ABSTRACT

Recently, there has been considerable interest in the petroleum industry on multiphase flow through pumps. Pumping gassy fluids has applications in both pipeline and electric submersible pumps. The present study discusses a dynamic and a correlational model for three electric submersible pumps (ESP's).The dynamic model is applicable to pipeline centrifugal pumps (since the flow physics is exactly the same) provided a correlation for bubble size is known.

INTRODUCTION

Centrifugal forces dominate gravitational forces inmultiphase impeller flow. As such, it has been shown byinvestigators that the diffuser performance can beignored and impeller behavior determines centrifugalpump behavior. The basic model presented in this paperis develoiped for electric submersible pumps (ESPs) butis equally applicable for gas-liquid flow through anycentrifugal (radial or axial-type) pumps. This paperheavily draws on parts of papers presented elsewhere bythe same authors'*3*4*.5 T he interest in developing anoffshore multiphase pump hasbeen mainly due to major economic benefits associatedwith laying a single multiphase pipeline (as opposed toone gas and one liquid pipeline). Development andmodelling multiphase pipeline pumps is becoming moreimportant as the oil companies are venturing further intothe sea for oil. Data from an Amoco-Centrilift study' wereused to validate the model developed. There have beenquite a few studies in the nuclear industry3 on modelingimpellers. It has been shown' that none of the models ofthe nuclear industry can be used for ESPs in gassywells, This is because the nuclear industry models arepump specific, applicable to substantially lower voidfractions, and do not consider the effects of inletpressure. The development of a general model formultiphase flow through centrifugal pumps would be acomplicated task primarily because of complex pumpgeometries. A two-dimensional multiphase pump modelwould require the knowledge of phasic holdup andvelocity profiles in a pump and this data is not at all easyto measure. The results show very encouragingperformance prediction for both the radial and axialpumps. Limited success was also obtained in correlating the pump pressure increase (Model 2). Performance ofaxial and radial pumps is also compared.

PREVIOUS WORK

Most of the work on multiphase flow through ESPs has been done in the nuclear industry. A comprehensive review is given by Sachdeva3. The nuclear industry models cannot be used because of reasons cited earlier and will not be discussed. The work in the nuclear industry can be summarised into three main phases: Black Box Methods: These were largely unseuccessful. Investigators realised thatat least a qualitative idea of the flow physics inside the pump was required. Experimented Studies: Various U.S. and Japanese investigators3 qualitatively studied the movement of the gaseous phase within the impeller. It was found that bubbly flow caused lower head degradation than slug/churn-turbulentflow. Investigators also found that the gas slug growth around the impeller eye led to unstable flow towards the left-hand side of the pump (rate-head) curve.

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