Choosing Between Vertical Lineshaft and Submersible Turbine Pumps: What's Right for Your Jobsite?
Pump and motor technologies have evolved to address changing customer demands over recent years. This white paper takes an objective look at what both systems provide – and outlines the critical questions professionals should ask when selecting the best option based on application, operational and maintenance considerations.
Application Considerations
What flow rate is required?
Flow rates may be the deciding factor when choosing between a vertical lineshaft turbine (VLST) and a submersible pump. Submersibles are typically operated at higher speeds than VLSTs, enabling them to move more water with a slimmer diameter pump. Some submersible turbines feature a bowl size up to 11" with flow rates up to 2,250 gpm. In contrast, a VLST has bowls as large as 42" and flow rates up to 40,000 gpm.
Bottom line: Submersibles are ideal for higher speeds.
Is the well straight or misaligned?
For a misaligned or crooked well, professionals will want a submersible turbine, as a VLST requires a straight well to prevent damage to the pump shaft and bearings. Before installing a VLST, many engineers require a plumbness test. This confirms that the well casing has no bends and that a VLST can be installed without damage to the bowl assembly, shaft, bearings or discharge head.
Bottom line: VLSTs require a straight well.
Is the application a top feeding well or open water?
If the well receives its water supply from the top formations, this is considered a top feed well. For a top feed well, a VLST is the right choice since it assures adequate cooling. When using a submersible, professionals will need to use a flow sleeve to help water flow past the motor and keep the motor cool. Improper motor cooling is the #1 cause of failure on a submersible motor.
Bottom line: VLSTs will perform optimally in top feed wells.
What role does price play in your decision?
With both types of pumps, pricing can depend on several variables: How deep are you setting the system? What size wire will you need? What horsepower do you require? Smaller horsepower motors are typically less expensive in a submersible application. In larger applications, a VLST motor may be less expensive, but a permanent magnet submersible motor may save the customer electricity consumption costs.
Bottom line: Several factors can affect cost beyond the initial price
of the equipment
Is a power source readily available or will you need a diesel-driven backup option?
VLST pumps with a gear drive can work with a diesel backup. You can also install a generator to operate an electric-driven lineshaft or submersible pump.
Bottom line: There are options for both.
Operational Considerations
Is the customer looking for a more efficient motor?
Many municipalities are now requiring high efficiency pumping systems. VLST above ground motors operate with +90% efficiency that has trumped induction submersible motor efficiency for decades; however, some manufacturers now offer permanent magnet submersible motors with equivalent +90% efficiency. These newly designed motors help level the playing field making this a tougher decision.
Bottom line: There are options for both.
How will you operate, control and protect the system?
A variable frequency drive (VFD) can operate either a VLST or submersible turbine. VFDs provide soft start capability to reduce the inrush current at startup and prolongs motor life when compared to a standard across-the-line pump starter, and real time monitoring via apps on a mobile device. You can program the drive set-up, monitor real time voltage, amperage, pressure settings and more within these apps. The first time a VFD goes into a fault mode, the unit has paid for itself. This type of advance notification prevents catastrophic damage to the pump – and a costly replacement.
A reduced voltage soft starter provides a more basic level of operational options and protection. These starters are designed to reduce the voltage and subsequently the inrush current during motor starting. The lower inrush current lessens the torque generated by the motor during startup, to reduce wear on mechanical parts and improves system lifetime. Some utilities and localities require that a method of reduced voltage starting be used on motors over a given horsepower to reduce demand fluctuations and peak current draw.
Bottom line: If you want to extend the life of the motor and overall system, a soft starter or VFD is important. VFDs are preferred for both types of motors.
Maintenance Considerations
How accessible is the motor?
VLST motors are more accessible than a submersible motor. Most of these motors are rewindable and easy to service. Submersible motors require more labor to access them and are not always rewindable.
How often will the pump require maintenance?
Submersible turbine pumps have fewer moving parts and require less scheduled maintenance. VLSTs have shafts, couplings and bearings that need to be lubricated.
Bottom line: Submersibles are easier to maintain but are harder to access.
Which is easier to install?
Because VLSTs have more parts, they require a more involved installation process than a submersible motor.
Need Help Pinpointing the Best Solution?
Whether you need to specify an entire system or replace a single part, there are online tools and resources that can help you pinpoint the ideal solution. Franklin Electric’s FE Select allows you to easily size, configure, price and generate a quote that includes downloadable assets like performance curves.
