Investigating the Efficiency of Centrifugal Pumps via FEA

Investigating the Efficiency of Centrifugal Pumps via FEA

Motsi Ephrey Matlakala Department of Mechanical and Industrial Engineering Technology

University of Johannesburg Johannesburg, South Africa [email protected]

Daramy Vandi Von Kallon

Department of Mechanical and Industrial Engineering Technology University of Johannesburg Johannesburg, South Africa

[email protected]

Abstract

The area of significance to a centrifugal pump design is the impeller geometric parameters that help achieve pump performance. The recent investigation conducted at Rand Water show that the loss of hydraulic performance of centrifugal pumps used at their pumping stations is as a result of the detachment of the impeller neck rings on the impellers. A design modification to the casing wear rings or impeller neck rings was recommended to smooth the entry of water into the eye of the impeller by reducing the gap between the casing wear ring and the impeller neck ring. Various tests are performed on the centrifugal pumps at the Zuikerbourch Rand Water Pumping Station and show that design parameters such as the diameter of the impeller, blade angles, number of blades, suction diameter and discharge diameter can have considerable influence on the performance of the centrifugal pumps. A model and simulation to predict the efficiency losses of these pumps is proposed. Computation Fluid Dynamic (CFD) simulations are carried at the different parameters and it was observed that it saves time and costs of the company when designing the centrifugal pump.

Keywords Centrifugal pumps, pump performance, pump design parameters, efficiency and impeller.

1. Introduction Rand Water is the largest bulk pure water supplier in Africa and is one of the largest in the world, providing bulk potable water to more than 11 million people in Gauteng, parts of Mpumalanga, the Free State and North West provinces of the Republic of South Africa. Rand Water has two purification plants, the Vereeniging and Zuikerbourch station, to purify water for human consumption (Bachus & Custodio, 2003). After the purification process, the water is pumped to booster stations of Rand Water which are based at Mapleton, Palmiet, Zwartkopjes, and Eikenhof. The Booster Stations pump the water to local and metro municipalities, mines and industries. The municipalities, e.g. Johannesburg Water, in turn, supply the water, at a cost, to consumers or individual households (Anon., 2018). 1.1. Problems Associated with the Centrifugal Pumps This research is based on investigating the efficiency losses of the Rand Water centrifugal pumps installed at Zuikerbourch Rand Water Pumping Station. The pump-set has two stages connected in series. Over the years the hydraulic efficiency of the pump has dropped to a point at which it consumes more energy to deliver the required head and flowrate. A model and simulation to predict these efficiency losses is used to develop better design and operation practices for the centrifugal pumps (Matlakala, et al., 2019; Xiao & Tan, 2020). It is desired to investigate possible ways to improve the efficiency and reliability of centrifugal pumps through better design, installation, operation and maintenance practices. 2. Design

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The experimental analysis performed indicates that the centrifugal pump has two stages. The focus in our design and calculations are on a single stage of centrifugal pump specifically on stage 1 and the assumptions are made based on the specifications in Table 1 (Berg, 2017; Matlakala, et al., 2020; Peng, et al., 2020).

Table 1. Centrifugal Pump Specification

Duty Flow Rate (Ml/d) 200

Duty Generated Head (Set)(Revised) (m) 198

Guaranteed Demand (set) (kW) 5203.69 Guaranteed Efficiency (%) 86.23

Duty Generated Head (Stage 1) (m) 50

Duty Speed (Stage 1) (rev/min) 730 Rated Voltage (Stage 1 and 2) (kV) 11000

Rated Speed 1st Stage (rev/min) 745

Duty Generated Head (Stage 2) (m) 148

Duty Speed (Stage 2) (rev/min) 1490.00 Rated Speed (Stage 2) (rev/min) 1490

Suction Dia. (mm) 1400 Delivery 1st Stage Dia. (mm) 1000

Delivery 2nd Stage Dia. (mm) 900

Density (kg/m^3) 1.00

Gravity (m/sec^2) 9.79

Pump-set Utilization

(%)

52.15

2.1. Determination of Pump Design Parameters Calculations for a single-stage centrifugal pump with a Radial 6-Vanes impeller are carried out. Impeller diameters of 2400mm, 2200mm and 2000mm were used and the affinity law was applied to test the efficiency of the centrifugal pump (Matlakala, et al., 2019; Randall & Whitesides, 2008). Impeller is radial rotor type that increases the acceleration of the fluid (Chandrasekaran, et al., 2020). The results are summarised in Table 2. Figure 1 indicates that the flowrate of the pump and head are directly proportional as the impeller diameter decreases. The efficiency of the pump increases with the decrease of the impeller diameter, see Figure 2.

Table 2. Impeller design parameters

Impeller Diameter (mm) Flow Rate (m3/s) Head (m) Power (kW) Power Output (kW) Eff (%)

2400 2,315 50 1550.73 1135.51 73% 2200 2.122 42.014 1194.46 1135.51 95% 2000 1.929 34.722 897.413 1135.51 127%



Figure 1: Head Vs Flow |Rate Figure 2: Efficiency Vs Impeller Diameter

A series of the three tests are conducted where the blade angles of the impeller is varied at 38ᵒ, 44ᵒ, and 53ᵒ to determine the effect of the blade angle parameter on the centrifugal pump performance. The calculations are based on the condition that the flow coefficient is 0.11, therefore, the radial flow velocity is Vf2 = 0.11U2. The results obtained are summarised in Table 3. The head of the centrifugal pump increases with increase of angle blades of the impeller, see Figure 3. The variation of the blade angle of the centrifugal pump increases the performance of a pump. Figure 4 shows that the efficiency of the pump improves with increase of the blade angle of the impeller.

Table 3. Impeller Blade Angle Design Parameters

Blade Angle (°) Head (m) Flow Rate (m3/s) Water Power (kW) Power Input (kW) Efficiency (%)

32 52.319 2.315 1188.17 1550.73 76.62 42 59.889 2.315 1360.08 1550.73 87.70 53 65.659 2.315 1491.31 1550.73 96.16

Figure 3: Impeller blade angle Vs head Figure 4: Efficiency Vs Impeller blade angle

Spme calculations of the model is carried out considering the different number of blades of the impeller varying at 6 blades, 8 blades and 10 blades (Matlakala & Kallon, 2019). The results are summarised in Table 4. Figure 5 shows the relationship between the efficiency and flow rate of the centrifugal pump that the flowrate is directly proportional to the efficiency. Figure 6 shows the relationship between the number of blades and the efficiency of the centrifugal pump. It can be seen that the efficiency of the pump drops with increasing number of blades.

Table 4: Number of Blades of the Impeller

No. of Blades Flow rate (m3/s) Head (m) Water power (W) Rotational shaft power (W) Efficiency (%)

6 2.723 50 1335.05 1550.73 86.09 8 2.684 50 1316.61 1550.73 84.90

10 2.647 50 1298.17 1550.73 83.71

Figure 5: Efficiency Vs Flow Rate Figure 6: Efficiency Vs number of blades

Calculations of the model are carried out considering different diameters of the suction varying at 1400 mm, 1300 mm and 1200 mm. This is done to analyze the performance of the centrifugal pump with different diameters of the suction of the pump (Matlakala, et al., 2019). The results are summarised in Table 5. The theory of centrifugal pump says that



the suction diameter should always be bigger than the delivery diameter. The efficiency of the pump improves with increase in suction diameter, see Figure 7. The bigger the diameter of the suction, the more it allows the pump to prime which will lead to higher efficiency of the pump. Figure 8 shows that the increase of suction diameter also increases the head of the pumps, therefore, water power of the pump will increase.

Table 5. Pump Suction Diameter Design Parameters

Suction Diameter

Flow Rate (m3/s)

Head (m)

Power Out (kW)

Power In (kW)

Efficiency (%)

1400 2.315 49.361 1120.97 1550.73 72.26 1300 2.315 49.301 1119.63 1550.73 72.20 1200 2.315 49.242 1118.29 1550.73 72.11

Figure 7: Efficiency Vs pump suction diameter Figure 8: Suction diameter Vs head

Important calculations of the model are carried out considering different diameters of the discharge of varying at 1000 mm, 900 mm and 800 mm. The calculations are done to analyze the performance of the centrifugal pump with different discharge diameters, provided that the flowrate and the suction diameter remain the constant (Matlakala & Kallon, 2019). The discharge diameter of the casing of the pump does not have much effect or improvement on the performance of the centrifugal pump. The calculation results for the variation of discharge diameter are summarised in Table 6. The efficiency of the pump improves with the decrease in discharge diameter, see Figure 9. Figure 10 shows that the increase in discharge diameter decreases the head of the pumps, when tested at constant suction diameter.

Table 6: Pump Discharge Diameter Design Parameters

Discharge Diameter

Flow Rate (m3/s)

Head (m)

Power Out (kW)

Power In (kW)

Efficiency (%)

1000 2.315 49.36 1120.97 1550.73 72.26 900 2.315 49.57 1125.79 1550.73 72.59 800 2.315 49.98 1135.03 1500.73 73.19



Figure 9: Head Vs discharge diameter Figure 10: Efficiency Vs discharge diameter

3. Simulations The flow simulations of the model are carried out in this section considering the different design parameters of the centrifugal pump. The design parameters of the centrifugal pump include the impeller diameters, blade angles of the impeller, suction diameter and discharge diameter. To analyse the performance of the centrifugal pump the simulation is done using Computational Fluid Dynamics (CFD) in ANSYS FLUENT. The simulation is carried out varying the number of blades and blade angles of the impeller. The study parameters of the centrifugal pump are assumed to be:

• Rotational Speed = 1800 rpm • Pressure outlet = 15000 Pa • Velocity of the Impeller = 0.6 m/s

3.1. Effect of Variation of Impeller Diameter Impeller trimming adjusts the centrifugal pump head and flows to the actual needs. Trimming of impeller improves the performance of the centrifugal pump. The trimming of the impeller should not be more than 75% of a pump original diameter (Matlakala, et al., 2020; Matlakala, et al., 2019). The simulation was performed with the impeller diameter varying at 350mm, 335mm and 320mm. The results obtained are shown in Figures 11 to 13 and summarized in Table 7. The Best Efficiency Point (BEP) of the pump is at the maximum flow rate with efficiencies of 0.203m3/s and 91.59% respectively, see Figure 14.

Figure 11: Simulation of Impeller Diameter 350 mm.





Table 7. Simulation Results for Impeller Diameter

Impeller Diameter

(mm)

Maximum Pressure

(kPa)

Maximum Velocity (m/s)

Flow Rate (m3/s)

Head (m)

Water Power (W)

Efficiency (%)

350 20.59 8.79 0.203 2.099 4156.79 91.59 335 14.08 7.39 0.170 1.435 2389.62 52.66 320 14.02 5.52 0.127 1.429 1777.33 39.17

Figure 14: Pump Curve for Impeller Diameter.

3.2. Effect of Variable of Number of Impeller Blades The effect of variation of number of impeller blades is determined at constant speeds of the impeller and at constant blade angle. The simulation of the centrifugal pump is performed with the impeller blades varying at 6 Blades, 8 Blades and 10 Blades (Matlakala & Kallon, 2019). The results obtained are shown in Figures 15 to 17 and summarized in Table 8. The efficiency increases gradually as illustrated in Figure 18 within the range selected for the number of blades of the impeller. The Best Efficiency Point is achieved at maximum flow rate with efficiencies of 0.167m3/s and 91% respectively, see Figure 18.

30

40

50

60

70

80

90

100

1

1.5

2

2.5

3

3.5

4

4.5

0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21

Flow Rate (m/s3)Ef

ficie

ncy

(%)

Head

(m),

Pow

er (k

W)

Head, Power, Efficiency Vs Flow Rate

Head Power Efficiency

BEP



Figure 15: Simulation of 6 Blades.



Table 8: Simulation Results for Number of Impeller Blades

Number of Impeller Blades

Maximum Pressure (kPa)


Flow Rate (m3/s)

Head (m)

Water Power (W)

Efficiency (%)

6 24.76 7.25 0.167 2.524 4122.605 90.845 8 14.40 6.91 0.159 2.536 3948.313 87.005 10 19.83 7.09 0.163 2.189 3497.548 77.072

Outlet (Discharge) High Pressure

N = 1800rpm Low Velocity



Figure 18: The Pump Curve for Number of Blades. 3.3. Effect of Variation of Blade Angle of the Impeller The effect of variation of angle of impeller blade is determined at constant speed of 1800 rpm and constant number of 6 blades. Simulation of the centrifugal pump was performed with the impeller blades angle varying at 44°, 53° and 60°. The results obtained are shown in Figures 19 to 21 and summarized in Table 9. The Best Efficiency Point is achieved at maximum flowrate and efficiencies of 0.166m3/s and 94% respectively as illustrated in Figure 22.

Figure 19: Simulation of 53o Blade Angle.


0

1

2

3

4

5

75

80

85

90

95

0.1625 0.163 0.1635 0.164 0.1645 0.165 0.1655 0.166 0.1665 0.167 0.1675

Head

(m),

Pow

er (k

W)

Effic

ienc

y (%

)

Flow Rate (m3/s)

Head (m), Power (kW) & Efficiency(%) Vs Flow Rate (m3/s)

Efficiency Head

BEP




Table 9. Simulation results for impeller blade angles

Angles of the Impeller Blade



Flow Rate (m3/s)

Head (m)

Water Power

(W)

Efficiency (%)

38° 24.76 7.25 0.167 2.523 4122.605 91.136 44° 24.88 7.42 0.170 2.536 4239.722 93.724 53° 21.48 7.14 0.164 2.189 3522.213 77.862

Figure 22: Pump Curve for Blade Angles.

3.4. Effect of Variation of Suction Diameter of the Pump

Centrifugal Pump operators struggle with pump operation because fluid flows from a high-pressure area to a low-pressure area. Pump inlet condition is often overlooked and is a reason most of the pumps fail. Simulation of the centrifugal pump are performed with the suction diameter varying at 171mm, 185mm and 200mm. The results obtained are shown in Figures 23 to 25 and summarized in Table 10. The Best Efficiency Point of the pump occurs at the maximum efficiency of 90.84% and flowrate of 0.167m3/s respectively, see Figure 26.

70

75

80

85

90

95

100

1.5

2

2.5

3

3.5

4

4.5

0.1638 0.1643 0.1648 0.1653 0.1658 0.1663 0.1668 0.1673

Flow Rate (m/s3)Ef

ficie

ncy

(%)

Head

(m),

Pow

er (k

W)

Head, Power & Efficiency Vs Flow Rate


BEP



Figure 23: Simulation of Suction Diameter 171 mm.



Table 10. Simulation Results for Section Diameter

Suction Diameter

(mm)



Flow Rate (m3/s)

Head (m)

Water Power (W)

Efficiency (%)

171 24.76 7.25 0.167 2.524 4122.60 90.84 185 26.31 7.78 0.209 2.682 5502.18 121.24 200 29.08 7.79 0.245 2.964 7116.76 156.82

Outlet (Discharge)

High Pressure

Outlet (Discharge)

Inlet (Suction)

Rotational Direction 1800rpm



Figure 26: Pump curve for Suction Diameter. 3.5. Effect of Variation of Discharge Diameter of the Pump

The discharge diameter of the centrifugal pump plays a major role in the performance of the pump. If the pump performs far from its duty point, the delivery valve becomes critical to adjust the performance of the pump. Simulation of the centrifugal pump are performed with the discharge diameter varying at 148 mm, 158 mm and 168 mm. The results obtained are shown in Figures 27 to 29 and summarized in Table 11. Figure 30 shows that the efficiency of the centrifugal pump increases with the flowrate of the pump steadying at a maximum flowrate and efficiency of 0.155 m3/s and 76% respectively after which it then decrease with increasing flowrate.

Figure 27: Simulation of 148 mm Discharge Diameter.

Figure 28: Simulation of 158 mm Diameter.

020406080100120140160180

2

3

4

5

6

7

8

0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25

Effic

ienc

y(%

)

Head

(m),

Pow

er (k

W)

Flow Rate (m/s3)



BEP

Outlet (Discharge)

High Pressure

Outlet (Discharge)

Low Velocity

N=1800rpm



Figure 29: Simulation of 168 mm Diameter.

Table 11: Simulation Results for Discharge Diameter

Discharge Diameter (mm)



Flow Rate

(m3/s)

Head (m)

Water Power

(W)

Efficiency (%)

148 24.76 7.25 0.125 2.524 3088.182 68.268 158 23.31 7.44 0.148 2.356 3372.608 74.556 168 21.99 7.01 0.155 2.242 3417.061 75.538

Figure 30: Pump Curve for Discharge Diameter.

4. Discussion This paper outlines the design procedure and testing of a centrifugal pump model in Ansys before the actual design can be manufactured. The simulation results are shown in counterflow through the centrifugal pump in the midplane view. The simulation performed in different parameters including the variation of the impeller diameter, impeller blade angles, number of blades, suction diameter and discharge diameter. In the results, only the maximum pressures and velocities were considered as critical variables to help determine the head, flowrate and efficiency of the pump.

67686970717273747576

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

0.123 0.127 0.131 0.135 0.139 0.143 0.147 0.151 0.155 0.159

Flow Rate (m/s3)

Effic

ienc

y (%

)

Head

(m),

Pow

er (k

W)



BEP



5. Conclusion The objective of the research was to investigate the factors that cause excessive power consumption during pump operation and to determine the causes of efficiency losses in the centrifugal pumps at Rand Water. The focus in this research was to determine the factors that affect the hydraulic efficiency of the pump and conduct tests for centrifugal pump performance and compare the results of the operation duty point with the previous tests. The centrifugal pump was designed and tested at different parameters which include the impeller diameter, blade angle, number of blades, suction and discharge diameter of the centrifugal pump. It was found through simulations and calculations that the design stage of the pump is critical for the hydraulic performance of the pump. The calculation results showed that the discharge and suction diameters of the centrifugal pump does not have much effect on the hydraulic efficiency of the pump. It is concluded that the suction area should exceed the discharge area for the pump to be efficient. The affinity law is considered to be efficient for improving the hydraulic performance of the pump. To increase the number of blades an additional material will be required, therefore, more cost. The weight of impeller will be higher thus more power will be required to operate the pump (Li, et al., 2020). Depending on the size of the pump, the number of blades of impeller should be balanced. Again the pump was found to be more efficient at blade angles less than 90°. The size of the centrifugal pump designed required that the angle of the blades should be less in order for the hydraulic efficiency to improve. The design and analysis methods are useful to generate performance and flow predictions. Therefore, the design can be optimised to reduce energy consumption, increase pump operating life and provide better system flexibility.

6. References Anon., n.d. Rand Water Board. [Online] Available at: http://www.randwater.co.za/AboutUs/Pages/CoreBussiness.aspx [Accessed 24 February 2018]. Bachus, L. & Custodio, A., 2003. Know and Understand Centrifugal Pumps. First Edition ed. AE Amsterdam: Elservier. Berg, Q. V. D., 2017. Assets Inspection Report Pump-Set 20, Johannesburg Chandrasekaran, M., Santhanam, V. & Venkateshwaran, N., 2020. Impeller design and CFD analysis of fluid flow in rotodynamic pumps. Materials Today: Proceedings, 27 July.pp. 1-5. Li, M. Y. et al., 2020. Design and experimental investigation of a phase change energy storage airtype solar heat pump heating system. Applied Thermal Engineering, Issue 179, pp. 1-12. Matlakala, M. E., Kallon, D. D. V., Simelane, S. P. & Mashinini, P. M., 2019. Impact of Design Parameters on the Performance of Centrifugal Pumps. s.l., s.n., pp. 197-306. Matlakala, M. E. & Kallon, D. V. V. Effect of Discharge Diameter on Centrifugal Pump Performance. SAIIE, 2019. Pp. 721-730. Matlakala, M. E. & Kallon, D. V. V. Influence of impeller Blade Count on the Performance of Centrifugal Pump. SAIIE, 2019. Pp 603 - 612. M.E. Matlakala, D.V.V. Kallon, K.F. Mogapi, I.M Mabelane, D.M. Makgopa. Influence of Impeller Diameter on Performance of Centrifugal Pump.IOP Conference Series: Metarials Science and Engineering.655, (1). 2019 , pp. 009-012. M.E. Matlakala, D.V.V. Kallon, K.F. Nkoana, B.D. Mafu, S.B. Mkhwanazi. Effect of Suction Diameter Variations on Performance of Centrifugal Pump. OIC. Pp. 170-173. Matlakala, M. E., Kallon, D. V. V., Simelane, S. P. & Mashinini, P. M. A Computational Model for the Efficiency of Centrifugal Pumps. Dissertation Submitted to the University of Johannesburg. 2021 Peng, G., Fan, F., Huang, X. & Ma, J., 2020. Optimal hydraulic design to minimize erosive wear in a centrifugal slurry pump impeller. Engineering Failure Analysis, Volume 120, pp. 1-13. Randall, W. & Whitesides, P., 2008. Basic Pump Parameters and the affinity Laws. PDHonline Course M125 (3 PDH), p. 4. Xiao, W. & Tan, L., 2020. Design method of controllable velocity moment and optimization of pressure fluctuation suppression for a multiphase pump. Ocean Engineering, Volume 220, pp. 1-13.



7. Biographies Mr Motsi Ephrey Matlakala is a South African holder of a M-Tech in Mechanical Engineering from the University of Johannesburg. Mr Matlakala is currently working as a Mechanical Engineering Graduate at Rand Water Zuikerbourch Pumping Station since June 2017. During his Masters studies, He published five (5) papers and was also selected as a reviewer of paper in two (2) conferences, locally and internationally. Mr Matlakala is preparing to enrolling PhD in Mechanical Engineering with the University of Johannesburg. Mr Matlakala is a member of South African Institute of Mechanical Engineering SAIMECH and Candidate Technologist with ECSA. Mr Matlakala’s primary research areas are System Analysis and Dynamics, Optimization, Computational Fluid Dynamics, Finate Element Analysis and Water Research. Dr Daramy Vandi Von Kallon is a Sierra Leonean holder of a PhD degree obtained from the University of Cape Town (UCT) in 2013. He holds a year-long experience as a Postdoctoral researcher at UCT. At the start of 2014 Dr Kallon was formally employed by the Centre for Minerals Research (CMR) at UCT as a Scientific Officer. In May 2014 Dr Kallon transferred to the University of Johannesburg as a full-time Lecturer and later a Senior Lecturer in the Department of Mechanical and Industrial Engineering Technology (DMIET). Dr Kallon has more than twelve (12) years of experience in research and six (6) years of teaching at University level, with industry-based collaborations. He is widely published, has supervised from Masters to Postdoctoral and has graduated seven (7) Masters Candidates. Dr. Kallon’s primary research areas are Acoustics Technologies, Mathematical Analysis and Optimization, Vibration Analysis, Water Research and Engineering Education.



Investigating the Efficiency of Centrifugal Pumps via FEA

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