Wet Gas Compressors - Stability and Range

Abstract

The oil and gas industry is a large user of turbomachinery. The demand for oil and gas is consistently growing, and changing market conditions require innovative solutions. Operation and optimization of turbomachinery in a variety of applications is therefore of great interest. Moreover, potentially extreme environmental procedures mean that innovative design and operational attributes must be used. The development and implementation of new subsea technology is an important focus area to increase petroleum production and recovery from existing fields. The technology will contribute to the exploitation of small and remote fields and access to fields in very deep water. A traditional centrifugal compressor may not be applicable for wet gas compression, because the liquid may cause mechanical damage due to erosion and corrosion of the compressor components; in addition, the design is not optimized for multiphase flow. In this research, particular emphasis is given to the wet gas compressor as a more fundamental knowledge of the impact of liquid is essential with regard to the understanding of aerodynamic and fluid dynamic compressor behaviour. It is also of vital importance to identify the mechanisms leading to unsteadiness, like surge and rotating stall. A key element is the investigation of the machine performance and working range when a liquid phase is present. In the present work, attention has been given to the experimental validation of an advanced facility, equipped with special windows designed for flow inspection. The experimental tests were performed on an air-water multiphase open-loop test rig, which consists of a single-stage centrifugal compressor, with a standard three-dimensional impeller that was tested in dry and multiphase conditions. The main objective of this research is the study of the stability and the range of the wet gas compressor at a low mass flow rate. Focus of interest is on the effect of the presence of a liquid phase. Thus the performance evaluation from dry to wet operating conditions is fundamental. Moreover the study of the flow path inside the impeller and the diffuser has great importance in order to clarify the impact of the liquid. In this case the visualization of the fluid has a central role. The analysis of the working range has been supplemented by the investigation at off‒design conditions, in particular in the area characterized by low mass flow rate. An accurate examination of the dynamic pressure trends from dry to wet working conditions is carried out. The results show the performance deterioration owing to the content of liquid with respect to dry working conditions. However the presence of a liquid has a positive influence on the machine stability and operating range. Through a fast Fourier transform analysis, the stabilizing effect on instabilities onset is highlighted, compared to normal operations. In addition, the flow evolution inside the impeller and diffuser is studied. The main flow patterns, typical of a centrifugal compressor, are revealed also in wet conditions: the impeller flow presents a jet-wake structure, while in the diffuser a logarithmic spiral path is observed. Also the characteristic behaviour, like reverse flow that creates a “doughnut” formation, towards the suction pipe, is documented. The outcomes that improve the knowledge about the wet gas compressor are a baseline to define a method in order to conduct multiphase tests and provide a starting point to design a wet tolerant system.