Line Shafts vs. Submersibles: Some Big Advantages for the Sub

In high volume groundwater pumping applications, the pump is almost always located underground. Even so, the type of pump and the method of driving it can be very different. The two most common pumping systems for high volume groundwater applications are vertical line shaft turbines (VLSTs) and submersible turbines.

VLSTs use an aboveground motor with a drive shaft connecting the motor to the submerged pump. These pumps are suitable for some applications, primarily in shallow sets.

Submersible turbines use a submersible motor coupled to the submerged pump so that both are located together in the well. In most applications, a submersible makes more sense. Some of the main reasons follow.

Size and Complexity

A VLST has more components and they are spread out over a greater distance, making handling and installing VLST pumping systems cumbersome. Since a submersible is installed as a complete, compact unit, it is far easier to handle and install. Another factor that contributes to the larger size of VLSTs is that in most cases, they operate at half of the RPM of a submersible. To compensate, a VLST requires more stages or larger turbine bowls to generate the same performance as the equivalent submersible.

Since a VLST requires a drive shaft to couple the motor and pump, it has many more moving parts than a submersible. And, since the shaft extends the length of the well, it must be stabilized with bearings at specified intervals. If it is oil-lubricated, the shaft rotates within a bearing tube stabilized by retainers, with the bearing tube itself mounted inside the water delivery pipe. This bearing tube must be sealed along its entire length. Otherwise, oil can leak and contaminate the water source.

A submersible turbine system is much simpler. The motor and pump are coupled directly together, so no shaft stabilization is needed. No additional moving parts are necessary.

Larger Well Casing

To accommodate the larger diameter of a VLST and its associated drive shaft hardware, the well casing must be larger than with a submersible. This can dramatically increase installation costs. In addition, a submersible may be installed in a well that isn’t completely straight, whereas a well with a VLST must be completely straight to avoid shaft fatigue and premature failure.

Pump House Requirements

The need for a pump house is a significant consideration for any installation. Since a VLST motor is aboveground, it usually necessitates a pump house to protect it from weather and vandalism, as well as to suppress noise. This adds considerable construction costs to the installation.

On the other hand, a submersible turbine system does not require a pump house at all. Its underground location reduces risk of damage from weather, vandalism, and accidents. In addition, a submersible does not generate the noise of a VLST, so it can be placed almost anywhere.

Continuing Maintenance

Over time, nothing will be easier and cheaper to maintain than a submersible pump/motor combination. A VLST has more moving parts that call for routine maintenance, requiring that an inventory of spare parts be kept on hand. In addition, a VLST requires regular lubrication, packing, and shaft/impeller adjustments. This simply isn’t the case with a submersible. It is designed to spend its entire life under water with no additional maintenance.

Efficiency and Starting

It is true that aboveground motors are slightly more efficient than submersible motors on the drawing board. However, in actual VLST applications, this motor efficiency is lost due to frictional losses in the line shaft. These losses are typically one to two horsepower per 100 feet of depth and generally exceed the electrical losses in the drop cable for a submersible pump/motor.

In a VLST installation, the motor must not only get the pump moving, but also the entire drive shaft and its associated hardware. Submersible installations have a lower moment of inertia, which means faster and easier starting.

Subs Just Make More Sense

Although every installation is different, the long-term costs of a VLST generally outweigh those of a submersible for several reasons. First, the pump requires more material to accommodate its larger diameter and/or additional stages. Second, a VLST requires a larger drop pipe to make room for the drive shaft. The deeper the well, the longer and costlier this becomes. Finally, a VLSTs installation and maintenance costs exceed those of a submersible. The longer, straighter borehole is more expensive to drill, and the complexity of the installation adds labor time. When it is necessary to pull a VLST, the difficulty in doing so inflates already high maintenance costs.

All of these factors contribute to the logic of making a submersible pumping system your first choice. Franklin Electric’s submersible turbines, motors, and control panels are designed to work most efficiently together. They are backed by an experienced submersible support team that includes the Franklin hotline and on-site field service network, both of which are offered at no charge to users of Franklin Electric products. No other company offers such a complete system solution exclusively for you. Overall, submersibles and Franklin Electric just make more sense.