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Kabul University Water Pumps
Faculty of Engineering
Civil Engineering Department What is a pump?
• It is the heart of a hydraulic system
• It converts mechanical energy into
Hydraulics – CE 353
hydraulic energy.
• The mechanical energy is delivered to
the pump via a prime mover such as an
electric motor.
Chapter 5 Part I
Types Turbo-hydraulic Pumps
• Turbo-hydraulic Pumps (pumps depending on dynamic • Centrifugal (Radial flow) Pumps
forces)
• Hydrostatic or Positive-displacement Pumps • Propeller (Axial Flow) Pumps
• jet (Mixed-Flow) Pumps
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Centrifugal (Radial flow) Pumps Centrifugal (Radial flow) Pumps
The fundamental principle of the centrifugal pump Modern centrifugal pumps basically consist
of two parts:
was first demonstrated by Demour in 1730.
The rotating element, which is commonly called the impeller, and a
shaft.
The housing, which encloses the rotating element and seals the
pressurized liquid inside.
Centrifugal (Radial flow) Pumps Centrifugal (Radial flow) Pumps
The theory of centrifugal pumps is based on the principle of angular The angular momentum (or momeni: of momentum) with respect to a
momentum conservation. fixed axis of rotation can thus be defined as the moment of the linear
Physically, the term momentum, which usually refers to linear momentum with respect to the axis:
momentum, is defined as the product of a mass and its velocity, or
angular momentum = (radius) (momentum)
momentum = (mass) (velocity) = (radius) (mass) (velocity)
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Centrifugal (Radial flow) Pumps Centrifugal (Radial flow) Pumps
The principle of conservation of angular momentum requires that the The angular momentum (or moment of momentum) for a small fluid
time rate of change of angular momentum in a body of fluid be equal to mass per unit time (ρdQ) is
the torque resulting from the external force acting on the body. This
relationship may be expressed as ֍
Centrifugal (Radial flow) Pumps Centrifugal (Radial flow) Pumps
For the total fluid mass that enters the pump per unit time
֍
simplified to
The torque applied to a pump impeller must equal the difference of
angular momentum at the inlet and outlet of the impeller
The power input to the pump (Pi in bold to differentiate it from pressure,
P) can be computed as
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Centrifugal (Radial flow) Pumps Centrifugal (Radial flow) Pumps
The output power of a pump is usually An alternative way of determining Hp
expressed in terms of the pump when a pump operates between two
discharge and the total energy head reservoirs, the pump output power
that the pump imparts to the liquid may be expressed as
(Hp).
We can see that the total energy head
that the pump imparts to the liquid is
The polar vector is generally used in analyzing the vane geometry and its
relationship to the flow. The efficiency of a centrifugal pump
depends largely on the particular
design of the vane blades and the
pump housing.
also on the conditions under which
the pump operates.
The efficiency of a pump is defined by
the ratio of the output power to the
input power of the pump:
Theoretically, the energy loss at the inlet reaches its minimum
value when water enters the impeller without whirl.
This is achieved when the impeller is operated at such a speed
that the absolute water velocity at the inlet is in the radial
direction.
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The efficiency of a centrifugal pump The efficiency of a centrifugal pump
A hydraulic pump is usually driven The overall efficiency of the pump
by a motor. system is thus
The efficiency of the motor is
defined as the ratio of the power
applied to the pump by the motor
(Pi) to the power input to the motor
(Pm):
or
The efficiency of a centrifugal pump The efficiency of a centrifugal pump
Efficiency values are always less the total energy head at the
than unity because of friction and entrance to the pump
other energy losses that occur in
the system.
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The efficiency of a centrifugal pump The efficiency of a centrifugal pump
the total energy head at the The difference between the two is
discharge location the amount of energy that the pump
imparts to the liquid:
Example5.1
A centrifugal pump has the following characteristics: ri
= 12 cm, r0 = 40 cm βi = 118°, β0 = 140°. The width of
the impeller vanes is 10 cm and is uniform throughout.
At the angular speed of 550 rpm, the pump delivers 0.98
ri = 12 cm, r0 = 40 cm βi = 118°, β0 = 140°
m3/s of water between two reservoirs with a 25-m
elevation difference. If a 500-kW motor is used to drive • impeller vanes width = 10 cm and is
the centrifugal pump, what is the efficiency of the pump uniform throughout
• angular speed = 550 rpm
and the overall efficiency of the system at this stage of 25-m 0.98 m3/s
operation?
500-kW motor
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Solution Solution
Tangential velocity
Radial velocity
Solution Solution
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Propeller (Axial Flow) Pumps Propeller (Axial Flow) Pumps
• A rigorous mathematical analysis for designing • Linear impulse is defined as the integral of the product of
propellers based on the energy-momentum relationship the force and the time from t‘ to t’’, during which the force
is not available. acts on the body:
• However, the application of the basic principle of impulse
momentum provides a simple means of describing their
operation. • If a constant force is involved during the time period, T,
then the impulse may be simplified to
Propeller (Axial Flow) Pumps
The principle of impulse momentum requires that the linear
impulse of a force (or force system) acting on a body during
a time interval be equal to the change in linear momentum
in the body during that time.
or The factor, (mass)/(time), can be expressed as the mass involved per
unit time (i.e., mass flow rate) or
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The velocity change is therefore the change of the fluid between the Applying the impulse momentum relationship between sections 1 and
two ends of the control volume: 4:
F is the force exerted on the fluid by the propeller and the right-hand
side drops out when the pump is installed in a flow conduit of uniform
diameter
Ignoring losses and the Bernoulli principle between sections 1 and 2
and sections 3 and 4 results in
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Propeller (Axial Flow) Pumps Example 5.2
Propeller pumps are generally used for low- A 10-ft-diameter propeller pump is installed
head (under 12m), high-capacity (above 20 L/ to deliver a large quantity of water between
s) applications. two reservoirs with a water surface elevation
However, more than one set of propeller difference of 8.5 ft. The shaft power supplied
blades may be mounted on the same axis of to the pump is 2,000 hp. The pump operates
rotation in a common housing to form a at 80-percent efficiency. Determine the
multistage propeller pump. discharge rate and the pressure just
upstream of the pump if the pressure just
downstream is 12 psi. Assume the pipe size
remains uniform throughout.
Solution Solution
The energy imparted to the flow by the pump is
And
[1 hp = 1.98E6 ft-lb/s]
Assuming that friction losses are negligible for this short pipe
Solving the above equation yields
For Ke = 0.5 (entrance coefficient) and Kd = l.0 (exit coefficient), we
have
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Centrifugal Pump Characteristic Curves Centrifugal Pump Characteristic Curves
• Pump characteristic curves (or performance curves) • The pump head is the energy head added to the flow by
which are produced and supplied by reputable the pump.
manufacturers, are graphical representations of a • The brake horsepower is the power input required by the
pump's expected operational performance. pump in power units
• Generally display the variation of the pump head, the • The efficiency is the ratio of the power output to the
brake horsepower, and the efficiency with the pump's power input
generated flow rate.
Centrifugal Pump Characteristic Curves shutoff head
• The pump head at zero discharge is called the shutoff
head. rated capacity
• The discharge corresponding to the maximum efficiency
is called the rated capacity.
• The characteristics of a given pump vary with the
rotational speed.
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Centrifugal Pump Characteristic Curves
• However, if the characteristics are known for one
rotational speed, then the characteristics for any other
rotational speed with the same impeller size can be
obtained using the affinity laws.
Single Pump and Pipeline Analysis Single Pump and Pipeline Analysis
• A single pump placed in a pipeline to move water from To analyze this system for the flow rate, neglecting the
one reservoir to another reservoir or to a demand point minor losses, we can write the energy equation as
represents the most common pump-application scenario.
• Determining the flow rate that is produced in these
pump-pipeline systems requires knowledge of both
pump operation and pipeline hydraulics.
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Example 5.3 Solution
Consider the pump-pipeline system shown below. The
reservoir water surface elevations are known: EA = 100 ft
and EB = 220 ft. The 2.0-ft-diameter pipe connecting the
two reservoirs has a length of 12,800 ft. and a Hazen-
Williams coefficient ( CHW) of 100.
a.The pump characteristics are known (columns l and 2 in
the following table) and are plotted in Figure 5.10 (a).
Determine the discharge in the pipeline, the velocity of flow,
and the energy grade line.
b.Suppose the pump characteristics given in part (a) are at
a rotational speed of 2,000 rpm. Determine the discharge in
the pipeline and the pump head if the pump runs at 2, 200
rpm.
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Solution
Pumps in Parallel or in Series Pumps in Parallel or in Series
Manifold
• The optimum efficiency of a pump can be
Qtotal
obtained only over a limited range of operation
Qtotal =Q1+Q2+Q3 (i.e., discharges and total heads).
Pump Pump
• Therefore, it is often advantageous to install
Pump
several pumps in parallel or series
configurations in pumping stations to efficiently
Q1 Q2 Q3
operate over a broad range of expected flow
Pump Pump Pump rates and required system heads.
Q
Q Htotal =H1+H2+H3
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Pumps in Parallel or in Series
• When two pumps are installed in parallel, neglecting the
minor losses in their resident branch lines, the energy
head added to the flow by the two pumps must be the
same to satisfy the energy equation of the resident
pipeline system. Manifold
Qtotal
Qtotal =Q1+Q2+Q3
Pump Pump Pump
Q1 Q2 Q3
