Turbine Lifecycle Management using NDT Techniques

Table of Content

• Process Flow of Turbine Operation
• Types of Turbines
• Key Turbine Components Subject to NDT
• Common Failure Mechanisms in Regular Turbines
• NDT Techniques for Turbine Monitoring
• Impact of NDT on Turbine Lifecycle Management
• Key Takeaways

Turbines are integral to modern power generation, harnessing various energy sources to produce electricity. The global turbines market was valued at USD 99.7 billion in 2023 and is projected to reach USD 138.99 billion by 2031, reflecting a CAGR of 4.24% from 2024 to 2031 (Markets & Data, 2023). This growth is highlighted by the increasing demand for renewable energy and efficient power sources.

The COVID-19 pandemic initially disrupted the global turbines market, causing a decline in installations in 2020 due to supply chain interruptions and travel restrictions. However, the market has shown resilience, bouncing back as governments implement stimulus measures and the demand for renewable energy surges.

As turbines become prevalent in various industries, the need for thorough and regular inspections has become paramount. By employing advanced NDT Techniques, industry professionals can identify and address issues before they escalate, thereby extending the lifespan of turbine components and reducing downtime.

Process Flow of Turbine Operation

The operation of turbines, irrespective of their nature, operating fluid, or application, follows a general flow, which includes:

• An energy source (Fuel, Steam, Water, Wind) that determines the initial energy input.
• Fuel combustion, steam generation, or kinetic energy harnessing to cause an energy conversion.
• The energy conversion that causes the Turbine Blades to rotate.
• The resulting rotation drives a generator or mechanical process.
• A final output is obtained as electricity generation or mechanical output.
• The spent energy is then expelled or recycled for efficiency.

Types of Turbines

Image Credit: Wikimedia

Turbines have helped the world move both literally and figuratively. Numerous varieties are available in the market today, classified generally based on the nature of the fluid used in its operation. Some of the turbines used in modern industries include:

1. Solar Turbines:

Solar turbines are fundamental to solar thermal power plants, converting solar energy into mechanical work or electricity. 

• Operation: These turbines operate in Concentrated Solar Power (CSP) systems, using mirrors or lenses to concentrate a large area of sunlight onto a small area. The sunlight heats a working fluid (such as molten salt). The heated fluid generates steam, which drives the turbine to produce electricity.
• This mechanism involves the principles of Thermodynamics, Optics, and Heat Transfer.
• The high temperatures in these systems require monitoring using advanced NDT methods to check for thermal stresses, material degradation, and structural integrity.
• Solar Turbines are used in the Renewable Energy industry, in Concentrated Solar Power (CSP) plants.
• They are also used in Power Generation, to integrate with grid power for peak load management and in High-temperature applications in manufacturing.

2. Transonic Turbines:

Image Credit: SemanticScholar

• Transonic turbines operate at speeds that approach or exceed the speed of sound commonly used in high-performance aerospace and power generation applications. 
• The extreme conditions created in the transonic flow require specialised NDT to detect shockwave-induced stresses and aerodynamic heating.
• Operation: In Transonic Turbines, high-speed air flows through turbine blades. In turn, kinetic energy is converted to thermal energy. This turbine uses the mechanism of Performance Optimization, in managing shockwaves and aerodynamic heating.
• This mechanism involves the principles of Aerodynamics, Thermodynamics, and Material Sciences.
• Transonic Turbines aid in the Aerospace industry with Jet engines, space propulsion systems, Power Generation, High-efficiency gas turbines, and experimental and advanced propulsion systems.

3. Industrial Gas Turbines:

• Industrial gas turbines are designed for higher efficiency and reliability, converting natural gas or other fuels into mechanical energy or electricity. 
• Their operational demands require meticulous NDT Methods to detect high-temperature corrosion, thermal fatigue, and material degradation.
• Operation: In an Industrial Gas Turbine, fuel is mixed with compressed air and ignited. High-pressure, high-temperature gases expand through turbine blades, causing rotation. Mechanical energy from turbine rotation is then converted into electricity via a generator.
• This mechanism involves the principles of Thermodynamics, Fluid Dynamics, and Heat Transfer.
• The Oil and Gas Industry, Power Generation, and Industrial Manufacturing often use Industrial Gas Turbines.

4. Gas Turbines:

• Gas turbines operate on the principle of high-temperature combustion driving turbine blades. 
• Rigorous NDT is essential to detect thermal fatigue, stress corrosion, and material degradation.
• Operation: In Gas Turbines, air is compressed, mixed with fuel, and then ignited. High-pressure gases expand through turbine blades, causing rotation. The spent gases are expelled, completing the cycle.
• The principles of Thermodynamics and Fluid Dynamics are used in Gas Turbines.
• The applications of gas turbines are generally within the Oil and Gas, Aerospace, and Power Generation Industries.

5. Steam Turbines:

Image Credit: EnergyEducation

• Steam turbines convert thermal energy from steam into mechanical work, commonly used in power plants, refineries, and industrial processes. 
• The high-pressure, high-temperature conditions make them prone to erosion, corrosion, and creep, necessitating NDT protocols.
• Operation: In Steam Turbines, steam is generated by heating water. Steam expands through turbine blades, causing rotation. The steam is then condensed into water for reuse.
• The mechanism of steam turbines uses the principles of Thermodynamics and Heat Transfer
• Steam Turbines are used in Power Generation, Refineries and Industrial Processes.

6. Nuclear Turbines:

• Nuclear turbines are integral to Nuclear Power Plants, converting steam into mechanical energy using nuclear reactors. 
• Nuclear Turbines are used in nuclear power plants.
• Operation: Nuclear Turbines use fission to heat water and generate steam. This steam expands through turbine blades, after which it is condensed and recycled.
• This turbine uses the principles of Nuclear Physics and Thermodynamics.

7. Hydroelectric Turbines:

• Hydroelectric turbines harness kinetic energy from water flow to generate electricity. 
• Under operation, they face unique challenges such as cavitation, corrosion, and structural stress.
• Operation: In Hydroelectric Turbines, water is directed through penstocks, driving the turbine blades. The mechanical energy is converted into electricity using a generator.
• This method uses the principles of Kinetic Energy Conversion and Hydrodynamics.
• Additional Types of water turbines include: 

1. Pelton Turbine: An impulse turbine used for high-head hydroelectric sites.

2. Francis Turbine: A reaction turbine widely used for medium head and flow.

3. Kaplan Turbine: A Francis turbine variation, used in low-head, high-flow applications.

4. Turgo Turbine: A modified form of the Pelton wheel, for medium head sites.

5. Tyson Turbine: This is a conical water turbine with helical blades. It's used for specific low-head applications.

6. Cross-Flow Turbine: Also known as Banki-Michell or Ossberger turbine, suitable for small hydro project

• Hydroelectric urbines are used in Power Generation and Water Management, among other applications.

Image Credit: NavyHistory

8. Marine Turbines:

• Marine turbines are used in propulsion and electricity generation on ships, with water flow driving the turbines.
• They face challenges such as biofouling, corrosion, and mechanical wear.
• The principles used in Marine Turbines include Fluid Dynamics and Material Science.
• They are used in Military Vessels and Commercial Shipping.

9. Wind Turbines:

• Wind turbines convert kinetic energy from the wind into mechanical energy and then electricity. 
• They are used in the Renewable Energy industry.
• Operation: Wind turns the turbine blades, which drive a generator to produce electricity.
• The principles involved in wind turbines include Aerodynamics and Energy Conversion.

The complex mechanisms used in these turbines require advanced NDT methods to ensure efficiency and a long lifespan. This must be done while addressing industry-specific challenges and maintaining operational safety.

Also Read: Wind Turbine Performance Optimisation through Blade Inspection

Key Turbine Components Subject to NDT

Given the high operational demands and harsh conditions turbine components endure, advanced NDT methods are essential to detect defects prematurely preventing potential failures. Key components include the following:

Image Credit: MaritimeProfessional

1. Rotor and Blades:

Blades are the primary energy conversion elements in turbines, subjected to immense stress from high-speed rotation and varying loads. Rotors and Blades can encounter subsurface defects such as cracks, voids, and delaminations, and Surface wear, such as erosion, and fatigue.

NDT Techniques to assess rotors and blades:

• Phased Array Ultrasonic Testing: To detect subsurface flaws with accurate detection and characterisation of internal defects.
• X-ray Computed Tomography: Provides a detailed 3D imaging of internal structures. This results in a comprehensive analysis of complex geometries and internal conditions.
• Close Visual Inspection: Inspecting rotor and blade components visually from a close proximity to identify surface defects and wear.
• Boroscopy: Using borescopes to inspect internal parts of rotor and blade components that are not easily accessible.
• Inspection with Drones: Utilising drones equipped with cameras to inspect hard-to-reach areas of the rotor and blades, such as tips and edges, to detect damage and defects.

Image Credit: Olympus-ims

2. Stators and Vanes:

Stators and vanes direct fluid flow within the turbine, experiencing significant thermal and mechanical loads. Stators and vanes are prone to surface cracks, thermal fatigue, corrosion and material degradation due to high temperatures and fluid flow.

NDT Techniques to assess stators and vanes:

• Eddy Current Testing: To identify surface and near-surface cracks and material defects on conductive surfaces. 
• Thermographic Testing: To detect thermal anomalies indicative of defects. This technique provides real-time imaging of temperature variations, highlighting areas of concern.

3. Combustion Chambers:

Combustion chambers operate under extreme conditions, making them prone to cracking and material degradation. The defects in combustion chambers may include thermal cracks, material erosion, stress corrosion, high-temperature oxidation, and fatigue.

NDT Techniques to assess combustion chambers:

• Acoustic Emission Testing: Monitors real-time changes and crack propagation.
• Infrared Thermography: This can be used to identify hot spots and thermal fatigue.

4. Casings and Housing:

Casings provide structural support and containment, requiring thorough inspection to ensure operational safety and integrity. Casings and Housings may encounter internal cracks, weld defects, structural deformities, surface corrosion, and material fatigue.

The NDT techniques used to inspect casings and housings:

• Radiographic Testing: To reveal internal defects through X-ray imaging.
• Magnetic Particle Testing: This can detect surface and near-surface discontinuities in ferromagnetic materials.

5. Bearings and Shafts:

Bearings and shafts safeguard the rotational integrity and smooth operation of turbines. They can be prone to wear, misalignment, surface fatigue, cracking, and spalling in bearing surfaces.

The NDT Techniques used to inspect bearings and shafts:

• Vibration Analysis: To monitor mechanical vibrations to detect imbalances and wear through vibration signatures.
• Oil Analysis: Can assess lubricant conditions to identify wear particles and contaminants and obtain insights into the condition and health of rotating components.

6. Nozzles and Buckets:

These components direct and harness fluid flow, facing erosion and thermal stresses. Nozzles and buckets often encounter erosion, cracking, and material loss due to fluid impact. They may also be subject to thermal fatigue and deformation.

The NDT Techniques used to assess nozzles and buckets:

• Shearography: To detect subsurface defects by measuring surface deformation.
• Laser Doppler Vibrometry: Assesses dynamic behaviour and integrity, providing precise measurements of vibrational characteristics and structural integrity.

7. Seals and Gaskets:

Seals and gaskets ensure airtight and fluid-tight connections within the turbine system. These components may experience wear, deformation, leakage, and material degradation due to thermal and chemical exposure.

The NDT Techniques used to inspect Seals and Gaskets:

• Leak Testing: To identify and locate leaks using methods like helium leak detection.
• Ultrasonic Testing: This helps detect internal and surface flaws with accurate defect detection and sizing.

8. Fasteners and Bolts:

Image Credit: Eddyfi

Fasteners and bolts maintain the structural integrity and assembly of turbine components. Fasteners and bolts may experience stress corrosion cracking, fatigue, material failure, improper torque, and tension leading to loosening.

The NDT Techniques used to assess fasteners and bolts:

• Eddy Current Testing: To detect surface and near-surface defects.
• Ultrasonic Bolt Tension Measurement: This can measure the tension and integrity of bolts.

Common Failure Mechanisms in Regular Turbines

To effectively plan and execute an NDT Inspection on turbine components, the failure mechanisms and flaws that frequent the turbine structure and their operation should be thoroughly understood.

The key issues afflicting turbines may include:

• Stress Corrosion Cracking: This initiates from tensile stress and corrosive environments, detected using AE and ET.
• Thermal Fatigue: Thermal fatigue is a result of temperature fluctuations, identifiable through TT and PAUT.
• Erosion and Corrosion: These are caused by high-speed particles and chemical reactions, monitored using RT and MT.
• Material Degradation: Material degradation can occur due to prolonged high-temperature exposure, assessed using CT and TT.
• Vibration-Induced Damage: Vibration-induced damage rises from operational imbalances and can be tracked using VA and LDV.
• Foreign Object Damage (FOD): FOD is caused by external debris, detectable using VI and RT.

NDT Techniques for Turbine Monitoring

Advanced NDT techniques can help industries mitigate failure due to flaws and defects. Identifying an appropriate NDT technique for the specific test subject, its operating environment and material characteristics is imperative for any operator choosing to perform an inspection.

NDT Techniques that can be vital in the inspection of turbines include:

1. Phased Array Ultrasonic Testing (PAUT):

PAUT allows for precise control of the ultrasonic beam, enabling detailed imaging of complex structures and detection of subsurface defects.

2. X-ray Computed Tomography (CT):

This provides high-resolution 3D imaging of internal structures, revealing even the smallest of flaws within turbine components.

3. Acoustic Emission (AE) Testing:

AE can monitor real-time crack formation and growth by detecting high-frequency stress waves emitting from active defects.

4. Infrared Thermography (IRT):

This method detects temperature variations on the surface of components, identifying hot spots indicative of thermal fatigue and defects.

5. Eddy Current Testing (ET):

ET is sensitive to surface and near-surface defects, making it ideal for detecting cracks and corrosion in conductive materials.

6. Radiographic Testing (RT):

RT uses X-rays or gamma rays to produce detailed images of internal flaws in welds, castings, and complex geometries.

7. Laser Doppler Vibrometry (LDV):

LDV measures the vibrational characteristics of components, providing insights into structural integrity and dynamic behaviour.

8. Shearography:

This NDT process detects subsurface defects by measuring surface deformation under stress and is useful for identifying delaminations and other internal flaws.

9. Vibration Analysis (VA):

VA analyses mechanical vibrations to detect imbalances, misalignments, and wear in rotating components such as bearings and shafts.

10. Oil Analysis:

This method assesses the condition of lubricants and detects wear particles and contaminants, offering early warning of potential failures in mechanical systems.

To compare the above techniques, we must consider the following factors:

• Detection Capability: The ability to identify and characterise defects, including subsurface flaws, cracks, corrosion, and material degradation.
• Resolution: The level of detail the technique can provide, impacting the ability to detect small or fine defects.
• Inspection Speed: The time required to perform the inspection, affecting downtime and operational efficiency.
• Applicability: The versatility of the technique for different turbine components and materials, including static and rotating parts.
• Cost: The overall expense of deploying the technique, including equipment, training, and operational costs.

Impact of NDT on Turbine Lifecycle Management

Implementing NDT methods makes a significant enhancement to turbine lifecycle management. Turbine operators can achieve several critical benefits by integrating NDT techniques into routine maintenance and inspection processes, which may include:

1. Proactive Maintenance:

Effective NDT practices enable proactive maintenance, which can aid in: 

2. Early Detection of Defects:

Identifies potential issues before they lead to catastrophic failures.

3. Scheduled Maintenance:

Allows for planned repairs and replacements during scheduled downtimes.

4. Extended Component Lifespan:

Prolongs the service life of critical turbine parts through timely interventions.

5. Risk Mitigation:

Identifying potential failures early can help mitigate risks associated with turbine operations, which results in:

6. Enhanced Safety:

Prevents accidents and hazardous incidents by detecting structural weaknesses.

7. Operational Reliability:

Maintains consistent turbine performance through continuous monitoring.

8. Regulatory Compliance:

Ensures adherence to safety standards and industry regulations.

9. Cost Savings:

Optimised maintenance schedules and early fault detection contribute to significant cost savings, improving the overall economic viability of power generation projects. This leads to:

10. Reduced Downtime:

Minimises operational interruptions, leading to higher productivity.

11. Lower Repair Costs:

Prevents extensive damage that would require expensive repairs or replacements.

12. Efficiency Gains:

Enhances overall efficiency by maintaining optimal turbine performance.

13. Data-Driven Decision Making:

NDT techniques provide valuable data that supports informed decision-making processes, aiding in:

14. Condition Monitoring:

Offers real-time insights into the health of turbine components.

15. Trend Analysis:

Enables the analysis of wear patterns and failure trends to predict future issues.

16. Maintenance Optimisation:

Informs the development of tailored maintenance strategies based on actual component conditions.

17. Environmental Benefits:

NDT contributes to the sustainability of turbine operations by promoting efficient resource use, ensuring:

18. Resource Conservation:

Reduces material waste through precise defect identification and targeted repairs.

19. Energy Efficiency:

Ensures turbines operate at peak efficiency, reducing energy consumption.

20. Reduced Emissions:

Minimises the environmental impact by preventing unplanned outages and inefficient operations.

21. Enhanced Component Performance:

Regular NDT inspections help maintain and enhance the performance of turbine components. This helps in:

22. Performance Monitoring:

Tracks the effectiveness of repairs and maintenance efforts.

23. Quality Assurance:

Ensures that new and repaired components meet stringent quality standards.

24. Optimised Performance:

Maintains the integrity of turbine parts, ensuring they function at their best.

Non-Destructive Testing can significantly enhance the yield of turbines used across various industries, elevating their overall usefulness and operational efficiency. Every operator should prioritise training and continually update their knowledge to maintain their products, structures, and machinery effectively. Staying abreast of the latest NDT technologies and methodologies is essential for ensuring the reliability and safety of turbine operations.

Key Takeaways

• Effective NDT methods enable early defect detection, scheduled maintenance, and extended component lifespan, ensuring turbines operate smoothly and efficiently.
• By identifying potential failures early, NDT techniques enhance turbine safety and reliability, ensuring compliance with industry standards and regulations.
• NDT optimises maintenance schedules, reduces downtime, lowers repair costs, and improves overall economic viability, making power generation projects more sustainable and efficient.

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