Control Systems. Molecular Weight Compensation Consideration in Compressor Surge Control

Abstract

Tutorial 10B: The effects of gas molecular weight variation on a centrifugal compressor’s surge line and computation of the operating margin from surge for the purposes of antisurge control is discussed using different methods. This paper will review the accuracy of most commonly used coordinate system (reduced polytropic head vs reduced flow) for molecular weight compensation used by some OEMs and third party surge control vendors using test results from a FPSO application employing Main gas, Injection gas and Export gas compressors, and present an alternative highly accurate molecular weight compensation method that can be used by surge controllers to account for shifts in the surge point under varying molecular weight conditions. This alternate method has been field tested on several compressor control applications in FPSOs, refineries, LNG and petrochemical plants.