Water Well Journal
Issue link: https://read.dmtmag.com/i/767379
electrical service size and voltage. The estimated horsepower can be obtained through the following formula: Brake horsepower (BHP) = GPM × TDH 3960 × PE (1) (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: BHP = 250 GPM × 250 feet TDH = 3960 × .75 62,500 = 21.04 BHP (15.7 kW) 2970 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 determina- tions 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 in- stallation 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 evalua- tions where a project either never got off the ground, was tem- porarily 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 proj- ect 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 in- volved in the potential project with the facts needed to make an informed decision whether or not to proceed with the proj- ect. Generally, a preliminary design rarely proceeds past the sketch phase, providing just enough detail to enable individu- als 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 usu- ally 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. WATER WORKS continues on page 34 Figure 2. Submersible pump/motor cutaway view. Not to Scale Twitter @WaterWellJournl WWJ January 2017 33