Page 457 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 437

Flow

Acoustic particle
velocity excites
shear layer

Standing
acoustic
wave
d

FIG. 11.5 Feedback mechanism of flow-excited acoustic resonances of closed side branches
(Ziada and Shine).

and the acoustic response (this is shown for a side-branch closed piping stub
flow disturbance in Fig. 11.5).
The Strouhal number is a nondimensional parameter that relates disturbance
frequency to flow speed, size of the flow disturbance, and geometric configu-
ration. The vortex-shedding frequency is related to the Strouhal number by the
equation:
fd
St ¼
U
There is currently no modeling method available to accurately capture
vortex-shedding excitation and predict the amplitude of resonance for the
majority of typical compressor station piping disturbances such as those created
by well-rounded tee connections to dead legs. Significant work is being per-
formed in other fields to capture this phenomenon for sharp-edged, small depth
cavities, but the phenomenon is more complex for geometric with rounded
edges or larger dimensions. Therefore, most engineering approaches either
determine a frequency separation margin to completely avoid resonance or
use conservative estimations of the amplitudes by implementing an excitation
2
formula often based on the dynamic component of the flow, ρv .
Low-frequency vibrations in centrifugal compressor piping systems can be
caused by broadband excitation forces that occur due to FIT. The FIT is an