Surge Avoidance for Compressor Systems

This tutorial explains the surge phenomenon as well as the precursors of surge and the damage that surge can cause to the equipment. Then, methods to keep the compressor system from surge, i.e., modern surge control systems, are discussed. This includes the necessary instruments, algorithms used, as well as the piping layout required. Sizing considerations for valves and other system components, in particular methods to correctly estimate allowable upstream pipe volumes, are described. Surge control systems for single body and multibody or multisection compressors will be explained, and attention will be given to the integration of the surge control system with other aspects of the station and unit control. In a look forward, new methods for surge detection and detection of system changes prior to surge are covered. INTRODUCTION Operation of centrifugal gas compressors can be defined by three operating parameters: speed, head, and flow. Centrifugal compressors have a maximum head that can be achieved at a given speed. At that peak head there is a corresponding flow. This is a stability limit. Operation of the compressor is stable provided the head is lower (less resistance in series with the compressor) and the flow is greater than these values. That is, the system is stable, as long as reductions in head result in increases in flow. Surge occurs when the peak head capability of a compressor is reached and flow is further reduced. Depending on the dynamic behavior of the compression system, system surge can occur at somewhat higher or, seldom, lower flows than the peak head capability. This is a particular issue in systems with low frequency pulsations (Kurz, et al., 2006). When the compressor can no longer meet the head imposed by the suction and discharge condition (which are imposed by the compression system), flow reverses. When a compressor approaches its surge limit, some of its components (diffusers, impeller) may start to operate in stall. Stall occurs when the gas flow starts to separate from a flow surface (Figure 1). Changing the operating point of a compressor always involves a change in incidence angles for the aerodynamic components. Just as with an airfoil (Figure 1), increasing the incidence angle will eventually lead to stall. Stall in turbomachines often appears as rotating stall, when localized regions of separated flow move along the diffuser at speeds below the rotational speed of the impeller (Day, 1991). Surge is the ultimate result of system instability. Figure 1. Progression of Stall. (NACA4412 airfoil at increasing angle of attack, based on data by Nakayama, 1988) 123 SURGE AVOIDANCE FOR COMPRESSOR SYSTEMS