KEYWORDS: Combustion Turbine, Evaporative Cooler, Langelier Saturation Index, Ryznar Stability Index, Puckorius Scaling Index, Reclaimed Water

Transcription

1 Emerging Water Chemistry Issues in Combustion Turbine Evaporative Coolers DANIEL J. ROBINETTE, P.E. AND CHARLIE NICHOLS, P.E., CH2M HILL, 9127 S. Jamaica St., Englewood, CO REPORT IWC-08-XX KEYWORDS: Combustion Turbine, Evaporative Cooler, Langelier Saturation Index, Ryznar Stability Index, Puckorius Scaling Index, Reclaimed Water ABSTRACT: Combustion turbine manufacturers have developed stringent water quality guidelines for evaporative coolers. This paper will explore why these water quality guidelines exist, how they could be improved, and why they are difficult to meet in this day of water scarcity and reclaimed water use. Figure 1. Combined Cycle Power Plant 1.0 INTRODUCTION Evaporative Cooler Locations Evaporative coolers are used extensively in the power industry to increase power output from combustion turbines. The evaporative cooling process, and associated makeup water treatment, is similar to that of cooling towers, but the makeup and circulating water quality requirements and allowable chemical treatment for evaporative coolers are much more constrained. These higher constraints, as mandated by combustion turbine manufacturer s water quality guidelines, make it extremely difficult to find lowcost, readily-available makeup water sources of adequate quality. Furthermore, as will be shown, the guidelines limit the cycles of concentration that can be attained to a maximum of 5. This paper will present the issues involved and three case studies. It is the hope of the authors that the message presented herein will resonate with combustion turbine manufacturers, engineering firms and evaporative cooler end-users to spur more research in this area. Figure 2 shows an external view of the combustion air inlet duct and the location of the internal media banks and circulating water basin (sump). Figure 2. Inlet Air Duct Housing Evaporative Cooler Evaporative Cooler Media Banks Basin 2.0 BACKGROUND PHYSICAL DESCRIPTION - Figure 1 shows a 580 megawatt, 2 x 1 (two combustion turbines on one steam turbine) combined cycle power plant and points out the location of the evaporative coolers (EC) in the combustion air inlet ducts. Each of the combustion turbines generates 145 megawatts with 2,845,500 lbs/hr (dry basis) of air flowing through each EC. 1

2 Figure 3 shows an internal view of the lowest of the three EC media banks illustrated in Figure 2. Each bank of media measures 44 ft wide by 12 ft tall. The media is approximately 18 inches thick relative to the flow path of combustion air the direction of which is left to right in Figure 2 and right to left in Figure 3. Figure 3. Lower Level EC Media Bank (1 of 3 levels) approximate measure of how concentrated the circulating water is relative to the makeup water concentration. Figure 4. Evaporative Cooler Water Usage Water Flow, % of Evaporation Cycles of Concentration Makeup Water Blowdown Some general principles from Figure 4 are worth noting: 1) Makeup water consumption approaches infinity as COC approaches 1.0 reflecting infinite blowdown, 2) Blowdown is equal to evaporation at COC of 2.0, 3) Makeup is twice evaporation at COC of 2.0, and 3) COC of 2.0 represents a turning point, or elbow in the curve where successively higher unit increases in COC above 2.0 net successively smaller and smaller incremental decreases in water consumption and blowdown. It is therefore desirable to operate the evaporative cooler with at least 3 COC from a water conservation standpoint and there is very little to be gained by operating the evaporative cooler above 5 COC unless wastewater disposal costs are very high. The evaporative cooler basin that serves all three media banks is located below the grating and media shown in Figure 3 and measures 4.6 ft wide x 3 ft deep x 44 ft long. For a typical GE 7FA combustion turbine, the sump holds approximately 4,000 gals of water. Circulating water is distributed from the sump over each bank of media in a cross-flow configuration with the combustion air flowing from right to left in Figure 3. Drift eliminators are located on the downstream side of the media. PROCESS DESCRIPTION Evaporative coolers operate much like cooling towers from a mass balance standpoint. Thus, makeup water and blowdown in evaporative coolers are related to cycles of concentration (COC) as displayed in Figure 4. COC is defined as the makeup water flow rate divided by the blowdown water flow rate and is an From the above discussion, it is clear that EC makeup water and blowdown quantities are keyed to evaporation and COC. Evaporation depends on local climate conditions and is determined using standard psychrometric calculations applied to the flow of combustion air. For the EC depicted in the above photographs, that are located in an arid climate, the evaporation rate is approximately 50 gpm each. The case studies given later show that typical EC cycles of concentration range from 2 to 5. Applying this to the evaporation rate of 50 gpm, gives blowdown flow rates from gpm respectively. For the EC depicted here, the basin contains approximately 4,000 gallons of water, thus, the mean residence time (MRT) of water in the system is approximately 320 minutes at 5 COC and 80 minutes at 2 COC. These MRTs are relatively short compared to cooling towers that are usually equipped with much larger basins and can have 2

3 MRTs as high as five days or more. MRT is an important parameter for ECs because the use of treatment chemicals is discouraged by the EC water quality guidelines, and MRT has a significant impact on the quantification of mineral precipitation*. ph-alkalinity RELATIONSHIP For the ensuing discussion on water chemistry guidelines, it is important to understand how the ph and M Alkalinity are related for water that is assumed to be at or near equilibrium with atmospheric CO 2. Figure 5 shows three curves that demonstrate the semilogarithmic relationship between ph and M Alkalinity. Figure 5. ph and M Alkalinity Relationship M Alkalinity, ppm CaCO Operating Evaporative Cooler Theoretical Equilibrium Curve Cooling Tower Survey by Kunz et al ph The Theoretical Equilibrium Curve represents the ideal case where the circulating water reaches equilibrium with atmospheric CO 2. The equation for the Theoretical Equilibrium Curve is, : phe = x Log(T Alk) Eq. (1) where: phe = Equilibrium ph T Alk = Total Alkalinity expressed as CaCO 3 The equilibrium carbon dioxide concentration that corresponds to a given phe and Total Alkalinity can be determined using mass action and mass balance equations (Kunz et. al. 1971). It can be shown that the carbon dioxide so determined is equivalent to the free CO 2 referred to in the graph entitled Effect of Bicarbonate Alkalinity and CO 2 on ph given on page 109, Section 72 of the Siemens Handbook. Additional information that describes what is meant by free CO 2 concentration and the equilibrium relationships involved can be found in Stumm & Morgan, pages ** The curve labeled Operating Evaporative Cooler in Figure 5 is from actual data from a single evaporative cooler, and shows only slight deviation from the theoretical equilibrium curve. The curve labeled Cooling Tower Survey from Kunz et. al. is from a survey of numerous cooling towers and provides further verification of the ph/m Alkalinity semilogarithmic relationship. 3.0 WATER CHEMISTRY GUIDELINES Typical water chemistry guidelines for evaporative coolers are given in Table 1. These guidelines have been established by combustion turbine manufacturers for two primary reasons: 1) to protect the gas path of the combustion turbine from fouling and corrosion, and 2) to extend the life of internal components of the evaporative cooler, particularly the media. UNDERSTANDING THE GUIDELINES A major portion of the guidelines were derived to prevent the phenomenon known as media sagging. Figures 6 and 7 depict brand new media, and media that has sagged, respectively. * Hawthorn, D. Scaling is a Production Process. IWC #95-14, Presented at the International Water Conference, Pittsburgh, Pennsylvania, October Puckorius, P.R. and J. M. Brooke, A New Practical Index For Calcium Carbonate Scale Prediction In Cooling Tower Systems, NACE - CORROSION 90, Paper No. 99, April 23 27, 1990, Las Vegas, Nevada, pg 3. General Electric Company, Water Supply Requirement for Gas Turbine Inlet Air Evaporative Coolers, GEK A, Copyright 1999, Revised, January 2002, pg 8. Siemens (Formerly U.S. Filter / Permutit), Water and Waste Treatment Data Book, 15 th Printing, Copyright 1991 by the Permutit Company, Inc., pg 109. ** Stumm, W., Morgan, J.J., Aquatic Chemistry, 3rd ed., John Wiley & Sons, Inc.,1996. Kunz, R.G., A.F. Yen and T.C. Hess, Coolingwater Calculations, Chemical Engineering, August 1977, pp

4 Figure 6. New Evaporative Cooler Media Figure 7. Sagged Evaporative Cooler Media The manufacturer s guidelines suggest that aggressive water, as defined by the non-scaling side of the scale for the specified calcite scaling indices has a tendency to leach phenolic resin from the media. The phenolic resin is added during media manufacture to add stiffness and impart strength to the media. The purpose of the circulating water calcite indices specifications, along with the minimum makeup water specifications is to assure that only non-aggressive circulating water is used in order to prevent the sagging phenomenon. These specifications virtually preclude using low-calcium makeup water sources such as demineralized water and reverse osmosis permeate. The supposition that low-calcium water necessarily causes leaching of the phenolic resin is somewhat controversial however, because substantiated case histories exist where low-calcium water has been used without causing the media to sag for six to twelve calendar years of operation. Table 1 Typical Evaporative Cooler Water Chemistry Guidelines Specification Limits Makeup Water Circulating Water Interpretation by Authors Specifications Primarily for Protection of Combustion Turbine Alkali Metals mg/l Na + < 550 For protection of hot gas path Chloride mg/l Cl < 300 For protection of compressor Copper, Total mg/l Cu < 0.05 < 0.5 Iron, Total mg/l Fe < 0.1 < 1.0 Heavy Metals mg/l Fe+Mn+Cu+V+Pb < 0.2 total < 1.0 total For protection of hot gas path Manganese, Total mg/l Mn < 0.05 < 0.2 Specifications Primarily for Protection of Evaporative Cooler Oil & Grease mg/l < 2.0 < 10 To prevent media fouling Silica mg/l SiO 2 < 100 To prevent silica scale Solids, Total Suspended mg/l < 5 < 30 To prevent media fouling Conductivity, Specific µs/cm 50 minimum < 5000 Solids, Total Dissolved mg/l ions 30 minimum < 3000 Alkalinity, M (Total) mg/l CaCO 3 15 minimum < 500 The purpose of makeup water minimum specifications are to extend media life and 4

5 Table 1 Typical Evaporative Cooler Water Chemistry Guidelines Specification Limits Makeup Water Circulating Water Calcium mg/l CaCO 3 15 minimum < 500 ph Calcite Indices: Interpretation by Authors protect susceptible materials of construction from corrosion. Langelier Saturation Index (LSI) LSI is the most conservative calcite index because it does not account for supersaturation. Actual ph is used in determination of LSI. Ryznar Stability Index (RSI) RSI takes supersaturation into account, thus it gives more realistic results than LSI. Actual ph is used in determination of RSI. Puckorius Scaling Index (PSI) PSI is the same as RSI except that ph is calculated from CO 2 /M Alk equilibrium curve. No internal treatment normally allowed except in special cases. Internal Chemical Treatment Pending approval by gas turbine manufacturers the following may be allowed: 1) Non-oxidizing biocide and 2) Crystalmodification polymers. 1 In cases where 100% pretreated water (such as reverse osmosis permeate or demineralized water) must be used, the guidelines recommend controlling makeup water ph at ISSUES WITH THE EVAPORATIVE COOLER WATER QUALITY GUIDELINES Although the guidelines are very well documented, it is a challenging task to use the guidelines because of the following issues. AMBIGUITY - One of the confusing aspects of the water quality guidelines is the inclusion of makeup water limits in addition to circulating water limits that are based on different cycles of concentration (COC). COC values calculated from makeup and circulating water constituents in Table 1 range from 4 to 10. CONFLICTING SPECIFICATIONS - Due to the number and complexity of the specifications, it is often difficult to meet all of the specifications simultaneously. It is not uncommon for attempts to bring one parameter into specification, to drive another parameter out-of-specification. NUMEROUS CALCITE INDICES CAUSES CONFUSION - Inclusion of three different calcite indices in the guidelines brings about conflicts because it is often the case that the LSI is out-ofspecification when the RSI and PSI are within the control range. DIFFERENT CONTROL RANGES FOR RSI AND PSI - Both RSI and PSI use the same scale for evaluating results relative to predicting the formation or dissolution of calcite. This scale has a neutral point of 6.0. As RSI or PSI increases above 6.0, the water becomes progressively more calcite-dissolving. An RSI or PSI above 6.0 has also been correlated to corrosion of carbon steel which increases as the indices increase. As RSI or PSI decreases below 6.0, the water becomes progressively more calciteforming. Based on the above, it would seem that the control ranges for the RSI and PSI in the evaporative cooler water chemistry guidelines should be the same. However, from the explanatory text within the guidelines of a major combustion turbine manufacturer, it is clearly the intention of the authors to prevent scale formation by maintaining a scaling index which is slightly on the scale-dissolving 5

6 side of neutral. The PSI control range supports this intention, but not the RSI which straddles the neutral point between scale-dissolving and scaling. An example is given in one combustion turbine manufacturer s guidelines of how to calculate the PSI. The example uses a multiplication factor of 0.67 to adjust the total alkalinity in the cycled-up circulating water downwards in the PSI calculation. The guidelines do not state however, why this factor is applied, and the authors of this paper see no scientific reason that one third of total alkalinity would disappear unless an acid were added. Nevertheless, it can be shown that the application of the 0.67 factor when calculating PSI of the circulating water, has the effect of increasing the result by approximately 0.5 units above that of RSI calculated using full total alkalinity (no multiplication factor) with all else being equal. Therefore, the author s of this paper propose that this multiplication factor, may be the reason that the authors of the guidelines adjusted the RSI control range from down to which puts the lower half of the control range below the neutral point and into the scaling region. Said another way, the RSI control range may have been adjusted to be 0.5 units below that of the PSI to increase the likelihood that both RSI and PSI will be within their respective control ranges simultaneously because the 0.67 multiplication factor used for calculating PSI causes an offset of approximately 0.5 units. The authors of this paper contend that the RSI and PSI control ranges should be the same, because both indices use the same evaluation criteria for scale-forming and scale-dissolving tendency. Therefore, the authors propose that the 0.67 multiplication factor be eliminated so that the RSI and PSI are calculated on the same basis of total alkalinity. HIGH LEVEL-OF-DIFFICULTY - Calculation of the calcite indices can be laborious when designing the water treatment for the evaporative cooler. During the design stage, the actual ph of the circulating water is unknown, and yet determination of the LSI and RSI require the direct entry of ph. It therefore must be estimated, but curves such as those given in Figure 5, are not given in the guidelines. PSI has the advantage that ph is not a direct input variable, but instead uses equilibrium ph (phe) in its formula. phe is calculated from total alkalinity using Equation 1 given earlier. NO CLEAR PRIORITY FOR DECISION-MAKING WHEN SPECIFICATIONS CONFLICT - In cases where two specifications conflict, it is frequently necessary to make a choice as to which specification should be adhered to while the other specification is not. However, the guidelines do not provide sufficient information to make a decision. For instance, if a specification for the gas turbine were to conflict with a specification for the evaporative cooler media, most would agree that the combustion turbine specification should take priority because the cost/value of the combustion turbine is much greater than the evaporative cooler. However, the guidelines do not provide information on what the relative values would be. OMISSION OF IMPORTANT PARAMETERS The water quality guidelines currently make no provision for several important specifications. Tricalcium Phosphate In order to use reclaimed water (tertiary treated municipal wastewater treatment plant effluent) containing phosphate as makeup to the evaporative cooler, the end-user must be able to predict if tricalcium phosphate will form. However, the guidelines currently do not include such information. When phosphate is present in the makeup water it is very difficult to have the calcite indices in compliance without causing a potential phosphate deposition issue. Sulfate As mentioned in the guidelines of one combustion turbine manufacturer, sulfur oxides can cause hot gas path corrosion, but sulfate is conspicuously absent as a specification. 5.0 CASE STUDIES Case studies are presented below that show the impact of progressively worse makeup water quality on evaporative cooler operation. The case studies also serve as examples of how to calculate various parameters governed by the typical water quality guidelines. The following common basis will be used in each of the cases. Evaporation: 50 gpm Circulating Water Temperature: 77 F It is the goal of each case to determine the maximum cycles of concentration that can be achieved (without external pretreatment) while complying with all of the turbine manufacturer s water quality guidelines, as indicated in Table 1. CASE 1: IDEAL MAKEUP WATER FOR EVAPORATIVE COOLING - The optimum makeup 6

7 water composition would conform to the following criteria: 1) The lowest TDS possible (while meeting criteria 2) in the cycled-up circulating water in order to minimize turbine fouling and corrosion thereby maximizing turbine life, 2) The optimum calcite indices that result in scalefree operation of the evaporative cooler media and maximize media life. 3) The maximum cycles of concentration (to minimize blowdown). Determining the composition of this ideal makeup water starts by examining the makeup water constituents that have minimum concentration specifications: M Alkalinity greater than 15 mg/l as CaCO 3, Conductivity greater than 50 µs/cm ph between 7 and 8.5 Total Dissolved Solids greater than 30 mg/l Calcium greater than 15 mg/l as CaCO 3 (6 mg/l as Ca) An evaluation of these parameters indicates that there are actually only two independent constituents Off-Spec Result calcium and M Alkalinity. The rest of the parameters are dependent on the two primary constituents which is essentially an aqueous solution of calcium bicarbonate, Ca(HCO 3 ) 2, in equilibrium with the atmosphere. Furthermore, when stoichiometric and/or electroneutrality requirements are imposed, there becomes only one independent variable, either calcium or M Alkalinity. M Alkalinity is the logical choice because it affects the minimum specified ph. M Alkalinity is determined by trial-and-error until the corresponding equilibrium ph, determined from Eq. 1 is above the minimum of 7 required. This occurs at an M Alkalinity of 50 mg/l as CaCO 3. An amount of calcium that is stoichiometrically equivalent to the amount of M Alkalinity is then found. The final step is to determine the cycles of concentration (COC) by cycling-up the makeup water until the calcite indices are within the desired range (RSI = ). It was found that 5 COC puts the RSI at 6 as desired. Equation 1 was used to estimate ph in the RSI formula, so PSI and RSI are essentially equal. Table 2 shows the results of the optimization relative to the specifications. Table 2 Case 1 Results Ideal Makeup Water (Calcium Bicarbonate in Pure H 2O) Makeup Water at 77 F Circulating Water at 5 COC and 77 F Case 1 Spec. Case 1 Spec. Specifications Primarily For Protection of Combustion Turbine Alkali Metals mg/l Na < 550 Chloride mg/l Cl 0 0 < 300 Copper, Total mg/l Cu 0 < < 0.5 Iron, Total mg/l Fe 0 < < 1.0 Heavy Metals mg/l Fe+Mn+Cu+V+Pb 0 < 0.2 total 0 < 1.0 total Manganese, Total mg/l Mn 0 < < 0.2 Specifications Primarily For Protection of Evaporative Cooler Oil & Grease mg/l 0 < < 10 Silica mg/l SiO < 100 Solids, Total Suspended mg/l 0 < 5 0 < 30 Conductivity, Specific µs/cm minimum 578 < 5000 Solids, Total Dissolved mg/l ions minimum 403 < 3000 Alkalinity, M (Total) mg/l CaCO minimum 250 < 500 Calcium mg/l CaCO minimum 250 < 500 ph Calcite Indices: Langelier Saturation Index (LSI)

8 Off-Spec Result Table 2 Case 1 Results Ideal Makeup Water (Calcium Bicarbonate in Pure H 2O) Makeup Water at 77 F Circulating Water at 5 COC and 77 F Case 1 Spec. Case 1 Spec. Ryznar Stability Index (RSI) Puckorius Scaling Index (PSI) As can be seen, there is only one off-spec result, LSI, but, as mentioned earlier, LSI is overly conservative and is therefore superseded by the RSI and PSI. The COC of 5 will result in the following amounts of blowdown and makeup water that are derived from the standard evaporation rate of 50 gpm used in these case studies: Evaporation: 50 gpm 5 COC: 12.5 gpm 5 COC: 62.5 gpm One of the issues with the specifications is illustrated by Case 1. In order for the circulating water calcite indices to be within their control ranges, the value of the calcite indices for the makeup water must be well into the corrosive water range (greater than 7). Thus, rapid corrosion of carbon steel and galvanized steel makeup water piping would be expected. This example shows that the theoretical maximum cycles of concentration obtainable under the turbine manufacturer s guidelines is five as limited by the minimum ph of 7 and corresponding M Alkalinity at equilibrium. CASE 2: CITY OF DENVER DRINKING WATER FOR MAKEUP - Case 1 was based on an ideal situation and established the maximum COC that can be achieved in theory at five. In Case 2, a typical drinking water composition is used to illustrate the practical consequences that arise from higher-than-ideal alkalinity and calcium concentrations. Because of its relatively low TDS, calcium and alkalinity, drinking water is a preferred source of makeup water for evaporative coolers because it usually results in COC greater than 3. Table 3 shows the results of the cycle-down (from the theoretical maximum of five) that stops at 3.6 COC when compliance with the specifications is achieved. Off-Spec Result Table 3 Case 2 Results City of Denver Drinking Water Makeup Water Circulating Water at 3.6 COC Case 2 Spec. Case 2 Spec. Specifications Primarily For Protection of Combustion Turbine Alkali Metals mg/l Na < 550 Chloride mg/l Cl < 300 Copper, Total mg/l Cu < 0.05 < 0.5 Iron, Total mg/l Fe < 0.1 < 1.0 Heavy Metals mg/l Fe+Mn+Cu+V+Pb < 0.2 total < 1.0 total Manganese, Total mg/l Mn < 0.05 < 0.2 Specifications Primarily For Protection of Evaporative Cooler Oil & Grease mg/l < 2.0 < 10 Silica mg/l SiO < 100 Solids, Total Suspended mg/l < 5 < 30 Conductivity, Specific µs/cm minimum 1,220 < 5000 Solids, Total Dissolved mg/l ions minimum 853 < 3000 Alkalinity, M (Total) mg/l CaCO minimum 238 < 500 Calcium mg/l CaCO minimum 302 < 500 8

9 Off-Spec Result Table 3 Case 2 Results City of Denver Drinking Water Makeup Water Circulating Water at 3.6 COC Case 2 Spec. Case 2 Spec. ph Calcite Indices: Langelier Saturation Index (LSI) Ryznar Stability Index (RSI) Puckorius Scaling Index (PSI) The maximum COC for Case 2 is 3.6. This is less than the theoretical maximum COC of 5 from the previous example and reflects that as the calcium and alkalinity of the makeup water increase, the COC of the circulating water is forced downward. The COC of 3.6 will result in the following amounts of blowdown and makeup water: Evaporation: 50 gpm 3.6 COC: 19.2 gpm 3.6 COC: 69.2 gpm CASE 3: RECLAIMED WATER FOR MAKEUP -It can be concluded from Cases 1 and 2 that as the quality of the makeup water degrades relative to the ideal, COC must decrease to stay in compliance with the turbine manufacturer s guidelines. This point is further illustrated by Case 3, where the drinking water quality used for Case 2 is degraded as it would be after passing through a typical residential area and into the sewer system followed by processing in a municipal wastewater treatment plant to eventually become reclaimed water. Along the way, the water picks up TDS, as well as the objectionable individual constituent phosphate. Table 4 shows the results of the reduction in COC to comply with the specifications. Off-Spec Result Table 4 Case 3 Results City of Denver Reclaimed Water Makeup Water Circulating Water at 2.0 COC Case 3 Spec. Case 3 Spec. Specifications Primarily For Protection of Combustion Turbine Alkali Metals mg/l Na < 550 Chloride mg/l Cl < 300 Copper, Total mg/l Cu < 0.05 < 0.5 Iron, Total mg/l Fe < 0.1 < 1.0 Heavy Metals mg/l Fe+Mn+Cu+V+Pb < 0.2 total < 1.0 total Manganese, Total mg/l Mn < 0.05 < 0.2 Specifications Primarily For Protection of Evaporative Cooler Oil & Grease mg/l < 2.0 < 10 Silica mg/l SiO < 100 Solids, Total Suspended mg/l < 5 < 30 Conductivity, Specific µs/cm minimum 1,623 < 5000 Solids, Total Dissolved mg/l ions minimum 1,135 < 3000 Alkalinity, M (Total) mg/l CaCO minimum 267 < 500 Calcium mg/l CaCO minimum 244 < 500 ph Calcite Indices: Langelier Saturation Index (LSI) Ryznar Stability Index (RSI)

10 Off-Spec Result Table 4 Case 3 Results City of Denver Reclaimed Water Makeup Water Circulating Water at 2.0 COC Case 3 Spec. Case 3 Spec. Puckorius Scaling Index (PSI) The maximum COC for Case 3 is two. This is well below the theoretical maximum COC of 5 and is substantially less than the COC of 3.6 from the previous case using drinking water makeup. Again, the low COC are a reflection of the fact that as the calcium and alkalinity of the makeup water increase, the COC of the circulating water is forced downward to stay in compliance with the specifications. The COC of 2.0 will result in the following amounts of blowdown and makeup water: Evaporation: 50 gpm 2.0 COC: 50 gpm 2.0 COC: 100 gpm But what about the phosphate that is a common residual constituent of reclaimed water? Currently, the manufacturer s guidelines do not include specifications that would limit phosphate concentration, so for this case study, it is assumed that the phosphate concentration in the reclaimed water, 1.9 mg/l PO 4 (typical), would cycle-up to 3.8 mg/l PO 4 in the evaporative cooler unless precipitation of tricalcium phosphate would occur. To determine if precipitation of tricalcium phosphate will indeed occur, an equilibrium model was used to determine the saturation index of tricalcium phosphate. Results from the equilibrium model indicate that the water is supersaturated with tricalcium phosphate with a saturation ratio of 36. Saturation Ratio (SR) is defined as the ratio of ion activity product to solubility product constant. Thus, the SI of tricalcium phosphate is: Where: SR Ca3(PO4)2 = a Ca 3 x a PO4 2 Ksp Ca3(PO4)2 SI = Saturation Index a = activity, moles/liter Ksp = Solubility Product Constant This level of supersaturation assures that tricalcium phosphate will deposit on the evaporative cooler wetted parts and could lead to catastrophic consequences. 6.0 NEED FOR ADDITIONAL RESEARCH This paper has described several issues with the current evaporative cooler water chemistry guidelines that should be addressed. Targeted research in the areas described below would be very beneficial. Ultimately, the guidelines could be updated based on the research, and would be of benefit to turbine manufacturers, engineering design firms and the end-user. MEDIA LIFE EXPECTANCY AND CALCITE INDICES - In discussions with a manufacturer of the most commonly used evaporative cooler media, an expected media life of three to five years has been given as typical. On the other hand, the standard warranty period covering defects in the media is just one year. The cost of media replacement is approximately $50,000 for a 500 MW Power Plant. Therefore, the expected O&M cost of media replacement is about $10,000 - $17,000 per operating year. The authors have taken issue with use of LSI/RSI/PSI as surrogate parameters correlated to media life because the basis of these indices is to predict calcite precipitation or dissolution in water, and the media strengthening agent does not contain calcite, but typically contains a phenolic resin. Therefore, the use of calcite scaling indices to determine media life is brought into question, and additional research or supporting evidence from media manufacturers is needed to validate or nullify the correlation of media life to calcite indices. The advantage of such research would be the possible elimination or relaxation of the calcite indices control ranges that ultimately limit the maximum COC to 5. The use of 304 stainless steel rather than galvanized steel and yellow metals would alleviate the concern of corrosion to metallic structural and piping components. In parallel, the EC suppliers are encouraged to develop media that is not subject to the burdensome constraints noted in this paper. 10

11 PROPOSED NEW METHOD FOR PREDICTING MEDIA LIFE - It is well known that low-tds water such as condensate, reverse osmosis permeate and demineralized water have an almost universal tendency to dissolve susceptible substances, and this seems to be the case with the phenolic resin agent that gives the media its strength. But it also appears that the aggressiveness of water towards the strengthening agent is mitigated when the chemistry is controlled within the LSI/RSI/PSI limits. However, the authors believe that these calcite indices must be disassembled to discover what parts of these surrogate parameters truly apply. As part of the research effort, the authors would like to put forth the proposition that M Alkalinity and consequently ph are the two most important variables contained within the LSI/RSI/PSI that have a primary effect on the dissolution rate of the media strengthening agent (phenolic resin). If this is indeed the case, then it should be possible to develop a correlation of expected media life as a function of M Alkalinity/pH of the circulating water. The specified circulating water ph range typical of turbine manufacturer s guidelines is which corresponds to equilibrium M Alkalinities of mg/l as CaCO 3. However, the M Alkalinity upper limit from the guidelines is only 500 mg/l as CaCO 3 which would correspond to an equilibrium ph of approximately 8.5. In discussions with a leading manufacturer of evaporative cooler media, two cases were described to illustrate the impact of ph/alkalinity on media life. In the first case, the evaporative cooler had achieved three years of successful operation at a ph of 7.8 and alkalinity of 160 mg/l as CaCO 3. In the second case, the evaporative cooler was located in an industrial area, and was drawing in contaminated air that caused circulating water ph to drop to approximately 5. The media in this second case only lasted six months before sagging occurred. With these considerations in mind, it is apparent that the current water chemistry guidelines for evaporative cooler circulating water ph range may not provide adequate protection of the media due to the lower ph limit of 7.0. From Figure 5, this would only provide about 40 mg/l of alkalinity as a buffer against ph dropping below 7. It is therefore recommended that the circulating water ph range be changed to to maximize media life. The corresponding equilibrium M Alkalinity control range would then be mg/l as CaCO 3. case histories are needed that provide substantiated end-user experience with low-tds, low-calcium water as makeup to the evaporative coolers. To this end, the authors are currently developing two such case histories that have operated for 12 and 7 years on 100% demineralized water and 100% RO permeate respectively. It is planned to present these case histories at next year s IWC. One advantage of such documentation would be to encourage re-examination of the water quality guidelines that disallow the use of low-tds, lowcalcium water. ENABLE THE GUIDELINES FOR RECLAIMED WATER - It would be an obvious advantage to enhance the guidelines so that reclaimed water can be used in evaporative coolers with a minimum of risk. MAKE THE GUIDELINES EASIER TO USE, MORE CONSISTENT AND LESS AMBIGUOUS - The current guidelines are very restrictive and difficult to use. Spreadsheet and/or on-line tools should be developed to automatically perform calculations. The Langelier Saturation Index (LSI) is the most conservative of the scaling indices and is often out of range when the RSI and PSI are within range. Therefore it should be noted that the LSI is superseded by the RSI and PSI. Additional research resulting in an equal control range for the PSI and RSI is needed. There should be two sets of control ranges, one for evaporative coolers that contain galvanized steel and yellow metal components, and another less-restrictive set for evaporative coolers made of corrosion-resistant 304 stainless steel. ADD SULFATE SPECIFICATION TO PROTECT THE HOT GAS PATH The authors have recently encountered two cases where the issue of sulfur oxide buildup on the gas turbine blades has been a serious concern. However, the current guidelines do not impose a sulfate limit. Consideration should be given to including such a limit. ALLOW THE USE OF LOW-TDS, LOW-CALCIUM MAKEUP WATER - Well documented, long-term 11