Centrifugal pumps has a prominent vibration frequency at blade passage frequency, BPF (number of blades multiplied with the rotational speed). If the level increases at BPF it may be an internal problem in the pump, for example bad alignment or damaged blades. BPF harmonics can also appear.
Following FFT contains a high frequency broad banded noise, which indicates cavitation caused by low inpouring pressure.
Cog wheel pumps usually have a prominent vibration component at gear mesh frequency, i.e. the number of cogs multiplied with the rotational speed. If the level at the gear mesh frequency changes, due to for example the appearance of harmonics and side bands, it can be a sign of a broken or in any way damaged cog.
V=number of pump blades
T=number of cogs
S=number of threads
|Source of vibration||Exciting frequency||Dominating direction||Amplitude||Spectral characteristic||Comments|
|Not rotating looseness||1X, 2X, 3X||radial||constant||narrow band||The multiples can stretch to 10X|
|Rotating looseness (rotor, pump wheels etc.)||1X||radial||Vary from start to start||narrow band||Sometimes 0,5 X multiples|
|Centrifugal pumps with (V) blades||Blade passage= VX||radial||Varying level on blade passage frequency||multiples||Large pumps give highest amplitude at blade passage. For smaller pumps strongest amplitude at multiples on blade passage.|
|Cog wheel pumps with (T) cogs||Cog frequency = TX||radial|
|Rotor touching||0,5 X, 1X||radial||constant||narrow band||Can excite rotor critical speed.|
|Screw pumps||SxX||radial||S=number of threads|
|Cavitation||random||radial||varying||broad band||Random noise up to 20 kHz.|