EPRI Closed Cooling Water Chemistry Guideline, Revision 1

Closed Cooling Water Chemistry Guideline, Revision 1

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Closed Cooling Water Chemistry Guideline, Revision 1 Revision 1 to TR-107396, Closed Cooling Water Chemistry Guideline 1007820

Final Report, April 2004

EPRI Project Manager T. Eckert

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • 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 Water Technology Consultants, Inc.

ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax). Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2004 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS This report was prepared by Water Technology Consultants, Inc. 27515 Mountain Park Road Evergreen, Colorado 80439 Principal Investigator K. A. Selby This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Closed Cooling Water Chemistry Guideline: Revision 1 to TR-107396, Closed Cooling Water Chemistry Guideline, EPRI, Palo Alto, CA: 2004. 1007820.

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PRODUCT DESCRIPTION

This Closed Cooling Water Chemistry Guideline addresses the use of chemicals to mitigate corrosion, fouling, and microbiological growth in the closed cooling water (CCW) systems of nuclear power plants. The chemical additives used for these purposes depend on plant design characteristics, water quality, operating parameters, and the specifications of Nuclear Steam Supply System (NSSS) suppliers. The list of chemicals is not as extensive as that for service water systems but can be confusing to utility CCW system engineers and chemists. Both generic chemicals and proprietary blends are used in CCW systems. Acquiring chemicals with sufficiently low levels of contaminants (for example, halides and metals) to meet NSSS suppliers’ original specifications is an issue in some nuclear plants. This concern relates particularly to many microbiological control agents. The use of chemicals in CCW systems will not resolve problems originating from poor design or maintenance practices that result in excessive system leaks. Excessive leakage makes consistent chemical control very difficult and offsets the ability of the chemicals to provide adequate protection. Results and Findings The use of operating ranges for the various treatment chemicals discussed in this guideline will allow a nuclear power plant to limit corrosion, fouling, and microbiological growth in CCW systems to acceptable levels. The recommended frequencies of analysis for Control and Diagnostic Parameters document satisfactory chemical control of the systems. This revision of the Closed Cooling Water Chemistry Guideline provides recommendations for closed cooling systems of all manufacture and design. Challenges and Objectives This guideline will be of value to nuclear power plant chemical personnel, engineering personnel with closed cooling system responsibility, maintenance personnel, and management personnel. The guideline provides the technical basis for a reasonable but conservative set of chemical treatment and monitoring programs. It will assist station management in endorsing a chemistry program to optimize equipment life, reduce chronic operating problems, and provide assistance with issues regarding regulatory requirements and plant life extension. Because of the wide range of operating conditions for closed cooling water systems and the differences in materials of construction and chemical treatment regimes, each station must develop its own site-specific chemistry program.

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EPRI Perspective This first revision of the Closed Cooling Water Chemistry Guideline has been endorsed by the utility chemistry community and represents another step in developing a more proactive chemistry program to limit or control closed cooling system degradation with increased consideration of corporate resources and plant-specific design and operating concerns. Each utility should examine its plant-specific situation to determine which recommendations should be implemented. Approach A committee of industry experts—including utility specialists, Institute of Nuclear Power Operations representatives, water treatment service company representatives, consultants, a primary contractor, and EPRI staff—collaborated in reviewing available data on closed cooling water system corrosion and microbiological issues. From these data, the committee generated water chemistry guidelines that should be used at all nuclear plants. Recognizing that each nuclear plant owner has a unique set of design, operating, and corporate concerns, the Guidelines Committee developed a methodology for plant-specific optimization. Keywords Closed cooling systems Corrosion Nitrites Chromates Molybdates Hydrazine

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ACKNOWLEDGMENTS The following individuals were ongoing members of Plant Support Engineering’s Closed Cooling Water Chemistry Guideline Revision 1 Task Group. As such, they have made significant contributions to the development of this guideline by attending the majority of the task group meetings, reviewing and commenting on various drafts, and writing portions of the document. Ms. Debra J. Bodine, Chairperson

Tennessee Valley Authority

Mr. Stephen Preece Ms. Josée Chalut Mr. John P. Terry Mr. Jack Bills Mr. Edward C. Frese Mr. Robert A. Martin Mr. Richard P. Gerdus Mr. William Dunphy Ms. Barbara Homyk Mr. Daryl Marse Mr. Timothy Eckert Mr. S. Chris Baker Mr. Thomas Britt Dr. Robert Winters Ms. Equilla Minga Mr. Edward A. Hinshaw Mr. Frank Sperlak Mr. Kenneth H. Kingston Mr. Gregory C. Broxterman Mr. Peter Ten Eyck Mr. Trevor Rebel Mr. Daniel W. Miller Mr. Michael L. Millinor

British Energy Generation, Ltd. Buckman Laboratories, Inc. Buckman Laboratories, Inc. Constellation Nuclear Dominion Generation Duke Energy Corp. Entergy Nuclear Northeast Entergy Nuclear Northeast Entergy Nuclear Northeast Entergy Operations, Inc. EPRI Charlotte Exelon Nuclear Exelon Nuclear FirstEnergy Nuclear Operating Co. Institute of Nuclear Power Operations Nuclear Management Co. Nuclear Management Co. Omaha Public Power District Nalco Company Nalco Company Pacific Gas & Electric Co. PPL Susquehanna, LLC Progress Energy, Inc. vii

Mr. Rocky H. Thompson Dr. Richard W. Lutey Dr. Robert Litman Mr. Stephen E. (Chip) Citzler Mr. Timothy M. Sarette Mr. Michael Perry Mr. Dennis D. Rickertsen Mr. Joel R. Robinson Mr. Robert L. Theimer Mr. K. Anthony (Tony) Selby

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Progress Energy, Inc. R. W. Lutey & Associates, Inc. Radiochemistry Laboratory Basics South Texas Project Nuclear Operating Co. Southern California Edison Co. Southern Nuclear Operating Co. Southern Nuclear Operating Co. Southern Nuclear Operating Co. TXU Generation Co., LP Water Technology Consultants

EPRI Licensed Material

CONTENTS

1 INTRODUCTION ....................................................................................................................1-1 1.1

Background..................................................................................................................1-1

1.2

Terms and Definitions ..................................................................................................1-2

1.2.1

Control Parameters .............................................................................................1-2

1.2.2

Diagnostic Parameters........................................................................................1-2

1.2.3

Investigative Parameters.....................................................................................1-2

1.2.4

System Ranking by Tiers ....................................................................................1-3

Tier 1 System...............................................................................................................1-3 Tier 2 System...............................................................................................................1-3 Intermittent System ......................................................................................................1-3 1.2.5

Remaining Plant Systems ...................................................................................1-3

1.2.6

CCW Action Levels .............................................................................................1-4

CCW Action Level 1.....................................................................................................1-4 CCW Action Level 2.....................................................................................................1-5 1.3

Contaminant Specifications .........................................................................................1-5

1.4

Purpose .......................................................................................................................1-6

1.5

Scope...........................................................................................................................1-6

1.6

Assumptions ................................................................................................................1-7

1.7

Management Responsibility and Program Ownership.................................................1-7

1.7.1

Water Chemistry Management Philosophy.........................................................1-7

1.7.2

Generic Management Considerations.................................................................1-8

1.7.3

Training of CCW Chemistry Control Personnel...................................................1-9

1.8

Roles and Responsibilities of Plant Personnel ..........................................................1-10

1.8.1

Operations.........................................................................................................1-10

1.8.2

System Engineering ..........................................................................................1-10

1.8.3

Chemistry ..........................................................................................................1-10

1.8.4

Maintenance......................................................................................................1-10

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2 DESCRIPTIONS OF CLOSED COOLING WATER SYSTEMS .............................................2-1 2.1

Purpose of Closed Cooling Water Systems.................................................................2-1

2.2

Closed Cooling Water System Design.........................................................................2-1

2.3

Variations of Closed Cooling Water System Design....................................................2-2

2.4

System Materials .........................................................................................................2-3

2.5

Classification of Closed Cooling Systems....................................................................2-6

3 PROBLEMS IMPACTING CHEMISTRY IN CLOSED COOLING WATER SYSTEMS..........3-1 3.1

Corrosion .....................................................................................................................3-1

3.1.1

Mechanisms ........................................................................................................3-1

3.1.2

Impact of Corrosion.............................................................................................3-1

3.2

Microbiological Growth.................................................................................................3-2

3.3

Fouling .........................................................................................................................3-2

3.4

Operational and Mechanical Considerations ...............................................................3-3

4 TECHNICAL BASIS OF TREATMENT CHEMICALS USED FOR CORROSION CONTROL .................................................................................................................................4-1 4.1

Introduction ..................................................................................................................4-1

4.2

Makeup Water Sources ...............................................................................................4-2

4.3

Chemicals Used for Corrosion Control ........................................................................4-3

4.3.1

General ...............................................................................................................4-3

4.3.2

Commonly Used Ferrous Alloy Corrosion Inhibitors ...........................................4-4

Critical Concentration of Nitrite, Molybdate, and Chromate – Technical Basis............4-4 Nitrites..........................................................................................................................4-5 Molybdate ....................................................................................................................4-7 Nitrite/Molybdate ........................................................................................................4-10 Chromate ...................................................................................................................4-13 Hydrazine...................................................................................................................4-14 Silicates .....................................................................................................................4-16 4.3.3

Copper Alloy Corrosion Inhibitors .....................................................................4-17

4.4 Closed Cooling Water System Corrosion Control Without Chemicals (Pure Water Systems)...................................................................................................................4-19 4.5

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Blended Glycol Formulations .....................................................................................4-21


5 TREATMENT PROGRAM AND MONITORING RECOMMENDATIONS ..............................5-1 5.1

CCW Action Levels ......................................................................................................5-1

5.1.1

CCW Action Level 1 ...........................................................................................5-2

5.1.2

Action Level 2.....................................................................................................5-2

5.2

Nitrite-Based Programs................................................................................................5-2

5.3

Molybdate-Based Programs ........................................................................................5-4

5.4

Nitrite/Molybdate Programs .........................................................................................5-5

5.5

Chromate-Based Programs .........................................................................................5-6

5.6

Hydrazine-Based Programs.........................................................................................5-7

5.7

Silicate-Based Programs .............................................................................................5-8

5.8

Pure Water Systems ....................................................................................................5-9

5.9

Blended Glycol Formulations .....................................................................................5-10

6 TECHNICAL BASIS OF CHEMICALS USED FOR MICROBIOLOGICAL AND FOULING CONTROL ................................................................................................................6-1 6.1

General ........................................................................................................................6-1

6.2

Non-Oxidizing Biocides................................................................................................6-2

6.2.1

Glutaraldehyde....................................................................................................6-2

6.2.2

Isothiazolone .......................................................................................................6-4

6.2.3

Methylene Bis(Thiocyanate)................................................................................6-5

6.2.4

DBNPA................................................................................................................6-6

6.2.5

THPS...................................................................................................................6-7

6.2.6

Ionine Quaternary Compounds ...........................................................................6-8

6.2.7

Dodecylguanidine Hydrochloride (DGH) ...........................................................6-10

6.2.8

Carbamates.......................................................................................................6-10

6.2.9

Quaternary Ammonium Salts (Quats) ...............................................................6-10

6.2.10 6.3

Tris-Nitro ........................................................................................................6-10

Oxidizing Biocides .....................................................................................................6-10

6.3.1

Hydrogen Peroxide ...........................................................................................6-10

6.3.2

Chlorine Compounds ........................................................................................6-11

6.4

Other Treatments.......................................................................................................6-11

6.5

Methods Used for Fouling Control .............................................................................6-12

6.5.1

General .............................................................................................................6-12

6.5.2

Suspended Solids Deposition ...........................................................................6-12 Water-Borne Foulants from Outside the System ..................................................6-12

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Foulants Generated Within the System ................................................................6-13 6.5.3

Chemical Foulant Control Methods...................................................................6-13

Dispersants ................................................................................................................6-13 Surfactants (Biodispersants)......................................................................................6-13 6.5.4

Feed and Bleed.................................................................................................6-14

Disadvantages ...........................................................................................................6-14 6.5.5

Drain and Refill..................................................................................................6-15

6.5.6

Sidestream Filtration and Sidestream Demineralization ...................................6-15

Advantages ................................................................................................................6-15 6.5.7

Other Mechanical Means of Suspended Solids Control....................................6-15

7 TECHNICAL BASIS OF MONITORING PARAMETERS.......................................................7-1

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7.1

Introduction ..................................................................................................................7-1

7.2

Testing Parameters......................................................................................................7-2

7.2.1

Corrosion Inhibitors .............................................................................................7-2

7.2.2

pH........................................................................................................................7-2

7.2.3

Percent Glycol.....................................................................................................7-2

7.2.4

Conductivity.........................................................................................................7-3

7.2.5

Cation Conductivity .............................................................................................7-3

7.2.6

Chloride and Fluoride (Halides) ..........................................................................7-3

7.2.7

Nitrate..................................................................................................................7-4

7.2.8

Ammonia .............................................................................................................7-4

7.2.9

Iron and Copper (Corrosion Products) ................................................................7-4

7.2.10

Microbiological Activity.....................................................................................7-5

7.2.11

Radionuclides ..................................................................................................7-5

7.2.12

Specific Microorganisms ..................................................................................7-6

7.2.13

Total Organic Carbon (TOC) ...........................................................................7-6

7.2.14

Dissolved Oxygen ............................................................................................7-6

7.2.15

Sulfate..............................................................................................................7-7

7.2.16

Calcium and Magnesium .................................................................................7-7

7.2.17

Refrigerant Chemicals .....................................................................................7-7

7.2.18

Reserve Alkalinity (RA) ....................................................................................7-7

7.2.19

Visual Evaluation of CCW Fluids .....................................................................7-8

7.3

Monitoring Frequency ..................................................................................................7-8

7.4

Investigative Parameters .............................................................................................7-8


8 ADDITIONAL MONITORING TECHNIQUES .........................................................................8-1 8.1

Corrosion.................................................................................................................8-1

8.1.1

Monitoring Techniques........................................................................................8-2

Corrosion Coupons ......................................................................................................8-2 Electrochemical Means................................................................................................8-3 Electrochemical Noise .................................................................................................8-4 8.2

Fouling .........................................................................................................................8-4

8.3

Microbiological Growth.................................................................................................8-5

8.3.1

Colony Counting Techniques ..............................................................................8-5

Adenosine Triphosphate (ATP)....................................................................................8-6 Dip Slides.....................................................................................................................8-7 Total Aerobic Plate Counts ..........................................................................................8-8 PetriFilm....................................................................................................................8-8 Test Kits .......................................................................................................................8-8 8.3.2

Sessile Bacteria Monitoring Techniques .............................................................8-8

Stainless Steel Corrosion Coupons .............................................................................8-8 Screened Coupons ......................................................................................................8-9 Glass Bead Holders .....................................................................................................8-9 Metal Specimen Holder................................................................................................8-9 Inspections...................................................................................................................8-9 8.3.3 8.4

Monitoring for Specific Microbial Species ...........................................................8-9

Non-Chemistry Monitoring .........................................................................................8-10

8.4.1

Inspections ........................................................................................................8-10

8.4.2

Nondestructive Evaluations (NDEs)..................................................................8-10

Ultrasonic Testing ......................................................................................................8-10 Electromagnetic Automatic Scanning ........................................................................8-11 Eddy Current Testing .................................................................................................8-11 Radiographic Testing .................................................................................................8-11 8.4.3

Leakage ............................................................................................................8-11

8.4.4

Heat Transfer ....................................................................................................8-12

8.5

Trending.....................................................................................................................8-12

8.6

Chemical Analysis......................................................................................................8-12

8.6.1

Ion Chromatography (IC) ..................................................................................8-13

8.6.2

Ion Selective Electrodes (ISE) ..........................................................................8-13

8.6.3

Atomic Absorption (AA) and Inductively Coupled Plasma (ICP) .......................8-14

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8.6.4

Ultraviolet/Visible Spectroscopy (UV/Vis) .........................................................8-14

8.6.5

pH and Conductivity ..........................................................................................8-15

8.6.6

Vendor Test Kits................................................................................................8-15

8.6.7

X-Ray Fluorescence (XRF) of Filtered Material ................................................8-16

9 TROUBLESHOOTING AND CORRECTIVE ACTIONS .........................................................9-1 9.1

Introduction ..................................................................................................................9-1

9.2

Nitrite-Treated Systems ...............................................................................................9-1

9.2.1

System Leakage .................................................................................................9-1

9.2.2

Water In-Leakage................................................................................................9-2

9.2.3

Bacterial Growth..................................................................................................9-2

9.2.4

Nitrate-Forming Bacteria Growth ........................................................................9-3

9.2.5

Ammonia-Forming Bacteria Growth....................................................................9-4

9.2.6

Acid-Forming Bacteria Growth ............................................................................9-4

9.2.7

Increase in System Corrosion Rates...................................................................9-5

9.2.8

Oil In-Leakage.....................................................................................................9-5

9.2.9

Contaminant Ingress ...........................................................................................9-6

9.3

9.3.1

System Leakage .................................................................................................9-6

9.3.2

Water In-Leakage................................................................................................9-7

9.3.3

Bacterial Growth..................................................................................................9-7

9.3.4

Increase in System Corrosion Rates...................................................................9-8

9.3.5

Oil In-Leakage.....................................................................................................9-8

9.3.6

Contaminant Ingress ...........................................................................................9-8

9.4

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Molybdate-Treated Systems ........................................................................................9-6

Nitrite/Molybdate-Treated Systems..............................................................................9-9

9.4.1

System Leakage .................................................................................................9-9

9.4.2

Water In-Leakage................................................................................................9-9

9.4.3

Bacterial Growth................................................................................................9-10

9.4.4

Nitrate-Forming Bacteria Growth ......................................................................9-10

9.4.5

Ammonia-Forming Bacteria Growth..................................................................9-11

9.4.6

Acid-Forming Bacteria Growth ..........................................................................9-12

9.4.7

Increase in System Corrosion Rates.................................................................9-12

9.4.8

Oil In-Leakage...................................................................................................9-13

9.4.9

Contaminant Ingress .........................................................................................9-13


9.5

Chromate-Treated Systems.......................................................................................9-14

9.5.1

System Leakage ...............................................................................................9-14

9.5.2

Water In-Leakage..............................................................................................9-14

9.5.3


9.5.4

Oil In-Leakage...................................................................................................9-15

9.5.5


9.6

Hydrazine-Treated Systems ......................................................................................9-16

9.6.1

Hydrazine Decomposition .................................................................................9-16

9.6.2

System Leakage ...............................................................................................9-16

9.6.3

Water In-Leakage..............................................................................................9-17

9.7

Silicate-Treated Systems ...........................................................................................9-17

9.7.1

System Leakage ...............................................................................................9-17

9.7.2

Water In-Leakage..............................................................................................9-18

9.7.3


9.7.4

Oil In-Leakage...................................................................................................9-19

9.7.5


9.7.6

Bacterial Growth................................................................................................9-19

9.8

Pure Water Systems ..................................................................................................9-20

9.8.1

System Out-Leakage ........................................................................................9-20

9.8.2

Water In-Leakage..............................................................................................9-20

9.8.3

Acidophilic Bacteria Growth ..............................................................................9-21

9.8.4

Ammonia-Forming Bacteria Growth..................................................................9-21

9.8.5


9.8.6

Oil In-Leakage...................................................................................................9-22

9.8.7


9.9

Inhibited Glycol Systems............................................................................................9-23

9.9.1

System Leakage ...............................................................................................9-23

9.9.2

Water In-Leakage..............................................................................................9-23

9.9.3

Glycol Breakdown .............................................................................................9-24

9.9.4


9.9.5

Oil In-Leakage...................................................................................................9-25

9.9.6


9.10

Flow Charts ...........................................................................................................9-25

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10 METHODOLOGY FOR PLANT-SPECIFIC TREATMENT OPTIMIZATION ......................10-1 10.1

Introduction ...........................................................................................................10-1

10.2

Determining Chemical Treatment Needs ..............................................................10-1

10.2.1

Metallurgy and Design ...................................................................................10-1

10.2.2

Makeup Water Source ...................................................................................10-2

10.2.3

Operating Conditions .....................................................................................10-3

10.2.4

System Conditions .........................................................................................10-4

10.2.5

Previous Treatment Regime ..........................................................................10-4

10.2.6

Compatibility ..................................................................................................10-5

10.2.7

Impact on Other Plant Systems .....................................................................10-5

10.2.8

Plant Life Extension Experience and Issues..................................................10-6

10.3

Environmental Impact and Health Effects .............................................................10-6

10.4

Nonchemical Treatment Considerations ...............................................................10-6

11 REFERENCES ...................................................................................................................11-1 A POSITION PAPER ON IMPURITY CONCENTRATION LIMITS FOR CLOSED COOLING WATER SYSTEMS................................................................................................. A-1 A.1

Executive Summary .................................................................................................... A-1

A.2

Introduction ................................................................................................................. A-2

A.2.1

Current Impurity Limits ....................................................................................... A-2

A.2.2

Role of Water Quality in SCC............................................................................. A-3

A.3

Technical Position....................................................................................................... A-3

A.3.1

Factors Affecting SCC of Stainless Steel Materials ........................................... A-3

A.3.2

Failure Modes with Stainless Steel System Materials........................................ A-4

SCC of Stainless Steel Tubing in Heat Exchangers ................................................... A-5 Chloride Ion Concentration in the Crevice .................................................................. A-5 Summary .................................................................................................................... A-6 A.4

Side Effects or Risks................................................................................................... A-6

A.5

Strategy ...................................................................................................................... A-6

A.6

Recommendations ...................................................................................................... A-7

A.7

References ................................................................................................................. A-7

B CHEMISTRY-RELATED PROBLEMS IN CLOSED COOLING WATER SYSTEMS ........... B-1 B.1

Corrosion Mechanisms ............................................................................................... B-1

B.1.1

xvi

General Corrosion.............................................................................................. B-1


B.1.2

Localized Corrosion ........................................................................................... B-1

Pitting Corrosion ......................................................................................................... B-1 Crevice Corrosion ....................................................................................................... B-2 Stress Corrosion Cracking (SCC) ............................................................................... B-2 Galvanic Corrosion ..................................................................................................... B-2 Microbiologically Influenced or Induced Corrosion (MIC) ........................................... B-3 Flow-Accelerated Corrosion (FAC) ............................................................................. B-3 Erosion........................................................................................................................ B-3 B.2

Impact of Corrosion..................................................................................................... B-4

B.2.1

Failures .............................................................................................................. B-4

B.2.2

Fouling ............................................................................................................... B-4

B.3

Microbiological Organisms.......................................................................................... B-5

B.3.1

Nitrogen – Biogeochemical Cycle ...................................................................... B-5

Step 1. Fixation of Molecular Nitrogen........................................................................ B-5 Step 2. Ammonification ............................................................................................... B-6 Step 3. Nitrification/Nitrosification ............................................................................... B-6 Step 4. Nitrate Reduction/Denitrification ..................................................................... B-7 Step 5. Recycling and/or Short Cuts........................................................................... B-9 B.3.2

Aerobic Mechanisms.......................................................................................... B-9

Iron and Manganese Bacteria..................................................................................... B-9 Sulfur-Oxidizing Bacteria .......................................................................................... B-10 B.3.3

Anaerobic Mechanisms.................................................................................... B-10

Sulfate-Reducing Bacteria ........................................................................................ B-10 Nitrifying and Denitrifying Bacteria............................................................................ B-10 B.3.4

Microbiologically Influenced/Induced Corrosion (MIC)..................................... B-10

B.3.5

Impact of Microbiological Fouling..................................................................... B-11

B.4

References ............................................................................................................... B-11

B.5

Bibliography ............................................................................................................. B-11

xvii


LIST OF FIGURES Figure 2-1 Typical Closed Cooling Water System......................................................................2-2 Figure 9-1 Troubleshooting Flow Chart for Sudden Shift in Inhibitor Concentration................9-26 Figure 9-2 Troubleshooting Flow Chart for Contaminant Increase ..........................................9-27 Figure 9-3 Troubleshooting Flow Chart for pH Change ...........................................................9-28 Figure 9-4 Troubleshooting Flow Chart for Change in Conductivity ........................................9-29 Figure 9-5 Troubleshooting Flow Chart for Increase in Corrosion Rate or Corrosion Product.............................................................................................................................9-30

xix


LIST OF TABLES Table 2-1 Approximate Percent (%) Composition of Metals Commonly Used in Closed Cooling Systems ................................................................................................................2-4 Table 2-2 Characteristics of Some Nonmetallic Materials Used in Closed Cooling Systems .............................................................................................................................2-5 Table 4-1 Typical Corrosion Inhibitors in Closed Cooling Water Systems .................................4-3 Table 5-1 Operating Ranges and Monitoring for Nitrite-Based Programs .................................5-3 Table 5-2 Operating Ranges and Monitoring for Molybdate-Based Programs ..........................5-4 Table 5-3 Operating Ranges and Monitoring for Nitrite/Molybdate Programs ...........................5-5 Table 5-4 Operating Ranges and Monitoring Frequencies for Chromate-Based Programs ......5-6 Table 5-5 Operating Ranges and Monitoring Frequencies for Hydrazine-Based Programs............................................................................................................................5-7 Table 5-6 Operating Ranges and Monitoring for Silicate-Based Programs ...............................5-8 Table 5-7 Operating Ranges and Monitoring for Pure Water Systems......................................5-9 Table 5-8 Control Ranges and Monitoring for Proprietary Blended Glycol Programs..............5-10 Table 5-9 Control Ranges and Monitoring for Nonproprietary and Plant-Specific Blended Glycol Programs...............................................................................................................5-11 Table 7-1 Contaminant Concentration Versus Calculated Cation Conductivity .........................7-3 Table 8-1 CCW Corrosion Coupon Rate Guidelines, Mils per Year (mpy) ................................8-3 Table 10-1 Critical Concentration of Chloride and Sulfate, Above Which Pitting of Armco Iron in Chromate or Nitrite Solutions Occurs (25oC) ........................................................10-2 Table A-1 Recommended CCW Impurity Limits ....................................................................... A-1 Table A-2 Impurity Concentration Levels Allowed by NSSS Vendors ...................................... A-2 Table B-1 Suggested Critical Velocity Limits for Condenser Tube Alloys in Sea Water .......... B-4

xxi


1

INTRODUCTION

1.1

Background

The cooling water needs in electric utility power plants are not limited to steam surface condensers. Other cooling needs include a variety of applications such as component cooling systems, emergency diesel generator jackets, enclosure coolers, and lubricating oil coolers. These cooling needs can be met by closed cooling water (CCW) systems. In nuclear power plants, the CCW systems provide cooling to both safety-related and non-safety-related heat exchange equipment. This report provides guidance on chemistry controls in these systems to minimize corrosion, fouling, and microbiological growth. It is written in a format similar to several other EPRI guidelines for chemistry control at nuclear power plants. However, this guideline does not come under the aegis of the NEI 97-06 [1] document that specifies adherence to guidelines. The purpose of the NEI 97-06 mandate in the other guidelines is to ensure steam generator long-term reliability at PWRs. This is done by describing control parameter limits that are to be adhered to without change. The systems described in this guideline are unrelated to the NEI 97-06 document requirements and, as such, do not fall under those requirements. Corrosion or fouling of the heat exchange equipment can interfere with its intended function. This is especially important in safety-related equipment. CCW piping consists of carbon steel in many (if not most) CCW systems. Carbon steel is subject to corrosion unless protected. In most cases, carbon steel corrosion control in CCW systems is achieved by adding corrosion inhibitor chemicals. In nuclear plants, these inhibitors have included chromates, nitrites, molybdates, hydrazine, and silicates. Inhibited glycol (ethylene glycol or propylene glycol) is also used in some systems. In addition, some plants achieve satisfactory control of corrosion without chemicals in pure water systems. Many plants used chromates initially but have converted to other inhibitors. Chromates are excellent corrosion inhibitors for carbon steel and have the added advantage of being toxic to microbiological organisms. Environmental issues led to a decline in the use of chromate inhibitors in the mid 1980s. This resulted in the replacement of chromates with alternate carbon steel corrosion inhibitors in many plants. These alternate inhibitors (nitrites, molybdates, silicates, etc.) can achieve comparable corrosion inhibition, but also introduce more variables because they do not inhibit the growth of microbiological organisms. EPRI has published a report that addresses the effectiveness of chromate and chromate replacements [2]. This Closed Cooling Water Chemistry Guideline addresses the use of chemicals to mitigate corrosion, fouling, and microbiological growth in nuclear CCW systems. The chemical additives used for these purposes depend on plant design characteristics, water quality, operating 1-1

EPRI Licensed Material Introduction

parameters, and specifications of the nuclear steam supply system (NSSS) suppliers. The list of chemicals is not as extensive as that for service water systems [3], but can be confusing to utility CCW system engineers and chemists. Both generic chemicals and proprietary blends are used in CCW systems. Acquiring chemicals with sufficiently low contaminant levels (for example, microbiological control chemicals) to meet some NSSS suppliers’ original specifications is an issue in some nuclear plants (refer to Section 1.3). The use of chemicals in CCW systems will not resolve problems originating from poor design or poor maintenance practices that result in excessive system leaks. Excessive leakage makes consistent chemical control very difficult and offsets the ability of the chemicals to provide adequate protection. The guidance provided here is targeted at reducing corrosion, microbiologically influenced corrosion mechanisms, and system fouling. It also provides examples of troubleshooting based on changes in chemical parameters and system operation. Deviating from the specifications outlined in this guideline, with the understanding and approval of plant management, is allowed, provided that sound technical justification and/or performance monitoring supports the deviation.

1.2

Terms and Definitions

1.2.1 Control Parameters Control Parameters specify the desired operating chemistry of the system. Control Parameters are those parameters that have a demonstrated relationship to closed cooling water system materials degradation. Examples of Control Parameters are pH, corrosion inhibitor concentration, and corrosive contaminants. There are limits associated with Control Parameters. Deviation from Control Parameters should be with the understanding and approval of plant management. 1.2.2 Diagnostic Parameters Diagnostic Parameters are important to monitor program effectiveness, identify programmatic problems, and assist in problem diagnosis. These parameters are routinely monitored to ensure system health or detect adverse trends. Diagnostic Parameters vary depending on the type of treatment program in use. Depending on the treatment program, Diagnostic Parameters might include conductivity, ammonia, nitrate, chloride iron, copper, sulfate, and microbiological activity. There are not limits associated with Diagnostic Parameters. 1.2.3 Investigative Parameters These nonroutine parameters are part of a toolbox of tests that can be used to respond to a problem or perceived problem. In certain cases, Investigative Parameters can be used along with Control and Diagnostic Parameters for system troubleshooting. Examples of Investigative Parameters include, but are not limited to, individual bacterial species, dissolved oxygen, total 1-2


organic carbon (TOC), and calcium and magnesium. Section 9 provides more discussion regarding the use of these Investigative Parameters in conjunction with the Control and Diagnostic Parameters. 1.2.4 System Ranking by Tiers This guideline ranks CCW systems by tiers. The purpose of ranking the systems by tiers is to provide for more frequent chemical analysis of safety-related systems or systems whose unavailability will challenge continued plant operations. The tier ranking of a system does not impact Control or Diagnostic Parameters or impact nominal system concentrations or Action Levels. Tier 1 System This constitutes a cooling or heating system whose unavailability will challenge continued plant operation. At a minimum, all systems that are safety related, or supporting a safety-related function, are designated as Tier I systems. Other circuits can be designated as Tier I by the station at its discretion. Tier 2 System This constitutes all remaining systems. Such systems do not usually challenge continued operation if unavailable. Intermittent System An Intermittent (intermittently operated) System can be either Tier 1 or Tier 2, depending on its intended function. It is operated on an intermittent basis as needed or for routine surveillance. Assignment as a Tier 1 or Tier 2 system does not impact the control ranges or parameters monitored but might impact the frequency of monitoring. 1.2.5 Remaining Plant Systems There can be closed cooling systems associated with a plant that are not normally supervised by plant technical personnel. Examples of these systems are office building and warehouse cooling and heating systems. These systems fall outside the scope of this guideline. However, there might be plant CCW systems that are in Level B storage. It might be necessary to address these in the site-specific strategic plan. Level B storage refers to ANSI 45.2.2 storage levels for nuclear components. Level B has the requirements of temperature within the range of 40–140°F (4–60°C), no control on humidity, indoors (walls and ceilings intact), and above the floor.

1-3


1.2.6 CCW Action Levels CCW Action Levels have been established for these guidelines so that the importance of out-ofspecification conditions can be effectively communicated to management. The CCW Action Levels have been written to parallel the actions that are taken in primary and secondary chemistry systems. However, because closed cooling systems operate at temperatures and pressures that are significantly lower than the primary or secondary systems of power plants, the timeframes in which significant effects can be realized are much longer. Revision 0 of this guideline did not provide guidance on the significance of an out-of-specification condition to corrosion in that system, or how soon the condition had to be corrected. This made it difficult to plan appropriate timeframes and work schedules for correcting these conditions. CCW Action Level 1 conditions are those that can be addressed using the 12-week work schedule concept, with no discernable increases in corrosion rates or system efficiency. CCW Action Level 2 communicates a more serious condition, requiring action outside the normal 12-week work schedule. The CCW Action Levels apply only to Control Parameters, which have defined limits. Diagnostic Parameters are used as a means to help identify the source of the off-normal condition, and are generally not a direct function of corrosion inhibition. Diagnostic Parameters have no CCW Action Levels. It is important to note that it is expected that these timeframes should be used as not-to-exceed dates, and that corrective actions will be completed as soon as work schedules permit. The various Control Parameters can have a negative synergistic effect on system corrosion rates, if two or more are outside the normal operating range at the same time (see discussion under the Action Level subsections). CCW Action Level 1 The normal operating ranges recommended in this guideline represent a conservative interpretation of closed cooling water chemistry as generally practiced in the nuclear power and other industries. Action Level 1 denotes a condition where system chemistry Control Parameters are outside the normal operating levels. The recommended action is to increase monitoring frequency, as appropriate, and return the parameter to within the prescribed normal operating range within 90 days. If the parameter has not returned to the normal operating range within 90 days, CCW Action Level 2 is entered. This timeframe is based on only one Control Parameter being in the Action Level range. If more than one is beyond established limits, a negative synergistic effect might result. Therefore, if more than one parameter is in CCW Action Level 1, it is recommended that more aggressive corrective actions be initiated than if only one parameter is in Action Level 1. Until parameters are returned to normal, increased sampling frequency should be considered part of those actions.

1-4


CCW Action Level 2 Values exceeding the CCW Action Level 2 threshold could initiate short-term system materials degradation. The recommended action is to return the parameter to within the prescribed control band within 30 days. If the parameter has not returned to the normal range after 30 days in CCW Action Level 2, and system operation is to continue, then a risk assessment (engineering evaluation) shall be performed indicating that the out-of-control parameter will not affect the long-term reliability of the system. The evaluation should include a sunset date for the out-ofcontrol condition. This timeframe is based on only one Control Parameter being in the Action Level 2 range. If more than one is beyond established limits, a negative synergistic effect might result. Therefore, if more than one parameter is in CCW Action Level 2, it is recommended that more aggressive corrective actions be initiated than if one parameter is in the action level. Until parameters are returned to normal, increased sampling frequency should be considered part of those actions.

1.3

Contaminant Specifications

Nuclear power plants in the United States have been supplied by four Nuclear Steam Supply System (NSSS) suppliers—Babcock & Wilcox, Combustion Engineering, General Electric, and Westinghouse. Although each of the NSSS suppliers has individual designs, the basic CCW system and its respective materials are fairly consistent. Each NSSS supplier has issued some type of guideline covering water quality parameters in CCW systems; however, these guidelines are inconsistent between suppliers. An example is chloride specification, which ranges from <10 ppm (General Electric) to ≤150 ppb (Westinghouse). A goal of the Closed Cooling Water Chemistry Task Group is to establish impurity limits that will protect system metallurgy but that are reasonable and achievable. With respect to 300 series stainless steels, impurity limits for chloride and fluoride are recommended to be 10 ppm each. These are shown in Table A-1 in Appendix A. As reflected in the Diagnostic Parameters for the various treatment programs, these limits are also considered suitable for the protection of carbon steel in systems using nitrite, molybdate, and chromate treatment programs. Lower limits are needed in systems using a hydrazine treatment program or systems using no added chemicals (pure water systems). The position paper from which the above limits are derived is located in Appendix A. The position paper also recommends periodic flushing to avoid sediment buildup in the bottom of exchangers and that the use of chlorinated solvents for degreasing or other cleaning of closed cooling water system components be prohibited.

1-5


1.4

Purpose

The purpose of the Closed Cooling Water Chemistry Guideline is to provide CCW system engineers and chemists with guidance for maintaining system chemistry in CCW systems. This guidance addresses: •

Establishing the technical basis for chemical treatment programs

•

Establishing site-specific programs

•

Establishing recommended control ranges for the various treatment programs

•

Establishing Control and Diagnostic Parameters for each type of treatment program

•

Defining Action Levels to protect the CCW systems

•

Establishing monitoring frequencies

•

Establishing appropriate trending programs

•

Providing guidance for troubleshooting and corrective actions

•

Determining compatibility with other system materials

•

Providing industry experience

•

Providing support for plant life extension

This guideline was developed for nuclear power plants; however, most of the information in the report is applicable to fossil fueled power plants. They also have CCW systems that are similar in design and experience many of the same problems.

1.5

Scope

The scope of the Closed Cooling Water Chemistry Guideline is to: •

Describe CCW systems

•

Provide appropriate definitions

•

Describe water-related problems in CCW systems

•

Provide a detailed guide to chemical selection

•

Provide guidance on selecting and monitoring a total water treatment program

•

Provide guidance on evaluating performance

Although the methods and technologies described in this guideline are extensive, they are not allinclusive. There are existing methods that are not described in this guideline, and other methods continually being developed that could be applicable.

1-6


1.6

Assumptions

The systems discussed in this guideline are limited to closed cooling water systems. However, there are closed low-temperature hot water systems (<82°C, 180°F) in some plants that are similar in design and operate in a similar manner to closed cooling systems. The chemical treatment programs described for closed cooling systems in this guideline are also generally applicable to closed hot water systems but should be evaluated based on system design. This guideline does not address steam surface condenser circulating water systems, open or raw water types of systems, although the treatment programs might be the same or similar. The guideline does not address Stator Cooling systems because these are addressed in other EPRI reports [4].

1.7

Management Responsibility and Program Ownership

It is recognized that a specific CCW monitoring program applicable to all plants cannot be defined due to differences in design, experience, management structure, and operating philosophy. However, there are standard programs that are used across the industry and most systems can use one of these standard programs. The overall goal is to minimize corrosion and maximize the availability and operating life of CCW systems. To meet this goal, an effective water chemistry control program is essential and should be based upon the following: •

Recognition of the long-term benefits of, and need for, proper water chemistry control to avoid or minimize corrosion degradation of piping, heat exchangers, and other components

•

Clear management support for operating procedures designed to establish and maintain water chemistry control (as provided in Sections 4, 5, 6, and 7)

•

Adequate resources of staff, equipment, funds, and organization

•

Careful evaluation of site-specific needs and circumstances

•

Continuing review of plant and industry experience and research

•

Initiating revisions to the program as appropriate

1.7.1 Water Chemistry Management Philosophy Management endorsement and support of the CCW strategies is key for successful implementation. Without it, failure is likely. Management should require predefined methods to establish and maintain good chemistry for asset preservation. Nuclear station management is periodically faced with the problem of either keeping a unit available to produce power and meet short-term power demands, or shutting down to repair system problems that affect good control of water chemistry, thus helping to ensure the longterm integrity of system components. The water chemistry philosophy must be effectively communicated to management so that it is understood that operating with off-normal chemistry might eventually result in loss of availability of that unit. Limiting the magnitude and duration of 1-7


off-normal chemistry can maximize plant availability and system integrity. A program document, which describes this philosophy, can effectively communicate these concepts to utility management. This document identifies any deviations that a plant takes from the control parameters in these guidelines, either permanently or for interim periods when plant conditions do not allow a prompt return to the control parameter band. Such deviations should be based on an engineering evaluation, which identifies how the proposed deviations do not have an effect on system operation. This would be of particular importance for those systems that fall under the Maintenance Rule. The information presented in this section is based on the observation that operating and maintenance philosophies regarding chemistry can significantly affect the integrity and longevity of CCW systems components. The philosophy and policies discussed reflect the desirability of operating in a proactive rather than a reactive mode. The costs associated with maintaining CCW chemistry within these industry recommendations are likely to be less than those associated with the repair or replacement of heat exchangers and piping, and the outages associated with those efforts. Although it is recognized that variety among individual utility organizations exists, there are basic goals and functions common to all. This section addresses key management considerations but makes no attempt to specify how they should be integrated into a specific organizational structure. Additional organizational and administrative guidelines are presented in the INPO Guidelines for Chemistry at Nuclear Power Stations, Rev. 2 (INPO 88-021, Rev. 2) October 1995. Utility personnel are encouraged to consider both the recommendations in this section and INPO recommendations when developing or revising their site-specific programs. 1.7.2 Generic Management Considerations This section lists and discusses the considerations that are common to most utilities, including the elements of organizations that are needed to carry out a water chemistry program effectively. Utility-specific implementation policies and procedures should assign responsibilities to specific positions within the organization. One major element of these guidelines is the need for every level of management to understand the importance of the site-specific performance goals and effective chemical treatment presented in Sections 4, 5, 6, and 7. Effective chemical treatment has great benefits for the utility company by having a positive impact on CCW system integrity. An important ingredient of a successful management plan for CCW chemistry control is a set of documents that specify operating guidelines. The documents should: •

State the need for the document

•

State the goal regarding CCW chemistry and station operation

•

Highlight management support for the document

•

Assign responsibility for: – –

1-8

Preparation and approval of procedures to implement policy Implementation of procedures


– – – –

Assessment of the effectiveness of chemistry control in minimizing component degradation Monitoring, analysis, and data evaluation for the chemistry program Surveillance and review functions Implementing corrective actions

Utility personnel responsible for plant design in chemistry-related areas should: •

Understand CCW systems materials and the operational chemical treatment relationship with the system materials

•

Ensure that the system design is reviewed by knowledgeable plant operating personnel, vendors, and/or consultants as appropriate

Procedures should include: •

A plant-specific monitoring program to ensure long-term system integrity

•

Control limits, sampling frequency, specific corrective action responses, and corrective action notification and responsibilities

•

Standardized analytical procedures to ensure accurate laboratory results

•

Provisions for data review

1.7.3 Training of CCW Chemistry Control Personnel A program for training all personnel involved with CCW chemistry control should be established. This program should incorporate the latest information available from EPRI, other utilities, NSSS suppliers, water treatment vendors, and industry expert consultants. Some instruction in the basics of the program should be considered for all employees who, by virtue of their job responsibilities, can affect water chemistry. The training programs should be designed for the level and qualifications of personnel being trained. The following elements should be included: •

A clear statement of the policy regarding CCW chemistry control, including clarification of the impact of this policy upon the various responsibility areas.

•

Identification of the impact of poor chemistry control.

•

Identification of the chemical treatment control ranges for each program in use in the plant.

•

Techniques for recognizing unusual conditions and negative trends, particularly for the station chemists and laboratory technicians. Potential corrective actions and their consequences should be thoroughly discussed.

•

The interaction of system operations, engineering, radiation protection, maintenance, and chemistry departments.

1-9


1.8

Roles and Responsibilities of Plant Personnel

1.8.1 Operations Operators are most familiar with system operational parameters of flow, pressure, and temperature. These characteristics can be indications of system concerns such as fouling of heat exchanger surfaces. It is important that changes be effectively communicated to the system engineer and chemistry personnel so that system chemistry changes can be assessed as part of the concern. Changes in system alignment (for example, idle pump to operate or alternate train placed into service) should be reported to the chemistry staff so that system chemistry changes can be taken into account. 1.8.2 System Engineering The visual inspection of system component internals is one of the most effective ways of assessing corrosion mechanisms in a plant system. System engineers are cognizant of ongoing work for standby trains of equipment and should routinely inspect system internal surfaces for unusual indications of system wear or corrosion. Their review of system thermal performance often provides useful information to chemistry personnel in assessing the effectiveness of chemical treatment. Plant chemistry staff should be notified of system inspections in order to participate in the evaluation using data from sampling and analysis of debris, deposits, or corrosion films as available. It is also important to note that if parts substitution arises due to obsolescence or wear of system components, it is valuable to have chemistry personnel assess the compatibility of new system materials with the corrosion control agents used in the system. 1.8.3 Chemistry Maintaining corrosion control agents in a specified concentration band most effectively controls general corrosion of plant systems. Large swings in concentrations of these control agents can be detrimental to the protective corrosion film that has been established in the system. Identification of trends, or changes in trends of these control agents, can provide insight to the system engineer or operators about potential system problems. Proper assessment of contaminant concentrations can also provide valuable information regarding system integrity. Thus, routine sampling and analysis, which is evaluated critically, helps to maintain long-term system integrity. 1.8.4 Maintenance The repair, replacement, or modification of a plant system can provide opportunities for plant staff to assess the health of a system. It is also an opportunity for unwanted materials to inadvertently enter the system. Many new or replacement components are manufactured and shipped with coatings or materials to preserve their integrity in the dry state. These materials must be removed prior to installation in the system. Maintenance procedures should address the types of chemicals that might be used to perform routine work on system components and how excesses of these materials are to be removed. It should be procedurally required that any solvents or chemicals that are used in system maintenance must be effectively removed prior to reassembling the component. Maintenance personnel can also provide valuable insight to operators, engineers, and chemists about changes in corrosion surfaces of the system. 1-10


2

DESCRIPTIONS OF CLOSED COOLING WATER SYSTEMS

2.1

Purpose of Closed Cooling Water Systems

There are a number of Closed Cooling Water (CCW) system designs. Some systems cool plant components that contain radioactive fluids and serve as barriers between radioactive fluids and the service water system. Other CCW systems are on the evaporator side of a refrigeration chiller (chiller systems) and some cool the jackets of emergency diesel generators. Some of the CCW systems cool safety-related components while others cool non-safety-related components. Lowtemperature (<180ºF, 82ºC) closed hot water systems (often for building heat) are often included, based on system design. A closed system is defined as “a heating or refrigerating piping system in which circulating water or brine is completely enclosed, under pressure above atmospheric, and shut off from the atmosphere, except that the expansion tank could be open to the atmosphere” [5]. A closed system has also been defined as “one in which the water is circulated in a closed loop with negligible evaporation or exposure to the atmosphere” [6].

2.2

Closed Cooling Water System Design

Figure 2-1 illustrates a typical Closed Cooling Water system. The individual heat exchangers are part of a closed loop. Heat from the closed loop is removed by a heat sink heat exchanger (raw water heat exchanger or refrigeration chiller). The scope of the Closed Cooling Water Chemistry Guideline does not, however, include the treatment of the service water side of the heat sink heat exchanger. This is covered in the Service Water System Chemical Addition Guideline [3]. Low-temperature closed hot water systems are also represented by Figure 2-1. As such, the principles described in this guideline can also be applied to low-temperature closed hot water systems.

2-1

EPRI Licensed Material Descriptions of Closed Cooling Water Systems

Figure 2-1 Typical Closed Cooling Water System

2.3

Variations of Closed Cooling Water System Design

In addition to water-cooled systems, there are some air cooled CCW systems. There is also at least one nuclear plant with a hybrid design that operates as a closed loop only during normal operation. Under emergency or shutdown conditions, the flow is diverted through an open recirculating cooling tower. This introduces contaminants during those periods of open recirculation operation. In addition, valve leakage could also introduce contaminants when the system is operated in the isolated mode. Although most CCW systems share the same basic design, there are numerous design variations. These include: •

Variations in flow rates

•

Idle (stagnant) heat exchangers

•

Low-flow heat exchangers

•

Variations in thermal load

•

Makeup water variations

•

Radiation fluxes

•

Variations in oxygen content

•

Shared heat exchangers (cross ties)

2-2


2.4

System Materials

The materials of construction chosen for nuclear CCW systems are a function of engineering factors, economic factors, and water quality factors. In most plant designs, the piping is typically unlined carbon steel or stainless steel. The heat exchanger tubesheets, channels, and channel heads are typically bare or coated carbon steel or stainless steel. Heat exchanger tube materials are typically a copper alloy (for example, Admiralty brass or copper-nickel) or a stainless steel. In addition to these materials, emergency diesels might use cast iron, aluminum, and soldered connections. Other materials that might be included in a CCW system include nonmetallic materials (gaskets, valve seats, pump seals, packing) and sacrificial anodes. Table 2-1 is a reference for many different types of metals used in CCW systems. The approximate concentrations of the composite materials for these alloys are given, as well as the general Unified Numbering System (UNS) designation. Per ASTM E527, “The unified numbering system (UNS) establishes 18 series of numbers for metals and alloys. Each UNS number consists of a single letter-prefix followed by five digits. In most cases the letter is suggestive of the family of metals identified; for example, A for aluminum, P for precious metals, and S for stainless steels.” Some alloys have trace elements (usually considered <0.01% by weight), which are purposely included, while, in other alloys, they are present because they are not excluded as part of the manufacturing process. The more important of these elements are included as notations in Table 2-1. This table can prove a useful aid to plant personnel who are planning system changes. New or replacement components for systems should have a UNS number specification. This can help plant personnel in assessing if the new or replacement part is suitable for the chemical control currently used in the CCW system. Alternatively, the component might be a significant enough change of material so that the chemical control agent should be changed to suit the new component. It might also be useful in assessing if there are other components in the CCW system that could form galvanic couples with the new material.

2-3

EPRI Licensed Material Descriptions of Closed Cooling Water Systems Table 2-1 Approximate Percent (%) Composition of Metals Commonly Used in Closed Cooling Systems Metal (1)/UNS (3)

Fe

Carbon Steel C00001-99999

98

Cast Steelc F11000-12000

98

Cu

Ni

Cr

Zn

~70

8–10

18–20

Stainless Steel (400) S40000

80

13

6

Stainless Steel (6% Molybdenum Austenitic) S31254 N08367 N08026

49 49 49

0.8 0.1 3

18 24.5 35

20 20.5 24

90/10 Cu/Ni C70600

0.4-1.0

89.0

9.5

70/30 Cu/Ni C71500w

0.5

68.0

30.0

0.01

71 71

28 28

60

39

C44500 (Metal D) Naval Brass (phosphorized) C46700 Phosphobronze C60000

Mo 6.1 Mo 6.1 Mo 6.1 0.1

0.1–0.3

<0.1– 0.7

0.4–1.0

0.05– 0.3

Pb trace

Sn=1.0, Pb=0.02, As=trace Sn, P trace Sn+P to 1%

Sn=10, P=0.1

99+

39

Pb+As+P+Sb up to 1% Traces of Ag, P, or As, depending on grade 5 7 8

2.5

95 91 92

1.4

31

66

85

5

Titanium R50250-R56322

Si=1.00 max

Sn=4.3, P=0.3

Copper C10000-14000

Aluminum-Nickel-Bronze C63000

2

95 60

Monel 400 N60000

Other (2)

90

Muntz Metal C36000

Aluminum Bronze C60800 C61300 C61000

Al

Carbon = 0.5

Stainless Steel (304) S30400

Admiralty Brass C44300 (Metal B)

Mn

Carbon-0.15 10 Up to 11

Up to 8

3–6

Ti 75-98, Many different alloys and range of percentages. Also alloys with V and Mo.

(1) UNS refers to the Unified Numbering System for Metals. This system was developed jointly by ASTM and SAE and is published under ASTM E527/SAE J1086. Some materials have a range of numbers listed. This signifies that there are several different alloys with similar material composition. The composition values in this table are the most commonly encountered in CCW systems. (2) Although these represent minor constituents, they are intentionally added to the metals to provide certain characteristics. (3) Listings of specific alloy compositions corresponding to specific UNS numbers can be found in “Unified Numbering System for Metals and Alloys,” ASTM DS 56H. w

= wrought, c = cast

Note: Weld materials are not included in the above table.

2-4


Table 2-2 lists names of some nonmetallic components commonly used in CCW systems. It also shows their generic chemical composition, temperature limits, and some chemical incompatibilities. The temperature limits are for the materials in pure water. Note that increased temperature might increase the susceptibility of these materials to degradation with certain chemicals. This chart should be used as a general guide, and new or replacement components with nonmetallic parts should be assessed for compatibility with system chemicals during the work control process. The incompatibilities are based on 48-hour exposure tests, usually at room temperature. Nonmetallic components can also provide a colonization site and food source for certain types of bacteria. Any change to the chemical regime or disinfection of a CCW system due to bacterial contamination should also take these system parts into account. Table 2-2 Characteristics of Some Nonmetallic Materials Used in Closed Cooling Systems Material Name (1) Neoprene

Composition

Temperature Limit, oF (oC) (2)

Use

Incompatibilities (3)

Chloro-butadiene polymer

250 (121)

Lining for piping or heat exchanger channel heads

Freon 11, diesel fuel, ammonia, morpholine

Viton

Perfluorinated hydrocarbon (carbon, hydrogen, fluorine)

350 (177)

Gaskets, O-rings

Ammonia, hydrazine, amines

EPDM

Hydrocarbon polymer consisting of ethylene and propylene chain components

250 (121)

Gaskets, O-rings, tubing, component linings

Teflon


600 (316)

Valve packing, gaskets, O-rings

Buna N

Hydrocarbon polymer with a nitrile functional group

350 (177)

Gaskets, O-rings

Certain Freons

Kalrez


425–600 (218– 316))

O-rings, gaskets

Solutions of some diamines >200oF (93oC) might cause more rapid deterioration

Polyvinyl chloride (PVC)

Saturated hydrocarbon backbone (carbon, hydrogen, chlorine)

390 (199)

Piping, valves

Sodium hypochlorite at 5% (embrittlement with long-term exposure), ethanolamine

Polypropylene

Carbon, hydrogen

250 (121)

Hoses

Concentrated sulfuric acid, ethanolamine, hydrogen peroxide

(1) Information regarding the standard abbreviations for these materials can be found in Standard Practice for Rubber and Rubber-Lattices-Nomenclature, ASTM D1418. (2) These are conservative limits for these materials. (3) A material’s incompatibility with specific CCW system chemicals should be verified with the material manufacturer.

2-5


Material composition and chemical reactivity information for Table 2-2 is taken from the following product literature: •

Dupont Dow Elastomers Product Bulletin H-68249, “Chemical Resistance and Fluid Compatibility, Including All Chemicals Under the Clean Air Act.”

•

Montell Polyolefins, Technical Bulletin ID# Tl-010 (1998), “Pro-Fax Polypropylene— Chemical Resistance.”

•

DuPont Dow Elastomers, Technical Information Bulletin, VT-500.1, (R3) (1997), “Viton®—Fluid Resistance Guide.”

•

Little Giant Pump Company, “Chemical Resistance Chart” (also found in Cole-Parmer 2003– 2004 Product Catalog).

•

Compatibility of PWR Gasket and Packing Materials and Ion Exchange Resins with Organic Amines, EPRI TR-100794, Revision 1, 1994.

2.5

Classification of Closed Cooling Systems

In some nuclear plant designs, there are a limited number of CCW systems. In other designs, there might be numerous CCW systems (in excess of 20 per unit in some designs). Some of the CCW systems are far more important to plant safety and reliable operation than others. This guideline ranks CCW systems by tiers. The purpose of ranking the systems by tiers is to provide for more frequent chemical analysis of safety-related systems or systems whose unavailability will challenge continued plant operations. The tier ranking of a system does not impact Control or Diagnostic Parameters, nominal system concentrations, or Action Levels. Tier 1 – A cooling or heating system whose unavailability will challenge continued plant operation. At a minimum, all safety-related systems are designated as Tier 1 systems. Other circuits might be designated as Tier 1 by the station at its discretion. This classification ensures sampling and testing of key parameters on a more frequent basis. Tier 2 – All remaining plant systems. Such systems usually do not challenge continued operation if unavailable. Intermittent – An intermittently operated system can be either Tier 1 or Tier 2, depending on its intended function. It is operated on an intermittent basis as needed or for routine surveillance. Assignment as a Tier 1 or Tier 2 system does not impact the control ranges or parameters monitored but might impact the frequency of monitoring. There might be closed cooling systems associated with a plant that are not normally supervised by plant technical personnel. Examples of these systems are office building and warehouse cooling and heating systems. These systems fall outside the scope of this guideline. However, there can be plant CCW systems that are in Level B storage. It might be necessary to address these in the site-specific strategic plan. Level B storage refers to ANSI 45.2.2 storage levels for nuclear components. Level B has the requirements of temperature within the range of 40–140ºF (4–60ºC), no control on humidity, indoors (walls and ceilings intact), and above the floor. 2-6


3

PROBLEMS IMPACTING CHEMISTRY IN CLOSED COOLING WATER SYSTEMS

Potential problems that could impact chemistry in CCW systems include corrosion, microbiological growth, and fouling. Scale formation, which is common in open recirculating systems, is typically not a problem in CCW systems because fresh water is not usually added on a regular basis. Mechanical problems can have a significant impact on chemistry in CCW systems. Chief among these is leakage from the system, which results in the addition of untreated water to the system. The following provides an overview of the chemistry-related problems in CCW systems. More detailed discussions are presented in Appendix B.

3.1

Corrosion

3.1.1 Mechanisms There are a variety of corrosion mechanisms that can occur in CCW systems. These include: •

General corrosion

•

Localized corrosion – pitting

•

Localized corrosion – crevice corrosion

•

Localized corrosion – underdeposit

•

Stress corrosion cracking

•

Galvanic corrosion (dissimilar metals)

•

Microbiologically influenced corrosion (MIC)

•

Flow-assisted corrosion (FAC, also known as erosion-corrosion)

3.1.2 Impact of Corrosion Excessive corrosion in closed cooling systems will eventually lead to failures, which are most likely to take the form of through-wall penetration. In extreme cases, a critical piece of equipment might be lost, which brings about a costly unscheduled outage such as the mid-cycle shutdown of a nuclear power plant. 3-1

EPRI Licensed Material Problems Impacting Chemistry in Closed Cooling Water Systems

Active corrosion within a CCW system can be of significant consequence long before actual failures occur due to the generation of corrosion products. These corrosion products are not readily removed by blowdown as in an open cooling water system. Unless a cleanup program is undertaken, the corrosion products generated within the closed cooling water system do not leave. When they accumulate on heat transfer surfaces, a loss of efficiency will result. Corrosion products are much more voluminous than the base metal from which they were produced and can restrict flow. Accumulations in low-flow or stagnant areas of the system can become breeding grounds for bacteria. In addition, corrosion can occur beneath the deposits.

3.2

Microbiological Growth

Microbiological organisms will be found in virtually all Closed Cooling Water systems. If not controlled, these organisms can cause both corrosion and fouling of equipment in the system. Microorganisms are usually classified according to their ability to grow in the presence or absence of oxygen. Organisms that require oxygen in their metabolic processes are termed aerobic, and those that thrive in oxygen-free environments are termed anaerobic. Microorganisms are also classified according to whether they are free-floating in the bulk water (planktonic bacteria) or are attached to component surfaces (sessile bacteria). Growth of sessile organisms can contribute to fouling through the accumulation of slime masses. These can decrease or block flow in equipment. Slime masses can indirectly contribute to corrosion because their metabolic waste products accumulate below the slime mass, in contact with the metal surface. These waste products contain acidic and heteroatom compounds, which contribute to underdeposit corrosion. Microbiological growth can also directly impact corrosion by contributing to microbiologically influenced corrosion (MIC). Certain bacteria, such as acid producing bacteria (APB) and sulfate reducing bacteria (SRB), create a localized corrosive environment. Bacteria also create problems in nitrite-treated systems by consuming nitrite. Nitrifying bacteria consume nitrite and produce nitrate. Denitrifying bacteria can consume nitrite and produce ammonia or nitrogen. These nitrite conversions can lead to a low or high pH corrosive environment and loss of capability to inhibit corrosion.

3.3

Fouling

Fouling of CCW systems can be caused by the accumulation of corrosion products, microbiological growth, or materials introduced from outside the system. Fouling due to corrosion products and microbiological growth were discussed previously. Another mechanism for fouling is the introduction of materials from outside the CCW system. Most common is the use of untreated or raw water for makeup to systems with significant water losses. The impurities in the makeup water can deposit in the system as mineral scale.

3-2

EPRI Licensed Material Problems Impacting Chemistry in Closed Cooling Water Systems

3.4

Operational and Mechanical Considerations

Chemical treatments should not be used to compensate for mechanical problems, operational problems, errors, or other deficiencies that substantially increase makeup requirements. Systems described as “closed” often are not. Water losses usually are usually unnecessary and can be reduced or eliminated. Some causes of water losses from closed cooling systems are: •

Cross connections to other systems operating at lower pressures

•

Expansion tank discharges

•

Valve, packing, fitting, piping, seal, and other leakage

•

Stress corrosion cracking of rolled tubes

The addition of makeup water can dilute the system chemistry below the concentrations at which the corrosion inhibitors are effective. If the makeup water is demineralized, it is free of dissolved solids but might contain dissolved oxygen. If the makeup source is service water or potable water, impurities such as calcium, magnesium, chloride, sulfate, and suspended solids are introduced. Also, excessive makeup to a closed system, regardless of makeup source, can introduce microbiological activity due to its presence in the makeup water. Each system has a corrosion inhibitor decay curve (concentration versus time), the slope of which is a composite effect of inhibitor decomposition and dilution. Changes to the slope of this curve can provide early warning of system leakage increases or contaminant ingress. Other mechanical areas of concern in CCW systems are stagnant or low-flow areas, dissimilar metals, and temperature fluctuation effects. Stagnant and low flows often do not allow corrosion inhibitors to maintain adequate contact with metal surfaces. Deposition is also likely to occur in stagnant and low-flow areas. Dissimilar metals, in contact with each other, can result in galvanic corrosion. Temperature fluctuations can increase system stresses and impact microbiological growth. Many CCW systems have side loops or redundant loops that are operated infrequently. Some examples of these are cooling loops for radwaste equipment, heating and cooling systems (sometimes part of one system), and letdown degassifier coolers. These loops can remain inactive for weeks to months and will be stagnant at their ambient temperature. The concentration of inhibitor might change (depending on the inhibitor and the length of time isolated from the main loop), while the concentration of contaminants remains constant. In addition, the amount of suspended corrosion products might increase. Thus, when the side loop is put back into service, the concentrations for all these parameters will change slightly. Knowing when these side loops are put into or taken out of service can provide important information to the chemist trying to understand changes in inhibitor or contaminant concentrations. For example, if a side loop will be removed from service for about six months, it might be desirable to increase the system inhibitor concentration to the high end of the normal range, ensuring that inhibitor decay does not fall out of the control band for the stagnant loop.

3-3


4

TECHNICAL BASIS OF TREATMENT CHEMICALS USED FOR CORROSION CONTROL

4.1

Introduction

Chemical treatment in CCW systems is used for the purposes of: •

Minimizing corrosion

•

Controlling microbiological growth

•

Controlling the deposition of suspended solids

•

Preventing scale

A variety of water treatment chemicals is available for the control of corrosion, microbiological growth, and suspended solids deposition in CCW systems. These chemicals can be applied either as individual products or as blends of two or more products. The selection of these agents is dependent upon a number of factors. These include, but are not limited to, system design, system metallurgy, system operating conditions (including temperature and flow velocities), environmental discharge limitations, radwaste considerations, and makeup water characteristics. Corrosion inhibitors are materials that react with a metal surface forming a thin film or passive layer to inhibit the corrosion process. Some inhibitors function at the cathodic areas, while others function at the anodic areas; thus, they are often referred to as cathodic and anodic corrosion inhibitors. Some organic inhibitors, such as azoles, form an inhibiting film on the metal surface and could function in both cathodic and anodic modes. Microbiological control is a key factor in CCW system operations. Microbiological control agents include non-oxidizing biocides, oxidizing biocides, and biostatic agents. In nuclear CCW systems, non-oxidizing biocides are typically used. The selection and application of biocides are often driven by safety and environmental regulatory concerns. In nuclear power plant CCW systems, residual contaminants in biocides are also important factors in selection and use. Deposit inhibitors are low molecular-weight anionic polymers that help keep particulate material in suspension. They also prevent suspended solids from settling on heat transfer surfaces or in low-flow areas. These inhibitors are used less in CCW systems than in open recirculating systems. They do, however, have application in those CCW systems where the accumulation of suspended solids is a potential problem. Scaling is not usually a problem in CCW systems unless there is excessive system leakage and raw water is used for makeup. Scale inhibitors are usually not part of a CCW treatment program. 4-1

EPRI Licensed Material Technical Basis of Treatment Chemicals Used for Corrosion Control

In addition to the chemicals discussed, there are other chemicals that are added to the treatment programs to control pH of the CCW systems. The purpose of pH control is to keep pH in a range where corrosion is minimized and treatment chemicals are most effective [7]. These might be chemicals such as sodium (or potassium) hydroxide, sodium bicarbonate, sodium carbonate, sodium tetraborate (borax), amines, and phosphates (glycol systems). Water treatment service companies are constantly working to develop new treatment programs for CCW systems. Although this guideline incorporates the typically used treatment programs that are commercially available, others might have been developed following the publication of this guideline. Such new programs could be used, provided a thorough evaluation is performed prior to introducing them into station systems.

4.2

Makeup Water Sources

Demineralized water is the preferred makeup water source in CCW systems. Industry experience has shown that other water sources might be acceptable as a makeup water source if all of the following parameters are considered and periodically analyzed: •

Conductivity: High conductivity can be an indication of a corrosive environment.

•

Alkalinity: Waters with no alkalinity can depress the pH of the system. In almost all cases, the lowering of pH will increase the potential for general corrosion in the system. The counterpoint to this is that the addition of makeup water containing alkalinity might decrease general carbon steel corrosion; however, alkalinity, calcium hardness, and pH should be evaluated for scaling potential under the actual CCW system chemistry and conditions. This relates to calcium carbonate scaling.

•

Calcium Hardness: The system should be evaluated for potential hardness scales (other than calcium carbonate scales). Such scales include, but are not limited to, calcium phosphate, calcium phosphonate, calcium molybdate, and calcium fluoride.

•

Chloride, Fluoride, Sulfate: Chloride, fluoride, and sulfate can contribute to corrosion.

•

Manganese, Iron, Copper: Manganese, iron, and/or copper can cause corrosion and/or deposition problems.

•

Residual Chlorine and Other Oxidants: Makeup water with residual chlorine (or other oxidants) can be corrosive to system materials and degrade chemical additives.

•

Ammonia: Ammonia can be aggressive to copper alloys.

•

Total Organic Carbon (TOC): Total organic carbon can contribute to microbiological growth.

•

Suspended Solids: Suspended solids can lead to deposition.

As long as the above items are considered, other water sources, particularly potable water, can be successfully used in CCW systems.

4-2


4.3

Chemicals Used for Corrosion Control

4.3.1 General Corrosion control requires knowledge of the metallurgy of the system, an understanding of the susceptibility of the metals to corrode under the operating conditions encountered, and an understanding of the limitations of the specific corrosion inhibitors applied. It should also be noted that corrosion inhibitors require metal surfaces free of deposits in order to be effective. The purpose of a corrosion inhibitor is to reduce the flow of electrons that are an integral part of the corrosion process. Cathodic inhibitors act by reducing the flow of electrons at the cathodic areas. They are typically not applicable to CCW systems, mainly because of system pH levels. Anodic inhibitors act by reducing the flow of electrons at the anodic areas. Anodic inhibitors are often considered pitting prone because underfeed of the inhibitor can cause pitting. Hydrazine concentrations in the range of 5–50 ppm provide a reducing environment in the closed cooling water system. Although reaction with oxygen is not rapid at the system’s normal operating temperatures (~75–150ºF [24–66ºC]), maintaining this concentration range will reduce dissolved oxygen to the 1–10 ppb range. Additionally, long-term exposure to hydrazine concentrations of this range will maintain iron oxides in the magnetite form. This helps to minimize general corrosion rates. Inhibitors described as film formers create a thin protective layer on the metal surface. Because the film is created on the entire metal surface, they are considered neither anodic nor cathodic. The corrosion inhibitors used for protection of copper and copper alloys are film formers. Typical corrosion inhibitors used in CCW systems are shown in Table 4-1. Table 4-1 Typical Corrosion Inhibitors in Closed Cooling Water Systems1 Anodic

Film Formers

Reducing Agents

Nitrite

Tolyltriazole

Hydrazine

Molybdate

Benzotriazole

Chromate

Mercaptobenzothiazole

Polysilicate

1

See Section 4.5 for a discussion of inhibitors used in glycol systems.

4-3


4.3.2 Commonly Used Ferrous Alloy Corrosion Inhibitors The typical corrosion inhibitors for mild carbon steel in power plant CCW systems are: •

Nitrites

•

Molybdates

•

Chromates

•

Reducing agents

•

Silicates

Combinations of these chemicals (nitrite plus molybdate) are also frequently used. By combining inhibitors, the total inhibitor concentration required is often less than if either inhibitor was used alone. The above inhibitors will almost always require the addition of a copper corrosion inhibitor if there is copper or a copper alloy in the system. They will also usually require the addition of a pH control agent to adjust pH into a range that will assist the inhibitor with corrosion control. Critical Concentration of Nitrite, Molybdate, and Chromate – Technical Basis Considerable research in the 1950s and 1960s indicated that the critical concentration of passivators such as chromate (CrO42-), nitrite (NO2-), and molybdate (MoO42-) lies somewhere in the range of 10-3 to 10-4 M [8]. The critical concentration is defined as the point below which passivity is lost in anodic areas and pitting may be initiated. The presence of chloride ions and elevated temperatures can increase this critical concentration. It should be noted that these experiments were conducted with demineralized water under laboratory conditions. Actual plant conditions are different from laboratory conditions and must be taken into account. It is noted that there is an order of magnitude difference in the range cited by Uhlig. For purposes of this guideline, the critical concentration of nitrite, molybdate, and chromate is considered to be the more conservative 10-3 M. This translates to 46 ppm of nitrite as NO2-, 160 ppm of molybdate as MoO42-, and 116 ppm of chromate as CrO42-. The mechanisms of other inhibitor systems (silicates, reducing agents, pure water) are different and do not follow the same critical concentration guidelines. The working control range for an inhibitor should be technically justified, sufficiently conservative, but not overly narrow. This guideline will recommend a normal operating range based on applicable research and industry experience. The discussion of each inhibitor will indicate exceptions that might justify establishing a lower or higher control band if technically justified and documented. The characteristics, advantages, and disadvantages of various corrosion inhibitors are identified in the following paragraphs.

4-4


Nitrites Mechanism

Nitrite forms a passivating iron oxide film at the anode. Nitrite complexes with iron surfaces to form an extremely thin and highly tenacious corrosion product layer in a manner similar to chromate. Applicable Metallurgy

Ferrous alloys [9]. Normal Operating Range

For the purposes of this guideline, the normal operating range for nuclear power plants is 500– 1500 ppm as NO2-. Applicable Concentration – Low: The lower concentration of 500 ppm as NO2 is based on general industry experience and offers a conservative inventory of inhibitor to compensate for potential losses due to leaks, microbiological activity, or oxidation. In other industries, operation at levels down to 300 ppm as NO2- is not unusual, particularly in chilled water systems that typically operate in the range of 45–50oF (7–10oC). Applicable Concentration – High: The upper concentration of 1500 ppm as NO2- is based on general industry experience; however, this is not a level that could be considered harmful for the system components. Nitrite concentrations up to 4000 ppm as NO2- were tested in one study and did not appear to damage carbon pump seals [10]. Operation in excess of 1500 ppm as NO2should take into consideration such factors as: •

Interference with analytical tests for impurities (particularly chlorides and fluorides [see the discussion on analytical methods in Section 8])

•

Impact on radwaste

•

Providing potential for increased biological activity

Associated Additives and Conditions

Nitrite is usually combined with a pH control agent (sodium hydroxide or a buffering compound such as sodium tetraborate [borax]) and a copper corrosion inhibitor such as an azole. Borax often cannot be used in BWR plants because of the potential for it to pass through the radwaste treatment facility and be added to the reactor feedwater. In this case, other pH control agents or buffers are used. A pH range of 8.5 to 11.0 is applicable to nitrite-treated systems where an azole is maintained at recommended concentrations or no copper alloys are present in the system [11]. This pH range 4-5


represents a change over the pH range recommended in Rev. 0 of the Guideline (Sections 8.5 through 10.5). This change has been made based on operating experience at South Texas Project, input from technical experts in the Task Group, and the beneficial biostatic aspects of increased pH. An azole is needed for copper corrosion inhibition. Applicable control ranges for copper corrosion inhibitors are discussed in Section 4.3.3. As noted in Section 4.3.3, an increased azole concentration (>25 ppm) is recommended when operating at a pH above 10.5. Nitrates are sometimes added to nitrite formulations to make the inhibitor package applicable to systems containing aluminum. Nitrates are also produced by the oxidation of nitrites, primarily by microbiological activity. Advantages

•

Nitrite is the most widely used inhibitor component. These treatments are effective inhibitors that do not require oxygen to form the inhibitor film. The success of nitrite-based inhibitors is well documented in the industry.

•

Nitrite is more resistant to the effects of chloride than is chromate, but is somewhat less resistant to the effects of sulfate [12] (see Section 9, Table 9-1 for more detail). Neither chloride nor sulfate should be present at significant concentrations in plants where demineralized water is used as makeup.

•

Nitrite can be used at higher levels (up to 4,000 ppm) than chromate because it has less of a tendency to abrade carbon pump seals [10].

•

Nitrite has lower aquatic toxicity than chromate; however, it might also be subject to discharge restrictions.

Disadvantages

•

Nitrite does not inhibit microbiological growth and, in fact, is a nutrient for certain types of bacteria.

•

Because nitrite is used at high concentrations, it adds to the difficulty of processing radwaste. The high concentration reduces the run length of radwaste ion exchange beds.

•

Under certain conditions, concentrations of nitrate (a byproduct of nitrite-treated systems) in the range of 10,000 ppm (1%) have been associated with stress corrosion cracking of carbon steel [13,14]. Concentration mechanisms underneath deposits might serve to achieve these levels even if nitrate concentration in the bulk water is lower.

•

The ammonia that might be produced in nitrite-treated systems can attack copper alloys under oxidizing conditions. Admiralty brass and other brasses might exhibit stress corrosion cracking in the presence of ammonia and oxygen.

•

Water drained from systems treated with nitrite should not be reused in the system unless steps are taken to eliminate biological effects. Nitrite serves as an excellent food source for biological activity. Reusing water that has been drained from the system increases the potential for biological contamination.

4-6


Radwaste Considerations

Nitrite-containing wastewater will compete with anionic radioisotopes for ion exchange sites, resulting in reduced removal capacity for other ions such as iodine. Consumption of cation resin results from the presence of the nitrite counter-ion (sodium or potassium). In addition to the exhaustion of resin sites, the introduction of CCW water will shift the inlet pH to the demineralizers. Nitrite-containing CCW water can: •

Act as a regenerant solution for the anion and cation resin, causing the sloughing or bleed-off of radionuclides.

•

Precipitate some metal ions (notably iron) by shifting inlet demineralizer pH from pH of 4–5 to 7–8. This will foul resin bead surfaces, choking off resin capacity, and might lead to increased ∆p on beds causing premature changeout.

•

Change the chemical form of 58/60Co2+ and 125SbO+1 and +3 to hydroxy-colloids, which are neither exchangeable nor filterable.

Nitrite is added as sodium salts because of solubility characteristics. This can present a challenge to those plants that recycle water back to the primary system because the sodium ion will bleed through ion exchangers in the radioactive waste processing systems and can add to 24Na formation in the primary coolant system. Waste from closed cooling loops is often processed through the radioactive waste processing systems. If significant changes are made to the corrosion control agents used (either concentrations or types), the plant Updated Final Safety Analysis Report (UFSAR) should be consulted regarding changes to chemical processes and the Radwaste Process Control Program. The presence of tetraborate in many commercial nitrite formulations is also of concern at BWR plants because reuse of water containing boron might cause reactivity changes and tritium production when used for makeup in reactor cooling water systems. Borates can also affect the analysis of contaminants like fluorides and chlorides if the water is recycled. High concentrations of nitrates can react with organic material to create an exothermic reaction. Nitric acid is known to react with ion exchange materials and create a potentially explosive situation. Molybdate Mechanism

Molybdate operates by a similar passivation mechanism as chromate and nitrite. It forms a passivating film at the anode, consisting of ferric, ferrous, and molybdenum oxides. Molybdenum is in Group VI A in the periodic table, along with chromium. It shows inhibitive effects on the general and localized corrosion of ferrous and nonferrous metals.

4-7


It is generally accepted that the corrosion inhibition of iron is achieved by adsorption of molybdate onto the porous oxide surface. The mechanism is thought to be a process of ion exchange, followed by the formation of an insoluble film reported to be ferric molybdate (FeMoO4). The adsorption produces a layer that resists the corrosive effects to other anions, particularly chlorides and sulfates. Role of Dissolved Oxygen

It is commonly believed that formation of the passive film requires the presence of dissolved oxygen or another oxidizing agent, but it is not clear how much oxygen might be required. There are conflicting references on this subject. Vukasovich provided a literature review on the subject [15]. One of the most commonly cited references is Pryor and Cohen [16]. This study states that oxygen is the primary passivator with adsorbed molybdate acting in film repair to maintain a passive oxide film. This study concluded that molybdate is ineffective in deaerated solutions. It has been pointed out by May [17], however, that the Pryor and Cohen study compared an oxygen-saturated solution versus an oxygen-free solution. The molybdate grew a film of interference in deaerated water but the solution became colored with iron. The measured corrosion rate in the deaerated water was 0.09 mils per year. Uhlig and King also indicate that both oxygen and molybdate adsorb on the corroding iron surface [18]. Other studies have indicated that oxygen does not play a central role in the molybdate corrosion mechanism. Robertson indicated adsorption of molybdate is at the metal-solution interface and formation of a molybdate complex with iron having protective properties [19]. Cartledge identified the precipitation of ferrous-ferric molybdates, which block corroding anodic sites [20]. Meziane indicated that the inhibiting effect of molybdates in the range of 10-4 to 10-3 M was increased by the presence of dissolved oxygen. However, passivity was also obtained in deaerated conditions [21]. The bulk of the literature indicates that oxygen does play some role in the molybdate corrosion inhibition mechanism. What is not clear are the amount of oxygen that might be needed and the impact of molybdate concentration and pH level on inhibitor effectiveness. Most closed cooling systems are not completely oxygen free. There are several points in a typical system for oxygen ingress. These include the air-water interface in expansion tanks, oxygen saturated makeup water, and pump seals. Some oxygen can be beneficial but there is no available information that establishes a critical oxygen concentration. Based on available empirical evidence, adequate molybdate concentrations and pH ranges will provide satisfactory corrosion protection. Applicable Metallurgy

Molybdate is applicable to ferrous alloys and aluminum.

4-8


Normal Operating Range

For the purposes of this guideline, the normal operating range for nuclear power plants is 200– 1000 ppm as MoO42-. Applicable Concentration – Low: The lower concentration of 200 ppm as MoO42- is based on industry experience and offers an inventory of inhibitor to compensate for system leaks. Because molybdate is not subject to loss by microbiological activity or oxidation, less of an inventory is needed than for nitrite. Initial (passivation) dosages should be in the upper part of the typical operating range (600–1000 ppm as MoO42-). Applicable Concentration – High: There has not been an upper acceptable dosage established for molybdate, based on pump seal degradation, as there has been for chromate. However, the impact of high dosages on overall system condition has not been investigated and it is prudent to stay within the recommended operating range. Operating in excess of 1000 ppm as MoO42should take into consideration such factors as: •

Interference with analytical tests for impurities (particularly chlorides and fluorides [see discussion of analytical methods in Section 8])

•

Impact on radwaste


Molybdate is often used with sodium hydroxide, sodium carbonate, sodium bicarbonate, an amine, or a borate to control pH. A pH range of 9.0–11.0 is applicable for molybdate-treated systems where an azole is maintained at recommended concentration or no copper alloys are present in the system. Molybdate is added as the sodium molybdate salt. This ionizes in water to form MoO42(molybdate) ions. Molybdate ion is the conjugate base of the weak acid H2MoO4. The molybdate ion is the species that participates in the passivation process. If the pH of the solution is lowered, two effects occur. First, the molybdate ion becomes protonated, forming the H2MoO4 species (which does not participate in the passivation process). Secondly, the molybdate ion forms polyhydroxy complexes below pH 7.0 and polymeric molybdate species at approximately pH 6.0. This transformation to species other than the MoO42- species below pH 7.0 reduces the efficacy of the added molybdate [22]. An azole is needed for copper corrosion inhibition. Applicable control ranges for copper corrosion inhibitors are discussed in Section 4.3.3. Advantages

•

Molybdate does not exhibit the toxicity of chromate.

•

Molybdate does not act as a nutrient for microbiological growth.

•

Concentration of molybdate in the system is only reduced via system demand and leakage.

•

Once formed, the protective film is tenacious and provides good protection against the effects of chlorides and sulfates [23]. 4-9


Disadvantages

•

Molybdate does not inhibit microbiological growth as does chromate.

•

Incomplete understanding of the role of dissolved oxygen on film formation.

•

Molybdate is more costly than nitrite but this might be offset by not being lost through microbiological activity.

•

Molybdate might have some environmental restrictions.

•

Molybdate can be activated to 99Mo, which decays to 99Tc. This might, however, be less significant than the activation of sodium to 24Na (molybdate is present as the sodium salt so both radionuclides have the potential to be formed).


Molybdate-containing wastewater will compete with anionic radioisotopes for ion-exchange sites, resulting in reduced removal capacity for other ions such as iodine. The counter-ion (sodium or potassium) will increase consumption of cation resin. Large volumes of resin are, therefore, required to process wastewater from molybdate-treated systems. If tetraborate is used as a pH buffer, the reactivity management concerns discussed previously will also be of concern to BWR plants. Borates can also affect the analysis of contaminants such as fluorides and chlorides if the water is recycled. Molybdate is added as sodium salt because of solubility characteristics. This can present a challenge to those plants that recycle water back to the primary system because the sodium ion will bleed through ion exchangers in the radioactive waste processing systems. This can add to 24 Na formation in the primary coolant system. Waste from closed cooling loops is often processed through the radioactive waste processing systems. If significant changes are made to the corrosion control agents used (either concentrations or types), the plant UFSAR should be consulted regarding changes to chemical processes and the Radwaste Process Control Program. Nitrite/Molybdate Mechanism

A nitrite/molybdate combination inhibitor operates by the same mechanisms described for the individual components. Sodium nitrite is an oxidizing salt that can accelerate the rate at which the ferric-molybdate film is formed on the metal surface. The combination of nitrite and molybdate is synergistic in that nitrite maintains the surface oxide film and molybdate protects the surface oxide from sulfate and chloride attack [23].

4-10


Applicable Metallurgy

Nitrite/molybdate is applicable to ferrous alloys and aluminum. Normal Operating Range

Wide ranges of nitrite-to-molybdate ratios are provided by various suppliers. For the purposes of this guideline, the normal operating range for nuclear power plant CCW systems is 50–1500 ppm NO2- and 160–1000 ppm MoO42-. Applicable Concentration – Low: The lower concentrations of 50 ppm NO2- and 160 ppm MoO42- are based on general industry experience and offer a conservative inventory of inhibitor to compensate for potential losses due to leaks, microbiological activity, or oxidation. Applicable Concentration – High: The upper concentrations of 1500 ppm NO2- 1000 ppm MoO42- are based on the upper concentrations for the individual nitrite and molybdate programs. Operating in excess of these levels should take into consideration such factors as: •

Interference with analytical tests for impurities (particularly chlorides and fluorides [see discussion of analytical methods in Section 8])

•

Impact on radwaste

•

Providing potential for increased biological activity


Sodium tetraborate (borax), sodium hydroxide, sodium carbonate, or sodium bicarbonate can be added for pH control in order to achieve a pH range of 8.5–11.0. Borax often cannot be used in BWR plants because some might pass through the radwaste treatment facility and be added to the reactor feedwater. A pH range of 8.5–11.0 is applicable to nitrite/molybdate-treated systems. This is the same pH range as for nitrite-treated systems. An azole is needed for copper corrosion inhibition. Applicable control ranges for copper corrosion inhibitors are discussed in Section 4.3.3. Nitrates are sometimes added to nitrite/molybdate formulations to make the inhibitor package applicable to systems containing aluminum. Nitrates are also produced by the oxidation of nitrites, primarily by microbiological activity. Advantages

•

Nitrite and molybdate exhibit low aquatic toxicity.

•

The total dissolved solids content of the closed cooling water might be lower than if either is used alone when using the low-concentration end.

4-11


•

The nitrite/molybdate combination is less prone to system upsets. If nitrite is lost through microbiological action, the molybdate remains and provides protection.

•

Nitrite and molybdate are synergistic and can be operated near the bottom of the ranges applicable to nitrite or molybdate.

Disadvantages

•

The nitrite might be a nutrient for microorganisms.

•

Both nitrite and molybdate might have some environmental restrictions.

•

Water drained from systems treated with nitrite should not be reused in the system unless steps are taken to eliminate biological effects. Nitrite serves as an excellent food source for biological activity. Reusing water that has been drained from the system increases the potential for biological contamination.


As previously discussed, both nitrite and molybdate will compete with anionic radioisotopes. Anion resin has a high relative selectivity for nitrite and molybdate in comparison to anionic radioisotopes such as iodine. Sodium or potassium used as counter-ions to nitrite and molybdate will consume cation resin and inhibit removal of cationic radioisotopes. Large volumes of resin are therefore required to process wastewater from molybdate-treated systems. If tetraborate is used as a pH buffer, the reactivity management concerns discussed previously, will also be of concern to BWR plants that might reuse borated water. Borates can also affect the analysis of contaminants such as fluorides and chlorides if the water is recycled. Molybdate can be activated to 99Mo, which decays to 99Tc (a very long-lived radionuclide). Molybdate and nitrite are both added as sodium salts because of their solubility characteristics. This can present a challenge to those plants that recycle water back to the primary system because the sodium ion will bleed through ion exchangers in the radioactive waste processing systems. This can add to 24Na formation in the primary coolant system. Waste from closed cooling loops is often processed through the radioactive waste processing systems. If significant changes are made to the corrosion control agents used (either concentrations or types), the plant UFSAR should be consulted regarding changes to chemical processes and the Radwaste Process Control Program. High concentrations of nitrates can react with organic material to create an exothermic reaction. Nitric acid is known to react with ion exchange materials and create a potentially explosive situation.

4-12


Chromate Chromate is a strong oxidizing agent that accelerates the oxidation of ferrous ions to ferric ions so that a thin adherent iron oxide quickly forms at the anodic surface. Metal oxides caused by this reaction become passive and are relatively inert to further oxidation or corrosion. Chromatebased inhibitors provide excellent corrosion protection for CCW systems but offer challenges because of toxicity concerns. At the levels recommended for corrosion control, chromate is also an effective microbiological control agent. Mechanism

Chromate forms a passive surface layer of ferric and chromic oxides. Applicable Metallurgy

Chromate is applicable to ferrous alloys, copper alloys, zinc, aluminum and soft solder. Normal Operating Range

For the purposes of this guideline, the normal operating range for nuclear power plants is 150– 300 ppm as CrO42-or as specified by NSSS Supplier. Applicable Concentration – Low: The lower concentration of 150 ppm as CrO42- is based on industry experience and offers an inventory of inhibitor to compensate for system leaks. Because chromate is not subject to loss by microbiological activity or oxidation, less of an inventory is needed than for nitrite. Applicable Concentration – High: The 300 ppm maximum is due to the degradation of carbon pump seals caused by high chromate levels (above 500 ppm) [10]. One NSSS vendor recommends 1000 ppm CrO42- on the initial fill. Operating in excess of 1000 ppm as CrO42should take into consideration such factors as: •

Interference with analytical tests for impurities (particularly chlorides and fluorides [see the discussion of analytical methods in Section 8])

•

Impact on radwaste


Although chromate-based inhibitors are effective over a broad pH range, sodium or potassium hydroxide is often added to adjust the pH into a range of 8.0 to 11.0. Potassium dichromate or disodium phosphate can be used for pH reduction. Because chromate is a good copper alloy corrosion inhibitor, the addition of an azole is not required. Chromate, however, is compatible with the azoles.

4-13


Advantages

•

High level of effectiveness as a corrosion inhibitor

•

Action as a biocide or biostat

Disadvantages

•

Due to the aquatic and mammalian toxicity of chromate, care must be given to maintaining a leak-free system. Leaks (for example, pump or valve packing leaks) will flow to a waste collection system and require processing. In the event that part (or all) of the system must be drained, the chromate-containing water must be routed to an appropriate wastewater treatment facility or be collected and reused if at all possible. However, chromate is easier to reuse because the inhibitor is less likely to become microbiologically contaminated.

•

Above approximately 500 ppm as CrO4=, chromate can degrade carbon pump seals [10]. No information is available on its impact on metallic or ceramic pump seals, based on the fact that one NSSS supplier recommends 1000 ppm CrO42- on initial fill.


Because chromate will compete with anionic radioisotopes, large volumes of resin might be required to process wastewater from chromate-treated systems. The relative selectivity of anion resin for chromate is high in comparison to anionic radioisotopes such as iodine. Similar effects are seen on cation resin from the counter-ion used with chromate (typically potassium or sodium). Additionally, the resin generated by radwaste processing of chromate waste might also be considered a mixed waste, depending on the extent of its loading on the resin. Determination of the mixed waste implications of spent chromated resin should be made on a case-by-case basis. Handling of chromated wastewater could also be a concern from a personnel safety standpoint. Waste from closed cooling loops is often processed through the radioactive waste processing systems. If significant changes are made to the corrosion control agents used (either concentrations or types), the plant UFSAR should be consulted regarding changes to chemical processes and the Radwaste Process Control Program. Hydrazine Mechanism

Hydrazine reacts with iron oxide and copper oxide to form protective oxide (passive) films [24]. The corrosion inhibiting effect of hydrazine is based essentially on the activation and acceleration of the oxidation of Fe0 to Fe(II) and subsequent formation of a protective layer of magnetite on the surface of the steel.

4-14



Hydrazine has been successfully used in all-ferrous and mixed metallurgy systems. For mixed metallurgy systems, the hydrazine and pH operating ranges require site-specific optimization to minimize the total corrosion product concentration in the coolant. Normal Operating Range

For the purposes of this guideline, the normal operating range is 5–50 ppm. Levels up to 200 ppm are used in some all-ferrous alloy systems. Applicable Concentration – Low: The lower concentration of 5 ppm is based on plant experience. Applicable Concentration – High: The upper concentration of 50 ppm is based on plant experience in plants with mixed metallurgy. The upper concentration of 200 ppm in plants with all-ferrous metallurgy is also based on plant experience. Associated Additives and Conditions

Hydrazine is either used without additives or with the addition of an amine for pH control. The applicable pH ranges are 8.5–9.6 when copper alloys are present, or 8.5–10.5 when in all-ferrous alloy systems. Advantages

•

Hydrazine does not add any dissolved solids to the water being treated.

•

Impact on radwaste ion exchange is minimal.

•

Plants using hydrazine have not experienced microbiological problems.

Disadvantages

•

Hydrazine decomposes to ammonia, which can attack copper alloys under high-oxygen conditions.

•

Discharges to the environment are regulated due to hydrazine toxicity.

•

Oxygen ingress can result in localized corrosion.

•

The passive or protective oxide film formed by hydrazine is loosely bound.


Hydrazine-treated systems typically have a lower specific conductivity than chromate-, nitrite-, and/or molybdate-treated systems. Radwaste processing of hydrazine-containing wastewater is generally not as difficult as with other treatment methods. 4-15


Large volumes of hydrazine-treated water can change the chemical form of 58/60Co2+ and 125 SbO+1 and +3 (due to pH change) to hydroxy-colloids, which are neither exchangeable nor filterable. Silicates Mechanism

The most frequently used silicate inhibitors are the glassy amorphous polymeric silicates. Silicates react with dissolved metal ions at the anode. One likely mechanism for their corrosion protection is the fact that they hydrolyze in water to form a negatively charged colloidal particle that migrates to the anodic area [25]. Uhlig indicates that silicates facilitate passivation by making conditions more favorable for adsorption of oxygen [26]. In alkaline waters, a silicate with a formula of Na2O · 3.3SiO2 is often used, although other silicates, such as solid glassy silicates, have been used. The gel formed at anodic sites is highly tenacious and the inhibiting properties of silicates increase with increasing temperature and pH. Silicates have surface-active properties that allow them to penetrate deposits of corrosion products and inhibit active corrosion. Applicable Metallurgy

Silicates protect copper, aluminum, and ferrous alloys. Some diesel engine manufacturers prohibit silicates. Silicates have been used in nuclear plants but less frequently than the other inhibitors. Normal Operating Range

The normal operating range is 20–60 ppm as SiO2 above the inorganic silica content of the water. However, dosages up to 200 ppm are not uncommon. Applicable Concentration – Low: The lower concentration of 20 ppm above background is based on general and plant experience. Applicable Concentration – High: The upper concentration of 60 ppm above background (and sometimes up to 200 ppm) is based on general and plant experience. Associated Additives and Conditions

Silicates do not perform well at pH levels below 7.0, but are satisfactory in low dissolved solids water above pH 7.5 and as high as 11.0. Polysilicates are in an alkaline medium and increase the pH of CCW water. Sodium hydroxide can be used to provide additional pH control. Azoles are usually not required but can be added for additional copper alloy corrosion protection.

4-16


Advantage

•

The toxicity of silicates is low.

Disadvantages

•

Film formation is slow (days or weeks versus hours).

•

Polysilicates form a fragile film (easily destroyed).

•

Silicates appear to be primarily anodic inhibitors, but have not been reported to contribute to pitting.

•

Limited shelf life of the polysilicate product.


Polysilicates are non-ionic and will act as inert species when processed with ion exchange. Few radwaste processing considerations apply to polysilicate-containing wastewater at PWR plants. At BWR plants, however, polysilicates are of concern because they will break down to orthosilicates when used for reactor coolant makeup. 4.3.3 Copper Alloy Corrosion Inhibitors Chemicals used for copper corrosion control are nitrogen containing organic compounds (azoles). Three chemicals of this type are used: tolyltriazole (TTA), benzotriazole (BZT), and mercaptobenzothiazole (MBT). After the first quarter of 2004, there might be no U.S. domestic manufacturers of MBT. MBT is still available in the U.S. via the European market. There is also a halogen-resistant azole available. This is a modification of the TTA molecule that has been chlorinated, improving resistance to degradation by oxidizing biocides. Because this molecule contains chlorine, its use will add chloride to the CCW water. Mechanism: Azoles react with copper ion to form a thin film that reinforces the oxide film on the copper layer [27]. MBT is the fastest film former of its type. Azoles are general film formers— the film is formed at both the anode and the cathode [28]. Although TTA and BZT form a protective film more slowly than MBT, they are more stable under conditions of high temperature. BZT and TTA are used in both open recirculating and CCW systems. Some suppliers blend TTA or BZT with MBT to incorporate both rapid filmforming properties and stability into a single formulation. The azoles also react with copper present in the bulk water. The copper-azole complex is a fine particulate that can be removed with a 0.45-µ filter [29].

4-17



All three azoles are effective for copper alloys. MBT has been identified as the most effective for soft solder connections in emergency diesel generators [30]. Normal Operating Range

For the purposes of this guideline, the nominal system concentration for CCW systems in nuclear power plants is 5–100 ppm azole. Applicable Concentration – Low: The lower concentration of 5 ppm is based on general industry experience and offers a conservative inventory of inhibitor to compensate for potential losses due to leaks and system demand. One study confirmed the applicability of a 10-ppm azole dosage [31]. This study did not test TTA concentrations below 10 ppm. The 5-ppm azole lower limit recognizes that a protective film exists on the metal and a maintenance dosage is required. TTA/BZT will protect the zinc in brasses although the information available involves a laboratory study using city water [32]. The study tested brass with a higher zinc content (37%) than Admiralty (27%) and suggested a range of 20–50 ppm azole. Applicable Dosage Range – High: There has not been an upper dosage established from the standpoint of ineffectiveness or risk to system components. The recommended maximum of 100ppm azole is based on economics and TOC considerations. Concentrations in excess of this would offer little benefit. At 10–100 ppm, the azoles are effective in the pH range of 8.5–11.0. TTA dosage should provide a residual concentration of 25 ppm azole or greater for an operating pH of 10.5–11.0 [11]. Associated Additives and Conditions

The azoles are typically used as additives to ferrous alloy inhibitors. Advantages

•

The presence of an azole reduces deposition of copper ion on steel.

•

Azoles are low in toxicity compared to chromate.

Disadvantages

•

MBT produces disulfide and sulfate upon degradation.

•

Decreasing commercial availability of MBT.

4-18



At typical use concentrations (5–100 ppm azole), radwaste processing of filming inhibitors such as TTA, BZT, or MBT is not of great concern. Because the inhibitors are sparingly soluble in water, high concentrations can lead to fouling of ion exchange resins. Because MBT degrades to sulfate, slightly higher sulfate concentrations could result in water treated with MBT. Continuous input of these chemicals to radwaste will increase the amount of organic loading on anion resins.

4.4 Closed Cooling Water System Corrosion Control Without Chemicals (Pure Water Systems) Corrosion control of CCW systems without chemicals has been successful. These systems typically have low operating temperatures (<100oF) and use demineralized water for makeup. Boiling water reactors have used demineralized water in CCW systems to prevent system chemicals from leaking into floor drains and entering the radioactive waste treatment and water recycle systems. A disadvantage is the lack of chemical buffering, which requires that system purity and integrity be maintained. The primary impurities of concern are anions that can accelerate a variety of corrosion mechanisms (general and pitting corrosion, as well as stress corrosion cracking of stainless steels). Oxygen control in these systems is a very important parameter as well. Most CCW systems contain copper or copper alloys. The mechanism of formation for copper corrosion films can take place in two separate oxygen control ranges. One of these regimes controls oxygen concentration to <100 ppb. For this range the cuprous oxide film is formed on the surface of the copper alloy. The other regime maintains the system in an oxygenated state at >2000 ppb. Here the cupric oxide film forms the protective barrier for the metal. Both of these oxygen control regimes can be used successfully, but they must be used separately. The rate of copper corrosion significantly increases when the system is allowed to operate in the oxygen concentration range between 100–2000 ppb. The corrosion film is in a state of flux between cuprous and cupric oxide and easily sloughs off the metal surface, causing additional corrosion. This will have the effect of reducing flow, increasing solids and copper content, and reducing heat transfer efficiency. Although stator cooling systems are not included in this guideline, most experiences with significant corrosion problems due to poor oxygen control are related to stator coolant systems [4]. In one case, the vent line outside a building became blocked with avian debris, prohibiting exchange of outside air with the system. The oxygen concentration slowly depleted, the copper corrosion rate increased, and system flow decreased [33]. In a separate case, oxygen intrusion into a low-oxygen system occurred in a time period of a few minutes, causing a 35% flow reduction and significant increase in copper corrosion products2.

2

Seabrook Station, February 1996.

4-19


Mechanism and Effectiveness

Low ionic impurities and dissolved oxygen in the cooling water allows the formation of a stable oxide film with acceptably low corrosion rates. Impurity control can be achieved with sidestream filters for particulates and demineralizers for ionic contaminants. “Feed and bleed” is also used for impurity control but this can change dissolved oxygen levels, potentially affecting the corrosion film. It is important to ensure that the makeup water to the system has approximately the same oxygen content as the system. Applicable Metallurgy

Corrosion control without chemicals (pure water systems) is applicable to ferrous and copper alloys. Control Range

System corrosion will increase with impurity concentrations. The impact of the individual impurities is additive. If more than one parameter is greater than the recommended range, effort should be expended to correct that condition. The recommended control values are: •

Conductivity ≤2 µS/cm

•

Chloride ≤50 ppb

•

Fluoride ≤50 ppb (for systems with stainless steel and temperature >150ºF [66ºC])

•

Sulfate ≤50 ppb

•

Dissolved oxygen <200 ppb (≤100 ppb or >2000 ppb based on stator cooling guidelines for systems with copper)


Chemical additives are not used for demineralized water CCW systems. In the event of microbiological activity, a biocide that does not increase system impurities should be used. Advantages

•

No chemical additive costs

•

System leakage has minimal impact on other plant systems

•

Low probability of biological activity due to pure water

Disadvantages

•

Lack of a corrosion inhibitor

•

Low buffering capacity

•

Little room for error

4-20


4.5

Blended Glycol Formulations

Blended glycol formulations are used in closed cooling systems that require protection against freezing. Glycol formulations are typically not used in systems that do not require freeze protection because heat transfer characteristics are not as good as water. In systems requiring freeze protection, the heat transfer characteristics have been considered in the original design. If converting from an aqueous coolant to a glycol-based coolant, the reduction in heat transfer characteristics should be evaluated. Corrosion inhibitors are added to glycol formulations to minimize corrosion rates. In most cases, glycol antifreeze solutions are generally supplied containing an appropriate proprietary inhibitor package. There are at least two plants that are purchasing glycol without inhibitors and adding inhibitors. If the intent is to purchase an inhibited glycol, care should be taken to specify an inhibited ethylene glycol solution and not simply ethylene glycol. Commercial glycol antifreeze solutions also commonly contain a minor percentage of di-ethylene glycol in addition to ethylene glycol. The corrosion inhibitor objectives for glycol coolants are similar to those of pure water corrosion inhibitors. Different approaches exist, but they are all equally appropriate in water or glycol solutions. One of the contributors to corrosion problems in glycol-water cooling systems is the slow breakdown of glycol into organic acids at elevated temperature, resulting in a loss of buffering capacity and a drop in pH. The inhibitor blend should retard glycol breakdown by buffering the solution to counter the effect of the organic acid breakdown products. Buffering capacity is usually provided by a high concentration of trisodium phosphate, sometimes fortified with sodium tetraborate and caustic soda to provide a system pH of 10.0– 10.5 (when the concentrate is diluted to 50% in the system). Typical phosphate concentrations range up to 20,000 mg/L as PO4 in the concentrate. Potassium phosphate salts are also used as buffers due to the greater solubility of the potassium salt at high pH. At these concentrations, the phosphate also serves as a corrosion inhibitor for steel. Tolyltriazole is generally added to the concentrate at concentrations of 500–3000 mg/L active, which is 100-fold higher than typical for non-glycol systems. The tolyltriazole serves several purposes. It inhibits copper alloy corrosion and controls solder corrosion by shutting down the cathodic reaction on the copper/solder galvanic couple. In a closed cooling system, a buildup of corrosion products can deplete tolyltriazole concentrations between extended maintenance intervals. Because the cost of the tolyltriazole is small compared to the glycol cost, a high initial concentration provides inexpensive insurance against premature tolyltriazole depletion. Tolyltriazole depletion (caused by corrosion products) is sometimes used as an indicator that the corrosion inhibitor is failing or at the end of useful life. The principal corrosion inhibitor for aluminum and cast iron in antifreeze solutions is silicate. Silicate can also be effective in inhibiting the copper/solder galvanic couple, should the tolyltriazole become depleted. Silicate is added to the glycol concentrate at 500–1100 mg/L as 4-21


SiO2. Some engine manufacturers require low silicate coolant to prevent silicate dropout (gelation), which creates deposits that plug radiators and heat exchange equipment. Sodium nitrate (NaNO3), at a concentration of 500–1500 mg/L (as NO3), can be used in addition to silicate to control aluminum pitting corrosion; however, it is more for use with ferrous metals. An antifoam, such as silicone or ethylene oxide/propylene oxide copolymer, is generally added to the glycol concentrate to inhibit foaming that might produce air pockets within the cooling system, cause false level indications, or result in foam overflowing the surge tank. A red, yellow, or green dye is generally included in the concentrate to provide a visual indication of the presence or condition of the glycol solution. For example, 30 mg/L of Alizarine Cyanone Green G-Extra plus 20 mg/L of Uranine C yellow will produce the characteristic yellow-green hue of most consumer and industrial glycol antifreeze solutions. Inexpensive soft solder (containing greater than approximately 92% lead) is very susceptible to corrosion when galvanically coupled to copper alloys. Nitrite at levels above 4000 mg/L as NO2 can greatly accelerate the solder corrosion rate, causing corrosion product (lead phosphate) blooms, which can restrict cooling flow in radiator tubes or cause the solder joint to fail. Sodium nitrite up to a concentration of 3,000 mg/L as nitrite (NO2), however, is often an additional component of heavy-duty diesel glycol coolant applications and is used to inhibit cavitationinduced pitting of the cast iron piston sleeve liners. The vibration of the pistons can induce cavitation bubble formation on the outside of the sleeve liners, often resulting in intense pitting of the liner. Nitrites are particularly effective at inhibiting this cavitation-induced pitting. Silicates and additional tolyltriazole can effectively counteract the impact of nitrite on solder corrosion. Heavy-duty diesel glycol formulations could also be formulated with polymeric dispersants, such as polyacrylate and sequestrants (for example, phosphonates), to complex the calcium in the water, which is used to dilute the glycol concentrate. These dispersants and sequestrants are also useful in complexing and dispersing ferrous corrosion products, but coolant filters are far more effective. Calcium phosphate deposits are prone to form in both the extreme hot and cold zones of the cooling system. Deposits in hot areas on the engine can cause hot spots to form near the firing areas, which can change engine timing. Deposits in cold areas, such as the heater core, can reduce heat transfer and plug the core in extreme cases. It is important to realize that the amount of calcium deposit is limited to the calcium introduced in the dilution water, which is small for most glycol containing closed cooling systems. For example, the total amount of calcium phosphate that could theoretically form from the calcium contained in 120 gallons of raw water with 100 mg/L of calcium hardness (as CaCO3) is only 0.3 lbs (0.14 kg). Solutions of ethylene and propylene glycol as low as 20% [34] to 25% [35] have been shown to be effective at controlling most types of microbiological growth. Because not all microorganisms are represented by these studies, it is possible for some growth to occur in this concentration range. Concentrations above 30% are readily recognized as maintaining biostatic conditions. Seabrook Station has used ethylene glycol systems at concentrations of 25–30%, with no measurable microbiological activity for over 15 years. Testing parameters and frequencies for inhibited glycol solutions are described in Section 5.9. 4-22


5

TREATMENT PROGRAM AND MONITORING RECOMMENDATIONS

When properly applied, there are several chemical treatment programs that have a documented history of success in nuclear power plant closed cooling systems. The technical basis of the various corrosion inhibitors is discussed in Section 4. This section will summarize these treatment programs, establish common control ranges, and identify where exceptions might exist. Deviating from the specifications outlined in this guideline, with the understanding and approval of plant management, is allowed provided that sound technical justification and/or performance monitoring supports the deviation.

5.1

CCW Action Levels

CCW Action Levels have been established for these guidelines so that the importance of out-ofspecification conditions can be effectively communicated to management. The CCW Action Levels have been written to parallel the actions that are taken in primary and secondary chemistry systems. However, because closed cooling systems operate at temperatures and pressures that are significantly lower than the primary or secondary systems of power plants, the timeframes for significant adverse effects on system materials are much longer. Revision 0 of these guidelines did not provide guidance on the significance of an out-of-specification condition, or how soon the condition had to be corrected. This made it difficult to plan appropriate timeframes and work schedules for correcting these conditions. CCW Action Level 1 conditions are those that can be addressed using the 12-week work schedule concept, with no discernable increases in corrosion rates or system efficiency. CCW Action Level 2 communicates a more serious condition, requiring action outside the normal 12-week work schedule. The CCW Action Levels apply only to Control Parameters, which have defined limits. Diagnostic Parameters are used as a means to help identify the source of the off-normal condition, and are generally not a direct function of corrosion inhibition. Diagnostic Parameters have no CCW Action Levels. It is important to note that it is expected that these timeframes should be used as not-to-exceed dates, and that corrective actions will be completed as soon as work schedules permit. The various Control Parameters can have a negative synergistic effect on system corrosion rates if two or more are outside the normal operating range at the same time. For these circumstances, consideration should be given to elevating to CCW Action Level 2 if two or more parameters are in CCW Action Level 1 conditions. 5-1

EPRI Licensed Material Treatment Program and Monitoring Recommendations

5.1.1 CCW Action Level 1 The normal operating ranges recommended in this guideline represent a conservative interpretation of closed cooling water chemistry as generally practiced in the nuclear power and other industries. Action Level 1 denotes a condition where system chemistry Control Parameters are outside the normal operating levels. The recommended action is to increase monitoring frequency, as appropriate, and return the parameter to within the prescribed normal operating range within 90 days. If the parameter has not returned to the normal operating range within 90 days, CCW Action Level 2 is entered. This timeframe is based on only one Control Parameter being in the Action Level range. If more than one is beyond established limits, a negative synergistic effect might result. Therefore, if more than one parameter is in CCW Action Level 1, it is recommended that more aggressive corrective actions be initiated than if only one parameter is in the Action Level. Increased sampling frequency, until parameters are returned to normal, should be considered as part of those actions. 5.1.2 CCW Action Level 2 Values exceeding the CCW Action Level 2 threshold could initiate short-term system materials degradation. The recommended action is to return the parameter to within the prescribed control band within 30 days. If after 30 days in CCW Action Level 2, the parameter has not returned to the normal range and system operation is to continue, then a risk assessment (engineering evaluation) shall be performed indicating that the out-of-control parameter will not impact the long-term reliability of the system. The evaluation should include a sunset date for the out-ofcontrol condition. This timeframe is based on only one Control Parameter being in the Action Level range. If more than one is beyond established limits, a negative synergistic effect might result. Therefore, if more than one parameter is in CCW Action Level 2, it is recommended that more aggressive corrective actions be initiated than if one parameter is in the Action Level. Increased sampling frequency, until parameters are returned to normal, should be considered as part of those actions.

5.2

Nitrite-Based Programs

Nitrite-based corrosion control programs use sodium nitrite as a carbon steel corrosion inhibitor in addition to a pH control agent. The pH control agent can either be sodium hydroxide or a sodium tetraborate buffer. A nitrite-based program will also contain a copper corrosion inhibitor (tolyltriazole or benzotriazole) if copper alloys are present in the system. Tables 5-1 and 5-2 list operating ranges and monitoring frequencies for nitrite- and molybdate-based programs.

5-2

EPRI Licensed Material Treatment Program and Monitoring Recommendations Table 5-1 Operating Ranges and Monitoring for Nitrite-Based Programs Parameter


Action Levels (AL 1 – 90 days AL 2 – 30 days)

Monitoring Frequency Tier 1 Systems (1)

Tier 2 Systems

Intermittent Systems

Weekly

Monthly

Monthly or as operated

Weekly

Monthly


Monthly

Monthly


Monthly

Monthly


Control Parameters Nitrite

pH

500–1500 ppm as NO2-

8.5–11.0

AL 1: <500 ppm AL 2: <300 or >4000 ppm AL 1: <8.5 or >11.0 AL 2: <8.0 or >11.5

Azole (2)

Chloride and Fluoride (4)

5–100 ppm azole (3)

AL 1: <5 ppm

≤10 ppm Cl

AL 2: >10 ppm

AL 2: <3 ppm

≤10 ppm F Diagnostic Parameters

Conductivity

Consistent with nitrite level and Evaluate trend

Weekly

Monthly


Nitrate (5)

Evaluate trend

Quarterly

Quarterly

Quarterly or as operated

Ammonia (2)

Evaluate trend

Quarterly

Quarterly

Quarterly

Chloride and Sulfate (6)

Evaluate trend

Monthly

Monthly


Total Iron, Total Copper (7)

Evaluate trend

Monthly

Monthly


Microbiological

Evaluate trend

Monthly

Monthly


Radioactivity (8)

Evaluate trend

Quarterly

Quarterly


(1) The weekly frequency for Tier 1 parameters can be reduced to up to once per month if steady-state chemical control has been maintained (all control parameters are maintained within the normal operating range) for at least one quarter and microbiological activity is under control. (2) Not required if it can be documented that there are no copper alloys in the system. (3) Azole to be in excess of 25 ppm if pH is in the range of 10.5–11.0. o

o

(4) Applies to systems that contain stainless steel where the temperature is >150 F (66 C). Strategic plan should consider that NSSS vendor specification might be more restrictive. (5) Some blended products might contain nitrate. This reduces the value of nitrate analysis and trending. (6) If not applicable as a Control Parameter, chloride should be a Diagnostic Parameter due to general corrosion concerns. (7) Iron and copper monitoring can be quarterly if corrosion rate monitoring is being performed. (8) Applicable to those systems with the potential to become contaminated.

5-3


5.3

Molybdate-Based Programs

Molybdate-based corrosion control programs use sodium molybdate as a carbon steel corrosion inhibitor in addition to a pH control agent. The pH control agent can be either sodium hydroxide or a sodium tetraborate buffer. In some cases, carbonate, bicarbonate, or an amine are used to avoid the use of borate. A molybdate-based program will also contain a copper corrosion inhibitor (tolyltriazole or benzotriazole) if copper alloys are present in the system. Table 5-2 Operating Ranges and Monitoring for Molybdate-Based Programs

Parameter


Action Levels

Monitoring Frequency

(AL 1 – 90 days AL 2 – 30 days)

Tier 1 Systems (1)

Tier 2 Systems


Weekly

Monthly


Weekly

Monthly


Monthly

Monthly


Monthly

Monthly


Control Parameters Molybdate pH Azole (2) Chloride and Fluoride (4)

200–1000 ppm = as MoO4 9.0–11.0 5 to 100 ppm as azole (3) ≤10 ppm Cl ≤ 10 ppm F

AL 1: <200 ppm AL 2: <160 ppm AL 1: <9.0 or >11.0 AL 2: <8.5 or >11.5 AL 1: <5 ppm AL 2: <3 ppm AL 2: >10 ppm Diagnostic Parameters

Conductivity

Evaluate trend

Weekly

Monthly



Evaluate trend

Monthly

Monthly


Iron and Copper (6)

Evaluate trend

Monthly

Monthly


Microbiological

Evaluate trend

Quarterly

Quarterly


Radioactivity (7)

Evaluate trend

Quarterly

Quarterly


(1) The weekly frequency for Tier 1 parameters can be reduced to up to once per month if steady-state chemical control has been maintained (all control parameters are maintained within the normal operating range) for at least one quarter. (2) Not required if it can be documented that there are no copper alloys in the system. (3) Azole to be in excess of 25 ppm if pH is in the range of 10.5–11.0. o

o

(4) Applies to systems that contain stainless steel where the temperature is >150 F (66 C). Strategic plan should consider that NSSS vendor specification might be more restrictive. (5) If not applicable as a Control Parameter, chloride should be a Diagnostic Parameter due to general corrosion concerns. (6) Iron and copper monitoring can be quarterly if corrosion rate monitoring is being performed. (7) Applicable to those systems with the potential to become contaminated.

5-4


5.4

Nitrite/Molybdate Programs

Nitrite/molybdate corrosion control programs use the combination of sodium nitrite and sodium molybdate as a carbon steel corrosion inhibitor in addition to a pH control agent. The pH control agent can be either sodium hydroxide or a sodium tetraborate buffer. In some cases, carbonate, bicarbonate, or an amine is used to avoid the use of borate. A nitrite/molybdate program will also contain a copper corrosion inhibitor (tolyltriazole or benzotriazole) if copper alloys are present in the system. Table 5-3 lists operating ranges and monitoring frequencies for nitrite/molybdate programs. Table 5-3 Operating Ranges and Monitoring for Nitrite/Molybdate Programs Parameter

Nitrite Molybdate pH


50–1500 ppm NO2160–1000 ppm MoO428.5–11.0



Tier 2 Systems


Weekly

Monthly

Weekly

Monthly

Weekly

Monthly

Monthly

Monthly

Monthly

Monthly

Monthly or as operated Monthly or as operated Monthly or as operated Monthly or as operated Monthly or as operated

Control Parameters AL1: <50 ppm AL2: <25 ppm or>4000 ppm AL1: <160 ppm AL2: <80 ppm AL 1: <8.5 or >11.0 AL 2: <8.0 or >11.5 AL 1: <5 ppm AL 2: <3 ppm AL 2: >10 ppm


5–100 ppm as azole (3) ≤10 ppm Cl ≤10 ppm F

Conductivity

Evaluate trend

Weekly

Monthly

Nitrate

Evaluate trend

Monthly

Monthly

Ammonia (2)

Evaluate trend

Monthly

Monthly

Chloride and Sulfate (5) Iron and Copper (6)

Evaluate trend

Monthly

Monthly

Evaluate trend

Monthly

Monthly

Microbiological

Evaluate trend

Monthly

Monthly

Radioactivity (7)

Evaluate trend

Quarterly

Quarterly

Azole (2)

Diagnostic Parameters Monthly or as operated Monthly or as operated Monthly or as operated Monthly or as operated Monthly or as operated Monthly or as operated Quarterly or as operated

(1) The weekly frequency for Tier 1 parameters can be reduced to up to once per month if steady-state chemical control has been maintained (all control parameters are maintained within the normal operating range) for at least one quarter and microbiological activity is under control. (2) Not required if it can be documented that there are no copper alloys in the system. (3) Azole to be in excess of 25 ppm if pH is in the range of 10.5–11.0. o

o


5-5


5.5

Chromate-Based Programs

Chromate-based corrosion control programs use a chromate salt as a carbon steel corrosion inhibitor in addition to a pH control agent. Potassium dichromate or disodium phosphate can be used for pH reduction. Because chromate is a good copper alloy corrosion inhibitor, the addition of an azole is not required. Chromate is compatible, however, with the azoles. Table 5-4 lists operating ranges and monitoring frequencies for chromate-based programs. Table 5-4 Operating Ranges and Monitoring Frequencies for Chromate-Based Programs Parameter




Tier 2 Systems


Weekly

Monthly


Weekly

Monthly


Monthly

Monthly


Control Parameters Chromate

150–300 ppm as CrO42- (1)

AL 1: <150 or >300 ppm AL 2: <115 or >500 ppm

pH

8.0–11.0

AL 1: <8.0 or >11.0 AL 2: <7.5 or >11.5


≤10 ppm Cl

AL 2: >10 ppm

≤10 ppm F Diagnostic Parameters

Conductivity

Evaluate trend

Weekly

Monthly



Evaluate trend

Monthly

Monthly


Iron and Copper (5)

Evaluate trend

Monthly

Monthly


Radioactivity (6)

Evaluate trend

Quarterly

Quarterly


=

(1) Some NSSS suppliers request an initial chromate concentration of 1000 ppm CrO4 . This indicates that no system components will be damaged by the high chromate level. This requirement constitutes a legitimate exception to the normal control range and should be accommodated. (2) The weekly frequency for Tier 1 parameters can be reduced to up to once per month if steady-state chemical control has been maintained (all control parameters are maintained within the normal operating range) for at least one quarter. o

o


5-6


5.6

Hydrazine-Based Programs

Hydrazine-based programs have been successfully used in all-ferrous and mixed metallurgy systems. The two types of systems require different hydrazine levels and operating pH ranges. Table 5-5 lists operating ranges and monitoring frequencies for hydrazine-based programs. Table 5-5 Operating Ranges and Monitoring Frequencies for Hydrazine-Based Programs

Parameter




Tier 2 Systems


Weekly

Monthly


Weekly

Monthly


Control Parameters Hydrazine, mixed metallurgy

5–50 ppm as N2H4

AL 1: <5 ppm

Hydrazine, all-ferrous metallurgy

5–200 ppm as N2H4

AL 1: <5 ppm

pH, mixed metallurgy

8.5–9.6

AL 2: <8.0 or >10.0

Weekly

Monthly


pH, all-ferrous metallurgy

8.5–10.5

AL 2: <8.0 or >10.5

Weekly

Monthly


≤150 ppb as ion (each)

AL 2: >1000 ppb

Monthly

Monthly


Monthly

Monthly


Chloride, Fluoride, Sulfate (2) Dissolved Oxygen

≤100 ppb

AL 2: <1 ppm AL 2: <1 ppm

AL 1: >150 ppb AL 1: >100 ppb AL 2: >1000 ppb Diagnostic Parameters

Cation conductivity (3)

Evaluate trend

Weekly

Monthly


Ammonia

Evaluate trend

Monthly

Monthly


Iron and Copper

Evaluate trend

Monthly

Monthly


Radioactivity (4)

Evaluate trend

Quarterly

Quarterly


(1) The weekly frequency for Tier 1 parameters can be reduced to once per two weeks if steady-state chemical control has been maintained (all control parameters are maintained within the normal operating range) for at least one quarter. (2) Weekly cation conductivity can be substituted for Cl, F, and SO4 if organic anions are not present. If cation conductivity is <1 µS/cm, Cl, F, and SO4 are considered to be within specification. (3) See section 7.2.5 for specific effects of some contaminants on cation conductivity. (4) Applicable to those systems with the potential to become contaminated.

5-7


5.7

Silicate-Based Programs

Silicates are attractive from an environmental standpoint but do not offer the same level of corrosion protection as nitrites, molybdates, chromates, and hydrazine. They should be used in applications where corrosion rate monitoring capability is installed. Table 5-6 lists operating ranges and monitoring frequencies for silicate-based programs. Table 5-6 Operating Ranges and Monitoring for Silicate-Based Programs

Parameter




Tier 1 Systems (1)

Tier 2 Systems


Weekly

Monthly


Weekly

Monthly


Monthly

Monthly


Monthly

Monthly


Control Parameters Silicate

pH

Azole (2) Chloride and Fluoride (4)

20–60 ppm as SiO2 above background 9.0–11.0 5 –100 ppm as azole (3) ≤10 ppm as Cl ≤10 ppm as F

AL 1: <20 or >200 ppm AL 2: <15 ppm AL 1: <9.0 or >11.0 AL 2: <7.5 AL 1: <5 ppm AL 2: <3 ppm AL 2: >10 ppm Diagnostic Parameters

Conductivity

Evaluate trend

Weekly

Monthly



Evaluate trend

Monthly

Monthly


Iron and Copper (6)

Evaluate trend

Monthly

Monthly


Microbiological

Evaluate trend

Monthly

Monthly


Radioactivity (7)

Evaluate trend

Quarterly

Quarterly


(1) The weekly frequency for Tier 1 parameters can be reduced to up to once per month if steady-state chemical control has been maintained (all control parameters are maintained within the normal operating range) for at least one quarter. (2) Not required if it can be documented that there are no copper alloys in the system. (3) TTA to be in excess of 25 ppm if pH is in the range of 10.5–11.0. o

o


5-8


5.8

Pure Water Systems

Pure water systems have been used successfully in several plants. Success relies on maintaining water purity and low or high dissolved oxygen levels. Table 5-7 lists operating ranges and monitoring frequencies for pure water systems. Table 5-7 Operating Ranges and Monitoring for Pure Water Systems

Parameter

Control Range



Tier 1 Systems

Tier 2 Systems


Weekly

Monthly


Weekly

Monthly


Monthly

Monthly


Monthly

Monthly


Monthly

Monthly


Weekly

Monthly


Control Parameters pH

5.5–8.0

Conductivity

<2 µS/cm

Chloride (1)

≤50 ppb

Fluoride (2)

≤50 ppb

Sulfate (1)

≤50 ppb

Dissolved Oxygen (3)

≤200 ppb

AL 1: <5.5 or >8.0 AL 2: <5.0 or >8.5 AL 1: >2 µS/cm AL 2: >5 µS/cm AL 1: >50 ppb AL 2: >100 ppb AL 1: >50 ppb AL 2: >100 ppb AL 1: >50 ppb AL 2: >100 ppb AL 1: >200 ppb AL 2: >500 ppb Diagnostic Parameters

Iron and Copper

Evaluate trend

Monthly

Monthly


Microbiological (4)

Evaluate trend

Monthly (4)

Monthly (4)

Monthly or as operated (4)

Radioactivity (5)

Evaluate trend

Quarterly

Quarterly


(1)

See discussion in Section 10.2.2 for systems containing no stainless steel.

(2)

Applies to systems that contain stainless steel where temperature is >150 F (66 C). Strategic plan should consider that NSSS vendor specifications might be more restrictive.

(3)

This represents the control range for ferrous systems or ferrous systems with copper alloys. Systems that have copper components should follow the Stator Cooling Water Guidelines for oxygen control. The Action Levels in this instance would be >100 ppb or <2000 ppb for AL 1, and >300 ppb or <1000 ppb for AL 2.

(4)

Pure water has a low nutrient value. Frequency of microbiological monitoring can be decreased with documented history of low microbiological activity.

o

o

(5) Applicable to those systems with the potential to become contaminated.

5-9


5.9

Blended Glycol Formulations

Blended glycol formulations are not monitored as frequently as other systems because of the stability of the glycol and the presence of relatively high concentrations of inhibitors. In addition, vendors of proprietary formulations typically will not provide details on correcting adverse conditions or trends outside of the scope of their service agreement. It is generally necessary to accept the sampling schedule they provide and follow their specific recommendations for adjusting the inhibitor package. However, analysis and trending of certain key parameters by the site can provide an early warning of incipient problems for which corrective actions can be taken either at the site or by notifying the vendor of the condition. Tables 5-8 and 5-9 list control ranges and monitoring frequencies for blended glycol programs. Table 5-8 Control Ranges and Monitoring for Proprietary Blended Glycol Programs

Parameter

Action Levels


(AL 1 – 90 days AL 2 – 30 days)

Monitoring Frequency Tier 1 Systems

Tier 2 Systems

Quarterly

Annually

Quarterly

Annually

Control Parameters Glycol, % Volume (Freeze Point) (1) pH

AL 1: <30 or >60 AL 2: <25 or >65

30-60 (2) 7.5-11.0 [36]

AL 1: <7.5 or >11.0 AL 2: <6.5

Diagnostic Parameters Appearance

Evaluate trend

Quarterly

Annually

Conductivity

Evaluate trend

Quarterly

Annually

Contaminants, Corrosives, or Scale Formers

(3)

Annually

Annually

Corrosion Inhibitors

(3)

Annually

Annually

Glycol Properties and Breakdown Products (organic acids)

(3)

Annually

Annually

Evaluate Trend

Annually

Annually

(3)

Annually

Annually

Corrosion Products Reserve Alkalinity

(1) Freeze point can be used in conjunction with, or in lieu of, %v glycol. Both are typically determined by index of refraction or specific gravity. (2) To be consistent with specific freeze protection requirements. (3) Formulation or engine manufacturers might provide recommendations for particular parameters to monitor based on makeup water source and other factors. Maintain in the recommended range.

5-10

EPRI Licensed Material Treatment Program and Monitoring Recommendations Table 5-9 Control Ranges and Monitoring for Nonproprietary and Plant-Specific Blended Glycol Programs

Parameter



Monitoring Frequency Tier 1 Systems

Tier 2 Systems

Quarterly

Annually

Quarterly

Annually

Quarterly

Annually

Control Parameters Glycol, % Volume (Freeze Point) (1) pH Corrosion Inhibitors

30–60 (2)

7.5–11.0 [37]

AL 1: <30 or >60 AL 2: <25 or >65 AL 1: <7.5 or >11.0 AL 2: <6.5

(3) Diagnostic Parameters

Appearance

Evaluate trend

Quarterly

Annually

Conductivity

Evaluate trend

Quarterly

Annually

Reserve Alkalinity

(3)

Quarterly

Annually

Contaminants, Corrosives and Scale Formers

(3)

Annually

Annually

Evaluate trend

Annually

Annually

(3)

Annually

Annually

Corrosion Products Glycol Properties and Breakdown Products (organic acids)

(1) Freeze point can be used in conjunction with or in lieu of, %v glycol. Both are typically determined by index of refraction or specific gravity. (2) To be consistent with specific freeze protection requirements. (3) Added inhibitors might be from a manufacturer other than the glycol manufacturer. Maintain inhibitors in range recommended by inhibitor manufacturer.

5-11


6

TECHNICAL BASIS OF CHEMICALS USED FOR MICROBIOLOGICAL AND FOULING CONTROL

6.1

General

An effective closed cooling water biocide should: •

Be effective against sessile as well as planktonic microorganisms and particularly effective against the strains of bacteria that contribute to fouling or MIC mechanisms in CCW systems

•

Be compatible with corrosion inhibitors, dispersants, and other chemicals used in the system

•

Be effective at the pH of the cooling water

•

Be able to penetrate and disperse biomass

•

Be persistent, maintaining its toxic properties while in the system. Ideally, it should degrade to compounds that do not interfere with CCW system operation

•

Be susceptible to a degradation or deactivation mechanism to facilitate discharge

•

Provide ease of handling to the greatest extent possible

Biocides used to control microbiological growth fall into two broad categories—non-oxidizing and oxidizing. Oxidizing biocides are chemicals whose effectiveness depends upon their ability to oxidize and, thus, destroy organic material. Oxidizing biocides are not frequently used in CCW systems and cannot be used with nitrite-based programs. A chemical that is not an oxidizing agent but is toxic to one or more classes of microorganisms (and in some cases macroorganisms) is classified as a non-oxidizing biocide. There are many different types used, each with certain limitations. Non-oxidizing biocides are chemicals that kill organisms via a metabolic mechanism, cell transpiration or ingestion/digestion disruption, reproductive disruption, suffocation, starvation, poisoning, or trauma. In many cases, it is necessary to use more than one non-oxidizing biocide in a CCW system. In a microbiologically active system, more than one species of bacteria is usually growing. A particular biocide is more effective against some types of organisms than others. The more resistant organisms might continue to grow in spite of an application of a specific biocide. The use of a second biocide on an alternating basis helps prevent the establishment of resistant

6-1

EPRI Licensed Material Technical Basis of Chemicals Used for Microbiological and Fouling Control

microflora. This does not mean, however, that a plant cannot achieve satisfactory microbiological control through the use of a single specific biocide. Selection and application of a biocide must be made with careful consideration. Some biocides contain constituents (for example, chloride) that are incompatible with some CCW system components. After a biocide is added, the active portion eventually degrades and loses effectiveness. Organic material remains that can then become a nutrient for bacteria or can add to TOC loading of the CCW system. Some of the non-oxidizing biocides available are outlined in the following subsections. There are other biocides that are also available but less commonly used. There is ongoing research into new non-oxidizing biocides. Non-oxidizing biocides provide an additional benefit. Subsequent to addition for biocidal purposes, the biocide remains a biostat for certain periods of time. The persistence is referred to as the half-life of the compound and depends on system pH and temperature. In many cases the degradation products have biostatic properties. A biostat is a compound that does not kill organisms but has a significant impact on the organisms’ capability to reproduce. There are conditions in the CCW system that can have a biostatic effect. An example is pH. At the higher levels of the CCW system pH operating ranges (for example, >10), microbiological growth rates are reduced.

6.2

Non-Oxidizing Biocides

There are numerous non-oxidizing biocides available for potential use in closed cooling water systems. A few have been commonly used in nuclear power plants. Others have the potential for use as long as their limitations are understood. For example, many of the non-oxidizing biocides are chloride contributors. This might be a limiting factor in CCW systems containing stainless steel but might be less important if stainless steel is not present. The following is a discussion of many of the non-oxidizing biocides available. It is not an allinclusive list. In addition, new biocides are frequently introduced. Several of the biocides are discussed in detail. Others are discussed in less detail but should be given consideration based on their merits as long as their limitations are evaluated. 6.2.1 Glutaraldehyde Glutaraldehyde is a linear, 5 carbon dialdehyde (1,5-pentanedial).

6-2


Mechanism and Effectiveness Glutaraldehyde disrupts the organism by cross-linking cell protein [38]. It is effective against bacteria, yeasts, and molds and is most effective over a pH range of approximately 6–9. Above a pH of 9, it starts to degrade but can achieve a rapid kill before degradation. The increasing pH does not impact its effectiveness, only the rate of degradation. San Onofre has reported good control when using glutaraldehyde in CCW systems operating in the 9.0–9.5 pH range3. Glutaraldehyde is effective against both aerobic and anaerobic microorganisms. It is effective against both planktonic and sessile organisms and against sulfate-reducing bacteria. This is one of the most common biocides used in the nuclear industry. Applicable Metallurgy Glutaraldehyde is noncorrosive to metals at recommended use levels. Dosage Range Typical dosages are 25–100 ppm active, but the manufacturer’s label might allow a broader range. The rate of degradation due to pH can be taken into account when selecting the initial concentration or considering the use of a biodispersant. Residual Contaminants Glutaraldehyde contains only carbon, hydrogen, and oxygen and does not contain significant levels of residual contaminants. Deactivation and Degradation Glutaraldehyde is deactivated by reacting with an amine (for example, ammonium hydroxide) at a neutral or elevated pH. Glutaraldehyde undergoes a reaction at pH >9.0 to form higher molecular weight aldehydes, which are not biocidal. Ultimately, these products are biodegradable to carbon dioxide at the elevated pH. Advantages •

Glutaraldehyde is compatible with common corrosion inhibitors.

•

Active concentrations can be measured with a field test kit. This kit is available from the biocide manufacturer (Dow Chemical), water treatment service companies, and some suppliers of chemical test kits. The user should confirm that nitrites will not interfere with the test. It should also be noted that the test kits are not glutaraldehyde-specific. Test kits will

3

Data provided to Task Group by Tim Sarette, Southern California Edison Co.

6-3


respond to other aldehydes as well. The intermediate step in glutaraldehyde decomposition at pH >9 is polymerization to other aldehydes (which are not biocidal). Thus, a false positive indication of biocidal concentration can be inferred when high system pH values exist. Disadvantages •

Glutaraldehyde ultimately decomposes to carbon dioxide and possibly to glutaric acid. This can result in pH depression in the CCW system.

•

Incomplete decomposition creates organics that might be nutrients for certain organisms.

•

Glutaraldehyde increases TOC in the CCW system and can cause foaming.

•

Glutaraldehyde is deactivated by ammonia or amine (1:1 molar basis required).

6.2.2 Isothiazolone This biocide is a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin3-one. It is usually supplied as a 1.5% solution (nonionic). Mechanism and Effectiveness Isothiazolone inhibits macromolecular synthesis (for example, protein, RNA, and cell wall). It is effective against planktonic and sessile bacteria. It is also effective against anaerobes, including sulfate-reducing bacteria (SRB). It is typically used in a pH range of 6.0–9.0, with 8.0 being optimum. Above a pH of 9.0, the rate of hydrolysis increases. The increasing pH does not impact its effectiveness, only the rate of degradation. Applicable Metallurgy Isothiazolone is noncorrosive to metals at recommended use levels. Dosage Range Typical dosages are 1–5 ppm active, but the manufacturer’s label might allow a broader range. The rate of degradation due to pH can be taken into account when selecting the initial concentration or considering the use of a biodispersant. Residual Contaminants This biocide contains magnesium chloride and nitrate as a stabilizer. The stabilizer will contribute chloride to the water. The biocide might also contain a significant concentration of copper (0.05–0.25 ppm copper at normal biocide dosage). There is a formulation of the biocide currently available that does not contain copper. The addition of 1.5 ppm active biocide will contribute 0.3 ppm chloride from the chloride that is part of the 5-chloro-2-methyl-4-isothiazolin-3-one molecule. 6-4


The magnesium chloride present as a stabilizer will contribute additional chloride. The supplier should be asked to quantify this chloride contribution prior to use. The compound also contains sulfur and can be a sulfate contributor. The addition of 1.5 ppm active biocide will contribute approximately 1 ppm of sulfate to the water. Deactivation and Degradation The rate of hydrolysis of isothiazolone increases with increasing pH. The increasing pH does not impact its effectiveness, only the rate of degradation. Advantages •

Low dosages are required.

•

It is a water-based material and does not contain residual organics.

•

Isothiazolone acts very rapidly.

•

It is very effective against nitrifying and denitrifying bacteria.

Disadvantages •

Sulfate and chloride contribution.

•

Possible copper contribution (depending on formulation).

•

No field test kit.

6.2.3 Methylene Bis(Thiocyanate) Methylene bis(thiocyanate) is the methylene ester of thiocyanic acid with a chemical formula of CNS-CH2-CNS. Note: methylene bis(thiocyanate) is commonly referred to by the acronym MBT. This is not to be confused with mercaptobenzothiazole (copper corrosion inhibitor), also called MBT. Mechanism and Effectiveness The thiocyanate fragment of the molecule blocks the transfer of electrons in the respiratory system of microorganisms [39]. It is very effective against bacteria and fungi. At pH levels above 7.5, the compound hydrolyzes but can be effective up to a pH of 8.5. At a pH of 8.5, the half-life of the biocide is approximately 4 hours [40]. At pH 9.5 it must be slug-fed to be effective. Applicable Metallurgy Methylene bis(thiocyanate) is noncorrosive to metals at recommended use levels. 6-5


Dosage Range Typical dosages are 1–8 ppm active. Residual Contaminants 1 ppm of the biocide will contribute 0.75 ppm of sulfate to the water. Deactivation and Degradation Methylene bis(thiocyanate) hydrolyzes to less toxic compounds. Advantages •

Effective at low dosages

•

A broad-spectrum biocide

•

Readily deactivated and hydrolyzed

•

Biofilm penetrator and not affected by the presence of other organics

Disadvantages •

Repeated applications can increase TOC content of CCW system

•

Not a water-based product

•

Contributes sulfate to the water

•

Solvent might contain chloride

•

No practical field test kit

6.2.4 DBNPA DBNPA is 2,2-dibromo-3-nitrilopropionamide. Mechanism and Effectiveness DBNPA interacts with enzymes and interferes with cell respiration and metabolism. It is very effective against bacteria and achieves a significant kill in 1–3 hours. It is effective against both aerobic and anaerobic bacteria. At pH levels above 7.5, the compound hydrolyzes but can be effective up to a pH of 8.3 or higher. As pH increases, the rate of hydrolysis increases. For example, at 37oC, the half-life at pH 7.32 is 7.5 hours, but, at a pH of 8.3, it is only 25 minutes. The rate of degradation due to pH can be taken into account when selecting the initial

6-6


concentration or considering the use of a biodispersant. In spite of the rapid rate of hydrolysis at elevated pH, it has been used successfully in nuclear CCW systems4. Applicable Metallurgy DBNPA is noncorrosive to metals at recommended use levels. Dosage Range Typical dosages are 0.5–10 ppm active. As noted, the rate of degradation due to pH can be taken into account when selecting the initial concentration or considering the use of a biodispersant. Residual Contaminants 1 ppm of DBNPA will contribute 0.7 ppm of bromide to the water. DBNPA is contained in a glycol solvent (47 to 60% polyethylene glycol) that might provide a nutrient source for bacteria. Deactivation and Degradation DBNPA degrades to carbon dioxide, bromide ion, and ammonia. Strong reducing agents, such as sulfites, deactivate DBNPA via debromination. Mercaptobenzothiazole (copper corrosion inhibitor) also deactivates DBNPA. Advantages •

DBNPA acts very rapidly.

•

Low dosages are required.

•

A field test kit is available.

Disadvantages •

The glycol solvent adds considerable TOC to the CCW system.

•

Degradation produces ammonia.

•

Bromide is added to water.

6.2.5 THPS THPS is tetrakis(hydroxymethyl) phosphonium sulfate. It is a non-foaming, water-soluble quaternary phosphonium compound with broad-spectrum biocidal efficacy.

4

Oconee and Catawba experience.

6-7


Mechanism and Effectiveness THPS disrupts cell wall integrity by denaturing protein. THPS is effective over a pH range of 3–10. THPS exposure results in rapid inhibition of lactate dehydrogenase activity in the metabolism of SRB. Applicable Metallurgy THPS is noncorrosive to metals at recommended use levels. Dosage Range Typical dosages are 20–100 ppm active. Residual Contaminants A 50 ppm dose of THPS (active) will contribute 12 ppm of sulfate and 3–4 ppm of phosphate. Deactivation and Degradation THPS is absorbed by activated carbon. It can be deactivated with ammonium bisulfite (and probably sodium bisulfite). Its effectiveness is reduced in systems with high turbidity or suspended solids because of adsorption to solids in bulk water. Advantages •

THPS exhibits low toxicity to mammals, fish, and Daphnia magna.

•

Halide free.

•

Field test kit available (minimum detection level 10 ppm active).

Disadvantages •

High dosages required

•

Sulfate contribution

•

Limited data on effectiveness in closed cooling systems

6.2.6 Ionine Quaternary Compounds These are low molecular weight polymeric, ionine quaternary nitrogen biocide/biostats. Their chemical description is poly (oxyethylene iminio-ethyleneiminio-ethylene dichloride); approximate nominal molecular weight is 5,000–10,000. The ionine quaternary biocides are algaecidal, bactericidal, and actively biostatic in closed cooling water environments. They are also effective against mollusks, hydroids, and other macrofouling organisms. They are non-foaming and not affected by pH. Some of these 6-8


compounds have National Sanitation Foundation (NSF) allowances as potable water biocide. They are also effective as a biodispersant, especially in preventing biofilm deposits. Mechanism and Effectiveness The mode of action is in disruption of cell membranes. The molecule penetrates the membrane, thereby interrupting ion and electron transport, and other transport mechanisms. Disruption of the cell membrane can also cause leakage of intracellular materials, thus causing death. Application Metallurgy Ionine quaternary biocides are noncorrosive to metals and other materials of construction at recommended use levels. Dosage Range Ionine quaternary biocides are typically used at 5–25 ppm active in closed cooling water systems. Residual Contaminants A 25-ppm (active) dose of an ionine quaternary biocide will contribute approximately 4 ppm of chloride. Deactivation and Degradation Activated carbon, corrosion products, and high levels of turbidity or suspended solids in bulk water absorb the ionine quaternary biocide. Advantages •

Ionine quaternary biocides persist under most situations in closed loop cooling systems, thereby providing significant biostatic activity in treated systems.

•

Field test kit available – minimum detection level approximately 0.6 ppm active.

•

Ionine quaternary biocides are compatible with most inhibitors used to control corrosion.

Disadvantages •

Contributes approximately 16% chloride w/w per addition level of active ingredient.

•

Shock dosage of an ionine quaternary biocide at maximum levels might limit the effectiveness of anionic compounds used for scale and deposit control at minimal dosage levels. 6-9


6.2.7 Dodecylguanidine Hydrochloride (DGH) This product contains chloride and is incompatible with many nuclear CCW systems because of chloride limitation. At typical use rates (25–50 ppm active), it would contribute 3–7 ppm of chloride to the CCW system. 6.2.8 Carbamates Carbamates can attack copper alloys and precipitate iron. They are more applicable to open recirculating systems. 6.2.9 Quaternary Ammonium Salts (Quats) These products contain chloride and are incompatible with many CCW systems because of chloride limitation. Because there are many different quat formulations on the market, the chloride contribution can vary. Quats can cause foam that could cavitate pumps. 6.2.10 Tris-Nitro Tris-nitro (2-nitro-2hydroxymethyl-1,3-propanediol) is unstable at a pH greater than 8.5 and releases formaldehyde and ammonia.

6.3

Oxidizing Biocides

Oxidizing biocides are rarely used in closed cooling systems. They are not compatible with nitrite-treated systems because the biocide will oxidize the nitrite to nitrate. Of the oxidizing biocides, only hydrogen peroxide is compatible with nuclear CCW systems. 6.3.1 Hydrogen Peroxide Hydrogen peroxide (H2O2) is an aqueous oxidizing agent. Mechanism and Effectiveness Hydrogen peroxide (H2O2) is a strong oxidizing agent and acts by destroying the protein groups in microbiological substances. Hydrogen peroxide is not compatible with nitrite-treated systems. It oxidizes the nitrite to nitrate. It can be used in chromate- or molybdate-treated or pure water systems, but, because it is an oxidizing agent, it might increase corrosion rates. Applicable Metallurgy It is compatible with all metallurgy, but oxidizing action could increase corrosion rates of ferrous and copper alloys. 6-10


Dosage Range Applicable dosage ranges for H2O2 in CCW systems have not been well established. Dosages in the range of 10–100 ppm have been effective in open recirculating and spray pond systems. Residual Contaminants None. Hydrogen peroxide degrades to water and oxygen, thus making it attractive from the standpoint of residual contaminants. Advantage •

Hydrogen peroxide does not add any residual contaminants to the system.

Disadvantages •

Hydrogen peroxide might cause foaming if it reacts with any significant amounts of biological material.

•

At Surry, the addition of approximately 1500 ppm as hydrogen peroxide to a bearing cooling water system generated sufficient gas to create oscillations in pump amperes and discharge pressure [41].

6.3.2 Chlorine Compounds The use of chlorine compounds will add significant levels of chloride to the CCW water. Like other oxidants, chlorine compounds are not compatible with nitrite.

6.4

Other Treatments

Most vegetative cells are killed when exposed to temperatures exceeding 62°C (145°F). However, specific bacteria (for example, Bacillus sp. and anaerobic Clostridium sp.) have the capability of forming endospores when exposed to adverse conditions such as an increase in temperature. Endospore-forms of these types of bacteria are resistant to thermal effects of temperatures exceeding 100°C (212°F). When the temperature is reduced to 62°C (145°F), the endospores germinate into reproducing vegetative cells and the population regenerates. Thermal treatment is not typically used in CCW systems as a remedial measure, but there are potential applications. Many emergency diesel generator jacket water systems achieve an operating temperature sufficient to kill many bacteria during the monthly surveillance operation.

6-11


6.5

Methods Used for Fouling Control

6.5.1 General Fouling in closed cooling systems results from the settling of suspended solids, usually corrosion products, in piping or on heat exchange surfaces. Such deposits shield a localized area of metal from the bulk cooling water flow, creating two different chemical environments or a differential concentration cell. The effect of localized corrosion or pitting attack can be severe on thin-walled heat exchanger tubes. Suspended material tends to drop out in low-flow areas of a cooling system or onto biologically active sites. The amount of suspended solids that chemical treatment can keep dispersed depends upon characteristics of the cooling system. When there are low cooling water velocities (<2 feet/second or <0.6 meters/second), and certainly when shell-side cooling is present, suspended solids levels above approximately 50 ppm in the cooling water, might drop out even if chemical treatment is used. In systems with consistently high-flow velocities (>2 feet/second or >0.6 meters/second), chemical treatments can effectively control suspended solids of several hundred ppm. 6.5.2 Suspended Solids Deposition Suspended solids are a cause of fouling in CCW systems. These solids settle on the cooling system surfaces, often being “glued” by microbiological slimes. Foulants originate from two sources: •

Water-borne foulants from outside the system

•

Foulants generated within the system

Water-Borne Foulants from Outside the System

Potential foulants are transferred to the closed cooling systems by some type of in-leakage. Both chemical and mechanical methods are used for control. In the case of closed cooling systems, known foulants must be removed from the makeup water. There is the potential for formation of mineral scale if raw water is used for makeup and if there is significant makeup. Such foulants cannot be handled effectively in the recirculating water with chemical treatment. The application of chemicals such as dispersants, sludge fluidizers, and surfactants will not deliver acceptable results unless employed with side-stream filtration, full-flow filtration, feed and bleed, or drain and refill. Oils and other contaminants can be introduced to CCW systems through leaks. In auxiliary cooling systems containing lubricating oils, leakage is typically from the higher pressure oil side to the lower pressure cooling water system. Leaks in service water system heat exchangers will also introduce foulants into the CCW system. 6-12


Foulants Generated Within the System

Corrosion of system materials, particularly carbon steel components, can result in the introduction of significant quantities of suspended material to the system. Although copper and copper alloy corrosion might not produce significant levels of suspended solids, the corrosion products can cause pitting of carbon steel. 6.5.3 Chemical Foulant Control Methods There are two types of chemicals that are used for suspended solids control in CCW systems— dispersants and surfactants. In many CCW systems, one of these could be part of the watertreatment program. Supplementary doses of dispersants or surfactants are usually applied only in response to an identified or perceived problem. There is often more than one technique that can be used. It is important that the foulant first be identified before a dispersant or surfactant is selected. Dispersants Low molecular weight acrylate-based polymers (carboxyl active groups) are typically used as dispersants in cooling water systems. Polyacrylic acid and sodium polyacrylate (sodium salt of polyacrylic acid) are the forms of polyacrylates used. In addition to straight polyacrylates, polyacrylamides and acrylamide/acrylate copolymers are also applicable. The low molecular weight dispersants (1000–2000 MW) are typically used, although up to 10,000 MW polymers have been successfully used. Several copolymers and terpolymers are also used. When dispersants are used in CCW systems, it is important that the suspended material be removed from the system. This can be done by feed and bleed or by filtering. Dispersants are manufactured by several producers of basic and specialty chemicals. In addition, some of the larger water treatment service companies manufacture their own polymeric dispersants. Treatment levels will vary depending upon the needs of the cooling system and water characteristics. The factors that influence dispersant dosage are suspended solids composition and concentration. Dispersant dosages of 2–3 ppm (as 100% active polymer) might be adequate for general duty if suspended solids loading is minimal (<20 ppm). At higher suspended solids loading (>20 ppm), 5–10 ppm might be needed. Surfactants (Biodispersants) The effectiveness of a biocide program can be enhanced or improved with the addition of a surfactant. These are nonionic chemicals applied in conjunction with a biocide for removing and dispersing microbiological slime deposits. A surfactant can help prevent attachment of microorganisms on surfaces by reducing the surface tension of the water. Surfactants also improve the effectiveness of biocide penetration into the biofilm or biomass when control is lost. This helps 6-13


to dislodge the microorganisms into the bulk water where they are more easily controlled with biocides. A surfactant must be compatible with the primary biocide and be approved for use in the specific system. Surfactants are used when oil or slime deposits are found in the cooling water system. These surfactants are wetting agents similar to low-foaming detergents. They act by emulsifying and dispersing the slimy or oily foulants. Even though the surfactants are of the low-foaming variety, they must be used carefully. Once the biomass in the system is released by the surfactant, the total concentration of organic material could result in foaming. When the biomass of sessile organisms is thus disturbed, the effectiveness of microbiocide applications is enhanced. Non-ionic surfactants are typically used because charged (anionic or cationic) structures might not be compatible with other components in the water treatment program. These surfactants are also called biodispersants by many manufacturers. They are very different from polymeric dispersants in that they are not highly charged and, therefore, do not act in the same way. Surfactants are manufactured by basic and specialty chemical suppliers. For water treatment, they are typically marketed through water treatment service companies. Because surfactants have many uses other than water treatment, they are also available through other sources. Dosages of 10–20 ppm or more are added as needed to clean up the cooling water equipment. Low dosages (less than 5 ppm) have been used as biopenetrants. Surfactants should only be used to address a specific problem rather than as a general addition to a water-treatment program. When used, they should be added at an adequate dosage to have a measurable effect. Care should be exercised, however, to prevent excessive foaming, which could impair pumping and heat transfer efficiencies. 6.5.4 Feed and Bleed Feed and bleed is commonly used in nuclear plants to reduce the concentration of a contaminant. This consists of bleeding some of the CCW system water to drain and replacing it with fresh water. Inhibitor is added before, during, or after the bleed to maintain the inhibitor concentration range. Disadvantages •

Significant quantities of water must often be removed and replaced in a CCW system to effect the desired reduction in suspended solids.

•

Water bled from the system can have a negative impact on radwaste.

•

The replacement water might destabilize the protective film and increase corrosion rates.

6-14


6.5.5 Drain and Refill This consists of completely draining fluid from the CCW system and replacing it with fresh water and inhibitor. 6.5.6 Sidestream Filtration and Sidestream Demineralization Sidestream filtration is often a more effective means of removing suspended material from CCW systems than feed and bleed. Advantages •

Only particulate matter is removed.

•

Specific particle size ranges can be targeted (depending on filter design).

•

No additional load on radwaste is created.

Sidestream filtration will not remove dissolved impurities; however, sidestream demineralization can be used to remove dissolved impurities. This can be accomplished by either a deep bed demineralizer or a precoat powdered ion exchange resin filter demineralizer. There are numerous factors that must be evaluated in considering the addition of sidestream filtration or sidestream demineralization to a CCW system. In a nuclear plant, this is not a simple and straightforward choice. The modification process is complex and expensive. Therefore, this guideline document cannot make any recommendation regarding the installation of sidestream filtration or sidestream demineralization beyond pointing out the potential advantages of the technology. 6.5.7 Other Mechanical Means of Suspended Solids Control In addition to chemical methods, there are mechanical means of mitigating suspended solids fouling. These include mechanical cleaning and flow reversal or backwashing. A detailed discussion of these methods is beyond the scope of this guideline.

6-15


7

TECHNICAL BASIS OF MONITORING PARAMETERS

7.1

Introduction

This section provides the technical basis for parameters monitored in closed cooling water systems. It provides the technical basis for parameters with limits and where Investigative Parameters may apply to specific treatment regimens. Corrosion, biological growth, and fouling in CCW equipment can interfere with its intended design function. Control of these processes is important in both closed and open recirculating cooling systems. The importance is greater, however, in closed systems because the foulants (corrosion products, slime masses, etc.) are retained in the system rather than being lost through systematic blowdown (bleed). The overall goal for a CCW system chemistry program is to control those conditions that might impact the intended function(s) of the CCW system or unacceptably shorten component life. Because CCW systems in nuclear plants vary in design, it is important to maintain flexibility in addressing specific performance targets and view those targets in the context of the total program. The purpose of monitoring CCW systems is to: •

Confirm that specified chemical parameters in the CCW system are being maintained

•

Confirm that performance goals are being met

•

Identify adverse trends

An appropriate monitoring program for a particular CCW system will be dependent on several factors. These include but are not limited to: •

System metallurgy

•

System design

•

System integrity (leaks)

•

Chemical treatment program

•

Site-specific history

7-1

EPRI Licensed Material Technical Basis of Monitoring Parameters

There are certain measurement tools that can be used to help determine the effectiveness of the treatment program. These tools may include: •

Chemical treatment and impurity measurements

•

Corrosion control measurements

•

Microbiological measurements

•

Non-chemistry monitoring

There is no single monitoring program that is appropriate for all CCW systems. Most programs will use chemical testing, but the frequency and range of parameters will vary. Microbiological measurements are helpful but do not provide information on deposits in the CCW system. Corrosion rate monitoring (coupons or electrochemical probes) might provide valuable information in some systems, but might not be meaningful in other systems. The necessity of performing specific measurements is a function of system design, operating conditions, and system history. Regardless of the specific measurements used, the trending of data is an important part of the overall CCW performance monitoring program.

7.2

Testing Parameters

7.2.1 Corrosion Inhibitors This test is a direct measurement of the corrosion control components in the treatment program. It is measured to ensure that there is an adequate level of protection for system metallurgy. Refer to Section 4 for the technical basis for corrosion inhibitors. 7.2.2 pH Refer to Section 4. CCW system pH is monitored because: •

pH has a direct impact on corrosion rate

•

pH might be an important indicator of microbiological activity in the system

•

pH can be an indicator of system in-leakage

In inhibited glycol systems, pH is monitored because the conversion of glycol to organic acids can cause a pH decrease and accelerate corrosion. 7.2.3 Percent Glycol In systems containing an inhibited glycol formulation, the percent glycol or freeze point (typically determined by index of refraction or specific gravity) is tested to determine that it is in the proper range. A glycol percentage that is too high or too low will not provide the required freeze protection. A glycol percentage that is below 30% can potentially act as a microbiological nutrient and glycol can be degraded. 7-2


7.2.4 Conductivity Conductivity is tested because it is an indirect measurement of the concentration of chemical treatment in the CCW system. Conductivity might also be a rapid indicator of system leaks or contaminant ingress. 7.2.5 Cation Conductivity Cation conductivity is a diagnostic parameter for Hydrazine-treated closed cooling water system chemistry programs and can also be used in pure water systems. Cation conductivity is an on-line continuous measurement and can be used as a quick and simple means of monitoring and controlling anionic contaminants without having to analyze for each anion. The values shown in Table 7-1 were calculated using the EPRI ChemWORKs Plant Chemistry Simulator 4.0. The table shows that controlling a closed cooling water system on hydrazine-based chemistry to a cation conductivity of 1.3 µS/cm or less would ensure that the limit of 150 ppb (see Table 5-5) would not be exceeded for sulfate, chloride, or fluoride. Similarly, for a pure water chemistry, controlling cation conductivity to 0.45 µS/cm would ensure that neither chloride nor sulfate would exceed the 50 ppb limit (see Table 5-7). Table 7-1 Contaminant Concentration Versus Calculated Cation Conductivity Contaminant, ppb

Calculated Cation Conductivity, µS/cm

-

3.17 µS/cm

-

0.60 µS/cm

150 ppb F 50 ppb Cl

150 ppb Cl 50 ppb

-

1.80 µS/cm

SO42-

0.45 µS/cm

SO42-

1.34 µS/cm

150 ppb

The presence of organic acids or other anions that are not specified in Section 5 will result in higher cation conductivities, which will make cation conductivity less useful for controlling sulfate, chloride, or fluoride. When cation conductivities are routinely above 1.3 µS/cm for hydrazine-based chemistry or 0.45 µS/cm for pure water-based chemistry, then the concentration of each anionic control parameter must be determined by chemical analysis. 7.2.6 Chloride and Fluoride (Halides) Chloride can contribute to carbon steel corrosion and chlorides and fluorides can be related to stress corrosion cracking of 300 series austenitic stainless steels. The chloride and fluoride limit for Hydrazine-treated systems is based on the Westinghouse limit of 150 ppb. The 50 ppb limit for pure water systems is based on consensus of the Closed Cooling Water Chemistry Guideline Task Group. For pure water systems without stainless steel, fluoride would be a diagnostic parameter. For chemistries other than hydrazine and pure water, refer to Appendix A (stainless steel) and Section 10.2.2 (carbon steel). 7-3


7.2.7 Nitrate This applies to the measurement of nitrate in nitrite or nitrite/molybdate-treated systems only. Nitrite can be converted to nitrate in one of two ways: •

Nitrate is produced by the conversion of nitrite to nitrate by nitrifying bacteria.

•

Although nitrate can be produced by the direct oxidation of nitrite by dissolved oxygen, the kinetics are very slow and high temperatures are required.

An increase in nitrate concentration is primarily an indicator of the presence of nitrifying bacteria. A nitrate increase can be used to diagnose this condition. Some CCW treatment formulations contain added nitrate. When this is the case, the nitrate test might have less meaning as a nitrifying bacteria indicator because of the statistical significance in measuring small changes to a large background concentration. Nitrates are generally not harmful in closed cooling water systems but have been reported to contribute to stress corrosion cracking of carbon steel at high concentrations. Under certain conditions, concentrations of nitrate in the range of 10,000 ppm (1%) have been associated with stress corrosion cracking of carbon steel [13,14]. Concentration mechanisms underneath deposits might serve to achieve these levels, even if nitrate concentration in the bulk water is lower. 7.2.8 Ammonia Ammonia can be a corrodent to copper and copper alloys under oxidizing conditions, and can impact pH. Ammonia can be produced by denitrifying bacteria in nitrite and nitrite/molybdatetreated systems. Microbiological testing is a confirming diagnostic tool. Ammonia can also be produced by hydrazine and amine decomposition in hydrazine- and amine-treated systems. In addition, ammonia can be present in potable water used for makeup (rarely used in nuclear plants). Ammonia is measured as a possible indicator of denitrifying bacteria. There is no need to measure ammonia in molybdate-, chromate- or silicate-treated systems. 7.2.9 Iron and Copper (Corrosion Products) The accumulation of corrosion products (iron, copper, and other metal oxides) in the CCW system is an indirect indication of corrosion. In nitrite-, molybdate-, nitrite/molybdate-, and hydrazine-treated systems, the pH generally favors insoluble iron oxide. However, the pH can vary over a wide range and, thus, the percentage of insoluble iron will vary. In pure water systems, both soluble and insoluble iron might exist. Each system establishes a normal concentration of these corrosion products based on the corrosion inhibitor(s) used, the materials of construction of the system, and their respective surface areas. Consequently, a specific number cannot really be assigned as a not-to-exceed value. It should also be noted that conditions in a system that might lead to pitting corrosion 7-4


would not be observable in these trends of metals because the overall increase in concentration from this effect would be very small. Additionally, it is very difficult to sample for dissolved versus insoluble species unless specifically engineered sample points (isokinetic sampling) are available. This means that the most reliable method of analysis to be used in trending will be total metal. The trends of the total concentration of these metals will provide general information about upsets to the system. Metal concentrations, which trend higher in bulk coolant, might be significant because they can foul heat exchanger tube surfaces, pump seals, and flow-measuring devices. This can result in loss of flow, increased ∆P, loss in efficiency, and increased maintenance for components. The values chosen for acceptable ranges or limits for iron and copper should be developed on a plant-specific basis. Factors to be considered in establishing the limits are: •

Components with thinnest nominal wall thickness yielding a maximum mm/yr corrosion rate.

•

Makeup water rate to the system.

•

Are maintenance crews identifying component repairs or failures due to the presence of solids?

•

Does the system have filtration or demineralization capabilities?

•

Is there an historical correlation between iron or copper levels and system parameters such as flow, pressure, or temperature?

•

Are component specifications being used as part of plant life extension justification?

7.2.10 Microbiological Activity An increase in microbiological activity, particularly in nitrite-treated systems, can lead to corrosion and fouling. In molybdate-treated systems, microbiological activity can reduce system pH. Microbiological activity is typically not a problem in chromate- or hydrazine-treated systems. 7.2.11 Radionuclides The presence of certain radionuclides in systems cooling a primary system component, can be an indication of leakage into the CCW system or neutron activation of corrosion inhibitors. Refer to NRC Information and Enforcement Bulletin 80-10. This bulletin directs operators of nuclear power plants in the United States to have a 10CFR50.59 screening performed for continued operation of a system, which does not normally contain radioactive materials, when it becomes radioactively contaminated.

7-5


7.2.12 Specific Microorganisms5 Detecting specific microorganisms or groups of microorganisms is an important Investigative Parameter. The need arises to isolate and characterize microorganisms found during a visual or microscopic deposit examination when determining the root cause for non-conformance. Several of the microorganisms associated with MIC and other microbiological problems have unique shapes and sizes and can be characterized by the use of the microscope alone. In other cases, they can be isolated or detected from a deposit at the site of the non-conformance by using selective culturing procedures. By culturing samples on selective culturing media, it is possible to determine the presence of slime-forming bacteria, sulfate-reducing bacteria, metal-oxidizing bacteria, and nitrite- or nitratemetabolizing bacteria. Fungi and algae (blue-green algae) can also be characterized by selective culturing techniques. The microorganisms most commonly looked for include: •

Sulfate-reducing bacteria (for example, Desulfovibrio sp.)

•

Metal-oxidizing bacteria (for example, Gallionella sp., Leptothrix sp.)

•

Acid-producing bacteria (for example, Clostridium sp., Vibrio sp.)

•

Slime-forming bacteria (for example, Siderocapsa sp., Pseudomonas sp., Bacillus sp.)

•

Nitrite- or nitrate-oxidizing bacteria and ammonia producing bacteria (for example, Nitrosomonas sp., Nitrobacter sp., Nitrococcus sp.)

7.2.13 Total Organic Carbon (TOC) Total organic carbon in CCW systems is produced by various sources, primarily azoles and biocides. TOC can provide food for bacteria and make microbiological control more difficult. There is no evidence that increased TOC can lead to increased system corrosion. Elevated TOC could be related to foaming but this would be more a function of the type of organic compound in the systems than TOC itself. The Entergy Waterford 3 plant experienced evidence of foaming when sampling a CCW system that had been aggressively treated with glutaraldehyde. No pump cavitation was observed, but the plant used TOC analysis as a troubleshooting tool. The analysis revealed that TOC in the system was in the range of 100–300 ppm above what would be expected because of the presence of TTA. TOC is considered an investigative parameter. 7.2.14 Dissolved Oxygen Dissolved oxygen plays a part in the corrosion process. In inhibited CCW systems (nitrite, molybdate, nitrite/molybdate, chromate, silicate), a passive film is established and the presence of dissolved oxygen does not appear to have a significant impact on corrosion rates. In fact, it is commonly believed that some dissolved oxygen is helpful in molybdate-treated systems (see Section 4.3.2). Unless there is significant makeup or the system is designed so that oxygen 5

Abstracted from the EPRI Microbiologically Influenced Corrosion Training Course, Ed.2002.

7-6


ingress is normal, dissolved oxygen is rapidly depleted and CCW systems will be relatively free of dissolved oxygen. In inhibited systems, dissolved oxygen is an Investigative Parameter. There is little value in routine measurement. In pure water and hydrazine systems, dissolved oxygen is a Control Parameter. In these systems, oxygen can play a significant role in copper and copper alloy corrosion because most systems do not employ a copper corrosion inhibitor. The limits for hydrazine-treated and pure water systems are based on the Stator Cooling Guidelines [4]. 7.2.15 Sulfate The sulfate ion can contribute to IGSCC in 300 series stainless steels. Sulfate can compete with the corrosion inhibitor in carbon steel. Sulfate contamination can come from poor quality makeup water, service water in-leakage, or biocide additions. Sulfate is a routine Control Parameter in pure water and Hydrazine systems and an Investigative Parameter in other inhibited systems. 7.2.16 Calcium and Magnesium In systems with raw water makeup, hardness ions (calcium and magnesium) can lead to the formation of mineral scales if there is excessive makeup. Calcium and magnesium can also be introduced by service water in-leakage. The presence of magnesium can be observed due to the use of isothiazolone, which has magnesium chloride added as a stabilizing agent. Calcium and magnesium are Investigative Parameters and are not measured unless raw water ingress is suspected. 7.2.17 Refrigerant Chemicals Refrigerant chemicals (for example, Freon) were listed in Table 5-1 in Revision 0 of the Closed Cooling Water Chemistry Guideline. In reality, the evaporator section of a chiller operates under a vacuum. Therefore, the refrigerant gas cannot make its way into the chilled water during system operation. Analyzing for refrigerant chemicals in chilled water would have little value, even as an Investigative Parameter. 7.2.18 Reserve Alkalinity (RA) Reserve Alkalinity is a term applied to inhibited glycol engine coolants to give an approximate indication of the amount of remaining alkaline inhibitors. The alkaline inhibitors provide buffering action and neutralize acids that are introduced into the coolant or created by the oxidation of ethylene or propylene glycol. Some inhibitors that contribute little or no RA can give excellent corrosion protection to certain metals but have little capacity to combat acid contamination. Therefore, caution should be used when monitoring RA. Reserve Alkalinity is not the sole dependable measure of the coolant’s ability to prevent corrosion, nor can it satisfactorily indicate the additional life of the coolant [42]. 7-7


7.2.19 Visual Evaluation of CCW Fluids The condition of the system fluid can be qualitatively assessed from visual examination of a sample. This is particularly true in glycol systems, but can be applied to other systems as well. The attributes that should be qualitatively evaluated on a fresh sample of coolant are: •

Color – Glycol formulations (as well as other inhibitor packages) will have a proper color and intensity for the concentration of inhibitor present. A visual comparison with an inhibitor concentration of the approximate concentration should be made to assess these parameters.

•

Turbidity or Clarity – Terms like translucent, hazy, and opaque are qualitative terms indicating the amount of suspended material.

•

Solids – Their color and approximate volume percent in the sample container after a short settling period without centrifugation.

•

Surface film – Visual examination of the sample surface for oils, congealed materials (lubricants), or other floating matter.

The visual observation of these characteristics should be performed in a well-lighted area (preferably the laboratory) with a white-to-neutral-colored background in order to see through the sample. This will provide for consistent results between analysts. Notations should be made in the Remarks or Comments section of the sample log.

7.3 Monitoring Frequency Sampling and analysis frequencies are discussed in the tables in Section 5. Emergency diesel generators fall into the category of intermittent systems. They are typically operated on a monthly surveillance schedule and are sampled at the time of operation. Unless plant experience indicates otherwise, it is important that these systems be sampled while they are operating or shortly after operation is terminated. Otherwise, the sample might not be representative. Note that diesel operation is likely to change any microbiological populations due to temperature fluctuations. Investigative Parameters are used to assess changes in the chemistry of CCW systems when there is no apparent cause for the change that can be found through analysis of the Control and Diagnostic Parameters. As such, there is no recommended sampling frequency.

7.4

Investigative Parameters

Investigative Parameters are typically not used until an anomalous result in the CCW system shows up. At that time, the cause of the change can be determined by using these parameters, although an actual value for the system might not exist. Simulation methods can be used to assess what the ideal values of these parameters might be when this type of situation arises. One such method is to use the makeup water source and add 7-8


fresh inhibitor solution to the system’s existing concentration, and then measure the value of the Investigative Parameter. This would then be compared to the value presently in the system. Another method might be to evaluate the presence of a potential contaminant based on chemical analysis of the system additives, provided by the manufacturer (for example, using the specification sheet, which provides information on potential contaminants). Perhaps the best way to describe how to evaluate the results of Investigative Parameter analyses is through the following examples: Case 1: A CCW system has a sudden decrease in the concentration of chromate. The chemistry analysis for chromate uses the colorimetric diphenylcarbazone method. It is also noted that the color of the sample has a green tinge. The initial response is to look for leaks and increased makeup causing the chromate concentration to drop. In parallel, some chemical analyses might be appropriate. Analysis of the sample for total chromium by atomic absorption analysis would show if the concentration of total chromium has changed. If it has not, analysis for TOC would be indicated. Chromium can exist in two different oxidation states in CCW systems: +3 (chromic ion) and +6 (either chromate or dichromate ion). Carbonaceous materials can be oxidized by chromate, changing its oxidation state to +3. The source of TOC might be a different makeup water source, chemical additive contamination, or maintenance activities. Case 2: Routine measurement of specific conductivity in a system that uses hydrazine is 25–35 µS/cm. A sudden increase in the conductivity of the CCW sample to 55 µS/cm is observed, as well as an increase in the sulfate concentration from 20–45 ppb. It seems apparent that there is ingress of raw water. Sodium, calcium, and magnesium analysis would help to identify if the increase in sulfate is commensurate with the amount of these ions present in the system. A calculation could also be performed to determine if the sulfate increase from raw water alone could account for the increase in conductivity.

7-9


8

ADDITIONAL MONITORING TECHNIQUES

8.1

Corrosion

The control of corrosion is the primary purpose of the CCW treatment program. Although this is the goal, it is often not easy to directly confirm satisfactory corrosion control; however, the treatment programs that are typically used in nuclear CCW systems have a well-regarded history of good corrosion control. The corrosion rates targeted in CCW systems are dependent on the alloy. Carbon steel usually has the highest corrosion allowance. Corrosion rate targets for copper alloys are limited by the secondary effect of inducing pitting corrosion of carbon steel. When copper alloys corrode, copper ions are transported in the water and might deposit on carbon steel surfaces as copper metal. This initiates small galvanic corrosion cells on the carbon steel and creates pits. Stainless steels are limited to very low general corrosion rates, consistent with the higher corrosion resistance of these materials. It is important to note that pitting attack on any metal is unacceptable. There are several methods available to determine corrosion rates. Methods without heat transfer involve inserting a metal specimen in the sample stream and measuring corrosion rate. This is done by either determining weight loss or by measuring an electrochemical phenomenon related to corrosion (linear polarization resistance or electrical resistance). Corrosion rate measurements are done under ideal conditions and do not represent a direct correlation with system conditions. Although corrosion rate measurements can provide helpful data, they might not reflect what is actually happening in an operating system. Industry experience has shown that corrosion rates will be low if inhibitor levels are consistently maintained, pH is held within recommended ranges, microbiological growth is controlled, and leakage is minimized. An alternative to direct corrosion rate measurements is an integrated monitoring and inspection program. This is a qualitative evaluation of the existence of a corrosion problem. Although the results are not quantitative, they might provide more realistic information about plant corrosion conditions than does a corrosion rate monitoring program.

8-1

EPRI Licensed Material Additional Monitoring Techniques

8.1.1 Monitoring Techniques Corrosion Coupons Corrosion coupons are metal specimens that are carefully prepared, weighed, and inserted into the water stream. They are exposed for a period of time (recommended minimum 90 days), removed, and re-weighed to determine a weight loss [43]. The weight loss is used to calculate a corrosion rate for the specific metal alloy. The corrosion rate is expressed in mils per year (mpy) or millimeters per year (mm/yr). Corrosion coupons are fairly easy to use. Although they do not exactly duplicate system geometry or heat transfer conditions, they provide a relatively repeatable means of estimating the rate of corrosion that will occur using a given material under a given set of conditions. Typically, a corrosion coupon rack is only operated at a single-flow rate. Actual system flows might vary over time and a coupon rack does not duplicate this condition. Corrosion conditions in the actual system, thus, might not be the same as predicted by the coupons. The corrosion rates determined by coupons are, therefore, only a guide as to what is occurring in the CCW system. When a corrosion coupon is inserted into a system, the initial corrosion rate is high until the protective film is formed. The corrosion rate then decreases over time and approaches an equilibrium value. If a corrosion coupon is exposed for a short period of time (for example, <30 days), the measured corrosion rate will be significantly higher than if the corrosion coupon is exposed under the same conditions for a longer period of time (for example, 90 days or more). There are advantages and disadvantages to shorter and longer exposure periods: •

A short exposure period can provide a rapid indication of the success of the corrosion control program.

•

A shorter exposure period allows fluctuations in corrosive conditions to be observed but yields corrosion rates that are higher than actually occurring in the system.

•

A longer exposure period is more representative of actual system conditions but does not provide any information on the impact of system upsets on corrosion rates.

A 90-day exposure period is often considered to be a valid compromise. The exposure period is long enough to allow for mature protective film formation but short enough to allow observation of any short-term upsets in corrosion rates. Longer exposure periods can be used in CCW systems but should be consistent from one test period to the next in order to eliminate exposure period as a variable in the test. The exposure period can also be impacted by ALARA considerations. Another valuable use of corrosion coupons is to remove the coupon after a period of exposure and visually inspect it. This will determine if a uniform protective film is being formed, therefore, detecting pitting attack.

8-2


Some suppliers provide pretreated (passivated) coupons. This consists of immersing the coupon in a strong corrosion inhibitor solution and coating the surface with a tightly adherent protective layer. This reduces the initial corrosion of the bare metal surface and produces a lower overall corrosion rate. The use of pretreated coupons is not covered in the ASTM standard. Pretreated coupons in CCW systems interfere with the basic purpose of the corrosion coupon, which is to verify that the corrosion inhibitor is forming a passive film before corrosion can occur. If acceptable from the standpoint of mechanical strength, the corrosion coupon rack should be constructed of plastic (PVC) piping because PVC does not corrode. If the rack must be constructed of a metallic material, stainless steel is the best, although carbon steels and other non-copper alloys can be used. Carbon steel can corrode and accumulate corrosion product deposits that can affect flow velocities. However, because corrosion rates are typically low in CCW systems, carbon steel coupon racks can provide satisfactory service. Copper-bearing alloys can result in copper plating out on a carbon steel coupon. This can cause galvanic corrosion and produce an erroneous result. Although corrosion coupon monitoring can be simple and relatively inexpensive in non-nuclear applications, this is often not the case in nuclear plants. This is due to the high cost of the system modifications required to install corrosion coupon racks and the large number of CCW systems in many plants. Acceptable corrosion rates in CCW systems are lower than would be established strictly to achieve acceptable equipment life based on metal loss due to general corrosion. In a CCW system, corrosion products (foulants) are not routinely and continuously discharged from the system as they are in an open recirculating system. The prevention of fouling, therefore, becomes a driving force in corrosion rate guidelines. When adequate inhibitor levels, pH levels, and microbiological control are maintained in a system, it should be possible to achieve the corrosion rates on corrosion coupons shown in Table 8-1. These measurements are desirable in some circumstances but are not requirements. Table 8-1 CCW Corrosion Coupon Rate Guidelines, Mils per Year (mpy)

Good Marginal Poor

Carbon Steel

Copper Alloy

Stainless Steel

<0.3

<0.2

<0.1

>0.3 to 0.5

>0.2 to 0.3

>0.5

>0.3

>0.1

Electrochemical Means Linear Polarization Measurements [44]: Linear polarization operates on the principle that a voltage impressed across the conductive interface boundary will result in a current flow that is directly proportional to the corrosion occurring on the metal electrode surface.

8-3


Available designs include models with two electrodes and three electrodes. The probes can be inserted into a section of pipe or installed in a bypass-type corrosion coupon rack. Probes with elements that are flush with the piping wall are also available. Most often, probes are used in a corrosion coupon rack installation because the probes are more readily accessible for examination and replacement. The method is applicable to all natural or treated waters. Since the linear polarization method depends on solution conductivity, errors can be introduced when dealing with low-conductivity waters (for example, deionized quality). The equipment manufacturer should be consulted on probe designs applicable to a specific water. Temperature and flow influence the corrosion rate measured by the probes. Changes in flow or temperature will cause changes in the apparent corrosion rate in the piping or coupon rack. If fouled, probes will produce erroneous results, especially if deposits bridge the opening between the probes. Electrical Resistance [44]: This technique measures the resistance of a section of wire that is exposed to the cooling water. As the wire corrodes, its cross-sectional area decreases, thus causing an increase in resistance. The resistance increase over time is converted to a corrosion rate. Electrochemical Noise (ECN) Electrochemical noise [45] corrosion monitoring derives information on the corrosion rate and mechanism from naturally occurring fluctuations of potential and current. Three electrodes are used. A zero-resistance ammeter is connected between two of them, and the third is used as a reference electrode to monitor the potential of the first two. Potential and current density measurements are typically taken once a second, and a single data record typically covers 5 minutes. The calculated corrosion rate has the same relation to actual metal loss of the equipment as does the rate calculated from Lindar Polarization Resistance (LPR). In addition, ECN is a more sensitive method than LPR, and generally has somewhat wider variation around the mean. In addition, various calculated parameters have been proposed as localization factors, which indicate whether the corrosion is general or localized (pitting, crevice, cracking, and so on).

8.2

Fouling

Fouling in CCW systems will most likely be the result of either corrosion or microbiological growth. Control of both of these mechanisms will prevent fouling of the system. There are no specific guidelines for the amount of fouling allowed in CCW systems. It should be minimal and not interfere with heat transfer. For plant performance monitoring purposes, the assumption should be verified that fouling on the CCW side is minimal. Fouling must be limited below the amount that would cause the fouling factor to be raised above design assumptions in heat exchanger sizing calculations. It might be possible with installed plant equipment (temperature, 8-4


pressure, and flow gauges) to assess relative fouling in certain portions of the system. Flow monitoring can also be performed using hand-held, adjacent-to-line, acoustic flow monitors. Measurements made over several months can assess changes in flow or temperature differentials. One of the most effective methods of determining the extent of fouling is through visual inspection of open system components. This can be done any time a system component is taken out of service for scheduled maintenance. The exposed system surfaces can be visually examined with small magnifiers, and samples of corrosion or fouling deposits can be taken and analyzed. It is critical that if microbiological fouling is suspected, personnel disassembling the equipment notify the visual inspectors prior to opening the system so that they can help to minimize inadvertent contamination. Maintaining a record of these inspections (video or photographs) as part of the work control process will help to provide an historical database of observations. This can assist in long-term assessment of system health.

8.3

Microbiological Growth

Control of biological growth is important in CCW systems because of its impact on heat transfer and corrosion. The accumulation of slime masses will cause direct fouling of heat exchange surfaces. Biological growth might also result in MIC-related failures. In addition, nitrifying and denitrifying bacteria will consume nitrite corrosion inhibitors and produce byproducts that are either less effective as corrosion inhibitors or are corrosive. The goal of a bacterial monitoring program is to control bacterial populations on surfaces within the system. If feasible, it would be preferable to measure the organisms that colonize on a surface (sessile organisms) rather than measuring the free floating (planktonic) organisms, particularly if the treated system might have creviced conditions or biological deposits. In some cases, determining sessile counts is feasible without modifying existing plant systems. Most microbiological testing is performed on the bulk water in the CCW system (planktonic organisms). The tests for planktonic bacteria are easier to perform than tests for sessile bacteria. Note that planktonic bacteria results might not have a direct relationship to sessile organisms. Sessile organisms, rather than planktonic organisms, cause problems such as MIC or denitrification in CCW systems. In many cases, however, planktonic bacteria tests do provide a good indication of trends in general microbiological control. 8.3.1 Colony Counting Techniques Various methods to measure planktonic bacteria are available. Due to differences in measurement technique, acceptance criteria for biological growth must be established for the technique used. The standard total aerobic plate count method will yield results in colony-forming units per milliliter of sample (CFU/ml). Other methods, such as adenosine triphosphate (ATP), will yield results in different units such as grams/ml of ATP or relative light units (RLUs). Appropriate limits need to be established as part of the site-specific strategic plan. For bulk water total aerobic plate counts, a limit of 103 (1,000) CFU/ml is often used. However, it should be 8-5


noted that this is an arbitrary figure and some systems have operated successfully at higher bacterial levels. These guidelines for aerobic bacteria suffer from the limitation that there might not be a relationship between the bacteria in the bulk water and microbiological deposits (sessile bacteria) on surfaces. Although valuable in many cases, bulk water testing can be considered a crude screening test. The typical total aerobic plate count limit of 103 CFU/ml represents a measurable but low level of total aerobic bacterial activity. Part of the total microbial activity in a system might consist of anaerobic bacteria, fastidious bacteria, molds, algae, and protozoa. The aerobic techniques will not quantify this activity. Tests are also often performed for sulfate-reducing bacteria (SRB) or total anaerobic bacteria. If measured by Most Probable Number (MPN) plate count techniques, it is recommended that SRB be maintained at zero in the bulk water. The SRB limit of zero is based on the assumption that any measurable SRB activity in the bulk water means that there are active SRB colonies in biofilms or underneath deposits. Adenosine Triphosphate (ATP) This is a bioluminescence method of determining the concentration of living organisms [46]. ATP is present in all living organisms and is not limited to bacteria. The analysis makes use of bioluminescence to measure living organisms with a photometer. ATP monitoring is a method of determining levels of biological activity not based on culturing organisms, but rather on measuring a cellular component. An advantage of the ATP technique is that it detects all microbiological species (often referred to as the total biomass) rather than specific groups of organisms (for example, aerobic bacteria, anaerobic bacteria algae, mold, and so on). Total ATP is a measurement of all ATP in the sample. This includes background as well as ATP due to live organisms. Free ATP is a measurement of the background ATP in the sample. Microbial ATP (Total minus Free) is the ATP due only to live organisms. ATP monitoring eliminates the need for extended incubation times associated with traditional culture-based monitoring procedures (including dipslides). This reduces service time and provides the real time assessment of microbial loading in an industrial system. Therefore, ATP analysis is a rapid technique that produces results in a few minutes. In addition, it detects both aerobic and anaerobic organisms, as well as algae and fungus, and for this reason does not suffer the limitations of culturing methods. Because ATP in the cell decays rapidly when the organism dies, ATP is a measure of live organisms. It is not truly quantitative for actual bacteria populations because different microbial species (bacterial, algal, or fungal species) contain different amounts of ATP and healthy organisms produce more ATP than stressed organisms. There is no single ATP reading that can be universally used as an indicator of whether microbiological activity is low or high. Each type of ATP measuring equipment (referred to as ATP luminometers) will provide different results. Each site will have to establish ATP guidelines 8-6


based on the test and equipment being used. ATP can be made more quantitative by utilizing ATP standards. This is because ATP reagents differ from batch to batch and change in reactivity over time. Therefore, the use of ATP standards is helpful in determining that both instruments and reagents are operating satisfactorily. Currently, ATP tests are performed with the use of a self-contained “pen” that allows for the rapid measurement of Total or Free ATP. ATP luminometers will provide a read-out in RLUs. Using RLUs as a quantitative indicator of microbiological activity is flawed because: •

Variations exist between luminometers and, as different luminometers are used, the obtained values can vary

•

The pens rely on an enzyme that can change over time and storage conditions, both of which can have a dramatic effect on the results, giving a false indication of good system control

•

Different manufacturer’s pens will have differing concentrations of enzyme and components, leading to different light output from the devices, even if the ATP levels are the same

By converting the readings to standard units of ng/ml (ppb) of ATP, the results will be transferable between plants, meters, and pen batches, and over time. This makes the data generally applicable, instead of being limited to only one site. The principle of running an ATP standard is based on the measurement of a standard concentration of Adenosine Triphosphate (typically 1 nanogram of ATP per ml of water), used as an assurance of reagent and instrument function. The concentration of ATP in the standard represents the quantity of ATP expected from an ideal pure culture population of 1 x 106 bacteria/ml. The standard should be refrigerated and used before the expiration date. An ATP quality test standard is performed by using the ATP standard in place of a water sample. Therefore, the representative pen (for Total ATP) is dipped into the vial containing the ATP standard (as opposed to the water sample) and the bioluminescence (RLU) is measured with the ATP meter. Knowing that the vial in question represents 1 ng of ATP/ml, the measured RLU from the meter is equal to 1 ng of ATP/ml. To convert a water sample RLU, the measured water sample RLU result is divided by the ATP standard measured RLU. For an example, if the 1 ng/ml ATP standard yields a result of 1000 RLU and the water sample yields a result of 500 RLU, the result recorded for the water sample is 0.5 ng/ml (500 RLU/1000 RLU). This should be done for the Free ATP test as well as for the Total ATP test. If both the Free and Total ATP tests are to be determined on the same water sample, it is important to test the Free ATP first. The Total pen contains a lysing agent, which will convert cellular ATP in the water sample to Free ATP and cause an artificially high Free ATP reading. Dip Slides Dip slides or paddle testers are a semi-quantitative method of testing for total aerobic bacteria. Some testers contain a medium for total aerobic bacteria on one side of the paddle and a medium for yeast and mold on the other side of the paddle. The main drawback with dip slides is that the sampling technique has a significant impact on the results. The results are particularly affected 8-7


by the amount of time the slide remains in the water. Another drawback is that the bacteria might not be able to grow on the particular nutrient media or at the particular incubation temperature. This is a good tool, however, for routine approximations of the presence of aerobic bacteria. Total Aerobic Plate Counts Total Aerobic Plate Counts [47] are determined by pour plate, spread plate, or a membrane filtration method. It provides an approximation of total numbers of aerobic bacteria by measuring the number of aerobic bacteria that can grow rapidly under the conditions provided. Many aerobic organisms are unable to grow on the nutrients provided or at the particular incubation temperature. Additionally, clustered groups of bacteria will count as a single Colony Forming Unit. Plate counts are more quantitative than dip slides, but require a higher skill level to perform. The method is not specific for any particular type of bacteria. PetriFilm This is a proprietary version of the total aerobic plate count where the incubation media is contained in a dry form on a film. The advantages are ease of use and compactness [48]. The technique requires less skill level than the Total Aerobic culturing techniques described but might still require serial dilutions. Test Kits A variety of bacterial test kits are available. These allow the determination of specific types of aerobic, anaerobic, and facultative anaerobic bacteria that might be of interest in a CCW system. Included might be slime-forming bacteria, nitrifying bacteria (for example, Nitrobacter), denitrifying bacteria (for example, Nitrobacter agilis), sulfate-reducing bacteria (for example, Desulfovibrio desulfuricans), anaerobic acid producing bacteria, and so on. 8.3.2 Sessile Bacteria Monitoring Techniques Sessile bacteria accumulate on surfaces, while planktonic bacteria are those that are free floating in the water. The sessile bacteria are of more concern than the planktonic bacteria, thus, it might be advantageous to monitor for sessile bacteria. There are several techniques for doing this. Stainless Steel Corrosion Coupons Stainless steel corrosion coupons can be used in a corrosion coupon rack as a surface to culture sessile organisms. The coupon can then be removed after a period of time, swabbed (or agitated) to remove the biofilm, and the organisms cultured or tested for ATP (biomass/cm2). Another variation is to sandwich two stainless steel coupons together to simulate a crevice.

8-8


Screened Coupons Screened coupons are similar to corrosion coupons except that a rigid stainless steel screen substitutes for the corrosion coupon. The screen accumulates biofilm. The accumulation of biofilm can be evaluated qualitatively, or the biofilm can be removed and colony-counting or ATP techniques can be used to estimate the relative amount of biofilm. Glass Bead Holders Glass bead holders contain glass beads that provide a surface for biofilm formation. After a suitable incubation period, several glass beads are removed and the biofilm is physically removed (usually ultrasonically). Colony-counting or ATP techniques can then be used to estimate the relative amount of biofilm. Metal Specimen Holder This is a flow-through device that holds metal plugs or buttons, which provide a surface for bacterial colonization. One trade name for such a device is the Robbins Device. The plugs can be removed for examination while the holder is in place. Microscopic examination of the biofilm or biomass can indicate the unique characteristics of the microflora. The microflora can also be enumerated and characterized using colony-counting or ATP techniques. The amount of biofilm or biomass can be determined by using gravimetric techniques. Plugs of multiple metallurgies can be used in this device to test the impact of metallurgy on colonization. Inspections While not an on-line monitoring technique, inspections represent an important opportunity to evaluate the extent of sessile bacteria accumulation and collect samples (for example, swabs) for identifying sessile bacteria species. 8.3.3 Monitoring for Specific Microbial Species Monitoring for specific microbial species might be warranted for two conditions: •

To identify specific bacteria types relative to a particular treatment approach (for example, nitrifying or denitrifying bacteria in a nitrite-treated system)

•

To determine the most appropriate biocide treatment regimen

8-9


8.4

Non-Chemistry Monitoring

8.4.1 Inspections Closed cooling water system inspections are a form of monitoring that provide a snapshot assessment of equipment status and water-treatment program performance at a particular time. Most of these inspections are performed in conjunction with normal maintenance activities. Over the course of several inspections, trend data are collected that can be used to predict potential problems and service life. The objectives of any CCW system inspection should include data collection and analyses to determine the potential for: •

Loss of structural integrity caused by corrosion

•

Excessive leakage caused by corrosion

•

Decreased flow capacity caused by corrosion and/or deposition

•

Reduced heat transfer caused by corrosion and/or deposition

Inspections performed to characterize the corrosion and deposition present in the CCW system should be designed to identify the forms of corrosion and deposition, their cause(s), and the rate of degradation that can be used in calculating equipment residual life assessments. Inspection planning, sample collection, and inspection documentation are important parts of the inspection process. Inspections are discussed in detail in other EPRI publications [49,50]. Inspection techniques will vary depending upon the type of CCW system equipment (piping, heat exchangers, valves, pumps, etc.) and its accessibility. 8.4.2 Nondestructive Evaluations (NDEs) A number of mechanical techniques can be used for on-line inspections of service water systems [49,50]. Three of the more common methods are ultrasonic testing (UT), eddy current testing, and radiographic testing. These techniques are also suitable for off-line inspections. Ultrasonic Testing Ultrasonic testing is a nondestructive method for characterizing flaws or wall thickness in metallic piping and components, usually from the outside surface. UT methods might be useful for detecting localized corrosion, provided appropriate inspection locations are selected and inspection grids are sufficiently fine. Small wall losses might not be detected by UT. Because UT could be applied from either the outside or inside of the component, it can be used where access is limited. For CCW system applications, inspections are most commonly performed from the outside. A major advantage of ultrasonic testing is that the system does not need to be de-watered. Often, the system can be examined while it is operating. 8-10


All materials can be inspected by UT; however, some very coarse-grained materials (for example, stainless steel welds or castings) might be difficult to inspect. Geometric discontinuities or dissimilar metal joints will produce UT reflections that will require interpretation by an experienced operator. Fortunately, piping and components are fairly standard configurations that are relatively easy to inspect. There are automated forms of UT testing that scan an area of piping. This can detect irregularities in pipe thickness and detect localized corrosion or pitting. UT methods (manual or automatic) do not provide information on the character or thickness of deposits. Electromagnetic Automatic Scanning This technique also provides a map of thickness variations in the scanned area of the piping. The technique does not provide information on the character or thickness of deposits. Eddy Current Testing Inspection of tubing or pipes by eddy current methods relies upon the capability to detect changes in the patterns of currents induced in the part being examined. Applying an alternating current (the exciting current) to a coil in the vicinity of the part to be inspected causes other currents (eddy currents) to flow in the part as a result of electromagnetic flux. Cracks or other flaws will disturb the flow of the eddy currents and these disturbances can be detected electronically. Eddy current testing is typically performed on non-magnetic materials, usually thin-walled heat exchanger tubing. Most often eddy current testing is done from the ID of the pipe or tube, thus, access to the ID is required. De-watering of the system is generally necessary and, as with UT, the success of the eddy current method is strongly dependent upon the training and skill of the operator (and signal interpreter). Radiographic Testing Radiographic testing (RT) is not typically used in CCW systems. In certain cases, it might have some application. RT consists of penetrating the structure with ionizing radiation and capturing an image of the structure on a film or electronic sensor (typically displayed on a viewing screen or television monitor). RT provides information on the character or thickness of deposits. Depending on the configuration of the scan, RT might miss pitting corrosion. 8.4.3 Leakage The corrosion and deposition characteristics of CCW systems change rapidly if they are not truly closed. Monitoring of system leakage is therefore an important part of the CCW monitoring program. Note, however, that small pinhole leaks and cracks might self-seal and not be detected by leakage monitoring. Leakage monitoring might be accomplished by metering system makeup 8-11


or by measuring the loss of chemicals over a period of time. The latter can be accomplished by chemical testing or conductivity measurements. In some cases, system leakage can be detected by tracking head tank level if makeup to the system is manual rather than automatic. Chemical treatment selection and application, dosage and demand, should always be controlled and adjusted, taking into consideration the relative and actual makeup water use rate and amount. Closed system losses should be trended and acted upon accordingly. 8.4.4 Heat Transfer Performance monitoring is typically part of the engineering program. It can be used to confirm that conditions in the CCW system are not degrading heat exchanger performance. System health reports should show trends of system parameters such as temperature, flow, pressure, and heat exchanger efficiency as a function of time. They should also evaluate any seasonal effects that might present themselves in the data.

8.5

Trending

The logging and trending of data is an important part of the CCW system monitoring program. The data to be trended will depend on the system design, chemical treatment program, and monitoring in place. Data to be trended might include Control Parameters, Diagnostic Parameters, and system losses. The purpose of trending is to detect, and often anticipate, problems caused by system leakage, microbiological growth, or the accumulation of system impurities. Logging of system parameters is important to provide a historical database for comparison purposes. Trending of these parameters is important to anticipate changes in system conditions. Calculations can be used as part of the trending regimen to assist with system evaluation. One valuable calculation is the conductivity-to-nitrite ratio in a nitrite-treated system. This can help detect whether a nitrite decrease is due to system leakage or microbiological activity. A leak will decrease both conductivity and nitrite and the conductivity-to-nitrite ratio will remain constant. Microbiological activity will decrease nitrite but leave conductivity unchanged, resulting in an increasing ratio. The nitrite-to-molybdate ratio can serve the same purpose in a nitrite/molybdate system.

8.6

Chemical Analysis

The wide variety of corrosion control agents, biocides, and pH adjust chemicals do not allow a comprehensive list of chemical analyses and detection ranges. However, each combination of chemicals provides analytical challenges, which need to be effectively addressed in order to successfully monitor trends of inhibitors, biocides, and contaminants. This section addresses some general chemical analysis concepts that are important for the analytes and some specific methods of analysis. This is not an all-inclusive list of methods that can be used for analysis, but 8-12


it includes the more frequently used methods. Any method should be matrix-tested to ensure that measurement results are not masked or biased by the presence of high concentrations of corrosion inhibitors. 8.6.1 Ion Chromatography (IC) This technique is routinely used to monitor concentrations of control agents such as nitrite, as well as contaminants like nitrate, chloride, fluoride, and sulfate (anions), or sodium, calcium, and magnesium (cations). The determination of nitrite will be mostly unaffected by all other species because nitrite is the major ionic species in the solution. However, because the concentration range of this ion can be so broad (500 to >1500 ppm), it will also act as an eluent in the chromatography process. This can have several effects on the analytical process, most notably, variability of the detection limit and the retention time of the analyte. The retention time shift should be determined by comparing the system chromatogram with chromatograms of standards used to calibrate the instrument, as long as the standards are run in the inhibitor matrix. The shift of retention time can also cause a broadening or sharpening of the chromatographic peak (dependent upon the eluent strength relative to the calibration). The detection limit is affected by the detector response to the inhibitor in the system, as well as the way the peak shape has deformed due to the presence of other ions in the system. Thus, if a conductivity detector is used, the detection limit will most likely increase at higher concentrations of anionic materials (due to higher conductivity). However, the limit of detection of the contaminant analytes is not nearly as important as being able to reproduce values close to the Action Levels. Thus, it would be important to establish a quality check for each inhibitor system at the upper end of the inhibitor concentration, ensuring that Action Level values can be achieved. For analysis of cations, the impact that borate or phosphate (added as part of the inhibitor package) can have on complexing with the metals should be evaluated under matrix match conditions. 8.6.2 Ion Selective Electrodes (ISE) This method of analysis is based on the electrochemical potential change of a half-reaction based on the concentration of a specific ion when measured against an electrode, which is selective for the half-reaction of that ion. Because the method measures potential and not an intrinsic parameter of the particular ion, it is subject to many different types of interferences, most notably other ions of similar charge and reduction potential. This method should also be verified applicable in the concentration used for the CCW system, for the ions being determined. Major solution constituents, such as nitrite, can be effectively assessed because the contaminant concentrations are generally insufficient to alter the potential for the nitrite electrode. This has been used as an alternative to ion chromatography for chloride, fluoride, and sulfate analysis. There are ion selective electrodes for metal ions as well.

8-13


Ammonia analysis by electrodes can be performed using one of two different types. Ammonium ion is detected as described previously and the sample is made acidic with hydrochloric acid. This method suffers from interference from sodium ion. Thus, analysis of systems where the corrosion inhibitor is added as sodium salt will need to be assessed for the extent of interference. The second electrode method is by gas sensing electrode (GSE). In this technique, a membrane that is sensitive to the passage of ammonia gas is immersed in a sample that has had sufficient NaOH added to it to convert all of the ammonia to gas. 8.6.3 Atomic Absorption (AA) and Inductively Coupled Plasma (ICP) Both of these methods are very useful at determining total metal ion concentration in solution. Analysis for iron, copper, magnesium, calcium, chromium, molybdenum, and sodium can all be routinely performed using these methods. Both methods measure an element-specific characteristic based on atomic structure of the metal, making the analysis very specific for an individual element. For the types of matrices that are experienced in CCW systems, there are few, if any, matrix effects that would affect the analytical result. It is important to note that, in each of these analyses, the result is for total metal content. This analysis does not yield information about the oxidation state of the material being examined, nor does it identify molecular species. This is best described by two different analyses. Chromate is a good anodic corrosion inhibitor. It is also a strong oxidizing agent. If a CCW system containing chromates were to experience an intrusion of oil, it is likely that the oil would be partially oxidized to carbon dioxide and water, while the chromate would be reduced to Cr+3. Depending on the concentration of Cr+3 and the pH, the Cr+3 might precipitate. The Cr+3 does not provide any anodic inhibition at all. Analysis of the water after the event would only indicate the total chromium, which would be a mixture of the two species. Such a result would lead to an erroneous conclusion about the appropriate level of corrosion inhibitor. Molybdates are also good anodic inhibitors. However, this species can also be converted to MoO3, which has no corrosion inhibitor value at all. This species can form small particulates that can become suspended. If total molybdenum analysis is performed, it again might indicate a satisfactory inhibitor level, when in fact it is low. In each of these instances, a spectrophotometric test is available for the specific molecular species, which provides the inhibitor function. It might be advisable to use the alternate test on a certain frequency (for example, annually) to ensure that no degradation occurs. This will be verification that the AA or ICP methods are fairly reproducing the inhibitor concentration required. 8.6.4 Ultraviolet/Visible Spectroscopy (UV/Vis) UV/Visible spectroscopy is useful for measuring the specific electronic transitions from valence shell electrons in molecular species. Hydrazine, silica, phosphate, boric acid, molybdate, nitrite, nitrate, and chromate are all species that can be determined by this method. 8-14


Chloride, fluoride, and sulfate can also be determined, but this practice has become less common with the advent of ISEs and IC units. In general, a chromogenic agent, which has molecular absorbance at a specific wavelength, is used in the analysis. These molecular bands are very broad and interferences from other colored matter in the same wavelength range are interferences. Sample turbidity can also provide interference in these analyses, so sample filtration might be necessary prior to analysis to ensure accurate measurements. 8.6.5 pH and Conductivity These two methods of analysis measure the total effects of all the dissolved matter in the solution on the parameter of hydrogen ion concentration (pH) and solution electrical conductivity. These tests are used as measures of gross contamination or loss of corrosion control agent. Together, these can be used on a more frequent basis than the element- or inhibitor-specific tests to ensure that system chemical composition remains relatively the same. This parameter is useful because the intrusion of an unknown contaminant will most likely affect these two tests, which would be a warning to begin an investigation. One particular intrusion, which would not necessarily show up as affecting these two analyses, would be from solvents or oils. In these cases, odor, color, or TOC would be helpful in diagnosing the source of the contaminant. Care must be taken when performing pH on inhibited glycol coolants. In the case of coolants greater than 50% volume glycol, pH values are not actual values according to the accepted definition, but are apparent pH values, which are useful in interpretation of coolant condition [37]. 8.6.6 Vendor Test Kits Most vendor test kits are made for field use as a first-order measure of the concentration of a particular analyte. The methods are almost always based on sound analytical methods that have been modified for a portable test kit. In almost all cases, the analyst is asked to interpolate scale readings (using a mini-meter) or judge a color against a standard set of colors (using a color comparator). For systems that are relatively stable, with no contaminant ingress and little change in process, these might be acceptable for routine analyses. However, if these are used for CCW systems, other laboratory methods should be used on a routine basis to ensure that trends are not developing that might prove harmful to the system. Vendor test kits can suffer from a myriad of interferences, the most notable are colored contaminants. A specific example occurs with the analysis of glutaraldehyde (a di-aldehyde). The reagent used is sensitive to the glutaraldehyde molecule yielding a certain color. Partial decomposition products of glutaraldehyde will also yield a color with this material, but not of exactly the same wavelength. The analyst must then discern, visually, whether or not the color of the material being tested is the same as that of the standard.

8-15


8.6.7 X-Ray Fluorescence (XRF) of Filtered Material XRF is a routinely used method of analysis for metals. It is very effective for analysis of insoluble materials filtered from a system. The instrument produces a highly focused electron beam on the solid material on the filter surface and this results in the production of characteristic x-rays of the elements. Most commercial instruments have the capability of analyzing in the range of 3–20 keV. This is the region for the Kα x-ray lines for all of the first-row transition elements and molybdenum. The Kα x-ray lines are the most sensitive for any element. This region can also be used for Lα lines of some of the higher atomic number elements, such as lead. This can potentially be used to differentiate between insoluble and dissolved metals. It is very important to ensure that the sample point and process provide a representative flow for the sampling of particulates in the fluid medium.

8-16


9

TROUBLESHOOTING AND CORRECTIVE ACTIONS

9.1

Introduction

This section is intended to provide general guidance for some of the more common chemistry trends that might be observed in closed cooling water systems. A separate section is included for each of the most popular chemical treatment programs encountered in the nuclear industry, but it is not intended to be a comprehensive list. Treatment programs covered in this section include nitrite, nitrite-molybdate, molybdate, chromate, hydrazine, glycol, and pure water. Due to the differences among closed cooling water system designs, materials of construction, fluid interface type, makeup water sources, etc., some of this information might not be applicable in a particular situation. It might be necessary to develop system-specific troubleshooting guidance to supplement this information. This information should be useful as a starting point in developing a site-specific comprehensive closed cooling water treatment and monitoring program. Several examples of troubleshooting flow charts are also provided as an alternate way of presenting the information provided in this section. The end-user might find these examples useful if it is desired to include such troubleshooting charts in site-specific procedures. Each system has a corrosion inhibitor decay curve (concentration versus time), the slope of which is a composite effect of inhibitor decomposition and dilution. Changes to this curve can provide early warning of system leakage increases or contaminant ingress.

9.2

Nitrite-Treated Systems

9.2.1 System Leakage Symptoms •

Decreasing pH

•

Loss of nitrite

•

Excessive makeup

•

Loss of conductivity (following makeup water additions to the system)

•

Constant conductivity-to-nitrite concentration ratio

•

Loss of azole 9-1

EPRI Licensed Material Troubleshooting and Corrective Actions

Corrective Actions •

Consider increasing sampling frequency for inhibitor

•

Locate and repair system leakage

•

Add inhibitor if necessary

•

Adjust pH if necessary

•

Consider use of dye or tracer to find leaks

9.2.2 Water In-Leakage Symptoms •

Possible pH change (depending on pH of water entering the CCW system)

•

Loss of nitrite

•

Loss or gain of conductivity (depending on conductivity of water entering the CCW system)

•

Constant or increasing conductivity-to-nitrite concentration ratio (depending on conductivity of water entering the CCW system)

•

Loss of azole

•

Increase in storage tank level

•

Increase in concentrations of impurities or radioactivity (depending on which impurities are present in water entering the CCW system)


Repair system in-leakage

•

Evaluate impact of contaminants from in-leakage on the system

•

Consider need for feed and bleed, drain and refill, or filtration

•

Increase inhibitor concentration if necessary

•

Consider need to analyze for dissolved oxygen

•


9.2.3 Bacterial Growth Symptoms •

ATP, total bacteria, or plate counts in excess of specified maximum

•

Possible decrease in pH

9-2


•

Possible flow restrictions

•

Loss of heat transfer capability

•

Rapid, unexplained rise in corrosion rates


Add biocide

•

Consider need for alternate biocide or alternating biocides

•

Adjust pH if required

•

Consider need for feed and bleed or drain and refill

•

Consider need for system cleaning

9.2.4 Nitrate-Forming Bacteria Growth Symptoms •

Decreasing pH

•

Loss of nitrite

•

Increase in nitrate

•

No significant change in conductivity (between chemical additions)

•

Increase in conductivity-to-nitrite ratio

•

Constant azole

•

Observation of deposit accumulation


Add biocide

•


•

Consider increasing monitoring frequency for nitrifying bacteria and/or nitrate concentration

•

Obtain and analyze surface samples for sessile bacteria if possible

•


•

Add nitrite inhibitor if necessary

•

Make pH adjustment if necessary

•


9-3


9.2.5 Ammonia-Forming Bacteria Growth Symptoms •

Increasing pH due to ammonia production

•

Loss of nitrite (nitrite might not decrease significantly; ammonia might come from the breakdown of organic nitrogen compounds)

•

Increase in ammonia

•


•

Increase in conductivity-to-nitrite concentration ratio

•

Constant azole


Add biocide

•


•

Consider increasing monitoring frequency for ammonia-forming bacteria and/or ammonia

•


•


•


•


•


9.2.6 Acid-Forming Bacteria Growth Symptoms •

Decreasing pH

•

Constant nitrite concentration

•

Small increase in conductivity

•

Constant conductivity-to-nitrite ratio

•

Constant azole


Add biocide

•


9-4


•

Increase monitoring frequency for aerobic and anaerobic acid-forming, endospore-forming bacteria

•

Obtain surface samples for sessile bacteria if possible

•


•


•


9.2.7 Increase in System Corrosion Rates Symptoms •

Increasing trends in total iron or total copper in bulk water

•

Increase in corrosion rates on weight loss coupons or electrochemical monitor (if used)

•

Visible corrosion, tuberculation, or pitting during inspections

•

Corrosion indications as a result of NDE evaluations

•

Occurrence of leaks


Consider increasing sample frequency for iron and/or copper analysis

•

Determine cause for increase in corrosion rates and develop corrective action plan

•

Evaluate effect of elevated iron and copper in system

•


•

Increase inhibitor concentrations if necessary

•

Review microbiological control status

9.2.8 Oil In-Leakage Symptoms •

Possible increase in sulfate and/or phosphate

•

Possible visible sheen on system samples

•

Possible unexplained decrease in TTA (oil can interfere with analytical test)

9-5



Initiate testing for TOC, oil and grease, sulfate and/or phosphate

•

Locate oil source and develop corrective action plan

•

Evaluate impact of oil in system

•

Remove via cleaning, feed and bleed or drain and refill

•

Evaluate if azole addition is necessary

9.2.9 Contaminant Ingress Symptom •

Increase in the contaminants found in the in-leakage source but not normally found in the treated system.


Identify source of contaminant and develop corrective action plan

•

Evaluate effect of elevated contaminant level in the system

•

If elevated contaminant will result in increased corrosion rate or degraded system performance, place the system into feed and bleed or drain and refill, or increase inhibitor concentration(s) to compensate

9.3

Molybdate-Treated Systems


Loss of molybdate

•

Decreasing pH

•

Excessive makeup

•

Decreasing conductivity (following makeup water additions to the system)

•

Loss of azole


Increase sampling frequency for inhibitor as appropriate

•


9-6


•


•

Adjust inhibitor concentrations if necessary


Possible pH change depending on pH of water entering the CCW system

•

Loss of molybdate

•

Loss or gain of conductivity (depending on conductivity of water entering the CCW system

•

Loss of azole

•



Locate and repair system in-leakage

•

Evaluate effect of contaminants on the system

•


•


•




•


•


•

Possible loss of heat transfer capability


Add biocide

•


•


•


•

Consider need for system cleaning 9-7



Increasing trend in total iron or total copper concentration

•


•

Visible corrosion or tuberculation during inspections

•

Occurrence of leaks


Increase sample frequency for iron and/or copper analysis

•

Determine cause for increase in corrosion rates and correct

•

Evaluate effect of elevated iron and copper in the system

•

Evaluate need for feed and bleed or drain and refill



•


•

Possible unexplained decrease in azole (oil can interfere with analytical test)


Initiate testing for TOC, oil and grease, and/or sulfate concentration

•

Locate oil source and correct

•

Evaluate impact of oil in the system and remove via cleaning or feed and bleed


9-8

Increase in contaminant found in the in-leakage source but not normally found in the treated system



Identify source of contaminant and correct.

•

Evaluate effect of elevated contaminant level in the system.

•

If elevated contaminant will result in increased corrosion rate or degraded system performance, place the system into feed and bleed or drain and refill, or increase inhibitor concentration(s) to compensate.

9.4

Nitrite/Molybdate-Treated Systems


Decreasing pH

•

Loss of nitrite and molybdate

•

Excessive makeup

•


•

Constant conductivity-to-nitrite or molybdate-to-nitrite concentration ratio

•

Loss of azole


Consider increasing the sampling frequency for inhibitor

•

Add inhibitor if necessary

•


•


•

Consider use of dye or tracer to find leaks



•

Loss of nitrite and molybdate

•


•

Constant or increasing conductivity-to-nitrite concentration ratio depending on conductivity of water entering the CCW system) 9-9


•

Constant molybdate-to-nitrite concentration ratio

•

Loss of azole

•

Increase in storage tank level

•




•

Evaluate impact of contaminants from in-leakage on system

•

Consider need for feed and bleed, drain and refill, or filtration

•


•

Consider need to analyze for dissolved oxygen

•




•


•


•

Loss of heat transfer capability


Add biocide

•


•


•


•


9.4.4 Nitrate-Forming Bacteria Growth Symptoms •

Decreasing pH

•

Loss of nitrite

9-10


•

Constant molybdate

•

Constant azole

•


•

Increase in nitrate

•

Increase in conductivity-to-nitrite or molybdate-to-nitrite ratio

•

Observation of deposit accumulation


Add biocide

•


•

Consider increasing the monitoring frequency for nitrifying bacteria and/or nitrate concentration

•


•


•


•

Make pH adjustment if necessary

•




•

Loss of nitrite concentration (nitrite might not decrease significantly; ammonia might come from the breakdown of organic nitrogen compounds)

•

Constant molybdate concentration

•

Increasing ammonia concentration

•


•

Increase in conductivity-to-nitrite or molybdate-to-nitrite concentration ratio

•

Constant azole


Add biocide

•

Consider the need for alternate biocide or alternating biocides 9-11


•

Consider increasing the monitoring frequency for ammonia-forming bacteria and/or ammonia concentration

•


•


•


•


•

Consider the need for system cleaning

9.4.6 Acid-Forming Bacteria Growth Symptoms •

Decreasing pH

•

Constant nitrite concentration

•

Constant molybdate concentration

•

Small increase in conductivity

•

Constant conductivity-to-nitrite and molybdate-to-nitrite concentration ratio

•

Constant azole


Add biocide

•


•

Increase the monitoring frequency for aerobic and anaerobic acid-forming, endosporeforming bacteria

•

Obtain surface samples for sessile bacteria if possible

•

Consider the need for feed and bleed or drain and refill

•


•

Consider the need for system cleaning


Increasing trends in total iron or total copper in bulk water

•

Increase in corrosion rates on weight loss coupons or electrochemical method (if used)

9-12


•

Visible corrosion, tuberculation, or pitting during inspection

•

Corrosion indications as a result of NDE evaluations

•

Occurrence of leaks


Consider increasing the sample frequency for iron and/or copper analysis

•


•


•


•

Increase inhibitor concentrations if necessary

•

Review microbiological control status



•


•

Possible unexplained decrease in TTA/BZT concentration (oil can interfere with TTA analysis)


Initiate testing for TOC, oil and grease, sulfate, and/or phosphate

•

Locate oil source and develop corrective action plan

•

Evaluate impact of oil in system

•

Remove via cleaning, feed and bleed, or drain and refill

•

Evaluate if azole addition is necessary


Increase in the contaminant found in the in-leakage source but not normally found in the treated system

9-13



Identify source of contaminant and develop corrective action plan

•


•

If elevated contaminant will result in increased corrosion rate or degraded system performance, place system in feed and bleed, drain and refill, or increase inhibitor concentration(s) to compensate Chromate-Treated Systems

9.5


Decreasing pH

•

Loss of chromate

•

Excessive makeup

•

Decreasing conductivity (following makeup water additions to system)

•

Loss of azole if present in the system



•


•


•

Adjust chromate if necessary



•

Loss of chromate

•


•

Loss of azole if present in the system

•


9-14



Locate and repair system in-leakage

•

Evaluate effect of contaminants in the system

•


•

Increase chromate concentration to compensate if necessary

•



Increasing trend in total iron or total copper concentration

•


•


•

Occurrence of leaks



•


•


•




•


•

Possible unexplained decrease in azole if used (oil can interfere with analytical test)


Initiate testing for TOC, oil and grease, sulfate, and/or phosphate concentration

•


•

Evaluate impact of oil in the system and remove via cleaning, feed and bleed, or drain and refill 9-15





Identify source of contaminant and correct.

•


•


9.6

Hydrazine-Treated Systems

9.6.1 Hydrazine Decomposition Symptoms •

Increasing pH, conductivity

•

Loss of hydrazine

•

Increase in ammonia


Evaluate need for feed and bleed to remove ammonia and replace with fresh demineralized water

•

Add inhibitor to maintain specified levels


Loss of hydrazine

•

Decrease in pH, conductivity

•

Increase in dissolved oxygen

9-16




•

Repair system leakage

•

Adjust inhibitor concentration as needed


Increased conductivity

•

Increase in concentration of impurities found in water entering the CCW system



•


•

Evaluate need for feed and bleed or increase inhibitor concentration to compensate if necessary

•


9.7

Silicate-Treated Systems


Decreasing pH

•

Loss of silicate

•

Excessive makeup

•


•

Loss of azole



•


•


9-17




•


•

Loss of silicate

•

Possible turbidity

•

Loss of azole

•




•


•

Evaluate need for feed and bleed, drain and refill, or increase silicate concentration to compensate if necessary

•



Increasing trends in total iron or total copper

•


•


•

Occurrence of leaks



•


•


•


9-18



Possible increase in sulfate and or phosphate

•


•




•


•

Evaluate impact of oil in the system and remove via cleaning, feed and bleed, or drain and refill

9.7.5 Contaminant Ingress Symptoms •


•

Possible turbidity


Identify the source of contaminant and correct.

•


•


9.7.6 Bacterial Growth Symptom •


•


9-19


Corrective Action •

Add biocide

•


•


•


•


9.8

Pure Water Systems

9.8.1 System Out-Leakage Symptoms •

Excessive makeup

•

Changes in oxygen concentration (depending on the level in makeup water)

•

Changes in copper concentration


If oxygen has changed, sample at various points of the system to determine point of ingress

•




•

Increase in conductivity

•

Increase in concentrations of impurities (depending on which impurities are present in water entering the CCW system)

•

Increase in dissolved oxygen content



•

Evaluate effect of service water contaminants on the system

•

Evaluate need to place system in feed and bleed or drain and refill

9-20


9.8.3 Acidophilic Bacteria Growth Symptoms •

Decreasing pH

•

Increase in conductivity (usually due to organic short-chain acids like acetic and formic) with no mineral contaminant ingress


Add biocide

•


•

Increase monitoring frequency for aerobic and anaerobic bacteria

•

Begin monitoring sulfate concentration more routinely, looking for trends

•


•


•

Determine source of bacteria ingress



•

Increase in ammonia

•

Increasing conductivity


Add biocide

•


•

Perform system flush

•

Increase monitoring frequency for ammonia-forming bacteria and ammonia

•


•


•

Determine source of nitrogen for ammonia formation

9-21




•


•


•

Occurrence of leaks



•


•


•



Possible increase in sulfate and or phosphate

•


•

Organic odor



•


•




9-22



Identify source of contaminant and correct

•


•

If elevated contaminant will result in increased corrosion rate or degraded system performance, place the system into feed and bleed or drain and refill

9.9

Inhibited Glycol Systems


Decreasing pH

•

Loss of glycol

•

Excessive makeup

•


•

Loss of azole



•


•




•

Loss of glycol

•


•

Loss of azole

•

Increase in concentrations of impurities (depending on which impurities are present in water entering the CCW system)

9-23




•


•

Place system into feed and bleed if necessary

•


9.9.3 Glycol Breakdown Symptoms •

Possible decreasing pH

•

Decrease in reserve alkalinity

•

Increase in glycolic and related acids


Evaluate need to drain and refill

•

Add replenishment package from manufacturer if appropriate



•


•


•

Occurrence of leaks



•


•


•

Place system into feed and bleed or drain and refill if necessary

9-24




•


•



Initiate testing for sulfate and/or phosphate

•


•





Identify source of contaminant and correct

•


•

If elevated contaminant will result in increased corrosion rate or degraded system performance, place the system into feed and bleed or drain and refill, or increase inhibitor concentration(s) to compensate

9.10 Flow Charts The following flow charts (Figures 9-1 through 9-5) can be used as guidance in troubleshooting a change in CCW system chemistry: •

Sudden shift in inhibitor concentration

•

Ingress of contaminants

•

Change in pH

•

Change in conductivity

•

Increase in corrosion 9-25


Specific charts might need to be developed for those systems where there are many system interfaces and/or complex system inhibitor chemistry.

Figure 9-1 Troubleshooting Flow Chart for Sudden Shift in Inhibitor Concentration

9-26


Figure 9-2 Troubleshooting Flow Chart for Contaminant Increase

9-27


Figure 9-3 Troubleshooting Flow Chart for pH Change

9-28


Figure 9-4 Troubleshooting Flow Chart for Change in Conductivity

9-29


Figure 9-5 Troubleshooting Flow Chart for Increase in Corrosion Rate or Corrosion Product

9-30


10

METHODOLOGY FOR PLANT-SPECIFIC TREATMENT OPTIMIZATION

10.1 Introduction A wide range of materials and operating conditions exist in power plant closed cooling water systems. Because of this, there is no single optimum water chemistry program that can be specified for all cases. As such, a CCW chemistry program requires development of a sitespecific strategic plan. This program or plan should consider system design, past history, monitoring capability, enhanced physical treatment equipment (such as sidestream filters and sidestream demineralizers) and operating experience. Deviating from the specifications outlined in this guideline is allowed, provided that sound technical justification and/or performance monitoring supports the deviation and the deviation has the understanding and support of management. Cost-benefit analysis is seldom a major factor in the selection of a CCW chemical treatment program. The chemical cost to treat closed systems is usually a relatively minor budget item.

10.2 Determining Chemical Treatment Needs Chemical treatment programs for closed cooling water systems are developed from six areas: •

System metallurgy and design

•

Makeup water quality

•

Operating conditions

•

System conditions (such as history of microbiological growth)

•

Previous treatment regime(s)

•

Compatibility and impact on other plant systems (for example, radwaste)

10.2.1 Metallurgy and Design The selection of specific metallurgy for closed cooling water systems will be based on many design factors in addition to corrosion resistance. Chemical treatment must be compatible with and protect all of the system components. Even though a specific metal or alloy might make up only a small part of the total system, corrosion and the transport of corrosion products can impact corrosion of other components. The corrosion inhibitor(s) must preferably protect all system 10-1

EPRI Licensed Material Methodology for Plant-Specific Treatment Optimization

metallurgy or, at least protect the major components from the impact of corrosion of the minor component. Because there are numerous operating factors that play a part in chemical treatment selection, it is important that the system engineer be consulted and made a part of the selection process. Most closed cooling systems will contain carbon steel (piping) and one or more copper alloys (heat exchanger tubing). In addition, one or more components containing stainless steel might be present. Some systems use metals such as aluminum, titanium, or monel. 10.2.2 Makeup Water Source Makeup water source selection is fundamental to proper, long-term closed system performance. Although closed system design calls for low makeup requirements due to a lack of evaporation, deconcentration, or significant leakage, the makeup source quality is a critical factor in the selection of a treatment program. Makeup water quality requirements can vary from demineralized water (typical) to potable water (rare but used in some nuclear plants). Another potential makeup water source is condensate, which might contain ammonia or other neutralizing amines. These amines can promote the corrosion of copper alloys. In some cases, a requirement for high-purity recirculating water might preclude the use of any water-soluble chemical treatment for corrosion control. In these cases, corrosion protection is limited to system metallurgy and design, and control of dissolved gases. Chloride and sulfate can have a significant impact on the inhibitor concentration required for complete inhibition. However, at the treatment levels commonly used, there is adequate protection unless chloride and sulfate levels are significant. Table 10-1 provides data from one study of chromate and nitrite-treated systems. Table 10-1 Critical Concentration of Chloride and Sulfate, Above Which Pitting of Armco Iron in Chromate or Nitrite Solutions Occurs (25oC) [12] Inhibitor, Concentration

10-2

Critical Concentration Cl-

SO4=

CrO4=

145 ppm

7 ppm

37 ppm

CrO4=

360 ppm

18 ppm

81 ppm

NO2-

33 ppm

127 ppm

14 ppm

NO2-

67 ppm

280 ppm

37 ppm

NO2-

333 ppm

>1213 ppm

304 ppm


There are a few things that should be noted about Table 10-1. First of all, the material used in these studies was Armco iron6 and not carbon steel. The data are therefore not directly applicable to carbon steel. In addition, only chromate and nitrite inhibitors are covered. Molybdate-based inhibitor systems were not part of this study. It is reasonable to expect molybdate-based systems to behave similarly. What the table shows is that the inhibitors offer reasonable protection from the effects of chloride and sulfates as long as adequate inhibitor levels are maintained. The impact of chloride and sulfate on carbon steel in nitrite-treated systems was addressed by Conoby and Swain [51]. This study concluded that the weight ratio of sodium nitrite to impurities should be kept above one to avoid attack on carbon steel. In addition, the minimum inhibitor concentration should exceed the sulfate concentration by 250–500 ppm and equal the chloride concentration at low levels of aggressive ions. This should be given consideration in those systems that use potable water for makeup. 10.2.3 Operating Conditions Some CCW systems are characterized by high metal skin temperatures. Reductions in heat transfer and restrictions in coolant flow from mineral, microbiological, or corrosion deposits cannot be tolerated in these systems. The conditions under which the system operates have an impact on selection of the chemical treatment agents. Normal circumstances demand that treatment components remain active and effective indefinitely, with supplementary treatment requirements limited to treatment of the makeup water. Careful consideration must also be given to low-flow and stagnant-flow conditions. In order for corrosion inhibitors to protect metal surfaces, there must be sufficient contact between the inhibitor and the metal surface. This is made more difficult by low- and stagnant-flow conditions. In these cases, higher inhibitor concentrations might be required than would be required under normal flow conditions. Many CCW systems have side loops or redundant loops that are operated infrequently. Some examples of these are cooling loops for radwaste equipment, heating and cooling systems (sometimes part of one system), and letdown degassifier coolers. These loops might remain inactive for weeks to months and will be stagnant at their ambient temperature. The concentration of inhibitor might change (depending on the inhibitor and length of time apart from the main loop), but the concentration of contaminants remains constant. In addition, the amount of suspended corrosion products might increase. Thus, when the side loop is put back into service, the concentrations for all these parameters will change slightly. Knowing when these side loops are put into or taken out of service can provide important information to the chemist when trying to understand changes in inhibitor or contaminant concentrations.

6

American Iron Co., 99.8–99.9% purity.

10-3


10.2.4 System Conditions System cleanliness is important for both new construction and older installations. New systems should be thoroughly cleaned and flushed before use in order to remove debris, mill scale, oil and grease, and other foreign materials that could interfere with proper application of specific corrosion inhibitors. With regard to existing systems, the results of previous inspections should be incorporated into treatment decisions. Corrosion deposits from various corrosion mechanisms, mineral scales, and foulants such as biomass, affect changes in piping and heat exchanger performance. Metal loss and reduced heat transfer might be the most significant, but friction increases and reduced-flow rate capacity are also important. The presence of biological deposits is another aspect of system conditions that has an impact on treatment selection. For example, the presence of biological deposits might limit the use of nitrite-based corrosion inhibitors because these inhibitors are a nutrient source. Cleaning methods include in-place chemical cleaning with acids, alkalis, dispersants, solvents, and chelating or sequestering agents. High-pressure water and various mechanical methods, such as brushing or scraping, can also be used to clean and restore closed system piping and heat exchangers to good operating condition. Program selection should take into account the performance of past inhibitor programs. Expected plant-life and life extension issues are also factors that should not be overlooked. Many CCW systems do not have optimum sample locations. Often sampling must be done from low-point drains. These are usually satisfactory for sampling soluble species. Samples collected for analysis of insoluble species might not be entirely representative. 10.2.5 Previous Treatment Regime The selection of chemical treatment for a closed cooling water system must take into account any previous treatment regime. The nature of the protective film, any deposits, and any residual treated water remaining in the system, can adversely affect replacement inhibitor performance. A transition plan is required any time there is a conversion from one program to another. (This is not limited to CCW systems.) An example is the conversion from a chromate- to a nitrite-based program. Chromate and nitrite are not compatible, so it is important that the chromate be thoroughly removed from the system before the nitrite is introduced. Converting from one type of treatment program to another can also release suspended solids (for example, corrosion products) into the system. This possibility should be considered and contingency plans developed to deal with this situation. Either sidestream filtration or feed and bleed will be required to remove suspended solids. Feed and bleed can significantly increase the load on radwaste. 10-4


Plants Hatch, Perry, and Surry have experienced significant levels of dissolved solids in CCW water following conversion from a nitrite to a molybdate program. Waterford 3 converted from nitrite to molybdate and did not experience any significant problems. Clinton did not experience any significant problems converting from nitrite to molybdate but had sidestream filtration in place shortly after the conversion. 10.2.6 Compatibility Seals, packing, flexible couplings, vibration dampers, expansion joints, piping, hoses, and any other system components containing materials other than metals might not require chemical corrosion protection but must be compatible with the treatments to be used to protect the rest of the system. Fiberglass and plastic pipe of various types are generally impervious to water treatment chemicals at use levels. These materials could be adversely affected, however, if exposed to higher concentrations of chemicals often used in pretreatment, cleaning, and initial passivation procedures. Compatibility of nonmetallics with chemicals is discussed in Section 2 and Table 2-2. Compatibility of closed cooling water system chemical treatment components is essential. The compatibility issue must be addressed whenever the inhibitor chemistry is changed and each time a treatment regime is designed. One area of potential incompatibility that is often overlooked is the accidental, incidental, or deliberate mixing of two or more systems. Cross-connections are always a possibility. Incompatibility can become a health and safety issue whenever the possibility of a cross-connection with a potable or other domestic water system exists. Isolation techniques or backflow prevention is a prime consideration in these cases. Incompatibility of treatment and treatment components can result from differences in ionic charge, oxidation/reduction potential, or general reactivity such as neutralization or adsorption. pH becomes a factor because most inhibitor components or regimes work best within a specific pH range. Therefore, chemical treatment ingredients must be compatible with one another and effective in the same operating environment of pH, temperature, and other characteristics of a particular system. These characteristics include flow rates, operating cycle, and the presence of glycol or other antifreeze compounds (glycol and chromate are incompatible). 10.2.7 Impact on Other Plant Systems The chemicals used in a CCW system can have an impact on other plant systems, especially radwaste treatment systems. In many cases, this is due to the high ionic loading that can be placed on radwaste ion exchange systems. There is also the possibility that activation products will be produced. Many of these effects are discussed in detail in Section 4 for each specific corrosion inhibitor. There are other plant systems that might be impacted by CCW systems. These include industrial waste treatment systems and sewage treatment systems. Industrial waste treatment systems could be impacted by components such as dispersants and impurities such as iron and copper. Sewage treatment systems can be impacted by biologically active substances such as biocides and 10-5


chromates. There is also the possibility that leakage might occur from or to primary or secondary loops in nuclear power plants. 10.2.8 Plant Life Extension Experience and Issues Plant life extension involves demonstrating adequate management of aging effects on plant systems and components, as required by 10 CFR 54.21. Closed-cycle cooling water systems are specifically required to be evaluated in accordance with ID number XI.M21 of NUREG-1801, Generic Aging Lessons Learned (GALL) Report. Therefore, maintaining conditions to minimize corrosion and microbiological growth in CCW systems is essential for effectively mitigating many aging effects. Several plants have met this requirement by ensuring that their CCW chemistry programs were aligned with Revision 0 of the EPRI Closed Cooling Water Chemistry Guideline (TR-107396). For those systems not in compliance with the guidelines, comprehensive justifications need to be incorporated into the station’s site-specific CCW chemistry program.

10.3 Environmental Impact and Health Effects All chemicals have potential health effects. These effects must be considered as part of an overall chemical handling program. Environmental impact considerations are also an important part of program selection; however, they are not the only consideration. If chromates are to be used in CCW systems, it is especially important that system leaks are minimized and plans are made for the containment and handling of CCW system water during equipment repair. Water drained from systems treated with nitrite or molybdate should not be reused in the system unless steps are taken to eliminate biological effects. Reusing water that has been drained from the system increases the potential for biological contamination.

10.4 Nonchemical Treatment Considerations These considerations include pure demineralized water systems and mechanical treatment methods such as sidestream filters and/or demineralizers. Pure water systems were discussed in Sections 4 and 5. Sidestream filters and demineralizers can be effective for removing suspended and dissolved solids from the CCW water and preventing deposition.

10-6


11

REFERENCES 1. Nuclear Energy Institute (NEI). “Steam Generator Program Guidelines,” Document 97-06, Rev. 1 (January 2001). 2. Chromate Substitutes for Corrosion Inhibitors in Cooling Systems. EPRI, Palo Alto, CA: 1987. NP-5569. 3. Service Water System Chemical Addition Guideline. EPRI, Palo Alto, CA: 1996. TR-106229. 4. Generator Cooling System Operating Guidelines: Cooling System Maintenance and Performance Guidelines During Start-Up, Operation, and Shutdown. EPRI, Palo Alto, CA: 2001. 1004004. 5. Fundamentals, The Handbook CD, American Society of Heating, Refrigeration and AirConditioning Engineers (ASHRAE), Atlanta, GA, 2001. 6. The NALCO Water Handbook. McGraw Hill, New York, NY, 1988, p. 38.10. 7. Corrosion Basics. NACE International, Houston, TX (1984). 8. H. H. Uhlig. Corrosion and Corrosion Control. John Wiley & Sons, 1985, p. 265. 9. Butler & Ison. Corrosion and Its Prevention in Waters. Reinhold Publishing Corp., New York, NY 1966, p. 165. 10. Investigation of the Effects of Corrosion-Inhibiting Treatment on Mechanical Seals in Recirculating Hot Water Systems, NACE International, Houston, TX. 7G181. 11. PMC Specialties Group Bulletin CORTT-7565-PH, “Effect of pH on the Effectiveness of COBRATEC TT-100 in Preventing Copper Corrosion.” 12. H. H. Uhlig. Corrosion and Corrosion Control. John Wiley & Sons, 1985, p. 232. 13. Corrosion Handbook, ASM International, Materials Park, OH. 14. OE 7031, “Stress Corrosion Cracking in Closed Loop Component.” 15. “Molybdate: The Versatile Inhibitor,” Paper 444, Corrosion 89, National Association of Corrosion Engineers International (1989). 16. M. J. Pryor and M. Cohen, J. Electrochemical Society, Vol. 100, p. 203 (1953). 11-1

EPRI Licensed Material References

17. R. May, G. E. Betz, private communication. 18. H. H. Uhlig and P. F. King, J. Electrochemical Society, Vol. 106, p. 1 (1959). 19. W. D. Robertson, J. Electrochemical Society, Vol. 98, p. 94 (1951). 20. G. H. Cartledge, Corrosion, Vol. 24, p. 223 (1968). 21. M. Meziane, F. Kermiche, C. Flaud, “Effect of Molybdate Ions as Corrosion Inhibitors of Iron in Neutral Aqueous Solutions,” British Corrosion Journal. Vol. 33, No. 4 (1998). 22. F. A. Cotton and G. W. Wilkinson. Advanced Inorganic Chemistry: A Comprehensive Text. Wiley Interscience Publishers, 1966. 23. C. M. Mustafa, S. M. Shahinoor, and I. Dulal, British Corrosion Journal. Volume 32, No. 2, p. 133 (1997). 24. Technical Information Report No. 10-298, “Oxidation and Degradation Products of Common Oxygen Scavengers,” Calgon Corporation, Pittsburgh, PA. 25. Corrosion Inhibitors, National Association of Corrosion Engineers, Houston, TX (1973). 26. H. H. Uhlig. Corrosion and Corrosion Control. John Wiley & Sons, 1985, p. 266. 27. Drew Principles of Industrial Water Treatment, Drew Industrial Division, Ashland Chemical Company, p.66 (1985). 28. E-Mail communication, Bruce Strickland, PMC Specialties, June 11, 2003. 29. E-Mail communication, Bruce Strickland, PMC Specialties, June 12, 2003. 30. United States Nuclear Regulatory Commission. IE Circular No. 80-11, May 1980. 31. Moccari, “Corrosion Inhibitor Evaluation for Materials Used in Closed Cooling Systems,” Materials Performance, p. 54, September 1999. 32. Holm, et al., “Surface Analytical Investigation of Triazole Inhibitor Layers on Brass,” Proceedings of the International Water Conference, p. 70 (1991). 33. Personal communication between S. Mathis and R. Litman and visit to Millstone III (1999). 34. J. S. Lee, R. I. Ray, K. L. Lowe, J. Jones-Meehan, and B. J Little, “An Evaluation of Microbial Growth and Corrosion of 316L SS in Glycol/Seawater Mixtures,” Biofouling. Vol 19 (Supplement), pp 151–160 (2003). 35. Dow Chemical Company, HVAC Focus, Winter 2001. 36. American Society for Testing and Materials. 2001 Annual Book of ASTM Standards, Designation: D 4985-00a, “Standard Specification for Low Silicate Ethylene Glycol Base 11-2

EPRI Licensed Material References

Engine Coolant for Heavy Duty Engines Requiring a Pre-Charge of Supplemental Coolant Additive (SCA),” Volume 15.05. 37. American Society for Testing and Materials. 2001 Annual Book of ASTM Standards, Designation: D 1287-91 (Reapproved 1997), “Standard Test Method for pH of Engine Coolants and Antirusts,” Volume 15.05. 38. R. G. Eagar and A.B. Their, “Control of Microbiological Fouling with Glutaraldehyde,” Technical Paper TP 87-1, Cooling Tower Institute Annual Meeting (1987). 39. J. W. McCoy. The Chemical Treatment of Cooling Water. Chemical Publishing Company, New York, NY 1974, p. 89. 40. The NALCO Water Handbook. McGraw Hill, New York, NY, 1988, p. 22.15. 41. OE 15981. 42. American Society for Testing and Materials. 2001 Annual Book of ASTM Standards, Designation: D 1121-98, “Standard Test Method for Reserve Alkalinity of Engine Coolants and Anitrusts,” Volume 15.05. 43. American Society for Testing and Materials. 2003 Annual Book of ASTM Standards, Designation D 2688-94 (Reapproved 1999), “Standard Test Methods for Corrosivity of Water in the Absence of Heat Transfer (Weight Loss Methods),” Volume 11.01. 44. American Society for Testing and Materials. 2000 Annual Book of ASTM Standards, Designation G 96-90 (Reapproved 1996), “Standard Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods),” Volume 03.02. 45. K. A. Selby and D. W. Townley, “Corrosion Rate Monitoring in Power Plant Systems: Methodology and Applications,” EPRI Service Water Reliability Improvement Seminar (July 2000). 46. J. W. Mc Coy. Microbiology of Cooling Water. Chemical Publishing Company, New York, NY 1980, p. 117. 47. American Public Health Association. 9215 Heterotrophic Plate Count, Standard Methods for the Examination of Water and Wastewater, 18th Edition, p. 9-32 (1992). 48. 3M Company, Minneapolis, MN. 49. Service Water System Corrosion and Deposition Sourcebook. EPRI, Palo Alto, CA: 1993. TR-103403. 50. Guide for the Examination of Service Water System Piping. EPRI, Palo Alto, CA: 1994. TR-102063. 51. J. F. Conoby and T. M. Swain, Materials Protection. Vol. 6, No. 4, p. 55 (1967). 11-3


A

POSITION PAPER ON IMPURITY CONCENTRATION LIMITS FOR CLOSED COOLING WATER SYSTEMS

A.1

Executive Summary

This position paper pertains to auxiliary closed cooling water (CCW) systems in nuclear power plants. Examples are component cooling systems, emergency diesel generator jackets, enclosure coolers, and lubricating oil coolers. The water used in these systems is of very high quality, typically deionized water or condensate. Nuclear Steam Supply System (NSSS) suppliers provide specifications concerning allowable impurity limits for the CCW water. However, the current impurity limits are inconsistent among the NSSS suppliers and, in some cases, are unrealistically conservative. (Chloride and fluoride limits vary from 10 ppm to 150 ppb.) This paper establishes the basis for new limits that will assure protection of CCW system materials but, at the same time, will be reasonable and achievable. Data and experience were gleaned from a number of recognized industry leaders in the fields of metallurgy and corrosion. Only stainless steel degradation is addressed, and primarily stress corrosion cracking (SCC) of stainless steel. The following impurity limits are recommended: * Table A-1 Recommended CCW Impurity Limits Impurity

Degradation Mechanism

Proposed Upper Limit

Chloride

Stainless Steel Stress Corrosion Cracking (SCC)

10 ppm @ < 180 °F

Fluoride

Stainless Steel Stress Corrosion Cracking

10 ppm @ < 180 °F

* Providing that these additional criteria are also met: -- Material stresses are kept below 90% of the yield strength. -- Dissolved oxygen is kept below 8 ppm. -- pH is kept greater than 7. SCC is much more likely in vertical exchangers than in horizontal exchangers. This is due to evaporation and increasing temperatures in the vapor zone, below the top tube sheet. The CCW system heat exchangers under consideration, however, are normally horizontal and are not prone to this failure mode (Ref. 1).

A-1

EPRI Licensed Material Position Paper on Impurity Concentration Limits for Closed Cooling Water Systems

To avoid sediment buildup in the bottom of exchangers, periodic flushing is recommended and filtration is beneficial. Also the use of chlorinated solvents should be prohibited. While it is not possible to be 100% certain that SCC failures will not occur in the CCW systems, the above impurity levels are conservative and represent a very low, acceptable risk.

A.2

Introduction

Nuclear power plants in the United States are supplied by four nuclear steam supply system (NSSS) vendors. These are Babcock & Wilcox, Combustion Engineering, General Electric and Westinghouse. Each of these vendors has a different overall plant design, however, the auxiliary closed cooling water (CCW) systems and their materials of construction, are similar. The CCW systems under consideration are distinct from the primary system and include component cooling systems, emergency diesel generator jackets, enclosure coolers, and lubricating oil coolers. The water used in these systems is of very high quality, typically deionized water or condensate. The materials of construction cover a wide range of alloys from low carbon steel to stainless steel. This paper pertains only to stainless steel and considers, primarily, degradation by stress corrosion cracking (SCC). Chloride and fluoride ions are known to cause SCC. A good practice is to keep them as low as possible to avoid failures. The current impurity limits for these halides, set by the NSSS vendors are, however, inconsistent and, in some cases, are unreasonably low. The goal of this paper is to establish chloride and fluoride limits that will assure the protection of system materials, but at the same time, be reasonable and achievable. A.2.1 Current Impurity Limits Nuclear Steam Supply System (NSSS) vendors provide specifications and guidelines concerning impurity concentration limits for specific closed cooling water systems of nuclear power plants. The current specifications for chloride and fluoride impurity concentrations in the Component Cooling Loop of the Auxiliary Coolant System are shown in the Table A-2. Table A-2 Impurity Concentration Levels Allowed by NSSS Vendors Westinghouse

General Electric

Combustion Engineering

Babcock & Wilcox

PWR

BWR

PWR

PWR

Chloride

150 ppb

10 ppm

1 ppm

1 ppm

Fluoride

150 ppb

(none provided)

(1 ppm total for all halides)

1 ppm

A-2


A.2.2 Role of Water Quality in SCC There is abundant evidence in the literature that chloride and fluoride play a primary role in causing SCC in stainless steel materials. Other factors, however, such as stress, temperature, pH and oxygen level, are also important. Maintaining an effective chemistry control program in an operating CCW system will reduce the chance of SCC.

A.3

Technical Position

SCC is a complex phenomenon that is affected by several factors in addition to chloride/fluoride. A tensile stress is required. Dissolved oxygen and temperature are very important. As the oxygen level and temperature decrease, the tendency for SCC decreases. Except for very alkaline conditions (caustic solutions), the probability of SCC decreases as the pH increases. In CCW systems, many of these factors are such that the probability of SCC is low. This is discussed below and references are provided. A.3.1 Factors Affecting SCC of Stainless Steel Materials Under nuclear power plant CCW conditions of operation, SCC of Type 304 and 316 Stainless Steel is influenced by many interrelated factors. These factors include: •

Metal alloy composition

•

Temperature

•

Stress Levels

•

Chloride concentration

•

Component surface conditions

•

pH

•

Oxygen

•

Other ionic species (such as sulfate, fluoride)

•

Microorganisms

•

Corrosion inhibitors/chemical additives

The parameters that are controlled in a CCW system include pH, microorganisms, ionic contaminants, and inhibitors. The pH in the CCW system water is controlled by the use of chemical additives. Chemicals such as chromate, nitrite or molybdate (carbon steel corrosion control) and azoles (copper alloy corrosion control) are added to reduce corrosion in the systems. Biocides are added as needed to reduce microbe populations. Monitoring of other parameters (that is, chloride, fluoride, sulfate), and heat exchanger performance, when possible, and taking prompt corrective actions when necessary, will provide assurance that detrimental corrosion is mitigated.

A-3


Stress. CCW systems are designed such that stress levels do not exceed 33 % of material yield strength. CCW system components will not exceed 90 % material yield strength even under accident conditions of operation. By establishing and maintaining a CCW system chemistry control program, there can be reasonable assurance that corrosion and SCC are under control in the CCW system components. It has been documented (Ref. 2,3,4) that with chloride concentration levels maintained below 200 ppm, SCC is not a major concern. Documented data also indicates that for an alkaline range (pH > 7.0), SCC is minimized (Ref. 6). Chemical additives, such as nitrite, molybdate, chromate, etc., provide a corrosion boundary for carbon steel to enhance the CCW components’ resistance to corrosion. Operating temperatures and component surface temperatures in the CCW for the majority of the system are <180°F. SCC is minimized at these temperatures. Although some references indicate that fluoride concentrations in the CCW water are not detrimental to system components and will not cause significant corrosion or SCC. (Ref. 9), fluoride is considered a stainless steel corrodent for purposes of this document. For pure water CCW systems (BWR), soluble impurity concentrations are limited and controlled by the CCW water-conductivity limit. The GE specification for conductivity of a pure water CCW system is 3.0 µS/cm. The GE specification for chloride concentration limit in a nitrite corrosion inhibitor solution is <10 ppm.(Ref. 10) Sensitization. Sensitization is a state of reduced corrosion resistance of austenitic stainless steels caused by a depletion of chromium in the grain boundary area. It occurs when high carbon containing stainless steels (~.08%C) are heated in the temperature range 950-1450°F. It is often observed after welding, in the heat- affected zone. Sensitized stainless steel is very susceptible to intergranular corrosion and SCC. Intergranular SCC of sensitized Type 304 stainless steel has been observed in the laboratory in pure water with 1-2 ppm oxygen in the temperature range 122 - 212°F (Ref. 6). However, sensitization can be avoided if low carbon grades of stainless steel (~.03%C) are used, for example, Types 304L and 316L. It is expected that these low carbon grades will be in place in the CCW systems under consideration and, hence, sensitization will not be an issue. A.3.2 Failure Modes with Stainless Steel System Materials SCC of Stainless Steel Tubing in Heat Exchangers The closed cooling water system heat exchangers in nuclear power plants are normally horizontal with the cooling water on the outside of the tubes. Experience indicates that there are three distinct failure modes of tubing when cooling water is on the shell side and that these are influenced by the metallurgy in use. The three failure modes are: Underdeposit-pitting corrosion and crevice corrosion and/or microbiological influenced corrosion (MIC) of the lower tubes due to crud buildup in the bottom of the exchanger A-4


Mechanism: The flow around the baffles on the shell side tends to deposit sediment and entrained matter in the cooling water in the bottom of the heat exchanger, often to the extent that the lower tubes are covered. Under deposit pitting/crevice corrosion and/or MIC, are common failure modes for Type 304 and Type 316 stainless steel and copper alloy tubes, covered by sediment/crud in such heat exchangers. Highly alloyed stainless steel tubing (that is, 6% molybdenum) is resistant. Crevice corrosion of stainless steel tubes at stainless steel tubesheets, from the shell side Mechanism: The tube to tubesheet crevice is a tight stationary crevice where crevice corrosion is likely for Type 304 tubes, in Type 304 tubesheets, in waters with > 150-200 ppm chlorides. It is also likely for Type 316 tubes, in Type 316 tubesheets, in water with > 1000 ppm chlorides. For these alloys in clad tubesheets, the carbon steel under the cladding is galvanically protective to stainless steel and prevents crevice corrosion, even when the chloride concentration exceeds the 150-200 ppm limits for Type 304, and the 1000 ppm limits for Type 316. For copper alloy metallurgy, crevice corrosion in solid or clad tubesheet-to- tube crevices is not a problem. Stress corrosion cracking of stainless steel tubes in stainless steel tubesheets Mechanism: SCC is a location-specific failure mode. The residual stresses from rolling the tubes into the tubesheet are high enough for SCC to occur. Eight (8) ppm of oxygen is adequate. If chlorides were able to reach critical concentrations in the crevice, SCC might occur. To avoid SCC, the following limits are proposed: chlorides < 10 ppm, tube inlet temperatures < 180F, oxygen < 8 ppm, pH > 7, and residual stress < 90% of yield. Chloride Ion Concentration in the Crevice Evaporation from hot surfaces is a leading mechanism for concentrating chlorides and initiating SCC. Stainless steel tubes in vertical heat exchangers, used in the chemical industry, suffer SCC in the vapor pocket just below the top tubesheet. Stainless steel downcomer piping suffers SCC just above the liquid level. Insulated stainless steel piping has suffered SCC when wet by rain or overhead leaks that filter through the insulation and evaporate from the hot surface. Time is a major factor. In many cases, 3 to 7 years are required for initiation of SCC from chloride concentration on hot surfaces. The higher the chlorides in the water or moisture evaporated from hot surfaces, the shorter is the time for initiation of SCC. There are no well accepted mathematically certain guidelines. SCC has been encountered in vertical heat exchangers, in waters with as low as 14-ppm chlorides, although it took seven years for the SCC to occur (Reference 8). However, in horizontal heat exchangers, with water on the shell side, the tubes are covered and there is no hot wall to evaporate water and concentrate chlorides. The literature mentions possible chloride ion concentration in crevices by occlusion and other mechanisms, so concentration by other than evaporation cannot be ruled out. Experience, however, suggests that the proposed 10 ppm chloride upper limit for Type 304 or Type 316 tubes in solid tubesheets, in A-5


horizontal heat exchangers with cooling water on the shell side, would be ultraconservative for both SCC and crevice corrosion. Experience also suggests that it would be wise to include a prohibition against the use of carbon tetrachloride and other chlorinated solvents in degreasing after tube rolling operation. Use of chlorinated solvent degreasers has resulted in SCC failures of heat exchangers when placed in service. For clad stainless steel tubesheets, the carbon steel would protect stainless steel from SCC as well as from crevice corrosion. SCC is not a problem with copper alloy tube and tubesheets. Summary The proposed guidelines should include recommendations that: •

Periodic flushing out of sediment that collects in the bottom of such units is recommended. Well-maintained filters in closed cooling water systems are beneficial.

•

The use of chlorinated solvents for degreasing after tube rolling operations be prohibited.

•

For Type 304 tubes in solid stainless steel tube sheets, and Type 316 tubes in solid stainless steel tubesheets, the proposed 10-ppm chloride limit is believed to be ultraconservative insofar as prevention of SCC and crevice corrosion is concerned.

•

For clad tube sheets, for more highly alloyed stainless steel tubes, and for copper alloy metallurgy, much higher chloride ion concentrations could certainly be tolerated.

A.4

Side Effects or Risks

Recommendations in this document are based on sound information and judgment. If the recommendations are followed, the likelihood of SCC is very small. Even if SCC were to occur, there are factors that still keep the risk of unexpected and damaging failures at a safe level. First, under the closed system conditions discussed here, SCC requires a long initiation time and proceeds very slowly. This will allow an inspection program to uncover the presence of cracking before failures occur. Secondly, it is very unlikely that the primary system will be contaminated via a failure in the closed cooling water system.

A.5

Strategy

This position paper provides materials, corrosion, and chemistry control data, to support increasing the limits for closed cooling water impurity (Chloride and Fluoride) concentrations. The operating conditions (Stress and Temperature) of the Component Cooling Water and Emergency Diesel Jacket Cooling Water Systems are considered the most detrimental for the systems’ materials of construction, corrosion control, and water chemistry control. The strategy is that, when operating each plant’s closed cooling water systems with an appropriate chemistry control program, the system materials of construction are provided with protection (that is, corrosion is under control and SCC is minimized). This provides an assurance of system and A-6


component integrity and reliability. The other auxiliary systems should receive adequate protection also, since they operate with less stress, lower temperature, and less detrimental conditions.

A.6

Recommendations

•

The impurity limits for CCW system water chemistry should be established at 10 ppm for chloride and 10 ppm for fluoride. The current chloride and fluoride limits can be increased from 0.150 ppm (W) and 1.0 ppm (B&W) to 10 ppm, based on information from ASM and NACE documents (referenced in Section A.7)

•

Periodic flushing is recommended to avoid sediment buildup in the bottom of exchangers. Well-maintained filters are beneficial (cost/benefit evaluation will be needed to support any system modifications).

•

It is recommended that the use of chlorinated solvents for degreasing or other cleaning of CCW components be prohibited.

A.7

References

1. Shell and Tube Heat Exchangers, A Symposium on Shell and Tube Heat Exchangers. Materials/Metalworking Technology Series AMERICAN SOCIETY FOR METALS, 1981 2. Metals Handbook Ninth Edition, Volume 11, Failure Analysis and Prevention. Prepared under the direction of the ASM Handbook Committee, 1985 3. Metals Handbook, Volume 1, Tenth Edition, Properties and Selection: Irons, Steels, and High Performance Alloys. ASM international, 1989 4. Handbook of Corrosion Data. ASM International, 1989 5. Critical Temperatures for Chloride Stress Corrosion Cracking of Type 300 Stainless Steel, C. P. Dillon, Materials Performance, December 1996 6. G. Cragnolino and D. D. MacDonald, “Intergranular Stress Corrosion Cracking of Austenitic Stainless Steel at Temperatures Below 100oC, A Review,” NACE, Corrosion 38. Vol. 38, No. 8 (1982). 7. Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel in Oxygenated Pure Water at Elevated Temperatures (Review), S. Szlarska-Smialowska and G. Cragnolino, CorrosionNACE Vol. 36, No. 12, December, 1980 8. Arthur H. Tuthill, P.E., correspondence to Tim Eckert, EPRI, Dec. 23, 1996 9. Localized Corrosion in Halides Other than Chlorides, Materials Performance, June 1993 10. General Electric Chemical and Radiochemical Specifications

A-7


B

CHEMISTRY-RELATED PROBLEMS IN CLOSED COOLING WATER SYSTEMS

B.1

Corrosion Mechanisms

In the discussion that follows, it is assumed that the reader has a basic familiarity with the corrosion process, including an understanding of the corrosion cell, galvanic series, and so on. It is also assumed that the reader has a basic understanding of microbiologically influenced corrosion (MIC). There are numerous references available for those readers who are interested in obtaining additional information. The discussion that follows is directed toward the specific types of corrosion most pertinent to closed cooling systems. B.1.1 General Corrosion General corrosion refers to a more or less uniform loss of metal from the surface of the metal exposed to the corrodent (in this case, closed cooling water). Anodic and cathodic areas are not fixed; therefore, any location can act as an anodic area at one moment and a cathodic area the next. This shifting of anodic and cathodic areas results in uniform loss of metal over the entire surface. Piping systems are designed with an allowance for general corrosion. B.1.2 Localized Corrosion Localized corrosion results when metal wastage is concentrated in a relatively small area. It is promoted by a separation between the cathodic and anodic areas. The metal loss forms a penetration rather than general thinning. It is, therefore, much more likely to cause an unexpected metal failure than is general corrosion. Pitting Corrosion Pitting is defined as localized corrosion where the depth is greater than the diameter of the affected area. Pitting is usually observed when the passive film breaks down on corrosionresistant alloys. Pits can grow and then become inactive, or they might continue to propagate all the way to through-wall penetration. Pitting corrosion is an autocatalytic process. Once a pit is formed, the environment inside the pit becomes much more aggressive than the bulk water. Chloride ions are attracted to the pit by positively charged metal ions. The metal chloride solution reacts with water to produce B-1

EPRI Licensed Material Chemistry-Related Problems in Closed Cooling Water Systems

hydrochloric acid, which is much more corrosive than the bulk water. The insoluble corrosion products build up and isolate the pit from the bulk water even more. The presence of copper ions in the water can also lead to pitting. The copper ion can plate out on a steel surface as copper metal. This sets up a localized galvanic cell and forms a pit. Crevice Corrosion Crevice corrosion is a broad term that encompasses at least three specific types of corrosion: •

Oxygen concentration cells

•

Ion concentration cells

•

Underdeposit corrosion

In crevice corrosion, the diameter of the affected area is greater than the depth. Typical situations in which crevice corrosion occurs are underneath deposits or underneath components, such as weld backing rings. In each of these types of corrosion, the separation of anodic and cathodic areas critical to localized corrosion is provided by a geometric discontinuity. This separation keeps the solution inside the crevice stagnant, which sets up a difference in ionic and oxygen concentration between the solution in the crevice and the bulk water. Once the area within the crevice becomes anodic, the same basic mechanisms described for pitting become active. Stress Corrosion Cracking (SCC) A metal becomes susceptible to SCC when it is simultaneously put under tensile stress and exposed to a corrosive environment. The required stress is generally 50% or more of the yield strength, although some material-environmental pairs might crack at much lower stresses. Corrosion texts contain lists of materials and environments where SCC has occurred, and the lists are constantly expanding as previously unexplained failures are attributed to SCC. The two most common SCC-susceptible material-environment pairs used in cooling systems are stainless steels exposed to waters high in chloride and some copper alloys exposed to ammonia. Galvanic Corrosion When two dissimilar metals are connected and exposed to a water solution, the less noble metal becomes anodic and the other cathodic, to set up what is termed a galvanic corrosion cell. The anode deteriorates rapidly, while the cathode is given greater corrosion resistance than it normally would have as a result of the electron transfer. If the anodic area is relatively large compared to the cathodic area, the rate of corrosion will be low. With a small cathode, the overall reaction is limited by the number of electrons that can be accepted at the cathode. The opposite situation (large cathodic area and small anodic area) results in rapid dissolution of the anode. In this case, the large cathodic area readily accepts all available electrons from the anodic area. B-2


Manganese deposits can also play a part in galvanic corrosion. This is characterized by pitting attack underneath the manganese deposits. Microbiologically Influenced or Induced Corrosion (MIC) Microbiologically Influenced or Induced Corrosion is one of the main problems in closed cooling water systems. The mechanisms for MIC are discussed in Section B.3. Flow-Accelerated Corrosion (FAC) Flow-Accelerated Corrosion or FAC (sometimes referred to as erosion-corrosion) exacerbates the general corrosion phenomenon. FAC is the removal of corrosion product films by the action of the environment. In CCW systems, this is most often caused by water velocity and impingement. It is usually a localized form of corrosion because it depends on the geometry of the system to direct water flow. Bends in pipes, elbows, tees, pump impellers, and valves are especially susceptible. Removing the corrosion product film greatly increases the corrosion rate above what it would be without erosion. On the other hand, erosion would not be as significant without corrosion taking place. Carbon steels and copper alloys are susceptible to flow-accelerated corrosion. The addition of a half percent or more of chromium to carbon steel, increases its resistance to attack. Under closed cooling water system conditions, stainless steels are virtually immune to the problem. If FAC is a suspected mechanism in high-flow areas of CCW systems, the EPRI CHECWORKS™ software [1] can be used to assess the corrosion observed. Erosion Erosion and FAC are two separate mechanisms. Erosion is strictly mechanical interaction with a metal. The erosion phenomenon that is most often a problem is inlet end tube erosion in heat exchangers. Copper alloy heat exchanger tubes are often subjected to erosion conditions, especially at the inlet end where the turbulence is greatest. Tube blockages can create high fluid velocities and impingement on the tube wall, which will remove protective films quickly. Some suggested maximum velocities are shown in Table B-1. Although these velocities are for condenser tube alloys in seawater, the data should also generally apply to other types of heat exchangers and other types of waters.

B-3

EPRI Licensed Material Chemistry-Related Problems in Closed Cooling Water Systems Table B-1 Suggested Critical Velocity Limits for Condenser Tube Alloys in Sea Water [2] Material

B.2

Recommended Maximum Velocity Feet/Second

Meters/Second

Copper

3

0.9

Admiralty brass

5

1.5

Aluminum brass

8

2.4

90-10 copper nickel

10

3.0

70-30 copper nickel

12

3.7

Type 316 stainless steel

No maximum velocity limit

Titanium

No maximum velocity limit

Impact of Corrosion

B.2.1 Failures Corrosion in closed cooling systems will eventually lead to failures, most likely to take the form of through-wall penetration. If this occurs, the system is no longer closed, and the water that leaks out of the system must be replaced along with any treatment chemicals. The cost of replacing water and treatment chemicals can become excessive. In extreme cases, a critical piece of equipment might be lost, bringing about an unscheduled outage. B.2.2 Fouling Active corrosion within a closed cooling water system can be of significant consequence long before actual failures occur due to the generation of corrosion products. These corrosion products cannot easily be blown-down as in an open cooling water system. Unless a cleanup program is undertaken, the corrosion products that are generated within the closed cooling water system remain in the system. When they accumulate on heat transfer surfaces, a loss of efficiency will result. Accumulations in low-flow or stagnant areas of the system can become breeding grounds for deleterious bacteria that can contribute to MIC. The formation of deposit tubercles has a secondary damaging effect. The tubercles become a site for localized underdeposit corrosion.

B-4


B.3

Microbiological Organisms

Microbiological organisms will be found in virtually all closed cooling water systems. If not controlled, these organisms can cause both corrosion and fouling of equipment in the system. Microorganisms are usually classified according to their ability to grow in the presence or absence of oxygen. Organisms that require oxygen in their metabolic processes are termed aerobic, and those that require oxygen-free environments are termed anaerobic. Microorganisms are also classified according to whether they are free-floating in the bulk water (planktonic bacteria) or are attached to component surfaces (sessile bacteria). Sessile bacteria are part of slime masses. These are of much more concern in cooling systems than are planktonic bacteria. Growth of sessile organisms can contribute to fouling through the accumulation of slime masses. Metal-oxidizing bacteria such as Gallionella sp. also contribute to severe fouling by producing tubercles (nodules) on carbon steel pipe ID surfaces. Tubercles can literally plug the pipe completely and stop flow. Tubercles are composed of ferric hydroxide/oxide formed as a result of oxidation of the ferrous metal by the metabolism of metal-oxidizing bacteria. Slime masses and tubercles can indirectly contribute to corrosion by providing sites for underdeposit corrosion. Although one would tend to perceive closed systems as being depleted in oxygen, this is not necessarily the case. Oxygen can enter with makeup water as well as other sources. Many systems require significant quantities of makeup water due to the loss of water from leaks. Also, many closed systems use tanks that are open to the atmosphere and/or have in-leakage of air at seals. B.3.1 Nitrogen – Biogeochemical Cycle Nitrogen is uniquely positioned in nature’s biogeochemical cycle of elements because there are so many microorganisms that have the ability to alter forms of nitrogen. The existence of life has an absolute need for the many end products of nitrogen metabolism by microorganisms; however, there are many problems that also result. A major problem occurs when denitrifying bacteria remove fertilizing nitrates from the soil. Another problem occurs where nitrite is used as a corrosion inhibitor and microorganisms convert it to nitrate. Undesirable transformations occur in natural and man-made environments, for example, waste treatment processes producing unpleasant odors, corrosive gases, and pH modifications. Many products of nitrogen transformations are very corrosive to metals and concrete. All of these are of interest. The following steps provide a summary of these microorganisms and the transformations they have the ability to perform. Step 1. Fixation of Molecular Nitrogen The natural reservoir for the N2 in our environment is the atmosphere. No substantial amounts are found in geological deposits. Atmospheric N2 is not readily available to biological systems, and is accessible to the biological ecosystem only through the process of nitrogen fixation. B-5


Nitrogen fixation is a process that requires a substantial amount of energy input. Small amounts of combined nitrogen are introduced to the ecosphere by volcanic activity, by atmospheric lightning discharges, and ionizing radiation. Chemical fixation of molecular N2 by man (HaberBosch synthesis) is limited but assuming an increasingly important role in the global nitrogen cycle. The primary mechanism of biogeochemical cycling of the element N is highly dependent on the activities of microorganisms. The first step involves nitrogen fixation by a very specific group of bacteria, usually found in soil environments (rhizosphere) or in aquatic environments (fresh or seawater). These bacteria include Rhizobium sp. (which accounts for the largest contribution of combined N2) in terrestrial habitats. In aquatic habitats, cyanobacteria (blue-green algae) such as Anabena sp. and Nostoc sp. are the most important in nitrogen fixation processes. Step 2. Ammonification The first biologically active compound resulting from N2 fixation is the formation of organic nitrogen (NH2), which is immediately converted to NH3 and incorporated into amino acids and other nitrogen-containing biochemicals, usually essential to microbiological metabolism and subsequently to the metabolism of higher life forms. Many microorganisms, plants, and animals are capable of ammonification through the reaction: 2(NH2) + CO + H2O + urease enzyme system Æ 2NH3 + CO2 The incorporated amino group can be transferred through transamination to form other amino acids, proteins, and other nitrogen-containing compounds by one group of organisms, and is subsequently used as sources of carbon, nitrogen, for synthesis and energy by other groups of organisms. Free ammonia in solution, or as a gas, will exist as a byproduct of this process. The amount depends on the ability of the environment to contain or trap the ammonia. When the environment is favorable for containment, many of the potential problems mentioned earlier can take place. Step 3. Nitrification/Nitrosification Nitrification is a process performed by autotrophic/chemolithotrophic bacteria that utilize nitrite (NO2) or ammonia (nitrosification) as an energy source, and CO2 as the primary carbon source. The reaction is: Nitrite-oxidizing bacteria (nitrifying) Æ NO2 + ½ O2 Æ NO3•

Nitrobacter sp. (also heterotrophic)

•

Nitrospina sp.

•

Nitrococcus sp.

•

Nitrospira sp.

B-6


Both nitrite- and ammonia-oxidizing groups are frequently called nitrifying bacteria. These microorganisms live in a wide range of environments and often show a tendency to attach to surfaces (sessile) forming tight clusters of cells commonly called cysts, and some produce biofilm. They are aerobic or micro-aerophilic bacteria. Ammonia-Oxidizing Bacteria (nitrosifying bacteria) Ammonia-oxidizing bacteria in contained environments (for example, CCW systems with minimal amounts of O2) can produce either alkaline or acidic byproducts that contribute to increased potential for MIC, especially with the corrosion of non-ferrous metals such as copper alloys.

Nitrosomonas sp. → NH3 + O2

→ HNO3

Acidic byproducts

→ NH3 (low O2)

→ NO2

Neutral/weak acidic byproducts

→ NH3 + O2

→ NO/NH2OH

Alkaline byproducts

Nitrosovibrio sp. Nitrosococcus sp. Nitrosomonas sp. Nitrosopira sp. Nitrosolobus sp.

Step 4. Nitrate Reduction/Denitrification This process includes at least three different metabolic pathways. The microorganisms that are involved with these reactions are chemosynthetic/heterotrophic (require an organic energy source). They are diversified by the characteristic that some are aerobic, micro-aerophilic, facultatively anaerobic, or anaerobic and, thus, have optional metabolism pathways. The metabolic pathways are defined by the chemical metabolites produced. Microorganisms often associated with MIC assimilate nitrate nitrogen to organic nitrogen/ammonia. These include (those in bold print are significant contributors to MIC): Aeromonas sp.

Edwardsiella sp.

Arthrobacter sp.

Clostridium sp.

Enterobacter sp.

Escherichia sp.

Thiobacillus sp.

Pseudomonas sp.

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Assimilation of Nitrate Nitrogen A metabolic pathway is: Bacteria Æ NO3 + metalloprotein/reduced cofactors/catalysts Æ NO2 Æ NO intermediates (?) Æ NH2OH Æ NH3 Æ amino acids Æ proteins/ATP Æ microbiological metabolism. This pathway is always associated with a significant increase in the amount of biomass produced and might result in biofouling problems. This pathway is also very common with metabolism of yeast, fungi, and all higher plants. Dissimilation of Nitrate Nitrogen A metabolic pathway is: Bacteria Æ NO3 + reductase enzyme catalyzed reduction Æ NO2 Æ ? and possibly ammonification. This is a pathway that can occur under both aerobic and anaerobic conditions. It is an alternative pathway to O2 respiration for some microorganisms. In most cases, this process is associated with an increase in NO2 concentration, often NH3, and biomass. Although not directly involved with corrosion, the metabolic pathway increases the potential for MIC. Typical microflora include: Aeromonas sp.

Enterobacter sp.

Arthrobacter sp.

Eschericia sp.

Bacillus sp.

Micrococcus sp.

Citrobacter sp.

Nocardia sp.

Denitrifying Bacteria These microorganisms are responsible for the actual removal of nitrogen compounds, as such, from the microenvironment. The byproducts of this metabolic pathway are usually metabolites for other pathways or a release of gas that contributes to gassing problems in closed loop cooling systems. The process is essentially an anaerobic function at a site with excessive deposition of nitrate sludge and other organic biomass, usually in a system where nitrite oxidation by microorganisms had occurred for an extended time. The gassing problem can be justification for bleed and feed or flush and fill procedures. Denitrification does not directly increase the potential for corrosion (MIC); however, it does increase difficulties in maintaining stable operating and control conditions. A metabolic pathway is: denitrifying bacteria Æ NO3 Æ denitrification Æ NO2 Æ NO Æ NO2 (gas) Æ +/- N2 (gas)

B-8


Typical microflora include: Alcaligenes sp.

Hyphomicrobium sp.

Bacillus sp.

Pseudomonas sp.

Nitrosomonas sp.

Thiobacillus sp.

Step 5. Recycling and/or Short Cuts It is important to remember that this biogeochemical cycle is a continuously recycling process. It does not typically reach an endpoint. Some stages are limited by the accumulation of byproducts that either affect the metabolism of the microorganisms or change the microenvironment. It is a dynamic process where several stages of the cycle can occur simultaneously. Several of the stages are reversible and there are short cut pathways that bypass the dominant pathways. The recycling process ensures that all living organisms are supplied with a form of nitrogen necessary for their existence. This is particularly significant when one considers that the primary nitrogen reservoir exists as a non-metabolizable compound, N2. This diversity contributes to potential problems in both terrestrial and aqueous environments. The diversity has provided situations where great benefits are obtained but, when not managed, the nitrogen biogeochemical cycle can contribute to problems such as environmental pollution, MIC, plugging and fouling, biofouling, and loss of heat transfer in cooling water systems. Suggested Reading Microbial Ecology, Fundamentals and Applications – Fourth Ed.. By R. M. Atlas and R. Bartha, Chapter 11, Biogeochemical Cycling: Nitrogen, Sulfur, Phosphorus, Iron, and Other Elements. Pub. Benjamin/Cumming Science. Imprint - Addison Wesley Longman. Inc., Menlo Park, CA, 1998. B.3.2 Aerobic Mechanisms Iron and Manganese Bacteria Pseudomonas sp. bacteria are aerobic slime-formers. They contribute to MIC by scavenging oxygen and forming deposits that can harbor sulfate-reducing bacteria (SRB). Pseudomonas sp. can also reduce Fe+3 to Fe+2, which can contribute to corrosion by exposing bare metal. Gallionella sp. bacteria are associated with the pitting of carbon and stainless steels. They are obligate-aerobic iron bacteria that excrete high levels of iron and manganese in their waste. The morphology of stainless steel pits caused by these microbes is characterized by a small opening leading to a cavernous corroded area. Corrosion deposits on carbon steel are hemispherical. Deposits can be voluminous enough to occlude carbon steel pipes. Crenothrix sp., Leptothrix sp., Clonothrix sp., and Sphaerotilus sp. are aerobic, filamentous iron-oxidizing B-9


and/or manganese-oxidizing bacteria. They oxidize soluble iron to the insoluble Fe(OH)3. Hemispherical tubercles, which are typically brown or reddish brown, are formed by these bacteria. Black tenacious deposits of MnO2 can also be formed. Sulfur-Oxidizing Bacteria Thiobacillus sp. and Ferrobacillus sp. are aerobic bacteria that oxidize sulfide to sulfate and form sulfuric acid. They often coexist with sulfate-reducing bacteria. They appear to be much less damaging to metals than they are to concrete. Thiothrix sp. and Beggiatoa sp. are aerobic bacteria that oxidize H2S to elemental sulfur. The yellow sulfur reaction product is often visible when these bacteria are present. Their major contribution to MIC is the formation of slime in which SRB can live. B.3.3 Anaerobic Mechanisms Sulfate-Reducing Bacteria The sulfate-reducing bacteria (SRB), Desulfovibrio sp. and Desulfomaculum sp., are responsible for the largest economic impact of all the bacteria associated with MIC. These anaerobic bacteria convert sulfate to sulfide. The sulfide is very aggressive to cast iron, carbon, and low-alloy steels, stainless steels, high-nickel alloys, and copper alloys. The pitting of copper alloys is significantly increased in the presence of as little as 0.2 ppm sulfide. SRB damage to stainless steel and carbon steel is indicated by the presence of black iron sulfide (FeS). Pitting in highnickel and copper-based alloys can often be attributed to sulfide attack. The presence of SRB in deposits can readily be detected by a simple field test. The addition of an acid (for example, 10% citric acid) to the deposit will liberate hydrogen sulfide gas (rotten egg odor) if SRB have been present. Nitrifying and Denitrifying Bacteria Nitrifying bacteria such as Nitrosomonas sp. and Nitrobacter sp. oxidize nitrite to nitrate. Denitrifying bacteria reduce nitrite and nitrate to nitrogen gas and ammonia. These bacteria include facultative anaerobic bacteria such as Pseudomonas sp and others listed in Section B.3.1. B.3.4 Microbiologically Influenced/Induced Corrosion (MIC) Microorganisms working in conjunction with traditional corrosion mechanisms can significantly alter the kinetics of the reaction, hence, the name microbiologically influenced/induced (MIC) corrosion. The corrosion-inducing aspect of the MIC name usually refers to the establishment of a deposit or crevice by microbiological growth, thereby initiating corrosion that otherwise would not occur. Microorganisms influence corrosion in many ways, such as: •

The excretion of metabolic byproducts that accelerate corrosion

•

Concentrating halides, which rapidly corrode ferrous materials

B-10


•

Interfering with the cathodic half-reaction under oxygen-free conditions, resulting in increased anodic dissolution

•

Oxidation of metal anions to less soluble forms (for example, Fe+2 to Fe+3) by iron-oxidizing bacteria

•

Destruction of protective coatings

•

Destruction of corrosion inhibitors

B.3.5 Impact of Microbiological Fouling Slime-forming bacteria secrete a sticky film that provides them with protection from the environment and reinforces their attachment to system surfaces. This film, in its pure state, feels slippery to the touch and will curl and peel if allowed to dry. The biofilm collects other particles from the water and can grow into a very thick film. Formation of biofilms inside heat exchanger tubes can lead to a rapid degradation in performance. In severe cases, the fouling becomes so extensive that the deposit can only be removed by shutting down and cleaning the equipment.

B.4

References

1. CHECWORKS™ Flow-Accelerated Corrosion: User Guide. EPRI, Palo Alto, CA: 1997. TR-103198-P1. 2. F. L. LaQue. Marine Corrosion Causes and Prevention. John Wiley & Sons, p. 147.

B.5

Bibliography

Sourcebook for Microbiologically Influenced Corrosion. EPRI, Palo Alto, CA: 1989. NP-5580. Detection and Control of MIC: An Extension of the Sourcebook for MIC in Nuclear Power Plants. EPRI, Palo Alto, CA: 1993. NP-6815D. MIC Training Course, EPRI-PSE, Edition 8/02, Charlotte, NC. Service Water System Corrosion and Deposition Sourcebook. EPRI, Palo Alto, CA: 1993. TR-103403. Guideline for the Evaluation and Treatment of Corrosion and Fouling in Fire Protection Systems. EPRI, Palo Alto, CA: 1999. TR-109633. A Practical Manual on Microbiologically Influenced Corrosion, Volume 1, NACE International, Houston, TX: 1993. A Practical Manual on Microbiologically Influenced Corrosion, Volume 2, NACE International, Houston, TX: 2002. B-11


The Nalco Water Handbook, Nalco Chemical Company, McGraw-Hill, Inc., New York. Betz Handbook. GE Betz, Trevose, PA. Fontana and Greene. Corrosion Engineering. McGraw Hill, Inc., New York. 1978. James W. McCoy. Microbiology of Cooling Water. Chemical Publishing Company, New York, NY 1980.

B-12

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EPRI Closed Cooling Water Chemistry Guideline, Revision 1

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