Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
EXPERIMENT NO. 6
CENTRIFUGAL PUMP CHARACTERISTICS
1. AIM
Study and analysis of centrifugal pump characteristics.
2. Objectives
1. To study the operating characteristics of a centrifugal pump.
2. To study the operating characteristics of two centrifugal pumps operated in series and in
parallel.
3. Theory
Pumps are the fluid moving machineries which increase the mechanical energy of the fluids to
be displaced. The energy increase may be used to increase the velocity, the pressure or the
elevation of the fluids. A large number of pumps, differing widely in principle and mechanical
construction, have been developed to meet a wide variety of operating conditions. For selection
of pumps for a specific application requires the knowledge of operating conditions of the
system and applicability of different available pumps.
The mechanical energy of the liquid is increased by centrifugal action. Centrifugal pumps
are classified as single suction and double suction pumps depending upon the suction from
either one side or from both sides respectively.
In a single suction centrifugal pump the liquid enters through a suction connection concentric
with the axis of a high-speed rotary element called the impeller, which carries radial vanes
integrally cast in it. Liquid flows outward in the spaces between the vanes and leaves the
impeller at a considerably greater velocity with respect to the ground than at the entrance to the
impeller. In a properly functioning pump the space between the vanes is completely filled with
liquid flowing without cavitation. The liquid leaving the outer periphery of the impeller is
collected in a spiral casing called the volute and leaves the pump through a tangential discharge
connection. In the volute, the velocity head of the liquid from the impeller is converted into
pressure head. The power is supplied to the fluid by the impeller and is transmitted to the
impeller by the torque of the drive shaft, which usually is driven by a direct-connected motor
at constant speed, commonly at 1750 rpm. Another common type uses a double-suction
impeller, which accepts liquid from both sides. Also, the impeller itself may be a simple open
spider, or it may be enclosed or shrouded.
4. Experimental Set up
4.1. Requirements
Centrifugal pump test rig, bucket, watch.
4.2. Schematic Diagram of Experimental Setup
The schematic diagram of the experimental setup is shown below:
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Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
P2
V2 P4
V8
V4
V5
P1 C1 V3 C2
V1
P3
V-Notch
Measuring Tank
Sump Tank
OPERATION OPEN CLOSE
SINGLE PUMP : V1, V4, V5 V2, V3, V8, V9
SERIES : V1, V2, V5 V3, V4, V8, V9
PARALLEL : V1, V3, V4, V5 V2,V8,V9
5. Experimental Procedure
First, prime the pump by pouring water through valve V8. Subsequently, the following
sequence of operations are to be carried out:
5.1. Calibration of V-Notch
1. Measure the width of the V-notch at the top and depth of the V-notch.
2. Fill the storage tank with water.
3. Open valves V1, V2 and V5, and close valves V3, V4 & V9.
4. Start running the pumps.
5. Fill the channel with water until water starts spilling over the notch to the outlet.
6. Stop the water supply by closing the bench supply valve (V5) with the pump in running
condition.
7. Allow the water above the crest height to spill over the notch.
8. When the water level is at the level of the crest of the notch, bring the point gauge exactly
to the water surface, and note the reading (say R1) on the longer scale which coincides with
the “zero” of the shorter scale. (This is the datum level for subsequent height-measurements
when the water flows in the channel.)
9. Open valve V5 completely and wait for the level (or head) of water in the channel to
stabilize.
10. Bring the point gauge to the surface of the water by rotating the knob attached to the scale,
and note the reading (say R2) on the longer scale that coincides with the “zero” of the shorter
scale.
11. Collect the water in the bucket for an arbitrary but known duration, and determine the flow
rate of water by dividing the volume (or mass) of water collected by the time of collection.
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Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
12. The difference (R2-R1) is called the head which is related to the volumetric flow rate by the
following equation:
5
8
Q C d tan 2 g h 2 ; Where, h = R2-R1
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13. Adjust the water flow rate at seven different values from the maximum to zero flow rate
in the channel by manipulating valve V5 and determine the water flow rate and the
corresponding head.
14. Calculate the discharge coefficient (Cd) in each case, and take the arithmetic average of
these values as the Cd of the notch.
5.2. Pump Characteristics (Single Pump)
1. After carrying out the steps 1 to 3 mentioned in “Calibration of V-Notch”, open the valve
V5 fully.
2. Note the RPM of the pump from the control panel.
3. Note the gage pressure on the outlet line (P4 in the figure) and on the inlet line (P1 in the
figure). The difference of the readings in P4 and P1 is the head delivered by the pump.
4. Note the power supplied to the pump from the display on the control panel by pressing the
appropriate button (labeled W).
5. Note the head of the V-notch.
6. Repeat steps 2 to 5 for seven different flow rates of water spanning the whole range of flow
rate up to zero flow by manipulating V5.
5.3. Pump Characteristics (Pump in Series)
1. Fully open the valves V1, V2 and V5, and close valves V3 and V4.
2. Note the RPM of the pump from the control panel.
3. Note the gage pressure on the outlet line (P4 in the figure) and on the inlet line (P1 in the
figure).
4. Note the power supplied to the pump from the display on the control panel by pressing the
appropriate button (labeled W).
5. Note the head of the V-notch.
6. Repeat steps 2 to 5 for seven different flow rates of water spanning the whole range of flow
rate up to zero flow by manipulating V5.
7. Repeat steps 1 to 6 for three different RPMs.
5.4. Pump Characteristics (Pump in Parallel)
1. Fully open the valves V1, V3, V4 and V5, and close valve V2.
2. Note the RPM of the pump from the control panel.
3. Note the gage pressure on the outlet line (P4 in the figure) and on the inlet line (P1 in the
figure).
4. Note the power supplied to the pump from the display on the control panel by pressing the
appropriate button (labeled W).
5. Note the head of the V-notch.
6. Repeat steps 2 to 5 for seven different flow rates of water spanning the whole range of flow
rate up to zero flow by manipulating V5.
7. Repeat steps 1 to 6 for three different RPMs.
6. Observations
6.1. Calibration of V-Notch
Width across the top of the V-notch =
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Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
Depth of the V-notch =
Initial point gauge reading at zero flow rate (R1) =
Head of
Point gauge Amount of Duration of
S No the V-notch
reading, R2 Water collected collecting water
(R2 – R1)
6.2 Pump Characteristics*
1. RPM =
2. Initial point gauge reading at zero flow rate (R1) =
3. Length of Pipe in single pump operation =
4. Length of Pipe in series operation =
5. Length of Pipe in parallel operation =
6. Elevation =
7. Diameter of the suction pipe =
8. Diameter of the discharge pipe =
Point gauge Head of Gauge pressure reading Power to
S No
Reading, R2 The V-notch P1 P4 the pump
* Make separate tables for different configurations (single/series/parallel) of the pumps and for different RPMs.
7. Model Calculations
For steady incompressible flow of a liquid of density , the developed head is given as
P2 P1 V22 V12
H ( Z 2 Z1 ) h f (1)
g 2g
where subscripts 1 and 2 signify the values at the suction and delivery ports of the pump; P is
the pressure; V is the velocity; Z is the elevation; and hf is the friction head (that is, the frictional
losses) in the line between the suction and delivery ports. hf is given by
V
2
L
hf 4 f Kf (2)
D 2g
where f is the friction factor, L is length of the pipe, D is the diameter of the pipe, Kf is the loss
factor for fittings. Friction factor (f) depends on roughness and type of flow (laminar/turbulent)
and read from friction factor chart. Alternatively, it can be calculated from the equations:
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f ; for laminar flow
N Re
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Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
The friction factor for turbulent flow can be calculated by using Colebrook equations (either
implicit form or explicit form) given below:
7.1. Implicit Forms of Colebrook Equation
There are at least three forms of the Colebrook Equation that can be found in current
literature on hydraulics. These are:
2 log10
1 2.51
f 3.7 D N Re f (3)
2
1.74 2 log10
1 18.7
f D N Re f (4)
1 D 9.3
1.14 2 log10 2 log101
f (5)
N Re f
D
where,
f is the Friction Factor and is dimensionless
ε is the Absolute Roughness and is in units of length
D is the Inside Diameter and, as these formulas are written, is in the same units as ε.
NRe is the Reynolds Number and is dimensionless.
Note that ε/D is the Relative Roughness and is dimensionless.
These three equations are referred to as “Implicit” Equations. “Implicit” means that “f”, the
Friction Factor, is “Implied or understood though not directly expressed”2. Simply stated, the
equations ARE NOT in the form of “f = ………”. These are sometimes referred to as
“equivalent” but the results will vary when calculated to the fourth significant digit.
These equations can be solved for “f” given the Relative Roughness (ε/D) and the Reynolds
Number, (NRe), by iteration. Such iterations can be performed using an electronic spreadsheet.
A spreadsheet, “Friction Factor Formulas for Cheresource.xls” is available at
http://www.cheresources.com/colebrook1.shtml presented for demonstration. The spreadsheet
contains four worksheets. The first “Tab” is labeled “Iterations”. The Iterative solutions are
generated by breaking the formulas in two parts, that which is left of the equal sign and that
which is right of the equal sign (See row 20 as an example). The Iteration then tests values of
“f” that will result in the difference between the two sides to be zero or very close to zero. (A
complete explanation was published in the ASHRAE Journal of September, 2002: see
Reference 4 for details)
7.2. Explicit Forms of Colebrook Equation
There are four Explicit forms of Colebrook Equations reported in literature. (Ref: available on
internet at http://www.cheresources.com/colebrook1.shtml). Out of these the Serghide’s
solution is the most accurate one, which is given below:
Serghide’s Solution (see Reference 3 for details).
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Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
f ( A ( B A) 2 /(C (2 B) A))2 (6)
where
A 2 log10[( / 3.7D) (12 / N Re )]
B 2 log10[( / 3.7D) (2.51A / N Re )]
C 2 log10[( / 3.7D) (2.51B / NRe)]
Serghide can be used across the entire range of the Moody Diagram. Its accuracy is
unparalleled amongst the Explicit Equations evaluated here. It appears to be based on Eqn. 3,
as do all the Explicit Equations presented. There is less deviation between Serghide and Eqn.
3 then there is between Eqn. 3 and either Eqn. 4 or Eqn. 5.
1
f
4.07 log N Re
f 0.6 ; (7)
Eqn. 7 is the von Karman equation for turbulent flow which can be used only if the pipe is
smooth. In our case, the pipe is not smooth and hence this equation cannot be used. Colebrook
equation has to be used.
Power developed by the pump (or the power delivered to the fluid) Pf is given as
Pf QH g (8)
where Q is the volumetric flow rate of the liquid.
The mechanical efficiency of the pump is given as
Pf
(9)
PB
where PB is the total power supplied to the pump drive from an external source.
Operating (or performance) characteristics of a pump is commonly illustrated by plots of actual
head developed H, power consumption Pf, and efficiency versus the volumetric flow rate.
8. Results & Discussion
8.1. Calibration of V-notch
S No Volumetric flow rate Head of the V-notch Cd
Average Cd =
8.2. Pump Characteristics (RPM = )
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Chemical Engineering Department, Birla Institute of Technology & Science Pilani, Pilani Campus
Volumetric flow Head developed by Power delivered
S No Efficiency
rate the pump by the pump
1. Plot head, power delivered to the pump, power delivered to the liquid, and efficiency
versus the capacity of the pump (i.e., flow rate of water).
2. Suggest the operating point from the efficiency versus flow rate plot.
9. Conclusions
10. Precautions
Do not run the pump with the delivery valve closed for a long time to avoid damage of the
pump.
References
1. Warren, L McCabe, Smith, J C , and Harriott, P, Unit Operations of Chemical Engineering,
6th edition, McGraw Hill, New Delhi, India, 2000.
2. Babu, B.V.,"Pumps: Selection & Trouble Shooting", IPT (Indian Plumbing Today), Vol.
2005 (No. 6), pp. 41-49, November-December, 2005.
3. T.K.Serghide’s implementation of Steffenson’s accelerated convergence technique,
reportedly to have appeared in Chemical Engineering March 5, 1984.
4. Lester, T. “Calculating Pressure Drops in Piping Systems.” ASHRAE Journal Sept. 2002.
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