5. CENTRIFUGAL PUMP
Objectives:
a)
To determine the characteristics of a centrifugal pump including total head,
efficiency and NPSH versus flowrate.
b)
To determine the size of a geometrically similar pump that would be needed to
against a total head of 100 feet of water at peak efficiency using the same RPM.
power,
pump
Introduction:
Centrifugal pumps are the most common type of fluid mover in the chemical industry. A
fundamental understanding of the operation and performance of a centrifugal pump is of primary
importance to any engineering student.
A centrifugal pump converts 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 pumped.
The energy changes occur by virtue of two main parts of the pump, the impeller and the volute
or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy.
The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.
All of the forms of energy involved in a liquid flow system are expressed in terms of feet of
liquid i.e. head.
The process liquid enters the suction nozzle and then into the eye (center) of an impeller.
When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward
and provides centrifugal acceleration. As liquid leaves the eye of the impeller, a low-pressure
area is created causing more liquid to flow toward the inlet. Because the impeller blades are
curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This
force acting inside the pump is the same one that keeps water inside a bucket that is rotating at
the end of a string.
The key idea is that the energy created by the centrifugal force is kinetic energy. The
amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the
impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be
the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid. This
kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the
flow. The first resistance is created by the pump volute (casing) that catches the liquid and
slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is
converted to pressure according to Bernoulli’s principle.
Theory:
If we consider the inlet and discharge of the pump under test as the boundaries of a
control volume then we may apply Bernoulli's Theorem of continuity to the fluid within that
boundary (Armfield, 1980).
The head generated by the machine is:
Machine Head = g ΔH
J/kg
(1)
where ΔH is the pump differential head (m) and g is gravitational acceleration 9.807 m/s2.
Hydraulic power:
The hydraulic power of the pump is the product of machine head and flow, thus hydraulic
Power Nh,
Nh = g•Q• ΔH•ρwater
W
(2)
where Q is the flowrate (m3/s) and ρwater is the density of water kg/ m3
Power Input to Pump:
The dynamometer output power (brake horsepower) No is given by:
No=T*n
W
where, T = dynamometer torque
n = dynamometer rotational speed
60
nm = n * 2Π
2Π
n = nm * 60
(3)
N-m
rad/s
RPM
(4)
rad/s
(5)
W
(6)
Substituting in equation (3):
2Π
No=T * nm* 60
The power absorbed by the pump therefore, is the dynamometer output less transmission losses,
thus:
N p = No- NL
W
(7)
NL represents the transmission losses between the pump and the dynamometer motor and is the
power absorbed by bearing friction, air drag, etc. The value of the power
loss will vary between rigs and on the same rig will vary with motor speed.
The efficiency of the pump:
η=
Nh
No
× 100%
(8)
Pump Differential Head:
The measurement of pump differential head is effected by means of the two Bourdon
type pressure gauges.
It should be noted that the suction and discharge pipes are of different nominal bores thus
generating a velocity head across the pump which must be accounted for when measuring the
differential head.
The differential head can be calculated:
Pd − Ps Vd2 Vs2
∆H =
+[ − ]+ Z
ρ water g 2 g 2 g
m
(9)
where Ps is the pressure at the inlet of the pump; Pd is the pressure at the outlet of the pump; and
Z is the vertical difference between the inlet and outlet. Vs is the velocity at the inlet and Vd is
the velocity at discharge (m/s).
From a mass balance:
D4
V 2 = Vs2 s
d
D4
d
m2 / s2
(10)
Pd − Ps Vs2 Ds4
∆H =
+ [ − 1]
ρ water g 2 g Dd4
m
(11)
So,
In the case where:
Suction pipe NB, Ds=2.0"
Discharge pipe NB, Dd=1.5"
Since:
Q2
2
Vs =
(m/s)2
(12)
As2
where Q is the flowrate (m3/s) and As is the cross section area of the inlet pipe (m2).
then:
Pd − Ps
Q 2 Ds4
[
∆H =
+
− 1]
ρ water g 2 gAs2 Dd4
m
(13)
Net Positive Suction Head (NPSH):
The net positive suction head is the equivalent total head of liquid at the inlet of the pump
(suction) (Hs) minus the vapour pressure p.
NPSH = H s −
p
ρg
(14)
Where:
Vs2
Hs =
+
ρ water g 2 g
Ps
(15)
Pump Discharge
1. The basic method of measuring the pump discharge on the test rig is by means of the
volumetric measuring tank. The discharge is directed into the tank for a known period of time
and the rise in water level during that period noted, then:
Aδd
Q=
m3/s
(16)
t
where A = area of measuring tank, m2
δd = change in water level in tan, m
t = time, s
2. Venturi:
The pump discharge may be measured by means of the perspex venturi tube after the tube
has been calibrated. The venturi is being used in conjunction with a Dwyer Differential Pressure
transmitter.
The venturi demonstrates the principle of Bernoulli's continuity equation, thus flowrate Q
is related to the difference in pressure across the Venturi meter,
Q = CA2
2(− ∆P )ρ water
1− β 4
m3/s
(17)
where A2 is the cross-sectional area of the throat of the Venturi, C is the Venturi coefficient, and
β is the ratio of throat diameter to inside pipe diameter (pump outlet pipe diameter for the case
being studied).
In the case of an actual venturi, small losses occur due to viscous shear and friction effects, thus
reducing the theoretical flow through the device into Equation (17). A calibration curve for a
particular venturi tube will therefore show curves of theoretical discharge, predicted by the
equation, and actual discharge determined by
volumetric measurement.
Nomenclature:
A
As
D
H
Hatm
Hgs
Hgd
Hs
Hvs
ΔH
L
n
N
p
Q
t
T
V
W
δd
Suffix:p
o
L
h
s
d
1, 2
Constants:
g
ρwater
ρm
area of measuring tank m2.
cross section area of the inlet pipe, m2.
diameter of pipe, m
head, m
the barometer reading, m.
the reading of a gauge at the inlet of the pump, m.
the reading of a gauge at the outlet of the pump, m
the equivalent total head of liquid at the inlet of the pump
(suction), m
the velocity head at the inlet, m.
pump differential head, m.
length of dynamometer torque arm, m.
rotational speed rad/s.
power, w
the vapor pressure, mmHg
flowrate m3 /s
time, s.
torque kg-m
velocity. m/s.
weight applied to torque arm, Kg
change in water level in tank, m
pump input
dynamometer motor output
dynamometer transmission losses
hydraulic output
inlet (suction)
discharge
differential manometer limbs
Gravitational acceleration = 9.807 m/s2
density of water, 103 kg/ m3
density of mercury, 13.57 x 103 kg/m3
Apparatus:
The centrifugal pump used in this experiment is the Armfield R2-00. The pump
is of cast iron construction and is provided with an open impeller. On the pump cover plate
tappings are provided at various radii so that the increase in pressure across the impeller may be
determined. These tappings are brought to a manifold with valves for pressure sampling as
required.
The pump is driven by a trunnion mounted variable speed 1.6 kW DC motor. The pump
set is mounted on a substantial bed plate. The equipment includes a combined
transformer/rectifier and speed controller.
The rig includes the tanks necessary for carrying out performance testing. The main
reservoir is approximately 1.36m x 0.66 m x 0.53 m fabricated in G.R P. and fitted with a drain
valve. On this tank is mounted the volumetric measuring tank which incorporate a level indicator
and scale. A quick acting drain valve is provided together with an emergency overflow. A
manually operated diverter is included so that water discharged by the pump can be returned
either directly to the sump or to the measuring tank as required. To carry out flow measurement
it is necessary for a stop watch to be used. This system allows level measurements to be taken in
still water and, hence, increases the accuracy of flow measurement.
The pump suction pipe is fabricated in PVC with pressure tapping. The pump delivery
pipe work incorporates a gate type throttle valve. Pressure and suction electronic indicators are
supplied complete with small bore pipe work and valves to allow multiple pressure readings.
A perspex Venturi has been upgraded and now runs with pressure transmitters and
indicators. This Venturi is modeled on the requirements of B.S. 1042 Part 1- 1964 having a
nominal bore of 1.5" and a throat diameter of 1.28". The Venturi operates in conjunction with a
25 psi Dwyer differential transmitter and Omega DP32 indicator. This instrument allows pump
flows up to 60 GPM (5 L/sec.) to be determined, after the instrument has been calibrated.
A 50 psi Differential Pressure transmitter is also available. This instrument allows the
differential heads developed by the pump up to 30 ft to be determined. Tappings are provided on
the pump and the supply includes all necessary fittings and connecting flexible tube.
Specification:
Inlet pipe diameter
Outlet pipe diameter
Venturi throat diameter
Impeller outside diameter
Blade width
Number of blades
Blade type
Impeller type
Radius of strain gauge
2.0"
1.5"
1.28"
127 mm
11.4 mm
6
Backward curving
Open
1144.2 mm
Shaft Speed
Rating
Motor type
Electrical Supply
0 - 3000 RPM
1.6 kW at 2900 RPM.
Variable speed
220V/single phase/50-60 Hz
Relationship between Torque and voltage for strain gauge (when using x 10
amplification):
Torque=1.5861*Volts*g*0.1442
Procedure:
Start UP Procedure
a) Be sure suction side and discharge side valves are closed.
b) Turn on Main Power.
c) Turn on priming pump and slightly open discharge valve.
d) Adjust pump speed to approximately 15%.
e) Open suction side valve SLOWLY. Repeat as necessary.
f) Open discharge side valve SLOWLY.
g) Turn the Venturi Drain valve until line is drained of air.
h) Turn the Pressure Guage Drain(s) to Vent until the line is drained of air, and then
turn the valve to the right until suction lines are airless. Then turn valve to Suction so
the line is static.
Shut Down Procedure
a)
b)
c)
d)
Close discharge side valve.
Close suction side valve.
Reduce motor speed to 0 RPM using controller.
Switch motor off.
Experimental Procedure
a) Calibrate the venturi meter by making at least 8 runs from a low flowrate to a high
flowrate. The venturi meter is calibrated using the measuring tank and stopwatch.
b) At 8 or more discharge rates collect the data necessary to characterize the
pump
including the pressures across the pump, venturi pressure drop, motor
rotating speed and the
Torque Gauge Reading. The safety bar should always be inserted when force measurements
are not being made.
It is removed when making force measurements and the motor arm is SLOWLY
allowed to touch the strain gauge mechanism.
Report:
1) To determine various characteristics and parameters of a centrifugal pump.
include graphs of total pump differential head, hydraulic power, brake
horse
efficiency and Net positive suction power versus discharge flowrates.
These
power,
2) To determine the size of a geometrically similar pump that would be needed to pump
against a total head of 100 feet of water at peak efficiency using the same
RPM. What
flowrate is generated by the big pump at this condition? If energy
costs 10.2 cents/kwhr, how much does it cost to operate the big pump each year?
References:
Armfield Technical Education Co. Ltd., “Instructional Manual for Centrifugal Pump Test Rig
R2-00”, 1980.
Other references related to this lab:
Perry, R. H., Green, D. W. and Maloney J. O., “Perry’s Chemical Engineering Handbook”,
McGraw-Hill, 1997.
Sulzer Pump Division, Sulzer Brothers Ltd., “Sulzer centrifugal pump handbook”, Elsevier
Applied Sicence, London and New York, 1989.
Lobanoff, V. S. and Robert, R. R., “Centrifugal Pumps – Design & Application”, Gulf
Publishing Company, Houston, 1985.
Karassik, I. J., “Centrifugal Pump Clinic”, Dekker, New York, 1989.
Brown, G. G., “Unit Operation”, Wiley, New York, 1950.
Coulson, J.M. and Richardson, J. F., ‘Chemical Engineering” Vol. 1. 3rd Edition, p.133-144,
1977, (TP145C45).