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For example, if we look at a typical pump curve for
a (15) HP – (85) GPM submersible pump – we can
select a pump cut-in and cut-out point on the curve
which will provide optimum pump operation.
The pump curve shows pump performance as
its pressure rises from cut-off (
(
0
)
head in test) to
maximum head in feet.
On this curve, for a (15) Hp (85) GPM pump we can
see that at approximately (100) feet the pump stops
pumping and will pump up to (660) feet of head.
What we want to determine is at which point on the
curve do we want to have the pump cut-in, and at
which point on the curve do we want it to cut-out.
The pump operates in its best efficiency range at
the mid-range of the curve.
The cut-in point can’t be too low on the curve, as this
would cause inefficient operation. We should choose
a point just before the curve drops off rapidly…and
at a point still within the best efficiency range (60% to
80%).
The pump curve for the pump selected indicates the
total pressure range of the pump or its dynamic head
measured in feet of head.
The pump must have enough head, or pressure
differential to:
Bring water from the level of the well to the
well head at the surface of the ground.
Provide head capacity to pressurize system
piping to overcome elevation (static height)
and friction (friction loss) and deliver required
pressure.
Since the Well-X-Trol
®
will control the pump
operation throughout system pressurization, we
must select our pump cut-in pressure and pump cut-
out pressure.
For example:
In a well with water level (240) feet down in the well,
system pressurization by pump will be shown on the
pump curve at some point above the intersection of
the line indicating (240) feet of head.
In other words, at (240) feet of head we can assume
that the pump will have brought water from the
pumping level of the well to the surface and pressure
in the system piping will be (0) PSIG.
We now must select a pressure range (min. to
max.) to establish pressurized delivery of water to
the system and that we overcome elevation (static
height) and friction loss of the piping.
We must select a minimum pressure at the Well-
X-Trol
®
location that will insure enough pressure
to keep water flow under pressure at the top of the
system…overcoming both the static height and the
resistance to flow through the piping.
We’ll do this by pre-charging the Well-X-Trol
®
to
a pressure (P
1
) equal to the minimum pressure
required at the Well-X-Trol
®
location. In addition,
we’ll select a pump cut-in pressure (P
2
) which is
at the same pressure to start the pump whenever
system pressure drops to this point.
We’ll assume that a minimum system pressure
at the Well-X-Trol
®
location of (30) PSIG. This
pressure will be adequate to insure system
pressurization to overcome elevation and friction
loss and provide adequate pressure at the fixture.
To find this pump cut-in point on the pump curve,
we’ll have to convert (30) PSIG to feet of head.
To convert PSIG to feet of head, we must multiply
PSIG by (2.31).
30 PSIG x 2.31 = 69.3 or 70 feet of head
To locate this point on the pump curve, we must
add 70 feet of head…the pump head required to
pressurize the system to 30 PSIG…to the feet of
head required to lift the water to the surface:
Minimum System Pressure = 70 Ft. of Head
Plus Lift = 240 Ft. of Head
* Pump cut-in point on curve = 310 Ft. of Head
We have selected the point on the curve which will
be the pump cut-in setting (P
2
) of 30 PSIG on the
pump switch, and we will pre-charge the Well-X-
Trol
®
to 30 PSIG (P
1
).
Now we will select the maximum pump cut-out point
on the curve which will allow the widest possible
pressure range without impairing pump performance
and efficiency.
We will do this by moving up the curve to find a
point that:
Is just before the curve begins to “flatten” out
And is still within the upper limits of best
efficiency range of the pump
In this example, that point would be at the intersection
on the curve of the horizontal line indicating (410) or
(420) feet of head… let’s say (420) feet.
This is the point on the curve which will be the
maximum pump cut-out for this particular pump.