Part 14(a)—Submersible Pump Design, Part 1
By Ed Butts, PE, CPI
With regard to pumping from a deep well, there has probably been no more worthwhile and valuable improvement over the past 50 years than the submersible pump.
This type of pump combines the concept of a multi-stage vertical turbine pump, using an axial/mixed flow design, with a multi-horsepower and phase electrical motor designed to operate under hundreds of feet of water. This enables a reasonably low-cost and efficient lifting of water from water wells sometimes thousands of feet deep.
In the next three installments of The Water Works we will shift gears somewhat and vary our previous discussions of using a vertical turbine pump (VTP) to outline the alternate and currently popular method of using submersible pumps to deliver water for municipal, irrigation, industrial, commercial, and general water supply applications.
Knowledge of the inherent conditions of service in capacity and head is critical information needed by the designer throughout the entire design phase.
Disclaimer: The proper application of larger horsepower submersible pumps is a specialized engineering task requiring consideration of many individual factors, particularly the electrical power supply. The reader is cautioned to exercise appropriate judgment in the use and application of the following information, seek technical assistance whenever warranted, and follow all manufacturers’ guidelines—especially when considering very deep well or large horsepower applications. Please also follow all application and installation specifications and recommendations and all applicable regulatory codes.
Submersible Pump Concepts
The use of a submersible pump to deliver water and other fluids from deep wells, possibly thousands of feet deep, is not a new or unknown concept. This method of pumping has been in widespread use to date for more than 75 years and has overtaken the use of VTPs for many common potable water applications in recent years.
Other than the typically higher operating speed of 3600 RPM, the principle behind the use of a submersible centrifugal type of pump, equipped with several stages of individual impellers and diffusers commonly assembled in a single stack, closely mirrors many of the design and construction features of a VTP.
Submersible pumping units for commercial, industrial, irrigation, and municipal potable water supply are now available in capacities from just a few gallons per minute to more than 10,000 GPM; heads (TDH) that range from 50 feet to more than 1000 feet; motor and pump design speeds from less than 1200 RPM up to 3600 RPM; voltages from 240 volts single phase to more than 4600 volts three phase; motor horsepower from 3 HP to more than 2000 HP; pump and motor unit diameters from 4 inches to more than 24 inches; and discharge assemblies including several that are below and above ground configurations (Figure 1).
It is easy to see almost any conceivable freshwater and wastewater applications can be fitted with a submersible pump.
The basic hydraulic design of each pump is virtually identical using the well-known concept of multiple stages of combined impeller/diffuser assemblies with many of the smaller diameter (6-inch and 8-inch) VTP bowl assemblies undergoing modifications at twice their originally intended design speed for submersible applications.
Thanks to the affinity laws, these units are now commonly used for the higher capacity, head, and horsepower requirements found in many deeper or smaller diameter wells. There are various design factors associated with the successful design of a submersible pump for larger applications and we will detail each one separately.
(Basic Pump Selection)
Just as with a vertical turbine pump, the preliminary selection of the pump and motor, based on the limitations of the source and available power, is generally the first step to take with a larger submersible pump installation as well.
Knowledge of the inherent Conditions of Service (COS) in capacity and head is critical information needed by the designer throughout the entire design phase, but particularly important when initially evaluating the possible size range of the prospective pump and motor. Although a working knowledge of the source, usually a water well, is also vital, the COS is generally accepted as the most important single factor to initially understand if the job can even be done. Knowledge of the inherent conditions of service in capacity and head is critical information needed by the designer throughout the entire design phase.
Through a knowledge of the COS and consulting with the power utility and relevant pump curves, the designer can generally and quickly determine the approximate horsepower requirements, bowl and motor diameter range, approximate riser pipe size and type, available well clearance, and the operating voltage and phase required for the project. A submersible pump and motor each have unique construction features needed for the higher speed and deeper well service, as illustrated in a cutaway drawing (Figure 2).
Once knowledge of the intended capacity and head are estimated, the designer should determine the estimated horsepower that will be required to enable estimating the needed electrical service size and voltage. The estimated horsepower can be obtained through the following formula:
(1) Pump efficiency. For submersible pump estimating between 0 to 100 GPM, use a PE of 60% (.60); for 100 to 200 GPM, a PE of 65% (.65); and for flows above 200 GPM, use an estimated PE of 75% (.75)
Therefore, for an assumed project of 250 GPM at 250 feet TDH, the estimated horsepower requirement is:
Through use of this formula, the designer and owner are able to provide important data to the power utility which will enable an estimate of the necessary power supply, including voltage and current capacity. These preliminary determinations are used to inform the client of the projected installation elements and limits, which then leads to the preparation of an estimated cost, often referred to as an “engineer’s estimate.”
It must be remembered this type of water supply project usually includes many ancillary factors beyond the pump installation alone (refer to Figure 1). These include: riser pipe and drop cable, on- and off-site piping, operational and safety valving, electrical service (which may or may not include a standby generator) and controls, and a secure protection means (usually some form of a pump house). Therefore, ensuring the client is fully informed of the project’s potential design elements, limitations, and estimated costs can prevent much wasted work later if the client opts to suspend or cancel development.
I have been involved with numerous preliminary evaluations where a project either never got off the ground, was temporarily or indefinitely suspended, or had to be scaled back simply because one or two elements of the project either could not be reasonably satisfied or the estimated costs for the completed project were higher than the client expected or budgeted.
Obviously, in cases where all aspects of the potential project are known, this step may not be needed. But in many larger water supply projects a preliminary evaluation, design, and cost estimate is worth the cost and time.
A preliminary evaluation typically involves three to four individuals, often engineers or support staff, who are trained and experienced in examining hydraulic and pumping criteria and modeling needed in preparing conceptual designs along with project cost estimates to assist the various people involved in the potential project with the facts needed to make an informed decision whether or not to proceed with the project. Generally, a preliminary design rarely proceeds past the sketch phase, providing just enough detail to enable individuals with the data necessary to evaluate conceptual designs.
Typically, there are three crucial items to evaluate in a preliminary design concerning a potential submersible pump installation.
The first surrounds the available electrical power supply in the area, as routing high voltage primary power supplies to a region for the simple task of operating a single well pump is generally considered cost-prohibitive. The second factor usually involves source (the well) limitations, such as inadequate capacity or too limiting of a well diameter for the required flow, sand, or well hydraulic. The final item is water quality issues including cascading water, air entrainment, or the presence of regulated or unregulated contaminants (arsenic, nitrates, iron, hardness, or manganese).
Each of these potential factors and all potential solutions with assigned cost estimates provided can be examined in the preliminary report before proceeding to a final design process.
Final Pump Selection
Once the preliminary design and cost estimate has been submitted to the client and accepted, the next design step involves final selection of the pump and motor. Where a VTP is somewhat a unit comprised of many pieces of various types of different equipment assembled and installed together at the site to create a single pumping unit, a submersible pump is largely comprised of the pump and motor often assembled into a single unit at an offsite factory or manufacturer’s facility and shipped to the site for installation.
The selection of the submersible pump and motor must be performed so the final product is a cohesive unit, designed and intended to operate together as a single pumping machine. Based on the preliminary design, an intimate knowledge of the system, pump, and motor parameters and limitations are usually well understood by this stage.
The steps in Table 1 comprise a selection outline and each step is generally indicated and examined during the preliminary or final submersible pump selection. Not all the steps will apply to every installation, but each one is nonetheless important to a successful and long-lasting submersible pump installation and each should be considered.
Although the pump’s hydraulic and mechanical design is obviously important, the primary factors to a successful and long-lasting submersible pump installation usually rest on two individual—but equally important—factors.
(#1) The installation and placement of the pump to ensure there is always adequate submergence over the inlet (pump suction) to prevent cavitation or vortex conditions as well as the proper location of the motor within the well in order to be sure adequate water velocity and volume passes over the motor or an alternate method of motor cooling will occur to protect the motor at all flow rates.
(#2) A full evaluation of the current or proposed electrical installation to verify there is or will be adequate starting and running voltage available, along with a full and balanced three-phase power supply from the utility (where applicable) or generator, and the use of proper motor controls, including (at a minimum) short circuit, overload, and phase loss and reversal protection, and low water level protection.
I cannot stress enough the relative success of a submersible pump and motor installation is heavily vested in the designer’s careful and diligent procedures in each of the steps in Table 1 —not only to ensure a satisfied customer, but to provide a degree of self-satisfaction.
We will review each step in sequential order in the next two installments. Until then, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.