Reason why the orifice plate must be installed in front of the control valve
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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. Throttling orifice plates are always installed upstream of the control valve. Otherwise, it will not serve as a current limiter.
In general pipelines, the fluid flows at an optimal velocity; after the pressure is reduced by a pressure regulator, the pipe diameter increases accordingly based on this optimal velocity. If the flow meter is installed behind the pressure regulator, its nominal diameter will increase, which is not economical
I read the posts from the two people above and learned something from them. I used to only know that measuring elements such as orifice plates should be installed upstream of control valves, and that this is related to the straight pipe section and measurement accuracy, but the more detailed theoretical basis was unclear.
Well said; I’d like to add a bit myself. 1. The pressure loss of the orifice plate is not very low; it is relatively high compared to other flow meters. For details, see the flow meter comparison materials (source: Industrial Control Network). However, this ratio before and after the control valve is indeed very small. http://bbs.hcbbs.com/thread-424441-1-1.html 2. The orifice plate operates by measuring the pressure difference before and after it. Therefore, if it is installed after the control valve, due to the regulating function of the control valve, the pressure before the orifice plate will fluctuate and become unstable, resulting in unstable measurements and measurement errors. 3. Nothing is absolute. If it is necessary to install the orifice plate behind the control valve, I think that is also acceptable. As long as an adequate installation distance is ensured. I hope everyone can add more.
This post was last edited by zhaohh3211 on 2012-2-6 at 15:12. The explanations for the first and second floors are combined together, providing a clear explanation of this issue.
I think it’s just a matter of convention; given the need to take into account the flow conditions of the medium in orifice plate applications, it is installed in front of the control valve to improve measurement accuracy. As for whether it can be installed behind the control valve, I think there’s no problem as long as there is enough straight pipe section. But since they are all default rules, they are installed at the front; just as the pressure sensor is installed in front of the temperature sensor. In fact, when considering online flow control, there is no such thing as a front or back – aren’t there process valves, control valves, sensors, and various devices that affect the flow state of the medium in front of the orifice plate?
I think it’s just a matter of convention; given the need to take into account the flow conditions of the medium in orifice plate applications, it is installed in front of the control valve to improve measurement accuracy. As for whether it can be installed behind the control valve, I think there’s no problem as long as there is enough straight pipe section. But since they are all default rules, they are installed at the front; just as the pressure sensor is installed in front of the temperature sensor. In fact, when considering online flow control, there is no such thing as a front or back – aren’t there process valves, control valves, sensors, and various devices that affect the flow state of the medium in front of the orifice plate?
I agree with the views from floors 1, 2, and 4 – the orifice plate must indeed be installed in front of the control valve! !
We usually install orifice plates in front of control valves. The reasons given by the original poster are very comprehensive; indeed, as the liquid passes through the control valve, the pressure drops which leads to an increase in the vapor phase, and two-phase flow can even occur. Additionally, the orifice plate requires long straight sections before and after it (usually 10 units in front and 5 units behind) to reduce vortex formation. Therefore, it is necessary to place the orifice plate in front of the control valve; this is just my humble opinion for discussion purposes.
I would like to ask the original poster: I’ve seen a PID controller used by a foreigner, in which the flow-limiting orifice plate is installed behind the control valve. Could you tell me how the foreigner came up with this design? I’ve also never really understood it