The function of orifice is to reduce the pore size in the proper place of the pipe. When the liquid passes through the contraction, the beam will become thinner or contracted. The smallest cross section of the flow beam appears at the lower end of the actual shrinkage hole, which is called the contraction section. At the contraction section, the flow velocity is the largest, and the flow velocity increases with the decrease of the pressure at the contraction section. When the flow beam expands into larger regions, 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 the energy consumption. If the pressure PVC section shrinkage flow down to the corresponding liquid temperature of saturated steam pressure below PV, the stream is dissolved in the water and steam gas, mixed gas and steam to form small bubbles, the lower the pressure, the bubble more. If the pressure downstream of the orifice is still below the saturated vapor pressure of the liquid, the bubble will continue to produce in the downstream pipe and the liquid and vapor mixture exists. This phenomenon is known as flash vaporization (P2). If the downstream pressure recovery than the liquid saturation vapor pressure, the bubble under high pressure, rapid condensation and rupture in the bubble burst, local hole, high pressure water at high speed to the raw bubble occupies the space, forming an impact force. Because the gas bubble in the steam and steam can not be completely dissolved in an instant and condensed under impact is divided into small bubbles, then by condensation compression, high pressure water, so formed repeatedly, and produce a sand flow through the pipeline similar to we can imagine the noise, this kind of phenomenon called cavitation. The surface of the runner material forms fatigue and is seriously damaged under the action of water hammer. We call the formation, development and rupture of bubbles, so that the entire process of material damage is called cavitation.