Numerical investigations on effect of wear-ring clearance on performance of a submersible well pump

Abstract

A typical submersible well pump was investigated in this article. The whole flow field of submersible well pump was numerically simulated by computational fluid dynamics software. The influence of clearance of wear-rings on the external characteristic and internal flow field was analyzed through comparing the calculation results with experimental results. The result of the numerical simulation shows that changing clearance of front wear-ring has a greater impact on pump performances than changing clearance of back wear-ring, and the head and efficiency of pump decrease with the increase in the size of clearance. Especially when the size of clearance is larger than 0.5 mm, decreasing becomes more obvious. When the front and back wear-ring size of the clearance comes to 1.0 mm, the efficiency decreases from the highest point of 75.31% to 65.44% at rated flow, and the head of pump decreases about 3.5 m. When the size of clearance is 0.2 mm, reverse-flow will appear in the front shroud cavity of the impeller, and leakage from back wear-ring through the balance hole into the impeller, which has a little influence on the flow field of the impeller inlet.

Introduction

Submersible well pumps are the main equipments to extract groundwater, which are widely used in rural areas, factories, mines, tap-water company, geological exploration, oil, and other geothermal fields. The gap between the wear-rings of impeller of submersible well pump has great influence on the performance of the pump, and the size of the gap directly affects the efficiency and head of the pump.^1–3 There is a clearance between static ring installed in the pump body and rotated ring in the impeller shroud; the existence of the clearance not only produce a volume loss but also disturb the flow field structure inside the impeller channel, so as to affect the overall performance of the submersible well pump. But the clearance between the wear-rings is small, and the flow is complex, so the research is difficult.^4–6 The submersible well pump works in a limited space; the experimental method is used to obtain the flow characteristics of the wear-ring, which requires high-quality equipment and high cost; and it is difficult to obtain the flow characteristics of the balance hole area with the experimental method.^7–9

Baskharone ¹⁰ used computational fluid dynamics (CFD) methods to simulate the leakage flow and study its influence on the internal clearance leakage flow of multistage pump, and compared with the existing impeller leakage model. A research method by Li et al. ¹¹ to carry out the hydraulic performance through changing the size of wear-ring clearance by test, and the effect of different wear-ring gaps on the performance through changing the viscosity of oil, was studied. A labyrinth wear-ring was used for numerical simulation by Hirano et al., ¹² which analyzed the influence of the wear-ring clearance leakage and flow on force of impeller.

This article presents a simulation based on a computational domain including front and back wear-ring gaps, inter-stage clearance, and gains the simulation results by changing front wear-ring gap or back wear-ring gap, compares and analyzes the numerical simulation results with the test results, and explores the influence of deference size of wear-ring gap on characteristics and inner flow field.

Physical model and numerical method

Physical model

The type of pump model in this article is 200QJ80-22 with two stages as shown in Figure 1(a), for which the rated flow is 80 m³/h, rated head is 22 m, rated speed is 2850 r/min, impeller diameter is 124 mm, impeller inlet diameter is 79 mm, the impeller shroud diameter at wear-ring clearance is 89.7 mm, outlet angle is 25°, and the number of blades is 6.

Figure 1.

2D (a) assembly drawing and (b) structural grids.

The main reason of the wear-ring gap flow is that there is a pressure difference between the inlet and outlet of the gap. In addition to the liquid that has a certain viscosity, the impeller does rotary motion, so it will form a shear flow in circumferential direction.

Numerical model

In this article, the three-dimensional (3D) modeling of the inlet section, the impeller, the space guide vane, the front and back sealing rings, the clearance, and the outlet section is carried out through the Pro/E software platform. The whole mesh generation process was carried out in the ICEM 13.0 software, and the two-stage whole flow field was meshed with structural grid. The sketch of the impeller-structured mesh is shown in Figure 1(b). In this study, the grid sizes are less than 2.0 or the grid numbers are large than 3.12 × 10⁶. Figure 1 gives a general view of the model pump 2D assembly drawing and the mesh in the impeller.^13,14

The rotating part and the stationary part with the multi-reference frame model in FLUENT software were based on CFD platform. The channel flow field was set as 3D incompressible steady viscous flow, using standard k − ε two-equation turbulence model. SIMPLE algorithm was used for the pressure and velocity coupling. Turbulence flow near the wall is treated as the standard wall condition. The velocity at the inlet of the first impeller is set to be irrotational, the center section of the inlet pressure is set as the reference point of pressure, and the relative pressure is 0; the outflow is set as free flow. It is assumed that the solid wall is no-slip, which means the velocity in different directions on the wall is 0. The convergence accuracy is 10⁻⁵, and the rotational speed is 2850 r/min. ¹⁴

Test results

The experimental study of the model is able to not only verify the reliability of CFD numerical calculation but also analyze the difference in the performance of different clearances. So, the prototype model was manufactured and all the sizes of clearance are 0.50 mm, and tested by test rig shown in Figure 2. Experiments were done in an open-type pump system, which have the identification from Jiangsu Province of China. The test rig is composed of two parts, namely, the data acquisition system and the water circulation system. ¹⁵ The data acquisition system changes all kinds of physical quantities at different conditions, while the water circulation system supplies the necessary environment for submersible well pump. The test rig is shown in Figure 7. A turbine flowmeter was used to measure the flow Q and the precision of turbine flowmeter is ±0.3%. Speed n is measured by a tachometer (PROVA RM-1500, Taiwan). During the experiment, only one dynamic pressure transmitter (CYG1401) was used to measure the outlet pressure. The precision of CYG1401 is ±0.2%.

Figure 2.

Test ring of submersible well pump.

The overall measurement uncertainty is calculated by the square root of the sum of the squares of the systematic and random uncertainties, ¹⁶ and the calculated result of expanded uncertainty of efficiency is 0.5%.

In the pump performance test, pump efficiency was normally defined as follows

$$\eta =\frac{\rho gQH}{P_S}\times 100\%$$

(1)

where $\eta$ is the pump efficiency, Q is the rated flow (m³/s), H is the head of the pump (m), and P_s is the output power of the motor (W)

$$H=\frac{P_{\mathrm{out}}-P_{\mathrm{in}}}{\rho g}$$

(2)

where $P_{\mathrm{in}}$ is the inlet total pressure and $P_{\mathrm{out}}$ is the outlet total pressure (the $P_{\mathrm{in}}$ and $P_{\mathrm{out}}$ unit is Pa).

The test results were compared with the results of numerical simulation. The gap values used in simulation are in agreement with the test pump. The results are shown in Figure 3.

Figure 3.

Comparison of numerical and test results.

Figure 3 showed the comparison between the numerical results and the test results. As can be seen from Figure 3, the numerical results are consistent with the changing trends of experimental data. The numerical results are higher than the test results, and the main result is that the numerical calculation does not take into account the mechanical loss of the bearing and the volume loss caused by seal gap leakage, making the numerical value of the head and efficiency higher than the test value. In general, the comparison of numerical results with the experimental results shows the accuracy and feasibility of the numerical calculation.

Calculation result

The performance of the whole pump is closely related to the size of the seal-ring clearance of impeller shroud, and its influence is mainly manifested in the external characteristics and the inner flow field, so it is very important to analyze the external characteristics and the inner flow field of the whole pump. In this article, there are five kinds of clearance values of impeller shroud wear-ring which are selected to calculation by CFD, and the five kinds of clearance values are 0.2, 0.35, 0.50, 0.70, and 0.10 mm, respectively. And the size of inter-stage clearance remains constant throughout the numerical simulation (“b1” represents the wear-ring clearance of the impeller front shroud, and “b2” represents the wear-ring clearance of the impeller back shroud).

The influence of the change in the front wear-ring clearance and the back wear-ring clearance on the performance at the design conditions is shown in Figure 2. As can be seen from Figure 4, when the size of b1 is 0.2 mm, the size of b2 increased from 0.2 to 1.0 mm, the efficiency dropped by about 7 percentage points, and the head reduced to 2.6 m. When the value of b2 is 0.2 mm, the efficiency decreased by 8 percentage points and the head reduced 2.9 m with the value of b1 increasing from 0.2 to 1.0 mm. Compared to the previous two conditions, when the value of both b1 and b2 increased from 0.2 to 1.0 mm at the same time, the efficiency and head decreased more significantly, the efficiency decreased from 75.31% to 65.44%, and the head reduced from 24.7 to 21.2 m.

Figure 4.

Relative changes in (a) head and (b) efficiencies in different clearances at rated flow.

It can be clearly seen from the figure that when the value of the wear-ring clearance is between 0.2 and 0.5 mm, efficiency and head have a smaller downward trend; while the value is greater than 0.5 mm, efficiency and head are in a sharp decline. Because when the wear-ring clearance increases to a certain degree, larger volume will result in a drop in the performance, larger leakage flow impacts the inlet area of the impeller and balances the hole area of the impeller channel, the flow field of these area is disordered, and the hydraulic loss increases. So, the efficiency decreased significantly with the larger sizes of the wear-ring clearance. From the numerical results, the effect of change in the sizes of the front wear-ring clearance on the efficiency and head is greater than the change in the sizes of the back wear-ring clearance.

Figure 5 shows the results of efficiency and head under different clearance values of both b1 and b2 at different design conditions. It can be clearly seen from Figure 5 that the smaller the clearance sizes, the higher the efficiency and head of the pump; it can also be seen from Figure 5(a) that the maximum efficiency point of the pump shifts to the small flow rated point, probably in the 70–75 m³/h flow to reach the maximum value. And Figure 5(b) shows that in the small flow condition, the head goes down gently when the wear-ring clearance value increases from 0.2 to 0.5 mm.

Figure 5.

Comparison of performance characteristic under different clearances: (a) pump efficiency and (b) head.

Flow field

Front wear-ring clearance area flow field

Changes in the wear-ring clearance will affect the internal flow field of the pump. Therefore, in order to explore the internal flow field by changing wear-ring clearance sizes, take the flow field of the impeller as the research object in this article and take the front wear-ring flow field area to analyze the design condition.

The streamlined distribution of the first and second wear-ring clearance regions is, respectively, shown in Figures 6 and 7; it can be seen clearly from the chart that the flow in the front impeller shroud cavity generates a rotary motion when the clearance is 0.2 mm. Impeller exit near the front impeller shroud cavity area also has a reverse-flow. Due to the outflow from the impeller of high-pressure water flow into the pump cavity, the front wear-ring clearance is smaller, and the leakage of liquid is greatly hindered. So, the front impeller shroud cavity is full of reflux, but the loss has no effect on the main flow of the pump. Smaller leakage flow did not have a greater impact on the impeller inlet, and the second impeller front wear-ring clearance area is also the case. With the increase in gap sizes, the resistance by the wear-ring clearance to flow from the front impeller shroud cavity into the impeller inlet is reduced. The reverse-flow in the front impeller shroud cavity gradually disappears. In the front impeller shroud cavity was basically no reverse-flow when clearance value up to 0.7 mm, but the more flow through the front wear-ring clearance back to the impeller inlet, instead, the large leaks have a great impact on the impeller inlet so that the hydraulic loss and volume loss is more serious. Larger clearance leakage made the flow field of impeller disorderly, and the hydraulic efficiency and the volume efficiency were decreased, so as to decrease the efficiency and the head of the pump.

Figure 6.

2D streamline distribution in area of front wear-ring of first stage at rated flow.

Figure 7.

2D streamline distribution in the area of front wear-ring of second stage at rated flow.

Figures 8 and 9 show the static pressure contour distribution in the area of front wear-ring of first stage and second stage at rated flow, respectively. From the figure, we can clearly observe the pressure changes in the clearance areas with clearance sizes increasing. The greater the clearance size, the more obvious the pressure gradient in the clearance. The low pressure zone in the guide vane cavity outward diffusion, and in the outlet area of clearance, high pressure is spreading out as the clearance sizes increasing, in this case, not only the volume loss is serious, but also affects the pressure uniform distribution of the flow field inside the impeller. In the range of machining, smaller clearance sizes are better, because the smaller clearance sizes can reduce the hydraulic loss and volume loss, so as to improve the performance of the pump.

Figure 8.

Static pressure distribution in the area of front wear-ring of first stage at rated flow.

Figure 9.

Static pressure distribution in area of front wear-ring of second stage at rated flow.

Balance hole area flow field

In order to balance the axial force of the impeller, the impeller back shroud has balance holes generally. And the leakage from the balance hole will cause the flow field of impeller inlet to disorder and increase the hydraulic loss. Figure 10 shows the comparative analysis of static pressure contour in the balance hole area, where the clearance values of both impeller front shroud and inter-stage clearance are 0.2 mm, and the clearance values of back shroud are 0.20, 0.35, 0.50, 0.70, and 1.00 mm, respectively.

Figure 10.

Comparison of balance hole area static pressure under different back wear-ring clearances at rated flow: (a) first stage and (b) second stage.

It can be seen from Figure 10 that when the size clearance of impeller back shroud is 0.2 and 0.35 mm, the leakage through the balance hole has less impact on the flow field of the impeller inlet and has few hydraulic losses. The leakage increased with the increasing sizes of wear-ring clearance; the flow through the balance hole has a greater impact on flow field of the impeller back shroud, and the flow field inside the impeller is disordered and has great hydraulic loss. Larger leakage volume also led to increased volume loss, and the efficiency and head also decreased significantly. The results shown in Figure 10 are consistent with the results shown in Figure 4. Therefore, it is very important to strictly control the sizes of the clearance for the inner flow field structure and the optimum design of hydraulic performance.

Conclusion

The numerical results are consistent with the experimental results. The numerical calculation does not take into account the mechanical loss and the volume loss, so the test value is slightly lower than the numerical value.

The wear-ring clearance sizes of the impeller are taken as 0.20, 0.35, 0.50, 0.70, and 1.00 mm, respectively, and the numerical results of whole pump under different clearances were obtained. With the increase in the wear-ring clearance of impeller back shroud, the leakage increases; the flow through the balancing hole has a greater impact on the impeller back shroud and has great hydraulic loss, which seriously disorders the flow field inside the impeller.

The reverse-flow is formed when the front wear-ring clearance is small. In the export of clearance, the speed increases as the clearance increases. With the increase in the clearance sizes, the reverse-flow in the impeller front shroud cavity gradually disappears, while the leakage of wear-ring clearance increases; it has a great effect on flow field of the impeller inlet.