DEGASSED CATION CONDUCTIVITY MEASUREMENT

Transcription

1 (Presented at EPRI's 8th International Conference on Cycle Chemistry in Fossil and Combined Cycle Plants with Heat Recovery Steam Generators - June 20-23, 2006 Calgary, Alberta Canada) DEGASSED CATION CONDUCTIVITY MEASUREMENT Otakar Jonas, P.E., PhD. Lee Machemer, P.E. Jonas, Inc Faun Road Wilmington, DE Abstract This paper discusses the measurement and interpretation of degassed cation conductivity (DCC). There are three designs of DCC instruments: reboiling before or after cation exchange and nitrogen sparging after the cation column at ambient temperature. Each of these designs yields different results. Reboiling before cation exchange only removes free CO 2. However, cation exchange converts all salts to acids, which are more volatile than salts, and reboiling after cation exchange removes most carbonates and portions of the volatile acids. In addition, the following instrument design and water chemistry factors influence the measured DCC: reboiling vs. sparging, purity of the sparging gas, intensity and length of degassing, removal of vapors, temperature compensation from ~100 C to 25 C (for reboiled DCC), effectiveness of cation exchange, and ph and type of alkalizing agents, salts, and acids in the sample. The main question is about the meaning of the various DCC measurements in relation to corrosion and flow-accelerated corrosion. Introduction Specific conductivity (SC) and cation conductivity (CC) are core parameters for monitoring cycle chemistry. The instruments are accurate, reliable, and inexpensive. SC is a measure of the concentration of a mix of unknown dissolved chemical species, usually dominated by ammonia or amines. CC measures a mix of anions and H + after removal by ion exchange of all (most?) cations and converting salts to the corresponding acids. In an effort to expand the range of cycle chemistry monitoring and get information on the concentration of CO 2, use of degassed cation conductivity (DCC) was patented in 1958 [1]. Initially, there were a few users of this method but its use has increased during the last decade, mostly because of the concerns about FAC, expanded use of organic chemicals that decompose forming CO 2, and in an effort to meet turbine steam cation conductivity limits by substituting DCC for CC. 1

2 Most users of DCC, including the authors, had been under the impression that the DCC instruments remove only the free CO 2, a less corrosive chemical species in steam, condensate, and feedwater. DCC was included as a monitoring parameter in the first comprehensive U.S. cycle chemistry guidelines [2]. After the extensive monitoring project [3], where DCC exhibited poor accuracy (Figure 1) and there was a suspected removal of roughly 1/2 to 2/3 of formic and acetic acids, it was not recommended in the guidelines that followed [4-7]. At the time of the monitoring project [3], the instrument was tested for retention of HCl and HF, but not for organic acids. Figure 1 Degassed Cation Conductivity; Measured vs. Calibration Solutions Supplied from Standard Sample Synthesizer - Six Mixtures Representing Typical Power Plant Water and Steam [3] The use of degassed cation conductivity has become popular in many combined cycle plants due to their inability to meet manufacturer turbine steam cation conductivity limits. Instead, they apply the same limits to degassed cation conductivity in an effort to ignore the effects of high air inleakage, poor makeup water purity (aerated), and organic water treatment chemical decomposition products (carbon dioxide and organic acids) on cycle chemistry control. Cation conductivity is a critical control parameter for modern water and steam systems [2-7]. The cation conductivity of the superheated or reheated steam has a recommended normal limit of between 0.15 and 0.35 µs/cm, depending upon the steam cycle design and selected water treatment [2-7]. Carbon dioxide (Figure 2) and organic acids (Figure 3) often significantly contribute to the measured cation conductivity, sometimes resulting in cation conductivity readings above limits even when the concentrations of mineral acids are low. There are several sources of carbon dioxide in the boiler water and condensate including makeup water (and 2

3 aerated makeup storage tanks), air inleakage, decomposition of carbonates in the boiler, and decomposition of organic compounds [8]. Figure 2 Contribution of Carbon Dioxide to Cation Conductivity [9] Figure 3 Contributions of Chloride, Sulfate, and Organic Acids to Cation Conductivity [9] 3

4 Degassed cation conductivity is the cation conductivity of a solution after the carbon dioxide has been removed either by sparging the solution with a non-reactive gas (typically nitrogen) or by heating close to boiling at ambient pressure. The measurement is supposed to provide an indication of the concentration of corrosive salts and acids in the sampled stream without the influence of carbon dioxide. Degassed cation conductivity is commonly measured on superheated steam samples, however, it can also be applied to saturated steam, condensate, and feedwater sampled after the deaerator. An example of field CC and DCC data is in Figure 4. The data for two superheated steams (HP and LP) are from commissioning of a three drum combined cycle unit using phosphate boiler water treatment with deaeration only in the condenser. As can be seen, there is a large difference between CC and DCC, particularly for the LP steam. DCC Instruments Currently, there are several possible arrangements and two methods commonly used for removing carbon dioxide (degassing) in order to measure degassed cation conductivity: Reboiling the sample is heated to 100ºC after the cation exchange column to remove volatile species, sometimes followed by cooling of the sample for analysis [12, 14 to 19] Sparging the sample is nitrogen sparged after the cation exchange column to remove volatile species [20] The instrument arrangements include: Reboiler or sparger before or after cation exchanger Sample cooler after reboiler before DCC measurement at ambient temperature No sample cooler after reboiler, DCC measurement at ~100 C One example of a DCC flow diagram for degassing after a cation column is shown in Figure 5. An instrument for monitoring degassed cation conductivity was patented by Larson and Lane in 1958 [1] and has been utilized to determine the purity of steam and condensate after removal of ammonia and carbon dioxide. The Larson and Lane patent includes the use of the ion exchange bed without pre-boiling of the condensate, and the other of which provides for re-boiling of the condensate to remove a major portion of the carbon dioxide before the ion exchange treatment to remove ammonia and/or amines. Re-boiling following the ion exchange treatment is provided in the lower unit to reduce the carbon dioxide contact to a minimum. 4

5 Figure 4 Cation Conductivity and Degassed Cation Conductivity for HP and LP Superheated Steam. Commissioning of a 3 Pressure Combined Cycle Unit (Courtesy of Jonas, Inc.) 5

6 Figure 5 Flow Diagram for One Type of DCC Instrument [12] Reboiling Heating the sample to its boiling point increases the volatility of some species, causing them to enter the vapor phase and be removed from the sample. The typical reboiler design has the sample flow into the reboiler, where it is heated to slightly below 100ºC for a short time period, then exits the reboiler and the conductivity of the solution is measured. The residence time in the reboiler is designed to allow enough time for significant removal of volatile species while maintaining a relatively fast response time to changes in sample chemistry. 6

7 Sparging When a sample is sparged with a non-reactive gas (such as nitrogen), the partial pressure of the volatile impurities causes a fraction of the impurity to diffuse into the nitrogen bubbles to achieve equilibrium. As more nitrogen comes in contact with the liquid, more of the volatile impurities are removed. In the case of carbon dioxide, when it is dissolved in water, a fraction of it forms carbonate, a weak acid that increases the conductivity. However, when the nitrogen removes the dissolved carbon dioxide in the water, the carbonate converts to carbon dioxide in an effort to maintain equilibrium. This results in more carbon dioxide being removed, until all of the carbon dioxide in the water is eliminated. As long as sufficient contact time is provided, complete removal of the carbon dioxide can be achieved. Differences between Instruments and Causes of Errors The main difference in DCC measurements is between the instruments with degassing before the cation column and after. This difference depends on ph which determines the concentration of CO 2 gas. The second difference is between the degassing at close to 100 C and at ambient temperature (Figure 6). Gas stripping has a higher CO 2 removal efficiency than reboiling [15]. Figure 6 Removal of CO 2 vs. ph [21] 7

8 The following are causes of errors and other differences: Removal of organic acids by degassing [3] open cycle vs. volatility. An example of a removal of other volatile acids besides the carbonic acid is shown in Table 1, where the DCC is lower than that predicted by subtraction of the CO2 contribution. The data are from a combined cycle unit using an organic oxygen scavenger where the volatile acids removed were probably formic and acetic acids. Incomplete removal of CO 2 removal depends on ph (Figure 6) Incomplete cation exchange amines, oxygen scavengers Intensity and length of reboiling and sparging Rate of vapor removal Purity of the sparging gas (NO X, SO 2, CO 2, organics) Temperature compensation from ~100 C to 25 C vs. chemical species present (see Figure 7 for the large conductivity difference which needs to be compensated for) In some DCC designs [16, 17, 18] that include a reboiler, the degassed cation conductivity is typically measured at close to 100ºC to eliminate the cost of an additional sample cooler. Computerized temperature compensation is then used to adjust the measurement to the equivalent conductivity at 25ºC [13]. The temperature compensation used is independent of the impurities present, which makes the compensation susceptible to errors. In order to avoid these problems, the conductivity should be measured at 25ºC. Table 1 CC and DCC for Three Samples Obtained from a Combined Cycle Plant (Courtesy of Jonas, Inc.) Superheated Steam Cation Conductivity (µs/cm) Degassed Cation Conductivity (µs/cm) Cation Conductivity after Subtraction of CO 2 Contribution (µs/cm) LP IP HP

9 Figure 7 Temperature Effects on the Conductivity of High Purity Water [10] Conclusions 1. Degassed cation conductivity is an interesting but controversial steam and condensate monitoring parameter. Different instruments give different values which cannot be practically interpreted, the instruments are difficult to calibrate, and the measured values are non-conservative in relation to turbine corrosion and feedwater and extraction piping flowaccelerated corrosion where CO 2 and organic acids can play a major role. 2. There are two different designs of degassed cation conductivity analyzers: one with the degassing (reboiling or sparging) before the cation exchanger and one with degassing after. Degassing after the cation exchanger is not recommended because it removes an unknown mix of acids and the resulting degassed cation conductivity information cannot be properly interpreted. Also, the conductivity measured at ~100 C and compensation to 25 C can introduce errors. 3. To monitor the cation conductivity of samples without CO 2, the degassing should be before the cation exchange, as originally intended by the inventors of the method. Depending on sample ph, this would leave some CO 2 in the sample as carbonate and bi-carbonate, such as in the actual condensate. In saturated and wet steam, CO 2 and carbonates will be distributed between the gas and liquid phases. 9

10 4. The actual removal of chemical species in the degasser depends on their volatility, time and intensity of degassing, temperature (100 C vs. ambient), removal of the vapor, and the cation exchanger. In instruments with N 2 sparging, impurities, such as NOx, CO 2, SO 2, can be introduced. The volatility depends on the chemical equilibrium in the liquid phase. About 7% of acetate could be removed by reboiling. In cases where amines and certain organic oxygen scavengers are used, their ion exchange in the cation column may not be complete, introducing additional errors. 5. In relation to the purpose of the degassed cation conductivity measurement to monitor only corrosive impurities, such as chlorides and sulfates, degassed cation conductivity is a nonconservative parameter because, at lower temperatures, CO 2, carbonates, and organic acids are also corrosive. Their corrosive effects include stress corrosion cracking of carbon and low alloy steels and enhancement of flow-accelerated corrosion by lowering ph. 6. In monitoring condensate and feedwater in most plants, free CO 2 is already removed by condenser and deaerator deaeration. References 1. T. Larson, R. Lane. Apparatus and Method for Determining Steam Purity. U.S. Patent 2,832,673. April 29, Interim Consensus Guidelines on Fossil Plant Chemistry. EPRI, Palo Alto, CA: June CS Monitoring Cycle Water Chemistry in Fossil Plants: Volume 1. EPRI. Palo Alto, CA: October GS Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment. EPRI, Palo Alto, CA: April TR Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: December TR Cycle Chemistry Guidelines for Fossil Plants: Phosphate Treatment for Drum Units. EPRI, Palo Alto, CA: December TR Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: February O. Jonas. "Beware of Organic Impurities in Steam Power Systems." Power. Sept PowerPlant Chemistry Practice 006 and 007. PowerPlant Chemistry. January (1). 10. D. Gray. Advances in Cycle Chemistry Conductivity Measurement. PowerPlant Chemistry. June (6). 10

11 11. A. Bursik. Carbon Dioxide and Fossil Plant Cycle Chemistry. Proceedings of International Water Conference. Pittsburgh, PA IWC "Standard Test Method for On-line Determination of Anions and Carbon Dioxide in High Purity Water by Cation Exchange and Degassed Cation Conductivity." Annual Book of ASTM Standards. ASTM. New York. Volume D Standard Test Method for Electrical Conductivity and Resistivity of a Flowing High Purity Water Sample. Annual Book of ASTM Standards. ASTM. New York. Volume D R.W. Lane. Cation and Degassed Conductivity Sentry Equipment Leaflet. March N. Drew. Evaluation of Degassed After-Cation-Exchange Conductivity Techniques. PowerPlant Chemistry. June (6). 16. High Purity Condensate Monitoring. Application Data Sheet, Rosemount Analytical. ADS /Rev.B. August Condensate Analysis. Product Application Data. Foxboro Company. PAD P Martek Dissolved Carbon Dioxide Analyzer. Product Leaflet. Martek Instruments DCCP: Degassed Cation Conductivity Panel. Product Leaflet. Sentry Equipment Rev. 5. April Sparger Assembly. Product Leaflet. Waters Equipment Betz Handbook of Industrial Water Conditioning. Betz Laboratories, Inc. Trevose, PA O. Jonas. Steam, Chemistry, and Corrosion in the Phase Transition Zone of Steam Turbines. EPRI. Palo Alto, CA: February TR