Structure and measurement principle of the internal Venturi flow meter

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1. Introduction Differential pressure flowmeters, as a traditional measurement method, have a history of nearly a hundred years; today they remain the most widely used industrial online flow measurement instruments. 　　The throttling elements used in differential pressure flow meters are mainly standard orifice plates, nozzles, and venturi tubes. Given the various shortcomings in the technical performance of these throttling elements, and in order to meet the requirements of practical industrial measurement, several modified throttling elements based on orifice plates and nozzles have been invented over time, such as annular orifice plates, orifice plates with conical inlets, quarter-circle orifice plates (nozzles), wear-resistant orifice plates, wedge flow meters, and V-Cone flow meters. These modified throttle elements can compensate for certain shortcomings in the technical performance of standard throttle elements under specific measurement conditions, but they have significant limitations in terms of usage, and their measurement accuracy is also low. The V-Cone flowmeter from the United States is a modified throttle element with relatively good performance, yet the issue of unstable discharge coefficients caused by \"sharp edge erosion\" has not been completely resolved. 　　The internal venturi tube, a patented product developed by Dalian Sonika Electronics Co., Ltd., is a modified version of the traditional venturi tube. It combines the advantages of an annular orifice plate, a wear-resistant orifice plate, and a venturi tube; its technical performance is superior to that of orifice plates, nozzles, and conventional venturi tubes, as well as that of other existing modified throttling elements. The internal Venturi flow meter is a new generation of differential pressure flow measurement device with unique properties, and it has achieved good results in industrial measurement applications. 2. Structure and Measurement Principle 2.1 The internal Venturi tube consists of a circular measurement tube 1 and a special-type core 2 placed inside the measurement tube (see Figures 1 and 2). The special-shaped core is a rotating body coaxial with the measuring tube; its generatrix is the axial cross-section of a classical truncated Venturi tube with an infinitely thin imaginary wall. In other words, the radial outer surface of the rotating body consists of three parts: a conical surface 6 in the front section, a cylindrical surface 7 in the middle section, and a conical surface 8 in the rear section. The special-type core is positioned by its support shafts 9, 10 and the support rings 3, 4 that are coaxial with the measuring tube, and is fixed in place by braking elements (for products with a small diameter, there is no front support shaft; only the rear shaft together with the rear support ring is used for positioning and fixing). The support ring is composed of a coaxial inner ring and an outer ring, along with 3–4 support ribs that connect the inner and outer rings together. At specific positions on the wall of the measurement tube, a pressure tapping fitting 5 (or a remote flanged pressure tapping interface) is provided, while both ends of the measurement tube are equipped with standard flanges for connecting to the process pipelines on site. 2.2 In terms of the basic measurement principle, the internal Venturi tube shares the same measurement principle as traditional differential pressure flow meters such as the conventional Venturi tube; both are flow measurement methods based on the law of conservation of energy – Bernoulli’s equation and the equation of continuity for flow. The basic direct measurement value remains the differential pressure before and after the throttling element. As mentioned above, a stepped annular cavity (annular gap) is formed between the outer surface of this special-type core body and the inner surface of the measuring tube. The pattern of flow across the axial surface of this annular cavity is similar to that in a conventional Venturi tube; as a result, the changes in the fluid stream and the throttling process when the fluid flows through the internal Venturi tube are essentially the same as those that occur when the fluid flows through a conventional Venturi tube. 　　Leveraging the excellent technical properties of the internal venturi tube, it will play an important role in addressing the currently recognized challenges related to the metering of so-called \"high-flow, low-pressure, high-humidity gases\", as well as the metering of dirty fluids such as gas and unclean natural gas ； For those measurement applications where traditional differential pressure instruments such as orifice plates are still necessary, but there is a strong need to improve measurement accuracy and the quality of metering, the internal Venturi tube represents an ideal replacement for them.