The reason why the orifice plate must be installed in front of the regulating valve

①In the theoretical derivation of the flow measurement of the throttling device, there are two assumptions in theory, that is, the fluid flow is continuous and the law of conservation of energy is followed during the fluid flow. This tells us that the orifice plate cannot be installed where the gas and liquid two phases coexist, so as not to increase the measurement error; ②Some liquids (such as liquefied hydrocarbons) are partially vaporized after the pressure is reduced by the regulating valve, forming two-phase coexistence ①In In the theoretical derivation of the throttling device to measure the flow rate, there are two assumptions in theory, that is, the fluid flow is continuous and the law of conservation of energy is followed during the fluid flow. This tells us that the orifice cannot be installed where the gas and liquid phases coexist, so as not to increase the measurement error; ②Some liquids (such as liquefied hydrocarbons) are sometimes partially vaporized after the pressure is reduced by the regulating valve, forming two-phase coexistence; ③ Therefore, the flow orifice should be installed in front of the regulating valve as much as possible. This installation method is reasonable. When the orifice plate and other throttling devices and flow measuring instruments are installed on the same pipe as the regulating valve, they are generally required to be installed upstream of the regulating valve, because most of the flow measuring instruments need to have a certain straight pipe section. The longer the straight pipe section, the greater the measurement accuracy. high. After the fluid flows through the regulating valve, the flow state will change, which will affect the measurement of the flow meter. The orifice plate is a differential pressure flow meter, with a small pressure drop, which has little effect on the subsequent regulating valve. The pressure drop of the regulating valve is much larger than that of the orifice plate (generally accounting for 30% of the piping system). During the adjustment process, the downstream pressure fluctuates greatly, which affects the accuracy of the orifice plate. Of course there is also the problem of straight pipes. Especially the gas pipeline has the biggest problem. The flow-limiting orifice plate calculates the hole diameter of the orifice plate according to the formula, and then according to the value and k value, check the 'γc-k-d0/D relationship table' of this appendix to obtain the critical flow rate pressure ratio (γc). When the pressure ratio between the front and rear of the plate is P2/P1≤γc, the liquid flow can be restricted to a certain value. See the calculation of the orifice plate for flow restriction. The restrictor orifice must be installed upstream of the regulating valve. Otherwise, the current limiting operation will not