IMPROVING CENTRIFUGAL COMPRESSOR PERFORMANCE BY OPTIMIZING DIFFUSER SURGE CONTROL (VARIABLE DIFFUSER GEOMETRY) AND FLOW CONTROL (INLET GUIDE VANE) DEVICE SETTINGS

摘要

Surge control problems have been around as long as the centrifugal compressor itself. Many different approaches have been taken to improve operating range to surge (both in head and flow) depending on what type of surge mechanism is present in the compressor system. Compressor surge triggered by diffuser stall can be suppressed by variable diffuser geometry whereas surge from impeller stall can be eliminated by the use of variable-geometry inlet guide vanes. A given compressor duty in terms of flow and pressure ratio can be realized by an infinite number combinations of inlet guide vane / variable diffuser geometry settings. These various realizations of the same duty point have different compressor efficiencies. This paper describes a method that allows for optimal inlet guide vane/variable-geometry diffuser positioning (for a diffuser surge driven system) using two or three pressure measurements along the flow path, i.e. impeller inlet pressure, impeller exit/diffuser inlet pressure and diffuser exit pressure. Maximum obtainable diffuser pressure recovery can be used to determine the onset of surge. These maximum pressure recovery values are a function of variable-geometry diffuser setting only and are over most of the operating range independent of flow, head or inlet guide vane setting and can quickly be determined experimentally by pressure measurements. During operation, the known maximum pressure recovery value can be compared to the one determined from real time pressure measurements, and a determination as to the optimal setting of the diffuser can be made. Test data shown in this paper suggest that for the most efficient operation of a compressor, the diffuser should be positioned such that its pressure recovery value is close to its maximum. This in effect brings surge close to the operating point, but with careful control and safety factors it is surmised that stable operation is possible.