Water Well Journal
Issue link: http://read.dmtmag.com/i/529509
volume of water against a given amount of total head is referred to as the water horsepower formula (WHP). This formula applies to all centrifugal pumps: WHP = GPM × TDH (feet) 3960 (3960 is a factor derived from 33,000 ft-lb per minute of theo- retical horsepower divided by 8.33, the weight of one gallon of water) For example, a water system needing to move 1000 GPM against a vertical lift of 50 feet from a well and delivering 60 psi with an estimated friction loss total of 18.8 feet (use 10% of the sum of the combined lift and pressure): Vertical lift from well = 50.00 feet (+) 60 psi = 60 psi × 2.31 = 138.60 feet (+) Friction losses = 18.80 feet TDH = 207.40 feet WHP = 1000 GPM × 207.4 feet TDH = 52.373 WHP 3960 This is the theoretical horsepower needed for this job, not the actual power. To calculate that, we need to expand upon our above calculation and include the effects of the pump's efficiency at the operating point as a decimal. The revised number becomes the brake horsepower (BHP). For a pump with 80% operating efficiency (.80): BHP = 52.373 WHP = 65.46 BHP .80 This is the amount of horsepower required from an electric motor or engine to perform this job. In the horsepower avail- able from most electric motors, a 75 HP motor will be needed. For those of you in the metric world, you can convert horse- power to kilowatts (kW) by multiplying the BHP number shown above by .746: kW = 65.46 BHP × .746 = 48.838 kW Please remember the above procedure is for estimating only and should not be applied for actual equipment selection without a precise determination of the system's friction losses. I firmly believe this formula, even though somewhat technical in nature, should be understandable and recognizable to all in- dividuals working in or with pumps—for no other reason than to be able to speak intelligently with engineers and clients re- garding how much power is needed to get a job done. So the next time someone tells you he has a 10 HP pump that can deliver 2000 GPM and lift it 100 feet of head at the same time, you can do a quick calculation and tell him he is about 40-50 HP short. Reading Pump Curves All pumps must operate within their respective operating range in order to perform as expected. This means the next ENGINEERING continues on page 44 WWJ July 2015 43 Twitter @WaterWellJournl Figure 1. Example of a typical pump curve.