Re Configuring and Restaging Centrifugal Compressors for Depleting Reservoir Pressures

Abstract

Compressor stations are essential to export natural gas through pipelines. Gas reservoirs deplete over the years, as a consequence, gas compressor stations experience a lower-than-normal suction pressure. Hence it becomes increasingly difficult for the existing compressor hardware to export gas to destination. Variable speed compressors are designed with certain amount of flexibility to accommodate changes in inlet pressure conditions. However significant changes in inlet operating conditions, because of reservoir depletion, adversely affect natural gas export through centrifugal gas compressors. This prompts operating companies to develop alternate solutions to export low pressure gas. In the process, compressor modification opportunities need to be identified, developed and be applied to facilitate gas export at revised inlet conditions, either by changing pressure ratio across the compressor or gas flow rate or both. Rearranging parallel compressors to series operation is one of the likely methods of achieving higher pressure ratio for gas export. Restaging of existing compressor may also be required to optimize compressor operation. Cost effective projects are delivered by lining up the compressors differently and by changing compressor internal stator and rotor components to suit new operating conditions, avoiding off-design operation, and allowing the modified compressor to operate in an envelope with increased operational efficiency. This is also known as upgrade or re-rating of compressors. The paper is about rearranging and restaging existing gas compressors in onshore gas compressor stations, to export natural gas, when the gas wells experiencing significant pressure depletion. The paper explain method of establishing maximum gas export rate, highest pipeline back pressure for gas export and the compressor discharge pressures at battery limits. The adequacy checks are performed on compressors, motors, and all associated auxiliary systems.