Multi-Stage Centrifugal Pump Impeller Hydraulic Model Modification and Performance Analysis

Injecting oilfield waste into suitable receiving formations is an effective and environmentally acceptable method to dispose of drilling and production waste including cuttings, drilling fluids, produced water, emulsions and other produced waste. Traditionally, solid waste is degraded to less than 300 microns and suspended as water-based slurry containing 20% solid matter. Rheological properties are controlled so that the slurry can be injected, typically using triplex pumps, under high pressure (1000 to 5000 psi) through a casing annulus or tubular into hydraulic fractures. At locations, where injection pressures are within limits and the disposal well and domain may cope, or be benefitted with higher rates of injection, increases in the rate of disposal is beneficial to operations due to the increased efficiency allowing faster ROP's or faster evacuation of waste storage devices. This paper discusses the suitability and application of multistage centrifugal pumps for drilling waste injection and provides some data obtained during real time waste injection operations using multistage centrifugal pumps in parallel installation with traditionally used triplex pumps. Wear data in conjunction with engineering economic evaluation will be discussed. Slurry viscosity and particle size data will be presented to show the effect of pump shear on slurry properties and the implications for waste injection. A multistage centrifugal pump proved capable of pumping waste slurry continuously at relatively high rates for a limited trial period of time. A multistage centrifugal pump provides a continuous flow without the pulsation normally associated with using a reciprocating plunger-style pump and the need to have such pulsation dampened. Laboratory testing and realtime subsurface injection data at an offshore installation indicates that multistage horizontal centrifugal pumps may offer some benefits compared to traditional reciprocating triplex plunger pumps. Some operations requiring relatively high flow rates with flow media containing solids may benefit from implementation of this style pump providing the net power requirement and equivalent downhole pressure requirement are within the pump design range, and there is available footprint at the location. Oilfield waste can be disposed of in an environmentally responsible manner under zero-discharge conditions with the performance of the pump ensuring operational reliability and injection assurance. Operations on a global scale may benefit from this new application for multistage pumps. Decreased downtime together with slurry stability and controlled injection assures operational performance and cost effectiveness, enabling oilfield waste to be disposed in a safe, controlled and environmentally responsible manner.

Based on the three-dimensional Reynolds-Averaged N-S equations and the standard k-ε turbulence model, the inner flow of a multistage vane water centrifugal pump was simulated. The distribution law of inner flow was analyzed by using the SIMPLEC algorithm and multiple reference frame (MRF) model provided by CFD software Fluent. The pressure and velocity distribution law in impeller and vane diffuser are obtained. Results indicated that the lowest pressure area , where cavitation was easy to occur, was in the inlet of the first impeller blade back. There is reverse flow in outlet of impeller. An area of low pressure can be seen in the inlet of return guide vanes. The guide vane could reduce fluid velocity and eliminated fluid rotating component.

When predicting the performances of a centrifugal pump by eddy-resolving computational fluid dynamics (CFD) methods such as Large-Eddy Simulation (LES), it is important that the phenomenon of interest and associated flow eddies are accurately computed. Despite recent performance improvement of high-end computers, it is still a challenge to compute a whole multi-stage pump with leakage parts by applying fully-resolved LES and to directly compute active eddies in the turbulent boundary layers, due to grid size requirements. As a part of the analysis of the flows in a multi-stage centrifugal pump by LES, present research focuses on the flow in the pump inter-stage bush. It aims at determining the flow regime, the associated eddies in the bush, and a better understanding of the flow in a grooved inter-stage bush to allow future design optimisations. To assess the existence of the turbulent boundary layer for different Reynolds numbers, focus was placed on the appearance of streamwise vortices which are the smallest active eddies in the turbulent boundary layer. It was observed that these vortices start to develop from Reω=1800 although the velocity profile looks of a laminar boundary layer. Finally, based on CFD results, the effectiveness of the stage bush in terms of minimizing the leakage flow is investigated for the different Reynolds numbers. It is found that the current design shows the maximum effectiveness for Reω=900, and its effectiveness decreases at a higher Reynolds number.

In centrifugal pumps, axial thrust bearings are required to compensate for the axial loading on the pump impeller(s). These forces are mainly the result of unequal distributions of pressure on projected areas at both sides of the impeller(s). Axial loading tends to be highest in multistage pumps in which all impellers face the same direction. Balancing devices are often used to reduce the loading of the axial bearings. Common types of balancing devices are the balance drum and the balance disc. Both of them have their advantages and disadvantages. A stepped balance drum is a new concept for a balancing device that can be used in any type of turbomachinery. It combines the benefits of the balance drum and the balance disk without the drawbacks of both. In this paper, the new concept is presented along with a semi-empirical model of its performance. A CFD analysis is also presented which serves to verify the analytical model. An optimization procedure of a stepped balance drum is given using a multistage centrifugal pump as an example.

Radiated noise is unavoidable in the operation of multistage centrifugal pumps. In order to study the characteristics of radiated noise of multistage centrifugal pump, radiated noise test system of multistage centrifugal pump was built. Radiated noise experiments were carried out for a medium specific speed multistage centrifugal pump and its modification, which was devised with a new hydraulic design method in the field of sound optimization. The experiment validation on the sound field was performed in a semi-anechoic room and the acoustic parameters of the multistage pump at the different flow rates were gathered with the LMS system(a complete set of solutions for vibration and noise test). The results show that: As the speed is lower than the rated speed, the sound pressure level (SPL) of the radiated noise of centrifugal pump increases with the increase of rotating speed. And the radiated noise of multistage pump rises linearly with the increase of rotating speed. The maximum value of sound pressure level appears around the first stage and the minimum value appears around the middle stage at different flow rates. As the flow rate increases, SPL measured around vane diffuser increases and then decreases. In addition, discrete noise occupies the dominant position in the form of noise for both pumps at large flow rate. Compared with the prototype pump, the intensity of radiated noise of the modified pump weakens in varying degrees in the range of the whole variable speed. Furthermore, the radiated noise exhibits a dipole characteristic at low rotating speed.

As the centrifugal pump is running, the fluid usually flows into the impeller along pump shaft, and the fluid flows out radially by the force of the impeller. The force is mutual, so the impeller is also subjected to the reaction force of the fluid, but the distribution of this force on the blades is uneven. In addition, the front and rear shrouds of the impeller are asymmetric, which are the main causes of axial force. This paper adopts numerical calculation method studying the mechanism of axial force of impeller at all stages of multistage pump at various working conditions, and exploring the formation mechanism of shroud pressure differential force and blade twisting axial force and its variation laws of similarities and differences, analyzing the steady state and transient characteristics between axial force and hydraulic property of double-casing multistage pump. The results show that the rotational angular velocity of the fluid in the front and rear pump chamber at each stage impeller is distributed along the axial direction in three regions, the regions are pump body boundary layer, core region, and impeller boundary layer. The working surface and back surface of the blade twist have the high and low axial force area, and its distribution is staggered, at the same number of stages, the greater the flow rate, the smaller the blade twisting axial force. The shroud pressure differential force with the increase of impeller stages presents a linear increasing trend, conforms to the principle of linear superposition of cover pressure differential force. The total axial force pulsation of multi-stage pump is related to the number of secondary impeller blades, its primary frequency coincides with the secondary impeller blade frequency, increasing the flow rate can reduce the multi-stage pump axial force pulsation amplitude. The pulsation period of single-stage impeller head and efficiency are related to the number of impeller blades, the smaller the number of impeller stages, the stronger the pressure dynamic, and static interference effect of the impeller inlet and outlet. Rotation of the secondary impeller causes dynamic and static interference, which is the main reason for the pulsation of the axial force coefficient in double-casing multistage pumps, the pulsation intensity is related to the periodic generation and shedding of the blade vortex. The results of the study can be used as a reference for optimizing the axial force of double-casing multistage pumps.

The axial force balancing capacity of a balance drum is a key factor affecting the life of multi-stage centrifugal pumps. In this paper, a double shell segmental multistage pump is taken as the research object. The hydraulic performance and axial force performance are set as the optimization objectives, and the performance data are obtained by numerical simulation with FLUENT software. The BP neural network is used to establish the prediction model of structural parameters of the balance system, hydraulic performance and residual axial force performance, and it is used as the adaptive value evaluation model of genetic algorithm to solve the optimal value in the sample space. The results show that the radial clearance of the balance drum and the balance tube orifice flowmeter, the axial width of the balance cavity are the significant factors affecting the hydraulic performance and axial force performance of the multistage pump. When the radial clearance of the balance drum is 0.1mm, the clearance of the orifice flowmeter is 1.95mm, and the axial width of the balance cavity is 55mm, the multi-stage pump has the best hydraulic performance and the smallest residual axial force. The vortex band in the balance cavity can increase the amount of the fluid spin and enhance the axial force balancing capacity of the balance drum. The greater the area occupied by the negative high-rotation fluid in the balance cavity, the stronger the ability of the balance drum to balance the axial force. The test results show that compared with the prototype multistage pump, at nominal flow rate, the head and efficiency of the optimized model are increased by 0.71% and 1.63% respectively, and the bearing temperature and vibration speed of the multi-stage pump are significantly reduced.

9 - Turbo-compressor individual stage and overall performance

Cavitation behavior is very important in pumps for long time operation. However, there is difficulty in predicting the cavitation phenomena of pumps by Computational fluid dynamics (CFD). In order to accurately ascertain cavitation behavior, a comparison between CFD and experimental data is a significant and essential process. The purpose of this study is to analyze cavitating behavior in multistage centrifugal pumps numerically and experimentally. For this investigation an experimental set up was used to obtain cavitation performance results. The CFD method was used to investigate the multistage centrifugal pump performance under developed cavitating conditions. The Reynolds-averaged Navier-Stokes (RANS) equations were discretized by the finite volume method. The two-equation SST turbulence model was adopted to account for turbulent flows. Numerical data were validated with experimental data and a good comparison of results was achieved. Numerically, cavitation performances were obtained for different pump stages and the effects on cavitation were described according to different NPSH (Net positive suction head). The occurrence of cavitation was also described according to NPSH3% in the head drop lines and water vapor volume fraction on the impeller blade. The rapid drop in head at low NPSH was captured for different flow conditions. It was found that for stage to stage performance, the head drop changes could be related to losses inside the pump. It was also shown that the simulation results can truly represent the development of the attached sheet cavitation in the impeller.

In order to study the effect of different numbers of impeller blades on the performance of mixed-flow pump “saddle zone”, the external characteristic test and numerical simulation of mixed-flow pumps with three different impeller blade numbers were carried out. Based on high-precision numerical prediction, the internal flow field and tip leakage flow field of mixed flow pump under design conditions and stall conditions are investigated. By studying the vorticity transport in the stall flow field, the specific location of the high loss area inside the mixed flow pump impeller with different numbers of blades is located. The research results show that the increase in the number of impeller blades improve the pump head and efficiency under design conditions. Compared to the 4-blade impeller, the head and efficiency of the 5-blade impeller are increased by 5.4% and 21.9% respectively. However, the increase in the number of blades also leads to the widening of the “saddle area” of the mixed-flow pump, which leads to the early occurrence of stall and increases the instability of the mixed-flow pump. As the mixed-flow pump enters the stall condition, the inlet of the mixed-flow pump has a spiral swirl structure near the end wall for different blade numbers, but the depth and range of the swirling flow are different due to the change in the number of blades. At the same time, the change in the number of blades also makes the flow angle at 75% span change significantly, but the flow angle at 95% span is not much different because the tip leakage flow recirculates at the leading edge. Through the analysis of the vorticity transport results in the impeller with different numbers of blades, it is found that the reasons for the increase in the values of the vorticity transport in the stall condition are mainly impacted by the swirl flow at the impeller inlet, the tip leakage flow at the leading edge and the increased unsteady flow structures.

The integrated pump-gate is a hydraulic facility that integrates a pumping station and a gate, playing a vital role in urban drainage systems, flood control, and other scenarios. Although integrated pump-gates are widely used, their internal flow presents different forms depending on the application scenarios, such as backflow, vortices, and cavitation. These effects markedly influence the pump’s hydraulic performance, operational stability, and overall reliability. This study investigates the cavitation characteristics and internal flow fields within the complex geometry of the integrated pump-gate and numerically simulates the cavitation phenomenon using the SST turbulence model. Specifically, the influence of the impeller, guide vanes, and structural supports on the cavitation performance and internal flow state was analyzed. The results show that the geometric characteristics of the impeller’s leading edge significantly influence the cavitation structure. Regarding cavitation performance, NPSHc was determined to be 5.3 m. At the leading edge of the guide vanes, cavitation usually occurs at the axial diffusion position of the flow channel, and the degree of cavitation is affected by the relative position of the guide vanes and the impeller blades. The structural supports and protrusions significantly affect the vortex structures in the flow field, with protrusion-induced vortex clusters dominating the guide vane region.

PurposeRadial vibration of horizontal centrifugal pump has a close association with radial exciting forces. The purpose of this paper is to analyze the unsteady radial force in multistage centrifugal pump with double volute in detail and investigate the relevance of static pressure, radial force and radial vibration.Design/methodology/approachThe unsteady numerical simulation with realizable k-ε turbulence model was carried out for a multistage centrifugal pump with double volute using computational fluid dynamics codes Fluent. The performance tests were conducted by use of a closed loop system and performance curves from numerical simulation agree with that of experiment. Vibration tests were carried out by vibration probes instrumented on the bearing cover of pump near no-driven end. Fast Fourier transform was used to obtain the frequency components of radial forces on the impellers from numerical simulation, which are compared with ones of radial vibration from experiment in Y and Z direction. And the static pressure distributions in the impeller were analyzed under different flow rates.FindingsThe symmetrical double volute can effectively balance radial forces. The maximum radial force and vibration velocity appear at 0.6 Q among the three flow rates 0.6 Q, Q and 1.2 Q. The frequencies corresponding to relatively large amplitude of vibration velocities and radial forces on the impellers in Y direction are blade passing frequency of the impellers. Blade passing frequency of first-stage impeller and shaft frequency are predominating in Z direction. It indicates that the radial vibration of centrifugal pump is closely related to the unsteady radial force.Originality/valueThe unsteady radial forces of the impeller in multistage centrifugal pump with double volute were comprehensively analyzed. The radial forces should be considered to balance during the design of multistage centrifugal pump.

The transient process of a centrifugal pump existed in a variety of occasions. There were a lot of researches in the external characteristic in startup process and stopping process, but internal characteristics were less observed and studied. Study of the internal flow field had significant meanings. The performance of a pump could be evaluated and improved by revealing the flow field. In the other hand, the prediction of external characteristic was based on the correct analysis of the internal flow. In this paper, theoretical method and numerical simulation were used to study the internal characteristic of a centrifugal pump. The theoretical study showed that the relative flow in an impeller was composed of homogeneous flow and axial vortex flow. The vortex intensity was mainly determined by angular velocity of impeller, flow channel width and blade curvature. In order to get the internal flow field and observe the evolution of transient internal flow in the impeller, Computational Fluid Dynamics(CFD) were used to study the three-dimensional unsteady incompressible viscous flows in a centrifugal pump during starting period. The Dynamic Mesh (DM) method with non-conformal grid boundaries was applied to get the external characteristic and internal flow field. The simulate model included three pumps with different blade numbers and the same blade curvature. The relative velocity vector showed that there was a big axial vortex in impeller channel. At the beginning, the vortex was raised in the pressure side of the impeller outlet and with time went on, it shifted to the middle flow channel of the impeller and the vortex intensity increased. When the speed and flow rate reached a definite value, the influence of the axial vortex began to get smaller. The vortex developed faster when the flow channel got narrower. Due to the evolution of axial vortex, the slip factor during starting period was smaller than that in quasi-steady condition. As a result, transient head was lower than quasi-steady head in startup process.

In a centrifugal pump the interaction between the rotating impeller pressure field and the stationary diffuser pressure field yields pressure fluctuations as the result of a modulation process. These fluctuations may induce hydroacoustic pressure fluctuations in the exit chamber of the pump and could cause unacceptable vibrations. This paper presents a methodology for the prediction of hydroacoustic pressure fluctuations resulting from rotor-stator interaction in a multistage centrifugal pump. The method consists in the one-way coupling of incompressible CFD and hydroacoustic simulations. In a first step the rotorstator pressure fluctuations are calculated using a commercial 3D-RANS CFD-code (CFX 10) for different flow rates. The acoustic simulations are performed in two consecutive steps. Initially a free oscillation analysis using white noise pressure fluctuations is performed, which provides hydroacoustic eigen frequencies and mode shapes of the outlet casing. In a second step the spatially distributed pressure fluctuations from the CFD simulation are used to perform a forced oscillation analysis. This approach allows the prediction of possible standing waves in the hydraulic collection elements in the last stage of multistage pumps.

In the depths of the ocean, where minerals lie buried beneath layers of sediment, multi-stage pumps serve as the workhorses of deep-sea mining, lifting extracted materials through immense vertical distances. To ensure their efficiency and reliability, a comprehensive hydrodynamic analysis is required, focusing on vortex-induced vibrations and fluid–structure interactions. In this study, numerical simulations were conducted using computational fluid dynamics to investigate the internal flow characteristics of a multi-stage deep-sea pump. The distribution and formation mechanisms of vortices were analyzed using the omega vorticity identification method, which allowed for the capture of vortex structures and the evaluation of entropy production in the impeller and guide vanes across varying flow rates. The results revealed that at low flow rates, complex vortex structures, including flow separation and reflux vortices, were predominant at the impeller inlet. As the flow rate increased, these vortices gradually dissipated, leading to a more stable wake at the impeller outlet, while separation and reflux vortices became more pronounced at the guide vanes' inlet and outlet. A reduction in the blade passage vortex structure was also observed with increasing flow rates. Further analysis demonstrated that the stretching and twisting term played a crucial role in the formation, evolution, and dissipation of vortex structures at the guide vanes' inlet. Meanwhile, the Coriolis force was identified as a key factor in vortex generation at the impeller inlet and within the blade passage. These findings provide valuable insights into the vortex dynamics governing deep-sea pump hydraulics, offering a foundation for optimizing their design to enhance operational performance and ensure more efficient deep-sea resource extraction.