Controlling Diffuser Stall Effects in Centrifugal Compressor Systems, 18-9148Printer Friendly Version
Inclusive Dates: 07/01/99 - 01/01/01
Background - This project seeks to advance understanding and SwRI's engineering service capabilities related to rotating stall in the diffusers of high-performance centrifugal compressors. Centrifugal compressors impart angular momentum to the gas flowing radially through a bladed wheel (the impeller). The gas decelerates after exiting from the impeller into the diffuser, and thereby converts kinetic energy into pressure. Diffuser rotating stall tends to occur in high-head, low-flow compressors. When the tangential flow velocity from the impeller substantially exceeds the radial velocity, the flow tends to separate, and the diffuser exhibits pressure modulation in time and space, potentially leading to rotor vibration and piping pulsations. This poorly understood phenomenon represents one of several limiting factors that must be considered in applying these compressors for oil and gas production and processing.
Approach - The project is combining three technical tracks to enhance knowledge of diffuser stall and its severity: computational fluid dynamics (CFD), compressor system testing, and analysis of available data from literature. The CFD uses time-transient, three-dimensional solution of the Navier Stokes equations to model flow in the entire diffuser subject to highly tangential flows at inlet. The testing will take place in a facility designed to allow the two-way interaction between a centrifugal compressor and pulsations in the attached piping. In addition, a substantial existing database showing stall pulsation over a range of conditions has been subjected to regression analysis, resulting in quantitative relationships.
Accomplishments - The CFD analysis has proven its ability to predict stall in the diffuser. It is now being applied to reproduce literature data defining the conditions when stall occurs (onset conditions). With the resultant confidence in the CFD predictions, the CFD will provide a means to extrapolate onset conditions to the high pressures typical of advanced applications, but beyond the bounds of available data. The test facility is in advanced stages of assembly. The compressor is in place, with piping attached and bolted up, while the speed increaser and motor are also in place and undergoing shakedown tests prior to stall testing.
The data analysis effort has successfully developed relationships between compressor stage pressure rise, and the most severe dynamic stall pressure to be expected, if and when stall occurs. It has also provided a quantitative basis for predicting the vibrations of a rotor under the influence of stall. Thus the project has already provided the minimum knowledge needed to help operating companies determine the likelihood and worst-case consequences of stall. The CFD and facility tests will add validation and extrapolation capability.