Page 455 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 435

FIG. 11.4 Transient operating ellipse of centrifugal compressor when operating with pulsating
flow from mixed compression [4].

To evaluate the effect of pressure pulsations on the operating point of a cen-
trifugal compressor, the piping impedance of the entire system must be consid-
ered in conjunction with the compressor operating map to evaluate the
attenuation or amplification of pulsations at the centrifugal compressor flanges.
The piping impedance is a combination of resistive impedance (i.e., due to fric-
tional losses), acoustic inertia (due to the mass of the gas in the pipe), and stiff-
ness (due to the compressibility of the mass in the pipe). The impedance curve is
overlaid on the compressor performance map to convert the pressure pulses to
velocity fluctuations thus creating a transient compressor map. The resulting
operating point oscillates when exposed to pulsating flow in a path described
by an ellipsoidal shape on a head/flow centrifugal compressor map as shown
in Fig. 11.4. Experimental testing showed that 30% or more of the area defined
by the transient operating ellipse is across the surge line, the centrifugal com-
pressor will experience the effects of surge [4]. Modifications to the piping
impedance, compressor surge margin, or additional attenuation of the recipro-
cating compressor pulsations can be made to avoid the possibility of the cen-
trifugal compressor experiencing surge.

Flow-Induced Excitation Analyses: FIV, AIV, and FIT

Flow-induced excitation is used to describe an excitation source that is purely
flow related rather than from a machinery source. Well-known piping sources
of flow-induced excitation in compressor systems are high velocity and/or high-
density flow, changes in the piping geometry such as a tee or protrusion and
valves or orifice plates with high-pressure differentials. The excitation can
be of low amplitude over a wide range of frequencies, broadband excitation,