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CENTRIFUGAL PUMP CHARACTERISTICS

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Title: CENTRIFUGAL PUMP CHARACTERISTICS


1
CENTRIFUGAL PUMP CHARACTERISTICS
  • CE 370

2
Types of Pumps
  • There are different types of pumps which are used
    for different purposes in water and wastewater
    transportation
  • Low-lift pumps are used to lift water and
    wastewater from a source to the treatment plants,
  • high-service pumps are used to pump water and
    wastewater under pressure,
  • Booster pump are used to increase pressure in
    water distribution systems,
  • Well-pumps are used to lift water from wells,
  • Recirculation pumps are used to recirculate water
    within a system, and
  • Pumps are also used to feed chemicals, sample
    water and wastewater and fight fires.

3
Centrifugal Pumps
  • Centrifugal pumps are commonly used in the field
    of water and wastewater for a variety of
    applications
  • Lift and transport water,
  • Move sludge,
  • Well pumping, and
  • Wastewater lift stations.
  • Centrifugal pumps are common ones due to
  • Simplicity
  • Compactness
  • Low-cost
  • Ability to operate under wide conditions.
  • Centrifugal pumps are mainly composed of
  • The impeller, which is a rotating member with
    vanes
  • Surrounding case, see figure.

4
Centrifugal Pumps
  • The name "Centrifugal" is driven from the force,
    the pump depends on, to transport water. That
    force is the centrifugal force. The impeller can
    be
  • Closed, which is used to transport water and
  • Open, which is used to transport wastewater
    containing suspended solid.
  • How centrifugal pumps operate??
  • The impeller is driven at high rotational speed,
  • The impeller throws the water into the volute
  • The water channels through the nozzle to the
    discharge.

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6
Pump Head-Discharge Curve
  • Pump manufacturers establish head-discharge
    curves at constant impeller speed and based on
    experiments. The head given in those curves
    represent the head at a given discharge when the
    inlet static water level is at the elevation of
    the pump centerline and excluding the head losses
    developed in the suction and discharge pipelines,
    see figure. The figure shows the following
  • Discharge is controlled by a valve,
  • Discharge pressure is measured by a pressure
    gauge, and
  • Discharge flow rate is measured by a flow meter.
  • In the mean time, power input and efficiency of
    the pump are measured and determined.

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8
Pump Head-Discharge Curve
  • When valve is closed, the discharge pressure
    reaches a value called "the shutoff pressure".
    As the valve is gradually opened, the discharge
    flow will increase and the pump head will
    decrease, see figure.
  • The figure shows the pump efficiency increases
    with the increase in discharge flow, until it
    reaches an optimum value and starts to decrease.
  • Centrifugal pumps are made to operate close to
    the optimum efficiency. When pumps are operated
    beyond optimum conditions, problems such as
    cavitation and water hammering start to occur.
    When pumps are operated near to shutoff pressure,
    problems such as vibration and hydraulic losses
    could happen. Operating the pump at 60 to 120
    percent of the efficiency was considered to be a
    good practice.

9
Pump Characteristics
  • Usually manufacturers produce characteristics
    curves for pumps with different impeller diameter
    and operating speeds. For a given impeller
    diameter running at different speeds, the
    discharge is directly proportional to to the
    speed, head is proportional to the square of
    speed, and power input is proportional to the
    cube of the speed as shown in the following
    equations
  • where
  • Q discharge, gallons per minute (liters per
    second
  • H head, feet (meters)
  • Pi power input, horsepower (kilowatts).

10
Pump Characteristics
  • If the pump operates at constant speed but at
    different impeller diameter, the effect of
    discharge, head and power input becomes as
    follows
  • where D impeller diameter, inches (centimeters)

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12
Power and Efficiency
  • The power input of a pump is equal to the work
    done per unit time in lifting water to a higher
    point. Discharge and head can be related to
    power input by the following equation
  • where
  • Po power output, horsepower
  • Q discharge, gallons per minute
  • H head, feet
  • w unit weight of water 8.34 lb/gal
  • 550 foot pounds / second per horsepower
  • 60 seconds per minute
  • 3960 550 ? 60 / 8.34

13
Power and Efficiency
  • In metric units, the power output of a pump is
  • where
  • Po power output, kilowatts (kilonewton ? meter
    per second)
  • Q discharge, liters per second
  • H head, meters
  • w unit weight of water 0.0098 kN per liter

14
Power and Efficiency
  • The efficiency of the pump is the ratio of the
    power output to the power input and can be
    expressed in the form of
  • where
  • Ep pump efficiency, dimensionless
  • Pi power input, horsepower (kilowatts)
  • Po power output, horsepower (kilowatts).

15
System Characteristics
16
System Characteristics
  • When a pump is put into service in a water
    distribution system, its flow of discharge faces
    resistance due to
  • Static head and
  • Friction head loss
  • Static head is the difference in elevation
    between suction and discharge points. If a
    simple water system was considered, see the
    following figure.
  • Water is pumped from an inlet point
  • Water is discharged either into
  • elevated storage (outlet 1)
  • load center (outlet 2)
  • both (outlets 1 and 2)

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18
System Characteristics
  • If water is discharged from outlet 1, then the
    system will have the hydraulic grade line 1,
    while if water is discharged from outlet 2 then
    the system will be described by the hydraulic
    grade line 2.
  • The head-discharge curves for the system are
    shown in the following figure.

19
Constant-Speed Pumps
  • A constant-speed pump operates at a
    head-discharge point defined by the intersection
    of the pump head-discharge curve and the system
    head-discharge curve, see the following figure.

20
Constant-Speed Pumps
  • When the entire water is pumped to the elevated
    tank, the constant-speed pump will operate at
    point A, while when the water was entirely
    withdrawn from outlet 2, the pump will operate at
    point B.
  • The curve shows that Q2 gt Q1 why? Because the
    operating pressure (static and friction head
    losses) at point B is less than that at point A.
  • In small water systems, pumping stations may
    consist of two constant-speed pumps that operate
    intermittently. Water is pumped directly to an
    elevated tank, which in turn supplies the
    distribution system. Pump operation is
    controlled by the level of water in the elevated
    tank.
  • In larger systems installation of al least three
    pumps is desirable in order to cover the extremes
    in water demand and to provide a standby pump.
    Water is pumped directly to the distribution
    system and elevated tanks are connected to the
    piping system.
  • Parallel operation of constant-speed pumps is
    shown in the following figure.

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22
Constant-Speed Pumps
  • Pumps 1 and 2 are identical, while pump 3 has a
    greater capacity and higher shutoff head. The
    pumps are operated individually or in combination
    based on the water demand. When two or more
    pumps operate at one time, the combined
    head-discharge curve can be obtained by adding
    the rates of discharge at the same head of the
    individual curve.

23
Variable-Speed Pumps
  • In cases where variable discharge rates are
    required, discharge of pumps running at
    constant-speed can be controlled by using a valve
    at the pump outlet. Restricting discharge causes
    the impeller to recirculate water in the casing,
    reducing the efficiency and damaging the pump
    bearings and impeller. In order to maintain
    constant pump discharge head over a wide rage of
    flow rates, variable-speed pumps are used.
    Head-discharge curves for a pump operating at two
    impeller speeds are shown in the following figure.

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25
Variable-Speed Pumps
  • The pump can operate on an infinite number of
    curves between the minimum and maximum speeds,
    the dashed lines are of equal efficiencies.
  • The demand-head curve is shown in the following
    figure.

26
Variable-Speed Pumps
  • The curve illustrates the required boost head by
    the pump versus flow demand of the system. As
    water demand increases, pump discharge pressure
    decreases, so impeller speed increases. On the
    other hand, as water demand decreases, pump
    discharge pressure increases, so impeller speed
    decreases.
  • Combining the head-discharge and head-demand
    curves will result in producing the following
    figure. Also the speed and efficiency curves can
    be plotted.

27
Variable-Speed Pumps
  • At low water demand rates, the variable-speed
    drive should be prevented from running at very
    low speeds. So, when water demand is less than
    the minimum rate, pumps are designed to
    recirculate water through the pump in order to
    prevent the pump from damage.
  • The recommended minimum discharge rate is between
    25 and 35 per cent of the pumping rate at the
    best operating efficiency. The recirculation
    techniques are shown in the following figure.

28
Variable-Speed Pumps
  • The first system uses a flow meter to operate a
    modulating valve in a by-pass to maintain a
    nearly constant flow through the pump when the
    demand is less than the minimum recommended rate
    of discharge.
  • The second system employs a back-pressure
    regulating valve in the by-pass. This system can
    be used if the discharge curve turns downward.
    Variable-speed pumps operate in combination and
    may function by either load sharing or staggered
    operation.
  • In load sharing, all pumps operate at the same
    speed and discharge at equal rates. In staggered
    operation, one or more of the pumps operate at
    optimum efficiency (constant speed) while only
    one pump is varied to meet changing demand.
  • Variable-speed pumps can also be used in
    combination with constant-speed pumps.

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