What not to do
It is important to remember that cooling water systems rarely fail due to general corrosion--it would require close to 25 years of 10 mils per year of general corrosion (an unacceptably high rate) to cause a standard 1/4-inch piece of carbon steel piping to fail. Far and away the majority of the piping and heat exchanger tube failures in open or closed cooling water systems are caused by localized pitting. And localized pitting is the result of under-deposit corrosion.
Deposits are caused by a number of factors, including accumulations of silt and debris, calcium carbonate precipitation, and the growth of biofilms. Whatever the cause, the net result is the same: The area underneath the deposit becomes the anode and the area outside the deposit becomes the cathode; once the anode and cathode are established, corrosion is off to the races. Because it can be so localized (under a deposit), actual pitting or the potential for pitting in a specific area can be difficult to accurately access.
In rapidly fluctuating commodity markets as we have today, some might be tempted to cut back on treatment levels, to run closer to a minimum residual instead of in a safer range, or to increase the pH of the cooling water to reduce sulfuric acid costs. It also may be tempting to reduce biocide treatments to save money. Don’t fool yourself--these actions can have huge consequences on heat transfer, turbine backpressure, and corrosion in the cooling system that will far outpace the increased cost of chemicals.
Cutting back on treatment or applying treatment levels inconsistently will eventually lead to catastrophic failures that cost more to repair than any treatment regimen, regardless of rising chemical costs. Nevertheless, there are some things that you can do to ensure you are using the right chemicals to keep your system clean and reliable. The first step is to assess your current treatment practices and look for some better (and perhaps even less expensive) alternatives.
Below are two examples of lessons learned the hard way by a plant owner.
Case study #1: Pay me now or pay me later. It was a jump in the cost of molybdenum that contributed to problems in the mild steel piping in a chilled water system. As the cost of the metal increased and treatment costs kept pace, the decision was made to cut expenses by using much less treatment than is typical in these systems. The result was massive corrosion (Figure 1) that required replacement of major sections of the chilled water piping.
1. Out of balance. Failure to maintain proper levels of treatment resulted in a short-term gain and long-term loss for this piping system. Courtesy: M&M Engineering Associates Inc.
Once corrosion cells are established, they are very difficult to stop even if treatment is reestablished at the proper level later on. Corrosion cells also set up excellent homes for metal-munching bacteria. It also takes time for the symptoms to become obvious, so things may appear to be going great for a while, but sooner or later, the corrosion comes home to roost.
Case study #2: Microscopic real estate boom. If you fail to maintain either an effective biocide treatment regime or prevent scaling, then you should expect failures. Remember that microbiological fouling and inorganic deposits go hand in hand. Bacteria are looking for some real estate where they can settle down and multiply. Whether they start out as silt and debris or conventional calcium carbonate scale, deposits on cooling water piping look like brand new condos to acid-producing and sulfate-reducing bacteria. And once they move in, they are very difficult to evict.
The inorganic deposits protect the bacteria from biocides by absorbing or reacting with the biocide first. Similarly, once bacteria become established on piping, they form a biofilm that consists of a substance that traps debris and helps deposits to form. The combination can corrode most metals--copper, carbon steel, and even stainless steel can succumb to the effects of bacteria.
Figure 2 shows copper nickel tubing from a heat exchanger that failed due to bacteria deposits. When the tube deposits (which included calcium carbonate and silt) were removed from the sample, there was an area of pure copper metal around the failure. Preferential attack of one component of an alloy is a characteristic of microbiological-influenced corrosion. DNA analysis of the deposit proved that there were significant populations of corrosion-causing bacteria in the deposit.
2. Surface blemish. This heat exchanger tube failed because the bacteria did their work well. Courtesy: M&M Engineering Associates Inc.
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