Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
Referring back to the January column of The Water Works, there are actually three distinct flow rates the pump must be designed for: the average day flow = 156 GPM, the maximum day demand = 392 GPM, and the primary design condition = 500 GPM. This wide variance in the flow rate means this pump must be comfortably able to operate across this same wide and dis- parate flow range. This is a somewhat simple design for head since either a control valve or VFD will be used to control dis- charge pressure. When the preliminary design was performed, we deter- mined the conditions of service of the pump to be 500 GPM at 269′ TDH. However, since our original estimate of the column friction loss was higher than reality by 9′, the TDH will be lowered to 260′ to coincide with this lower value. Additionally, due to the available size of the well's "pump chamber" (the well casing I.D. = 12″) and required capacity of 500 GPM, we also know we can safely use either an 8″ or 10″ diameter bowl assembly. The basic difference between the two will become apparent by the number of stages that will be needed to generate the total head. In our example, a 10″ diameter bowl was selected. This size bowl fits well and provides adequate clearance in the 12″ well casing. A 10″ bowl typically displays the highest effi- ciency between an 8″ and 10″ diameter, and fewer stages are required to generate the needed head—a substantial savings in capital investment. Once we know the basic bowl diameter, we can then go to the various methods of selecting a pump. Typically, in the "old days" a VTP selection was conducted through a pump catalog or through a distributor. In today's world we now have access to various pump selection software programs that enable us to select a sample pump, compare dif- ferent models from the same manufacturer, or evaluate alter- nate selections from other manufacturers. In normal practice, I would have generally examined various 10″ bowl curves from up to two or three different manufacturers, if possible. As much as I like to tout professionalism and always pick- ing the single right pump for our clients, at some point the realities of life and commercial practice must creep into the equation. The reality in this case is that most of us simply don't have the time or access to all the pump brands and mod- els that might be available. The fact of the matter is most brands of pumps have a territorial dealer network and are rep- resented by a specific dealer in a specific geographic region. We are typically limited to picking a pump from the single or maybe two or three brands they represent. In spite of this limitation, in a competitive situation I still try to compare the pump I have selected to the other pumps I know or surmise have also been selected. In this instance, however, let's assume we can pick from several different brands. WATER WORKS continues on page 56 Figure 1. Pump #1 with seven stages, open impellers. K = 7 lb./ft Max. lateral = .75″ WWJ July 2016 55 Twitter @WaterWellJournl