Throttle orifice

The function of orifice plate is to reduce the diameter of orifice at the proper place in the pipe. When the liquid passes through the orifice, the flow beam will become thin or shrink. The smaller cross section of the beam appears downstream of the actual necking, which is called the contraction section. At the condensation section, the flow velocity is larger, and the increase of flow velocity is accompanied by the decrease of pressure at the condensation section. When the beam expands into a larger region, the velocity decreases and the pressure increases, but the downstream pressure does not completely recover to the upstream pressure, which is due to the larger internal turbulence and energy consumption. If the pressure PVC at the condensation section falls below the saturated vapor pressure PV at the corresponding temperature of the liquid, the vapor and dissolved gases in the stream will escape and form small bubbles in which the vapor and gas mix. The lower the pressure, the more bubbles. If the downstream pressure P2 of the orifice plate is still lower than the saturated vapor pressure of the liquid, bubbles will continue to form in the downstream pipeline, and the liquid-vapor two-phase mixing exists. This phenomenon is flash evaporation. If the downstream pressure restores to higher than the saturated vapor pressure of the liquid, the bubbles rapidly condense and break down under the action of high pressure. At the moment of bubble breakdown, local cavities are generated, and high-pressure water flows to the space occupied by the original bubbles at a very high speed, forming an impact force. Because the gas and vapor in the bubbles do not dissolve and condense completely in an instant, they are separated into small bubbles under the impact force and compressed and condensed by high-pressure water. This is repeated many times and produces a kind of noise similar to what we can imagine when sand flows through the pipeline. This phenomenon is called cavitation (see figure 2). The surface of the runner material is subjected to water hammer pressure, causing fatigue and serious damage. We call cavitation phenomenon the whole process of bubble formation, development and rupture so that the material is destroyed.