Vibration Analysis and Condition Monitoring for Compressors

Vibration analysis is the cornerstone of compressor condition monitoring. It provides early detection of mechanical degradation, enables precise fault diagnosis, and supports data-driven maintenance decisions. This article presents a comprehensive guide to vibration monitoring techniques specifically applied to reciprocating and centrifugal compressors.

1. Fundamentals of Compressor Vibration

All rotating and reciprocating machinery generates vibration as a byproduct of mechanical motion. The key to effective vibration analysis is distinguishing between normal operational vibration and vibration signatures that indicate developing faults.

• Forces: unbalance, misalignment, aerodynamic, mechanical looseness
• Response: structural resonance, bearing stiffness, foundation flexibility
• Transmission: housing, frame, piping – each modifies the vibration signal
• Measurement: sensor type, location, and mounting affect data quality

2. Measurement Strategy and Sensor Placement

Optimal sensor placement is critical for capturing meaningful vibration data. The measurement locations should be chosen to capture the dominant vibration modes of each machine component.

Location	Sensor Type	Faults Detected	Measurement Axis
Frame/bearing housing	Velocity or accelerometer	Unbalance, misalignment	Horizontal, Vertical, Axial
Cylinder body	Accelerometer	Valve impacts, piston slap	Radial
Crosshead guide	Accelerometer	Crosshead looseness, wear	Horizontal
Crankcase	Velocity	Main bearing, crankshaft	Radial
Piping/nozzle	Velocity	Pulsation, mechanical resonance	Tri-axial

3. Vibration Severity Assessment

ISO 10816 provides widely accepted vibration severity guidelines for machines with operating speeds between 120 and 15,000 RPM. These criteria define evaluation zones that help maintenance personnel assess machine condition.

Zone	Description	Action
Zone A	New machine condition – vibration within typical new values	No action needed
Zone B	Long-term operation acceptable – vibration within normal range	Monitor trends
Zone C	Warning – vibration warrants investigation	Plan maintenance
Zone D	Danger – vibration threatens machine integrity	Immediate action

Figure 1: Vibration analysis equipment with accelerometer sensors on compressor bearing housing

4. Spectral Analysis Techniques

The vibration spectrum decomposes the overall vibration into its frequency components, revealing the specific sources of mechanical excitation. Each fault type produces a characteristic spectral pattern.

4.1 Common Spectral Patterns

Frequency Component	Fault Indication	Confirmatory Evidence
1x running speed	Unbalance or bend in shaft	Phase stable across bearings
2x running speed	Mechanical or thermal misalignment	High axial vibration
3x-6x running speed	Looseness or play in running gear	Harmonic series present
Sub-synchronous (<1x)	Oil whirl, rub, or bearing instability	Orbit analysis confirms
High frequency (5-20 kHz)	Bearing element impacts	Envelope spectrum analysis

4.2 Reciprocating Compressor Specifics

Reciprocating compressors produce unique vibration signatures due to the intermittent nature of gas compression forces. Analysis requires crank-angle referenced measurement and time waveform examination in addition to spectral analysis.

• Use crank angle encoder for phase-referenced vibration analysis
• Examine time waveform for impact events at valve opening/closing
• Monitor crosshead knocking as indicator of clearance issues
• Track frame flexibility vibration for foundation integrity assessment
• Compare cylinder-to-cylinder vibration balance for uniformity check

5. Orbit Analysis and Shaft Centerline

Orbit analysis displays the dynamic path of the shaft center within the bearing clearance. Two proximity probes mounted 90 degrees apart provide X-Y displacement data that reveals bearing wear, oil whirl, rubs, and shaft misalignment.

• Circular orbit: normal condition with adequate oil film
• Elliptical orbit: misalignment or preload condition
• Inner loop orbit: oil whirl or sub-synchronous instability
• Flattened orbit: bearing wear with increased clearance
• Erratic orbit: rub condition or severe looseness

6. Envelope Analysis for Bearing Fault Detection

Envelope analysis (demodulation) is a powerful technique for early-stage bearing fault detection. By extracting the high-frequency impact energy modulated by bearing defect frequencies, envelope analysis can identify bearing degradation months before traditional vibration levels exceed alarm thresholds.

Bearing Defect	Characteristic Frequency	Calculation Basis
Outer race defect	BPFO	N/2 x (1 – d/D cos a) x RPM
Inner race defect	BPFI	N/2 x (1 + d/D cos a) x RPM
Ball defect	BSF	D/2d x (1 – (d/D cos a)^2) x RPM
Cage defect	FTF	1/2 x (1 – d/D cos a) x RPM

7. Continuous Monitoring vs. Periodic Survey

The choice between continuous online monitoring and periodic walk-around surveys depends on equipment criticality, failure development speed, and available resources. Critical compressors warrant permanent online systems, while less critical units may be adequately monitored through periodic route-based surveys.

• Online systems: real-time alerts, continuous trending, automated diagnostics
• Route-based: lower cost, requires trained technician, periodic data gaps
• Hybrid approach: online for critical, route-based for auxiliary equipment
• Wireless sensors: emerging technology for cost-effective continuous monitoring
• Integrate vibration data with process data for comprehensive analysis

Source: Compressor Technology Editorial Reference