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The reason why the orifice plate must be installed in front of the control valve is as follows: ① In the theoretical derivation for measuring flow rate using a throttling device, there are two assumptions: namely, that the fluid flow is continuous, and that the law of conservation of energy applies during the fluid flow process. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, to avoid increasing measurement errors ； ②Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases ； ③Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable. When throttling devices such as orifice plates, flow measurement instruments, and control valves are installed on the same pipeline, it is generally required that they be placed upstream of the control valve, as most flow measurement instruments need a certain length of straight pipe section; the longer this straight pipe section, the higher the measurement accuracy. The flow state of the fluid changes after passing through the control valve, which affects the measurement of the flow meter. Orifice plates belong to differential pressure flow meters; the pressure drop is very small, so they have little impact on the control valve downstream. The pressure drop of control valves is much greater than that of orifice plates (typically accounting for 30% of the pipeline system’s pressure loss), and this leads to significant pressure fluctuations downstream during regulation, affecting the accuracy of the orifice plate. Of course, there are also issues with straight pipe sections. The gas pipeline issue is the biggest, especially. For orifice plates used for flow restriction, the orifice diameter is calculated using a formula; thereafter, based on this value along with the k value, the critical flow rate pressure ratio (γc) is determined by referring to the “γc-k-d0/D relationship table” in this appendix. When the pressure ratio P2/P1 before and after each orifice plate is ≤ γc, the liquid flow rate can be restricted to a certain value. See the calculations for orifice plates used for flow restriction. Throttle orifice plates are always installed upstream of the control valve. Otherwise, it will not serve as a current limiter.