Three Cs of successful cooling water treatment
There are three principles that, if followed, ensure the success of any cooling water treatment. Conversely, ignore them and your treatment system is bound to fail, resulting in corrosion and, ultimately, metal failure.
Clean. Keeping metal surfaces clean is the most important of the three principles. It is practically impossible for any chemical treatment to overcome dirty cooling water. Sometimes the dirt in your cooling water is just that--airborne dust, flyash, or coal dust that gets entrained in the cooling water. Cooling towers, after all are pretty good air filters. These particles soak up and inactivate chemical treatments, settle in condenser tubes and in cooling tower basins, deposit on tower fill, and in general provide areas for deposits to form and bacteria to hide out and flourish. Other forms of “dirt” in your cooling water are the living kind: clumps of bacteria that may come in with the makeup water, grow in pretreatment equipment such as softeners and clarifiers, or slough off from portions of the tower. The third form of “dirt” includes corrosion products such as rust and precipitates, including calcium carbonate.
Minimizing the accumulation of airborne particulate matter may be as simple as better housekeeping in the area of the towers. Better control of ash piles or dust suppression on roads and the coal pile may also produce significant benefits (Figure 3).
3. Airborne contaminants. The close proximity of the ash-loading area has caused particulate accumulation on this tower support. Courtesy: M&M Engineering Associates Inc.
Many have benefited from the use of side stream filtration, such as gravity multimedia filters, to remove suspended materials from the cooling water. These filters can produce a drastic reduction in particulates in the cooling water, regardless of its source, with a commensurate reduction in biocide and chemical demand in the tower.
Correct. The correct cooling water treatment is one that keeps the cooling piping and equipment clean and free of corrosion. It is not necessarily the treatment that contains the least expensive chemicals or uses the least acid or biocide. The correctness of the treatment can be seen in periodic inspections of the equipment.
Nothing in water treatment remains constant except change. It is critical that you are using the correct chemical treatment for your system. Changes in water sources, additional cooling demand, or changes in operating practices can result in a need for a significant change in cooling water treatment. Pitting rates are very flow and temperature sensitive. While stagnant conditions will generally reduce the dissolved oxygen differential between what is inside and outside a deposit, they give bacteria, particularly anaerobic bacteria, just the right conditions to multiply. Systems that cycle, or alternate between stagnant and flowing conditions, can be the most difficult to treat.
Over the years, the general industry trend in cooling water treatment has been to allow the pH of the cooling water to increase. This practice is based on the assumption (disputed by some) that corrosion rates for steel decrease significantly as the pH of the water increases. The potential for calcium carbonate precipitation also increases at higher pH, but some recent developments in polymers have shown promise in controlling precipitation even at these higher pHs. Some treatments now claim that they do not require the addition of any acid for pH control. This sounds very tempting when acid costs are increasing.
It is important to remember that the effectiveness of bleach as a biocide drops dramatically as the pH of the cooling water increases. The savings in acid can be swamped by increased costs for additional biocides to control bacterial growth at the higher pH. Bromide is used in addition to bleach to treat these more alkaline cooling waters, and it can also be expensive. (See “Biofouling control options for cooling systems” in POWER, September 2007.)
There are also a number of non-oxidizing biocides that can be very effective, but they typically have very strict requirements for their proper application and often require special handling precautions. These chemicals are typically also very expensive to apply on a regular basis. Before changing one area of the treatment, it is important to consider the effect that doing so will have on the rest of the cooling water chemistry and to account for the costs and benefits to the whole system. It may turn out that, even at today’s price, the additional sulfuric acid required to achieve the lower-pH cooling water will produce the best results with the lowest overall chemical treatment costs.
Consistent. Corrosion thrives on disparities. Consistent application of any cooling water chemical treatments is critical to the treatment’s success. Once corrosion cells are established under deposits, whether they are inorganic deposits or microbiological slime layers, they are very difficult to remove. Deposits change the surface chemistry at the metal/water interface regardless of the bulk water chemistry. Pitting corrosion often cannot be stopped until the deposit (or biofilm) is completely removed.
Cooling water chemistry and the consistent concentration of corrosion inhibitors is critical to controlling corrosion. The ability of treatment chemicals to prevent calcium carbonate precipitation depends on the amount of calcium and pH in the cooling water and the amount of residual chemical. On-line and/or grab sample testing must be routinely performed to ensure that all these parameters are within limits. Because many of these polymers are difficult to analyze for, assumptions are made that one parameter included in the treatment, for example, orthophosphate, is a good indicator of the amount of treatment in the system. However, if there are variable amounts of phosphate in the makeup water, this assumption may not be valid. These presumptions need to be reviewed for a treatment to be effective.
Regular treatments with biocides are also important, whether a system is operating or not. Bleach can control biological activity, but it does not stop it completely. So biocides, adjusted for the season and local conditions, need to be consistently applied, even if that means running the circulating water pumps for an hour or two.
Go with the pros
There will always be legitimate differences of opinion, particularly among water treatment vendors, about what the correct treatment regimen is for a particular cooling water system. Some differences are legitimate and are based on a detailed understanding of a particular cooling water system. Obviously, the experience and training of the person making the recommendations should weigh heavily in your decision.
There will also always be “new and improved” treatment regimes. Research into cooling water treatment polymers is very active, and there are often real innovations that permit one water treatment company to present a chemical treatment system that truly is superior to yesterday’s technology. However, in some cases “new” just means a repackaging of treatment chemicals that have been around for years and is different just for the sake of being different.
It is important to remember that new is not always better. In our rush to get the latest and greatest, we sometimes throw away some tried and true producers. If you are not sure if you are getting the straight story, you may want to consult with an independent cooling water specialist for recommendations of generic chemicals that you can buy from any vendor.
Because conditions can change in any cooling water system, the service you get from your chosen chemical treatment vendor can be of more value than the chemicals themselves. Having the correct treatment also means having an experienced set of eyes regularly examine, test, and make recommendations for change, as needed.
--David G. Daniels (david_daniels@mmengineering.com) is a principal of M&M Engineering Associates and a contributing editor to POWER.
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