Since the turn of the century, steam turbine generators have earned an enviable reputation for economy and reliability in converting heat energy to electrical energy under the most exacting service conditions. Southwest Research Institute (SwRI) provides engineering expertise and extensive laboratory facilities to meet the challenges imposed in the design, assembly, and reliable operation of turbines of all sizes.

Capabilities include:

• Nondestructive evaluation
  
• Materials degradation studies
  
• Remaining life assessment
  
• Structural integrity analysis
  
• Failure analysis
  
• Materials evaluation and testing
  
• Field hardness testing and replication
  
• Vibration problem diagnosis
  
• Telemetry testing

Remaining Life Assessment

Turbine Rotors

Critical turbine components must be evaluated to assure safe operation during their lifetime. Acccurate life assessment procedures, coupled with a knowledge of specific rotor material properties, prevent costly premature retirement of rotors.

A wide range of finite element programs is used at SwRI to perform structural evaluations and remaining life assessment, including:

• ANSYS: Structural analysis
  
• NASTRAN: Structural analysis
  
• STRAP/SAFER: EPRI rotor integrity and life analysis
  
• BIGIF: EPRI fracture mechnics
  
• ADINA/ADINA-T: Structural/thermal analysis
  
• ABAQUS: Nonlinear structural analysis
  
• NESSUS: Probabilistic structural analysis

Computational facilities include VAX, MICROVAX, IBM, and Cray computers.

A high pressure steam turbine rotor in service for more than thirty years was subjected to remaining life assessment evaluation. Such evaluations are essential to determine how far beyond the design life these rotors can safely and reliably be operated.

Test specimens, used to obtain critical material properties, are trepanned from aged rotor periphery at plane of balance.

Specimen blanks after trepanning.

Low pressure (LP) steam turbine disc cracking caused by stress corrosion is experienced worldwide in rotors used in both nuclear and fossil fuel power plants. Cracking occurs in LP rotor discs in keyways, in the blade attachment areas of the rims, on bore surfaces, and on web surfaces. Under a cooperative industry research project, funded by a consortium of electric utilities, SwRI has developed the technology required to make remaining life predictions for shrunk-on low pressure turbine discs. Heat transfer analysis and stress analysis determine shrink-fit stresses, thermal stresses, and stress due to blade and disc mass. Disc integrity and remaining life assessment are conducted and recommendations are provided to help determine run, replacement, or reinspection intervals.

Low pressure (LP) steam turbine disc cracking caused by stress corrosion is experienced worldwide in rotors used in both nuclear and fossil fuel power plants. Cracking occurs in LP rotor discs in keyways, in the blade attachment areas of the rims, on bore surfaces, and on web surfaces. Under a cooperative industry research project, funded by a consortium of electric utilities, SwRI has developed the technology required to make remaining life predictions for shrunk-on low pressure turbine discs. Heat transfer analysis and stress analysis determine shrink-fit stresses, thermal stresses, and stress due to blade and disc mass. Disc integrity and remaining life assessment are conducted and recommendations are provided to help determine run, replacement, or reinspection intervals.

Axial view reveals crack locations in discs with axial-entry fir tree type blade attachment grooves.

Diagram shows typical stress corrosion cracking locations on steeple and rim area of low pressure steam turbine rotor.

Finite element grid is used for disc keywey elastic-plastic analysis.

Stress corrosion crack propagates in radial directions from the keywey crown of a shrunk-on disc.

Nondestructive Evaluation

SwRI offers numerous nondestructive evaluation techniques for inspection of turbine rotors, disc steeples, and keyways, including periphery ultrasonic, dye penetrant, magnetic particle, and eddy current inspection services.

The Institute developed the turbine rotor examination/evaluation system (TREES), an automated rotor bore ultrasonic examination system for steam turbines that incorporates features such as focused search units and a volumetric cell structure to produce unique examination capabilities and excellent flaw resolution. Focused beam search units provide a reliable means of detecting and sizing flaws without using conventional signal amplitude techniques. An automated data acquisition and processing computer system processes the data.

The Institute can custom assemble the TREES package and provide boresonic inspection services.

SwRI has extensive laboratory and field experience in developing and implementing internationally recognized inspection and evaluation procedures. Under a utility consortium program, an ultrasonic disc rim inspection system was developed that detects and sizes cracks in blade attachment areas without blade removal.

SwRI also conducts hardness measurements, replication, and microstructural assessments in the field using portable equipment.

On-site hardness measurements are made on a low pressure turbine rotor and on a stationary diaphragm.

The SwRI turbine rotor examination/evaluation system (TREES) possesses unique examination capabilities and excellent flaw resolution achieved by computer-designed focusing lenses attached to ultrasonic transducers. Six lens configurations are employed to examine zones within the rotor with beam diameter sizes from 0.031 to 0.125 inch.

Steam turbine discs can be inspected for cracking in the rim area without need for deblading by use of a new SwRI-developed ultrasonic inspection system.

Vibration Control and Rotor Balancing

Using advanced techniques for field measurement, signal processing, diagnostic analysis, and other predictive tools, SwRI can identify and solve rotor, blade, and structural dynamics problems that put steam turbine generators out of commission. These techniques are used to solve field problems, while providing confidence at the design stage that turbine trains will exhibit low vibration levels.

• Laboratory analysis of vibrational problems with rotating machinery and piping
  
• Steam turbine-generator rotor balancing
  
• Custom design of instrumentation and components for special applications
  
• Component testing of valves, pumps, pressure vessels, and instruments

This on-site impact modal analysis equipment measures blade resonance and mode shapes. Shown here is a high pressure turbine rotor from a 580 MW unit undergoing static blade resonance testing.

A low power laser and SwRI-built optical target system continuously monitors changes in alignment between turbine and generator to define and solve vibration problems caused by misalignment.

A portable spectrum analyzer is used in vibration surveys of turbine generator rotors. SwRI regularly performs these surveys to assist in rotor balancing.

Failure Analysis and Prevention

SwRI uses a multidisciplinary approach to metallurgical failure analysis on a wide range of steam turbine and boiler components. Metallurgy, corrosion engineering, stress analysis, fracture mechanics, and nondestructive evaluation are combined to identify mechanisms and root causes of failure. Such analyses help predict and monitor remaining component life.

The Institute maintains a certified hot laboratory for handling materials contaminated by radiation.

Cracks in this turbine disc were initiated at the bore and propagated by stress corrosion to critical size prior to catastrophic failure.

Various steam turbine components are subjected to failure analysis to identify cause of failure and to recommend corrective actions. Components include low pressure and high pressure turbine blades, bolts, steeples, and discs.

The scanning electron microscope is coupled to an image analysis system to study fracture morphology and to perform energy dispersive spectroscopic and image analysis of samples.

Material Testing and Evaluation

Static strength, creep, fatigue, impact, fracture toughness, crack growth rate, macro- and micro-hardness, and other testing and evaluation techniques are available in the Institute's extensive laboratories.

Controlled test environments include temperature, humidity, vacuum, high pressure, and immersion in chemically controlled gaseous or aqueous media. Specialized test capabilities include controlled multi-axial stress conditions, strain rates continuously variable from 1O^-6 to 10^-4 s^-l, coupled thermal and mechanical testing, programmed spectrum loading, and direct uniaxial loading to 1,650°C (3,000°F).

Sophisticated computer-controlled servo-hydraulic test systems are available, and many physical and thermal-physical property measurements are made according to ASTM standards. Specialized test equipment can be designed and assembled to meet unique test requirements.

Strain-controlled low cycle fatigue tests are performed at high temperatures.

A wide range of specimen designs is available for materials testing.

Measurement of high temperature creep crack growth properties of Class C and D turbine rotor materials are obtained to contribute to a data base used in remaining life assessment of HP and IP rotors.

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