Examination of Heat Recovery

Technical Report

Examination of Heat RecoverySteam Generator (HRSG) Plants

Assessment of Fiber-Optic Techniques

EPRI Project Manager S. Walker

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1395 • PO Box 10412, Palo Alto, California 94303-0813 • USA

800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Examination of Heat Recovery Steam Generator (HRSG) Plants Assessment of Fiber-Optic Techniques

1008092

Final Report, November 2005

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:

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Electric Power Research Institute (EPRI)

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Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.

Copyright © 2005 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS

This report was prepared by

Electric Power Research Institute (EPRI) 1300 W.T. Harris Blvd Charlotte, NC 28262

Principal Investigator S. Walker

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Examination of Heat Recovery Steam Generator (HRSG) Plants: Assessment of Fiber-Optic Techniques. EPRI, Palo Alto, CA: 2005. 1008092.

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PRODUCT DESCRIPTION

Previous EPRI reports have documented problems associated with operation and maintenance of complex heat recovery steam generators (HRSGs). The EPRI report Heat Recovery Steam Generator Tube Failure Manual (1004503) provides information about known HRSG tube failures and necessary steps that can be taken to diagnose and prevent similar problems. The EPRI report Delivering High Reliability Heat Recovery Steam Generators (1004240) provides guidance for continued and reliable operation of HRSGs from initial design, fabrication, and operation through lessons-learned experience.

As HRSGs age, regardless of the care taken to ensure the use of suitable materials, optimal heater design, and applicable water chemistry guidelines, components begin to fail. Therefore, it becomes necessary to apply nondestructive evaluation (NDE) techniques to inspect, monitor, and help mitigate HRSG failures. The EPRI report Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators (1004506) provides information on various NDE techniques available and their possible applications to detect and characterize location-specific forms of damage in HRSGs. The EPRI report Electromagnetic Nondestructive Evaluation (NDE) for Heat Recovery Steam Generators (1008093) provides additional information concerning electromagnetic NDE for HRSGs.

Access to tubes from both the outer and inner surfaces and tube-to-header welds has been a major limitation in applying suitable NDE techniques. Consequently, this report focuses on the introduction of various visual NDE techniques for field implementation. Specific visual NDE techniques covered in this report include the use of fiberscopes, borescopes, and lecturescopes. Each of these can be applied from the inside of finned and nonfinned tubing and piping that might be subject to various forms of corrosion, pitting, hydrogen damage, and cracking.

Results and Findings This report provides an overview of suitable visual examination techniques that use fiber optics to perform flaw detection in tubing and piping materials that are encountered in HRSGs. The described NDE techniques will be applicable from the internal surfaces of HRSG components if access to the components is either available or made available. Applying the appropriate NDE methods to each of the components and damage mechanisms is key to maintaining safe continued operation.

Challenges and Objectives Because of access limitations, the most prevalent NDE method currently used for HRSGs is visual examination of tubes and headers from either the inside or outside of the tube. Unfortunately, if indications are found that do not penetrate completely through the wall, no

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quantitative information can be obtained by the visual method to assist with run, repair, or replace decisions.

Applications, Values, and Use This report provides a review of visual examination equipment that is commercially available and that might be suitable for HRSG applications through provision of information regarding tube wall loss caused by corrosion and fatigue damage. With available access, these techniques are easy to implement and require no surface preparation before examination.

The specific visual examination techniques presented in this report are applicable from the inside surface of finned and nonfinned tubing and piping that might be experiencing various forms of corrosion, pitting, hydrogen damage, or cracking.

EPRI Perspective This report enhances the previous NDE guidelines provided in the following EPRI reports:

• Electromagnetic Nondestructive Evaluation for Heat Recovery Steam Generators (1008093)

• Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators (1004506)

• Heat Recovery Steam Generator Tube Failure Manual (1004503)

• Delivering High Reliability Heat Recovery Steam Generators (1004240)

Approach The goals of this activity were to investigate existing visual examination technology, primarily using fiber optics, for application to HRSG components—in particular, for finned tubing and header-to-tubing junctures. Several tools were identified that should be successful in the examination of these components. While reviews of the technology and application in other plant types for similar components reveals applicability, it is necessary to deploy the technology in a current HRSG to verify its capability with greater certainty.

Keywords Combined-cycle power plants Corrosion Failure reduction Heat recovery steam generators (HRSGs) Nondestructive evaluation (NDE) Visual examination

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ABSTRACT

EPRI reports have been published to document problems associated with operation and maintenance of complex heat recovery steam generators (HRSGs). The EPRI report Heat Recovery Steam Generator Tube Failure Manual (1004503) provides known HRSG tube failures and necessary steps that can be taken to diagnose and prevent similar problems. The EPRI report Heat Recovery Steam Generators (1004240) provides guidance for continued and reliable operation of HRSGs from initial design, fabrication, and operation through lessons-learned experience.

As HRSGs age, regardless of the care taken to ensure selection of suitable materials, optimum heater design, and applicable water chemistry guidelines, components begin to fail. Therefore, it becomes necessary to apply nondestructive evaluation (NDE) techniques to inspect, monitor, and help mitigate HRSG failures. The EPRI report Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators (1004506) provides information on various NDE techniques available and their possible applications to detect and characterize location-specific forms of damage. The EPRI report Electromagnetic Nondestructive Evaluation for Heat Recovery Steam Generators (1008093) provides specific guidance on the use of electromagnetic NDE techniques for examination of HRSGs.

In preparing the EPRI report Electromagnetic Nondestructive Evaluation for Heat Recovery Steam Generators (1008093), it became clear that access to tubes from both the outer surface and the inner surface and for tube-to-header welds was a major limitation in applying suitable NDE techniques. In addition, if access to the inside surface of tubes is provided, a simple visual examination can be performed much more simply and in a less expensive manner, while still providing adequate information concerning the condition of the tubing.

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CONTENTS

1 INTRODUCTION TO VISUAL EXAMINATION ......................................................................1-1 Technique .............................................................................................................................1-1 Equipment .............................................................................................................................1-2 Implementation......................................................................................................................1-3

Access ..............................................................................................................................1-3 Diagrams and Plans .........................................................................................................1-4 Temperature .....................................................................................................................1-4 Internal Conditions............................................................................................................1-4 Nearby Operations ...........................................................................................................1-5 Protection for Equipment ..................................................................................................1-5 Establishing a Baseline ....................................................................................................1-5 Care in Choosing Inspection and Documentation Angles ................................................1-5

Applications...........................................................................................................................1-6 Debris Removal.....................................................................................................................1-7

2 ASSESSMENT OF DAMAGE ................................................................................................2-1 Introduction ...........................................................................................................................2-1 Features of Failure ................................................................................................................2-1

3 VISUAL EXAMINATION EQUIPMENT: OPTICAL AND MECHANICAL AIDS.....................3-1 Introduction ...........................................................................................................................3-1 Optical Aids ...........................................................................................................................3-1 Basic Elements of Visual Examination ..................................................................................3-2

Object Being Examined ....................................................................................................3-2 Object Distance ...........................................................................................................3-2 Object Size ..................................................................................................................3-3 Discontinuity Size.........................................................................................................3-3 Reflectivity ...................................................................................................................3-4

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Entry Port Size.............................................................................................................3-4 Object Depth................................................................................................................3-5

Optical Instruments...........................................................................................................3-6 Fiberscopes .................................................................................................................3-6 Borescopes..................................................................................................................3-9 Lecturescope .............................................................................................................3-14 Photographic Equipment Systems.............................................................................3-14 Ultraviolet Light Sources............................................................................................3-15 Extend-a-Scope .........................................................................................................3-15 Remote Visual Inspection Systems ...........................................................................3-15 Accessories ...............................................................................................................3-16

Illumination .....................................................................................................................3-17 Recording Method ..........................................................................................................3-18

Permanent Method ....................................................................................................3-18 Subjective Method .....................................................................................................3-18 Mechanical Aids.........................................................................................................3-18

4 REMOTE AND SPECIALIZED VISUAL EXAMINATION EQUIPMENT ................................4-1 Introduction ...........................................................................................................................4-1 Remote Visual Examination of Components.........................................................................4-1

Lights and Illumination......................................................................................................4-1 Lenses ..............................................................................................................................4-2 Videocassette or Digital Recorder ....................................................................................4-2 Printers .............................................................................................................................4-2 Monitors............................................................................................................................4-3 Cables ..............................................................................................................................4-3 Calibration ........................................................................................................................4-3

Special Requirements ...........................................................................................................4-4 Television System Examinations......................................................................................4-4

Fiber-Optic Examinations ......................................................................................................4-5 Test Requirements ................................................................................................................4-5

Illumination .......................................................................................................................4-5 Personnel .........................................................................................................................4-6

Photographic Techniques for Recording Results ..................................................................4-6 Depth of Field ...................................................................................................................4-6

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Lighting .............................................................................................................................4-8 Film...................................................................................................................................4-9 Digital Cameras ..............................................................................................................4-10 Replication......................................................................................................................4-11

Summary.............................................................................................................................4-12

5 SUMMARY .............................................................................................................................5-1

6 REFERENCES .......................................................................................................................6-1

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LIST OF FIGURES

Figure 1-1 Recommended Photographic Angles to Highlight Tube Plastic Deformation (Left) and Overheating (Right) ...........................................................................................1-6

Figure 1-2 Examples of Debris Found in HRSG Tubing ............................................................1-8 Figure 2-1 Example of Flow-Accelerated Corrosion in Vertical Low-Pressure Evaporator

Tubing ................................................................................................................................2-3 Figure 2-2 Example of Flow-Accelerated Corrosion in Horizontal Low-Pressure

Evaporator Tubing..............................................................................................................2-4 Figure 2-3 Visual and Metallographic Characteristics of Two-Phase Flow-Accelerated

Corrosion............................................................................................................................2-5 Figure 3-1 Object Distance ........................................................................................................3-3 Figure 3-2 Object Size ...............................................................................................................3-3 Figure 3-3 Discontinuity Size .....................................................................................................3-4 Figure 3-4 Reflectivity ................................................................................................................3-4 Figure 3-5 Entry Port Size..........................................................................................................3-5 Figure 3-6 Object Depth.............................................................................................................3-5 Figure 3-7 Viewing Angle Versus Depth of Field .......................................................................3-6 Figure 3-8 Fiberscope ................................................................................................................3-6 Figure 3-9 Light Reflection Within a Glass Fiber........................................................................3-7 Figure 3-10 Glass Fiber Construction ........................................................................................3-8 Figure 3-11 Image Guide Usage................................................................................................3-8 Figure 3-12 Borescope.............................................................................................................3-10 Figure 3-13 Borescope Functionality .......................................................................................3-10 Figure 3-14 Direction of View...................................................................................................3-11 Figure 3-15 Field of View .........................................................................................................3-11 Figure 3-16 Measuring Field of View .......................................................................................3-12 Figure 3-17 Fiberscope Magnification......................................................................................3-13 Figure 3-18 Mini-Borescope.....................................................................................................3-13 Figure 3-19 Fiber-Optic Accessories .......................................................................................3-14 Figure 3-20 Remote Visual Inspection System ........................................................................3-16 Figure 3-21 Fiberscope Accessories........................................................................................3-16 Figure 3-22 Illumination Is Reduced by 50% at an Interface ...................................................3-17 Figure 4-1 Calibration Standard for Remote Visual Examination ..............................................4-4

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Figure 4-2 Depth of Field ...........................................................................................................4-7 Figure 4-3 Principal Plane of Focus for Measuring Dimension A–B Off the Print ......................4-8 Figure 4-4 Bounce Lighting........................................................................................................4-9 Figure 4-5 Digital Camera........................................................................................................4-11

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LIST OF TABLES

Table 4-1 Film Speeds Available .............................................................................................4-10 Table 4-2 Focal Length ............................................................................................................4-11

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1 INTRODUCTION TO VISUAL EXAMINATION

As heat recovery steam generators (HRSGs) age and fail due mainly to corrosion and fatigue, applying the appropriate nondestructive evaluation (NDE) techniques to detect, characterize, and assist in mitigating location-specific forms of damage becomes key to maintaining safe and continued operation.

Because of access limitations, the most prevalent NDE method currently used for HRSGs is visual examination of tubes and headers from either the outside surface or the inside surface. Unfortunately, if indications are found that do not penetrate completely through the wall, no quantitative information can be obtained by visual methods to make the necessary but difficult decision to run, repair, or replace the affected segment.

This assessment of visual examination techniques using fiber-optic NDE reviews the visual examination equipment that is commercially available and suitable for HRSG applications by providing information with regard to tube wall loss caused by corrosion and fatigue damage. With available access, these techniques are easy to implement and do not require surface preparation prior to examination.

Specific visual examination equipment covered in this report includes fiberscopes, borescopes, and lecturescopes. Each is applicable from the inside of finned and nonfinned tubing and piping suffering from various forms of corrosion, pitting, hydrogen damage, and cracking [1].

Technique

Visual examination is the most effective and informative method of finding flaws and degradation among the various HRSG components. Visual examination is the principal NDE method used to inspect HRSG tubes, headers, drums, and piping systems throughout the manufacture, assembly, and operation of the unit. Quality visual examination is one of the least appreciated and possibly the most demanding NDE activity currently being used in power plant inspection. In the past, visual inspection consisted of “looking over” a component; however, now it typically involves a great deal of training and practical experience before an examiner is qualified for this examination method.

The qualified visual examiner must have a good working knowledge of a vast array of plant components, metallurgy, potential flaw types for various components, failure mechanisms, and so on. The examiner must be capable of recognizing the presence of different forms of damage and classifying the damage in terms of severity and implications relative to ongoing operation of the component, based solely on what normally is limited visual evidence. The American Society

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Introduction to Visual Examination

for Nondestructive Testing (ASNT) currently recognizes visual examination as a specific examination discipline, and the requirements for acquiring certification are extremely demanding.

Although not an NDE technique in itself, debris removal is often a substantial part of a visual examiner’s job. It is common for items such as screwdrivers, grippers, welding rods, nuts, and bolts to drop into a component during maintenance operations. If not removed, they could cause considerable damage when the plant is brought back to power. Various devices and attachments have been developed to facilitate this task [1].

Equipment

Visual examination encompasses a vast array of tools to access specific locations and to observe surfaces for damage that indicates certain detrimental conditions. These tools can range from the simple to the sophisticated and can include the following:

• Inspection mirrors

• Magnifying glasses

• Portable microscopes

• Fiberoptic devices of various lengths, diameters, and tip articulations

• Rigid borescopes (also known as endoscopes), which can view forward or sideways and be either fixed or rotating

• Videoimagescopes (similar to fiberscopes in appearance, but they use charged coupled device (CCD) chips rather than optics to transmit the image

• Miniature cameras (analog and digital)

Many devices are available to carry the examination equipment to the point of interest. The simplest method is to connect a camera to a stiff but flexible rod to gain access to the interior of components such as headers, but the complexity can increase to purpose-built crawlers and manipulators for steam pipes, ducts, drains, and similar components.

To facilitate a visual examination, it is becoming more common to display the image on a monitor. This not only prevents the examiner from becoming tired after peering through a fiberscope for long periods, thereby increasing the risk of missing a flaw, but also enables colleagues and plant personnel to view the image and offer their advice and interpretation. Recording the image on videotape is an attractive option because an assessment can be made off-line. If examinations are performed in subsequent years, flaw growth can be assessed from the multiple video recordings.

If video recording is performed, it is essential that the tape be annotated while the examination progresses. Many parts of power plants look similar from the inside. In order to provide the best guidance for subsequent inspections or operations, it is vitally important to know precisely where the camera/viewer was looking. As an example, consider the case of a visual examination for

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ligament cracks in a header when subsequent ultrasonic examination is used to size any cracks that are found by visual examination. It is easy to mistakenly count the rows, particularly in headers that have staggered rows. Without an accurate visual examination record, there is a risk that the ultrasonic examiner could perform NDE on the wrong ligament.

A video-typewriter is usually used to caption the screen and permanently label the videotape. A microphone can also be used to provide an audio commentary, but this is prone to error if the examiner errs in row counting or loses his or her bearings, and loud plant noises can sometimes cover up the commentary.

Several systems appearing on the market use PC technology to assist in the examination, particularly in the interpretation and assessment functions. For example, some systems allow the user to input a wire mesh model of a component (for example, a header), and then this model is superimposed over the live image and moves as the camera/viewer moves. This enables a better sense of perspective to be realized so that near and distant features can be scaled correctly.

PC software has appeared in recent years that can capture video images and convert them into appropriate formats for use in word processing packages and databases such as EPRI’s Boiler Maintenance Workstation™ (BMW) for Windows. This greatly enhances the speed and value of an examination report. Digital cameras are also available that will transfer images directly to a PC.

Photographs can provide a permanent record of both the macroscopic positions of a component system and the surface features that might change during exposure to stress, temperature, and environment [1].

Implementation

Visual examination uses a wide range of equipment and provides access to many types of components; therefore, there is no universal procedure that describes how it should be implemented. However, several common factors should be considered when planning any visual examination [1].

Access

No examination is possible unless the examiner can gain access to the component. It might not be just the camera/viewer that needs access, but the examiner might also need a light source to illuminate the scene and a carriage or transport vehicle to move the examination head. This places constraints on the diameter of the entry port, how straight it is initially (there might be a bend to pass before entering the component proper), and how much room exists between the port and the adjacent component or wall.

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The challenges to performing a complete visual examination of an operational HRSG that has seen some amount of service are difficult because of the tight spaces and close tolerances. The higher efficiency HRSGs are more densely packed with pressure parts and assembled in such a complex manner in order to extract the maximum amount of steam from the exhaust gases.

Access to tubes within the HRSG enclosure is limited. The best access is at the leading- and trailing-edge tubes. Some access is also available at the tight spaces between modules. Tube ends located outside the gas path are, in some designs, somewhat more accessible due to the absence of fins near the headers. However, the close proximity of multiple headers in a component, enclosed header vestibules, and the short tube lengths outside the gas path also make direct visual examination difficult. Mirrors are often necessary to inspect these locations. Inspections of transfer pipes between headers, header vents, drains, and supports are usually available in the upper and lower vestibule enclosures. Inspections of risers, downcomers, piping, and drums outside the vestibule must contend with insulation and lagging coverings [1].

Diagrams and Plans

It is important for visual examiners to have detailed layout and dimensional information about the component because this will invariably determine the examination system to be used. Good diagrams also help examiners assess what they are seeing. It is easy for examiners to lose their sense of scale when looking at an image on a screen: what may appear to be an enormous hole might in reality be a small pit [1].

Temperature

Many optical devices rely on glue to cement the lenses and prisms of the component together, and the glue becomes soft as the temperature rises. Eventually, the device can cease to function because the optics slip out of alignment or the image clouds over. A practical rule of thumb is to never let the device exceed 120°F (50°C). Because many fiberscopes and endoscopes are intended for the medical market and are not expected to survive in the harsh conditions found in an HRSG, it is unwise to plan a visual examination during the first week of an overhaul or during a breakdown unless there is some forced cooling [1].

Internal Conditions

Dust and grit can enter delicate camera parts or prevent crawler wheels from gripping properly. Standing water might not be a problem if the water is shallow, but sometimes the heat from the illuminating light source can cause the lens to steam up [1].

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Nearby Operations

When examining long pipe runs, the examiner must be aware of any other operations that might be happening in the vicinity. Welding and grinding operations can impair the quality of the image by interfering with the signals transmitted along the cables. Also, welding operations can raise the temperature of the component locally and out of sight of the examiner who is located at the access port [1]. See the Temperature subsection above for information about the effects of temperature.

Protection for Equipment

A visual examination can involve a large amount of equipment (camera, cables and drum, monitor, videocassette recorder, video typewriter, crawler controller, and other equipment), all of which is delicate and will be impaired by the typical conditions found in most HRSGs. Care should be taken to prevent dust, grit, and water from falling onto the equipment from above by erecting a tent or tarpaulin sheet over it [1].

Establishing a Baseline

Comparisons offer the best approach to visual examination of HRSG components. Therefore, it is important to establish a baseline condition from which comparisons can be made to identify service-induced damage. Keep in mind that the objective of comparison is to locate differences created by stress, temperature, or environmental deviations. Baseline measurements of header/harp positions, tube alignment, and concentricity of enclosure penetrations will help identify movements that occur during startup and shutdown, indicative of high stresses. The initial HRSG examination should document all areas of slight misalignment resulting from manufacture and erection so that these anomalies will not be mistaken for service-induced damage [1].

Care in Choosing Inspection and Documentation Angles

The viewing direction of a set of photographs or video recordings should be selected with the forethought of how changes might occur. For instance, bent tubes entering a header should be photographed at a slight angle that would emphasize slight misalignment due to plastic deformation. Photographs taken to emphasize subtle differences in operating temperature should be taken nearly straight on in order to highlight the surface condition of the components and the natural emissivity of the tube surfaces. The camera flash is more effective in bringing out subtle discoloration effects due to differences in operating temperature, scale formation, oxidation, and exfoliation that can indicate overheating. Figure 1-1 illustrates these two photographic techniques [1].

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Figure 1-1 Recommended Photographic Angles to Highlight Tube Plastic Deformation (Left) and Overheating (Right)

Applications

Visual examination can be applied to any component in which the damage manifests itself at accessible surfaces to a degree that permits detection by visual means. Considerable ingenuity can often be exercised to produce a viewing and transport system for most plant components.

It is strongly recommended that a complete set of photographs or videotapes of all accessible areas of each component be generated during the commissioning phase of the HRSG life cycle.

Visual examination of drum internal surfaces offers a unique perspective on the water quality maintained during HRSG operation. A visual observation of wall loss and damage to separators, particularly in low-pressure (LP) drums, is a key indicator of flow-accelerated corrosion (FAC). Drum deposits, residues, pitting, and the color of the internal oxide are all indicative of the potential for internal corrosion damage. Visual examination of evaporator tubing can detect heavy internal deposits, which can be weighed and analyzed via tube sampling during planned inspections. Dividing the total weight per unit area by the effective service hours since the last inspection or cleaning will establish a rate of formation. Changes to this rate of formation can be representative of the risk for corrosion-activated damage and deterioration. Analysis of internal tube deposits should be done to evaluate the possibility of active underdeposit corrosion mechanisms (hydrogen damage, acid phosphate corrosion, or caustic gouging).

Each drum—low-pressure (LP), intermediate-pressure (IP), and high-pressure (HP)—will develop its own natural rate of deposition. Photographs of the drum’s internal surfaces can be used to track the amount of pitting and the formation of oxides. The coloration of the drum internals is affected by the oxidation reducing potential, the pH, and the presence of impurities or additives. In a multipressure HRSG, each drum can exhibit different characteristics. For

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example, if a reducing all-volatile treatment controls the feedwater entering the LP drum, the LP drum might exhibit black magnetite deposits (indicating that the LP evaporator and economizer circuits might be prone to FAC). This would signal the need for extra examination of the LP evaporator outlet bends and the drum steam separator cylinders. Variations can also be present on either end of the drum or above and below the water line in the steam drum.

Steam drum internal piping systems are used to feed boiler water, inject chemicals, extract samples, and remove unwanted solids. Visual examination should verify that the chemical injection, feedwater inlet, and blowdown piping holes are free of obstructions and deposits. The orientation of the blowdown holes is crucial for optimum removal of solids entrained in the circulating water systems.

Visual examination is also a valuable tool for assessing the extent of degradation when HRSG tube failures have occurred. Fatigue failures can exhibit evidence of deformation, misalignment, and rubbing. These signs should be sought out during repair of a leak, and adjacent tubing should be evaluated to prevent additional failures. Analysis of any external corrosion or overheating will also benefit from careful documentation by visual examination. Internal corrosion failures are not likely to exhibit any signs of deterioration on the external surfaces, and a drum internal surface inspection might be valuable [1].

Debris Removal

The removal of debris from inside a component is not an NDE technique, but it is a task that often befalls the visual examiner. The first task is to find the debris so that all of the techniques that are used to find component flaws can be brought to bear on locating unwanted material. After the debris is located, the second task is to retrieve it. Examples of debris that can be found in HRSG tubing are shown in Figure 1-2. The upper left photo shows shot blast media; the upper right photo shows exfoliated oxide scale, and the bottom photo shows a rock-like particle.

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Figure 1-2 Examples of Debris Found in HRSG Tubing

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Many devices have been constructed to achieve debris removal; some devices are purpose-made, and others can be improvised when required. Examples of debris-retrieval equipment include the following:

• Vacuums

• Magnets (for steel objects)

• Claws and grippers

• Impalers (for softer debris such as rags or foam)

• Baskets, nets, and cages that drop over the object

• Rotating spirals (Rotating a horizontal-axis spiral forward over an object traps the debris between the turns, and then the spiral can be pulled backward to remove the debris.)

• Sticky tape

• Suction cups

• Blowers (to move the debris to a section having easier access)

Care must always be taken to prevent the examination equipment itself from becoming stuck inside a component [1].

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2 ASSESSMENT OF DAMAGE

Introduction

After HRSGs are placed into service, it is desirable to initiate a proactive (predictive or preventive) tube failure program. This requires the combined use of periodic NDE, on-line monitoring, predictions about life and cost, and engineered corrective actions. There are various nondestructive and destructive methods to assess HRSG tube condition. The EPRI report Heat Recovery Steam Generator Tube Failure Manual (1004503) provides information on the locations and types of damage mechanisms encountered in HRSGs [5], and the EPRI report Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators (1004506) provides an overview of some NDE methods, along with guidance on where and how to perform the examinations [6].

Features of Failure

After the EPRI report 1004503 [5] has been consulted for types and locations of damage, it is important to review the EPRI report 1004506 [6] for the NDE method that should be applied for detection and assessment of the specific damage mechanism in each location.

When visual examination is suggested for the damage type, the equipment and recommendations included in Sections 3 and 4 of this report should be put to use.

Failures due to cracking are readily detectable with visual examination. In addition, wall thinning due to erosion or corrosion that results in a change in the surface condition is amenable to visual examination.

Severe leaks and ruptures, while relatively rare, are of great concern because they have resulted in personnel injuries and fatalities at nuclear plants, conventional plants, and industrial plants. As a consequence, flow-accelerated corrosion (FAC) is among the most extensively studied failure mechanisms, and the available information on it is applicable to HRSGs.

FAC is a damage mechanism that has caused metal losses and failures in piping, HRSG tubing, steam drum walls and internals, and other components composed of carbon or low-alloy steel. FAC damage occurs only under specific conditions of flow, water chemistry, geometry, and material, and it is most troublesome over a relatively narrow temperature range.

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Assessment of Damage

In HRSG units, FAC occurs under both single-phase (water) and two-phase (water and steam) flow conditions. Because water is necessary to dissolve the oxide layer, FAC does not occur in lines transporting dry or superheated steam [5].

The latest statistics for HRSG tube failures indicate that FAC is the second most important failure mechanism. Failures have essentially occurred in LP evaporators and economizers, but a few failures have also occurred in HP economizers. FAC occurs across the temperature range 160–570ºF (70–300ºC) with a maximum near 300ºF (150ºC); therefore, the regions of concern include economizer tubes at inlet headers, LP evaporator tubes especially at bends, LP drum internals, and horizontal LP evaporator tubes at bends. It should, however, be noted that IP evaporator tubes can also move into the susceptible range if a triple-pressure HRSG is operated at a reduced pressure [8].

Two-phase FAC has been recognized as a worldwide problem since about 1970. Since the mid-1980s, single-phase FAC has been acknowledged as a major problem in the balance-of-plant and secondary piping of U.S. and foreign nuclear and fossil plants [7].

Both single-phase and two-phase FAC can occur in the LP evaporator circuits, and it is imporant to recognize exactly which type is occurring [8].

With FAC, the normally protective iron oxide layer (magnetite) on carbon or low-alloy steel dissolves into a stream of flowing water or a water-steam mixture. The oxide layer becomes thinner and less protective, and the corrosion rate increases. Eventually, a steady state is reached in which the growth of the oxide and the iron oxide dissolution rates are equal and stable overall corrosion rates are maintained. In some areas, the oxide layer can be so thin as to expose an apparently bare metal surface. More commonly, however, the corroded surface exhibits a shiny black color typical of magnetite. The corrosion and oxide dissolution process reduces the component thickness until it fails due to ductile overload [5].

Damaged surfaces show certain characteristics during visual examination. Figures 2-1 and 2-2 illustrate examples and show the characteristics. In cases of single-phase FAC, the damaged surface typically exhibits an orange-peel appearance, which is sometimes more apparent if examined at low magnifications, as shown in Figure 2-1(a) and Figure 2-2(a). Other typical characteristics of single-phase FAC are chevron marks toward the extremities of the damage (in areas of slower FAC damage). In cases where two-phase flow is present, the appearance of FAC is more scalloped or wavy, as indicated in Figure 2-1(b) and Figure 2-3. Sometimes this appears as “tiger stripes” (alternate bands of rapid FAC and slow or nonexistent FAC). In many cases, both types can occur in the same tube region. Single-phase (water) FAC can and does occur in circuits where two-phase flow predominates; such areas have commonly been at tight 180º bends, such as shown in Figure 2-2(a). Often in these areas, the re-establishment of two-phase flow after the bend area is accompanied by blistered or boxlike magnetite as is clearly shown in Figure 2-2(b), (d), and (e) about one tube diameter downstream of the single-phase FAC [5].

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Another important feature to note is the total lack of any protective magnetite on the tube surface in areas of severe FAC, as shown in Figure 2-1(c) and Figure 2-2(c). Metallographically, FAC preferentially attacks the pearlite colonies of the carbon steel tube microstructure as shown in Figure 2-1(d) [5].

Figure 2-1 Example of Flow-Accelerated Corrosion in Vertical Low-Pressure Evaporator Tubing

Note: In Figure 2-1, (a) shows single-phase FAC, (b) shows two-phase FAC, and (c) and (d) show the lack of any protective magnetite on the tube surface. The arrows in (d) point to the preferential FAC attack of the pearlite colonies in the carbon steel.

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Figure 2-2 Example of Flow-Accelerated Corrosion in Horizontal Low-Pressure Evaporator Tubing

Note: In Figure 2-2, (a) shows a region of single-phase FAC at a tight 180º bend, where the flow is from left to right; (b), (d), and (e) show typical formations of “boxlike” and “blistered” magnetite; and (c) shows the lack of protective magnetite in the severe FAC areas.

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Figure 2-3 Visual and Metallographic Characteristics of Two-Phase Flow-Accelerated Corrosion

Note: Figure 2-3 illustrates the scalloped or wavy appearance that is typical in tubes damaged by two-phase FAC.

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3 VISUAL EXAMINATION EQUIPMENT: OPTICAL AND MECHANICAL AIDS

Introduction

The human eyes are the most important tool when performing visual examination; however, there are many situations where they are not sensitive enough, not accurate enough, or not able to adequately access the area to be examined. Numerous types of mechanical and optical equipment are available to supplement the eyes and to allow performance of a more complete examination. This section presents detailed design, operation, and characteristics of some of this equipment to facilitate its more effective use.

Optical Aids

Mirrors, magnifiers, borescopes, fiberscopes, binoculars, and remote visual inspection systems all aid in performance of visual examinations. Borescopes and fiberscopes provide the most common NDE methods used today for internal examinations.

Currently, there are no nationally recognized standards for fiberscopes, borescopes, and other remote visual inspection systems or for the examination techniques that use them. Uniform standards, specifications, accepted procedures, and examiner qualifications are all prerequisites to formal recognition of visual examination as a viable NDE method. In addition, one criterion for acceptance is the ability to provide reproducible results in hard copy form (for example, photograph, videotape, or digital image). Although such systems have long been available, most users still rely entirely on the subjective determination by the individual examiner with no actual record of what was seen. Digital cameras make visual examinations more objective. These cameras allow the examiner to take numerous photos and store the images digitally on a card or disk. The stored digital image can be downloaded, printed, and attached to the visual examination report.

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Visual Examination Equipment: Optical and Mechanical Aids

Basic Elements of Visual Examination

Visual examination is composed of five basic elements:

• Examiner

• Object being examined

• Optical instruments

• Illumination

• Recording method

Each element requires interaction with the others and affects the final results of the examination. This discussion focuses on fiberscopes and borescopes because they are the most widely used.

Object Being Examined

The object (work piece) to be examined must be considered. This critical aspect determines the specifications for the instrument and the illumination required. Some of the factors to be considered are the following:

• Object distance

• Object size

• Discontinuity size

• Reflectivity

• Entry port size

• Object depth

Object Distance

The object distance (see Figure 3-1) is important in determining the illumination source required as well as the required objective focal distance for the maximum power and magnification.

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Figure 3-1 Object Distance

Object Size

When combined with distance, the object size (see Figure 3-2) determines what lens angle or field of view is required in order to observe the entire surface, particularly with side-viewing borescopes.

Figure 3-2 Object Size

Discontinuity Size

The size of any discontinuities considered significant or critical (see Figure 3-3) determines the magnification and resolution required. For example, greater resolution is required to detect hairline cracks than to detect severe undercut.

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Figure 3-3 Discontinuity Size

Reflectivity

Due to changes in reflectivity (see Figure 3-4), light-absorbent or dark surfaces, such as those coated with carbon deposits, require higher levels of illumination.

Figure 3-4 Reflectivity

Entry Port Size

The entry port size (see Figure 3-5) determines the maximum diameter of the instrument that can be inserted into the work piece.

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Figure 3-5 Entry Port Size

Object Depth

If portions of the object are in different planes, the scope must have sufficient focus adjustment or depth of field to sharply visualize these different planes. Figure 3-6 shows varying depths of an object such that the focus must be adjusted between viewing the near surface and the far surface of the object.

Figure 3-6 Object Depth

The combination of all of these factors determines the optical and physical characteristics of the proper instrument for each particular examination problem regarding diameter, length, illumination, direction of view, field of view, magnification, resolution, and depth of field. Many hundreds of combinations are possible. However, some of the characteristics are essentially

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contradictory and at times require compromise. For example, Figure 3-7 shows how a wide field of view reduces magnification but has a greater depth of field, whereas a narrow field of view produces higher magnification but results in a shallow depth of field. This is explained further in the paragraphs that describe the particular instruments.

Figure 3-7 Viewing Angle Versus Depth of Field

Optical Instruments

Fiberscopes

The industrial fiberscope is a flexible instrument used to view inside small areas and around hard-to-reach areas. Its multilayered sheath protects two fiber-optic bundles, each of which is composed of tens of thousands of glass or quartz fibers. One bundle serves as the image guide, while the other bundle assists in illuminating the object (see Figure 3-8).

Figure 3-8 Fiberscope

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Fiberscopes usually have a controllable bending section near the tip so that the observer can direct the scope during the examination and scan an area inside an object. Fiberscopes are made in a variety of diameters and lengths, with a choice of distals for different viewing directions. Fiberscopes are used primarily to examine around corners such as curved tubing and piping, pumps, and valves, as well as other components where direct visual access is impossible.

According to the laws of physics, light travels only in straight lines. However, fiber optics enables us to bend light around corners without actually contradicting this basic principle. When high-quality optical glass is drawn into thin fibers, it is quite flexible. Therefore, it is possible to transmit light in a curved path without defying physical laws. This concept is shown in Figure 3-9. The fibers are only 0.000256 to 0.00118 in. (6.5 to 30 microns) in diameter, or roughly one-fourth the thickness of a human hair.

Figure 3-9 Light Reflection Within a Glass Fiber

Because a single, thin fiber is unable to transmit a satisfactory amount of light, thousands of these fibers are arranged in a bundle for transmitting sufficient light and image.

In order to prevent the light from diffusing, each individual fiber consists of a central core of high-quality optical glass or quartz coated with a thin layer or cladding of doped glass or quartz with a different refractive index (see Figure 3-10). This cladding acts like a mirror; most of the light that enters the end of the fiber is reflected internally as it travels and is prevented from escaping or passing through the sides to an adjacent fiber in the bundle.

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Figure 3-10 Glass Fiber Construction

Although the light is now effectively trapped within each fiber, not all of it emerges from the opposite end. No system ever provides 100% efficiency. Some of the light is absorbed along the way by the medium itself. The amount of absorption depends on the length of the fiber and the optical quality of the medium. For example, plastic fiber transmits light, but it is less efficient than glass or quartz and is unsuitable for use in fiberscopes. Quartz, on the other hand, is an efficient transmitter of light.

The fiber bundle called the image guide assembly in Figure 3-11 is used to carry the image formed by the objective lens at the tip of the scope back to the eyepiece. This is a coherent bundle, meaning that the individual fibers must be precisely aligned so that they are in identical relative positions to one another at their terminations.

Figure 3-11 Image Guide Usage

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Image guide fibers range from 0.000256 to 0.000669 in. (6.5 to 17 microns) in diameter. Their size is one of the determining factors of resolution, although the preciseness of alignment is important.

A real image is formed on the highly polished object end of the image guide. Many fiberscope manufacturers provide distal tip focusing. This focusing is done by remote control at the viewing end of the scope. These scopes also have a diopter adjustment at the eyepiece to compensate for eyesight differences.

The other fiber bundle, called the light guide bundle, is used to carry the light from the external, high-intensity source to illuminate the object and is noncoherent. These fibers are generally about 0.00118 in. (30 microns) each in diameter. The size of the entire bundle is determined by the diameter of the scope.

Fiberscopes usually have a controllable bending section near the tip so that the observer can direct the scope during examination and be able to scan an area inside an object. Fiberscopes are made in a variety of diameters and lengths. The diameter of the scopes can be as small as 1/32 in. (0.795 mm) and as large as 3/4 in. (19 mm). Lengths of glass scopes can be up to 45 ft (14 m) and quartz scopes can be up to 300 ft (90 m).

Good-quality fiberscopes are expensive but cost effective. They are used whenever it is necessary to examine around corners, such as within curved tubing, or when no entry port is available that would permit a direct line of sight to the area requiring examination.

Borescopes

A lens optic device is called a borescope. It differs from the fiberscope in that it is not flexible. Modern borescopes use a fiber-optic light-guide system—similar to the fiberscope—around the outside perimeter of the borescope.

In contrast to fiberscopes, the borescope (see Figure 3-12) is a rigid instrument that can be used to examine the inside of tubes, pipes, and other components. Originally invented to examine the bores of rifles and cannons, it was a thin telescope with a small lamp at the tip for illumination. This illumination system is today considered obsolete due to both its inadequacy and its safety hazard. Modern borescopes use a fiber-optic light guide system as in the fiberscope.

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Figure 3-12 Borescope

The image is brought to the eyepiece by an optical train that consists of an objective lens, sometimes a prism, relay lenses, and an optical lens (see Figure 3-13). The image formed is not a real image, but an aerial one; that is, it is formed in the air between the lenses. This means that it is possible to provide both diopter correction for the observer and control of the objective focus with a single adjustment of the focusing ring at the eyepiece. This focus control greatly expands the depth of field over nonfocusing or fixed-focus designs, while at the same time compensating for the wide variations in eyesight among the population of users.

Figure 3-13 Borescope Functionality

Because borescopes lack flexibility and the ability to scan areas, the specifications regarding length, direction of view (see Figure 3-14), and field of view (see Figure 3-15) become more critical to achieve a valid examination. For example, the direction of view should always be specified in degrees rather than in words or letters. Tolerances should also be specified. Some manufacturers consider the eyepiece to be 0º and, therefore, a direct view scope would be 180º. Other manufacturers start with the scope tip as 0º and then count back toward the eyepiece.

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Figure 3-14 Direction of View

Figure 3-15 Field of View

In order to use a borescope efficiently, the direction of view and field of view must be known. This is accomplished by placing the scope carefully on a 1-in. (25-mm) grid, prepared as in Figure 3-16, and viewing through the scope, looking to the edges of the scope. The maximum angle line seen at the edge of the scope is the field of view. The direction that the scope must be placed on the grid is used to determine the direction of view. It is not necessary to be overly critical in positioning the scope. This simple procedure will give both the direction of view and the field of view.

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Figure 3-16 Measuring Field of View

Figure 3-17 shows actual photos that were taken through scopes that were identical in every respect except for the field of view. Each photo was taken at the same distance from the object, but with fields of view of 20º, 40º, 60º, and 80º. Note how the lens angle affects the area seen and the magnification.

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Figure 3-17 Fiberscope Magnification

A relatively recent variation of the rigid borescope is the mini-borescope (see Figure 3-18), in which the relay lens train is replaced with a single solid fiber. Light passing through this fiber actually bends, and at a specific interval, an image is formed. This solid fiber is approximately 0.040 in. (1 mm) in diameter, making possible high-quality, high-resolution viewing in extremely small areas of examination.

Figure 3-18 Mini-Borescope

Other recent changes in the industry relative to fiber optics have made available nearly every type of examination equipment that could be needed to inspect critical, hard-to-reach areas of components. A brief description of some of the equipment follows.

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Lecturescope

The lecturescope is a flexible, dual-eyepiece viewer for all flexible fiberscopes. It allows two examiners to view the same image simultaneously. Separate diopter controls on each eyepiece ensure clear images for both examiners, regardless of their individual eyesight.

Photographic Equipment Systems

Automatic exposure, photographic fiber-optic systems (see Figure 3-19) enable easy fiberscope and borescope photography. Built-in automatic spot metering eliminates the need for exposure adjustment and ensures high-quality inspection photographs. The system also features an illuminated liquid crystal display (LCD) viewfinder, which allows metering for manual exposure settings. A built-in diopter control compensates for eyesight variations, and shutter speeds can be adjusted up to 1/2000th of a second with outstanding accuracy.

Figure 3-19 Fiber-Optic Accessories

This system also provides a connection for a Polaroid attachment, allowing the examiner to switch back to a 35-mm format with the flip of a switch. The use of the Polaroid instant camera allows the examiner to ensure that the setup is correct before taking the 35-mm picture.

This fiber-optics system uses high-resolution monitors for viewing by many people. This configuration is warranted when space is limited or when access is reduced. A videotape recorder provides a permanent record of the entire inspection. This system is available in black and white as well as color and can be used with both fiberscopes and borescopes.

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Ultraviolet Light Sources

This light source easily switches from white light to ultraviolet light for use primarily with the flexible fiberscope. This system helps examiners locate otherwise undetectable minute flaws by switching from white light to ultraviolet and filtering out all light except the ultraviolet wavelength range.

Extend-a-Scope

The extend-a-scope system offers inspection personnel ease and versatility for viewing internal industrial areas up to 50 ft (15 m) away. As many as five interchangeable viewing heads are provided. These screw easily onto the probe end to permit full scanning of the entire inspection area. Each of the heads has a 35° field of view. This particular system is extremely useful when inspecting several locations along the length of a pipe or components.

Remote Visual Inspection Systems

Remote visual inspection systems, such as the videoprobe, rely on microelectronics to transmit an image to a video monitor for display. The image is picked up and transmitted by a tiny electronic sensor that is embedded in the moveable distal tip of the probe. This sensor, a charged coupled device (CCD), acts like a miniature camera, sending image signals to the video processor where they are assembled and relayed to the video monitor for display.

Fiber optics is applied in order to carry light down to the area being examined. In terms of image resolution, fiber optics is rapidly approaching its limits due to the problems of constructing thinner fibers for coherent imaging bundles. Glass fibers naturally deteriorate with time. When individual fibers in the bundle break, defects referred to as the “salt-and-pepper effect” appear in the image. Some glass imaging bundles also absorb the blue wavelengths of light, thereby altering color in the observed image.

By using electronics to transmit the image back to the viewer, a videoprobe can eliminate these problems. There is no image fiber to break or deteriorate, and there are no broken fibers to cause dark spots that might intrude on the field of vision. Another advantage with this fiber-optic device is that images can be stored on a computer and transferred over phone lines, allowing off-site personnel to perform analysis of the image.

Remote visual inspection systems can have a diameter as small as 1/4 in. (6.35 mm) and can be up to 50 ft (15 m) in length. These systems are expensive, but the costs are more reasonable today than they were five years ago. Today, a high-quality remote visual inspection system can cost as little as U.S. $16,000. One commercially available system is shown in Figure 3-20.

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Figure 3-20 Remote Visual Inspection System

Accessories

Many accessories are available for fiberscopes and borescopes. Polaroid cameras, 35-mm cameras, and both super-8 and 16-mm cinema cameras can record the examination. Closed-circuit television (CCTV) displays with or without videotape are common, although color television is less common. Also available are attachments at the eyepiece that permit dual viewing or right-angle viewing for convenience where headroom is minimal. Several of these accessories are shown in Figure 3-21.

Figure 3-21 Fiberscope Accessories

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Illumination

Fiber-optic light guides are capable of transmitting the light from high-intensity projection lamps of 150 watts (W) or more through the scope to provide good lighting, and so tiny lamps at the scope tip are now obsolete. But fiber-optic lighting has not solved all of the problems. Certain applications and test objects present difficult problems when the light must be projected 1–2 ft (30–60 cm) into a carbon-coated cavity such as the combustion chamber of an industrial turbine. Also, smaller entry ports limit the total scope diameter, which consequently reduces the size of the light guide bundle within it. One can imagine that if the total outside diameter of a scope is only 0.080–0.120 in. (2–3 mm) including the wall thickness of the probe, there is not much room inside to fit both the lens or image guide system and the light guide.

One way to keep the illumination system as efficient as possible is to make the light guide fibers continuous from light source to scope tip. Fifty percent of the light is lost wherever there is an interface or connection (see Figure 3-22).

Figure 3-22 Illumination Is Reduced by 50% at an Interface

Another way to increase brightness is to use more powerful lamp sources. This is not quite as simple as merely using a 500-W tungsten lamp instead of a 150-W lamp. Due to its much larger filament size, the 500-W lamp cannot be focused to a fine spot at the end of the fiber bundle; therefore, any gain in output is minimal. Also, the 500-W lamp produces so much heat that it soon melts the end of the fiber bundle binder and burns it, forming a carbon layer that reduces the light output to almost nothing. Therefore, when using a lamp of greater than 150 W, it is necessary to use either a mercury-arc or xenon lamp. These can produce significantly more light, but because they require a high-voltage starter and intricate electronics, they are potentially a greater safety hazard. They are also larger, heavier, and more expensive.

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Recording Method

The information received from the visual examination can be recorded by either the permanent method or the subjective method.

Permanent Method

The permanent method produces a visual record by means of a photograph, videotape, cinema film, or a graphic image on a computer. Naturally, this has distinct advantages. The permanent results can be compared to a set of “normal” or “abnormal” standards. Comparison can be made with records of prior examinations to determine whether crack growth or other progressive changes have occurred. Several persons can study the record, and expert opinions can be obtained. Eye fatigue is reduced, and corrections for faulty vision are more easily accomplished. Thus, the decision can be more objective.

Subjective Method

With the subjective method, the examiner makes an immediate decision based solely on what he or she actually sees and relies solely on memory for any comparisons. This is the most commonly used method today, making standardization difficult if not impossible. A person’s visual acuity and competence are the paramount factors that determine whether the examination was valid. The percentage of accuracy is, of course, less than when using the permanent method.

Mechanical Aids

Many different types of measuring devices are commonly available for use in visual examination. These include such devices as steel rules, micrometers, calipers, and welding gages of numerous types. Information on the basic functions and proper use of these devices is typically included in the operating manuals or instruction sheets.

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4 REMOTE AND SPECIALIZED VISUAL EXAMINATION EQUIPMENT

Introduction

Visual examinations of HRSGs and associated systems are performed to determine the condition of critical inner surfaces. These areas include the high-stress points at the junction of the tubes with the headers, surfaces that protect all or part of the system from corrosion, and the inner surfaces of the associated systems, including the piping (especially at elbows and bends), heat exchangers, pump internals, and valve bodies.

Examinations can be conducted with the component empty, such as in the case of an HRSG, or full of water with components in place. Pipe runs, heat exchangers, and pumps should be completely disassembled for the most effective examination. This is not, however, the most efficient way to conduct an examination. Unless the components are disassembled, it is virtually impossible for the examiner to be close enough to the object of interest to perform an unprotected or unaided visual examination.

Remote Visual Examination of Components

Lights and Illumination

The characteristics of observed objects become more visible with increased light or illumination. The trade-offs are that more heat is generated with brighter wattage and shorter bulb life can be expected.

A common lighting specification for a lighting attachment to cameras or in conjunction with pan, tilt, and zoom cameras is as follows:

Two 35-W lamps with focused dichroic reflectors:

• 35-W flood: 1500 candlepower (cp) at 30º (half-angle) beam spread

• 35-W wide: 3000 cp at 20º (half-angle) beam spread

• 35-W spot: 8500cp at 10º (half-angle) beam spread

Turbo lamp mode: 90 W

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Remote and Specialized Visual Examination Equipment

Lenses

Different lenses can be selected to give a specific field of view, focal length, and magnification. A zoom lens can be selected with variable focal length. Zoom lenses can provide a range of detailed close-up shots or broad overall views. The field of view can be made narrower or wider depending on the setting. An assembly of lenses can be arranged with three groups of elements. The front-focusing objective group of lenses can be adjusted over limited distance with an external focus ring to fine-focus the lenses. Between the front and rear groups of lenses is a movable zoom group of lenses. The zoom group, the focusing group, and the rear stationary relay group of lenses determine the final image size when it comes to a focus on the camera sensor.

Videocassette or Digital Recorder

A videocassette recorder (VCR) consists of the scanner, transport system, servo controls, frequency response, and signal processing components. The scanner sweeps across the videotape, laying down a series of magnetic tracks with a video head. The transport manually threads and guides the tape through the cassette and into the VCR. The servo controls keep the video tracks at precise angles, start the tracks at precise times, and keep the signals in synchronization. The VCR processes the video signal before and after magnetic tape recording. The video tape contains video and audio channels of recorded video signals that have been frequency modulated. Different formats and speeds deliver the different resolutions commonly available today. Super video home system (S-VHS) has been the standard until lately. A 1/2-in. (1.27-cm) tape contains a maximum of 400 horizontal lines of image. As price continues to come down, digital recording will be assisted with “time-compressed analog” technology and bring digital recording media into common usage. Reducing the large number of bytes making up a digitized image will make digital image recording practical. Then 550 horizontal lines of information per frame will be normal.

During applications, the scan should be stopped, and the camera held perfectly still for at least 10 seconds to allow enough time for the reviewers to evaluate the image. One minute is preferable. Photographs can now be made from the tape, and computer-enhanced analysis can be conducted, if required. The examiner must keep in mind the importance of time during the examination. If a determination of the on-screen condition cannot be made readily, photographs should be taken for further evaluation so that the examination can continue.

Printers

Printers are typical components of visual inspection systems. High-quality digital printers make the hard copy prints useful for viewing before the final report is completed. Modern electronic files can transmit an image into a report with high-quality resolution. The best resolution remains the live monitor image at the time of acquisition.

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Monitors

The more lines of resolution the camera can capture and the video recorder can process, the more lines the monitor has to work with. But if the monitor has fewer lines of resolution than the rest of the video system, the higher-quality image is lost. On the other hand, if the monitor has more lines of resolution than the camera, video recorder, and processor, it cannot improve the quality of the image. It is important that the monitor be matched with the rest of the system for optimum performance. The size and shape of the landing beam in the monitor, the luminance distribution with the phosphorescent spot, the number of scan lines, and the bandwidth of the circuit all limit the ability of the monitor to display image detail.

Cables

Coaxial cable is by far the most common material used for video signal transmission. In a coaxial cable, a center conductor is surrounded by heavy wall insulation, around which is a braided or foil shield. The distance that a signal can be sent over a coaxial cable is related to the size of the central conductor as well as the quality and amount of the shield wire. A heavier wire will result in less signal loss. The most commonly used coaxial cable is RG-59U for distances up to 100 ft (30 m). RG-11U would be used for runs up to 1000 ft (300 m). Exceptionally long distances, in excess of 1000 ft (300 m), would use fiber-optic cable.

Today, optical fibers are being used to transmit video signals. The most critical part of any fiber-optic system is the connector. The face of the fiber at each connection must be polished to prevent signal loss. The connection must be aligned with another fiber to send or receive light.

The many elements of a visual examination system must be matched in order for the system to perform optimally. The lighting, scene reflectivity, sensitivity, lens quality, number of pixels, cable length, horizontal lines of resolution in the camera and the monitor, and number of recorded lines of resolution in the recording medium (VHS or digital) and the final report all play a major role in the degree of detail observed and recorded.

Calibration

Equivalent resolution to the direct visual method must be obtained when using remote visual equipment. The verification of resolution of the monitoring system is performed using a tight-wire standard target. The target is a wire that can be held before the camera at the point of the examination (see Figure 4-1). An ideal standard would contain a sufficient number of wires of different sizes to enable the limits of resolution of the system to be clearly established.

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Figure 4-1 Calibration Standard for Remote Visual Examination

Special Requirements

Television System Examinations

This section reviews some special requirements when using remote visual inspection equipment to perform the required examinations.

The most important aspect of using remote equipment is to ensure the availability of the required equipment. Because an equipment failure can result in an aborted examination, it is essential that the performance of remote equipment be verified before the examination is attempted. All television systems should be checked with cables that will be used in the examination, not just

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with test cables. A pressure test should be made on any underwater cameras. A camera that checks leak-tight at the surface might not be leak-tight at the working depth.

All camera mounts should be assembled to the camera in order to ensure compatibility. If cameras depend on the use of underwater tools, the availability of these tools should be verified.

Fiber-Optic Examinations

When using fiberscopes, borescopes, or associated equipment, remember that these instruments are delicate and must always be handled with great care by personnel who are trained in the use and care of the equipment.

Test Requirements

The following aspects should be considered when determining proper instrument specifications and illumination requirements:

• Objective distance is important in determining the required illumination source, as well as the required objective focal distance, for the maximum power and magnification.

• When combined with distance, the object size determines what lens angle or field of view is required to observe the entire surface. This is particularly important with side-view borescopes.

• The size of discontinuities that are considered to be significant determines the magnification and resolution required.

• Light-absorbent or dark surfaces, such as those coated with carbon deposits, require higher levels of illumination. Clean these areas to remove excess buildup where possible.

• The entry port size determines the maximum diameter of the instrument that can be inserted into the work piece.

• If portions of the object are at different depths, the scope must have sufficient focus adjustment or depth of field to visualize these different depths sharply.

Illumination

In most fiberscope, borescope, or television monitoring examinations, the light levels are relatively low compared to normal daylight conditions. Fortunately, light guides for fiber optics are capable of transmitting the light from high-intensity projection lamps of 150 W or more through the scope to provide good lighting. However, monitoring can be more complicated, so the examiner should plan as far in advance as possible to ensure that proper and adequate lighting is available for internal vessel examinations.

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Personnel

The examiner should ensure that appropriate compliance is maintained regarding examiner certification. The examiner should also ensure that each individual who is using special remote equipment is properly trained and is capable of using and maintaining the equipment before allowing the examination to proceed.

Photographic Techniques for Recording Results

The purpose of this section is to provide some useful techniques that will yield better results when photographing various plant components in conjunction with visual examination; it is not intended to provide an in-depth study of photographic techniques.

Depth of Field

Depth of field can be defined as the overall sharpness of focus apparent in a photograph. When trying to photograph a subject, only a single plane through the subject is actually in focus. This plane is called the principal plane of focus. When working at higher magnifications, this effect becomes even more significant. In a typical 35-mm camera, the lens diaphragm provides a degree of control over the thickness of the principal plane of focus or depth of field (see Figure 4-2).

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Figure 4-2 Depth of Field

Focusing should normally be done with the lens diaphragm completely open for best accuracy and image brightness. This step ensures that the principal plane of focus has been established. If the lens diaphragm opening is now reduced, portions of the subject—both in front of and in back of the principal plane of focus—appear sharper. Continuing to close down the lens diaphragm increases this effect. By adjusting the lens diaphragm, the depth of field can be controlled effectively, and focus of the final picture can be predetermined.

Using a standard 35-mm camera and lens, the best control over depth of field can usually be obtained by focusing one-third into the region or area of the discontinuity of interest. This is because the depth of field or area of sharpness (using a 55-mm lens) extends farther behind the principal plane of focus than it does in front of it. As magnification is increased (90-mm and 120-mm lenses), the reverse is true. A good general principle when considering depth of field is

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that it is affected by only two main factors: lens diaphragm opening and the image or subject magnification.

Because the most photographed discontinuities are three-dimensional, there is another factor to consider. The magnification will be exact only at the principal plane of focus. Where measurements of overall size of a discontinuity are to be made directly off the final print, the principal plane of focus must be at the widest part of the subject (see Figure 4-3).

Figure 4-3 Principal Plane of Focus for Measuring Dimension A–B Off the Print

Lighting

In general, when photographing plant components or discontinuities, lighting should be provided from the top of the subject being photographed. Lighting should also be provided from one direction on most three-dimensional objects to avoid ambiguity in relief. If supplementary lighting is required, it should be slightly weaker and more diffused than the main lighting source.

A common problem in trying to photograph plant components (for example, piping welds) is a hotspot caused by unwanted reflections of the flash unit on the subject itself. Such reflections can usually be eliminated by moving the flash unit to direct the specularly reflected light away from

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the lens. Another effective method of eliminating subject reflections is to bounce the flash off a piece of white cardboard (see Figure 4-4).

Figure 4-4 Bounce Lighting

Film

The actual size of the negative directly affects the quality of any printed enlargements. The larger the negative, the better the enlargement. Selecting film speed is another important decision. Several factors influence this decision, among them the amount of light available on the subject and how large a print is required from the film negative.

High-speed film requires less light but produces “grainy” prints. This graininess is increased as the size of the enlargement is increased. Slow-speed films are used where fine detail is required. The drawback to using a slow-speed film is that it requires more light on the subject.

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There are two systems for rating film speed. In the United States, the ASA number (International/American) is used, and in Europe, the DIN number (German) is used. Table 4-1 shows the various ASA and DIN speed numbers available for both black-and-white and color films.

Table 4-1 Film Speeds Available

Color Black and White Speed

ASA DIN ASA DIN

25 15 32 16

32 16 64 19 Slow

— — 80 20

64 19 100 21

80 20 125 22

100 21 160 23 Medium

125 22 — —

160 23 200 24 Fast

200 24 400 —

Digital Cameras

Digital cameras (see Figure 4-5) are becoming a mainstay for visual examinations. These cameras allow examiners to take numerous photos and store the image digitally on a card or disk. Unlike film, the camera uses a sensor, a charged coupled device (CCD), or a complementary metal oxide semiconductor (CMOS) to convert light into an electrical charge. An analog-to-digital (A/D) converter will convert this information into digital information. The digitized information is then manipulated by a microprocessor and displayed on an LCD. This is one of the great advantages of the digital camera: instant feedback. Because these images are electronic, they can be inserted into reports, downloaded to printers, or saved on discs for years. Most cameras have either serial, parallel, SCSI, or USB ports that allow the image to be downloaded directly to a computer. In addition, many cameras have small, removable, solid-state flash memory devices that allow the image to be saved.

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Figure 4-5 Digital Camera

Digital cameras collect the available light and focus it on the sensor. The focal length is the distance between the lens and the surface of the sensor. As the focal length is increased, greater magnification occurs, and objects appear to get closer. Just the opposite occurs when the focal length is decreased: the object appears further away, but the camera captures a wider field of view. Table 4-2 equates the focal length of a digital camera with that of a 35-mm film camera.

Table 4-2 Focal Length

Focal Length

Digital Camera

35-mm Camera

View Typical Uses

5.4 mm 35 mm Objects look smaller and further away.

Wide-angle: landscapes, large buildings, groups of people

7.7 mm 50 mm Objects look about the same distance as what your eye sees.

Normal shots of people and objects

16.2 mm 105 mm Objects are magnified and appear closer. Telephoto shots, close-ups

Digital cameras can have an optical zoom, a digital zoom, or both. An optical zoom actually changes the focal length of the lens. As a result, the image is magnified by the lens. This effect is a true zoom that improves the quality of the pictures. A digital zoom is a computer trick that magnifies a portion of the information that hits the sensor. The camera uses only half of the information received by the sensor and disregards the rest, using interpolation techniques to add detail to the picture. The same effect can be achieved by taking a picture without using the zoom and enlarging the picture with your computer.

Replication

In replication of a component surface, the replica must equal the quality of the component surface. Therefore, replication materials must be selected carefully. The first thing to consider when preparing for replicating a surface is how to contain the replicating medium. If the medium

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is a softened plastic, containment can be limited to uniform pressure on the plastic during replication. However, with a casting medium such as a dental replication material or castable silicon, other containing devices might be necessary. Because casting is the most common method of replication, this discussion will be limited to that method.

After damming the area to be replicated, a release agent should be applied to the surface. The release agent usually is an oil-based material, such as petroleum jelly, dissolved in a suitable non-residue solvent. The release agent should be sparingly sprayed or painted on the surface. Enough time must be allowed for the solvent to evaporate before putting the replicating medium on the surface.

Most of the castable materials available today are not acceptable for replication because they shrink too much during the curing process or because they can chemically attack metal surfaces. It is best to obtain materials that shrink less than 0.5% and that are chemically compatible with the surface being replicated. Dental reproduction materials meet these requirements well; however, they tend to cure quickly, usually in 5 to 15 minutes. There are also castable silicon materials available that cure in 30 minutes to 24 hours that work well. It is important to remember that all of these materials increase in viscosity with time. As a result, the activator should not be added until the examiner is ready to cast the surface.

When mixing the activating agent into the replication medium, care must be taken to mix well and not to introduce gas bubbles. Gas bubbles can collect on the component surface and create a hole in the replica. The casting medium should be poured onto the component’s surface slowly and uniformly. Force should not be used to cause the medium to flow across the surface because gas bubbles can get trapped on the surface. After completing the pour, the replica should not be disturbed until it is fully cured.

After curing, damming material is carefully removed from the surface before the replica is removed. The replica should be carefully lifted from the surface, starting at a corner. If resistance is encountered, a new location should be tried. This process is continued until the replica is removed. The replica should then be carefully placed on its back and allowed to age for at least 24 to 48 hours before handling.

The replica is a negative of the component surface. If a duplicate positive surface is required, a second casting must be made, using the negative replica and following the same rules just discussed.

Summary

The lead examiner must be either somewhat or completely involved in the overall remote visual examination plan and activities. In either case, the examiner must be knowledgeable of the equipment that will be used, as well as the characteristics and precautions of the examinations to be performed. While remote and specialized inspection equipment has been highlighted, the examiner should be familiar with all components to be examined, as well as the equipment to be used, to ensure proper training and supervision.

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5 SUMMARY

This report provides an overview of suitable visual examination techniques for performing flaw and damage detection in tubing and piping materials in HRSG plants. The described visual examination techniques are applicable from the inside surface of components if access to components to be tested is either available or made available. Applying the appropriate NDE methods to each of the components and damage mechanisms is key to maintaining safe continued operation.

Realizing the attributes of the various damage mechanisms and the strengths and limitations of the examination equipment is key to the performance of a meaningful visual examination.

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6 REFERENCES

1. Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators. EPRI, Palo Alto, CA: 2004. 1004506.

2. Nondestructive Evaluation of Component Interiors. EPRI, Palo Alto, CA, 1990. NP-6832.

3. M. J. Fletcher. “A Review of Optical Inspection Methods,” Insight, Vol. 38, No. 4, April 1996.

4. NDE Guidelines for Fossil Power Plants. EPRI, Palo Alto, CA: 1997. TR-108450.

5. Heat Recovery Steam Generator Tube Failure Manual. EPRI. Palo Alto, CA: 2002. 1004503.

6. Interim Guidelines for the Nondestructive Examination of Heat Recovery Steam Generators. EPRI. Palo Alto, CA: 2004. 1004506.

7. Flow-Associated Corrosion in Power Plants, Revision 1. EPRI/Electricité de France. Palo Alto, CA: 1998. TR-106611-R1.

8. Guidelines for Controlling Flow-Accelerated Corrosion in Fossil and Combined Cycle Plants. EPRI, Palo Alto, CA: 2005. 1008082.

9. Electric Power Research Institute, Visual Examination Technology 102.

10. Bill Bailey, “The Case of Eye Test Standardization,” Material Evaluation, Vol. 40, No. 8, ASNT, Columbus, OH.

11. Brank Becher, “An Analysis of Optical Factors in the Total Internal Visual Inspection System,” Olympus Corporation of America, New Hyde Park, NY.

12. ASME Boiler and Pressure Vessel Code, 1995 Edition, Section V, “Nondestructive Examination,” American Society of Mechanical Engineers, New York, NY.

13. American Society for Nondestructive Testing, Nondestructive Testing Handbook, 2nd Edition, Volume 8, Visual and Optical Testing, 1993.

14. Everest VIT, “Ca-Zoom PTZ-4.2 with Unitized Pan & Tilt,” Specification, 2001.

15. Everest VIT, “Remote Visual Testing Training Course Manual,” 1999.

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Program:

Heat Recovery Steam Generator (HRSG) Dependability

1008092

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Examination of Heat Recovery

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