Continuous monitoring for and correction of centrifugal-fan imbalances during operation is an especially cost-effective way of eliminating vibration problems.

From the time centrifugal fans first entered the marketplace, they have been subject to vibration-related problems. These problems range from simple unbalance conditions caused by mass variations on the fan rotor to more complex issues related to shaft alignment, bearing fatigue and resonance. In many cases, excessive vibration levels in fans lead to unplanned outages to perform maintenance. While these outages are necessary, they also can be very costly from both a maintenance and lost-production standpoint.

Some levels of vibration are acceptable—and standards have been established for these acceptable levels at corresponding operating speeds. The chart in Fig. 1 reflects commonly accepted criteria for vibration levels in most rotating equipment. To effectively deal with vibration issues in such equipment, however, it is necessary to implement a condition-based maintenance program that can identify problems before they turn catastrophic.

Condition-based maintenance
A condition-based maintenance program requires an initial review of the following common causes:

Shaft misalignment…
Proper alignment between a drive motor shaft and a fan shaft needs to be 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, misalignment conditions will lead to excessive levels of axial vibration. Because 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.

Resonance…

Resonance problems are often two-fold on large fan assemblies. The first component to address is critical speed. Mapping of critical speed typically is handled during new fan design. Most fans operate below first critical speed. The factors in avoiding critical speed in fan design include overall rotating mass, span between bearings and necessary operating speed to produce the required airflow. If a fan operates above first critical speed, careful attention must be paid to vibration levels as the fan accelerates to operating speed and coasts down to a stop from operating speed. Excessive levels of vibration while passing...(Read whole article)