Reducing Gas Flaring by Improving Turndown Ratio of Centrifugal Compressor

Abstract

Abstract The Oil and Gas Gathering and Processing facilities at West Kuwait field of Kuwait Oil Compnay operates turbine driven multistage (LP/HP) centrifugal compressor for transportation of produced gas. The main challenge for these facilities is to operate it at lower production capacities at many instaces due to various reasons. The crude oil processing units of these facilities were able to operate smoothly for as low as 25% of design capacity. However, the turn down ratio for gas compressors was limited to 55%. This limitation was resulting in offloading of compressor and subsequent gas flaring when facility was operated at lower production rates. The main reason for compressor offload was availability of single Anti-surge control valve & control system to prevent surge and also to control capacity. This was limiting the lower operating range of compressor. The non-availability of required High Pressure or Low Pressure Gas was resulting in frequent opening of Anti-surge valve which was offloading HP stage of compressor. A modification was done to install a recycle valve in parallel to Anti-surge valve. Moreover, a control system was also modified for independent control of compressor capacity. This has significantly increased the turndown ratio of compressor. This paper shares a field experience of limitation of compressor operating range without dedicated recycle valve for capacity control and provides guideline for designing and installing independent recycle control system for capacity control to increase lower operating range of centrifugal compressor. It was observed that actual turndown ratio of compressor improved from 55%to 20% after installing an independent recycle valve for capacity control. This improvement has increased capability to operate Gathering Centre at lower production rates without gas flaring. The paper justifies need for installation of a dedicated recycle valve for capacity control in parallel to Anti surge valve to increase turndown ratio of multistage centrifugal compressor.