1. ISO 10816-7 — standard specific to rotodynamic pumps
The ISO 10816 standards series covers mechanical vibration evaluation on machines by measurement on non-rotating parts. Part 7 (ISO 10816-7) is specifically dedicated to rotodynamic pumps — category that includes radial centrifugal pumps (FBCN, FBOT), mixed-flow and axial-flow pumps. The standard establishes measurement procedures, standardized transducer points and vibration limits to classify the machine's mechanical state in operation.
The standard monitoring quantity is RMS vibration velocity in mm/s — chosen because velocity is proportional to vibration kinetic energy and correlates better with bearing and seal fatigue damage than acceleration or displacement alone. The standard defines two pump categories (Category I for horizontal stationary pumps with power ≥ 200 kW; Category II for smaller or vertical pumps) and four severity zones.
2. Severity zones A, B, C and D
Zone A represents machines in excellent mechanical state — typically what is expected of a new pump after FAT (Factory Acceptance Test) or a well-serviced pump. Zone B represents the upper limit of acceptable continuous operation — no intervention urgency, but the trend should be monitored. Zone C indicates unsatisfactory mechanical condition — continuous operation is tolerable only until the next planned maintenance shutdown; the cause must be investigated.
Zone D is the risk range — the equipment may fail at any time, and shutdown for diagnosis is mandatory.
| Zone | Category I (large pumps) | Category II (smaller pumps) | Interpretation |
|---|---|---|---|
| A | ≤ 2.5 mm/s | ≤ 3.5 mm/s | Excellent — new or recently serviced pump |
| B | ≤ 4.0 mm/s | ≤ 5.0 mm/s | Acceptable for continuous operation |
| C | ≤ 6.6 mm/s | ≤ 8.3 mm/s | Unsatisfactory — investigate and correct at next maintenance |
| D | > 6.6 mm/s | > 8.3 mm/s | Not acceptable — shutdown for diagnosis |
3. Standardized measurement points — DE, NDE and axial
Measurement follows a universal convention in three main positions per bearing: vertical, horizontal and axial. Bearings are identified as DE (Drive-End — where the motor couples) and NDE (Non-Drive-End — opposite side, where the impeller sits near the casing). In a typical FBCN pump, this results in up to six measurement positions: DE-V, DE-H, DE-A, NDE-V, NDE-H, NDE-A.
The severity value compared to the ISO 10816-7 table is the highest RMS value observed across all positions — the pump is classified by the worst position, not the average.
Directional spectrum interpretation provides the first cause clue: predominantly radial vibration (vertical + horizontal) with peak at rotation (1×) suggests unbalance; vibration with peak at 2× suggests misalignment; elevated axial vibration suggests angular misalignment or irregular axial thrust; broadband at high frequency suggests cavitation or bearing wear. This analysis is done with FFT instruments (Fast Fourier Transform) — modern vibration analyzers.
4. Seven most frequent causes of elevated vibration
Industrial pump maintenance engineering classifies vibration causes into seven recurring groups, each with a distinct spectral signature:
- Residual rotor unbalance — dominant peak at 1× rotation, predominantly radial. Originating from cavitation erosion, solid deposits on the impeller or material loss after poorly executed maintenance. Solution: rebalance per ISO 21940 G2.5 (quality grade adopted by FB Bombas on all impellers).
- Pump-motor misalignment — peak at 2× rotation and elevated axial vibration. Frequent cause after motor replacement, coupling maintenance or foundation movement. Solution: laser alignment with appropriate tolerance (parallel ≤ 0.05 mm; angular ≤ 0.05 mm/100 mm).
- Cavitation — broadband at high frequency (above 1 kHz), characteristic "gravel" noise. Caused by insufficient NPSHa or operation outside the preferred range. Solution: review the suction line, check NPSH and adjust operating point to POR (70-120% BEP).
- Internal recirculation from operation outside POR — low-frequency vibration (sub-synchronous) and pressure fluctuation. Frequent in pumps operating well below BEP (flow < 50%). Solution: install minimum recirculation valve or resize the pump.
- Bearing clearance or wear — peaks at bearing characteristic frequencies (BPFI, BPFO, BSF, FTF) and progressive acceleration growth. Solution: bearing replacement; investigate contamination, lubrication or irregular axial thrust.
- Foundation or structure resonance — peak at structural natural frequency, amplitude very sensitive to small speed variations. Solution: stiffen foundation, alter assembly mass or change operating speed away from natural frequency.
- Pipe strain — vibration that grows after piping tightening, often asymmetric. Caused by piping forcing the pump out of alignment. Solution: loosen all flanges, check face flatness and re-tighten with piping supported by its own supports.
5. Factory baseline — hydraulic bench and ISO 21940 G2.5
Every FBCN, FBOT and FBEI Series pump leaves the FB Bombas factory in Cabreúva-SP with two minimum quality requirements that define the vibration "baseline": (1) dynamic rotor balancing per ISO 21940 grade G2.5 — ensuring residual unbalance is proportional to rotating mass at levels adequate for continuous operation; (2) hydraulic bench test, where performance curve and operating point are validated. In FBCN pumps, the bench test includes bearing vibration measurement as additional quality check.
This factory baseline is the reference for field regression diagnosis: if a pump that delivered zone A at FAT begins to show zone B/C months later, the diagnosis focus is finding what changed in the installation or equipment (pipe strain, misalignment, cavitation initiated by process change, bearing deterioration). Requesting the FB Bombas FAT report during acquisition is a good predictive maintenance practice.
6. Routine monitoring program
In industrial plants with critical pumps, ISO 10816-7-based vibration measurement routine is recommended: monthly measurement on pumps in continuous operation within POR; biweekly measurement on pumps operating near AOR limits; weekly measurement on pumps that showed zone C in previous measurement. For each pump, record temporal histogram of measurements — an ascending trend is more informative than an isolated value, even if the absolute value is still in zone B.
When maintenance engineering identifies a pattern suggestive of internal problem (not attributable to the installation), FB Bombas technical support can review the spectral report, compare with FAT baseline and indicate components to inspect — significantly shortening diagnosis time and the probability of unnecessary parts replacement.
