INTRODUCTION

Compressor surge is a phenomenon of considerable interest; yet it is not fully understood. It is a form of unstable operation and should be avoided. It is a phenomenon that, unfortunately, occurs frequently in the process industry, sometimes with damaging results. Surge has been traditionally defined as the lower limit of stable operation in a compressor, and it involves the reversal of flow. This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually, a part of the compressor is the cause of the aerodynamic instability, although it is possible for the system arrangement to be capable of augmenting this instability. Compressors are usually operated at a working line, separated by some safety margin from the surge line. Surge is often indicated by excessive vibration and an audible sound; however, there have been cases in which surge problems that were not audible have caused failures.

Extensive investigations have been conducted on surge. Poor quantitative universality or aerodynamic loading capacities of different diffusers and impellers, and an inexact knowledge of boundary-layer behavior make the exact prediction of flow in turbomachines at the design stage difficult. However, it is quite evident that the underlying cause of surge is aerodynamic stall. The stall may occur in either the impeller or the diffuser. When the impeller seems to be the cause of surge, the inducer section is where the flow separation begins. A decrease in the mass flow rate, an increase in the rotational speed of the impeller,...