Determining the Net Positive Suction Head of a Magnetic Drive Pump *

Cavitation accompanied by metal removal, vibration, reduced flow, noise, and efficiency loss can occur in the operation of a pump if the suction pressure drops below a critical value. Cavitation can be avoided by maintaining or exceeding the required net positive suction head, NPSH_r. This paper describes a simple and inexpensive laboratory set-up for determining the NPSH_r and developed head for a centrifugal pump in a closed circuit by observing the onset of cavitation as the temperature of the feed reservoir is increased. A secondary, but also very important, purpose of the exercise is to provide students with an opportunity to observe cavitation in a laboratory setting. In examining the results from the experiment, NPSH_r ranged from 0.8 to 3.3 m (2.6–10.7 ft), which is reasonable for small pumps, and increased with pump capacity. The developed head ranged from 2.2 to 5.9 m (7.2–19.2 ft) and decreased with pump capacity. The experiment could be improved by employing a 3600 rpm pump with a larger impeller, which would then cavitate at a lower suction temperature and permit a wider range of operating flow rates and the ability to generate a pump curve.