Water Well Journal
Issue link: http://read.dmtmag.com/i/728580
cal speeds can be addressed by the pump's supplier or manu- facturer at the time of the order placement and designed "out of the system." In addition to the critical speed concern, a further examina- tion of our example indicates reconsideration of the lineshaft diameter is also warranted. Although all the design criteria in- dicate use of 1-inch lineshaft is acceptable, further investiga- tion and a designer's instinct also shows that the installation would be better served if the lineshaft size (diameter) was increased to 1.25 inches. This increase in size is performed at a nominal increase in cost and only requires .45 more horsepower for the entire in- stallation, or a total motor load of 40.78 HP—well under the maximum allowable level of 46 HP. The additional strength and rigidity afforded from upsizing the lineshaft to 1.25 inches will pay marked dividends throughout the projected service life of the installation, especially while operating on a VFD. This drives to the heart of my earlier statement to always go back and review the design factors and always use your experience and judgment when designing water systems and well pumps. Your initial design decision may not be the best or final decision. Any concern with the alternate use of a control valve in- volves the potential for higher than desired horsepower draw at low flow, valve cavitation, and thermal heating of the water behind the control valve. In our example, the system design is predicated on the use of interim pressurized storage to avoid continuous operation through the use of either device, so these issues are not expected to be of concern. Life Cycle Cost Analyses A life cycle cost (LCC) analysis includes the following elements: LCC = C IPC + C IN + C E + C OL + C M + C DT + C ENV +/- C DD where: C IPC = Initial purchase cost (system or element) C IN = Installation and commissioning costs C E = Energy cost to operate C OL = Ongoing operating labor costs C M = Maintenance costs; labor and materials C DT = Equipment downtime and loss of production C ENV = Environmental costs C DD = Decommissioning, disposal, and salvage costs. Fortunately, I have access to several economic evaluation software programs and spreadsheets to assist with determining the best economic options. In this case I used an electronic spreadsheet. During the separate life cycle analyses, we included the following factors: VFD installed costs: 40 HP, 460 VAC, VFD ($5000) + installation and wiring costs ($3400) = $8400. CV installed costs: 6-inch CV ($1200) + installation costs ($2200) = $3400 (difference over a motor starter). Pump costs (for pump option #1): $15,900: 10-inch × 7 stage VTP set at 150 feet on 6-inch × 1-inch water lubricated 40 HP motor. Pump and motor installation costs: $2000 for all pump options (included in installation cost estimates). Maintenance costs: $1000 per year for all options. No salvage value or decommissioning costs assumed. Operational hours per day: Assumed five hours per day of operation at reduced flow. This roughly equates to the average daily demand of 35,000-40,000 GPD at 156 GPM. Power costs (per kilowatt hour): $0.12—no other fees or charges added. During the individual life cycle cost analysis, the reviews were based on three separate life spans for each device: In each example, use of a variable frequency drive over a control valve provided the lowest life cycle cost. Once again, it is important to note these relationships applied to this exam- ple only and may or may not also apply to other life cycle cost scenarios. Each prospective installation must be evaluated individually and by using the pertinent factors fairly and objectively. A more efficient pump will generally recover a cost differ- ence over a less expensive pump in enough time of operation from the difference in energy costs alone. The life cycle costs for Pumps #1, #2, and #4 are reflected in Table 1 with VFD operation (using an adjusted TDH) and were based on a 20- year service life. This life cycle cost analysis was conducted to assist with the selection of Pump #1 over the other three possible units. The spreadsheet results are shown in the table. In a potential installation such as this example, with a 10- inch bowl diameter VTP installed in a 12-inch well casing, there are limited opportunities to use a two-pump system where a smaller pump could be used for the lower demands and as a backup to the larger pump due to size and physical restraints. Any serious consideration of reverting to no less than a two-pump system with a VTP would require serious consider- ation of a moderate to large volume atmospheric water storage vessel with one or two booster pumps at a potential cost ex- ceeding $50,000 with the well pump feeding directly into the tank, which may or may not be considered by the client. This type of scenario would generally be met with far more client interest and feasibility if the installation was lower in cost and was performed using two submersible pumps with provisions to allow online swapping of pumps. We will discuss this type of option in the submersible pump example in this series. Summary Performing an economic analysis—a life cycle cost (shown here), equivalent uniform annual cost, capitalized cost, or waterwelljournal.com 64 October 2016 WWJ Control valve Variable frequency drive LCC LCC 10-year life $84,807.27 $80,173.48 (lowest LCC) 15-year life $125,510.91 $116,060.23 (lowest LCC) 20-year life $166,214.55 $151,946.97 (lowest LCC) WATER WORKS from page 62