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Marine Auxiliary Machinery

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Marine Auxiliary Machinery Chapter 3 Lesson 5 Centrifugal Pump operations Learning Objectives After successfully completing this lesson, you will be familiar with ... – PowerPoint PPT presentation

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Title: Marine Auxiliary Machinery


1
Marine Auxiliary Machinery
  • Chapter 3 Lesson 5
  • Centrifugal Pump operations

2
Learning Objectives
  • After successfully completing this lesson, you
    will be familiar with
  • Pump and system curve
  • Series pumping
  • Parallel pumping
  • Test

3
Review - Centrifugal Pumps
4
Series Pumping - Multistage
  • Series pumping can be considered as a series of
    centrifugal pumps arranged to supply one another
    in series and thus progressively increase the
    discharge pressure.
  • The illustration shows a cross-section through a
    typical vertical multistage single entry
    centrifugal pump used deep-well cargo pumping.

5
Series pumping - operation
  • In series operation, the discharge of one pump
    feeds the suction of a second pump. When two or
    more pumps are operated in series, the flow
    through all of the pumps is equal. Since whatever
    flows through one pump has to flow through the
    next pump in series, provided there are no side
    streams.

6
Purpose for pumping in series (1)
  • Commercial equipment
  • One purpose for operating pumps in series is to
    insure that commercially available equipment can
    be used in a particular system, while at the same
    time reducing system costs.

7
Purpose for pumping in series (2)
  • Long pipelines
  • The system with a long pipeline and a large
    amount of friction loss is across the entire
    pipeline.
  • This would result in a pump with an extremely
    high head, and thus an extremely high horsepower.

8
Purpose for pumping in series (3)
  • NPSHA for the second pump
  • In this case, the first pump of two in series
    might be a fairly low head pump (and thus a
    fairly low horsepower pump).
  • But, in calculating NPSHA for the second pump in
    series, would have an additional term, namely the
    TDH (Total discharge head) of the first pump.

9
Series Pumping Pump Curves (1)
10
Series Pumping Pump Curves (2)
  • Each of the pumps has H-Q curve
  • A combined pump curve can be generated.
  • This color curve is the combined curve for two
    pumps.
  • At arbitrarily selected values of flow, the TDH
    at this flow is doubled.
  • This color curve is for three identical pumps in
    series.
  • System head curves
  • The intersection of PC with SHC determines the
    total flow.

11
Non identical pumps in Series
  • The combined curve is generated using the same
    procedure as for identically sized pumps.
  • It is common in pipelines to have several
    differently sized pumps, allowing the operators
    the widest possible range of flow and/or
    variation of products pumped.

12
Pressure relief system
  • If a valve in the line downstream of the last
    pump in a series installation is inadvertently
    closed completely.
  • All of the pumps in the line move to their
    shutoff head.
  • The pressure in the system is considerably
    higher than normal
  • Possibly higher than the design pressure
  • A pressure relief system should be incorporated
    into the system

13
Parallel pumping (1)
  • The primary purpose of operating pumps in
    parallel is to allow a wider range of flow than
    would be possible with a single fixed speed pump
    for systems with widely varying flow demand.
  • Examples of applications for parallel pumping
    include
  • Municipal water supply
  • Wastewater pumps
  • Pumps in water chilled heating, ventilating and
    air conditioning systems (HVAC)
  • Main process pumps in variable capacity process
    plant
  • Condensate pumps in a steam power plant

14
Parallel pumping (2)
  • Usually there are no more than three or four
    pumps operating in parallel.
  • When parallel pumps are being considered
  • The pumps must be carefully matched to each other
  • To insure that the pumps are always operating at
    a healthy point on their H-Q curves
  • To insure that the system is such that true
    benefits are achieved form the parallel pumping
    arrangement
  • However this does not always turn out be to the
    case.

15
Parallel pumping (3)
  • A combined pump curve must be developed depicting
    the head-flow relationship for the pumps while
    pumping in parallel.
  • Once these two curves are constructed, the rule
    for total system flow is that the total flow
    through the system is represented by the
    intersection of the system head curve with the
    combined pump curve.

16
Flow for two pumps in parallel (1)
  • The resultant flow when two pumps are operated in
    parallel in system is not double the flow which
    one pump alone produces when operating by itself,
    a sometimes mistaken impression.

17
Flow for two pumps in parallel (2)
  • If the system curve were completely flat, then
    the flow double.
  • But, because the system head curve curves up due
    to friction, two pumps in parallel dont deliver
    double the flow of a single pump alone in the
    system.
  • Putting the third pump on line even further
    diminishes the increment of flow.

18
1pump in a system with 3 pumps (1)
  • With two identical pumps operating
  • The point ab is obtained by moving left from
    point b.
  • Point ab represents where each pump is operating
    on its own H-Q curve when two pumps are operated
    in parallel.

19
1pump in a system with 3 pumps (2)
  • Similarly, the dashed line moving left from
    point c intersects the pump curve at the point
    ac, where it would operate when three pumps
    operate in the system.
  • In general with parallel pumping, each pump runs
    out the furthest on its own H-Q curve when that
    pump operates alone in the system.
  • The pumps run the furthest back on their own H-Q
    curves when the maximum numbers of pumps are
    operated in parallel in the system.

20
Parallel Non-identical Pumps
  • The curves of the two different pumps are
    combined in curve C
  • The combined curve C actually follows curve A for
    a while.
  • Point c determines the total flow of the two
    pumps.
  • The horizontal dashed line going left from point
    c intersects the individual H-Q curves at the
    point a and b respectively.

21
Parallel System Mismatch (1)
  • Two non-identical pumps in parallel
  • The system curve Y is steeper
  • The pump A would operate at point a
  • The pump B would never be able to develop enough
    head.
  • It would cause the pump B
  • To be running at full speed but delivering no
    flow
  • Or caused pump B to operate at a very low flow.

22
Parallel System Mismatch (2)
  • This type of mismatch of non-identical pumps in a
    system should obviously be avoided.

23
Steep system head curve (1)
  • When the system head curve is very steep,
    operating a second pump in parallel with the
    first produces only a marginal increase in flow.
  • If these same pumps were piped in series, rather
    than in parallel, they would produce a higher
    flow through the system.
  • Only two such pumps piped in series deliver more
    flow through the system than three pumps
    operating in parallel do.

24
Steep system head curve (2)
  • The general rule is
  • If the system head curve is relatively steep,
    series pumping is probably more effective than
    parallel pumping for increasing the flow range of
    the pumps.
  • If fairly flat system head curves are concerned
    parallel pumping is probably more effective
    producing wide flow range than series.

25
Steep system head curve (3)
  • If the system head curve is too steep, a
    situation which could be caused by an under-sized
    piping system or by some other undersized
    component in the system which acts as a
    bottleneck.
  • In this case, capital would have been better
    spent, if possible, in flattening the system head
    curve, reducing bottlenecks, and increasing
    piping sizes, rather than in adding additional
    parallel operating pumps to the system.

26
Question 1
27
Question 1
28
Question 2
29
Question 3
30
Question 4
  • The main reason for operating pumps in parallel?
  • Is that a greater discharge pressure can be
    achieved.
  • Is to allow a wider range of flow than for a
    single arrangement
  • Is that the combined electrical load on the pumps
    is lower than an equivalent single pump.

31
Question 5
  • Which one of the following statements is true?
  • Centrifugal pumps cover the lower ranges of
    specific speeds, axial flow the higher, and mixed
    flow the intermediate values
  • Centrifugal pumps cover the higher ranges of
    specific speeds, axial flow the intermediate, and
    mixed flow the lower values.
  • Centrifugal pumps cover the intermediate ranges
    of specific speed, axial flow the higher, and
    mixed flow the lower values.

32
Question 6
  • Under what system conditions does it turn out
    that operating pumps in parallel achieves very
    little benefit?
  • If the piping system is of different size for
    each pump.
  • If the piping system is oversized, causing a lack
    of discharge pressure.
  • If the piping system is undersized or some
    component in the system acts as a bottleneck

33
Question 7
  • The main reason for operating pumps in series is?
  • To avoid cavitation in high-pressure system.
  • To share the power absorbed using many small
    motors instead of one large motor, which is more
    energy efficient.
  • So that commercially available pumps (i.e. small)
    can be used to avoid extremely high discharge
    pressures which would be necessary with only one
    pump
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