Pressure loss in pipes, pipe fittings and valves

Pipe systems are composed of various pipe elements, each with specific properties. When determining pressure losses a distinction is made between pure friction losses in straight pipe elements and the additional losses in pipe fittings and other components such as valves. Unlike in straight pipe elements, further losses occur in pipe fittings due to flow separation or secondary flow, in addition to the friction losses caused by the surface roughness.

The pressure loss in a pipe fitting depends on the type of deflection and is referred to as resistance coefficient Zeta . Resistance coefficients are determined by experiment via a pressure measurement of inlet p1 to outlet p2 of the pipe fi tting and are given as guide values in tables. The resistance coefficient indicates that pressure difference there has to be between the inlet and outlet in order to maintain a certain flow rate through a pipe element.

Pressure difference in straight pipe elements

The pressure difference Δp from inlet to outlet of a straight pipe element results from the friction factor λ, the pipe length L, the density of the fluid ρ and the square of the mean fluid velocity v divided by the pipe inner diameter di.

Pressure differential in pipe fittings

The pressure differential Δp between inlet and outlet of a pipe fitting is determined by the resistance coefficient Zeta, the density of the fluid ρ and the square of the mean fluid velocity v.

Adding all the pressure losses in the various pipe elements gives the system characteristic of the pipe system. The necessary pump head as a function of the flow rate results from the system characteristic.