Water Pumps

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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.

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• Turbo-hydraulic pumps are
• centrifugal pumps,
• Propeller pumps,
• and jet pumps.

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• 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).

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• 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.

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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.

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• 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.

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Cross section of a centrifugal pump
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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.

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• 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.

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Jet (Mixed-Flow) Pumps
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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.

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• 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

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Selection of a Pump
Head, discharge, and power requirement of
different pumps
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Pump selection Chart
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Characteristic curve
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Characteristic curve
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Characteristic curve
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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.

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• 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.

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• 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.

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Pumps in Parallel or Series
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• 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.

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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.

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• 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.

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• 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.

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• 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 ) gt Hs

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Cavitation in Water Pumps
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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 low-capacity,
  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.

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• 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.

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• 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)

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• 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.

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