Why is it reasonable to use an orifice plate to measure flow rate, with the orifice plate installed in front of the control valve?

1. When using a throttling device to measure flow rate, it is required that the fluid flow be continuous, and the law of conservation of energy must be obeyed during the flow process. Therefore, orifice plates cannot be installed in areas where gas and liquid coexist, to avoid increasing measurement errors. 2. Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, 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 orifice flow meter requires a stable flow field to measure flow, which is why a certain length of straight pipe is necessary. Control valves can affect the stability of the fluid and thus impact the accuracy of measurement by the orifice meter; therefore, the orifice meter should be installed upstream of the control valve.

It is reasonable to install the orifice plate in front of the control valve

Disadvantages of installing an orifice plate behind a control valve: In the design calculations for an orifice plate, one of the parameters is pressure – specifically, the pressure of the fluid in front of the orifice plate, which is a constant value. This fact must be taken into account; in other words, if the pressure deviates from this value, errors will inevitably occur in the measurements. If the orifice plate is installed behind the control valve, the pressure before the control valve is generally also a fixed value (the design value). The control valve regulates the flow of the medium, creating a certain pressure difference (a pressure difference is inevitable; otherwise, regulation would not be possible), and this pressure difference is not constant. The pressure difference generated by the control valve depends on its opening degree: the greater the opening degree, the smaller the pressure difference, and the smaller the opening degree, the larger the pressure difference. If the orifice plate is located behind the control valve, then the pressure value before the orifice plate will keep changing, which inevitably leads to measurement errors. After the fluid is throttled by the control valve, the flow pattern (not the flow velocity) becomes disrupted, whereas an orifice plate requires a stable flow pattern; unstable flow patterns also increase errors. These two points are key to understanding. Therefore, the orifice plate must be installed in front of the control valve!

The answer above is correct; the most important thing is to maintain stable pressure before the orifice plate

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It’s difficult to have a fully filled pipe behind the control valve, right? It’s generally hard to obtain accurate readings when the pipe isn’t full.

1. Regarding pressure issues, orifice plate measurement relies on differential pressure, so it does not cause any deviations in the measurement results. 2. As for flow pattern issues, there is a distance of approximately 30–50D between the orifice plate and the control valve, which is sufficient to eliminate turbulence; therefore, there is no right or wrong way to install it.