Experiment No. 10
CENTRIFUGAL PUMP TEST RIG
(VARIABLE SPEED WITH D.C. MOTOR)
Aim:
1) To determine:
● Pump input
● Shaft power
● Discharge
● Total head
● Pump output
● Overall efficiency
2) To plot the following performance characteristics:
● Head v/s discharge
● Pump efficiency v/s discharge
Introduction:
The hydraulic machines, which convert the mechanical energy into hydraulic energy,
are called pumps. The hydraulic energy is in the form of pressure energy. If the
mechanical energy is converted into pressure energy by means of centrifugal force
acting on the fluid, the hydraulic machine is called centrifugal pump.
Theory:
Centrifugal pump is a mechanical device, which consists of a body, impeller and a
rotating mean i.e. motor, engine etc. Impeller rotates in a stationary body and sucks
the fluid through its axes and delivers through its periphery. Impeller has an inlet
angle, outlet angle and peripheral speed, which affect the head and discharge.
Impeller is rotated by motor or IC engine or any other device.
The centrifugal pump acts as a reversed of an inward radial flow reaction turbine.
This means that the flow in centrifugal pump works on the principle of forced vortex
flow, which means that an external torque rotates a certain mass of liquid, the rise in
pressure head of rotating liquid takes place. The rise in pressure head at any point of
the rotating liquid is proportional to the square of tangential velocity (i.e. rise in
pressure head = V2/2g or ω2r2/2g) of liquid at that point. Thus at the outlet of the
impeller where radius is more, the rise in pressure head will be more and the liquid
will be discharged at the outlet with high pressure head. Due to this high pressure
head, the liquid can be lifted to a high level.
Experimental Setup:
Centrifugal pump test rig consists of a sump tank, a centrifugal pump, a DC motor
and a measuring tank. To measure the head, pressure and vacuum gauges are
provided. To measure the discharge a measuring tank is provided. Flow diversion
system is provided to divert flow from sump tank to measuring tank and from
measuring tank to sump tank. A valve is provided in pipeline to change the rate of
flow.
Fig.1: Block Diagram of centrifugal pump test rig apparatus
Requirements:
1) Electricity supply
2) Water supply
3) Drain required
Procedure:
1. Clean the apparatus and make tanks free from dust.
2. Close the drain valves provided.
3. Fill sump tank 3/4 with clean water and ensure that no foreign particles are there.
4. Open flow control valve given on the water discharge line and control valve given
on suction line.
5. Ensure that all ON/OFF switches given on the panel are at OFF position.
6. Set the desired RPM of motor/pump with the speed control knob provided at the
control panel.
7. Operate the flow control valve to regulate the flow of water discharged by the
pump.
8. Operate the control valve to regulate the suction of the pump.
9. Record discharge pressure by means of vacuum gauge, provided on discharge
line.
10. Record suction pressure by means of vacuum gauge, provided at suction line.
11. Record the power consumption by means of energy meter, provided in panel with
the help of stop watch.
12. Measure the discharge by using measuring tank and stop watch.
13. Repeat the same procedure for different speeds of pump.
14. Repeat the same procedure for different discharge with constant speed.
Precautions:
1) Never run the apparatus if power supply is less than 180 volts and above 230
volts.
2) Never fully close the delivery line and bypass line valves simultaneously.
3) Always use clean water.
4) Always keep apparatus free from dust
Observation:
Data:
Energy meter constant (EMC) = 3200 Pulses/kW hr
Area of measuring tank (A) = 0.128 m2
(ρ) = 1000 kg/m2
Density of fluid
Motor efficiency (ηm) = 0.8(assumed)
Acceleration due to gravity (g) = 9.81m/s2
Height of pressure gauge from vacuum gauge (hpg) = 1 m
Observation Table:
S.
No.
N
(RPM)
Pd,
(kg/cm2)
PS,
(mmHg)
R1
(cm)
R2
(cm)
Here,
N = Speed of pump
Pd = Delivery pressure
Ps = Suction pressure
R1 = Final level of water in measuring tank
R2 = Initial level of water in measuring tank
t = Time taken to fill measuring tank (R)
tp = Time taken by pulses of energy meter (P)
Calculations:
Pump input
Ei =
𝑃
𝑡𝑃
×
3600
𝐸𝑀𝐶
(kW)
Here P is Pulses of energy meter
Shaft output ES
= 𝐸𝑖 × η𝑚 (𝑘𝑊)
Rise of water level in measuring tank
Discharge Q =
𝐴×𝑅
𝑡
(m3/sec)
R=
𝑅2−𝑅1
100
(𝑚)
t
(sec)
tp
(sec)
Total head
Pump output
H = 10×⎡⎢𝑃𝑑 +
⎣
E0 =
𝑃𝑆
760
ρ×𝑔×𝑄×𝐻
1000
Overall efficiency η0 =
Pump efficiency ηp =
𝐸0
𝐸𝑖
𝐸0
𝐸𝑠
⎤+ ℎ
⎥
𝑝𝑔
⎦
(m of water)
(kW)
×100%
×100%
Results:
Plot head vs. discharge & pump efficiency vs. discharge.
Sources of errors:
Report the possible sources of error found in the experiment and the observations.
Discuss exactly how these affect the experimental data (i.e. will the readings be
larger or will be they be smaller due to the presence of a particular source).
Discussion:
1) Discuss the factors which affects the performance of pump.
2) What is cavitation and net positive suction head (NPSH).
3) State the affinity laws as applicable to centrifugal pump
References:
1) R.S Khurmi,” Hydraulics, Fluid Mechanics and Hydraulic machines”, 11th ed.,
S. Chand & Company Ltd., ND, 2008, Page 582-585, 587, 590.
2) Y.A.Cengel, J.M. Cimbala,”Fluid Mechanics”, 2nd ed., Tata McGraw-Hill, ND,
2007, page 180-182.