Water Well Journal
Issue link: http://read.dmtmag.com/i/668983
of these vapor bubbles as they reach a point of higher pressure. The NPSH for a bowl is a specific value calculated for that particular pump. It is basically the water head needed by the bowl to prevent cavitation. At sea level, atmospheric pressure is equal to 14.7 psi, which when multiplied by 2.31, yields 33.95 feet of theoretical head. Since there is no such thing in nature as a perfect vacuum, 25 feet of head is the commonly used value of available atmospheric pressure at sea level. This is referred to as the NPSH available (NPSHA). Common NPSH required (NPSHR) values for turbine pumps range from 5–10 feet. Based on a 10-foot NPSHR value, this pump would be able to actually lift water 25 feet minus 10 feet = 15 feet below the impeller. Deep well turbine pumps generally do not lift water below their intake and, as it is with all centrifugal pumps, the first (lowest) impeller must be primed or flooded with water when starting the pump— regardless of where the water level is while operating. Always remember the NPSHA must be greater than the NPSHR to prevent cavitation. For most deep well VTP designs, as long as sufficient submergence over the bowl is maintained and the water level doesn't drop below the top of the bowl, NPSH will generally not be a factor. I recommend using the top of the bowl assembly rather than the inlet for the minimum submergence requirement to be safe. In most cases, this value will provide an adequate safety factor to protect the pump from both vortex and NPSH problems. One last point on NPSH and air entrainment: While the noise associated with the NPSH problems and air entrainment sounds similar and the damage to the pump (impeller and pump bowl pitting) is often the same, the two conditions are different from each other. NPSH problems are related to suction and inlet conditions only, while air problems and introduction can occur even when the NPSHA is more than adequate. The correction of one problem may or may not solve the other. Sand Conditions This is an important factor to remember. Column (riser pipe) size is based on a combination of offsetting factors—a minimum discharge velocity must be maintained to prevent settling of any large pumped sand or gravel back into the bowl during or after operation. However, friction loss should be kept to values as low as possible to lessen the impact on the WATER WORKS continues on page 44 Figure 1. Column friction loss chart WWJ May 2016 43 Twitter @WaterWellJournl