Influence of resistance element on the accuracy of flow measurement

Abstract: The information on the influence of resistance parts on the accuracy of flow measurement is provided by excellent flowmeter and flowmeter manufacturers. 1. In the process flow, elbows are often used to change the direction or height, which is the most common type of pipeline resistance. Whether it is fully developed turbulent flow or free flow (drawn from the atmosphere), the flow passes through the elbow. will become more complicated and the flow velocity distribution will not. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the details of the article on the influence of resistance parts on the accuracy of flow measurement. 1. In the process flow, elbows are often used to change the direction or height. They are the most common type of pipeline resistance. Whether it is to fully develop turbulent flow or“free flow”After passing through the elbow (drawn from the atmosphere), the flow will become more complicated, the flow velocity distribution is no longer symmetrical to the axis, and accompanied by vortices (such as square tubes and secondary flow), which will affect the accuracy of flow measurement It mainly depends on the radius of curvature of the elbow and the size of the flow rate. When the fluid flows through the elbow, centrifugal force will occur, the outer wall pressure increases and the flow velocity decreases; the inner wall pressure decreases and the flow velocity increases, and when the fluid flows from the elbow to the straight pipe, the flow effect is opposite, the outer velocity increases, and the inner velocity decreases. At the same time, due to the inertia of fluid flow, a larger vortex area will be generated on the inside and a smaller vortex area will be formed on the outside (as shown in Figure 1). When the radius of curvature of the elbow decreases or the diameter of the pipe increases, the effect of separation will be enhanced. At 1/3 of the elbow, the flow will intensify and deteriorate, but under the action of fluid viscosity, the vortex will gradually weaken, and the flow velocity distribution will tend to In equilibrium, the flow situation will basically improve after about 10D. Therefore, it is not suitable to install a flow meter within 10D behind a single elbow. Combination of multiple elbows: The above situation is that the turbulent flow is fully developed before a single elbow, which is rare in industrial sites. It is common to have multiple bends or a combination of bends and other resistance elements, so just doing a few single-head tests is far from solving the problem of insufficiently developed turbulence. This complex combination increases the strength of the secondary flow, vortex and centrifugal force, the flow will worsen, and the larger the pipe (D>400mm) the more difficult it is to improve. According to professional estimates, the length of the straight pipe should be at least greater than 20D, otherwise the accuracy will be difficult to reach 5%. Increasing the distance between the elbows may weaken the mutual influence. Professionals suggest that the distance between the two elbows should not be less than 5D. 2. In order to change the diameter of the process pipeline, the reducing pipe is usually used, and there are mainly two forms of expanding pipe and shrinking pipe. For a long time it has the same effect on the flow field without distinction (such as American APIANSI1991-1999). However, the author believes that if the pipe shrinkage is properly handled, it will not only not destroy the flow field, but also can eliminate the vortex and improve the flow field. The following two situations are now discussed: abrupt pipe: that is, the change of pipe diameter does not transition, and it suddenly expands (such as Figure 2) or sudden shrinkage, vortices will be generated and the flow field will be destroyed. The greater the change in pipe diameter, the stronger the damage. Gradient tube: one is the gradually expanding tube, the fluid passing through the gradually expanding tube is a process of converting kinetic energy into potential energy, such as the expansion angle does not exceed 10°, this transition is gradually stable, the fluid will not be separated, nor will it produce a large pressure loss, such as the expansion section of Venturi. However, it is often not allowed to have such a long expansion section in the process, especially when the diameter of the pipe is large. so that the shrinkage angle is as large as 60°There will be no separation (such as the inlet of the Venturi tube), and it will also eliminate the vortex to improve the flow field. Many manufacturers have recognized this and use it as a low-cost and high-efficiency rectifier, such as the Venturi vortex. Street flowmeter, Albert flowmeter, gas ultrasonic flowmeter of American Colotron, and so on. According to the data, the influence of the latter on the accuracy of the flow meter will be about 20 times larger than that of the former. 3. Valves are usually used in the process industry to change the flow rate. Valves come in various forms and are a complex resistance element that not only brings vortices to the flow, but also deteriorates the flow velocity distribution (see Figure 3). Quite a few valves are not fully open in use, and the smaller the opening, the more serious the deterioration of the flow field. Moreover, in the control of the fluid, the valve is often used as a pressure-reducing device, which causes the fluid to produce pressure loss, the pressure drops sharply, and it is easy to generate cavities, which further increases the error and harm of the flow meter. In view of the above reasons, in the process layout, the valve should be arranged downstream of the flow meter as much as possible; if it must be arranged upstream, the flow meter should be at least 5D away from the valve with a straight pipe length of more than 5D. The valve is arranged on the bypass of the pipeline, and a less disturbing ball or shuttle valve is installed upstream of the main pipeline where the flow meter is installed. Professionals are especially reminded to carefully arrange the relative position between the flow meter and the valve, otherwise it will cause intolerable measurement errors.

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