Centrifugal pump

1.0 ABSTRACT

The purpose of this experiment was to find the basic operation of the centrifugal pump which is to convert energy of a prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being bei ng pumped. We also identified the performance of the centrifugal pump by calculating hydraulic power and the efficiency (which is a function of the output for an electric motor and as a function of flow rate for the centrifugal pump) at constant speed and compare compare the results with flow rate.

2.0 INTRODUCTION

A pump is a device used to move gases, liquids or slurries. A pump moves liquids or gases from lower pressure to higher pressure, and overcomes this difference in pressure by adding energy to the system (such as a water system). A gas pump is generally called a compressor, except in very low pressure-rise applications, such as in heating, ventilating, and air-conditioning, where the operative equipment consists of fans or blowers. Centrifugal pump A centrifugal pump is a rot dynamic pump that uses a rotating impeller to increase the velocity of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber, from where it exits into the downstream piping system. Centrifugal pumps are used for large di scharge through smaller heads.

 Figure 1.Basic element of a centrifugal pump

Centrifugal pump works A centrifugal pump works by the conversion of the rotational kinetic energy, typically from an electric motor or turbine, to an increased static fluid pressure. This action is described by Bernoulli’s principle. The rotation of the pump impeller imparts kinetic energy to the fluid as it is drawn in from the impeller eye (center) and is forced outward through the impeller vanes to the periphery. As the fluid exits the impeller, the fluid kinetic energy (velocity) is then converted to(static) pressure due to the change in area the fluid experiences in the volute section. Typically the volute shape of the pump casing (increasing in volume), or the diffuser vanes (which serve to slow the fluid, converting to kinetic energy in to flow work) are responsible for the energy conversion. The energy conversion results in an increased pressure on the downstream side of the pump, causing flow. Cavitation’s is the problems in the pump. It is defined as the phenomenon of formation of vapor bubbles of a fl owing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two

classes of behavior: inertial (or transient)cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the processing which a bubble in a fluid i s forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers etc. Due to the general complexity of flow through a centrifugal pump ,the actual performance of the pump cannot be predicted on a completely theoretical basis .Actual pump performance is determine experimentally through test on the pump and the result are presented as pump performance curve .Performance characteristics for a given pump geometry and operating speed are usually given in the form of plots of head rise ,efficiency and power versus flow rate (commonly referred as capacity).This information is most helpful to the engineer responsible for incorporating pump into a given pipe flow system

3.0 APPARATUS

The unit was constructed on a stable stainless steel base plate, comprises of a fixed speed centrifugal pump, a water sump tank and all required pipe works. It was installed with pressure gauges and flow meter for pump characteristic studies. The pump casing was made of transparent material; therefore the pump mechanism can be clearly visualized

F igure 1: Equipment Assembly

4.0 Procedures

1.

That the circulation tank was made sure it was filled with water up to at least the end of the pipe output.

2. The suction valve was made sure it was open. 3. The power supply to the pump was switched on. 4. The delivery valve was regulated to achieve the desired operating condition. 5. The delivery valve was slowly regulated until the flow rate reached 90 LPM. 6. The pressure reading on the pressure gauges was observed. Flow rate and pressure values were recorded when stable condition was achieved. 7. The observation was repeated by decreasing the flow rate as follows.

5.0 RESULTS

a)

Suction Pressure (P1)

Rota meter

Delivery Pressure (P2) 2

(FI1) LPM

cmHg

kgf/cm

kgf/cm

90

7.5

-

0.20

80

5

-

0.30

70

2.5

-

0.40

60

0

0.000

0.50

50

-

0.025

0.60

40

-

0.025

0.65

30

-

0.050

0.75

2

Change (1 kfg/cm )= 98.0665 kPa and (1 cmHg) = 1.333 kPa. 

 



 

Also change Rota meter 20  = 20  



   

.

 b) Drawing out the calculation of the hydraulic power, Pi= Q  P  (W)

Rota meter (FI1)

Water flow rate, Q

Difference in pressure, ΔP

LPM

(m /s)

3

(Kpa)

90

1.50x10

80

1.33 x10

70

1.17 x10

60

1.00 x10

50

8.33 x10

40

6.67 x10

30

5.00 x10

Hydraulic power, Pi (W)

-

09.61

14.42

-

22.75

30.26

-

35.89

41.63

-

49.03

49.03

-

56.39

46.80

-

61.30

40.46

-

63.74

31.87

3

Where, Q = water Flow rate (m /s)

C) Drawing out the calculation of the pump efficiency,  =

 Pi  P 

 100%

Pi = Hydraulic power (watt) and P = Motor power (watt)= 180 Watt

Rota meter (FI1) LPM

Pump efficiency,    (%)

90

08.01

80

16.81

70

23.13

60

27.24

50

26.00

40

22.48

30

17.71

6.0 DISCUSSION

We can see from the graphs and results that the pressure differences decreases with the flow rate, because centrifugal pump has no fixed volume at fixed inlet and casing. When the flow rate of the fluid increases, The pump head will decrease for a given impeller’s diameter. In order to increase the pump head the discharge flow rate will have to decrease The centrifugal pump has a very large capacity compared to other pump, and in turn more energy is consumed. Capacity (flow rate) is proportional to impellers speed and head. T he larger the impeller’s diameter or the faster speed, will increase the pressure head due to the higher exit velocity and converted to the head. The pressure becomes low due to more stages of impellers because of the friction loss between inlet and outlet. From the efficiency and flow rate graph we can see that efficiency is highest when the rota meter reads 60 lpm.Efficiency is neither good at high flow rate or lower flow rate. Centrifugal pump transfer fluids using impellers to kinetic energy into pressure energy, but it’s not a direct energy transfer. Hydraulic efficiency considered an efficiency energy that changes the fluid to pressure energy. Efficiency is a comparison (ratio) between the power coming out of the system and that put into the system. When the efficiency is high, the system is minimizing those losses. The efficiency decreases and this might related to shock loss of eye impeller due to an unsatisfactory match between inlet flow and impeller blade turbulence or due to friction loss in impeller blade passages. The overall efficiency on the centrifugal pumps increases or gain a good percent due to the pump over sizing and considerations of system flow rate when the system is designed or operating  parameters. The efficiency can be reached on special designs which can operate best efficiency  point (BEP) when the pump performance curve and system-operating system curve matches  perfectly.

7.0 CONCLUSION

After looking at the result we can say that ,higher the flow rate lesser the pressure difference. And we can conclude that the machinery works most efficient, when the flow rate and other conditions are optimum (as in 60 lpm) rather then very high or very low.

8.0 REFRENCE

Mukesh Sahdev, Centrifugal Pumps: Basic Concepts of Operation, Maintenance, and Troubleshooting (Part- I). [ONLINE] Available : http://www.mech.hku.hk/bse/cpd/watersystem/centrifugalpumps.pdf . [Accessed on 8 oct 2014].

Carl R. (Rod) Nave. (2000). Bernoulli's equation. Available: http://hyperphysics.phyastr.gsu.edu/hbase/pber.html. [Accessed on 08 oct 2014] st

Rama Durgaiah, 2002, Fluid Mechanics and Machinery , 1  Edition, New Age International (P) Ltd, India.

Erik Oberg. (2012). Centrifugal Pump Cavitation. Available: http://www.engineersedge.com/pumps/cavitation.htm. [Accessed on 7 Oct 2014]