1. Preamble (Overview)

To Orifice, nozzles and cultural Venturi tube as the representative of Differential Flowmeter industry has been successfully applied to the more centuries. In the accumulation of substantial practical experience on the basis of standardized differential pressure flow meter on the latest international standards ISO5167-1, ISO5167-2, ISO5167-3 and ISO5167-4 in March 2003 by the International Organization for Standardization (ISO) formally announcedִ. The so-called "standardized" is not established experimental calibration of differential pressure and flow, and estimate their measurement error. At present, only the total flow meter is the only such a flow meter to achieve this standard flowmeter.

Orifice flowmeter due to standardization and simple structure, secure easy to copy, high universality, low prices and access to a wide range of large number of applications. However, the orifice flowmeter because of its own structural defects have some major drawbacks such as the outflow of factors of instability, poor linear, non-repeatability, accuracy is not affected due to many factors, sewage and prone to easy plot Wear, pressure loss greater range than the (range of) small, on-site installation conditions, the direct request of the paragraph is too long, and so on.

Pressure to produce savings device has been optimized to improve the work without interruption, their size, cut expenditure of the geometric forms and parameters, the entrance edge profile, from pressure and throttle, the replacement of orifice work has been engaged in the development of . For example: In order to measure the flow of dirt fluid has developed the largest non-standard equipment to reduce expenditure are: lack orifice round, eccentric orifice, wedge-shaped orifice, wearable orifice, the circular orifice, Bend (elbow) And so on. This perfect, and improve the work until the mid-1980s to develop a qualitative leap, namely: fluid cutting expenditure will shrink to the pipeline near the central axis to fundamentally change the concept of using a coaxial installed on the pipeline in a sharp V-shaped cone will be Fluid gradually to reduce expenditure contraction of the pipeline within the side wall. By measuring this V-shaped within the cone before and after the differential pressure to measure flow. This V-shaped within the cone (VNZ) flowmeters Differential Flowmeter opened a new chapter. After 10 years of repeated testing and application, people generally understand it and accept it as a more effective flow meters. Practice has proved that: VNZ flow meter can be used in a short straight of the conditions, to a broader range than the cleansing fluid or dirt to achieve a more effective flow of accurate measurement. This chapter will be detailed proof of its working principles, structure with two systems composed of key technical and performance indicators; advantages and disadvantages of a typical product analysis and application of typical examples and experience.

2. Principle

2.1the basic principles of Differential Flowmeter

Overall, the work of Differential Flowmeter based on the following facts: If the fluid flows through a contraction (expenditure) items, the fluid will be accelerated. This fluid will enable it to accelerate the momentum increased, while in accordance with the law of conservation of energy in the fluid, it was speeding up the static pressure will certainly be a relatively lower value. Energy conservation law tells us: in a closed system, the fluid of the total energy is a constant. To further quantitative analysis, see Figure 1 below

Figure 1

In a cross-section, the average velocity of fluid is V1, its density is 1, pipeline in a cross-section of the cross sectional area is A1; When the fluid flow through cross-section 2:00, the corresponding average velocity is V2, density is 2 , The cross sectional area is A2, under the principle of continuity of fluid flow following formula: V1 A1 1 = V2 A2 2 (1)

If the fluid is the liquid, can be considered in the contraction before and after its density unchanged, that is,

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Therefore, the volume of liquid flow:qVV1A1V2A22