Understanding microbiologically influenced corrosion (MIC) as it relates to fouling in heat exchanger tubes.
One insidious nemesis of chiller operations is microbiologically induced corrosion (MIC). MIC is a serious concern and is a major cause of fouling in heat exchanger tubes, surface condensers and industrial shell and tube heat exchangers. The results of MIC costs the global economy billions in tube failure repair, total heat exchanger failure, major process interruption, reduced equipment life and costly recurring equipment inspection. All these outcomes require their own unique management systems and as a result, MIC has become a key target for heat exchanger cleaning operations.
What is MIC?
What is MIC and how is related to condenser cleaning systems? MIC, as related to shell and tube heat exchangers, is corrosion that is enabled by the presence of bacteria that can form and reside inside and under a film (Biofilm) that coats the inside surface of heat exchanger tubes. The biofilm is similar to a clear film that coats the typical dog watering dish. You can’t really see it until you run your finger on the dish surface and then the film is exposed through contrasts. MIC can reside in and under the biofilm and it often becomes shielded from corrosion inhibitors used for its control. MIC often results in localized, accelerated corrosion (pitting) and leads to premature tube failure.
How MIC Impacts Metallic Systems
Microbiologically influenced corrosion (MIC) can potentially affect all metallic systems in contact with ambient temperature seawater or freshwater as a result of the presence of particular organisms in microbial films. As show in Figure 1 above, this is particularly problematic in water cooled heat exchangers where damage, also called microbe induced corrosion, can wreak havoc on chiller efficiency. As Ashwini Sinha writes in Aspects of Failure of Condenser Tubes and Their Remedial Measures at Power Plants, “MIC, in addition to being a specific damage mechanism, can also substantially increase the galvanic, crevice and pitting corrosion rates on power plant components.”
MIC attack generally shows up in the pitting that occurs on surfaces in contact with deposits containing active biofilms. Along with deposit materials, this may also include sticky exopolymers associated with both organic matter. When pitting eats its way completely through the wall of the tube, the condenser fails. Whether from reduced efficiency or outright failure, the monetary cost to utility operators, power generators, hospitals, and universities is substantial. Fouling in heat exchanger tubes is found in up to 97% of operating shell and tube exchangers and estimates place the cost of this unchecked fouling to the U.S. economy alone at over $70 billion annually.
“The final case study involves a Cu chiller tube that experienced a leak after six years of service, and four years after recycled water had been introduced as cooling water tower makeup (fig 2). The recycled ” grey ” water was treated with Cu corrosion inhibitors and for bacteria control by free chlorine. Green deposits and scaling were found throughout the inside of the tubes. Localized pitting corrosion was observed under the deposits. Analysis of treated cooling tower water and untreated water was also performed, which included relatively high chlorine levels being detected. The author theorized that in this case the chlorine was ineffective as a biocide, and that the corrosion inhibitor may have increased bacterial growth. Overall, the conclusion was that the poor water treatment was the cause of the MIC that led to the piping failure. Additionally, once a biofilm is present on the tubes, the chlorine is unsuccessful in controlling bacteria growth within the biofilm.” – Microbiologically Influenced Corrosion of Copper and Its Alloys, A Review
Problems associated with fouling in heat exchanger tubes.
The problem MIC presents is interesting and the most prevalent current approach to MIC prevention is the employment of chemical treatment regimens. Professional chemical treatment is essential to water management in chilled and condenser circuits but by itself, it is often not effective at eliminating the biofilm that creates the MIC process. In fact, as found by Javed, some chemical inhibitors actually served to facilitate bacteria growth. Lastly, numerous studies also highlight the potential for reclaimed (grey) water to enhance the bacteria growth environment inside the condenser thus leading to a more permissive MIC environment.
A better approach to cleaning heat exchangers.
Current heat exchanger cleaning practices are generally not effective to remove biofilm during operations because manual condenser tube cleaning and chemical cleaning require the complete shutdown of the heat exchanger. Once the heat exchanger is put back in service, the biofilm again tries to take hold. However, there is a solution that prevents MIC and general fouling during the operation of the heat exchanger. The Helios Tube Cleaning System uniquely provides condenser tube cleaning, automatically and continuously, in the operating heat exchanger. Through frequency-based injection of sponge rubber cleaning balls into an operating exchanger, the Helios is a heat exchanger cleaning system that physically prevents the growth of biofilm on the surface of the heat exchanger tubes.
Automatically injected on a programmed schedule, the cleaning balls are sized to be slightly larger than the tube diameter, and as the balls travel through the tubes and conform around the tube enhancements they ensure bio fouling is removed from all angles of the interior tube surface. The result is a heat exchanger cleaning system that eliminates tube fouling completely and permanently.
The recurring management problems and unplanned maintenance events the Helios Tube Cleaning System can solve during heat exchanger cleaning include tube fouling elimination, fouling-related waste elimination, manual heat exchanger cleaning avoidance, microbiologically induced corrosion cessation and manual cleaning damage risk reduction all add up to making the Helios a gamechanger for heat exchanger cleaning.