General principles for electromagnetic flowmeter selection
(1) Whether the medium to be tested is a conductive liquid or slurry, thereby determining whether an electromagnetic flow meter is selected;
(2) The conductivity of the measured medium determines the type of electromagnetic flowmeter—whether it is high conductivity or low conductivity;
(3) The nominal diameter of the large, small and common flow process pipelines required by the process, determine whether the flow rate of the medium is at a more economical flow point, whether the pipeline needs to be reduced, and then determine the diameter of the flowmeter;
(4) Determine whether to use an integrated or split flowmeter, and the degree of protection of the flowmeter, etc., based on the layout of the process piping.
(5) Selecting the electrode type according to whether the measured medium is easy to crystallize or crusting;
(6) selecting an electrode material according to the corrosiveness of the measured medium;
(7) The corrosiveness, wear and temperature of the measured medium determine the lining material to be used;
(8) The high working pressure of the measured medium determines the nominal pressure of the flow meter;
(9) The insulation of the process piping determines the type of grounding ring.
Vortex flowmeter working principle
The working principle of the vortex flowmeter is to arrange a vortex generator in the fluid, so that the vortex is alternately generated on both sides of the body, and the vortex column is asymmetrically arranged downstream of the vortex generator to generate a certain frequency, by the formula f= St*v/(1-1.27d/D)*d, (St is the Strauhal number, which is a dimensionless number, related to the vortex generator and Reynolds number; v is the flow velocity; d is the incident head width; D is the nominal diameter) to get the flow rate.
In general, the vortex flowmeter output signal (frequency) is not affected by changes in fluid properties and composition, which means that the meter factor is only related to the shape and size of the vortex generator and the Reynolds number. Its advantages are: simple and firm structure, convenient installation and maintenance; suitable for a variety of fluids, liquid, gas, steam and some mixed phases are applicable; high precision, generally up to ± 1% R; flow range is wide, up to 10 : 1 or 20:1 or more; low head loss; no zero drift; relatively cheap price; disadvantage: not suitable for low Reynolds number Re <20000, limited use of high viscosity, low flow rate, small diameter The requirements for the environment are high, and places with vibration should be eliminated as much as possible, and the upstream side needs to have a long straight pipe section; the meter factor is lower, and the larger the diameter, the lower the diameter. The signal resolution is reduced, so the aperture should not be too large, generally used in DN15~DN300mm.
Mass flow meter
Since the volume of the fluid is affected by parameters such as temperature and pressure, it is necessary to give the parameters of the medium when the flow rate is expressed by the volume flow. In the case of changing media parameters, it is often difficult to achieve this requirement, resulting in distortion of the meter display value. Therefore, mass flow meters have been widely used and valued. Mass flow meters are available in both direct and indirect versions. Direct mass flow meters are measured using principles directly related to mass flow. Currently used mass flow meters such as calorimetric, angular momentum, vibratory gyro, Magnus effect and Coriolis force. The indirect mass flow meter is obtained by directly multiplying the density meter by the volumetric flow rate to obtain the mass flow rate.
In modern industrial production, the operating parameters such as temperature and pressure of the flowing working fluid are continuously improved. In the case of high temperature and high pressure, due to the material and structure, the application of the direct mass flowmeter is difficult, and the indirect quality is encountered. Flowmeters are often not suitable for practical applications because they are limited by the range of humidity and pressure. Therefore, a temperature-pressure-compensated mass flowmeter is widely used in industrial production. It can be regarded as an indirect mass flow meter. Instead of using a density meter, it uses the relationship between temperature, pressure and density. It uses a temperature and pressure signal to calculate the density signal by function, and multiplies it by the volume flow. Mass Flow. At present, temperature and pressure-compensated mass flowmeters have been put into practical use. However, when the measured medium parameters vary widely or rapidly, it will be difficult or impossible to correctly compensate, so further study the mass flow rate applicable in actual production. Meters and densitometers are still a topic.
Chen's above-mentioned common structural principle of flowmeters is much better than various types of flowmeters, such as various helium flowmeters and trough flowmeters for open channel flow measurement; flowmeters suitable for large-caliber flow measurement; measuring laminar flow Laminar flowmeter; related flowmeter for two-phase flow measurement; and laser method, nuclear magnetic resonance flowmeter and various tracer methods, dilution method flow measurement, etc. With the development of technology and practical application needs, the new flowmeter will continue to emerge more types of flowmeters.