Fans and Pumps: Blade Pass, Vane Pass, Cavitation and Flow-Related Vibration

From Mechanical Maintenance to Vibration Analysis - Part 9

In Part 8 - Resonance Diagnosis, we learned that a normal forcing frequency can become severe when it excites a natural frequency. Fans and pumps create important forcing frequencies of their own, especially blade-pass and vane-pass frequencies.

These machines also interact continuously with air, gas or liquid. A spectrum therefore reflects more than the rotor and bearings. It can contain evidence of inlet restrictions, poor operating point, recirculation, turbulence, cavitation, impeller or blade condition, and structural response.

Blade pass and vane pass are forcing frequencies, not automatic fault diagnoses. Their meaning comes from amplitude, harmonics, sidebands, waveform, operating point and process condition.

Begin with speed and component geometry

Before interpreting a peak, record the actual shaft speed and count the blades or vanes. Convert speed from revolutions per minute to hertz:

Running frequency (Hz) = rpm / 60

Then calculate the relevant pass frequency:

Blade-pass frequency (BPF) = number of blades x running frequency
Vane-pass frequency (VPF) = number of impeller vanes x running frequency

A fan with 8 blades operating at 1,200 rpm has a running frequency of 20 Hz and a blade-pass frequency of 160 Hz. A pump with 6 vanes operating at 1,480 rpm has a running frequency of 24.67 Hz and a vane-pass frequency of approximately 148 Hz.

The pass frequency normally exists because each blade or vane repeatedly moves past a stationary part of the housing. Pressure and flow vary during every pass. A visible peak is therefore expected on many healthy machines. What matters is whether it has changed and what other evidence accompanies it.

What makes a pass-frequency peak important?

A blade- or vane-pass peak deserves investigation when one or more of the following occur:

• Its amplitude rises significantly from a comparable baseline.
• Harmonics of the pass frequency increase.
• Running-speed sidebands appear or grow around the pass frequency.
• The response becomes strongly directional or changes with flow, damper position or load.
• The time waveform becomes distorted or contains repeating bursts.
• Pressure, flow, sound, temperature or power changes at the same time.

Possible causes include dirty or damaged blades, worn vanes, rotor or housing eccentricity, a loose impeller, diffuser or inlet problems, non-uniform blade spacing, flow restrictions, operation away from the intended duty point, or resonance near the forcing frequency. The spectrum alone rarely separates all of these possibilities.

Sidebands and harmonics add diagnostic information

Sidebands indicate modulation. If peaks around BPF or VPF are spaced by 1X running speed, something that changes once per revolution may be modulating the pass event. Possibilities include eccentricity, uneven clearance, a loose impeller, one damaged area, or another once-per-revolution flow disturbance.

For example, a six-vane pump at 24.67 Hz has VPF near 148 Hz. Peaks near 123.3, 148 and 172.7 Hz form 1X-spaced sidebands. This is evidence of modulation; it does not identify the physical cause by itself. Inspect the impeller, clearances and casing, compare radial directions, review phase where useful, and check whether the pattern changes with operating point.

Harmonics of BPF or VPF can result from a non-sinusoidal, strongly distorted or impacting force. Their presence may be normal for some designs, especially if stable. Trend comparable conditions and compare similar machines before deciding that the amplitude is abnormal.

Fan vibration: combine aerodynamics with mechanical checks

Fan vibration can be influenced by rotor unbalance, alignment, bearings, belts and looseness, but it may also respond strongly to the air system. Useful checks include:

• Blade condition: inspect for dust buildup, corrosion, erosion, cracks, bent blades and missing balance weights.
• Inlet condition: check filters, screens, elbows, dampers and obstructions that can create uneven flow into the wheel.
• Outlet and duct condition: look for restrictions, abrupt transitions, flexible-connection problems and unstable damper operation.
• Operating point: record flow, pressure, damper position, motor load and speed. A changing process may change vibration without a new mechanical defect.
• Structure: compare BPF and its harmonics with natural frequencies. An aerodynamic force can become severe when resonance amplifies it.

On variable-speed fans, collect vibration and process data across a controlled speed range when the procedure permits. If BPF moves with speed but the response becomes large only near one fixed frequency, resonance becomes a strong competing hypothesis.

Pump vibration: start with the operating point

A centrifugal pump should be evaluated with its pump curve and system condition. The best efficiency point (BEP) is the flow at which a particular pump operates most efficiently for a given speed and impeller diameter. The manufacturer normally defines an allowable operating region around it.

Operating too far from the intended region can create unstable hydraulic forces, recirculation, pressure pulsation, higher radial load, noise and vibration. The exact acceptable region is pump-specific; do not turn a general percentage from a training chart into a universal limit.

Always record the operating state with the vibration reading:

• Suction and discharge pressure.
• Flow rate and valve positions.
• Liquid level and temperature.
• Speed, motor current or power, and pump configuration.
• Whether strainers, filters or parallel pumps have changed.

A route measurement taken at a different flow condition may not be directly comparable with the previous trend.

Cavitation: bubble formation and collapse

Cavitation begins when local liquid pressure falls low enough for vapour bubbles to form. As the bubbles move into a higher-pressure region, they collapse. Repeated collapse can generate noise, random impacts, vibration and material damage.

Common contributors include insufficient available net positive suction head, a restricted suction line or strainer, low tank level, excessive liquid temperature, poor inlet geometry, air ingress, excessive speed, or an operating point that demands more flow than the suction system can supply.

Typical evidence may include:

• A gravel-like or crackling sound.
• Random high-frequency bursts in the time waveform.
• A raised broadband noise floor or broad spectral humps.
• Changes at vane-pass frequency and its harmonics.
• Unstable suction pressure, flow or discharge pressure.
• Loss of performance and, with prolonged exposure, impeller pitting.

These symptoms are not unique to cavitation. Air entrainment, rubbing, a damaged bearing, process solids, resonance and sensor-mounting problems can produce similar evidence. Confirmation requires process checks and, where possible, inspection.

NPSH is a system check, not a vibration label

The pump manufacturer specifies net positive suction head required (NPSHr) for a defined test condition. The system provides net positive suction head available (NPSHa). Reliable operation requires sufficient margin between the available and required values under real operating conditions, according to the manufacturer's guidance and the applicable engineering standard.

A vibration analyst does not confirm adequate NPSH from a spectrum alone. Work with operations or engineering to verify suction pressure, vapour pressure at the actual liquid temperature, static head, suction-line losses, speed and flow. Never throttle a pump's suction valve as an improvised diagnostic test unless an approved procedure specifically permits it.

Recirculation and turbulence can resemble other faults

Internal recirculation can occur when flow separates and circulates within the impeller or casing, often during operation away from the intended region. It may produce low-frequency pulsation, broadband energy, pressure fluctuation and changes near VPF.

External turbulence can result from obstructions, sharp elbows close to the inlet, abrupt area changes, partially closed valves, dirty filters or disturbed inlet flow. In fans and pumps it may appear as broad, unstable low-frequency energy rather than a clean mechanical order.

A practical distinction is responsiveness to process change. If vibration follows flow, pressure, valve or damper position while shaft speed remains nearly constant, a flow-related mechanism becomes more likely. Make only approved operational changes and remain inside the manufacturer's limits.

Do not confuse cavitation with a bearing defect

Evidence	Cavitation or flow issue	Rolling-element bearing defect
Relationship to process	Often changes with flow, suction condition or liquid temperature	May change with load, but normally follows bearing geometry and speed
Waveform	Random or irregular high-frequency bursts	More regularly repeating impacts may be present
Spectrum	Broadband rise, humps and possible VPF changes	Calculated defect families and harmonics may appear, especially in enveloped data
Supporting checks	Pressure, flow, NPSH review, sound and impeller inspection	Envelope spectrum, lubrication condition, ultrasound and bearing inspection

The strongest diagnosis explains both vibration and process behaviour. If the evidence does not separate the alternatives, state the uncertainty and recommend the next discriminating test.

Worked diagnosis: six-vane process pump

A six-vane centrifugal pump operates at 1,480 rpm, or 24.67 Hz. The calculated vane-pass frequency is approximately 148 Hz. The latest measurement shows a higher VPF peak, a raised high-frequency noise floor and irregular bursts in the acceleration waveform. Operators also report a gravel-like sound and unstable discharge flow.

The analyst compares the data with the previous baseline and checks the operating condition:

• Speed is unchanged.
• Suction pressure is lower than during the baseline measurement.
• Liquid temperature is higher.
• Flow has increased after a process change.
• The suction strainer differential pressure is above its normal range.

Interpretation: the vibration and process evidence support a suction-side flow problem with cavitation as the leading hypothesis. The VPF increase alone would not be enough to reach that conclusion.

Action: operations returns the pump to an approved stable condition and the responsible team inspects the suction path and strainer. Engineering verifies the system's NPSH margin. After the confirmed restriction is corrected, suction pressure, flow, sound and vibration are remeasured under the same operating state. The broadband energy and VPF amplitude return near baseline.

How System 1 and other software help

Condition-monitoring software can calculate and trend BPF or VPF, display spectra and waveforms, create narrow-band alarms, trend broadband high-frequency energy, compare multiple measurement locations, and correlate vibration with speed, flow, pressure, temperature and load.

Online systems are especially useful when a flow problem is intermittent. Time-synchronised process and vibration trends can show whether vibration changes before, after or at the same time as the process condition.

Software still cannot guarantee an exact diagnosis. Blade count, vane count, actual speed, sensor position, frequency range, sampling, mounting and process tags must be correct. The analyst must test competing explanations and close the loop after action.

A practical fan-and-pump workflow

1. Verify the measurement. Confirm sensor position, direction, mounting, units, Fmax and actual speed.
2. Calculate forcing frequencies. Mark 1X, harmonics, BPF or VPF, bearing frequencies, belt frequencies and electrical components where relevant.
3. Record the operating condition. Include flow, pressure, valve or damper position, temperature, level, load and configuration.
4. Compare with a valid baseline. Use the same speed, load and process state whenever possible.
5. Read spectrum and waveform together. Look for harmonics, sidebands, broadband energy, modulation, impacts and instability.
6. Inspect the machine and flow path. Check blades, impeller, clearances, filters, strainers, ducts, piping, supports and visible damage.
7. Rank competing causes. Separate mechanical, hydraulic or aerodynamic, structural and measurement explanations.
8. Choose a discriminating test. Correlate process tags, use an approved operating change, review phase or resonance data, and apply other condition-monitoring methods.
9. Verify after correction. Repeat vibration and process measurements under the same condition.

Final takeaway

• BPF and VPF are calculated forcing frequencies, not automatic fault names.
• Amplitude change, harmonics, sidebands and operating condition give the peaks meaning.
• Cavitation commonly produces noise, random impacts and broadband high-frequency energy, but it must be confirmed with process evidence.
• Fans and pumps should be diagnosed as complete machine-and-process systems.
• A good report states the evidence, uncertainty, recommended check and verification result.

Coming in Part 10

Part 10: Electric Motor Vibration - Line Frequency, Pole Pass, Rotor Bars and Electrical/Mechanical Separation. We will connect spectral patterns with motor load, current data, speed and mechanical checks.

Discussion question: Have you seen a pump or fan repaired mechanically when the main cause was actually the operating condition or flow path?

References and further learning

• Mobius Institute, Vibration Analysis Category II Course Manual, Chapter 16: Pumps, Fans and Compressors.
• Emerson Process Management, Basic Vibration Analysis - Course 2031, Chapter 1: Introduction to Vibration, including pump-pass frequency and the fault guide.

Educational note: The patterns and examples in this article are learning guidance, not universal alarm limits, NPSH calculations, operating procedures or acceptance criteria. Follow approved site safety procedures, pump and fan manufacturer requirements, process limits and qualified engineering judgement.