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ENGR 120 Using Pump Curves to Select Pumps
Lets close the loop.
How does the pump testing that you have
done connect to real-world engineering?
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MotorPumps come in a variety
.
Here are a few examples
Drive shaft connectedPump
.
Pump
to tractor PTO
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This is a submersible pump consisting of
4 stages connected in series. Each stage
Water out
is a centrifugal pump in itself. The entire
pump is submerged under water.
Pum sta es
In this submersible pump water flows from
one stage to the next. The pressure (or head)
increases as water moves through the pump,
but the discharge remains constant.
This is analogous to batteries connected
in series. The total voltage is the sum of
the voltage from each battery, but the current
remains constant.
Water in
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Procedure for Selecting a Pump
(2) Select the design discharge for the pump
(3) Check pump manufacturers catalogs and select a pump that
will operate at maximum efficiency near the design discharge.
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System Head Curve
(1) Static lift vertical distance between the static water surface
and the ground surface
(2) Static discharge vertical distance between the ground surfaceand the ultimate point of use
(3) Well drawdown decrease in water level in the well in response
to pumping
flows through the pipe, valves, bends, etc.
(5) Operating head pressure (or head) required at the point of use.
For example, irr igation sprinklers require a certain amount ofpressure to operate correctly; a well system for a home typically
pumps water into a pressure tank which then supplies water to the
.
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These are all dynamic they increase with
increasing discharge from the pump.
System Head Curve
H(
ft) Operating Head
tal
Head,
Friction Loss
T
Static Discharge
Static Lif t
Discharge, Q (gpm)
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Operating head is determined by whatever is required atthe ultimate point of use (sprinkler, pressure tank, etc.)
Friction loss, hf, can be calculated from the Darcy-Weisbach equation
where
Well drawdown, s, can be calculated from the Jacob equation
where
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Select a submersible pump that will deliver 900 gpm to a water tank.
Static lift = 20 ft
SE
a c sc arge =
Dynamic head is discussed on the next slide.
Static
Discharge
motor
Static lift
Drawdown
pump
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Lets assume we are able to calculate the frict ion loss, hf, from the
Darcy-Weisbach equation. Note that hf varies with velocity of the water.
This means it also varies with pump discharge, Q.
, , .
this also varies with pump discharge, Q.
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We calculate and plot the total system head curve asthe sum of static lift, static discharge, well drawdown,
.
curve will be a function of pump discharge.
Lets assume we have done these calculations and
plotted the system head curve as shown of the following
graph.
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System
Head Curve
Now we superimpose this curve on the pumpcurves obtained from the pump manufacturer.
Static L ift + Static Discharge
The pump curves from the manufacturer were determined in
essentially the same way you determined your pump curves.
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System
Static Lift + Static Discharge
Operating Point
If we operate this pump at 1600 RPM, it will deliver 900 GPM
against a total system head of 120 ft.Efficiency is 71% (slight ly less than peak effic iency of 72%.)
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System
Static L ift + Static Discharge
What wil l happen if we operate the pump at 1200 RPM ?New Operating Point
The total system head drops to 75 ft, and the pump willdeliver only 550 GPM. The effic iency drops to 69%.