On Peculiar Flow Characteristics in Hydraulic Orifices

A key component of hydraulic fluid power systems - the standard orifice and, consequently, all equivalent components - apparently has, to this day, some mysteries yet to be unveiled. Knowledge on cavitation-induced liquid flow choking or saturation, which is a well founded topic in some areas of the wide field of hydraulics, e.g. water distribution piping systems, is practically neglected when assessing the design of typical mineral-oil-based power generation and control systems, for both mobile and industrial applications. This conclusion holds true at every level of study, from the technical reference literature adopted by designers to the more popular textbooks and journal papers. Moreover, the rare works addressing the phenomenon are focused on the underlying physical mechanics, completely missing any kind of evaluation of the functional consequences, especially the need to "revise" the standard quadratic law of turbulent flow. Prompted by one of these works, a preliminary experimental activity has been carried out, aimed at determining the actual flow characteristic of standard screw-in orifices used in fluid power pilot circuits. The results confirmed the undoubted presence of flow saturation; based on that, a suitable theoretical description was developed, and some practical applications are outlined in the paper. Finally, few open questions are listed, which need to be answered.