Centrifugal pumps: cavitation and misalignment in the vibration signal

A centrifugal pump driven by a coupled motor is the most common machine set in industry and one of the most confusing in vibration analysis, because problems in the motor, the coupling, the pump and the liquid itself all mix into the same signal. This article separates the signatures: what cavitation does to the spectrum, how to tell misalignment from unbalance, where looseness shows up, and where to measure on a motor-pump set so the data is actually useful.

It is written for the technician about to install sensors on pumps and for the manager who wants to understand why the pump "sounds like it is pumping gravel" and what that costs.

Where to measure on a motor-pump set

A horizontal set has four bearings: motor non-drive end, motor drive end, pump coupling end and pump outboard end (or the thrust bearing, if there is one). ISO 20816-3 calls for measuring on the bearing housing, in two radial directions (vertical and horizontal) and axially when there is a thrust bearing or misalignment is suspected. A triaxial sensor on each bearing covers everything; the acceptable minimum is the two bearings next to the coupling, one on each side, because that is where misalignment and coupling problems show up first.

Two notes from the standards:

• A pump with its impeller mounted on the motor shaft (close-coupled) is not evaluated under ISO 20816-3 but under ISO 10816-7, which is specific to rotodynamic pumps.
• Vertical pumps usually have a flexible support, and the standard calls for using the point with the highest reading as one of the evaluation points.

Record the operating flow and pressure along with each measurement. A pump running away from its design point vibrates more, and readings can only be compared under similar conditions.

Unbalance: 1x, radial

An impeller with buildup, uneven erosion or a broken vane generates a force that rotates with the shaft. In the spectrum it is a dominant peak at 1x running speed, in the radial direction, with little axial vibration. The force grows with the square of the speed, and the amplitude is similar horizontally and vertically, adjusted for the support stiffness in each direction. An impeller that has lost material (erosion) or gained it (buildup) shows up here, and calls for opening the pump.

Misalignment: 2x, and axial

Misalignment between motor and pump is the most frequent cause of vibration in a coupled set, and the cheapest to fix. The signature:

• Parallel (offset): a high peak at 2x running speed in the radial direction, often larger than 1x, with 3x present. Radial measurements on either side of the coupling are out of phase (180°).
• Angular: strong axial vibration, with 1x dominant and 2x and 3x present. High axial vibration, comparable to or greater than radial, is the most reliable sign.

In practice the two come together. The rule of thumb: high 2x radial and high axial, on both sides of the coupling, is misalignment until proven otherwise. Confirm with laser alignment and measure again: the 2x drops immediately. Uncorrected misalignment loads the bearings, the seal and the coupling, and what you pay for later is bearings and seals.

Mechanical looseness: harmonics and half running speed

A loose base, a slack anchor bolt, soft foot, excessive bearing clearance: looseness lets the machine "knock" on every revolution, and the spectrum shows a series of 1x harmonics (2x, 3x, 4x, up to 10x), sometimes with components at 0.5x and multiples of half running speed. An amplitude that differs greatly between horizontal and vertical at the same position also points to looseness. Looseness is the first thing to eliminate, because it amplifies everything else.

Vane pass frequency

Every centrifugal pump has a peak at number of vanes × running speed, the vane pass frequency (VPF). A 6-vane impeller at 1,750 rpm produces a VPF at 175 Hz. That peak is normal. It becomes a problem when its amplitude grows: impeller-to-volute clearance out of spec, an eccentric impeller, operation far from the best efficiency point or a partial blockage. Compare with the baseline; the absolute value says little.

Cavitation: broadband noise

Cavitation is the formation and collapse of vapor bubbles in the impeller when suction pressure drops below the liquid's vapor pressure. Each collapse is a small impact, thousands per second, at random points. That is why its signature is not a peak but a rise in the broadband noise floor, at high frequencies, unrelated to running speed. In the field literature, cavitation noise typically appears between 5 kHz and 11 kHz (300,000 to 650,000 cpm), above the band where mechanical defects sit. In the time domain, the waveform shows irregular high-frequency spikes; the pump "sounds like it is pumping gravel."

What sets it apart from the other defects: it goes away when suction conditions improve. Open the suction valve, lower the liquid temperature, reduce the flow, and the noise floor drops. Misalignment and unbalance do not behave that way.

Cavitation is not a mechanical defect: it is an operating condition. The fix is on the suction side (NPSH available lower than NPSH required, a clogged strainer, low level, a long suction line), not in replacing parts. But it destroys the impeller by erosion and overloads the bearings, which is why monitoring treats it as an alert.

The right sensor for a pump

The standard's band (10 Hz to 1 kHz) covers unbalance, misalignment, looseness and vane pass. Cavitation and early-stage bearing defects call for high-frequency response and rigid mounting (stud or epoxy, not a magnet on paint). If the pump has a history of cavitation, choose the sensor by its maximum frequency, not by its price; confirm in the proposal the usable range with the mounting you chose.

On our platform, the RMS per axis and the temperature of each bearing feed the trend and the change-of-level alerts (email and WhatsApp), and the on-demand FFT spectrum is where the engineer confirms whether the 2x, the 1x, the harmonics or the noise floor is the problem.

What to do now

1. Install sensors on the two bearings next to the coupling, at a minimum; on all four if the pump is critical.
2. Record the baseline with the pump at its normal operating point, noting flow and pressure.
3. If the trend rises: spectrum. 2x and axial, align. 1x radial, balance or open the impeller. Harmonics, tighten the base. High noise floor, check the suction.
4. Measure again after each correction and save it as the new baseline.

If you want to know what your plant needs, talk to one of our engineers.

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Sources: ISO 20816-3:2022 (clauses 4.2 and 5.3, and scope) and ISO 10816-7:2009 for rotodynamic pumps; Mobius Institute, vibration analysis dictionary (misalignment, unbalance, looseness); CBM Connect, Warning Signs of Pump Cavitation; Power-MI, Misalignment.