HRSG Inspection Planning Guide

Second Edition

P. Jackson D. Moelling

J. Malloy M. Taylor

Copyright © 2006 by Tetra Engineering Group, Inc. All Rights Reserved. Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. ISBN 0-9719616-2-X Cover Design by Natalie Vasa

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TABLE OF CONTENTS FOREWORD ..................................................................................................................... iii 1. OVERVIEW ............................................................................................................ 1-1 2. REASONS FOR INSPECTING HRSGs................................................................. 2-1

2.1. Statutory Requirements.................................................................................... 2-1 2.1.1. General..................................................................................................... 2-1 2.1.2. United States & North America............................................................... 2-1 2.1.3. European Union ....................................................................................... 2-2

2.2. Improving Reliability and Equipment Life...................................................... 2-2 3. PLANNING & PREPARATION FOR THE INSPECTION................................... 3-1

3.1. General............................................................................................................. 3-1 3.2. Safety First!...................................................................................................... 3-1 3.3. Identify and Locate HRSG Components ......................................................... 3-2 3.4. The Inspection Plan.......................................................................................... 3-4

4. WHAT TO LOOK FOR - DAMAGE AND DEGRADATION MODES IN THE HRSG............................................................................................................................... 4-1

4.1. Overview.......................................................................................................... 4-1 4.2. Pressure Boundary Damage............................................................................. 4-1

4.2.1. Corrosion-Fatigue .................................................................................... 4-4 4.2.2. Creep/Creep-Fatigue ................................................................................ 4-7 4.2.3. Graphitization .......................................................................................... 4-8 4.2.4. Deposition and Under-Deposit Corrosion ............................................... 4-9 4.2.5. Erosive Wear and Flow Accelerated Corrosion (FAC) ......................... 4-10 4.2.6. External Corrosion and Oxidation ......................................................... 4-15 4.2.7. Acid Dewpoint Corrosion ...................................................................... 4-17 4.2.8. Fatigue.................................................................................................... 4-18 4.2.9. Pitting..................................................................................................... 4-21 4.2.10. Stress Corrosion Cracking ..................................................................... 4-22 4.2.11. Thermal Overstress ................................................................................ 4-25 4.2.12. Tensile Overload.................................................................................... 4-27 4.2.13. Thermal Quench..................................................................................... 4-29 4.2.14. Wear....................................................................................................... 4-30 4.2.15. Weld Defects.......................................................................................... 4-31

4.3. Drums, Deaerators, Feedwater and Blowdown Tanks................................... 4-35 4.3.1. Drums..................................................................................................... 4-35 4.3.2. Deaerators and Feedwater Tanks ........................................................... 4-36 4.3.3. Blowdown Tanks ................................................................................... 4-36

4.4. Structures ....................................................................................................... 4-41 4.4.1. Duct, Casing and Insulation................................................................... 4-41 4.4.2. Supporting Structural Steel .................................................................... 4-44 4.4.3. Flow Conditioning Devices ................................................................... 4-47 4.4.4. Hangers and Supports ............................................................................ 4-50 4.4.5. Expansion Joints .................................................................................... 4-54 4.4.6. Penetrations and Seals............................................................................ 4-56 4.4.7. Stack, Silencers and Dampers................................................................ 4-59

4.5. Steam Piping Attemperator Sprays................................................................ 4-60 4.6. Emissions Equipment and Controls ............................................................... 4-63 4.7. Burners........................................................................................................... 4-65

5. Selecting Inspection Frequency ............................................................................... 5-1 5.1. Introduction...................................................................................................... 5-1

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5.2. Discussion........................................................................................................ 5-1 5.3. Statutory Limits ............................................................................................... 5-1 5.4. Service Based (EOH) Planning........................................................................ 5-1 5.5. Condition Based Planning................................................................................ 5-1

5.5.1. Shift Inspection ........................................................................................ 5-1 5.5.2. Monthly Scheduled Inspection ................................................................ 5-1 5.5.3. Annual Inspections................................................................................... 5-2 5.5.4. Major Overhaul........................................................................................ 5-3

6. Inspection Technology for HRSGs.......................................................................... 6-1 6.1. Overview of Inspection Methods..................................................................... 6-2 6.2. Visual and Optical Techniques ........................................................................ 6-3

6.2.1. Visual Inspection (VT) ............................................................................ 6-3 6.2.2. Borescope Inspection ............................................................................... 6-8 6.2.3. Surface Replication................................................................................ 6-12 6.2.4. Inspection of Welding............................................................................ 6-12

6.3. Ultrasonic Inspection (UT) ............................................................................ 6-14 6.4. Radiographic Inspection (RT)........................................................................ 6-17 6.5. Liquid Penetrant Testing (PT) ....................................................................... 6-19 6.6. Magnetic Particle Testing (MT, WFMT)....................................................... 6-20 6.7. Eddy Current Testing..................................................................................... 6-23 6.8. Laser Profilometry ......................................................................................... 6-23 6.9. Acoustic Emission (AE) ................................................................................ 6-25 6.10. Infrared Thermography (IR) ...................................................................... 6-25 6.11. Destructive Examination of Components .................................................. 6-27 6.12. Chemical Analysis ..................................................................................... 6-28 6.13. In-Situ Materials Testing ........................................................................... 6-28

7. Recommended Techniques for Inspection............................................................... 7-1 8. Case Studies ............................................................................................................. 8-1

8.1. Visual Inspection for HRSG Condition Assessment ....................................... 8-1 8.2. Flow Accelerated Corrosion Baseline ............................................................. 8-5 8.3. RT of Cracked Reheater Tubes........................................................................ 8-6 8.4. WFMT of HP Evaporator Feeder Piping ......................................................... 8-6 8.5. Evaporator Tube Inspection with Laser Profilometry ..................................... 8-8 8.6. PT of HP Drum Internal Welds ....................................................................... 8-9

9. REFERENCES ...................................................................................................... 9-10 10. APPENDICES ................................................................................................... 10-1

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FOREWORD Inspection is part of routine maintenance for any Heat Recovery Steam Generator (HRSG). Visual inspections are performed at regular intervals in accordance with the requirements of regulatory bodies and insurers. In the US statutory inspections are mandated typically every year although some jurisdictions allow justification for longer intervals. Additional inspections are sometimes performed to establish the baseline condition of the HRSG (often early in life, but not always) or to perform less frequent special inspections to confirm component integrity.

The high costs associated with unexpected forced outages for merchant units or for units in deregulated markets encourage efforts to anticipate problems in HRSG components that are susceptible to service related damage. Damage mechanisms vary in severity based on materials selected by the HRSG OEM, local operating stresses and the interaction between vibration and corrosion mechanisms. In addition to fundamental design differences, modern HRSGs are also subject to a wider range of duty in power applications. Baseload operation was normal for smaller HRSGs, but today’s deregulating electric power market requires many new large HRSGs to operate in cycling duty. This has resulted in many units experiencing damage much sooner than would be expected with baseload operation. Some cycled units have experienced tube failures and leaks after only a few thousand hours of operation. The more severe duty from cycling operation reveals design weaknesses earlier at many newer combined cycle plants, as well as not infrequently fabrication-related deficiencies

Emerging HRSG designs require components to operate at higher pressures in the future to produce more steam at higher temperatures. The result is greater risk of damage due to creep, corrosion fatigue and from increased thermal stresses, often associated with poor draining of condensate on startup or flow maldistributions. Heavy supplemental firing by duct burners has also substantially increased the metal temperatures of superheater components due to radiant heat fluxes. If water-side deposits have accumulated on downstream tubes, underdeposit corrosion can cause failure after only a few years of operation.

Flow Acceleration Corrosion (FAC) and erosive wear occurs in both single and two-phase flows, preferentially affecting carbon steel pressure parts at geometric discontinuities where locally high flow rates coupled with adverse water chemistry conditions result in a failure to maintain protective oxide films. The result has been premature tube failures at numerous combined cycle plants. FAC and erosive wear are problems that affect older as well as newer HRSGs.

This Inspection Planning Guide provides general guidance for planning inspections of HRSG components, where this is taken to mean all significant piping and structures within the HRSG OEM supply boundary including accessories such as duct burners and emissions control equipment. Valves and process instrumentation are not included.

The objective is to give the inspector the means to effectively anticipate and identify problem areas, recognize active damage mechanisms and respond to changes in modes of plant operation. This is particularly important for HRSGs that are being cycled if they were designed for baseload operation. Additional detailed information on HRSG tube failure mechanisms can be found in the companion volume to this guide - the HRSG Tube Failure Diagnostic Guide [1].

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This 2nd edition is updated with additional valuable information from the hundreds of field inspections performed by Tetra Engineering staff of horizontal and vertical gas path HRSGs since the last edition was released. There is expanded coverage on inspecting drums and deaerators, more material on failure modes and mechanisms, and additional appendices. More consideration is given to inspections of vertical gas path units and there are further case studies from field inspections.

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1. OVERVIEW The HRSG inspection is part of the process to maintain high reliability and efficiency in the combined cycle plant operation. As such the inspection planning process must accommodate several differing requirements:

• Statutory or other mandatory inspection requirements

• Preventive maintenance requirements

• Problem tracking and resolution requirements

When developing the plan one must try to answer the following questions:

1) When to inspect? When to perform inspections is a combination of several factors:

• Statutory requirements on inspection frequency • Timing of unit outages for scheduled maintenance • Urgency of known problems in the HRSG

(cracking, underdeposit corrosion, FAC, erosive wear, cold end corrosion, etc.)

• Availability of inspection support 2) Where to inspect? The scope of inspection locations in and around the

HRSG will depend on a number of factors • Available time for inspection (including

cooldown/heatup time) • Effort required to gain access such as draining

drums, removing baffles, installing scaffolding/skyclimbers (if required)

• Estimate of potential damage risk to key components

• Status of known problems 3) What to look for

in the inspection? A pre-inspection review of operating and maintenance data can have great value in answering this question. For example, an analysis of water chemistry logs over the period leading up to the outage is invaluable in determining if there is an increased potential for certain types of degradation within the HRSG.

4) What inspection techniques should be used?

The choice of appropriate inspection techniques will depend on allowable time, accessibility and the type of problems under consideration.

5) What are the post inspection requirements?

Post-inspection requirements are as important as the inspections themselves. These include reporting, recordkeeping, input to further repair actions or engineering assessments.

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Eddy current probes can detect thinned areas but are not well proven in HRSG applications. Radiography can also be used in some locations to assess thinning damage; see Section 6.4 for more information.

Figure 4-7. Thinning at IP Evaporator Tube Upper Bends (2-Pressure HRSG)

Figure 4-8. Mid-Span LP Evaporator Tube Leak Caused by FAC

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4.4. Structures HRSG Structures include:

• Supporting Structural Steel

• Gas Tight Ducting

• Insulation and Insulation Liner

• Expansion Joints

• Casing Penetrations

• Piping Supports and Hangers

• Stack

• Silencers

• Stack Dampers and Bypass Dampers

HRSG Structures are exposed to weather, hot turbine exhaust gases, steam and water leaks and emissions control chemicals. Deadweight, wind, thermal and seismic stresses also are applied to the HRSG. These components are typically inspected on an opportunistic basis, for example during annual statutory inspections by the insurance company’s boiler inspector or during scheduled work on other major components such as the GT.

4.4.1. Duct, Casing and Insulation Early HRSGs had frequent problems with the duct and casing/liners. HRSG OEMs have responded to these problems with improved materials, improved bolting patterns and related improvements to designs of other components such as duct burners that have been responsible for damage to liners in certain configurations.

Casing liner plates are often found damaged in the transition duct at the exit of the gas turbine. This is particularly common in ducts with abrupt geometry changes such as 90° angles in the duct or other sharp angled transitions. Another common location for liner damage is at the transition from the circular to rectangular duct. Casing liner plates have often cracked and require weld repairs in these areas.

These problems have resulted in insulation movement and loss with the result being hot spots. Improvements have included ceramic fiber insulation, injected insulation and improved liner designs that allow more thermal expansion. Liner plate buckling due to overheating is relatively common at older units due to overheating, particularly in hard-fired units.

When casing liner plates are breached, there is always the possibility that insulation will be torn loose, particularly in areas with high gas velocity. The result has been gross blockage of downstream finned tubes, obstruction of CO catalyst and/or SCR catalysts. These obstructions affect the intended function of the downstream components, in addition to the potential for creating local hot spots where the insulation has been removed.

Loss of washers and restraining nuts is relatively common as indicated in Figure 4-46. This problem often originates when the spot weld on the nut has broken or was never performed. With time the nut or washer will vibrate off or the washer will wear through the bolt or liner plate. As it is usually difficult given the time at hand to check the condition of each washer/bolt, look for wear marks made by the vibrating/rotating washer on the liner plate.

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7. RECOMMENDED TECHNIQUES FOR INSPECTION Figure 7-1 provides a cross-reference to recommended techniques for different parts of the HRSG and related critical components to detect and characterize different damage mechanisms. An inspection matrix is provided for all major portions of the HRSG. Destructive examination is not included. It is assumed that this is performed only on failed components or on un-failed similar components after failure of a nearby component.

Prior to performing an inspection, plans should be made to identify higher risk locations within each HRSG area that will be inspected. This may entail engineering assessments to estimate locations:

1. Areas with excessive local temperature that may experience creep and creep-fatigue damage after significant operating hours

2. Components with the highest local stress that may result in fatigue or corrosion fatigue

3. Areas expected to have the greatest risk of FAC wear damage or erosive wear damage

4. Locations susceptible to cold end corrosion

Using these types of assessments provides a way to prioritize the inspection locations while maintaining a manageable scope within the limits imposed by the outage window.

A quick guide on which NDE technique to use for a particular type of defect is provided in Appendix E.