Meter selection and flow measurement method considerations

Abstract: The selection of meters and the consideration of flow measurement methods are provided by excellent flowmeter and flowmeter manufacturers. If the measurement requirements are higher than the above, and the indicated flow error is between 2-10%, install a flow meter. If it is considered that a differential pressure instrument is selected according to the following selection steps, it is not necessary to install a flow sensor such as an orifice throttle body, and the flow rate of the elbow can be used. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following are the details of the article on the selection of meters and the considerations for flow measurement methods. If the measurement requirements are higher than the above, and the indicated flow error is between 2-10%, install a flow meter. If it is considered that a differential pressure instrument is selected according to the selection steps below, it is not necessary to specially install flow sensors such as orifice throttling. 1.2 After the initial selection of measurement and distribution, it is determined that the flow meter must be installed, and then the use requirements and various conditions are further understood in detail. First, according to the type and characteristics of the fluid, the exclusion method is adopted in the“primary election”The measurement scheme that cannot and should not be adopted shall be considered and analyzed in the second step. 1.3 Analysis Factors According to the schemes determined in the preliminary selection, collect samples, technical data and selection manuals from the manufacturers of the preliminary selection instruments, so as to fully understand the specifications and performance of the instruments; Environmental conditions and economic factors are considered in five aspects. According to the questions raised in the following sections, one by one is analyzed and compared in a list. The order of consideration is different from the initial selection according to the measurement purpose and focus. Generally, it starts with 'performance requirements and instrument specifications', and then considers other factors as shown in Figure 1. If the applicable object believes that 'economic factors' are the main factors (such as large pipe diameter transportation requires low pumping costs, and commercial accounting requires measurement errors to cause small losses), then when considering the other factors of 'performance requirements and instrument specifications', Sometimes it's time to revert to considering migration. 2 Considerations of fluid properties 2.1 General introduction Section 1.2 Preliminary selection of measurement methods After a number of applicable schemes are initially determined according to the type of fluid, the detailed fluid physical properties, many parameters and attributes should be obtained from the process department, and the adaptation to the selected scheme should be further considered sex. In addition to the fluid category, the temperature, pressure, density, viscosity and lubricity, corrosiveness and abrasiveness of the fluid are usually considered, and some applications also have hygienic requirements. Some measurement methods also consider special parameters in the physical properties of the fluid, such as the use of electromagnetic flowmeters to understand the conductivity of the liquid. All kinds of flow instruments are always affected by one or several parameters in the physical properties of the fluid, so the physical properties of the fluid largely dominate the type of the instrument to be selected. The selected measurement method and instrument should not only adapt to the properties of the fluid to be measured, but also consider the influence of a certain parameter change in the physical properties of the fluid during the measurement process. Common fluid density, viscosity, vapor pressure and other parameters can be found in the manual to assess the suitability of the fluid parameters and the selected instrument specification under the conditions of use. However, it is often encountered that the exact composition, temperature and pressure changes, and flow characteristics of the fluid are not very clear. It is difficult for the user to determine the value of the physical properties of the fluid. At this time, the manufacturer should consult the manufacturer to estimate whether the instrument to be selected can be used. . 2.2 Fluid temperature and pressure The working pressure and temperature of the fluid must be carefully defined, especially when measuring gases. Excessive density changes caused by temperature and pressure changes may change the selected measurement method. For example, when temperature or pressure changes cause large changes in flow characteristics and affect the measurement accuracy and other performance, it is necessary to make temperature and (or) pressure corrections. In addition, the structural design and material selection of the flow meter housing are also determined by the temperature and pressure of the fluid, so the maximum and minimum values ​​of pressure and temperature must be known exactly. When the pressure and temperature fluctuate greatly, the selection should be particularly careful. When measuring the gas flow, it is also necessary to determine the temperature and pressure of the upper limit of the flow of some instruments (such as differential pressure type), whether it is under the working condition or the standard state? 2.3 Fluid density and specific gravity Most liquid applications, its density and The specific gravity is relatively constant, and generally does not need to be corrected unless the temperature changes greatly and causes a large density change. In gas applications, the range and linearity of some instruments depend on the gas density, and it is usually necessary to know the values ​​under standard conditions and in use conditions for selection. There is also the conversion of the value of the flow state to some recognized reference value, which is widely used in the storage and transportation of petroleum gas. Low-density gas presents difficulties for some measurement methods, especially instruments (such as turbine flow meters) that use gas momentum to drive the detection element to work. 2.4 Viscosity and Lubricity Instrument performance tends to vary with Reynolds number, which is related to fluid viscosity. Unlike liquids, the viscosity of gas does not change significantly due to changes in temperature and pressure, its value is generally low, and the difference between various gases is small.

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