Water or gas flow in the pipes, gradually loses it pressure, because of the different resistances it meets. These resistances are due to straight pipe resistance or to single casual conditions such as direction changes, section reductions, etc. Therefore the total pressure drop for a pipe system is calculated according to the following formula:

Δp= Δp1 + Δp2

where:
Δp1 is the pressure drop due to straight lengths
Δp2 is the pressure drop due to single localized resistances