When a medium flows through a pipeline system, the energy of flow – equal to pressure energy + kinetic energy + potential energy – decreases in the direction of flow. Part of the flow energy is converted by the action of friction into forms of energy (e.g. internal energy, acoustic energy) that cannot be utilized technically. Flow energy losses in pipelines have a draining effect on the pressure energy, resulting in pressure loss. Using the extended Bernoulli equation, it is possible to express the pressure loss between two points (1 and 2) as follows (pressure loss = p_V):

p_v = s · g (z₁-z₂) + (w₁²-w₂²) + (p₁-p₂)

s = Density of the medium

g = Acceleration of gravity

z = Height coordinates

w = Flow velocity

p = Static pressure

In the special case of a horizontal pipeline (z₁ = z₂) with constant pipe diameter (w₁ = w₂), the pressure loss equals the static differential pressure p₁-p₂. The following formula with the coefficient of loss z is generally valid:

p_V = z w².

Experimentally determined z -values exist fornumerous pipeline elements. The pressure loss of a straight pipeline with round cross section is calculated by:

p_V = l · · · w².

l = Coefficient of friction

I = Length of pipe

d = Diameter of pipe