The operational stability of three multistage centrifugal compression systems was studied experimentally. Two of the tested machines are four-stage centrifugal compressors arranged in two cylinders and used for air separation. The third machine is a ten-stage compressor in two cylinders and used for oxygen production. The measurement features conventional probes for describing performances and per-stage pressure transducers for recording dynamic characteristics. The performance measurement was conducted for the entire machine and also per stages. The signal of dynamic pressures was analyzed in both time and frequency domains. The result of per-stage performances clearly indicated which stage is first to lose the operational stability and how far it happens before the whole system runs into the stability boundary. This result is further demonstrated by the commencing time and location of stages of the strengthened dynamic pressure oscillation which differs from the most dangerous location in terms of maximum amplitude of the pressure oscillation during surge. The knowledge thus obtained for the operational stability should serve a good basis for monitoring and control purposes when running the machine in multistage compression environment.

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