Optimization Design of Centrifugal Pump Impeller

Abstract: Impeller is the main hydraulic component that affects the performance of centrifugal pumps, which involves the overall energy efficiency and operational reliability of pumps that people are concerned about. This article briefly discusses how to improve the suction and hydraulic performance of a centrifugal pump by optimizing its impeller from a qualitative perspective, combined with experience and research results from peers, for reference only.

Author: Xie Xiaoqing, Ding Peng, Li Yanjie, 2
1. Shanghai Electric Kaishibi Nuclear Power Pump and Valve Co., Ltd
2. Jilin Yuqi Pump Industry Co., Ltd

Keywords: centrifugal pump impeller optimization suction performance hydraulic performance

A friend wants me to talk about the optimization design of centrifugal pump impellers. To this end, it is first necessary to clarify the purpose of optimization: to improve inhalation performance? Improving pump efficiency? Adjust the rise amplitude of the Q-H curve... Then optimize according to specific needs.
The main hydraulic component that affects the performance of a centrifugal pump is the impeller, and in addition, it also includes flow passage components such as the volute/guide vane that are matched with it. In fact, the author has partially covered the optimization design of centrifugal pump impeller in many articles on the WeChat official account "Pump Salon", such as "Comprehending Cavitation and Its Impact on Centrifugal Pump", "Comprehending Suction Specific Speed of Centrifugal Pump", "Influence of Impeller Geometric Parameters on Centrifugal Pump Performance", and so on.
Fluid mechanics belongs to a semi theoretical and semi empirical discipline, and there are still many areas where accurate design/simulation/prediction cannot be achieved, such as the inability to accurately simulate the true flow state of fluids and their impact on pump performance under different structures, temperatures, and pumping media. Therefore, this article can only briefly discuss how to optimize the impeller of a centrifugal pump to improve its suction and hydraulic performance from a qualitative perspective, combined with experience and research results from peers. For reference only.

Improve suction performance
I often see journal articles from various experts introducing the types, causes, and solutions of damage caused by cavitation. However, for ordinary engineers and on-site operators, the diagnosis and avoidance/elimination of cavitation phenomena are not simple and often difficult to correct.
The impeller blades have two types of bending: forward bending and backward bending. Due to the effectiveness of backward curved blade impellers in maximizing power, imparting high rotational force to the fluid, and preventing detachment, centrifugal pumps typically use backward curved blade impellers.
For the pump body, the cavitation behavior and suction performance of the pump are largely influenced by the geometric shape and area of the impeller inlet (eye). Many geometric factors at the inlet of the impeller can affect cavitation, such as inlet and hub diameter, blade inlet angle and upstream liquid flow incidence angle, number and thickness of blades, blade throat area, surface roughness, blade leading edge contour, etc. In addition, it is also related to the outer diameter of the impeller blades and the size of the gap between the guide vanes (for guide vane pumps) or the volute tongue (for volute pumps).
Over the years, many authors have studied and reported on the effects of some of the above factors on pump cavitation. Excellent tutorials covering all aspects of cavitation can be found in the literature of Schiavello and Visser (2008). Palgrave and Cooper (1986) conducted visual research on cavitation and proposed a general expression for estimating NPSHi based on inlet angle and inlet diameter. Schiavello et al. (1989) conducted a visual study on a cavitation test rig and compared the effects of impeller designs with different tip and hub impact free designs on their suction performance. Hergt et al., 1996, recorded the suction performance of impellers with different impeller diameters, blade inlet angles, and blade numbers.

1) Impeller inlet diameter/inlet area
In order to improve the suction performance of centrifugal pumps, designers generally achieve this by increasing the inlet diameter of the impeller. Today, this design method is still being used in the engineering design of centrifugal pumps.
When the shaft diameter is the same and the diameter clearance at the impeller ring is the same, the better the suction performance (the larger the impeller inlet area, the higher the suction specific speed value), the larger the clearance area at the impeller ring, which means that the leakage amount is greater and the pump efficiency is lower.
However, for the method of improving suction performance by increasing the impeller inlet diameter, special attention must be paid to not causing the suction specific speed value to seriously exceed the value specified in relevant standard specifications (such as UOP 5-11-7), otherwise it will cause the stable operating range of the pump to become very narrow.

2) Blade leading edge shape
Ravi Balasubramanian et al. studied the leading edge shapes of different impeller blades and found that the use of parabolic profiles can improve the suction performance of the impeller as long as the mechanical and manufacturing constraints of the leading edge blade thickness are met. The suction performance of the elliptical contour takes second place, and this shape is the default contour selection for the leading edge, as it can easily meet the mechanical and manufacturing limitations of the blade leading edge thickness [1].
3) Curvature radius of the inlet part of the impeller cover plate
Due to the influence of centrifugal force on the liquid flow at the inlet of the impeller at the turning point, the pressure near the front cover plate is low and the flow velocity is high, resulting in uneven distribution of impeller inlet velocity. Properly increasing the curvature radius of the inlet part of the cover plate is beneficial for reducing the absolute velocity at the front cover plate (slightly ahead of the blade inlet) and improving the uniformity of velocity distribution, reducing the pressure drop at the pump inlet, thereby reducing NPSHR and improving the pump's cavitation resistance performance.

4) Position of blade inlet edge and shape of inlet part
The blade inlet edge extends in the lateral suction direction of the hub, which adopts a swept back blade inlet edge (the inlet edge is not on the same axial plane, and the outer edge is staggered backwards by a certain angle), which can enable the liquid flow on the hub side to receive the action of the blade in advance and increase pressure.
The inlet edge of the blade extends forward and tilts, resulting in different circumferential velocities at each point. Generally, the axial velocity is approximately evenly distributed along the inlet edge, resulting in different relative liquid flow angles at each point on the inlet edge. In order to comply with this flow situation and reduce impact loss, the blade inlet should be made into a spatial twisted shape, which is why many low specific speed impeller blades are also made into twisted blades at present.
5) Blade inlet angle of attack
The design condition adopts a slightly larger positive angle of attack to increase the inlet angle of the blade, reduce the bending at the inlet of the blade, reduce the squeezing of the blade, increase the inlet flow area of the blade, and thus improve the suction performance. At the same time, it will also improve the operating environment under high traffic to reduce traffic loss. However, the angle of attack should not be too large, otherwise it will affect efficiency [3].

6) Blade inlet thickness and smoothness
Reduce the thickness of the blade inlet appropriately and round the blade inlet to make it close to streamline. Reducing blade thickness not only expands the area of the impeller suction channel, reduces flow velocity, and increases pressure (the shape of the blade inlet is very sensitive to pressure drop), but also improves the surface smoothness of the impeller and blade inlet, reducing resistance loss. These measures are all beneficial for improving the suction performance of the pump.
7) Balancing hole
The balance hole on the impeller has a certain destructive effect on the main flow entering the impeller due to leakage (the area of the balance hole should not be less than 5 times the sealing gap area to reduce the leakage flow rate and thus reduce the impact on the main flow). Research has shown that opening a balance hole on the impeller will reduce the intensity of eddy currents behind the impeller, some of which may even disappear, and improve the suction performance of the pump [4].
8) Impeller outlet diameter
A small decrease in impeller diameter will only slightly increase NPSHR. But when the diameter decreases by 5% to 10%, NPSHR will significantly increase, because a decrease in blade length will increase specific blade loads, thereby affecting the velocity distribution at the impeller inlet.
Precautions:
1) Try to avoid using the method of increasing the inlet area of the impeller to improve suction performance - to avoid serious exceeding of the suction specific speed [for example, for BB2 pumps, it is usually controlled within 14400 (m3/h, m)] [5], otherwise it is very easy to cause inlet backflow and expand the unstable operation area of the pump.
2) Blade passage syndrome cavitation should be avoided. This cavitation damage is caused by the small gap between the guide vanes (for guide vane pumps) or the volute tongue (for volute pumps) and the outer diameter of the impeller blades. When the liquid flows through this small channel, the increase in flow rate of the liquid causes a decrease in liquid pressure, local vaporization, and the generation of bubbles, which then rupture at higher pressures, leading to cavitation.

Improving hydraulic performance
There are many factors that affect the hydraulic performance of pumps, and the main factors that affect the hydraulic efficiency of impellers are various losses. Specifically:
1) Number of vanes
For centrifugal pumps, generally speaking, increasing the number of blades can improve the liquid flow situation and appropriately increase the pump head. However, increasing the number of blades will reduce the flow area of the channel, leading to an increase in flow velocity and friction loss of the blades. Therefore, increasing the number of blades too much not only reduces efficiency and deteriorates the cavitation performance of the impeller, but may also lead to a hump in the pump performance curve [6]. In addition, an increase in the number of blades will flatten the upward trend of the head characteristic curve (from the rated point) to the dead center; On the contrary, as the number of blades decreases, the head characteristic curve becomes steeper. Usually, 5-7 blades are selected for centrifugal pump impeller with a large number of blades.
2) Long and short vanes
Research has shown that any combination of short and long blades in a pump impeller will be beneficial for improving pump efficiency, as it can effectively prevent the development of any wake flow called wake flow due to the uneven distribution of flow velocity near the impeller inlet [7].

3) Twisted blade
Experiments have shown that pumps with twisted blades have higher efficiency near design operating points and in high flow areas than pumps with curved blades. At the same time, pumps with twisted blades have a higher shut-off head than circular blades (which can change the upward trend of the head characteristic curve to the shut-off point, especially for low specific speed centrifugal pumps, which can effectively improve/eliminate humps).
4) Impeller outlet diameter
The API 610 standard does not allow pumps to reach the maximum impeller diameter and requires cutting the impeller to meet the required performance of the pump. If the pump selection is too large, cutting the impeller is a relatively economical and effective method to reduce the generated pressure and flow rate. Although cutting the impeller is more efficient than using a throttle valve to meet the required operating conditions, due to the shortened impeller blades, the gap between the impeller blades and the pump casing increases, resulting in lower efficiency than full size impellers.
For radial flow impellers, their diameter should not be reduced to more than 70% of the maximum design diameter. A decrease in the diameter of the pump impeller will also change the width of the outlet channel, the angle of the blade outlet, and the length of the blade. The more the impeller diameter decreases from the maximum diameter, the more the pump efficiency will decrease with the cutting of the impeller, and the highest efficiency point will shift towards the direction of small flow.

The impact of other parameters on pump performance
1) Impeller blade width
As the blade width increases, the liquid pressure decreases, so the head will decrease with the increase of the impeller blade width; The effect of blade width on the efficiency of the optimal efficiency point is usually not significant (as the blade width increases, the efficiency of the optimal efficiency point may slightly increase), but the efficiency zone will shift towards the direction of low flow rate as the blade width decreases. The effect of efficiency is more significant at larger volumetric flow rates, in other words, as the blade width increases, the efficiency curve rapidly decreases to the right of the optimal efficiency point.
2) Impeller outlet blade angle
The larger the outlet blade angle, the higher the head at a given speed, but the cost is lower efficiency and wear performance. The lower outlet blade angle increases efficiency and blade length, but at the cost of reducing head. Therefore, the outlet blade angle usually needs to be optimized to achieve a balance of these factors [8]. The head increases with the increase of outlet blade angle, which can be explained by the increase in outlet cross-sectional size relative to the increased outlet blade angle, resulting in a decrease in liquid pressure drop in the flow channel between the blades. Reference [9] suggests that the highest efficiency value decreases with an increase in outlet blade angle. When the outlet blade angle is small, the efficiency of the pump on the right side at the highest efficiency point will rapidly decrease.
3) Impeller outlet splitter blade
Adding a splitter blade on the outlet side of the impeller will increase the head and hydraulic efficiency of the pump, and as the length of the splitter blade increases, the magnitude of the increase in head and efficiency will be greater [10]. The length of the splitter blade usually does not exceed 0.5 times the original blade length, which is related to the size of the impeller, the shape of the blades, and the number of blades.
4) Impeller blade outlet edge trimming
Grinding the back of the impeller outlet blade expands the area of the impeller outlet channel, thereby increasing the flow rate of the impeller. As the outlet channel area expands, the head will also increase, and the optimal efficiency point of the pump will shift towards the high flow side.
Special instructions
With the rapid development of computational technology and analysis software such as ANSYS, numerical simulation and computational fluid dynamics (CFD) have become one of the better tools for studying and evaluating the optimal characteristics of water pumps. This type of simulation is very useful in predicting and estimating many characteristics of pump performance, and provides many solutions before any further steps.
Therefore, in the optimization design process of centrifugal pump impellers, the support of CFD is indispensable. Usually, several different hydraulic schemes are designed first; Then use CFD for simulation analysis; Finally, based on the analysis results, select a solution that best meets the design requirements.
Using CFD for simulation analysis is an effective method for designing and estimating hydraulic designs, which can reduce time, reduce costs, and improve design accuracy. It can reduce errors to a large extent and provide alternative solutions.