Resistance to impact on the flow measurement accuracy

Abstract: the resistance of a influence on the flow measurement accuracy information produced by excellent flow meter, flow meter manufacturer to offer you the quotation. 1, the elbow in the process, to change direction or highly often use elbow, is a common device in pipelines is a kind of zui, whether it be a fully developed turbulence or free flow ( From the atmospheric drawn) After elbow flow will become more complex, velocity distribution is not. More flow meter manufacturers choose model price quote you are welcome to inquire, here is resistance to the article details the influence of the flow measurement accuracy. 1, the elbow in the process, to change direction or highly often use elbow, is a common device in pipelines is a kind of zui, whether it be a fully developed turbulence or & other The free flow & throughout; （ From the atmospheric drawn) After elbow flow will become more complex, velocity distribution no longer axis of symmetry in, and accompanied by whirlpool ( Such as the square tube and secondary flow) Effects on the flow measurement accuracy mainly depends on the size of the bend radius of curvature and velocity. When the fluid flows through the pipe, centrifugal force, will happen in the outer wall pressure velocity decrease; Wall pressure reduce rising velocity, and the elbow flow straight pipe, the effect of flow on the contrary, the lateral velocity increasing, the inside velocity decreases, and at the same time because of inertia, the fluid flow in the vortex of the medial will produce larger area, the lateral form small vortex area ( As shown in figure 1) 。 When the curvature radius, bend or pipe diameter increases, the separation effect will increase, in bend a third place, intensifying flow, but under the effect of fluid viscosity, the vortex will gradually weaken, velocity distribution will tend to balance, after about 10 d basic improve flow situation. Within a single bend after 10 d, therefore, should not install the flow meter. Bend combination, the above situation is single bend before for fully developed turbulent flow, in the industrial field is rare. Common is more elbow or bend resistance and other combinations, so only a few single head test far cannot solve the problem of the fully developed turbulence. The combination of this complex, increasing the secondary flow, vortex and the strength of the centrifugal force, the flow will worsen, the larger the pipe ( D> 400mm） Improved the more difficult. Estimated according to professional personage, straight pipe length should be at least more than 20 d, otherwise it is difficult to achieve 5% accuracy. Increase spacing between bend is likely to weaken the influence of each other, professional advice, the distance between the two bend should not be less than 5 d. 2, variable diameter pipe in order to change the process piping diameter usually adopt variable diameter pipe, expansion and shrinkage tube two main forms. For a long time it equally the influence of flow field, no difference ( APIANSI1991 - such as the United States 1999). 。 But the author thinks that, shrink tube, if properly handle, not only not destroy the flow field, and can eliminate vortex, the improvement of flow field, now to the discussion of the following two conditions: mutation tube: namely the change of the pipe diameter without transition, abruptly expanding ( As shown in figure 2) Or sudden narrowing, will produce whirlpool, destroy the flow field, the greater the diameter change, the more destructive. Gradient tube: one is divergent tube, fluid through a divergent tube is a process of kinetic energy is converted into energy, such as the expansion Angle of not more than 10 & deg; , the transformation is gradient stable, fluid will not separation, also won't produce bigger pressure loss, such as the expansion of the venturi. But often there is no such a long process expansion period, especially when larger diameter, especially; Second, reducer, fluid through the reducer is a process of potential energy into kinetic energy, is easy to implement. That big contract Angle to 60 & deg; Will not produce separation ( Such as venturi tube imported) , but will also eliminate vortex improve flow field, many manufacturers have realized this, and use it as a kind of low cost, high efficiency rectifier, such as venturi, Albert flowmeter, vortex flowmeter the recreation of gas ultrasonic flowmeter, and so on. According to the data, the latter's influence on the flow meter accuracy from the former to about 20 times larger. 3, the valve in the process industry often USES the size of the valve to change flow. The various forms of the valve, is a more complex resistance, not only bring flow vortex, and degradation velocity distribution ( As shown in figure 3) 。 Quite a number of valve in use are not fully open, the opening is smaller, the more serious the deterioration in flow field. And, in the fluid control valve as a decompression device, often make the pressure loss of the produced fluid, pressure drop sharply, also easy to produce holes, to further increase the error and harm of the flow meter. For these reasons, on the process layout, should as far as possible arrange the valve on the downstream side of the flow meter; If necessary arrangement in the upstream, flow meter distance valve should have at least more than 5 d straight pipe length, such as cannot assure, can arrange the valve on the bypass pipe, and the installation flow meter installed upstream of the main interference a smaller ball valve or shuttle valve. Professional special remind carefully arrange the relative position between the flow meter and valve, otherwise it will cause the error of measurement cannot tolerate. 4, manifold in the process industry, such as part of the fluid flowing from the main line or some fluid from the outflow in the pipeline to adopt manifold.

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