Basics Of Vibration Monitoring: A Primer

MACHINERY REQUIRES ATTENTION TO PERFORM AT DESIRED LEVELS OF PERFORMANCE APPROACH TOWARDS EQUIPMENT (Fig. 1):

CONDITION MONITORING :

1. A) VIBRATION MONITORING: ON LINE MONITORING and OFF LINE MONITORING
2. B) LUBRICATING OIL MONITORING: OIL PROPERTIES ANALYSIS and WEAR DEBRIS ANALYSIS

ONLINE MONITORING

• PERMANENTLY MOUNTED PROBES
• ALL ROTARY EQUIPMENT ABOVE 500 KW
• BENTLY NEVADA MAKE SYSTEM
• SHAFT VIBRTION AS WELL AS CASING VIBRATION

OFFLINE MONITORING

• PERIODIC DATA COLLECTION
• PORTABLE DATA ANALYZERS ARE USED
• ALL ROTARY EQUIPMENT ABOVE 22 KW
• CSI MAKE SYSTEM

CONDITION:

PHYSICAL CONDITION:

• APPEARANCE
• COLOUR
• TEMPERATURE
• VIBRATIONS
• SOUND
• LOOSENESS

INTERNAL CONDITION:

• VIBRATION SPECTRUMS
• FREQUENCY ANALYSIS
• ROTOR POSITION
• BEARING METAL TEMPERATURE
• OIL CONDITION – OIL DEGRADATION, METALS, WEAR PARTICLES IN OIL

Three Stages in Vibration Monitoring:

• Identification of parameter and machine
• How to measure? Instrumentation requirement
• When to measure? Frequency of CM

BASICS  OF  VIBRATION

What is Vibration (Fig. 2)?

Vibration can be defined as cyclic or an oscillating motion of a component from its mean position.

Units of Measurement:

• Displacement : m, mm, microns. mils
• Velocity : m/sec, mm/sec , in/sec
• Acceleration : m/sec2, g

What causes Vibration?

• Unbalance
• Misalignment
• Bent shaft
• Looseness
• Eccentricity
• Resonance
• Anti-friction bearing
• Journal bearing
• Aerodynamics and hydraulic problem
• Electrical problem
• Gear problem
• Belt-drives problem

Units of Vibration:

• Amplitude: It is the magnitude of vibration signal. How much is it vibrating? Size (severity) of the problem.
• Frequency: How many times oscillation is occurring for a given time period? What is vibrating? Source of the vibration.
• Phase Angle: The Phase Angle is the angle (in degrees) the shaft travels from the start of data collection to when the sensor experiences maximum positive force. How is it vibrating? Cause of the vibration.

Units of Amplitude

• Displacement: The distance a structure moves or vibrates from its reference or rest position.
• Velocity: Rate of change of displacement. It is the measure of the speed at which the mass is vibrating during its oscillation.
• Acceleration: It is the rate of change of velocity. The greater the rate of change of velocity the greater the forces (F=ma) on the machines.

When To Use Displacement / Velocity / Acceleration (Fig. 3)?

Systems / Tools for Monitoring

Transducers (Fig. 4): It is a basic device, which converts mechanical motion into electrical signal which can be amplified, filtered, analysed and displayed to indicate the vibrations and allow diagnosis of the overall machinery health.

Seismic Sensor –

• Works on piezo-electrical / moving coil principle
• Indirect measurement of shaft vibration.
• Directly mounted on machine casing / bearing house.
• Absolute vibration in terms of mm/sec or g

The VELOCITY PICK-UP is a contact type transducer.

Within the velocity pick-up, a spring-mass suspension system is used, which is designed to have low frequency. It is a permanent magnet-mass suspended on a spring and surrounded by a coil attached to the protective housing. A damping fluid is used to damp the natural frequency Velocity pick-up is limited to low frequency (between 10 Hz and 1500 Hz) for practical purposes.

Accelerometer (Fig. 5)

The accelerometer consists of a stack of piezoelectric crystals (such as quartz) on which a mass is attached.

When a piezoelectric crystal is stressed, it produces an electric voltage output which is proportion to the stress/force. When the accelerometer is attached to a vibration body, the crystal is stressed by the inertia of the mass caused due to the vibration. Electrical voltage output is proportion to the vibration acceleration.

VIBRATION SIGNALES:

• A frequency that is an integral multiple (′2, ′3, etc.) of a fundamental (′1) frequency.
• Sub-harmonic. A frequency that is an integral submultiple (′1/2, ′1/3, etc.) of a fundamental (′1) frequency.
• Vibration components (on rotating machinery) that are related to shaft speed.
• Sub-sychronous. Components of a vibration signal whose frequency is less than 1′ shaft speed.

Fig. 6: Dominant frequency vs Nature of Fault

•  Unidirectional vibrations. i.e. severity is more in radial directions as compared to axial.
• Phase difference is 90° in radial directions.
• Highly sensible w.r.t. machine rpm. It is directly proportional to machine speed.

FREQUENCY SPECTRUM (Fig. 7)

is a plot of frequency V/s amplitude. The frequency axis may be scaled in direct frequency units Hz or in order of shaft rotative speed ie. 1X , 2X etc. depending on requirement. Baseline data can also be superimposed on the existing data to view changes in rotor response from known conditions.

BODE PLOT (Fig. 8)

is a transient data plot and display is either 1X or 2X vibration amplitude and phase with respect to shaft rotative speed. This plot is only available as startup or shutdown data.

This plot is useful in determining the slow roll speed range, balance resonant frequencies, synchronous amplification factor, heavy spot location and rotor mode shape. Click here to know about causes and effects of piping vibration.

Fig. 8: Sample Bode Plot

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