Wet Gas Compressor Performance

Abstract

The world s energy demand is increasing and more attention is given to increase the recovery of gas fields. Installing a compressor near the wellhead can contribute to this. Constantly changing reservoir conditions requires the system to handle small amounts of liquid water and condensate. Wet gas compression is a cost efficient and compact alternative to liquid separation on the seabed. A wet gas compressor test facility exists at NTNU`s thermal laboratory and research on wet gas fundamentals is conducted. The facility consists of an open-loop impeller test rig with ambient air and water as test fluids. It is of vital importance to analyze and document the test rig and compressor performance under different operating conditions.The pressure and temperature profile through the compressor inlet system has been thoroughly validated in this thesis, as it is essential to understand and validate the flow through the inlet system to be able to establish accurate performance characteristics.Through experimental work in the test rig and utilizing HYSYS and CFD software, it has been demonstrated that the inlet measuring instruments are located in the static pressure and temperature recovery zone downstream the orifice meter. This will constitute large impacts on the reliability of performance parameters. It has been validated that condensation can occur slightly downstream the orifice throat during wet gas operation. This will affect the accuracy of the flow rate measurements. It was also determined by measurements that the measured ambient reference condition might not be representative for the compressed air as large temperature gradient in the laboratory was determined.Dry and wet performance characteristics have been established. A HYSYS model was used to perform the calculations and Aspen Simulations Workbook was used to transfer test data to the model.Documentation of steady-state stability constituted a large part of the performance analysis in order to acquire representative test points. The steady-state criterion of ±0.5 % in polytropic efficiency fluctuations for test points was not completely fulfilled for all points during dry gas operation.There are large challenges associated with obtaining reliable wet gas temperature measurements at the compressor inlet and discharge and this made the wet gas analysis a challenge. The inlet homogeneous wet gas temperature was therefore determined by HYSYS calculations utilizing ambient measurements as input. The temperature is very sensitive to variation in relative humidity, and this ensured that steady test points were not obtainable. Shaft power was therefore used as input instead of discharge temperature. The wet gas performance characteristics yielded a significant degradation of compressor performance parameters, including pressure ratio. This can be attributed to the additional internal losses associated with the multiphase annular flow pattern. Due to the low density ratio for ambient air- water mixtures, the degradation rate is more severe than what has been discovered for wet gas tests utilizing real hydrocarbon fluid-mixtures.An uncertainty analysis was conducted in order to validate the instrument accuracy and performance parameters accuracy and reliability. Finally, measures for how to reduce loading errors in the test rig were proposed.