Leakage flow is referred as the volume flow in a centrifugal pump that reaches the outside from the shaft seal. Such leakage losses occur, e.g. at the sealing face between the rotating pump shafts and the non-moving pump housing, but they also occur, among other things, with multi-stage pumps between two stages.
The sign for leakage flow is QL. The magnitude of the leakage flow depends not only on the viscosity of the medium transported, but also on the size of the pressure difference between the delivery chamber and its environment. The shaft seal has the task of reducing the leakage flow to a minimum in order to confine the wear of the sealing face.