Vane pass frequency is one of the more common and easily identified vibration monitoring components, present primarily in fans and pumps but also identifiable in compressors and other similar rotating equipment.  Vane press frequency is dependent upon the primary running speed of the machine, as with almost every other machine fault, and the number of blades, vanes, pistons or lobes present on the machine.  To calculate it, simply multiply the primary running speed by the number of blades, vanes or other similar component.

Vane pass frequencies will always be present in the types of machines mentioned above, and usually have prominent vibration signals in the frequency spectrum.  Forces are generated within the machine by pressure variations as the rotating lobe, vane or blade passes a stationary housing, causing turbulence.  This is inherent to the machine design and generates a non-uniform disturbance in the fluid or gas flow.  If this amplitude increases, it is a sign of machine wear and is most often associated with internal problems such as erosion of the impeller, in the case of centrifugal pumps, flow-related issues or potential misalignment.

Other, less common causes for high blade pass frequency amplitudes are rotor or housing eccentricity, non-uniform variable pitch blades, improper performance parameters that can cause cavitation, looseness of the impeller on the shaft, and pump starvation.  Impeller looseness, specifically, will cause sidebands at a distance of 1X, or the primary running speed of the machine, which can aid in identification.

Properly diagnosing blade pass frequencies and regularly monitoring their vibration amplitudes is crucial for maintaining quality in a pump, fan or compressor application.  Blade health and appropriate performance parameters play an integral part in ensuring the process runs smoothly and is efficient as can be.