure industrial boilers may be a bit broader, especially on the high end, because conditions are less harsh.  The ASME industrial boiler water guidelines, listed as Reference 5 here and also referenced in Part 1 of this series, give alkalinity limits rather than pH for lower-pressure boilers, although obviously pH and alkalinity are related.  

The second major function of phosphate is to mitigate the effects of hardness ingress.  Phosphate, and the hydroxide alkalinity produced from the reaction shown in Equation 1, will induce precipitation of calcium and magnesium as soft sludges that can be blown down from the boiler.  This aspect was particularly important in the early power industry when hardness ingress was common.  Upsets from modern high-purity makeup systems are rare, thus reducing the need for phosphate treatment to control hardness ingress.  However, for those units with water-cooled condensers, a condenser tube leak(s) will introduce numerous impurities to the boiler feedwater including the anions chloride and sulfate.  At the high temperatures in the boiler, these ions can potentially concentrate underneath deposits and form acidic compounds that attack tube metal and may also lead to hydrogen damage.  Phosphate can curb immediate problems, giving operators and maintenance personnel a small amount of time to shut down the unit and make repairs.  This can be critical in preventing serious corrosion that might quickly cause boiler tube failures.  Unfortunately, too many times plant management has not recognized that phosphate cushioning is only a temporary measure and have continued to operate a unit with a condenser tube leak.  As the author can directly attest, this solution is not viable. (6)

Phosphate treatment for high-pressure boilers underwent several transitions during the 20th century, whose discussion is beyond the scope of this article.  However, one issue definitely needs mention.  The sodium phosphates are reversely soluble at temperatures above about 250°F, and beyond this temperature will begin to precipitate (hide out) on boiler tube walls. 

Solubility of tri-sodium phosphate as a function of temperature. Boiler water chemistry.
Figure 5. Solubility of tri-sodium phosphate as a function of temperature.

Care is necessary in monitoring and implementing phosphate treatment programs for high-pressure units.  Modern guidelines suggest maintaining a low phosphate residual to minimize hideout.  Additional details are available in Reference 3.

Often problematic in industrial steam generators with extensive condensate return systems is transport of iron oxides to boilers from condensate system corrosion.  The iron oxide particulates will deposit on boiler tubes, typically on the hot side, where the deposits inhibit heat transfer and serve as sites for under-deposit corrosion.  Part 2 of this series outlined methods to reduce condensate system corrosion, but proper boiler water treatment is also important to minimize iron oxide deposition.  Phosphate programs for industrial boilers often include polymer conditioning agents that assist in keeping solids in suspension for removal via the boiler blowdown. In some cases, all-polymer programs without any phosphate have proven quite successful, as the following extract outlines.

All-polymer [treatment was] developed several decades ago with the goal of maintaining boiler tube cleanliness similarly to chelant programs, but with no risks of chelant corrosion. They are typically designed for systems with low hardness feedwater (<1.0 ppm calcium), and a pressure range of 900 to 1,200 psi. As the name all-polymer indicates, the chemistry contains no inorganic phosphates or other alkalis. Well-designed programs have delivered outstanding performance with reduced corrosion risk. Deposit control success involves several interrelated mechanisms including sequestration; dispersion to keep precipitates very small in size and suspended in the boiler water; and crystal modification that alters the normal crystalline structure and inhibits formation of strong crystal bonds to metal surfaces.

These polymers typically have one or more of three active groups attached to the polymer backbone; carboxylates (R-COO), sulfonates (R-SO3), or a non-ionic species such as amide, which contains an oxygen and amine molecule. Specific compounds include polyacrylate (PA), polyacrylic acid (PAA), acrylic acid/acrylamido methyl propane sulfonic acid, (AA/AMPS), polymethacrylate (PMA), and polyacrylamide (PAM). Some common additives to these formulations include organic phosphates like hydroxyethylidene diphosphonic acid (HEDP) and a newer compound, polyisopropenyl phosphonic acid (PIPPA). PIPPA serves as an iron dispersant at pressures up to 1450 psi.

The correct formulation will vary depending on makeup water purity, boiler pressure and operating characteristics, condensate return chemistry, and other variables. Polymers that work well for systems with sodium softened makeup may not be appropriate for boilers with high-purity makeup and condensate return. Though the corrosion risk of these compounds as compared to chelants is greatly minimized, some corrosion may still be possible especially in higher purity waters where overfeed could attack base metal. (3)

Conclusion – Boiler Water Chemistry Control

As is evident from the discussion above, a variety of boiler water treatment methods is possible, with case-by-case evaluations being the best path forward.  Very important is to have the correct analytical instrumentation in place to accurately monitor system chemistry.  We will cover this topic in a later installment of the series.  A key takeaway is that the harsh conditions in boilers, and especially higher-pressure units, can greatly exacerbate corrosion and scale formation.  Boiler water chemistry also significantly influences steam chemistry, which will be outlined in Part 4.            


  1. B. Buecker, “Preventative vs. Reactive Maintenance:  Don’t Neglect Makeup Water and Condensate Return Treatment – Part 1”; Maintenance World, February 2024.
  2. B. Buecker, “Preventative vs. Reactive Maintenance:  Don’t Neglect Makeup Water and Condensate Return Treatment – Part 2”; Maintenance World, March 2024.
  3. Water Essentials Handbook (Tech. Ed.: B. Buecker). ChemTreat, Inc., Glen Allen, VA, 2023.  Currently being released in digital format at
  4. K. Kraetsch and B. Buecker, “Advanced methods for controlling boiler tube corrosion and fouling – Part 2”; Hydrocarbon Processing, November 2021.
  5. Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers, The American Society of Mechanical Engineers, New York, NY, 2021.
  6. B. Buecker, “Condenser Chemistry and Performance Monitoring”; from the proceedings of the 60th International Water Conference, October 18-20, 1999, Pittsburgh, Pennsylvania. 


Brad Buecker

Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as a senior technical publicist with ChemTreat, Inc. He has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, IL, USA) and Kansas City Power & Light Company's (now Evergy) La Cygne, KS, USA, station. His work has also included eleven years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant.

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 is a member of the ACS, AIChE, AIST, ASME, AWT, the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and Power-Gen International.

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