I. Equipment Vibrations
Reliability team in a typical oil refinery will measure the baseline vibration signature of a rotating equipment and interpret its FFT spectral to ascertain if an equipment is correctly installed. These baseline vibration data facilitates the reliability team to monitor the equipment health during the equipment lifetime and advise the maintenance to perform pre-emptive maintenance as and when needed.
- Vibrations That Occur at Synchronous Speed of Equipment:
- Unbalanced Rotating Parts
- Eccentric Rotor
- Gear Tooth Defect
- Belt Drive Misalignment
- Coupling Misalignment (nX)
- Bent Shaft (nX)
- Structural Looseness (e.g. baseplate bolting loose, uneven surface under baseplate, occurs at 1X, 2X, nX, etc.)
- Vibrations That Occur at Sub-synchronous Speed of Equipment:
- Bearing Looseness (i.e. excessive clearance between bearing & shaft, occurs at 0.5X) – shows as truncated time waveform
- Hunting Tooth
- Rotor Rubbing
- Vibrations That Occur at Non-synchronous Speed of Equipment:
- Pole Pass Frequency
- Electric Line Frequency
- Electric SCR Frequency
- Oil Whirl (radial only)
- Oil Whip (radial only)
- Belt Frequency (radial only)
- Vibration Transducers:
- Accelerometer (cheap, amplitude is dependent of running speed), up to ~20kHz (unit: m/s2 , in/s2 or G)
- Velocity Transducer(expensive, amplitude is dependent of running speed), 10 ~ 10,000Hz (unit: mm/s or ips)
- Proximity Probe (measures displacement, amplitude is independent of running speed), capable of low freq response (down to zero Hz, unit: mil or micron)
- Machinery Diagnostic Data:
- Bode Plot
- Orbit Plot
- Polar Plot
II. Piping Vibrations
- Low Frequency Broadband:
- Flow Induced Turbulence (FIT) (typically < xx Hz)
- High Frequency Broadband
- Acoustic Induced Vibration (AIV) (typically > xxx Hz)
- Discrete Frequency Response
- Flow Induced Vibration (FIV) / Deadleg
- Pressure Pulsation by Reciprocating Equipment
- Centrifugal Compressor Vibration (1X & 2X vane pass frequency)
- Typically, excitation frequencies below xx Hz are damaging to piping & structures. Certain engineering companies specified that piping natural frequency should be less than xx Hz to be considered a safe design. However, it is actually an unconservative design when Flow Induced Vibration is present in the system.
- FFT Spectrum Analysis:
- FFT spectra shall include frequencies from 5 Hz to 2Z times running speed (where Z is the highest no. of impeller vanes in any stage of a multi-stage pump).
- Need to keep a lookout for sidebands & harmonics of the fault frequency
- Assess changes in the amplitude of peaks to assess the severity of fault condition.