Centrifugal Pumps. E-book. Formato PDF Robert L. Daugherty   -  Forgotten Books, 2017  - 

For the sake of simplicity the water is shown as entering the impeller in Fig. 1 with a positive pressure by which is meant a pressure that is greater than that of the atmosphere. However, the pressure at this point is usually less than that of the atmos phore in which case we call it negative. Likewise the axis of rotation need not necessarily be vertical as shown.