【Daily Question 20090207】Why is a orifice plate installed in front of a control valve?

①In 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. 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 by the flow meter. Orifice plates belong to differential pressure flow meters; the pressure drop is very small, resulting in 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.

Because the pressure of the medium after passing through the control valve changes depending on the degree of opening of the valve. Therefore, the orifice plate must be installed in front of the control valve. Based on experience, installing it in front of the control valve results in relatively stable system pressure, higher measurement accuracy, and fewer fluctuations ； Behind the control valve, the pressure fluctuates significantly as the valve is opened and closed. This can lead to discrepancies between the process conditions specified in the calculations for the throttling device used to select the orifice plate (especially since pipeline pressure has a significant impact on the measurement data) and the actual operating conditions, resulting in inaccurate measurements and errors in the readings ； Therefore, it is best to install it in front of the control valve. This post was last edited by chmyh on 2009-2-7 17:58]

Fluid flow is continuous, and the law of conservation of energy applies during fluid flow. Orifice plates should not be installed in areas where vapor and liquid coexist, to avoid increasing measurement errors. Some liquids will partially vaporize after pressure reduction through a control valve, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve.

Because fluid flow is continuous, and the law of conservation of energy applies during fluid flow. Therefore, orifice plates cannot be installed in areas where vapor and liquid coexist, to avoid increasing measurement errors. After the pressure of some liquids is reduced by a control valve, they sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve.

In the theoretical derivation for measuring flow rate using throttling devices, it is assumed that fluid flow is continuous and that the law of conservation of energy applies during this flow. This means that the orifice plate of the flow meter should not be installed in areas where both gas and liquid phases coexist, in order to avoid increased measurement errors. Some liquids, such as hydrocarbons, may partially vaporize after passing through a control valve, resulting in a mixture of gas and liquid phases; therefore, the flow orifice plate should be installed as close as possible in front of the control valve

Why is the orifice plate installed in front of the control valve? Answer: Because fluid flow is continuous, and the law of conservation of energy applies during fluid flow. Therefore, orifice plates cannot be installed in areas where vapor and liquid coexist, to avoid increasing measurement errors. After the pressure of some liquids is reduced by a control valve, they sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve.

A regulating valve is installed behind the orifice plate; generally, the orifice plate can serve to throttle flow and reduce pressure, providing rough regulation. When used in conjunction with a control valve, it can lower and stabilize the pressure before the valve, thereby reducing the load on the control valve and preventing it from operating too frequently.

I have also asked many people about this issue, and most of them believe that the flow meter (orifice plate) is installed in front of the control valve to ensure measurement accuracy and prevent the effects of modulation from affecting the measurement. In fact, that is not the case, because in practice it is also possible to find situations where the control valve is installed at the front and the metering device (orifice plate) is installed at the back. According to a person from the design institute, the reason why control valves are generally placed behind the metering device (orifice plate) is that if they are installed in front of the flow meter, and since control valves function to reduce pressure, the pipe diameter will increase after regulation. If the flow meter is placed behind the control valve, its nominal diameter will also increase, which raises the cost; therefore, the flow meter is generally placed in front of the control valve.

1. In the theoretical basis for measuring flow rate using throttling devices, there are two assumptions: that the fluid flow is continuous, and that the law of conservation of energy applies during the flow process. This tells us that orifice plates should not be installed in areas where gas and liquid coexist, as this could increase measurement errors. Some liquids may partially vaporize after passing through a control valve, resulting in a two-phase mixture; therefore, the orifice plate for measuring flow rate should be installed as close as possible to the control valve.

Installed in front of the control valve, it ensures relatively stable system pressure, high measurement accuracy, and minimal fluctuations； Behind the control valve, as it opens and closes, the pressure fluctuates significantly, resulting in low measurement accuracy.

Answer: Because fluid flow is continuous, and the law of conservation of energy applies during fluid flow. Therefore, orifice plates cannot be installed in areas where vapor and liquid coexist, to avoid increasing measurement errors. After the pressure of some liquids is reduced by a control valve, they sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve.