Water Well Journal
Issue link: http://read.dmtmag.com/i/728580
I n this installment of The Water Works we will expand upon our example pump selection with some additional technical considerations, as well as examine which option for flow control is best—a variable frequency drive or an inline control valve. As a reminder, this example was based on the initial use of a vertical turbine pump (VTP) with 150 feet of 6-inch × 1- inch column and lineshaft setting, installed in a 250-foot-deep well and capable of supplying water to a new subdivision with a projected average-day demand of between 35,000-40,000 gallons per day and a maximum day demand estimated between 90,000-95,000 GPD. In this case, this translates to three separate design flows: 156 GPM for the calculated minimum flow for average day consumption, 392 GPM for the system's maximum day demand, and 500 GPM for the combined duty of irrigation demand coincident with the maximum day demand. The majority of each separate demand is estimated to occur over a period of just four to five hours per day. We will detail much of the remaining well pump selection criteria, including the economic analysis prepared to justify the pump selection along with the flow control method. Two additional points must be made. Usually on most water system designs the source pump (well or booster pump) capacity is based on a singular primary condition of service (COS) since interim storage or additional sources are gener- ally planned to be available to supply any relative shortage of production. However, on a water system design where there is little or no storage and a single source planned such as this ex- ample, I try to efficiently plan the well and pump, considering the full range of demands, and design the pump around the mid-range flow with the pump hopefully capable of extending out to the maximum demand and also working comfortably at the lowest projected flow rate. In this example, I determined designing the VTP for the maximum flow rate of 500 GPM was the most practical. Finally, even though we have tried to design the pump to be as efficient as possible at all three duty points, the reality is water systems do not operate on a specific timetable with predictable flow rates throughout a given day. The fact of the matter is virtually all potable water systems, including this example, can conceivably need to provide anywhere between an almost zero flow rate (usually coinciding with the demand that occurs in the middle of the early morning hours, 2-4 a.m.) up to the maximum demand that can occur during the peak water demand period while grounds irrigation is also required. In these cases, the system designer must carefully evaluate each individual water system demand and decide if a means of flow control or water storage equipped with booster pumping is required to provide water during the low or high demand periods or if the pump can safely operate at the projected low flows. In most cases some type of interim low-flow provisions must be incorporated into the system design, such as a second pump not only to save wear and tear on the primary well pump and related components, but also to save the higher power costs associated with this type of relatively inefficient operation. In our example, where the water system will be connected to a supplementary and backup water supply, we have deter- mined the well pump should be designed for all of the pro- jected system demand conditions, although we also know the water system will at times only require a demand of 20 GPM or less. For this reason, as a prudent designer, we have in- formed our client that an additional device and a moderate volume of water storage, likely in the form of a hydropneu- matic pressure vessel, is warranted to be added to the installa- tion to protect the pump and system against these low flows and extreme pump cycling conditions. The two choices we are examining for flow control for our imaginary client are a variable frequency drive (VFD) or a control valve (CV). A thorough analysis and review of both types of flow control devices were outlined in the October and November 2003 issues of Water Well Journal ("Control Valves vs. Variable Frequency Drives: Facts and Fallacies"). Please refer to these two columns for a full discussion of the backgrounds and associated pros and cons of each method. The Example Design In the last installment of The Water Works, from a careful and complete review of four potential vertical turbine pumps, we selected Pump #1 as our well pump. The efficiency of our ED BUTTS, PE, CPI THE WATER WORKS ENGINEERING OF WATER SYSTEMS Part 13—Vertical Turbine Pump Design (3) WATER WORKS continues on page 60 This will help the designer and client make informed judgments on a potential purchase well before it is actually made. waterwelljournal.com 58 October 2016 WWJ