Boiler Tube Failures Overview

Boiler Tube Failures

• Boiler tube failures are a significant cause of forced outages in fossil fuel power plants
• These failures contribute to a 6% overall availability loss across units
• Failures are often linked to poor initial design, fabrication practices, fuel changes, operation, maintenance, and cycle chemistry

Presentation Layout

• Introduction to boiler tube failures
• Statistics on tube failures
• Cost analysis of tube failures
• Types of tube failures and associated failure mechanisms
• Factors influencing tube failures
• Methods for detecting tube failures
• Guidelines to prevent and control tube leaks
• Best practices for preventing tube leaks

Boiler Pressure Component Failure

• Boiler pressure components are complex, critical, and vulnerable in fossil fuel power plants
• Failures in these components historically lead to the highest percentage of lost availability
• These failures are often associated with poor initial design, fabrication practices, fuel changes, operation, maintenance, and cycle chemistry

Statistics of Tube Failures

• Statistical data on boiler tube failures is from the CEA
• A significant portion of data is related to water wall, superheater, economizer, and reheater failures across various capacity units (MW) across various types of boilers
• Data shows the frequency and impact of outages on different types of units

Boiler Tube Failures Post Overhaul

• Percentage of failures within a week after overhaul. Data collected between April 2007 and December 2011 shows varied failure rates (percentage) across different power plants

Boiler Tube Failure Zones

• Regions of the boiler known for higher failure rates. The percentages of failures differ with differing locations in the boiler system

Cost of Boiler Tube Failures

• Costs consist of repair expenses, startup oil costs, and lost production costs.
• Approximate repair costs are $200,000 per day
• Hourly downtime costs are associated with the lost generation
• Startup oil costs are roughly $7,500,000 per 150KL startup

Prominent Reasons for Tube Failures

• Poor water quality: With increasing operating pressure, feedwater quality is crucial.
• Changing coal quality: Adversly impacts boiler operability, and reliability.
• Cycling operation: Design issues and rapid changes affect the boiler's reliability and are likely sources for corrosion and damage
• NOx emissions: Deep staging combustion for NOx reduction has been implicated in serious waterwall fire corrosion in high-sulfur coal-fired supercritical units.
• Ageing: Many fossil fuel units are exceeding their design life, leading to increased stress and failure risks.

Economizer Effects

• Finned economizer tubes are difficult to inspect and measure due to poor access for thickness measurement and shielding.
• Economizer tube bends close to casing walls are also difficult to access

Typical Failure Mechanisms

• Boiler tubes operate in high pressure and temperature environments
• Failures can be caused by mechanical/thermal processes including creep, fatigue, erosion, and corrosion

Creep Failures - Overheating

• Short-term: Results in a ductile rupture where the fracture surface looks like a thin edge. These occur more commonly during startup.
• Long-term: Tube metal accumulates significant stress affecting the boiler tubes over long usage cycles

Fatigue

• Damage caused by repeated stresses
• Corrosion fatigue and thermal fatigue are common types of fatigue in tubes
• Tubes experience varying temperatures and flow causing damage from internal cracking and external surface cracking, amongst other effects

Erosion and Corrosion

• Erosion: Particle strikes and metal removal
• Corrosion: Material deterioration due to chemical attack. Types of corrosion include internal corrosion to external corrosion in the different areas of the boiler

Internal Corrosion-Hydrogen Damage

• Excessive hydrogen deposition along tube surfaces contributes to failures
• pH fluctuations, condenser leaks, and acidic deposits contribute to hydrogen damage and deterioration

Waterwall Fireside Corrosion

• Corrosion occurs on external waterwall tube surfaces.
• Reducing atmospheres during combustion are a leading cause, particularly in pulp and paper industry boilers.
• Improper burner operation can exacerbate corrosion.

Dissimilar Metal Weld (DMW) Failure

• Occurs often at the weld fusion line and may fail catastrophically
• Caused by high stresses at the interface and differing expansion properties between the various types of materials

Waterside Corrosion Fatigue

• Tube damage from thermal fatigue and corrosion
• Boiler design, water chemistry, oxygen content, and boiler operation affect the extent, occurrence, and severity of corrosion fatigue.
• Breakdown of protective magnetite scale that exposes the tubes to corrosion
• Locations with attachments (e.g., buckstays, seal plates, scallops) are more prone to this specific effect

Factors Influencing Boiler Tube Failures

• Data summaries the respective influence of various causes including design, operation, and maintenance issues

Water-Touched Tubes

• Detailed table showing influences of design, operation, and maintenance causes

Critical Considerations during Boiler Design

• Material selection is critical based on expected operating temperatures.
• Economizers and waterwall sections usually employ mild or medium carbon steel.
• Superheater and reheater sections often utilize low alloy ferritic steel, while highly stressed zones employ austenitic stainless steel.
• ASME codes are based in part on mid-wall tube temperatures

Metal Temperature Limits

• Data table including various tube steel types, ASME specifications, and maximum allowable temperatures for various manufacturers

Material and Fabrication Flaws

• Defects introduced during manufacturing, storage, or installation, These flaws include forging laps, inclusions, lack of fusion, deep tool marks, gouges, and dents
• Flaws lead to failures, especially in high-temperature zones

Control of Boiler Tube Leak During Design

• Adequate furnace sizes, alignment of coils, uniform spacing, material selection, and maintenance locations must be considered during this stage

Engineering and Erection Stage Aspects

• Tube material procurement and technical specifications must be sound according to codes
• Design reviews and mechanical details should consider reliability
• Fabrication processes should use conservative procedures for new materials.
• Inspection and documentation of weld locations and areas with changed tube material should be detailed
• Alignment, spacing, support, and quality of weld joints is critical.

Manufacturing and Erection Stage Aspects

• Quality checks throughout manufacturing, alignment of coils/panels, spacing across coils and walls, erection quality, and quality of site weld joints

Commissioning and Operation Stage

• Removal of temporary supports/structures
• Hydro test of pressure parts (and other checks)
• Quality of various chemical cleaning processes, and steam blowing
• Repeat CAVTs (Cold Air Velocity Testing) after correction of issues
• Optimizing soot blowing operations, and other various checks

Techniques to Detect Boiler Tube Leaks

• Extensive Dye Penetrant Testing for identifying flaws
• Oxide scale measurement to help determine the lifetime remaining on tubes by measuring the temp effects
• Modern techniques for flaw detection

NDT Techniques

• Describing Remote Field Electromagnetic Technique (RFET) as a suitable means for identifying internal flaws
• Low Frequency Electromagnetic Technique (LFET) allows scanning of tube surfaces for inspecting flaws

Time of Flight Diffraction (TOFD) Technique

• Describing how ultrasonic inspection can identify cracks in thicker components
• Checking for cracks and lack of fusion

CFD Modeling and CAVT Testing

• Using CFD modeling to predict flow profiles to detect potential issues
• Performing CAVT (Cold Air Velocity Test) to assess air flow, and using thermography to assess for issues
• Identifying potential blockage issues in water-wall tube walls

Robotic Inspection Using Magnetic Flux

• Using robotic inspections for detecting thickness loss in water wall tubing
• Boroscopic inspections are used for identifying crack issues

Monitoring Techniques

• Water temperature excursion monitoring
• Chemical parameter monitoring for pH, Na, DO, NH3 & PO4
• Dissolved oxygen level monitoring
• Detecting boiler tube leaks through Acoustic Steam Leak Detection

Operation and Maintenance Guidelines to Address Boiler Tube Leaks

• Procedures (and considerations) to be taken for controlling boiler tube leaks (including DMW, excessive fly-ash deposition)
• Specific actions to be taken during various stages of operations and maintenance

Approach Methodology to Prevent Boiler Tube Leaks

• Flowchart for approach method including pre-checks for possible BTL or other potential issues, and the various tracks
• Steps from before and during outage to after and during post-outage checks

Measures to Control Boiler Tube Leaks

• Procedures that should/must be maintained to reduce the risk
• Considerations for utilizing the steps effectively

This quiz explores the critical issue of boiler tube failures in fossil fuel power plants, highlighting their impact on availability and safety. It covers the statistics, types of failures, and methods for detection and prevention. Gain insights into the causes and best practices to mitigate these failures effectively.