Flow rate escaping from pressure compartments in spite of seals, particularly in systems with purely metallic seals, without contributing any useful work. Apart from the geometric variables associated with the site of the leak, the size of any leakage will depend on the pressure difference between the pressure compartment and its environment, as well as the viscosity of the medium escaping. With leakage from an eccentric ring gap (flow within gaps) of the kind associated, for example, with piston pumps and valve control spools, the following applies as per Hagen-Poiseuille's law:
where d m = average diameter of the sealing point, h = size of the gap, l = length of the sealing point, η = dyn. viscosity, e = eccentricity. A leakage in continuously adjustable valves is the flow rate from the pressure line to the return line where sealed working lines are involved. With continuously adjustable directional valves, it changes with the input signal and reaches its highest level at hydraulic zero (see DIN 24 311).