Centrifugal Pump Vibration Analysis: Measurement Points, Faults, Limits

Centrifugal pump vibration is measured on the pump and motor bearing housings in the horizontal, vertical and axial directions at steady operation. The overall RMS velocity is compared with the ISO 10816-7 zones, and the cause is found in the spectrum: 1x means unbalance, 2x and axial content mean misalignment, a series of harmonics means looseness, non-synchronous peaks mean bearings, and vane pass means hydraulics.

Measurement points and directions

A typical horizontal motor + coupling + pump set has four bearing locations. A layout of pump components is in the article Centrifugal pump design: six components and how to monitor each.

Point	Location	Directions
1	Motor bearing, fan end	Horizontal, vertical
2	Motor bearing, coupling end	Horizontal, vertical, axial
3	Pump bearing, coupling end	Horizontal, vertical, axial
4	Pump bearing, free end	Horizontal, vertical

Rules that decide whether the data can be trusted:

• Mount the sensor on a rigid part of the bearing housing, not on a thin cover, and use the same marked spot every time.
• Measure at steady operation and record flow, discharge and suction pressure. Pump vibration depends strongly on the operating point: in a Hydraulic Institute example, the overall level at 71 % of best efficiency point (BEP) flow was nearly double that at 93 %.
• Check background vibration with the pump stopped. According to ISO 10816-7 (as summarized by Europump), if background exceeds 25 % of the running level, corrective action may be needed to reduce its effect.
• For vertical lineshaft pumps, ISO 9905 and ISO 9908 call for readings at the top flange with a rigid coupling and near the top pump bearing with a flexible coupling.

Pump vibration limits per ISO 10816-7

ISO 10816-3 is no longer used for pumps: Europump states it has been superseded by part 7, ISO 10816-7:2009 "Rotodynamic pumps for industrial applications". It covers rotodynamic pumps above 1 kW with more than three impeller vanes. The rigid versus flexible support split used in part 3 is not made; instead, pumps are divided into two categories:

• Category I — pumps requiring high reliability, availability or safety: toxic and hazardous liquids, critical applications, oil and gas, special chemicals, nuclear and power plants.
• Category II — pumps for general or less critical duty, for example non-hazardous liquids. Vertical suspended pumps above 600 rpm are usually Category II.

Zone limits for RMS vibration velocity on non-rotating parts (bearing housings), mm/s:

Zone	Condition	Cat. I, ≤ 200 kW	Cat. I, > 200 kW	Cat. II, ≤ 200 kW	Cat. II, > 200 kW
A	New machines in the preferred operating range	2.5	3.5	3.2	4.2
B	Unrestricted long-term operation	4.0	5.0	5.1	6.1
C	Limited operation	6.6	7.6	8.5	9.5
D	Risk of damage	> 6.6	> 7.6	> 8.5	> 9.5
—	Recommended alarm (≈ 1.25 × upper limit of B)	5.0	6.3	6.4	7.6
—	Recommended trip (≈ 1.25 × upper limit of C)	8.3	9.5	10.6	11.9

Alarm and trip should be released only when the level stays above the setting for about 10 s, to avoid false alarms. The preferred and allowable operating ranges (POR and AOR) are stated by the pump manufacturer; POR is generally 70–120 % of BEP flow. Outside the allowable range vibration may be higher; this is tolerated short-term, but continuous operation risks damage.

The standard is under revision: ISO has registered the ISO 20816-7 project, intended to replace ISO 10816-7:2009. Check which edition your plant applies. Oil and gas pumps to ISO 13709 (API 610) are accepted against that standard's limits: for overhung and between-bearings pumps up to 300 kW per stage and 3600 rpm, overall bearing housing vibration on test must not exceed 3.0 mm/s in the preferred operating range and 3.9 mm/s in the allowable range.

Faults and their vibration signatures

Fault	Spectrum frequency	Direction	Other signs
Unbalance	1x, almost no harmonics	Radial, horizontal ≈ vertical	Sinusoidal waveform, one event per revolution; little axial except overhung rotors
Parallel misalignment	2x, often higher than 1x	Radial	Periodic waveform, one or two events per revolution
Angular misalignment	1x and 2x (sometimes 3x)	Axial	Usually combined with parallel
Mechanical looseness	Series of 1x harmonics, fractional harmonics (½x etc.), raised noise floor	Radial, often vertical	Unstable phase, random impacting in the waveform
Rolling bearing defect	Non-synchronous peaks (BPFO, BPFI, BSF, FTF) and harmonics	Radial	First high-frequency low-amplitude peaks; later "mounds" and "grass" in the spectrum
Cavitation, recirculation	Non-synchronous broadband vibration	Radial	Random impacts in the waveform, dependence on flow and suction head
Hydraulics at the volute tongue	Vane pass Z × n and harmonics	Radial	Grows away from BEP; if it changes with speed or flow, check for resonance

How to read the spectra themselves is covered in How to read an FFT spectrum: five vibration signatures. More on the symptoms of cavitation is in Pump cavitation: signs and diagnostics.

A few clarifications:

• Unbalance and misalignment can look similar. Misalignment is finally confirmed by an alignment measurement, and unbalance by the response to a trial weight.
• Looseness is easier to see on a logarithmic amplitude scale: small harmonics get lost next to the 1x peak on a linear scale.
• Bearings: high-frequency peaks are best viewed in the acceleration spectrum. If the bearing geometry is unknown, frequencies are approximated as BPFO ≈ 0.4 × N × n and BPFI ≈ 0.6 × N × n, where N is the number of rolling elements and n is running speed.
• Vane pass frequency equals the number of impeller vanes multiplied by running speed. Unsteady hydraulic radial forces act at this frequency and are present in all pump types.

Example frequency calculation

Pump at 1480 rpm, 6-vane impeller, bearing with 9 balls:

Component	Calculation	Frequency
1x	1480 / 60	24.7 Hz
2x	2 × 24.7	49.3 Hz
Vane pass	6 × 24.7	148 Hz
BPFO (estimate)	0.4 × 9 × 24.7	≈ 89 Hz
BPFI (estimate)	0.6 × 9 × 24.7	≈ 133 Hz

Exact bearing frequencies are taken from the manufacturer's catalog for the bearing designation.

Pump survey procedure

1. Inspect the set: seal leaks, cracked welds, grout and anchor bolts, coupling guard.
2. Record the operating point: flow, suction and discharge pressure, speed.
3. Take overall RMS velocity at all points and directions and compare with the ISO 10816-7 zones for your category and power.
4. At the points with the highest levels, record a velocity spectrum, an acceleration spectrum and a time waveform.
5. Identify 1x, its harmonics, vane pass and bearing frequencies; treat the rest as non-synchronous content.
6. If needed, repeat at another flow: hydraulic components (cavitation, recirculation, vane pass) depend on flow and suction head.
7. Save the result as a baseline and repeat on a route: the trend matters more than a single value.

Which instrument to use for pump vibration analysis

• Spectrum analysis, route collection and in-place balancing — Bently Nevada vb7: per the manufacturer's datasheet, two channels with simultaneous recording, up to 6400 lines, Fmax up to 40 kHz, laser speed sensor, two-plane balancing.
• Bearing ultrasound, lubrication checks and vibration on one route — SDT340: contact and airborne ultrasound sensors, an accelerometer, a built-in infrared temperature sensor and tachometer.
• Pumps where a sensor is hard to mount (submersible, in pits) — Artesis AMT Pro: analysis of motor currents and voltages; the manufacturer lists pump cavitation, unbalance, misalignment and bearing faults among detectable faults.

KEG TRK supplies these instruments in Kazakhstan and helps select a kit for your pump fleet. Price on request.

Sources

• Europump. Pump Vibration Standards Guidelines, 1st edition, July 2013 (ISO 10816-7, ISO 13709, ISO 5199).
• Institute for Standardization of Serbia. ISO/AWI 20816-7 project status.
• Hydraulic Institute. Pump Vibration & Troubleshooting Strategies, Part 2.
• Emerson. Vibration Analysis for Machinery Health Diagnosis.
• ACOEM. Finding Unbalance and Misalignment on a Spectrum.
• Fluke Reliability. Linear vs. Non-Linear Systems Explained, Part 2.
• KSB. Centrifugal Pump Lexicon: Radial thrust.
• Bently Nevada. vb7 Portable Data Collector, Analyzer and Balancer, datasheet 116M5255 Rev. F.
• SDT Ultrasound Solutions. SDT340.
• NVMS (Artesis distributor). Artesis AMT Pro.