Backflow in Twin-Screw-Type Multiphase Pump

Abstract

Summary The performance of a twin-screw-type multiphase pump was investigated from the viewpoints of backflow in a gap along the twin-screw shafts and of scaleup parameters. Although both the backflow and the scaleup parameters have been recognized as important factors in developing multiphase pumps, they have not yet been clarified. The twin-screw pump was equipped with pressure sensors, set in the multiphase-test facility, and experimented with under various conditions to clarify the relationship between backflow rates and factors such as differential pressure, gas-void fractions (GVF's), and the rotation speed of the shaft. A physical model was proposed with the empirical relationship of pressure distribution along the screw, and was successfully associated with scaleup parameters, such as the geometrical data of the twin-screw pump. Then it was used successfully to simulate the backflow in twin-screw pumps on relatively broad experimental conditions, judging from the comparison between the model and the experimental data. Introduction Literature Review. The recent trend of developing marginal and deepwater fields requires more economical methods of production and transportation of produced fluids than was required in the past. Almost all the fluid produced from oil wells is a mixture of gas and liquid (multiphase flow). A new concept, the multiphase production system1 (which can handle the multiphase fluid as it is), has been proposed to minimize or even eliminate offshore platforms. One of the methods of this new concept is to use a multiphase pump to pressurize the produced fluids without separation. Various types of multiphase pumps are being developed.2–4 As produced fluids contain not only crude oil, gas, and water, but also sands and corrosive gas, there are many issues to be resolved besides the inherent problems of the pumps themselves for the multiphase flow.5–7 Multiphase pumps are divided into two general types: rotodynamic and positive-displacement pumps. The twin-screw pump8,9 and the axial flow pump10,11 are viewed as representatives of the multiphase pump. This paper focuses on the twin-screw-type multiphase pump because of its high gas-handling capacity. It has already been introduced in the market (or is nearly completed in development stages) in European and North Sea oil fields; the following are some examples of these.Subsea booster system project - twin-screw pump with buffer tank, diaphgragm pump, radial flow pump, and othersMultiphase system - twin-screw pumpBornemann - twin-screw pumpWeir - inverse moineau screw pump Although there were many field trials, there have been few implementations of this practical usage. This is not only because of the relatively short durable tests but also because of unsolved problems for practical usage. From the latter viewpoint, this paper concentrates on solutions for practical use of the twin-screw-type multiphase pump. Although previous studies12,13 have recognized the twin-screw pump as having a high gas-handling capacity, the mechanism of backflow in the gap along the pump shaft has not been clarified yet, nor has the relationship between the backflow and the scaleup parameters of the pump. Since 1993, we have investigated the performance of the multiphase twin-screw pump, which has a gas-handling capacity up to 90%.14,15 We have already begun a new research program to clarify these subjects from 1996. If the anticipated problems such as pulsation, deflection, and resonance are encountered in this study, they will be included in the issues to be solved. Information obtained by placing pressure sensors along the pump shaft will help to clarify the mechanism of the backflow and provide more information on the scaleup parameters for up to 2 years. After this, field tests will be conducted to verify the performance for 2 additional years. Problem Description. It has not been fully argued in the literature that the pump capacity for multiphase fluids can be calculated using numerical analysis with geometrical information on the twin-screw pump. For the single-phase liquid flow, the total flow rate can be calculated easily because of the constant backflow rate of the single-phase liquid, as shown in Fig. 1. Then the total flow rate at delivery is identical to the total flow rate at suction in the single-phase flow because of a lack of compressibility. On the other hand, for the multiphase flow, it is difficult to calculate the total flow rate inside the pump because of the entity of compressible gas in the multiphase fluids. The real total flow rate decreases as the fluids are transported from suction to delivery. Furthermore, the fluids in the backflow may consist of liquid and/or gas. Because it was difficult to measure the components of backflow directly, the gas content along the pump screw was determined by measuring the pressure along the pump screw, and the backflow was calculated from the pressure distribution along the screw indirectly. Although some studies measured the pressure distribution along the pump screw, few were conducted in a quantitative manner. The following describes an elaborate experimental program conducted to generate pertinent data for the internal flow in the pump. A model developed to predict the backflow in the pump is presented and evaluated using the experimental data. Finally, the key parameters for designing the twin-crew multiphase pump are discussed. Pump Design. Design Parameters. Fig. 2 shows that the twin-screw pump is designed with intermeshing screws on parallel shafts, operating inside close-fitting bores. Generally, left- and right-side screws are mounted on each shaft to boost fluids from both ends of the pump into the middle. This arrangement has the advantages of doubling the pump capacity and balancing the axial hydraulic thrust created by the discharge pressure generated. This design also ensures that the shaft seals require only suction pressure instead of the full discharge pressure of the pump. The screws are kept in mesh by precision timing gears on each shaft, preventing screw contact. The bore centers are somewhat eccentric because of the deflection of the pressurized shaft. The integral screw-and-shaft design provides greater strength, higher torque capacity, higher volumetric capacity, and less deflection for reduced wear and long life. Therefore many design parameters for the twin-screw pump (such as pitch, screw length, clearances, screw profiles, shaft diameters, casing style, sealing methods, and materials) should be optimized for each application to ensure the best pump fit, providing maximum efficiency and reliability. The cross sections in which the multiphase fluids flow back are calculated from the scaleup parameters.

Vibration Modeling and Experimental Results of Two-Phase Twin-Screw Pump

“…The axial force buildup is roughly constant in the case of the single phase and becomes increasingly parabolic with increasing the GVF. This trend is consistent with the previous test [5,13].…”

“…He showed that for an initial circumferential velocity of the fluid entering the seal equals half the rotor surface velocity, the circumferential velocity of the fluid along the seal remains constant and generally has markedly less effect on the total leakage than the pressure-driven flow, especially for a short seal (low length over diameter ratio). In the case of twin-screw pump, discounting the coquette flow effect is consistent with the experimental data that show leakage is independent of pump speed [13].…”

Experimental Study of Two-Phase Pumping in a Progressive Cavity Pump Metal to Metal

“…The increase of volumetric efficiency when the gas content is increased at the suction port (see figures 6 and 7), is a phenomenon that has also been documented for twin screw pumps [8,9,10,11,12]. A model proposed for twin screw pumps supposes that internal clearances are always full of liquid, because the phases are separated under the action of centrifugal forces.…”

Theoretical and experimental analysis of a multiphase screw pump, handling gas–liquid mixtures with very high gas volume fractions

“…Several former investigations [1][2][3][4][5][6][7] showed already, that screw pumps present a promising method to meet the requirements on the pump performance and the reliability onshore and offshore. Nevertheless, there is still a demand on secure simulation tools for extreme operating conditions like very high gas volume fractions up to nearly 100%.…”