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According to the working principle and characteristics of electromagnetic flowmeters, combined with the maintenance experience of nuclear power plant peripheral systems, the failure phenomena of magnetic flow meters in different places and environments are analyzed, and the experience and precautions of such meters in the application are summarized.
The magnetic flow meter is used to generate a uniform DC or AC magnetic field. The DC magnetic circuit is realized by permanent magnets. Its advantage is that the structure is relatively simple and the interference from the AC magnetic field is small, but it is easy to polarize the electrolyte liquid passing through the measurement catheter, so that the positive electrode is surrounded by negative ions, and the negative electrode is surrounded by positive ions. Enclosure is the polarization phenomenon of the electrodes and causes the internal resistance between the two electrodes to increase, which seriously affects the normal operation of the meter. When the diameter of the pipe is large, the permanent magnet is correspondingly large, heavy and uneconomical. Therefore, the magnetic vortex flow meter generally uses an alternating magnetic field and is excited by a 50HZ power frequency power supply.
Nuclear power plant BOP (Balance of Plant, auxiliary part of nuclear power plant) peripheral system instrumentation and control equipment has the characteristics of a large number, various types, and many manufacturers involved. Among them, as a common instrument for measuring the flow and velocity of conductive fluids, magnetic flow meters are widely used in peripheral systems. 1 The working principle of magnetic flowmeter An electromagnetic flowmeter is a flow measuring instrument made by the principle of electromagnetic induction. It is often used to measure the current volume flow, cumulative volume flow and velocity of conductive liquids. It is mainly composed of a magnetic circuit system, a measuring tube, an electrode, a casing, and a lining. And converters and other components. As the application entity of the law of electromagnetic induction, during the operation of the electromagnetic flowmeter, when the conductive measured medium flows perpendicular to the direction of the magnetic field lines, an induced electromotive force is generated in the direction perpendicular to both the medium flow and the magnetic field lines, and the volume of the measured medium The flow is directly proportional. The induced potential output by the transmitter is a weak AC signal, which contains various interference components, and the internal resistance of the signal varies as much as tens of thousands of ohms. Therefore, in actual industrial applications, the converter is required to be an AC millivolt converter with high input impedance and capable of suppressing various interference components, and convert the induced potential into a (4-20) mA DC unified signal, which is transmitted through cables and wiring. To the terminal of the control cabinet, it is sent to the upper computer through DCS, PLC and other control systems for display, adjustment, control and processing.
Application characteristics of electromagnetic flowmeter
(1) There are no moving parts and bluffing fluid inside the electromagnetic flowmeter, no pressure loss, no mechanical inertia, and the response is very sensitive.
(2) The electromagnetic flowmeter has a large measuring range of pipe diameter, which is suitable for various pipelines and working conditions.
(3) The electromagnetic sanitary turbine flow meters has a uniform scale, and the volume flow of the fluid has nothing to do with the temperature, pressure, density, viscosity, flow state and other factors of the medium. It can be calibrated with water before being used to measure the volume flow of other conductive media (this step is usually It is completed before the equipment leaves the factory), no additional correction is required.
(4) Coating or lining can usually be used inside the electromagnetic flowmeter to solve the problem of measuring the flow of corrosive media. However, excessively high temperature will cause expansion, deformation and deterioration of the pipeline lining material, and the maximum operating pressure depends on the strength of the pipeline, the sealing condition of the electrode and the flange specifications. Generally, in the working environment of domestic electromagnetic flowmeters, the temperature and pressure should be controlled at 120 ℃ and 1.6 MPa. The actual working conditions of the site should be combined when selecting the instrument.
(5) When the liquid flow rate is too low or the distribution is uneven, the electromagnetic flowmeter usually has zero drift and large measurement deviation. Therefore, there must be a straight pipe section of appropriate length in front of the electromagnetic flowmeter to ensure the symmetry of the flow velocity distribution to a certain extent. If a similar situation occurs during system operation, the rationality of the electromagnetic flowmeter installation location should be considered on the premise of excluding damage to the instrument body.
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