Water Well Journal
Issue link: http://read.dmtmag.com/i/529509
The third affinity law states: "The input horsepower of an impeller varies as the cube of a change in speed." As in our previous example, the relationship is now the ratio of the two speeds multiplied by itself three times, then multiplied by the original horsepower: 2400 RPM/2950 RPM = (.8136) 3 = .8136 × .8136 × .8136 = .5386 × 47.78 HP = 25.7386 HP (new HP input) As you can see on our variable speed curve, the above cor- rections coincide with the revised conditions at 2400 RPM (COS #2). There are also some subtle shifts with the pump efficiency as well when you modify the speed—but for the purposes of this article, remembering how to modify a pump's perform- ance by using the affinity laws will help you. You'll be able to determine the important aspects of using any type of any variable speed controller, such as a variable frequency drive (VFD) where the proper use of this relation- ship will go a long way towards determining the change in performance with a change in speed or with a change in the impeller diameter. These are what I regard as the vital aspects of pumps you should know or at least know how to find. Next month, we'll look into the aspects of electric motors you should know. Until then, work safe and smart. DACUM Codes To help meet your professional needs, this article covers skills and competencies found in DACUM charts for drillers and pump installers. PI refers to the pumps chart. The letter and number immediately following is the skill on the chart covered by the article. This article covers: PIA-4, PIC-5, PIF-6 More information on DACUM and the charts are available at www.NGWA.org. waterwelljournal.com WWJ Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 35 years experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com. Learn How to Engineer Success for Your Business Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. The hardback book features 37 complete "Engineering Your Business" columns and the figures and tables that originally ran with the articles. The columns cover the groundwater industry and are divided into six areas: contractor classroom, getting started, troubleshooting pumps, rehabilitation, business basics, and tricks of the trade. Visit NGWA's Online Bookstore at www.NGWA.org for more information. ENGINEERING from page 47 vi and ser et y k Mar h es ic duc o our pr ts c H O J T J U S F W E " H S P " 8" ( / N ! T F M B T t / X X X t H S P A WA NG 48 July 2015 WWJ