Electromagnetic flowmeters are used in a very large amount in the current chemical production and other industrial occasions. Generally, as long as liquid measurement is involved, "the first thing to consider is electromagnetic flowmeters, because of their own measurement characteristics, electromagnetic flowmeters. The advantages in the measurement process are quite obvious. The advantages of the electromagnetic flowmeter are mainly manifested in high measurement accuracy, tetrafluoro lining acid and alkali corrosion resistance, high pressure resistance, and the liquid crystal display of the meter head displays the cumulative flow, instantaneous flow, and flow rate. Moving parts, no blockage, full digital processing, strong ability to carry interference, there are two types of pipeline split and plug-in, which can measure liquid-solid two-phase flow, high-viscosity liquid flow and salt, strong acid, strong alkali and other corrosive liquids There are many classification methods for electromagnetic flowmeters, and there are many middle-class products corresponding to each classification. We must have a basic understanding of this point when selecting products, which can guide us very well. To ensure the correct use of the product, this article makes 6 conventional classifications for electromagnetic flowmeters, which will be introduced to you below.
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1. Classification by sensor and converter assembly method
Split-type electromagnetic flowmeter: The split-type electromagnetic flowmeter is a commonly used form of electromagnetic flowmeter. The sensor is connected to the pipeline, and the converter is installed in the instrument room or near the sensor that is easy for people to approach, with a distance of tens to hundreds of meters. In order to prevent the intrusion of external noise, Signal cables are usually double-shielded. When measuring liquids with low conductivity and the gathering exceeds 30m, in order to prevent signal attenuation caused by part of the capacitance of the cable, the inner shield also requires a shield drive with a low impedance source at the same potential as Xinxian. Separate type The converter can be far away from the harsh environment on site, and it is more convenient to check, adjust and parameterize the electronic components.
Integrated electromagnetic flowmeter: The sensor and the converter are assembled together to directly output a standard signal of DC current (or frequency), which actually becomes an electromagnetic flow transmitter. The integrated type shortens the wiring of the signal line and excitation line between the two and simplifies the electrical Connection, instrument price and installation cost are relatively cheap, and small-diameter instruments are mostly used. With the commercialization of two-wire instruments, integrated instruments will develop rapidly. However, if it is installed in an inaccessible place due to the limitation of pipeline arrangement, maintenance will be inconvenient. In addition, the electronic components of the converter are installed on the pipeline, which will be greatly restricted by the fluid temperature and the vibration of the pipe.
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2. Classification according to whether the electrode of the flow sensor is in contact with the liquid to be measured
Contact electrode-----The electrode in contact with the liquid is the traditional structure of EMF, usually a pair of electrodes, and two pairs of electrodes are used in the large-diameter electromagnetic flowmeter instrument industry. Partial tube EMFs are also available with 3 pairs of motors and bar electrodes.
Non-contact electrode----The large-area electrode is close to the outer surface of the lining (or insulating measuring tube), picks up the flow signal by capacitive coupling, and can measure the electromagnetic flowmeter of the capacitive detection method of the non-contact electrode. . The preamplifier is placed in the sensor only by the electrodes, and the excitation frequency is higher than the usual EMF, which is 50/2Hz, and there are also those that exceed 100Hz. This type of meter does not generate electrode surface effect noise such as electrode passivation, oxidation and catalyst action, and there is almost no flow noise and slurry noise. The inner surface of the lining is covered with non-conductive layers such as grease or thin insulating structural layers, which will not affect the measurement. However, if the covering layer is a conductive film, the meter will have no indication. Some domestic instrument manufacturers call this type of instrument an electrodeless electromagnetic flowmeter.
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3. Classification by flow sensor and pipeline connection
Flange connection---Flange connection is a traditional connection method. There are connecting flanges at both ends of the sensor, which are fixed with the pipe flange by bolts, which can be installed in one direction. The large-diameter sensors of the electromagnetic flowmeter all use this connection method, and the volume and weight are larger than the clamp connection method. The length between the two flange faces of the DN15-600mm electromagnetic flowmeter has been unified by the domestic standardization organization (ISO 13359; 1998).
Flange clamp connection: The connection method developed in recent years for flange clamp connection. The sensor of the electromagnetic flowmeter itself cannot be flanged, and is connected to the pipe system between the two flanges of the pipeline with longer bolts. This type of sensor is small in size and light in weight, and has strong adaptability to different pressure specifications and standard pipe system flange hole spacing; but it is only suitable for small pipe diameters (below 200mm), and can withstand low liquid working pressure.
Clamp connection: The clamp connection can realize the quick disassembly and installation of the electromagnetic flowmeter, which is convenient for daily frequent cleaning.
Threaded connection: This type of electromagnetic flowmeter is mostly used in medicine, food and other industries, as well as in places such as pharmaceutical proportioning and injection. Threaded connection is also widely used in petroleum, geological exploration and other high-pressure water injection or cement slurry flow measurement above 16-20MPa. The thread shape is trapezoidal.
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4: Classification by output signal connection and excitation (or power supply) wiring system
Four-wire electromagnetic flowmeter-----Four-wire system is that the output signal line and power line (or excitation wire between the flow sensor and the sensor) of the traditional electromagnetic flowmeter are composed of two groups of four-wire system with two wires each. , is the current main format.
Two-wire electromagnetic flowmeter - current temperature, pressure/differential pressure, flow and material level and other parameters field instruments tend to the development of two-wire instruments that share wires for output signals and power supplies. The two-wire instrument does not require the mains power supply, and the electromagnetic flowmeter is often installed in remote places without mains supply. Using the two-wire system can save the cost of the mains wiring project. The design idea of the power supply of the two-wire electromagnetic flowmeter is divided into zero signal output current (ie 4mA) supply, greater than zero signal output supply and battery (or solar cell) supply. Battery-powered electromagnetic flowmeters and electromagnetic water meters are suitable for deployment in places where it is difficult to introduce city power, such as away from urban water distribution pools or discharge points after sewage treatment in suburban areas. Some models have a battery life of 1-2 years, while others are as long as 8-10 years.
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5. Classification by flow sensor structure
Pipeline electromagnetic flowmeter: The pipeline-type structure is the structure of a traditional electromagnetic flowmeter, and the flow sensor is connected to the pipeline system with a measuring pipe section.
Insertion type electromagnetic flowmeter: The insertion type flow sensor is essentially an electromagnetic flow rate sensor. The excitation coil and the electrode are assembled into a rod shape and inserted into the opening on the pipe to be measured. The measured flow rate is equal to the pipe area preset by the converter. Multiply the hydrochloric acid to obtain the flow rate. In addition to the single-point "point flow rate", there are side multi-point "runoff flow". This type of instrument is suitable for large-scale pipelines, because it measures the local flow rate to estimate the fluid flow, and the measurement accuracy is lower than that of the short-pipe type. It is usually only used for process control and is not suitable for trade accounting and measurement, but the price is relatively cheap.
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6. Classification by use
Universal type: In metallurgy, petrochemical, papermaking, printing and dyeing, textile, water supply, sewage treatment and other industries generally use split type, large-diameter flange connection; medicine, biology, fine chemicals and other industries commonly used micro-diameter, small-diameter instruments , The electromagnetic flowmeter with integrated clip-on connection or threaded connection is suitable for many places.
Explosion-proof type: explosion-proof electromagnetic flowmeter is used in places with flammable atmosphere. Due to the large current energy of most electromagnetic flowmeters, they usually involve explosion-proof type, sand-filled type, potting type and air-tight type. But now some electromagnetic flowmeters have greatly reduced excitation power, and there are also intrinsically safe types, that is, the so-called safe spark type.
Sanitary type: Cheese, food, medicine, biochemistry and other industries require regular steam sterilization, the sensor should be easy to disassemble and clean, and the connection part with the pipeline requires a hygienic type structure with quick loading and unloading, and the material in contact with the liquid should be non-toxic harmless. The sanitary electromagnetic flowmeter was developed to meet the above requirements.
Submersible type: It is used in pits installed under the ground and can withstand short-term water immersion, which is equivalent to IP67 or NEMA6 enclosure protection level; there are also electromagnetic flowmeter products equivalent to IP68 protection level, which can withstand long-term diving.
Submersible type: It is used to measure the water flow under the free water surface of open channel or underdrain, such as industrial drainage and sewage. The sensor is located at the lower part of the open channel interception baffle and is immersed in water for a long time. It is used to install multiple diverter models with the same shape as the sensor at the moment of large flow to expand the flow.
