Feedwater Heater Source Book

Life Cycle Management Sourcebooks— Volume V olume 10: Feedwater Feedwater Heaters

Technical Report

Life Cycle Management Sourcebooks—Volume 10: Feedwater Heaters 1009073

Final Report, December 2003

EPRI Project Manager J. Carey

EPRI  3412 Hillview Avenue, Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 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 Framatome ANP

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 © 2003 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS This report was prepared by Framatome ANP 400 South Tryon St. Charlotte, NC 28285 Principal Investigators J. Hamlen P. Clay This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Life Cycle Management Sourcebooks—Volume 10: Feedwater Heaters, EPRI, Palo Alto, CA: 2003. 1009073.

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PRODUCT DESCRIPTION

EPRI is producing a series of Life Cycle Management Planning Sourcebooks, each containing a compilation of industry experience and data on aging degradation and historical performance for a specific type of system, structure, or component (SSC). This sourcebook provides information and guidance for implementing cost-effective life cycle management (LCM) planning for feedwater heaters.

Background Industry’s cost for producing LCM plans for the many important SSCs in operating nuclear plants can be reduced if LCM planners have an LCM sourcebook of generic industry performance data for each SSC they intend to address—see Life Cycle Management Planning Sourcebooks—Overview Report (EPRI 1003058). The general objective of EPRI’s LCM sourcebook effort is to provide system engineers with generic information, data, and guidance they can use to generate a long-term equipment reliability plan for plant SSCs. These consist of aging and obsolescence management plans that are optimized for plant performance and economics. The long-term equipment reliability plan or LCM plan for the plant SSC combines industry experience and plant-specific performance data to provide an optimum maintenance plan, schedule, and expense profile throughout the plant’s remaining operational life. Objectives To provide plant engineers or their expert support with a compilation of generic information, data, and guidance needed to create a plant-specific LCM plan for the mechanically passive components of nuclear power plant feedwater heaters. Approach Experts in the maintenance and aging management of feedwater heaters followed the Life Cycle Management Planning Sourcebooks—Overview Report . They defined the scope of the physical system (the boundary) and the scope of the components to be included in the study. They then compiled information and data on historical industry performance of selected feedwater heaters within the scope and presented technical guidance for preparing plant-specific LCM plans. Prior to publication, the sourcebook was reviewed by EPRI LCM utility advisors. Results This sourcebook contains information on typical boiling water reactor (BWR) and pressurized water reactor (PWR) feedwater heaters. Information includes performance issues, reliability information, failure rate determination, component aging mechanisms, aging management maintenance activities, equipment upgrades, and replacements. Typical alternative plans for life cycle management are delineated. The sourcebook includes an extensive list of references.

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EPRI Perspective This report—together with the LCM process defined in the EPRI LCM Implementation Demonstration Project (EPRI 1000806)—should enable the preparation of plant-specific LCM plans for feedwater heaters with substantially less effort and cost than if planners/engineers had to gather all the generic information themselves. This sourcebook captures both industry experience and the expertise of the authors related to feedwater heaters. Using this sourcebook, planners and engineers need only to add plant-specific data and information to complete an economic evaluation and overall LCM plan for feedwater heaters. EPRI plans to sponsor additional LCM sourcebooks for other plant SSC types. The process of using sourcebooks as an aid in preparing LCM plans will improve as the industry gains experience in this area. EPRI welcomes constructive feedback from users. As EPRI integrates LCM analysis with development of equipment reliability technology starting in 2004, it also plans to incorporate lessons learned in future LCM sourcebook revisions.

Keywords Life cycle management Aging management Nuclear asset management Feedwater heater Heat exchanger Tube System reliability Component reliability

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ACKNOWLEDGMENTS

The technical contributions, review, and comment by the following individuals greatly enhanced this document and are appreciated: Patrick Clay, Framatome ANP Bert Mayer, Framatome ANP Cliff Munns, Framatome ANP Mitch McFarland, Framatome ANP EPRI Expert Reviewers Alan Grunsky, EPRI Sharon Parker, EPRI George Sliter, EPRI Gary Toman, EPRI

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CONTENTS

1 EXECUTIVE SUMMARY ..................................................................................................... 1-1 2 LCM SOURCEBOOK INTRODUCTION .............................................................................. 2-1

2.1

Purpose of LCM Sourcebook .................................................................................... 2-1

2.2

Relationship of Sourcebook to LCM Process ............................................................ 2-1

2.3

Basis for Selection of Feedwater Heaters for an LCM Sourcebook ........................... 2-1

3 BASIC INFORMATION ON FEEDWATER HEATERS ........................................................ 3-1

3.1

Safety and Operational Significance.......................................................................... 3-1

3.2

Feedwater Heater Function and Configurations ........................................................ 3-1

3.3

Feedwater Heater System and Component Boundaries...........................................3-14

3.4

Scope of Equipment Covered by the Feedwater Heater Sourcebook.......................3-14

4 HISTORICAL PERFORMANCE DATA FROM INDUSTRY OPERATING EXPERIENCE ......................................................................................................................... 4-1

4.1

Nuclear Industry Experience ..................................................................................... 4-2

4.1.1

Qualitative Data ................................................................................................ 4-4

4.1.2

Quantitative Data (EPRI, INPO EPIX/NPRDS, NRC)........................................ 4-8

4.1.3

Maintenance Rule............................................................................................4-12

4.1.4

EPRI PM Basis Templates ..............................................................................4-14

4.2

Generic Communications and Other Reports ...........................................................4-18

4.2.1

NRC Communications .....................................................................................4-19

4.2.2

INPO SEE-IN Communications .......................................................................4-20

4.2.3

INPO NPRDS/EPIX Data.................................................................................4-23

4.2.4

Experience in Fossil Power Generation and Industrial Facilities ......................4-26

4.3

Codes and Standards (Mandatory, Guidance, Associations, Societies) ...................4-28

4.4

Plant Performance Enhancement Program Technical Library ..................................4-30

4.5

Generic Industry Failure Rate Estimation Summary .................................................4-30

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5 GUIDANCE FOR PLANT-SPECIFIC SSC CONDITION & PERFORMANCE ASSESSMENT ....................................................................................................................... 5-1

5.1

Compiling Feedwater Heater Operating and Performance History ............................ 5-1

5.1.1 Feedwater Heater Condition Reviews (Work Orders, Failures Trends, Lost Power Generation, Trips, Preventive Repairs, Replacements, Refurbishments) ............ 5-2 5.1.2

Review of Diagnostic Tests and Monitoring Devices/Data ................................ 5-3

5.2

Review of Current Maintenance Plans .....................................................................5-11

5.3

Current Condition and Performance Assessment.....................................................5-11

5.4

Condition Monitoring Technologies, Tests, and Diagnostics.....................................5-13

5.4.1

System Condition Monitoring...........................................................................5-13

5.2.4

Component Condition Monitoring ....................................................................5-15

5.5

Water Chemistry and Corrosion ...............................................................................5-17

5.6

Feedwater Heater Repairs (Corrective Maintenance)...............................................5-18

5.6.1

Tube Leak Detection Methods.........................................................................5-19

5.6.2

Tube Repair.....................................................................................................5-20

5.6.3

Tube Sleeving .................................................................................................5-21

5.6.4

Tubesheet and Tubesheet Joints.....................................................................5-21

5.6.5

Shell Repairs ...................................................................................................5-22

5.6.6

FW Heater Cleaning ........................................................................................5-23

5.6.7

Spare Parts .....................................................................................................5-23

5.7

Heater Access..........................................................................................................5-24

6 GENERIC AGING AND OBSOLESCENCE ASSESSMENT ............................................... 6-1

6.1

Aging Effects and Mechanisms Review (Aging Matrix).............................................. 6-1

6.2

Expected Lifetimes of Feedwater Heaters................................................................. 6-6

6.3

Technical Obsolescence ........................................................................................... 6-6

6.4

Expected Lifetimes for Preventive Replacement or Redesign ................................... 6-8

7 GENERIC ALTERNATIVE LCM PLANS ............................................................................. 7-1

7.1

Plant Operating Strategies and Types of LCM Planning Alternatives ........................ 7-1

7.1.1

Plant Strategy 1: Operate plant for currently licensed period of 40 years ......... 7-1

7.1.2

Plant Strategy 2: Operate plant for 60 years with license renewal.................... 7-2

7.1.3 Other Plant Strategies: (Power Uprate, Early Decommissioning, >60 Years) 7-2

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7.2

Examples of Suitable Alternative LCM Plans............................................................. 7-3

7.3

Hypothetical Illustration of an LCM Planning Alternative for Feedwater Heaters........ 7-4

7.3.1

Alternative A – Base Case................................................................................ 7-4

7.3.2

Alternative B – Step Increase in NDE Examinations of the FW Heaters ........... 7-5

7.3.3

Alternative C – Major Maintenance (Tube Rebundle, etc.)................................ 7-5

7.3.4

Alternative D – FW Heater Replacement .......................................................... 7-5

7.3.5

Composite Evaluation....................................................................................... 7-6

7.4

Additional Evaluation of FW Heater Options.............................................................. 7-7

8 GUIDANCE FOR ESTIMATING FUTURE FAILURE RATES .............................................. 8-1 9 INFORMATION SOURCES AND REFERENCES................................................................ 9-1 10 ACRONYMS.....................................................................................................................10-1

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LIST OF FIGURES Figure 2-1 (a) LCM Planning Flowchart – SSC Categorization and Selection ......................... 2-3 Figure 2-1 (b) LCM Planning Flowchart – Technical and Economic Evaluation ....................... 2-4 Figure 2-1 (c) LCM Planning Flowchart – Implementation ....................................................... 2-5 Figure 3-1 Basic Outline of Plant Feedwater Heater Arrangement (items 1 through 6 are the feedwater heaters)..................................................................................................... 3-2 Figure 3-2 Cutaway View of a Typical Feedwater Heater (double-zone) ................................. 3-3 Figure 3-3 Single Zone Feedwater Heater .............................................................................. 3-4 Figure 3-4 Double Zone Feedwater Heater............................................................................. 3-4 Figure 3-5 Long and Short Drains Cooler Outline.................................................................... 3-6 Figure 3-6 Vertical Channel-Down Feedwater Heater (showing condensing and subcooling zones) ................................................................................................................. 3-8 Figure 3-7 Vertical Channel-Up Feedwater Heater.................................................................. 3-9 Figure 3-8 Bolted Pass Partition.............................................................................................3-11 Figure 3-9 Welded Pass Partition...........................................................................................3-12 Figure 4-1 SYSMON Access Screen – System 18 – Extraction Steam, Feedwater Heaters & Drains............................................................................................................4-12 Figure 4-2 SYSMON Outline Plan..........................................................................................4-13 Figure 5-1 Eddy Current Flowchart (from EPRI 1003470) ....................................................... 5-8 Figure 5-2 Summary of FW Heater Problems and Possible Causes (from EPRI 1003470) ....5-16

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LIST OF TABLES Table 4-1 Frequency of Initiating Events for Plant Faults ........................................................ 4-3 Table 4-2 Summary of Industry Database Search Results for FW Heaters (Operating Experience) ....................................................................................................................4-11 Table 4-3 Feedwater Heater PM Basis Template...................................................................4-15 Table 4-4 Failure Locations, Degradation Mechanisms, and PM Strategies...........................4-16 Table 4-5 LER Evaluation (from INPO SEE-IN database) ......................................................4-21 Table 4-6 NPRDS Feedwater Heater Search Results - Summary..........................................4-23 Table 4-7 NPRDS Feedwater Heater Search Results –Specific Components and Consequences ...............................................................................................................4-24 Table 4-8 EPIX System/Component Search Results for Feedwater Heaters..........................4-25 Table 4-9 EPIX Word Search Results for Feedwater Heaters (unique items).........................4-26 Table 4-10 Damage Incidence for FW Heaters (Fossil) –From Survey (EPRI 7417) .............4-27 Table 4-11 Operating Experience Data for FW Heater Failure Rate Determination................4-31 Table 5-1 Feedwater Heater Responses (to plant changes) –from EPRI TR-107422-V1 ......5-14 Table 5-2 Troubleshooting Feedwater Heater Problems (from EPRI 1003470)......................5-15 Table 5-3 Recommended Spart Parts for FW Heaters (from EPRI 1003470).........................5-23 Table 6-1 FW Heater Summary –Aging Mechanisms & Effects.............................................. 6-2 Table 6-2 Application of Obsolescence Evaluation Criteria ..................................................... 6-7 Table 7-1 FW Heater Composite Evaluation for LCM Planning (generic) ................................ 7-6

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1

EXECUTIVE SUMMARY

This Life Cycle Management (LCM) Planning Sourcebook for feedwater heaters will help plant engineers and/or expert consultants in preparing a life cycle management plan (a long-term reliability plan) for the feedwater heaters at their plant. The generic information and guidance presented in this document are expected to help plant engineers focus on areas where there may be significant opportunities for cost-effective improvements in long-term plans. Use of this sourcebook will help reduce the cost of preparing a plant-specific LCM plan for feedwater heaters. Guidance consists mainly of generic industry information, data, and references on feedwater heaters. This sourcebook identifies component aging mechanisms together with the maintenance activities to manage them, as well as any applicable obsolescence issues and available management options. Guidance is provided on how to build alternative LCM plans that can be considered for long-term planning for the critical components in the system. This sourcebook provides sample generic LCM plan alternatives to serve as starting points for plant-specific applications. Depending upon the level of detail desired for the plant-specific LCM plan, the generic data in this sourcebook may allow plant engineers to identify areas where significant cost-effective improvements or reductions in maintenance activity can be realized and where long-term planning for emerging obsolescence issues can be developed. This sourcebook also contains lessons learned from the LCM Planning Demonstration Project by providing guidance in the evaluation of plant-specific data, selection of plant operating strategies, and feasible alternative LCM plans. A comprehensive reference listing is included to give the utility engineer additional resources for consultation. In a companion document, the “LCM Planning Sourcebook Overview Report” (EPRI 1003058) [Ref. 1], LCM planning process-oriented information is presented as well as additional generic guidance for the preparation of plant-specific LCM plans. The sourcebook and overview report are meant to be used in conjunction, with the sourcebook addressing “what to evaluate” and the overview report addressing “how to evaluate”. The Feedwater Heater reliability issues identified and addressed by this study are : •

Tube Vibration, Erosion, Cracking, and Corrosion

•

Shell Erosion and Corrosion

•

Tube-to-Tubesheet Joint Problems

•

Drains Cooler Zone Erosion

•

Impingement Plate (baffle, shield, drain inlet, etc.) Erosion 1-1

Executive Summary

•

•

•

Tubesheet Erosion and Inlet Tube Erosion Adequacy of Feedwater Heater Performance Monitoring, Test Methodology & Frequency, and Repair Techniques Secondary Side Water Chemistry

The most sensitive components are the tubes, and their expected reliability and lifetime are based on the material involved. Sections 5 and 6 address these issues. Internals problems are often identified when the tubes are being inspected, and their failures (erosion, cracking) will often cause vibration problems leading to tube failure. Problems with FW heater shells (minor cracks and small areas of thinning) are relatively simple to repair via standard welding techniques if identified early, but a shell that has lost much of its thickness over a large area is a major corrective maintenance problem. The potential candidate approaches for formulating feedwater heater LCM plans as alternatives are identified in Section 7. These alternatives are options for consideration beyond the plant current base case for SSC inspection and maintenance. They include the following cases: •

•

•

•

Adjusting the frequency of time-directed maintenance to enhance the reliability of the feedwater heaters (this includes more frequent inspections of the tubes and shell). Increasing the scope of NDE (more tubes, shell welds, major portions of the shell etc.) Performing major maintenance on FW heaters that have experienced problems (replacing individual tubes, tube re-bundles, internals repair, etc.) Replacement of an entire FW heater

The potential for license renewal will also drive plants to consider when to replace feedwater heaters, depending upon refurbishment budgets and current heater reliability. The expected lifetime of stainless steel tubes is approximately 20 years (see Section 6.4 of this sourcebook), and engineers will need to plan accordingly as they perform LCM planning.

1-2

2

LCM SOURCEBOOK INTRODUCTION

2.1

Purpose of LCM Sourcebook

As indicated in the Life Cycle Management (LCM) Sourcebook Overview Report [Ref. 1], an LCM sourcebook is a compilation of generic information, data, and guidance an engineer typically needs to produce a plant-specific LCM plan for a system, structure, or component. The sourcebook will enable plant engineers or outside experts to develop an LCM plan with substantially less effort than if starting from scratch. The engineer need only compile and then add plant-specific data and information to complete an economic evaluation and LCM plan for the feedwater heaters (the subject of this report). It must be recognized that not all generic information in a sourcebook applies to every plant. Designs, layouts, and operational conditions are different from plant to plant. Some of the data can assist in the establishment of a benchmark when performing plant-specific LCM planning. The data may also show indicators or precursors to problems not yet experienced at a given unit. Caution and guidance is therefore provided in the plant-specific guidance sections of this report (Sections 5, 7, and 8) for the application and use of the generic feedwater heater information. These sections also contain useful tips and lessons learned from the EPRI LCM Plant Implementation Demonstration Program [Ref. 2] and various plant-specific LCM plans.

2.2

Relationship of Sourcebook to LCM Process

The process steps for LCM planning are detailed in the EPRI LCM Sourcebook Project Overview Report [Ref. 1]. The LCM Planning flowcharts in Figures 2-1 through 2-3 of this feedwater heater sourcebook are the same as Figure 1-1 of the LCM Sourcebook Overview Report. The chart is segmented into the four elements of the LCM planning process: 1) SSC categorization/selection, 2) technical evaluation, 3) economic evaluation, and 4) implementation. Process step numbering has been maintained consistent with the LCM Sourcebook Overview Report.

2.3

Basis for Selection of Feedwater Heaters for an LCM Sourcebook

The feedwater heaters were selected for the preparation of an LCM sourcebook by EPRImember utility advisors for the following reasons: •

Feedwater heaters are present in both BWR and PWR plants

•

They are important to power production and plant efficiency

2-1

LCM Sourcebook Introduction

•

They are subject to significant operating stress and degradation

•

They have a history of chronic maintenance problems

•

Their technical data are available in a number of previous EPRI technical reports

•

Improvements in feedwater heater operation directly affect the plant thermal cycle.

Section 3.4 contains further information on the nature of components and their scope with respect to reliability and LCM planning.

2-2


Figure 2-1 (a) LCM Planning Flowchart – SSC Categorization and Selection

2-3


Figure 2-1 (b) LCM Planning Flowchart – Technical and Economic Evaluation

2-4


Figure 2-1 (c) LCM Planning Flowchart – Implementation

2-5

3

BASIC INFORMATION ON FEEDWATER HEATERS

3.1

Safety and Operational Significance

Though feedwater heaters are non-safety related, they are important to power generation because they serve to help optimize plant thermal efficiency. Loss of feedwater heating may impose severe stress on the plant in terms of increased heat flux in the fuel and increased stress (as well as associated fatigue) on equipment such as feedwater nozzles, headers, and piping. Loss of feedwater heating will impact plant thermal performance. Most repairs can be performed online, but do require power reductions and heater isolations. If a Low-Pressure (LP) feedwater heater must be removed from service, an entire string must be isolated. The remaining string(s) will then be required to supply the entire feedwater flow requirements. Depending upon the plant design, the unit may need to be derated to approximately 80% of the rated power. If a High-Pressure (HP) feedwater heater string is removed from service, the unit may have to be derated to approximately 50% of rated power to make the necessary repairs. In addition, the repairs are often conducted in high temperature environments, under cramped conditions and with high humidity, making working conditions difficult. In terms of power generation and maintaining capacity factor, feedwater heater operation is very important to nuclear plants, both financially and with respect to thermal efficiency. Changes in power plant output cause changes in feedwater heating. The volume of feedwater flow (and the velocity) directly affects the amount of heat transfer taking place in the feedwater heaters. The primary references for this section are EPRI 1003470 (Feedwater Heater Maintenance Guide) [Ref. 11], EPRI NP-4057, Volume 3 (Nuclear Plant Feedwater Heater Handbook) [Ref. 23], and the Heat Exchanger Institute (HEI) Standards for Closed Feedwater Heaters [Ref. 30]. There are a variety of materials used in feedwater heater components. EPRI 1003470 [Ref. 11] provides a detailed description of these materials and their technical properties, particularly in Section 8. Typical materials are discussed later in this section. Unless specifically noted, the information in this sourcebook applies to both PWRs and BWRs equally.

3.2

Feedwater Heater Function and Configurations

This Section addresses Step 7 of Figure 2-1a, and provides a brief description of feedwater heaters and basic information on the feedwater heating systems in typical BWR and PWR nuclear power plants. An outline of basic feedwater heater arrangement in a typical plant is shown in Figure 3-1.

3-1

Basic Information on Feedwater Heaters

Figure 3-1 Basic Outline of Plant Feedwater Heater Arrangement (items 1 through 6 are the feedwater heaters)

The purpose of the feedwater heating system is to increase plant thermal efficiency by preheating the condensate/feedwater prior to its entering the reactor vessel (BWR) or steam generator (PWR). The system is comprised of various components and controls, which provide the following functions: •

•

•

Maintain proper water levels in the feedwater heaters and drains to provide maximum efficiency of the system. Supply heating steam to the feedwater heaters. Provide a flow path for the return of the condensed extraction steam drains to the feedwater and condensate systems.

•

Allow collection from the feedwater heater drains.

•

Prevent the entrapment of non-condensable gases.

Nuclear units require very large feedwater flows. There are generally five or six low-pressure and one high-pressure heater in each string of feedwater heaters. There may be two or three complete or partial strings of feedwater heaters. For example, there may be three low-pressure strings and two high-pressure heater strings for each unit. The numbering of each feedwater heater varies according to specific plant nomenclature. Some plants use the turbine extraction th stage (the steam supply for the heater), such as the 14 stage, etc. Some plants number the

3-2


heaters in numeric order for each string. For the purposes of this sourcebook, nuclear units are assumed to have between 12 and 21 FW heaters, depending upon design and vintage. A basic cutaway outline of a typical feedwater heater is shown below.

Figure 3-2 Cutaway View of a Typical Feedwater Heater (double-zone)

The feedwater heaters themselves are simply shell-and-tube heat exchangers, whereby the extraction steam (shell side) is used to heat the feedwater (tube side). The system consists of low pressure (LP) and high pressure (HP) feedwater heaters. The LP heaters are located on the suction side of the feedwater pumps and are used to heat condensate from the condenser hotwell before the water enters the feedwater pumps. The HP heaters are located on the discharge side of the feedwater pumps and are used to further heat the feedwater before it enters the reactor vessel (BWR) or the steam generators (PWR). The extraction steam system removes steam from various points on the HP and LP turbines for use in preheating the condensate and feedwater to improve overall thermal cycle efficiency. The condensate drained from the HP heaters is used along with the extraction steam from the LP turbine to heat the feedwater in the LP heaters. Some LP feedwater heaters are actually located in the condenser neck. Most BWRs are arranged to cascade all heater drains back to the condenser, allowing the condensate to be demineralized prior to being returned to the reactor vessel (as feedwater). Conversely, PWRs typically pump part of the higher-stage heater drains flow forward to the next higher stage heater (see Fig. 3-1). Nuclear plant high pressure feedwater heaters are generally of the two-zone design (condensing and drains subcooling zones). Nuclear plant low pressure feedwater heaters are generally either two-zone or single-zone (condensing only). Most feedwater heaters are oriented horizontally, but some plants use vertically-oriented heaters, especially in plants with limited floor space. BWRs are more likely to have vertically-oriented feedwater heaters. The main portions of a typical feedwater heater are as follows: the channel, the condensing zone, and the drains subcooling zone. The channel portion provides for the feedwater inlet and outlet nozzles. There are two types of channel design configurations: full access and manway 3-3


access designs. The elliptical head and hemispherical head are manway access designs, and the bolted removable cover and removable cover with a full opening are full access designs. Channels are designed to minimize the effects of erosion on the tubesheet and to provide convenient access for tubesheet plugging and other related maintenance. Channels also provide a ‘surge chamber’ that allows the feedwater flow to achieve a more uniform flow distribution before entering the tubes. These portions of a feedwater heater are shown (as outlines) in Figures 3-3 and 3-4, below.

Figure 3-3 Single Zone Feedwater Heater

Figure 3-4 Double Zone Feedwater Heater

3-4


The condensing zone is the largest zone in the feedwater heater. Steam is condensed as it traverses through the condensing zone and passes over the tubes carrying the feedwater. Drains from higher-pressure feedwater heaters flow into the condensing zone from the drains inlet nozzle (the drains cascade to the upstream heater, which is a lower pressure heater). An impingement plate is installed just inside this nozzle to protect the tubes from these flashing drains. The vent system typically consists of one or more perforated vent pipes installed along the length of the tube bundle. Many other designs are also used to accomplish this function. Non-condensible gases collect in these pipes and then pass through shell vent connections to the dearator of the main condenser, and are vented. BWR unit feedwater heater vent capacity is usually higher due to the radioactive hydrogen and oxygen produced in the reactor. The most common method of subcooling nuclear power plant feedwater heater drains is to provide the feedwater heater with a full-pass partial-length drain subcooling zone (see Figure 3-5 for a view of the short and long drains cooler designs). This zone is designed to achieve crossflow of the drains on the outside of the tubes (which carry the feedwater). The cross-flow design maximizes the heat transfer rate because the film coefficient is reduced by the perpendicular flow. In the horizontal heater, the full-pass short-end design requires the use of a siphon between the nominal level in the heater in the condensing zone and the flooded level of the drain cooler zone. This siphon results in a pressure lower than the shell pressure all around the inside of the drain cooler zone shroud. In addition, it requires that a minimum pressure drop exist across the entrance to the drain cooler zone to avoid flashing. The velocities through the shell side of the drain cooler zone must be minimized to keep the pressure drop low in order to avoid tube vibration. In the long drain cooler design (Figure 3-5), the entire drain cooler shroud is flooded and there is no pressure differential. In addition, a siphon is not required to reduce the possibility of flashing. The long and short drain designs are shown in the following figures.

3-5


Long Drains

Short Drains

Figure 3-5 Long and Short Drains Cooler Outline

The pressure drop through the full-length drains cooler zone, for the same given fluid velocity, is greater than the pressure drop through the short-end cooler zone (due to the greater length). Feedwater heaters can be classified based on orientation: horizontal, vertical channel-up, or vertical channel-down (see Figures 3-6 and 3-7 for the vertical orientation). The majority of nuclear plant heaters are horizontal, while the majority of fossil heaters are vertical [Ref. 11]. A vertical feedwater heater has considerably less fluid capacitance (volume of liquid contained in a unit level change) than a horizontal feedwater heater. In some cases, for channel-down applications, the shell of the heat exchanger is enlarged to increase the capacitance (sometimes called a ‘belly band’). This design change has also been made to original equipment at some plants. Considerations for vertical FW heaters involve proper distribution of steam and condensate within the shell. A vertical channel-up feedwater heater that includes drains cooling is subject to difficulties with flashing during varying flow conditions (i.e., changes in plant load). In this design, the drains cooler shrouding encloses several rows of tubes, and water level is maintained at the U-bend end 3-6


of the shell. These drains must be lifted to the top of the unit, and the potential for flashing is high. A horizontal heater provides more drains storage capacity, is easier to maintain, and requires more floor space (and supports). A vertical channel-up heater uses less floor space, has problems in the drains cooler zone design, is difficult to maintain, and is unable to have water drained from the tubes. A vertical channel-down heater uses less floor space, has problems with the drains cooler zone design, and is the most difficult heater to maintain. Appendix B of the Heat Exchange Institute (HEI) Closed Feedwater Heater Standards [Ref. 30] provides a detailed description of vertical feedwater heater issues. The feedwater heaters are controlled by a series of level controllers and other instrumentation controlling the vents, drains, and pumps. Feedwater heater controls have been addressed in a separate EPRI LCM sourcebook, EPRI 1007425 [Ref. 6]. It is important to note that the level within a feedwater heater is not the same at all locations along the tube length. The level can vary, depending upon the position of the steam inlet (or inlets), the design of the internals, and the flow through the unit. The level can vary significantly from the tubesheet to the U-tube end of the heater, especially if the drain cooler velocities are high. Maintaining a suitable level at the drain cooling zone inlet on short-end designs (operating under a siphon) is essential, in order to prevent flashing and subsequent erosion/damage. It is especially important to maintain the level well above the entrance at all times. The short-end drains cooler is capable of satisfactory performance if properly designed and operated, but it does offer more of a challenge than the long drains cooling design (due to the smaller space and therefore more sensitive parameter control). Accurate knowledge of the liquid level is very important in the drains cooling zone of any applicable feedwater heater. The following figures show the vertical channel-down and the vertical channel-up configuration for feedwater heaters. Further discussion about the unique difficulties associated with vertical feedwater heaters is also found in EPRI 1003470, Appendix B (which addresses heaters in fossil plants).

3-7


Figure 3-6 Vertical Channel-Down Feedwater Heater (showing condensing and sub-cooling zones)

3-8


Figure 3-7 Vertical Channel-Up Feedwater Heater

3-9


The feedwater heaters contain a number of sub-components (which are addressed in this report), including tubes, tubesheets, baffles, shield plates (or impingement plates), tube tie rods and spacers, tube supports, the drain nozzles, and the steam inlet and outlet nozzles. The heat exchanger shell is an ASME pressure vessel and houses all the internals. The channel head is removable so that access to the tubes and tubesheet can be provided. The tubes are typically Utubes which allow a double-pass for the feedwater through the shell. The components (and subcomponents) detailed in this report are as follows: •

Shell

•

Channel and Manways

•

Pass Partitions, Plate and Cover

•

Tubesheet

•

Tubes

•

Tube Supports and Baffles

•

Shell Relief Valves

•

Vents

•

Nozzles

The shell is a pressure vessel that provides support for the feedwater heater internal components and allows for the distribution of the extraction steam with a negligible pressure loss. The shell is a level control reservoir for the heater drains system. The shell also acts as a cooler for the condensed steam. The channel serves as the inlet plenum for the feedwater/condensate into the heat exchanger. The channel acts to redirect the flow from one tube pass to the next. The shape of the channel is dictated by the feedwater pressure and by tube end accessibility considerations. The pass partition divides the channel into feedwater inlet and outlet passes. There are three types of pass partitions: welded, bolted, and floating. The pass partition cover can be welded or bolted, depending upon maintenance access concerns. During plant operation, one half of the channel is hotter than the other half. The radial growth of the hotter side is greater than the colder side due to thermal expansion. This effect results in bending stresses at the pass partition weld junction. Over time, cyclical pressure and thermal stress may lead to cracking around the perimeter of the pass partition. A floating pass partition is welded only to the tubesheet face and to the feedwater outlet nozzle. This is the most flexible design, and is used to reduce stresses in the channel and the channel-to-tubesheet connection. Figures 3-8 and 3-9 show bolted and welded pass partitions.

3-10


Figure 3-8 Bolted Pass Partition

3-11


Figure 3-9 Welded Pass Partition

The tubesheet is a plate with holes drilled in it for the tubes. Some tubesheet holes have tube hole grooves that increase the holding force of the expanded tubes. Low-pressure feedwater heaters tubesheets are generally composed of carbon steel plate, while high-pressure heater tubesheets are generally composed of forged carbon steel. The tubesheets of nuclear highpressure heaters are typically 10-15 inches thick. Low-pressure tubesheets are typically 6-10 inches thick. Nuclear plant high-pressure heater tubes are always seal welded and roller expanded to the tubesheet. Low-pressure heater tubes for PWRs are typically roller expanded to a sufficient depth to meet ASME requirements, while the low-pressure tubes for BWRs are generally welded due to access limits associated with the anticipated high radioactivity levels. The tube design pressure is required by ASME and other governing codes to be equal to or greater than the maximum normal pressure that will be experienced during operation. The design minimum tube wall thickness for U-tube stainless steel is 0.035 inches and 0.028 inches for straight stainless steel tubes. The minimum radius of the U-bends should be 1.5 times the tube diameter.

3-12


Baffles and tube support plates are pinned supports for the tubes (in the shell). When properly designed, the support plates can prevent damage to the tubes from excessive vibration. The baffles direct the flow through the bundle. Baffles are used in the drains cooler zone to increase the rate of single-phase heat transfer. The shell relief valve is designed to protect the shell when the tube design pressure is greater than the shell design pressure. The relief valve protects the shell in the case of tube or tubesheet failure. BWRs must have a design to prevent a general discharge to the plant. The relief valve should be sized to take the flow of one clean heater tube rupture (two open ends discharging feedwater). The vents on a feedwater heater remove non-condensible gases from the shell and tube side of the heater during start-up and normal operation. There are inlet and outlet nozzles on feedwater heaters for the connections to the feedwater piping, the heater drains piping, the extraction steam piping, vent lines, and relief valves. There are also instrument taps (as necessary). In addition, there are other appurtenances to the shell, such as manways, flanges, hand-holds, lifting lugs, etc., but except for leakage events, these are not addressed in this report. These individual items are discussed in detail in EPRI 1003470 ( Feedwater Heater Maintenance Guide) [Ref. 11] and in EPRI NP-4057, Volumes 1-3 ( Nuclear Plant Feedwater Heater Handbook ) [23]. Much of the information and most of the figures in this LCM Planning Sourcebook were taken from these two references. The normal flow path for the feedwater in a nuclear power plant is typically described in the following steps: •

•

•

•

Condensate flows from the condensate demineralizers through the tubes of the fifth and fourth point drain coolers. The condensate flows through the tubes of the sixth through the second point heaters (in that order). The feedwater flows through the (reactor) feedwater pumps The feedwater flows through the first point heater through to the reactor (BWR) or steam generators (PWR).

Both the first- and second-point feedwater heaters (high-pressure heaters) typically have inlet and outlet motor-operated isolation valves. Each of these two feedwater heaters has a bypass line with a motor-operated isolation valve. The feedwater is successively heated by extraction steam, starting with low pressure saturated steam from the LP turbine to the sixth-point feedwater heater. Higher pressure saturated extraction steam is used for subsequent feedwater heaters. The first-point feedwater heater receives high pressure saturated extraction steam from one of the stages of the HP turbine. Different plants use different numbering systems when classifying their feedwater heaters, such

3-13


that heater #1 is the first heater after the condensate pumps and heater #6 (for example) is the closest to the steam generators (or the reactor vessel). As outlined in Figure 3-1, extraction steam from the HP and LP turbines is sent to the shell side of the feedwater heaters. As heat energy from the extraction steam is transferred to the feedwater (which flows in the tubes), the steam is condensed and is collected in the drain receiver (at the bottom of the feedwater heater). The energy given up to the feedwater causes the steam to condense and collect. The drain path depends upon the location of the heater or drain receiver in the heater string. The typical drain paths are shown in Figure 3-1. As previously mentioned, for BWRs, the drain path for all the FW heaters is typically directly back to the condenser, while for PWRs, the drain path is often forward to next higher-stage heater. Drain flow paths, water levels, FW heater controls, and interlocks are discussed in further detail in EPRI 1007425 ( LCM Planning Sourcebook, Vol.6: Feedwater Heater Controls) [Ref. 6].

3.3

Feedwater Heater System and Component Boundaries

Considering the flowpath to be condensate to feedwater, the feedwater heaters are bounded on the upstream side by the condensate system and also the extraction steam (sub-)system. The downstream boundary is the feedwater system itself - prior to the FW isolation valves to the reactor [BWR] or the steam generators [PWR]. The heater drains system also forms part of the downstream boundary (for the drain flow back into the condenser). The low-pressure heaters are typically part of the condensate system and the high-pressure heaters are typically part of the feedwater system. Differences from one plant to the next may exist with respect to the actual piping boundaries and system titles. Differences also exist between BWR and PWR plants, although with respect to the feedwater heaters themselves, the shell and tube design is the same. Figure 3-1 shows the typical outline for the feedwater heater arrangement.

3.4

Scope of Equipment Covered by the Feedwater Heater Sourcebook

Per the guidance contained in INPO document AP-913, “Equipment Reliability Process” [Ref.32], if a failure of a component or its structural supports defeats or degrades an important function or a function that is redundant to an important function, then it is considered a critical component. As discussed in Section 3.1, feedwater heaters are not safety-related, so the critical nature of the feedwater heater components is related to their importance for plant productivity. The scope of this report includes only the passive mechanical components and sub-components of nuclear plant feedwater heaters. The scope covers the most common mechanical components contained in feedwater heaters found in both BWRs and PWRs. These include: •

Shell

•

Tubes

•

Tubesheet

•

Tube-to-Tubesheet Joints

3-14


•

Tube Supports

•

Baffles

•

Impingement Plates

•

Nozzles

•

Vents

Active components such as instrumentation, controls, and valves, are excluded. Feedwater heater controls are addressed separately in EPRI report 1007425 [Ref. 6]. Non-metallic materials, such as insulation and gaskets, are not addressed by this sourcebook. In addition, FW heater supports are considered to be structural components, and are not addressed by this sourcebook. The tubes are the most vulnerable component with respect to damage and are of the highest concern with respect to surveillance and maintenance, so they are considered the most critical components. As discussed in Section 4, erosion and wall thinning in FW heater shells has also been observed, and one event (in 1999) involved the catastrophic rupture of a FW heater shell due to erosion. Other heater damage has been noted due to cracked or broken impingement plates or baffles (or end supports). Most of the surveillance and maintenance focus is on the tubes, but the other components will be addressed as well (see Section 5 for a review of condition monitoring and repair techniques).

3-15

4 HISTORICAL PERFORMANCE DATA FROM INDUSTRY OPERATING EXPERIENCE

This section addresses Step 9 in the LCM planning flowchart (Figure 2-1b). The information compiled in this section is to be used for a comparison or benchmark of plant-specific conditions and operating experience. The qualitative data is intended as a checklist of potential conditions affecting plant-specific performance, while the quantitative failure data may provide insight into the potential for plant-specific enhancements and may help identify where improvements can be made. For example, if the plant-specific component failures are much lower (e.g., by a factor of 3) than the generic data indicates, then one might conclude that the existing maintenance plan is effective and that further improvements will be difficult to achieve. On the other hand, the discrepancy between realized and industry typical failure rates might suggest that the current maintenance on the equipment could be relaxed if high reliability is viewed as the result of excessive maintenance practices that are not cost-effective. Similarly, if the plant-specific component failure rates are substantially higher than the generic industry failure rates presented in this section, or if the contribution of feedwater heaters to lost power production significantly exceeds the generic values (for BWRs and PWRs), then equipment replacement or major changes to current maintenance practices might be needed. Also, if the reliability of an SSC falls below a certain level, replacement or other major maintenance efforts will be required to satisfy Maintenance Rule performance criteria. It should be noted that this section addresses failures and failure data rather than repair practices and data. Some repair (and maintenance) techniques will be mentioned, but, in general, repair times will be available from specific plant records and will depend upon plant-specific maintenance practices. Monitoring techniques are addressed in Section 5. The Mean-Time-ToRepair (MTTR) will have an impact on system availability (with respect to the Maintenance Rule). A further discussion of specific maintenance (preventive or repair) and troubleshooting is found in Section 5. Computing (or estimating) failure rates for components is not an exact science. Hundreds, sometimes thousands, of event reports must be reviewed to determine which items are to be directly characterized as failures for the components under review. In addition, for a subject like feedwater heaters, the term failure will not mean the same thing among different utilities. Unlike electronic components (instrumentation), which are replaced when they fail (and are relatively easy to benchmark), many instances of feedwater heater failure are repaired without direct component replacement. A catastrophic failure of a shell is obviously a failure. However, the plugging of a single tube (possibly including adjacent tubes as insurance) may not be considered a failure, but just good preventive maintenance.

4-1

Historical Performance Data from Industry Operating Experience

Because these events are characterized differently by different plants, any failure rate which is estimated from INPO failure data (i.e., EPIX) is going to be lower than a true failure rate experienced in the field. Likewise, some FW heaters may be inspected and/or repaired when the plant is shut down for other reasons, and items identified during such work may not be classified as failures, even though corrective maintenance is needed. Also, there are significant differences between EPIX and NPRDS reports – these differences are discussed in Sections 4.1.2 and 4.2.3. There is a large amount of data from various sources related to industry experience presented in Section 4 of this sourcebook; this data is tabulated and evaluated in the appropriate sub-sections, and is summarized in Section 4.5, where the generic industry failure rate for FW heaters is addressed. Section 4.5 presents the failure rate values necessary for unit-specific LCM planning.

4.1

Nuclear Industry Experience

This section contains information on failure data and the associated failure mechanisms for nuclear power plant feedwater heaters. The information is presented in two forms: qualitative and quantitative. The qualitative information describes the degradation mechanisms and effects that apply to nuclear plant feedwater heaters, while the quantitative information identifies industry failures and failure mechanisms (by listing specific events). The quantitative information may be used to determine a generic industry failure rate, although subject to uncertainty. The listing of the failures may be used to provide an understanding of the industry experience as system and component engineers at specific plants evaluate LCM planning for feedwater heaters at their units. The listing of events is detailed for each source where the information was found, and is presented to highlight the applicable feedwater heater subcomponent involved (shell, tubes, baffles, shield plates, etc.). EPRI 1007425 ( Life Cycle Management Planning Sourcebooks, Volume 6: Feedwater Heater Controls) [Ref. 6] presents this information for feedwater heater controls, so any problems with instrumentation and control schemes will be listed there. A comprehensive review of the available industry operating experience has been done to extract the salient information and to present the data that the plant system or component engineer may utilize in assessing the plant-specific performance of a feedwater heater. A variety of industry databases were reviewed to identify problems that have been reported with feedwater heaters at nuclear power plants. The industry sources used in this review include EPRI (previous technical reports), INPO (EPIX/NPRDS), and NRC. The primary EPRI technical report referenced in this Section is EPRI 1003470 ( Feedwater Heater Maintenance Guide ) [Ref. 11]. To begin the operating experience review, the proper place is to consider system failure contributions to plant events. This information is available in NUREG/CR-5750 [Ref. 41], with the understanding that this reference uses the words ‘fault’, ‘failure’, ‘transient’, and ‘initiating event’ in generally the same sense (in conflict with the typical safety analysis usage of these terms). To provide an industry benchmark for generic historical SSC performance used in nuclear plant PRA analysis, the NRC conducted a review of industry-wide plant initiating events for the period 1987-1995 (from NUREG/CR-5750) [Ref. 41]. The relevant data for the initial plant faults from this report are tabulated separately for BWRs and PWRs in Table 4-1. Assuming that each fault contributes equally to plant shutdowns (i.e., all plant trips are considered equivalent fault or failure events with respect to consideration as data points), an estimate of the percentage of total plant failures contributed by a specific SSC may be estimated. The table shows this ‘system contribution factor’, on such a percentage basis. 4-2


This average generic SSC contribution can be used as an initial generic benchmark for the SSCspecific sourcebook (as a first step in the determination of an industry failure rate). These factors, after comparison with plant-specific SSC failure rates, can be used in economic analysis calculations to determine the impact on lost power generation attributed to the SSC under review. For SSCs not represented in Table 4-1, it may be assumed that the generic industry failure rate is negligible with respect to lost power generation. For feedwater heaters, the data must be evaluated carefully because they are not specifically mentioned as a separate component, but (as the table shows) feedwater problems are one of the leading items causing plant faults. The following table presents data from NUREG/CR-5750 [Ref. 41], which compiled information on the rates of initiating events for US nuclear units from 1987-1995. The data is based primarily on operating experience derived from Licensee Event Reports. While the table does not explicitly address feedwater heaters, it is clear that feedwater problems represent a significant contribution factor (approximately 19% in BWRs and approximately 27% in PWRs) towards plant faults (transients). If the loss of condensate flow is also considered, the percentages rise almost another 3%. Table 4-1 Frequency of Initiating Events for Plant Faults SYSTEM-CATEGORY ALL EVENTS

Initial Plant Faults IPF

Contribution % Factor

BWR 658

PWR 1327

BWR 100

PWR 100

LOSS OF OFFSITE POWER

4

13

0.607

0.980

LOSS OF VITAL BUS

7

3

1.060

0.226

LOSS OF IAS

13

13

1.976

0.980

FIRE

10

21

1.520

1.583

INADVERTANT CLOSURE OF MSIVs

16

5

2.432

0.377

LOSS OF CONDENSER VAC.

27

13

4.103

0.980

TOTAL LOSS OF FW FLOW

24

62

3.647

4.672

LOSS OF NON-SAFETY BUS

5

20

0.760

1.507

LOSS OF AC I&C BUS

12

19

1.824

1.432

LOSS OF NON-SAFETY COOLING WATER

16

34

2.432

2.562

PARTIAL MSIV CLOSURE

11

36

1.672

2.713

PARTIAL LOSS OF FEED WATER FLOW

45

240

6.839

18.086

PARTIAL/TOTAL LOSS OF CONDENSATE FLOW

18

36

2.736

2.713

EXCESSIVE FEEDWATER

49

61

7.447

4.597

VALID RPS TRIPS

64

132

9.726

9.947

6

88

0.912

6.631

TURBINE TRIPS

173

284

26.292

21.402

MANUAL REACTOR TRIPS

55

48

8.359

3.617

SPURIOUS SSAs

14

22

2.128

1.658

OTHER TRIPS

89

177

13.523

13.338

REACTIVITY IMBALANCE

4-3


Note that the word ‘fault’ in this table does not automatically refer to an accident event, but rather to an initial plant transient (see Table 2-1 of NUREG/CR-5750 for an outline of all ‘faults’ evaluated in the report). Some of them are accident events and some are just general transients. NUREG/CR-5750 evaluates both. As noted in the discussion on LERs, probably only a small number of these events (or faults) are directly related to feedwater heaters. Other issues (valve problems, drain problems, feedwater pump and feedwater recirculation problems) predominate, but some of these events are related to feedwater heaters. When considering feedwater heater failure rates and the impact to overall plant operation, it should be remembered that the greatest negative impact that feedwater heater performance can have on the plant is the cost of replacement power (for a sudden component failure). Feedwater heater repairs involve detailed and often difficult maintenance evolutions, and replacement of a feedwater heater tube bundle or of the heater itself is considered only as a last resort, once other options have been exhausted (tube plugging, sleeving, etc.). With respect to this data for LCM planning, one step would be to take the overall FW system initiating event rate (19% for BWRs and 27% for PWRs), review the plant-specific data to remove contributions from FW heater controls problems, and then multiply the remaining value by the number of trips, transients, and power reductions in a given period to determine a rate specific to feedwater heaters. In order to remove the I&C portion of the FW event data, one could use the data in Table 4-2, where (with the exception of the NPRDS information), only about 10% of the failures linked to FW heaters are specific to the heaters themselves. In summary, the NUREG/CR-5750 data confirms that FW systems and components contribute significantly to industry productivity losses and warrant the preparation of formal LCM plans. However, it would be difficult to determine a specific failure rate for feedwater heaters from the data, and any estimates would have a large uncertainty range. Other information has been reviewed to develop an estimated industry generic failure rate for feedwater heaters, as discussed later in this section.

4.1.1 Qualitative Data Qualitative data indicate several different failure modes from different aging/degradation mechanisms for feedwater heaters. The qualitative data typically list and describe aging mechanisms and/or effects, and may also describe various types of degradation. In the terminology of aging management, life cycle management, and license renewal (see, for example, SAND96-0344, NUREG-1800, or NUREG 1801), an aging mechanism results from exposure to a stressor, usually an environmental or cyclical condition, and the mechanism causes an aging effect in the SSC, which leads to degradation and ultimately to failure, in a specific mode. In SAND93-7070 [Ref. 7], The Aging Management Guideline for Commercial Nuclear Power Plants – Heat Exchangers, the stressors applicable to heat exchangers in nuclear power plants are summarized as: •

Mechanical

•

Hydraulic

4-4


•

Chemical

•

Electrical

•

Environmental (Thermal, Moisture, Radiation)

Mechanical stresses primarily involve fabrication issues and piping connection loads. Vibration and thermal expansion are also considered mechanical issues. Hydraulic stressors are loads imposed on heat exchanger components as a result of the pressure and flow of fluid through the heat exchanger. The magnitude of the hydraulic stress is dependent upon the characteristics of the process fluid, the design parameters of the heat exchanger, and the severity of operation or duty that the heat exchanger is expected to encounter over its service life. The fluid pressures exert stress on the heat exchanger shell, nozzles, tubes, tubesheets, channel heads, and partitions (including impingement plates). Erosion is considered a hydraulic issue. Process fluids react with the heat exchanger materials (internal components) and, depending upon the specific mechanical composition, can cause the components to become stressed. This stress can be localized and/or uniformly distributed throughout the heat exchanger internal surfaces. The water chemistry issues related to feedwater heaters are discussed in further detail in Section 5 of this report. The only electrical stressor for heat exchangers is galvanic corrosion resulting from a coupling of materials remote from one another on the galvanic series chart (anode to cathode). Galvanic corrosion is addressed in EPRI 1003470 [Ref. 11]. For the purposes of this report, it is not considered to be a major degradation mechanism, because dissimilar metal couples are only occasionally used in power plant functions (EPRI CS-3184) [Ref. 25]. Industry experience with galvanic corrosion in feedwater heaters is limited. The environmental stressors are detailed as follows: thermal stressors involve internal process fluid temperature effects on heat exchanger components, moisture stressors involve external humidity effects on the applicable heat exchanger components, and radiation stressors involve the cumulative radiation effects on the applicable heat exchanger components. These stressors are not considered as significant (particularly in comparison with the degradation mechanisms and effects resulting from the mechanical, hydraulic, and chemical stressors). The aging mechanisms and effects (i.e., the categories of problems) resulting from the important stressors are summarized as follows (from Tables 1-5 and 2-2 of the Heat Exchanger AMG): •

Fatigue

•

Corrosion

•

Stress Corrosion Cracking

•

Erosion

•

Embrittlement

•

Wear

•

Stress Relaxation

4-5


•

Creep

•

Fouling

In the text of the report (SAND93-7070, also the Heat Exchanger AMG), some of these aging mechanisms and their sub-types are dismissed as non-significant (thermal embrittlement, creep, thermal fatigue, irradiation-assisted stress corrosion, and neutron embrittlement). The remaining categories are included for evaluation in the AMG. It should be noted that the scope of the Heat Exchanger AMG does not include feedwater heaters, and steam is not addressed as a shell-side fluid. But the principal discussions related to shell-and-tube heat exchangers are the same for feedwater heaters, and the evaluations in the Heat Exchanger AMG provide an introduction into other industry technical reports on feedwater heater maintenance, operation, and problem/failure review. Note that fouling, while an important subject in the Heat Exchanger AMG and in other general heat exchanger technical reports (such as EPRI NP-7552 [Ref. 16]), is not a serious concern for feedwater heaters because the feedwater is boiler-quality water with strict chemistry controls (as opposed to raw or filtered water, which is used in many critical heat exchangers in a nuclear plant). Fouling is not a significant degradation mechanism for feedwater heaters. The Heat Exchanger AMG details the sub-components in the following groupings (from Table 32 of the report): •

Tubes / Coils

•

Tube Sheets

•

Shell / Nozzles / Internals

•

Waterbox / Channel Head / Divider Plates

•

Pressure Boundary Fasteners / Support Assembly

The Heat Exchanger AMG details typical heat exchanger materials as follows (Table 3-3): •

Tubes / Coils

Austenitic Stainless Steel, Admiralty Brass, Copper-Nickel, Titanium, Copper

•

Tube Sheets

Carbon Steel, Austenitic Stainless Steel, Muntz (copper alloy)

•

Shell / Nozzles / Internals

Carbon Steel, Austenitic Stainless Steel

•

Baffle Plates


•

Divider Plates


•

Fasteners

Forged Stainless Steel, Forged Carbon Steel

•

Waterbox / Channel Head


This breakdown compares well with previous EPRI analysis of feedwater heaters . For example, EPRI CS-1776 ( Failure Cause Analysis – Feedwater Heaters ) [Ref. 18] details feedwater heater problems in the following categories (from Table 2-1):

4-6


•

Steam Impingement

•

Tube Inlet Erosion

•

Drains Subcooler Flashing / Level Control Problems

•

Tube Vibration

•

Tube Plugging Problems

•

Corrosion

•

Unknown

•

Internal Failure

•

Head / Shell Leak

•

Tube-Tubesheet Joint Failure

•

Foreign Material Impingement

•

Tube Defects

The focus of this EPRI report was on fossil units, but the issues are the same for nuclear units. The primary difference between feedwater heaters in fossil units and those in nuclear units is that the fossil units typically have superheated steam and the nuclear units do not. Therefore, the fossil units must have a de-superheating zone, which is not addressed in this sourcebook. In EPRI NP-4057, Vol. 3, ( Nuclear Plant Feedwater Heater Handbook ) [Ref. 23], the problems with feedwater heaters are summarized as follows (from Section 3): Problems in the Condensing Zone •

Inadequate Venting Leading to Corrosion Problems and Thermal Deficiencies

•

Tube Damage Resulting from Inadequate Impingement Plates

•

Erosion Resulting from Inadequate Impingement Plates

•

Lack of Flash Chambers Causing Shell Erosion and Tube Damage

•

High Velocity Wet Steam Causing Erosion of Shells and Support Plates

Problems in the Drain Cooler Zone •

Erosion of Tubes and/or Baffle Plates Resulting from Inadequate Water Level and/or Inadequate Anti-Vortexing Design

•

Erosion Resulting from Excessive Pressure Drop and Flashing

•

High Drain Cooler Velocity Causing Vibration Damage, Baffle Plate Erosion

•

•

High Drain Cooler Velocity Resulting from Excessive Duty (i.e., a lower pressure FW heater being isolated) Inadequate Level Control Resulting in Erosion due to Flashing or Excessive Flow 4-7


•

Erosion and Tube Damage Resulting from Inadequate Plate Thickness

•

Erosion and Steam Flow from Tube/Plate Leakage

With respect to feedwater heater materials, EPRI CS-3184 ( Corrosion-Related Failures in Feedwater Heaters) [Ref. 25] contains an evaluation of feedwater heater tube corrosion problems written from a materials standpoint. The following materials are addressed in separate sections of that report: •

Carbon Steel

•

Admiralty Brass

•

Copper-Nickel Alloys

•

Monel Alloy 400

•

Type 304 Stainless Steel

•

Titanium

In EPRI 1003470 ( Feedwater Heater Maintenance Guide) [Ref. 11], the feedwater heater problems are categorized as follows: •

Corrosion (general, galvanic, crevice, pitting, intergranular, stress, and flow-accelerated)

•

Erosion (tube inlet, tubesheet, impingement)

•

Vibration

Vibration is further broken down to address two flow-induced excitation mechanisms which affect feedwater heater tubes: vortex shedding and fluidelastic whirling. The common tube damage from vibration is labeled as follows: fretting, collision, and cracking. In summary, it is clear that the environments and operating conditions in which feedwater heaters operate can cause significant problems such as fatigue, erosion, corrosion, cracking, and vibration. If not properly monitored, inspected, and maintained, the feedwater heaters will incur damage over time, causing a negative impact to plant efficiency. The following section of this report outlines specific numerical industry data on feedwater heater problems and failures.

4.1.2 Quantitative Data (EPRI, INPO EPIX/NPRDS, NRC) Quantitative failure data is available for feedwater heaters, their structures, and their components from a number of sources such as EPRI, INPO (EPIX/NPRDS, Plant Events database, O&MR, SEE-IN, and LER), and NRC (LER and Generic Communications). It should be recognized that such quantitative data has often been gathered for risk assessment or other safety-related evaluations. As such, the data tends to reflect functional failures rather than degradation failures (or discoveries), even though the degradation may reflect chronic problems and should be the primary focus of maintenance activities. It should also be noted that not all component failures at all plants are going to be listed in generic industry databases (i.e., this search cannot be considered exhaustive). The NPRDS database covers events/items from approximately late 1976 to late 1996 and the EPIX database (the successor to NPRDS) covers items from 1997 to the 4-8


present. The INPO Plant Events Database contains summaries of events since 1991. Significant events are further described in the INPO SEE-IN process (under SER, SEN, and SOER). The NRC ADAMS search covers a period from late 1998 to 2003, with more documents being added as the database is developed. As described in the following paragraphs, a refining process is needed to conduct a useful search of these industry databases. Extraneous items must be removed and actual SSC events must be addressed. Searches of the INPO databases (NPRDS, EPIX, and Plant Events) identified many items not directly associated with feedwater heater components, such as condenser problems, valve problems, secondary side I&C problems, secondary side piping problems, etc. For example, the INPO NPRDS search by system (checking the Feedwater, Condensate, and Extraction Steam systems) and component (checking ‘Heat Exchanger, Condenser, Steam Generator’) returned 924 failure records, while the NPRDS word search on “feedwater AND heater” returned 2076 records. Obviously, many of the word search results involved other equipment (with only a tangential mention of or connection to feedwater heaters). A review of the first 450 records (out of the 2076 items) showed that only 157 of these 450 records (~35%) were related to the component grouping of ‘Heat Exchanger, Condenser, Steam Generator’. The EPIX search results showed a similar pattern: the detailed system/component search listed 38 failure records, while the word search on “feedwater AND heater” listed 276 records. Most of these 276 failures did not involve feedwater heaters directly, but the feedwater heaters were mentioned in the abstract as being affected by or included in the event. One purpose of the quantitative review is to cull through all this information to present only the data specific to feedwater heaters. It should be noted that searches of failure data for all Life Cycle Management studies, no matter what system or component is being reviewed, must employ the same ‘refining’ process. Searches of INPO records for LERs (Licensee Event Reports) show the same pattern, as do searches of the NRC databases (such as an ADAMS search). With respect to the feedwater heaters, the search of the INPO LER database (by system - feedwater & condensate, and by component -heat exchanger) identified 5 LERs for a period covering 1984 to 2003, while the NRC ADAMS search (a word search keyed to “feedwater heater” and covering 1999 to 2003) identified 35 LERs. A search of the INPO SEE-IN records, looking specifically for LERs with the keywords “feedwater heater”, identified 281 records from 1984 to 2002. Not all of these items are actually LERs and not all of them directly relate to the feedwater heaters. So a more detailed evaluation of the records is necessary to accurately identify items directly related to feedwater heaters and their sub-components. In addition, EPRI report 1003470 ( Feedwater Heater Maintenance Guide) [Ref. 11] contains a summary of feedwater heater operating experience. The report states that there were 166 records found in the NPRDS database for failure items involving feedwater heater shells and internal components (from 1977 to 1996). The EPIX search described in the report lists 5 items (related to tube leaks) that were identified (for 1997 to 2001). Although EPRI 1003470 does not explicitly state it, this total of 171 failure-related items can be characterized as a generic industry failure rate of 0.0632 feedwater heater failures per plant per year, assuming that there are 104 operating reactors in the US and using a 26-year period (171 divided by 104 divided by 26). 4-9


This value is a general estimate, but it provides a starting point, using applicable industry information. Also, EPRI 1003470 [Ref. 11] identified 29 items pertaining directly to feedwater heaters in the INPO Plant Events database. The breakdown of these items is as follows: •

17 events involved inspection and discovery of shell thinning (near the nozzle inlet area)

•

9 events involved tube leakage

•

2 events dealt with manways and gaskets

•

1 event dealt with partition plate repairs

These events are described in further detail in Section 5 of EPRI 1003470. In the review conducted for the preparation of this LCM Sourcebook, the INPO Plant Events database identified 209 records which were keyed on “feedwater heater” (for a period from 1991 to 2003). The detailed review reduced the number directly involving the feedwater heater components to 47, most of which involved either tube leaks (17%) or shell thinning (38.3%). Other items identified included impingement plate problems, tube plugs which broke away or were not properly installed, and vent problems (corrosion). These 47 events represent a failure rate of 0.0377 failures per plant per year, computed by dividing the number of events by the number of operating reactors and dividing again by the number of years the database covers. This value of 0.0377 is approximately half the value found from the NPRDS/EPIX data of EPRI report 1003470, but it represents a search of a different INPO database. With respect to NRC Generic Communications, an ADAMS search identified 18 Information Notices tagged to feedwater heaters, 1 Information Bulletin tagged to feedwater heaters, and 3 Generic Letters tagged to feedwater heaters. In addition, 35 LERs were tagged to feedwater heaters (covering 1999-2003). The EPIX/NPRDS, Plant Events, SEE-IN, O&MR records, LERs, and NRC Generic Communications items are evaluated in detail in the following sub-sections of this LCM Sourcebook. These documents provide the industry operating experience for the LCM planning evaluation of the feedwater heaters. They also provide the basis for establishing a generic industry failure rate for feedwater heaters, to be discussed in Section 4.5. In Section 4.2.3 and Tables 4-6 and 4-7, specific data for the FW heater components (tubes, shell, etc.) are listed.

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Table 4-2 Summary of Industry Database Search Results for FW Heaters (Operating Experience)

Source (Database)

Records Found in Initial Search

Items Applicable to FW Heaters

ADAMS LER Search

35

0

INPO System Search for LERs

5

4

INPO SEE-IN LER Search (word search)

281

23

INPO Plant Events (word search)

209

47

INPO NPRDS (word search)

2076

N/A

INPO NPRDS System/Component Search

924

850

INPO EPIX (word search)

276

27

INPO EPIX System/Component Search

38

20

INPO SER (word search)

32

4

INPO SEN (word search)

7

2

INPO O&MR (word search)

18

1

NRC Generic Communications –IN

18

1

NRC Generic Communications –GL

3

0

NRC Generic Communications –IB

1

0

Note that the NPRDS items from the keyword search were not reviewed, due to the large amount of compiled material. The total number is listed here for completeness. Those items found from the system/component search were reviewed in detail. From the table above, it is clear that the vast majority of items identified through database searches do not impact the feedwater heaters and their mechanical components. Only the NPRDS review (for the system/component search) accurately focused in on the FW heaters themselves. It should be noted that the NPRDS system/component search was the most comprehensive and detailed search conducted, and it provided the most useful results. EPIX (the successor to NPRDS) is based more on Maintenance Rule (10CFR50.65) requirements than NPRDS, meaning that events (failures) in non-safety systems were not listed as often as in NPRDS. In addition, EPIX items are typically not identified unless there is a change in plant status (trip, power reduction, etc.). The NPRDS and EPIX items are addressed in Section 4.2.3. After a review of these items, those related to feedwater heater mechanical components are included in the industry generic failure rate estimation. In order to execute successful LCM planning on feedwater heaters, the items culled from the totals above need to be included as the operating experience history. Plant-specific operating experience augments this data to estimate failure rates used in LCM economic evaluations.

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4.1.3 Maintenance Rule Feedwater heaters are components that typically are assigned to (one of, or possibly all) three different plant systems: Condensate (LP feedwater heaters), Feedwater (HP feedwater heaters), or the Heaters & Drains system. Some plants may also include heater data in the Extraction Steam system. Under the EPRI SYSMON (system monitoring) program outline [Ref. 38], feedwater heaters are addressed in system 18: Extraction Steam / Feedwater Heaters & Drains. The EPRI SYSMON data is found in Reference 38. For Extraction Steam, Feedwater Heaters, and Drains, the SYSMON goal is no more than 3% unplanned capacity loss or no more than three (3) unplanned capacity loss events with greater than or equal to a 20% power reduction over a 24-month period, and also no common cause unplanned scrams and no more than two (2) scrams over a 24-month period. The reporting requirements include a Quarterly Report Card Update and a Maintenance Rule performance update. The overall system monitoring and trending requirements for the Extraction Steam / Feedwater Heaters and Drains system are shown in the System Monitoring Plan for System 18 (part of the SYSMON database). The latest version of SYSMON has been obtained from EPRI and the access screen for feedwater heaters is shown as follows:

Figure 4-1 SYSMON Access Screen – System 18 – Extraction Steam, Feedwater Heaters & Drains

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The SYSMON flowchart in Figure 4-2 describes the process.

Figure 4-2 SYSMON Outline Plan

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4.1.4 EPRI PM Basis Templates EPRI TR-106857-V32 ( Preventive Maintenance Basis, Volume 32 – Heat Exchangers – Tube Type) [Ref. 19] was prepared in 1998 to provide the overall PM Basis project with input for shell and tube heat exchangers. A large fraction of US nuclear plants are reducing PM costs and improving equipment performance by more closely matching PM tasks with the functional importance of their equipment. To succeed, utilities require information on the most appropriate tasks and task intervals for the major types of equipment in their plants, accounting for the influence of duty cycle and service conditions. Previously, this data did not exist in an accessible form, often resulting in arbitrary and unsuitable tasks and leading to intervals which actually increased maintenance costs and diminished reliability. The PM Basis Project was developed in response to requests by EPRI nuclear members to provide a technical basis beyond vendor information and to provide recommendations for assigning PM tasks to groups of plant components. The Preventive Maintenance Basis project provides utilities with the technical information necessary to build PM tasks and PM task intervals associated with over 40 specific component groupings (such as valves, electric motors, pumps, HVAC equipment, etc.) beyond the fundamental vendor information supplied with their products. In order to make the PM Basis information more user-friendly, EPRI combined all the various SSC PM templates into a single database – PM Basis Version 4.0, with the Vulnerability Analysis Module [Ref. 35]. The Preventive Maintenance Basis Database provides the nuclear plant maintenance community with an essential reference for PM task selection and evaluation for common major components. It provides the rationale for each task, the influence of equipment failure locations, the failure mechanisms, the timing of failures, and information on task content and task intervals. Under the EPRI PM Basis Table, the feedwater heaters are evaluated as a separate category of heat exchanger. They have their own boundary description, their own vulnerability evaluation, and their own PM Basis Template. There are four separate tasks listed for feedwater heaters: •

Performance Monitoring

•

NDE Inspection

•

Internal Inspection

•

Operator Rounds

Each task has its own frequency, depending upon criticality, duty cycle, and the severity of the service conditions. In addition, cleaning and leak testing were also included as separate items (see Table 4-3). The template also contains a listing of potential failure locations for the feedwater heaters, including degradation mechanism and influence (cause). There are 24 separate listings, which cover the tube joint, the tubesheet, the tubes, the nozzles, the closure devices, and the internal hardware (plates, supports, etc.). Each of the separate listings is coded for the four tasks detailed above, in terms of importance.

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The template defines the boundary of the feedwater heater as comprising all components from nozzle to nozzle, including the shell and internal components, and excluding the control systems and devices, the relief valves, and the insulation, along with the main steam reheaters (MSR). The PM template helps to build and/or evaluate the maintenance performed on the feedwater heaters at any given plant. It provides a quick overview to the system or maintenance engineer of the minimum recommended maintenance for feedwater heaters. The overall PM Basis template for tube-type heat exchangers is as follows, with the feedwater heater information shown. Columns 1 through 8 refer to the different heat exchangers considered in the EPRI program – column 1 represents the high-pressure feedwater heaters and column 3 represents the low-pressure feedwater heaters. Other heat exchangers addressed by the template include the Service Water heat exchangers, the Safety Injection oil coolers, the Diesel Generator Jacket Water heat exchangers and lube oil coolers, the RHR heat exchangers, and various plant area coolers (including control room A/C). Table 4-3 Feedwater Heater PM Basis Template

Equipment Description

Critical:

Duty Cycle:

PM Task

Service Conditions:

Yes

1 (HP) X

2 X

3 (LP) X

4

X

Low Severe

X X

X

Mild

6

7

8

X

X

X

X

X

X

X

X

X

X X

X

X

No High

5

X

X X

Performance Monitoring

Task Interval

1M

1M

NDE Inspection

Task Interval

5Y

5Y

Internal Inspection

Task Interval

5Y

5Y

Task Interval

2Y

AR

Task Interval

2Y

4Y

X

External Inspections (air-cool only) Cleaning

Leak Testing

The indicated frequencies are 1 Month, 5 Years, 2 Years, 4 Years, and As Required.

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EPRI TR-106857-V32 [Ref. 19] contains a discussion of each PM task, including information about possible failure locations for consideration, the progression of the observed degradation, fault discovery, and the tools and equipment needed for the task. The report also provides definitions for the terms used in the template (critical, duty cycle, service conditions, etc.). There is also a detailed discussion about degradation mechanisms for the various sub-components and possible PM strategies for each item. The PM strategy to address degradation is outlined in the following table, which lists failure locations, degradation mechanisms, and the PM strategy for tube-type heat exchangers. Items related to MIC and fouling are not addressed because they are not as applicable to FW heaters as other heat exchangers. Table 4-4 Failure Locations, Degradation Mechanisms, and PM Strategies Failure Location Tubes

Degrad. Mechanism Erosion (internal)

Degrad. Influence High flow velocity

Degrad. Progression Continuous

Failure Timing 1 Random

Foreign Material Suspended Solids

Discovery / Prevention Eddy Current Inspection Cleaning

PM Strategy NDE

Random

Random

Internal Inspections

Continuous

Random

Cleaning

Continuous

Random

Continuous or Random Continuous

Random Random

Eddy Current

NDE

Random Continuous

Random 2 Random

Inspection

Perform. Monitoring

FAC Non-uniform scale deposits Erosion (external)

Fluid impingement

Corrosion:

Foreign Material Water Chemistry and Fluid Quality

MIC Galvanic Chemical

Defects

2

Eddy Current Tube Materials

NDE ∆P, ∆T

*(these are not as applicable for FW Heaters as other HX) Manufacturing Installation

4-16

1

Random

Random

Cleaning


Inspection

Cleaning NDE

Eddy Current


Failure Location

Degrad. Mechanism Cracking

Degrad. Influence Vibration

Degrad. Progression Continuous or Random

Failure Timing Random

Fatigue

Discovery / Prevention Eddy Current

PM Strategy NDE

Inspection

Perform. Monitoring

Monitoring Leak Test Vibration Inspection Audible Noise

Tube Joint (welded)

Tube Joint (rolled)

Defect

Defect

Galvanic Attack Internals:

Looseness

Baffles, Supports, Tie Rods, Spacers, Diffusers, Impingem. Plates Tubesheet

Failed Welds

Improper Installation

Random

Random

Leak Test Pressure Testing

Abusive Transient

Ultrasonic Testing

Plugging

Chemistry Monitoring Pressure Testing


Random

Random

Abusive Transient

Ultrasonic Testing

Plugging

Chemistry Monitoring Inspection

Water Chemistry

Continuous or Random

Random

Materials Water Hammer

Random

Random

Vibration

Continuous

Random

Audible Noise

1

Leak Test

Leak Test

Internal Inspections Inspection

Video Insp.

Operator Rounds

1

Thermal Stress

Continuous

Random

Impingement (Tube Failure)

Random

Random

Corrosion

Water Chemistry

Random

Random

Cathodic Protection


Cracking

Materials Plugging

Random

Random

Inspection

Leak Test

Pressure Test

Internal Inspection

Erosion 2

1

4-17


Failure Location Shell

Degrad. Mechanism FAC

Degrad. Influence Flow Rates

Inlet and Outlet Nozzles

Corrosion

Water Quality

Failure Timing 3 Random

Discovery / Prevention Inspection

PM Strategy Internal Inspections

UT Thick. NDE

MIC *(not as applicable for FWH) Gasket and O-ring leaks

Closure Devices Channel, Partitions, Manways, Flanges Notes: 1) 2) 3)

Degrad. Progression Continuous

Eddy Current


Random

Random

Materials

Random

Age

Continuous

(should be failure free for 7 yrs.)

Inspection Thermal Performance

Perform. Monitoring Operator Rounds

Random on a scale of months t o years Random but can be very rapid (wi th sudden damage) Random, but detectable within 2 or 3 years

This table should also be used with the consideration of various PM, PdM, and CM schemes (discussed in Section 5).

4.2

Generic Communications and Other Reports

The NRC issues several hundred documents to the public each day. Many of these are specific to one licensee, but some are intended for the entire industry (or a portion thereof). Generic Communications are NRC-issued documents which address industry operating experience and are transmitted to one or more class of licensee. The most typical types of generic communications include Information Notices, Generic Letters, Information Bulletins, and Regulatory Issue Summaries. This report details the review of Information Notices, Information Bulletins, and Generic Letters with respect to feedwater heaters. Information Notices are issued to provide significant recently identified information about safety, safeguards, or environmental issues. They do not require a response. Information Bulletins provide notice of recently identified issues in more detail, and typically they do require a response. Generic Letters request that addressees: (1) perform analyses or submit descriptions of proposed corrective actions regarding matters of safety, safeguards, or the environment and submit in writing that they have complied with the requests with or without prior NRC approval of the action; (2) submit technical information that the NRC needs to perform its regulatory function; or (3) submit proposed changes to technical specifications. By a generic letter, the NRC may also provide licensees with staff technical or policy positions not previously transmitted or not broadly understood. The NRC website provides two methods for searching Generic Communications – a word search of the Generic Communications or an ADAMS search. 4-18


As previously listed, a review of NRC generic communications indicated the following items (with respect to feedwater heaters): •

18 NRC Information Notices

•

4 NRC Generic Letters

•

1 NRC Information Bulletin

•

35 LERs identified in ADAMS (2000-2003)

4.2.1 NRC Communications A review of the 35 LERs identified in the ADAMS word search (using “feedwater heater” and keying on LERs), which covered a period from 1999 to 2003, indicated that none of these items directly addressed feedwater heaters or their sub-components. Two of these items involved level control issues and vent problems which were part of secondary-side transients, but these issues are more properly assigned to the feedwater heater instrumentation and controls. In fact, of the 35 items, 10 were directly related to feedwater heater instrumentation and control problems, usually hi-level problems. Three (3) of the items were related to condensate or condenser problems, five (5) were related to feedwater pump or feedwater valve problems, two (2) were related to recirculation M-G set problems, and two (2) were related to EHC problems. Four of the items were duplicates (or revisions/corrections), making a total of 30 unique items for the period. A review of the 3 NRC Generic Letters (79-15, 88-20, 89-08) identified during the ADAMS search reveals that none of these Generic Letters has any direct relationship with feedwater heaters. GL 79-15 addresses operating problems and unresolved issues with steam generators in PWRs, GL 88-20 addresses severe plant vulnerabilities and individual plant examinations, and GL 89-08 addresses erosion/corrosion-induced pipe wall thinning (with mention of several incidents involving feedwater piping). Information Bulletin 88-07 described power oscillations at a BWR following a plant trip, and added that one of the issues which could cause power oscillations was the loss of feedwater heaters. But the feedwater heaters themselves were not mentioned in the event, so this Bulletin does not directly apply. The ADAMS search identified 18 Information Notices with some relationship to feedwater heaters. Also, a word search of the NRC Generic Communications revealed 5 additional Information Notices. A review of these notices indicated that only one item (IN 99-019) was applicable to feedwater heaters. This IN describes the FW heater shell rupture in May 1999 already addressed in other documents.

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4.2.2 INPO SEE-IN Communications INPO SEE-IN (Significant Event Evaluation Information Network) documents consist of several types of reports which communicate lessons learned from industry events. These types of reports include SOERs (Significant Operating Experience Reports), SERs, (Significant Event Reports), SENs (Significant Event Notifications), O&MRs (Operations and Maintenance Reminders), and LERs (NRC Licensee Event Reports). These reports differ from plant notifications (NPRDS/EPIX) and other plant event reports because they contain some level of analysis of the problem, as opposed to just an identification and description. The INPO LER search by system and component (looking under Feedwater, Condensate, and Extraction Steam, and for Heat Exchanger) identified 5 LERs, covering a period from 1984 to 2003. One of these items (#331-03001) involved a reactor trip and water chemistry problems caused by a condenser failure and did not relate directly to feedwater heaters. The remaining 4 items are as follows: •

•

•

•

#461-95001 - Lack of Understanding of Feedwater Heater Train Operation During Removal of FW Heater from Service Results in Decreasing Feedwater Temperature, Loss of Heater Level Control, and Manual Scram #313-89020 - Manual Actuation of Emergency Feedwater Pump Necessitated by Feedwater Flow Diversion Through Feedwater Heater Tube Leaks and a Feedwater Recirculation Valve #336-84012-1 - Manual Reactor Trip Necessitated by a Leak in the No. 5A Feedwater Heater due to a Steady Rise in Level #281-84010 - Reactor Trip – 6A Feedwater Heater High Level

The INPO SEE-IN LER word search (keyed on “feedwater heater”) identified 281 items, covering a period from 1984 to 2003. Of these 281 items, a review indicates that 23 were related to feedwater heater mechanical components or subcomponents. These items are listed in the following table.

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Table 4-5 LER Evaluation (from INPO SEE-IN database)

Date

Plant Type

Description

5/14/99

PWR

Steam leak from the rupture of the shellside of the feedwater heater (combination of steam impingement & flow-accelerated corrosion).

6/18/95

PWR

A high differential temperature across the feedwater heater caused tubesheet deflection (3 tubes popped out).

1/5/95

BWR

Low pressure feedwater heater string isolation to correct degradation related to suspected tube leaks.

11/8/93

BWR

Plant had been shutdown to repair a tube leak on a feedwater heater. Maintenance personnel had successfully completed the feedwater heater repairs.

9/22/93

PWR

During the secondary plant response, the piping welds on the inlet of the feedwater heater relief valves failed.

7/27/93

PWR

During the secondary plant response after the reactor trip, the tube side relief valve for the feedwater heater ruptured and an upstream small bore pipe failed at an elbow upstream of the valve.

3/1/93

PWR

Extraction line to the feedwater heater ruptured, causing a 3 by 6 inch hole in the line (programmatic failure of the erosion/corrosion (E/C) program).

10/16/91

BWR

A feedwater heater string was out of service due to a heater leak.

9/12/91

PWR

Reactor at 93% for repair of tube leaks in the low pressure feedwater heater.

12/15/90

BWR

Planned maintenance outage to repair low pressure feedwater heater tube leaks.

11/21/90

PWR

One feedwater heater developed a gasket leak due to slight water hammer resulting from the unit trip. The feedwater heater head bolts were retorqued to eliminate the leakage.

4/10/90

BWR

Weld failure on the relief valve line on the north feedwater heater.

5/30/89

PWR

Feedwater flow diversion through feedwater heater tube leaks.

9/1/88

PWR

Design deficiency in the feedwater heater which caused it to become "airbound" (originally intended to have a continuous operating vent, but the vent was never installed).

4/30/88

PWR

To reduce the production of sludge, all major copper components (e.g., feedwater heater (HX) tubes, etc.) in the secondary system have been replaced with stainless steel components.

Sludge-lancing and crevice flushing were done to reduce the sludge and remove to contaminants from the tubesheet crevice.

To reduce the caustic environment in the tube crevice and prevent tube support plate denting, a secondary system boric acid addition program has been implemented.

4-21


Date

Plant Type

Description

2/4/87

PWR

High CF pressure resulted in damage to the feedwater heater relief valves. An investigation revealed that a relief valve was leaking since it had a broken weld at its base.

12/30/86

BWR

Failure of three main condenser tubes caused by 2A feedwater heater stainless steel lagging which separated at the spot welds and fell on the condenser tubes (believed to be the spot welds used to attach the lagging to the feedwater heater, accelerated by the effects of steam flow from the #1 bypass valve).

7/2/86

PWR

A cracked weld on the high pressure feedwater heater relief valve piping that resulted from overpressurization.

4/24/86

BWR

Through-wall failure in the 6th stage feedwater heater extraction steam line at the downstream reducer leading to the 6th stage feedwater heater. The failure was caused by wet steam erosion.

5/31/85

PWR

The expansion joint for main feedwater heater cracked at a weld.

12/11/84

PWR

Low vacuum due to a leak in a moisture separator reheater cross-under pipe manway gasket and 2 small leaks in elbows on feedwater heater vent lines.

11/28/84

PWR

An orderly shutdown necessitated by a leak in the feedwater heater. All feedwater heater tube leaks were plugged.

5/5/84

PWR

Feedwater heater was later determined to have leaking tubes which were plugged.

4/18/84

PWR

Feedwater heater was inspected and found to have one (1) leaking tube and thirty-one (31) leaking tube plugs.

Most of the remaining items dealt with feedwater heater controls and other secondary side problems (pipe failures, turbine and condenser problems, etc.). The 23 items identified in the LER database can be used to compute a failure rate. Using a base of 104 operating reactors for the period from 1984 to 2003, and covering a period of 20 years, the failure rate would be 0.0111 failures per unit per year (23 divided by 104 divided by 20). This value is smaller than the other values previously estimated from the NPRDS/EPIX databases (0.0632), and the INPO Plant Events database (0.0377). This is not unexpected because LERs do not have to be submitted if a failure impacts production, but does not affect plant safety. Many issues will not be categorized as LERs. Therefore, the failure rate determined from review of LERs on FW heaters (0.0111) is not useful for the purposes of this sourcebook, and will not influence the generic industry failure rate determination (see Section 4.5 for further detail). The search of SERs, SENs, and O&MRs identified the following items: There were 7 items identified under SEN (significant event notification) via a word search for ‘feedwater heater’. Of these, two addressed the same event: the feedwater heater shell rupture at a PWR in May 1999. One of the items involved a steam line rupture, one involved a condenser tube rupture, and on involved the rupture of a drain line from a moisture separator reheater. One

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item involved isolation of extraction steam with an unrecognized loss of FW heating, and one item involved a scram due to excessive power oscillations. There were 32 items identified under SER (significant event report) via a word search for ‘feedwater heater’. A review indicated that only 4 items are related to feedwater heaters, and none of these involved specific feedwater heater mechanical component failures. There were 18 items identified in the INPO O&MR database via a word search for ‘feedwater heater’. Of these, only one item was related to feedwater heaters mechanical components, O&MR 431, Secondary Plant Pressure Vessel Wall Thinning. This item involves the FW heater shell rupture previously addressed during the LER and SEN review.

4.2.3

INPO NPRDS/EPIX Data

As previously described, there were four searches done of the INPO NPRDS/EPIX databases. NPRDS was investigated using a word search (for “feedwater AND heater”) and using a system/component search (focused on the condensate, feedwater, heater drains, and extraction steam systems and on heat exchanger as the component), and EPIX was searched in the same manner. The word searches yielded significantly more items than the system/component searches, but this almost certainly reflects the occurrence of the words “feedwater heater” in the text of items which do not directly involve feedwater heaters. The review and evaluation of these items is addressed in this sub-section. The numbers of items identified are listed in Table 4-2. The NPRDS items identified via a word search on “feedwater heater” were not individually reviewed due to the amount. The NPRDS items identified through a system/component search were individually reviewed (924 total items). The EPIX responses were reviewed individually, for both the word search and the system/component search, with 38 items listed for the system/component search and 276 items for the word search. The NPRDS data is tabulated in Tables 4-6 and 4-7. Note that the items are considered for the years 1983 through 1996 – the previous years were dropped because only 11 items were identified (too few data points for 1976-1982). Note also that the data do not show how much lost power generation was experienced, only whether the event caused a power reduction or a plant shutdown (the records do not identify lost megawatt hours). The average number of plants (95) was chosen based upon plant start-up dates. Table 4-6 NPRDS Feedwater Heater Search Results - Summary

Total Items

HP / LP

Applicable Items

924

292 / 632

839

Time Span

Avg. Number of Plants

Failure Rate

14 years

95

0.631

These results are an order of magnitude greater than those found for the INPO Plant Events database, the LER data, and the INPO EPIX database. This is not unexpected, because the 4-23


NPRDS failure criteria were different than the other databases, and included many failures the others do not identify. These results are discussed further in Section 4.5, where the failure rates for FW heaters are evaluated in more detail. The following list details the NPRDS feedwater heater records by year. 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996

-

19 44 62 51 64 90 105 101 79 72 74 30 59 35

Table 4-7 NPRDS Feedwater Heater Search Results – Specific Components and Consequences

Tube Leak / Tube Thinning

Manway / Flange Leak

Shell Leak

505 (60.2%)

204 (24.3%)

33 (3.9%)

Nozzle Leak 19 (2.3%)

Plug Leak 54 (6.4%)

FWH Internals Damage 49 (5.8%)

Power Reduction 123 (14.7%)

Note that the total is greater than 100% because many items involved more than one finding. Also, the items with power reductions are an estimate only because often the NPRDS field for plant impact often conflicted with the text of the record. The percentage of failures involving power reductions and shutdowns will be used in Section 4.5 in the determination of the generic industry estimated failure rate. For the manway/flange leaks, the records were primarily for worn, damaged, or misapplied gaskets, with a small percentage (about 10%) involving weld cracks, seal leaks, or improper bolt tightness. For the 38 EPIX system/component search items, 18 involved relief valves, vent valves, or controls and are not included in the final total. The 20 EPIX items involve one shell failure (already identified), four other cases of external leakage (shell leakage), one problem with a manway flange, one problem with a divider plate, and 13 items involving tube leakage or unavailability (due to surveillance test results). It should be noted that 7 of the 20 items were submitted by the same multi-unit site, which supports the previous statement that not all utilities report failures to the same degree or with the same frequency (for one site to submit more than 1/3 of the total is quite unusual, given that there are 104 operating plants of varying age in the overall data field).

4-24


The EPIX results are presented in tabular form in Table 4-8, showing the impact on power generation from the various failures. Table 4-8 EPIX System/Component Search Results for Feedwater Heaters

Date

BWR/ PWR

Type (LP / HP)

Description LP FWH tube leakage to design limitations LP FWH tube leakage to design limitations LP FWH tube leakage to design limitations LP FWH tube leakage to design limitations LP FWH tube leakage to design limitations LP FWH tube leakage to design limitations

attributed

Lost Megawatt Hours

3/24/97

PWR

LP

6/08/97

PWR

LP

6/24/97

PWR

LP

8/01/97

PWR

LP

10/26/97

PWR

LP

2/24/98

PWR

LP

10/06/98

BWR

HP

1/15/99

PWR

LP

2/08/99

PWR

LP

2/20/99

PWR

LP

4/02/99

PWR

LP

Tube Leak

29,285

5/14/99

PWR

LP

Shell Rupture

78,029

7/10/99

BWR

HP

Tube Leakage

14,552

7/15/99

PWR

LP

Tube Leakage

21,049

10/07/99

PWR

LP

Shell Thinning

None

12/06/00

PWR

HP

Socket weld leakage on shell

None

1/25/01

PWR

HP

Tube Leakage

4/30/01

BWR

LP

FAC found in steam inlet

None

5/01/01

BWR

LP

Damaged plugs found

None

4/11/03

BWR

LP

Tube Failures

attributed attributed attributed attributed attributed

Tube Leakage Tube leakage due to inadequate design of drain cooler endplate Tube leakage due to inadequate design of drain cooler endplate Tube leakage due to inadequate design of drain cooler endplate

15,433.2 11,345.3 23,215.6 13,970 10,896.6 None 19,927 17,028 4212 4363

12,341

72,243

Total MW-Hours Lost

347,889.7

Avg. MW-Hours Lost (for 20 events)

17,394.5

A review of the 276 EPIX word search (on feedwater AND heater) results indicates the following: 27 items which are directly related to the passive mechanical components of FW heaters. The remainder of the items address controls or other secondary side equipment. If these 27 items are used to estimate a failure rate for FW heaters, then 27 is divided by 104 (assumed 4-25


number of operating reactors) divided by 6 years, which gives a result of 0.0433 FW heater failures per unit per year. A more detailed review of these 27 items shows that only 7 were not previously identified in the EPIX system/component search. These 7 items are listed in tabular form below (note that the word search items often did not specify whether a LP FWH or a HP FWH was involved, so that tag is left out). Table 4-9 EPIX Word Search Results for Feedwater Heaters (unique items) Lost Megawatt Hours

Date

BWR/PWR

Description

7/20/97

PWR

Tube Leakage

43,392

10/16/98

PWR

Tube Leakage

4896

12/27/98

PWR

Missing Tube Plugs

1/18/99

BWR

Shell Thinning/Leakage

73,081

6/16/99

BWR

Tube Leakage

19,993

10/10/00

BWR

Tube Leakage

55,923

6/21/01

BWR

Shell/Weld Leakage

0

0

Total MW-Hours Lost

197,285

Avg. MW-Hours Lost

28,184

Of the 27 items listed in Tables 4-8 and 4-9, 19 (or 70%) are attributed to tube leakage, while 5 (or 18.5%) are attributed to shell thinning or shell damage. The remaining items involve plug problems and inlet FAC.

4.2.4 Experience in Fossil Power Generation and Industrial Facilities EPRI CS-1776 ( Failure Cause Analysis – Feedwater Heaters ) [Ref. 18] was developed as part of the EPRI Fossil Plant Performance and Reliability Program. Data from the Edison Electric Institute (EEI) was used, along with data from a special survey conducted by EPRI and EEI to gather information on feedwater heater problems. After the data was received and reviewed, a study team visited several utilities to gather additional information and to clarify the data. In addition, three major FWH vendors and a major A/E firm were visited to obtain their views on feedwater heater failures. CS-1776 provides a summary of the survey results and also discusses the problem areas for feedwater heaters. The most serious areas identified were the following: •

Tube vibration

•

Drains subcooler flashing and level control

•

Tube plugging

4-26


•

Corrosion

•

Tube inlet erosion

•

Steam impingement

The report addressed each of these areas in a separate section. Channel head-to-shell leakage was also identified as a serious problem, although it was found that other problems usually caused this damage. Feedwater heater failures for unknown reasons comprised a large problem area, reflecting the difficulty in diagnosing failures in a closed feedwater heater. EPRI sponsored a feedwater heater survey in 1991 (EPRI GS-7417, Feedwater Heater Survey) [Ref. 24] to ensure that the state of the art knowledge was collected, to extend current technical knowledge and experience, and to meet the higher demands of future power plants. This survey was part of the development project, “Feedwater Heaters for Improved Coal-Fired Power Plants”. An initial step in the project was to review the problems associated with HP Feedwater Heaters. A detailed questionnaire was prepared and distributed. The survey was distributed to 35 power plants in the USA and 16 power plants in Europe. The units were all fossil-fired. A brief summary of the survey results are as follows: Table 4-10 Damage Incidence for FW Heaters (Fossil) – From Survey (EPRI 7417)

Defective Component

United States

Europe

Hxch Tubes

83%

25%

Tube-to-Tubesheet Joint

23%

12%

Tubesheet & Channel Head

23%

6%

Other Components

37%

6%

8%

6%

Replaced FW Heaters

Table 4-10 shows that 83% of US fossil plants (from the survey) have experienced tube problems at some time or another. EPRI has also issued a number of documents associated with improving the heat rate for fossil power plants (some of this information would be applicable to nuclear plants as well). Those reports include EPRI TR-109546 [Ref. 26]. In addition, there is a two-volume EPRI set available on plant thermal performance, EPRI TR-107422 (Thermal Performance Engineer’s Handbook) [Refs. 13 & 14].

4.3

Codes and Standards (Mandatory, Guidance, Associations, Societies)

The following codes and standards are applicable to feedwater heaters, and are regularly used in the design, specification, operation, inspection, maintenance, repair, and modification of feedwater heaters:

4-27


•

HEI (Heat Exchange Institute) Standards for Closed Feedwater Heaters, Sixth Edition, 1998

•

TEMA (Tubular Exchanger Manufacturer’s Association) Standards, 6 Edition, 1978

•

ASME Codes/Standards (Boiler & Pressure Vessel Code, Section VIII, Div. 1)

•

ASME Performance Test Code PTC 12.1 – Closed Feedwater Heaters

th

Feedwater heaters are pressure vessels and their design and construction must meet the requirements of the ASME Boiler & Pressure Vessel Code, Section VIII, Division 1, including applicable addenda and case rulings. All feedwater heater units are to be stamped with the ASME Code symbol. Both the HEI and TEMA standards reference the ASME Code (as they describe code requirements for the heat exchanger equipment). The HEI Standards for Closed Feedwater Heaters combines present industry standards, typical purchaser requirements, and manufacturer’s experience to provide an overall view of design criteria for closed feedwater heaters. The standards outline practical information on equipment nomenclature, dimensions, testing, and performance. Use of the standards enables effective communication between the purchaser and the manufacturer. The standards represent the collective experience of the section members (manufacturers) and they provide a guide in the preparation of feedwater heater specifications and selection. The standards are organized to provide definition of technical terms, a discussion on feedwater heater performance, a listing of mechanical design standards for sub-components (including code requirements), material design standards, guidance on replacement specifications, a discussion of feedwater heater protection, a discussion of typical installations, an overview of feedwater heater channel types, and an outline of typical feedwater heater internals. Appendices address various issues such as feedwater heater operation, maintenance, and the various zones within a feedwater heater. The TEMA Standards are presented in three classes, for “R”, “C”, and “B”, each reflecting acceptable designs for various service applications (power plant heat exchangers fall into the “C” category). The standards are compiled by the Technical Committee of the Association. The standards present information on the nomenclature for heat exchangers, data on fabrication tolerances, general fabrication and performance information, a discussion on installation, operation, and maintenance, mechanical standards for the three different classes described, materials specifications, thermal standards, data on physical properties of fluids, general technical information, and recommended good practices for heat exchangers. The standards also address topics such as tube orientation, thermal expansion, heat exchangers supports, impingement protection, and heat exchanger testing. TEMA is comprised of manufacturers of various types of shell and tube heat exchanger equipment. ASME PTC 12.1 provides procedures, direction, and guidance for determining the performance of a closed feedwater heater with respect to the following: 1. Terminal Temperature Difference (TTD), which is the difference between the saturation temperature (corresponding to the steam inlet pressure and the feedwater outlet temperature) 2. Drain Cooler Approach (DCA), which is the difference between the drain outlet temperature and the feedwater inlet temperature

4-28


3. Tube side (feedwater) pressure loss through the heat exchanger 4. Shell side pressure loss Note: FW heater performance monitoring is discussed in further detail in Sections 5.1.2 and 5.4 of this sourcebook. This code applies to all horizontal and vertical heaters except those with partial pass drain cooling zones (short-end drain coolers). Designs with partial pass drain cooling zones are horizontal heaters with submerged drain cooling zones and vertical heaters with the channel-up design. In those designs, only a portion of the feedwater passes through the drain cooling zones; therefore, there are two feedwater flow streams with different temperature profiles. The heater design is based on specific operating conditions that include flow, temperature, and pressure. It is necessary to predict the heater performance by adjusting the design parameters for the test conditions. PTC 12.1 presents methods of calculating the predicted heater performance. It is a relatively new code, having been published in 2000. Other organizations have codes and standards that apply to feedwater heaters to a more limited degree, such as the following: •

•

•

•

•

ANSI (American National Standards Institute) – standards for dimensional requirements for materials such as piping, tubing, flanges, etc. AWS (American Welding Society) – procedures for welding and post-weld heat treatment that are not already covered by ASME or HEI SSPS (Steel Structures Painting Council) – standards for removal of mill scale, blasting, surface preparation, and the application of coatings or paint ASTM (American Society of Testing and Materials) – standards for the physical and chemical properties of heat exchanger materials, fabrication tolerances, and materials testing that is not addressed by ASME ASNT (American Society of Non-Destructive Testing) – standards to address the NDE performed on the feedwater heaters (including testing and personnel qualifications)

In addition, there are requirements associated with pressure vessels and nuclear insurance which must be part of the overall feedwater heater maintenance and engineering file at each power plant. The system or component engineer is normally responsible for this area. Feedwater heaters are addressed in the NEIL (Nuclear Electric Insurance Limited) Loss Control Standards [Ref. 44] in two places: first, as part of the condensate/feedwater systems, where credit is given under the boiler & machinery loss control program for high integrity tubing observation, testing, inspection, and surveillance; and second, where partial credit is applied for original tubing that has been replaced with non-copper bearing tubing (i.e., stainless steel).

4-29


4.4

Plant Performance Enhancement Program Technical Library 2

EPRI maintains a Plant Performance Enhancement Program (P EP) technical library which contains a variety of documents on feedwater heaters. The documents range from evaluations of drain cooler problems at specific plants to overall descriptions of feedwater heater replacements (at specific plants) to test program reviews and evaluations. These files are available to EPRI members via the EPRI website or by contacting EPRI directly. EPRI TR-107422-V2 (Thermal Performance Engineering Handbook, Volume 2) [Ref. 14] lists over 100 separate files applicable to feedwater heaters.

4.5

Generic Industry Failure Rate Estimation Summary

The review of plant operating experience to determine a failure rate for FW heaters has shown that the data have a wide disparity, and any estimation of a generic industry failure rate is highly dependent upon how the data are evaluated and which sets are the most credible for the purposes of LCM planning. The following table summarizes the wide range of values determined from the operating experience review. The number of failures are those found in the search that are directly related to FW heater mechanical components, and the failure rate is the value determined by dividing the number of events by the applicable number of operating nuclear units, and then dividing again by the number of years (inclusive) that the database covers.

4-30


Table 4-11 Operating Experience Data for FW Heater Failure Rate Determination

Source

NPRDS/EPIX data from EPRI report 1003470

No. Failures

171

Time Span

1976 – 2002

Failure Rate

Observations

0.0632

The failures are listed in EPRI 1003470 to show failure types and affected FW heater components. The failure rate was computed (in this sourcebook) from the tabulations of 1003470. The data is detailed, but the failure records were not individually evaluated. These records are representative of typical FW heater failures

0.0377

This data is taken from the INPO Plant Events database, which contains industry event summaries prepared by INPO as part of the INPO event screening process. INPO does not guarantee the accuracy of this data. This information is considered typical of operating experience data.

0.0111

This data considers only events which merit an LER, and are not representative of FW heater failures in the industry. The failure rate from the data is too low for a true generic failure rate because not enough events are included. This value does not influence the determination of the generic industry failure rate.

0.0433

The data from EPIX is limited because the reporting standards do not include most secondary-side plant equipment. EPIX is directed toward the Maintenance Rule, which is focused on safety. FW heaters are not safety-related and their failures will not typically be reported in EPIX.

0.631

The NPRDS data is the most comprehensive and detailed data for FW heater failures. All typical failure modes are included, and the results allow for the broadest estimation of failure rate. However, NPRDS records also include many failures found during outages that do not impact plant operation, and also includes many minor failures that were repaired without any plant impact. NPRDS also includes older data when nuclear plants were discovering the failure modes for FW heater components. The NPRDS failure rate is therefore too high to represent a generic rate for the industry.

INPO Plant Events 47

INPO LER database

INPO EPIX review

INPO NPRDS review

23

20

839

1991 – 2003

1984 – 2003

1997 – 2003

1983 – 1996

4-31


The reasons for the broad range of failure rates have been explained in the specific sub-sections that address the sources of the data. In order to determine or select a meaningful value for a generic industry failure estimate for FW heaters, these differences need to be considered. In view of the wide range of failure rates in Table 4-11, it is appropriate to estimate a range, rather than a single value, for a generic industry failure rate. This allows for the consideration of all the data sources, without ignoring or relying too heavily on any single one. In this manner, all events are considered (to the appropriate degree) – both the events that actually impact the plant’s power production and the events discovered during surveillance but do not impact plant operation. For this LCM sourcebook, the balance is struck by estimating a generic industry failure rate of between 0.05 and 0.1 failures per unit per year. The lower value of 0.05 is chosen as being representative of the EPRI 1003470 NPRDS/EPIX data, the INPO Plant Events data reviewed for this sourcebook, and the EPIX data reviewed for this sourcebook. The value chosen is higher than the individual failure rates found for the last two sources because they do not include all failures impacting production. The upper value of 0.1 was chosen based on the percentage of NPRDS failures (about 15% from Table 4-7) that involved power reductions (from Table 4-11, 0.631 times 0.15 equals 0.095), This estimated failure range addresses the uncertainty involved in reviewing the disparate industry failure data. The range allows for the differences in failure definition, differences in failure reporting, and differences in how failures are categorized with respect to maintenance. The range also accounts for differences in FW heater age and service, because older FW heaters are expected to have a higher failure rate and will require additional surveillance and more maintenance. The experience information analyzed above is available only on a per-plant basis and not on a per-heater basis. Because plant-specific LCM planning is normally done on a component basis, the extent to which available information can provide an estimate of failure rate for an individual FW heater is explored in the following paragraphs. One way to estimate a per-heater failure rate is to simply divide a generic per-unit rate estimated in this report by the plant-specific number of heaters. Although this would be an improved generic estimate (because it would appropriately give higher or lower per-unit rates depending on the number of plant-specific heaters), it would contain an element of inaccuracy because the per-unit generic estimate does not account for the per-unit number of heaters on which the failure count is based. To improve the accuracy of the generic estimate for LCM planning purposes we need to know the total population of heaters covered by the data sources. The average number of heaters per unit would then be divided into the per-unit failure rates to give the average per-heater failure rate. A way to obtain the exact population of heaters and the average per-unit number of heaters is to research documents such as Safety Analysis Reports for all plants. This activity was beyond the scope of this project. It could, of course, be performed by any future plant-specific LCM planner. 4-32


For our purposes, an estimate of the average number of heaters per unit was obtained by interviewing several plant experts familiar with US plants. They came up with a range of perunit heaters of from 12 to 21, with the average of 16.5, assuming a uniform distribution of values, which is only approximately true. (An estimate of the total is then 16.5 x 104 = 1716 FW heaters in US plants). The average divided into the per-unit range of 0.05 to 0.1 leads to the perheater range of 0.003 to 0.006 failures per year. This would be the range of values to be used by a FW heater LCM planner regardless of the number of heaters in his or her plant. Note that these estimates still include the uncertainty caused by not knowing the exact number of FW heaters in the population. A point-value evaluation of the net present value of various alternative plant-specific LCM plans would use a value between 0.003 to 0.006 as “generic” information to be combined with plantspecific failure data to give the value or values of failure rate to be used in the calculations with a point-value LCM tool such as EPRI’s LcmVALUE or LcmPLATO. This could be done using either a single value (such as the average, 0.0045) or a higher/lower value within the range to account for judgments of how well the industry average applies to the plant under evaluation. For example, if the plant currently has a preventive maintenance program or a tube/shell material type superior to the “average” plant, then a lower value in the range would be used to combine with the plant-specific failure rate. Another approach might be to use the upper and lower values of the range (i.e. a sensitivity study) and see if this changes the choice of the optimum LCM alternative. This study’s finding that there is a significant range of industry experience-based failure rates (so that forecasts of future failure rates would be even more uncertain) is the same finding as in most of the nine other LCM sourcebooks produced to date. For this reason, the fidelity of LCM planning efforts can be improved (over that provided by the point value approach) by going to the next level of analytical technology and treating uncertainty explicitly using the probabilistic approach embedded in “decision analysis.” In this approach, the single values used in deterministic evaluations are replaced by probability density functions (PDFs) as inputs. It has been shown (e.g., in the LCM uncertainty analysis performed for the South Texas main generator [Ref. 46]) that failure rates are among the most important drivers of LCM decisions. As a first order approximation to a FW heater PDF, one might use a uniform distribution between 0.003 and 0.006 failures per year (perhaps adjusted to account for plant-specific data). An improved distribution could be obtained by conventional methods for combining statistical data with expert judgment (for a FW heater, the judgment would be elicited from a few experts such as the plant FW heater component engineer, a secondary-side system engineer, a manufacturer technical representative, or an expert consultant). EPRI is exploring various existing tools for performing plant-level LCM with uncertainty and will use the findings to guide future work in this area [Ref. 47].

4-33

5

GUIDANCE FOR PLANT-SPECIFIC SSC CONDITION & PERFORMANCE ASSESSMENT

This section addresses Steps 8, 10, and 11A in the LCM planning flowchart (Figure 2-2) and provides guidance for the plant-specific LCM planning for feedwater heaters. Also included in this section is a compilation and description of available and useful condition or performance monitoring programs, along with a brief overview of the most common repair techniques used in the industry for feedwater heaters. This information is useful for LCM planning when maintenance strategies are being evaluated and alternative LCM plans are being developed. Individual plants may also discover that their program can be enhanced by including some of these techniques (if they are not currently utilized). •

•

•

In Step 8, the plant-specific operating and performance history is compiled, as discussed in Section 5.1 below. Step 10 comprises a compilation and review of the plant-specific maintenance program for feedwater heaters (see Section 5.2) leading to the establishment of a complete inventory of the current maintenance tasks and providing a basis for determining if enhancements or changes are desirable (or necessary). In Step 11A, the intent is to characterize the present plant-specific physical condition and performance of the feedwater heaters and the implementation of effective preventive maintenance procedures, diagnostics, and component condition monitoring. The assessment of the maintenance tasks should pay close attention to whether and how the tasks address any deviations identified in the SSC performance assessment and SSC condition review. The deviations may be positive in that plant-specific SSC performance and conditions are superior to the industry average, which may indicate that unnecessary or too frequent PM is being performed, or the deviations may be negative, which indicate a need or opportunity for improvement. Details of the condition and performance assessment are discussed in Section 5.3.

5.1

Compiling Feedwater Heater Operating and Performance History

The operating/performance history and the age of the FW heaters at any given plant have a major bearing on the LCM planning evaluation and the choices made for the future, and they provide the basis for the condition and performance assessment. In conjunction with the performance review, a thorough assessment of the existing equipment is of paramount importance in making realistic decisions with respect to the feasibility of maintenance options and strategies. To make the optimal decisions, the history must be known and correctly evaluated. The following are the recommended steps in assembling the operating and performance history for the FW heaters:

5-1

Guidance for Plant-Specific SSC Condition & Performance Assessment

•

•

•

•

•

•

Assembling the maintenance history for the FW heaters, particularly for the past 10-15 years. This includes maintenance on the shell (if any), the tubes, the tubesheets, the tube supports, the baffles and shield plates, the drains and vents, and the inlet/outlet nozzles. The maintenance history includes preventive maintenance, predictive maintenance, corrective maintenance, and a listing of work orders. Trending the historic failure rates to identify the specific components that exhibit unusual performance challenges (either design or operational problems). This will primarily involve the tubes and the baffle and shield plates. Compiling and reviewing performance test results, surveillance test results, etc., to determine if trends exist in the FW heaters. This includes a review of available data on thermal performance. Reviewing the Maintenance Rule performance parameters and trends, the system & component health reports, the monitoring status, the goals, and the discussions of specific failures, along with the effectiveness of corrective action. Reviewing the plant trip and power reduction history to determine and list the events caused by the FW heaters and sub-components. For those events, the lost power generation, the extended outages, the power reductions, the corrective actions, etc. provide the input into the historical cost of FW heater failure. These results provide a basis for projecting future performance in the LCM planning. The results will be negative if the performance is declining and additional corrective action is not implemented and positive if new PM or PdM tools are applied, or if equipment replacement is begun. Review of design changes and technology upgrades (in maintenance or operation) that have been instituted.

•

Review of the availability of spare parts and maintenance equipment.

•

Review of any component replacement work.

•

Review to establish the feasibility of obtaining major component replacement in the most efficient manner possible (particularly for older equipment).

The following sections/paragraphs detail specific areas for review and provide specific guidance on industry methodology to determine the current condition of the feedwater heaters. Also included is a review of current repair techniques for feedwater heater components.

5.1.1 Feedwater Heater Condition Reviews (Work Orders, Failures Trends, Lost Power Generation, Trips, Preventive Repairs, Replacements, Refurbishments) To develop a clear picture of past feedwater heater performance from which projections can be made, a thorough review of the maintenance history is needed. The maintenance history is captured at most plants in Work Orders, usually located within a database (most plants typically have electronic records from the mid-1980s to the present). Work Orders are written to implement preventive, predictive, and corrective maintenance and also to implement other activities, such as surveillance tests, overhauls, inspections, design changes, and replacements.

5-2


With respect to feedwater heater LCM, the most important Work Orders are those which implement corrective actions as a result of problems or failures (including work from design changes due to problems or enhancements). These Work Orders often contain information concerning the root cause, whether repetitive problems were involved, the cost and man-hours spent in the corrective action, and the reason why the problem was not detected in an early stage. This information is used to identify additional preventive maintenance (PM) or predictive maintenance (PdM) activities, potential enhancements to the current maintenance program, and/or the need for upgrades, replacement, or new design. The basic premise is that performance can only be improved by preventing problems (the ability to detect degradation and failure before loss of function occurs) and by determining the actions which could prevent future problems. The Work Order review also provides detailed information as to the annual frequency of occurrence of problems and failures presently experienced by the feedwater heaters. The problem occurrence rate is one of the most important inputs for calculating the costs of corrective maintenance and failure (due to lost power production, new equipment, regulatory risk, EPIX reporting, MR monitoring, etc.) when performing economic modeling of LCM alternatives. The Work Order review can also be used to trend the annual corrective and preventive maintenance activities over the past several years to determine if the rate of problem occurrence is increasing or decreasing, and if the ratio of corrective to preventive Work Orders (and their implementation costs) are changing. An effective PM program should show a gradual decrease in corrective Work Orders and associated costs. This data can lead to the identification of additional corrective and/or preventive actions that may be justified to improve the feedwater heater programs. These actions would then be part of the alternative LCM plans for the feedwater heaters (to be evaluated on a plant-by-plant basis). In the Work Order review, consideration must be given to the impact of FW heater maintenance on plant reliability, with respect to the different types of heaters involved (high-pressure or lowpressure), with respect to plant lost power generation (if an entire heater string is taken out of service), and any overall maintenance strategy for doing work on-line versus off-line.

5.1.2 Review of Diagnostic Tests and Monitoring Devices/Data There are a number of applicable EPRI reports that address feedwater heater testing and performance monitoring. They are briefly described (with applicable information) in the following paragraphs. EPRI 1003470 ( Feedwater Heater Maintenance Guide) [Ref. 11] provides an overview of general maintenance activities, preventive maintenance tasks, non-destructive examination testing, and destructive testing and analysis (see Section 6 of the reference for details). The report also addresses performance monitoring (and inspections). Performance monitoring looks at the overall integral performance of the feedwater heater. Performance deterioration that is detectable is likely to be caused by vibration, corrosion, erosion, scaling or other deposits, or control system problems (addressed in EPRI 1007425) [Ref. 6].

5-3


Performance monitoring is also addressed in EPRI GS-6935 ( Feedwater Heaters Maintenance and Repair Technology: Reducing Outage Cost ) [Ref. 17]. This report provides an overview of some newer surveillance and inspection techniques for feedwater heaters (such as sonic pulse testing). The report also details the advantages of on-line acoustical monitoring to identify tube leaks and shell cracks. For an efficient outline of general heat exchanger issues, EPRI TR-106471 ( Heat Exchangers: An Overview of Maintenance and Operations) [Ref. 15] is useful. The report does not specifically focus on feedwater heaters or nuclear plants, but it does provide helpful information on heat exchanger performance, heat exchanger components, non-destructive testing, and general information on repairs. EPRI NP-4057, Volumes 1-3 ( Nuclear Plant Feedwater Handbook ), particularly Volume 3 [Ref. 23], outlines routine heater performance analysis by focusing on the critical parameters to be measured regularly. The points listed are: •

Shell-side pressure

•

Shell-side steam saturated temperature

•

Feedwater temperature at the inlet and outled nozzles

•

Drain temperature exiting the shell

•

Feedwater flow

As previously detailed in Section 4.3, this data can be used to determine the uncorrected terminal temperature difference (TTD) and the drains cooling approach (DCA) for comparison with design values. Note that there is a difference between the uncorrected TTD (which is the saturated temperature for the actual shell-side steam pressure minus the actual feedwater outlet temperature) and the corrected TTD (developed in PTC 12.1), which is the saturated temperature for design steam pressure minus the feedwater outlet temperature corrected to design conditions. The TTD is essentially a measure of the heat transfer capability of the feedwater heater. The higher the TTD above the design value, the poorer the performance of the heater. The actual measurements can only be compared with the values obtained during performance testing to determine if significant changes are taking place. If for the same conditions, the TTD is substantially higher, then the feedwater heater has problems. In general, a TTD that is significantly higher than design (even if conditions are not identical) is cause for concern. A tabular representation of FW heater performance monitoring is shown in Table 5-1. NP-4057 points to ASME PTC 12.1 [Ref. 39] for detailed, comprehensive, and accurate performance testing. The DCA (drains cooling approach) is the difference between the drains outlet temperature and the feedwater inlet temperature. A DCA that is significantly higher than the design value is cause for concern. For proper analysis of the data, it is desirable to know the water level within the shell and to know how it was determined (and where the instrument taps are).

5-4


EPRI GS-6935 ( Feedwater Heaters Maintenance and Repair Technology: Reducing Outage Cost ) [Ref. 17] contains a detailed outline of maintenance tasks, on-line monitoring, maintenance accessibility, inspection and testing, corrective maintenance and repairs, and post-maintenance / pre-operational testing. This reference also addresses recording maintenance data, and has a section devoted to heater venting maintenance. The purposes of the report are: •

•

•

To report to utilities on the state-of-the-art feedwater heater maintenance and repair technology To develop feedwater heater procurement specifications and guidelines based upon the best sample specifications and experience available To develop additional recommendations for EPRI and the Utility Advisory Group for future projects to improve the maintenance of feedwater heaters

The report was written based on material collected from numerous sources, principally an industry literature search (conducted from 1982 – 1988), which reviewed technical reports, articles, symposia, and other data specifically related to feedwater heater maintenance. Also included in the source review were the study team’s own records and experience, a utility maintenance and engineering records search (related to feedwater heaters), interviews with power plant personnel, expert interviews, and comments from EPRI and the Utility Advisory Group. The report strongly concluded that feedwater heater maintenance plans must be effectively established and implemented in order to obtain the optimum performance. The report challenged utilities to apply strong forward-looking programs to maintain the feedwater heaters in their plants. The details of such maintenance (and performance monitoring) are presented in the following paragraphs. The report highlighted the following on-line monitoring techniques: •

Acoustic Monitoring (for tube leaks, crack detection, etc.)

•

On-Line Level Monitoring (normal liquid level annunciation)

•

On-Line Thermal Performance Monitoring (TTD, DCA)

The report highlighted the following inspection and test techniques (for easily accessible areas): •

•

•

Visual Inspection – external (vents and drains) Visual Inspection - internal (channel interior surface, pass partition plate, tubesheet face, tube-to-tubesheet joints, and tube interiors) Fiber Optic Inspections (using various instruments)

Inaccessible areas of the feedwater heater (such as the shell interior) should be performed with boroscopes, fiberscopes, and video boroscopes. Erosion on the shell and/or baffle/impact plates and tube damage should be considered and investigated when conducting these types of inspections. Acoustic leak detection is most valuable when monitored on-line. Early leak detection is based on the fact that even the smallest leak creates discrete noises from the turbulence of the leak jet. The sensors are piezoelectric pressure transducers installed near the feedwater inlets and 5-5


mounted so that the water-borne noise is acoustically coupled to the transducers. EPRI has sponsored a number of programs to demonstrate the various configurations of test equipment, and several plants (nuclear and fossil) have identified tube leaks, baffle erosion, and shell cracks well before scheduled outages and before the damage became severe enough to impact plant operation [Ref.17]. The leak testing of tubes can be accomplished from either the tube or the shell side using several different testing methods. All leaking tube and/or tube-to-tubesheet joints should be identified both on the tubesheet (with an oil-based marking pen) and on a tubesheet layout drawing. The most widely used methods for leak detection from the shell-side are as follows: •

Pressurized Air

•

Water

•

Vacuum

•

Halogen Gas

Though less frequently utilized for general leak testing, the following tube-side techniques are effective: •

Pressurized Air

•

Vacuum

•

Hydrotest

•

Sonic Pulse

As with leak testing, the determination of the tube leak location within the tube span may be accomplished from either the shell side or the tube side via several different methods, such as vacuum testing, pressurized air, water, and sonic pulse testing. In some cases, the orientation of the feedwater heater will feature a specific type of test. Individual tube hydrotesting has also been useful in examining tubes that have presented indications of problems through other observation (i.e., eddy current testing). An individual hydrotest will show tubes that are near failure. An individual hydrotest can be used to verify eddy current results, although tubes may be damaged by the test. In that case, the tube will need to be stabilized with rods (and plugged). Sonic pulse testing involves the transmission of an audible sound wave down the inside of an empty tube under inspection. Echoes from this sound wave are returned from any anomalies in the tube’s inner wall and the echo characteristics provide information about the locations and types of the anomalies. This method detects anomalies on the inner tube wall. Electromagnetic testing (ET) is the technique used most often to examine feedwater heater tubes. It can be performed rapidly with a high confidence in the test results, and the data is repeatable (and can be stored and retrieved easily). Selection of the appropriate ET technique is dictated by the magnetic properties of the tubing material. For non-magnetic alloys (304 SS), eddy current testing (ECT) is widely used. For shell thickness determination and verification, pulsed eddy current testing is used (and is described later in this section). 5-6


The most commonly used NDE technique for determining the condition of feedwater heater tubes is eddy current testing (ECT). Eddy current testing is extremely fast, repeatable, good for baseline screening, and accurate in sizing known defects. However, ECT is very operatordependent. Quantification of certain types of defects is sometimes difficult, and other NDE methods must be used in conjunction with ECT to obtain optimum results. Eddy current testing is described in detail in EPRI 1003470 ( Feedwater Heater Maintenance Guide) [Ref. 11]. Eddy current testing utilizes an internal probe specifically designed for the tube diameter and material being tested. The probe consists of an alternating current coil which establishes an electric field that produces a field of eddy currents in the tube. This field is interrupted by tube wall flaws such as wall thinning, cracking, and pitting, and these interruptions cause an impedance change in the coil. This impedance change is reflected as a Lissajous signal and is measured in terms of signal magnitude and/or signal phase. In-place tube bundles are inspected from the tube inside diameter by pulling the test probe through the tube at a constant speed. Tubing can be inspected at rates greater than 1 foot per second. During this process, the data can be saved to a magnetic disk for review by a qualified analyst. The defect type, size, depth, and wall thickness can be determined by comparison to a known calibration standard. Figure 5-1 shows a flowchart rendition of the eddy current process.

5-7


Figure 5-1 Eddy Current Flowchart (from EPRI 1003470)

5-8


The tubes are scanned through the absolute and/or differential test modes to achieve full volumetric inspection along the entire tube length. This is accomplished by fully inserting the probe and pulling the probe back along the tube length at specified test speeds. The inspection objective is to detect and analyze all potential tube flaws. Eddy current testing offers the following advantages: •

It is relatively fast (high digital sample rates allow for test speeds up to 260 in. per sec.

•

It has a good sensitivity to material property changes.

•

It has a good sensitivity to surface cracks.

•

The test equipment is portable and state of the art.

•

The test equipment is easily configured and applied for automatic examinations.

•

Data storage and retrieval are easy.

•

Multi-frequency, multi-channel equipment allows for efficient monitoring.

Eddy current limitations include the following: •

Eddy currents are influenced by the presence of material flaws and material property variations such as conductivity, permeability, geometry, etc.

•

Eddy currents are influenced by flaw orientations that make the testing direction-dependent.

•

Eddy current testing is mainly used for testing thin materials (i.e., tubes).

•

•

The sensitivity level for flaw detection is not uniform throughout the test piece. Optimum detection occurs at the test surface closest to the test coil and decreases exponentially below the surface. Good eddy current signal interpretation requires a high level of operator training and awareness.

Because conventional eddy current testing cannot be performed on magnetic tubing material, several alternative eddy current schemes need to be considered: •

•

•

•

Full-Saturation ET : This procedure uses a magnetic saturation technique which renders ferromagnetic tubing non-magnetic. This technique is applicable for tubes made of 439 stainless steel, Seacure, Monel, and Al29-4C with a wall thickness of up to 0.035 inches (0.9mm). Partial-Saturation ET : This technique uses an ET probe with equipped with permanent magnets to partially saturate the tube wall. This technique is applicable to carbon steel tubes and thick-walled ferritic stainless steel tube materials. Remote-Field ET: This technique provides a volumetric inspection of ferromagnetic tubing without magnetically saturating the tube wall. This technique is applicable to all ferromagnetic tubing, including carbon steel, Monel, and ferritic stainless steel. Flux Leakage: A pair of induction coils are used to detect tube flaws, one for inside and outside diameter flaws, and one for just inside diameter flaws. By positioning on

5-9


differentially connected induction coil between the magnetic poles, differences in flux leakage along the wall are monitored. Changes in the total flux caused by gradual wall thinning are easily noted. The most important considerations in planning ECT examinations are to prepare a comprehensive tube sampling scheme to adequately assess the current feedwater heater conditions, to perform the necessary corrective measures, including tube pulls, to mitigate any existing problems, and to calculate the remaining life once the test results are complete. In order to have a successful ECT examination, the following tasks should be considered: •

Gather FW heater design drawings, records of past operation, maintenance/repair records, chemistry data, and previous ECT examination reports

•

Establish a tube sampling scheme consistent with an existing numbering system

•

Establish a tube plugging criteria

•

Prepare for repair and remedial measures (i.e., tube cleaning) if necessary

EPRI 1003470 [Ref. 11] contains a detailed section that addresses each of these items and provides data on tube plugging criteria. Please refer to Section 6.3.4 for further detail on ECT. EPRI GS-6935 also contains detailed information on eddy current testing [Ref. 17]. EPRI 1003470 also contains a discussion of pulsed eddy current testing, which is used to evaluate the thickness of FW heater shells. Pulsed eddy current testing senses the presence of defects by inducing eddy currents in the insulation jacket and outside surface of the shell. The change in the magnetic field is monitored as the currents diffuse and permeate the shell and the wall of the insulation jacket. The shell thickness is measured by recording the time it takes for the currents to diffuse and permeate. Pulsed eddy current testing has been increasingly used in FW heater applications because of the following features: •

The methodology measures wall thickness through the insulation, eliminating the cost of insulation removal

•

The probe can penetrate a carbon steel shell up to several inches thick

•

The probe requires no direct contact with the shell and can be applied on-line

•

The testing is not affected by the geometry of the shell, any coatings, or the type of insulation

•

The probe is tolerant of misalignment and motion

•

The probe is not sensitive to the user’s skill (unlike tube testing)

The technique focuses on a relatively large area which limits its application (or usefulness) to the identification of widespread corrosion/erosion. corrosion/erosio n. But this is the case with flow-accelerated corrosion (FAC). The limitations of pulsed eddy current testing include the need to assess a broad area of the shell, possible underestimation of wall loss, the possibility that localized damage could be overlooked, the need to re-calibrate the equipment at various locations around the shell, and the difficult working conditions (when used on-line) due to heat and noise. 5-10


The results of pulsed eddy current testing indicate that the technology will show damage after wall loss exceeds about 20%.

5.2

Review of Current Maintenance Plans

Once the plant-specific maintenance history has been compiled, the current maintenance activities need to be identified. When using the word ‘maintenance’ in LCM planning, it must be noted that this includes preventive, predictive, and corrective actions, whether required by regulations (testing, inspection, surveillance, monitoring, sampling, EQ, etc.), or by applicable codes/standards (IEEE, NFPA, ASME, etc.) or by insurance requirements, or by state/local government requirements, or by plant procedures, programs, or policies. Collecting the associated activity parameters (frequency, affected components, labor hours, material costs, etc.) will provide key input to developing a base case for LCM planning. This base case will not only create an inventory of the current activities but will provide a benchmark for comparison to industry practice and a basis from which the need for additional activity, enhancements, or even task reductions can be considered. One way to assemble this information is shown in Table 7-1, where the LCM alternatives can be compared. Further detail will be included in plant-specific LCM tables because actual estimated costs can be developed, based upon known factors (labor rates and material costs) and supported by yearly budgets, estimates, and future projections. Section 3.3 of EPRI 1003058 (Life Cycle Management Planning Sourcebooks – Overview Report) [Ref. 1] provides guidance on collecting plant-specific information about the current maintenance plan, both in terms of technical scope and economics. The guidance for system engineer interviews and the data detailed in Table 3-7 (Listing of Typical Plant Programs) and Table 3-8 (Inventory of SSC Maintenance Tasks and Parameters) will be particularly helpful in the review. Section 3.8 of EPRI 1003058 details the collection of data for economic evaluations, which is the fulcrum of any plant-specific LCM plan. Often, the most difficult data to quantify are the costs associated with various plant maintenance activities.

5.3

Current Condition and Performance Performan ce Assessment

The generic performance data and information presented in the preceding sections can be used for plant-specific LCM planning in several ways. In particular, for plants without a large basis of experience, the generic data provides a basis for a sound component-specific PM program. The data may also be used for comparison trending or performance projection when attempting longterm LCM planning. The data may also be used to evaluate future degradation and/or failure estimations. The steps involved in plant-specific performance and condition assessment (including benchmarking) can be summarized as follows: •

At the system level, benchmark the feedwater heater contribution to the total plant lost power generation against the BWR/PWR specific average (Table 4-1). This will provide a preliminary assessment as to the current and past plant system and component health and will indicate if the plant feedwater heaters perform at, above, or below the present industry standards with respect to lost power generation (and any associated impact to plant safety). The results of this benchmarking provide a basis for projecting future trends (negative or positive) in the LCM planning for the feedwater heaters. This step also allows for the 5-11


establishment of a plant-specific failure rate (which may be used in future LCM planning documents). •

•

•

•

•

•

•

At the component level (for an individual feedwater heater and its sub-components), a review should be conducted of all the plant transients, power reductions, and reactor scrams since plant operation began. This review should focus on the cause of the event, the principal systems or components involved, and whether the feedwater heaters were a direct or indirect contributor to the event. In many transients, the feedwater heaters are affected even if they are not involved in the root cause, and their response to the event should be noted and understood. At the component level, compare the plant-specific failure rates with those discussed in Section 4.1 of this sourcebook. If this is not possible, review in detail the previous 10 years of feedwater heater corrective action work orders to develop the plant-specific failure history. Compare the EPRI SYSMON data for feedwater heaters (discussed in Section 4.1.3 of this sourcebook) with the plant-specific parameters established under the Maintenance Rule to verify that the plant parameters are reasonable and representative and appropriate for the feedwater heaters. The SYSMON goal is no more than 3% unplanned capacity loss or no more than three (3) unplanned capacity loss events with greater than or equal to a 20% power reduction over a 24-month period, and also no common cause unplanned scrams and no more than two (2) scrams over a 24-month period [Ref. 38]. Compare plant-specific feedwater heater maintenance tasks against the industry recommendations, shown in Table 4-3 of this sourcebook, in the tables found in Section 4.3 of EPRI 1003470 [Ref. 11], and also in Tables 6-3 and 6-4 of EPRI 1003470 to identify opportunities for addition or deletion of PM or PdM activities and any adjustments to task intervals. Evaluate any design-related deficiencies (such as shield plate geometry problems leading to erosion) causing plant-specific issues with the feedwater heaters. Compare plant-specific failures (i.e., their causes) to the generic failures and causes discussed in Section 4 to identify problems not yet experienced at the plant. Evaluate plant-specific maintenance practices with respect to industry experience and standards to identify opportunities for enhancements.

EPRI has issued a number of technical reports that address feedwater heater assessment. These reports provide guidance in performing maintenance, and also in evaluating component conditions. For example, EPRI 1003470 ( Feedwater Heater Maintenance Guide) outlines feedwater heater performance, troubleshooting, failure modes, condition-based maintenance, repairs, and replacements. EPRI NP-7552 ( Heat Exchanger Performance Monitoring Guidelines) provides discussions of heat exchanger selection criteria, a description of the effects of fouling, a description of performance monitoring methods, and an analysis of the results of heat exchanger performance monitoring. EPRI NP-4057 ( Nuclear Plant Feedwater Heater Handbook ) provides 3 volumes which address feedwater heater design & operation, feedwater heater specifications, and feedwater heater maintenance. EPRI TR-106741 ( Heat Exchangers: An Overview of Maintenance and Operations) provides an overview of heat exchanger performance, monitoring, and repair. Finally, EPRI GS-6913 ( Feedwater Heaters: Replacement

5-12


Specification Guidelines) provides improved guidance for the development of specifications for the procurement of replacement feedwater heaters.

5.4

Condition Monitoring Technologies, Technologies, Tests, and Diagnostics

Condition monitoring for individual components and performance monitoring for the FW heaters as a whole are critical processes to maintain heater reliability at the highest levels. Performance monitoring will include an overview of the FW heaters as a whole and also is part of the plant’s thermal performance monitoring program.

5.4.1 System Condition Monitoring System condition monitoring consists of observing the system parameters and trends during operation and comparing this to typical operation to determine if any degradation of performance is occurring. Levels, pressures, and temperatures are monitored continuously for the feedwater heaters and alarms are experienced during process excursions. In digital control systems, operating parameters can be trended using statistical process control techniques and system performance can be predicted. In older analog systems, external recording devices are used to record the system parameter values (responses) and to develop baselines for comparing performance trends. These methods can be used to predict future performance and system degradation, allowing corrective maintenance to be performed prior to critical situations. The FW heaters play an important role in the overall thermal performance of the plant. The methods mentioned earlier (in Sections 4.3 and 5.1.2) for FW heater performance monitoring include the TTD (terminal temperature difference) difference ) and the DCA (drain cooler approach). Also of importance is the TR (temperature rise) across the FW heater. To reiterate, the terms are defined as follows: •

•

•

The TTD of a FW heater is the difference between the temperature of the extraction steam o and the temperature of the FW outlet. The design TTD for most plants varies from 5 F to o 8 F. A high TTD indicates that the FW heater thermal performance is deficient. The DCA of a FW heater is the difference between the temperature of the subcooled condensate leaving the heater and the temperature of the feedwater entering the heater. It can be a direct measure of the heat transfer capability of the heater. For most plants, the design o DCA is typically about 15 F. Also, the TR across the FW heater (the difference between the feedwater inlet and outlet temperatures) gives an indication of how well the heater is operating.

The data can be assembled in tabular form to provide the FW heater response to changes in these parameters. Compared to the design values for the FW heater, increases in TTD, decreases in TR, and changes in DCA are indications of one or more problems.

5-13


Table 5-1 Feedwater Heater Responses (to plant changes) – from EPRI TR-107422-V1 Typical Responses to Plant Power Changes Power Change

∆T

TTD

DCA

Power Increase

Increase

Increase

Increase

Power Decrease

Decrease

Decrease

Decrease

Typical Responses to FW Heater Problems

Problem

∆T

TTD

DCA

Inadequate Venting

Decrease

Increase

Decrease

Level Increase

Decrease

Increase

Decrease

Level Decrease

Increase

Decrease

Increase

Tube Fouling

Decrease

Increase

Increase

Tube Leak

Decrease

Increase

Decrease

High FW Flow

Decrease

Increase

Increase

Plugged Tubes

Decrease

Increase

Increase

Trending of these variables allows for the development of a historical “health” report for each FW heater, and is an important part of a successful preventive maintenance program. EPRI has developed a model to assist with evaluation of overall thermal performance, the Nuclear Thermal Performance Advisor (NTPA). The NTPA is a PC-based program based on expert system technology and on the EPRI Thermal Performance Diagnostic Manual. The NTPA is an interactive heat rate diagnostic system that includes nine plant diagnostic areas: •

Condenser

•

High-Pressure FW Heaters

•

High-Pressure Turbine

•

Low-Pressure FW Heaters

•

Low-Pressure Turbine

•

Main Steam System

•

Moisture Separator Reheaters

•

Nuclear Steam Supply System

•

Turbine Control Valves

5-14


5.2.4 Component Condition Monitoring Individual components within a system may also be monitored using predictive technology. For the feedwater heaters, a survey of applicable component condition monitoring techniques was performed, and the following programs/activities were determined to be of potential use. For detailed information, review EPRI 1003470 ( Feedwater Heater Maintenance Guide) [Ref. 11], EPRI TR-106741 ( Heat Exchangers: An Overview of Maintenance and Operations) [Ref. 15], EPRI NP-7552 ( Heat Exchanger Performance Monitoring Guidelines) [Ref. 16], and EPRI GS6935 (Feedwater Heaters Maintenance and Repair Technology) [Ref. 17]. Much of this information has already been addressed earlier in this Section (in 5.1.2), where specific methods of condition monitoring are detailed. The following paragraphs outline the process in a broader fashion. The first step in the evaluation of condition monitoring data is to have a template (or structure) which makes the information available for proper interpretation. This involves the outline of troubleshooting methodology in response to problems or changes in the FW heaters. The following table presents a template for troubleshooting FW heater problems. Table 5-2 Troubleshooting Feedwater Heater Problems (from EPRI 1003470) Problem Experienced

Temperature rise of FW gradually decreases Pressure drop across FW heater gradually increases Temperature rise of FW is less than specified Temperature in shell-side condensate is less than specified

Area of Investigation

Possible Solutions

Fouled heat transfer surfaces.

Check the water chemistry.

Deposits of scale or corrosion on the interior surfaces of the tubes.

Clean the tubes.

Air-binding non-condensable gases are accumulating in the steam space of the FW heater. The gland seals at the lowpressure end of the turbine may be operating incorrectly.

Avoid contamination. Check the vents. Check the gland seals. For heaters operating at vacuum, check all joints.

Vent lines are cool Temperature rise of FW is less than specified

FW flow is bypassing.

Check pass partition welds. Check diaphragms and gaskets.

Pressure drop across the heater decreases Shell floods with condensate with outlet control valve fully open

A possible leak at the partition plate allows water to bypass the tubes. Ruptured tube or a leak between the FW side and the steam space of the heater.

Condensate level is unstable Level is erratic and surges Water Hammer Vibration

Control devices are not working properly or the back-pressure in the drain line is too high. The FW heater is overloaded, the temperatures are too high, or the flow rates are causing pulsations in system piping.

Check for erosion. Shut down FW heater to avoid water backing into extraction steam lines. Repair any leaks. Repair the control devices. Reduce back-pressure. Check all parameters (flow rates, pressure, temperature) against design specifications.

5-15


Once a problem has been identified and checked, then the cause(s) need to be determined. This involves a review and a comparison of the specific problem event to known failure modes, which would include the previously identified aging mechanisms (and effects). A summary of FW heater problems and a flowchart of possible causes are listed in the following figure.

Figure 5-2 Summary of FW Heater Problems and Possible Causes (from EPRI 1003470)

5-16


EPRI NP-7552 [Ref. 16] provides a detailed summary of heat exchanger performance monitoring methods.

5.5

Water Chemistry and Corrosion

EPRI NP-4057 [Ref. 23] contains a discussion on water chemistry for feedwater heaters. The discussion references EPRI CS-3184 ( Corrosion-Related Failure in Feedwater Heaters) [Ref. 25] for additional technical information. In conventional nuclear plants, the fluid under consideration is demineralized water in various o o phases from liquid to slightly superheated steam with a temperature range of 100 F to 550 F o o (38 C to 288 C). The pressure range is from almost full vacuum to 2000 psig (13,993 kPa). Some of the impurities present in feedwater are calcium and magnesium salts, totally dissolved and suspended solids, as well as corrosion products and contaminants. Silica, aluminum, iron, and copper deposits are commonly found, as we as residuals of oxygen, hydrazine, ammonia, and hydroxide. For BWRs, ammonia, hydrazine, and hydroxide are not likely to be present, but oxygen levels are likely to be substantially higher than in PWRs, and free hydrogen is also present. There are two basic treatments used to control feedwater chemistry. PWRs now use allvolatile treatement while BWRs rely upon high purity, demineralized water. The water chemistry factors which influence feedwater heater tube corrosion are steam contaminants (e.g., chloride ions, halide salts, and dissolved oxygen). Other critical factors are fluid temperature and pH achieved by adjustment with ammonia. Systems with all-steel and stainless steel components generally run with a pH between 9.2 and 9.8 to minimize carbon steel corrosion, thereby limiting the entrainment of corrosion products in the condensate. Problems are encountered with mixed metal systems containing both copper and carbon steel alloys. These systems are generally run at a pH of between 8.8 and 9.2 which is not optimum for the control of corrosion in either material, but represents a useful compromise. Oxygen contributes to uniform corrosion, stress corrosion cracking, intergranular corrosion (particularly exfoliation), and inlet-end erosion in PWR units. Steam surface condensers reduce oxygen levels to 7 ppb or less, but steam surface condensers are only designed to maintain this oxygen removal at full load and with minimal air in-leakage. High concentrations of oxygen can occur at low loads and during outages. Temperature levels are critical to various tube materials because of their inherent material properties. There are many problems and inadequacies with the metals used in feedwater heaters and none of the materials is totally immune to corrosion. The nature and aggressiveness of the environment determines the degree and type of corrosion. All of the following major categories of corrosion have been experienced in feedwater heater systems: •

Uniform corrosion (general attack) – copper alloys

•

Crevice corrosion

•

Pitting

•

Intergranular corrosion

5-17


•

Selective leaching or parting (copper alloys)

•

Erosion-corrosion

•

Stress corrosion (copper alloys)

It is important to recognize that the water chemistry can vary considerably throughout the feedwater system. Variations in pH from 7.2 to 10.4 have been noted. For this reason, the whole system should be monitored as necessary to protect the feedwater heaters. For further information, a much more complete evaluation of water chemistry issues is found in EPRI TR-102134-R5 ( PWR Secondary Water Chemistry Guidelines) [Ref. 12]. Also, as previously mentioned, additional detail about corrosion problems in feedwater heaters may be found in EPRI CS-3184 ( Corrosion-Related Failures in Feedwater Heaters) [Ref. 25]. This report evaluates feedwater heater corrosion problems on a materials basis, with separate sections for carbon steel, admiralty brass, copper-nickel alloys, Monel alloy 400, type 304 stainless steel, and titanium. There is also a detailed discussion of water chemistry in power plants. In developing LCM plans for feedwater heaters, it is recommended that the system or component engineer be familiar with the specific plant specifications with respect to secondaryside water chemistry. A past history of corrosion problems in the feedwater heaters (and in the secondary-side piping, valves, etc.) may indicate a weakness in the plant chemistry program.

5.6

Feedwater Heater Repairs (Corrective Maintenance)

Making timely and appropriate repairs to feedwater heaters is very important because components cannot always be replaced (and should not be) when problems are first identified. Part of a successful surveillance and maintenance program is the implementation of proper repairs to damaged or degraded equipment. Feedwater heaters operate in stressful environments and will show evidence of this stress as the plant ages, even if all the sub-components function properly. EPRI 1003470 [Ref. 11] contains a section that addresses feedwater heater repair (Section 7). The items addressed include the following: •

Tube Leak Detection Methods

•

Tube Repairs

•

Tube-to-Tubesheet Repairs

•

Diaphragm, Pass Partition, Channels, and Covers

•

Miscellaneous Repairs

•

Spare Parts

Each of these items (and also shell repair) is briefly discussed in the following paragraphs. For detailed information, refer to EPRI 1003470 [Ref. 11], and also EPRI TR-106741 [Ref. 15], which addresses general heat exchanger repair.

5-18


5.6.1 Tube Leak Detection Methods To detect feedwater heater leaks, plant operators commonly monitor heater water levels, the terminal temperature differences between the water and the steam, and stem positions on level control valves. However, given the volume of water passing through the feedwater heaters, leaks must be quite large before becoming observable in this fashion. Leaks of this magnitude will affect plant efficiency and can damage adjacent tubes. Once a tube leak is detected, determining which heater in a train is leaking can be difficult. The right heater must be identified so that it may be taken off-line with a minimal impact to the plant as a whole. Tube leak detection is discussed in detail in Section 5.1.2, with the common methods listed. The following methods may be used to detect tube leaks: •

Detailed Operations Indications

•

Acoustic Leak Detection (on-line and off-line)

•

Shell/Tube Pressurization (off-line)

•

Eddy Current Testing off-line (see Section 5.1.2 for detail)

The use of detailed operational indications involves marking the stem positions of the normal drain valve during each shift. This method can be enhanced by developing a valve flow vs. stem position calibration curve. Indications of leakage in the feedwater heater can also be found from an increased water level in the heater (which should be checked frequently), an increasing terminal temperature difference (TTD), the actuation of a high-level alarm, the opening of the high-level dump valve to the condenser, a discrepancy between the condensate and feedwater flow indicators, an increasing air demand signal for the air-operated control valves, and an excessive pressure drop across the integral drains cooler. Acoustic leak detection is possible because when a leak develops in a pressurized heater tube, the turbulence associated with the water escaping from the leak generates pressure disturbances in the surrounding medium (i.e., the shell). These disturbances (or sound waves) are propagated through the shell-side medium and may be detected by the use of accelerometers or piezoelectric equipment (or by pressure transducers). Both low-frequency (2-20 kHz) and high-frequency (20-175 kHz) detection monitoring is used, with the high-frequency equipment being more effective and reliable. Manufacturers have developed dual-frequency sensors because the lowfrequency data is also helpful. Shell pressurization is widely used in leak detection, and is discussed in detail in Section 5.1.2. Identifying the exact location of a tube leak within the tube can be difficult; however, the location is important in determining the probable cause of the failure. In U-tube feedwater heaters, the following details should be determined and documented: the pass in which the failure occurred, the distance from the failure location to the tubesheet, and the overall length of the failure (if possible).

5-19


5.6.2 Tube Repair Tube repair includes several options: tube plugging, tube sleeving, tube pulling, and plug removal. Time constraints influence how a leaking tube is repaired. For example, a tube may be plugged with a temporary tube plug and some adjacent tubes may also be plugged (a practice known as ‘insurance’ plugging, although this is performed less currently than in previous years)) and then at the next planned outage, the temporary plugs can be removed and the actual condition can be determined with eddy current testing. Only the defective tubes would then be plugged with a permanent plug. Plug designs may be categorized as follows: •

Hammer-in taper-type plug

•

Elastomeric plug

•

Mechanical-type plug

•

Welded-type plug

The hammer-in taper plugs are relatively easy to install, are inexpensive, and can be removed with a pipe wrench or by drilling them out. The plug material must be compatible with the tube to be plugged and should be softer than the tube material. The design allows for only limited contact between the plug and the tube wall. Hammer-in plugs are typically limited to use on LP FW heaters. Mechanical seal plugs typically contain jaw segments and an O-ring which allow for greater sealing capability. The plugs are driven into the tube with a driving nut and an Allen wrench. They can be installed fairly easily and give reliable performance in both LP and HP applications. They are more expensive than hammer-type plugs. Welded plugs can be either seal-welded in the tube or explosively expanded into the tube. Seal welding involves a mechanical plug (tapered or hollow) that is inserted into the tube like the hammer-in method and is then welded to the tubesheet for a permanent seal. Seal welding is difficult at times due to the heater orientation and because the surfaces need to be very clean to build a good weld. Explosive welding involves the insertion of a thimble plug (with a small detonator) into the tube. The plug then explodes against the tube, forming a seal across the entire length of the plug. Explosive welding has matured as a plugging technique, so that safe and reliable bonds can now be made without damage to the tubesheet ligaments. Explosive plugging offers greater integrity and better long-term reliability than other techniques, it is relatively easy to perform (even in awkward orientations), it can be less expensive than regular seal welding, and minimum preparation is needed. Its limitations include the need for licensed personnel to perform the work and the permanency of the plugs (they are typically pure nickel and are difficult to drill out). Each plugging technique has its limitations and special considerations for where it might be used. If permanent repairs cannot be decided upon and made in a timely fashion, it is better to use a high integrity temporary plug instead.

5-20


Complete detail on the various tube plug schemes, along with an outline for making the decisions and proper planning necessary to successfully plug tubes, may be found in EPRI 1003470, Section 7 [Ref. 11]. In addition to plugging, the section also discusses tube stabilizing (to prevent any future damage to adjacent tubes. The section also addresses tube failure reporting and the maintenance of tube plugging records. EPRI GS-6935, Section 9 [Ref. 17] also addresses tube plugging in detail, with an emphasis on excellent procedural control and a good description of the plugging techniques and their selection criteria. As a part of license renewal programs (and refurbishments), some utilities are reviewing their maintenance histories and considering feedwater heater restoration by decreasing the number of tubes that have been plugged. For example, non-defective tubes that were plugged for insurance purposes can be examined and then returned to service if undamaged. Also, some leaks at the tube-to-tubesheet joint can be sleeved and the tube returned to service. Comprehensive testing (i.e., a hydrotest) is needed to verify that restoration is a viable option, with a post-maintenance test to certify the work.

5.6.3 Tube Sleeving Tube sleeves can be used to cover damaged inlet tube ends and to seal a tube leak at almost any point along the tube. Often, permanent repairs can be made without losing the use of the tube. Tube sleeving involves the following considerations: •

Tube sleeving can be the optimum repair for inlet end erosion

•

Tube sleeving can restore a leaking tube to operation

•

Tube sleeving can strengthen tubes at the supports (where vibration and thinning occur)

•

Tube sleeving requires detailed advance planning for the vendor and the tools needed

•

Tube sleeving requires skilled installation

Sleeves can be installed by mechanical expansion, explosive welding techniques, or hydraulic expansion. For tube defects located near the tube end, a roller-expanded method may be used. Complete detail on tube sleeving is available in EPRI 1003470, Section 7 [Ref. 11]. This report also contains a few detailed examples of repairs made at nuclear plants.

5.6.4 Tubesheet and Tubesheet Joints When the pressure on one side of the tubesheet is considerably higher than the pressure on the other side, leaks will occur in the smallest leak paths. The consequent erosion, which resembles the holes bored by worms, is sometimes called wormholing (or wire-drawing). Wormholing can occur in the following locations: tube-to-tubesheet welds, the tube and tube hole surfaces, and the ligaments between the tube holes and the tubesheet. If the tubes are more resistant to abrasion than the tubesheet, the ligaments can be severely damaged, even to the point of rendering the tubesheet unsafe. When an inspection discloses the washing away of the tube-to-tubesheet welds of if there has been leakage from the high-pressure side to the low-pressure side through the joint, then it is 5-21


probable that there is wire-drawing damage to the ligament. This possibility is one reason for the full-depth expansion of tubes into the tubesheet holes. Eroded tube-to-tubesheet welds should always be examined to determine if cracks or any porosity has been exposed. The metal surface should be removed to the bottom of the crack or the porosity, and then the space can be repaired via welding. If the wormholing has progressed into the ligaments, then the damaged areas need to be ground out. Most of tube should be removed from the tubesheet (leaving enough to anchor the tube in place). Weld metal is then applied to the tubesheet until the original thickness is restored. In the course of performing eddy current or visual examinations of feedwater heaters, it is sometimes necessary to pull a tube from a tube bundle for further examination. EPRI 1003470 provides a description of how tubes are pulled from heaters. There are two types of tube-to-tubesheet joints used for feedwater heaters: expanded-only, and welded and expanded. Expanded-only tube joints are used for low-pressure feedwater heaters. An alloy weld overlay is generally not required for an expanded-only joint. High pressure feedwater heaters generally use a welded and expanded tube joint with an overlay on the face of the tubesheet to match the tubing material. Again, EPRI 1003470 presents a detailed overview of tube-to-tubesheet joint repairs (in Section 7.3). Repairs to the diaphragm, the pass partition, the channel covers, and other gasketed covers are detailed in Section 7.4 of EPRI 1003470 (Feedwater Heater Maintenance Guide).

5.6.5 Shell Repairs Shell repairs are made as the result of erosion or cracking or other problems discovered through non-destructive examination of the FW heater shell (or through visual inspection). Typical repairs include weld build-ups of wasted areas, repairs on major shell welds, and welding repairs made to expansion joints (if applicable). Weld repairs are also made for problems identified on nozzles, manways, and other shell connections (vents, etc.). Cracks in the shell (at the tubesheet joint, for example) may be ground out prior to welding repairs. Shell repairs are addressed in detail in EPRI 1003286 ( Repair Technology for Degraded Pressure Vessels and Heat Exchanger Shells: Repair and Replacement Applications Center Task 91) [Ref. 45]. This report assists plant personnel in making informed repair or replacement decisions that take into account the continued operation of the vessel, the consequences of possible outage time and loss of production, and the satisfaction of all applicable ASME Boiler & Pressure Vessel Code requirements. Heads, channels, and gasket joints need to be replaced in the same orientation in which they were removed. The gasket should be aligned just as it was prior to removal and the manufacturer’s directions need to be followed with respect to bolting torque, torque pattern, and the number of torque passes. The head (or flange) ought to seat the gasket, not crush it. A large amount of gasket material protruding from the edge of a flange joint indicates over-tightening. Lubricant must be applied to bolts prior to tightening. The bolts are then tightened at the specified torque and in a staggered pattern consistent with the manufacturer’s direction, with the proper number 5-22


of passes being used. EPRI TR-106741 [Ref. 15] addresses shell repairs and issues related to flanges and other shell features.

5.6.6 FW Heater Cleaning Cleaning is a corrective maintenance activity that is employed after significant degradation has been observed. It is not a preventive maintenance activity for FW heaters. Cleaning will not extend the life of the tubes directly, but will remove deposits from the internal surface. Cleaning should involve the following tasks: an evaluation of the type of degradation that has occurred, a determination of the appropriate cleaning method (providing the best results and limiting any potential damage to the heater), and a consideration of chemical cleaning as an option. Industry literature (see EPRI GS-6935, Appendix A) [Ref. 17] indicates that mechanical cleaning of heat exchanger tubes does not appreciably cause thinning of the tube wall.

5.6.7 Spare Parts The recommended spare parts for feedwater heaters are listed below. Table 5-3 Recommended Spart Parts for FW Heaters (from EPRI 1003470) Parts

Tube Plugs

Quantity

Comments

2% - 3% of tubes depending upon heater condition

Includes welding supplies if welded plugs are used

10% of total bolts

Includes both a bolt and a nut

Two sets

Includes gaskets for pass partition covers.

One set

Includes special welding supplies, if necessary

One set of roll expansion tools for each tube diameter and gauge

Drivers and spare rolls optional

Bolting:

Manway cover, Channel cover, Pass Partition cover Gaskets Diaphragm Tube Expanders

Accessories (gauge glass, control valve, temp. and press. instrumentation)

------

As recommended by the manufacturer

The decision to replace a feedwater heater (or to re-tube or rebundle the heater) is explored in Section 8 of EPRI 1003470. A materials list is also presented that details material specifications and plant environment. It is noted that, in general, feedwater heaters are designed with approximately 5% excess heat transfer surface area. Historically, a feedwater heater is 5-23


approaching a replacement or major overhaul decision point when 10% of the tubes are plugged [Ref. 23]. Decisions for run/repair/replacement and remaining life assessment are detailed in Section 8 of EPRI 1003470 [Ref. 11]. The run/repair/replacement decision is also addressed in Sections 7 and 8 of this LCM Sourcebook.

5.7

Heater Access

It is normal practice in the industry to obtain detailed design instructions from the manufacturer for the disassembly and reassembly of each heater. As detailed in EPRI 1003470 [Ref. 11], the typical information provided includes the following: •

•

Torque values and tightening pattern sequence, type of gasket material, gasket surface preparation, and the alignment of mating components for bolted designs Special care needed in removal and re-installation instructions for welded diaphragm configurations

•

Special welding requirements for torus ring closures

•

Handling instructions for the various manway covers

Most heaters a U-tube configurations and have both tube ends in the same channel separated by a partition plate. Many older heaters, particularly high-pressure hemi-head designs, provide inadequate clearances for maintenance in the channel area. In addition, some partitions were fully welded, which requires cutting and re-welding to gain access to both passes. Overall, the industry has moved away from such designs. Utilities should consider how much working clearances and accessibility impact the maintenance and testing for their feedwater heaters. Modifications which improve access and working clearances should be considered in conjunction with LCM planning, and accessibility should definitely be factored into any heater replacement decisions in the future. One possibility to consider is the installation of neck flange and bolted cover arrangements, to allow full access with easily removable manways. An additional recommendation (from EPRI NP-4057-V3 [Ref. 23]) is to substitute flanged fullaccess covers for the elliptical heads.

5-24

6

GENERIC AGING AND OBSOLESCENCE ASSESSMENT

This section addresses steps numbered 11B and 11C in the LCM planning flowchart (see Figure 2-1b). The intent is to help characterize the aging of passive SSCs, the wear-out of active components, and the obsolescence of SSCs. This discussion will serve both to help address the need for and the timing of any replacement of FW heaters in the LCM planning process, and to identify potential environmental or service conditions that affect the rate of degradation or that may require special plant-specific attention. It is noted that obsolescence is not considered for FW heaters because obsolescence is normally related to electronic components, instruments, analog equipment, and piece parts that are no longer commercially available . However, the aging mechanisms and effects for the FW heaters are tabulated and evaluated (but obsolescence is not listed). FW heaters and obsolescence are addressed in Section 6.3.

6.1

Aging Effects and Mechanisms Review (Aging Matrix)

Table 6-1 presents a brief overview of the aging mechanisms and effects (and the common plant programs for aging management) for FW heaters and their various materials. This table is not comprehensive, because it does not mention plant-specific programs or frequencies, but it is detailed enough to begin to develop an understanding of aging management for FW heaters. A plant-specific LCM plan will contain the details of each plant program. In terms of license renewal, FW heaters are usually excluded from the LR scope and are therefore not typically listed in aging management matrices (for various heat exchangers). However, an aging matrix for a FW heater will be similar to an aging matrix for other plant heat exchangers that are included in license renewal. Table 6-1 follows the format and information given in GALL report (NUREG-1801) [Ref. 29] for heat exchangers.

6-1

m a r g g c i n o r f i i g P c A t e p l n S a e r c m e i p e t y g a e T a H n / a t n M l a P ) s ( t c e f f E g n i g A

t n e m s s e s s A e c n e c s e l o s b O d n a g n i g A c i r e n e G

s t c e f f E & s m s i n a h c e M g n i g A – y r a m m u S r e 1 t 6 a e e H l b a W T F

e g a m a D e v i t a l u m u C

e ) u g s ( i t a m F s i l n a a r h m c e h e M T / g l n a i c g i n A a h c e M

, m a , r e g c o r n g P a , g n i n l n i t g e n t s o t r n i e t t s e T n i s T a o e e T e c C M c n e l y n a r i v a t n a s t l o l m i n i i t e r o m e c e v n v r f r h e r u u e C P F S P

l a i r e t a M f o s s o L

n o n i s o i r s o r o o r r C o / n n i C n o o o i s i s o g s n o r i o r r t r r o i t o C P E C c / & l i e n a r n c i o ) i e a v s C n l v I o e a e r C r A G G C M E F (

l ) a l i a l e r i e ) e c t t p S a y y o l l M t ( n o a b r w a l o C ( t r a P / t n e n o p m s o e C b u T

) e v o b a s a e m a s (


g n i t s e T e c n a m r o f r e P

c g i f i n i t c s e e p T S e r e c t n a a e l l i / H e t v r n a u l S P

g n i l u o F

e e g g a a m m a a D D e e v v i t i t a l a l u u m m u u C C

n o i t a t i p i c e r P , s e t a l u c r i t a r e a W P

e u g i t a n F o l i t a a r m r b e i h V T d / e l c a u c d i n n I a - h w c o e l F M

, m a , r e g c o , r n g g P a , n n i n l e g i t t o t n s r n i e t t s e T n i s T a o e e T e c C M c n e l y n a r i v a t n a s t l o l m i n i i t e r o m e c e v n v r f r h e r u u e C P F S P


n o n i s o i r s o r o o r r C o / C n o g i s n i o t r t i E P / & e n c i o ) i v s C e I o r C r A C M E F (



g n i t s e T e c n a m r o f r e P

g n i t s e T e c n a l l i e v r u S


g n g i n k i l u c r o a F C


) C C S ( g n i k c a r C n o i s o r r o C s s e r t S

n o i t a r b i V d e c u d n I w o l F


l e e t S ) y s l o s l e l a n h i g a i t S h (

s e b u T

2 6


3 6




l ) a l i a s y r i c l o l e t p A a y M t ( r e p p o C t r a P / t n e n o p m s o e C b u T



n o n i s o i r s o r o o r r C n o / o C n i o s g i s o n r o r i t r t o i E C P c / & i n n e c i o ) a i C v v l e C s a r I o r A G C M E F (



l a i r e t a M n s i e t e i g r n e a p h o r C P

e e g g a a m m a a D D e e v v i t i t a l a l u u m m u u C C




g n i h c a e L e v i t c e l e S

e u g i t a n F o l i t a a r m r b e i h V T d / e l c a u c d i n n I a - h w c o e l F M

n o n i s o i r s o r g o n o i r l r C o n o / h n i n o C o s o m i i s o g s r n o o r i o r r t r W r o i t o P E d C C c / & n l i e n a a r n c i o ) r i e a v v C s C a n l e a e r I o r A e G G C M E F ( W

g n i t s e t T s e e T c n s a s l l e i n e d v r r u a S H







) C C S ( g n i k c a r C n o i s o r r o C s s e r t S

l e e ) t S y o n l l o a b r w a o C l (

t e e h s e b u T

n o i t a t i p i c e r P , s e t a l u c i t r a P


g n i k c a r C n o i s o r r o C s s e r t S








n o n i s o i r s o r g o n o i r l r C o o / h n C n o o m i i s g s r n o o i o r t r W r i t E o C P / & d n l e n a a r i c ) o e v i C r a n e C s o e e r I r A F G C M E ( W

l l ) e a l i e a t r i c S ) e t p s y o a y s l l M t ( l e a n h i g a i t S h (

t r a P / t n e n t o e e p h m s o e C b u T




g n i l u o F

e g a m a D e v t g i a n i l k u c m a r u C C

n o i t a t i p i c e r P , s e t a l u c i t r a P

) C C e S ( u g g i t n i a k F c l a r a C m r e n h o T i s / o r r l a o i c C n a s h s c e r e t S M


n o d ( n g ) c s i a i f i n e s r c a c e n t o f o i r p i t n u s S c o l e M a r e p l n t s a r a n r e e I u t / H l t a c x e t n u u r e l s t a i h P V S t





n o n i s o i r s o r o o r r C o / n n i C n o o o i s i s o g s n o r i o r r t r r o i t o C P E C c / & l i e n a r n c i o ) i e a v s C n l v I o e a e r C r A G G C M E F (

e u g i t a F l a m r e h T / l a c i n a h c e M

n o n i s o i o s r r o o r r C o / n n i C n o o o i s i s o g s n o r i o r r t r o i t r o C P E C c / & l i e n a r n c i o ) i e a v s C n l v e a e r o r A G G C E F (

l e e ) t S y o n l l o a b r w a o C l (

l e e t S n o b r a C

s l a n r e t n I / s e l z z o N / l l e h S

& d a t e e a H l l P e r n e n i a d v h i C D

4 6

Generic Aging and Obsolescence Assessment

General Notes: 1) Mechanical / Thermal Fatigue is dependent on the number of cycles (> 7000 is of concern) at o o system temperature > 220 F (carbon steel) and > 270 F (stainless steel) 2) Galvanic corrosion is significant when materials used are not close together on the galvanic chart (in the presence of an electrolyte and where sacrificial anodes are not used) – typically not of concern for FW heaters 3) Crevice/Pitting corrosion may exist in stagnant or low f low areas where bulk fluid contaminants may concentrate 4) MIC (microbiologically-induced corrosion) can attack any material but is s ignificant for treated water only if a biocide is not used (not an issue for FW heaters, which use boiler-quality water), although layup conditions should be monitored 5) Wear may occur at interface joints due to some relative motion from flow-induced vibration and/or thermal expansion 6) Particulates, precipitation, and MIC are plausible if the supply originates at the bottom of a tank or reservoir and no biocide is used (does not apply for FW heaters) o

7) The threshold temperature for stress-corrosion cracking in stainless steel is 140 F in the presence o of bulk fluid halides or sulfates and 200 F in the presence of O2 8) Copper alloys that have less than 15% zinc or 8% aluminum (aluminum bronze) are susceptible to crevice/pitting corrosion as well as selective leaching (if not inhibited, such as admiralty brass containing >1% tin)

Table 6-1 is typical for aging mechanism evaluation performed for license renewal. Table 4-4, while similar, is more directed towards degradation and failure. An aging mechanism review is at the heart of LCM maintenance planning. There are five (5) documents pertinent to such a review for feedwater heaters: The Aging Management Guideline for Commercial Nuclear Power Plants – Heat Exchangers (SAND93-7070) [Ref. 7], Standard Review Plan for the Review of License Renewal Applications for Nuclear Power Plants (NUREG-1800) [Ref. 28], The Generic Aging Lessons Learned (GALL) (GALL) report (NUREG-1801) [Ref. 29], Nuclear Energy Institute (NEI) Guidance Document NEI 95-10 [Ref. 42], and EPRI 1003056 ( Non-Class 1 Mechanical Implementation Guideline and Mechanical Tools, Re vision 3) [Ref. 43]. This last reference was developed to provide guidance on implementing the requirements of license renewal for applicable mechanical SSCs. With respect to License Renewal, feedwater heaters are passive but they do not require aging management evaluation because they are typically scoped out of consideration. Heat exchangers that are within the scope of license renewal are evaluated for their pressure boundaries in the tubes and shells. The aging mechanisms and effects for these heat exchangers are monitored and controlled by existing plant programs, as detailed in the License Renewal Application and the technical aging management review document. The NEI guidance document can be used as a starting point in the aging management review of the feedwater heaters (by reviewing the heat exchanger group with plant-specific data). The GALL report (NUREG-1801) [Ref. 29] outlines the required aging management programs for heat exchangers. Passive SSCs susceptible to aging are identified together with typical materials, environments, and aging mechanisms and effects. The active components of the heat 6-5


exchangers are excluded from aging management review (they are addressed under the performance monitoring required by the Maintenance Rule under 10CFR50.65). EPRI 1003056 [Ref. 43] provides details on how to implement license renewal programs (surveillance, maintenance, procedures, inspections) for in-scope mechanical equipment.

6.2

Expected Lifetimes of Feedwater Heaters

A new feedwater heater today would be expected to last 20 years or more (the lifetime of the materials in the tubes); however, that has not been the case in the past (due to age-related degradation, specific design problems, material problems, etc.). Section 4 of this document provides an overview of FW heater failures and problems as identified in various industry databases. The expected lifetime is based upon the expected lifetimes of the various materials in the FW heater. The tubes are the most sensitive sub-components and will usually be the limiting factor in overall heater lifetime. Shell thinning due to erosion is also a factor, although good surveillance programs should identify shell thinning before the problem is serious (see Section 5.1.2). But it is worth noting that 38% of the items involving feedwater heaters from the INPO Plant Events database were discoveries of shell thinning. From NPRDS, the percentage of items involving the shell was about 4%; however catastrophic shell failures can impact a plant for months, due to FW heater isolation, possible power reductions, potential damage and personnel injury, and root cause investigations. As noted in Section 5.6, a FW heater is approaching time for retubing, rebundling, or complete replacement when approximately 10% of the tubes have been plugged [Refs. 11 and 23].

6.3

Technical Obsolescence

Many systems in nuclear power plants (particularly those with electronic instrumentation) are susceptible to technical obsolescence. For LCM maintenance planning, these systems or components may have to be replaced or modified to account for the unavailability of spare parts. In these cases, the likelihood and timing of the need for replacement will be determined by the failure (or degradation) rate of the equipment. The feasibility and cost of reverse engineering the obsolete components also needs to be considered. For feedwater heaters, obsolescence is not a concern because they are strictly mechanical components which can be repaired over long periods of time and replacements can be fabricated (if necessary), and although they are subject to aging, they are not expected to cause any operational vulnerabilities based on the unavailability of replacement components. The obsolescence of the feedwater heater controls (which is an important issue) is addressed in EPRI 1007425 [Ref. 6]. To ascertain whether a given system or component is susceptible to technical obsolescence, the evaluation methodology provided in the EPRI LCM Sourcebook Overview [Ref. 1] is utilized (as a first step). Table 6-2 provides for the weighting of the various issues with obsolescence. The ranking guidance for the table is presented below, along with a short evaluation of the scoring for feedwater heaters.

6-6


Table 6-2 Application of Obsolescence Evaluation Criteria Technical Obsolescence Evaluation Criteria

Score

1.

Is the SSC still being manufactured and will it be available for at least the next nex t five years?

5.0

2.

Is there more than one supplier for the SSC for the foreseeable future?

3.0

3.

Can the plant or outside suppliers manufacture the SSC in a reasonable time (i.e., within a refueling outage)?

3.0

4.

Are there other sources or contingencies (from other plants, shared inventory, stock-piled parts, refurbishments, secondary suppliers, imitation parts, commercial dedications, etc.) available in case of emergency?

3.0

Is the SSC frequency of failure per year times the number of SSCs in the plant times the remaining operating life (in years) equal to or lower than the number of stocked SSCs in the warehouse?

3.0

6.

Can the spare parts inventory be maintained for at l east the next five years?

3.0

7.

Is the SSC immune to significant aging degradation?

1.0

8.

Can new designs, technology, and concepts be readily integrated with the existing configuration (hardwaresoftware, digital-analog, solid-state, miniaturized electronics, smart components, etc.)?

3.0

Is a technical upgrade desirable, commensurate with safety, and cost-effective?

3.0

5.

9.

Total Obsolescence Score:

Yes 5 3

3

3

3 17

Ranking Guidance for Table 6-2 •

•

•

If the total score is < 6 (RED), then the SSC obsolescence is serious and will have an immediate impact on LCM planning. Potential options to address obsolescence and contingency planning should be identified. If the total score is between 6 and 10 (YELLOW), the SSC may have long-term concerns with obsolescence. If the total score is >10 (GREEN), then obsolescence is not of concern for the SSC.

The estimated score (on a generic industry basis) for feedwater heaters is greater than 10, with firm ‘YES’ answers for questions 1, 2, 4, 6 (for smaller sub-components), and 9. Question 5 is not really applicable because plants do not keep spare feedwater heaters in stock (although repair parts for sub-components are stocked). Therefore, obsolescence is not an issue for feedwater heaters. Some plants may need to address issues with with procurement and test equipment 6-7


availability on a going-forward basis, especially with respect to the potential for license renewal (an extended 20 years of plant operation). For the feedwater heater controls, EPRI 1007425 estimated that for most instrumentation, the total score will probably be below 6.

6.4

Expected Lifetimes for Preventive Replacement or Redesign

EPRI 1003470 [Ref. 11] addresses component lifetimes on a material basis in Section 8. Early heater tubing materials (the tubes are the most sensitive component) were admiralty brass, copper, and copper-nickel alloys. These materials have high thermal conductivity and some corrosion resistance. In the 1960s, as fossil units grew in size and supercritical units were built, carbon steel and Monel (70/30 nickel copper) became popular for high-pressure heater applications. The emergence of copper deposits in boilers and on turbine blades forced the industry to move towards stainless steel and carbon steel materials. In the nuclear industry, stainless steel tubes began to replace all copper alloys in feedwater heaters. The most common types of tube materials are 304-SS (TP304N), ASME SA286-TP349, AL-6XN, and Seacure. The most recent tubing material introduced is ASME SA213 T-22. Type 439 SS (ferritic) is also being selected more for FW heater tubes. EPRI 1003470 provides a comparison for these materials and feedwater heater lifetimes. The average life expectancy for a carbon steel tubed heater is approximately 12 years. The average life expectancy for a T-22 tubed heater is 15 to 20 years. The average life expectancy for a 304-SS tubed heater is 20 years. The expected lifetimes of other heater components (shell, impingement plates, supports, etc.) is dependent on the maintenance and surveillance performed on the heater (and sometimes on the design). For example, impingement plates which are installed as completely perpendicular to the direction of steam flow will not last as long as plates which are curved or are installed at an angle. The lifetime of the tubes is also dependent upon the quality and the spacing of the tube supports (vibration problems from poor support design are a common issue). Likewise, design problems which cause flashing in the drains cooler zone will lead to rapid tube damage and also erosion of supports and the shell itself. Detailed material evaluations and comparisons may also be found in EPRI GS-6913 [Ref. 20]. The values given are indicative and are for planning purposes only. Depending upon the effectiveness of each plant’s maintenance and condition monitoring programs (along with the water chemistry program), these values could vary widely.

6-8

7

GENERIC ALTERNATIVE LCM PLANS

This section addresses Steps 12 to 17 in the LCM planning flowchart (Figure 2-2). The EPRI LCM Demonstration Project Report [Ref. 2] summarizes alternative LCM plans as follows: “Following the assessment of aging and reliability, potential alternative LCM plans should be identified. The objective here should be to explore whether there are potentially better ways of addressing the aging management of the SSC. These inputs can come from plant staff, but input should also be solicited from outside experts and industry benchmarking projects.”

The following guidance for these steps includes the identification of possible plant operating life strategies and the development of alternative LCM plans that are compatible with or integral to the strategies identified. Also provided is a hypothetical illustration of alternative plans for feedwater heaters, with the attendant discussions of the logic used to build the alternatives along with the derivatives of the assumptions. With an expected FW lifetime of approximately 20 years (less than the overall plant lifetime), utilities will need to plan for several FW heater replacements for the duration of plant operation.

7.1

Plant Operating Strategies and Types of LCM Planning Alternatives

The determination of LCM planning alternatives will be driven to a large extent by the plant operating strategies that (implicitly or explicitly) are being followed or evaluated, and by the current reliability of the FW heaters. Accordingly, the set of LCM planning alternatives to be evaluated is quite plant-specific. Typical plant operating strategies and standard approaches to LCM planning alternatives are presented and discussed below.

7.1.1 Plant Strategy 1: Operate plant for currently licensed period of 40 years This strategy requires minimizing risk during the remaining operating period until the plant’s license expires, and identifying the limiting SSCs which could result in premature power reduction or equipment replacements possibly forcing an economic decision regarding early decommissioning. Feedwater heaters would certainly qualify as a limiting component, given their importance to power production and their influence on the plant’s overall heat rate. LCM plan alternatives under this strategy include: •

LCM Plan Alternative 1A: A base case to determine the cost of the activities performed under the current maintenance plan, assuming that the activities will continue as-is until the

7-1

Generic Alternative LCM Plans

end of the licensed plant life. This case also assumes the continuation of the existing maintenance program without any major capital investments, unless absolutely necessary. •

•

•

LCM Plan Alternative 1B: An alternative plan in which the current maintenance plan is optimized and low-cost activities are added to provide improvements in reliability. LCM Plan Alternative 1C: An alternative plan in which the current maintenance plan is optimized and an aggressive PM program is implemented to reduce equipment failures, lost power production, and regulatory risk. LCM Plan Alternative 1D: An alternative plan in which the current maintenance plan is optimized and older components are replaced with more reliable equipment. For the feedwater heaters, this could include new tubes, new shield plates, and other internal subcomponent replacements. In the most extreme situation, it would involve an entirely new feedwater heater.

7.1.2 Plant Strategy 2: Operate plant for 60 years with license renewal This strategy recognizes the potential for license renewal and extended operation of the plant. Major investments will be required to achieve extended operation. These investments can only be justified by the additional revenue generated in the period of extended operation (20 years). LCM planning alternatives considered for feedwater heaters under this strategy should include the following: •

•

LCM Plan Alternative 2A: A rigorous preparation for license renewal with an aggressive aging management program, system/component performance enhancements, and timely component replacements and/or upgrades. This LCM plan includes replacement of feedwater heater components in a timely manner. This can even include replacement of the entire FW heater. LCM Plan Alternative 2B: Preparing for license renewal with an aggressive PM and PdM program, but delaying plans for major capital improvements until the actual extended license is implemented (i.e., year 39 of plant life). This alternative involves the position that the expected lifetime for a feedwater heater is at least 20 years (with proper maintenance and surveillance). This plan could include consideration of cleaning the FW heater tubes.

Every plant should prepare a base case for 60 years of operation (to address the remaining lifetime). This will include actions to reach the limit of 60 years, which will probably include the replacement of several FW heaters on some sort of prioritization (rolling) schedule.

7.1.3 Other Plant Strategies: (Power Uprate, Early Decommissioning, >60 Years) Many plants have pursued various types of power uprate programs to increase the megawatts thermal and megawatts electric output. With respect to feedwater heaters, this has caused some unique difficulties because power uprate programs often involve increased feedwater flow (leading to higher velocities in the FW heater tubes, and the potential for subsequent tube vibration and erosion). Heat transfer rates are also affected, putting more demands on FW heater operation. This alternative must be reviewed carefully with respect to the FW heaters in order to obtain the maximum benefit to the plant. INPO issued SER (Significant Event Report) 5-02 7-2


[Ref. 40] in August 2002 to address problems with power uprates at nuclear units involving inadequate analysis, design, or implementation which caused over 40 plant events between 1997 and 2002. Many of these events involved secondary-side equipment like feedwater heaters. In terms of license renewal and power uprate and implementation projects, the impact on the feedwater heaters needs to be carefully considered to avoid any problems or any acceleration of degradation. With respect to early decommissioning, this alternative would probably lead to the continuation of the current maintenance plan for the FW heaters. Future equipment replacements would probably be considered only if the failure rate was so high that overall unit operation was strongly impacted (frequent shutdowns, large drops in power production, a large drop in overall plant heat rate). The input of the plant Thermal Performance Engineer is critical in evaluating how the feedwater heaters fit into the overall maintenance and operation of the plant. Section 8 of EPRI 1003470 [Ref. 11] gives guidance on major repair and replacement decisions.

7.2

Examples of Suitable Alternative LCM Plans

For each alternative LCM plan proposed, detailed maintenance activities and schedules need to be identified. Each plan will involve some mixture of the LCM approaches shown in steps 13 to 17 of Figure 2-2. The plans might involve the following: •

•

•

•

•

Adjusting the frequency of time-directed maintenance to enhance the reliability of the feedwater heaters (or to reduce maintenance costs) Considering that diagnostic predictive maintenance switch from a time-directed basis to a condition-directed basis Performing preventive and non-invasive maintenance activities on-line if feasible. Note: some of the movement in this area has caused unforeseen problems in the industry. EPRI and INPO documents should be reviewed to determine what activities actually may be performed on-line. Plant operating procedures that specify equipment that may be taken out of service should also be reviewed to establish a clear pathway for on-line maintenance of feedwater heaters and associated equipment. Addressing tasks that facilitate or enable operating changes which minimize or equalize component wear. Adding preventive and predictive maintenance activities that may enhance the reliability of the feedwater heaters. A number of these activities are discussed in Sections 5.1 and 5.2; other suggestions may be sought from other power plants or industry working groups. If improvements in heat transfer capability are needed, FW heater cleaning could be considered.

One alternative that will almost certainly be considered, if only as a base case for comparison with actual changes, is the option of continuing the current scheme for the feedwater heaters. This case is presented with the presumption that existing maintenance practices will continue and that present failure rates will apply. However, in characterizing this base case, the wear-out of the major components (and the need for replacement) and the acceleration of the failure rates as components reach their end-of-life condition must be considered. 7-3


Another option to be considered for FW heaters involves the cleaning of the tubes, particularly if performance monitoring has shown that thermal performance of a given heater has declined. This can be done during an outage with no impact on plant operation. The improvement in heat transfer capability can be determined via a comparison of data from before and after the tube cleaning. In developing and reviewing each alternative task for LCM planning, all costs and benefits should be considered and utilized in the analysis. This includes the cost of labor and materials, the cost of planning itself, the costs associated with lost generation (declining thermal efficiency, heater derating, and plant trips) and replacement power, and any potential regulatory costs (due to NRC involvement or financial oversight from state regulators). The base case for the feedwater heaters includes all the current surveillance and maintenance done on the feedwater heaters at any given plant. For the purposes of LCM planning, these costs will include engineering and craft labor, equipment costs, and vendor costs for surveillance testing and inspections. For some plants, a certain number of FW heaters are inspected each outage, for other plants, problems heaters are already identified, and the outage work involves checking the condition of the current degradation. Other plants (plants which have recently replaced FW heaters) may not require such detailed inspections, and their average costs may be lower. It should be noted that over the life of the plant, the predictive and preventive maintenance costs may be able to be controlled and remain fairly constant, but corrective maintenance costs will increase with time. It should be noted that an engineering evaluation needs to be performed on FW heaters for any decision involving power uprate or FW heater upgrades. Due to increased flow requirements, some existing heaters may not be acceptable under such plans, and additional changes may be necessary.

7.3

Hypothetical Illustration of an LCM Planning Alternative for Feedwater Heaters

A simple hypothetical case outline is evaluated to illustrate the process of developing LCM Planning Alternatives for the feedwater heaters. The various alternatives mentioned above are evaluated here.

7.3.1 Alternative A – Base Case Alternative A (the base case) is to continue the current maintenance program and to enhance or upgrade the program only if absolutely necessary. Major efforts, such as tube rebundling or FW heater replacement, are considered only if required. This alternative would be selected if a plant already has a successful FW heater maintenance and surveillance program, as verified by the LCM economic analysis showing no benefit from increased PM and/or PdM activities. The cost would be (approximately) the same as the plant’s current program, with minor adjustments in the future for possible tube plugging, and additional

7-4


NDE as the FW heaters age. For plants with above industry average problems or failure rates, this option is clearly not recommended (although plant-specific data will reveal the situation).

7.3.2 Alternative B – Step Increase in NDE Examinations of the FW Heaters This alternative considers a step increase in the NDE examination of the FW Heaters, which means an increase in eddy current testing (of the tubes and the shell), and additional examinations of the tubesheet, the impingement plates, the channel head, and the inlet/outlet nozzles. The purpose of this alternative is to offer increased surveillance so that costly surprises (i.e., tube failures) can be avoided. As a rough estimate, this alternative involves an increase in the overall maintenance spending for the FW Heaters, due to more frequent (and more detailed) inspection. Part of the increase will involve engineering time to develop more detailed documents to track the condition of the FW Heaters (drawings, evaluation reports for NDE data, etc.). There may also be increased engineering costs as the component engineer and possibly the thermal performance engineer (and even system engineers) educate themselves further about feedwater heaters. There may also be additional costs related to tube plugging, sleeving, and other maintenance actions (arising from better surveillance data).

7.3.3 Alternative C – Major Maintenance (Tube Rebundle, etc.) This alternative considers a major repair effort on a damaged FW Heater. This effort will involve a large outlay to correct problems that may have been known for some time, but have not been completely addressed. This effort may involve re-tubing or rebundling. Re-tubing a heater involves replacing only the tubes. Rebundling a heater involves replacing the tubes, the tubesheet, the tube supports, and the baffle plates. Retubing is typically an option for lowpressure, straight tube heaters only. Rebundling is usually an option for just low-pressure heaters as well. Individual tubes can be replaced in high-pressure heaters, although this is not typical. It should be noted that the difference in cost between a rebundle and a complete FW Heater replacement is usually only about 5% or less. This factor should be considered when a heater is experiencing repeated degradation problems.

7.3.4 Alternative D – FW Heater Replacement This alternative involves the complete replacement of a FW Heater. Such a determination is made when the present heater is at its limit with respect to tube plugging, and further degradation is expected. Complete replacement is also a consideration if the shell is damaged (eroded or cracked) beyond a reasonable repair (i.e., a localized weld repair or patch). As a rough estimate, when 10% of the tubes in a FW heater are plugged, the heater is approaching its end of life. Further plugging would only increase the negative economic impact on the plant. Section 8 of EPRI 1003470 [Ref. 11] contains detailed information on how to plan for the procurement of a new FW Heater, including a detailed list of materials for all the sub-

7-5


components involved. The technical and commercial properties (cost) are also presented. EPRI GS-6913 [Ref. 20] also addresses FW heater replacement in detail.

7.3.5 Composite Evaluation A composite evaluation of the alternatives is listed in Table 7-1. This composite shows the attributes for consideration and provides a summary which can be used as a starting point for plant-specific evaluations (with plant-specific technical data and economic analysis). As discussed in Section 4 of this LCM Sourcebook, as estimated industry generic failure rate for feedwater heaters has been determined to be approximately 0.05 to 0.1 failures per plant per year. This value reflects the fact that nuclear plants typically have 12 or more feedwater heaters. See Section 4.5 for an explanation of the generic industry failure rate estimation. Table 7-1 presents a comparison of the various hypothetical alternatives. Table 7-1 FW Heater Composite Evaluation for LCM Planning (generic) Attribute

Plant Impact (Lost Generation) Initial Cost Future Work Needed Composite

Alternative B

Alternative C

Alternative D

Higher Impact –

Highest Impact –

Additional Outage Work – Some Benefit

Major Outage Item – Good Benefit if a Heater is Failing

Major Outage Item – Good Benefit if a Heater is Failing

+20% of Alternative A

$700,000*

$800,000*

Minimal Impact –

Yes Better than Base Case

No, Not if Shell is Properly Maintained Needed if Heater Tubes are Failing Regularly

Not Expected Needed if Heater is Degrading Beyond Repair

*Note that these values are the cost of the tubes and the replacement heater only. The design and modification costs, construction costs, and testing costs are not included. These can often be significant (depending upon heater accessibility, location, and orientation), and may cost much more than the new tubes or replacement heater itself. Also, there will be a difference between the cost of a low-pressure FW heater and a high-pressure FW heater, and this difference can be significant (due to higher pressures and temperatures and more stringent ASME Section VIII construction criteria). In general, it takes about 1 to 2 months to prepare a new FW heater specification (longer if more time is needed to assemble data or if the engineering data is not readily available). The lead time for ordering a tube rebundle is approximately 6 months, and the lead time for an entirely new heater is approximately 8-10 months. Depending upon FW heater accessibility and orientation, the actual rebundle or replacement time (schedule) may vary from 2 to 6 weeks.

7-6


The values for a heater rebundle cost and a full heater replacement were taken from communication with mechanical component engineers in the industry who have specified new FW heaters and other heater repairs. These values are indicative and are given as general estimates only. Actual cost is highly dependent upon FW heater location, accessibility, and orientation. The expectation for LCM planning is that a FW heater maintenance scheme will be based upon an evaluation of the current status of the plant equipment, rather than a complete change in direction with respect to plant maintenance and surveillance. For a plant with FW heater degradation problems, Alternatives C and D will very likely be selected if the current equipment is negatively affecting operation. For plants with good equipment and a well-developed maintenance and surveillance program (and a pro-active repair program), a choice in line with Alternative B will likely be considered, even if the base case is working well (for future successful heater operation and possible consideration of license renewal). The potential for license renewal will also drive plants to consider FW heater replacements in a controlled manner, in order to achieve the 60-year lifetime. As mentioned in Section 6, the average expectancy for carbon steel tubed heaters is about 12 years. The average life expectancy for T-22 tubed heaters is 15 to 20 years, and the average life expectancy of a type 304-SS tubed heater is about 20 years. This data will figure into any major repair/replacement decisions.

7.4

Additional Evaluation of FW Heater Options

In assessing the most economical approaches to major FW heater maintenance decisions, the age of the FW heater, the design performance, and the impact on outage time for a heater replacement must be evaluated. First, the age is important because any heater older than 15 years will be facing increasing difficulties going forward. Even a FW heater with 15 years of satisfactory performance will show aging degradation at an increasing rate. Second, the performance of the specific heater design will require evaluation to determine if reliable performance can continue. Some heaters have design problems with poor tube supports (or supports placed too far apart), problems with undersized or poorly oriented impingement plates, problems with poor separation for the drains cooler zone (leading to steam entry and/or flashing), and problems with pass partition plates. These problems can be repaired (to varying degrees), but any new heater specification should avoid the problems of the past. Third, the impact on outage duration for a FW heater replacement can be significant depending upon the amount of work necessary for installation (as previously mentioned). It is also important to consider how much of the total labor will be from the fabrication shop, and how much will be performed at the site.

7-7


EPRI NP-4507-V3 [Ref. 23] contains a detailed discussion of cost evaluation, which considers shop labor, field labor, replacement power cost, and material costs for heater rebundles and heater replacements. All of these factors need to be addressed in plant-specific LCM planning.

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8

GUIDANCE FOR ESTIMATING FUTURE FAILURE RATES

This section addresses a part of step number 18 of Figure 2-2. It must be recognized that failure rates are a main driver of the LCM planning process, and knowledge about estimating future failure rates will directly impact decisions about component maintenance and replacement. In addition, as components age, their vulnerability to the applicable aging mechanisms increases. General guidance for estimating SSC future failure rates can be found in Section 2.6 of the LCM Planning Sourcebook Overview Report (EPRI 1003058) [Ref. 1]. Below are some useful ideas for estimating failure rates in FW heater LCM planning studies. •

•

•

•

•

Sections 6.1 and 6.2 provide a discussion on the estimated life of FW heaters. This data is not conclusive, because there is no set figure for the duration of a FW heater lifetime, but it is a beginning point in estimating the expected remaining life of a FW heater (based upon operating experience gained to date). If “in-kind” replacements are made, existing failure rates may be applied for the future (and possibly extended, given improvements in technology and maintenance). Specific communications with equipment manufacturers is key in developing this information (with respect to LCM planning). The generic industry failure rate discussed in Section 4 may be used in conjunction with plant-specific data to establish a plant-specific failure rate for feedwater heaters. Plants with FW heater trending programs can extract failure data (for tubes and other subcomponents) and compute failure rates directly. Data can be plotted to determine if the aging effects previously identified are being exhibited and/or if the current PM programs are effective. A sufficient duration of time for data collection is necessary (at least 10-15 years). A review of Corrective Work Orders (WOs) can provide a means of following the FW heater problems and may be used to compute failure rates. The criterion of failure must be clearly defined. This WO review should encompass a long enough span of time to generate meaningful results (for FW heaters, more than 10-15 years, in order to observe the aging effects and to evaluate the effectiveness of plant maintenance). Repetitive problems must be identified and categorized as such in LCM planning. The data presented in Section 4 provides a background for understanding how FW heater problems can impact plant operation. It is important to differentiate between the FW heaters themselves and the FW heater controls. A separate EPRI LCM sourcebook (EPRI 1007425) has been prepared for FW heater controls, and should be reviewed for these components. Often, problems in the control scheme (or with the instrumentation) for the FW heaters leads to major component problems (erosion, vibration, corrosion, etc.) in the future.

8-1

Guidance for Estimating Future Failure Rates

•

•

•

•

•

The feedwater heaters are installed with strings of heaters in the plant (for LP and HP applications). A single string out of service would rarely cause the loss of the entire system function (and plant shutdown), but it would require some degree of power reduction. While component failure (and the repair/replacement impact) must be addressed in LCM planning, the consideration of lost power generation may be limited to just power reductions (small or large) rather than complete shutdown. Maintenance Rule programs and PRA evaluations emphasize functional failures rather than degraded performance. Feedwater heater LCM plans should consider both. PRA models may assume that the probability of catastrophic failures of passive heat exchangers is insignificant compared to the probability of other component failures. However, these passive failures (as discussed in Section 4) are of considerable concern in LCM planning (due to the impact of lost power production and the cost of replacement power). The complete failure of a feedwater heater shell must be addressed in LCM planning, especially if plant data shows evidence of wall thinning. The effects of any power uprate on a nuclear unit must be considered when evaluating future failure rates. INPO SER 5-02 addresses the fact that over 40 plant events have occurred from 1997 to 2002 as a result of inadequate analysis, design, or implementation of nuclear plant power uprates. Many of these events involved feedwater heaters and related secondary-side equipment. The effects of power uprate on the feedwater heaters (increased FW flow, increased heat transfer demand, etc.) must be reviewed and considered on a going-forward basis. The potential for license renewal (with an extended 20 years of plant operation) will introduce new considerations in LCM planning, particularly for components that normally experience age-degradation at a slow rate, but can experience damage to vulnerable components in just one cycle (like feedwater heaters). LCM planning for the period of extended operation must account for possible feedwater heater replacement if the plant undergoes a general refurbishment prior to (or at the beginning of) the 20 year period.

In summary, failure rate predictions for feedwater heaters are made using the guidance of the LCM Sourcebook Overview Report [Ref. 1]. The generic industry failure rate is discussed in Section 4 of this document and in Section 5 of EPRI 1003470 [Ref. 11]. This rate is based upon available data and is uncertain. The plant-specific PRA and Maintenance Rule records will be an important source of information in determining a plant-specific failure rate (along with all the maintenance records, of course). The LCM planning process should be fairly complete with carefully defined activities for each of the LCM alternative plans (as they are developed). In this way, the influence of new or additional PM activities, implementation of component replacements, and plant modifications (power uprate, license renewal, etc.) can be appropriately considered in estimating future failure rates for input into LCM economic evaluations.

8-2

9

INFORMATION SOURCES AND REFERENCES

1. EPRI Technical Report 1003058, “Life Cycle Management Planning Sourcebooks – Overview Report”, December 2001. 2. EPRI Technical Report 1000806, “Demonstration of Life Cycle Management Planning For Systems, Structures, and Components”, With Pilot Applications at Oconee and Prairie Island Nuclear Stations, January 2001. 3. EPRI Technical Report 1003059, “Life Cycle Management Planning at V.C. Summer Nuclear Plant: Main Condenser, Radiation Monitoring System, and Nuclear Safety-Related HVAC Chilled Water”, December 2001. 4. EPRI Technical Report 1003060, “Life Cycle Management Planning at Wolf Creek Generating Station: EDG, Main Steam, and Feedwater Isolation Valves, and Reactor Protection System”, December 2001. 5. EPRI Technical Report 1006686, “Nuclear Power Plant Life Cycle Management Planning Implementation Demonstration: Reference Manual and Tutorial for the LcmPLATO Database Application, Version 1.0”, January 2002. 6. EPRI Technical Report 1007425, “Life Cycle Management Planning Sourcebooks – Volume VI – Feedwater Heater Controls”, March 2003. 7. SAND93-7070, “Aging Management Guideline for Commercial Nuclear Power Plants – Heat Exchangers”, June 1994. 8. EPRI Technical Report NP-1558, “A Review of Equipment Aging Theory and Technology”, September 1980. 9. EPRI Technical Report TR-100844, “Nuclear Power Plant Common Aging Terminology”, February 1992. 10. EPRI Technical Report TR-106853, “The Maintenance Engineer Fundamentals Handbook: An EPRI Course”, October 1996. 11. EPRI Technical Report 1003470, “Feedwater Heater Maintenance Guide”, May 2002. 12. EPRI Technical Report TR-102134-R5, “PWR Secondary Water Chemistry Guidelines – Revision 5”, May 2000.

9-1

Information Sources and References

13. EPRI Technical Report TR-107422-V1, “Thermal Performance Engineer’s Handbook – Volume 1”, March 1998. 14. EPRI Technical Report TR-107422-V2, “Thermal Performance Engineer’s Handbook – Volume 2”, October 1998. 15. EPRI Technical Report TR-106741, “Heat Exchangers: An Overview of Maintenance and Operations”, March 1997 16. EPRI NP-7552, “Heat Exchanger Performance Monitoring Guidelines”, December 1991. 17. EPRI GS-6935, “Feedwater Heaters Maintenance and Repair Technology: Reducing Outage Cost”, August 1990. 18. EPRI CS-1776, “Failure Cause Analysis – Feedwater Heaters”, April 1981. 19. EPRI Technical Report TR-106857-V32, “Preventive Maintenance Basis – Volume 32: Heat Exchangers – Tube Type”, July 1998. 20. EPRI GS-6913, “Feedwater Heaters: Replacement Specification Guidelines”, August 1990. 21. EPRI NP-4057, Volume 1, “Nuclear Plant Feedwater Heater Handbook – Volume 1: Primer”, June 1985. 22. EPRI NP-4057, Volume 2, “Nuclear Plant Feedwater Heater Handbook – Volume 2: Design and Procurement Guidelines”, June 1985. 23. EPRI NP-4057, Volume 3, “Nuclear Plant Feedwater Heater Handbook – Volume 3: Operation and Maintenance Guidelines”, June 1985. 24. EPRI GS-7417, “Feedwater Heater Survey”, August 1991. 25. EPRI CS-3184, “Corrosion-Related Failures in Feedwater Heaters”, July 1983. 26. EPRI Technical Report TR-109546, “Heat Rate Improvement Reference Manual”, July 1998. 27. EPRI Technical Report 1003103, “EPRI-Lite”, December 2001. 28. NUREG-1800, “Standard Review Plan for Review of License Renewal Applications for Nuclear Power Plants”, April 2001. 29. NUREG-1801, “Generic Aging Lessons Learned (GALL) Report”, April 2001. th

30. HEI Standards for Closed Feedwater Heaters, 6 Edition, Heat Exchange Institute, 1998. 31. EPRI 1006998, “Life Cycle Management Plan for Feedwater Heater and Moisture Separator Controls at Hope Creek Generating Station”, March 2003. 32. INPO AP-913, Rev. 1, “Equipment Reliability Process Description”, November 2001.

9-2

Information Sources and References th

33. Standards of the Tubular Manufacturer’s Association, 6 Edition, 1978. 34. NUREG-1377, Rev. 4, “NRC Research Program on Plant Aging: Listing and Summaries of Reports Issued Through September 1993”, published December 1993. 35. EPRI Technical Report 1003282, “PM Basis Version 4.0 with Vulnerability Analysis Module”, July 2002. 36. EPRI Technical Report TR-106858-R1, “Preventive Maintenance Basis – Project Overview Report”, November 1998. 37. EPRI Technical Report 1007079, “Reliability and Risk Significance”, December 2001. 38. EPRI 1000260, “SysMon 2.0 User’s Manual – System Monitoring by Systems Engineers, 37 System Templates”, July 2000. 39. ASME Performance Test Code PTC 12.1, “Closed Feedwater Heaters”, published 2000. 40. SER 5-02, “Lessons Learned from Power Uprates”, INPO, August 2002. 41. U.S. Nuclear Regulatory Commission, “Rates of Initiating Events at U.S. Nuclear Power Plants 1987-1995”, NUREG/CR-5750, February 1999. 42. NEI 95-10, Rev. 3, “Industry Guideline for Implementing the Requirements of 10 CFR Part 54 – The License Renewal Rule”, April 2001. 43. EPRI 1003056, “Non-Class 1 Mechanical Implementation Guideline and Mechanical Tools, Revision 3”, November 2001. 44. NEIL (Nuclear Electric Insurance Limited) Loss Control Standards, Updated September 2002. 45. EPRI 1003286, “Repair Technology for Degraded Pressure Vessels and Heat Exchanger Shells: Repair and Replacement Applications Center Task 91”, December 2002. 46. EPRI 1009159, “Life Cycle Management Plan for Main Generator & Exciter at the South Texas Project Generating Station”, September 2003. 47. EPRI 1009731, “LCM Economic Risk Tools Demonstration”, January 2004.

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10

ACRONYMS

ABB

Asea Brown Boveri

AE

Acoustic Emission

AMG

Aging Management Guideline

AMR

Aging Management Review

ANSI

American National Standards Institite

ASME

American Society of Mechanical Engineers

ASNT

American Society of Non-Destructive Testing

ASTM

American Society of Testing Materials

AVT

All-Volatile Treatment

AWS

American Welding Society

B&PV

Boiler & Pressure Vessel

B&W

Babcock & Wilcox

BOP

Balance-of-Plant

BWR

Boiling Water Reactor

Btu

British Thermal Units

CS

Carbon Steel

CE

Combustion Engineering

DCA

Drains Cooler Approach

ECP

Electrochemical Potential

ECT

Eddy Current Testing

EEI

Edison Electric Institute

EPIX

Equipment Performance & Information Exchange

EPRI

Electric Power Research Institute

ET

Electromagnetic Testing

FAC

Flow-Accelerated Corrosion

FMAC

Fossil Maintenance Applications Center

FFT

Final Feedwater Temperature

10-1

Acronyms

FIV

Flow-Induced Vibration

FW

Feedwater

FWH

Feedwater Heater

GADS

Generating Availability Data System

GARS

Generating Availability Reliability System

GE

General Electric

GL

Generic Letter

HEI

Heat Exchange Institute

HP

High Pressure

ID

Inner Diameter

IGA

Inter-Granular Attack

IGSCC

Inter-Granular Stress Corrosion Cracking

INPO

Institute of Nuclear Power Operations

IRIS

Internal Rotary Inspection System

IRT

Infrared Technology

ITHT

Individual Tube Hydrostatic Test

LCM

Life Cycle Management

LER

Licensee Event Report

LP

Low Pressure

LR

License Renewal

MIC

Microbiologically-Influenced Corrosion

MPFF

Maintenance Preventable Functional Failure

MR

Maintenance Rule

MSR

Moisture Separator Reheater

MT

Magnetic Particle Testing

MTTR

Mean-Time-to-Repair

NEIL

Nuclear Electric Insurance Limited

NERC

North American Electric Reliability Council

NDE

Non-Destructive Examination

NMAC

Nuclear Maintenance Applications Center

NPAR

Nuclear Plant Aging Reasearch

NPRDS

Nuclear Plant Reliability Data System

NPV

Net Present Value

10-2

Acronyms

NRC

Nuclear Regulatory Commission

O&MR

Operations & Maintenance Reminder

OD

Outer Diameter

OE

Operating Experience

OPEC

Operating Plant Experience Code

pH

Percent Hydrogen

PM

Preventive Maintenance

PdM

Predictive Maintenance

ppm

Parts per Million

PT

Liquid Penetrant Testing

PTC

Performance Test Code

PWR

Pressurized Water Reactor

RCM

Reliability-Centered Maintenance

RFEC

Remote Field Eddy Current

RVT

Remote Visual Testing

SCC

Stress Corrosion Cracking

SEE-IN

Significant Event Evaluation Information Network

SEN

Significant Event Notification

SER

Significant Event Report

SOER

Significant Operating Experience Report

SS

Stainless Steel

SSC

System, Structure, Component

SYSMON

System Monitoring (an EPRI tool for system engineers)

TGSCC

Trans-Granular Stress Corrosion Cracking

TEMA

Tubular Exchanger Manufacturer’s Association

TR

Temperature Rise

TTD

Terminal Temperature Difference

UCLF

Unplanned Capacity Loss Factor

UT

Ultrasonic Testing

VT

Visual Inspection Testing

W

Westinghouse

10-3

Feedwater Heater Source Book

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