Instability and Surge in Dual Entry Centrifugal Compressors

Abstract

While the operating characteristics of a compressor with a dual-entry impeller are similar to those of the more conventional single-entry machine, its stall behavior can have peculiarities of its own. These arise from the fact that the two impeller halves operate in parallel; each has inlet conditions that may occasionally be imperfectly matched. These conditions can result in instability. The necessity to eliminate these difficulties has stimulated considerable work at AiResearch Manufacturing Company of Arizona. As a result, the mechanism of the phenomenon has become well understood and brought well under control. The more fundamental aspects of this research are described herein.

The Stability of Pumping Systems—The 1980 Freeman Scholar Lecture

“…These will: (1) modify the stability criterion somewhat, moving the point of instability to the left of the peak, and (2) change the shape of the oscillations shown for the nonlinear calculations, with the tendency being to make the waveforms less like the relaxation oscillation situation. A further example of flow transients that can arise in pumping systems in parallel has been documented in [181]. This was a phenomenon that occurred in a dual entry centrifugal compressor, where a large amplitude, sudden flow shift occurred, accompanied by a substantial pressure change.…”

“…The machine is thus operating in rotating stall on the turbine side and at a high flow rate on the accessory side. Reference [181] also cites instances in which the flow at T 2 might not be stable, in which case this side encounters mild surging.…”

“…Piping systems in the process industries, for example, can use several compressors in parallel. The boiler feed systems described in [181] and [86] (in which large amplitude pulsations were encountered) had three multistage centrifugal pumps in parallel, and exhaust gas systems for power plants can also have several fans operating in parallel.…”

Frequency Response of Blower/Duct/Plenum Fluid Systems

“…(10) (H) (12) where ( ) 0 indicates the nominal operating point value, Q 0 the nominal flow through system.…”

“…(1)- (12) are combined, the basic plenum equation may be written in Laplace transform notation which is convenient for frequency response analysis as (13) where r P =C-and S is the Laplace transform variable. The quantity r p is the basic plenum chamber time constant, which is a function of the plenum capacitance and of the inlet (duct) and exit resistances.…”