Pressure distribution field within the separation chamber of hydrocyclones is qualitatively and quantitatively analyzed in the context of the design of compact plants for the on-board treatment of ballast water. Once the qualitative analysis has been carried out, the conclusions arising from this analysis have been evaluated by defining a parameter termed "asymmetry coefficient", which provides quantitative information about the behaviour within the hydrocyclone. The influence of the vortex finder both on flow patterns and on air-core precession is successfully studied. As well as giving an insight into hydrocyclone flow lines behaviour, this article attempts to improve operational efficiency by modifying several design parameters in order to provide a stabilization of flow lines (reducing turbulence). (c) 2007 Elsevier B.V. All rights reserved.