Components of large industrial fan impellers have natural resonant frequencies, which, if excited during operation, may cause vibratory stress and fatigue. The result may be cracking or destruction of certain components. Fortunately, several simple field tests performed with fairly inexpensive equipment can measure component vibration response and anticipate frequencies that will be excited after installation.
One such testthe impact, or impulse excitation techniqueoffers several advantages over the swept-sine wave excitation test. It can suggest corrections to prevent natural frequency excitation.
Natural frequency excitation
Component parts of an industrial fansuch as the shroud (rim), webplate (center disk) or impeller bladehave a natural frequency of vibration. Geometry, material of construction and points of connection to other components determines this resonant frequency.
It's possible, but not desirable, to excite any of the possible natural frequencies by some outside force. A simple example would be the blade of an axial flow impeller vibrating as a cantilever beam. This vibration type is not to be confused with unbalance, bearing vibration or shaft natural frequency. Consider this example.
A large sintering plant had a 98-inch diameter axial flow impeller with several cracked blades. In fact, one blade tore completely loose from the hub. A vibration analysis revealed three natural frequencies or modes:
At 92 Hz, the blade vibrated as a cantilever with its base anchored and the entire blade flexing up and down (see Figure 1).
At 124 Hz, the blade exhibited one node near the transition point between the thicker and thinner sections of the blade (see Figure 2).
At 372 Hz, the blade exhibited a node at the base of the blade and another just beyond the transition point between the thicker and thinner sections (see Figure 3).
The 10-bladed impeller operates between 680 rpm and 750 rpm, which produces a blade passage frequency between 113 Hz and 125 Hz. The blade passage frequencyits operationally induced frequencycoincided with the second resonant frequency. As a result, the blades developed cracks at the node location. A redesigned blade featuring a beefier cross section raised the resonant frequency to 152 Hz, which solved the problem.
Determining the natural frequencies of the component parts is only part of the job. One also must anticipate which frequencies are likely to be excited. Among the variables to consider are:
Blade passage frequency.
Rotating stall caused by partially closed inlet dampers.
Stack- induced and duct-induced vibration.
Mechanical drive vibration caused by bearing problems, coupling misalignment and other problems.
Shaft torsional frequency, especially in variable speed drive applications.
Impeller test preparation
Preparing an impeller for testing requires suspending it on soft supports so the shaft centerline is horizontal. There should be no contact between the fan and the floor or other obstruction. The test area should be free of ambient noise and vibration.
Usually, the following centrifugal fan locations are tested:
Shroud outside diameter, midway between two blades.
Shroud inside diameter, midway between two blades.
Web outside diameter, midway between two blades.
Blade inlet, midway between hub and shroud.
On axial fans, only the blades are tested, but they should be tested at several points to determine mode shapes at each resonance.