Part 1 (Pumps & Systems, August 2012) discussed an example in which neglecting corrections for velocity head and gauge elevation did not introduce significant errors in the calculations. Part 2 explores an application for which the error would be unacceptably large if velocity head and gauge elevation were ignored.
This application involves a vertical turbine pump with a discharge gauge reading 30 psig, or 30 x 2.31 = 70 feet. The elevation difference between the gauge and the water level is 803 - 790 = 13 feet. When added to correct the pump head, the new head is 70 + 13 = 83 feet.
Back-reading the pump curve in Figure 2, a 20,000-gallon-per-minute (gpm) operation is observed based on the non-corrected reading. However, only a 9,000-gpm flow is observed if the head is adjusted properly by taking into account the gauge’s elevation difference. In this case, a quick-and-dirty method would imply a pump operating near its best efficiency point (BEP) and with no potential problems. Using the 13-foot elevation correction, the pump actually operates at 9,000 gpm—far away from the BEP—and has a potential for problems.
It is important to know when approximate methods will provide a reasonably close answer versus when taking the simple route can mean errors in the calculations and trouble with the application.