A Revised Compressor Polytropic Performance Analysis

The compressor polytropic head and efficiency analysis are based on the assumption that the compression process follows the path of a constant polytropic exponent n. Both the ASME PTC10-97 and the ISO 5389 refer to the polytropic analysis by John M. Schultz. The procedure utilizes a head correction factor and two compressibility functions to obtain a solution of the integral Δhp = ∫vdp. Present computer technology renders possible a direct integration of the compression path where the variation in actual gas properties along the path is included. This method eliminates the averaging of gas properties which the Schultz procedure includes. This paper reports deviation in compressor performance using the Schultz procedure with different average gas properties. The implementation of a direct integration procedure, employing actual gas properties from the new GERG-2004 equation of state, is given. The GERG-2004 equation of state has proven to give accurate density values both in the vapour and liquid phases. Depending on how the polytropic compression analysis is implemented, the work has revealed up to 4% deviation in polytropic head and efficiency for some specific compressors. This adds an extra uncertainty in compressor performance verification. Even though the API 617 allows up to 4% deviation, some compressors have to meet a more stringent demand, for instance 2% at the Sno̸hvit LNG plant. Future challenges within oil and natural gas production are related to wet gas compressors. The present paper points out the advantages in using a direct integration method for wet gas performance predictions as this takes phase changes along the compression path into account.