Maintenance

Centrifugal compressors have significant maintenance advantages: few contacting parts, no oil in the compression chamber, and long service intervals. However, they require continuous monitoring and specialized maintenance.

Maintenance Advantages

Characteristic	Benefit
No touching rotors	Less wear
No oil in compression	Clean air, fewer filters
High-quality bearings	Long life
Robust design	Long intervals
Fewer moving parts	Fewer failures

Maintenance Program

Daily Inspections

Check	Method	Limit
Oil pressure	Gauge	2-4 bar typical
Oil temperature	Sensor	40-65°C
Bearing temperature	Sensor	less than 95°C
Vibration	Monitor	less than 7 mm/s
Noise	Auditory	No changes
Leaks	Visual	None
Process pressures	Instruments	Normal range

Periodic Maintenance

Interval	Component	Action
2,000-4,000 h	Inlet filters	Inspection/change
4,000-8,000 h	Gearbox oil	Analysis
8,000-16,000 h	Gearbox oil	Change
Annual	Intercoolers	Cleaning
Annual	Instrument calibration	Verify
20,000-40,000 h	Seals	Inspection/replacement
40,000-100,000 h	Bearings	Inspection/replacement
80,000-120,000 h	Complete overhaul	Rebuild

Vibration Monitoring

Vibration monitoring is the primary indicator of centrifugal compressor health.

Measurement Points

              ┌─────────────────────────────────────┐
              │           COMPRESSOR                 │
              │                                      │
    Motor ════╪════ Bull Gear ════╪════ Impeller    │
              │         │         │                  │
              │    [V1] [V2]  [V3][V4]              │
              │     ↓    ↓     ↓   ↓                │
              │   Measurement points                │
              └─────────────────────────────────────┘

    V1, V2: Bull gear bearings
    V3, V4: Impeller bearings (per stage)

Sensor Types

Type	Measurement	Application
Accelerometer	Acceleration (g)	High frequency
Velocimeter	Velocity (mm/s)	Medium frequency
Proximity	Displacement (µm)	Low frequency, position
Axial	Axial position	Rotor thrust

Vibration Limits

Per ISO 10816-3 for large machines:

Zone	Velocity (mm/s RMS)	Action
A (new)	less than 2.8	Normal
B (acceptable)	2.8 - 7.1	Monitor
C (alert)	7.1 - 11.2	Investigate
D (danger)	greater than 11.2	Stop

Frequency Analysis

Frequency	Probable Source
1X RPM	Unbalance
2X RPM	Misalignment, looseness
Gear mesh frequency	Gear wear
High frequency	Bearings
Sub-synchronous	Instability, surge

Typical vibration spectrum:

Amplitude
    │
    │  ▓
    │  ▓
    │  ▓     ▓
    │  ▓     ▓         ▓
    │  ▓     ▓    ░    ▓    ░
    └──┴─────┴────┴────┴────┴───── Frequency
      1X    2X   GMF  2GMF  BPFO

    1X: Shaft speed
    GMF: Gear Mesh Frequency
    BPFO: Ball Pass Frequency Outer

Axial Position Monitoring

Rotor axial movement is critical:

                    Proximity
                     sensor
                         │
                         ▼
    ────────────────────[○]────────────────
                         │
              ┌──────────┴──────────┐
              │     Rotor collar    │
              └─────────────────────┘
                    ← Gap →
                   (typical 0.5-1.5 mm)

Condition	Gap	Action
Normal	Nominal +/- 0.1 mm	None
Alert	Nominal +/- 0.3 mm	Investigate
Danger	Nominal +/- 0.5 mm	Stop

Oil Analysis

Parameters to Monitor

Parameter	Limit	Indicates
Viscosity	+/- 10% of new	Degradation
TAN (acidity)	less than 0.5 mg KOH/g	Oxidation
Water	less than 200 ppm	Contamination
Fe particles	less than 100 ppm	Gear wear
Cu particles	less than 50 ppm	Bearing wear
Particle count	ISO 17/15/12	Cleanliness

Analysis Frequency

Condition	Frequency
Normal	Quarterly
New oil	500 h after change
Problem suspected	Immediate
Post-overhaul	500 h, 2000 h

Common Problems and Diagnosis

Diagnostic Table

Symptom	Probable Cause	Verification	Solution
High 1X vibration	Unbalance	Spectrum	Balance
High 2X vibration	Misalignment	Laser	Realign
Gear noise	Wear	Borescope	Replace
High bearing temp	Lubrication	Oil flow	Check pump
Frequent surge	Control	Calibration	Adjust
Low efficiency	Intercooler fouling	Approach	Clean
Abnormal axial position	Thrust wear	Sensor	Overhaul

High Vibration Decision Tree

                    High vibration
                          │
              ┌───────────┴───────────┐
              │                       │
          1X dominant            2X dominant
              │                       │
              ▼                       ▼
         Unbalance              Misalignment
              │                       │
       ┌──────┴──────┐         ┌──────┴──────┐
       │             │         │             │
   Sudden        Gradual   Angular       Parallel
       │             │         │             │
       ▼             ▼         ▼             ▼
   Blade loss   Impeller    Check         Check
               fouling     couplings      bases

Overhaul

When to Overhaul

Indicator	Limit
Operating hours	80,000-120,000 h
Increasing vibration	Upward trend
Degraded efficiency	greater than 5% loss
Oil analysis	Metals out of limit
Axial position	Significant change

Typical Overhaul Scope

Major overhaul includes:
├── Complete disassembly
├── Dimensional inspection
│   ├── Impellers
│   ├── Diffusers
│   ├── Seals
│   └── Bearings
├── Replacement of:
│   ├── Bearings
│   ├── Seals
│   ├── O-rings and gaskets
│   └── Worn parts
├── Cleaning of:
│   ├── Intercoolers
│   ├── Casing
│   └── Oil system
├── Rotor balancing
├── Reassembly
├── Testing:
│   ├── Pressure
│   ├── Vibration
│   └── Performance
└── Documentation

Duration and Cost

Aspect	Typical Range
Overhaul duration	2-4 weeks
Cost (% of new)	15-25%
Personnel required	4-8 technicians
Post-overhaul testing	24-72 hours

Planning

Plan overhaul 6-12 months in advance. Coordinate with plant shutdowns and ensure spare parts availability.