Pumps and Pumping Stations

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Pumps and Pumping Stations

• Pumps add energy to fluids and therefore are
  accounted for in the energy equation
• Energy required by the pump depends on
• Discharge rate
• Resistance to flow (head that the pump must
  overcome)
• Pump efficiency (ratio of power entering fluid to
  power supplied to the pump)
• Efficiency of the drive (usually an electric
  motor)

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Pump Jargon

• (Total) Static head difference in head between
  suction and discharge sides of pump in the
  absence of flow equals difference in elevation
  of free surfaces of the fluid source and
  destination
• Static suction head head on suction side of
  pump in absence of flow, if pressure at that
  point is gt0
• Static discharge head head on discharge side of
  pump in absence of flow

Total static head
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Pump Jargon

• (Total) Static head difference in head between
  suction and discharge sides of pump in the
  absence of flow equals difference in elevation
  of free surfaces of the fluid source and
  destination
• Static suction lift negative head on suction
  side of pump in absence of flow, if pressure at
  that point is lt0
• Static discharge head head on discharge side of
  pump in absence of flow

Total static head
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Pump Jargon
Total static head (both)
Note Suction and discharge head / lift measured
from pump centerline
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Pump Jargon

• (Total) Dynamic head, dynamic suction head or
  lift, and dynamic discharge head same as
  corresponding static heads, but for a given
  pumping scenario includes frictional and minor
  headlosses

Energy Line
Dynamic discharge head
Total dynamic head
Dynamic suction lift
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• Example. Determine the static head, total dynamic
  head (TDH), and total headloss in the system
  shown below.

El 730 ft
ps ?6 psig
El 640 ft
pd 48 psig
El 630 ft
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• Example. A booster pumping station is being
  designed to transport water from an aqueduct to a
  water supply reservoir, as shown below. The
  maximum design flow is 25 mgd (38.68 ft3/s).
  Determine the required TDH, given the following
• H-W C values are 120 on suction side and 145
  on discharge side
• Minor loss coefficients are
• 0.50 for pipe entrance
• 0.18 for 45o bend in a 48-in pipe
• 0.30 for 90o bend in a 36-in pipe
• 0.16 and 0.35 for 30-in and 36-in butterfly
  valves, respectively
• Minor loss for an expansion is 0.25(v22 ? v12)/2g

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• Determine pipeline velocities from v Q/A..
• v30 7.88 ft/s, v36 5.47 ft/s, v48
  3.08 ft/s
• Minor losses, suction side

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1. Minor losses, discharge side

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1. Pipe friction losses

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1. Loss of velocity head at exit

1. Total static head under worst-case scenario
   (lowest water level in aqueduct, highest in
   reservoir)

1. Total dynamic head required

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Pump Power

• P Power supplied to the pump from the shaft
  also called brake power (kW or hp)
• Q Flow (m3/s or ft3/s)
• TDH Total dynamic head
• ? Specific wt. of fluid (9800 N/m3 or 62.4
  lb/ft3 at 20oC)
• CF conversion factor 1000 W/kW for SI, 550
  (ft-lb/s)/hp for US
• Ep pump efficiency, dimensionless accounts
  only for pump, not the drive unit
  (electric motor)

Useful conversion 0.746 kW/hp
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• Example. Water is pumped 10 miles from a lake at
  elevation 100 ft to a reservoir at 230 ft. What
  is the monthly power cost at 0.08/kW-hr,
  assuming continuous pumping and given the
  following info
• Diameter D 48 in Roughness e 0.003 ft,
  Efficiency Pe 80
• Flow 25 mgd 38.68 ft3/s
• T 60o F
• Ignore minor losses

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Find f from Moody diagram
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Pump Selection

• System curve indicates TDH required as a
  function of Q for the given system
• For a given static head, TDH depends only on HL,
  which is approximately proportional to v2/2g
• Q is proportion to v, so HL is approximately
  proportional to Q2 (or Q1.85 if H-W eqn is used
  to model hf)
• System curve is therefore approximately parabolic

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• Example. Generate the system curve for the
  pumping scenario shown below. The pump is close
  enough to the source reservoir that suction pipe
  friction can be ignored, but valves, fittings,
  and other sources of minor losses should be
  considered. On the discharge side, the 1000 ft of
  16-in pipe connects the pump to the receiving
  reservoir. The flow is fully turbulent with D-W
  friction factor of 0.02. Coefficients for minor
  losses are shown below.

K values K values
Suction Discharge
1 _at_ 0.10 1 _at_ 0.12
1 _at_ 0.12 1 _at_ 0.20
1 _at_ 0.30 1 _at_ 0.60
2 _at_ 1.00 4 _at_ 1.00
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• The sum of the K values for minor losses is 2.52
  on the suction side and 5.52 on the discharge
  side. The total of minor headlosses is therefore
  8.04 v2/2g.
• An additional 1.0 v2/2g of velocity head is lost
  when the water enters the receiving reservoir.
• The frictional headloss is

Total headloss is therefore (8.041.015.0)v2/2g
24.04 v2/2g. v can be written as Q/A, and A
pD2/ 4 1.40 ft2. The static head is 34 ft. So
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System curve
Static head
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Pump Selection

• Pump curve indicates TDH provided by the pump
  as a function of Q
• Depends on particular pump info usually provided
  by manufacturer
• TDH at zero flow is called the shutoff head
• Pump efficiency
• Can be plotted as fcn(Q), along with pump curve,
  on a single graph
• Typically drops fairly rapidly on either side of
  an optimum flow at optimum efficiency known as
  normal or rated capacity
• Ideally, pump should be chosen so that operating
  point corresponds to nearly peak pump efficiency
  (BEP, best efficiency point)

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Pump Performance and Efficiency Curves
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Pump Selection
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Pump Efficiency

• Pump curves depend on pump geometry (impeller D)
  and speed

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Pump Selection

• At any instant, a system has a single Q and a
  single TDH, so both curves must pass through that
  point ? operating point is intersection of
  system and pump curves

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Pump System Curve

• System curve may change over time, due to
  fluctuating reservoir levels, gradual changes in
  friction coefficients, or changed valve settings.

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Pump Selection Multiple Pumps

• Pumps often used in series or parallel to achieve
  desired pumping scenario
• In most cases, a backup pump must be provided to
  meet maximum flow conditions if one of the
  operating (duty) pumps is out of service.
• Pumps in series have the same Q, so if they are
  identical, they each impart the same TDH, and the
  total TDH is additive
• Pumps in parallel must operate against the same
  TDH, so if they are identical, they contribute
  equal Q, and the total Q is additive

Adding a second pump moves the operating point
up the system curve, but in different ways for
series and parallel operation
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• Example. A pump station is to be designed for an
  ultimate Q of 1200 gpm at a TDH of 80 ft. At
  present, it must deliver 750 gpm at 60 ft. Two
  types of pump are available, with pump curves as
  shown. Select appropriate pumps and describe the
  operating strategy. How will the system operate
  under an interim condition when the requirement
  is for 600 gpm and 80-ft TDH?

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• Either type of pump can meet current needs (750
  gpm at 60 ft) pump B will supply slightly more
  flow and head than needed, so a valve could be
  partially closed. Pump B has higher efficiency
  under these conditions, and so would be preferred.

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• The pump characteristic curve for two type-B
  pumps in parallel can be drawn by taking the
  curve for one type-B pump, and doubling Q at each
  value of TDH. Such a scenario would meet the
  ultimate need (1200 gpm at 80 ft), as shown below.

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• A pump characteristic curve for one type-A and
  one type-B pump in parallel can be drawn in the
  same way. This arrangement would also meet the
  ultimate demand. Note that the type-B pump
  provides no flow at TDHgt113 ft, so at higher TDH,
  the composite curve is identical to that for just
  one type-A pump. (A check valve would prevent
  reverse flow through pump B.) Again, since type B
  is more efficient, two type-B pumps would be
  preferred over one type-A and one type-B.

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• At the interim conditions, a single type B pump
  would suffice.
• A third type B pump would be required as backup.