Common Causes of Vibration in Centrifugal Fans

Ever since centrifugal fans have been manufactured they have been subject to vibration related problems. These problems range from simple unbalance conditions caused by mass variations on the fan rotor to much more complex issues related to shaft alignment, bearing fatigue, or resonance issues. In many cases excessive vibration levels in fans lead to unplanned, forced outages to perform maintenance.

Once to this stage, these outages are necessary to maintain safety. However, most often, they are costly both from a maintenance and lost production standpoint. Standards have been set as to what are acceptable vibration levels for corresponding operating speeds.

Other sources that outline acceptable balance and vibration levels for fans include ANSI/AMCA 204-96, Balance Quality and Vibration Levels for Fans; ISO 14694:2003, and Industrial Fans; Specifications for Balance Quality and Vibration Levels.

Below are brief discussions of the most common causes of vibration in centrifugal fans along with the corresponding symptoms and methods for correction.

Shaft Misalignment

Proper alignment between a drive motor shaft and a fan shaft is an important step that needs to be properly addressed during new fan installation or if a shaft/rotor assembly is replaced. Misalignment between a drive motor shaft and fan shaft typically results in a 1X and 2X harmonic component of vibration. Often times, misalignment conditions will also lead to excessive levels of axial vibration. Since most fans are not equipped with axial vibration probes this is often not detected unless the 2X vibration component exists. Misalignment can be caused by careless installation of new equipment, but is more commonly caused by bent shafts or improperly seated bearings. Misalignment should be able to be detected prior to startup of a fan by using a dial or laser alignment system to verify proper alignment between the drive motor shaft and fan shaft. However, a bent fan shaft may not be detected by the alignment system, which may allow the above symptoms to persist.

Resonance

Resonance problems are often twofold on large fan assemblies. The first component that has to be addressed is critical speeds. Critical speed mapping is typically a task that is addressed during new fan design. Most fans are designed to operate below first critical speed. The factors in avoiding critical speed in fan design are overall rotating mass, span between bearings, and necessary operating speed to produce the required airflow. If a fan operates above first critical speed then careful attention has to be paid to vibration levels as the fan accelerates up to operating speed and, more importantly, coasts down to a stop from operating speed. Excessive levels of vibration while passing through a critical speed can lead to severe damage to bearings, seals, and other related equipment. The second factor, structural resonance, can be much more challenging to predict. Every structure has a natural frequency at which it will resonate. If a fan operates at a structural resonance point that is not corrected it can also lead to component failures. Structural resonance can occur at 1X operating speed or at a harmonic frequency (2X, 3X, etc.). Structural resonance will vary depending on operating speed and can be easily identified by performing a signature plot mapping vibration amplitude, versus frequency, versus rotational speed.

Mechanically Loose Connections

Looseness in any mechanical connection between bearing caps, bearing pedestals, or foundations can cause excessive vibration levels or amplify an already existing unbalance problem. In most cases, a mechanically loose connection will yield harmonic levels of vibration (2X, 3X, etc.) and may also yield sub-harmonic levels of vibration (X/2, X/3, etc.). Vibration caused by mechanically loose connections is often misdiagnosed due to the presence of sub-harmonic vibration levels.

A second type of vibration caused by mechanically loose connections can take place if there is looseness in the connection between the fan rotor and fan shaft. In many cases this will induce an extremely high unbalance related vibration level that is not necessarily at 1X operating speed. This type of vibration can be very difficult to determine, but easily corrected if found. In most cases, properly designed interference fits between the rotor hub and fan shaft can be implemented to avoid this condition.

Cracked Shafts or Rotors

Crack propagation in either a fan shaft or rotor can lead to one of the most dreaded failure modes in any type of rotating equipment. If undetected, a crack in a shaft or rotor can eventually lead to catastrophic failure of the fan. Early crack detection can take place if vibration trending and analysis takes place on a piece of equipment. The common symptoms of a crack propagating in a fan are both an emergence and growth of a 2X component of vibration along with a change in the phase and amplitude of the 1X vibration component.

Rotor Mass Unbalance

Rotor mass unbalance is the most common cause of excessive vibration in most rotating equipment and fans. The primary symptom of rotor mass unbalance is a high 1X vibration level. Rotor mass variation leading to an unbalanced condition is typically caused by three primary factors.

1. Variations in manufacturing can lead to
unevenly distributed mass in the fan rotor.

2. Exposure to high air stream temperatures
can cause uneven growth of the fan rotor.

3. Deterioration of the fan rotor caused
by either high speed particle collisions….

Article by Andrew J. Winzenz. Continue reading at uptimemagazine.com

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