Machine Health Monitoring
Industrial machinery is the backbone of manufacturing, power generation, oil & gas, and many other sectors. Ensuring reliable machine performance is crucial for safety, productivity, and cost optimization. One of the most effective strategies for predictive maintenance is machine health monitoring primarily through vibration and temperature measurements.
By analyzing mechanical vibrations and thermal behavior, engineers can detect early signs of wear, imbalance, misalignment, or lubrication failure before catastrophic breakdowns occur.
Brief Introduction of type of Machinery that is generally used in Process Plants
In a Process Plant, mainly there are following Machinery that exists;
- Pumps
- Description: Devices that move liquids (e.g., crude oil, chemicals, water, or slurries) using rotating impellers or pistons to generate flow via centrifugal or positive displacement action.
- Compressors
- Description: Machines that increase gas or vapor pressure using rotating or reciprocating mechanisms, essential for gas handling and pressure management.
- Subtypes: Centrifugal compressors (for continuous high-volume flow), reciprocating compressors (for variable loads), screw compressors (for process gases), and axial compressors (for large airflow).
- Turbines
- Description: Convert fluid energy (steam, gas, or water) into mechanical rotation to drive equipment or generate power.
- Subtypes: Steam turbines (common in plants with boilers), gas turbines (for power generation or cogeneration), and turbo-expanders (for gas expansion and energy recovery).
- Electric Motors and Generators
- Description: Motors convert electrical energy to rotational power; generators produce electricity, often driven by turbines.
- Subtypes: Induction motors (for general use), synchronous motors (for precise control), and alternators/generators (for backup or primary power).
- Fans and Blowers
- Description: Rotate blades to move air or gases at low pressure, supporting ventilation and cooling.
- Subtypes: Axial fans (for high-volume airflow), centrifugal blowers (for moderate pressure), and induced draft fans.
- Gearboxes and Agitators/Mixers
- Description: Gearboxes adjust speed or torque via rotating gears; agitators use impellers to mix fluids or solids.
- Subtypes: Planetary or helical gearboxes; propeller, turbine, or anchor agitators.
- Applications: Modifying motor output for pumps or compressors; mixing reactants in reactors, blending chemicals in storage tanks, or homogenizing slurries.
- Extruders and Specialized Rotating Equipment
- Description: Extruders force material through dies using rotating screws; other specialized units include rotary kilns or centrifuges.
- Subtypes: Single-screw or twin-screw extruders; high-speed centrifuges for separation.
Why do we need to monitor the Machine health?
Monitoring machine health is essential for reliability, safety, and cost efficiency.
Machine health must be monitored because rotating equipment is prone to faults such as imbalance, misalignment, bearing wear, gear damage, and looseness. These issues may start small but, if left undetected, can quickly escalate into severe failures.
Machine health monitoring provides early warning signs that allow engineers to take corrective action before problems become catastrophic.
How the Machine health is monitored-A Brief Description
Machine health is monitored by continuously measuring key physical parameters that indicate the operating condition of equipment. The two most widely used methods are vibration monitoring and temperature monitoring:
- Vibration Monitoring
Sensors such as accelerometers, velocity transducers, and proximity probes are installed on rotating parts (motors, pumps, turbines, compressors as explained earlier). These measure displacement, velocity, acceleration, phase, and frequency of vibration(To be discussed in detail later in this article). By analyzing vibration signals, engineers can detect faults like imbalance, misalignment, bearing wear, gear damage, or looseness.
The Machinery or associated part of a machinery usually has Axial and Radial movements.
Vibrations in a Machinery is measured in Axial, Radial and Tangential Directions.
Genertally Strongest Signals are in the Radial Direction.
[A Drawing to indicate ther measurement points is shown below ]
Position 1 and 4 indicate Radial horizontal position while Position 2 and 5 Radial Vertical Direction.
Position 3 and 6 indicate Axial direction.
Key Vibration Parameters
Vibration can be described using different measurable quantities. Each provides unique insights:
1. Displacement
What it is: The actual physical movement of a machine part (in microns or mils).
Why measure: Best for detecting low-frequency, large-amplitude issues such as unbalance, shaft bow, or misalignment.
How measured: Typically with proximity probes (eddy current sensors) mounted near the shaft.
Proximity Probes
An eddy current proximity probe is a non-contact sensor used to measure the displacement, position, or vibration of a conductive target (usually a metal surface).
- Working principle: It operates by generating an alternating magnetic field from a coil. When placed near a conductive surface, this field induces circulating currents (eddy currents) in the target. These eddy currents change the coil’s impedance, which is converted into a voltage proportional to the distance between the probe tip and the target.
- Key features:
- Non-contact measurement
- High accuracy for small displacements
- Works only with conductive materials
- Commonly used in turbines, compressors, and rotating machinery for shaft vibration and clearance monitoring
An Eddy Current Proximity Probe is always mounted with a small gap between the probe tip and the conductive target (e.g., a rotating shaft).
This gap is called the proximity gap or initial gap.
The probe is biased with a gap voltage, which is the DC voltage corresponding to the distance between the probe tip and the target surface.
Typical gap voltages are in the range of –2 V to –18 V, depending on probe calibration.
The sensor’s output then fluctuates around this gap voltage as the shaft vibrates or moves, giving displacement information.
2. Velocity
What it is: The rate of change of displacement (mm/s or in/s).
Why measure: Directly relates to mechanical stress on components. Most standards (ISO 10816, ISO 20816) use velocity RMS values for overall machine condition assessment.
How measured: With velocity transducers or calculated from accelerometer signals.
Velocity Transducers
Velocity transducers are used in machinery condition monitoring to measure vibration levels in terms of velocity. Two main designs are commonly used:
Above images are a curtesy of Bentley Nevada
Moving-Coil Velocity Sensors
Design & Operation:
A coil of wire moves relative to a permanent magnet.
The relative motion induces a voltage proportional to vibration velocity (self-generating, no external power needed).
Advantages:
Strong, low-noise signal.
Ideal for low-frequency applications.
Limitations:
Orientation sensitive: e.g., a vertical-design sensor used horizontally may cause coil drag and response errors.
Mechanical moving parts make them more delicate.
Above images are a curtesy of Bentley Nevada
Piezoelectric Velocity Transducers
Design & Operation:
Essentially accelerometers with onboard signal integration circuitry.
Vibration deforms a piezoelectric crystal → generates a charge difference proportional to acceleration.
Internal electronics amplify and integrate the signal into velocity units.
Advantages:
Solid-state design, no moving parts → robust and reliable.
Integration occurs within the sensor → reduces noise pickup along field wiring.
Limitations:
Require external power supply for amplifier and integrator circuits.
3. Acceleration
What it is: The rate of change of velocity (g or m/s²).
Why measure: Best for detecting high-frequency faults such as bearing defects, gear mesh faults, or looseness.
How measured: With piezoelectric accelerometers, the most common vibration sensors.
Accelerometers
Accelerometers use a piezoelectric crystal element to sense vibration. When subjected to vibration, the crystal undergoes periodic deformation, generating a tiny charge difference across its faces. This charge, proportional to the acceleration, is then amplified into a usable voltage or current signal that can be transmitted via field cables to a vibration monitoring system.
Types of Piezoelectric Accelerometers:
Compression Type (older design):
The sensing crystal is compressed between a reference mass and the sensor base. While effective, it is more prone to thermal stress and base distortion.
Shear Type (modern design):
The crystal is mounted on a central post, surrounded by a ring-shaped reference mass, and preloaded by a clamping band. This configuration greatly reduces susceptibility to thermal effects and mechanical distortion, making it more stable and reliable in demanding environments.
Above images are a curtesy of Bentley Nevada
4. Phase
What it is: The timing relationship between vibrations at different locations or between vibration and shaft rotation.
Why measure: Useful for diagnosing unbalance, misalignment, and resonance. Phase analysis helps in balancing and verifying root cause.
How measured: Using proximity probes (for shaft reference) along with vibration sensors.
The Proximity Probe is used against a flywheel tied to the shaft, the notch as shown in the figure below generates a pulse at 360 deg. Revolution, that’s why the term Key-Phasor
Key Phasor (KPH):
A Key Phasor is a once-per-revolution reference signal used for measuring phase angle and rotational speed of a shaft.
It is typically obtained by machining a single notch or projection on the shaft (or probe target area).
As the shaft rotates, this feature passes the probe once per revolution, generating a distinct pulse at the same angular position each time.
When the Key Phasor pulse is combined with the vibration probe signal, it allows us to determine the absolute phase angle—that is, the exact angular position of the shaft at which maximum vibration occurs.
Applications:
Determining phase relationships between vibration signals and shaft position.
Measuring shaft speed.
Essential in rotor dynamics analysis, balancing, and condition monitoring of rotating machinery
5. Frequency
What it is: The number of vibration cycles per second (Hz).
Why measure: Identifies specific fault patterns. For example:
- 1× running speed → unbalance
- 2× running speed → misalignment
- Bearing characteristic frequencies → bearing wear
How measured: Through FFT (Fast Fourier Transform) analysis of vibration signals collected by accelerometers or velocity sensors.
Temperature Monitoring in Machine Health Monitoring
1. Importance of Temperature Monitoring
Temperature is one of the most direct indicators of machine condition, as excessive heat almost always means an abnormal operating condition. Continuous monitoring helps detect problems like:
- Lubrication failure (dry or contaminated oil/grease).
- Overloading (mechanical or process-related).
- Friction and misalignment (mechanical stresses in rotating parts).
- Electrical faults (motor windings, insulation breakdown).
- Cooling system inefficiency (blocked passages, fan/pump issues).
2. Devices Used for Temperature Monitoring
a) Thermocouples
- Based on the Seebeck effect (voltage from two dissimilar metals at different temperatures).
- Wide range: –200 °C to +1800 °C (depending on type).
- Rugged, fast, and inexpensive.
- Application: Turbines, exhaust systems, thrust bearings, boilers.
b) Resistance Temperature Detectors (RTDs)
- Resistance of metals like platinum increases predictably with temperature.
- High accuracy: ±0.1 °C to ±0.5 °C.
- Range: –200 °C to +600 °C.
- Application: Bearing housings, compressors, pumps, and precision equipment.
c) Infrared (IR) Sensors
- Non-contact — detect infrared radiation.
- Very fast, ideal for moving or hard-to-reach components.
- Sensitive to surface emissivity and environmental conditions.
- Application: Rotating shafts, couplings, motors, and electrical switchgear.
3. Thrust Bearing Temperature Measurement
Thrust bearings carry axial loads in rotating machinery (steam turbines, gas turbines, compressors, generators). Because they operate under high stress, temperature monitoring is vital:
- How it’s done:
- Thermocouples or RTDs are embedded in the bearing shoes or bearing pads.
- The temperature of the babbitt metal lining (bearing surface) is measured.
- The hottest pad (usually trailing edge of loaded pads) indicates bearing health.
- Why it matters:
- Lubrication failure → causes rapid temperature rise.
- Overloading or misalignment → uneven heating of pads.
- Cooling oil issues → steady increase in bearing metal temperature.
- Excessive thrust load → dangerous rise in thrust bearing pad temperature, a critical trip parameter in turbines.
- Typical limits:
- Alarm level: ~80–90 °C
- Trip level: ~95–105 °C (varies by OEM)
4. Faults Detected by Temperature Monitoring
- Lubrication issues: Sudden increase in bearing or thrust pad temperature.
- Overloading: Continuous overheating in loaded bearings.
- Electrical insulation breakdown: Hot spots in motors, generators, transformers.
- Misalignment: Localized heating of couplings, thrust pads, or journal bearings.
- Cooling system failure: Progressive rise in all monitored points.
5. Benefits of Temperature Monitoring
- Early detection of lubrication and load problems.
- Protection of thrust bearings, which are critical and costly components.
- Enables predictive maintenance, avoiding catastrophic failures.
- Increases machine reliability, availability, and safety.
Common Machine Parts and Sensor Placement
Sensors are mounted on rigid, accessible surfaces close to vibration sources. Typical placements include:
- Bearings (Journal, Thrust, or Rolling Element Bearings):
- Why Here?: Bearings support the rotating shaft and are primary sites for wear, cavitation, or lubrication failure, which generate high-frequency vibrations.
- Placement Details: Radial sensors on the bearing housing (horizontal and vertical directions); axial sensors for thrust bearings to measure end-play or thrust position.
- Common in Machines: Pumps, compressors, turbines.
- Shaft/Rotor:
- Why Here?: Direct measurement of shaft orbit, runout, or bow, detecting imbalances or rubs.
- Placement Details: Non-contact sensors probe the shaft surface near couplings or mid-span; often paired with a keyphasor (reference mark) for phase reference.
- Common in Machines: Turbines, generators, high-speed rotors in compressors.
- Casing/Housing:
- Why Here?: Captures overall machine vibration transmitted through the structure, useful for low-frequency faults like misalignment.
- Placement Details: On the outer casing near bearings or couplings, in three axes (X-Y radial, Z axial).
- Common in Machines: All rotating equipment, especially enclosed units like motors.
- Couplings:
- Why Here?: Monitors angular misalignment or coupling wear, which causes torsional vibrations.
- Placement Details: Sensors on adjacent machine housings, aligned to capture relative motion.
- Common in Machines: Coupled systems like motor-pump or turbine-compressor trains.
- Gearbox or Impeller Areas:
- Why Here?: Detects gear mesh frequencies or impeller vane passes, indicating tooth wear or cavitation.
- Placement Details: On the gearbox housing or volute near the impeller.
- Common in Machines: Centrifugal pumps, gear-driven compressors.
In a typical setup (e.g., Bentley Nevada systems), 4-8 sensors per machine provide comprehensive coverage: 2 radial proximity probes per bearing for displacement, plus accelerometers on the casing for high-frequency data.
Vibration Monitoring Techniques and Their Application in Different Machines
Vibration monitoring is one of the most powerful methods for assessing machine health. Every rotating machine produces a unique vibration signature, and changes in vibration patterns often signal developing faults such as imbalance, misalignment, bearing wear, gear damage, or looseness. Different techniques are applied depending on the machine’s size, speed, and criticality.
1. Vibration Monitoring Techniques
a) Time-Domain Analysis
- Measures vibration amplitude (displacement, velocity, acceleration) over time.
- Useful for detecting sudden shocks, impacts, and overall vibration severity.
- Applied in portable vibration meters for quick checks.
b) Frequency-Domain Analysis (FFT – Fast Fourier Transform)
- Converts time signals into frequency spectra.
- Helps identify fault types (e.g., imbalance → 1× running speed, misalignment → 2×, bearing defects → high-frequency peaks).
- Widely used in predictive maintenance.
c) Phase Analysis
- Examines the relationship between vibration and shaft rotation.
- Useful for diagnosing imbalance, misalignment, and resonance conditions.
- Common in balancing procedures.
d) Envelope (Demodulation) Analysis
- Extracts high-frequency signals caused by bearing or gear defects.
- Excellent for early-stage fault detection in rolling element bearings.
e) Orbit and Shaft Centerline Analysis
- Uses proximity probes to plot shaft movement (orbit shape).
- Applied in large critical machines like turbines and compressors.
f) Wavelet Analysis & Advanced AI Techniques
- Breaks vibration signals into multiple frequency bands.
- Useful for complex machines with varying speeds and loads.
- Increasingly used in machine learning–based predictive maintenance systems.
In rotating machinery—such as pumps, compressors, turbines, and motors commonly found in complex process plants like refineries—vibration sensors are essential for monitoring machine health. They detect oscillations caused by imbalances, misalignments, bearing wear, or other faults, allowing predictive maintenance to prevent downtime. Sensors are strategically placed on key parts where vibrations are most indicative of issues, typically at points of high mechanical stress or motion. Placement follows standards like API 670 for machinery protection systems, ensuring orthogonal (90° apart) mounting for accurate radial and axial measurements.
Below, I’ll detail common machine parts for sensor placement, the types of sensors used, and why they’re chosen. Then, I’ll include pictorial examples (descriptions with links to diagrams from reliable sources, as direct embedding isn’t possible here—feel free to click for visuals).
Types of Vibration Sensors Used
The choice depends on the frequency range, machine speed, and measurement type (displacement, velocity, or acceleration). Here’s a summary:
| Sensor Type | Measurement Unit | Frequency Range | Typical Placement | Pros | Cons | Common Use in Rotating Machinery |
| Proximity Probes (Eddy Current/Displacement Sensors) | Mils or microns (peak-to-peak) | Low (0-1,000 Hz) | Shaft/bearing (non-contact) | Accurate for slow speeds; insensitive to surface; good for journal bearings | Requires clear gap; lower sensitivity to high frequencies | Turbines, compressors (radial/axial shaft position) |
| Velocity Sensors (Electrodynamic) | Inches/sec (in/s) or mm/s (RMS) | Medium (10-1,000 Hz) | Bearing housing/casing | Strong signal at low freq.; easy install; ideal for overall vibration | Less sensitive to high freq.; heavier | Pumps, motors (general monitoring) |
| Accelerometers (Piezoelectric or MEMS) | g’s or m/s² (peak) | High (up to 15,000 Hz) | Casing/bearing housing | Wide freq. range; compact; detects bearing/gear faults | Needs signal conditioning; sensitive to mounting | All machines (high-freq. faults like bearing wear) |
- Selection Criteria: Proximity for turbo-machinery (>1,800 RPM); velocity for mid-range (e.g., 600-3,600 RPM pumps); accelerometers for broad diagnostics. In refineries, a mix is common (e.g., proximity on turbine shafts, accelerometers on pump casings).
2. Types of Vibration Measurements by Machine Capacity
A. Small-Capacity Machines (Pumps, Fans, Small Motors)
- Characteristics: Low to medium power (<100 kW), lower criticality.
- Monitoring Approach:
- Handheld vibration meters (velocity RMS).
- Accelerometers for high-frequency fault detection (bearing defects).
- Periodic checks instead of continuous monitoring.
B. Medium-Capacity Machines (Compressors, Blowers, Industrial Gearboxes)
- Characteristics: Medium power (100–1000 kW), moderate criticality.
- Monitoring Approach:
- Accelerometers mounted permanently for continuous data.
- FFT-based spectral analysis for gear/bearing issues.
- Phase analysis for alignment and balancing.
- Online condition monitoring systems often implemented.
C. Large-Capacity & Critical Machines (Turbines, Generators, Large Compressors, Marine Engines)
- Characteristics: High power (>1 MW), critical for plant operation.
- Monitoring Approach:
- Proximity probes for shaft displacement and orbit analysis.
- Multiple sensors (displacement, velocity, acceleration) combined.
- Continuous online monitoring with redundant systems.
- Advanced diagnostics like orbit plots, waterfall diagrams, and modal analysis.
- Integrated with plant-wide Distributed Control Systems (DCS) or specialized vibration monitoring platforms (e.g., Bently Nevada).
Bibliography
- Mills, Simon R. W. (2010). Vibration Monitoring and Analysis Handbook.
- Randall, Robert Bond. Vibration-based Condition Monitoring.
- Kuttner, Thomas & Rohnen, Armin. Practice of Vibration Measurement.
- Norton, M. P. Fundamentals of Noise and Vibration Analysis.
- International Organization for Standardization. (2003). ISO 17359: Condition monitoring and diagnostics of machines – General guidelines. Geneva: ISO.
- Wikipedia – Various articles on vibration and machine condition monitoring.
- Several online resources, including technical websites and YouTube tutorials.
