Water Well Journal
Issue link: http://read.dmtmag.com/i/617280
There are two common design variables we must look at regardless of which type of well pump we use. The first is the required pump capacity and the second is the needed pumping head. Step 1: Determine the Required Pump Capacity The first step in designing a functional water system is to determine the required capacity from the source, bearing in mind there is an upper limit to the safe source capacity, which in this case is 1000 GPM. Always remember this initial step is not always as easy as it may seem to be. Not only do you need to evaluate and cal- culate all of the needed daily volume and flow rate from the source—in this case a well— but you must ensure there will also be enough water to handle the ancillary loads such as fire protection, irrigation demands, any commercial or industrial uses, and the volume of lost water through leakage, inaccurate metering, pirated water, and other uses. In our example, we will assume the developer has negoti- ated an agreement with the neighboring city to purchase water for fire protection and peak demands. But all other water sys- tem demands—the maximum daily demands, grounds water- ing, and the average leakage and lost water—to the extent possible must be accommodated from the source. This can be a tricky step since you must do your best to de- liver as much as possible from the source without overpump- ing the source or exceeding the capacity of the source pump. Given that fire protection and peak demands will be automati- cally delivered from the supplementary source, it is not nearly as critical to design the source for those volumes, but you still want to do a good job for the client. So size your source accordingly. This is where some engineering and thought must enter the picture. As I indicated in a previous article, the typical home consumes most of its water during a four-hour window in the day, from 7-9 a.m. and 7-9 p.m. In our example, we shall as- sume the daily demand of residential usage also occurs during those same four hours each day (240 minutes). Although not totally realistic, it provides a means of a design. Domestic water demands Average daily demands: 500 residents at 75 GPCD = 37,500 GPD/240 minutes = 156 GPM. Maximum day demands: 500 residents at 188 GPCD = 94,000 GPD/240 minutes = 392 GPM. + Common grounds irrigation 5 acres at .25 inch per day of consumptive use (including efficiency) × 27,150 gal/acre-inch= 33,938 gal. This formula indicates the common areas of the grounds will need around 34,000 gallons of water per day to avoid stressing the grass and landscaped areas. The landscape archi- tect has informed us the irrigation cycles are set so the water must be delivered over six hours (360 minutes) each day. Therefore: 33,938 GPD ÷ 360 minutes = 94 GPM. + Estimated 3% of water loss/day Adding the two flow requirements = Maximum day demands: 392 GPM + Irrigation demands: 94 GPM = Subtotal: 486 GPM × 1.03 = 500 GPM This capacity is well under the maximum well capacity of 1000 GPM, so the well is deemed safe to use. As I previously cited, all the other water demands such as fire protection and water system peak demands are intended to be handled from the supplementary and backup water connection to the neigh- boring city. It is important to note there are numerous formulas and calculation methods used to determine the required capacity needed for a subdivision, city, or development. In our case, the 100 homes are estimated to consume around 4 GPM on a maximum day, with the peak demands coming from the adja- cent city. With no disputing information available, this value is fairly close to what is seen in practice. Typically, the larger the water system, the lower the aver- age water demand observed for each home. For example, when the size of the water system approaches 1000 homes or more, the average use per residence can fall to 1 GPM or less. Rarely are water system designs this cut and dried. Almost inevitably, the added duties placed on the water system for fire protection and peak water system demands must also come from the water system itself. This is often in the form of elevated water storage or ground-level reservoirs on higher ground, such as a nearby hill or with a booster pump station on flat ground within the system. In our case, the pumping rate from the well will include a safety factor to increase the flow above 500 GPM to allow for 40 January 2016 WWJ waterwelljournal.com WATER WORKS from page 39 Figure 2. Well profile example