Advanced cooling water treatment concepts (Part 2)

This installment examines the general chemistry of phosphate replacement technologies that have proven effective in many applications.

Editor's note: This is the first of a multi-part series by Brad Buecker, President of Buecker & Associates, LLC.

Read Part 1 here.

Part 1 of this series offered an overview of the most prominent cooling water scale/corrosion inhibitor treatment programs since the middle of the last century. Treatment evolved from the very effective but ultimately hazardous acid/chromate chemistry to phosphate/phosphonate/zinc treatment that utilized "controlled" precipitation reactions and mildly basic pH to reduce the corrosion and scaling potential of cooling water. The latter programs can be complicated to control, with sometimes a fine line between corrosion and scaling conditions. The photo below of a two-pass heat exchanger offers a dramatic example.

At the inlet end (the lower tubes of this exchanger), corrosion was evident. At the warmer outlet side (the top half), deposition was troublesome, as is clearly visible. The phosphate/phosphonate program was not particularly effective at mitigating corrosion or scaling depending on location and temperature within the heat exchanger.

In this installment, we will examine the general chemistry of phosphate replacement technologies that have proven effective in many applications; with less uncertainty than phosphate/phosphonate treatments, and with reduced environmental impact from discharge chemistry.

As the reader will recall from Part 1, from a corrosion-control standpoint, phosphate/phosphonate programs largely rely on deposition of reaction products to inhibit anodic and cathodic reactions. A common corrosion cell in aerated water is shown below.

While carbon steel oxygen corrosion is probably the most common mechanism, many other corrosion mechanisms are possible. Space limitations prevent a detailed discussion of most of these mechanisms in this article, but I hope to outline some of the most important in a future Power Engineering article. Continuing with the main topic; reliance on precipitating chemistry to depolarize anodic and cathodic reactions can often be very challenging, where variable conditions can lead to other problems such as the scale formation shown in Figure 1. Accordingly, modern programs have emerged to establish a direct protective film on metal surfaces. The important features of the organic molecule(s) in many formulations are active sites that directly attach to metal surfaces with the hydrophobic organic chain extending outwards.

One compound with which this author is familiar goes by the general chemical name of reactive polyhydroxy starch inhibitor (RPSI), (1) where active oxygen-containing groups on the molecules attach to the metal surface with the organic portion shielding the metal. This chemistry and similar technologies have significantly grown in popularity and use over the last decade or so, with now several thousand applications or more. Results indicate that proper application of the chemistry, which does not require high concentrations, can often lower carbon steel corrosion rates to less than 1 mil per year (mpy, where a mil is 0.001 inches). This is well within the projected lifetime of typical carbon steel components.

Data from Reference 1 also indicates good corrosion protection of 300-series stainless steel metals from chloride pitting and cracking, which brings up a subject that this author has been planning to address. For several years, I was heavily involved in reviewing water treatment design specifications for new combined cycle power plants. In numerous instances, the design engineering firm would specify either 304 or 316 stainless steel for steam surface condenser tubes, apparently without giving any thought to cooling water chemistry and potential problems from impurities.

A primary case in point is that stainless steels form an oxide layer which protects the base metal, but where chloride in sufficient concentrations will penetrate the oxide layer and initiate pitting. For years, the recommended maximum chloride limits for these steels ranged from 500 ppm for 304 SS to 3,000 ppm for 316L (L stands for low-carbon content) SS at ambient temperature. Research has subsequently shown that these limits were too high, and one noted materials expert suggests 100 and 400 ppm, respectively, for clean tubes. (2) Deposits increase the corrosion potential. Some makeup waters have chloride levels that exceed these guidelines before even being cycled up in a cooling tower. (3) Pitting is an insidious corrosion mechanism, and has been known to cause failure within months and sometimes even weeks of materials that should last for decades. Another element that can cause severe stainless steel corrosion is manganese. We will examine that issue in a future article.

Two primary takeaways come from this example. First, design engineers for major projects that have water and process fluid systems need to consult with or have on staff chemistry and corrosion experts who can select the correct materials. It is typically much easier to select proper materials in the design phase than to deal with operational issues after installation. Second, and of direct importance to this discussion, is that the film-forming chemistry highlighted above may offer a solution at existing facilities in which material replacement would be cost prohibitive.

Another benefit of this modern cooling water treatment alternative is environmentally related. Phosphorus is a primary, and often limiting, nutrient for microbiological growth in cooling systems and in receiving bodies of water, including retention ponds for cooling tower blowdown. The following two figures from Reference 1 show a before and after photo of the retention pond at an industrial facility, in which treatment was changed from a polyphosphate/zinc program to a non-phosphorus (non-P) film-forming program.

Results such as these are often an additional driving factor for program change, particularly in locations where environmental regulations limit or perhaps even prohibit phosphorus discharge from point sources. (Agricultural runoff is a different issue that cannot be addressed here.) Furthermore, regulations continue to tighten on discharge of other elements and compounds, which in this case often includes zinc; a common corrosion inhibitor in phosphate/phosphonate programs.

As Part 1 outlined, phosphate/phosphonate programs provide double duty as both corrosion and scale control methods. For the advanced non-P programs now, polymers with active groups serve for scale control. Figure 6 outlines the general structure of and common active groups on the polymers.

These compounds function via a variety of mechanisms to control scale formation, including:

An often-important factor for deposit control is to enhance the ability of the polymers to penetrate deposits. This is especially true for organics, including oils and greases, as these compounds bind deposits together. Surfactants can assist in breaking down these materials. Nonionic surfactants are similar to detergents by having a hydrophilic (water loving) functional group and a lipophilic (oil loving) chain. As the lipophilic end binds with oils, the hydrophilic end attaches to water molecules to remove the oil. Structural modifications to the lipophilic and hydrophobic active sites allow for specialized properties.

Polymers of variable chain lengths are available, where a thorough analysis of the water constituents is necessary to select chain size and the most efficient active groups. Also, some compounds may cause foaming, and these issues need to be considered during product selection. And, of course, sometimes field adjustments are necessary, as laboratory testing may differ from the actual full-scale application.

Modern methods are available to move recirculating water chemistry control beyond the complicated phosphate/phosphonate programs that held sway for four decades. However, the chemistry cannot be applied blindly or without monitoring, with the expectation that all problems will instantaneously be solved. Cases are well known where corrosion coupons indicate good performance, but locations within the system become heavily fouled or corroded. Temperature effects and other factors may be at work in these locations. Even more importantly, microbiological fouling can completely offset any effects of the scale/corrosion inhibitors. Micro-fouling is often the proverbial "800-pound gorilla in the room" when it comes to cooling water difficulties. In the next several parts of the series, we will review these issues.

This discussion represents good engineering practice developed over time. However, it is the responsibility of plant owners, operators, and the technical staff to implement reliable programs based on consultation with industry experts. Many additional details go into the design and subsequent use of these technologies than can be outlined in a single article.

References

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 [email protected].