与被测电解质液体接触的金属电极会发生电化学反应(也就是电极材料腐蚀的过程)，电极上会产生极化电压。 Electromagnetic reactions between the electromagnetic flowmeter and the metal electrode in contact with the electrolyte liquid being measured (that is, the process of electrode material corrosion), and a polarized voltage will be generated on the electrode. When the metal material reacts electrochemically with the medium, an oxide protective film is formed on the surface to balance the electrochemical reaction. When the slurry containing solid particles or fibrous fluid flows through the electrode, the protective film on the electrode is rubbed to break the balance of the electrochemical reaction, and the protective film on the electrode surface is to be re-formed.化 Voltage. When measuring very low-conductivity fluids, oscillating DC polarization voltages also occur. This phenomenon is called "flow noise" for low conductivity measurements. When the conductivity of the measured fluid is low to a certain extent, such as the flow of media such as alcohol and pure water, the displacement current cannot be ignored as the charge in the capacitor moves. Applying the following formula can approximate the magnitude of flow noise.
The fluid flow rubs against the surface of the lining of the electromagnetic flowmeter so that the charge near the collecting electrode moves with it, so that a variable polarization noise is induced on the electrode. Obviously, if the dielectric constant of the medium is high, the displacement current is large, and the electric charge moving near the electrode is also increased, and the flow noise increases as the dielectric constant E of the medium increases. According to the application conditions of the electromagnetic flowmeter, the condition for ignoring the displacement current is we / σ << 1. It can be seen that as the fluid conductivity σ decreases, the displacement current will increase. The fluid conductivity σ is inversely proportional to the voltage of the flowing noise. In a highly viscous fluid, the charge is not easy to overcome the binding force of the fluid and is released to the vicinity of the electrode. Therefore, the viscosity coefficient of fluid motion is inversely proportional to the flow noise. It should be noted that the flow signal voltage induced at this time is applied to the fluid medium as an applied electric field of the dielectric fluid. The higher the flow velocity of the fluid, the higher the signal voltage induced on the electrode, that is, the greater the intensity of the external electric field applied to the dielectric fluid, thereby intensifying the movement of the charge, that is, increasing the flow noise. It is found in practical use that the magnitude of the fluid velocity and the magnitude of the flow noise have an exponential function. In addition, it is known from physics that the frequency of the external electric field has a great influence on the polarization of the dielectric. Since polarization is a process, there is a phenomenon of polarization relaxation in time. When the frequency of the electric field increases, it is too late to turn to polarization (the polarization process of the dielectric composed of polar molecules), that is, the electric dipole is too late to make a corresponding turn with the change of the electric field, so the turning polarization does not actually exist. The dielectric constant will be greatly reduced. Therefore, when the excitation frequency is high, the polarization phenomenon is weakened, and the flow noise is also reduced. However, it should also be noted that excessively high excitation frequencies increase the dielectric loss of the dielectric. Under the action of an applied voltage, a part of the electrical energy in the dielectric is converted into thermal energy. The high-frequency external electric field heats up the dielectric during repeated polarization. The higher the frequency, the more pronounced the heating. Therefore, the excitation frequency generally used for low conductivity measurement flowmeters may be 100 ~ 400Hz. DC interference potentials tend to drift randomly. The asymmetrical DC interference potential of the two electrodes to ground will transform the DC common mode voltage into a DC differential mode voltage. Excessive dc differential mode voltage can block the amplifier and destroy its linearity. In the converter measurement circuit, the capacitor isolation and sampling signal cutting circuit can make the DC interference voltage output very small. 实际应用时，也要采取措施防止被测管道内壁受介质腐蚀产生大的电位差。 In the practical application of the electromagnetic flowmeter , measures must also be taken to prevent the inner wall of the pipe under test from being corroded by the medium to produce a large potential difference. At this time, it is necessary to take measures to equipotentially connect the front and rear metal pipes to reduce the occurrence of DC interference potential.
At present, for industrial measuring instruments, different types have their own corresponding high temperature ranges. For example, the thermometer uses the range of 1500 to 2000 ° C as the high temperature range. For valves such as control valves and on-off valves, when referring to high temperatures, it means a temperature of 400 to 500 ° C or more. The flowmeter also stipulates different high temperature ranges according to the principle structure. The current practical upper limit of industrial temperature is about 540 ℃ for the differential pressure flow meter of the throttling mechanism, about 200 ℃ for the positive displacement flowmeter, and about 400 ℃ for the area type flowmeter. For representative high temperature fluids that need to be measured but have some problems in measurement, we list the following three. (Vortex flowmeter, integrated orifice flowmeter) High-temperature gas, high-temperature steam as heat-conducting medium or energy source, strong thermal-conducting coal for high-speed breeder reactor-liquid metal sodium, etc .; It is these three fluids. Due to the difference in accuracy and materials used, the main points required in measuring the flow are different, and it is difficult to describe the common points in common. Therefore, in this chapter, only the practical problems of the throttling flow meter used for high temperature gas and high temperature steam and the electromagnetic flow meter for measuring liquid metal are described as the center to describe the flow measurement of high temperature fluid. For molten iron and molten steel, there are two methods for measuring the flow: one is the method of measuring the cross-sectional area of the flume using the high-frequency skin effect; the other is the electromagnetic flowmeter. Both methods can be used in continuous steelmaking equipment. In order to perform direct, non-contact measurement, various aspects of research and discussion have been conducted, but in fact, batch processing is generally performed by converting the flow rate to liquid column height or weight. Therefore, it is not described in this chapter. In addition, when measuring the flow rate of liquid metal sodium, the ultrasonic method of installing a detector outside the piping was tested, and it will be practical soon, and it will not be described here. Application examples of various devices for measuring the flow of high-temperature fluids with a throttling mechanism The departments that use high-temperature fluids are mainly electric power, steel, and chemical industries.
人们从不同原理结构的多种类型的流量计中,首先研究了节流机构的差压流量计是否适用，而实际上应用也最多。 When measuring the flow rate of liquid, water vapor, gas, etc., electromagnetic flowmeters have first studied whether the differential pressure flowmeter of the throttling mechanism is suitable from various types of flowmeters with different principles and structures. most. This is because it has a simple structure and has rich data and practical experience. As a result, the throttling device, as a process detection instrument, is more valued than any other instrument. With the goal of high efficiency and large capacity, power generation equipment for enterprises and power plants for various plants are used to increase the steam temperature and pressure of boilers to critical temperatures and pressures. This kind of steam is high temperature and high pressure superheated steam, its temperature reaches 568 ° C, and its pressure reaches 240 kgf / cm2. The flow of high-pressure steam flowing from a boiler to a steam turbine is generally measured with a throttle device such as a nozzle. In steelmaking and ironmaking, the temperature of the combustion air is increased and then blown into a furnace such as a blast furnace, a converter, and a sintering furnace. The flow of these combustion air can be measured with throttling devices such as venturi tubes. In addition, the gas discharged from these furnaces also belongs to the high temperature range, and the flow rate can be measured with a venturi tube. Because the exhaust gas is high-temperature gas, it is corrosive and contains dust. When selecting materials, we must consider these characteristics and conditions of use, and choose materials that are corrosion-resistant and suitable for the purpose of use. In the chemical and petrochemical industries, for example: hydrogen production equipment and reforming furnaces using steam reforming methods, the temperature inside the reaction tube is also increased by about 800-900 ° C. Petroleum refining contact upgrading device. The temperature is 420-580 ° C, and the pressure is 15-50 kgf / cm2. Although the furnace temperature in these devices is high, it is because the flow rate before entering the furnace is measured and controlled. It is not necessary to measure such hot fluids. The above are just a few examples. There are many more high-temperature fluids to be measured, and the list is endless. When considering the use of a throttling device to measure the flow of high-temperature fluids, as well as the selection of valve materials, it is important to pay attention to pressure while paying attention to temperature. With the proper materials selected, flow rates in the high temperature range can be measured. However, from the point of view of material strength, the tensile strength and yield point decrease with increasing temperature. When the temperature reaches about 400 ° C or higher, creep occurs. Therefore, this temperature can be regarded as the highest temperature in the throttling device. .