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Ryznar Index Bringing a New Meaning to "Water Balance"
Despite religious attention to maintaining an LSI within widely accepted ranges (0.0 to 0.3), many facilities experience calcium losses from their pool surfaces. This is not from the slow dissolution of calcium, but rather from the rapid dissolving of calcium from acid cleaning the surfaces. Staining of the pool surfaces from metal oxide deposition is too often the result of corrosion byproducts released from the metals making up the the pools filter, pool water heater, and recirculating system. Also, considerable maintenance costs are attributed to replacing corroded equipment. Mistakenly, many believe Langelier Saturation Index (LSI) is an indicator of the corrosiveness of water toward metal, particularly iron based metal. The LSI is an indicator of the solubility of calcium carbonate in water. It does not provide a measurement of the corrosiveness of water toward metal. In pool water applications where pH is properly controlled (generally around 7.5), the potential for calcium carbonate precipitation in the bulk water of the pool is non-existent. Referring to figure 1, we can see that as pH values common to pool water applications (7.2 - 7.8), CO2 based alkalinity is in the bicarbonate form. No carbonate ions exist. So calcium carbonate cannot form. While a negative LSI may indicate calcium may be eluted from materials containing calcium carbonate, having a positive LSI does not indicate calcium carbonate will form. It indicates it will not dissolve. With an LSI of 0.3 and a pool water pH of 7.5, calcium carbonate will not form since carbonate ions do not exist. However, if calcium carbonate is added to the water, it would not dissolve. Since we cannot form calcium carbonate at a pH of 7.5, why be concerned about operating our pools at a higher LSI like 1.0? Under the right conditions you could. However in the heat exchanger and carbon steel piping, the potential for corrosion exist. Corrosion cells are made up of two half cells. these half cells are called the cathode and the anode. The cathode is there the highest oxidation potential exists. The anode is where the lowest oxidation potential exists. Anodes can be created from imperfections in the metal during manufacturing ir installation (crevices), and or any number of deposit forming materials which cover the surface of the metal. For example, if a droplet of sun tan lotion coated even a small amount of the metal surface, oxygen is deprived from reaching the metal surface at that location. When a location of low potential forms, the area where oxygen is in plentiful supply possesses a high electrical potential compared to the anode where the oxygen supply has been cut off. The cathode draws electrons (oxidizers consume electrons) from the anode. The anode releases metal ions, while the cathode under goes a series of electrochemical reactions. At the cathode, oxygen and water molecules are chemically and or physically adsorbed to the surface of the metal. As electrons are supplied, the oxygen, water, and electrons from hydroxide ions. Hydroxide ions (those HOˉ ions supplied by caustic and lime) raise the localized pH of the water at the cathode. Water containing calcium and alkalinity (bicarbonate at a pool water pH of 7.5) contact the localized high pH at the cathode. Referring to figure 1, we can again see that as the pH rises, the species in which CO2 exist in water changes. As the pH rises above 8.3, the concentration of bicarbonate, and carbonate alkalinity formation begins. When the saturation limit and calcium carbonate is reached, it begins shifting to carbonate alkalinity. When we reach the saturation limit of calcium carbonate, it precipitates onto the cathode. the precipitation of calcium carbonate onto the cathode is a good thing since it restricts the flow of oxygen to the cathode. This reduces the oxidation potential, and slows or shuts down the half cell reactions. When either one of the half cell reactions stops, the entire corrosion reaction stops due to the need for electro-neutrality. While the formation of calcium carbonate helps protect the equipment from corrosion, more is not necessarily better. We want to control the amount of calcium carbonate which precipitates at the cathode. therefore we want to achieve a balance between maintaining adequate cathodic protection, without inducing excessive scaling. The Ryznar Index is an empirical took used to determine the tendency and relative severity of water to scale or corrode. The Ryznar chart was based on evaluating thousands of water systems and numerous water applications. The benefits of Ryznar over LSI is its ability to indicate whether or not there is a scaling condition or a corrosive condition in dynamic systems containing mild steels. In those cases where little concern for corrosion exist, if nothing else, Ryznar should open our eyes to the range for which we can safely and effectively operate our pool water chemistry. Example: Let's assume we have a pool water balance with the following parameters: pH - 7.5, pool temperature 82ºF, Calcium - 250ppm, "M" alkalinity - 100ppm, TDS 2000ppm. The pH of the saturation )pH) for this water is 7.4. Based on these parameters the LSI equates to 0.1. The pool manager would probably feel comfortable with his pool water balance. However, the calculated Ryznar index (RI = 2pHs - pH) tells another story. With a Ryznar index of 7.3, the Ryznar chart (fig. 2) clearly indicates a corrosive condition for both hot and cold water systems. While many pools operate with LSI values which imply proper water balance, they often experience pool staining and reduced equipment life. Ryznar can be a powerful tool used to address this void in the water balance equation. Summary: To protect the pool surfaces from eluting calcium, sufficient calcium levels must be maintained in the pools bulk water, while maintaining proper control of the waters pH. To protect the metal equipment, as well as to reduce the potential of metal staining caused by corrosion byproducts, pool managers should adopt the Ryznar index. Target a Ryznar index of 6.3 - 6.7, with 6.5 being the ideal target. Adjust the water balance by correcting the alkalinity (typically 60 -150ppm) while maintaining adequate calcium (in the range of 400 - 800ppm). These ranges should be used as guidelines. Adjust based on the specific needs of your operation, incoming water chemistry, and your pool water chemistry. (Note: For salt water pools, use the Stiff-Davis Index to correct for increased calcium carbonate solubility, Nalco Water Handbook, McGraw Hill Publishing) Use good sense. If your equipment (such as the heat exchanger) appears to be corroding, and your calcium level is 300ppm with an alkalinity of 100ppm (assume pH at 7.5), one option is to operate at higher calcium levels while maintaining the same alkalinity and pH to lower the Ryznar index to 6.5. First make sure you're not feeding chlorine or acid into the heater! |
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