What is Blade Tip Timing? Non-Intrusive Blade Monitoring • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is Blade Tip Timing? Non-Intrusive Blade Monitoring • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is Blade Tip Timing? Non-Intrusive Blade Monitoring

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Understanding Blade Tip Timing

Definition: What is Blade Tip Timing?

Blade tip timing (BTT, also called non-intrusive stress measurement system or NSMS) is an advanced measurement technique for monitoring individual turbine, compressor, or fan blade vibration and stress using stationary optical or capacitive sensors that detect precise arrival times of blade tips as they pass sensor locations. By comparing actual arrival times to expected times (based on rotor speed), BTT systems calculate blade deflection, vibration frequency, amplitude, and can detect blade resonances, cracks, and abnormal vibration on individual blades without requiring instrumentation mounted on the rotating blades themselves.

BTT is the primary method for blade health monitoring in gas turbines (aircraft engines, industrial turbines) and critical for detecting blade fatigue, resonance conditions, and foreign object damage that could lead to catastrophic blade failure and engine destruction.

Operating Principle

Time-of-Arrival Measurement

  1. Sensors Positioned: Multiple sensors (typically 2-8) around casing circumference
  2. Expected Arrival: Calculate when blade tip should arrive at each sensor based on rotor speed
  3. Actual Arrival: Sensor detects blade tip passage with microsecond precision
  4. Time Difference: Deviation from expected = blade deflection
  5. Multiple Sensors: Multiple time measurements per revolution resolve vibration
  6. Blade-by-Blade: Each blade tracked individually

Deflection Calculation

  • Time deviation × blade tip velocity = tip displacement
  • Displacement indicates blade bending/vibration
  • Microsecond timing resolution → micrometer displacement resolution

Sensor Types

Optical Sensors

  • Laser or LED light source
  • Photodetector senses reflected light
  • Most common BTT sensor type
  • Good accuracy and reliability

Capacitive Sensors

  • Detect blade tip by capacitance change
  • Conductive blade required
  • Less affected by contamination than optical
  • Shorter sensing distance

Eddy Current Sensors

  • Similar to proximity probes
  • Detect metallic blades
  • Rugged and reliable

Applications

Gas Turbine Engines

  • Aircraft engine development and certification
  • Industrial turbine commissioning
  • Compressor and turbine blade monitoring
  • Flutter and resonance detection

Steam Turbines

  • LP turbine blade monitoring
  • Detect blade damage or resonance
  • Long blade vibration assessment

Large Fans and Compressors

  • Induced draft fans in power plants
  • Axial compressor stages
  • Critical blade condition monitoring

Information Provided

Individual Blade Behavior

  • Each blade tracked separately
  • Identify which specific blades vibrating
  • Detect cracked blades (different frequency)
  • FOD (foreign object damage) detection

Vibration Frequencies

  • Blade natural frequencies during operation
  • Detect resonance conditions
  • Flutter identification
  • Forced response characterization

Stress Assessment

  • Blade deflection indicates bending stress
  • High-cycle fatigue monitoring
  • Compare to design limits
  • Predict remaining blade life

Advantages Over Strain Gauges

No Rotating Instrumentation

  • Strain gauges require installation on blades
  • Need slip rings or telemetry (complex, expensive)
  • BTT uses stationary sensors only
  • Lower cost and complexity

All Blades Monitored

  • Strain gauges typically on 1-2 blades
  • BTT monitors every blade in stage
  • Identifies outlier blades
  • Complete population assessment

Permanent Capability

  • Can be permanently installed
  • Continuous or periodic monitoring
  • Strain gauges often test-only

Challenges

Complex Signal Processing

  • Under-sampled data (few points per revolution)
  • Sophisticated algorithms needed
  • Aliasing challenges
  • Requires specialized software

Installation Requirements

  • Must access blade path
  • Casing modifications may be needed
  • Precise sensor positioning
  • Calibration for specific blade geometry

Environmental Issues

  • Contamination on optics (exhaust, oil)
  • High temperature affecting sensors
  • Vibration of casing affecting measurements

Blade tip timing is a specialized but powerful technique for non-intrusive blade vibration measurement in turbomachinery. By precisely timing blade tip arrivals at multiple sensor locations, BTT systems monitor individual blade health, detect resonances and cracks, and prevent catastrophic blade failures in gas turbines and other bladed rotating machinery where blade integrity is critical to safe, reliable operation.

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