JOMO KENYATTA UNIVERSITY OF AGRICULTURE AND TECHNOLOGY

2014

PERFORMANCE OF A

CENTRIFUGAL PUMP EXPERIMENTAL ANALYSIS OF A ROTORDYNAMIC

PUMP

MIMISA DICKENS EN251-0305/2011

C I V I L , C O N S T R U C T I O N A N D E N V I R O N M E N T A L E N G I N E E R I N G D E P A R T M E N T

Abstract

The results of an experiment carried out to investigate the theory of a Rotor dynamic pump and to

determine the relationship between the head, discharge, the input power and the efficiency of a

centrifugal pump under the prescribed revolution speed are presents with much focus on the specific

aspects mentioned. This paper represents experimental study work carried out on centrifugal pump.

Vibrations and noise are both pre dominant due to hydraulic effects. The pump system used allows for

parametric variation of discharge. Data acquired are manually compiled and analyzed, reduced and

presented into a form that can be typically used to analyze pump characteristics. Reduced data is used

in determining the characteristic curve of the pump and to indicate the relationship between the

efficiency and flow rate and power.

Introduction

Centrifugal pumps are classified as rotary type of pumps in which a dynamic pressure developed enables

the lifting of water to great heights. The history of pumps dates back to the ancient day of technological

development in Egypt where the locals used water wheels with buckets mounted on them to move

water for the purposes of irrigation. It was not until the late 1600’s that true centrifugal pumps were

developed by Denis Papin, a French boy, who developed the hydraulic device though with straight

vanes. John G. Appold introduces the curved vane in 1851 thereby improving the efficiency of the

hydraulic device. It has been rapidly superseding the other types of pumps over the years and is

seemingly the most used kind of pump. It is most suited for situations requiring moderate to high flow

rates and modest increases in pressure. They are majorly used in municipal water supply systems,

circulating water heating and cooling systems applied in buildings, pump system in dish and cloth

washing machines and for pumping cooling water in automobile engines. Positive displacement pumps

are more suited for high pressure-low flow applications. Flow rate is a function of rotational speed and

has negligible dependence on pressure rise. They are also used to supply oil under very high pressure for

hydraulic actuators such as those on large earth moving machines.

Below is a sketch of a typical centrifugal pump.

Fluid which flows into the impeller within the

inner radius is given a significant momentum and kinetic energy thus enabling it to flow radially

outwards at a higher momentum and kinetic energy. As it leaves the outer radius of the impeller, it is

slowed down leading to a significant increase in pressure that was initially aimed for the system.

The actual head (H) produced by a centrifugal pump if dependant on the flow-rate (Q). The head-flow

relationship can be easily determined by selecting appropriate impeller geometry. Pumps are normally

designed in a way that head reduces with an increase in flow for the purpose of a stable flow rate when

the pump is connected to a piping system. A typical head-flow curve for a pump is as indicated below.

The result of the application of the mechanical energy equation applied on two sections of a piping

system proves that

𝐻𝑃 + 𝑉1

2

2𝑔+

𝑃1

𝛾+ 𝑍1 = ℎ1 +

𝑉22

2𝑔+

𝑃2

𝛾+ 𝑍2

Where 𝐻𝑃 is the pump head and ℎ1 = total head loss in the piping length under study. The others are the

pressure head and the velocity head of the system.

For any given pump operating at a given rotational speed, there is always only one operating point

where the geometry of the impeller blades is an optimum. When this is combined with the other forces

an efficiency of the system is obtained which is a function of the rate of flow. The efficiency is the ratio

of the fluid work (power outputted) to the shaft power input of the pump.

This relation can be shown as 𝜂 = 𝑌𝑄𝐻𝑃

𝑃𝑠ℎ𝑎𝑓𝑡

The performance of a pump is highly dependent on the impeller and casing geometry, the

rotational speed, the size of the pump and the properties of the flowing speed. However, it is

not necessary to vary all these factors in order for one to be able to determine the performance

of a pump. Two geometrically identical pumps with flow rates adjusted so that the ratio of

tangential to radial fluid velocities is the same are said to be homologous. Homologous pumps

are known to have geometrical similarity and are also known to have the following

dimensionless parameters the same.

𝜋1 = 𝑄

𝑁𝐷3

𝜋2 = 𝑔𝐻

𝑁2𝐷2

Where N is the angular speed and D the diameter. The Diameter is taken as a measure of the length

scale of the pump in question. A larger diameter indicates that all the other dimensions of the pump are

relatively larger. The outer diameter of the impeller is normally used. These relationships make it

possible to estimate the performance of a pump of known diameter by testing another pump with a

different diameter. It also becomes possible to determine the effects of a changing angular speed. This

scaling, however, is not perfect and a few errors are expected of it.

Materials Used

For the effectiveness of the process, a number of equipment and materials had to be availed. These are

as they have been listed below.

o A Centrifugal pump

o An Electric board comprised of an ammeter, a voltmeter and a power factor meter

o A V-notch with a hook gauge

o Pressure gauges on a suction pipe and a delivery pipe

o A thermometer

The apparatus were arranges in a set-up as shown below.

The set-up was arranged such that when the operation was started, all values are read simultaneously

for effectiveness of the process. The set-up was checked for correct layout with a few tests after which

the actual experiment was started and data collected.

Procedure

The temperature of the water was first measured after which the crest level of the v-notch was

measured using the hook-gauge. The operation of the pump was started with the gate valve closed after

which the gate valve was slowly opened and a small discharge set. The head above the v-notch was

measured using the hook gauge after it was clear that the flow had become steady. The readings of the

pressure gauges, voltmeter, ammeter and the power factor meter were recorded on the data sheet. The

procedure was repeated after the discharge was increased with the gate valve.

Theoretical Knowledge pertaining to the experiment

A pump is a device that supplies energy to a fluid. The effect of supplying energy can be s tudied via the

mechanical energy equation.

∆𝐸 + ∆𝑝

𝜌+

𝑔

𝑔𝑐∆𝑧 + ∆ [

𝑉2

2𝛼𝑔𝑐

] + ∑𝐹 = 𝑄ℎ − 𝑊𝑠

The equation neglects all shearing stresses. The power supplies in a system originate from a change in

pressure since pressure at section 2 is greater than the pressure at section 1. It also originates from the

change in level, change in kinetic energy and frictional changes.

In this experiment, it is assumed that there are no internal energy changes, no kinetic energy change,

zero heat generation and zero significant change in height. As such, the energy balance changes to

∆𝑝

𝜌+ + ∑𝐹 = − 𝑊𝑠

The actual shaft work done, therefore, is the total work done minus the frictional losses.

𝑊𝑠 = 𝑊𝑇 − ∑𝐹

Definition of terms related to the study

1. Net positive suction Head (NPSH) – this is the difference between the static head at the suction

inlet and the head at the inlet at the vapour pressure.

2. Cavitation- this is the formation of bubbles around the impeller blades at low pressure areas

which move and collapse at high pressure areas. This collapse causes micro-jets orientated

towards the blade at extremely high pressure. This impact causes severe erosion of the impeller

blades in the presence of this phenomena, noise and great vibration will be detected.

3. Efficiency – this is generally the ratio of the work done by the pump against the electrical energy

supplied by the pump.

Results and Tables:

Fundamental Data

Properties of water

Temperature 20°C

Density (ρ) 998.203 kg/m3

Specific weight (w) 9788.379 N/m3

Properties of centrifugal pump

Revolution speed (N) 48.0 rev/s

Difference of the elevation of gauges (HG) 0.290 m

Properties of V-notch

Half angle of V-notch (θ) 45°

Coefficient of discharge (CdV) 0.576

Coefficient (KV) 1.360

Crest level (hook gauge) 0.224 m

Efficiency of motor (ηmo) 0.8

Operation Data

Stage

V-notch Electric board Pressure gauges Gross Head

(H)

m

Actual power

(P)

× 103watts

Efficiency

(ηo)

Specific speed

(Ns)

Reading

m

Head (HV)

m

Discharge (Q)

× 10−3m3

/s

Voltage

(V)

V

Current

(A)

A

Power factor

(Pt) (cos φ)

Input powe

r (PS)

× 103watt

Reading Pressure head

Head differe

nce

(p2 − p1

w)

m

Gauge 1 (p'1) cmHg

kg/cm2

Gauge 2

(p'2) kg/cm2

Gauge 1 (p1/w)

m

Gauge 2 (p2/w)m

1 0.163 0.061 1.250 380 4.0 0.75 1.580 0.30 2.20 3.005 22.040 19.034 19.324 0.236 0.149

7 349.34

2 0.156 0.068 1.640 380 4.0 0.78 1.643 0.28 2.17 2.805 21.739 18.934 19.224 0.309 0.187

8 401.71

3 0.142 0.082 2.619 385 4.2 0.80 1.792 0.25 2.15 2.505 21.539 19.034 19.324 0.495 0.276

3 505.65

4 0.137 0.087 3.036 385 4.3 0.81 1.858 0.24 2.12 2.404 21.238 18.834 19.124 0.568 0.305

9 548.76

5 0.130 0.094 3.684 385 4.4 0.82 1.925 0.21 2.10 2.104 21.038 18.934 19.224 0.693 0.360

2 602.13

6 0.123 0.101 4.409 385 4.6 0.83 2.037 0.17 2.09 1.703 20.938 19.235 19.525 0.843 0.413

7 651.07

7 0.116 0.108 5.213 385 4.8 0.84 2.151 0.12 2.04 1.202 20.437 19.235 19.525 0.996 0.463

2 707.95

8 0.109 0.115 6.099 380 4.9 0.85 2.193 0.02 2.00 0.200 20.036 19.836 20.126 1.202 0.547

9 748.55

9 0.098 0.126 7.664 380 5.3 0.85 2.372 0.00 1.85 0.000 18.533 18.533 18.823 1.412 0.595

3 882.27

10 0.065 0.159 13.710 385 6.5 0.86 2.982 0.00 1.11 0.000 11.120 11.120 11.410 1.531 0.513

5 1717.69

Calculations:

Discharge

Q = KVHV

52

KV =8

15CdV√2g tan θ

Where HV = head above V-notch,

CdV = coefficient of discharge of V-notch,

θ = half angle of V-notch,

KV = coefficient of V-notch.

Sample calculation

Q = KVHV

52

Q = 1.360 ∗ 0.06152

Q=1.250

Input power

The motor in the hydraulics laboratory is a three-phase motor. The power supplied to the shaft of the pump (Ps) is known as follows:

PS = √3AVPtηmo … . (13.3)

Where A = current (Ampere),

V = voltage (volt),

Pt = power factor(= cos φ),

ηmo = efficiency of motor.

Sample calculation

PS = √3 ∗ 4.0 ∗ 380 ∗ 0.75 ∗ 0.8

PS =1.580

Gross head

H =p2

ρg−

p1

ρg+ HG

Sample calculation

H = 22.040 − 3.005 + 0.290

H = 𝟏𝟗.𝟑𝟐𝟒𝐦

Overall efficiency

ηO =ρQgH

PS

× 100(%)

Sample calculation

ηO =998.203 ∗ 1.250 ∗ 9.81 ∗ 19.324

1.580× 100(%)

ηO = 𝟏𝟒.𝟗𝟏%

1) Specific speed

The specific speed of a pump is defined as

NS =NQ

12

H34

NS = 𝟑𝟒𝟗.𝟑𝟒

Discussions

From the results herein obtained and recorded, a graph relaying the characteristic curves of the

pump under study was developed. The curve indicated the peak capabilities of the pump in terms of head and efficiency thereby indicating the performance properties.

Graph obtained from the results is as below.

From this graph, it is correct to indicate that the maximum efficiency of the pump is about76%. The pump is also capable of producing a maximum head of about 24metres

During the design of the pump basis is placed on the power so that the power so that the

functionality of the pump produces the highest power output, which is usually not too far from the range of acceptable efficiencies. This is quite evident in the graph obtained.

y = -0.1132x2 + 0.9751x + 21.842

y = -0.7885x2 + 15.476x - 1.3098

10

30

50

70

90

110

130

150

0

5

10

15

20

25

0 2 4 6 8 10 12 14

Effic

ien

cy (%

)

Head (m

)

Discharge (m3/s)

Perfomance Curves of Pump

Head Input Power Efficiency Poly. (Head) Poly. (Efficiency)

Similarly, specific speed for this pump was determined so as to estimate its peak values too. The graph obtained was as below.

The specific speed obtained for the highest efficiency value is quite low for this size of pump.

This is an indication of the presence of some error in the values obtained.

The peak specific speed obtained from this graph is 0.56

Conclusion

The experiment was generally a success since characteristic curves of the pump were obtained. The graph obtained indicates the peak capabilities of the pump.

However, the values obtained were not specifically accurate and. a such, the graphical representation too.

Errors must have occurred in the system as a result of leakages in the system, friction and erroneous and inaccurate recording and reading of values during the experiment. Possibly, errors

could also have been as a result of incorrect calculations and compilation of otherwise correctly recorded data.

As such, a lot of care was taken during the collection of data and during the compilation of the results so as to reduce the margin of error expected at the final issue of the report.

It is further recommended that the experiment be repeated if more accurate data is required for the study of the performance characteristics of the centrifugal pump. This can be performed

under controlled conditions that ensure little margin of error. Such a condition would include reducing the size of groups involved in the exercise.

y = -401.91x2 + 456.35x - 48.108

0

10

20

30

40

50

60

70

80

90

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Effi

cie

ncy

%

specific speed Ns

graph of 𝜂O vs. Ns

References

1. Kumar, S., Gandhi, B. K., & Mohapatra, S. K. (2014). Performance Characteristics of Centrifugal Slurry Pump with Multi-Sized Particulate Bottom and Fly Ash Mixtures.

Particulate Science and Technology, (just-accepted). 2. Marrero, T. R. Project-based Learning: Centrifugal Pump Operations. 3. Kumar, S., Gandhi, B. K., & Mohapatra, S. K. (2014). Performance Characteristics of

Centrifugal Slurry Pump with Multi-Sized Particulate Bottom and Fly Ash Mixtures. Particulate Science and Technology, (just-accepted).