Experimental study on the internal flow field of deep well pumps

Studying the flow field in the pump, especially the mixed-phase flow field, is the key to improving the performance of deep well pumps. As PIV technology, LDV technology and ultrasonic technology become increasingly mature, people can already use these advanced flow field testing technologies to perform high-precision measurements without disturbing the flow field. In Canada, some people have used these advanced technologies to study the flow field of deep well pumps. The Ocean Mechanics Laboratory of the University of Petroleum (Beijing) has applied PIV (Particle Imaging Velocimetry) to conduct experimental research on this problem. The main content is to measure the flow inside the pump at different flow rates. Field distribution rules, and compare the similarities and differences of the flow fields in deep well pumps during single-phase and mixed-phase flows.
1. Particle imaging velocity measurement and image processing technology
The basic principle of particle imaging velocimetry (PIV) technology① is to use the scattering effect of particles scattered in the fluid on light to optically record the position of the particles in the flow field at different times, thereby obtaining the displacement of the particles. Based on the particle Following the flow field, the velocity and instantaneous motion parameters of the fluid at the location of the particle are measured.
Using PIV technology, numerous particle conditions in the flow field can be recorded on the same image through multiple exposures in chronological order, or on different graphics through high-speed cameras. Using the relevant assumptions and laws of physics and mechanics, and according to the corresponding mathematical model, parameters reflecting the characteristics of the flow field (particle displacement, velocity, etc.) can be obtained through a series of numerical operations. Usually, a PIV system mainly consists of a lighting system, a PIV image recording and storage system, and a PIV processing system.
2. Small gas-liquid two-phase deep well pump simulation test device
Deep well pumps work underground, and their working medium is not single-phase, so it is difficult to actually test the flow field of deep well pumps working on site. In addition, due to the complex formation conditions of oil wells in each oil field, it is difficult to find general rules. Therefore, a small gas-liquid two-phase deep well pump simulation test device was established in the laboratory, as shown in Figure 1.
The simulation test device is mainly composed of a hydraulic control system and a pneumatic control system. In order to conduct visual research, the deep well pump model pump barrel and plunger are made of plexiglass. The pump diameter is 57 mm, the plunger length is 0.3 m, the simulated stroke is 0~0.6 m, and the number of strokes is 0~6 times/ s, the working condition of internal pressure is 0.7 MPa.
3. Test process
​ Use industrial white oil with a density and viscosity similar to crude oil as the test medium, and use GDX501 polyethylene beads with a density close to that of white oil as tracer particles. Use a strong sheet light source generated by a 10 W neon laser generator and the corresponding optical path system as the illumination source for PIV photography, and use video or photography methods to capture PIV images. According to different working conditions, PIV images of the deep well pump barrel, pump valve, plunger and other parts under single-phase and gas-liquid two-phase flow media conditions were recorded and photographed for further analysis and processing.
4. Experimental results and analysis
Since there have been corresponding research results on the flow field at the fixed valve position of deep well pumps, and the PIV image processing program for gas-liquid two-phase flow is imperfect, this article focuses on the analysis of the deep well pump moving valve and column when the flowing medium is a single-phase fluid. The flow field at the plug site.
4.1 Movement rules of deep well pumps and valves
In the experiment, it was found that the movement rules of the deep well pump valve are not exactly the same as people’s previous understanding of it. In addition to linear movement in the vertical direction, its movement is also accompanied by two types of rotational movement. When the plunger movement speed is small, the valve ball rotates up and down around the horizontal axis; when the plunger movement speed is large, the valve ball rotates horizontally around the vertical axis and revolves along the inner corner of the valve seat, that is, the central axis of the valve seat hole circle. Its rotational angular speed is related to the movement speed of the plunger. The greater the movement speed of the plunger, the greater the rotational angular speed of the valve ball. The special movement form of the valve ball is mainly related to the particularity of the valve ball and valve seat structure and the impact of the fluid. The deep well pump valve is a spherical valve. When the fluid flows around it, the detachment of the boundary layer will occur at the rear of it, and a lateral exciting force will be generated at the same time. Due to the symmetry of the valve ball, this transverse excitation force will move repeatedly around the “equator” of the valve ball, so that the valve ball is not always located on the 3 axis cnc machining of the valve seat hole, but deviates a distance and is close to the valve seat. Rotating on the corners is the phenomenon of “revolution”.
In addition, due to the instability of the fluid flow and the deviation of the valve ball, the asymmetry of the fluid flow relative to the valve ball will occur, which will cause a certain deflection of the valve ball, so that the lateral force does not act on the center of the ball, but on the horizontal plane. There is a certain eccentricity, which causes the valve ball to rotate on the horizontal plane, which is the phenomenon of “rotation”. The above conclusions are obtained in the case of pure liquid. In gas-liquid mixed-phase flow, due to the existence of bubbles, the flow field disturbance is more severe, and the bubbles have a certain impact on the valve ball. At this time, the valve ball movement is more complicated. In addition to rotational motion, there is also violent up and down beating.
4.2 PIV image processing results of single-phase flow swimming valve ball
From the flow field velocity vector of the swimming valve of the deep well pump, it can be seen that the flow field around the swimming valve ball is not symmetrically distributed, and the boundary layer of the left valve gap continues until near the top of the valve ball before falling off. This shows that the force exerted by the fluid on both sides of the fixed valve gap on the valve ball is unbalanced, causing the valve ball to rotate. As the number of strokes increases and the fluid flow rate increases, the boundary layer of the valve ball detaches earlier, and the asymmetry of the flow field around the valve ball still exists, so the eccentricity of the valve ball becomes stronger, which is consistent with what was observed during the test. The movement rules of the valve ball are consistent.
It can be seen that due to the lateral impact force of the fluid on the valve ball, the valve ball deviates from the axis, coupled with the influence of its “rotation”, the valve ball has a certain lag time when opening and closing, thereby reducing the pump’s Pumping efficiency is reduced, resulting in loss of pump stroke. In addition, due to the non-streamline shape of the valve seat, the suction resistance increases, the lag time of the valve ball increases, and the disturbance of the valve ball increases. This kind of drift and disturbance of the valve ball has a great relationship with the shape of the valve ball and valve seat. In order to make the valve ball move as close as possible to the ideal up and down vertical movement, and in order to reduce the overflow resistance of the valve gap, the valve can be improved. The design of the cover and valve seat allows the valve cover to limit the jumping height of the valve ball. While ensuring the maximum flow area, try to make the valve ball only move vertically up and down, and design the valve cover and valve seat into streamlined shapes to reduce flow resistance. These improvements can reduce the disturbance of the valve ball and shorten the lag time of opening and closing, thereby achieving the purpose of improving pump efficiency.
In addition, it can be seen from the curl of the flow field in this part that there are two obvious vortices between the bottom end of the plunger at the suction port of the swimming valve and the pump barrel. This is because when the plunger makes a downward stroke, there is a certain area at the bottom of the plunger, which forces the liquid at the bottom to flow downward during the downward movement. At this time, the swimming valve ball is in an open state, and the liquid at the lower end of the swimming valve ball is pressed into the swimming valve gap and enters the inner cavity of the plunger, causing liquid backflow at the bottom of the plunger, thereby forming a vortex. These two vortices greatly increase the flow resistance of the liquid and also increase the degree of disturbance of the swimming valve ball. In order to eliminate vortex and reduce overflow resistance, the cross-sectional area of the bottom of the plunger can be reduced as much as possible and its shape can be bell-shaped, thereby reducing overflow resistance.
4.3 PIV image processing results at the outlet of the single-phase flow plunger
​ Velocity vector diagram of the flow field at the outlet of the plunger top under the action of single-phase flow. It can be seen from the figure that the outlet at the top of the plunger presents the following flow field characteristics: the pipe flow inside the plunger is in a symmetrical flow state, and the streamline distribution is relatively uniform. This shows that the tube flow inside the plunger is stable and roughly in a laminar flow state, which can be seen more clearly from the curl of the flow field at the plunger outlet. However, at the plunger outlet, due to the reduction of the flow cross section and the change of the cross-sectional shape, the fluid generates a horizontal velocity component at the plunger outlet, especially a vortex at the corner, thereby generating negative pressure and increasing the flow rate. flow resistance. As the number of strokes increases, the vortex at the corner of the plunger outlet also continues to strengthen, and the flow resistance at the outlet also increases accordingly. In order to reduce the generation of vortex, it can be seen from the analysis of this part that if the plunger outlet corner is designed to be streamlined or chamfered at this part, it should minimize the vortex, thereby reducing the flow resistance at this part.
5 suggestions
The following improvements should be made to deep well pumps:
(1) The valve ball is a major component of a deep well pump and a wearing part. It determines the pump’s efficiency and pump inspection cycle. It is recommended to use eccentric spherical valve balls for deep well pumps with miscible flow, drop-shaped valve balls for deep well pumps with narrow flow channels, and tapered valve balls inlaid with sealing rubber for deep well pumps containing sand.
(2) Under the condition of ensuring the maximum flow area, the flow cross-sectional shape of the valve ball cover should be as streamlined as possible to reduce the flow resistance.
(3) When designing the structure of the plunger, consideration should be given to designing the suction port at the lower end of the plunger into a streamlined or bell-mouth type to reduce its suction resistance. While ensuring that the plunger outlet has the largest flow cross-section, the plunger outlet flow channel is also designed to be streamlined to reduce the flow resistance at the plunger outlet.
Author affiliation: Lin Sheng, Fang Huacan, Department of Mechanical and Electrical Engineering, University of Petroleum, Beijing 102200
Introduction to the author: Lin Sheng (1971-), male, received a master’s degree from the University of Petroleum (Beijing) in 1996 and is currently pursuing a doctorate at the Institute of Mechanics, Chinese Academy of Sciences. ①Dong Shouping. Particle Imaging Velocimetry (PIV) and its image processing technology. Internal information of the Department of Mechanical and Electrical Engineering, University of Petroleum (Beijing).