Assumptions:

• The fluid is incompressible and nonviscous.
• There is no energy loss due to friction between the fluid and the wall of the pipe.
• There is no heat energy transferred across the boundaries of the pipe to the fluid as either a heat gain or loss.
• There are no pumps in the section of pipe under consideration.
• The fluid flow is laminar and steady state.

Bernoulli's Equation is basically a statement of the conservation of energy per unit volume along the pipe.

Energy Density or Energy per unit Volume (SI: J/m³):

For an ideal fluid flow the energy density is the same at all locations along the pipe. This is the same as saying that the energy of a unit mass of the fluid does not change as it flow through the pipe system.

* A compressed fluid or gas has the ability to do work if it is allowed to expand, i.e. it has stored energy. The magnitude of the pressure P is equal to the Potential Energy per unit volume due to the Hydrostatic Pressure in the fluid. Note that the unit of pressure even can be expressed at a unit of energy density, Pa = N/m²=(N^. m)/(m^{2 .} m) = J/m³.
* The kinetic energy density can be though of a pressure exerted by the fluid due to its motion.
* We have already seen that gravitational potential energy density, rgh, is just the pressure of a fluid due to its weight.