Abstract
For the purpose of adjusting a flow rate in piping systems, the valves are widely used in industry, none the less for undesirable energy losses due to flow contraction. The valve is a sort of devices that forces fluid flow to contract and to enlarge in a pipe, and an orifice is a typical example of it. In such a device, fluid flow exhibits various phenomena, such as separation, contraction and expansion, and if its sectional area ratio of orifice to pipe is small, the injurious effects of cavitation, i.e. noise, vibration and increasing head loss, will be induced. There are a large number of papers on cavitation in valves and orifices, where the cavitation numbers are defined differently by various researchers. However, there are very few attempts to generalize the conditions of cavitation occurring in valves and orifices which have similar flow and cavitation conditions. An analogy can be found between the phenomena of cavitation occurring in such devices, on the basis of experimental results on the most standard ones, namely sharp-edged and cylindrical orifices. On a simple assumption, the critical condition of cavitation occurrence in such ones can be well prescribed universally by the cavitation number which is determined with the static pressure and velocity head upstream of those devices. Furthermore, based on experimental data of not only these orifices but also two-dimensional gate and butterfly valves, it is concluded that an empirical equation on desinent cavitation number is well expressed with area ratio and contraction coefficient which is calculated from two-dimensional potential theory with free stream line.
