Hydraulic
or aerodynamic forces
- combined into a single category since they are
similar in nature and involve moving a fluid. In the case of a fan or lp
blower, a very compressible "fluid" (a low pressure gas) is being moved.
In the case of a compressor or hp blower, a less compressible "fluid" (a
high pressure gas) is being moved. In the case of a pump, a non-compressible
fluid (an actual "fluid") is being moved. The less compressible the fluid,
the more susceptible the component is to flow-related vibration problems.
Some of the problems we will discuss include vane or blade pass frequency,
cavitation, recirculation, internal clearance problems and flow turbulence
/ surging. The most common and well known of these frequencies, vane or
blade pass frequency - which is simply the number of vanes, blades, lobes,
etc. x RPM - is briefly discussed here.
-
Low Pressure (Centrifugal)
Fans & Blowers - Not normally susceptible
to these problems to a significant degree.
-
BPF - Rarely a
problem unless the frequency excites a resonant frequency in the downstream
ductwork. This normally is translated into a noise problem and, less often,
a structural problem. Rarely does it cause a mechanical problem such as
accelerated bearing or component wear.
-
Flow Turbulence
- Can cause low frequency, broadband vibration (below or just higher than
1x rpm).
-
High Pressure
Blowers and Compressors - Far more susceptible
due to much tighter clearances and much higher pressures.
-
Pumps
- Problems are the most severe since it is a non-compressible fluid
that is being moved. Tight clearances and high pressures compound the potential
problems. Not only can mechanical clearance problems cause large vibrational
problems (wear rings, impeller / housing / diffuser clearances) but operating
a pump at different pressures and flow rates than its design point can
cause excessive and even destructive vibration.
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