Open cooling tower systems are well recognized as existing under much greater environmental stresses, and therefore traditionally suffer a greater corrosion loss. Compared to a closed chill water or secondary piping system which will typically show a 1-2 mil per year (MPY) or less corrosion rate, open systems generally range from 3-5 MPY to well above. See our photo gallery of various corrosion types.

     In addition to a difference in wall loss and corrosion rate, closed systems rarely suffer the more severe corrosion attack caused by microbiologically influenced corrosion (MIC), under deposit pitting, and galvanic corrosion at dissimilar metals. While a severely corroded closed piping system may show a 5-8 MPY corrosion rate, severe problems at an open water system can produce a corrosion rate of 25-50 MPY, and more.

     Clearly, many forces are at work within different types of piping systems to create such variances in corrosion rate. Some are well outside the control of any building owner or plant operator, or of the chemical water treatment contractor whose responsibility it is to provide adequate corrosion control. Review a summary of piping quality, operating, and design changes which have occurred.

     Viewing the operating differences between both closed and open piping systems offers some basic explanation of this variation in corrosion rate:

• The cooling tower of an open piping system acts as a giant air filter and gas scrubber, and operates under almost identical principals for pollution control units used for industrial applications. Therefore, a substantial volume of airborne dirt and particulates are captured and introduced into an open system, but not a closest system.
  
  
• The greater volume of dirt, iron oxide, and other particulates which are captured by open systems then becomes the initiating cause for higher corrosion rates. This in turn will often result in extremely damaging under deposit or cell corrosion problems for cooling tower, process water, or other open piping systems. Such externally introduced deposits are not a problem for closed systems.
  
  
• Microbiological growths, while occasionally a problem in closed systems, are a continuous threat to any open piping system due to abundant oxygen, and a never ending supply of new food and microorganisms collected through the effect of the water washing through the air.
  
  
• An open piping system exposes its contents to some degree of sunlight for part of its cycle - thereby providing needed energy to algae and other light dependant microorganisms.
  
  
• Due to the aerating action of the cooling tower, water in an open system is typically oxygenated to its maximum limit - thereby providing abundant oxygen for microbiological growths and metal oxidation.
  
  
• Condenser or open water systems operate at higher temperatures by approximately 50-70 º F. over a closed chill water system. Such higher temperatures are in the optimal growth range for many microorganisms. Higher operating temperatures are also recognized for increasing the corrosion rates in most operating environments by speeding up the various chemical processes.
  
  
• The biocides prescribed for open water use carry with them relatively short half lives in order that their discharge in the blowdown does not negatively affect the bacterial cultures critical to the operation of local wastewater treatment plants. Therefore, controlling microbiological growths becomes a much greater problem for open systems due to both higher organic loading and restrictions on the available countermeasures.
  
  
• The air scrubbing action of the cooling tower absorbs many gaseous materials and pollutants, often lowering pH significantly. Nutrients scrubbed from area kitchen exhausts often fuel excess biological activity and MIC, while the sulfur dioxide fume from a boiler or power plant exhaust can produce acidic pH conditions of 6.0 or lower.
  
  
• Cooling tower or open water systems require a percentage of their make-up water to be blown down or discharged from the system in order that calcium carbonate, magnesium carbonate, and other hardness components do not concentrate sufficiently to produce high total dissolved solids (TDS) and a scaling condition. A constant blowdown from the system, however, also means a substantial loss of both corrosion inhibitor and microbiocide.

     An obvious question, therefore, becomes the inversely proportional degree of chemical water treatment protection provided between open circulating systems, which exist at such greater threat, and closed systems, which often present little if any concern. And the answer lies in the last difference between such systems, as cited above.

     Typically, an open condenser or process water system will be chemically treated at a concentration of 8-10 PPM of molybdate, phosphonate, or some other rust inhibitor. Yet a closed system may be maintained at 200 PPM or greater of the same product. Notwithstanding the 20 to 1 or greater difference in chemical concentration for a typical closed system, perhaps 80% or more of an annual water treatment budget still pays for the chemical used at the open water side.

     This is due to the high volume of blowdown necessary for any open system - which is greatly dependant upon the characteristics of the make-up water. Blowdown rates can range from 5-10% of the make-up, to as much as 30% or more in hard water or areas of high total dissolved solids (TDS). In moderately sized open water systems, the chemical treatment of tens of thousands of gallons of water per day may be required.

     While the true evaporation of water leaves any treatment chemicals behind, cooling tower blowdown, and the loss of solid water droplets through the fan discharge, or by wind drift and overspray requires adding chemical to maintain desired treatment levels.

     Considering for illustration a typical evaporation rate based upon 30 gallons per minute per 1,000 tons of refrigeration, we can then estimate that a 5,000 ton system will make-up approximately 150 gallons per minute (GPM) or 216,000 gallons during a 24 hour period. Assuming a high allowable cycles of concentration or low to moderate blowdown rate of 10% then means the necessity to chemically treat a new 21,600 gallons of water every day, or 151,000 gallons per week. A 20% blowdown requirement would double that amount.

     Alternate methods of calculating cooling tower evaporation, as well as useful formulas for cycles of concentration, blowdown and make-up are rate are provided in the below table:

     Chemically treating such a large volume of new make-up water at the higher 200-300 PPM levels necessary to produce reliable 1 MPY corrosion control would require an enormous expenditure given the high cost of today's corrosion inhibitors and microbiocides - typically exceeding $1,000 per 55 gallon drum. Paying possibly 20 times the cost of an existing water treatment contract is virtually prohibitive regardless of the expected benefit in extending system life.

     For this reason primarily, lower chemical concentrations are usually specified for open water systems. It is not because of any lesser need or some scientific reasoning often suggested by water treatment contractors - but simply because of the high costs involved.

     With the competitive nature of the chemical treatment business, cost always plays an extremely important role. Likely set by the previous history of chemical contracts, or by the budget demands of the client, water treatment specialists often need to meet a pre-determined price range. The interests of one chemical supplier to recommend and charge for higher and more appropriate inhibitor levels, against others that would not, would guarantee business failure given the tight and competitive market which exists.

     So in effect, the level of chemical provided, and the degree of corrosion control produced, is not entirely dependant upon the chemical supplier, the effectiveness of the inhibitor chemicals, nor any other plant engineering actions - but by the budget constraints of the client to some degree.

     Through some reasoning, likely not well defined or publicized, the higher corrosion rates produced at lesser inhibitor levels have been judged as acceptable by both client and water treatment contractor alike. It is simply a trade off between pipe service life, the possibility of operating problems, and money.

     In reality, most large diameter piping systems will last their intended lifespan at those higher 3-5 MPY corrosion levels. However, such higher corrosion levels always present greater opportunity for more severe operating problems to develop, and narrow the margin of safety between trouble free operation and disaster.

     Higher corrosion rates also lead to an inevitable build-up of deposits and other secondary operating problems. See Technical Bulletin C-4 about the problems associated with interior deposits.

     For smaller diameter piping, and especially where threaded schedule 40 pipe is involved, any higher corrosion rate exceeding 1 MPY will inevitably shorten pipe life. The more recent use of thinner schedule 20 and schedule 10 pipe in many applications almost eliminates any margin of safety where corrosion is concerned - virtually guaranteeing a failure unless extraordinary corrosion control exists.

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