Fossil Plant Cycle Chemistry

Effective December 6, 2006, this report has been made publicly available in accordance with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. Export Administration Regulations. As a result of this publication, this report is subject to only copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.

Fossil Plant Cycle Chemistry Instrumentation and Control— State-of-Knowledge Assessment SED

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Fossil Plant Cycle Chemistry Instrumentation and Control— State-of-Knowledge Assessment 1012209

Final Report, March 2007

EPRI Project Manager K. Shields

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 • PO Box 10412, Palo Alto, California 94303-0813 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Electric Power Research Institute (EPRI) Scientech, LLC

NOTICE: THIS REPORT CONTAINS PROPRIETARY INFORMATION THAT IS THE INTELLECTUAL PROPERTY OF EPRI. ACCORDINGLY, IT IS AVAILABLE ONLY UNDER LICENSE FROM EPRI AND MAY NOT BE REPRODUCED OR DISCLOSED, WHOLLY OR IN PART, BY ANY LICENSEE TO ANY OTHER PERSON OR ORGANIZATION.

NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail [email protected]. Electric Power Research Institute, EPRI, and TOGETHER…SHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc. Copyright © 2007 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS This report was prepared by Electric Power Research Institute (EPRI) 3420 Hillview Avenue Palo Alto, CA 94304 Principal Investigators K. Shields B. Syrett Scientech, LLC 2650 McCormick Drive Suite 300 Clearwater, FL 33759 Principal Investigators D. Meils J. Witherow This report describes research sponsored by the EPRI. The report is a corporate document that should be cited in the literature in the following manner: Fossil Plant Cycle Chemistry Instrumentation and Control—State-of-Knowledge Assessment. EPRI, Palo Alto, CA: 2007. 1012209.

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PRODUCT DESCRIPTION

Effective monitoring of the purity of water and steam is an integral part of a productive cycle chemistry monitoring program. EPRI’s cycle chemistry guidelines for fossil plants identify a group of core monitoring parameters that are considered the minimum requirements. Meeting the core monitoring requirement is part of EPRI’s cycle chemistry benchmarking criteria for plant cycle chemistry programs. In addition to the core parameters, many other chemistry parameters may be measured—either routinely or as needed for diagnostic and troubleshooting purposes. On-line monitoring of cycle chemistry is preferable to grab sample analysis. The EPRI report Reference Manual for On-Line Monitoring of Water Chemistry and Corrosion: 1998 Update (TR-112024) marked the last time this subject had been addressed. That state-of-art assessment was identified as the initial activity in a new project that considers both the need and opportunity for advancements in monitoring technology. Results and Findings This report describes the available technology options for monitoring a number of cycle chemistry parameters. The main focus is on methods available and in common use at fossil stations; however, information is also provided on a number of recently developed techniques as well as others that show promise. The report also considers techniques to monitor electrochemical corrosion potential and corrosion rate—two parameters not generally monitored in fossil plants and for which further development could lead to improved monitoring tools for fossil plants in the future. Challenges and Objectives The real-time monitoring of cycle chemistry supports operator oversight of water and steam purity, minimizing the time needed to 1) identify out-of-specification chemistry conditions and 2) implement appropriate corrective actions. It also provides a warning of plant equipment malfunction, facilitates the control of chemical additions, optimizes maintenance and repair schedules, and improves corrosion control. Fossil plant personnel involved in the evaluation, selection, operation, and maintenance of chemistry analyzers will find the information in this report useful. Information addressing the individual EPRI core monitoring parameters should be of particular value to users whose plants do not currently meet all of the core requirements. Applications, Value, and Use Despite the many advances in fossil plant cycle chemistry monitoring that have been achieved over the last 30 years, the current techniques have their limitations. In addition, resources needed to obtain, install, operate, and maintain analyzers to provide the information needed to control cycle chemistry are often an area of concern. Familiarity with the technology involved is therefore important when limited resources are invested in chemistry surveillance and control. v

EPRI Perspective Chemistry analyzers now in use require that samples be collected and conditioned. Information provided by the analyzers is useful for assessing conditions under which the potential exists for corrosion or other chemistry-related damage to cycle components. However, this information is indirect in that it does not determine the presence or magnitude (that is, rate) of corrosion. Research efforts are now investigating corrosion in boilers and turbines to enable a better understanding of its mechanisms and root causes in order to further the development of cycle chemistry guidelines. These future guidelines could likely be more effectively implemented if improved monitoring techniques were available. Therefore, investigations are planned to identify and develop monitoring techniques for the direct measurement of chemistry environments in which corrosion or deposition activity occurs. Approach The project team collected information from various sources for this report. These included the published literature, instrument manufacturer personnel and materials, consultants knowledgeable in chemistry instrumentation, the Internet, and relevant EPRI reports. Keywords Cycle chemistry Core monitoring parameter Instrumentation Analyzer Corrosion Surveillance

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CONTENTS

1 CYCLE CHEMISTRY MONITORING IN FOSSIL PLANT CYCLES ......................................1-1 1.1 Introduction ....................................................................................................................1-1 1.2 Monitoring Requirements and Choice of Cycle Chemistry.............................................1-1 1.3 Cycle Chemistry Monitoring Approaches .......................................................................1-3 1.3.1 Core Monitoring Parameters ..................................................................................1-3 1.3.2 Diagnostic Monitoring Parameters .........................................................................1-4 1.3 Future Cycle Chemistry Guidelines and Monitoring Approaches...................................1-5 1.4 References .....................................................................................................................1-7 2 AIR IN-LEAKAGE ..................................................................................................................2-1 2.1 Effects of Air In-leakage on Unit Performance and Cycle Chemistry .............................2-1 2.2 Monitoring Methods........................................................................................................2-2 2.2.1 Rotameters .............................................................................................................2-2 2.2.2 Early Advanced Air Removal Monitoring Systems .................................................2-3 2.2.3 Multisensor Probe Design ......................................................................................2-3 2.3 References .....................................................................................................................2-7 3 CARRYOVER IN DRUM BOILERS........................................................................................3-1 3.1 Introduction to Carryover................................................................................................3-1 3.2 Consideration of Carryover in EPRI Cycle Chemistry Guidelines ..................................3-2 3.3 Determination of Total Carryover from Drum Boilers .....................................................3-4 3.4 Sampling and Data Collection Considerations ...............................................................3-4 3.5 References .....................................................................................................................3-5 4 CONDUCTIVITY .....................................................................................................................4-1 4.1 Purpose and Use ...........................................................................................................4-1 4.2 Description .....................................................................................................................4-1 4.3 Technical Considerations ...............................................................................................4-9

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4.3.1 Cell Constants ........................................................................................................4-9 4.3.2 Cell Constant Determination or Verification..........................................................4-10 4.3.3 Cell Construction and Installation Considerations ................................................4-10 4.3.4 Cell Polarization....................................................................................................4-11 4.3.5 Temperature Effects .............................................................................................4-11 4.3.5.1 Solute Effects................................................................................................4-12 4.3.5.2 Solvent Effects ..............................................................................................4-12 4.3.6 Cation and Degassed Cation Conductivity Temperature Compensation .............4-13 4.3.7 Cation Conductivity Column Connections, Size, Flow and Flow Rate Considerations................................................................................................................4-13 4.3.8 Cation Conductivity Resin Exhaustion, Regeneration and Rinse-in .....................4-13 4.4 Interferences ................................................................................................................4-14 4.4.1 Organic and Strong Acids Interferences...............................................................4-14 4.4.2 Sample Line Interference .....................................................................................4-14 4.5 Calibration ....................................................................................................................4-15 4.6 Calibration Checks .......................................................................................................4-15 4.7 Alternative Methods for Determining Conductivity .......................................................4-15 4.4 End User Considerations .............................................................................................4-21 4.9 References ...................................................................................................................4-21 5 OXYGEN.................................................................................................................................5-1 5.1 Purpose and Use ...........................................................................................................5-1 5.2 Description of Methods...................................................................................................5-1 5.2.1 Galvanic Method.....................................................................................................5-2 5.2.2 Polarographic Method ............................................................................................5-3 5.2.3 Equilibrium Method.................................................................................................5-4 5.3 Technical Considerations ...............................................................................................5-5 5.3.1 Membrane Replacement ........................................................................................5-5 5.3.2 Electrode Cleaning .................................................................................................5-5 5.3.2.1 Polarographic..................................................................................................5-5 5.3.2.2 Galvanic ..........................................................................................................5-6 5.3.2.3 Equilibrium ......................................................................................................5-6 5.3.3 Temperature and Pressure Compensation for Sensors .........................................5-6 5.3.4 Flow Rate Sensitivity ..............................................................................................5-8

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5.3.5 Response Time ......................................................................................................5-8 5.3.5.1 Response Time After Air Calibration...............................................................5-8 5.3.5.2 Response Time Due to Changes in Dissolved Oxygen Concentration in the Process Stream .....................................................................................................5-9 5.4 Interferences ..................................................................................................................5-9 5.4.1 Oxygen Contamination ...........................................................................................5-9 5.4.2 Sample Conditioning ............................................................................................5-10 5.4.3 Electrolyte.............................................................................................................5-10 5.4.4 Stray Current ........................................................................................................5-10 5.4.5 Membrane Fouling, Positioning and Tension .......................................................5-10 5.4.6 Hydrogen ..............................................................................................................5-10 5.4.7 Other Cycle Chemistry Additives..........................................................................5-11 5.4.8 TDS ......................................................................................................................5-11 5.5 Calibration ....................................................................................................................5-11 5.5.1 Polarographic .......................................................................................................5-11 5.5.2 Galvanic................................................................................................................5-11 5.5.3 Equilibrium............................................................................................................5-12 5.5.4 Air Calibration for All Sensor Types......................................................................5-12 5.6 Calibration Checks for All Sensor Types......................................................................5-13 5.6.1 Zero Point for All Sensor Types............................................................................5-14 5.6.2 Maintenance for All Sensor Types........................................................................5-15 5.7 Alternative Methods......................................................................................................5-15 5.7.1 Luminescent Oxygen Sensors [17].......................................................................5-15 5.7.1.1 µg·kg-1–Resolution Optical Sensor to Monitor Dissolved Oxygen ...............5-18 5.8 End User Considerations .............................................................................................5-19 5.9 References ...................................................................................................................5-20 6 OXIDATION-REDUCTION POTENTIAL ................................................................................6-1 6.1 Purpose and Use ...........................................................................................................6-1 6.2 Description of Method ....................................................................................................6-1 6.3 Technical Considerations ...............................................................................................6-4 6.3.1 Voltmeter Selection ................................................................................................6-4 6.3.2 Reference Electrodes .............................................................................................6-5 6.3.3 ORP Sensing Electrode..........................................................................................6-5

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6.4 Interferences ..................................................................................................................6-7 6.5 Calibration ......................................................................................................................6-7 6.5.1 Calibration Checks .................................................................................................6-9 6.6 End User Considerations ...............................................................................................6-9 6.7 References ...................................................................................................................6-10 7 pH ...........................................................................................................................................7-1 7.1 Purpose and Use ...........................................................................................................7-1 7.2 Description .....................................................................................................................7-1 7.3 Technical Considerations ...............................................................................................7-2 7.3.1 Sensing (Glass) Electrode......................................................................................7-2 7.3.2 Reference Electrode...............................................................................................7-3 7.3.3 Temperature Effects ...............................................................................................7-5 7.3.3.1 Electrode Effects.............................................................................................7-6 7.3.3.2 Solution Additive Effects .................................................................................7-7 7.4 Interferences ..................................................................................................................7-8 7.4.1 Interfering Ions........................................................................................................7-8 7.4.2 Interfering Stray Currents .......................................................................................7-8 7.5 Calibration ......................................................................................................................7-9 7.6 Calibration Checks .......................................................................................................7-10 7.7 Alternative Methods for Determining pH ......................................................................7-11 7.8 End User Considerations .............................................................................................7-14 7.9 References ...................................................................................................................7-15 8 SODIUM..................................................................................................................................8-1 8.1 Purpose and Use ...........................................................................................................8-1 8.2 Description of Method ....................................................................................................8-1 8.3 Technical Considerations ...............................................................................................8-4 8.3.1 Sensing Electrode ..................................................................................................8-4 8.3.2 Reference Electrode...............................................................................................8-7 8.4 Interferences ..................................................................................................................8-8 8.5 Calibration ......................................................................................................................8-9 8.6 Calibration Checks .......................................................................................................8-11 8.7 Alternative Methods......................................................................................................8-12

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8.8 End User Considerations .............................................................................................8-13 8.9 References ...................................................................................................................8-13 9 AMMONIA ..............................................................................................................................9-1 9.1 Purpose and Use ...........................................................................................................9-1 9.2 Description of Methods...................................................................................................9-2 9.2.1 Background ............................................................................................................9-2 9.2.2 Conductivity Approximation ....................................................................................9-3 9.2.2.1 Conductivity Approximation Limitations ..........................................................9-3 9.2.3 Ion Selective Electrode...........................................................................................9-4 9.2.4 Colorimetry .............................................................................................................9-5 9.3 Technical Considerations ...............................................................................................9-6 9.3.1 Ion Selective Electrodes (ISE)................................................................................9-6 9.3.1.1 Ammonium Sensing Electrode........................................................................9-6 9.3.1.2 Reference Electrode .......................................................................................9-6 9.3.2 Colorimetric Analyzers............................................................................................9-7 9.3.2.1 Colorimetric Limitations...................................................................................9-8 9.3.2.2 Sample Considerations.................................................................................9-11 9.3.2.3 Time Delay....................................................................................................9-11 9.3.2.4 Sample Temperature ....................................................................................9-11 9.3.2.5 Sample Volume ............................................................................................9-12 9.4 Interferences ................................................................................................................9-12 9.4.1 ISE Interferences..................................................................................................9-12 9.4.2 Colorimetric Interferences ....................................................................................9-13 9.5 Calibration ....................................................................................................................9-13 9.5.1 ISE Calibration......................................................................................................9-13 9.5.2 Colorimetric Calibration ........................................................................................9-14 9.6 Calibration Checks .......................................................................................................9-14 9.7 Alternative Methods......................................................................................................9-15 9.7.1 Direct Nesslerization.............................................................................................9-15 9.7.2 Titrimetric Ammonia Determination ......................................................................9-16 9.7.3 ISE – Gas Permeable Membrane.........................................................................9-16 9.8 End User Considerations .............................................................................................9-17 9.9 Field Experience...........................................................................................................9-17

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9.9.1 Sample Delivery ...................................................................................................9-17 9.9.2 Analyzer Malfunction ............................................................................................9-18 9.10 References .................................................................................................................9-18 10 CHLORIDE .........................................................................................................................10-1 10.1 Purpose and Use........................................................................................................10-1 10.2 Description of Method ................................................................................................10-2 10.3 Technical Considerations ...........................................................................................10-4 10.3.1 Sensing Electrode ..............................................................................................10-4 10.3.2 Reference Electrode...........................................................................................10-4 10.3.3 Temperature Considerations ..............................................................................10-5 10.4 Interferences ..............................................................................................................10-6 10.5 Calibration ..................................................................................................................10-6 10.6 Calibration Checks .....................................................................................................10-9 10.7 Alternative Methods ...................................................................................................10-9 10.7.1 ASTM D512 Test Method A—Mercurimetric Titration ........................................10-9 10.7.2 ASTM D512 Test Method B—Silver Nitrate Titration .........................................10-9 10.7.3 Standard Methods: Method 4500 - Cl D. Potentiometric Method [11] .............10-10 10.7.4 Standard Methods: Method 4500 - Cl E. Automated Ferricyanide Method [11]................................................................................................................................10-10 10.7.5 Ion Chromatography.........................................................................................10-10 10.8 End User Considerations .........................................................................................10-10 10.9 References ...............................................................................................................10-11 11 HYDRAZINE .......................................................................................................................11-1 11.1 Purpose and Use........................................................................................................11-1 11.2 Description of Methods...............................................................................................11-2 11.3 Technical Considerations ...........................................................................................11-2 11.3.1 Colorimetry .........................................................................................................11-2 11.3.1.1 Colorimetric Limitations...............................................................................11-3 11.3.2 Amperometry ......................................................................................................11-4 11.3.2.1 Two Electrode Method ................................................................................11-4 11.3.2.2 Three Electrode Method .............................................................................11-5 11.3.3 Iodide Ion Selective Electrode Method ...............................................................11-6 11.4 Interferences ..............................................................................................................11-7

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11.4.1 Colorimetry .........................................................................................................11-7 11.4.2 Amperometry ......................................................................................................11-8 11.4.3 Ion Selective Electrode Method..........................................................................11-8 11.5 Calibration ..................................................................................................................11-8 11.5.1 Colorimetry .........................................................................................................11-8 11.5.2 Amperometry ......................................................................................................11-9 11.5.3 Ion Selective Electrode Method..........................................................................11-9 11.6 Calibration Checks ...................................................................................................11-10 11.7 Alternative Methods .................................................................................................11-10 11.8 End User Considerations .........................................................................................11-11 11.8.1 Recognizing Instrument Malfunctions...............................................................11-12 11.8.1.1 Sample Delivery........................................................................................11-12 11.8.1.2 Analyzer Malfunction.................................................................................11-12 11.9 References ...............................................................................................................11-13 12 HYDROGEN .......................................................................................................................12-1 12.1 Purpose and Use........................................................................................................12-1 12.2 Discussion of Methods ...............................................................................................12-1 12.2.1 Electrochemical / Polarographic .........................................................................12-2 12.2.2 Thermal Conductivity..........................................................................................12-2 12.2.3 Gas Density Meter..............................................................................................12-2 12.3 Technical Considerations ...........................................................................................12-3 12.3.1 Electrochemical / Polarographic .........................................................................12-3 12.3.2 Thermal Conductivity..........................................................................................12-5 12.3.3 Gas Density Meter..............................................................................................12-8 12.4 Calibration ..................................................................................................................12-9 12.5 Calibration Check .......................................................................................................12-9 12.6 Alternative Methods ...................................................................................................12-9 12.7 End User Considerations .........................................................................................12-10 12.8 References ...............................................................................................................12-11 13 ION CHROMATOGRAPHY ................................................................................................13-1 13.1 Purpose and Use........................................................................................................13-1 13.2 Description of Method ................................................................................................13-1

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13.2.1 Ion Exchange Chromatography..........................................................................13-3 13.2.2 Ion Exclusion Chromatography ..........................................................................13-7 13.2.3 Ion Pair Chromatography ...................................................................................13-7 13.3 Technical Considerations ...........................................................................................13-7 13.3.1 Sample Injection .................................................................................................13-7 13.3.2 Column Selection .............................................................................................13-10 13.3.2.1 Concentrator Column................................................................................13-11 13.3.2.2 Guard Column...........................................................................................13-11 13.3.2.3 Separator Column.....................................................................................13-11 13.3.2.4 Eluent Suppressor Column .......................................................................13-12 13.4 Eluent Selection .......................................................................................................13-13 13.5 Detectors ..................................................................................................................13-17 13.5.1 Interferences.....................................................................................................13-19 13.6 Calibration ................................................................................................................13-21 13.6.1 Calibration Checks ...........................................................................................13-21 13.7 Alternative Methods .................................................................................................13-21 13.8 End User Considerations .........................................................................................13-22 13.9 References ...............................................................................................................13-22 14 IRON AND COPPER ..........................................................................................................14-1 14.1 Purpose and Use........................................................................................................14-1 14.2 Description of Methods...............................................................................................14-1 14.3 Technical Considerations ...........................................................................................14-2 14.3.1 Integrated Sampling ...........................................................................................14-2 14.3.1.1 Wet Chemistry Analysis of Integrated Samples..........................................14-3 14.3.1.2 XRF Analysis of Integrated Samples ..........................................................14-3 14.3.2 Turbidity..............................................................................................................14-4 14.3.3 Particle Counter..................................................................................................14-5 14.3.4 Dynamic Light Fluctuation Based Particle Monitors ...........................................14-6 14.3.5 Acoustic Detection..............................................................................................14-6 14.3.6 Colorimetric ........................................................................................................14-7 14.3.6.1 Colorimetric Limitations...............................................................................14-8 14.4 Calibration ..................................................................................................................14-9 14.4.1 Wet Chemistry Analysis......................................................................................14-9

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14.4.2 XRF Analysis ....................................................................................................14-10 14.5 Calibration Check .....................................................................................................14-10 14.6 Alternate Methods ....................................................................................................14-11 14.7 End User Considerations .........................................................................................14-11 14.8 References ...............................................................................................................14-12 15 PHOSPHATE......................................................................................................................15-1 15.1 Purpose and Use........................................................................................................15-1 15.2 Description of Method ................................................................................................15-1 15.3 Technical Considerations ...........................................................................................15-3 15.3.1 Colorimetric Limitations ......................................................................................15-3 15.3.2 Sample Considerations ......................................................................................15-4 15.3.3 Time Delay .........................................................................................................15-5 15.3.4 Sample Temperature..........................................................................................15-5 15.3.5 Sample Volume and Introduction of Chemicals..................................................15-5 15.3.6 Light Intensity .....................................................................................................15-5 15.4 Interferences ..............................................................................................................15-6 15.4.1 Particulate Matter ...............................................................................................15-6 15.4.2 Sample Discoloration..........................................................................................15-6 15.4.3 Other Substances...............................................................................................15-6 15.5 Calibration ..................................................................................................................15-7 15.6 Calibration Check .......................................................................................................15-7 15.7 Alternative Methods ...................................................................................................15-7 15.8 End User Considerations ...........................................................................................15-8 15.9 Field Experience.........................................................................................................15-8 15.9.1 Sample Delivery .................................................................................................15-9 15.9.2 Analyzer Malfunction ..........................................................................................15-9 15.10 References .............................................................................................................15-10 16 SILICA ................................................................................................................................16-1 16.1 Purpose and Use........................................................................................................16-1 16.2 Description of Method ................................................................................................16-1 16.3 Technical Considerations ...........................................................................................16-3 16.3.1 Colorimetric Limitations ......................................................................................16-3

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16.3.2 Sample Considerations ......................................................................................16-4 16.3.3 Time Delay .........................................................................................................16-5 16.3.4 Sample Temperature..........................................................................................16-5 16.3.5 Sample Volume and Introduction of Chemicals..................................................16-5 16.3.6 Light Intensity Check ..........................................................................................16-6 16.4 Interferences ..............................................................................................................16-6 16.4.1 Ortho-phosphate.................................................................................................16-6 16.4.2 Non-reactive Silica..............................................................................................16-6 16.4.3 Particulate Matter ...............................................................................................16-7 16.4.4 Reagent Contamination......................................................................................16-7 16.4.5 Sample Discoloration..........................................................................................16-7 16.4.6 Sample Temperature..........................................................................................16-7 16.5 Calibration ..................................................................................................................16-7 16.6 Calibration Checks ................................................................................................16-8 16.7 Alternative Methods ...................................................................................................16-9 16.8 End User Considerations ...........................................................................................16-9 16.9 Field Experience.......................................................................................................16-10 16.9.1 Sample Delivery ...............................................................................................16-10 16.9.2 Analyzer Malfunction ........................................................................................16-10 16.10 References .............................................................................................................16-11 17 TOTAL ORGANIC CARBON .............................................................................................17-1 17.1 Purpose and Use........................................................................................................17-1 17.2 Forms of Organic Carbon and Description of Method................................................17-2 17.3 Technical Considerations ...........................................................................................17-3 17.3.1 High Temperature Combustion Method .............................................................17-3 17.3.2 Persulfate Oxidation ...........................................................................................17-4 17.3.3 Conductometric-type TOC..................................................................................17-6 17.4 Calibration ..................................................................................................................17-8 17.5 Calibration Check .......................................................................................................17-8 17.6 Alternative Methods ...................................................................................................17-9 17.6.1 Closed-Loop Photocatalytic Oxidation................................................................17-9 17.6.2 TOC Data Comparison .....................................................................................17-11 17.7 End User Considerations .........................................................................................17-11

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17.8 References ...............................................................................................................17-12 18 ELECTROCHEMICAL CORROSION POTENTIAL ...........................................................18-1 18.1 Purpose and Use........................................................................................................18-1 18.2 Description of Method ................................................................................................18-2 18.2.1 Use of Reference Electrodes..............................................................................18-3 18.3 Technical Considerations ...........................................................................................18-4 18.3.1 Reference Electrode Selection ...........................................................................18-4 18.3.2 Quasi-Reference Electrodes Used in Corrosion Rate Probes............................18-5 18.3.3 Reference Electrode Issues ...............................................................................18-6 18.3.4 Voltmeter Selection ............................................................................................18-7 18.4 Calibration and Maintenance Procedures ..................................................................18-7 18.4.1 Reference Electrodes .........................................................................................18-7 18.4.2 Electrochemical Corrosion Potential Probe ........................................................18-8 18.5 Field Experience.........................................................................................................18-8 18.5.1 ECP Measurements in Condensers ...................................................................18-8 18.5.2 ECP Measurements in Boiling Water Reactors ..................................................18-9 18.6 Possible Future ECP Measurements in Fossil Plants ................................................18-9 18.7 References ...............................................................................................................18-10 19 CORROSION RATE ...........................................................................................................19-1 19.1 Purpose and Use........................................................................................................19-1 19.2 Description of Traditional Methods.............................................................................19-1 19.3 Advantages of On-Line Corrosion Monitoring ............................................................19-2 19.4 Description of Physical Methods ................................................................................19-3 19.5 Description of Electrochemical Methods ....................................................................19-5 19.5.1 General Considerations......................................................................................19-6 19.5.2 Linear Polarization Resistance (3-Electrode) .....................................................19-8 19.5.3 Linear Polarization Resistance (2-Electrode) ...................................................19-11 19.5.4 Electrochemical Impedance Spectroscopy.......................................................19-13 19.5.5 Coupling Current (Zero Resistance Ammetry) .................................................19-14 19.5.6 Electrochemical Noise ......................................................................................19-15 19.5.6.1 Electrochemical Potential Noise ...............................................................19-15 19.5.6.2 Electrochemical Current Noise .................................................................19-17

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19.5.6.3 Electrochemical Noise Resistance............................................................19-18 19.5.7 Galvanic Corrosion Monitoring Using Zero Resistance Ammetry.....................19-18 19.5.8 Coupling Current Between Two or More Metals...............................................19-19 19.5.9 Coupling Current Between Segmented Weld Electrodes .................................19-20 19.5.10 Electrochemical Methods in Combination ......................................................19-20 19.6 Calibration ................................................................................................................19-21 19.7 End User Considerations .........................................................................................19-21 19.8 Possible Future Corrosion Rate Monitoring in Fossil Plants ....................................19-22 19.9 References ...............................................................................................................19-22

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LIST OF FIGURES Figure 2-1 Multisensor Probe.....................................................................................................2-5 Figure 2-2 Multisensor Probe Instrument Schematic .................................................................2-6 Figure 3-1 Representative Drum Boiler Mechanical Carryover..................................................3-3 Figure 4-1 A Typical Cation Conductivity Flow Diagram............................................................4-4 Figure 4-2 Chloride (Cl) Concentration vs. Specific Conductivity ..............................................4-5 Figure 4-3 Sulfate (SO4) Concentration vs. Specific Conductivity..............................................4-6 Figure 4-4 Carbon Dioxide (CO2) vs. Specific Conductivity .......................................................4-7 Figure 4-5 Typical Degassed Cation Conductivity Schematic Diagram.....................................4-8 Figure 4-6 Relationship Between Ammonia Concentration mg/L (ppm) and Specific Conductivity (µS/cm) at 25°C (77°F) ................................................................................4-16 Figure 5-1 Typical Oxygen Sensing Probe ................................................................................5-2 Figure 5-2 Solubility of Dissolved Oxygen (mg/L (ppm)) vs. Temperature (°C) .........................5-7 Figure 5-3 Response of a Polarographic Oxygen Sensor to a 20 µg/L (ppb) Oxygen Addition Generated by a Faraday Cell .............................................................................5-14 Figure 5-4 Principle of Optical Oxygen Detection Using Fluorescent Dye...............................5-16 Figure 5-5 Fluorescence Density Decay Time as a Function of Oxygen Concentration..........5-16 Figure 5-6 Phase Shift of Modulated Signals...........................................................................5-17 Figure 5-7 Stern-Volmer Calibration Curve..............................................................................5-18 Figure 5-8 Luminescence Sensor Design................................................................................5-19 Figure 7-1 Flowing Junction Reference Electrode Head Cup Configuration..............................7-5 Figure 7-2 Combination Electrode .............................................................................................7-6 Figure 7-3 Standardization (Zero Intercept) Adjustment............................................................7-9 Figure 7-4 Slope (Span) Adjustment........................................................................................7-10 Figure 7-5 Specific Conductivity, Ammonia, pH at 25°C..........................................................7-12 Figure 7-6 Ammonia Concentration vs. pH for Various Carbon Dioxide Concentrations.........7-13 Figure 8-1 Measured Sodium Concentration vs. Concentration of Sodium Added....................8-3 Figure 8-2 Response Time of Sodium Ion Selective Sensors: Time vs. Sodium Concentration in µg/L (ppb) Before and After Etching .......................................................8-5 Figure 8-3 Elapsed Time After Known Standard Addition: Time vs. Sodium Concentration in µg/L (ppb)................................................................................................8-6 Figure 8-4 Sodium Analyzer Response: Elapsed Time After Addition of 20µg/L (ppb) Sodium vs. Sodium Concentration.....................................................................................8-7

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Figure 8-5 Calibration Slope (Millivolt Response) of Sodium Ion Selective Electrode at Varying pH Values .............................................................................................................8-9 Figure 8-6 Verification Results Immediately After a 200 and 2,000 µg/L (ppb) Sodium Calibration ........................................................................................................................8-10 Figure 9-1 Percent of Ammonia and Ammonium Ions as a Function of Solution pH .................9-3 Figure 9-2 Relationship between Ammonia, Specific Conductivity and pH in the Absence of Carbon Dioxide at 25ºC..................................................................................................9-4 Figure 9-3 Chemical Formula and Structure for Phenol ............................................................9-7 Figure 9-4 Chemical Formula and Structure for Salicylic Acid...................................................9-8 Figure 9-5 Absorbance vs. Concentration ...............................................................................9-10 Figure 10-1 Thermo Orion Electrode Reservoir System ..........................................................10-5 Figure 10-2 Calibration Curve for Low Level Chloride SIE ......................................................10-8 Figure 11-1 Absorption vs. Concentration ...............................................................................11-4 Figure 12-1 Polarographic Hydrogen Sensing Electrode with “Guard Ring”............................12-4 Figure 12-2 The Wheatstone Bridge Circuit Showing Schematically How the Reference Gas Flows Over Filament Resistances Ra and Rc, while the Sample Flows Over Filament Resistances Rd and Rb. Ra = Rb = Rc = Rd when Reference and Sample Gases have the Same Thermal Conductivity ...................................................................12-6 Figure 12-3 Diagram of Gas Chromatograph with Thermal Conductivity Detector..................12-7 Figure 12-4 Hydrogen Sensor with Thermal Conductivity Detector .........................................12-8 Figure 13-1 Schematic of a Single Column Ion Chromatograph .............................................13-4 Figure 13-2 Illustration of How Ions Elute (Leave the Separator Column) at Different Rates Resulting in a Separation of Ionic Species in the Flowing Eluent ..........................13-5 Figure 13-3 Gradient Separation of Common Anions Using a Hydroxide Gradient.................13-6 Figure 13-4 Anion Analysis Using Ionpac® AS17 Separator Column with Eluent Generator; Method 1 ........................................................................................................13-9 Figure 13-5 Anion Analysis Using Ionpac® AS17 Separator Column with Eluent Generator; Method 2 ......................................................................................................13-10 Figure 13-6 Chemistry and Ion Movement in Continually Regenerated Eluent Suppressor.....................................................................................................................13-14 Figure 13-7 The KOH Generator Cartridge Consists of a KOH Generating Chamber and + K Electrolyte Reservoir Connected by a Cation Exchange Connector .........................13-16 Figure 13-8 Carbonate Removal Device................................................................................13-17 Figure 13-9 Comparison of Spectrum with (Top Spectrum) and without Carbonate Removal Cevice ............................................................................................................13-18 Figure 13-10 Chromatogram of a Sample Containing 0.022 µg/l Sodium and 3000µg/l Ethanolamine .................................................................................................................13-20 Figure 14-1 Particle Counter Schematic, Illustrating 1 Particle at 3 microns and 2 Particles at 1 micron.........................................................................................................14-5 Figure 14-2 Dynamic Light Fluctuation Schematic ..................................................................14-6 Figure 14-3 Illustration of Self Absorption at High Concentrations ..........................................14-9

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Figure 15-1 Typical Beer’s Law Absorption (A) vs. Concentration Curve ................................15-3 Figure 16-1 Graph Showing 100 Percent Absorption ..............................................................16-4 Figure 17-1 High Temperature Oxidation TOC Analyzer Flow Diagram..................................17-5 Figure 17-2 Persulfate Oxidation, Ultraviolet Lamp TOC Analyzer Flow Diagram...................17-7 Figure 18-1 An Analogy between Measuring Mountain Height with Respect to Sea Level and Measuring ECP with Respect to a Reference Electrode...........................................18-4 Figure 19-1 Equivalent Electric Circuit (Upper Figure) and Corresponding Schematic Diagram of the 3-Electrode Corrosion Probe Immersed in the Corrosive Environment (Lower Figure)...........................................................................................19-10 Figure 19-2 View of the Exposed End of a Corrosion Probe with Three Concentric Flushmounted Electrodes Arranged to Minimize Solution Resistance Errors.........................19-11 Figure 19-3 Equivalent Electric Circuit for the 2-Electrode Corrosion Probe Immersed in the Corrosive Environment.............................................................................................19-12

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LIST OF TABLES Table 1-1 EPRI Cycle Chemistry Core Monitoring Parameters for Conventional Fossil Units ...................................................................................................................................1-3 Table 2-1 Example Diagnostic Capabilities of a Five-Probe Air In-leakage Measurement System ...............................................................................................................................2-6 Table 2-2 MSP Probe Indications for Various Probe Positions..................................................2-7 Table 4-1 Typical Conductivity Range Limits as a Function of Cell Constant............................4-9 Table 4-2 Equivalent Conductances of Separate Ions at Various Temperatures ....................4-18 Table 6-1 To Convert ORP or ECP Values Measured Using Reference Electrode #1 to Values on Reference Electrode #2 Scale, Add the Indicated Conversion Factor to the Measured Potential ......................................................................................................6-6 Table 6-2 Expected ORP Values for Reference Quinhydrone Solutions at pH 4 and pH 7.......6-8 Table 7-1 Various Solution Additive Temperature Correction Factors for Power Plant Steam/Water Cycle pH Measurements ..............................................................................7-7 Table 7-2 Example Calculated pH by Differential Conductivity from One Instrument Supplier ............................................................................................................................7-14 Table 8-1 Typical Results from Known Addition Method for Calibration Check in the nanograms/L (ppt) range..................................................................................................8-12 Table 9-1 Precision Data for Manual Phenate Method Based on Triplicate Analyses of Ammonium Sulfate.............................................................................................................9-9 Table 9-2 Precision and Bias Data; Direct Nesslerization .......................................................9-15 Table 10-1 Boiler Water Chloride Limits in µg/L (ppb) @ 15858 kPa, (2300 psi) ....................10-2 Table 12-1 Some Performance Ranges for Aqueous EC and TC Detectors .........................12-10 Table 17-1 Summary of Some Typical On-Line Instrument Capabilities ...............................17-12 Table 19-1 Typical Examples of the Relationship Between the EPN “Fingerprint”, Statistical Parameters, and the Corrosion Mechanism in Progress ...............................19-17

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1 CYCLE CHEMISTRY MONITORING IN FOSSIL PLANT CYCLES

1.1 Introduction Technology advancements made during the twentieth century enabled substantial improvement to be made in the efficient and cost effective production of electric power from fossil fuels. In conventional designs in which steam is produced and expanded in a turbine, the ability to provide water at higher purity levels than previously possible enabled designers to increase the operating temperatures and pressures of fossil units. These improvements were accompanied by increasingly tighter chemistry limits and a need for reliable monitoring approaches that could be used for surveillance and control purposes. This need shifted the emphasis on chemistry monitoring from infrequent collection and analysis of grab samples to reliance on analyzers that provided semi-continuous or real time monitoring of the chemistry parameters of interest. Also, a trend of reductions in fossil plant staff size and experience levels have dictated increasing reliance on the performance of chemistry analyzers and the reliability of chemistry monitoring data. This is of some concern in all situations and, in particular, those where responsibility for day to day chemistry activities has been assigned to plant operators without extensive relevant education or training. Advancements in data transmission, display, storage and assessment (including expert systems) as well as general communications have made these changes in staffing possible. However, effective use of these capabilities remains completely dependent on the suitability of the cycle chemistry treatments selected and the associated monitoring capability of the selected analyzers.

1.2 Monitoring Requirements and Choice of Cycle Chemistry Over the last 20 years EPRI has taken a leadership position in the area of cycle chemistry control in fossil generating units. The third generation of EPRI Cycle Chemistry Guidelines was introduced over the period 2001-2005. The guidelines are widely accepted as the de facto worldwide standard; available options for feedwater and boiler water treatment and operational chemistry target values and action levels for conventional fossil units are summarized in a series of reports [1-3]. Effective cycle chemistry programs require proper selection of treatments consistent with unit characteristics. Treatment control and optimization requires provision of sampling and analysis capabilities consistent with the treatments selected for use. The current guidelines indicate that 1-1

EPRI Proprietary Licensed Material Cycle Chemistry Monitoring in Fossil Plant Cycles

there are three options for the feedwater treatment and four choices for boiler water treatment. The feedwater treatments are designated and may be generally characterized as follows. •

Reducing all-volatile treatment, AVT(R), in which the feedwater must be dosed with a reducing agent to minimize corrosion of copper alloy components

•

Oxidizing all-volatile treatment, AVT(O), in which no reducing agent is added or needed since there are no copper alloys in the feedwater system

•

Oxygenated treatment, OT, in which the feedwater is dosed with oxygen; OT can only be used in cycles where the feedwater meets necessary purity criteria, employs condensate polishing and no copper alloys are present following the condensate pump discharge.

The available boiler water treatments are identified and generally characterized as follows. •

All-volatile treatment [1], AVT, in which the feedwater treatment is either AVT(R) or AVT(O) as appropriate and no further chemical treatment is applied within the boiler.

•

Phosphate Continuum Treatment [2], PC, in which a drum boiler is dosed principally with trisodium phosphate (TSP); dosing of the boiler water with sodium hydroxide is also allowed as a supplement to TSP when needed for pH control. Low and high level TSP dosage variants of PC, termed PC(L) and PC(H), respectively may be considered depending on the characteristics and needs of the unit.

•

Caustic Treatment [2], CT, in which a drum boiler is dosed with sodium hydroxide.

•

Oxygenated Treatment [3], OT, in which the feedwater dosed with oxygen as needed for OT is used in the boiler without further treatment; OT may be used in cycles with once-through or drum type boilers, however, with the latter feedwater oxygen dosing must be more carefully controlled so as to avoid possible corrosion in the boiler.

Additional details concerning selection, use and optimization of each treatment is provided in the corresponding guideline report. These reports also provide guidance concerning required sample points and chemistry parameters that should be monitored for the various chemistry treatment options. This guidance is based on worldwide experience in ensuring minimized levels of corrosion, impurity ingress and deposition as needed to eliminate chemistry related damage in boilers and turbines while simultaneously minimizing, or even eliminating, the need for operational chemical cleaning of boilers. The chemistry parameters that may be monitored under the cycle chemistry guidelines fit into two general categories: 1) those parameters which all fossil plants should have for optimum chemistry control (termed the core parameters) and 2) those parameters which are regarded as diagnostic parameters that may be monitored as needed for troubleshooting or during commissioning. The rationale for each important sample point and chemistry parameter combination is discussed in detail in the individual guideline reports to explain its designation as an EPRI core or optional (diagnostic) monitoring requirement. The findings of EPRI cycle chemistry research for conventional fossil plant cycles are also suitable for application to combined cycle units with heat recovery steam generators since the 1-2

EPRI Proprietary Licensed Material Cycle Chemistry Monitoring in Fossil Plant Cycles

underlying science is the same. A separate guideline report [4] is available that considers differences in design and operation of combined cycle unit as they effect the chemistry.

1.3 Cycle Chemistry Monitoring Approaches This report provides a review and discussion of on-line analyzer techniques currently available and in use in fossil power plants, updating many technical presentations from a 1998 report [5]. The focus is on analyzers used for fossil plant chemistry monitoring as described in the latest guidelines [1-4]. Monitoring techniques that may be considered for use in future plants are also presented as a precursor to possible future research being planned by EPRI to address possible improvements in monitoring and control of cycle chemistry in fossil steam-water cycles. 1.3.1 Core Monitoring Parameters An overall summary of the EPRI Core Monitoring Parameters and sampling points, adapted from References 1-3, is provided in Table 1-1 [1-3]. Except in situations where a specific boiler design, feedwater chemistry or boiler water chemistry is indicated in the notes, the core requirements indicated apply to all conventional plant designs and chemistries. Table 1-1 EPRI Cycle Chemistry Core Monitoring Parameters for Conventional Fossil Units [1-3] Parameter

Monitoring Points

Cation Conductivity

Condensate Pump Discharge, Condensate Polisher Outlet or Economizer Inlet, Reheat (or Main) Steam, Boiler Water a

Specific Conductivity

Treated Makeup, Boiler Water b

pH

Boiler Water c

Dissolved Oxygen

Condensate Pump Discharge, Economizer Inlet, Boiler Water d

Sodium

Condensate Pump Discharge, Economizer Inlet, Boiler Water e

Oxidation-Reduction Potential (ORP)

Deaerator Inlet f

Air in-leakage

Condenser Air Removal System Exhaust

Carryover

Calculated from Boiler Water and Saturated Steam Readings g

Notes: a – Drum boiler units only; measured on blowdown for boilers on PC or CT, and either blowdown or downcomer for boilers on AVT or OT b – blowdown of drum boilers on PC and CT c – Drum boiler units only; measured on either pH or downcomer of units on AVT; on either blowdown or downcomer of units on PC or CT; on downcomer of units on OT d – Only required in drum boiler units on OT; measured on downcomer e – Only required in drum boiler units on PC or OT; measured on downcomer f – Only required on units employing AVT(R) as feedwater chemistry g – Based on either on-line monitor readings (if available) or analysis results for grab samples

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EPRI Proprietary Licensed Material Cycle Chemistry Monitoring in Fossil Plant Cycles

The Core Monitoring Parameters are considered the minimum level of surveillance needed for all conventional fossil generating units. They are purposely included as an integral part of the EPRI Cycle Chemistry benchmarking criteria used to assess individual cycle chemistry programs. In general, use of on-line analyzers for continuous analysis of chemistry is preferred. In the ensuing discussions of on-line analyzer technology, greatest emphasis has been placed on the EPRI Core Monitoring Parameters, which are presented in Sections 2-8 of the report. •

Section 2, Air in-leakage

•

Section 3, Carryover in Drum Boilers

•

Section 4, Conductivity; includes discussions of specific, cation and degassed cation conductivity

•

Section 5, Dissolved Oxygen

•

Section 6, Oxidation-Reduction Potential (ORP)

•

Section 7, pH

•

Section 8, Sodium

1.3.2 Diagnostic Monitoring Parameters In customizing their cycle chemistry programs, the organization may elect to monitor one or more of the diagnostic parameters with on-line analyzers. The more common monitoring techniques typically used for diagnostic purposes are presented in Sections 9-17 of the report. These are listed as follows. •

Section 9, Ammonia

•

Section 10, Chloride

•

Section 11, Hydrazine

•

Section 12, Hydrogen

•

Section 13, Ion Chromatography; can be used to monitor various ions in water and steam.

•

Section 14, Iron and Copper; includes a discussion of indirect means of monitoring suspended solids and particles in water and steam

•

Section 15, Phosphate

•

Section 16, Silica

•

Section 17, Total Organic Carbon

For many of the parameters and techniques more than one approach may be available. In some instances, particularly when chemistry data are needed for diagnostic purposes, collection of grab samples for laboratory analysis may be sufficient to satisfy the needs of the operator. These considerations are included in the Section 2-17 discussions. 1-4

EPRI Proprietary Licensed Material Cycle Chemistry Monitoring in Fossil Plant Cycles

1.3 Future Cycle Chemistry Guidelines and Monitoring Approaches Conventional sampling and analysis for chemistry surveillance and control in fossil generating units requires extraction of a representative sample of the fluid and subsequent conditioning to standard conditions, which are also assumed when establishing limit values for the various parameters of interest. Present day on-line analyzers are sensitive to inlet sample parameters such as flow, temperature, pressure, etc. and available designs can compensate for some but not all deviations from standard conditions. Thus effective sample conditioning is of great importance when using available monitors for cycle chemistry surveillance and control. The challenges of effective sample extraction and conditioning are considerable. Economic considerations often lead to compromises in design of sample conditioning system. For example, it is generally preferable with respect to provision of representative samples that the length of sample lines be minimized. However, such practice is frequently not followed as the cost of centralized sample conditioning and analysis facilities is less than that of remote sample stations. Direct measurement of fluid properties would eliminate the need of sample conditioning and associated error sources. This is now possible for some parameters such as pH and specific conductivity but only over a limited range of temperatures. Effective use of such instrumentation around the steam water cycle would require significant expansion of the working temperature range and development of new control limits that correspond to such temperatures. This need is greatest in high temperature areas such as boilers and turbines in which direct monitoring is not currently possible. The purpose of chemistry monitoring is to protect the components in contact with steam and water from damage and efficiency loss. The mechanisms and associated root causes responsible for most chemistry related damage and efficiency are now very well understood but the precise conditions at which corrosion and deposition activity begin are still not known. The past and present chemistry guidelines all serve to provide the operator with a warning as to when corrosion and/or deposition activity are thought to place the unit at risk. A focal point of improved chemistry guidelines is to provide operators with direct indications that the chemistry in boiler water supports corrosion activity since chemistry related boiler tube failures continue to represent a substantial source of availability loss across the industry. In consideration of this, EPRI has initiated laboratory research to better understand the environmental conditions in boilers which lead to corrosion damage and tube failure by mechanisms such as hydrogen damage. This work involves making direct measurements of surface potential and pH at temperature while adjusting the chemical composition of the water. Initial work will consider volatile treatments at various contaminant levels and subsequently consider the effect of solids based treatment chemicals used in drum type boilers. The apparatus used to make the laboratory measurements is not suitable for use in a working boiler. Underdeposit corrosion activity in boilers is also dependent on tube surface cleanliness. Research has been undertaken to develop a comprehensive model that can be used to understand the 1-5

EPRI Proprietary Licensed Material Cycle Chemistry Monitoring in Fossil Plant Cycles

various parameters which influence deposition. The ultimate aim of this work is to consider chemical activity under various deposit conditions so as to define the conditions under which corrosion activity becomes significant. The latest cycle chemistry guidelines include target values for boiler water in drum-type units that minimize the risk of corrosion damage. However, these values are empirical in that they reflect operating experience in fossil plants. Similarly, allowable levels of boiler tube deposition used to set criteria on the advisability of chemical cleaning are empirical, and represent levels of waterside solids that field experience suggests will not lead to tube damage and failure. It is envisioned that future chemistry guidelines will reflect the findings of research by EPRI in the corrosion and deposition areas. For waterside corrosion, the ultimate chemistry surveillance capability would be to measure corrosion activity directly in boilers and turbines, the major components at greatest risk of corrosion damage. However, there is currently no means to monitor corrosion rates under fossil plant operating conditions, in particular those which exist in these high temperature components. Sections 18 (on Electrochemical Corrosion Potential (ECP)) and 19 (on Corrosion) of this report review the science of corrosion monitoring and requirements for practical use of corrosion measurement techniques in fossil plants. From inspection of the Section 18 and 19 discussions it becomes clear that the potential for use of ECP and corrosion monitors in steam-water cycles exists but this remains a developing area. There are constraints on the applicability and reliability of available corrosion measurement devices that must be resolved in order to produce instruments suitable for use in the more demanding components of working fossil plants. Possible changes in cycle chemistry guidelines to prevent or control corrosion, based on the currently used indirect direct measurement methods, could initially involve modification of chemistry limits for impurities in the boiler water. These revised chemistry limits could perhaps be further refined to reflect the state of deposits on heat transfer surfaces as the effects of deposits on corrosion activity become more fully understood. Assuming that a robust corrosion monitor capable of operating in the boiler and/or turbine environment could be produced, extensive testing would be required as part of the development process. Location of its use would need further consideration due to concerns about compliance with design codes, installation costs, and possible consequences of probe damage during service. These concerns are significant and probably dictate installation of the monitor in a location external to the unit so it can be reliably isolated if needed. Such an approach may in fact hold many advantages over use of in situ probes.

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EPRI Proprietary Licensed Material Cycle Chemistry Monitoring in Fossil Plant Cycles

1.4 References 1. Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187. 2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188. 3. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925. 4. Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438. 5. Reference Manual for On-Line Monitoring of Water Chemistry and Corrosion: 1998 Update, EPRI, Palo Alto, CA: 1999. TR-112024.

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EPRI Proprietary Licensed Material

2 AIR IN-LEAKAGE

2.1 Effects of Air In-leakage on Unit Performance and Cycle Chemistry Fossil unit operation requires that the condenser be operated under vacuum. The air removal system is designed to remove noncondensable gases from the condenser. In so doing vacuum conditions are established during fossil unit startup and maintained during unit service. Excessive air in-leakage rates can result in reduced condenser vacuum, thereby reducing turbine backpressure and the efficiency of the cycle [1]. Air ingress can also increase the concentration of dissolved oxygen in the steam-water cycle. All EPRI Cycle Chemistry Guidelines for conventional fossil and combined cycle plants now feature a Target Value of ≤10 ppb for dissolved oxygen at the condensate pump discharge [2-5]. Meeting the target provides maximum flexibility in treatment and generally ensures that contamination by carbon dioxide, which contributes to measured cation conductivity, is minimal. By meeting the target for dissolved oxygen at the condensate pump discharge and by using ammonia as the feedwater pH conditioning agent, there is generally no need to rely on techniques other than cation conductivity as the primary indicator of cycle contamination by inorganic ions such as chloride and sulfate that are of concern in boilers and turbines. Condensate dissolved oxygen and carbon dioxide levels are affected by the extent and location of air in-leakage in the condenser and in other parts of the cycle that operate at subatmospheric pressure. Air in-leakage in excess of that removed by the condenser air removal system will result in increased condensate dissolved oxygen and carbon dioxide levels, which may cause corrosion within the condenser. Contamination of the feedwater by oxygen and carbon dioxide may also lead to an increase in corrosion-product generation within the feedwater part of the cycle [1]. Effective monitoring and control of condenser air in-leakage is considered so important that it has been designated a Core Monitoring Parameter in all EPRI cycle chemistry guidelines [2-5]. Control of condenser air in-leakage to reduce condensate oxygen to ≤10 ppb is of greatest importance in those units that must employ AVT(R) so as to minimize corrosion of copper alloys and corrosion product transport in the feedwater part of the cycle. Protection of copper alloys in the feedwater environment requires that a reducing condition is established and maintained at all times. EPRI research has shown and field experience has confirmed that use of hydrazine or another reducing agent to establish a negative oxidation-reduction potential (versus silver/silver chloride; see Section 6) condition consistent with the requirements of AVT(R) cannot be reliably

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EPRI Proprietary Licensed Material Air In-leakage

achieved when the condensate oxygen exceeds 10 ppb, regardless of the concentration or type of reducing agent applied. In units that employ oxidizing feedwater treatments, maintaining low levels of cycle air inleakage is not needed for protection of copper alloys in the feedwater. However, control of this parameter is desirable to limit dissolved oxygen and carbon dioxide contamination, which provides maximum flexibility in control of the feedwater treatment with minimum interference in measurement of two important chemistry monitoring parameters, namely conductivity and pH. In fossil unit cycles with condensate polishers, carbon dioxide will be exchanged by the anion media, potentially increasing the required frequency of regeneration (in deep bed system designs) or replacement of the precoat media (in precoat filter/demineralizer system designs). The Heat Exchange Institute recommends that the condenser design restrict air in-leakage to no more than 1.0 scfm per 100 MW of generating capacity. Constant vigilance must be exercised to prevent air in-leakage to the cycle from those equipment elements under vacuum. Particular problem areas include pump seals, valve bonnets, threaded joints, and especially the expansion joint between the turbine and condenser. Monitoring and limiting the amount of air in-leakage (condenser air removal system flow) is essential for proper control of dissolved oxygen and carbon dioxide in the cycle. Such monitoring will determine when an exhaustive effort must be made to find and fix the source of air leakage. Air in-leakage control is a continual process over the working life of any steam generating unit. The time and effort required to control air inleakage is justified by the beneficial effects on unit startup times, efficiency and compliance with cycle chemistry guidelines [1].

2.2 Monitoring Methods The earliest and still most common means of monitoring air removal at the condenser air removal system exhaust is by means of rotameters. Other monitoring techniques are also available. Some of these devices measure the volumetric flow rate while others measure volumetric flow plus some additional parameters [1]. 2.2.1 Rotameters Typically, a rotameter installed at the exhaust of the air removal system is relied upon to quantify air in-leakage. This was the standard approach at the time plants were designed and constructed whether the air removal devices employed were steam jet air ejectors or mechanical vacuum pumps. Manual rotameter readings were intended to be taken at fixed intervals, typically once a day. However, low manpower levels in many fossil plants often prevent this. Air in-leakage can increase and remain at potentially damaging levels for hours or possibly days before the excess in-leakage is discovered, since turbine backpressure may not have increased significantly or noticeably.

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EPRI Proprietary Licensed Material Air In-leakage

Under certain conditions, rotameters provide adequate flow rate measurements of air in-leakage. However, many factors can affect the accuracy of the readings so obtained, such as debris, exhauster pulsations, water carryover, etc. Rotameters also have limited range, generally 10:1. Therefore, most problematic air in-leakages cause the rotameter to be pegged against the high level stop. Rotameters receive essentially very little maintenance and are checked very infrequently at many plants. In these situations, the operator has no idea of the extent of the air in-leakage, given the fact that the turbine backpressure rise may appear to be rather small and insignificant [1]. 2.2.2 Early Advanced Air Removal Monitoring Systems Rotameters have long been recognized as subject to a number of limitations and deficiencies. Their accuracy is questionable and readings are often unstable and subject to either high or low bias [1-6]. The data are only available intermittently and an operator is required to adjust valve settings and collect readings; there is no means to collect readings in real time or automatically transmit them to the control room operator. In an attempt to mitigate these inherent problems with rotameters, some improved instrument designs, which featured continuous readouts of removal rates, were developed for use in power plants [1,6,7]. Some of these designs had some serious drawbacks, such as not being able to distinguish between water vapor and air; and, operating under the principle that the condenser exhaust gases were always at saturation, when in fact, they were not. Some of these devices required a high level of maintenance, which often exceeded the resources of the plant [1]. 2.2.3 Multisensor Probe Design Subsequent developments have produced more advanced instrumentation, which employs a number of sensors that collectively overcome these earlier limitations and is available commercially [1,7,8]. One such instrument features a multisensor probe (MSP) which is installed in the exhaust line from the condenser. This device is depicted in Figure 2-1 [1,8]. This instrument has the ability to directly measure the mass properties of gas in the condenser vacuum line, as well as compute other parameters based on thermodynamics. The instrument combines a gas analyzer with a total mass flow meter. The instrument utilizes a velocitymeasuring device, and other facilities for measuring temperature, pressure, and relative saturation. By including the measurement of pipe diameter, the instrument can separate the effects of non-condensable gas from water vapor and compute individual and combined properties of the gas mixture. As a result, the following parameters of the gas flowing in the exhauster suction line are either directly measured or are determinable by calculation using direct measurement results and the piping diameter: •

Air in-leakage rate

•

Volumetric flow rate

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EPRI Proprietary Licensed Material Air In-leakage

•

Total mass flow

•

Water vapor mass flow rate

•

Water vapor/air mass ratio (relative saturation)

•

Water vapor density

•

Air density

•

Partial pressure of water

•

Partial pressure of air

•

Total pressure

•

Temperature

Air in-leakage, as measured by the MSP is the sum of all leak sources entering the subatmospheric pressure system and subsequently flowing through the exhauster line upstream of the sensor location. The MSP, however, does not measure air entering the low-pressure system beyond (downstream of) the probe, such as at valves, input connections, and shaft seals. The MSP does measure the exhauster capacity, which is useful to gauge the capacity of the exhausters to remove the gas mixture from the condenser [4]. Evaluation and analysis of condenser system performance and measurement of in-leakage is best accomplished with the use of multiple probes or alternatively by collection of data from desired locations with single portable probe. An example system utilizes five MSP probes as shown schematically in Figure 2-2. As indicated in the example depicted in the figure, each probe is identified by a number that is used to categorize performance information and to help pinpoint possible leak locations, as illustrated in Table 2-1 and Table 2-2 [4].

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EPRI Proprietary Licensed Material Air In-leakage

Figure 2-1 Multisensor Probe

2-5

EPRI Proprietary Licensed Material Air In-leakage

3

2

4

1

5

Exhauster 2

Exhauster 1

Condenser Figure 2-2 Multisensor Probe Instrument Schematic

Table 2-1 Example Diagnostic Capabilities of a Five-Probe Air In-leakage Measurement System Leak Location

Probe Indications of Air In-Leakage (SCFM) and Plant Dissolved Oxygen (DO)

Below water line, left side of condenser

Much higher than normal DO, 1>2 1 + 2 = 3 = 4 + 5, 4=5

Above water line, right side of condenser

Slightly higher or normal DO, 2 > 1, 1 + 2 = 3 = 4 + 5, 4=5

Small leak or faulty exhauster down stream of probe 4

Slightly higher or normal DO, 1 = 2, 1 + 2 = 3 = 4 + 5, 4 4 (back flow at 4)

Center joint seal, LP bearing seal, or other central location

Slightly higher or normal DO, 1 = 2, 1+2=3=4+5

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EPRI Proprietary Licensed Material Air In-leakage Table 2-2 MSP Probe Indications for Various Probe Positions Leak Location

Probe Indications of Air In-Leakage (SCFM)

Normal tight system

1 = 2.5 SCFM 2 = 2.5 SCFM 3 = 5 SCFM 4 = 0 SCFM (exhauster 2 not in service) 5 = 5 SCFM

Abnormal, need to locate and fix leaks (central joint seal)

1 = 15 SCFM 2 = 18 SCFM 3 = 33 SCFM 4 = 16.5 SCFM 5 = 16.5 SCFM

Abnormal, need to locate leaks and fix vacuum pump shaft seal in exhauster 2

1 = 20 SCFM 2 = 20 SCFM 3 = 40 SCFM 4 = 20 SCFM (Differences due to reversed flow 5 = 60 SCFM through shaft seal being sensed)

2.3 References 1. Condenser Tube Failures: Theory and Practice: Volume 1: Fundamentals. EPRI, Palo Alto, CA: 2005. 1010188. 2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188. 3. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188 Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925. 4. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925. 5. Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438. 6. K. Weaver, M. Twerdochlib and J. Bellows, “On-line Monitoring of Steam Turbine Air inleakage”, presented at the Joint ASME-IEEE Power Generation Conference, Philadelphia Pennsylvania, September 25-29, 1998: ASME, NY, NY. 88-JPGC-PWR-3. 7. J. A. Ferrison and P. Baker, “Air Inleakage Measurement Experiences with the Chell Air Ingress Monitor”, presented at the EPRI Condenser Technology Conference, Saint Petersburg Florida, September 30, 1983.

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EPRI Proprietary Licensed Material Air In-leakage

8. J. W. Harpster, “On Understanding Mechanisms that Control Dissolved Oxygen in Condenser Condensate.”, Proceeding of the 21st Annual Electric Utility Workshop, University of Illinois, Champaign, IL, May 8-10, 2001.

2-8

EPRI Proprietary Licensed Material

3 CARRYOVER IN DRUM BOILERS

3.1 Introduction to Carryover In cycles with once through boilers, the steam purity is essentially equal to that of the boiler feedwater providing that impurity levels are maintained below those at which deposition (and possible subsequent dissolution of the deposited solids) could occur. In cycles with drum type boilers the composition of the drum steam is dependent on the phenomenon known as carryover. Carryover is defined as the ratio of concentration of a chemical species in the saturated steam exiting the boiler drum to the concentration of the same species in the boiler water (usually measured in the blowdown). Total carryover (T) is defined in Equation 3-1 as follows. T=M+V

Equation 3-1

In this equation, M is mechanical carryover (due to steam moisture) and V is the vaporous carryover (due to volatile partitioning into the vapor phase). Mechanical carryover represents any boiler water fine droplets or mist that exits the boiler drum with the saturated steam. It is dependent on factors such as boiler pressure, drum water level, and the design and integrity of the internal separator devices employed to prevent boiler water from entering the steam. Vaporous carryover represents those impurities which partition from the liquid to the vapor phase during the boiling process. The partitioning tendency of individual chemical species is dependent on operating temperature and pressure. These partitioned impurities cannot be removed by the steam separator devices which are provided to control mechanical carryover. The extent to which a species partitions to steam often exceeds its solubility in steam, which can lead to deposition in superheaters, reheaters and steam turbines. For example, silica has long been recognized as a constituent subject to vaporous carryover. If not controlled, silica in steam will form deposits on the low pressure turbine. EPRI research investigating the properties of copper around the cycle determined that copper is highly volatile in boiling water, even at low pressures. However, the low solubility of copper results in significant deposition of copper in the primary superheater.

3-1

EPRI Proprietary Licensed Material Carryover in Drum Boilers

3.2 Consideration of Carryover in EPRI Cycle Chemistry Guidelines While previous EPRI chemistry guidelines suggested that unit specific steam purity guidelines be established based on assessment of predicted mechanical and volatile carryover relationships, the latest guidelines [1-3] include target values that reflect results of over a decade of research to define vaporous carryover based on a series of experimental assessments of the various constituents of interest and concern [4-10]. This work provides very accurate predictions of vaporous carryover over the operating pressure ranges of interest and considers the interaction of other species in the boiler water. For potentially corrosive contaminants such as sodium, chloride and sulfate, the vaporous carryover is pressure- and solution pH-dependent; in general, volatile carryover of these impurities is very minor except at high pressure providing the boiler water pH is maintained in expected ranges. By contrast, earlier volatility studies were based on single component solutions, at impurity concentrations in the liquid phase which greatly exceeded those allowable in boiler water [11]. Thus the latest control curves for boiler water are much more reliable with respect to vaporous carryover. However, they are based on assumed levels of mechanical carryover, which has a dominant influence on total carryover for most impurities. The primary exceptions are silica and copper. Figure 3-1 indicates the relationship used to define mechanical carryover in the guidelines [2]. Note that the values in this curve include a safety margin. Plant operating experience has shown that the performance guarantees of the manufacturer are generally readily met during performance testing normally conducted during commissioning of new units. However, performance often declines as a consequence of deterioration of the steam separator devices over time. Also, carryover is sometimes observed to increase during unit startup as a consequence of variations in steam drum water level. Such steam purity excursions may go unnoticed in cases where the unit sampling and analysis system is not in service during startup. Regrettably, a substantial number of steam purity problems in fossil units with drum boilers are ultimately determined to have been caused by carryover. These events are noted across the worldwide fleet of units. Disturbingly, it is very clear that many of these events could have been identified sooner had the plant staff assessed the carryover activity but this is not checked in many boilers after completion of the initial performance test conducted during commissioning of the unit. To address this gap in chemistry monitoring, the most recent EPRI cycle chemistry guidelines have designated carryover from drum boilers as a Core Monitoring Parameter. Carryover is the only core parameter that is a relationship of measured parameters. It is determined by calculation, using data measured in boiler water and saturated steam. Also, it need not be checked continuously or even with on-line instrumentation. The most recent cycle chemistry guidelines [1-3] indicate assessment of carryover at six month intervals so as to provide plant operators with an early indication of an ongoing or developing problem with steam purity. This approach also allows for inspection of plant operating and chemistry records and for planning of boiler drum internal inspection activities when assessment carryover assessment findings suggest such action is warranted.

3-2

EPRI Proprietary Licensed Material Carryover in Drum Boilers

Total carryover is usually measured by analysis of sodium in the boiler blowdown and saturated steam. Sodium is generally the dominant cation of possible concern in the boiler water, particularly when the boiler water is treated with trisodium phosphate or caustic. Sodium can be monitored reliably and at low concentrations by either on-line analyzers or laboratory analysis of grab samples. Section 8 of this report provides information on sodium analysis options. 4.8

6.2

7.6

9.0

Drum Pressure (MPa) 10.3 11.7 13.1 14.5

15.8

17.2

18.6 19.6

0.3

Mechanical carryover (%)

0.2

0.1 0.09 0.08 (Note: This curve includes a safety factor of 2)

0.07 600 700

900

1100 1300 1500 1700 1900 2100 2300 2500 2700 2850 Drum Pressure (psig)

Figure 3-1 Representative Drum Boiler Mechanical Carryover [2]

Carryover is also a Core Monitoring Parameter in combined cycle plants with drum-type generating circuits [12]. In multi-pressure HRSG designs, drum carryover should be monitored in each circuit. 3-3

EPRI Proprietary Licensed Material Carryover in Drum Boilers

3.3 Determination of Total Carryover from Drum Boilers Total carryover, the ratio of sodium concentrations in the saturated steam and boiler water, may be determined by calculation as shown in Equation 3-2.

T=

Na in saturated steam Na in blowdown

Equation 3-2

During carryover assessment, it is preferable to evaluate a broad range of operating conditions, particularly in cycling and peaking units.

3.4 Sampling and Data Collection Considerations Calculation of total carryover requires determination of a low volatility parameter (typically sodium though other parameters, such as chloride or sulfate, could be considered as an alternative or in addition to sodium) in the saturated steam and boiler water of the unit. Ideally, the readings should be collected as close to simultaneously as practical. When one or more on-line analyzers will be used to collect the data the outputs of the instruments should be checked and calibration performed if needed. If a single online analyzer will be used for both sample points, sufficient time must be provided to allow for switching of samples and stabilization of readings. In cases where on-line analyzers are not used, grab samples should be collected as close to simultaneously as practical. The boiler blowdown is the preferred sample point for carryover assessment as it is more representative of the liquid phase which may enter the steam with the saturated vapor. It should have a somewhat higher concentration of the constituent of interest than a boiler downcomer sample, which will tend to be slightly diluted by the feedwater entering the boiler. For carryover assessment purposes, saturated (drum) steam is the only acceptable sample point for steam. Superheated and reheat steam samples are not representative for carryover assessment since impurities in the saturated steam may form deposits prior to the superheater and reheater. Also, impurities from previously deposited solids may later dissolve and be present in the superheated and reheat steam. In addition, the composition of superheated and reheat steam is influenced by use of feedwater for attemperation. Monitoring of the saturated steam provides verification of compliance with the boiler manufacturer’s performance guarantee for steam purity, which often applies only to the saturated steam. Depending on boiler design the saturated sample source may be from single or multiple steam leads (off-take lines). The boiler operator should ensure that a representative sample is taken. In older boilers with multiple steam lead arrangements, some of the leads may be isolated and no longer operable. In such cases, resultant steam purity data does not provide complete assurance that contamination is not entering the steam path.

3-4

EPRI Proprietary Licensed Material Carryover in Drum Boilers

With proper sampling and analysis, it is thus possible to effectively monitor the total carryover of impurities into the high pressure turbine. As carryover is influenced by boiler water pH, the operator should note this at the time the sodium or other constituent readings are checked. Also, if a downcomer sample of boiler water must be used, the condition of the feedwater should be noted. These additional details will be helpful in interpretation of results including comparison of findings from carryover assessments made over time. Such an approach greatly improves the chance for identifying and correcting problems before target values for impurities allowed in the reheat steam are exceeded. The saturated steam chemistry data and trends over time can also be related to the performance of the steam drum moisture separators by measuring the carryover of impurities from the boiler into the steam. Excessive carryover may indicate poor moisture separator performance. When the design includes multiple steam leads and these can be sampled individually, it should be possible to identify areas of greatest concern. The latest cycle chemistry guidelines are based on findings of a series of EPRI sponsored investigations of volatility [4-10]. In the case of constituents such as silica and copper, volatile carryover is a significant contributor to total carryover over a broad range of operating pressures [8,9]. However assessment results for most impurities including sodium, chloride and sulfate has confirmed that mechanical carryover of has the greatest impact upon total carryover and steam purity, particularly at boiler drum operating pressures below 2500 psi (17.2 MPa). As discussed here and in the individual cycle chemistry guidelines, total carryover from drum boilers is considered now a Core Monitoring Parameter which should be routinely checked at intervals of about every six months so as to minimize the risk of steam contamination and possible damage to the turbine.

3.5 References 1. Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187. 2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188. 3. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925. 4. Behavior of Ammonium Salts in Steam Cycles. EPRI, Palo Alto, CA: December 1993. TR102377. 5. Volatility of Aqueous Sodium Hydroxide, Bisulfate and Sulfate. EPRI, Palo Alto, CA: February 1999. TR-105801.

3-5

EPRI Proprietary Licensed Material Carryover in Drum Boilers

6. Vapor-Liquid Partitioning of Sulfuric Acid and Ammonium Sulfate. EPRI, Palo Alto, CA: February 1999. TR-112359. 7. Volatility of Aqueous Acetic Acid, Formic Acid and Sodium Acetate. EPRI, Palo Alto, CA: July 2000. TR-113089. 8. Behavior of Aqueous Electrolytes in Steam Cycles: The Solubility and Volatility of Cupric Oxide. EPRI, Palo Alto, CA: November 2000. 1000455. 9. The Volatility of Impurities in Water/Steam Cycles. EPRI, Palo Alto, CA: 2001. 1001042. 10. Vapor-Liquid Partitioning of Phosphoric Acid and Sodium Phosphates. EPRI, Palo Alto, CA: 2003. 1007291. 11. Assessment of the Ray Diagram. EPRI, Palo Alto, CA: 1996. TR-106017. 12. Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

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EPRI Proprietary Licensed Material

4 CONDUCTIVITY

4.1 Purpose and Use Conductivity, more specifically cation conductivity is an EPRI Guideline Core Monitoring parameter [1-4]. As such, cation conductivity is continually monitored on-line to indicate the ionic concentration in the solution being monitored and to warn of contaminant in-leakage. Three types of conductivity measurements are discussed:

•

Specific conductivity

•

Cation (acid) conductivity

•

Degassed cation conductivity

These three types of conductivity are also monitored for one or more of the following reasons:

•

To facilitate the correlation of a water chemistry parameter (e.g., pH, conductivity, ammonia correlation).

•

To check the accuracy of water chemistry control (such as ammonia or pH).

•

To warn of condensate polisher malfunction.

•

To provide feedback signal for automated process control.

•

To facilitate demineralizer system operation and/or regeneration.

•

To monitor for the intrusion of volatile contaminates (e.g., CO2 or volatile organics).

•

To monitor the laboratory pure water system.

•

To determine the concentration of ions in solution.

The data generated by the on-line monitoring equipment are used by plant chemistry and operations department personnel. The goal for plant personnel is to identify the intrusion of impurities into the steam-water cycle and take appropriate corrective actions to minimize the duration and severity of the upset conditions.

4.2 Description On-line conductivity monitoring equipment measures the ability of water to conduct electricity (i.e., the electrical conductivity of water). Water conducts electric current by the movement of ions dissolved in the water. Consequently, the conductivity value can be used to assess the 4-1

EPRI Proprietary Licensed Material Conductivity

purity of water or, in cases where the ions are corrosive, conductivity can be used to assess the corrosivity of water. Conductivity of power plant waters in the boiler or steam generator cycle is usually measured in units of micro Siemens per centimeter (µS/cm), formerly called micro mho per centimeter, which, by definition, is the inverse of resistivity in units of megohm-cm. The conductivity, C, of an aqueous solution depends on the concentration (Q) of the dissolved substance, the degree of dissociation (α) into positively and negatively charged ions, and the charge on those ions which is dependent on their valence (n):

C = α QnF ( I + + I − )

Equation 4-1

Where F is Faraday’s constant (96,463 coulombs), and I+ and I- represents the mobility of the positive and negative ions, respectively. The mobility is dependent on the type of dissolved substance. If more than one substance is present, Equation 4-1 is used to determine the contribution of each substance, and then the total conductivity is obtained by adding the individual contributions together. Three types of conductivity are routinely measured in power plants: specific conductivity, cation conductivity, and degassed cation conductivity. Common to all three types is the measurement of the resistance of a known volume of water in a water sample cell. There are two common approaches to making these measurements: either with an on-line flow-through cell or dip cell fitted with electrodes. For either application, electrode sensors are immersed in the water and the resistance between the electrodes is measured directly. In this section, only on-line flow-through cell measurements are discussed further. Electrode materials must be corrosion-resistant in the expected environment. Typical materials include nickel, stainless steel and titanium. The body of the flow-through cell may be fabricated from such materials as stainless steel (e.g. type 316) or polyethersulfone (PES). Specific conductivity is one of the most frequently monitored parameters in power plant waters. In its most common form, the technique involves measuring the electrical conductivity (or conductance) of a volume of water which occupies the space between two electrodes of known dimensions. For instance, if the electrodes are flat, parallel plates a distance L apart, and each has a surface area, A, facing the opposite plate, the conductivity, C, of a water sample between the two electrodes is given by:

C = 1/R = (κ × A)/L

Equation 4-2

or by:

κ = L/(A × R) = (L × C )/A

4-2

Equation 4-3

EPRI Proprietary Licensed Material Conductivity

where R is the resistance of the sample and κ is the specific conductivity. By definition, κ is the conductivity of a column of the sample water having a length of 1 cm and a cross-section of 2 1 cm measured at 25°C (77°F). In on-line conductivity monitoring equipment, the electrodes are incorporated into a flowthrough conductivity cell allowing the water sample to be constantly (or frequently) refreshed, thereby allowing the measured conductivity value to be continually updated. Cation conductivity is another common conductivity measurement which can provide useful additional information when compared with specific conductivity. Here, before measuring the conductivity, the water sample first passes through a cation resin bed which exchanges any cations (positive ions) in the water for hydrogen ions (Figure 4-1). For instance, if the water contains NaCl, the dissolved Na+ ions are held in the bed and H+ ions are released in their place. Thus, NaCl (equivalent conductance 126 S-cm2/equivalent) is converted into HCl (equivalent conductance 426 S-cm2/equivalent) yielding a conductivity increase of about a factor of 3. In a similar manner, other salts are converted to their corresponding acids with an accompanying approximate 3-6 fold increase in conductivity. The conductivity of the water leaving the cation resin bed—the cation conductivity—is measured in a second flow-through cell, similar to the cell used for specific conductivity measurements. Any acids in the sample would, of course, pass through the cation bed unchanged because the positive ions involved are already H+ ions. Conversely, if the water contains chemicals that are alkaline in nature, such as NH4OH or NaOH, the NH4+ or Na+ ions would be replaced by H+ ions and result in the formation HOH (that is H2O, or water) with a corresponding large decrease in conductivity. If the cation conductivity is lower than the specific conductivity, it would reflect the removal of alkaline species by the cation bed, such as those that might be used for water treatment or demineralizer regeneration (e.g., NH4OH, NaOH respectively). If the cation conductivity is higher, it would indicate the presence of salts, like NaCl, in the original water sample, possibly caused by cooling water in-leakage in the condenser. Interpretation of cation conductivity values becomes more difficult when opposing effects of salts and alkaline species are seen in the same sample. Carbon dioxide (CO2) may also be in the sample and will be converted to carbonic acid as the sample stream passes through the cation column. Elevated cation conductivity from carbon dioxide may not be a corrosion concern and its contribution to cation conductivity may be removed using a degassed cation conductivity monitor.

4-3

EPRI Proprietary Licensed Material Conductivity

Figure 4-1 A Typical Cation Conductivity Flow Diagram [11] Source: Adapted from Reference 11, Courtesy American Society for Testing and Materials

Various cation conductivity measurements can be related to the concentrations of specific chemical species. Figures 4-2, 4-3, and 4-4 show conductivity as a function of a single species; HCl, H2SO4 and H2CO3 respectively [5]. Degassed cation conductivity is the third type of conductivity measurement. During startup the main concern is the presence of salts in the steam/water cycle. The detection of these salts is enhanced as they are converted to strong acids in the cation column as previously discussed but the presence of carbon dioxide also increases cation conductivity thereby reducing the sensitivity of the conductivity measurement to dissolved salts. To eliminate the effects of carbon dioxide and to minimize the hold times, the water may be degassed prior to measuring the cation conductivity. The resulting degassed cation conductivity of the feedwater or steam more accurately reflects the effects of the dissolved salts of interest during startup. This is achieved by delivering the water sample leaving the cation bed to an electric reboiler where the sample is heated to near boiling temperatures to drive off the dissolved gases. The conductivity of the water leaving the reboiler is then measured in a third flow-through cell. With the contribution from dissolved gases removed, this conductivity measurement can be employed as an extremely sensitive detector of small condenser leaks or malfunctions in the condensate polisher or demineralizer. Figure 4-5 shows a schematic diagram of a typical degassed cation conductivity monitor.

4-4

EPRI Proprietary Licensed Material Conductivity

Figure 4-2 Chloride (Cl) Concentration vs. Specific Conductivity [5] Source: Adapted from Reference 5, Courtesy American Society for Testing and Materials

4-5

EPRI Proprietary Licensed Material Conductivity

Figure 4-3 Sulfate (SO4) Concentration vs. Specific Conductivity [5] Source: Adapted from Reference 5, Courtesy American Society for Testing and Materials

4-6

EPRI Proprietary Licensed Material Conductivity

Figure 4-4 Carbon Dioxide (CO2) vs. Specific Conductivity [5] Source: Adapted from Reference 5, Courtesy American Society for Testing and Materials

4-7

EPRI Proprietary Licensed Material Conductivity

Figure 4-5 Typical Degassed Cation Conductivity Schematic Diagram [5] Source: Adapted from Reference 5, Courtesy American Society for Testing and Materials

For accurate results, the temperature of the water sample entering this cell must be closely controlled, and the instrument must be capable of correcting for temperatures near boiling. Temperature compensation is very important in degassed cation conductivity measurements, and is complicated by the fact that measurements must be performed near the boiling point of water. With adequate insulation of the cell and temperature measuring thermometer, and with careful adjustment of the reboiler temperature controller, the reboiler can provide a supply of flowing water at a constant temperature of about 95°C. For these acidic solutions at temperatures near boiling temperature correction factors are complex. End users should evaluate the proposed correction factors to assure the instrument suppliers have accommodated for these unique conditions into their temperature correction factors (See discussion which follows on Cation Conductivity Temperature Compensation).

4-8

EPRI Proprietary Licensed Material Conductivity

Other methods of stripping the dissolved gases from the sample (e.g. sparging with high purity nitrogen) are also available, but they are more labor-intensive and not as widely used.

4.3 Technical Considerations Water samples taken from the steam/water cycle typically have low ionic strength (i.e., 0.1 to 100 µS/cm specific conductivity), leading to several analytical challenges. These challenges can be overcome provided appropriate conductivity cells with appropriate cell constants are selected; appropriate temperature compensation is applied; cation conductivity resin and columns are properly designed and used; carbon dioxide and organic acid interference is understood and, appropriate cell constant checks are performed. Without appropriate consideration to these factors, conductivity measurements may be erroneous. 4.3.1 Cell Constants The dimensions of the cell determine the value of the cell constant. In equation 2, for instance, the cell constant is the geometric factor, L/A. In practice, electrodes are seldom plate-shaped, rather they are rings, cylinders or pins; but, whatever design is used, the geometry of the system determines the cell constant. The units of the cell constant are reciprocal length, e.g. cm-1. The dimensions of the cell, and hence the value of the cell constant, must be selected to provide enough sensitivity over the conductivity range of interest. Low conductivity measurements (high resistance) require a low cell constant—that is, large electrodes that are relatively close together. Conductivity cells for high conductivity (low resistance), on the other hand, have a high cell constant where electrodes are smaller and spaced further apart. In most commercial instruments, the value of the cell constant must be manually selected on the associated instrumentation before the measurements are made. Typical conductivity range limits are listed in Table 4-1 as a function of cell constant. Table 4-1 Typical Conductivity Range Limits as a Function of Cell Constant Cell constant (cm-1)

Optimum Conductivity Range (µS/cm)

Optimum Resistively Range (ohm-cm)

0.01

0.055 to 20

50,000 to 18,000,000

0.10

0.5 to 200

5000 to 2,000,000

1.00

10 to 200

500 to 100,000

10.00

100 to 20,000

50 to 10,000

100.0

1000 to 200,000

5 to 1000

4-9

EPRI Proprietary Licensed Material Conductivity

4.3.2 Cell Constant Determination or Verification Cell constants stated by the manufacturer with each cell are typically determined experimentally. Cells are manufactured to specific geometries to represent a specific cell constant. Subsequently, the cell constant may be influenced by minor dimensional variations during manufacture, shockinduced dimensional changes during shipping, mounting orientation, or flow velocity. Therefore it is recommended that the cell constant be determined with a stable solution of a known specific conductivity. The cell constant is calculated based on the observed specific conductivity compared to the expected specific conductivity [7]. For low conductivity measurements (cell constant 0.01) a stable solution with a known specific conductivity in a range of 0.55-20 µS/cm would be required to calculate the cell constant. Solution with specific conductivity 100 µS/cm specific conductivity). That reference conductivity cell is then used to determine or verify the lower cell constant cell (0.01) by comparison of a low conductivity sample stream as described ASTM D-5391 [8]. The lower cell constant cell being determined or verified should be connected to the same low conductivity (i.e., in the range of 0.55-20 µS/cm specific conductivity) sample stream as, and in close proximity to, the reference cell to avoid sample contamination and to minimize temperature differences between the cells. 4.3.3 Cell Construction and Installation Considerations Cells used for low ionic strength water applications (1.5 mm, 0.06 in., between plates) or be located downstream of installed resin traps. Cells should never be installed downstream of pH monitors due to sample contamination from KCl ions released through the pH reference electrode. Cells should be designed to minimize the capacitance of the cell and the lead wires. Lead wires should be limited to 7 m (20 ft.). Excessive capacitance can lead to a positive bias conductivity. 4.3.4 Cell Polarization R (or C) in Equation 4-2 is determined by measuring the current resulting from applying a known potential across the electrodes of the conductivity cell. An important factor in determining the accuracy of conductivity measurements is the extent of polarization caused by the passage of this current. If a direct current (DC) potential were to be applied across the electrodes, positively charged ions in the sample water would move to the cathode and negatively charged ions would move to the anode. This movement and accumulation of ions at the electrodes would result in polarization of the electrodes, which would oppose (be of opposite polarity to) the applied DC potential, and would result in a reduction in the current. In commercial 2-electrode systems, the effects of polarization are greatly reduced by applying an alternating current (AC) potential instead of a DC potential. The reversing polarity of the AC current counters the tendency for ions to accumulate at one electrode or the other. In principle, the higher the frequency the lower is the polarization effect, but capacitance effects in the connecting cables provide an upper limit to the frequency that can be used in a practical system. While the use of an AC potential to energize the conductivity cell is standard practice, it may not completely eliminate polarization. However, the effects of polarization in a 2-electrode conductivity cell can be minimized by ensuring that the optimum cell constant is selected, as previously discussed, so that the current density flowing between the electrodes is as low as possible without sacrificing adequate measurement sensitivity. 4.3.5 Temperature Effects Specific conductivity, cation conductivity, or degassed cation conductivity measurements are affected by the temperature of solution being measurement. By convention, conductivity is routinely reported corrected to 25°C (77°F). Even minor temperature differences from 25°C (77°F) can have a significant impact on observed conductivity measurements. An error of only 0.1°C in the temperature measurement can result in an error of 0.0006 µS/cm in water that has a conductivity of 0.055 µS/cm; an error of over 1 percent. Because of this error, samples outside 25 ± 0.2°C must have an appropriate temperature compensation factor applied. In many modern microprocessor-based conductivity instruments, temperature is measured directly in the conductivity cell and the instrumentation automatically corrects the conductivity 4-11

EPRI Proprietary Licensed Material Conductivity

value for temperature. To assure accurate temperature measurement, microprocessor-based conductivity instruments may also feature a simple electronic temperature calibration feature. However, a more precise temperature measurement calibration, using separate certified thermometers, may provide improved performance. The conductivity at 25°C (77°F) is normally the value calculated and displayed by the instrument. However, it is important to note that the temperature coefficient, “a”, in Equation 43 is dependent on the nature and concentration of the ions present (solute) in the solution being monitored, temperature coefficients can range from about 0.5% per °C to 3.0% per °C in strong electrolyte, highly conductive solutions [9]:

κ

T

=

κ

25

[1 + a(T - 25)]

Where: κ T and respectively.

κ

25

Equation 4-4

are the specific conductivities (µS/cm) at temperatures T°C and 25°C,

4.3.5.1 Solute Effects For power plant steam/water cycle specific conductivity measurements > 10 µS/cm, the coefficient “a” in Equation 4.3 is typically set to 2% per °C different from 25°C. The 2% temperature coefficient is appropriate for the most common ions present in a power plant steam/water cycle. In more sophisticated, microprocessor-controlled instruments, the value of “a” in Equation 4-3 can be adjusted to suit the particular solution being monitored. 4.3.5.2 Solvent Effects For conductivity measurements > 10 µS/cm, hydrogen (H+) and hydroxyl ions (OH-), formed by dissociation of the solvent (water), contribute very little to the overall conductivity of the solution. However, for conductivity measurements < 10 µS/cm, the relative importance of these ions increases until in ultra high purity waters approaching 0.055 µS/cm, they become the dominant factor as the only contributors to conductivity. At 25°C (77°F), pure water has a conductivity of 0.055 µS/cm. In these pure waters, the linear relationship given in Equation 4.3 is no longer valid, and more accurate empirical equations must be used. The three most widely used equations are known as the General Electric (GE), Marsh and Stokes, and Truman Light equations [10]. Some microprocessor-controlled instruments provide all three of these options or a proprietary option to allow the user to select the preferred temperature compensation equation.

4-12

EPRI Proprietary Licensed Material Conductivity

4.3.6 Cation and Degassed Cation Conductivity Temperature Compensation Sample temperature compensation for cation conductivity requires a unique correction factor for the acidic composition of the cation effluent. Cation conductivity samples have had their cations + + replaced with H ions and therefore are acidic solutions. The H ions have a direct impact on the ionization of water. The additional H+ ions tend to suppress the ionization of water. This interaction and its variation with temperature require more sophisticated temperature compensations to provide accurate cation conductivity measurement, particularly at temperatures outside 25 ± 0.2 °C [9]. End users should evaluate cation conductivity monitors to assure appropriate temperature compensation is applied. 4.3.7 Cation Conductivity Column Connections, Size, Flow and Flow Rate Considerations Cation columns come in a variety of shapes and sizes, making it important to match the appropriate sample flow with the cross-sectional area of the column. Sample flow through the column may be either upward or downward. In either case, air needs to be vented from the column to avoid incorrect readings. The columns must be fitted with an end screen to distribute flow across the cross section of the column and to prevent resin fines from leaking out of the column. Upward flow provides for automatic venting of air during initiation of sample flow which may be helpful for establishing faster accurate readings during startup of cycling plants. However, the cation resin in the column must be packed full to prevent fluidizing or channeling within the cation column. For downward flow cation columns fluidizing is not a concern since the resin is compressed in the cation column from the sample flow. However, channeling or incomplete ion exchange may occur when flow velocity through the column is too fast or too slow. Fast downward flow causes channeling where a portion of the flow to move faster than the bulk flow through the column thereby not allowing adequate contact time for complete ion exchange. The most common problem with cation conductivity is not having adequate flow velocity to maintain the turbulence to bring the sample into close contact with the resin resulting in incomplete exchange. Columns should have an inside diameter of < 60 mm, (2.3 in) and be designed to have a flow velocity of > 300 mm/min, (1 ft/min) and be constructed of non-leaching materials. Tubing used to connect the sample stream to the cation column should be minimized and ideally not be permeable to gases (i.e., CO2) [11]. 4.3.8 Cation Conductivity Resin Exhaustion, Regeneration and Rinse-in The measurement of cation conductivity involves passing the sample stream through a bed of strong acid cation exchange resin. Resin used in the cation column should be sulfonated styrenedivinylbenzene 8% cross-linked, strong acid gel resin in the hydrogen form. The resulting cation4-13

EPRI Proprietary Licensed Material Conductivity

exchanged sample stream is then directed to the conductivity cell for measurement. Successful measurement of cation conductivity requires very careful packing of the resin bed to avoid channeling of the flow. In previous validation studies [12] nuclear grade “indicating resin” (type TDC-1 hydrogen form cation resin) was used, and the data indicated that the beds began to lose efficiency as soon as the top of the bed began to change color. Since the beds must be replaced as soon as the efficiency begins to drop, the color warning provided by these indicating resins is helpful but not an absolute indication of resin efficiency. For new columns a rinse-in period is needed to establish a stable reading. Residual regeneration chemicals and ions that leach out from the resin while the resin is not in use must be flushed out of the resin column before a stable accurate measurement can be obtained. A rinse-in time of a few minutes to several hours may be experienced depending on the condition of the resin being rinsed. If a stable measurement has not been achieved after a few hours of rinse-in the resin may be unacceptable for use. Consult the cation resin supplier’s information on rinse-in times. Alternatively, two resin columns can be used in series with a conductivity cell after each column; a difference in the two conductivity readings signals depletion of the upstream column. If the cation resin is not replaced before exhaustion, cations in the sample will not be exchanged completely, leading to an error in the cation conductivity measurement. This series flow schema also assures the second column is being adequately rinsed to minimize flush time when it is placed in service [12].

4.4 Interferences 4.4.1 Organic and Strong Acids Interferences It is worth noting that interpretation of the difference between cation conductivity and degassed cation conductivity values could be complicated by the presence of low molecular weight organic acids, such as formic acid and acetic acid. However, studies have shown that the actual loss of formic or acetic acid from a degassed cation conductivity system via the system vent is so low that this loss can be disregarded [13]. Further it has been shown that the strong acids formed from neutral salt conversion in the cation column (i.e., HCl or H2SO4 ) are not volatile at degassed cation conductivity tempertures [14]. 4.4.2 Sample Line Interference For low ionic strength water measurements (< 10 uS/cm) long sample lines should be avoided. Ions in the sample steam tend to interact with the surface of the sample lines. These interactions have ion-exchange like characteristics (so called inorganic ion exchange). To the extent practical, all joints on sample lines should be air-tight. Sample line flow velocity should be maintained > 2 m, (6 ft.) per second to prevent sample deposition. Sample flow through the cell should typically be 200 mL/min, unless otherwise specified by the supplier.

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EPRI Proprietary Licensed Material Conductivity

4.5 Calibration The analytical capabilities of on-line conductivity instruments should be checked periodically to demonstrate calibration stability. One method exists for verifying instrument stability; the Line Method [15]. For the Line Method, a calibrated separate conductivity meter, typically a portable conductivity meter, is used to analyze the same sample steam as the installed on-line instrument. The results of the comparison are evaluated against the acceptance criteria (e.g., the measured values agree within ± 0.1 µS/cm). Provided the on-line analyzer agrees within the acceptance criteria the on-line instruments calibration has not changed. If the results are outside the acceptance criteria the on-line instrument should be recalibrated.

4.6 Calibration Checks The conductivity measurement involves two main components; the conductivity cell and the conductivity measuring instrument. The determination and verification of cell constants has been previously discussed. Evaluating a conductivity measuring instrument requires the use of precision resistors with a tolerance of ± 0.1% [8]. The precision resistors are used in place of the measuring cell and temperature compensator. The conductivity check resistance in ohms equals -1 the cell constant (cm ) divided by the conductivity to be indicated (S/cm). The difference between the stated precision resistors resistance and the instrument read out converted to resistance should be compared to the instrument suppliers recommended acceptance criteria. If the difference between these two resistances is outside the instrument suppliers recommended acceptance criteria the conductivity measuring instrument should be recalibrated or replaced. Additional maintenance and calibration activities are recommended in EPRI Report GS-7556 [11].

4.7 Alternative Methods for Determining Conductivity Because of the interrelationship between pH, specific conductivity and ammonia, knowing any two of these variable can lead to an estimation of the third. If ammonia and pH are known then specific conductivity may be estimated. Figure 4-6 shows the relationship between ammonia and specific conductivity.

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EPRI Proprietary Licensed Material Conductivity

Figure 4-6 Relationship Between Ammonia Concentration mg/L (ppm) and Specific Conductivity (µS/cm) at 25°C (77°F) [1] Source: Reference 1

Specific conductivity is a measure of the concentration of ions in solution. Assuming only one ion is in solution then specific conductivity can be calculated from the equivalent conductance [6] for that ion and the conductivity of the solvent, water. The equivalent conductance, Λ, is defined as the conductivity of 1.0 gram equivalent of solute contained between two electrodes spaced exactly 1.0 cm apart. The solution volume and the electrode area are not specified. The 4-16

EPRI Proprietary Licensed Material Conductivity

equivalent conductance is obtained by multiplying the conductivity (specific conductance), κ, by the volume, V, (in milliliters) containing exactly 1.0 gram equivalent weight.

Λ =κ V

Equation 4-5

Since the normal concentration (normality), N, of a solution is the number of equivalents per liter (or 1000 mL, which is 1000 cm 3 ) equivalent conductance is:

Λ=

1000κ N

Equation 4-6

The equivalent conductance ( Λ ) has units of S - cm 2 - equivalent -1 . Conductivity (specific conductance) can be obtained from equivalent conductance as follows:

κ (µ S − cm ) = −1

(

)

Λ S − cm 2 − equivalent −1 N ( equivalent/L ) 3

1000 cm /L

106 µ S/S Equation 4-7

κ = 1000 Λ N Equation 4-6 shows that specific conductivity can be determined from equivalent conductance and the solution normality. The concentration of ions has an effect on conductance; the higher the concentration the lower the equivalent conductance because of the interaction of ions in carrying charges (i.e., “activity” effects). The equivalent conductance at infinite dilution is the value used to calculate the conductivity of most power plant solutions in which ionic compounds are present in the mg/L (ppm) level. Table 4-2 gives the equivalent conductance values for selected ions at infinite dilution. The solution conductivity is the sum of the individual component conductivities, including that of the solvent. This can be stated as follows:

κ (S − cm −1 ) = ∑ Where

Λc Λ Cc + ∑ a Ca Ea Ec

Equation 4-8

Λ c = equivalent conductance of an individual cation, S - cm 2 − eq −1

E c = gram equivalent weight of an individual cation, g - eq −1

C c = concentration of an individual cation in mg/L (ppm) Λ a = equivalent conductance of an individual anion, S - cm 2 − eq −1

E a = gram equivalent weight of an individual anion, g - eq −1 C a = concentration of an individual anion in mg/L (ppm)

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EPRI Proprietary Licensed Material Conductivity

This equation is the same as:

κ (µ S − cm −1 ) = ∑1000 Λ c N c +

∑1000Λ N a

Equation 4-9

a

Where N c and N a are the normalities of the individual cations and anions, respectively. The following example illustrates the use of equivalent conductance of ions in calculating conductivity. Table 4-2 Equivalent Conductances of Separate Ions at Various Temperatures Equivalent Conductance, S-cm2 - equivalent-1 (at infinite dilution) Ion

0°C

18°C

25°C

50°C

75°C

100°C

Ammonium ( NH +4 )

40.02

64.5

74.5

115

159

207

½ Calcium (Ca+2)

30.0

51.0

60.0

98.0

142

191

½ Barium (Ba+2)

33.0

55.0

65.0

104

149

200

Hydronium (H3O+)

240

314

350

465

565

644

Potassium (K+)

40.4

64.6

74.5

115

159

206

Sodium (Na+)

26.0

43.5

50.9

82.0

116

155

41.1

65.5

75.5

116

160

207

Hydroxide (OH )

105

172

192

284

360

439

Nitrate ( NO −3 )

40.4

61.7

70.6

104

140

178

½ Sulfate (SO −42 )

41.0

68.0

79.0

125

177

234

Chloride (Cl-) -

Example: Calculate the conductivity of a 1 mg/L (ppm) NaCl (sodium chloride) solution at 25°C (77°F) from data in Table 4-2. Solution: a. The first step is to calculate the ionic mg/L (ppm) values:

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ppm Na + = ppm NaCl x

= 1 ppm x

Atomic weight of Na Molecular weight of NaCl

22.99 g Na/mole NaCl 58.44 g NaCl/mole NaCl

= 0.393ppm ppm Cl = ppm NaCl x

= 1 ppm x

Atomic weight of Cl Molecular weight of NaCl

35.45g Cl/mole NaCl 58.44 g NaCl/mole NaCl

= 0.607 ppm b. The equivalent weights must be determined: The equivalent weight for Na + is its atomic weight since it has a valence of +1.

E c = 22.9898 g/eq The equivalent weight for Cl − is the atomic weight because it has a valence of -1.

E a = 35.453 g/eq The conductivity can be calculated from the ionic equivalent conductivities from Table 4-2 assuming 25°C (77°F) Λ c = 50.9 S − cm 2 − equivalent −1 Λ a = 75.5S − cm 2 − equivalent −1

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EPRI Proprietary Licensed Material Conductivity

) ∑ ΛE

(

κ µS − cm −1 =

c

Cc +

c

=

Λa

∑E

Ca

a

50.9 75.5 x 0.393 + x 0.607 22.9898 35.45

= 0.870 µS − cm −1 + 1.23 µS − cm −1 = 2.163 µS − cm -1 Solvent Effects: The Conductivity of Pure Water Water ionizes to yield hydronium and hydroxyl ions:

2H 2 O

→ ←

H 3O + + OH −

Equation 4-10

The hydronium and hydroxyl ions contribute to the conductivity of the solution. At 25°C (77°F), the conductivity of pure water is:

κ (µS − cm −1 ) = κ H O+ + κ OH− 3

= 1000 Λ H O+ N H O+ + 1000Λ OH− N OH− 3

3

= (1000 µS − 1cm −3 − S−1 x 350 S − cm 2 − eq −1 x 1.004 x10−7 eq/l )

+ (1000 µS − 1cm -3 − S-1 x192 S − cm 2 − eq −1 x1.004 x10−7 eq/1)

= 0.0351 µS − cm −1 + 0.0193 µS − cm −1 = 0.0544 µS − cm −1 Thus, a sample of pure water will always still have a background conductivity because of the ionization of water:

κ m = κ s + 0.0544 µ S − cm −1 (25°C)

Equation 4-11

where κ m is the measured conductivity and κ s is the conductivity of the ionic constituents in the sample (i.e., the solute). 4-20

EPRI Proprietary Licensed Material Conductivity

4.4 End User Considerations The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment provided by the manufacturer or supplier should be considered when selecting a suitable on-line conductivity monitor. In general, manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application. Other on-line conductivity monitor considerations for pure water include:

•

Select conductivity cells with appropriate cell constants

•

Assure appropriate temperature corrections are applied for each conductivity measurement

•

Select cation columns with appropriate flow characteristics

•

Ensure cation resin is properly regenerated and rinsed for use

•

Be aware of potential carbon dioxide or organic acid interferences with cation and degassed cation conductivity

•

Minimize sample line lengths and use appropriate tubing for connections

•

Validate conductivity cell constants periodically

4.9 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.

4.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

5.

ASTM D4519-94, “Standard Test Method for On-Line Monitoring of Electrical Conductivity to Determine Anions and Carbon Dioxide in High Purity Water”. American Society for Testing & Materials, Philadelphia, PA.

6.

John Dean ed., Lange’s Hand Book of Chemistry, 12th edition, McGraw-Hill Book Company, 1979, Section 6, pp. 34.

7.

ASTM D1125-91, “Standard Test Methods for Electrical Conductivity and Resistivity of Water”. American Society for Testing & Materials, Philadelphia, PA. 4-21

EPRI Proprietary Licensed Material Conductivity

8.

ASTM D5391-93, “Standard Test Method for Electrical Conductivity and Resistivity of a Flowing High Purity Water”. American Society for Testing & Materials, Philadelphia, PA.

9.

D. Gray, “Cation Conductivity Temperature Compensation”, Ultrapure Water, Volume 16, No. 4, April 1999, pp. 60-63.

10.

O. Zabarsky, “Temperature Compensated Conductivity—A Necessity for Measuring HighPurity Water,” Ultrapure Water, pp. 56-60 (January/February 1992).

11. ASTM D D6504-00, “Standard Practice for On-Line Determination of Cation Conductivity in High Purity Water”. American Society for Testing & Materials, Philadelphia, PA. 12.

Monitoring Cycle Water Chemistry in Fossil Plants, Vol. 3 Project Conclusions and Recommendations, by A.F. Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon. EPRI, Palo Alto, CA: October 1991. GS-7556, Volume 3.

13.

M Gruszkiewicz, “Degasssed Conductivity—Comments on an Interesting Reasonable Plant Cycle Chemistry Monitoring Technique, Part 1: Degassing of Low-Molecular-Weight Organic Acids in Technical Degassed Cation Conductivity Monitors”, PowerPlant Chemistry, Volume 6(3), 2004, pp. 177-184.

14.

M. Gruszkiewicz, “Degasssed Conductivity—Comments on an Interesting Reasonable Plant Cycle Chemistry Monitoring Technique, Part 2: Degassing of Carbon Dioxide in Technical Degassed Cation Conductivity Monitors and Temperature Conversion of Cation Conductivity Measured at nearly 100°C to 25°C ”, PowerPlant Chemistry, Volume 6(5), 2004, pp. 279-289.

15.

ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis”., American Society for Testing & Materials, Philadelphia, PA.

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5 OXYGEN

5.1 Purpose and Use Dissolved oxygen (DO) is an EPRI Core Monitoring Parameter [1-4]. As such, the reason for continuous on-line monitoring of DO is to check the acceptability of water chemistry, thereby ensuring that dissolved oxygen is maintained at acceptable levels—with lower levels in feedwater for cycles employing reducing all volatile treatment-reducing (AVT (R)) and somewhat higher levels when using oxidizing all volatile treatment-oxidizing (AVT(O)) or oxygenated treatment (OT). Dissolved oxygen may also be monitored for one or more of the following reasons:

•

To check the accuracy of water chemistry control, so ensuring that corrosion rates are kept at acceptable low levels.

•

To facilitate the correlation of a water chemistry parameter with plant operating variables, with an aim to optimizing operations (e.g., condenser air removal or deaerator operations).

•

To provide feedback stimulus for automated process control, e.g., for oxygen control on oxygenated treatment (OT).

•

To monitor for condensate pump seal leakage.

•

During and following changes in feedwater treatment.

The data generated by continuous on-line monitoring of dissolved oxygen is used by plant chemistry and operations department personnel. The goal for plant personnel is to maintain dissolved oxygen within prescribed limits.

5.2 Description of Methods In typical fossil power plant steam/water cycles, dissolved oxygen concentrations may be temporarily high during start-up but rarely exceed 400 µg/L (ppb) (parts per billion) during normal operation and are often much less. On-line monitoring techniques that are particularly accurate over the concentration range of 0-400 µg/L (ppb) are therefore of great interest. Three approaches are discussed below. Two are based on diffusion—the galvanic and the polarographic methods—and the third approach is based on establishing equilibrium between dissolved oxygen concentrations on the inside and outside of the sensing probe. 5-1

EPRI Proprietary Licensed Material Oxygen

All three methods make use of a sensing probe immersed in the water sample of interest. In each case, the probe consists of two metal electrodes—an anode and a cathode—in contact with an internal electrolyte that is separated from the water sample by a semi-permeable membrane. Because all three methods measure current (amperes) flowing between the cathode and anode, they may be termed amperometric methods. Figure 5-1 shows a typical oxygen sensing probe.

Figure 5-1 Typical Oxygen Sensing Probe [5] Source: Adapted from Reference 5, Courtesy Mettler-Toledo Thornton

5.2.1 Galvanic Method In the galvanic method (also known as the Mackereth cell method), the anode material is typically composed of lead and the cathode is a much more noble material, such as silver, gold or platinum. The galvanic method is polarographic although it does not require external polarization. Consequently, these electrodes have different electrochemical potentials in the internal electrolyte, and they form a galvanic cell if they are electrically coupled through an external ammeter. The reactions that take place at, say, a platinum cathode and a lead anode in a potassium iodide internal electrolyte are as shown in equations 5-1 and 5-2. 5-2

EPRI Proprietary Licensed Material Oxygen

Cathode:

O 2 + 2H 2 O + 4e - → 4OH -

(Reduction Reaction)

Equation 5-1

Anode:

2Pb + 4I - → 2PbI 2 + 4e -

(Oxidation Reaction)

Equation 5-2

The lead is oxidized to lead iodide and the oxygen is reduced to hydroxyl ions. This results in the flow of an electrochemical current between the anode and cathode. The charge is carried by free electrons in the wires and ammeter within the external circuit, and by charged ions, such as I- and OH-, in the internal electrolyte. Another design [5] uses a sodium bicarbonate electrolyte, a platinum cathode and a lead anode. The electrochemical reactions can be summarized as shown in equations 5-3 and 5-4. Cathode:

O 2 + 2H 2 O + 4e - → 4OH -

Anode:

2Pb + 4OH - → 2PbO + 2H 2 0 + 4e - (Oxidation Reaction) Equation 5-4

(Reduction Reaction) Equation 5-3

In this case, there is no consumption of electrolyte and only the anode is oxidized. Consequently, with a suitable electrolyte composition the anode surface can be kept clean so maintenance frequencies can be extended to upwards of three years. In either case, the magnitude of this current is controlled by the rate at which oxygen arrives at the cathode surface, which is determined by the rate of oxygen diffusion through the semipermeable membrane and the internal electrolyte. The rate at which oxygen diffuses through the semi-permeable membrane is, in turn, proportional to the dissolved oxygen content of the water sample flowing past the membrane. Thus, the electrochemical current flow is proportional to the dissolved oxygen content in the water sample. The current can be converted with amplifiers and appropriate circuitry to provide a current or voltage output for a strip chart recorder, data logger, or control equipment. 5.2.2 Polarographic Method In the polarographic method (also known as the Clark cell method), the anode and cathode materials have different nobilities but not as widely different as in the galvanic method. The anode material is typically silver and the cathode is a more noble material, such as gold or platinum. Consequently, the natural galvanic effect is much smaller, and another approach is used to achieve the necessary sensitivity to oxygen. The potential difference between the cathode and anode (the cell voltage) is increased and held constant by external instrumentation. This cell voltage is typically held constant at a value in the range 0.8 to 1.0 volt. The current flowing between the cathode and anode is measured and, as in the galvanic method, the current is directly proportional to the dissolved oxygen concentration in the water sample flowing past the probe. The reactions that take place at, say, a gold cathode and a silver anode in a buffered potassium chloride internal electrolyte are as shown in equations 5-5 and 5-6. 5-3

EPRI Proprietary Licensed Material Oxygen

Cathode:

O 2 + 2H 2 O + 4e- → 4OH −

(Reduction Reaction)

Equation 5-5

Anode:

4Ag + 4Cl - → 4AgC1 + 4e -

(Oxidation Reaction)

Equation 5-6

The silver is oxidized to silver chloride and the oxygen is reduced to hydroxyl ions. This results in the flow of an electrochemical current between the anode and cathode. The charge is carried by free electrons in the wires and ammeter within the external circuit, and by charged ions, such as Cl- and OH-, in the internal electrolyte. The magnitude of this current is controlled by the rate at which oxygen arrives at the cathode surface, which is determined by the rate of oxygen diffusion through the semi-permeable membrane and the internal electrolyte. The rate at which oxygen diffuses through the semi-permeable membrane is, in turn, proportional to the dissolved oxygen content of the water sample flowing past the membrane. Thus, the electrochemical current flow is proportional to the dissolved oxygen content in the water sample. The current can be converted with amplifiers and appropriate circuitry to provide a current or voltage output for a strip chart recorder, data logger, or control equipment. 5.2.3 Equilibrium Method In the equilibrium method, the anode and cathode materials are typically platinum. Equilibriumtype probes [6] rely on establishing equilibrium between dissolved oxygen in the sample and inside the probe. When the probe is immersed in the sample, oxygen diffuses through the membrane and is reduced at the cathode as shown in Eq. 5-7. Simultaneously, an equal amount of oxygen is generated at the anode as shown in Eq. 5-8. The diffusion continues until the partial pressure of oxygen on both sides of the membrane is equal and balanced. The current necessary to maintain this equilibrium is converted to the dissolved oxygen concentration in the solution being monitored. Cathode:

O 2 + 4H + + 4e - → 2H 2 O

(Reduction Reaction)

Equation 5-7

Anode:

2H 2 O → O 2 + 4H + + 4e -

(Oxidation Reaction)

Equation 5-8

As in the diffusion-type approaches, oxygen is reduced at the cathode. However, in equilibrium sensors a measuring circuit causes electrical current to flow through the anode resulting in an oxidation reaction that produces oxygen. The net reaction of oxygen being reduced at the cathode and oxygen being produced at the anode leads to no net consumption of oxygen or water in the overall reaction. The equilibrium is maintained until the dissolved oxygen concentration changes in the sample stream and then a new equilibrium is reached. The resultant current flow is proportional to the dissolved oxygen concentration in the sample stream. The current can be converted with amplifiers and appropriate circuitry to provide a current or voltage output for a strip chart recorder, data logger, or control equipment.

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EPRI Proprietary Licensed Material Oxygen

5.3 Technical Considerations Water samples taken from the steam/water cycle typically have low dissolved oxygen (i.e., a few µg/L (ppb) to several hundred µg/L (ppb)), leading to several analytical challenges. These challenges can be overcome provided appropriate membranes are utilized, electrodes are kept clean, appropriate temperature and pressure compensations are applied, flow rate sensitivity is understood and interferences are minimized. Response times are optimized and calibration and calibration checks are performed as required. Without appropriate consideration to these factors dissolved oxygen measurements may be erroneous. 5.3.1 Membrane Replacement The semi-permeable membrane is usually polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), ethylene-tetrafluoroethylene copolymer, Teflon®, high density polyethylene, silicone or a similar material. Most often one type of membrane is used more typically with one type of sensing electrode. Polarographic type sensing electrodes tend to favor the ‘fluoro-type’ membranes. Galvanic type sensors tend to favor the high density polyethylene membranes. Equilibrium-type sensing electrodes tend to favor silicone membranes. One manufacturer [5] claims the thicker Teflon® membrane design reduces the amount of oxygen permeating the membrane thereby reducing sensitivity to coatings and low flow. Teflon® membranes are thicker but allow for faster transfer rate for oxygen. Since conventional fluoro-type membranes allow large volumes of oxygen through the membrane, the potential for depletion of oxygen near membrane surface requires a higher flow rate to assure oxygen levels are not depleted near the sensing membrane surface. Fouling of the fluoro-type membrane surface leads to slower permeation and lower readings. For fluoro-type membranes, the relative tension, thickness and smoothness of the membrane affect its gas permeability. Membrane replacement is required on fluoro-type commercially available DO analyzers after periodic electrode cleaning. Some manufacturers provide aids for installing membranes in a specific configuration. Other manufacturers provide a disposable DO probe to eliminate the need for, and variability in, membrane installation [7]. 5.3.2 Electrode Cleaning 5.3.2.1 Polarographic For polarographic sensors, the calibration slope can change if the anode turns gray from the deposition of silver that is dissolved in the electrolyte as silver chloride (an anodic reaction product). Polarographic type sensors therefore require periodic cleaning to remove silver chloride deposits from the anode.

5-5

EPRI Proprietary Licensed Material Oxygen

5.3.2.2 Galvanic Depending on the type of electrolyte and the size of the anode, galvanic sensing electrodes may require periodic cleaning. Specifically, galvanic sensing electrodes with a potassium iodide or potassium chloride based electrolyte may require infrequent cleaning of the anode when the electrolyte is being replenished. Deposits on the anode should be removed using a soft cloth and gentle cleaning action. Electrodes with sodium bicarbonate electrolyte and large surface lead (Pb) anode may require infrequent cleaning since the anode product, lead oxide (PbO), deposits over a large area. Cleaning for these large surface area anodes should be only be done per the instrument suppliers recommendations when the anode appears fouled. Electrodes with sodium bicarbonate electrolyte and platinum electrodes do not require cleaning [5]. 5.3.2.3 Equilibrium Equilibrium sensors are not typically cleaned or refurbished. Once they have reached their end of life they are replaced with a new sensor. 5.3.3 Temperature and Pressure Compensation for Sensors Oxygen has a temperature-dependent solubility in water at atmospheric pressure as shown in Figure 5-2. If the curve were extrapolated, the solubility at 100°C would be zero as water vapor pressure reaches 100% at boiling—again, at atmospheric pressure. The entire curve would be shifted upwards if the pressure over the water were increased. Working in the opposite direction, deaerators depend on partial vacuum and/or elevated temperatures to remove all dissolved gases. DO sensors are electrochemical devices that take advantage of the gas permeability of polymer membranes to separate the sensing electrodes from the sample. This separation enables a sensor to provide a controlled environment for the electrodes and electrolyte while allowing oxygen to enter from the sample and react. It keeps the electrochemistry fairly well contained and clean. The diffusion rate of oxygen through a membrane is proportional to the partial pressure of oxygen in the sample. Of course the membrane material and thickness also affect the diffusion rate, but they are fixed and those properties are normalized in calibration.

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EPRI Proprietary Licensed Material Oxygen

Figure 5-2 Solubility of Dissolved Oxygen (mg/L (ppm)) vs. Temperature (°C) [5] Source: Adapted from Reference 5, Courtesy Mettler-Toledo Thornton

The oxygen which permeates the membrane reacts at the cathode, producing a current in direct proportion to the quantity of oxygen. That current is the measurement signal which matches the oxygen partial pressure and the concentration of DO, at least at constant temperature. To derive a concentration measurement from partial pressure with varying temperature, the signal must be temperature compensated, based on the relationship in Figure 5-2. That is, the DO concentration in water that a partial pressure represents depends on temperature. The sensor’s RTD (resistance temperature detector) signal is used by the instrument microprocessor to temperature compensate the measurement. In addition, the instrument must compensate for the temperature-dependent diffusion rate of oxygen through the membrane material. Some utility personnel prefer to avoid the need for temperature compensation by ensuring that the temperature of the water sample is maintained at 25°C (77°F) as it passes over the oxygen sensor; in this way, sources of error associated with the temperature compensation algorithm can be avoided. During calibration, air is used as the standard and its oxygen partial pressure depends on the ambient barometric pressure at the time of calibration. Compensation for actual barometric pressure during calibration assures this dependency is accounted for. Pressure compensation is important because the tension in the semi-permeable membrane affects its diffusion characteristics. Compensation is usually handled electronically by the instrumentation, but when the water sample pressure is low (say < 345 kPag (50 psig)), pressure compensation can be done mechanically instead. This may be achieved by incorporating a 5-7

EPRI Proprietary Licensed Material Oxygen

flexible membrane in the walls of the oxygen probe housing so that the external water sample pressure can be transmitted immediately to the internal electrolyte, thereby maintaining equal pressure on both sides of the semi-permeable membrane. Another strategy to eliminate the need for pressure compensation is to provide an overflow head-cup on the sample supply line to assure a low constant pressure on the membrane. 5.3.4 Flow Rate Sensitivity For accurate measurements, it is essential that the oxygen content of the water at the outer surface of the semi-permeable membrane is representative of the bulk water passing through the flow cell. For diffusion type sensors, oxygen diffuses out of the sample through the semipermeable membrane and is reduced in the sensor. Therefore, a continuous flow of fresh sample must be supplied to the membrane surface. Minimum flow rates (e.g., 50-200 mL/min) are specified by the manufacturer of the monitoring equipment to ensure that the rate of oxygen removal by the sensor is negligible compared with the oxygen supplied to the flow cell. For equilibrium type sensors, oxygen diffuses out of the sample and into the semi-permeable membrane only until equilibrium is reached. Therefore, the sample must only be supplied to the membrane at a flow rate sufficient to allow for representative sampling of the process being monitored. In either case, an upper flow rate limit may also be specified, above which the water flow becomes so turbulent that the membrane vibrates and induces convective transport of oxygen and other species through the internal electrolyte. This condition would lead to an overestimate of the dissolved oxygen concentration. If there is a period when flow to the analyzer may be completely interrupted (such as when a unit is not in service), it is advisable to avoid prolonged sensor response to high oxygen levels by terminating power to the analyzer. 5.3.5 Response Time Generally it is better to have a fast responding sensor. Fast response time minimizes the time a sensor is out of measuring range after calibration and minimizes the time to see a change in DO concentration in the process stream. 5.3.5.1 Response Time After Air Calibration Most modern sensors can achieve 90% reduction from air saturated conditions within 60 seconds after calibration and be fully within normal operating ranges within 20-30 minutes. For a polarographic-type oxygen sensor, an auxiliary ring-shaped cathode (“guard ring electrode”) may be installed to surround the main centrally located disk-shaped cathode. One purpose of the auxiliary cathode is to reduce residual oxygen present in the electrolyte after air calibration. The auxiliary cathode is also designed to reduce or eliminate the adverse effect of 5-8

EPRI Proprietary Licensed Material Oxygen

interfering species that may initially be present in the internal electrolyte or that are created by the anodic reaction. In the absence of the auxiliary cathode, these species would diffuse to the main cathode and contribute to the total measured current, thereby leading to an overestimate of the dissolved oxygen concentration. With the auxiliary cathode in place, these species are removed before they can reach the main cathode. For all types of oxygen sensors, sample line and sample flow cell considerations should be optimized to minimize the time it takes to purge residual DO out of the sample chamber and lines. After air calibration it is important to understand that all of the sample lines may be full of air, and that all the residual oxygen must be purged out and/or dissolved into the sample liquid, and then dissipated before the readings will read the correct “system” value. The larger volume of the sample line that is opened to air during the air calibration, the longer is the recovery time. Also, the slower the flow rate after calibration, the longer the recovery time. So it is best to first purge the air out with a relatively low (gentle) flow rate, then after the lines are full of water, increase the flow rate to reduce the time required to dissipate the remaining DO. 5.3.5.2 Response Time Due to Changes in Dissolved Oxygen Concentration in the Process Stream Once residual DO is purged from the sample chamber and lines, a change in the process stream DO can be detected by all types of sensing electrodes in a matter of a few minutes. Response time tends to increase for polarographic type sensors over a calibration cycle due to silver deposition on the cathode. See section on Interferences which follows for more detailed discussion of this phenomenon.

5.4 Interferences 5.4.1 Oxygen Contamination Oxygen (in the form of air) is an omnipresent contaminant. Sample line and instrument sample chamber leaks are likely sources of oxygen contamination. All sample lines and flow chambers should be air-tight and be constructed of stainless steel (e.g., Type 316SS). Polyvinylidene flouride (PDVF) or Nylon tubing are suitable for temporary connections if needed. One helpful strategy to determine if dissolved oxygen readings are being contaminated is to adjust sample flow. Flow rate increases of up to 50% may be appropriate for this diagnostic check; however, do not exceed instrument suppliers maximum flow rate. If increasing flow produces a decrease in DO, then some of the DO is likely coming from a leak since, as the sample flow increases and the source leak remains the same, the resultant DO goes down because it is being diluted by the additional sample flow. If the DO does not decrease with an increase of sample flow then the DO reading is correct. If the DO increases as the sample flow rate is increased then the original sample flow rate was most likely too low.

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EPRI Proprietary Licensed Material Oxygen

5.4.2 Sample Conditioning Hot samples should be cooled as close as possible to the sampling point to inhibit the reaction of dissolved oxygen with hydrazine or other reducing agents as the sample flows through the sample line to the monitoring instrument. New sample lines should be conditioned in accordance with ASTM D 3370 [8]. 5.4.3 Electrolyte Polarographic type sensors and galvanic type sensors that use an iodide or chloride salt containing electrolyte consume electrolyte during operation. Periodic recalibration is necessary to adjust for this consumption and for other factors such as the electro-deposition of silver on the cathode. Periodic electrolyte replacement is also required. 5.4.4 Stray Current For polarographic sensors, dissolved silver ions from the anode may contribute to stray current from the anodic oxidation reaction shown in equation 5-9.

4Ag + 4Cl - → 4AgC1 + 4e -

Equation 5-9

5.4.5 Membrane Fouling, Positioning and Tension Deposits on the permeable membrane (e.g., iron oxides) will slow instrument response and can lead to a negative bias. For equilibrium sensors this has less impact on DO measurements since only a small amount of oxygen is needed to establish equilibrium. Buildup of organics in sample lines or sample chambers will impact DO measurements. Affected surfaces should be cleaned with a dilute biocide. Membrane tension and positioning can affect sample read out. Manufacturer’s instructions for membrane installation should be followed. 5.4.6 Hydrogen For sample streams containing hydrogen, i.e., stator cooling water, dissolved hydrogen can cause significant negative errors in equilibrium probes. Equilibrium type probes are not recommended for these applications. Galvanic type probes are not affected by dissolved hydrogen (H2) and are recommended for use when hydrogen is present. Polarographic type sensors must be compensated for dissolved hydrogen concentrations in the range of 25-50 cc/kg at STP in the sample stream.

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EPRI Proprietary Licensed Material Oxygen

5.4.7 Other Cycle Chemistry Additives Volatile additives in the sample stream (reducing agents, ammonia, hydrogen and other neutralizing amines) may pass through the probe and cause unwanted reactions leading to negative errors. The instrument supplier’s manual should be consulted for relevant cautions and limitations. 5.4.8 TDS Although not typically an issue with power plant steam water cycle samples, high dissolved solids (> 1000 mg/L (ppm)) affect the solubility of oxygen. Some analyzers provide a salinity correction which allows the user to input a correction factor for salinity in parts per thousand [6].

5.5 Calibration Because of the difference in operating principle for each of the three sensing electrodes, there are different calibration needs. 5.5.1 Polarographic Polarographic sensing electrodes require calibration each time the anode and cathode are cleaned and each time the internal electrolyte is replenished. The time between calibrations is dependent on the DO concentration being measured. Higher DO concentrations generally mean that more frequent calibrations are necessary. For instance based on the acceptability of calibration check results, monthly or quarterly calibrations may be required. 5.5.2 Galvanic For galvanic sensors that use a salt containing electrolyte (e.g., iodide or chloride) the anode is oxidized as follows:

2Pb + 4I - → 2PbI 2 + 4e -

Equation 5-10

(Oxidation Reaction with potassium iodide salt electrolyte) Periodic cleaning of the anode is required. Additionally since electrolyte is consumed in this galvanic sensing electrode design, periodic replenishment of electrolyte is necessary. The time between calibrations is dependent on the DO concentration being measured. Higher DO generally means more frequent calibrations. Monthly or quarterly calibrations may be required. For galvanic sensors using a sodium bicarbonate electrolyte, the electrolyte is in dissociation equilibrium with water which produces hydroxide ions. 5-11

EPRI Proprietary Licensed Material Oxygen

Na + + HCO3- + H2O ←⎯⎯→ Na+ + H2CO3 + OH-

Equation 5-11

The anode is oxidized as follows:

2Pb + 4OH - → 2PbO + 2H 2 O + 4e -

Equation 5-12

(Oxidation Reaction with sodium bicarbonate electrolyte) Hydroxide ions are also generated by the cathode counter reaction (Eq. 5-1) proportional to dissolved oxygen concentration. Since electrolyte is not consumed in this reaction, the electrolyte is not required to be replenished. With proper design the anode surface is large in order to reduce the affects of oxidation and extend the intervals between probe cleaning. The time between calibrations is dependent on the DO concentration being measured. Higher DO generally means more frequent calibrations. Yearly or bi-yearly calibrations may be required. For galvanic sensors the anode can be cleaned with simple scrubbing, the electrolyte is not a proprietary solution and can be made in-house, and the replacement of the Teflon® membrane is easily accomplished without special tools or training. 5.5.3 Equilibrium For equilibrium sensors, since there is no consumption of the electrolyte and no oxidation of the anode frequent calibrations are not necessary. Consequently, based on the acceptability of calibration check results, only yearly or bi-yearly calibrations may be required. 5.5.4 Air Calibration for All Sensor Types In all of these cases, the on-line oxygen monitor allows span calibration by exposure of the probe to either air-saturated water or water vapor-saturated air. This is possible because the composition of air is constant over the entire globe, having 20.94% oxygen in dry air. This span calibration can be performed by plant personnel by manually turning a calibration knob or by pressing a button to activate a self-calibration function of the instrument. Some instruments can also be set to perform automatic span calibration periodically (such as every 7 days). During this procedure, the flow cell is automatically drained, thereby exposing the probe to moist air. Because barometric pressure has a direct affect on oxygen concentration in the atmosphere, a correction for barometric pressure is necessary during DO sensor calibration. Air, which contains 20.94% oxygen, will have an oxygen partial pressure of 159 mm Hg if the total pressure is 760 mm Hg. Data tables [9] of oxygen solubility are normally referenced to obtain this value. When conditions differ from this, an altitude or pressure correction must be made. The correction is made using equation 5-10.

5-12

EPRI Proprietary Licensed Material Oxygen

S=S’ (P-p)/ (760-p)

Equation 5-13

Where: S is the solubility of oxygen at the barometric pressure of interest, S’ is the solubility at 760 mm Hg at a given temperature, P is the barometric pressure in mm Hg, and p is the partial pressure of water vapor at the given temperature. Due to the large difference between the calibration concentration of oxygen and the routinely measured concentration of oxygen in power plant systems it may take hours for all trace oxygen to diffuse out of the probe allowing accurate measurements of the process stream.

5.6 Calibration Checks for All Sensor Types The analytical capabilities of on-line DO instruments should be checked periodically to demonstrate calibration stability. Three methods exist for verifying instrument stability; the Standard Injection Method [10], the Line Method [11] or by using a Faraday cell [12]. For the Standard Injection Method, a known DO near the expected DO of the process being monitored is analyzed by the on-line instrument and the results are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is considered to be acceptable. If the results are outside the acceptance criteria, the on-line instrument should be recalibrated. Introducing a known standard near the expected DO is no trivial task. One approach is to prepare a calibration gas comprising a low concentration of oxygen in nitrogen and to replace the normal sample flow to the DO analyzer with the calibration gas for the calibration period. However any source of air in-leakage would bias the test results high. A second approach [13] is to bubble a calibration gas into a capped, vented sample container with the appropriate DO probe immersed in the sample. The expected DO can be calculated based on atmospheric pressure, fugacity and solubility of the calibration gas. Again, any source of air in-leakage would bias the test results high. For the Line Method a calibrated separate DO meter, typically a portable DO meter, is used to analyze the same sample stream as the installed on-line instrument. The results of the comparison are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10%). Provided the comparison results agree within the acceptance criteria, the on-line instruments calibration is considered to be acceptable. If the results are outside the acceptance criteria, the on-line instrument should be recalibrated. Alternatively, a suitable wet chemistry method (Ampoules ASTM D 5543 [14], Indigo Carmine ASTM 888 [15]) may be used to determine DO in the sample stream and the results compared as above. For the third approach, a Faraday cell is used to generate a known concentration DO in the sample inlet by dissociating water. This is done by passing a current from one electrode through the sample water to a second electrode positioned some distance away from the first. The 5-13

EPRI Proprietary Licensed Material Oxygen

Faraday cell is designed so that the dissociated oxygen dissolves in the flowing sample. The corresponding dissociated hydrogen produced is not in sufficient quantity to affect instrument readings. The calculated increase in O2 can be measured with the DO meter. If there is agreement to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10, the on-line instruments calibration is considered to be acceptable. If the results are outside the acceptance criteria, the on-line instrument should be recalibrated. Figure 5-3 shows the response of a polarographic oxygen sensor to a 20 µg/L (ppb) oxygen addition generated by a Faraday cell.

Figure 5-3 Response of a Polarographic Oxygen Sensor to a 20 µg/L (ppb) Oxygen Addition Generated by a Faraday Cell [12] Source: Adapted from Reference 12

5.6.1 Zero Point for All Sensor Types Because the current flowing in an on-line amperometric dissolved oxygen analyzer is directly proportional to the dissolved oxygen concentration (more precisely, the partial pressure), a zero current should always represent zero oxygen. Thus, a zero-point calibration is not strictly necessary, although periodic correction for zero drift (to maintain electronic zero) may be necessary on some instruments.

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EPRI Proprietary Licensed Material Oxygen

5.6.2 Maintenance for All Sensor Types On-line oxygen monitors may be checked using substitute resistors in place of the probe. The instrument supplier’s instructions should be consulted for specific details. Since dissolved oxygen is typically corrected for the temperature of the sample stream, periodic verification of the temperature sensor is recommended. The instrument supplier’s instructions should be consulted for specific details. Temperature should be correct to within +/- 1°C (1.8°F). Calibration and maintenance procedures are typically described in literature supplied with the monitoring equipment by each manufacturer but, in some instances, it may be more appropriate to follow the maintenance and calibration activities recommended in EPRI Report GS-7556 [14].

5.7 Alternative Methods ASTM D5543-94 [15], and ASTM D 888-87 [16] provide alternative methods for determining DO. However, neither colorimetric method is suitable for on-line determination. Following is a discussion of a recently developed oxygen sensor technology. 5.7.1 Luminescent Oxygen Sensors [17] Optical sensing of oxygen originates from the work of Kautsky in 1939 where he demonstrated that oxygen can dynamically quench the fluorescence of an indicator (decrease the quantum yield). This principle has been reported in various fields of application such as monitoring aquatic biology in waste water tests for blood gas analysis and cell culture monitoring. The method is now recognized by ASTM for the measurement of oxygen in water. Compared to classical oxygen detection using electrochemical sensors, luminescent technology offers several advantages such as no oxygen consumption, independence from sample flow velocity, no electrolyte and low maintenance. Optical sensing of oxygen is based on the measurement of the red fluorescence of a dye/indicator illuminated with a blue light as show in Figure 5-4. The dye fluorescence is quenched by the presence of oxygen. The oxygen concentration can be calculated by measuring the decay time of the fluorescence intensity as shown by Figure 5-5. The higher the oxygen concentration is, the shorter the decay time will be. By modulating the excitation, the decay time is transformed into a phase-shift of the modulated fluorescence signal, which is independent of fluorescent intensity and thus of potential aging (Figure 5-6).

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EPRI Proprietary Licensed Material Oxygen

Figure 5-4 Principle of Optical Oxygen Detection Using Fluorescent Dye [17] Source: Adapted from Reference 17

Figure 5-5 Fluorescence Density Decay Time as a Function of Oxygen Concentration [17] Source: Adapted from Reference 17

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EPRI Proprietary Licensed Material Oxygen

Figure 5-6 Phase Shift of Modulated Signals [17] Source: Adapted from Reference 17

The oxygen partial pressure (pO2) is then linked to the corresponding phase-shift measurement (Φ) to build the sensor calibration curve (Figure 5-7). This curve is described by the SternVolmer equation, (Eq. 5-14), where Ksv is the indicator quenching constant (in mbar-1) representing the quenching efficiency of the oxygen and thus the sensor sensitivity, f0 is a constant and Φ0 is the phase-shift at zero oxygen representing the unquenched fluorescence decay time of the dye. The calibration curve thus relies on two parameters: the phase-shift at zero oxygen and the luminescent spot sensitivity, Ksv. The dissolved oxygen concentration is then calculated with Henry’s law using the water solubility curve as a function of the temperature.

pO 2 =

(Φ0 − Φ ) K SV [Φ − Φ 0 (1 − f 0 )]

Equation 5-14

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EPRI Proprietary Licensed Material Oxygen

Figure 5-7 Stern-Volmer Calibration Curve [17] Source: Adapted from Reference 17

5.7.1.1 µg·kg-1–Resolution Optical Sensor to Monitor Dissolved Oxygen The measuring system consists of four key components. First, a specific sensitive luminescent spot has been developed to reach µg/L (ppb) resolution (Figure 5-8). This exchangeable spot is fixed on an optical fiber probe connected to the measuring electronics. In the instrument, a high resolution digital phase meter is integrated together with the opto-electronics component (excitation light and light detection device). Finally, a flow chamber equipped with a solenoid valve is connected to the sample line. The role of the solenoid valve is to switch from the measurement sample to the calibration sample at user programmable intervals to either verify or calibrate the system. The calibration procedure is therefore completely automatic and operator independent.

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EPRI Proprietary Licensed Material Oxygen

Figure 5-8 Luminescence Sensor Design [17] Source: Adapted from Reference 17

5.8 End User Considerations The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line DO instrument. In general, manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application. Other on-line DO instrument considerations include:

•

Adequate temperature and pressure compensation

•

Appropriate membrane selection

•

Frequency of electrode cleaning

•

Sensitivity to flow rate

•

Hydrogen sensitivity

•

Frequency of electrolyte replacement

•

Membrane fouling considerations

•

The use of a guard ring in polarographic sensors to a)reduce residual oxygen present in the electrolyte after air calibration and b)reduce or eliminate the adverse effect of interfering species that may initially be present in the internal electrolyte or that are created by the anodic reaction

•

Response time

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EPRI Proprietary Licensed Material Oxygen

•

Ease of servicing/cleaning sensor internals and membranes

•

Appropriate air-tight sample lines

•

Ease of calibration and on-going verification of instrument performance

5.9 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.

4.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

5.

“A New Approach to Low-Level Dissolved Oxygen Measurements”, David Gray, Thornton Inc., Proceedings of the International Water Conference October 2001, Pittsburgh, PA.

6.

“Honeywell DL5 PPM and PPB Dissolved Oxygen Probe”, Personal Correspondence, Alan Hatcher, Honeywell, Dan Meils, Scientech, June 2006.

7.

Waltron LLC, µAI-9437, Dissolved Oxygen Instruction Manual.

8.

ASTM D3370-95a (2003) e1, “Standard Practices for Sampling Water from Closed Conduits”. American Society for Testing and Materials, Philadelphia, PA.

9.

th John Dean ed., Lange’s Hand Book of Chemistry, 12 edition, McGraw-Hill Book Company, 1979.

10.

Advanced Laboratory QA/QC Practices, Daniel Meils, SCIENTECH, Inc., 2006.

11.

ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis”. American Society for Testing and Materials, Philadelphia, PA.

12.

“Recent Progress in the Design and Operation of Fully Automated Oxygen Monitoring Systems.” H. Maurer, Swan Analytical Instruments, Scientech On-line chemistry Process Instrumentation Seminar, Clearwater, Fl, 1995.

13.

ASTM D 5462-02, “Standard for On-Line Measurement of Low-Level Dissolved Oxygen in Water”. American Society for Testing and Materials, Philadelphia, PA.

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14.

Monitoring Cycle Water Chemistry in Fossil Plants”, Vol. 3 Project Conclusions and Recommendations by A.F. Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon. EPRI, Palo Alto, CA: October 1991. GS-7556.

15.

ASTM D5543-94, “Standard Test Methods for Low-Level Dissolved Oxygen in Water, Test Method A—Color Comparator Test Method Using Self-Filling Glass Ampoules”. American Society for Testing and Materials, Philadelphia, PA.

16.

ASTM D 888-87, “Standard Test Method for Colorimetric Indigo Carmine, Test Method A”. American Society for Testing and Materials, Philadelphia, PA.

17.

Dunand, Frank A., Ledermann, Nicolas, Hediger, Serge, Power Plant Chemistry, 2006 8(10), 603.

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6 OXIDATION-REDUCTION POTENTIAL

6.1 Purpose and Use Oxidation-reduction potential (ORP) [also known as the reduction-oxidation (redox) potential] is listed as an EPRI Core Monitoring Parameter [1], in mixed metallurgy systems using all volatile treatment including a reducing agent—ATV(R). The purpose for monitoring ORP is to assure the feedwater is in a reducing condition as needed to minimize copper transport when operating with AVT(R) Chemistry. ORP has also been used to monitor changes to AVT(O) and OT but this is not required. This chemical treatment scheme refers to the feedwater treatment. In other power plant systems such as; cooling tower systems and drinking water / service water systems, ORP reflects the balance between oxidizing disinfectants and dechlorinating agents. Oxidizing biocides include gaseous chlorine, bleach, chlorine dioxide, chlorinated and brominated isocyanurates, calcium hypochlorite, bleach-bromide mixtures, halogenated hydantoins, and peroxide. Release of these oxidizing biocides into surface waters is potentially harmful to the environment and is closely controlled. As a result, chemical dechlorination is often required prior to any surface discharge. Many water purification components are also intolerant of oxidizing biocides (reverse osmosis membranes and ion exchange resins) and must be protected. Typical dechlorination products include sulfur dioxide, sodium meta-bisulfate, and activated carbon. ORP serves as a useful indicator that has been used to regulate the addition of oxidizing biocide to the system and as an indicator of proper dechlorination after the disinfection is complete. ORP is continually monitored on-line in units with copper alloy feedwater system components to monitor the chemistry environment. Negative ORP values indicative of a reducing environment are needed under all conditions to minimize copper corrosion and transport.

6.2 Description of Method This subsection presents basic background information on the principles of making corrosion potential (also called electrochemical corrosion potential (ECP)) and ORP measurements. Since these potentials have been measured in power plants only in recent years, it is appropriate to briefly review the terminology relating to these potentials. ORP and ECP are specific types of electrochemical potential measurements and are not synonymous. Both these types of measurements are quantifications of the potential difference between a metal electrode and a reference electrode immersed in the environment of interest. In the case of the ORP 6-1

EPRI Proprietary Licensed Material Oxidation-Reduction Potential

measurement, the metal electrode is a noble metal (e.g. Pt) and, in the case of the ECP measurement, the metal electrode is a metal used to fabricate one of the power plant components of interest. The value of the measured potential is determined by the specific oxidation and reduction reactions occurring at the metal/environment interface. ORP values and ECP values can differ by hundreds of millivolts and can respond quite differently to changes in system conditions. The corrosion reactions occurring on the system component will control the value of ECP while the platinum electrode used for the ORP measurement—being a noble metal and presumed to be immune to corrosion—will not experience the same change. Instead, ORP is more a reflection of the oxidative power of the aqueous environment (see below). ORP is defined as electromotive force between a noble metal electrode and a reference electrode when immersed in a solution [2,3]. Generally the system is not in equilibrium and the redox 2+ 3+ species are generally not conjugately related (e.g. made up only of Fe and Fe ions that are related by the charge transfer reaction Fe3+ + e- → Fe2+ ). Indeed, complex aqueous systems contain many conjugate pairs such as Fe2+/Fe3+, H2/H+, HOCl/OCL- and O2/H2O in aerated water. The normal ORP convention is to use a platinum sensing electrode although gold or silver is sometimes used. In the case of an ECP measurement, the metal electrode may be the power plant component itself (such as a pipe or heat exchanger tube) or it may be a separate piece of metal (a “probe”) having a chemical composition similar to the component of interest and immersed in the environment of interest. When a metal, such as iron, is corroding in an environment, such as water containing dissolved oxygen, the primary oxidation reaction is irreversible and heterogeneous:

e.g., Fe → Fe 2+ + 2e _

Equation 6-1

This reaction, together with the primary reduction reaction, e.g., H 2O +

1

2

O 2 + 2e − → 2OH −

Equation 6-2

control the corrosion potential measured. Theoretically, each corrosion product is thermodynamically stable only in a specific corrosion potential range. A change in the corrosion 2+ reaction product from Fe to Fe3O4 to Fe2O3, for instance, would be reflected in a change in the ECP. When the metal is platinum or some other noble, non-corroding metal electrode, the potential measured can still be considered a corrosion potential of sorts but, more correctly, it is an ORP measurement. Under such circumstances, the primary oxidation reaction, occurring at the metal/environment interface is simply the reverse of the reduction reaction. For instance, the reduction reaction in Eq. 6-2 would be in equilibrium with the following oxidation reaction: 2OH - → H 2O + 1 2 O 2 + 2e −

6-2

Equation 6-3

EPRI Proprietary Licensed Material Oxidation-Reduction Potential

and the rates of the forward and backward reactions are usually quite small. Here, the oxidation and reduction reactions do not involve (irreversible) corrosion; rather they are reversible, homogeneous reactions reflecting the oxidative power of the environment. The platinum (or other noble metal) electrode merely provides a surface upon which the reversible reactions can be detected and monitored as an ORP. The oxidation reaction (e.g. Eq. 6-3) also occurs on corroding metals but, here, the corrosion reaction (e.g. Eq. 6-1) normally occurs at a much higher rate and it controls the value of the ECP. In summary, while ORP and ECP (corrosion potential) are measured in a similar way, the former reflects the oxidative power of the environment while the latter reflects the corrosion reaction occurring on the metal surface. The ECP may be dependent on the ORP in that an increase in ORP may be accompanied by an increase in the corrosion potential, but ORP and corrosion potential seldom have the same value. Two important reversible reactions that influence the ORP of a power plant environment can be written as follows:

H 2O + 1 2 O 2 + 2e- ↔ 2OH − , 2H+ + 2e- ↔ H2 ,

E = 1228 . − 0.0591pH + 0.0148 log pO 2

E = 0.000 − 0.0591pH − 0.0296 log p H 2

Equation 6-4 Equation 6-5

where E is the ORP at 25°C (77° F) measured with respect to a standard hydrogen electrode (SHE); and pO 2 and p H 2 are respectively the partial pressures of oxygen and hydrogen in the environment. The 2-directional arrows in Eq. 6-4 and Eq. 6-5 indicate the reversibility of the reactions. Similar relationships can be derived for other temperatures. From these equations, it is clear that environments containing dissolved oxygen have a higher ORP than those containing hydrogen ions (or hydronium ions) at all pH values. This means that oxygen is a more powerful oxidant than hydrogen ions. Furthermore, in both equations, the ORP is decreased by about 59 mV for each increase of 1 pH unit at 25°C (77° F). The ORP also decreases as the partial pressure of oxygen decreases and the partial pressure of hydrogen increases. ORP is only used to confirm reducing conditions with AVT(R) in units with copper in feedwater. As might be expected, the ORP is also influenced by the presence of reducing agents like hydrazine, and ORP measurements have been used to ensure that hydrazine is maintained at the correct level. However, ORP depends on far too many factors to sensibly exhibit and predict any sort of “cause and effect” relationship for the parameter of interest. The best example of this: if the sample contains oxygen in quantities of >10 µg/L (ppb), it is generally not possible to add enough reducing agent to establish a negative ORP indicative of a reducing environment. ORP is a function of the system pH, the partial pressures of hydrogen and oxygen, the mass transport properties, the flow rate, and the materials in the system.

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EPRI Proprietary Licensed Material Oxidation-Reduction Potential

6.3 Technical Considerations An ORP analyzer has similar components as a pH analyzer. The ORP analyzer is comprised of a volt meter, reference electrode and sensing electrode. The potential between t he reference electrode and the sensing electrode are measured by the volt meter and a readout in mV is displayed. 6.3.1 Voltmeter Selection

Most laboratory and process pH meters are also suitable for measuring ORP by utilizing an appropriate set of electrodes and choosing a suitable meter output scale. The ORP readout will be in millivolts (mV) units with a typical response of -1000 to +1000 mV being adequate. Temperature compensation of ORP measurement is related only to the electrode properties described by the Nernst Equation. 0 E = E + 2.3 (RT/nF) logQ

Equation 6-6

where: E

=

measured potential

E

=

potential when all components involved in the reaction are at unit activity and 25 °C (77° F)

R

=

gas constant

T

=

absolute temperature, °K (°C = T – 273.15)

F

=

Faraday constant

n

=

number of electrons involved in the reaction

Q

=

product of all the activities of the oxidants divided by the product of all the activities of the reductants

0

Automatic temperature compensation is normally not attempted in ORP measurements in cycle chemistry measurements. Every attempt should be made to measure the sample near the reference temperature of 25 °C (77°F). For readings not made at this standard temperature, the error is fairly minor and is not considered in power plant ORP measurements. These effects are on the order of 0.1 to 0.2 mV/ °C when using the most common Ag/AgCl, Sat. KCl reference electrode. It is important to mention that the ORP of an aqueous solution is almost always sensitive to pH variations, even to reactions that do not appear to involve hydrogen or hydroxyl ions. The ORP 6-4

EPRI Proprietary Licensed Material Oxidation-Reduction Potential

generally tends to increase with increasing hydrogen ion concentration (lowering pH) and decrease with increasing hydroxyl ion concentrations (elevating pH values). At 25°C (77°F), ORP values vary approximately 59 mV/pH unit—this property is useful during electrode calibration but is troublesome when taking readings in samples with varying pH values, such as a feedwater system. 6.3.2 Reference Electrodes

Reference electrodes used for ORP measurements must be maintained and periodically checked for proper operation. It is common practice to check reference electrode potentials against the potentials of other nominally identical reference electrodes. All measurements should agree within one millivolt and any reference electrode not meeting this standard should be withdrawn from service. Some reference electrodes can be rejuvenated and brought back into service simply by replacing the internal electrolyte, where possible. Other reference electrodes are not designed for rejuvenation and must be discarded, as in sealed gel type electrodes. Ideally, all reference electrodes should be calibrated periodically because their response characteristics tend to change with time. Here, calibration involves checking that the measured reference electrode potential [measured with respect to a standard hydrogen electrode (SHE)] is equal to the theoretical potential. The apparatus needed to make such a calibration using a hydrogen electrode as the standard is described in ASTM G5-87 [4]. The rate that a reference electrode degrades depends partly on the type of service it sees and the contaminants it picks up from the service environment (see Section 18). For instance, a silver/silver chloride (Ag/AgCl) electrode, which can be used at temperatures up to 300°C (572°F), is susceptible to irreversible damage if contaminated with sulfide. The formation of silver sulfide on the electrode surface causes a permanent change to occur that cannot be fixed by recalibration. Calibration may also be impossible if the silver chloride component has been reduced to silver by hydrogen that has diffused through the reference electrode internal electrolyte from the bulk environment. Dilution of the internal electrolyte (usually 3 molar or saturated potassium chloride solution) with the bulk environment may also occur, especially at high temperatures (e.g. above 200°C (363°F) and when there is a pressure differential between the inside and outside of the reference electrode chamber. Although ORP is not measured at these temperatures in power plant applications, dilution of the KCl will cause the reference potential to drift from the initial value, so periodic replacement of this solution is recommended in electrode designs that allow this. 6.3.3 ORP Sensing Electrode

Corrosion scientists and electrochemists in the research world typically report ORP values on the SHE scale even though the measurements are rarely actually made using an SHE. In practice, a more robust, more convenient reference electrode is used and the value measured is converted to the SHE scale either by calculation or by adjustment of instrument zero during calibration. In the power industry, however, ORP values are usually reported with respect to the reference 6-5

EPRI Proprietary Licensed Material Oxidation-Reduction Potential

electrode contained in the ORP probe. Such a practice is potentially confusing only because the reference electrode used is not routinely mentioned when reporting the ORP value. Fortunately, the great majority of ORP probes used in power plants use one type of reference electrode: the Ag/AgCl reference electrode. Also fortunately, the internal electrolyte in the reference electrode, which influences the electrode potential, is typically potassium chloride with a concentration ranging from 3M to a saturated solution. An increase in the chloride concentration over this range would decrease the reference electrode potential by no more than 11 mV for temperatures in the range 20-30°C (68-86°F) (see Table 6-1). Since errors of 11 mV would be considered small for most practical ORP measurements, knowledge of the precise KCl concentration is usually not an issue at near-ambient temperatures. Corrections for KCl concentration may be necessary if the ORP is measured at higher temperatures. Nevertheless, confusion has arisen when ORP values measured versus a Ag/AgCl reference electrode are inadvertently compared with others measured versus an SCE or SHE. Consequently, it is recommended that the reference electrode used for the measurement is always mentioned along with the measured ORP values (e.g., ORP = 0.100 V(Ag/AgCl, Sat. KCl)). Table 6-1 To Convert ORP or ECP Values Measured Using Reference Electrode #1 to Values on Reference Electrode #2 Scale, Add the Indicated Conversion Factor to the Measured Potential [3] Convert From Potential (mV) Versus Reference Electrode #1* (Listed Below)

Add the Conversion Factor Below to Convert To Potential (mV) Versus Reference Electrode #2* To SCE

To Ag/AgCl (3M KCl)

To Ag/AgCl (Sat. KCl)

To SHE

20°C 25°C 30°C 20°C 25°C 30°C 20°C 25°C 30°C 20°C 25°C 30°C (68° F) (77° F) (86° F) (68° F) (77° F) (86° F) (68° F) (77° F) (86° F) (68° F) (77° F) (86° F)

From SCE

—

—

—

+34

+35

+36

+45

+45

+45

+247 +244 +241

From Ag/AgCl (3M KCl)

-34

-35

-36

—

—

—

+11

+10

+9

+213 +209 +205

From Ag/AgCl (Sat. KCl)

-45

-45

-45

-11

-10

-9

—

—

—

+202 +199 +196

From SHE

-247

-244

-241

-213

-209

-205

-202

-199

-196

—

—

* Ag/AgCl = silver/silver chloride; SCE = saturated calomel electrode; SHE = standard hydrogen electrode. Note: the presence of liquid junction potentials may result in the listed conversion factors being in error by 1 or 2 mV.

When reporting and comparing comparing ORP measurements, they should be referred to the same reference electrode scale. As described above, the conversion factors shown in Table 6-1 can be used to convert from one reference electrode scale to another. For instance, an ORP of

6-6

—

EPRI Proprietary Licensed Material Oxidation-Reduction Potential

+100 mV measured at 25°C (77°F) versus a Ag/AgCl, Sat. KCl reference electrode can be converted to an ORP versus an SHE, by adding 199 mV:

Therefore,

ORP (Ag/AgCl, Sat. KCl) = 100 mV

Equation 6-7

ORP (SHE) = 100 mV + 199 mV = 299 mV

Equation 6-8

6.4 Interferences In general, ORP measurements are not subject to solution interferences from color, turbidity, colloidal matter and suspended matter. Deposits on the surface of the metallic portion of the sensing electrode can cause the electrode to be unresponsive or exhibit a memory effect. ORP measurements are temperature sensitive as defined by the Nernst Equation. However, the magnitude of the error is small compared to the variability of the system being measured and automatic temperature compensation is not normally employed. ORP measurements are also pH dependent as previously discussed. The pH/ millivolts correlation is approximately 59 mV per standard pH unit at 25°C (77°F). Again it is important to remember that ORP readings are by no means absolute determinations that can be correlated across multiple systems. While a +100mV (Ag/AgCl, 3M KCl) reading may be highly oxidizing in a feedwater system, a cooling tower at lower pH may have very little oxidizing power at this value. For true disinfecting conditions using chlorine, an ORP reading of +400 mV to +600 mV (Ag/AgCl, 3M KCl) may be desired.

6.5 Calibration The instrument used to measure ORP may be calibrated using standard solutions that have known ORP values. For instance, one possible set of standard ORP reference solutions are based on pH 4 and pH 7 buffer solutions, similar to those used for pH meter calibration. However, for ORP standards, quinhydrone (formulated to provide an equimolar solution of quinone and hydroquinone) is added to the buffer solutions, which forms a reversible oxidation-reduction couple when dissolved in water. Hydrogen ions participate in the reaction between the quinone and hydroquinone, Equation 6-11, creating a pH dependent equilibrium: C6 H 4 O 2 + 2H+ + 2e- ↔ C6 H 6 O 2

Quinone

Equation 6-9

Hydroquinone

E = E°-[2.303RT/(2F)] × log a hq /a q − [2.303RT/F] × pH

Equation 6-10

When the activity of quinone (aq) is equal to the activity of hydroquinone (aaq), the second term ( log a hq /a q ) on the right hand side of Eq. 6-10 drops out so that the electrode potential, E, is dependent only on pH. 6-7

EPRI Proprietary Licensed Material Oxidation-Reduction Potential

The quinhydrone solutions are prepared by dissolving 10 grams of quinhydrone in one liter of pH 4 or pH 7 buffer solution [5]. Quinhydrone is not very soluble, so only a small amount will dissolve in the buffer solution, changing it to an amber color. However, it is important that excess quinhydrone is used so that solid crystals are always present. The quinhydrone powder poses a moderate health risk, causing irritation of the lungs with prolonged exposure to the dust. The calibration solutions are fairly innocuous unless ingested in large amounts. These hazards are minimized or can be avoided by using safe handling practices. Refer to the relevant MSDS for appropriate information. For the pH 4 and 7 quinhydrone solutions, ORP values of about 263 mV and 86 mV, respectively, can be expected at 25°C (77°F) when measured against a Ag/AgCl, Sat. KCl reference electrode [2,5,6]. These values are consistent with a change of 59.0 mV per pH unit, which are consistent with the calculated value of 2.303 RT/F (= 59.2 mV at 25°C (77°F)) in Eq. 6-10. ORP values for the standard solutions at other temperatures and for other reference electrodes are provided in ASTM D1498-00 [2] and are reproduced in Table 6-2. Table 6-2 Expected ORP Values for Reference Quinhydrone Solutions at pH 4 and pH 7 ORP Value (mV) Reference Electrode*

pH 4 Buffer Solution

pH 7 Buffer Solution

20°C (68°F)

25°C (77°F)

30°C (86°F)

20°C (68°F)

25°C (77°F)

30°C (86°F)

Ag/AgCl

268

263

258

92

86

79

SCE

223

218

213

47

41

34

SHE

470

462

454

295

285

275

* Ag/AgCl = silver/silver chloride (Sat. KCl); SCE = saturated calomel electrode; SHE = standard hydrogen electrode.

The quinhydrone standards are easily made but they are stable due to reactions with air, primarily oxygen contamination, for only about eight hours and are not expected to yield highly reproducible ORP values. Nevertheless, ORP values in freshly prepared solutions are expected to be within 10 mV of the values listed in Table 6-2; and an ORP value measured with a probe in one of these standards can usually be considered acceptable if it lies within 30 mV of the listed value. If the electrode does not respond as expected or has a slow response, it should be cleaned and the calibration procedure repeated. Removal of oily or organic deposits can be achieved with a detergent or, if necessary, methanol or isopropyl alcohol. For removal of contaminants or mineral deposits, the electrode should be soaked in 10% nitric acid for 10 minutes. Alternatively, ASTM D1498-00 suggests the use of warm (70°C) aqua regia for about 1 minute [2]. As a last resort, the platinum surface can be polished with a 600 grit wet-dry emery cloth or a 1-3 micron alumina polishing powder to remove any stubborn coatings or particulates. The electrode will need to soak in purified water for at least 30 minutes after performing any of these cleaning procedures.

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EPRI Proprietary Licensed Material Oxidation-Reduction Potential

A single point calibration (or “standardization”) is often considered adequate. ORP meters, unlike pH meters, do not have “slope calibration” to allow adjustment of the response to a specified change in ORP. Nevertheless, it is usually good practice to verify that the ORP probe (platinum/reference electrode combination) is operating in a predictable fashion by measuring the ORP in a second standard solution. If the electrode potential does not change by the expected amount, the electrode should be cleaned and re-calibrated, as described above. Other ORP calibration solutions are available based on the ferrous/ferric cyanide or ferrous/ferric sulfate equilibrium reactions, but the quinhydrone standards are more widely used. 6.5.1 Calibration Checks

On-line ORP instruments should be checked periodically to demonstrate calibration stability. The Line Method [7] is appropriate for verifying instrument stability. Here, a calibrated separate ORP monitor is used to analyze the same sample steam as the installed on-line instrument. The two results are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-line instrument’s calibration is considered to be acceptable. If the results are outside the acceptance criteria the on-line instrument must be recalibrated.

6.6 End User Considerations Manufacturers of on-line pH monitors typically include the ability to monitor ORP in their instrument design. In fact, the two measurements are closely related and only differ in the design of the sensing electrode and manner in which the mV output signal is manipulated. For the most accurate ORP determinations (±5 mV precision), samples should be maintained at 25 ±1 °C (77 ±2 °F) to preclude ion activity discrepancies [2]. ORP sensing probes in power plant feedwater systems do not suffer from foreign material buildup. In cooling water or service water applications, ORP sensing probes are susceptible to foreign material plating the noble metal surface. Some of these applications require almost daily electrode cleaning, stabilization, and calibration verification. Systems are presented by several manufacturers that provide in-situ on-line cleaning of the probe. Calibration is performed with one or more pH buffers to which has been added a quinhydrone compound (see section 6.4). Most calibrations are only one point (the mV reading of a pH 4 buffer with added quinhydrone) adjustments but making a cross check with a second buffer is advisable. The span value of the output signal is also a user defined variable with many of the instruments. Full scale deflections are possible with as little as 100mV and can be ranged up to 1000mV. ORP readings are empirical at best but satisfactory for their intended use in fossil feedwater cycles. Absolute values are not transferable from one system to another and may undergo changes with various water quality parameters.

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EPRI Proprietary Licensed Material Oxidation-Reduction Potential

6.7 References 1. Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187. 2. ASTM D 1498-00; “Standard Practice for Oxidation-Reduction Potential in Water”. ASTM International West Conshohocken, PA 19426; 2006. 3. Barry Dooley, Digby Macdonald, and Barry C. Syrett, “ORP—The Real Story for Fossil Power Plants”, Power Plant Chemistry, 2003, Volume 5(1). 4. ASTM G5-87, “Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements”, ASTM, Philadelphia, PA. 1987. 5. L. McPherson, Chemical Engineering, p. 143-145, March 1994. 6. S. Filer, A.S. Tenney III, D. Murray and S.J. Shulder, “Power Plant ORP Measurements in High Purity Water”, NUS International Chemistry On-Line Process Instrumentation Seminar, Clearwater Beach, FL. November 1997. 7. ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis, American Society for Testing & Materials”. American Society for Testing and Materials, Philadelphia, PA.

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EPRI Proprietary Licensed Material

7 pH

7.1 Purpose and Use In boiler water, pH is an EPRI Core Monitoring Parameter [1-4]. As such, pH should continuously be monitored on-line to check the acceptability of the water chemistry, thereby ensuring that corrosion rates are kept at low levels. In fossil plant steam-water cycles pH may also be monitored for one or more of the following reasons:

•

To facilitate the correlation between two or more water chemistry parameter (e.g., pH, conductivity, ammonia correlation).

•

To provide a feedback signal for automated process control.

•

To warn of in-leakage of contaminants.

•

To warn of condensate polisher malfunction.

•

To troubleshoot or verify the accuracy of other on-line pH monitors.

•

To check the pH of water streams not routinely monitored continuously by on-line monitors.

The data generated by continuous on-line monitoring of pH is used by plant chemistry and operations personnel. The goal for plant personnel is to maintain pH within prescribed limits. The pH method and instrument described below is substantially different from the pH method and instrument used for environmental or other high ionic strength solutions. Although the technical understanding for the measurement of pH is essentially the same, other considerations for these high ionic strength samples are significantly different. For a discussion on environmental pH analysis refer to APHA Standard Methods for Examination of Water and + Wastewater, Part 4500-H [5].

7.2 Description The term pH is a measure of the acidity (or alkalinity) of an aqueous fluid. The factor that most influences the level of acidity is the activity of the hydrogen (H+) ions in the fluid. By definition, pH is the logarithm of the reciprocal of the H+ activity:

7-1

EPRI Proprietary Licensed Material pH

pH = log10 [1/H + ] = - log10 H +

Equation 7-1

In weak solutions typical for power plant on-line instrumentation applications, H+ activity is approximately equal to H+ concentration in moles/liter. The pH is obtained by measuring the potential difference between a pH-sensitive electrode and a reference electrode, both of which are immersed in the solution of interest. Measurement of pH is accomplished by means of an electrode that develops an electrochemical + potential directly related to the H activity of the solution in which the electrode is immersed. The purpose of the pH-sensing electrode is to provide a varying electrochemical signal corresponding to the H+ ion concentration in the solution being tested. Although it is not feasible to measure the electrochemical potential directly, it is possible to measure differences in potential with a voltmeter or electrometer. Consequently, a reference electrode is needed in addition to the pH-sensitive electrode to complete the circuit. The purpose of the reference electrode is to provide a constant electrochemical reference potential (a baseline) against which the potential of the pH-sensing electrode can be compared. The reference electrode + potential is not affected by H ion concentration. Therefore, when pH instrumentation measures the potential difference between the pH-sensitive + electrode and the reference electrode, the potential difference can be directly related to the H ion activity at the pH electrode surface. Instrument manufacturers supply the pH electrodes and reference electrodes separately or in a single combined unit. For low-conductivity pH applications separate electrodes provide some technical advantages discussed below. For routine pH applications combination electrodes are adequate.

7.3 Technical Considerations Water samples taken from the steam/water cycle typically have low ionic strength (i.e., 0.1 to 100 µS/cm), leading to several analytical challenges. These challenges can be overcome provided appropriate pH sensing and reference electrodes are selected; appropriate temperature compensation is applied; interfering ions are minimized; interfering stray current is minimized; and, appropriate calibration and calibration checks are performed. Without appropriate consideration of these factors it is not unusual for the pH electrode response to be slow, “noisy”, and non-reproducible under such circumstances. 7.3.1 Sensing (Glass) Electrode

For power plant cycle chemistry pH measurements, the sensing electrode is normally made from a special glass, fashioned into a thin-walled, small bulb, and fitted with a suitable electrical contact on its inner surface, the so-called ‘glass electrode’. The electrical contact takes the form 7-2

EPRI Proprietary Licensed Material pH

of a reference half cell, normally a silver wire partially coated with (or chemically converted to) silver chloride (Ag/AgCl), which dips into a pH-buffered solution held within the glass bulb. The external surface of the glass bulb is immersed in the sample water being measured. 7.3.2 Reference Electrode

While the actual potential measured will depend on the specific type of reference electrode used, the change in potential resulting from a known change in pH will be independent of the reference electrode. For instance, a change of 1 pH unit at 25°C (77°F) will produce a change in potential difference of 59.16 mV. The pH monitoring instrumentation is simply a voltmeter, more specifically a high impedance volt meter, calibrated to read pH in specific units (SU), instead of potential in millivolts (mV). Variations in reference electrode potentials are simply adjusted by an electronic zero adjustment. NOTE: By convention, the instrument is adjusted so that electronic zero (zero mV; also called the isopotential point) represents the mid-point of the pH scale; pH 7. For pH measurements to be accurate the reference electrode must provide a stable reference point against which the pH-sensitive electrode potential is measured and there must be electrolytic contact between these two electrodes. Reference electrodes are often composed of a silver (Ag) wire partially coated with silver chloride (AgCl, thus creating a Ag/AgCl junction) surrounded by a saturated potassium chloride (KCl) solution contained in a non-conductive glass tube. Electrolytic contact between the KCl solution on the inside of the glass tube and the water being measured on the outside of the tube is often maintained by means of a small porous plug or frit that passes through the tube wall. + A chemical potential gradient promotes migration of KCl solution (K and Cl ions) through the frit. As a result, the KCl solution becomes diluted over time, leading to a change in the electrochemical potential of the reference electrode and—without replacement of the saturated KCl solution or re-adjustment of the voltmeter zero setting—consequently leading to an inaccurate pH measurement. Dilution may also occur when the solution being monitored flows through the frit into the saturated KCl solution because of a positive pressure gradient through the frit. Similarly, a negative pressure gradient can drive the KCl solution into the solution being monitored. To prevent this type of dilution or loss of KCl solution, the electrodes must be placed in a vented sample flow-through sample chamber. Additionally, the reference electrode fill solution (saturated KCl) should be topped off routinely to maintain a level nearly full. The presence of a few KCl crystals at the bottom of the reference electrode tube helps ensure that the solution is always saturated. (Note: Consult the reference electrode manufacturer’s manual for specific instructions.)

For pH measurements of low ionic strength solutions (i.e., specific conductivity 10 µS/cm, as is the case in some boiler blowdown samples) a separate flowing junction reference electrode is not necessary since + the ionic strength in the solution is adequate to provide H ion mobility on its own. A typical combination electrode is shown in Figure 7-2. Combination electrodes incorporate the sensing and reference electrodes into one electrode assembly.

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EPRI Proprietary Licensed Material pH

Figure 7-1 Flowing Junction Reference Electrode Head Cup Configuration Source: Adapted from a Drawing Provided Courtesy of Honeywell Corporation

7.3.3 Temperature Effects

The potential difference measured between the pH sensing and reference electrodes is also influenced by temperature. As a consequence, the calibration of the instrument (the voltmeter) is affected by the sample temperature. The sensitivity to temperature results from electrode effects and solution additive (composition) effects. It is easy to compensate for the first effect and relatively easy to compensate for the second effects provided the solution additives are known.

7-5

EPRI Proprietary Licensed Material pH

Figure 7-2 Combination Electrode

7.3.3.1 Electrode Effects

One effect relates to the dependency of the glass electrode and reference electrode potentials to temperature: the potential difference changes according to the Nernst equation by 2.303 RT/F for each pH unit of change, where R is the gas constant, F is the Faraday constant, and T is the temperature in degrees Kelvin. For instance, a change of 1 pH unit at 0°C (32°F) will cause a change of 54.20 mV, whereas at 25°C (77°F) the change will be 59.16 mV, and at 100°C (212°F) the change will be 74.05 mV. Many modern pH monitoring systems measure the water sample temperature along with the pH and automatically compensate for the temperature. For on-line monitoring, the pH-sensitive electrode, the reference electrode, and the temperature sensor are often incorporated in a flow-through cell through which the water sample flows continuously or intermittently. In other systems, the temperature compensation must be performed manually, or the pH value must be corrected using calibration charts provided with the instrumentation. The temperature compensation only corrects for variations in the electrode slope factor of the Nernst equation; it does not account for the effect on the pH value to 25°C (77°F) of the temperature-dependent dissociation of chemical species in the water sample. This requires an additional compensation.

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EPRI Proprietary Licensed Material pH

7.3.3.2 Solution Additive Effects

The dissociation of pure water itself is dependent on temperature, such that the pH changes from 7.47 at 0°C (32°F) to 6.12 at 100°C (212°F). The dissociation of other chemical additives in the water is also dependent on temperature. Consequently, it is essential that the temperature of measurement be reported along with the pH value. Many utilities find the best solution to this problem is to ensure that the water sample contacting the glass and reference electrodes is maintained at 25°C (77°F) at all times. Otherwise the composition of the solution being measured should be used to determine the appropriate correction factor. The mathematical calculation for determining the corrected pH to 25°C is given in equation 7-2. pH@25C = pHm + (T-25) Correction Factor

Equation 7-2

Where: pH@25C =

the pH corrected to 25°C

pHm

=

the pH measured

T

=

the temperature of the pH sample being analyzed

=

the appropriate correction factor from Table 7-1

Correction Factor

Table 7-1 shows typical temperature correction factors used in power plant applications [9]. Table 7-1 Various Solution Additive Temperature Correction Factors for Power Plant Steam/Water Cycle pH Measurements Solution Additive

System Monitored

Correction factor per degree centigrade different from 25°

Ammonia

Condensate and feedwater (or boiler on AVT)

0.0323 SU

Strong bases

Boiler chemistry (PO4, NaOH treatment)

0.0332 SU

None

Ion exchange or water treatment effluent

0.0165 SU

Example:

Assume: An all volatile treatment (AVT) plant with ammonia as the primary pH additive Observed pH = 9.55 Observed temperature = 22°C Correction factor for ammonia = 0.0323 SU degree centigrade different from 25° 7-7

EPRI Proprietary Licensed Material pH

Solution: From Equation 7-2: pH@25 = pHm + (T-25) Correction Factor pH@25 = 9.55 + (22-25) 0.0323 pH@25 = 9.55 + (-3) 0.0323 pH@25 = 9.55 + (-0.969) pH@25 = 9.46

7.4 Interferences 7.4.1 Interfering Ions

Other errors may occur in the pH reading under some circumstances. For instance, at high pH values where the H+ ion concentration is very low, other positively charged ions may greatly outnumber the H+ ions and may have a significant effect on the potential of the glass electrode. A prime example is the sodium (Na+) ion error which is evident at pH values above about 10 if the Na concentration is greater than about 10 gram/liter or 10,000 mg/L (ppm). However, this level of Na+ concentration is not typically encountered in power plant cycle chemistry. None the less low sodium error electrodes are recommended for measurements over pH 10. 7.4.2 Interfering Stray Currents

The high resistance of the sensing electrode can also cause problems, particularly at low temperatures (below 10°C or 50°F) where the resistance is even greater. The high resistance can stimulate stray currents (AC pick-up) in the circuit unless the integrity of the insulation is rigorously maintained. Essentially no current can flow between the glass electrode and reference electrode during measurement if the pH value is to be measured accurately. This requirement dictates the use of a high impedance (perhaps >10,000 megohm) voltmeter and the need to minimize the length of the leads from the electrodes to the instrument. Low impedance outputs, available on many instruments, can be used to transmit the pH data over great distances without suffering degradation or interference from external voltage sources. Flow sample chambers provide a mounting point for the sensing and reference electrodes. The flow chambers for high purity pH measurement electrodes should be designed to prevent stray current that may affect pH measurements. Stray current may be created when high purity (low conductivity) water flows past glass electrodes, thereby creating a static-like charge. One strategy to avoid stray current interference is to manufacture the flow chamber from a conductive 7-8

EPRI Proprietary Licensed Material pH

material, such as 316L stainless steel, and to provide appropriate electrical grounding for the instrument. Electrical charges may also be developed when the electrodes are wiped with an absorbent paper towel. Electrodes should always be blotted not wiped to prevent this charge buildup.

7.5 Calibration The pH instrument may be calibrated by immersing the sensing and reference electrodes first in one buffer solution of known pH for standardization (zero intercept; neutral pH) adjustment shown in Figure 7-3 of known pH, then in another buffer of different pH to set the instruments slope (span) shown in Figure 7-4. For instance, buffer solutions of pH 7 and pH 10 may be used to calibrate the instrument in preparation for measuring the pH of solutions expected to be alkaline (basic), as in most power plant operations. The instrument uses these accurately known buffer values to set the slope and intercept of the electronically stored “curve” of pH versus potential. In principle the pH of the buffers used should span, or be close to, the pH of the solution being monitored. It is generally undesirable to standardize with pH 7 (isopotential point) and pH 4 (slope / span) buffers for power plant process samples since most samples are in the alkaline range. Furthermore, standardization in conventional buffers of relatively high ionic strength will increase the time for subsequent stabilization in a low ionic strength solution; and the possibility of sample contamination by the buffer will also be increased. Therefore, low ionic strength buffers are recommended for use with low conductivity water (i.e., specific conductivity < 10 µS/cm) samples.

Figure 7-3 Standardization (Zero Intercept) Adjustment

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EPRI Proprietary Licensed Material pH

7.6 Calibration Checks On-line pH instrument analytical capabilities should be checked periodically to demonstrate calibration stability. Two methods exist for verifying instrument stability; the Standard Injection Method [6] or the Line Method [7]. For the Standard Injection Method, a buffer of known pH, near the expected pH of the process being monitored, is analyzed by the on-line instrument and the results are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 0.1 pH unit). Acceptance criteria are either established based on statistically derived limits. i.e., ± 3 sigma [6] or based on some predetermined limits established from experience, i.e., ± 0.1 pH unit. Provided the on-line analyzer agrees within the acceptance criteria the on-line instrument’s calibration is deemed not to have changed. If the results are outside the acceptance criteria the on-line instrument must be recalibrated.

Figure 7-4 Slope (Span) Adjustment

For the Line Method a calibrated separate pH meter, typically a portable pH meter, is used to analyze the same sample stream as the installed on-line instrument. Provided the on-line analyzer agrees within the acceptance criteria (e.g., matches the results of the on-line analyzer within ± 3 sigma or ± 0.1 pH unit) the on-line instruments calibration is deemed not to have changed. If the results are outside the acceptance criteria the on-line instrument must be recalibrated. Additional technical discussions are presented in ASTM D5128-90 (1995) ε1, Standard Test Method for On-Line pH Measurement of Water of Low Conductivity [9].

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EPRI Proprietary Licensed Material pH

Additional maintenance and calibration activities for pH instruments are described in EPRI Report GS-7556 [8].

7.7 Alternative Methods for Determining pH Although the preferred method for monitoring pH in an on-line application is by direct measurement as described above there are alternative methods for determining pH in low ionic strength solutions. Specifically, if the amount of contaminants is low and the amount of the additive affecting pH is known, then the pH may be estimated. There are at least two techniques for this: determining pH from known ammonia concentration and specific conductivity, and determining pH from the calculated difference between specific conductivity and cation (acid) conductivity. Determining pH from a known ammonia concentration and specific conductivity can be accomplished by using a curves developed for these parameters as shown in Figure 7-5. This calculation is only valid in the absence of appreciable carbon dioxide. Example:

Assume: Specific conductivity at 25°C = 8 µS/cm Ammonia concentration = 1.5 mg/L (ppm) So: pH at 25°C = 9.50

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EPRI Proprietary Licensed Material pH

Figure 7-5 Specific Conductivity, Ammonia, pH at 25°C [1] Source: Reference 1

Carbon dioxide is often present in samples due to some air in-leakage. Determining pH from a known ammonia concentration, carbon dioxide and specific conductivity can be accomplished by using a family of curves developed for these parameters as shown in Figure 7-6.

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EPRI Proprietary Licensed Material pH

10 9 8 7 6 5 4

Ammonia (ppm NH3)

3 2

1 0.9 0.8 0.7 0.6 0.5

CO2 (ppm) 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.20 0.10 0.05 0.00

0.4 0.3 0.2

0.1 8.8

9.0

9.2

9.4

9.6

9.8

10.0

pH at 25 °C Figure 7-6 Ammonia Concentration vs. pH for Various Carbon Dioxide Concentrations [1] Source: Reference 1

Some instrument suppliers provide capability to calculate pH from specific and cation conductivity. This can be done relatively simply provided several assumptions are met. Determining pH from the calculated difference between specific conductivity and cation conductivity requires the following assumptions are met:

•

Two channel conductivity instrument is available providing specific conductivity and cation conductivity.

•

Only one pH adjusting chemical additive is used.

•

The correct non-linear temperature compensation for the corresponding additive affecting pH (ammonia, ethanolamine, morpholine, etc.).

•

No phosphates are present in the sample stream.

•

At pH < 8, impurities (such as NaCl) must be small compared to the additive affecting pH.

•

Optimally the pH range of the sample of interest should be between 7.5–10.5. 7-13

EPRI Proprietary Licensed Material pH

Provided these assumptions are met the calculated pH is remarkably close to the observed pH. An example in Table 7-2 shows very good agreement [10]. Table 7-2 Example Calculated pH by Differential Conductivity from One Instrument Supplier [10] Impurities

1000 µg/L (ppb) Na+, (4.35 x 10-5 M/L)

pH additive (Ammonium Hydroxide)

3.5 x 10-5 M/L

Specific conductivity

12.9 µS/cm

Cation conductivity

18.3 µS/cm

Theoretical pH

9.5

Calculated pH by differential conductivity

9.47

7.8 End User Considerations The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line pH instrument. In general, manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application. End users should also verify that the instrument meets the requirements for its intended purpose and use (i.e., it demonstrates the required level of accuracy and precision when installed and functioning at your facility). Other on-line pH instrument considerations for low ionic strength (10 µg/L (ppb). It is easier for manufacturing the electrode, but needs regular exposure to sodium in solution or it becomes “super-Nernstian,” resulting in erroneously low sodium results at low (10.5. Figure 8-5 [5] shows that the slope of the calibration line deviates from Nernstian response for pH < 9 for Sodium values between 10 and 100 µg/L (ppb). Therefore, higher pH is needed to get Nernstian response at sodium concentrations 10 µg/L (ppb), a pH of 9.0 or higher is acceptable. In this case a buffer solution containing ammonium hydroxide or morpholine may be introduced into the sample upstream of the sensing electrodes or if the sample stream itself has a pH >9.0 no external pH adjustment may be required. The sensing electrode is made of glass containing low concentrations of sodium. At extremely low sample concentrations of sodium, the dissolution of the electrode glass, containing sodium affects the instrument readout. At about 5 nanograms/L (ppt) the glass electrode establishes an equilibrium with the surrounding sample stream such that sodium ions in the sample stream are balanced with sodium ions being sloughed off the sensing glass. At sample stream concentrations below about 5 nanograms/L (ppt), the equilibrium shifts such that the sensing electrode is supplying sodium ions, by dissolution, to the sample to maintain an equilibrium of about 5 nanograms/L (ppt)) sodium. In effect, the measured sodium concentration will never be 100 µg/L (ppb) to minimize the potential for standard contamination. It is also necessary to check the flow of the reference probe electrolyte. Unstable signals very often have their origin in the reference system. Figure 8-6 [5] shows the performance of a sodium ion selective electrode following calibration with 200 and 2,000 µg/L (ppb) sodium solutions. The data in the figure show:

•

95.8%recovery at 2 µg/L (ppb),

•

98.7 % recovery at 20 µg/L (ppb), and

•

99.1 % recovery at 200 µg/L (ppb).

Figure 8-6 Verification Results Immediately After a 200 and 2,000 µg/L (ppb) Sodium Calibration [5] Source: Adapted from Reference 5, Courtesy Swan Instruments

A modification of this technique, the double known addition (DKA) method, involves the addition of a known amount of a standard solution to the sample on two successive occasions. The potential, ES, recorded by the instrument at the start of this procedure corresponds to the starting concentration, CS, as shown in equation 8-2. This equation is a simplified version of equation 8-1, in which CB is assumed to be small compared to CS, and 2.3026RT/nF equals a factor, S:

8-10

EPRI Proprietary Licensed Material Sodium

E = E O + S log (CS / CIso )

Equation 8-2

The addition of the standard increases the concentration by a known amount, dC1, to CS+dC1, and changes the measured potential to E1, as shown in equation 8-3: E1 = E O + S [log (CS + dC1 ) / CIso ]

Equation 8-3

Then, a second addition of a standard solution is made to the sample. This standard solution is preferably about ten times more concentrated than the first, causing a further increase in concentration, dC2, and a new potential, E2, as shown in equation 8-4: E 2 = E O + S [log(CS + dC1 + dC 2 ) / CIso ]

Equation 8-4

The instrumentation automatically solves the three equations (8-2 through 8-4) for the three unknowns, and EO and S are stored for subsequent use in the on-line monitoring mode. Some instruments also correct for the detection limit of the system, CB, shown in equation 8-1. This so-called blank correction improves detection of very low levels of sodium. If there are any questions regarding the effectiveness of the pH adjusting solutions, checking the pH of the sample discharge from the analyzer may be appropriate to ensure that the buffer is elevating the pH as desired. It may be more appropriate to follow the maintenance and calibration activities recommended in EPRI Report GS-7556 [7].

8.6 Calibration Checks On-line sodium instruments should be checked periodically to demonstrate calibration stability. Two methods exist for verifying instrument stability; the Standard Injection Method [8] or the Line Method [9]. For the Standard Injection Method, a known sodium standard, in the concentration range where the sodium calibration can be readily verified (typically 10 µg/L (ppb)), is analyzed by the online instrument and the results are compared to the acceptance criteria (e.g., the results should agree within ± 3 sigma or ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is considered still acceptable. If the results are outside the acceptance criteria, the on-line instrument must be recalibrated. One approach is to prepare a standard and introduce this standard to the sodium analyzer replacing normal sample flow for the test period with the standard. Unfortunately, any source of sodium contamination would bias the test results high. A second approach [6] involves injection of a standard as a known addition into the sample stream and the mixture is analyzed by the instrument. The results are compared to the acceptance criteria (e.g., they should agree within ± 3 sigma or ± 10%). Again, any source of 8-11

EPRI Proprietary Licensed Material Sodium

sodium contamination would bias the test results high. If the results are outside the acceptance criteria the on-line instrument should be recalibrated. Some instruments [5] provide fully automated calibration verification; a standard solution is always available. The standard is diluted down to 10 µg/L (ppb) for automatic verification. As the diluted solution is injected, the sensor will report the increase. If it is within limits of recovery and response time specified by the supplier, the instrument is considered to meet acceptance criteria. One instrument supplier has proposed a calibration check by a known addition method using a standard and dynamic on-line addition to provide a ppt (nanograms/L) level standard solution for low-level verification of on-line analyzers [6]. Assuming the process concentration does not change during the evaluation period, Table 8-1 results can be used to demonstrate the calibration has not changed by following the Standard Injection Method described above. Table 8-1 Typical Results from Known Addition Method for Calibration Check in the nanograms/L (ppt) range Spike (nanograms/L Na (ppt))

Recovery

10 ppt

90%

42 ppt

93%

64 ppt

103%

140 ppt

105%

1,100 ppt

104%

Note: baseline sodium concentration stated to be about 12 ppt.

For the Line Method, a calibrated separate sodium monitor, typically a portable sodium monitor is used to analyze the same sample steam as the installed on-line instrument. The two results are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10%). Provided the online analyzer agrees within the acceptance criteria, the on-line instrument’s calibration is considered to be acceptable. If the results are outside the acceptance criteria the on-line instrument should be recalibrated.

8.7 Alternative Methods Various other methods exist for determining sodium including; Ion Chromatography (IC), Atomic Absorption (AA) spectroscopy [10,11], Inductively Coupled Plasma (ICP), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and Graphite Furnace, flame photometry. However, IC is the only suitable alternative method for continuous on-line determination.

8-12

EPRI Proprietary Licensed Material Sodium

8.8 End User Considerations The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line sodium instrument. In general, manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application. Other on-line sodium instrument considerations include:

•

Appropriate pH adjustment for analysis range of interest to minimize H+ ion interference with sodium measurements

•

Ease and robustness of calibration for the intended use

•

Ease and robustness of calibration verification

•

Ease of reestablishing sensing electrode response time by etching

•

Appropriate formula for ion selective sensing electrode glass

•

Appropriate sensing electrode linearity characteristics in the analytical range of interest

•

Reference electrode stability

8.9 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.

4.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

5.

“On-Line Sodium Monitoring with Ion-Selective Glass Electrodes”, Dr. Peter Wuhrmann, Swan Analytical Instruments, ESKOM International Conference on Process Water Treatment and Power Plant Chemistry, November 1997.

6.

“Theoretical and Practical Aspects of Glass Electrodes in On-Line Applications, Steve West, Thermo-Orion, Scientech Chemistry On-Line Chemistry On-line Process Instrumentation Seminar, Clearwater Florida, November 2000.

8-13

EPRI Proprietary Licensed Material Sodium

7.

Monitoring Cycle Water Chemistry in Fossil Plants, Vol. 1 Monitoring Results, by A.F. Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon. EPRI, Palo Alto, CA: October 1991. GS-7556, Volume 1.

8.

Advanced Power Plant Chemistry QA/QC Practices, Scientech, LLC., Clearwater, FL, 2006.

9.

ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis”, American Society for Testing & Materials, Philadelphia, PA.

10. ASTM D4191-93, “Standard Test Method for Sodium in Water by Atomic Absorption Spectrophotometry”, 1992 Annual Book of ASTM Standards, Vol. 11.01 Water. American Society for Testing and Materials, Philadelphia, PA. 11. ASTM D6071-96(2000), “Standard Test Method for Low Level Sodium in High Purity Water by Graphite Furnace Atomic Absorption Spectroscopy”, 2000 Annual Book of ASTM Standards, Vol. 11.01 Water. American Society for Testing and Materials, Philadelphia, PA.

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EPRI Proprietary Licensed Material

9 AMMONIA

9.1 Purpose and Use Ammonia is not defined as a Core Monitoring Parameter under current EPRI Cycle Chemistry Guidelines [1-4] but is commonly checked, usually as a grab sample at a location such as the economizer inlet. Ammonia is one of several alkaline chemicals that may be added to increase the pH of boiler feedwater in order to control corrosion of carbon-steel and other ferrous alloys in the steam/water cycle. When system metallurgy contains copper alloys, accurate determination of ammonia content is necessary to minimize formation of the copper/amine complex which leads to rapid localized corrosion. Frequent ammonia analyses or on-line ammonia monitoring may be appropriate in these mixed metallurgy systems. The pH of the feedwater in all-ferrous systems depends on the mode of operation. In plants using all volatile treatment (AVT) chemistry control, the pH is typically 9.2-9.6, measured at 25°C (77°F). For units where single phase flow accelerated corrosion is a concern, the feedwater pH is often elevated to a minimum 9.4 with control ranges approaching 9.8 on the top end. For once-through units using oxygenated treatment (OT), the pH is 8.0-8.5, and, for drum units using OT, the pH is in the range 9.0-9.6. For systems with both ferrous and copper alloy components, OT is not recommended, and the pH range for AVT is usually reduced to 8.8-9.3. This lower pH range, corresponding to a lower ammonia concentration, is preferred for mixed metallurgy systems because copper alloys corrode increasingly more rapidly at higher ammonia concentrations, particularly when the oxidation-reduction potential (ORP, Section 6) is oxidizing [1]. Plants where ammonia is continually monitored on-line use the information to:

•

Check the accuracy of water chemistry control, so ensuring that corrosion rates are kept at acceptable low levels.

•

Facilitate the correlation of ammonia with other chemistry parameters (i.e., pH and specific conductivity).

The data generated by continuous on-line monitoring of ammonia is used by plant chemistry and operations personnel. The goal for plant personnel is to maintain ammonia within prescribed limits.

9-1

EPRI Proprietary Licensed Material Ammonia

9.2 Description of Methods Ammonia content can be approximated from Specific Conductivity curves when analyzing high purity water. There are two common methods for on-line determination of ammonia. One method utilizes an ion selective electrode which is sensitive to the ammonium ion concentration in the sample [5]. The second method is colorimetric based on Berthelot’s Phenate Method of detection and quantification [6]. 9.2.1 Background

For accurate ammonia determination, some form of bench analysis or on-line instrumentation is required. On-line analyzers provide frequent and accurate information and can be used at several locations in the steam/water cycle. Some plants measure the ammonia content in the saturated steam. Current EPRI cycle chemistry guidelines recommend monitoring ammonia (when in use) at the economizer inlet. Sampling of saturated steam, however, will aid in determining the amount of ammonia that has volatilized and could be transported with the steam to the condenser. This may be an important consideration in plants with copper alloy condenser tubes. +

When ammonia (NH3) dissolves in water, it equilibrates with hydronium ion (H3O ) or hydroxyl ion (OH-), depending on the pH, to form ammonia (NH3) or ammonium ions (NH4+). The relative amounts of ammonia and ammonium are a function of the solution pH (Figure 9-1). At pH values below 6 (high H3O+ concentrations), the first reaction is promoted and virtually all of the ammonia is converted to ammonium ions (Eq 9-1). At pH values above 12 (high OHconcentration), the second reaction shifts to the left (reactant side) and the ammonium ion concentration is virtually zero (Eq. 9-2). Approximately equal concentrations of ammonia and ammonium ions exist at pH 9.3 at 25 °C (77 °F)

←⎯⎯→ NH4+ + H2O

Equation 9-1

NH3 + H2O+ ←⎯⎯→ NH4+ + OH-

Equation 9-2

NH3 + H3O+

Ammonia may also be formed in the steam/water cycle due to the decomposition of other volatile amines (e.g. morpholine, cyclohexylamine, etc.) and hydrazine (N2H4), a chemical reducing agent frequently used in high-pressure boiler operations. At temperatures between 111270°C (232 and 518°F), hydrazine volatilization increases with only slight chemical breakdown. However, above 270°C (518°F), hydrazine undergoes rapid thermal degradation, defined by the following reaction:

3N 2 H 4 + Heat → N 2 +4NH 3

9-2

Equation 9-3

EPRI Proprietary Licensed Material Ammonia

Figure 9-1 Percent of Ammonia and Ammonium Ions as a Function of Solution pH

9.2.2 Conductivity Approximation

Clearly, the concentration of ammonia in the feedwater is a critical parameter for controlling corrosion in the cycle. If the pH of the water is controlled entirely by the concentration of ammonia present, the concentration can be estimated quite accurately by interpolation from a graph, knowing only the sample pH and conductivity (Figure 9-2). 9.2.2.1 Conductivity Approximation Limitations

The conductivity measurement is also influenced by the presence of other ionic species, and the pH measurement is affected by carbon dioxide and other chemicals that may be present. Consequently, all changes in conductivity or pH are not necessarily indicative of changes in the ammonia concentration. Furthermore, because pH is a logarithmic scale, as the ammonia levels increase to high values, the changes in pH become increasingly small and the sensitivity of measurement decreases. At low levels of ammonia, both pH and conductivity rapidly approach lower limits reflecting the values for the ammonia-free water, and again the sensitivity of measurement decreases. Consequently, at best, accurate estimates can be made in a fairly narrow range of ammonia concentrations using this approach; i.e. between 0.1 and 2.0 mg/L (ppm) ammonia. 9-3

EPRI Proprietary Licensed Material Ammonia

Figure 9-2 Relationship between Ammonia, Specific Conductivity and pH in the Absence of Carbon Dioxide at 25ºC [1] Source: Adapted from Reference 1

9.2.3 Ion Selective Electrode

One type of on-line ammonia monitor utilizes an ion selective electrode for analyte measurement [5,6]. The ion selective electrode detects NH4+ ions, not ammonia, so measurement must be preceded by an adjustment of the sample pH to a value that is low (pH> CB, (i.e. the concentrations of the standards are much larger than the chloride content at the detection limit) there are three equations with three unknowns: E = E O + S log (CS / CIso )

Equation 10-2

where S is temperature dependent slope and CIso is the chloride ion concentration in µg/L (ppb) in the sample that produces an electrode potential that is independent of temperature; the isopotential point. This is the reference point for temperature compensation. The addition of the standard increases the concentration by a known amount, dC1, to CS+dC1, and changes the measured potential to E1, as shown in equation 10-3: E1 = E O + S [log (CS + dC1 ) / CIso ]

Equation 10-3

Then, a second addition of a standard solution is made to the sample. This standard solution is preferably about ten times more concentrated than the first, causing a further increase in concentration, dC2, and a new potential, E2, as shown in equation 10-4: E 2 = E O + S [log(CS + dC1 + dC 2 ) / CIso ]

10-6

Equation 10-4

EPRI Proprietary Licensed Material Chloride

The instrumentation automatically solves the three equations (10-2 through 10-4) for the three unknowns, EO and S are stored for subsequent use in the on-line monitoring mode, and a calibration curve is established. This curve is stored in the microprocessor for determining unknown chloride values. The newest generation of monitor employs a two segment calibration method to cover a wide range of concentrations. The first segment is an approximately liner range of 0 to 125 µg/L (ppb) chloride. The second segment is a range of 75 µg/L (ppb) to 1000 µg/L (ppb) where the electrode response is logarithmic with changing concentrations. Calibration is carried out at three chloride levels with concentrations being in the range of 0-20 µg/L (ppb), 75–125 µg/L (ppb), and 100–1000 µg/L (ppb) ranges, starting with the lowest concentration. The first solution can be the “zero chloride” solution and the second and third levels can be generated by introducing known chloride concentrations into the sample chamber. Calibration parameters of both segments are computed by the microprocessor and effects from the temperature fluctuation are constantly corrected. Based on the potential measured on the sample solution, the microprocessor makes a judgment of which segment of calibration is to be used to read the chloride concentration. The electrode response to these two segments can be characterized by the following equations: Low Level:

E = Eo(T) + S1(T)*(C/C2)

Equation 10-5

High Level:

E = Eo(T) + S2(T)*log (C/C2)

Equation 10-6

where: E = measured electrode potential Eo(T) = temperature dependent potential value S1(T), S2(T) = temperature dependent slope values C = concentration (activity) of chloride ion in the sample C2 = concentration (activity) of chloride ion of the second standard By doing a three point calibration, the microprocessor determines the actual values of all the parameters and enables measurement of chlorides at both low and high levels. The monitor incorporates a cooling system to achieve the stated 5 µg/L (ppb) detection limits and uses its microprocessor to constantly update temperature corrections for data supplied by the automatic temperature probe (ATC) probe. Figure 10-2 demonstrates a typical Ion Selective Electrode calibration. Note the theoretical slope of 59.6 mV for each decade of concentration change. A typical working range comprises three or more decades of concentration where the electrode response conforms to the Nernst equation. Note the deviation from Nernst behavior and how the low detection limit is estimated by extrapolating the linear portion of the curve and baseline mV.

10-7

EPRI Proprietary Licensed Material Chloride

Figure 10-2 Calibration Curve for Low Level Chloride SIE [16]

10-8

EPRI Proprietary Licensed Material Chloride

10.6 Calibration Checks On-line chloride instrument analytical capabilities should be checked periodically to demonstrate calibration stability. Two methods exist for verifying instrument stability; the Standard Injection Method [9] or the Line Method [10]. For the Standard Injection Method, a known standard solution, near the mid-point of the calibration curve, is analyzed by the on-line instrument and the results are compared to the acceptance criteria. Acceptance criteria are either established based on statistically derived limits. i.e., ± 3 sigma or based on some predetermined limits established from experience, i.e., ± 10%. Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is deemed to be within acceptance limits. If the results are outside the acceptance criteria the on-line instrument must be recalibrated. For the Line Method a calibrated separate chloride analyzer, typically a bench top analyzer is used to analyze a sample from the same sample stream as the installed on-line instrument. Provided the bench top analyzer agrees within the acceptance criteria (e.g., matches the results of the on-line analyzer within ± 3 sigma or ± 10%) the on-line instruments calibration is deemed to within acceptance limits. If the results are outside the acceptance criteria the on-line instrument must be recalibrated.

10.7 Alternative Methods There are a multitude of wet chemical analyses available for chloride determination [7]. Although these methods are not currently used for on-line analyses, a listing is provided for reference. 10.7.1 ASTM D512 Test Method A—Mercurimetric Titration

This method describes a colorimetric end-point titration whereby a dilute mercuric nitrate solution is added to an acidified sample in the presence of di-phenylcarbazone-bromphenol blue indicator. The end point of the titration is the formation of the blue-violet mercury diphenylcarbazone complex. This test method is validated for chloride concentrations from 8 to 250 mg/L (ppm). Commercial test kits based on this method have a workable range of 10-8000 mg/L (ppm). 10.7.2 ASTM D512 Test Method B—Silver Nitrate Titration

This method describes a colorimetric end-point titration whereby a sample adjusted to pH 8.3 is titrated with silver nitrate in the presence of potassium chromate indicator. The end point is indicated by the persistence of the brick red silver chromate color. This test method is validated for chloride concentrations from 8 to 250 mg/L (ppm). Commercial test kits based on this method have a workable range of 5-1000 mg/L (ppm).

10-9

EPRI Proprietary Licensed Material Chloride

10.7.3 Standard Methods: Method 4500 - Cl D. Potentiometric Method [11]

Chloride is determined by a potentiometric titration with silver nitrate solution and a silver-silver chloride electrode system. During titration an electronic voltmeter is used to detect a change in potential between the glass reference electrode and the silver-silver chloride sensing electrode. The end point of the titration is that instrument reading in which the greatest change in voltage has occurred for a small and constant increment of silver nitrate added. Commercial test kits based on this method have a workable range of 5-1000 mg/L (ppm) 10.7.4 Standard Methods: Method 4500 - Cl E. Automated Ferricyanide Method [11]

Thiocyanate ion is liberated from mercuric thiocyanate by the formation of soluble mercuric chloride. In the presence of ferric ion, free thiocyanate ion forms a colored ferric thiocyanate, whose intensity is proportional to the chloride concentration. This method describes an automated process employed by the Technicon AutoAnalyzer II [12]. Using standard sample volumes, the detection limit for this instrument was published as 70 µg/L (ppb) chloride. 10.7.5 Ion Chromatography

An alternative on-line method for chloride determination, ion chromatography, discussed in Section 13 of this manual, uses a specific conductivity detector [13,14]. The trace anion method utilizes a sample pre-concentration column where the anions of interest are collected on an ion exchange column. After a suitable volume of sample has been concentrated, eluent is pumped through the concentrator column to remove the trapped anions. Eluent then flows through the analytical column where the anions are separated based on the retention characteristic of each anion relative to the eluent used. The eluent stream containing the anions of interest then passes through a suppressor device where the cations from the eluent are exchanged for hydrogen ions, converting the anions to their acid form. After the suppressor device, the eluent solution passes through a conductivity detector where the separated anions are detected. Detection limits for the anions are enhanced because the anions are in the acid form rather than the sodium salt. This method reports a single operator detection limit of 0.8 µg/L (ppb). The common practical range of the method is listed as 1-100 µg/L (ppb) for chloride. At least one instrument manufacturer has commercially available on-line ion chromatography equipment operating in the electric utility setting [15].

10.8 End User Considerations The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line sodium instrument. In general,

10-10

EPRI Proprietary Licensed Material Chloride

manufacturers determine these characteristics using their own in-house methods. End users should select the instrument that is best suited for the intended analytical application. Other on-line chloride instrument considerations include: •

Appropriate pH adjustment for analysis range of interest to minimize OH- ion interference with chloride measurements.

•

Ease and robustness of calibration for the intended use.

•

Ease and robustness of calibration verification.

•

Appropriate sensing electrode linearity characteristics in the analytical range of interest.

•

Reference electrode stability.

10.9 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.

4.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

5.

ASTM D5542-04, “Standard Test Method for Trace Anions in High Purity Water by Ion Chromatography”. 2004 Annual Book of ASTM Standards, Vol. 11.02 Water. American Society for Testing and Materials, Philadelphia, PA.

6.

ASTM D5996-05, “Standard Test Method for Measuring Anionic Contaminants in HighPurity Water by On-Line Ion Chromatography”. 2005 Annual Book of ASTM Standards, Vol. 11.02 Water. American Society for Testing and Materials, Philadelphia, PA.

7.

ASTM D512-04, “Standard Test Method for Chloride Ion in Water”. 2004 Annual Book of ASTM Standards, Vol. 11.01 Water. American Society for Testing and Materials, Philadelphia, PA.

8.

The Volatility of Impurities in Water/Steam Cycles. EPRI Palo Alto, CA: 2001. 1001042.

9.

Model 1817LL Chloride Analyzer Instruction Manual, Thermo Scientific Corporation, Beverly, MA. 2003 S-1817LL-E-0506 RevB.

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EPRI Proprietary Licensed Material Chloride

11. ASTM D512-04, “Standard Test Method for Chloride Ion in Water”. 2004 Annual Book of ASTM Standards, Vol. 11.01 Water, American Society for Testing and Materials, Philadelphia, PA. 12.

Standard Methods for the Examination of Water and Wastewater, 21st Edition. Baltimore, MD. 2005.

13.

ASTM D 5996. “Standard Test Method for Measuring Anionic Contaminants in HighPurity Water by On-Line Ion Chromatography”. ASTM International, West Conshohocken, PA. 2005.

14.

ASTM D 5542. “Standard Test Method for Trace Anions in High Purity Water by Ion Chromatography”. ASTM International, West Conshohocken, PA. 2004.

15.

Dionex Process Analytical DX-800 Product Information, Dionex Corporation, 1228 Titan Way, Sunnyvale, CA 94088-3603. 2004.

16.

Fossil Power Plant Chemistry, Scientech, LLC, 1060 Keene Road, Dunedin, FL 34698-6300, 2001.

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EPRI Proprietary Licensed Material

11 HYDRAZINE

11.1 Purpose and Use Hydrazine is a reducing agent that is frequently used in boiler feedwater treatment to promote oxygen removal. Hydrazine reacts with oxygen to form water and nitrogen and, under certain conditions (e.g. temperatures > 270°C (518°F)), it can also decompose to form ammonia and nitrogen. Hydrazine is also instrumental in promoting magnetite (Fe3O4) formation which provides an iron oxide surface that consumes dissolved oxygen. Reducing agents such as hydrazine are critical in mixed metallurgy systems (systems containing copper alloys) to prevent copper corrosion due to amines in the presence of dissolved oxygen. While hydrazine is not listed as an EPRI Core Monitoring Parameter [1-3], it is often monitored in mixed metallurgy feedwater cycles using reducing All Volatile Treatment—AVT(R). This chemical treatment scheme refers to the feedwater treatment and is not influenced by further addition of phosphate or caustic in the boiler drum. The prescribed location for this sample point is the deaerator inlet, although many plants sample the economizer inlet as well. Proper control of hydrazine as a reducing agent is necessary to protect the copper alloys in the mixed metallurgy system from oxygen attack. Overfeeding hydrazine can lead to excess ammonia in the system, which can also be damaging to the copper alloys. Hydrazine feedrate can be indirectly controlled by measurement of the oxidation reduction potential (ORP) of the system—discussed in detail in Chapter 6. Hydrazine is continually monitored on-line in the plant for the following reasons, listed in order of importance: •

To check the accuracy of water chemistry control, so ensuring that corrosion rates are kept at acceptably low levels.

•

To evaluation of other chemistry parameters (i.e., ORP and dissolved oxygen).

•

To provide feedback stimulus for automated process control.

The data generated by continuous on-line monitoring of hydrazine is used by plant chemistry and operations personnel. The goal for plant personnel is to maintain hydrazine within prescribed limits.

11-1

EPRI Proprietary Licensed Material Hydrazine

11.2 Description of Methods Several methods are available for monitoring hydrazine concentration on-line: ion chromatography, colorimetry, amperometry, and the ion selective electrode method. Since ion chromatography is not typically used for on-line surveillance and control of hydrazine, it will be discussed in subsection 11.7, Alternative Methods of Analysis. The colorimetric method is an automated version of the ASTMD 1385 [4] grab sample technique, based on the reaction of hydrazine with p-dimethylaminobenzaldehyde to produce a yellow reaction product in an acidic environment. The intensity of the yellow color, which is proportional to the hydrazine concentration in the sample, is measured with a spectrophotometer at a wavelength of 460 nm. The amperometric methods fall into two variants: a 2-electrode system in which an anode and a cathode are made from dissimilar metals, and a 3-electrode system in which anode, cathode and reference electrodes are connected to a potentiostatic circuit. In each case, the anode contacts the water sample containing hydrazine, and the potential difference between the anode and cathode promotes the oxidation of hydrazine at the anode surface. The current that flows between the anode and cathode as a result of the electrochemical reactions is proportional to the hydrazine concentration. The ion selective electrode (ISE) method involves adding iodine to the water sample which reacts with the hydrazine to produce iodide. The concentration of iodide produced, which is proportional to the concentration of hydrazine in the water sample, is measured with an iodide ion selective electrode (sometimes called specific ion electrode).

11.3 Technical Considerations 11.3.1 Colorimetry

Typically, the water flow rate to the analyzer inlet exceeds the minimum flow required and the excess flows to waste. This arrangement ensures that a fresh sample is always available to the analyzer. Often, the sample is also conditioned to regulate the inlet water pressure. At the beginning of each analysis cycle, the measurement cell is flushed thoroughly with a fresh sample. An initial sample blank absorbance reading is taken with the spectrophotometer at a wavelength of 458 to 460 nm to provide the zero reference measurement. Then a solution of p-dimethylaminobenzaldehyde in methyl alcohol and hydrochloric acid is added to the sample. A yellow color characteristic of p-dimethylaminobenzalazine is formed (Eq. 11-1), the intensity of which depends on the amount of hydrazine present. A second absorbance measurement is made at 458 to 460 nm and compared with the zero reference measurement. The difference between the two measurements, which is proportional to the hydrazine concentration in the sample, is displayed directly in units of hydrazine concentration.

11-2

EPRI Proprietary Licensed Material Hydrazine

2 ρ − (CH 3 )2 NC6 H 4CHO + N 2 H 4 → (CH 3 )2 NC6 H 4CHN − HCHC6 H 4 (CH 3 )2 + 2H 2O Equation 11-1

11.3.1.1 Colorimetric Limitations

All colorimetric tests such as hydrazine rely on the principle of Beer’s Law (also known as the Beer-Lambert Law) which states that the amount of light absorbed by a sample (A) is proportional to some absorption constant (a), the path-length of light (b), and the concentration of the analyte species(c). A=axbxc

Equation 11-2

The absorption constant is determined by the color species and test conditions of the method and remains constant as long as the reagents, test conditions, and wavelength of light do not change. The path length (the amount of sample through which the light passes) is also constant for a given sample cell. The mathematical formula then infers that the amount of light absorbed by a sample (i.e. the intensity of the color developed in the test) is directly proportional to the concentration of the analyte—in this case, hydrazine. As the hydrazine concentration increases, the intensity of the yellow color will increase and this color change can be quantified. Limitations on this analytical principle arise at both the low end and high end of the useable range. The low end limit is caused by the detection limit of the test—this is primarily a function of the photo-multiplier (detector) sensitivity and stability. How small a change can be detected by the sensor and how stable is the baseline (or the zero reading)? When trying to quantify a test right at the detection limit, negative readings are sometimes observed or duplicate readings are seen that have large relative errors although the absolute error may only be a few µg/L (ppb). As a result, most hydrazine analyzers will list a detection limit, an accuracy limit and a precision limit. This accuracy limit is typically an absolute value or some percentage of the reading. The colorimetric analyzers reviewed did not list an absolute accuracy value. A commonly used analyzer has the following specifications which demonstrate this technical consideration [5]. •

Minimum Detection Limit: 1 µg/L (ppb).

•

Accuracy: ±2% of reading.

•

Precision: ±1 µg/L (ppb) or ±2% of reading whichever is greater.

Evaluation of this specification shows that the uncertainty of the measurement is large relative to the measurement itself at or near the minimum detection limit. At lower concentrations (1–5 µg/L (ppb)) end users are cautioned about using data for critical decision making since the inaccuracy is large relative to the measurement itself. At concentrations >50 µg/L (ppb) the precision limit of ± 5% is used since it becomes larger than ± 1 µg/L (ppb). A different analyzer may have a different specification with a lower detection limit as a result of using another analytical method (changing the absorption constant) and using a longer path length sample. 11-3

EPRI Proprietary Licensed Material Hydrazine

Limitations on the high end of the analytical range (Figure 11-1) arise from a property called self-absorption. As the color intensity becomes more and more yellow, the amount of light coming into the sample cell is almost totally absorbed.

Figure 11-1 Absorption vs. Concentration

At some point there is 100 percent absorption, the line flattens out, and the analytical test is no longer usable. However, the area where linearity first starts to deviate is also an area of concern. The molecules imparting color (absorbing light) are so numerous that some molecules are shaded from the incident light and not accounted for. The subsequently curved line can no longer be used for accurate analytical determination. The ionic solution strength at higher concentrations also starts to affect the color developing molecules and leads to non-linear response. Most instruments therefore have a maximum analytical range which corresponds to the top portion of the straight-line relationship. Analytical readings above this linear region are disallowed and the analyzer produces an “over range” alarm. Turbidity or color in the water sample may interfere with this analysis, but this is not normally of concern in the sample streams that are typically analyzed in power plants. 11.3.2 Amperometry 11.3.2.1 Two Electrode Method

As indicated above, there are two types of amperometric methods: the 2-electrode and the 3-electrode methods, and details of both methods vary somewhat from one manufacturer to the next. For instance, in one manifestation of the 2-electrode technique, the water sample flows at a constant rate through a tube made from a porous ceramic [6]; other manufacturers make the tube from a semi-permeable membrane instead. A platinum wire anode is positioned along the axis of 11-4

EPRI Proprietary Licensed Material Hydrazine

this tube in direct contact with the water sample, and a silver wire cathode is wound around the outside of the tube. The electrodes and ceramic tube are surrounded by a non-porous outer jacket. The space between the jacket and tube is filled with a mixture of gel and silver oxide. The silver oxide, being in intimate contact with the spiral wound silver wire, becomes part of a composite cathode. Electrolytic contact between the platinum anode and silver/silver oxide cathode occurs because ionic transport is possible through the porous ceramic tube. The potential difference between the anode and cathode (essentially a galvanic couple) stimulates oxidation of hydrazine at the anode and reduction of silver oxide and plating of silver at the cathode. The resultant current depends on how rapidly hydrazine is transported to the anode and, for a given flow rate, flow geometry, pH, and temperature, the current is proportional to the concentration of hydrazine in solution. A thermistor or equivalent device may be incorporated into the measurement cell to provide automatic temperature compensation. Some manufacturers suggest that, for optimum performance, the sample should be made highly alkaline by adding sodium hydroxide to the water sample upstream of the measurement cell. The anode reaction is as follows: N2H4 + 4OH-

→

N2 + 4 H2O + 4 e-

Equation 11-3

The following reaction simultaneously occurs at the cathode: Ag2O + H2O + 2e-

→

2Ag + 2 OH-

Equation 11-4

11.3.2.2 Three Electrode Method

Like the 2-electrode method, the 3-electrode method is configured to generate a mass transport limited current that is proportional to the hydrazine concentration. Design details vary from one manufacturer to the next, but in one manifestation, the sample must be pre-conditioned if the conductivity of the water sample is < 8 µS/cm. If pre-conditioning is necessary, the water must flow through a bed of granulated marble to increase the conductivity and adjust the pH. The water sample then flows at a constant rate through a stainless steel tube that also serves as the counter-electrode (cathode) of the measurement cell. A gold-plated stainless steel sensing electrode (anode) is positioned along the axis of this tube; a silver/silver chloride reference electrode is mounted nearby, and brought into electrolytic contact with the water sample by means of an electrolyte bridge. The bridge is typically a plastic tube, filled with potassium chloride solution, and fitted at its end with a porous ceramic diaphragm to minimize mixing of the potassium chloride and the water sample. Another manufacturer pre-conditions (increases the conductivity of) the flowing water sample by injecting potassium nitrate; the water then flows to a specially designed flow cell in which a silver counter-electrode (cathode), a platinum sensing electrode (anode), and a calomel reference electrode are mounted. In all 3-electrode designs, the sensing electrode, counter-electrode and reference electrode are connected to a potentiostat. The potentiostat controls the electrochemical potential difference between the sensing electrode (anode) and reference electrode by supplying, and automatically adjusting, the flow of direct current between the counter- and sensing electrodes (cathode and 11-5

EPRI Proprietary Licensed Material Hydrazine

anode). Thus, the potentiostat replaces the stimulus of the galvanic couple in the 2-electrode method, and ensures that the electrochemical conditions prevailing at the anode surface lead to a mass transport limited current. One manufacturer that uses a platinum anode (working electrode) and stainless steel cathode (counter-electrode) reports a linear relationship between cell current and hydrazine concentration in the range of 0 to 500 µg/L (ppb) when the anode potential is controlled at +480 mV versus the Ag/AgCl reference electrode [5]. Following are the reactions at the two electrodes: Anode:

N2H4 + 4 OH-

Cathode: 4 H2O + 4 e-

→ →

N2 + 4 H2O + 4e2 H2 + 4 OH-

Equation 11-5 Equation 11-6

The reducing agent is oxidized on the platinum electrode into nitrogen and water. Simultaneously the counter electrode decomposes the water. For a given flow rate, flow geometry, pH, and temperature, the measured current is proportional to the concentration of hydrazine in solution (just as it is in the 2-electrode method). One advantage of the 3-electrode system is that the cathode needs very little maintenance whereas, in the 2-electrode system, the silver oxide gel mixture must be replaced when it has been consumed by the cathodic reaction. In addition, the 3-electrode system theoretically has a faster response because both anode and cathode are in direct contact with the water sample. Teflon® beads, driven by the sample flow, circulate on the surface of the platinum anode to prevent deposition. In all amperometric methods, the accuracy of measurements can be degraded by the presence of other oxidizable species in the water. Sulfite, for instance, must be removed by passing the sample through an anion bed before the hydrazine concentration can be measured. In addition, the concentrations of morpholine, cyclohexylamine, and ferrous ion should each be maintained below 1 mg/L (ppm) to avoid significant errors. Amperometric methods also respond to carbohydrazide (a hydrazine substitute) which may be of analytical interest or may be an interference depending on the type of reducing agent being used. The reaction of hydrazine in amperometric determinations is enhanced by an elevated pH. Amines such as ammonia, diisopropylamine (DIPPA), diethylamine (DEA), or monoethylamine (MEA) are used to condition the sample to a pH >10.2 before it enters the measuring cell. 11.3.3 Iodide Ion Selective Electrode Method

In this method, the pressure and flow rate of the water sample are controlled at the inlet of the analyzer. The water flows at a constant rate through special plastic tubing that is coiled in a reagent bottle containing both acid and iodine. Both substances diffuse through the tubing wall and dissolve in the sample, resulting in a reduction of the sample pH and, at the same time, a controlled addition of iodine [9]. The hydrazine in the water sample and the iodine react to form iodide, as follows: N 2 H 4 + 2 I 2 → 4H + + 4I - + N 2

11-6

Equation 11-7

EPRI Proprietary Licensed Material Hydrazine

The acid is added to stifle the following competing reaction which, if it occurred, would lead to production of iodide by a route that is unrelated to the hydrazine concentration: 3H 2O + 3 I 2 → 6H + + 5I - + IO 3-

Equation 11-8

Thus, each molecule of hydrazine in the original sample produces four iodide ions and the concentration of iodide is directly proportional to the original hydrazine concentration. The sample flows on to the measurement cell which houses an iodide ion selective electrode and a reference electrode. The iodide electrode responds logarithmically to iodide concentration, as described by the Nernst equation: E = E o + [2.3026 R T / n F] × log10 [CI /C Iso ]

Equation 11-9

where: E

=

the measured potential of the electrode pair (V)

Eo

=

the measured potential when the iodide concentration in the sample equals CIso (V)

R

=

the ideal gas constant (8.317 joules/°Kxmole)

T

=

temperature of the sample (°K)

n

=

the valence of the ionic species (equals 1 for iodide ion)

F

=

Faraday’s constant (96,486.7 coulombs/gram-equivalent)

CI

=

effective iodide concentration, i.e. activity, µg/L (ppb)

CIso =

the iodide concentration in µg/L (ppb) in the sample that produces an electrode potential that is independent of temperature: the isopotential point. This is the reference point for temperature compensation.

The measured potential, E, may be converted directly to units of hydrazine concentration and is usually displayed by the instrumentation in this way. It is clear from the Nernst Equation that the measured potential is dependent on temperature as well as iodide concentration. Typically, hydrazine analyzers monitor the temperature of the sample and automatically compensate for changes in temperature.

11.4 Interferences 11.4.1 Colorimetry

The colorimetric method, which is based on the acidified reaction of hydrazine with p-dimethylaminobenzaldehyde is relatively free from chemical interferences. It’s unlikely that the ammonia in the feedwater sample would be strong enough to overcome the hydrochloric acid in the color reagent but note that the color developing reaction is dependent on acidic conditions. As with any colorimetric test, yellow colored species that absorb in the same wavelength of light (460 nm) have the potential to interfere with quantification. Most on-line analyzers attempt to 11-7

EPRI Proprietary Licensed Material Hydrazine

avoid this interference by setting the baseline (zero) of the reading immediately before adding the color developing reagent. In this way, any background color will be accounted for prior to color development. The same caveat is true for suspended solids and turbidity in the sample. 11.4.2 Amperometry

The amperometric methods are influenced by other reducing agents such as carbohydrazide. Reducing agents are not necessarily a source of interference in that several instruments actually have a customizable program which allows readout in µg/L (ppb) carbohydrazide rather than hydrazine. Use of an amperometric method allows flexibility to use alternate reducing agents. The sensing electrode tends to suffer surface degradation over time and this often leads to sluggish response. Some manufacturers have designed a measuring cell with free-flowing Teflon beads that are agitated by the sample flow and serve to scour the platinum electrode surface clean. 11.4.3 Ion Selective Electrode Method

The ion selective electrode method is susceptible to any other reducing agents that would react with the iodine reagent to produce iodide, the analyte that is detected. Other halogen ions such as chloride also cause a sensor response. As in the amperometric method, molecules of carbohydrazide or other reducing agents can either be an interference source or the analyte species of interest. Use of an ISE method allows flexibility to use alternate reducing agents.

11.5 Calibration 11.5.1 Colorimetry

Most colorimetric on-line hydrazine monitoring instrumentation allows fully automated 2-point calibration. When activated, the instrument pumps calibration standards through the system instead of the normal water samples. The zero and span are then automatically adjusted to be consistent with the concentrations of hydrazine in the standard solutions. The frequency of calibration can be pre-programmed on some instruments. Calibration is traditionally performed by plant staff, typically by activating a built-in function of the instrument that provides an automatic two-point calibration. However, some older instrumentation requires manual calibration. Whatever the age of the instrumentation, a typical calibration would make use of demineralized water (0 µg/L (ppb) hydrazine) and a standard solution containing a known concentration, for instance, 200 µg/L (ppb) hydrazine. Another instrument performs its baseline calibration automatically (perhaps once every 8 hours) and the full scale calibration on another schedule (every two days). Performing additional calibrations while an individual is observing the instrument can be valuable for validity assurance. If sample valves, reagent lines, mixing mechanisms, light 11-8

EPRI Proprietary Licensed Material Hydrazine

source, detectors, or amplifiers are not operating properly, the instrument will likely not calibrate to manufacturer’s specifications. The individual commanding the manual calibration can then troubleshoot the analyzer and resolve the discrepancy. Calibration and maintenance procedures are typically described in literature supplied with the monitoring equipment by each manufacturer. Additional insight on maintenance and calibration is included in EPRI Report GS-7556 [7]. 11.5.2 Amperometry

Only one amperometric on-line hydrazine monitoring instrument allows fully automated 2-point calibration in a fashion very similar to the colorimetric design [8]. When activated, the instrument pumps calibration standards through the system instead of the normal process sample. The zero and span are then automatically adjusted to be consistent with the concentrations of hydrazine in the standard solutions. The frequency of calibration can be pre-programmed and manual calibration can also be initiated at any time. This instrument is also designed to accept a grab sample. All of the amperometric on-line hydrazine analyzers allow calibration to a sample value that is determined by a laboratory bench method. These reference methods are either the ASTM D13854 (p-dimethylaminobenzaldehyde) previously discussed or a proprietary Hach method [12]. Bench analysis of this sample and correlation of the on-line instrument to this reading would then theoretically establish the slope for the electrode response. The disadvantage of this approach is that hydrazine values in the process sample are often fairly low. The calibration obtained by correlating the bench test and the analyzer in this low range creates a compressed slope region. This narrow calibration range can result in large inaccuracies when the hydrazine content is high. It is often not feasible to elevate the process hydrazine content in order to determine a response closer to full scale on the analytical curve. 11.5.3 Ion Selective Electrode Method

The hydrazine analyzer based on the iodide ion selective electrode design is calibrated using the double known standard addition (DKA) method [9]. The calibration process makes use of a precisely controlled syringe called a dynamic calibrator. This syringe injects known volumes of hydrazine standard into the flowing sample stream and allows a two point DKA calibration. These two calibration points are at approximately 20 µg/L (ppb) and 200 µg/L (ppb) hydrazine. Prior to the addition of the first standard, the instrument measures the potential (E0) and stores this value in the microprocessor. As shown in Eq. 11-10, the measured potential (E) is equal to E0 when CS = zero: E = E O + S log (CS / CIso )

Equation 11-10

11-9

EPRI Proprietary Licensed Material Hydrazine

A known amount of Standard Solution 1 is added to the sample reservoir which increases the concentration (Cs) by a corresponding amount (dC1). The new potential (E1) is measured and stored when electrode stability is reached (Eq. 11-11). E1 = E O + S [log (CS + dC1 ) / CIso ]

Equation 11-11

Standard 2 (ideally 10 times more concentrated than Standard 1) is added which again increases the concentration in the sample reservoir by a corresponding amount (dC2). Again the new potential (E2) is measured and stored when stable (Eq. 11-12): E 2 = E O + S [log(CS + dC1 + dC 2 ) / CIso ]

Equation 11-12

There are now three equations (Eqs. 11-10, 11-11, and 11-12) with three unknowns coming from the Nernst Equation. The instrumentation automatically solves the three equations (11-10 through 11-12) for the three unknowns, and EO and S are stored for subsequent use in the on-line monitoring mode. The working range for the ion selective electrode method is 0-200 µg/L (ppb) with accuracy listed in two different ranges [9]. When the instrument is spanned at 0-50 ppb (µg/L), the accuracy is ±2 µg/L (ppb) or ±10% of the reading, whichever is greater. When spanned over the broader 0-200 µg/L (ppb) range, the accuracy is ±2 µg/L (ppb) or ±15% of the reading. Some analyzers can also be single point calibrated against an external sample analysis (off line calibration to a value determined on the bench) as in the amperometric design.

11.6 Calibration Checks On-line hydrazine instruments should be checked periodically to demonstrate calibration stability. The Line Method [10] is appropriate for verifying instrument stability for either ISE, colorimetric, or amperometric methods. For the Line Method, a calibrated separate hydrazine monitor, typically a bench top analyzer is used to analyze the same sample steam as the installed on-line instrument. The two results are compared to the acceptance criteria (e.g., agree within ± 3 sigma or ± 10%). Provided the online analyzer agrees within the acceptance criteria, the on-line instrument’s calibration is considered to be acceptable. If the results are outside the acceptance criteria the on-line instrument must be recalibrated.

11.7 Alternative Methods The wet chemistry method used for laboratory analysis is invariably based on the p-dimethylaminobenzaldehyde color complex [10]. When a solution of this indicator in methyl

11-10

EPRI Proprietary Licensed Material Hydrazine

alcohol is added to hydrazine in a diluted hydrochloric acid environment, a characteristic yellow color of p-dimethylaminobenzalazine, as shown earlier in Eq. 11-1, is formed. The intensity of the yellow color formed is proportional to the hydrazine present and is in good agreement with Beer’s Law in the range of 5–200 µg/L (ppb) hydrazine. The analytical wavelength for this determination is 458 nm with a 50 cm sample cell. Higher concentrations can be determined by making appropriate sample dilutions. Background color in the prescribed wavelength interferes with the test, as do turbidity and other dark colors. Some canceling of these effects is available through suitable manipulation of the colorimetric blank. Ion Chromatography is also available for an alternative method of hydrazine detection in the laboratory [11]. Hydrazine can be separated from other monovalent cations. The cation-resin column is packed with poly (styrene-divinylbenzene) based action exchanger with sulfonic acid functional groups. Silica-based columns are not suitable for hydrazine analysis. The mobile phase is made up of 3.2 mM nitric acid. One problem when using mobile phase nitric acid is that trace amounts of transition metals can irreversibly retain on the column. The symptom is identified by loss of retention time as the exchange sites are tied up by the strongly bound metal ions. To avoid this problem, a scavenger column is connected between the pump and injection valve to remove traces of transition metals form the mobile phase before it reaches the separation column. The scavenger column is packed with high capacity cation exchanger and eliminates frequent regenerations of the separation column and prolongs column life. Reported detection limits for this ion chromatographic analysis of hydrazine are 2 mg/L (ppm) with a 100µL injection volume. The limit of detection can be improved by increasing the injection volume.

11.8 End User Considerations The performance characteristics (quantification range, accuracy, precision, bias, drift) and design characteristics (cycle time, selection of reagents, reagent consumption, sample manipulation, sample conditioning, and chemical interferences) for the hydrazine monitor as provided by the manufacturer or supplier should be considered when selecting an suitable instrument. Other on-line hydrazine monitor considerations for feedwater/condensate samples include: •

Inlet sample flow and pressure requirements

•

Digital Control System (DCS) interface compatibility

•

Provision for adequate sample and spent reagent drain

•

Provision for instrument purge air / pressurization air if required

•

Ability to perform external validation with grab samples or standards

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EPRI Proprietary Licensed Material Hydrazine

11.8.1 Recognizing Instrument Malfunctions

Several measurement errors may occur when using a colorimetric hydrazine analyzer. They can be collected into two groups: Group 1, pertaining to sample delivery and Group 2, related to analyzer malfunction. A brief discussion of these malfunctions follows. 11.8.1.1 Sample Delivery

•

All analyzers must rely on sample delivery at adequate flow. One approach is to monitor the inlet sample pressure to assure adequate flushing between samples. Required inlet sample pressure is from 3.45 kPa ± 2.07 kPa (5 ±3 psi). Another manufacturer has a constant head system with an overflow weir in the sample reservoir. The reservoir also has a float switch installed to indicate a “no sample’ condition. Both designs effectively stop the analyses routine if sample flow is not detected. It is desirable to have an alarm contactor available to signal this “trouble” condition to the DCS and subsequently to the control room / lab. On systems without this relay, a hydrazine analyzer can effectively sit idle for long periods of time and the malfunction may go unnoticed.

•

As discussed previously, the colorimetric monitor attempts to cancel out the effects of suspended particulates in the sample by taking a “zero reading” immediately prior to color development. However, accumulation of particulates in the sample lines and colorimeter cells can cause very erratic and erroneous readings. Special precautions should be taken during cyclic operation and start-ups to filter the suspended material prior to the hydrazine analyzer.

11.8.1.2 Analyzer Malfunction

•

The most common problem with colorimetric analyzers has to do with liquid handling within the instrument. Depending on the analyzer design, the failure list includes pinch valves that leak through, sample lines that plug or develop pin holes, reagent lines that become unattached or develop leaks, eductors that become plugged and peristaltic pumps that develop leaks. Many of these malfunctions will be evident by liquid lying in the bottom of the analyzer or running out onto the floor. Less obvious malfunctions will be evident by the analyzer failing to calibrate, producing negative values, or yielding wildly fluctuating readings.

•

Reagent and standard replenishment continues to cause malfunctions in all designs. Purchase of prepared reagents and standards from the instrument supplier is a good practice. One instrument design has an internal reservoir of demineralizer water used for dilution of concentrated samples. Diligence is required to keep this diluent bottle filled and allow the analyzer to work properly.

•

Properly maintained and calibrated, amperometric analyzers seldom have malfunctions. The majority of these instruments have a pH buffering reagent which needs periodic replenishment. Fortunately, the reagent consumption is fairly small and requires attention only once a month. Most systems use incoming sample pressure to provide the motive force

11-12

EPRI Proprietary Licensed Material Hydrazine

for the reagent addition (either via an eductor that draws buffering vapor into the sample or via diffusion tubing that is immersed in the chemical) so external pumps and isolation valves are not required. It appears that surface cleanliness and stability of the noble sensing (working) electrode is a critical design consideration. Three of the four amperometric hydrazine analyzers that were reviewed have self-cleaning working electrodes to address this concern. The fourth analyzer (which is unique in that it does not require a pH buffering reagent) is not self-cleaning and requires electrode servicing on a three month schedule. Analyzers in this design category all measure and quantify weekly or monthly drift tolerances which also is an indication of the electrode condition. •

The iodide selective ion electrode analyzer has no moving parts, similar to the amperometric technology. Addition of the single reagent is made by way of diffusion tubing using only the inlet sample pressure and no isolation valves. Interferences to this process are minimal since the samples are fairly pure. Chloride ingress during a condenser leak on sea-water cooling could create erroneous hydrazine results since the electrode also responds to other halogen ions.

11.9 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All Volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSG’s). EPRI, Palo Alto, CA: 2006. 1010438.

4.

ASTM D1385, “Standard Test Method for Hydrazine in Water”. American Society for Testing and Materials, Conshohocken, PA 19428-2959. 2001.

5.

Hach 9186 Oxygen Scavenger Instrument Manual ,1st Edition; Hach Company, Loveland, CO 2003-04.

6.

Amatek / Thermox 7025 Hydrazine Analyzer; Bulletin P-57; Pittsburgh, PA.

7.

Monitoring Cycle Water Chemistry in Fossil Plants: Vol. 1 Monitoring Results, by A.F. Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon. EPRI, Palo Alto, CA: October 1991. GS-7556, Volume 1.

8.

Waltron Aqualyzer 9071 Hydrazine Analyzer Product Information Sheet; Waltron LLC, Whitehouse, NJ 08888-0070.

9.

Thermo Orion 1818AO Oxygen Scavenger Monitor Data Sheet; Thermo Electron Corporation, Beverly, MA, 01915. 2006.

11-13

EPRI Proprietary Licensed Material Hydrazine

10. ASTM D1385, “Standard Test Method for Hydrazine in Water”, American Society of Testing and Materials, Conshohocken, PA 19428. 2001. 11. Alltech Application Note: A0034; Alltech Associates, Inc. Deerfield, IL 60015. 1997. 12. Hach Method 8140, Iron Reduction Method. Hach Water Analysis Handbook.

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EPRI Proprietary Licensed Material

12 HYDROGEN

12.1 Purpose and Use Hydrogen is not widely monitored in fossil stations [1] but hydrogen analyzers may be used in a variety of power plant applications [1-3]. For instance, in the steam/water cycle of power plants, a layer of magnetite forms on the surfaces of carbon steel tubing and piping which protects these components from further corrosion. Under some circumstances, the magnetite is disrupted, leading to accelerated corrosion (the anodic reaction) at the sites of the disruption and an accompanying production of hydrogen (the cathodic reaction). Even though other possible sources of hydrogen exist, the concentration of hydrogen in the steam, feedwater, etc. has sometimes been measured to provide an indication of active corrosion in the feedwater or steam generator of nuclear plants [4]. Many large electrical generators are cooled with hydrogen gas due to its high thermal coefficient of heat transfer. Maintaining a high hydrogen purity (usually >98%) is necessary for efficient heat transfer. Hydrogen has a combustible range in air of 4%-75% and an explosive range of 13%–59%. Accurate on-line analysis of hydrogen purity is necessary as a safety issue both during normal operations and purging. The data generated by the monitoring equipment is used by the plant chemist and operations department personnel. The goal for plant personnel is to maintain hydrogen within prescribed limits for electrical generators.

12.2 Discussion of Methods Three types of hydrogen analyzers are in use: the polarographic sensor, the thermal conductivity sensor, and a vibratory gas density meter. Both electrochemical (polarographic) and thermal conductivity detectors have experienced widespread use in the past 20 years. Hydrogen analyzers based on polarographic detectors are used for quantifying dissolved hydrogen in aqueous samples. Hydrogen analyzers based on the thermal conductivity are used in gaseous hydrogen analysis (gas chromatography, generator hydrogen purity) and in one design of aqueous sensor [5]. An analyzer designed as a hydrogen purity meter for the gas cooled generator application uses a pair of vibrating piezo-electric elements to calculate the hydrogen concentration [6].

12-1

EPRI Proprietary Licensed Material Hydrogen

12.2.1 Electrochemical / Polarographic

The polarographic method, which is similar to the polarographic method for oxygen (see Dissolved Oxygen, Section 5), makes use of a probe which incorporates two metal electrodes— an anode and a cathode—in contact with an internal electrolyte that is separated from the water sample by a semi-permeable membrane. Hydrogen from the sample diffuses through the membrane and is oxidized at the anode within the probe. The current flowing between the anode and cathode is directly proportional to the partial pressure of hydrogen in the water sample flowing over the outer surface of the membrane. Because a current (amperes) is measured, the polarographic technique may also be termed an amperometric method. 12.2.2 Thermal Conductivity

This method employs a gas chromatograph in conjunction with a thermal conductivity sensor. Hydrogen purity meters for monitoring hydrogen cooled generators also use thermal conductivity sensors. Thermal conductivity (TC) sensors use the high heat transfer coefficient of hydrogen as their basis of operation. The variation in heat transfer ability between hydrogen and some reference gas causes a heated element (filament or thermistor bead) to cool at a differing rate. The electrical resistance of the heated element is proportional to the temperature of that element. Therefore, the resistance to current flow can be a direct measure of the thermal conductivity of the gas and indirectly can quantify the amount of hydrogen in the sample. Detailed discussion of this detector follows in the Technical Considerations discussion later in this section. In gas chromatography the various components of the gas mixture are separated by a packed media (i.e. a molecular sieve) and elute off the separation column at varying times. Each gas eluant is then passed through a detection device. Instruments using thermal conductivity detectors direct the gas through a series of filaments referred to as a Wheatstone bridge design. The thermal conductivity of the gas component is compared to the thermal conductivity of a known carrier gas and this difference in the ability to transfer heat (thermal conductivity) can be used to quantify the individual component. It is important to understand that in gas chromatographic analysis two distinct steps are involved. First is the separation of the gas mixture into the individual gas components by some form of selective column. The retention time of the component as it elutes off the separation column is indicative of the gas identity. After separation, a variety of detection methods can then be used to quantify the individual component; e.g. a thermal conductivity detector. 12.2.3 Gas Density Meter

A gas density meter can also be used to quantify hydrogen content. Hydrogen has the lowest density of all gasses. This physical property can be used to differentiate hydrogen from other gases and gas mixtures such as carbon dioxide and air. The gas density monitor under consideration uses vibrating piezo-electric driver elements to cause a thin walled cylinder to 12-2

EPRI Proprietary Licensed Material Hydrogen

vibrate [6]. The resonance frequency of this vibration varies with the density of the gas within the cylinder. A second set of elements located in the same plane senses this resonance frequency and correlates the frequency to the density of the gas filling the cylinder. The vibrating gas density meter is designed to determine the hydrogen purity during normal generator operations. It can also determine the amount of hydrogen in carbon dioxide during the generator purge sequence and the amount of carbon dioxide in air during the final purge step.

12.3 Technical Considerations 12.3.1 Electrochemical / Polarographic

The probe used in the polarographic method contains an anode and a cathode, separated from the water sample by a semi-permeable membrane. The anode material is typically platinum and the cathode is a material such as silver/silver chloride (produced by chloridizing a silver electrode). The semi-permeable membrane may be polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), SARAN™, Tedlar® polyvinyl fluoride (PVF) or a similar material; this membrane allows hydrogen to pass through it from the water sample to the internal electrolyte, but prevents the transfer of dissolved solids which, if it occurred, could interfere with the measurement. The internal electrolyte contacting the anode and cathode may be sodium chloride, potassium chloride, or other salt solution, and would normally be buffered. The potential difference between the cathode and anode (the cell voltage) is held constant by external instrumentation. This cell voltage is typically held at about -0.4 volt. The reactions that take place at, say, a silver/silver chloride cathode and a platinum anode in a buffered potassium chloride internal electrolyte are as follows: Cathode:

2AgCl + 2e- → 2Ag0 + 2C1-

(Reduction)

Equation 12-1

Anode:

H 2 + 2OH - → 2H 2O + 2e-

(Oxidation)

Equation 12-2

[or H 2 → 2H + +2e- ]

(Oxidation)

Equation 12-3

The hydrogen is oxidized to water (or protons) and the silver chloride is reduced to metallic silver. This results in the flow of an electrochemical current between the anode and cathode. The charge is carried by free electrons in the wires and ammeter within the external circuit, and by charged ions, such as Cl- or OH-, in the internal electrolyte. The magnitude of this current is controlled by the rate at which hydrogen arrives at the anode surface, which is determined by the rate of hydrogen diffusion through the semi-permeable membrane and the internal electrolyte. The rate at which hydrogen diffuses through the semi-permeable membrane is, in turn, proportional to the dissolved hydrogen content of the water sample flowing past the membrane. Thus, the electrochemical current flow is proportional to the dissolved hydrogen content of the water sample. The current can be converted with amplifiers and appropriate circuitry to provide a current or voltage output for a strip chart recorder, data logger, or control equipment. 12-3

EPRI Proprietary Licensed Material Hydrogen

Temperature and pressure compensation are common features of on-line hydrogen monitoring equipment. Temperature compensation is necessary because the permeability of the semipermeable membrane and the hydrogen diffusion rate through water increase as the temperature increases. Temperature compensation is accomplished by incorporating a suitable sensor, such as a thermistor, into the hydrogen probe housing, and including appropriate circuitry in the instrumentation. Optimum temperature compensation can be provided over a wide range by incorporating more than one thermistor, each accurate over part of this range. In this way, the value of dissolved hydrogen that would have been measured at a pre-set standard temperature, usually 25°C (77°F), is automatically calculated and displayed. Pressure compensation is important if the fluids and components within the probe are compressible because the tension in the semi-permeable membrane affects its diffusion characteristics. When necessary, compensation is usually handled electronically by inputting pressure sensor data to the instrumentation from an external pressure sensor in the sample line. Some monitors may have other special features. For instance, in one polarographic hydrogen sensor, an auxiliary ring-shaped anode (“guard ring electrode”), see Figure 12-1, surrounds the main centrally located disk-shaped anode [5]. The auxiliary anode, fabricated from platinum, is designed to reduce or eliminate the adverse effect of interfering species that may initially be present in the internal electrolyte or that are created by the cathodic reaction. In the absence of the auxiliary anode, these species would diffuse to the main anode and contribute to the total measured current, thereby leading to an overestimate of the dissolved hydrogen concentration. With the auxiliary anode in place, these species are removed before they can reach the main anode.

Figure 12-1 Polarographic Hydrogen Sensing Electrode with “Guard Ring” [7]

For accurate measurements, it is essential that the hydrogen content of the water at the outer surface of the semi-permeable membrane be representative of the bulk water passing through the 12-4

EPRI Proprietary Licensed Material Hydrogen

flow cell. Because hydrogen diffuses out of the water sample and into the semi-permeable membrane, a continuous flow of fresh water sample must be maintained over the membrane surface. Minimum flow rates (typically 20-50 mL/min) are specified by the manufacturer of the monitoring equipment to ensure that the rate of hydrogen removal by the sensor is negligible compared with the flux of hydrogen through the flow cell. An upper flow rate limit may also be specified, above which the water flow becomes so turbulent that the membrane vibrates and induces convective transport of hydrogen and other species through the internal electrolyte. This condition would lead to an overestimate of the dissolved hydrogen concentration. 12.3.2 Thermal Conductivity

Thermal conductivity determination of hydrogen content is an on-line technique for gaseous environments such as for hydrogen in the generator cooling systems. A critical feature of the thermal conductivity method is the Wheatstone bridge network comprising four filaments, connected together as shown in Figure 12-2. Each filament is located within its own cavity within a single metal block—the thermal conductivity cell block. Two of the cavities (containing filaments Rd and Rb, Figure 12-2) are connected and form part of a flow channel for the sample gas, and the other two cavities (containing filaments Ra and Rc) form part of a separate flow channel for the reference gas. In some applications, the reference flow channel is plugged at its inlet and outlet, and is filled permanently with a non-flowing reference gas. A constant voltage is applied to the bridge to heat the filaments. The heat from the filaments is dissipated by conduction through the surrounding gas to the adjacent block of metal which serves as a heat sink. By keeping the temperature of the heat sink constant at, say, 50°C (122°F), the temperature of each filament is controlled only by the thermal conductivity of the gas surrounding that filament. Since the resistance of the filament is determined by its temperature, the resistance is also controlled by the thermal conductivity of the surrounding gas. The thermal conductivity of a gas is dependent upon its molecular weight: the lower the molecular weight, the higher the mobility of the molecules, and the higher the thermal conductivity. In gas mixtures, the overall thermal conductivity is determined by adding together the products of the individual thermal conductivities of the gases and their partial pressures. Thus, a mixture of nitrogen and hydrogen has a higher thermal conductivity than nitrogen alone, because hydrogen has the lowest molecular weight of any gas. When the thermal conductivities of the sample and reference gases are identical, the heat is conducted away from all four bridge filaments equally, the bridge is balanced electrically, and no output is seen on the voltmeter (or other bridge indicator). When the thermal conductivities of the sample and reference gases are different, the resistances of the filaments exposed to the sample gas and reference gas are different, leading to an unbalance in the bridge which is detected by the indicator. If the concentration of only hydrogen in the sample gas mixture changes with time, the extent of the unbalance can be converted directly to the concentration of hydrogen.

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EPRI Proprietary Licensed Material Hydrogen

Figure 12-2 The Wheatstone Bridge Circuit Showing Schematically How the Reference Gas Flows Over Filament Resistances Ra and Rc, while the Sample Flows Over Filament Resistances Rd and Rb. Ra = Rb = Rc = Rd when Reference and Sample Gases have the Same Thermal Conductivity. [Solid lines = electrical circuit; dashed lines = sample or reference gas flow]

This same Wheatstone Bridge design is used in thermal conductivity detectors for gas chromatographs. Chromatography illustrated in Figure 12-3 is a technique for separating chemical substances that relies on differences in partitioning behavior between a flowing mobile phase and a stationary phase to separate the components in a mixture. The sample is carried by a moving gas stream through a tube packed with a finely divided solid that may or may not be coated with a film of a liquid. Because of its simplicity, sensitivity, and effectiveness in separating components of mixtures, gas chromatography is one of the most important tools in chemistry. It is widely used for quantitative and qualitative analysis of mixtures. The method consists of, first, introducing the test mixture or sample into a stream of an inert gas, commonly helium or argon, that acts as carrier. Liquid samples are vaporized before injection into the carrier stream. The gas stream is passed through the packed column, through which the components of the sample move at velocities that are influenced by the degree of interaction of each constituent with the stationary nonvolatile phase. The substances having the greater interaction with the stationary phase are retarded to a greater extent and consequently separate from those with smaller interaction. As the components elute from the column they can be quantified by a detector and/or collected for further analysis.

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Figure 12-3 Diagram of Gas Chromatograph with Thermal Conductivity Detector

Separation of a gas into its multiple components is an example of gas-solid chromatography (GSC). Gas-solid chromatography is based upon a solid stationary phase on which retention of analytes is the consequence of physical adsorption The choice of carrier gas depends on the type of detector that is used and the components that are to be determined. Carrier gases for chromatographs must be of high purity and chemically inert towards the sample e.g., helium (He), argon (Ar), nitrogen (N2), carbon dioxide (CO2) and hydrogen (H2). The carrier gas system can contain a molecular sieve to remove water or other impurities. The carrier gas will always have a thermal conductivity far removed from that of the analyte in question. Therefore, nitrogen and argon are typical carrier gases for hydrogen determination. The thermal conductivity detector described previously can now determine the difference between the eluting component and the pure carrier gas. The identity of the eluting component is determined by the retention time in the column and the concentration is quantified by the height of the peak or the area under it. Modern detectors create a millivolt signal which correlates to the area under the eluant peak and is totalized for quantification against a standard of known hydrogen concentration. On-line analyzers using an in-situ thermal conductivity sensor as in Figure 12-4 are also used for determining hydrogen content in both aqueous and gaseous samples

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Figure 12-4 Hydrogen Sensor with Thermal Conductivity Detector [7]

Rather than utilizing a true Wheatstone Bridge design with four elements and continually flowing reference gas, the small enclosed volume between the diffusion membrane and the thermal conductivity sensor is periodically flushed with a purge gas. After each purge, the gas to be measured diffuses through the membrane, changing the thermal conductivity of the gas surrounding the detector. It is the rate of change of the thermal conductivity that allows the concentration of the gas to be calculated [7]. Selection of membrane material allows flexibility for different sample environments and sensitivity limits. 12.3.3 Gas Density Meter

This hydrogen measurement technique is not intended for samples from the steam-water cycle. It measures the density of a gas sample and compares the resultant value to that of the pure material (be it a pure gas such as hydrogen or carbon dioxide, or a gas mixture such as air) [8]. The technology developments to use the density monitor as a hydrogen purity analyzer have come about since the year 2000. The principle of measurement relates to the relationship between gas density and frequency ratio. [6] The resonant frequency of the thin walled cylinder varies with the density of the gas flowing inside and outside of it. Hence, measuring the resonant frequency will provide the density of the flowing gas. Two pairs of piezo-electric elements are fitted to the cylindrical resonator along the same circumferential plane: one pair for drive and the other pair for detection. The former pair generates two vibration modes of the resonator independently. It is difficult to obtain the gas density from a single resonant frequency since the temperature effect is too large to be 12-8

EPRI Proprietary Licensed Material Hydrogen

compensated for completely. Hence, the density of the gas is based on the ratio between the resonant frequencies so as to reduce the temperature effects. Another merit of applying the frequency ratio is that the effect of piezo-electric element deterioration with age is reduced. Fundamentally, a vibratory gas density meter can only measure the density of the gas sample. [6] Nevertheless, if the densities of the individual components of a mixed gas containing two components are known, the concentration of those individual components can be obtained from the measured and corrected density of the mixed gas. The concentrations of component gases cannot be obtained from a mixed gas containing three or more components, except in special cases. Performance specifications for the vibratory gas density meter are available in the literature [8]. The structure of the measuring sensor is designed to minimize interference [6]. Multiple o-rings serve both to provide a gas tight seal and to dampen the resonator from external vibration. A thin platinum resistance temperature detector (RTD) is located in the gas path to accurately measure and compensate for the sample temperature. The variation in the vibration frequency of the resonator is a function of the gas density. This frequency change is used to determine the hydrogen purity of the gas sample.

12.4 Calibration Calibration is accomplished with a zero and span gas which are normally piped in parallel with the normal sample point and controlled via a multi-port switching valve. Automatic, semiautomatic and one-touch manual calibration modes are available. No calibration frequency was implied in the technical literature.

12.5 Calibration Check On-line hydrogen instrument analytical capabilities should be checked periodically to demonstrate calibration stability. One method is recommended for verifying instrument stability; the Standard Injection Method [10]. For the Standard Injection Method, a known standard gas, near the mid-point of the calibration curve, is analyzed by the on-line instrument and the results are compared to the acceptance criteria. Acceptance criteria are either based on statistically derived limits. i.e., ± 3 sigma or based on some predetermined limits established from experience, i.e., ± 10 %. Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is considered still acceptable. If the results are outside the acceptance criteria, the on-line instrument must be recalibrated.

12.6 Alternative Methods Although not a continuous monitoring method, a grab sample thermal conductivity approach is an improved version of ASTM D1588 [9] Method A, a standard method that was adopted in 1960, was used for over 20 years, but was withdrawn by ASTM in 1984. In the improved 12-9

EPRI Proprietary Licensed Material Hydrogen

version, dissolved and gaseous hydrogen are stripped from the water sample, nitrogen is added to the stripped gas mixture to adjust the pressure to atmospheric, and the gas mixture is cooled (or warmed) to a standard temperature. The thermal conductivity of this water-saturated mixture of nitrogen and hydrogen is compared with the thermal conductivity of a reference gas, such as water-saturated, hydrogen-free nitrogen. This is achieved by exposing the sample and reference gases to metal filaments that are heated by passing a current through them. The resistances of the filaments are controlled by their temperatures which are, in turn, determined by the thermal conductivities of the gases surrounding them. The thermal conductivity is a function of the partial pressure of hydrogen present, so the hydrogen concentration can be related to the measured resistances of the filaments.

12.7 End User Considerations Both the aqueous electrochemical (EC) and thermal conductivity (TC) sensors use a gas permeable membrane to separate the sample liquid from the sensing surfaces. The choice of the membrane material is a function of the desired detection limits, the measurement range, the sample matrix and the response time. Table 12-1 lists some performance ranges for both EC and TC detectors; the membrane materials have some commonality but are not perfectly matched so direct membrane comparison is not intended. The values are intended to be for example only and do not represent all available sensors. Table 12-1 Some Performance Ranges for Aqueous EC and TC Detectors EC Trace

EC Low Range

EC Mid Range

EC High Range

TC Trace

TC Mid Range

TC High Range

Membrane Material (1)

PFA

Tefzel® (ETFE)

Tedlar® (PVF)

Saran™ (PVDC)

Silicone rubber

PFA

ETFE

Measurement Range

0-75 µg/L (ppb)

0-300 µg/L (ppb)

0–3200 µg/L (ppb)

0 µg/L (ppb) to 32 mg/L (ppm)

0–1000 µg/L (ppb)

0–2000 µg/L (ppb)

0µg/L (ppb) to 10 mg/L (ppm)

Accuracy

±1% or 0.03 µg/L (ppb)

±1% or 0.09 µg/L (ppb)

±1% or 1 µg/L (ppb)

±1% or 10 µg/L (ppb)

±1% or 1µg/L (ppb)

±1% or 2 µg/L (ppb)

±1% or 8 µg/L (ppb)

Response time

2 sec

5 sec

6 sec

50 sec

12 sec

17 sec

17 sec

Minimum Sample Flow

50–220 ml/min

40–200 ml/min

20–70 ml/min

20–40 ml/min

250 ml/min

220 ml/min

200 ml/min

Cal Gas

1 % H2

10% H2

100% H2

100% H2

10% H2

100% H2

100% H2

(1) ETFE (ethylene-tetrafluoroethylene), PVF (polyvinyl fluoride), PVDC (polyvinylidene chloride), PFA (Perfluoroalkoxy)

The EC sensor relies on the surface of the platinum anode to generate an electric current in the presence of the hydrogen. For this to take place, an extremely clean metal surface is essential. If 12-10

EPRI Proprietary Licensed Material Hydrogen

any film, grease, or other impurity covers the metal surface this reaction may be impeded or even stopped. Also, the reaction that takes place on the chloridized silver cathode leads to loss of performance after a certain operational time. Periodic sensor maintenance must occur to restore original performance. The main steps involved include: •

Dechloridization of the cathode—removing the chloride film from the silver cathode surface.

•

Rechloridization of the cathode—a new layer of silver chloride is grown on the cathode’s surface.

•

Activation of the platinum anode—The center anode surface is polished and treated with nitric acid.

Both the electrochemical and thermal conductivity sensors utilize a membrane to separate the liquid sample from the detecting surfaces and allow hydrogen diffusion into the cell. Periodic replacement of this membrane is to be expected to assure proper sensor operation. Hydrogen purity meters using direct thermal conductivity sensors are available and come as OEM equipment with many hydrogen cooled generators. Gas chromatographic analysis using the thermal conductivity sensor is primarily used as a laboratory technique, but there is no reason that on-line GC could not be applied for a sample from the generator hydrogen cooling system. Using the GC for measuring dissolved hydrogen in an aqueous sample would require a membrane and sweep gas to deliver the sample to the column inlet. Automating this process may be fairly delicate. It is more likely that a manual sample scheme would be utilized. An aqueous sample is pulled in a pressurized container, the sample is purged with a gas, the pressure and /or volume is closely adjusted and the mixture is admitted to a gas chromatograph. The most likely application for the vibratory gas density meter is analyzing gaseous hydrogen (hydrogen purity) in a generator hydrogen cooling system. The system is relatively uncomplicated compared to an on-line gas chromatograph. Performance data was given in an earlier section. It should be noted that hydrogen purity analyzers are also available using thermal conductivity sensors as described earlier. The thermal conductivity of the process gas passes through one half of the sensor and is compared to the thermal conductivity of a reference gas.

12.8 References 1.

Guideline Manual on Instrumentation and Control for Fossil Plant Cycle Chemistry. EPRI, Palo Alto, CA: April 1987. CS-5164.

2.

PWR Primary Water Chemistry Guidelines: Revision 3, prepared by PWR Primary Water Chemistry Guidelines Revision Committee. EPRI, Palo Alto, CA: November 1995. TR105714.

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3.

BWR Water Chemistry Guidelines: 1996 Revision, prepared by BWR Water Chemistry Guidelines Revision Committee. EPRI, Palo Alto, CA: December 1996. EPRI Report TR-103515-R1.

4.

Generation and Retention of Helium and Hydrogen in Austenitic Steels Irradiated in a Variety of LWR and Test Reactor Spectral Environments, F. A. Garner, B. M. Oliver, L. R. Greenwood, D. J. Edwards and S.M. Bruemmer (Pacific Northwest National Laboratory)∗M. L. Grossbeck, (Oak Ridge National Laboratory) 2001.

5.

Orbisphere 3600 series Hydrogen Sensor; Sales Information by Hach Ultra Analytics, Geneva, Switzerland 2003.

6.

GD Series Vibratory Gas Density Meters, Yokogawa Technical Report No. 29 SUZUKI Jun-ichi 2000.

7.

Orbisphere Model 31xxx Sensor Datasheet, Hach Ultra Company—2005.

8.

Yokogawa GD402 Gas Density Meter; Product Bulletin GS 11T3E1-01E; Yokogawa Electric Corporation, Tokyo, Japan. 2000.

9.

ASTM D1588-60 (Reapproved 1974; withdrawn in 1984), “Standard Test Methods of Dissolved and Gaseous Hydrogen in Water”. American Society for Testing and Materials, Philadelphia, PA.

10. Advanced Power Plant Chemistry QA/QC Practices, Scientech, LLC, Clearwater, FL, 2006.

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13 ION CHROMATOGRAPHY

13.1 Purpose and Use Ion chromatography (IC) is a detection technique that is applicable to quantification of both cations and anions. Sodium is designated as an EPRI Core Monitoring Parameter [1-4] while anions such as chloride and sulfate are monitored as needed for trending and troubleshooting purposes. The reader is referred to individual chapters on sodium and chloride to survey the implications of improper contaminant control. Note that sodium is typically monitored on-line by the ion selective electrode (ISE) method. While ion chromatographs are more complex than many of the on-line instruments, they have the advantage of quantifying multiple analytes in one analysis test. Still, use of on-line IC is rare; it is much more common to rely on laboratory IC for analysis of grab samples for trace ion quantification. Ion chromatographs are used on-line in the plant for the following reasons: •

To warn of in-leakage of contaminants.

•

To warn of condensate polisher malfunction.

•

To check the accuracy of water chemistry control (such as sodium, ammonia, chloride, sulfate, phosphate), so ensuring that corrosion rates in both the boiler and steam system are kept at acceptable low levels.

•

To detect and quantify organic acids which may be the result of water treatment plant inadequacy, condenser in-leakage, or degradation of organic boiler treatment chemicals.

•

To facilitate the correlation of a water chemistry parameter with plant operating variables, with an aim to optimizing operations.

The data generated by continuous on-line monitoring of sodium is used by plant chemistry and operations department personnel. The goal for plant personnel is to maintain sodium below prescribed limits.

13.2 Description of Method In the mid-1970s, it became apparent that critical components in the steam/water cycle of power plants were undergoing localized corrosion because of the presence of very low concentrations (only a few parts per billion, µg/L (ppb)) of ions such as sulfate, sodium and chloride. The corrosive conditions were often transient and occurred at times when grab sample analyses were 13-1

EPRI Proprietary Licensed Material Ion Chromatography

not performed, so the corrosive conditions sometimes went unchecked for long periods. Shortcomings of the on-line instruments then available, such as conductivity meters, led to consideration of IC, a method first described in 1975 [5]. By the end of the 1970’s, rugged online ion chromatographs had been developed that were suitable for use in power plants [6]. Three plant demonstrations of on-line IC were supported by EPRI [7,8] in the 1980s and led to widespread acceptance of the technique by the electric power utility industry. Plants utilizing ion chromatography in this era were limited to bench top analyzers and grab samples. The ion chromatograph is unlike other on-line chemical analyzers discussed in this manual in that it is capable of monitoring the concentrations of many ionic species in a single water sample. For instance, it can monitor trace amounts of anions such as F-, Cl-, Br-, NO2-, NO3-, SO42-, PO43-, and CrO42-, or cations such as Na+, NH4+, K+, Ca2+, Cu2+, Ni2+, Zn2+, and Fe3+, or soluble silica. Ion chromatographs come in a wide variety of models, each customized for a specific task. In principle, a water sample is injected into the IC flow system and is pumped through a resinpacked column along with an eluent, a solution that facilitates the separation of the sample species in the column. The sample species are separated because they move at different rates through the column, depending on their relative affinities for the resin. The eluent and separated sample flow to a detector, such as a conductivity cell, that monitors the passage of the sample species and transmits a signal to a computer. The computer compares the sample data with data from standard solutions, and identifies the various species present, together with their concentrations. IC is a sub-set of a separation process called High Performance Liquid Chromatography (HPLC). HPLC separations rely on a stationary phase and a mobile liquid phase. The various components of the sample (analytes) are attracted to the stationary phase to a differing degree which effects a time-based separation (elution) from the stationary phase column. The liquid phase transports the analytes through the column and to the detecting device. The type and concentration of the liquid phase affects the rate of elution and subsequently the degree of separation. All IC techniques rely on the fact that the species being analyzed in the water sample are capable of separation in one of three basic types of separator columns: •

Ion exchange chromatography, also known as high performance ion chromatography (HPIC).

•

Ion exclusion chromatography, also known as high performance ion chromatography exclusion (HPICE).

•

Ion pair chromatography, also known as mobile phase ion chromatography (MPIC).

Although on-line IC equipment requires an elevated degree of maintenance and involves relatively complex procedures, it analyzes many ions simultaneously, and does so quite rapidly with very good sensitivity and selectivity. Furthermore, when the sample contains only trace amounts of ions, grab sample analysis tends to be imprecise because of contamination during sampling and transfer. The automated water sample handling feature of on-line IC allows much greater precision.

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13.2.1 Ion Exchange Chromatography

This is the most widely used type of IC in power plants. It involves an ion exchange process in the separator column, and it is useful for separating both inorganic and organic anions and cations. The first step in the analysis is injection of a known volume of the water sample into a flowing liquid, called the eluent or mobile phase (see Figure 13-1). The eluent carries the sample into a separator column that is packed with resin beads made from materials such as polymethacrylate or polystyrene-based resin crosslinked with divinyl benzene. The resin is formed with exchange functions or separation groups that interact either with negatively or positively charged ions, depending on whether analysis of anions or cations is desired. When analysis of anions is needed, the exchange function is generally a quaternary ammonium group whereas, in cation chromatography, it is a sulfonate group. An anion exchange resin, for example, may be pre-treated with a bicarbonate (HCO3-) solution to completely convert the fixed -N+R3 groups to the bicarbonate form (resin-N+R3 HCO3-). As the water sample passes through the column, the ions interact with the resin functional groups in different ways depending on their unique chemical and physical properties. For instance, an anion exchange resin in bicarbonate form can exchange a bicarbonate ion for, say, a chloride ion from the flowing water sample: resin − N+ R3HCO3− + Cl − → resin − N+ R3Cl − + HCO3−

Equation 13-1

Adsorption processes may also be involved in the process (especially for hydrophobic ions). The resin will bind the chloride ion temporarily, but eventually the eluent, flowing behind the sample, reverses the procedure by providing a replacement bicarbonate ion and causing the chloride ion to be released or eluted into the eluent. As a result, each type of ion is retained for a discrete, reproducible period of time (retention time) which is characteristic of that ion. This causes a temporal separation of the ions in the eluent so that, as it flows out of the bottom of the separator column, first one ion appears, then another, and so on, as illustrated in Figure 13-2. In this idealized example, the dead time, tm, is the time taken for a compound which does not interact with the separator column to migrate through the column with the eluent. The gross retention time, tms, is the time taken from first introducing the eluent at the top of the column to achieving the peak concentration of an interactive ion at the bottom of the column (peak 1 in Figure 13-2). The net retention time, ts, is equal to the gross retention time less the dead time (ts = tms – tm). Another ion may appear at a greater net retention time, as represented by peak 2 in Figure 13-2.

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EPRI Proprietary Licensed Material Ion Chromatography

Figure 13-1 Schematic of a Single Column Ion Chromatograph

By injecting standard solutions of each ion of interest into the chromatograph, the retention times are established for each, which allows a correlation to be made subsequently between the ts value and the type of ion present. The eluent then flows to a detector that allows the concentration peaks in Figure 13-2 to be detected by one of a number of methods. The detector is coupled to a microprocessor-controlled instrument that monitors the retention time and the height of (or area under) the concentration peak. Comparing the sample data with calibration data, the computer then identifies and quantifies the dissolved ions in the original water sample. The most widely used detector is a conductivity cell, but amperometric, fluorescence, and ultraviolet/visible absorbance detectors are also used on occasion.

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Figure 13-2 Illustration of How Ions Elute (Leave the Separator Column) at Different Rates Resulting in a Separation of Ionic Species in the Flowing Eluent

When the detector is based on conductivity, the higher the concentration of the ions in the eluent, the higher is the conductivity reading. The precise correlation between conductivity and concentration is dependent on the relative mobilities of the ions, and may be determined by calibration with standard samples. Although the separated ions contribute to the conductivity, the eluent itself is often highly conductive. Consequently, the effects of the ions of interest would be overwhelmed unless methods are employed to suppress the eluent conductivity. One approach is to pass the effluent from the separator column through a second column—a suppressor column—before it reaches the detection system. The function of the second column is to convert the highly conductive eluent into a weakly dissociated solution of low conductivity. For instance, in the case of a sodium bicarbonate eluent carrying chloride anions, the suppressor column would exchange the sodium ions for protons to produce weakly dissociated carbonic acid. In this way, the ability of the conductivity cell to detect chlorides is increased dramatically. The suppressor column is also invaluable when a technique called gradient elution is employed. For ion chromatography to be a practical analytical tool, the analyte peaks must be completely separated prior to detection and quantification. Several anions such as fluoride, acetate, formate, chloride and nitrite have short retention times on the stationary phase and a weak eluent (low solution strength) must be used to separate the peaks. However, this same solution strength would cause exceedingly long retention times for anions such as nitrate and phosphate. This slow elution would limit the practicality of the test for on-line utilization. Gradient elution describes a process where the early ions are separated under a low solution strength and then the eluent concentration is increased (other analytical schemes even change the chemical nature of the eluent) to shorten retention time of the later analytes. This eluent change however increases 13-5

EPRI Proprietary Licensed Material Ion Chromatography

the specific conductivity of the flow to the detector and elevates the background conductivity. Use of the suppressor column provides more stable and lower background conductivity to the detector which improves detection limits and response. Gradient elution is further enhanced by a device called an eluent generator. This system electrolytically produces high purity potassium hydroxide eluent from demineralized water. The eluent concentration can be changed by altering the carrier flow and the applied current. In addition to producing a highly repeatable eluent concentration, the KOH eluent is free of carbonate ions to provide the exceptionally flat detector baseline seen in Figure 13-3 [9]. Acidic eluent can also be produced with the eluent generator.

Figure 13-3 Gradient Separation of Common Anions Using a Hydroxide Gradient [9] Source: Reference 9, Courtesy Dionex Corp.

Problems can occur if the water sample contains foulants, such as corrosion product particulates. In such cases, the sample should first pass through a so-called guard column before reaching the separator, as illustrated in Figure 13-1. The guard column removes these species and prevents fouling and interferences in the separator column. If the sample contains a significant concentration of particulate matter, a 0.22 µm or 0.45 µm filter should be installed upstream of the columns to remove the particulates thereby preventing plugging of the columns. 13-6

EPRI Proprietary Licensed Material Ion Chromatography

Modern instruments require only 10 to 100 microliters of a water sample to identify ions present in the parts per billion (µg/L (ppb)) range. The detection limit can be reduced to a few parts per trillion (ppt) or less by enriching the samples in a concentrator column. This is done by passing relatively large volumes (25 milliliters or more) of sample through the column to accumulate the ions of interest before passing the eluent through the analytical column in the normal way. 13.2.2 Ion Exclusion Chromatography

This form of IC is not commonly used for analysis of water samples in power plants. It involves Donnan exclusion, steric exclusion, and adsorption processes on a totally sulfonated, high capacity, cation exchange resin. Ion exchange does not occur in the separation process. Donnan exclusion, for example, relies instead on the ability of undissociated species to penetrate the pores of the resin, while totally dissociated, inorganic acids cannot penetrate the Donnan membrane so are not retained. Thus, dilute hydrochloric acid, which is used as an eluent, is completely excluded from the resin because it is totally dissociated, whereas neutral water molecules and other undissociated species can diffuse unhindered in and out of the pores of the resin. Ion exclusion chromatography is able to separate weak organic acids from totally dissociated acids [19]; separating oxalic, formic and acetic acids from other long chain hydrophilic acids. Ion exclusion chromatography can also be used for determining carbonate and borate concentrations [20]. 13.2.3 Ion Pair Chromatography

This method of IC can separate metal complexes and surface active (exchangeable) anions and cations. However it is rarely used by the electric power utility industry. Ion pair chromatography involves pairing of two ions followed by adsorption on neutral, non-polar, macroporous resin, such as polystyrene/divinyl benzene. Ion pairing usually enhances the separation of ions with similar exchange properties. Although a couple of mechanisms have been proposed, the physicochemical phenomena on which the retention mechanism is based have not yet been fully explained [10].

13.3 Technical Considerations 13.3.1 Sample Injection

Delivery of the sample onto the separator column is accomplished either by direct injection or by sample pre-concentration. As a rule of thumb, direct sample injection onto the separator column (10 to 100 µL) can achieve ion detection limits in the 3-7 µg/L (ppb) levels. Increasing the sample volume lengthens the amount of time required to load the sample onto the separator column. The long injection time causes a broadening of the peak and leads to a poorer signal to noise ratio which is detrimental to low level quantification. The larger sample volume also 13-7

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causes a larger “water peak” (negative conductivity spike) which can mask some of the early eluting ions. Lower detection and quantification levels are achieved by loading the sample onto a preconcentrator column. Many ions can be quantified in the few µg/L (ppb) and sub µg/L (ppb) range using a concentrator column without complex chemical pretreatment. This short column (35 -50 mm (0.14-0.20 inches) in length) has a stationary phase similar, if not identical, to that on the separator column. The function of this short column is to strip ions from a measured volume of high purity sample. After the concentration step is complete, eluent flows through the concentrator column and the ions of interest move onto the larger separation column. The concentrator column eliminates the disadvantages of a large water dip and broad peaks that result from large direct sample injection volumes as were just discussed. However, there are also some technical issues with the concentrator column. Additional valves are required for directing a sample through the concentrator and then switching to eluent flow. Additional time must also be built into the sample cycle to allow for pre-concentration. It is important that the direction of sample concentration on the concentrator column and the analyte elution from the concentrator column occurs in opposite directions. This produces a rapid release of the ions onto the separator column. Another consideration is the exchange capacity of the concentrator column which can limit the amount of sample that is exchanged. The situation is further complicated when ions of widely different affinities are present in the sample. An ion with a high exchange affinity can act as an eluent and strip off an anion of lesser affinity. For example, anions such as sulfate can act as an eluent and displace more weakly retained ions such a chloride or fluoride when concentrations are close to the dynamic capacity of the concentrator column. Breakthrough volume, which is defined as the volume of sample that causes an ion of interest to be eluted from the concentrator column, must be determined for a simulated sample matrix to ensure quantitative retention of all anions and accurate results. Procedures for calculating the breakthrough volume are available in the technical literature. Exceeding the breakthrough volume on a cation analysis is very possible when high concentrations of amines (ammonia, mono-ethanolamine, etc.) are present with low amounts of sodium. The hydroxide formed by the neutralizing amine can also act as an eluent when concentrating anions. A high volume direct sample injection technique has recently been developed and incorporates the use of gradient effluent elution [11]. A large volume of sample (up to 1000µL) is injected onto the separator column and the column is allowed to stabilize with a very low eluent concentration for approximately 5 minutes. This displaces the water peak and slowly elutes the weakly attracted ions such as fluoride, acetate, and formate. Gradient increases to higher KOH concentrations are then used to separate the more strongly retained ions such as phosphate and sulfate. The two methods that were put forth have a separation time of 27 minutes (method 1) and 42 minutes (method 2) [11]. This analytical method is very well suited to on-line ion chromatography because valve manipulation is minimized compared to pre-concentration. The eluent generator is highly repeatable, is relatively free from carbonate contamination, and able to run unattended for long periods of time. Conductivity suppression produces a very flat baseline which promotes low detection limits. Chromatograms showing the results of this technique are presented in Figure 13-4 and Figure 13-5. 13-8

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Figure 13-4 Anion Analysis Using Ionpac® AS17 Separator Column with Eluent Generator; Method 1 [11] Source: Reference 11, Courtesy Dionex Corp.

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Figure 13-5 Anion Analysis Using Ionpac® AS17 Separator Column with Eluent Generator; Method 2 [11] Source: Reference 11, Courtesy Dionex Corp.

13.3.2 Column Selection

There are four types of columns used in typical cycle chemistry IC applications. They are the concentrator columns, guard columns, separator columns, and suppressor columns. The concentrator, guard, and separator columns are chemically similar for a given separation method. 13-10

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When analysis of anions is needed, the functional group is generally a quaternary ammonium group whereas, for cation separation, the active site is a sulfonate group. 13.3.2.1 Concentrator Column

The concentrator column is designed to improve the detection limit of a given analyte. Direct injection is often limited by the volume of sample—as the volume becomes larger, the water dip becomes more of a hindrance and peak shape often broadens. The concentrator column has a similar exchange functionality to the separator column but the liquid phase is the sample (usually fairly pure water). In most cases, the liquid phase of the sample won’t elute the analyte from the concentrator column and the low level ions accumulate near the entrance of the concentrator column. Exceptions to this inert behavior (liquids containing high concentrations of strongly absorbed cations such as amines or non-neutral pH solutions) can create a loss of the first anions to elute from the column. Determination of the breakthrough volume of the concentrator for a given sample matrix is always a consideration. As the sample flows through the concentrator column, the ions of interest occupy an exchange site on the resin surface and remain somewhat firmly attached. At the end of the concentration cycle, chemical eluent flows through the concentrator in a reverse flow and elutes off the trapped ions. Ideally, this elution should be a rapid event to place all of the ions onto the separator column in a plug flow. When concentrating 10 mL or less of sample, a sample loop is the preferred introduction method. Larger volumes of sample are measured as they flow through the concentrator column either with a rotameter (perhaps using the pressure of a process sample stream to facilitate flow) or with a calibrated pump pulling a known volume through the column. One should never use a pump to push a sample through the concentrator column due to the possibility of contamination. If the samples contain particulate material, the concentrator column will become fouled and a high pressure drop can develop. In this application, concentrator columns may have a short useful life. 13.3.2.2 Guard Column

The guard column is really part of the analyte separation process; it is exactly the same construction and material as the separator column but approximately 10%- 20% of the column length. Its function is to be a sacrificial component in the event of particulate contamination from the sample. While the concentrator column in principle performs this same filtering capacity, its filtering capability is compromised because of the reverse directional flow during the elution step. Any particulates that were deposited on the separation media can now be flushed toward the separator column. For this reason, a guard column is employed even in applications where a concentrator column is being used. 13.3.2.3 Separator Column

The separator column is responsible for sequentially eluting the analytes of interest in a very repeatable manner. Separation is always a compromise between degree of peak resolution and the time required for the last peak to elute. The improved performance of HPLC over classical 13-11

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liquid chromatography results from the reduction in particle size of the separation media (stationary phase). As the size decreases, more active sites (sometimes called theoretical plates in liquid chromatography terminology) are available for a given length of column. Typical separation columns have internal diameters of 4-10 mm (0.15-0.39 inches) and are 10-30 cm (3.9-11.8 inches) in length. Larger internal diameters and particle dimensions create more diffusion during the separation process and result in broader peaks. Recent advancements have resulted in micro-bore columns with internal diameters of 1-5 mm (0.04-0.20 inches) which are packed with stationary phase particles in the 3-5 µm range. Smaller diameters create faster and sharper separation with less eluent consumption but require higher operating pressures to overcome the internal flow resistance. 13.3.2.4 Eluent Suppressor Column

The final column in a typical IC analyzer is the eluent suppressor column. The most common detector for ion chromatography is one based on specific conductivity. This detector is highly sensitive, is universal for all charged species, and responds in a predictable way to increases in ionic concentration of the analyte. The largest limitation to the conductivity detector is the high background conductivity of the liquid phase eluent. This high background value tends to mask the small changes in conductivity that occurs when the analyte elutes off the separator column. This problem of high background conductivity was solved in 1975 with the introduction of the eluent suppressor column immediately following the separation column [12]. The suppressor column is packed with a second ion- exchange resin that converts the ions of the eluting mobile phase to a weakly ionized species without affecting the conductivity due to the analyte. For example, when cations are being determined in the ion chromatograph, hydrochloric acid is a common eluting reagent and the suppressor column is an anion resin in the hydroxide form. The product of the suppressor column reaction is water: H+ (aq) + Cl- (aq) + resin+OH- (s)

→

resin+Cl- (s) + H2O

Equation 13-2

The analyte ions (cations) are not retained or altered by this suppressor column and the background conductivity from the eluent is very low. For anion separations, the suppressor column is a cation exchange resin in the hydrogen ion form. Common anion eluents are sodium bicarbonate, sodium carbonate, and potassium hydroxide solutions. The conductivity of these eluents is suppressed as follows. The largely undissociated carbonic acid does not contribute significantly to the conductivity. The product of the potassium hydroxide suppression exchange reaction is water. Na + (aq) + CO3-2 (aq) + resin-H+ (s) → resin-Na+ (s) + HCO3-1

Equation 13-3

Na + (aq) + HCO3-1 (aq) + resin-H+ (s) → resin-Na+ (s) + H2CO3

Equation 13-4

K+ (aq) + OH- (aq) + resin-H+ (s) → resin-K+ (s) + H2O

Equation 13-5

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After continued operation, the active sites of the suppressor ion exchange resin become saturated with the eluent ions and must be restored to their hydrogen or hydroxide form. Typically this is accomplished with a regeneration cycle—the anion resin is regenerated with a caustic solution and the cation exchange resin is regenerated with an acid solution. This regeneration cycle is either performed external to the ion chromatograph or with switching valves that re-routes the eluent and admits regenerant to the suppressor column. In both cases, there is an interruption to the analytical cycle while the suppressor is being regenerated or reaching equilibrium if it is externally regenerated. Recent advancements include a continually regenerated eluent suppressor column [13]. This regeneration uses an electrochemical water splitting process where DC current is used to generate low level hydronium and hydroxide ions and the suppressor is continually removing eluent ions. Figure 13-6 shows the suppression mechanism inside a suppressor when KOH is the eluent for anion-exchange separations. Analyte ions elute from the separator column with potassium counter ions. Two electrodes, one beside each membrane (on the side opposite the eluent), hydrolyze water to hydronium and hydroxide ions. Hydrogen ions diffuse across the membrane next to the anode, neutralizing the hydroxide eluent to water, while potassium ions from the eluent diffuse across the other membrane, providing counter ions to the hydroxide being generated at the cathode. In effect, potassium hydroxide from the eluents is transferred across the membrane and does not reach the detector. The suppressor thus lowers the background conductivity from the eluent to almost zero and significantly improves the sensitivity of the conductivity detector.

13.4 Eluent Selection Determining which eluent is required for a given ion chromatographic analysis is primarily dependent on the ions of interest and their expected concentration. Recommendations often come from the instrument manufacturer as part of the methodology for a given separation column. In general, the higher the eluent molar concentration, the more rapid the analytes elute from the separation column. The goal for any ion chromatographic analysis is to have complete resolution between analyte peaks and minimize the time required for the last peak to elute from the separation column.

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Figure 13-6 Chemistry and Ion Movement in Continually Regenerated Eluent Suppressor [13] Source: Reference 13, Courtesy Dionex Corp.

The most common eluents for anion separation are alkaline solutions of sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium tetraborate, sodium carbonate, and sodium bicarbonate. Review of current literature shows ionic solution strength of these alkaline eluents ranging in concentration from 0.5 -65 millimolar. Cation separations are typically performed using methanesulfonic acid (MSA), pyridine dicarboxylic acid, ammonium hydroxide, disodium hydrogen phosphate, sodium iodide, nitric acid, ethylene diamine tetra-acetic acid (EDTA), boric acid, diaminoproprionic acid (DAP), pyridine dicarboxylic acid (PDCA), oxalic acid, and citric acid. Literature again shows eluent ionic strength to be in the 1-50 millimolar range. As previously discussed, gradient elution involves changing the eluent composition and /or concentration throughout the course of the analytical run. A low strength eluent is used for perhaps the first five minutes of the run to separate and deliver the low affinity analytes to the detector. At some precise and repeatable time, the eluent solution strength is increased to promote a faster elution of the more strongly attached analytes. In some analytical schemes, as many as four different eluent compositions are used throughout a single run. Gradient elution is made possible largely by the suppressor column described previously. Without the eluent suppressor, the increasing solution strength would produce correspondingly higher baseline conductivity. This baseline shift would make quantification of low level ions very difficult. Efficient eluent suppression results in a flat baseline even as the eluant strength increases and promotes accurate quantification. The quality of water used for eluent preparation is very important. Any contamination in the eluent will occupy ion exchange sites on the separator column and reduce peak resolution. A gradual increase in baseline will often occur when the eluent is contaminated. One 13-14

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decontamination technique is to install an eluent trap column in the flow path prior to introducing the eluent into the separator column. This trap will remove trace levels of anion contamination but it requires periodic regeneration to restore its active sites. A recent development in eluent technology is the eluent generator [13] composed of an electrolyte reservoir and an eluent chamber separated by an ion exchange connector. Passage of a current through this system allows electrolytic production of the eluent, as described below. This apparatus allows automatic production of high quality eluents on a real time basis (eluent storage and subsequent contamination is eliminated). Generators are available for a range of eluents including potassium hydroxide, sodium hydroxide, lithium hydroxide, sodium carbonate and sodium bicarbonate, methanesulfonic acid (MSA). By adjusting the electrolytic current supplied to the generator, differing solution strengths can be produced, greatly simplifying gradient elution. One generator can actually change the ratio of sodium carbonate / sodium bicarbonate for further eluent modification. The eluent generator cartridge is the heart of the eluent generation process. Figure 13-7 illustrates the operation principle of the KOH cartridge. The cartridge consists of a high pressure KOH chamber and a low pressure K+ ion electrolyte reservoir. The KOH chamber contains a cathode where the hydroxide ions are generated through an electrolytic water splitting reaction. The K+ electrolyte reservoir contains an anode and an electrolyte solution of K+ ions. The KOH generation chamber is connected to the electrolyte reservoir by means of a cation exchange connector that permits passage of the K+ ions from the electrolyte reservoir into the generating chamber while preventing the passage of anions from the electrolyte chamber into the generating chamber. The cation exchange connector also serves the critical role of a high pressure physical barrier between the low pressure electrolyte reservoir and the high pressure generating chamber. To generate KOH eluent, demineralized water is pumped through the KOH generating chamber and a DC current is applied between the anode and cathode of the eluent generator cartridge. Electrolysis of water occurs at both the cathode and anode of the device. Water is oxidized to form H+ ions and oxygen gas at the anode in the K+ electrolyte reservoir. H2O

→

2H+ + ½ O2 + 2e- (anodic reaction)

Equation 13-6

Water is reduced to form hydroxide ions and hydrogen gas at the cathode in the KOH generating chamber. 2H2O + 2e-

→

2OH- + H2 (cathodic reaction)

Equation 13-7

Hydrogen ions, generated at the anode, displace K+ ions in the electrolyte reservoir. The displaced K+ ions migrate across the cation exchange connector into the KOH generating chamber. These K+ ions combine with the OH- ions generated at the cathode to produce KOH solution, which is used as the eluent for the anion-exchange chromatograph. The concentration of the generated KOH is dependent on the current applied to the KOH generator and the flow rate through the KOH generating chamber. Therefore with a constant flow rate, the eluent 13-15

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generator will precisely control the applied current to accurately and reproducibly generate KOH at the desired concentration.

Figure 13-7 The KOH Generator Cartridge Consists of a KOH Generating Chamber and K+ Electrolyte Reservoir Connected by a Cation Exchange Connector [13] Source: Reference 13, Courtesy Dionex Corp.

This same principle is used for generation of NaOH and LiOH alkaline eluents and acidic eluents. A K2CO3 cartridge allows generation of various solutions of this potassium carbonate eluent. Adding the Electrolytic pH Modifier (EPM) to the potassium carbonate generator allows production of bicarbonate and carbonate ions. The ratio of carbonate to bicarbonate can be precisely controlled by adjustment of the current applied to the EPM. As previously discussed, regenerant contamination by trace levels of anions can be overcome with the use of a trap column prior to admission of the eluent onto the separation column. A conventional trap column needs to be periodically regenerated which hampers the continuous operation of an on-line analyzer. Recent developments include a Continually Regenerated Trap Column (CR-TC) [13] for use with hydroxide and MSA eluent generators which allows for operation with very low baseline drift during gradient elution, improved day to day reproducibility and continual on-line operation. The Continuously Regenerated Anion Trap Column (CR-ATC) [13] removes all anionic contamination (including carbonate) for anion exchange applications. The Continuously Regenerated Cation Trap Column (CR-CTC) [13] removes trace level cationic contamination (including ammonia) from MSA applications.

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Both samples and stored eluents absorb carbonate upon contact with air that can compromise the chromatographic separation. The large peak caused by carbonate elution can interfere with accurate determination of sulfite and nitrite which elute on the tail of the carbonate peak. Minimizing the carbonate interference leads to improved quantification. Use of a Carbonate Removal Device (CRD)[14] can reduce this carbonate interference. Figure 13-8 shows a diagram of the CRD.

Figure 13-8 Carbonate Removal Device [13] Source: Reference 13, Courtesy Dionex Corp.

The CRD makes use of a thin film of carbon dioxide-permeable coating on the exterior surface of a narrow bore capillary membrane tube. The CRD is inserted into the system flow path after the anion suppressor. Suppressed eluent including the analytes of interest are routed inside the coated capillary membrane tube. A self-sustaining source of alkaline solution originating from the suppressor sweeps the outside of the coated capillary membrane. The carbonic acid /carbon dioxide diffuses across the membrane, is quickly converted to carbonate by the alkaline material and is swept to waste. Figure 13-9 below shows the improvement when using the CRD [14].

13.5 Detectors The most common detector for on-line ion chromatography is based on measuring the conductivity of the eluent and the subsequent conductivity increase associated with each analyte as it elutes off the separation column. According to published standards [15], the detector should have a low dead volume (1µL). The temperature compensated or corrected flow-through conductivity cell should be capable of measuring from 0 to 1000 µs/cm. Temperature control should be at ±0.5 °C or better if the cell is temperature controlled. Conductivity detection is 13-17

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applicable to cations, anions, and several organic acids. When utilizing an eluent suppressor column, the range of conductivity measurement is significantly compressed and a more sensitive cell can be utilized.

Figure 13-9 Comparison of Spectrum with (Top Spectrum) and without Carbonate Removal Cevice [14] Source: Reference 14, Courtesy Dionex Corp.

Another detection module routinely used in ion chromatography but with less versatility than conductivity detectors is the post column reactor. In the post column reactor, color developing reagents are added to the eluting analytes. The degree of color intensity follows the LambertBeer Law as follows: A=axbxc

Equation 13-8

where the color absorbance (A) is proportional to the absorbance constant (a), the sample cell path length (b) and the concentration of the analyte (c). The post column reactor is then followed by a photometric detector operating either in the UV or visible wavelength range. This detector applies equation 13-8 to compare the unknown analyte concentration to a standard of known concentration and thus to quantify the sample. Classic ion chromatography analytical methods using the post column reaction and photometric detection include the speciation of chromium (III) and chromium (IV), speciation of nitrate and nitrite and quantification of organic compounds such as methylcarbamates.

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Fluorescence detectors make use of a physical characteristic of many organic compounds. When bombarded with incident light, these molecules emit a characteristic wavelength of light. The intensity of the emitted light is proportional to the quantity of molecules being bombarded. Therefore, the ion chromatograph separates a group of poly-aromatic hydrocarbons (PAH) into the specific components and the fluorescence detector quantifies the individual components. The difference between the UV / Vis detector described above and the fluorescence detector has to do with the orientation of the photomultiplier device (PMD). In the UV / Vis system, the PMD is in alignment with the incident light and the detector monitors the amount of light that is absorbed. The PMD in the fluorescence detector is at right angles to the incident light and measures the emitted light from the molecule. Electrochemical detectors are used in the analysis of organic acids. They measure the current produced by the oxidation of the compound at the gold electrode. The specific technique used is called pulsed amperometry. A possible application would be ion exclusion separation of organic acids followed by electrochemical detection. Only the conductivity detector is used in current on-line chromatography instruments in the power generation industry. 13.5.1 Interferences

Interference occurs when analyzing anions in power plant water samples when the feedwater is pH adjusted using volatile amines. The eluent used for anion separation is an alkaline liquid phase with closely controlled solution strength in the millimolar range. The strong amine in the sample can cause problems with breakthrough volumes if a concentrator column is used. If direct injection of a large sample volume is utilized, the pH shift caused by the amine can cause premature elution and insufficient peak resolution. To overcome this interference, ASTM D 5996 recommends that the sample for ion chromatographic anion analysis be taken from the effluent of a strong acid resin cation exchange column. There is a risk of sample and standard contamination when working with trace level analysis. Just the act of pulling a sample and transporting it to the IC instrument can contaminate the samples. On-line instruments are very advantageous since sample handling is kept to a minimum and the instrument pulls a sample from a flowing stream without contact with the environment. When analyzing trace ions via grab samples, the sample bottles should be rinsed with deionized (DI) water, filled with DI, and allowed to stand at least overnight before emptying, drying and filling with sample. For ultra-low level analysis, this process may need to be repeated until acceptable blank values are obtained. Poly-carbonate and poly-styrene tissue culture flasks with plug seals have proven to be an excellent material for trace ion grab samples. When a flask is not in use, it is best to keep it full of DI water and rinsed periodically. Disposable, powder-free PVC gloves should be worn when working on the IC system and care should be taken to minimize contact with all solutions, even when wearing gloves.

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Another potential interference is when one analyte is present at such a high concentration that it masks adjacent peaks on the chromatogram. The previous discussion about removal of the carbonate peak (using a Carbonate Removal Device) is an example of this type of interference when analyzing anions. Ideally, one manipulates the chromatographic separation such that the interfering ion elutes later than the analyte of interest, which minimizes the interference. The following chromatogram (Figure 13-10) shows the analysis of low level sodium in the presence of high concentrations of monoethanolamine [18]. The large tailing peak of the monoethanolamine is not a source of interference in this example since it elutes later than the analyte of interest. A boiler water anion analysis containing high phosphate concentration would also benefit from the slow elution time of the phosphate peak (Figure 13-4).

Figure 13-10 Chromatogram of a Sample Containing 0.022 µg/l Sodium and 3000µg/l Ethanolamine [18] Source: Reference 18, Courtesy Dionex Corp.

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13.6 Calibration Ion chromatography calibration usually involves analyses of known standards leading to the development of a 1-, 2-, or 3-point calibration curve. In some instruments, the calibration procedure is automated by using a built-in function; activation may be either manual or computer-controlled to occur at pre-set intervals. 13.6.1 Calibration Checks

On-line ion chromatography instruments should be checked periodically to demonstrate calibration stability. Two methods exist for verifying instrument stability; the Standard Injection Method [16] or the Line Method [17]. For the Standard Injection Method, a known standard composed of the analytes of interest (e.g., Cl, SO4, Na, etc.) in the concentration range where the calibration can be readily verified, is analyzed by the on-line ion chromatograph and the results are compared to the acceptance criteria (e.g., the results should agree within ± 3 sigma or ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is considered still acceptable. If the results are outside the acceptance criteria, the on-line ion chromatography must be recalibrated. Unfortunately, any source of contamination would bias the test results high. See the discussion on interferences in Section 13.4 for more information on sample handling. A second approach involves injection of a standard as a known addition into the sample stream and the mixture is analyzed by the ion chromatograph. The results are compared to the acceptance criteria (e.g., they should agree within ± 3 sigma or ± 10%). Again, any source of contamination would bias the test results high. If the results are outside the acceptance criteria the on-line ion chromatograph must be recalibrated.

13.7 Alternative Methods Ion chromatography’s greatest advantage over other on-line detection methods is its ability to analyze for several components in one analytical run. The majority of the components of interest in the power plant have on-line analyzers which are based on alternate methods. The reader is directed to the following sections for further information on these alternate methods. •

Sodium – Section 8 – ISE.

•

Ammonia – Section 9 – ISE and colorimetric.

•

Chloride – Section 10 – ISE.

•

Hydrazine – Section 11 – ISE, amperometric, colorimetric.

•

Phosphate – Section 15 – Colorimetric.

•

Organics – Section 17 – Combustion/Oxidation.

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Note that there are no alternative methods for on-line analysis of low level sulfate. Grab samples for low level sulfate are analyzed on a laboratory IC for quantification and verification of the online instrument. While Section 17 lists alternate methods for TOC determination, only IC has the ability to identify and quantify the individual organic components. Differentiating between organic carbon species may become increasingly important to meet future steam purity limits for the steam turbine.

13.8 End User Considerations The end user needs to understand the degree of maintenance and technical commitment required for on-line ion chromatographic analysis. Several new developments such as eluent generators, continuously regenerated suppressors and trap columns, and automatic calibration should significantly reduce the amount of day to day attention that is required. Without these enhancements, daily attention to suppressor regeneration is likely—this activity usually lends to several hours of down-time while the instrument returns to a stable condition. Most on-line instruments now provide for monthly supplies of eluent in their storage compartments.

13.9 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.

4.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

5.

H. Small, T.S. Stevens, and W.C. Bauman, “Novel Ion Exchange Chromatographic Method Using Conductometric Detection”, Analytical Chemistry, Vol. 47, No. 11, (1975).

6.

T.O. Passell, “Use of On-Line Chromatography in Controlling Water Quality in Nuclear Power Plants”, J. of Chromatography A, Vol. 671, p. 331. (1994).

7.

On-Line Chromatography at Three PWR’s, by M.N. Robles and J.L. Simpson. EPRI, Palo Alto, CA: September 1985. NP-4121.

8.

In-Plant Measurement of Corrosive Ions in Water, by M.N. Robles, J.L. Simpson, D. Dutina, and T.O. Passell. EPRI, Palo Alto, CA: September 1989. NP-6308.

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9.

Dionex EG50 Product Information Brochure.

10. J. Weiss, Handbook of Ion Chromatography, edited by E.L. Johnson. Dionex Corporation, Sunnyvale, CA (1986). 11. Determination of Trace Anions in High-Purity Waters by Ion Chromatography with the IonPac® As17 Using High-Volume Direct Injection with the EG40. Dionex Corporation, Sunnyvale, CA 2003. 12. H. Small, T. S .Stevens, W .C. Bauman, Anal, Chem,, 1975,47,1801. 13. Dionex Reagent-Free Ion Chromatography (RFIC®) System; Product Brochure, LPN1819 10M, Dionex Corporation , Sunnyvale, CA 2006. 14. Reducing Carbonate Interference in Anion Determinations with Carbonate Removal Device, Dionex Technical Note 62, Dionex Corporation, Sunnyvale, CA 2006. 15. ASTM D5996-05, “Standard Method for Measuring Anionic Contaminants in High-Purity Water by On-Line Chromatography”. ASTM International, West Conshohocken, PA. 2005. 16. Advanced Power Plant Chemistry QA/QC Practices, Scientech, LLC., Clearwater, FL, 2006. 17. ASTM D3864-96(2000), Standard Guide for Continual On-Line Monitoring Systems for Water Analysis, American Society for Testing & Materials, Philadelphia, PA. 18. Determination of Sodium at the Parts-Per-Trillion Level in the Presence of High Concentrations of Ethanolamine in Power Plant Waters—Dionex Application Note 152, Dionex Corporation, Sunnyvale CA. 19. Acclaim OA (Organic Acid) Analytical Column Product Manual; Dionex Corporation, Document 031996-01. 2004. 20. IONPAC® TBC-1 Borate Concentrator Column and IONPAC® ICE Borate Separation Column; Dionex Corporation LPN 1001 3M. 1998.

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14 IRON AND COPPER

14.1 Purpose and Use Iron and copper are not defined as Core Monitoring Parameters in current EPRI Guidelines [1-4] but it is suggested that samples be taken and analyzed periodically to measure corrosion product levels in the steam-water cycle. Performance of corrosion product monitoring campaigns is a useful means of verifying that the levels of corrosion product transport are consistent with those which can be attained when the feedwater treatment has been properly selected and optimized. Transport of feedwater corrosion products can cause problems in both fossil and nuclear power plants. Iron and copper corrosion products are of primary concern because of their relative abundance in power plant cycles. In fossil power plants, transported corrosion products can form deposits on water-wall tubes under high heat flux conditions, providing a site for concentration of impurities and potentially leading to tube failures by caustic gouging, hydrogen damage, and low cycle corrosion fatigue [5]. Corrosion product monitoring in the plant is conducted primarily for the following reasons: •

To facilitate the correlation of a water chemistry parameter with plant operating variables.

•

To check the accuracy of water chemistry control (such as reducing agent, oxygen, ammonia or pH), so ensuring that corrosion rates are kept at acceptable low levels.

The data generated by continuous on-line monitoring of iron and copper is used by plant chemistry and operations personnel. The goal for plant personnel is to maintain iron and copper within prescribed limits. Iron and copper may exist in dissolved, colloidal and suspended forms. Some of the available methods are capable of a total metal determination while others assess the concentration of the metal in a particular form. For example, it is common to test grab samples of boiler feedwater for suspended iron oxides during startup of fossil boilers of once-through design.

14.2 Description of Methods Several methods of analysis are available and in use. Monitoring techniques range from grab sampling and laboratory analysis to on-line automated techniques that often utilize the grab analysis methods, either periodically or with integrated samples. The on-line integrated sampling device allows concentration of filterable species on a 0.22 µm or 0.45 µm membrane and concentration of non-filterable colloidal and soluble species on cation (and, if needed, anion) 14-1

EPRI Proprietary Licensed Material Iron and Copper

ion exchange membranes. The accumulated waterborne particulates can then be analyzed in one of two ways. Either the filters are removed and the samples are digested for subsequent analysis by flame atomic absorption, graphite furnace atomic absorption, or atomic emission spectroscopy, or the particulates on the filters are irradiated on-line with X-rays that cause elements to emit a fluorescent radiation that can be correlated with elemental concentrations (an analytical technique known as X-ray fluorescence or XRF). On-line integrated sampling does not allow real time on-line analysis of iron, copper, and other corrosion products. But the on-line integration of the sample does circumvent the difficulties in obtaining representative samples for standard wet chemistry laboratory methods. As an example of these difficulties, a conventional grab sample containing iron particles with an average diameter of 5 µm and a density of 5 g/mL could give erratic results when directly analyzed by graphite furnace atomic absorption: if the average iron concentration were 10 µg/L (ppb), no more than one particle (and possibly none) would be present in the 20 µL aliquot typically injected into the furnace [5]. Since iron levels in most sample streams of interest are typically below the detection limit of direct XRF analysis, a concentration mechanism is required for XRF analysis as well. Other possible approaches to monitoring corrosion product particulates include: turbidity, particle counters, particle monitors, acoustic detection, and colorimetric.

14.3 Technical Considerations 14.3.1 Integrated Sampling

Several integrated sampling devices are commercially available. They require access to a representative water sample, flowing at rates high enough (flow velocity > 1 m/sec (3 ft/sec)) to avoid corrosion product particle dropout. Significant pressure drops in sample coolers should be minimized for the same reason. Detailed information on sampling procedures is available in EPRI reports CS-5164 and GS-7556 [6,7]. The sample flow is split so that water is delivered continuously to the integrated sampling device at a constant flow rate in the range of about 50 to 200 mL/min, and the remainder flows to the drain. The filtration unit holds a 0.45 µm or a 0.2 µm membrane to collect the particulate matter. If dissolved iron analysis is desired, one or more cation ion exchange membranes are positioned beneath the particulate filter to collect colloidal and soluble species passing through the membrane filter. A constant sample flow rate is maintained through the filter and ion exchange membranes by incorporating a pressure regulator downstream of the filtration unit. The volume of water passing through the integrated sampling device is measured with a sample flow totalizer before the sample flows to waste. For samples that require wet chemistry analysis of the collected material, sampling periods range from one to seven days, equivalent to total sample volumes in the range 144 to 1008 liters for a

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flow rate of 100 mL/min. The filter pad and ion exchange membranes are then removed from the filtration unit for chemical analysis. For samples analyzed using an XRF spectrometer analysis, the sampling period continues until it becomes necessary to change the filter for one of two reasons: either the amount of particles deposited reduces the flow rate below a useful range or the amount of iron in the deposit produces a fluorescent signal beyond the linear response curve of the detector. The total amount of flow through these samplers is critical when calculating or comparing results achieved from wet chemical or XRF spectrometer analysis [8]. 14.3.1.1 Wet Chemistry Analysis of Integrated Samples

For wet chemistry laboratory analyses of integrated samples of iron and copper, the membrane filter (carrying the particulates) and cation resin impregnated membranes (carrying the soluble metals) are digested separately in hydrochloric-nitric acid mixtures to produce the integrated and solubilized samples needed for reliable chemical analysis [9]. A concentration factor of 1000 or more is typical using this approach. Thus, these samples can be used to analyze separately for particulate loading and for dissolved corrosion products (iron and/or copper) in the original sample. Various standard laboratory methods are available for these analyses. For instance, the flame atomic absorption technique is described in D1068-05 Standard Test Methods for Iron in Water [10] and in D1688-02 Standard Test Methods for Copper in Water [11]. Graphite furnace atomic absorption techniques are addressed in EPA 236.2 for iron [12] and EPA 220.2 for copper [13]. Analysis by direct current atomic emission spectroscopy is covered in D4190-03 Standard Test Method for Elements in Water by Direct-Current Argon Plasma Atomic Emission Spectroscopy [14] and analysis of metals in drinking water by atomic emission spectroscopy utilizing inductively coupled plasma is described in EPA 200.7 [15]. Ion chromatography may also be used but requires post-column determination using a chelating agent and is detected with a suitable visible absorbency detector. 14.3.1.2 XRF Analysis of Integrated Samples

In the XRF analysis of an integrated sample, a sample is irradiated with X-rays of an appropriate energy to excite, through ionization, the elements of interest in the sample. Upon spontaneous de-excitation, sample elements emit fluorescent radiation, the component energies of which are characteristic for each element, and the intensities of which may be correlated with the elemental concentrations in the sample. The concentrations of particulate or dissolved metals, or both, in water streams are determined through accumulation on appropriate collection media (filters or ion exchange materials) and detection by x-ray fluorescence spectroscopy, providing real time determination of iron and other metals found in water streams. The water sample delivered into the monitoring system passes 14-3

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through a flow sensor, and then, to a flow cell assembly containing a membrane or resin filter, depending on the application of interest. For an application where only dissolved metals are to be analyzed, the sample needs to be filtered upstream of the sample chamber to prevent particulate contamination of the resin membrane surface. A sample bypass valve is used for flow control through the sample chamber. Two sample chambers in sequence can be used to determine both particulate and dissolved components of the metal(s) of interest. X-ray fluorescence is used to determine the concentration of the captured material. XRF analysis gives a measure of total elemental concentration independent of the oxidation state or molecular configuration of the element. Elements with atomic numbers 13 through 92 can be detected. The filter chamber is essentially a variation of the traditional corrosion product sampler used to collect integrated samples. The main difference in the design of the flow cell in the on-line monitor is that the sample enters the filter chamber in a way that allows an x-ray probe to be positioned in close proximity to the filter or resin membrane surface. Since even a small quantity of water covering a sample significantly attenuates the excitation and emission radiation, a computer controlled valve switching system is incorporated into the monitor. In one position, this valve allows sample flow to proceed through the monitoring unit and metals to accumulate on the filter or resin membrane while the total flow is monitored. In the other position, the valve introduces air or other another gas (e.g., nitrogen) to purge the filter chamber of liquid while the sample is diverted to waste. It is during the air purge that the x-ray measurement takes place. In this way, the monitor operates by continuously alternating between two modes: a sample accumulation mode and an analysis mode. Typical time assignments for these modes for sample concentrations in the low µg/L (ppb) range are five minutes each; thus, in one cycle, sample accumulates for five minutes followed by a five minute x-ray measurement. With various delays for valve switching operations, computer extraction of x-ray data, and data manipulation, the measurement cycle in this case lasts approximately 14 minutes. Single-channel instruments can be configured to monitor iron, copper, or any other solid species. A two-channel instrument (with one sample chamber per channel) can provide greater monitoring flexibility. For instance, one channel can be equipped with an iron-probe that contains curium-244 while the second channel is equipped with a sulfur-probe (for lighter elements) that contains iron-55. Alternately, both channels can use iron-probes so that one channel can be collecting a sample while the other channel is analyzing for iron [16]. 14.3.2 Turbidity

Turbidity is defined by ASTM as “the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample”. In simple terms, the interaction between light and suspended particles in a liquid is called turbidity. This interaction is affected by the size, shape, and refractive index of the particles, and the wavelength of the incident light. The colors of the suspended material and the sample fluid also affect the measurement. Turbidity is an “index” of water clarity. A common method used for turbidity measurement is nephelometry in which a detector measures light reflected, or “scattered” from 14-4

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particles at an angle 90 degrees to the incident (source) light. In liquids with only a few particles per milliliter, or if the particles are only slightly reflective, this is a very small signal. Since metal oxide particles are usually dark they absorb rather than reflect light so nephelometry is not a preferred method for this application. 14.3.3 Particle Counter

Particle counters detect particles in liquids, determine their size, and put them into “bins” of defined size ranges. There are various optical and electrokinetic methods used in particle counting. One of the simplest and most sensitive for particles above 1 micron is light blocking, or “extinction” (Figure 14-1). The light source is a laser diode. As sample flows through the sensor, particles create shadows on the detector which are converted to electrical (millivolts (mV)) “blips” proportional to the particle size. A high speed microprocessor interprets the data and directs information to “bins” of selected size ranges, e.g. 2-5 microns, 5-10 microns, etc. Since this is a continuous sample flowing at a constant rate, the data can be mathematically expressed in particles per milliliter, or “counts per mL” for each size bin. Particle sizes are expressed as diameters, although it’s obvious that all particles aren’t spheres, or even circular. In many practical applications this doesn’t detract from the usefulness of the data since an assumption can be made that size “distribution” would be the same if all particles were spherical or circular. A great advantage for light blocking counters in boiler cycle water is that the particles are usually opaque and dark making very distinct, easily detectable “shadows”. On line particle counters are being used in potable and industrial applications and are capable of detecting one particle per milliliter of 2 micron size [20].

Figure 14-1 Particle Counter Schematic, Illustrating 1 Particle at 3 microns and 2 Particles at 1 micron [21] Source: Adapted from reference 21

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14.3.4 Dynamic Light Fluctuation Based Particle Monitors

The particle monitor (PM) operates on the principle of “dynamic light fluctuation” (DLF), which is distinctly different from, the particle counter [20]. Figure 14-2 is a simple schematic of the measuring cell. As particles pass through the cell they cause disruptions, or “fluctuations” in intensity of the infrared LED light beam. More particles increase the magnitude of the fluctuations. The PM derives information on particulate concentration from the magnitude of these fluctuations, not from the total intensity of scattered or blocked light as in turbidimeters or particle counters. This information is converted to a reading proportional to the particulate concentration, and is displayed as the “particle index” (PI). The instrument does not measure particle size or number, but tracks “trends” in particulate concentrations. Sensitivity is equal to, or better than the particle counter. A unique feature of particle monitors is that the water sample never contacts the sensor cell optical components. Maintenance consists of simply replacing the plastic sample tubing. Also the PM does not require “calibration”, but uses a simple procedure that verifies instrument “sensitivity and repeatability”. The PM has the capability of detecting particulate concentration as low as one µg/L (ppb) and the capability of automatically obtaining a sample when the PI exceeds a predefined “threshold”. This enables plant personnel to analyze for specific contaminants and particulate concentrations when an “excursion” occurs [21].

Figure 14-2 Dynamic Light Fluctuation Schematic [21] Source: Adapted from reference 21

14.3.5 Acoustic Detection

This technique involves the use of a transducer that transmits a focused, ultrasound beam into the liquid containing particles. As this energy pulse strikes the corrosion product particles, some of it is reflected back to the transducer where it is detected. Each pulse lasts for 1 microsecond and 14-6

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200 pulses per second are transmitted. This allows about 500 microseconds to receive the reflected echoes to “count” particles. The instrument’s output consists of a digital meter, optional bar graph, and current or voltage process outputs. The output is a relative concentration indication. It is simply referred to as “counts,” but may often be correlated to mg/L (ppm), µg/L (ppb), percent solids, or some other unit meaningful to the user. Some newer instruments provide enhanced performance via computer control and advanced signal processing. Primary differences for these monitors include independent bubble and particle response, and particle size compensation [22]. 14.3.6 Colorimetric

Soluble copper concentration can be determined colorimetrically. Colorimetric determination requires the addition of chemical reagents to provide development of the proper color species. To operate properly, this instrument needs to add the reagents in the proper sequence and amount, assure adequate mixing and necessary reaction times, accurately determine the amount of light absorption at the proper wavelength, correlate this light absorption to some standard curve, and then prepare for the upcoming sample. The colorimetric on-line copper analyzer performs a batch-wise analysis of the sample using a chemical approach that is based on the Bathocuproine method [17,23]. Cuprous ions form a water soluble orange colored chelate with bathocuproine disulfonate. The sample is buffered at a pH of about 4.3 and reduced with hydroxylamine hydrochloride. The absorbance is measured at 484 nm. Soluble iron is determined colorimetrically by two methods. The first is based on the reaction of ferrous ions with a 1,10 phenanthroline reagent at a pH of 3.2 [34]. Ferric ions can also be determined with this method if a preliminary chemical reduction step is performed. Reagents such as hydroxylamine or ascorbic acid are used to convert ferric to ferrous ions. The orange-red complex formed by this reaction is then measured at 510nm. On-line iron analysis based on this method is available [25] but the 3 µg/L (ppb) detection limit gives marginal suitability for condensate / feedwater applications. Test kits for grab sample total iron determinations are also available using the phenanthroline color complex [26]. The stated iron detection limit for the test kit is 20 µg/L (ppb). A second analytical method is based on the addition of ammonium thioglycolate in an acidic pH to form a red / purple complex [27]. Color intensity is measured at 562 nm with a reported detection limit of 9 µg/L (ppb) and a working range to 1400 µg/L (ppb). The colorimetric test is specific to ferrous iron and a digestion step must be added for conversion of ferric iron; after the digestion, test results are given as total iron and speciation is determined by difference. On-line colorimetric determination for iron is not currently available for the concentration of interest in power plant condensate and feedwater applications. 14-7

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14.3.6.1 Colorimetric Limitations

All colorimetric tests rely on the principle of Beer’s Law (also known as the Beer-Lambert Law) which states that the amount of light absorbed by a sample (A) is proportional to some absorbtivity constant (a), the path-length of light (b), and the concentration of the analyte species(c). A = a x b x c The absorbtivity constant is dependent on the color species and test conditions of the method and remains constant as long as the reagents, test conditions, and wavelength of light do not change. The path length (the amount of sample through which the light passes) is also constant for a given sample cell. The mathematical formula then infers that the amount of light absorbed by a sample (i.e. the intensity of the color developed in the test) is directly proportional to the concentration of the analyte—in this case, copper. As the copper concentration increases, the intensity of the orange color will increase and the color change can be quantified. Limitations on this analytical principle arise at both the low end and high end of the useable range. The low end limit is caused by the detection limit of the test—this is primarily a function of the photo-multiplier (detector) sensitivity and stability. How small a change can be detected by the sensor and how stable is the baseline (or the zero reading)? When trying to quantify a concentration near the detection limit, it is not uncommon to see negative readings or duplicate readings that have large relative errors although the absolute error may only be 10 or 20 µg/L (ppb). As a result, most copper analyzers will list a detection limit, an accuracy limit, and a precision limit. This accuracy limit is typically an absolute value or some percentage of the reading, which ever is bigger. A typical analyzer has the following specifications which demonstrate this technical consideration [18]. •

Minimum Detection Limit: 1 µg/L (ppb).

•

Accuracy: ±2% of reading (µg/L).

•

Precision: ±2% of reading (µg/L).

Limitations on the high end of the analytical range, shown in Figure 14-3, arise from a property called self-absorption. As the color intensity becomes more and more blue, the amount of light coming into the sample cell is almost totally absorbed.

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Figure 14-3 Illustration of Self Absorption at High Concentrations

At some point there is 100 percent absorption, the line flattens out, and the analytical test is no longer usable. However, the area where linearity first starts to deviate is also an area of concern. The molecules imparting color (absorbing light) are so numerous that some molecules are shaded from the incident light and not accounted for. The absorption coefficient can change at high analyte concentrations and stray light due to monochrometer imperfections can also skew the linearity. The subsequently curved line can no longer be used for accurate analytical determination. Most instruments therefore specify a maximum concentration that can be measured accurately corresponding to the top portion of the straight-line relationship.

14.4 Calibration 14.4.1 Wet Chemistry Analysis

Membrane filter standards for iron are prepared from an atomic absorption standard for iron. Alternatively, users may make up their own standard solution. The atomic absorption (AA) standard is 1000 mg/L (ppm) iron dissolved in dilute acid solution. Sodium hydroxide or ammonia solution is used to adjust measured aliquots of this standard to a pH of 10 to 11. Under these conditions, iron is insoluble and filterable. The resulting iron suspension is filtered through a 0.45 micron membrane filter and the filtrate analyzed by AA after appropriate acidification and digestion. Any iron determined in the filtrate is subtracted from the amount contained in the volume passed through the filter to determine the amount deposited on the filter. The aliquots of AA standard may be diluted with high purity water prior to pH adjustment. The dilution provides more conveniently handled volumes, and the larger volumes will promote more uniform distribution of iron on the filter surface. The amount of dilution does not affect the total iron to be filtered. 14-9

EPRI Proprietary Licensed Material Iron and Copper

This procedure is repeated to prepare filter standards covering a range of total iron from zero (blank filter) to three milligrams. The lowest prepared standard should be near the minimum detectable amount (approximately 3 µg) distributed over the surface of a membrane filter with a diameter of 47 mm. This procedure, without adjustment to high pH, may be used to prepare cation resin membrane standards covering a range of total dissolved iron. For colorimetric analysis a series of standards are prepared and analyzed to determine the calibration curve for the on-line analyzer. On-line copper analyzers are equipped to automatically analyze standards and determine the calibration curve to be used in the on-line instrument. 14.4.2 XRF Analysis

Filter or resin membrane standards are analyzed using the on-line x-ray probe to collect intensity data under the exact conditions of on-line service. This includes installing the filter or resin membrane in the flow chamber of the on-line system and analyzing through the Kapton® window. The intensity data collected in this way is plotted against the known amounts of total iron and the equation for the best line through the data is determined by standard least squares regression techniques. For up to approximately 3000 µg/L (ppb) of total iron, the relationship is nearly linear. The resulting calibration equation is then used to convert intensity measurements of filters or resin membranes to iron mass during on-line operation. The calibration curve parameters may be inputted to the x-ray control electronics unit or directly to the PC depending on the software configurations of the monitor. In either case, the intensity measurements during an on-line session are converted automatically to mass values and both are stored in the data file, which is updated continuously during operation. This mass measurement (in micrograms, µg) is used with the incremental volume total (L) between measurements to give the desired concentration in µg/L (ppb).

14.5 Calibration Check For those devices which use off-line wet chemistry analysis of collected particulates, the validation should be on the analytical method (rather than the sample collection technique) and is typically defined by the lab’s QA/QC procedures. For the device that incorporates an on-line XRF monitor, validation is accomplished by removing the particulate-laden filter and conducting off-line analysis using a laboratory XRF system. For XRF analytical capabilities should be checked periodically to demonstrate calibration stability. One method exists for verifying instrument stability; the Line Method [19]. For the Line Method, a calibrated separate XRF instrument is used to analyze the same sample as the installed on-line XRF instrument. The results of two instruments are compared to the acceptance 14-10

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criteria (e.g., the differences agree within ± 3 sigma or the measured values agree within ± 10%). Provided the on-line analyzer agrees within the acceptance criteria, the on-line instrument’s calibration is deemed to be acceptable. If the results are outside the acceptance criteria the online instrument should be recalibrated.

14.6 Alternate Methods Historically, the quantity of particulate matter has been estimated by comparison with a set of filter pad “standards”. The test protocol was developed by Babcock and Wilcox, a boiler manufacturer, to predict the level of “cleanliness” in the condensate / feedwater cycle during plant start-up. Two series of test pads with increasing particulate loading were created; one using red iron oxide (i.e. hematite) and another using black iron oxide (i.e. magnetite) material. A photograph showing this sequence of increasing particulate loading was then used to as a comparison against the analytical sample. This comparison was used to monitor start-up and predict the likely corrosion product transport.

14.7 End User Considerations The performance characteristics (range of measurement, accuracy, precision, bias, drift, response time, and signal change) for the monitoring equipment, provided by the manufacturer or supplier should be considered when selecting a suitable on-line iron and copper instrument. End users should select the instrument that is best suited for the intended analytical application. Integrated corrosion product samplers are best suited for a comprehensive monitoring program. The main emphasis is on quantifying the filterable material although some users also install the ion exchange membrane. Other on-line iron and copper instrument considerations include: •

Analytical capability to digest and analyze samples from an integrated sampler.

•

Need for continuous or semi-continuous monitoring.

•

Need for speciation.

•

Suitability of method to meet the defined accuracy, precision, bias and drift requirements.

•

Ease and robustness of calibration for the intended use.

•

Ease and robustness of calibration verification.

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14.8 References 1.

Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment. EPRI, Palo Alto, CA: 2004. 1004188.

3.

Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA: 2005. 1004925.

4.

Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). EPRI, Palo Alto, CA: 2006. 1010438.

5.

Interim Consensus Guideline on Fossil Plant Chemistry, by A.F. Aschoff, Y.H. Lee, D.M. Sopocy, and O. Jonas. EPRI, Palo Alto, CA: June 1986. CS-4629.

6.

Guideline Manual on Instrumentation and Control, by R.D. Hopkins, E.H. Hull, K.J. Shields, and S. Yorgiadis. EPRI, Palo Alto, CA: April 1987. CS-5164.

7.

Monitoring Cycle Water Chemistry in Fossil Plants, Vol. 1, by A.F. Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon. EPRI, Palo Alto, CA: October 1991. GS-7556, Volume 1.

8.

Connolly, D. and S. Harvey. “On-Line Corrosion Product Monitoring of Nuclear Plant Feedwater”, Ultrapure Water, November 1995, pp. 28-31.

9.

PWR Secondary Water Chemistry Guidelines: Revision 4, prepared by PWR Water Chemistry Guidelines Revision Committee. EPRI, Palo Alto, CA: November 1996. TR102134-R4.

10. ASTM D1068-05, “Standard Test Methods for Iron in Water”, American Society for Testing & Materials, Philadelphia, PA. 11. ASTM D1688-02, “Standard Test Methods for Copper in Water”, American Society for Testing & Materials, Philadelphia, PA. 12. Iron, Method 236.2 (AA Furnace), Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020. U.S. Environmental Protection Agency, Cincinnati, Ohio, (Revised March 1995). 13. Copper, Method 220.2 (AA Furnace), Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020. U.S. Environmental Protection Agency, Cincinnati, Ohio, (Revised March 1995). 14-12

EPRI Proprietary Licensed Material Iron and Copper

14. ASTM D4190-03, “Standard Test Methods for Elements in Water by Direct-Current Argon Plasma Atomic Emission Spectroscopy”, American Society for Testing & Materials, Philadelphia, PA. 15. Inductively Coupled Plasma, Method 200.7 (AA Furnace), Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020. U.S. Environmental Protection Agency, Cincinnati, Ohio, (Revised March 1998). 16. Connolly, D. and P. Millett. “On-Line Particulate Iron X-Ray Monitor”, Ultrapure Water, February 1994, pp. 61-65. 17. Part 3500-Cu C, Bathocupric Method, Standard Methods for the Examination for Water th and Wastewater, 20 Edition, 1999, American Public Health Association, Washington, DC. 18. Waltron, LLC, Aqualert Division, 9040 Series Copper Analyzer, product information manual, Whitehouse, NJ. 19. ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis”, American Society for Testing & Materials, Philadelphia, PA. 20. Chemtrac PC 2400D Particle Counter and PM 2500 XRD Particle Monitor, Chemtrac Systems, Inc. Norcross, GA 30092. 21. “Continuous Metal Transport Monitoring using On-Line Particulate Determination”, L. Joseph Hancock, Richard A. Breckenridge—Arizona Public Service Company, presented at Eighth International Conference on Cycle Chemistry in Fossil and Combined Cycle Plants with Heat Recovery Steam Generators, Calgary, Canada 2006. 22. Acoustic Particle Counter, Innovative Dynamics Inc, Ithaca, NY. 23. Waltron µAI9045 On-Line Copper Analyzer, Waltron LLC, Whitehouse, NJ. 24. Standard Methods for the Examination of Water and Wastewater, 21st Edition. Method 3500 Fe B. Phenanthroline Method, 2005. 25. Model CFA-1026 On-Line Iron Analyzer, Scientific Instruments, 200 Saw Mill River Road, Hawthorne, NY 10532. 26. Hach Method 8008 FerroVer® Powder Pillows, Hach Company, Loveland, CO. 27. Hach Method 8147 FerroZine® Reagent, Hach Company, Loveland, CO.

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15 PHOSPHATE

15.1 Purpose and Use Phosphate is listed as an EPRI Guideline Core Monitoring Parameter [1,2]. The on-line monitoring equipment is used most frequently to monitor phosphate in the continuous blowdown of fossil boilers. Optimal phosphate concentration is both a function of the operating pressure of the steam/water cycle and the likelihood of boiler contamination. Phosphate is continually monitored on-line in the plant for the following reasons: •

To check the accuracy of water chemistry control (such as the sodium-to-phosphate molar ratio).

•

To facilitate the correlation of phosphate content with plant operating variables.

•

To warn of in-leakage of contaminants.

The data generated by continuous on-line monitoring of phosphate is used by plant chemistry and operations department personnel. The goal for plant personnel is to maintain phosphate within prescribed limits.

15.2 Description of Method Cycle chemistry guidelines for fossil power plants indicate that, when phosphate is used for pH control, the sodium-to-phosphate molar ratio should be controlled to minimize corrosion rates and scaling of heat transfer surfaces [1]. The resulting need for continuous surveillance of phosphate (and sodium) levels has led to the introduction of several on-line monitors. One relevant technique is on-line ion chromatography: further details of this approach can be found in Section 13. More frequently, an on-line colorimetric technique is used to monitor phosphate. Although the details of the method vary from one manufacturer to another, they all result in reaction of the phosphate with reagents to produce a colored solution. The intensity of the color, which is proportional to the concentration of phosphate, is then measured with a spectrophotometer. The colorimetric methods are based on reactions that are specific to the orthophosphate ion so other forms of phosphorus (organic phosphorus and such hydrolyzable forms as metaphosphate, pyrophosphate, and tripolyphosphate) will not be detected by this method. It is possible that a small percentage of the hydrolyzable phosphorus is converted to orthophosphate when an acidic reagent is added to the sample during the analysis procedure. 15-1

EPRI Proprietary Licensed Material Phosphate

Several manufacturers choose to add acidified ammonium molybdate and a reducing agent to produce a blue phosphomolybdate complex. Others add an acidified mixture of ammonium molybdate and ammonium metavanadate to produce a yellow molybdovanadophosphoric acid complex. The absorbance of the blue solution is measured with a spectrophotometer at 880 or 625 to 670 nm depending on the details of the method. In general the blue phosphomolybdate complex is a more sensitive method than the yellow molybdovanadophosphoric acid method. Similarly, the absorbance of the yellow solution is measured at 400 to 480 nm depending on the details of the method. One manufacturer uses the blue phosphomolybdate method for measuring phosphate in the low range (0-5000 µg/L (ppb) as PO4) and the yellow molybdovanadophosphoric acid method for measuring phosphate in a higher range (0-50 mg/L (ppm) as PO4). The on-line analysis methods are similar, though not identical, to the laboratory analysis techniques described in ASTM D515 Method A [3] (blue complex) and the now discontinued Method C [4] (yellow complex). Similar references are found in Standard Methods for the Analysis of Water and Wastewater 4500-P C (yellow complex) and 4500-P D (blue complex) [5]. However, unlike laboratory units, on-line analyzers are equipped with automatic fluid handling systems. A sample bypass flow system is provided so that the on-line analyzer has continuous access to a fresh sample of the water containing phosphate. The water sample is typically conditioned so that the pressure and temperature are within set limits before it enters the analyzer. Necessary reagents and standards are often purchased from the instrument supplier which eliminates the risk of field contamination during the preparation process. An instrument that uses the molybdovanadate reagent is described below as an example of the methods available. At the beginning of each cycle, the measurement cell is flushed thoroughly with fresh sample. Sulfuric acid is added to the sample to dissolve particulate matter and an anionic surfactant is added to minimize bubble formation on the measuring cell walls. An initial sample blank absorbance is measured at 480 nm to serve as a zero reference, and then the molybdovanadate reagent is added to react with the orthophosphate to form the yellow molybdovanadophosphoric acid complex. The sample is then allowed to stand undisturbed for a pre-set time to allow for proper color development. The light absorbance through this colored solution is measured at 480 nm. The difference between this absorbance and the initial blank absorbance is converted to the orthophosphate (PO4) concentration and displayed as mg/L (ppm) PO4. Silica is a potential interference in this analysis, but only at concentrations not typically found in fossil power plant samples, i.e. above 500 mg/L (ppm). Sulfide present in the sample can also interfere with quantification. Sulfides are not typically found in fossil power plant samples.

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15.3 Technical Considerations 15.3.1 Colorimetric Limitations

All colorimetric tests such as phosphate rely on the principle of Beer’s Law, as shown in Equation 15-1 (also known as the Beer-Lambert Law). This law states that the amount of light absorbed by a sample (A) is proportional to some absorption constant (a), the path-length of light (b), and the concentration of the analyte species (c). A=axbxc

Equation 15-1

The absorption constant is determined by the color species and test conditions of the method and remains constant as long as the reagents, test conditions, and wavelength of light do not change. The path length (the amount of sample through which the light passes) is also constant for a given sample cell. The mathematical formula infers that the amount of light absorbed by a sample (i.e., the intensity of the color developed in the test) is directly proportional to the concentration of the analyte—in this case, phosphate. As the phosphate concentration increases, the intensity of the color will increase and this color change can be quantified as illustrated in Figure 15-1.

Figure 15-1 Typical Beer’s Law Absorption (A) vs. Concentration Curve

Limitations on this analytical principle arise at both the low end and high end of the useable range. The low end limit is caused by the detection limit of the test—this is primarily a function of the photo-multiplier (detector) sensitivity and stability. How small a change can be detected by the sensor and how stable is the baseline (or the zero reading)? When trying to quantify a test right at the detection limit, sometimes negative readings or duplicate readings are seen that have large relative errors although the absolute error may only be a fraction of a mg/L (ppm). As a 15-3

EPRI Proprietary Licensed Material Phosphate

result, most phosphate analyzers will list a detection limit, an accuracy limit and a precision limit. This accuracy limit is typically an absolute value or some percentage of the reading. A commonly used analyzer has the following specifications which demonstrate this technical consideration [6]. •

Minimum Detection Limit: 0.2 mg/L (ppm).

•

Accuracy: ±0.5 mg/L (ppm) or ±5% of reading, whichever is greater.

•

Precision: ±0.5 mg/L (ppm) or ±5% of reading whichever is greater.

Evaluation of this specification shows that the accuracy uncertainty of the measurement is large relative to the measurement itself at or near the minimum detection limit. At lower concentrations (0.2-1.0 mg/L (ppm)) end users are cautioned about using data for critical decision making since the inaccuracy is large relative to the measurement itself. At concentrations greater than 10 mg/L (ppm) the accuracy limit of ± 5% is used since it becomes larger than ± 0.5 mg/L (ppm). A different analyzer may have a different specification with a lower detection limit as a result of using another analytical method (changing the absorption constant) and using a longer path length sample. At some point there is 100 percent absorption, the line flattens out, and this analytical approach is no longer usable. However, the area where linearity first starts to deviate is also an area of concern. The molecules imparting color (absorbing light) are so numerous that some molecules are shaded from the incident light and not accounted for. The absorption coefficient can change at high analyte concentrations and stray light due to monochrometer imperfections can also skew the linearity. The subsequently curved line can no longer be used for accurate analytical determination. Most instruments therefore have a maximum analytical range which corresponds to the top portion of the straight-line relationship. Analytical readings above this linear region are disallowed and the analyzer produces an “over range” alarm. 15.3.2 Sample Considerations

Process samples for phosphate analysis need to be representative of the sample stream being measured. As such, proper flushing of sample lines, temperature conditioning, particulate removal, and pressure conditioning are all required. Many analyzers have a failsafe feature which senses a loss in sample flow and triggers an alarm. In addition, because the sample analysis is based on a color development technique, proper flushing of the sample cell is required between samples to remove the previous test solution. This cell flushing step is usually accomplished by regulating the incoming sample pressure and allowing a predetermined time to completely exchange the new sample for the prior test solution. Inlet sample pressure is often regulated at 3.45 kPa ± 2.07 kPa (5±3 psi) to assure adequate flushing; sample pressure control becomes an issue when process stream pressure changes due to load changes or sample sequencing.

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15.3.3 Time Delay

Another technical consideration is the time lag required for reagent addition and color development which necessarily results in a batch type process. A typical phosphate analyzer has a nominal ten minute cycle from introduction of one sample to the next. This analysis delay could be troublesome if the process stream phosphate concentration is changing rapidly. Fortunately, in the typical fossil plant applications, the phosphate content is a slowly changing value and the batch process concept does not significantly hinder its usefulness. The amount of time required for color development is temperature dependent. Some colorimetric instruments try to account for varying temperatures by adding sample heaters or by modifying the delay time set aside for color development as a function of sample temperature. Most phosphate analyzers make allowances for this color development time and will accept inlet sample temperatures in the nominal 5-50°C (41-122°F) range. 15.3.4 Sample Temperature

Further sample temperature consideration must be given if the sample temperature is below the dew point of the surrounding environment. Under these conditions, moisture can condense on the surface of the sample cell, which will lead to unstable results. Many instruments are designed to accept an instrument air purge stream to lower the relative humidity of the sample compartment. Any leaks in the sample line or reagent lines within the compartment must also be immediately fixed to prevent condensation of water vapor on the sample cell. 15.3.5 Sample Volume and Introduction of Chemicals

A fixed volume of sample is required for the color determination. This is provided by a sample cell of known volume or a known volume sample loop (small bore tubing). The individual chemical reagents can either be added by a mechanical pump, an eductor which creates a low pressure region in the flowing liquid and sucks the chemical in, or through a pressurized system, which admits the reagent through a solenoid valve. These chemical reagents must be replenished on a regular basis—most instruments are designed for a 30-40 day supply of chemicals. Intimate mixing of the chemical and the sample is attained with either a magnetic stirrer or a turbulent flow mixing device in a loop flow technology. The proper wavelength of incident light is obtained by an interference filter and the differential light absorption is determined by a photomultiplier detector. This photometric response is then compared to a calibration curve derived using samples of known phosphate concentration. 15.3.6 Light Intensity

Many instruments also measure the intensity of light that passes through the sample before the color developing reagent is added (i.e. during the “zero reading”) to assure some minimum value

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EPRI Proprietary Licensed Material Phosphate

is obtained. If this light intensity is not adequate, an error message is often displayed that directs to user to “clean sample cell” or “replace lamp”.

15.4 Interferences 15.4.1 Particulate Matter

All colorimetric tests must also account for particulates in the sample which result in light scatter across the sample cell. The particulates cause an opacity or cloudiness in the sample which can hinder the analytical measurement. This light scatter can be misinterpreted as light absorption which will lead to erroneously high phosphate content. Most instruments try to minimize this interference by taking a “zero reading” or “base-line reading” of the sample immediately before adding the color developing reagent. This is certainly a valid and desirable step in the measuring process. However, the analytical reading is usually made after the sample has been allowed to stand undisturbed for some length of time. If particles that were suspended at the time of the “zero reading” have settled at the time of the analytical reading, an erroneous quantification will occur. Particulates can thus lead to inconsistencies in analytical results. While instruments claim their ability to not have particulate matter interference, the user should make every effort to filter particulates before doing the colorimetric test. 15.4.2 Sample Discoloration

Sample discoloration can also affect on-line phosphate analyzers. While establishing the “zero reading” as mentioned above tends to cancel out the effect of background color, discoloration in the same approximate wavelength (e.g. blue or yellow) can shift the calibration line and limit the maximum range of quantification. The detector senses that the instrument is no longer in the straight line region of the calibration curve and refuses to quantify the result; it gives an “overrange” message which disallows the test. 15.4.3 Other Substances

Positive interference is caused by silica and arsenate only if the sample is heated. Negative interference is caused by arsenate, fluoride, thorium, bismuth, thiosulfate, thiocyanate, or excess molybdate. Blue color is caused by ferrous ion but this does not affect results if ferrous ion concentration is less the 100 mg/L (ppm). Another potential interference in the molybdovanadate based phosphate test is the presence of sulfide. No provision is made for counteracting this negative interference in the on-line instrument but the presence of sulfide in most process streams should not occur. A bench test with a Bromine Water pre-step should be used as a cross check if sulfide is suspected to quantify the level of the interference [7].

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EPRI Proprietary Licensed Material Phosphate

15.5 Calibration On-line phosphate monitoring instruments have both manual and automatic calibration capabilities. A typical automatic calibration routine is to have a weekly verification using an internal standard. Frequently, this calibration uses a standard purchased from the instrument manufacturer/supplier. The automatic calibration is usually an analysis of a single standard within the linear range according to the Beer-Lambert Law to establish the response slope. The second point of the calibration line is the x-y origin prior to addition of the color development reagent. The standard concentration is traditionally rather high on the absorbance / concentration curve to assure stability of the standard. This can lead to accuracy issues if the analytical result of the process stream is only 1 or 2 % of the standard concentration. A recommended second step would be to introduce a freshly-made standard into the instrument closer to the analytical value of the process stream for accuracy verification. Several of the on-line phosphate monitors have provision to analyze a grab sample or verification sample independent of the internal solution which is used for the automatic calibration. These on-line results are then compared to those obtained from standard laboratory analysis techniques (such as ASTM D515) [3].

15.6 Calibration Check On-line phosphate instrument analytical capabilities should be checked periodically to demonstrate calibration stability. Two methods exist for verifying instrument stability; the Standard Injection Method [8] or the Line Method [9]. For the Standard Injection Method, a known standard solution, near the mid-point of the calibration curve, is analyzed by the on-line instrument and the results are compared to the acceptance criteria. Acceptance criteria are either based on statistically derived limits. i.e., ± 3 sigma or based on some predetermined limits established from experience, i.e., ± 10%. Provided the on-line analyzer agrees within the acceptance criteria, the on-line instruments calibration is considered still acceptable. If the results are outside the acceptance criteria, the on-line instrument must be recalibrated. For the Line Method, a calibrated separate phosphate analyzer, typically a bench top analyzer using ASTM D515 is used to analyze a sample from the same sample stream as the installed online instrument [3]. Provided the bench top analyzer agrees within the acceptance criteria (e.g., matches the results of the on-line analyzer within ± 3 sigma or ± 10%) the on-line instruments calibration is considered still acceptable. If the results are outside the acceptance criteria the online instrument must be recalibrated.

15.7 Alternative Methods While different manufacturers use slightly different color development reagents and protocol (adding ammonium molybdate and a reducing agent to produce a blue phosphomolybdate complex or adding an acidified mixture of ammonium molybdate and ammonium metavanadate 15-7

EPRI Proprietary Licensed Material Phosphate

to produce a yellow molybdovanadophosphoric acid complex) on-line phosphate analysis is typically done by colorimetric comparison. It should be noted that ion chromatography (IC) is also a widely recognized technique for anion analysis and phosphate content may be determined by this technique [10]. From a practical standpoint, ion chromatography is a more sophisticated analytical technique which relies on a partitioning of the various anions in a sample and a sequential arrival of each anion at the detector over a period of time. The technique can be highly sensitive with detection levels in the 100 µg/L (ppb) range for phosphate. IC would most likely be used for on-line phosphate determination when several trace anions were of analytical interest (e.g. acetate, formate, chloride, and sulfate) and obtaining a phosphate value would be incidental in the scan. The elution time (delay time from sample injection until arrival at the detector) for phosphate is typically from 10 to 30 minutes depending on chromatographic column conditions. Similarly, the phosphate concentration in most boiler samples is one to two orders of magnitude greater than the trace ions of analytical interest. Analytes of widely differing concentrations can often be a problem in IC as the high concentration causes severe tailing, destabilizes the column, and requires a long rinse time. Ion chromatography continues to be developed for the electric utility industry and may fill special needs in anion quantification.

15.8 End User Considerations The performance characteristics (quantification range, accuracy, precision, bias, drift) and design characteristics (cycle time, selection of reagents, reagent consumption, sample manipulation, sample conditioning, and chemical interferences) for the phosphate monitor as provided by the manufacturer or supplier should be considered when selecting a suitable instrument. Other on-line phosphate monitor considerations for boiler water samples include: •

Inlet sample flow and pressure requirements.

•

Physical space requirements and mounting configuration (surface mounted or recessed into a sample panel).

•

Digital Control System (DCS) interface compatibility.

•

Provision for adequate sample and spent reagent drain.

•

Provision for instrument purge air/pressurization air if required.

•

Ability to perform external validation with grab samples or standards.

15.9 Field Experience Several errors are most likely to occur with a colorimetric phosphate analyzer. They can be collected into sample delivery or analyzer malfunction errors.

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EPRI Proprietary Licensed Material Phosphate

15.9.1 Sample Delivery

•

All analyzers must rely on sample delivery at an adequate flow rate to assure adequate flushing between samples. The approach used by one manufacturer is to monitor the inlet sample pressure. Required inlet sample pressure is from 3.45 kPa ± 2.07 kPa (5 ±3 psi). Another manufacturer has a constant head system with an overflow weir in the sample reservoir. The reservoir also has a float switch installed to indicate a “no sample’ condition. Both designs effectively stop the analyses routine if sample flow is not detected. It is desirable to have an alarm contactor available to signal this “trouble” condition to the DCS and subsequently to the control room / lab. On systems without this relay, a phosphate analyzer can effectively sit idle for long periods of time and the malfunction will go unnoticed.

•

As discussed previously, the phosphate monitor attempts to cancel out the effects of suspended particulates in the sample by taking a “zero reading” immediately prior to color development. However, accumulation of particulates in the sample lines and colorimeter cells can cause very erratic and erroneous readings. Special precautions should be taken during cyclic operation and start-ups to filter the suspended material prior to the phosphate analyzer.

15.9.2 Analyzer Malfunction

•

The most common problem with colorimetric phosphate analyzers has to do with liquid handling within the instrument. Depending on the analyzer design, the list of possible problems includes pinch valves that leak, sample lines that plug or develop pin holes, reagent lines that become unattached or develop leaks, eductors that become plugged and peristaltic pumps that develop leaks. Many of these malfunctions will be evidenced by liquid lying in the bottom of the analyzer or running out onto the floor. Less obvious malfunctions will be evident by the analyzer failing to calibrate, producing negative values, or yielding wildly fluctuating readings.

•

Reagents and standards must be replenished as needed. Reagent and standard consumption without appropriate replenishment may cause malfunctions in all designs. One design also has an internal reservoir of demineralizer water used for dilution of concentrated samples. Diligence is required to keep this dilution bottle filled to allow the analyzer to work properly.

Calibration and maintenance procedures and schedules are typically described in literature supplied with the monitoring equipment by each manufacturer. In addition, guidelines for maintenance and calibration activities are recommended in EPRI Report GS-7556 [11].

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15.10 References 1.

Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment, EPRI, Palo Alto, CA:2004. 10004188.

2.

Cycle Chemistry Guidelines for Combined Cycle / Heat Recovery Steam Generators (HRSGs), EPRI, Palo Alto, CA: 2006. 1010438.

3.

ASTM D515-88, Standard Test Methods for Phosphorous in Water, Method A: Colorimetric Ascorbic Acid Reduction, American Society for Testing and Materials, Philadelphia, PA.

4.

ASTM D515-88, Standard Test Methods for Phosphorous in Water, Method C: Colorimetric Molybdovanadophosphate Method, (method discontinued in 1988) American Society for Testing and Materials, Philadelphia, PA.

5.

Standard Methods for the Examination of Water and Wastewater, 21st Edition, 2005.

6.

Hach Series 5000 High Range Phosphate Analyzer; Model 60001 Instrument Manual, Second Edition, Rev.2, 11/2000.

7.

Standard Methods for the Examination of Water and Wastewater, 21st Edition, 2005.

8.

Advanced Power Plant Chemistry QA/QC Practices, Scientech, LLC, Clearwater, FL, 2006.

9.

ASTM D3864-96(2000), “Standard Guide for Continual On-Line Monitoring Systems for Water Analysis”, American Society for Testing & Materials, Philadelphia, PA.

10. Application Note 146—Determination of Trace Anions in High Purity Waters by Ion Chromatography with the IonPac AS17 Using High Volume Direct Injection with the EG40, Dionex Corporation. 11. Monitoring Cycle Water Chemistry in Fossil Plants, Vol. 3 Project Conclusions and Recommendations, by A.F. Aschoff, D.M. Sopocy, D.T. Eglar, O. Jonas, J.K. Rice, C.C. Stauffer, and W.E. Allmon. EPRI, Palo Alto, CA: October 1991. GS-7556.

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16 SILICA

16.1 Purpose and Use While silica is not listed as an EPRI Guideline Core Monitoring Parameter [1-3] for fossil plants it is widely monitored for diagnostic and troubleshooting purposes. It is included as a Core Monitoring Parameter for combined cycle plants [4]. Elevated silica concentrations in steam can lead to silica deposition in the low pressure turbine so it is desirable to control the silica content of boiler water. Silica content is also the primary indication of anion resin exhaustion in both makeup and condensate polishing equipment: under most conditions, an increase in silica content at the anion resin outlet will occur prior to a change in either pH or conductivity. Silica is most frequently monitored in the plant for the following reasons: •

To warn of in-leakage of contaminants.

•

To facilitate the correlation of a water chemistry parameter with plant operating variables, with an aim to optimizing operations.

•

To check the accuracy of water chemistry control (for silica), so ensuring that carry-over and deposit rates are kept at acceptable low levels.

•

To warn of condensate polisher malfunction.

The data generated by continuous on-line monitoring of silica is used by plant chemistry and operations department personnel. The goal for plant personnel is to maintain silica below prescribed limits.

16.2 Description of Method On-line silica analyzers make use of the heteropoly blue method—a colorimetric analysis procedure described in ASTM D859-05 [5] in which the soluble silica reacts with molybdate ion to form a greenish-yellow silicomolybdate complex, which in turn is converted to a blue complex by reduction with 1-amino-2-napthol-1-sulfonic acid or ascorbic acid. A spectrophotometer is used to measure the absorbance of this blue sample at a wavelength of about 815 nm. The difference between this absorbance and that of a reagent blank is directly related to the silica content of the sample. According to ASTM D859, the useful measurement range is 20 to 1000 µg/L (ppb) but manufacturers of on-line analyzers have extended this range to 0 to 5000 µg/L (ppb).

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EPRI Proprietary Licensed Material Silica

The molybdate-reactive forms of silica measured by this method include dissolved simple silicates, monomeric silica and silicic acid, and an undetermined fraction of polymeric silica. These forms generally predominate in power generation cycles although non-reactive (see Section 16.4.2 Non-reactive Silica) colloidal (crystalloidal) silica is sometimes present in the make-up water supply. On-line silica analysis is similar to the laboratory analysis technique described in ASTM D859 but on-line analyzers are equipped with an automatic fluid handling system. A sample bypass flow system is provided so that the on-line analyzer has continuous access to a fresh sample of the water containing reactive silica. The analyzer transports the sample through the system with inlet sample pressure, with a peristaltic pump or with a positive displacement piston pump; reagents may be transported by similar pumps or by compressed air. The reagents are added sequentially in three stages and, at each stage, the solutions are mixed and are given a specific time to react before continuing to the next stage. Not all analyzers are equipped to null the effects of phosphate, so it is important to select a model that handles this interference if samples are likely to contain phosphate; for instance, for boiler water samples taken from drum units on phosphate treatment. As the sample enters the analyzer, it is mixed with ammonium molybdate solution and either hydrochloric or sulfuric acid. This acidic molybdate solution is allowed to react for a specific period (a few minutes) with any dissolved silica and phosphate to form molybdosilicic and molybdophosphoric acid complexes, respectively. For instance, hydrated silicic acid reacts with the acidified ammonium molybdate solution to form the molybdosilicic acid complex and ammonium sulfate byproduct as follows: H 8SiO 6 + 12(NH 4 ) 2 MoO 4 + 12H 2SO 4 → H8 [Si(Mo 2O 7 ) 6 ] + 12(NH 4 ) 2 SO 4 + 12H 2O Equation 16-1

Then oxalic acid is added and thoroughly mixed with the sample to halt the development of the molybdosilicic acid complexes and destroy any molybdo-phosphoric acid complexes. Citric acid may be used instead of oxalic acid, although it is not as efficient at masking the phosphate interference. A light absorbance measurement at a wavelength in the range of 810–820 nm is made on a reagent blank to provide a zero reference for the sample. Then 1-amino-2-napthol-1sulfonic acid or ascorbic acid is added to the sample to reduce the yellow colored molybdosilicic acid complex to a heteropoly blue colored solution. A second light absorbance measurement at the same wavelength is made and compared with the reference measurement. The difference between the two measurements determines the silica concentration, and the result is displayed directly in units of concentration (for instance, µg/L (ppb)). Phosphate, at concentrations >50 µg/L (ppb), is a primary interference in this analysis but it can be chemically eliminated by the addition of oxalic acid or citric acid.

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16.3 Technical Considerations 16.3.1 Colorimetric Limitations

All colorimetric tests such as silica rely on the principle of Beer’s Law (also known as the BeerLambert Law) which states that the amount of light absorbed by a sample (A) is proportional to some absorption constant (a), the path-length of light (b), and the concentration of the analyte species(c). A=axbxc

Equation 16-2

The absorption constant is determined by the color species and test conditions of the method and remains constant as long as the reagents, test conditions, and wavelength of light do not change. The path length (the amount of sample through which the light passes) is also constant for a given sample cell. The mathematical formula then infers that the amount of light absorbed by a sample (i.e., the intensity of the color developed in the test) is directly proportional to the concentration of the analyte—in this case, silica. As the silica concentration increases, the intensity of the blue color will increase and this color change can be quantified. Limitations on this analytical principle arise at both the low end and high end of the useable range. The low end limit is caused by the detection limit of the test—this is primarily a function of the photo-multiplier (detector) sensitivity and stability. How small a change can be detected by the sensor and how stable is the baseline (or the zero reading)? When trying to quantify a test at the detection limit, sometimes a negative reading is observed or duplicate readings may have large relative errors. As a result, most silica analyzers will list a detection limit, an accuracy limit, and a precision limit. This accuracy limit is typically an absolute value or some percentage of the reading. A commonly used analyzer has the following specifications which demonstrate this technical consideration [6]. •

Minimum Detection Limit :