All equipment with rotors are subject to imbalance. Usually the imbalance increases the noise and vibration of the equipment. Try removing a fan blade from your room fan and see how much noise it makes due to unbalance. Motors, blowers, pumps, fans, they all can become unbalanced at any time. Correcting for the imbalance varies between each type of equipment and depends primarily on the cause of the imbalance.

Motors and single stage pumps generally have a compacted, sleek rotor design, that does not go out of balance very easily, or often. Fans probably go out of balance most often due to their continuous operation in some very rough mediums of dirt, moisture, heat, cold, etc.... For this reason, fans are manufactured with access doors in the housings to allow cleaning, inspection, repair, and re-balancing of the rotating assembly. They are expected to go out of balance over time and to require in-field balancing.

Cast iron centrifugal air and gas blowers are not designed to be field balanced. There are no access doors. The rotors are balanced at the factory during manufacturer with the expectation the balance will not change. The cast aluminum, or fabricated steel, impellers have no wear points, so they have no reason to go "out of balance" other than via subjection to foreign material, or damage to the impeller. Unfiltered air can allow dirt, paper, & who knows what, to lodge in the impellers. Landfill gas applications always have plenty of moisture/dirt mix that builds up on the inside of the impellers.

The build up is relatively evenly distributed throughout the impeller while it is in operation. The imbalance is created when a chunk falls off! Then you have an imbalance with one point of the circle lighter than the rest. In this case, a simple test to determine rotor unbalance is to check for a heavy spot in the rotor. Set the shaft keyway at 12:00 o'clock, and see if the shaft falls to 6:00 o'clock. Repeat at various clock positions. A heavy spot confirms the rotor is not balanced, and then most likely the cause of the increased noise and vibration.  

There are field balancing techniques utilizing a balancing ring, or modifications to the coupling hubs, or the addition of other components to the shaft, but they are certainly not the manufacturer's approved procedure. The best way to balance a cast iron, multistage, centrifugal blower is in a balancing machine. The impellers are mounted on the shaft, with spacers and locknuts, and dynamically balanced as a complete unit. The rotor is then marked, and disassembled to permit re-assembly of the complete blower.  

All rotors have some measure of imbalance. It is just a matter of whether it is an "acceptable" amount of imbalance. Manufacturer's generally follow the ISO industry standards for tolerances that vary greatly depending on application, speed, and type of installation.

Vibration data collection and analysis is the best way to prevent imbalance from damaging your rotating equipment. Invest in PM, it will save you money!