What is the orifice plate? What is the function?
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Release Time：2022-08-02
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, the local resistance of the orifice plate will cause energy loss, because the pressure of the fluid is reduced. This phenomenon is thermodynamically known as throttling. This method is simpler than using a regulating valve, but it must be properly selected, otherwise the liquid is prone to cavitation and affects the safe operation of the pipeline.
When the pressure difference between the front and back of the pipe is large, the orifice plate is often used. The principle is: when the fluid flows in the pipe, the local resistance of the orifice plate will cause energy loss, because the pressure of the fluid is reduced. This phenomenon is thermodynamically called throttling. This method is simpler than using a regulating valve, but it must be properly selected, 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 small 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 pressing force increases, but the downstream pressing force will not fully recover to the upstream pressing force, which is the result of internal turbulence and greater energy consumption. If at the corresponding liquid temperature, the compression force 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 pressing force 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. Bubbles will quickly coagulate and collapse under high pressure if the downstream compression force is higher than the saturated vapor pressure of the liquid. 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 the impact force, and then compressed and condensed by high-pressure water. This is repeated many times, resulting in a sand similar to what we can imagine. The noise of the gravel flow passage pipe is called cavitation. The surface of the flowing material is fatigued and severely damaged under the action of water hammer pressure. We put the bubble formation, development and rupture of the whole process, resulting in material damage, known as cavitation phenomenon.
In order to calculate the differential pressure of the orifice plate, a new concept needs to be introduced, namely-differential pressure PS. When the pressure difference p across the orifice plate increases, the flow rate qm also increases. When the pressure p increases to a certain value, the pressure pvc at the constriction drops below the saturated vapor pressure pv of the fluid, and part of the fluid vaporizes, so that the pipe flow no longer increases with the increase of the pressure difference, forming the so-called choke phenomenon. At this time, the pressure difference between the two ends of the orifice plate is called the pressure difference PS of the blocking flow. When the actual pressure difference p of the orifice plate is less than its corresponding PS, flashing or cavitation can be avoided. When the pressure difference between the two ends of the pipe is large, multi-stage pressure reduction can be used, but the actual pressure difference p of each stage orifice plate should be less than the PS corresponding to the inlet of the stage.
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