Development of a Criterion for a Robust Identification of Diffuser Rotating Stall Onset in Industrial Centrifugal Compressors

Abstract

Recent studies showed that a prompt detection of the stall inception, connected with a specific model to predict its associated aerodynamic force, could provide room for an extension of the left margin of the operating curve of high-pressure centrifugal compressors. In industrial machines working in the field, however, robust procedures to detect and identify the phenomenon are still missing, i.e., the operating curve is almost ever cut preliminarily by the manufacturer by a proper safety margin; moreover, no agreement is found in the literature about a well-defined threshold to define the onset of the stall. In particular, in some cases, the intensity of the arising subsynchronous frequency is compared to the revolution frequency, while in many other ones it is compared to the blade passage frequency. A large experience in experimental stall analyses collected by the authors revealed that in some cases unexpected spikes could make this direct comparison not reliable for a robust automatic detection. To this end, a new criterion was developed based on an integral analysis of the area subtended to the entire subsynchronous spectrum of the dynamic pressure signal of probes positioned just outside the impeller exit. A dimensionless parameter was then defined to account for the spectrum area increase in proximity to stall inception. This new parameter enabled the definition of a reference threshold to highlight the arising of stall conditions, whose validity and increased robustness was here verified based on a set of experimental analyses of different types of full-stage test cases of industrial centrifugal compressors at the test rig.

Unstable Flow Structures Present at Different Rotational Velocities of the Centrifugal Compressor

“…Spectral studies allow one to distinguish new pressure oscillation peaks and associate them with certain flow structures. Bianchini et al uses the criterion based on the diffuser pressure power spectrum for the identification of the onset of vaneless diffuser rotating stall [33]. The inlet recirculation can be also identified based on its characteristic features.…”

Detection of vaneless diffuser rotating stall by means of dynamic pressure sensors and acoustic measurements

“…Notwithstanding this, the procedure of [8] was able to correctly reconstruct the pattern anyway. In addition, the stall onset parameter (ϡ) proposed by Bianchini [2] (Eq. 1) was measured for both sensors.…”

“…the increase of the rangeability of the stages towards lower minimum flow limits. To this end, enhancing the comprehension of effects those aerodynamic phenomena that precede surge is of capital importance [2]. In particular, since the use of vaneless diffusers is often suggested in all those cases where a good rangeability is needed, vaneless diffuser rotating stall (VDRS) has attracted the attention of researchers for many years [3,4].…”

“…However, nice prospects have been shown by each of the two methods. More specifically, the use of dynamic pressure sensors was shown to be able not only to reconstruct the stall pattern [16], but also to possibly represent a valuable tool for determining an automatic onset threshold [2]. On the other hand, acoustic measurements have been proposed as a valuable alternative [17], being less intrusive than flush-mounted pressure probes.…”

An experimental and numerical investigation on the impact of tip gap on the performance of vaned diffuser for industrial centrifugal compressors

“…The yellow vertical line represents the stall onset, while the red horizontal line the is the revolution frequency (1xREV). Stall onset is detected as soon as the measured frequency amplitude exceeds the blade passing frequency (BPF) amplitude [13][14]. From a perusal of joint time-frequency graphs, it is apparent that high gap values lead to a right shift of the yellow line, i.e., to a decrease of the minimum flow limit, and thus to an increase of the rangeability.…”