62187 Ch5 Water Pumps-I-Hwang

Report
Water Pumps
Water pumps are devices designed to
convert mechanical energy to hydraulic
energy. All forms of water pumps may be
classified into two basic categories:
 turbo-hydraulic
pumps,
 positive-displacement pumps.
 Turbo-hydraulic pumps are:
 centrifugal
 Propeller
 and
pumps,
pumps,
jet pumps.
 Analysis of turbo-hydraulic pumps:
is a problem involving fundamental principles
of hydraulics.
 Positive-displacement pumps:
move fluid strictly by precise machine
displacements such as a gear system rotating
within a closed housing (screw pumps) or a
piston moving in a sealed cylinder (reciprocal
pumps).
 Analysis of positive-displacement pumps
involves purely mechanical concepts and does
not require detailed knowledge of hydraulics.
 This chapter will only treat the first category,
which constitutes most of the water pumps
used in modern hydraulic engineering systems.
Centrifugal Pumps
 Modern centrifugal pumps basically consist of
two parts: 1. the rotating element ( commonly called
the impeller);
 2. the housing that encloses the rotating
element and seals the pressurized liquid
inside.
 The power is supplied by a motor to the shaft of
the impeller.
 The rotary motion of the impeller creates a
centrifugal force that enables the liquid to enter
the pump at the low-pressure region near the
center (eye} of the impeller and to move along
the direction of the impeller vanes toward the
higher-pressure region near the outside of the
housing surrounding the impeller.
 The housing is designed with a gradually
expanding spiral shape so that the entering
liquid is led toward the discharge pipe with
minimum loss while the kinetic energy in the
liquid is converted into pressure energy.
Cross section of a centrifugal pump
5.3 Jet (Mixed-Flow) Pumps
 Jet pumps capitalize on a high-pressure stream of fluid.
 The pressurized fluid ejects from a nozzle at high speed
into a pipeline transferring its energy to the fluid requiring
delivery.
 Jet pumps are usually used in combination with a
centrifugal pump, which supplies the high-pressure
stream, and can be used to lift liquid in deep wells.
 They are usually compact in size and light in weight.
 They are sometimes used in construction for dewatering
the work site.
 Because the energy loss during the mixing
procedure is heavy, the efficiency of the jet pump
is normally very low (rarely more than 25%).
 A jet pump can also be installed as a booster
pump in series with a centrifugal pump. The jet
pump may be built into the casing of the
centrifugal pump suction line to boost the water
surface elevation at the inlet of the centrifugal
pump as shown schematically in Figure 5.7. This
arrangement avoids any unnecessary installation
of moving parts in the well casing, which is
usually buried deep below the ground surface.
Jet (Mixed-Flow)
Pumps
5.4 Selection of a Pump
 The efficiency of a pump depends on:

the discharge, head, and power requirement of the
pump.
 The approximate ranges of application of each type
of pump are indicated in Figure 5.8.
 The total head that the pump delivers its discharge
against includes the elevation head and the head
losses incurred in the system.
 The friction loss and other minor losses in the
pipeline depend on:


the velocity of water in the pipe (Chapter 3),
and the total head loss can be related to the discharge
rate.
 For a given pipeline system (including the pump), a
unique H-Q curve can be plotted, as shown in
Example 5.3, by computing the head losses for
several discharges.
 In selecting a particular pump for a given system,
the design conditions are specified and a pump is
selected for the range of applications.
 The H-Q curve is then matched to the pump
performance chart (e.g., Fig 5.9 and 5.10) provided
by the manufacturer.
 The matching point , M, indicates the actual working
conditions.
 Selection process ----- example 5.3
Selection of a Pump
Head,
discharge, and
power
requirement of
different pumps
Pump
selection
Chart
Characteristic curve
Characteristic curve
Characteristic curve
5.5 Pumps in Parallel or in
Series
the efficiency of a pump varies with:
 the discharge rate of the pump and
 the total head overcome by the pump.
The optimum efficiency of a pump can
be obtained only over a limited range of
operation (see Figure 5.10).
To install a pumping station that can be
effectively operated over a large range
of fluctuations in both discharge and
pressure, it may be advantageous to
install several identical pumps at the
station.
 When several pumps are connected in
parallel in a pipeline, the discharge is
increased but the pressure head remains the
same as with a single pump.
 It should be noted that two identical pumps
operating in parallel may not double the
discharge in a pipeline because the total head
loss in a pipeline is (Hp a Q2).
 The additional resistance in the pipeline will
cause a reduction in the total discharge.
 Curve B in Figure 5. 1 1 schematically
shows the operation of two identical pumps
in parallel. The joint discharge of the two
pumps is always less than twice the
discharge of a single pump.
 Pumps connected in series in a pipeline will
increase the total output pressure, but the
discharge will remain approximately the
same as that of a single pump. A typical
performance curve for two pumps connected in series is shown
by curve C in Figure 5.11.
Pumps in Parallel or Series
 The efficiency of two (or more) pumps operating in
parallel or in series is almost the same as that of the
single pump based upon discharge.
 The installation can be arranged with a separate motor
for each pump or with one motor operating two (or
more) pumps.
 Multipump installations could be designed to perform
either in-series or in-parallel operations with the same
set of pumps. Figure 5.12 is a typical schematic of such an
installation.
 For series operations, valve A is opened and valves B
and C are closed; for parallel operations, valve A is
closed and valves B and C are open.
5.6 Cavitation in Water Pumps
 One of the important considerations in pump installation
design is the relative elevation between the pump and
the water surface in the supply reservoir.
 Whenever a pump is positioned above the supply
reservoir, the water in the suction line is under pressure
lower than atmospheric. The phenomenon of cavitation
becomes a potential danger whenever the water
pressure at any location in the pumping system drops
substantially below atmospheric pressure.
 To make matters worse, water enters into the suction line
through a strainer that is designed to keep out trash.
This additional energy loss at the entrance reduces
pressure even further.
 A common site of cavitation is near the tips of
the impeller vanes where the velocity is very
high.
 In regions of high velocities much of the
pressure energy is converted to kinetic energy.
This is added to the elevation difference
between the pump and the supply reservoir, hp,
and to the inevitable energy loss in the pipeline
between the reservoir and the pump, hL. Those
three items all contribute to the total suction
head, Hs, in a pumping installation as shown
schematically in Figure 5.13.
 The value of Hs must be kept within a limit so that the
pressure at every location in the pump is always above
the vapor pressure of water; otherwise, the water will be
vaporized and cavitation will occur.
 The vaporized water forms small vapor bubbles in the
flow. These bubbles collapse when they reach the
region of higher pressure in the pump. Violent
vibrations may result from the collapse of vapor bubbles
in water. Successive bubble breakup with considerable
impact force may cause high local stresses on the
metal surface of the vane blades and the housing.
These stresses cause surface pitting and will rapidly
damage the pump.
 To prevent cavitation, the pump should
be installed at an elevation so that the
total suction head is less than the
difference between the atmospheric
head and the water vapor pressure
head, or
(Patm/g - Pvap/g ) > Hs
Cavitation
in Water
Pumps
5.7 Specific Speed and Pump Similarity
 The selection of a pump for a particular service is based on the
required discharge rate and the head against which the discharge is
delivered.
 To lift a large quantity of water over a relatively small elevation (e.g.,
removing water from an irrigation canal onto a farm field) requires a
high-capacity, low-stage pump.
 To pump a relatively small quantity of water against great heights
(such as supplying water to a high-rise building) requires a lowcapacity, high-stage pump. The designs of these two pumps are very
different.
 Generally speaking, impellers of relatively large radius and narrow
flow passages transfer more kinetic energy from the pump into
pressure head in the flow stream than impellers of smaller radius and
large flow passages. Pumps designed with geometry that allows water
to exit the impeller in a radial direction impart more centrifugal
acceleration to the flow than those that allow water to exit axially or at
an angle. Thus, the relative geometry of the impeller and the pump
housing determine the performance and the field application of a
specific pump.
 Dynamic analysis (Chapter 10) shows that
centrifugal pumps built with identical proportions
but different sizes have similar dynamic
performance characteristics that are
consolidated into one number called a shape
number. The shape number of a particular pump
design is a dimensionless number defined as
 where w is the angular velocity of the impeller in
radians per second, Q is the discharge of the
pump in cubic meters per second, g is the
gravitational acceleration in meters per second
squared, and Hp is the total dynamic head in
meters that the pump develops.
 In engineering practice, however, the dimensionless
shape number is not commonly used. Instead, most
commercial pumps are specified by the term specific
speed. The specific speed of a specific pump design
(i.e., impeller type and geometry) can be defined in two
different ways. Some manufacturers define the specific
speed
 of a specific pump design as the speed an impeller
would turn if reduced in size enough to deliver a unit
discharge at unit head. This way, the specific speed may
be expressed as Eqn (5.22).
 Other manufacturers define the specific speed of a
specific pump design as the speed an impeller would
turn if reduced enough in size to produce unit power
with unit head. This way, the specific speed is
expressed as Eqn(5.23)
 Normally, the specific speed is defined at the optimum
point of operational efficiency.
 In practice, pumps with high specific speeds are
generally used for large discharges at low-pressure
heads,
 while pumps with low specific speeds are used to deliver
small discharges at high-pressure heads.
 Centrifugal pumps with identical geometric proportions
but different sizes have the same specific speed.
Specific speed varies with impeller type. Its relationship
to discharge and pump efficiency is shown in Figure
5.14.

similar documents