Cooling Testing and Control By Dave Christophersen, CWT Originally Published: CSCN November 2006 With the extremely high cost of molybdenum in recent years, its use as a corrosion inhibitor or tracing agent in cooling water products, where product consumption is significant, has become essentially cost prohibitive. Other corrosion inhibitors such as phosphates, zinc, silicates, and organophosphorous compounds are now used largely in the absence of molybdates. Also, the use of molybdenum has been restricted in some areas because of environmental concerns, mostly centered around concentration limitations in municipally generated sludges. Phosphates Where orthophosphate or polyphosphates are in use, testing for the phosphate is a good and accurate test. There are a number of phosphate procedures, but all tests determine orthophosphate. Other forms of phosphate such as polyphosphate or organo-phosphates must first be converted to orthophosphate to determine their concentrations with a phosphate test procedure. Control can become more complicated when there is phosphate in the makeup water. The form of the phosphate (orthophosphate, polyphosphate, or both) and the concentration range needs to be known so that it is accounted for in the cycled cooling water. Cooling Towers 248
Example Makeup water contains 0.5 ppm of orthophosphate and 0.4 ppm of polyphosphate as PO 4. The cooling tower is operated at five cycles of concentration and a cooling water product that contains 4 % of orthophosphate is being applied. The desired inhibitor product dosage is 100 ppm. At five cycles, there will be 2.5 ppm of orthophosphate from the makeup water orthophosphate, and 2.0 ppm of polyphosphate applied from the makeup water, but some of it will have reverted to orthophosphate. You should test for polyphosphate in the tower water initially and then periodically to determine the reversion rate for your system. Typically, we assume about a 50% reversion rate. The actual reversion rate will depend upon ph and retention time, and the specific type of polyphosphate. If when tested the polyphosphate showed to be 1 ppm in the cycled tower water, then the total orthophosphate from the makeup would be 3.5 ppm. 100 ppm of the inhibitor product would add 4 ppm orthophosphate, so a tested residual of 7.5 ppm or orthophosphate would indicate that 100 ppm of the product was in the system. Phosphate Concentrations Table 1: Phosphate Summary Orthophosphate (ppm) Polyphosphate (ppm) Makeup Water 0.5 0.4 as PO 4 Tower Water, 5 Cycles Before Reversion Tower Water, 5 Cycles After Reversion Orthophosphate From Total in Cycled Tower Water 2.5 2.0 3.5 1.0 4.0 7.5 1.0 Phosphonates Most all cooling tower products contain one or more phosphonates that are used for scale inhibition, corrosion inhibition, or both. Phosphonate testing is not as accurate as phosphate testing, but they can be used for controlling product feed. Phosphonates are subject to oxidation to orthophosphate by Cooling Towers 249
chlorine or bromine and are lost to precipitation with cations such as calcium. If the system is chlorinated or brominated, assume a 20 30% degradation to phosphate. The actual amount can be determined by testing for residual phosphonates and phosphate. There are several phosphonates tests that can be used: Hach UV digestion, then phosphate test. Boiling with acid and persulfate, followed by phosphate test. Palintest drop test. Taylor drop test. UV Digestion The Hach test procedure is the most accurate and has a reproducibility of about ± 10%. A persulfate reagent is used along with a UV light to decompose the organo-phosphate (phosphonate) to orthophosphate. An orthophosphate test such as the Hach PhosVer3 procedure then determines the amount of phosphate contributed by the phosphonates. Any orthophosphate already present before the digestion is subtracted from the total orthophosphate after digestion. This can be done by adding PhosVer3 reagent to the tower water that has not had the digestion and use this as the blank, or actually determine orthophosphate in the tower water and subtract it from the total orthophosphate determined after the persulfate digestion. The amount of phosphorus in each specific phosphonate molecule varies, so there is a specific conversion factor from orthophosphate to phosphonate. Each ppm of orthophosphate created by HEDP digestion = 1.085 ppm HEDP. The phosphorus content of PBTC is much lower. Each ppm of orthophosphate created from the digestion of PBTC = 2.84 ppm of the PBTC molecule. Hach Phosphonate Test Example: The cycled tower water has 6 ppm orthophosphate and a cooling water product that contains 2.5% PBTC and 1.8% HEDP is being applied at a desired dosage of 120 ppm. Assuming all of the phosphonates remain as phosphonates and have not been oxidized in the cooling tower by bromine or chlorine and assuming it has not been lost to precipitation, you should get 3.05 ppm of orthophosphate from the phosphonates after a persulfate / UV digestion. Cooling Towers 250
Table 2: Phosphate Summary From PBTC: 120 ppm x 2.5% = 3 ppm 3 ppm PBTC 2.84 ppm PBTC per ppm PO 4 = From HEDP: 120 ppm x 1.8% = 2.16 ppm 2.16 HEDP 1.085 ppm HEDP per ppm PO 4 = From orthophosphate in the tower water: Total orthophosphate in sample after digestion: Orthophosphate from phosphonate digestion: 1.06 ppm orthophosphate 1.99 ppm orthophosphate 6 ppm 9.05 ppm 3.05 ppm Boiling With Acid and Persulfate A digestion can also be accomplished by adding acid and persulfate, then boiling for about 30 minutes. If just acid were used, only polyphosphate would be hydrolyzed or reverted to orthophosphate. It persulfate is also added, the organo-phosphates and polyphosphates will be digested to orthophosphate. This test would be more applicable for samples that do not have polyphosphates, since the test will not distinguish between orthophosphate developed from phosphonates or polyphosphates. Phosphonate Drop Counts CROWN recommends the Palintest procedure, but either the Palintest or Taylor drop count test for phosphonates can be used. These procedures are less accurate and subject to interferences. It is best to determine the number of drops on a known product concentration and relate the number of drops to that concentration. It is advisable to also compare these results initially and periodically to the Hach UV digestion method. The Taylor procedure is based upon the procedure Monsanto created in the 1960 s for their Dequest 2000 AMP and Dequest 2010 HEDP products. The procedure drops the ph to around 2.0, but PBTC does not get picked up well at this low of ph. Where PBTC is in use, the Palintest method is preferred. The procedure buffers the ph to around 3.0 and is more effective at detecting the PBTC along with the HEDP and AMP. On the Palintest method, each 0.7 ppm of HEDP or AMP in the water should require one drop of titrant, and each 2.0 ppm of PBTC should require one drop. Polyphosphate and some organics will interfere with the test and show up as phosphonates. To account for this, a blank is run on the makeup water. If it Cooling Towers 251
takes two drops for the color change on the blank, then those two drops are subtracted from the test results of the treated water. Note that the blank results are not cycled up by the tower cycles. Polyphosphates revert to orthophosphate which does not interfere and experience has shown that cycling the blank should not be done. If the product contains polyphosphate and a residual in the cycled water, it will increase the number of drops required. If fluorides are in the cycled tested water at > 1.0 ppm, this causes a substantial interference that may disqualify the drop test procedure from being usable. It is advisable to check with the city supplier to see if they add fluorides and at what level. If high fluorides are present, an idea that may work is to first run the drop test procedure on the tower water to get a baseline number. Then take a sample of the cycled tower water and add 100 ppm of product and see how many drops are required. Subtract the number of drops used for the baseline from the drops required for the 100 ppm sample to determine how many drops represent 100 ppm of product as a basis for setting control limits. The Palintest end point is the drop when the color change from green/gray to blue/purple first occurs. Palintest Phosphonate Drop Count Example: The cooling water is treated with 140 ppm of a product that contains 2.5% PBTC and 1.8% HEDP. The product has a specific gravity of 1.16. There is no fluoride in the water. First, determine the interferences in the makeup water by running the test procedure on an untreated sample. On this example assume it took two drops. Next, make a 100 ppm solution. To do this add 1 gram or 0.86 ml (1mL/1.16 gram/ml) of the chemical product to 99 grams (99 ml) of makeup water. Mix this up well, then add 1 gram (1 ml) of this 1% solution to 99 grams (99 mls) of makeup water. This is now a 0.01% solution or 100 ppm of the product. This would place 1.8 ppm of HEDP and 2.5 ppm of PBTC in the solution. Run the phosphonates test on this solution, and for this example it required the theoretical number of drops of about 6. Cooling Towers 252
Table 3: Theoretical Phosphonate Titrant Usage From HEDP: 1.8 ppm 0.7 ppm HEDP / Drop From PBTC: 2.5 ppm 2.0 ppm PBTC / Drop From Blank: Total Drops: 2.5 Drops 1.25 Drops 2 Drops 5.75 drops, which will require 6 drops to see the color change. 140 ppm of product would be about (140 100) x 4 drops = 5.6 drops or 5-6 drops + 2 drops for the blank = 8 drops. This can be confirmed by making a 140 ppm solution and testing it. Azole, Zinc, or Silica Tests The Hach test procedures for azole, zinc, or silica can be used to check product dosage if the specific ingredient is in the applied product. Remember, as with phosphonates, the applied concentrations and actual residuals can be different. Azole residuals decrease as they film with copper. Zinc is lost as it precipitates at the cathode or in the bulk water. Silica is lost as it films metal surfaces. In establishing control ranges and dosages, take into account some of this loss. For example, we may apply azole at 2 ppm, but have a desired residual in the water of only 1 ppm. Mass Balance Chemical dosages should be confirmed by mass balances and compared to chemical testing. Mechanisms should be set up on each system to conveniently determine water makeup, cycles, water loss, and chemical consumption. The concentration in the recirculating water should be calculated from the actual product usage and blowdown or water loss. Mass Balance Example: The cooling tower is operating at five cycles of concentration. The makeup meter shows 120,000 gpd makeup. At five cycles, this is a water loss of 24,000 gpd. The product being fed contains 1.8% HEDP, 2.5% PBTC, 1.5% BZT, and 1% zinc; and the desired dosage is 100 ppm. Daily product use determined by drum level and confirmed with drawdown cylinder testing is 28 lbs per day. This is a calculated applied dosage of 140 ppm of product in the cycled cooling tower water (140/120 x 24,000/1000 = 28 lbs). Cooling Towers 253
Chemical testing showed 4 drops of phosphonates (6 drops from the test 2 drops for the blank), which was previously determined to represent 100 ppm product. Testing also revealed 1.5 ppm BZT and 0.8 ppm of zinc residuals in the water. All of the chemical tests show that some portion of the active component has been consumed or residuals would have been higher at 140 ppm of applied product. Component Expected Residuals with No Loss When Applied at 140 ppm Calculated Dosage Based on Actual Residual Phosphonate 8 drops 6 drops = 100 ppm BZT 2.1 ppm 1.5 ppm BZT = 100 ppm Zinc 1.4 ppm 0.8 ppm Zinc = 80 ppm Loss to System Reactions 40 ppm 40 ppm product 60 ppm Conclusion Mass balance is the most accurate way to determine applied dosage. If the product dosage was projected to be effective at 100 ppm, it is likely that this product is being overfed by 40%. Chemical testing suggests that there is more than sufficient residual of active components even after some loss to the system, so product dosage can be lowered and results monitored to confirm that desired results are maintained. There is expected to be some loss of active components as they react with the materials in the system and the impurities in the water. Where molybdate is used or has been used as a monitoring method for product control and consumption, generally its loss to the system is minimal. That means that if the product shown above contained 1% molybdate as Mo, Cooling Towers 254
it is likely that the test results would have been very close to 1.4 ppm Mo and the product dosage would have been decreased to 100 ppm to lower Mo to 1.0 ppm. Molybdate used as a tracer, then, would commonly yield a lower product usage rate because the other active components would not ordinarily be used to control the dosage. Cooling Towers 255