Understanding Fan Defects

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 are Fan Defects?

Fan defects are faults in industrial fans and blowers including blade damage (cracks, erosion, buildup), unbalance from material loss or accumulation, aerodynamic instabilities (stall, surge), structural problems (loose blades, cracked hubs), and bearing/drive failures. These defects create characteristic vibration patterns dominated by blade passing frequency and its harmonics, along with 1× unbalance vibration and low-frequency aerodynamic pulsations.

Fans are ubiquitous in industrial facilities (HVAC, process cooling, combustion air, material handling), and their failures can affect production, safety (ventilation), and energy efficiency. Understanding fan-specific defects and monitoring techniques enables effective maintenance and prevents catastrophic blade failures.

Common Fan Defects

1. Blade Damage and Erosion

Material Buildup

Cause: Dust, scale, or process material accumulating on blades
Effect: Creates mass unbalance, changes aerodynamics
Symptom: Increasing 1× vibration over time
Common In: Material handling fans, process exhaust fans
Solution: Periodic cleaning, upstream filtration

Erosion and Wear

Cause: Abrasive particles wearing blade surfaces
Effect: Material loss creating unbalance, performance degradation
Pattern: Usually asymmetric (leading edge more than trailing)
Detection: 1× vibration increase, reduced performance

Corrosion

Chemical attack on blade material
Creates pitting and material loss
Reduces blade strength
Can lead to cracks and blade failure

Blade Cracks

Locations: Blade root (hub attachment), leading edge, weld joints
Causes: Fatigue, corrosion, impact, vibration
Symptoms: Changing vibration pattern, possible 2× component
Danger: Can lead to complete blade separation

Missing or Broken Blades

Severe unbalance from asymmetric blade configuration
Very high 1× vibration
Abnormal blade passing frequency pattern
Immediate shutdown and repair required

2. Unbalance

The most common fan vibration problem:

Sources: Buildup, erosion, manufacturing tolerances, blade damage
Signature: 1× synchronous vibration
Correction: Field balancing often effective
Recurring: May require addressing root cause (erosion, buildup sources)

3. Aerodynamic Instabilities

Stall

Airflow separation from blade surfaces at off-design conditions
Random, turbulent flow creating broadband vibration
Reduced efficiency and performance
Common at low flow rates or high inlet resistance

Surge

Periodic flow reversal in system
Very low frequency (< 5 Hz) severe pulsations
Can damage fan and ductwork
Requires system modifications to eliminate

4. Structural and Mechanical Issues

Loose Blades: Set screws or welds failed, multiple harmonics
Cracked Hub: Hub structure failure, can be catastrophic
Worn Shaft: Allows fan wheel to shift, creating runout
Housing Resonance: Fan casing or ductwork resonating at BPF or harmonics

5. Drive and Bearing Problems

Belt drive issues (wear, misalignment, tension)
Bearing failures (common in dirty/hot environments)
Coupling problems (misalignment, wear)
Motor defects affecting fan operation

Vibration Characteristics

Blade Passing Frequency (BPF)

The key fan-specific frequency:

Calculation: BPF = Number of Blades × RPM / 60
Example: 12-blade fan at 1200 RPM → BPF = 240 Hz
Normal Amplitude: Depends on fan type (axial fans higher than centrifugal)
Elevated BPF: Blade damage, clearance problems, aerodynamic issues
Harmonics: 2×BPF, 3×BPF indicate blade problems or resonances

Unbalance (1×)

Most common high-amplitude component
Increases with buildup or erosion
Correctable through balancing
May recur if root cause not addressed

Aerodynamic Pulsations

Stall: Broadband increase, random fluctuations
Surge: 1-5 Hz severe pulsations
Turbulence: Broadband low-frequency (10-100 Hz)

Fan-Specific Considerations

Fan Types and Defect Patterns

Centrifugal Fans

Unbalance most common issue
BPF typically moderate amplitude
Buildup on backward-curved blades common
Seal and bearing problems from process contamination

Axial Fans

Higher BPF amplitudes normal
Blade tip clearance critical
Aerodynamic instabilities more common
Blade fatigue from alternating aerodynamic loads

Induced Draft (ID) Fans

Severe erosion from flyash and particles
High temperatures affecting materials
Corrosive environments
Frequent rebalancing required

Diagnostic Strategy

Initial Assessment

Measure overall vibration at bearings
Perform FFT analysis identifying dominant frequencies
Check for 1× (unbalance), BPF (blade issues), bearing frequencies
Assess performance (flow, pressure)
Visual inspection if accessible

Problem Identification

High 1×: Unbalance → balance or clean fan
High BPF: Blade damage, clearance issues → inspect blades
Broadband: Cavitation or stall → check operating point
Low Frequency: Surge or recirculation → system modification
Bearing Frequencies: Bearing wear → replace bearings

Prevention and Maintenance

Balancing

Field balance fan wheels in-situ
Rebalance after cleaning or blade repair
Use clip-on or bolt-on weights for adjustability
Document balance weights for reference

Inspection and Cleaning

Periodic inspection for buildup, erosion, damage
Clean blades during outages
Check blade attachment security
Look for cracks, especially at blade roots

Operating Practices

Operate near design point when possible
Avoid prolonged operation at extremes (very high or very low flow)
Control inlet conditions (minimize turbulence)
Implement protective coatings for erosive/corrosive service

Fan defects combine mechanical problems common to all rotating equipment with aerodynamic issues unique to fans. The blade passing frequency signature, combined with standard vibration analysis techniques, enables effective fan condition monitoring and guides maintenance decisions for these critical air-moving machines in industrial facilities.

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What are Fan Defects? Common Failures and Diagnosis

Published by admin on October 31, 2025 October 31, 2025