MultiBooster instability and surge - Ustabilitet og surge i MultiBooster
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Industry has lately turned the focus on subsea processing as a tool to enable new developments where the technology needs are increasing. Multiphase pumping ( boosting ) plays a key role in the increase of production and in oil and gas recovery. In addition, the technology has shown to be more profitable and environmental friendly compared to conventional methods, due to longer tie-back distances and the potential of zero gas flaring. However, the presence of gas leads to unwanted flow regimes generating additional performance losses and system instabilities. Safe and reliable operation requires a deep understanding of the physical mechanisms causing the unstable behavior. The main objective of the thesis has been to explore the multiphase booster instability and surging mechanisms to give an improved insight of the main influencing flow mechanisms.A literature study has been performed, reviewing previous studies considering surging in multiphase pumps. In addition, phase slip and other related mechanisms have been studied through bubble tracking and available correlations. Data processing and direct flow visualization with a high-speed camera have been employed in experimental tests. Additionally, a data processing system have been utilized in order to relate pressure pulsations to flow mechanisms. An extensive test campaign has been conducted through the test facility in the Thermal Energy Department Laboratory at NTNU. The facility features a mixed-flow rotodynamic single-stage multiphase pump, reproducing the full scale, MultiBooster, by Aker Solutions at Tranby (Oslo). The transparent pump casing permits an excellent optical access of the hydraulic channels, which allows a visual study of the flow field behavior. All tests have been performed at atmospheric inlet pressure, where gas volume fractions, flow rates, and rotational speeds are the varying parameters. Characteristic multiphase flow phenomena have been analyzed through flow visualization, bubble tracking, and data processing. The phenomena have shown to affect the machine negatively, due to the increased pressure variations and channel obstructions. The experimental work indicates that, bubble coalescence, gas pockets, phase slip, empty-of-gas channels, and recirculation zones plays a major role during the unsteady machine operation. The phenomena show an intermittent behavior, dependent on the operating condition and the specific pump design. As the surging is approached, the overall flow field irregularities show to intensify, accompanied with strong pressure variations. Moreover, this correlation has formed a foundation for detecting the surging inception, and will be presented in this thesis.