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| Franklin Application/Installation Data (AID)
... For The Professional Driller-Installer |
Vol. 19, No. 4 Jul/Aug 2001 |
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| LST or Submersible?
Go for the Sub! |
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| Line shaft turbines (LSTs) use an above-ground
motor to turn a drive shaft that reaches down the well casing to the
submerged pump. A submersible, of course, uses a combinedmotor and
pump assembly completely submerged underwater. Although LSTs have
their applications, in most cases a submersible motor makes more sense.
Here’s why: |
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| Submersible pumps
are easier to install. |
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| Complete Unit - Since a submersible
motor and pump is installed as a complete unit, it’s easier to
handle than an LST pumping system. |
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| Smaller – To compensate for its
lower RPM, an LST requires more stages, larger turbine bowls, and
larger casing. |
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| Less Complex – An LST has many
more moving parts than a submersible. The shaft extends the length
of the well, and must be stabilized throughout, with bearings at specified
intervals. If it is oil-lubricated, the shaft must rotate within a
bearing tube stabilized by retainers, with the bearing tube itself
mounted inside the water delivery pipe. |
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| An anti-reverse coupling is required in an
LST to prevent back-spin of the shaft (which can damage bearings and
cause bowls and shaft to unthread). To prevent water from flowing
back down the well where it can disturb sand and other debris, the
system should have check valves, although many LSTs do not. |
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Well Casing – To accommodate
the larger
diameter of an LST, along with the associated drive shaft hardware,
the size of the well casing must be larger than with a submersible.
In addition, a
submersible may be installed in a well that isn’t completely
straight. |
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| This leads to the equipment required for
a submersible installation being smaller and lighter. In many case,
the installation time for a submersible is less than half that necessary
for a line shaft pump. |
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| No Pump House – In many cases,
an LST will require a pump house, and this can be one of the largest
installation considerations. Since no pump house is required for a
submersible, this saves noise and construction costs, and dramatically
reduces the future risk of damage from weather, vandalism or accidents.
Having no pump house means having a cosmetically-clean installation.
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| Check Valves – Many submersible
pumps already have check valves built in, to prevent water from flowing
back down the pipe and causing the motor to backspin. |
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| Submersible pumps
are easier to maintain. |
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| Routine Maintenance - Over time, nothing
will be cheaper to maintain than the hassle-free submersible pump.
Whether it has a vertical hollow shaft or a standard horizontal motor
with a right-angle gear head, a line shaft turbine routinely
requires oil lube, packing, and impeller adjustments. On the other
hand, because Franklin submersible motors are designed to spend
their entire lives underwater, they have a constant source of lubrication
and require zero routine maintenance. |
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No Spare Parts – No routine maintenance
for
submersible motors means not having to keep an expensive inventory
of spare parts. |
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| Submersible pumps
are efficient. |
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| You may have heard that the LST motor is
more efficient than the submersible motor. As a general rule, above-ground
motors are slightly more efficient than submersible motors. However,
in actual LST applications, this motor efficiency is lost to losses
in the line shaft. These losses are typically 1 to 2 horsepower per
100 feet of depth, which generally exceed the losses in the submersible's
drop cable. Even though there will be some losses in the submersible
drop cable, these are minimal if the cable is properly sized. |
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| Lower Thrust Loads – With no
shaft to support, submersible installations have lower thrust loads
than LSTs. |
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| Fast Starting – Submersible motors
have a low moment of inertia for rotors, which means faster and easier
starting. |
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| No Noise – Because they reside
underground, submersibles do not generate the noise of an LST and
can thus be placed almost anywhere. |
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| Submersible pumps
are economical. |
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| Pump – Although the LST motor
is usually less expensive than a submersible motor, the pump itself
costs more due to the required larger diameter and number of stages. |
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| Drop Pipe – The cost of the hollow tube
and drive shaft may be up to seven times that of the drop pipe and
cable of a submersible. The deeper the well, the longer and costlier
the shaft. |
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Installation and Maintenance –
Installation and maintenance contribute heavily to the overall cost
of the LST. The longer, straighter bore hole is more expensive to
drill, and its extended installation time adds to labor costs. When
it is necessary to pull an LST from the well, the difficulty in doing
so inflates maintenance expenses that are, as mentioned earlier, already
high.
Thus, in nearly every installation where electricity is available,
submersible pumping systems are less expensive to purchase, install,
operate, and maintain than their line shaft turbine counterparts. |
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| Subs just make more
sense. |
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| All of these factors contribute to the logic
of making a submersible pumping system your first choice. For more
information on the advantages of a submersible, contact your pump
manufacturer or call our Submersible Service Hotline at 1-800-348-2420. |
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Introducing
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In our last issue, we told you
about Bob McClain's move from the Hotline to Field Service.
That left us with a vacancy on the Hotline that we needed
to fill with the right person and quickly! Lucky for us,
we found that person right here at home. Dewey Randall
comes to us from within Franklin Electric, where he has
spent the last 12½ years in the Marketing Department
of our Engineered Motor Products Division. Dewey's technical
knowledge of electric motors is excellent, and we look
forward to the fresh insight he will bring to the Hotline.
You can reach Dewey by calling the Submersible Service
Hotline (800-348-2420) or by e-mail drandall@fele.com.
Please join us in welcoming him to our Submersible Field
Service team. |
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Dewey Randall
Headquarters Service Engineer |
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