Understanding Vane Passing Frequency

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Optical Sensor (Laser Tachometer)

Complete vibration diagnostics and balancing kit from Vibromera, including four vibration sensors with cables, a signal interface module, laser tachometer with magnetic stand, digital scale, USB cable, and a carrying case. The system is designed for field rotor balancing and condition monitoring on industrial equipment.

Devices

Dynamic balancer “Balanset-1A” OEM

Definition: What is Vane Passing Frequency?

Vane passing frequency (VPF, also called impeller vane frequency or simply vane pass) is the frequency at which the vanes (blades) of a rotating pump impeller pass by a stationary reference point such as the volute cutwater (tongue), diffuser vanes, or casing features. It is calculated as the number of impeller vanes multiplied by the shaft rotational frequency (VPF = Number of Vanes × RPM / 60). This is the pump equivalent of blade passing frequency in fans.

VPF is the dominant hydraulic vibration source in centrifugal pumps, typically appearing in the range of 100-500 Hz for industrial pumps. Monitoring VPF amplitude and its harmonics provides critical diagnostic information about impeller condition, hydraulic performance, and clearance issues.

Calculation and Typical Values

Formula

VPF = Nv × N / 60
Where Nv = number of impeller vanes
N = shaft speed (RPM)
Result in Hz

Examples

Small Pump

5 vanes at 3500 RPM
VPF = 5 × 3500 / 60 = 292 Hz

Large Process Pump

7 vanes at 1750 RPM
VPF = 7 × 1750 / 60 = 204 Hz

High-Speed Pump

6 vanes at 4200 RPM
VPF = 6 × 4200 / 60 = 420 Hz

Typical Vane Counts

Centrifugal Pumps: 3-12 vanes (5-7 most common)
Small Pumps: Fewer vanes (3-5)
Large Pumps: More vanes (7-12)
High-Head Pumps: More vanes for energy transfer

Physical Mechanism

Pressure Pulsations

VPF arises from hydraulic pressure variations:

Each impeller vane carries fluid at high velocity
As vane passes volute cutwater, pressure pulse created
Pressure differential across vane changes rapidly
Creates force pulse on impeller and casing
With Nv vanes, Nv pulses per revolution occur
Pulsation frequency = vane pass rate = VPF

At Design Point (BEP)

Flow angle matches vane angle
Smooth flow, minimal turbulence
VPF amplitude moderate and stable
Optimal pressure distribution

Off Design Point

Flow angle mismatched to vane angle
Increased turbulence and flow separation
Higher pressure pulsations
Elevated VPF amplitude
Possible additional frequency components

Diagnostic Interpretation

Normal VPF Amplitude

Pump at best efficiency point (BEP)
VPF amplitude stable over time
Typically 10-30% of 1× vibration amplitude
Clean spectrum with minimal harmonics

Elevated VPF Indicates

Operating Off BEP

Low flow operation (< 70% BEP) increases VPF
High flow (> 120% BEP) also elevates VPF
Optimal operation at 80-110% of BEP

Impeller-to-Casing Clearance Issues

Worn wear rings increase clearance
Impeller shift from bearing wear
VPF amplitude increases with excessive clearance
Performance degradation (internal recirculation)

Impeller Damage

Broken or cracked vanes create asymmetry
VPF amplitude with sidebands at ±1× speed
Erosion or buildup on vanes
Foreign object damage

Hydraulic Resonance

VPF matches acoustic resonance in piping or casing
Dramatic amplitude amplification
Can cause structural vibration and noise
May require system modifications

VPF Harmonics

2×VPF and Higher

Multiple harmonics indicate problems:

2×VPF Present: Non-uniform vane spacing, impeller eccentricity
Multiple Harmonics: Severe hydraulic turbulence, vane damage
Excessive Amplitudes: Potential for fatigue failures

Subharmonics

Fractional VPF components (VPF/2, VPF/3)
Indicate flow instabilities
Rotating stall or separation cells
Common at very low flow rates

Monitoring and Trending

Baseline Establishment

Record VPF when pump new or freshly overhauled
Document at design operating point
Establish normal VPF/1× amplitude ratio
Set alarm limits (typically 2-3× baseline VPF amplitude)

Trending Parameters

VPF Amplitude: Track over time, increasing indicates developing problem
VPF/1× Ratio: Should remain relatively constant
Harmonic Content: Appearance or growth of 2×VPF, 3×VPF
Sideband Development: Emergence of ±1× sidebands around VPF

Operating Condition Correlation

Track VPF vs. flow rate
Identify optimal operating zone (minimum VPF)
Detect when operating point has shifted
Correlate with performance degradation

Corrective Actions

For Elevated VPF

Operating Point Optimization

Adjust flow to bring pump closer to BEP
Throttle discharge or adjust system resistance
Verify suction conditions adequate

Mechanical Correction

Replace worn wear rings (restore clearances)
Replace worn or damaged impeller
Correct bearing problems allowing impeller shift
Verify proper impeller position (axial and radial)

Hydraulic Improvements

Improve inlet piping design (reduce pre-swirl, turbulence)
Install flow straighteners if needed
Verify adequate NPSH margin
Eliminate air entrainment

Relationship to Other Frequencies

VPF vs. BPF

Terms often used interchangeably for pumps vs. fans
VPF: Preferred term for pumps (vanes in liquid)
BPF: Preferred term for fans (blades in air)
Calculation and diagnostic approach identical

VPF vs. Running Speed

VPF = Nv × (running speed frequency)
VPF always higher frequency than 1×
For 7-vane impeller, VPF = 7× running speed frequency

Vane passing frequency is the fundamental hydraulic vibration component in centrifugal pumps. Understanding VPF calculation, recognizing normal vs. elevated amplitudes, and correlating VPF patterns with operating conditions and pump condition enables effective pump diagnostics and guides decisions about operating point optimization, clearance restoration, and impeller replacement.

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What is Vane Passing Frequency? Pump Blade Diagnostics

Published by admin on October 29, 2025 October 29, 2025