When the pressure difference between the front and back of the pipeline is large, the orifice plate is often used. The principle is: when the fluid flows in the pipeline, due to the local resistance of the orifice plate, the pressure of the fluid decreases and the energy is lost. This phenomenon is thermodynamically called throttling. This method is simpler than using a regulating valve, but it should be appropriate, otherwise the liquid is prone to cavitation and affects the safe operation of the pipeline.

The function of the orifice plate is to reduce the aperture in the appropriate place of the pipe. When the liquid passes through the neck, the flow beam will become thinner or shrink. The cross section of the beam occurs downstream of the actual constriction and is called the constriction cross section. In the constricted flow section, the flow rate is large, and the increase in flow rate is accompanied by a substantial decrease in the pressure of the constricted flow section. When the flow expands to a larger area, the velocity decreases and the pressure increases, but the downstream pressure does not fully recover to the upstream pressure, which is the result of internal turbulence and greater energy consumption. If, at the corresponding liquid temperature, the pressure pvc of the converging section drops below the saturated steam pressure pv, the steam and gas dissolved in the water will escape from the steam flow, forming small bubbles mixed with the steam and gas. The lower the pressure, the more bubbles. If the pressure p2 downstream of the orifice plate is still lower than the saturated vapor pressure of the liquid, bubbles will continue to be generated in the downstream pipe, and a two-phase mixture of vapor and liquid will exist, which is called flash evaporation. If the downstream pressure is higher than the saturated vapor pressure of the liquid, the bubbles will quickly condense and collapse under the action of high pressure. At the moment when the bubbles burst, a local cavity will be generated, and the high-pressure water will flow to the space occupied by these original bubbles at a higher speed, forming an impact force. Since the gas and steam in the bubbles cannot be completely dissolved and condensed in an instant, they are divided into small bubbles under the action of impact force, and then compressed and condensed by high-pressure water. Repeated so many times, it produces a noise similar to what we can imagine of sand flowing through the pipe, a phenomenon called cavitation. Under the action of water hammer pressure, the surface of the flowing material is fatigued and severely damaged. We put the bubble formation, development and rupture of the whole process, resulting in material damage, known as cavitation phenomenon.

Flash and cavitation: The main difference between flash and cavitation is whether the bubbles burst. For the system pipeline with flash evaporation phenomenon, because the medium is two-phase flow of steam and water, the specific volume and flow rate of the medium are doubled, and the erosion surface is seriously worn, which shows that the erosion surface has a smooth and polished appearance. Flash evaporation also produces noise and vibration, but its sound level value is generally below 80 dB, which does not exceed the allowable range specified in the specification. Otherwise, cavitation will produce serious noise due to bubble collapse and high-speed impact, and the pipeline will produce large vibration. On the surface of a small area of the flow channel, the pressure formed by the impact force can reach hundreds or even gigapascals, and the impact frequency can reach tens of thousands of times per second, which may cause serious damage to the scouring surface in a short time. The performance is that the scouring surface will produce a rough surface similar to cinder. In addition, reactive gases, such as oxygen that escapes from the liquid, give off heat when condensed by the bubbles and can also chemically attack metals.

Whether it is flash evaporation or cavitation, it will cause different degrees of damage to the pipeline, which is detrimental to safe operation. Therefore, when selecting the orifice plate, these two situations should be avoided. Since the pressure downstream of the orifice plate is often higher than the saturated vapor pressure of the liquid, it is important to select the orifice plate to prevent cavitation. In practical engineering applications, it is feasible to use multi-stage throttling orifices in the decompression system. In order to prevent cavitation in the pipeline, the number and diameter of the orifice plate must be calculated according to the actual situation of the pipeline when selecting the orifice plate.