Process Engineering Division – Instrumentation and Automation Section – Daily Question – Question dated 2020-09-05
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Process Engineering Section – Instrumentation and Automation Section – Daily Question – Question from 2020-09-05: Discussion question: 226. When using an orifice plate to measure flow rate, why is it most appropriate to install the orifice plate in front of the control valve?
In the theoretical derivations for measuring flow rate using throttling devices such as orifice plates, it is required that the fluid flow be continuous, and the law of conservation of energy must be obeyed during the flow process. This requires that the orifice plate not be installed in areas where gas and liquid coexist, in order to reduce measurement errors. Some liquids may partially vaporize after pressure reduction through a regulating valve, resulting in a coexistence of gas and liquid phases. Therefore, it is reasonable to install the flow orifice plate in front of the control valve.
Flow measurement using an orifice plate is accurate only when the fluid flow is in a stable turbulent state. If a orifice plate is installed behind the control valve, the adjustment action of the control valve will affect the flow pattern of the fluid, resulting in significant measurement errors. Therefore, the orifice plate should be installed in front of the control valve. If it must be placed after the control valve, a sufficient length of straight pipe section at a distance from the control valve is required.
In the theoretical derivations for measuring flow rate using throttling devices such as orifice plates, it is required that the fluid flow be continuous, and the law of conservation of energy must be obeyed during the flow process. This requires that the orifice plate not be installed in areas where gas and liquid coexist, in order to reduce measurement errors. Some liquids may partially vaporize after pressure reduction through a regulating valve, resulting in a coexistence of gas and liquid phases. Therefore, it is reasonable to install the flow orifice plate in front of the control valve
In the theoretical derivations for measuring flow rate using throttling devices such as orifice plates, it is required that the fluid flow be continuous, and the law of conservation of energy must be obeyed during the flow process. This requires that the orifice plate not be installed in areas where gas and liquid coexist, in order to reduce measurement errors. Some liquids may partially vaporize after pressure reduction through a regulating valve, resulting in a coexistence of gas and liquid phases. Therefore, it is reasonable to install the flow orifice plate in front of the control valve.
In the theory of measuring flow rate using throttling devices, there are two assumptions: that the flow of the fluid is continuous, and that the law of conservation of energy applies during the flow of the fluid. This tells us that orifice plates should not be installed in areas where gas and liquid coexist, in order to avoid increasing measurement errors. After some liquids have their pressure reduced by a control valve, they may partially vaporize, resulting in the coexistence of two phases; therefore, it is reasonable to install the flow orifice plate as close as possible in front of the control valve.
In the theoretical derivations for measuring flow rate using throttling devices such as orifice plates, it is required that the fluid flow be continuous, and the law of conservation of energy must be obeyed during the flow process. This requires that the orifice plate not be installed in areas where gas and liquid coexist, in order to reduce measurement errors. Some liquids may partially vaporize after pressure reduction through a regulating valve, resulting in a coexistence of gas and liquid phases. Therefore, it is reasonable to install the flow orifice plate in front of the control valve.
The control valve is a throttling element, and vortices or turbulence are generated behind it, which affect orifice plate measurements (such as causing fluctuations)
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. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, to avoid increasing measurement errors ； 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; this installation method is reasonable.
When measuring flow rate using a throttling device, there are two theoretical requirements: the fluid flow must be continuous, and the law of conservation of energy must hold true 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. 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; this installation method is reasonable.
The orifice plate must be installed in front of the control valve. For flow regulation, the pressure before the valve is generally more stable than the pressure after the valve in most cases. So in most cases, the orifice plate is installed in front of the valve.