Machining and assembly faults, the corrosive phenomena of cavitation or slag accumulation, but also the simple construction tolerances of the individual elements, cause rotating machines to lose balance.
The first symptoms of this are vibrations and noise; the analysis of these "markers" in the case of a fire pump, during its operation, can tell a lot about its performance and condition and the coupling with the motor. Vibrations begin before overheating occurs critically, noises betray inefficient consumption and impurities of lubricants.
Typically, noises and vibrations can be related to fluid passing through the pump, the mechanical parts or the vibration of the pump structure itself (generally much less energetic than the first two types).
The first type of vibration is generally attributable to cavitation phenomena; the rapid vaporisation and condensation of fluids, in areas of abnormally low pressure, create micro-bubbles that collapse "shooting" very high pressure microjets onto the pump body. Friction and turbulence of the fluid reduce efficiency and the microjets, added together, can cause extremely harmful vibrations that can lead to misalignment of the shafts or wear of the bearings as well as problems in the supports and bearing parts of the pump body, of the bases, fatigue of the materials with consequent breakages, etc.
Fortunately, these phenomena leave a clearly detectable acoustic signature which allows a maintenance engineer to intervene early.
Even sudden changes in fluid speed (think of the starting of a fire alarm system) can create vibrations, noises and consequently harmful stresses to the components of the system. Varying suddenly the inertia of a fluid, which is not particularly compressible (sudden stops or flow starts, sudden changes of direction) can create pressure peaks that exceed the structural resistances of gaskets, membranes, valves, etc. or that, if repeated, can wear them out prematurely. In this case also, the acoustic investigation can detect these phenomena or analyse them with an appropriate level of detail.
To conclude, vibrations that individually might be negligible can mutually cancel each other out but also, more dangerously, enter into resonance and accumulate. The overlapping of
vibrational frequencies, for example, generates the so-called beat, detectable as a rhythmic variation of the volume - becoming exponentially more harmful.
It is therefore evident that it is not enough to measure the level of the vibrations and instead a thorough analysis of the spectrum is necessary. Vibrational analysis is a very broad and widely standardised non-destructive field of investigation, both from the point of view of the equipment necessary for the analysis, and for the modes (choice of reading points, acceptance criteria as well as acceptable vibration levels set for the various rotating machines, residual imbalance) and finally for the specific skills of the person performing the measurement.