Where to Measure
Collect vibration measurements at the bearing housings, in three directions:
- Radial horizontal (H) — sensitive to radial faults: imbalance, misalignment, radial looseness
- Radial vertical (V) — same; compare with horizontal (significant difference suggests looseness or soft foot)
- Axial (A) — sensitive to axial faults: angular misalignment, thrust bearing wear, axial looseness
Measure at both the drive-end (DE) and non-drive-end (NDE) bearing housings on both pump and motor. Store baseline readings when the machine is known to be in good condition.
ISO Vibration Severity Limits
ISO 10816-7 defines vibration velocity limits for centrifugal pumps in industrial use. RMS velocity in mm/s:
| Zone | RMS velocity (mm/s) | Action |
|---|---|---|
| A — New, excellent condition | <2.3 mm/s | No action |
| B — Acceptable long-term | 2.3–4.5 mm/s | Monitor |
| C — Alert — investigate | 4.5–7.1 mm/s | Schedule inspection |
| D — Danger — stop when possible | >7.1 mm/s | Urgent action required |
Note: These are general guidelines. Critical pumps often have tighter alert levels defined in their condition monitoring programme.
Fault Signatures
1. Imbalance
Signature: Strong 1× RPM peak in radial (horizontal and vertical) directions. Axial vibration low relative to radial. Phase difference between H and V measurements is ~90°.
Cause: Build-up of deposits on impeller, loss of balance weight, erosion of impeller vanes, new impeller not dynamically balanced, or worn wear rings causing impeller eccentricity.
2. Misalignment
Signature: Dominant 1× and 2× RPM peaks. High axial vibration — particularly at coupling. Phase analysis across the coupling shows 180° phase difference if angular misalignment is dominant. Offset misalignment shows strong radial components at 1×.
3. Cavitation
Signature: Broadband random vibration — the noise floor of the spectrum rises across a wide frequency range, not at a specific frequency. Often accompanied by audible crackling noise. Acceleration (g) or high-frequency envelope analysis is most sensitive.
4. Bearing Defects
Each bearing defect frequency is calculated from bearing geometry and shaft speed:
- BPFO (ball pass frequency outer) = Nb/2 × RPM × (1 − Bd×cos α/Pd) — outer race defect
- BPFI (ball pass frequency inner) = Nb/2 × RPM × (1 + Bd×cos α/Pd) — inner race defect
- BSF (ball spin frequency) — rolling element defect
- FTF (fundamental train frequency) — cage defect
These are usually detected using envelope (demodulation) analysis — the carrier frequency is typically 2–20 kHz. Early bearing defects show sub-harmonic sidebands around the carrier; as the defect progresses, the BPFO/BPFI peaks become clearly identifiable.
5. Looseness
Signature: Multiple harmonics of running speed (3×, 4×, 5× RPM and higher). Often asymmetric — horizontal and vertical are significantly different. Sub-harmonics (0.5×, 1.5× RPM) indicate severe looseness.
6. Blade Pass Frequency (BPF)
BPF = shaft RPM × number of impeller vanes. A pump with 6 vanes at 1,450 RPM has BPF = 145 Hz. Elevated BPF indicates hydraulic excitation from worn wear rings, damaged vanes, or off-BEP operation causing recirculation.
Trend, Don't Just Alarm
Single-point measurements are less useful than trends over time. Set up a trending system: collect measurements at fixed intervals (weekly for machines near alarm limits; monthly for normal machines). Plot overall vibration level and key spectral peaks against time. Rising trends indicate developing faults — the rate of increase tells you how much time you have before intervention is needed.