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Preventing Unprecedented Breakdowns
Nirmal Shah, Managing Director,H2O Innovation India Ltd Water usage in cooling towers for the removal of process heat is the highest amongst all other water usages and also poses many challenges. It receives significant attention at the hands of the management. This paper therefore is devoted to re-circulated cooling water systems in chemical, petro-chemical, petroleum refineries, fertilisers and power plants.

Water plays a very important role in all the industrial manufacturing activities. The importance of water in chemical industries can be gauged from the fact that on an average 30- 35 per cent of a Chemical plant’ investment is in water systems. Water usage in cooling towers for the removal of process heat is the highest amongst all other water usages and also poses many challenges. In chemical plants many reactions are either exothermic or the reactants need to be heated for the completion of the reaction. The excess heat generated is removed by circulating cooling water through the process equipment such as heat exchangers or condensers etc. Cooling towers are heat removal structures that transfer process heat waste to the atmosphere.

Hot water from the plant is circulated through the cooling tower structure where there is an intimate contact of hot water falling from top with the relatively dry incoming air. As a result there is an evaporation of some water and the temperature of hot water is reduced to the wet bulb temperature. The cooled water is brought back to the plant and continuous re-circulation takes place. Through the cooling tower structure considerable water is evaporated and lost to the atmosphere. Because of continuous water recirculation, the impurities in water get concentrated and water absorbs microbes and dust particles from the atmosphere. In order to limit the level of impurities in the circulated water, a continuous stream of water, called "blow down" water is let off from the cooling tower. The quantity of "blow down" determines the usage efficiency of water, called as "concentration cycles". The loss of water through "blow down", evaporation, drift and leakage is made up by adding fresh water called "make up" water.

The quality of "make up" water is a major factor that can affect plant performance.

Plant Management Perspective
• To operate the plant on continuous basis for long periods of time without any interruptions due to breakdown on account of water.
• Operate at the highest possible heat transfer efficiency.
• Economise on the use of water by operating at the highest possible concentration cycles.

Problems due to Inadequate Treatment • Corrosion of heat exchanger tubes and pipelines
• Scale formation on the heat transfer surfaces
• Biological fouling coupled with under film corrosion • Deposition of suspended particles on heat transfer surfaces • As the water circulates through the plant heat exchangers or condenser, the corrosive nature of water or the salts in water along with biological fouling create nasty problems for plant operations. Corrosion is caused by aggressive water such as deionised or desalinated water and scale formation takes place in high hardness waters. Fouling is accelerated by incorporation of suspended particles and bacteria from atmosphere. Also, dissimilar metal junctions cause galvanic corrosion. There is a snowballing effect – once corrosion / scale sets in, it reduces the cross-sectional area thereby reducing the flow of water, which in turn reduces velocity and lets particles settle.
• Loss in heat transfer efficiency
• Breakdown of equipment and forced shutdowns
• Constriction of water flow causing high temperature differential across the heat transfer surfaces

Possible Problems with Cooling Water Systems
• Corrosion
• Scale formation and
• Microbiological corrosion and fouling

A) Corrosion
Corrosion is an electro-chemical process requiring an anode, a cathode and an electrolyte. At any given time when metal is exposed to water (electrolyte) numerous anodic and cathodic sites will be present due to concentration differences, microbiological and other deposits on the surface, impurities and stress in metal. There are basically two classes of corrosion, namely general where the whole metal surface is affected, and localised where only a small area of the metal is affected. The first is relatively easier to deal with; the second, however, is more clandestine and can appear in a variety of different guises.

B) Scale Formation
All waters contain dissolved solids consisting of calcium and magnesium salts. The amount of these salts can vary depending upon the source of water and the season of the year. These salts tend to get deposited on the hot heat exchanger surface because of changes in the water properties such as pH, concentration of the ions and reaction with corrosion products. Furthermore the calcium salts exhibit inverted solubility, meaning that at higher temperatures the solubility decreases. Scale inhibitors are added to the water systems to prevent the deposits from precipitation on the heat transfer surfaces.

C) Microorganism Fouling
This is perhaps the worst type of problems that a cooling water system can face. This is also one of the least understood problems. Presence of microorganisms can drastically change the electrochemical conditions at the water and metal surface interface such as the pH, changes in the oxygen and ions concentration levels. These changes will cause acceleration of corrosion process in the localised areas thus causing pitting of the metal surface of the pipes, pumps, fittings and even vessels.
The growth of bacteria generates by-products like acids, alkali, reducing agents such as ammonia and hydrogen sulfide etc. These also change the ionic equilibrium in water. Tubercles are produced on the metal surface because of the microbial action. Furthermore the biofilm formed on the heat transfer surface will retard the heat transfer efficiency significantly. In short, the microbiologically induced corrosion is a very complex phenomenon and must be addressed properly to avoid frequent breakdowns and loss of production. To mitigate the microbiologically induced problems a regular dosage of oxidising and non-oxidising biocides is added to the cooling water systems. Chlorine and Chlorine Dioxide are the typical oxidising biocides and Isothiazolines, Quaternary ammonium compounds, gluteraldehydes etc. are the non-oxidising biocides used. In general, a well thought out treatment program suitable for each situation is prescribed to mitigate the complex problems of corrosion, scale control and microbiological fouling depending upon the water quality and customers requirements. A case study on the use of chlorine dioxide to mitigate the fouling and corrosion is given as under:

Conclusion
Due to unique properties of chlorine dioxide, use of the same in system having contamination of ammonia or organic matters is essential to control the fouling and Microbiological Induced Corrosion. It is now accepted that a proper combination of corrosion inhibitors, polymeric dispersant, combination of oxidizing and non-oxidizing biocides and use of bio-dispersant, is essential to maintained the metal surface free from deposits, scale, bio-fouling and corrosion.