Abnormal noises from an industrial centrifugal fan are essentially a warning signal from the equipment. The first step is to identify the type of noise, then take targeted corrective actions. Never continue operating blindly, as this may lead to serious failures such as impeller rupture or main shaft fracture.

Based on the sound characteristics and possible failure locations, abnormal noises can mainly be divided into the following five categories with corresponding solutions:

1. Sharp High-Frequency Metal Friction Noise (Rubbing Noise)

Symptoms:
A continuous or intermittent sharp “screeching” sound, often accompanied by local overheating of the casing, and in severe cases, smoke may be visible from the housing.

Possible Causes:

• Thermal expansion causing loss of clearance:
  When transporting high-temperature gases, thermal deformation of the casing may cause the impeller inlet ring and casing seal ring to rub against each other in radial or axial directions.
• Bent shaft or worn bearings:
  Rotor deflection or bearing wear may cause misalignment between the rotating impeller and stationary casing.
• Dust buildup or foreign objects:
  Hard particles stuck at the impeller rear side or inlet area may cause contact.

Solutions:

• Shut down the fan immediately if the rubbing is severe. After cooling, check and adjust the clearance between the impeller and inlet collector. The standard clearance is generally 0.5%–1% of the impeller diameter.
• For high-temperature fans, check whether the inlet/outlet expansion joints are restricted and release pipeline stress.
• Remove deposits from the impeller inlet ring and correct shaft straightness if necessary.

2. Low-Frequency Periodic “Knocking” Noise (Unbalance or Looseness)

Symptoms:
A repeated “thump-thump-thump” sound with a frequency matching the fan rotation speed. Vibration increases significantly, especially in the horizontal direction, and strong vibration can be felt near the foundation bolts.

Possible Causes:

• Uneven dust accumulation or wear on the impeller:
  This destroys the dynamic balance and is one of the most common problems in industrial applications.
• Loose rivets or cracked welds on the impeller:
  The blades may move or deform under high-speed rotation.
• Coupling pin wear or severe misalignment.

Solutions:

• Stop the fan immediately and inspect the impeller surface. If dust buildup is found, clean it thoroughly and perform dynamic balancing (field balancing or factory repair).
• Check impeller welds. If cracks are found, repair welding and proper heat treatment are required. Never operate with damaged components.
• Realign the coupling. For hot alignment, the deviation should generally be controlled within 0.05–0.10 mm.

3. Irregular “Rattling” or “Clunking” Noise (Bearing/Gear Failure)

Symptoms:
The noise is dull and impact-like, accompanied by a rapid increase in bearing housing temperature (above 75°C). Using a listening rod on the bearing housing reveals obvious impact pulses.

Possible Causes:

• Bearing cage failure or rolling element damage (spalling of balls or rollers).
• Lubricating oil deterioration, emulsification, or insufficient oil level, causing loss of oil film protection.
• For gearbox-driven fans: gear tooth pitting or tooth breakage.

Solutions:

• Shut down immediately. Do not continue running, otherwise shaft seizure and burnout may occur.
• Disassemble and inspect the bearings. Replace them with the same specification bearings (C3 clearance), preferably from reliable first-line brands.
• Clean the lubrication system completely, replace the oil, and ensure the oil level is within the specified range on the sight glass.

4. Periodic “Breathing” Noise (Surging)

Symptoms:
The noise is accompanied by severe pressure fluctuations in the duct system, unstable airflow, and large fluctuations of motor current.

Possible Causes:

• Increased system resistance or insufficient fan speed, causing the operating point to enter the surge area.
• Common in parallel fan operation or when variable frequency drive speed is set too low.

Solutions:

• This is a dangerous operating condition. Immediately open the vent valve or bypass valve to increase airflow and move the operating point away from the surge zone.
• Check whether inlet filters or pipelines are blocked. Clean or replace filters if necessary.
• Adjust the VFD minimum frequency setting to ensure the fan speed does not fall below 30%–40% of rated speed (depending on the fan performance curve).

5. Continuous “Humming” Electromagnetic Noise (Motor or Resonance)

Symptoms:
A continuous low-frequency humming sound that disappears immediately after shutdown. During operation, the fan produces abnormal noise and vibration in multiple directions.

Possible Causes:

• Insufficient foundation rigidity or loose anchor bolts causing resonance.
• Motor phase loss or broken rotor bars causing electromagnetic vibration.
• Fan operating speed close to the natural frequency (near critical speed).

Solutions:

• Tighten foundation bolts and add vibration isolation pads or reinforce the foundation with grouting if necessary.
• Use a clamp meter to check whether the three-phase motor currents are balanced. If the deviation exceeds 10%, inspect the motor windings.
• During shutdown, check the speed ratio between the fan and motor, and adjust the operating speed to avoid resonance peaks (normally avoid the resonance range within ±10% of the critical speed).

Important Safety Warnings

⚠️ Never touch rotating parts of the fan with your hands or tools while the equipment is running to “listen” for faults.

Cold Start After Maintenance:

After completing repairs, if the fan is handling high-temperature gases (above 200°C):

1. Manually rotate the fan shaft first to confirm there is no sticking or abnormal resistance.
2. Perform low-speed warm-up operation for about 5–10 minutes.
3. Increase to rated speed only after thermal expansion becomes stable.

If you have a vibration meter, record the current vibration velocity value (mm/s):

• Above 7.1 mm/s (ISO 10816-3 standard): The equipment is considered unacceptable and should be scheduled for maintenance.
• Above 11.2 mm/s: Emergency shutdown is required immediately.