This study presents a new gas-liquid model to predict electrical submersible pumps head performance. The newly derived approach based on gas-liquid momentum equations along pump channels has improved the Sachdeva model (Sachdeva, R., Doty, D. R., and Schmidt, Z., 1988, “Two-Phase Flow through Electrical Submersible Pumps,” Ph.D. dissertation, The University of Tulsa, Oklahoma; 1994, “Performance of Electric Submersible Pumps in Gassy Wells,” SPE Prod. Facil., 9, pp. 55–60) in the petroleum industry and generalized the Minemura model (Minemura, K., Uchiyama, T., Shoda, S. and Kazuyuki, E., 1998, “Prediction of Air-Water Two-Phase Flow Performance of a Centrifugal Pump Based on One-Dimensional Two-Fluid Model,” ASME J. Fluids. Eng., 120, pp. 327–334) in the nuclear industry. The new two-phase model includes novel approaches for wall frictional losses for each phase using a gas-liquid stratified assumption and existing correlations, a new shock loss model incorporating rotational speeds, a new correlation for drag coefficient and interfacial characteristic length effects by fitting the model results with experimental data, and an algorithm to solve the model equations. The model can predict pressure and void fraction distributions along impellers and diffusers in addition to the pump head performance curve under different fluid properties, pump intake conditions, and rotational speeds.
Dynamic multiphase flow behavior inside a mixed flow electrical submersible pump (ESP) has been studied experimentally and theoretically for the flrst time. The overall objectives of this study are to determine the flow patterns and bubble behavior inside the ESP and to predict the operational conditions that cause surging. An experimental facility has been designed and constructed to enable flow pattern visualization inside the second stage of a real ESP. Special high-speed instrumentation was .^elected to acquire visual flow dynamics and bubble size measurements inside the impeller channel. Experimental data were acquired utilizing two types of tests (surging test and bubble diameter measurement test) to completely evaluate the pump behavior at dijferetu operatiotial conditions. A similarity analysis performed for single-phase flow inside the pump concluded that viscosity effects are negligible compared to the centrifugal field effects for rotational speeds higher than 600 rpm. Therefore, the two-phase flow tests were peiformed for a rotational speeds ofóOO. 900, 1200, and 1500 rpm. Results showed formation of a large gas pocket at the pump intake during surging conditions.
Two-phase flow behavior prediction of centrifugal pumps is a hard task due to the complexity involved in modeling multiphase flow inside turbo machines. No models are currently available for this purpose. Some empirical correlations are available in the literature, but they are valid only for the tested pumps in the experimental range used to develop them. An experimental study has been conducted at The University of Tulsa Artificial Lift Projects – TUALP with a 22-stages GC6100 pump to gather data for pump performance under two-phase flow conditions. Air and water were used as working fluids. This study differs from other experimental works because the pressure changes were recorded stage-by-stage. The results of previous works have been reported as an average of the intake and discharge conditions, and depend on the number of stages used. Phenomena like surging and gas locking were observed during these tests and their boundaries have been mapped. It will provide some insight regarding when they appear, and the way they are revealed. The pressure increment and total hydraulic horsepower for the average pump and per stage as a function of the liquid flow rate, and each gas flow rate considered are presented. The average brake horsepower and efficiency for the pump are also plotted for the variables mentioned. The results indicate that the average behavior for the pump is significantly different from that observed per stage. Introduction Centrifugal pumps are dynamic devices which use kinetic energy to increase liquid pressure. They are successful with handling water and other incompressible fluids ranging from low to medium viscosities but are severely impacted by free gas or highly compressible fluids. Significant amounts of free gas may be found during hydrocarbons production. This motivated important research from the petroleum industry focusing on improving the successful application of ESP as an artificial lift method. The consequences of entrained gas on centrifugal pumps depend on the relative amount of gas and liquid present, and vary from a slight deterioration on performance up to a complete blockage known as "gas locking". Before gas locking occurs, another phenomenon known as surging takes place. Each pump is characterized by performance curves, which include the head developed, brake horsepower consumption and efficiency as function of the flow rate through the pump for a certain rotational speed (see Fig 1). Traditionally these curves are determined experimentally using water. The head characteristic curve is used to size the pump, while the brake horsepower information is useful to size the motor required to drive the pump. The sizing of a multi-stage ESP for water wells is fairly simple, and good accuracy of the predicted performance is achieved using the water performance information supplied by the manufacturer. The design of an ESP system using the water information for oil wells with high free gas fraction at pump intake conditions is a harder task, and is based on the prediction of performance curves by modification of the water curves. The leading parameter is the mixture density at the flow conditions of each stage. Applying this procedure, the ESP system often shows some degree of under or over sizing when operating. An accurate prediction of the performance for any pump handling free gas is challenging. Some empirical and mechanistic approaches have been attempted in the past. The main problem of the experimental approach is that the developed correlations are based on the average performance of the pump. These correlations become specific for the type and number of stages tested. On the other hand, theoretical models are difficult to develop since the geometry of the channels inside the pump is complex. The phenomena that take place in such channels are not well understood, and thus the use of empirical parameters to close the model is required.
Head deterioration observed in electrical submersible pumps (ESPs) under two-phase flow is mild until a sudden performance breakdown is observed in the pump head curve at a certain volumetric gas fraction. This critical condition is termed surging. Consequently, the head that the pump generates with two-phase flow depends on whether the stages operate under conditions before (mild performance deterioration) or after (severe performance deterioration) the surging point.The surging, for engineering purposes, can be predicted by published correlations, but the lack of a theoretical basis is a limiting factor for their application. Mechanistic models seem to be the proper alternative. However, the poor understanding of the physical mechanism that causes the surging hinders the development of such mechanistic models. This paper reviews some of these correlations and mechanistic models by comparing the correlation predictions against experimental data acquired in a closed loop with water and air using a commercial 24-stage ESP. The data cover a wide range of volumetric gas fractions, rotational speeds, and intake pressures. As a consequence of this analysis, a new correlation has been formulated. This correlation predicts the initiation of the surging as a function of rotational speed and fluid properties.
Dynamic multiphase flow behavior inside a mixed flow Electrical Submersible Pump (ESP) has been studied experimentally and theoretically for the first time. The overall objectives of this study are to determine the flow patterns and bubble behavior inside the ESP and to predict the operational conditions that cause surging. The theoretical study includes a mechanistic model for the prediction of the flow behavior inside the pump. The model comprises a one-dimensional force balance to predict occurrence of the stagnant bubbles at the channel intake. This model depends on two important variables, namely the stagnant bubble size and the bubble drag coefficient. The bubble size has been measured and a physically based correlation is presented. A new correlation for the drag coefficient is proposed as a function of rotational speed and Reynolds number. The model enables the prediction of the operational envelope of the ESP, namely the transition to surging.
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