It is interesting to mention that if there is a pump which delivers energy to the fluid and if there are viscous losses because of friction with the walls or expansion or contraction, the BE is modified as an engineering approximation :
Example: An inverse U-tube is used to drain water from a tank or reservoir. The bend of the inverted tube is 1m above the free surface of the water in the tank and 10 m above the ground where the water is discharged. Water exits the tube at atmospheric pressure. Determine the pressure in the bend.
(Fig. 20e)
To apply BE, assume , and neglect viscous losses (losses in the bend or due to wall-friction)
, and neglect viscous losses (losses in the bend or due to wall-friction) 
