Noise reduction of a multistage export / reinjection - Dresser-Rand

The blade passing frequency (BPF) is dependent on machine operating speed (RPM) and
impeller blade count ( N b ) and is given by:
BPF =
N
b ×
RPM
60
Knowing just the blade passing frequency is not enough to understand the compressor noise
characteristics. There are other features in the compressor noise spectrum and they vary from
machine to machine. These include the tonal prominence, peak bandwidth, and relative strength
among the noise peaks (first BPF and second harmonic). Each has essential information for the
design of the duct resonator array.
These noise characteristics have been mostly determined by experimental test data. Narrow
band FFT sound pressure level or narrow band sound intensity level measured from the
compressor provides all the noise characteristics in the spectrum, although the latter requires a
more sophisticated acoustic data acquisition system. The full octave and one-third octave sound
data are easier to obtain but do not provide all the acoustical details and frequency
characteristics.
Physical measurement, particularly with a new compressor design, is not always possible.
Under this circumstance, the computational approach becomes the only choice. Using an
unsteady computational fluid dynamics (CFD) model incorporating a transient sliding mesh
approach is required to simulate the impeller/diffuser aerodynamic interaction. Using this
analysis, the dominant tonal noise peaks of the compressor can be predicted. The current CFD
codes can not predict the broadband flow noise with good accuracy, this is determined
empirically.