Wet Gas Compression - Diffuser Instabilities

Abstract

The Norwegian petroleum industry has played an important role in the country's industrial development since the first oil production started in 1971 at Ekofisk. At present, however, natural gas has become an important part of the total petroleum output from the Norwegian continental shelf (NCS). This gas contains less than 5\% liquid on a volume basis, and is denoted wet gas. Today, a great part of the process equipment is being installed subsea for more cost-efficient production. To maintain the production rates as well as pressure decreases, boosting the wellstream is essential. Wet gas compression compresses the unprocessed wellstream, at the seabed, to increase the recovery of the well.

The purpose of this study is to conduct an experimental investigation to determine the instability and performance characteristics of a single-stage centrifugal compressor exposed to wet gas. Also a qualitative analysis was conducted in order to have a better understanding of the mechanisms of the wet gas flow, with special focus on the diffuser.

The compressor was tested at different rotational speeds and gas mass fractions (GMFs) in an open-loop test facility with ambient air and water. The stage consisted of a shrouded impeller with a vaneless diffuser surrounded by a symmetrical circular volute with increasing cross-sectional area. Liquid was uniformly injected into the impeller eye through multiple nozzles mounted in the inlet pipe.

High-response dynamic pressure sensors flush-mounted in the diffuser were used for quantitatively identify the instability inception and evolution. Changes in the instability characteristics at different GMFs and rotational speeds were revealed, both quantitatively and by visualization of the diffuser flow. Changes in performance characteristics were quantitatively revealed for the same variables. Analyses with reference to dry gas was performed.