First, the use of electromagnetic flowmeter
Electromagnetic flowmeters are widely used. Large-caliber instruments are mostly used in water supply and drainage projects. Small and medium sized mouthpieces are often used for liquids or liquids that are difficult to measure, such as paper pulp and black liquor, pulps from non-ferrous metallurgical industries, coal slurry from coal preparation plants, aggressive effluents from the chemical industry, and blast furnaces in the steel industry. The control of the cooling water in the tuyere and leakage monitoring, the flow measurement and control of the long-interval pipe coal for hydraulic protection. High-pressure electromagnetic flowmeters and slurry flowmeters are the first choice for field applications such as oilfields and coal dressings. Smaller diameters and smaller diameters are often used in sanitary applications such as the pharmaceutical industry, food industry, and biotechnology. Sanitary type electromagnetic flowmeter is developed by Jiangsu Jinhu Kexin Instrument Co., Ltd., specifically for the sanitary requirements of the pharmaceutical industry, food industry, and bioengineering.
Second, the accuracy of the electromagnetic flow meter grade and function The general-purpose electromagnetic flowmeter on the market has a large difference in function, some high precision, many functions, some of the low precision, simple function. High-precision instrument fundamental error (± 0.5% ~ ± 1%) R, low-precision instrument is (± 1.5% ~ ± 2.5%), the difference between the price of 1 to 2 times. Therefore, when the measurement accuracy is not very high (for example, non-commercial accounting is only aimed at restraint, and only high reliability and excellent repeatability are required), the use of high-precision meters is economically uneconomical.
Some models claim higher accuracy, and the fundamental error is only (±O.2%~±O, 3%)R, but there are strict devices to reduce the reference conditions, such as the temperature of 20~22°C. Straight pipe length requirements to identify more than 10D, 3D (wherever 5D, 2D), and even proposed flow sensor with the straight pipe before and after the integration of the flow in the standard installation for real flow calibration, in order to reduce the impact of improper clamping. Therefore, in the selection of a variety of models do not only look at the high goals pure, to specifically read the manufacturer sample or imitation for a comprehensive analysis.
The fundamental error of full-flow non-full-tube EMF is (±1.5% to ±2%) FS. The basic error of the piercing type instrument itself (ie, the test head) is usually ±2%R~±4% FS. Considering that the point flow velocity (or radial flow rate) is on behalf of the uniform flow velocity on the surface, the influence of changes in the velocity dispersion coefficient caused by the fluid change is affected. Can bring about 2% to 5% of the transition and smooth area measurement error, the overall measurement accuracy is even lower.
If it is used in large-diameter long-distance water pipes, electromagnetic flow detection heads are individually calibrated in the "calibration flume" due to superior speed dispersion (there are long enough forward straight pipe segments), and the limitations of fluid quality change are not large. The basic deviation may be close to or slightly larger than ±2%R under better preconditions such as a clear change in speed dispersion coefficient.
The utility of the electromagnetic flowmeter on the market is also very different, simply measuring the one-way flow, only output the imitate the signal to drive the rear instrument; multi-function meter has to measure the two-way flow, range switching, upper and lower limit flow alarm, empty tube And power cut off the alarm, small signal removal, flow display and aggregate total, active check and fault self-diagnosis, and host computer communications and activity configuration. Some models of serial digital communication function can choose a variety of communication interfaces and dedicated chip (ASIC) to connect HART peace talk system, PR0 å…€BuS, Modbus, CONFIG, FF field bus and so on.
Third, the flow rate of the electromagnetic flowmeter, full flow, bureau limit and caliber The selected instrument caliber is the same as the diameter, it should be determined by the flow. The flow rate of liquids and pipes with low viscosity, such as flow in the process industry, is generally 1.5 to 3 m/s. Electromagnetic flowmeters are used on such pipes. The sensor diameter can be the same as the pipe diameter.
The flow rate of the electromagnetic flowmeter at full flow can be selected within the limits of 1 to 10 m/s, and the limitations are relatively wide. The upper limit flow rate is not limited by reason, but the mortal recommendation does not exceed 5m/s. Unless the lining data can accept the flushing of the liquid flow rate, practical use rarely exceeds 7m/s, and exceeding 10m/s is even rarer. The lower limit of the flow rate for full flow is usually 1 m/s, and for some models it is 0.5 m/s. Some pipelines with low flow rate or low flow rate during the initial period of operation of the new construction project are considered from the perspective of measurement accuracy. The diameter of the instrument should be changed to smaller than the pipe diameter, and the different diameter pipes should be connected.
For fluids with easy adhesion, accumulation, scaling, etc., the selected flow rate should not be less than 2m/s, preferably 3 to 4m/s or more, and it has the effect of self-cleaning and avoiding adhesion accumulation. It is used for fluids with strong abrasion, such as pulp, and the common flow rate should be lower than 2～3m/s to reduce the wear on the lining and electrodes.
In the measurement of low-conductivity liquids that are close to the threshold, the lower flow rate (less than 0.5 to 1 m/s) can be selected, and as the flow rate progresses, the active noise is added and the output swing phenomenon is presented.
The limit of the electromagnetic flowmeter is relatively large. The mortality is not less than 20, and the meter with the active range switching function can exceed 50-100.
The caliber of the styling products that can be supplied domestically ranges from 10mm to 3000mm, although the practice still uses mostly small and medium sized calibers, but with most other flow meters. (such as volumetric, turbo, vortex, or Coriolis mass, etc.), large-diameter meters have a larger proportion. In a company's nearly 10,000 Taiwan meters, small caliber below 50mm, 65 ~ 250mm in the caliber, 300 ~ 900mm large diameter, over sized caliber over 1000mm accounted for 37%, 45%, 15% and 3%.
Fourth, the liquid conductivity The use of electromagnetic flowmeter is based on the premise that the liquid must be conductive, can not be less than the threshold (ie lower limit). When the conductivity is lower than the threshold, measurement error will occur until it cannot be used. If the threshold is exceeded, the measurement can be measured, and the indication error will not change much. The general-purpose electromagnetic flowmeter has a threshold of 10-4 (5×10-6)S/cm, depending on the model. When used, it also depends on the length of the signal line between the sensor and the converter and its dispersion capacitance. The manufacturer uses the length of the signal line corresponding to the regular conductivity in the simulant. Non-contact capacitively coupled large area electrodes can measure liquids with conductivity as low as 5 x 10-8)/cm.
The conductivity of industrial water and its aqueous solution is greater than 10-4S/crn, and the conductivity of acid, alkali and salt solution is between 10-4 and 10-1S/cm. There is no problem in application, and the low distilled water is 101S/cm. There is no problem. The low conductivity of finished petroleum products and organic solvents cannot be used. The figure above shows the conductivity of several liquids. The conductivity of some pure liquids or aqueous solutions was found to be low on the material, and it could not be used. However, practical examples may encounter instances where impurities can be used. Such impurities are good for adding conductivity. Regarding the aqueous solution, the conductivity in the material is measured for the ratio of pure water in the laboratory. The aqueous solution that can be used in practice can be proportioned with industrial water, and the conductivity will be higher than that of the water. It is also favorable for flow measurement.
There is also a case where, in the absence of ready-made conductivity data, the electromagnetic flowmeter installed in the pipeline cannot be measured. The sample is taken from the field line and taken offline to conduct the measurement using a conductivity meter and it is considered to be usable. This is because the sampled off-line process has a difference between the measured liquid and the pipeline; for example, the liquid has received CO2 in the atmosphere or N0x generates a trace amount of carbonic acid or nitric acid, which changes the conductivity.
Depending on the application experience, the liquid conductivity used in practice should preferably be at least one order of magnitude greater than the instrument manufacturer's regulatory threshold. Because the lower limit value of the standard rule of the manufacturer's instrument is the lowest value that can be measured under all kinds of preconditions for better application, it is subject to some application preconditions, such as the uniformity of conductivity, the connection signal line, and external noise, etc. Otherwise, you will see output swings. We encountered repeated measurements of low-level distilled or deionized water, which required conductivity close to the threshold of 5×10-6 S/cm, and exhibited output sloshing when used.
Five, electromagnetic flowmeter measurement of liquid containing mixed substances into the bubble-like flow of fine bubbles can still be the normal task, but measured volume containing the bubble volume of inclusion volume flow; if the gas content is added to constitute the bomb (block)-like flow, Because the electrode can be covered by the gas, the circuit is momentarily disconnected, and the output swing is not even a normal task.
Solid-liquid two-phase fluids containing non-ferromagnetic particles or fibers can also measure the volumetric flow rate of two phases. Fluids with higher solids content, such as drilling mud, drilling cement slurry, pulp, etc., are practically non-Newtonian fluids. Because the solids move together in the carrier fluid, there is slippage between the two, and there is a difference in speed. The single-phase liquid calibration instrument is used for the solid-liquid biphasic flow to catch additional errors. Although it has not yet been seen that electromagnetic flowmeters are used to report the effects of solids in solid-liquid two-phase fluids, there are reports that foreign countries have a 14% solids error within 3%; China's Yellow River Conservancy Commission Water Conservancy Science Seminar According to the laboratory's declaration, the flow rate of high-sand content water was measured, and the sediment volume was 17% to 40% (sand median particle size 0.35nm), and the instrumental measurement error was less than 3%.
In the slurry there are larger particles rubbed the surface of the electrode, in the lower frequency of the rectangular magnetic electromagnetic flowmeter will produce spike-like slurry noise, so that the traffic signal instability, we must use a higher frequency instrument or a comparison Instruments that strongly suppress the noise of the slurry can also be selected from mains AC excitation or dual frequency excitation instruments. The electromagnetic flowmeter for the slurry used by Kaifeng Weili Flow Meter Co., Ltd. in collaboration with Japan's Sammuh is very good at handling this problem.
Fluids containing a ferromagnetic substance change from a ferromagnetic material to a mortal electromagnetic flowmeter, and the measurement error occurs because the magnetic permeability in the measurement tube is changed by the content of the ferromagnetic material. However, an electromagnetic flowmeter compensated by a magnetic flux detection coil is disposed in the magnetic circuit to reduce the influence of the incorporation of the ferromagnetic body. Shanghai Guanghua Instrument Factory stated in the experimental report of the AC excitation instrument that the water contains slurry with a ratio of liquid to solid of about 4:1, grain size ≤ 0.15mm of iron ore, and an 80mm gauge meter for comparison of clean water and slurry flow. The instrument revealed a conversion of 7% to 10% of the performance value. The instrument equipped with a flux detection coil has an error within ±2% FS.
For the use of slurry containing ore particles, it should be noted that for the wear level of the sensor lining, additional errors occur in the expansion of the inner diameter of the measuring tube. Such sites should use ceramic linings or urethane rubber linings with good abrasion resistance. It is also recommended that the sensor devices be worn on vertical pipes so that the pipelines wear evenly and the defects in the lower half of the device wear out. It is also possible to install a nozzle-shaped sheath on the inlet side of the sensor cartridge, which extends the operation period.
6. Electromagnetic Flowmeter Liquid Adhesion and Precipitation When measuring the fluid that is easily attached to the pipe wall and precipitates substances, if a conductive substance with a higher conductivity than the liquid is attached, the signal potential will be short-circuited and the task cannot be shortened. The conductive layer should be the first to pay attention to the contamination of the electrode, for example, it is not easy to attach a pointed or hemispherical raised electrode, a replaceable electrode, a blade type cleaning electrode, etc. The scraper electrode can manually scrape off the scale outside the sensor. Foreign products have been equipped with an ultrasonic transducer on the electrodes to remove the outer scaly layer, but it is rare nowadays. There is also a temporary disconnection of the measurement circuit. A low-voltage, high-current flows between the electrodes in a short period of time to burn and remove adhesion layers of grease. In cases where adhesion is likely to occur, the flow rate can be increased to reach the self-cleaning target, and a more convenient and easy-to-clean pipe connection can also be accepted, and the cleaning sensor can be disassembled.
Non-contact electrode electromagnetic flowmeter attached non-conductive film layer, the instrument can still work, but if it is a high conductive layer, it can not be the same task.
Electromagnetic flowmeter on the contact type electrode attached to the conductive layer of the additional error â–³E as shown below
In the formula, σw and σf are distinguished as adhesion layer and liquid conductivity;
t - thickness of the adhesive layer;
d - measuring tube inner diameter.
The foreign body layer weakly attached to the liner wall is an iron oxide embroidered layer, or a fuel containing metal as the primary component, whose conductivity is greater than the liquid conductivity, and the measured flow rate will be lower than the actual flow rate; if it is a scale layer such as calcium carbonate, Its conductivity is lower than that of the liquid, and the measured flow rate is higher than the actual flow rate. If the adhesion layer conductivity is the same as that of the liquid, the additional error calculated according to the above figure is zero, but this is only limited to the premise that the thickness of the adhesion layer is small, as shown in the figure below, 2t/d is less than 10%. Since the smooth area is reduced when the same flow has an adhesion layer, but the uniform flow rate is added, they can cancel each other, and it can only be said that the additional error can be negligible.
Seventh, electromagnetic flowmeter and fluid contact parts data selection and fluid contact sensor parts have lining (or insulation material made of measuring tube), electrodes, grounding rings and gaskets, its data is resistant to turbidity, resistance Wear resistance and application of upper temperature limit the instrument's compliance with the fluid. Because of the small number of parts, the simple shape, and the sensitive data selection, the electromagnetic flow sensor is highly compliant with the fluid.
(1) Lining data (or measuring tube in direct contact with the medium)
Common lining materials include fluoroplastics, urethane rubber, neoprene rubber, and ceramics. Fluoroplastics include polytetrafluoroethylene (F-4), perfluoroethylene propylene (tetrafluoroethylene and hexafluoropropylene copolymer, F-46; foreign trade name Teflon FEP, commonly known as FEP), tetrafluoroethylene and ethylene copolymers (F-40) and the perfluoroalkyl vinyl ether copolymer (modified PTFE, foreign trade name Teflon FEP) effectively lining FRP, but also made its own measuring tube. This year, high-purity aluminum oxide (99.7% A12O3) ceramics were used to make Chen Li's products, but only small and medium caliber sensors were used.
Neoprene and FRP are used for non-aggressive or weakly erosive liquids, such as industrial water, waste water, and if acid or alkali, the price is the lowest price. Fluoroplastics have excellent resistance to chemical fouling liquids, but poor wear resistance cannot be used to measure ore slurry. Fluorine plastic evaluation is the earliest use of PTFE, because with the measurement tube only by pressure, no adhesion, can not be used for negative pressure pipeline, after the development of various types of modified, complete injection molding, and The measuring tube has strong connection force and can be used for negative pressure. Its type is:
Polyurethane rubber has excellent wear resistance, but the corrosion resistance of acid and alkali is poor. Its wear resistance is equivalent to 10 times that of natural rubber, which is used in coal slurry, pulp, etc.; medium temperature is lower than 40 ~ 60/70 °C. Alumina ceramics have excellent wear resistance and corrosion resistance to strong acids and bases. The wear resistance is about 10 times that of polyurethane rubber. It is used in aggressive slurries. However, brittleness occurs and the device is easy to ignore. Broken, can be used for higher temperatures (120 ~ 140/180 °C), but to avoid drastic changes in temperature, such as steam sterilization, the general temperature gradient can not be more than 100 °C, heating 150 °C to have 10min time.
Lining data selection and its characteristics and do not use fluid please refer to the above figure, s. 2, general-purpose electromagnetic flowmeter several data pressure and temperature can be combined with the limitations can see the right picture.
(2) Electrode and grounding ring data The resistance of the electrode to the measured medium is the first element to consider when selecting the data. Secondly, consider whether passivation and other external effects and noises can occur.
1) Select erosion-resistant data electromagnetic flowmeters Electrode corrosion resistance is very high. Do not allow erosion. Strictly speaking, only promise very low erosion rate, otherwise it will destroy the seal between the electrode and the lining. Leakage of the medium can damage the insulation and the instrument cannot be used until the entire electromagnetic flow sensor is destroyed. The commonly used metal materials include acid-resistant steels 1Cr18Ni12Mo2Ti, Hastelloy (corrosion-resistant nickel-base alloys) B, C, titanium, tantalum, and platinum-bismuth alloys, which can literally cover out all chemical liquids. There are also low-noise electrodes used in combination with slurries, such as conductive rubber electrodes, conductive fluorine plastic electrodes and porous ceramic electrodes, or metal electrodes covering these materials. The following table shows the general limitations of these data.
The limitations of erosion resistance shown in the above table are just an overview. The erosion of disparate data on various media can be found in the "Erosion Handbook". For example, the "Erosion Data and Material Selection Manual" contains a lot of data that can be used as reference when selecting electrode data to determine the preliminary plan. However, this can only be a preliminary plan. Whether it can comply with the premise of on-site application requires further investigation. Since most of the data retrieved from the manual is obtained under the premise of a “pure†implementation of the laboratory, the fluids used in practice often contain trace impurities, and are often intermixed with several media. They are erosive and simple. The erosion of a single medium is significantly different; whether the liquid can contain ablation gas and the velocity of the activity also affects the erosion rate quite horizontally. For example, Hastelloy B is resistant to 80% of 10% concentration, not inflated hydrochloric acid, but not to aerated hydrochloric acid. Therefore, the selection of electrode data in the guidelines should be determined by the user from the use of the medium in the use of other devices and past experience. Sometimes it is necessary to perform the necessary implementation, such as the erosive experiment where the liquid sample is taken on site and the standby data is used in the laboratory. The best practice is the on-site coupon. This is an aggressive experiment that is closest to the premise of practical use and can be compared to the conclusion that it is reliable.
The surface of the ruthenium-resistant chemical media is very extensive. The platinum-rhodium alloys are also very resistant to various types of acid-alkali salt solutions. However, they also have some chemical media that are not resistant to corrosion. The following table lists several media that are not resistant to corrosion. There is complementarity between the two.
2) Preventing Electrode Surface Effect Electrode corrosion resistance is the main factor in selecting the data, but sometimes the electrode electrode data has good resistance to the measured media, but it is inconsistent with the data, but also to prevent the appearance of the electrode. effect.
Electrode appearance effects are divided into three aspects: external chemical reactions, electrochemical and polarization scenes, and electrode catalyst effects.
Chemical reaction effects such as the electrode surface and the measured medium contact, constitute a passive film or oxide layer. They have a positive maintenance effect on wear and erosion resistance, but they can also add external contact resistance. For example, helium can be oxidized when in contact with water to form an insulating layer.
The symbol clarifies: A—prioritized data (in practice there is a very long operational life span);
B--believable data (long service life under most conditions);
×——Can't use it
Electrochemical potential shifts and polarization phenomena can create disturbing potentials and constitute noise. Slurry noise and active noise are equal to the display of electrode surface noise. Polarization potential is the electrode induced electromotive force in the polarity of the two electrodes, resulting in electrolyte polarization in the electrode surface. The low frequency rectangular wave excitation connects the advantages of DC excitation and AC excitation. Although the alternating excitation weakens the polarization potential by several orders of magnitude, it cannot completely eliminate the effects of polarization potential interference. The polarization potential is related to the nature of the liquid medium and the nature of the electrode data.
Slurry noise is the measurement of the liquid-solid two-phase conductive liquid fluxes such as mud fiber slurry. The solid particles (or bubbles in the liquid) rub across the surface of the electrode. The surface of the electrode contact with the electrochemical potential suddenly changes, and the output flow signal exhibits spike-like noise. .
Activity noise is measured when the lower conductivity (100 x 10-6S/cm adjacent and below) liquid flow, the electrochemical potential of the electrode changes on a regular basis, the occurrence of random noise added with the flow rate and the frequency j; caused by the instrument output appears shaken scene.
About the surface noise of the electrode, the data of the electrochemical and polarization potential of the liquid to be measured can be selected as well as the low noise electrode.
The measured medium undergoes chemical reaction under the catalytic effect of the electrode and affects the measurement. For example, when the platinum electrode electromagnetic flowmeter measures hydrogen peroxide, it will generate an aerosol on the surface of the electrode. When the flow rate is zero, the output will also shake.
The media for preventing or mitigating the appearance of the electrode—electrode data matching—has not been as aggressive as enough material, and as long as some limited experience remains, it is still to be accumulated in theory. Some examples are listed below.
Earthworms are resistant to water. However, if a helium-electrode electromagnetic flowmeter is used to measure the flow of water, the surface of the helium electrode will constitute an insulating layer, which will cause the instrument to lose power or show a great deal of noise after a period of operation. Sodium hydroxide and other lye can not use the helium electrode. In the process flow, even a very short time contact with water or "non-acid" liquid (such as cleaning the pipe system) will affect the normal operation of the instrument.
The platinum-bismuth alloy electrode or platinum electrode has superior corrosion resistance to hydrochloric acid, and the platinum electrode electromagnetic flowmeter is used for measurement of hydrochloric acid to obtain satisfactory results. However, the measurement of high concentrations of hydrochloric acid (above 10%) produces severe noise. When the platinum electrode is used to measure low-pressure hydrogen peroxide (pressure below 0.3 MPa), an aerosol occurs on the surface of the electrode due to the effect of the catalyst, and the electrical path is interrupted to affect the task.
Hastelloy B is resistant to the temperature and concentration of hydrochloric acid, and there are several excellent examples that are used. However, when the concentration exceeds a certain value, noise occurs. The change of the concentration of hydrochloric acid at the scene indicates that when the concentration is gradually added beyond 15% to 20%, the output of the instrument will be shaken up, and the concentration will be 20% at output swinging up to 20%. Acid solutions such as nitric acid and sulfuric acid also have similar theoretical experiences.
The water plant uses aluminum sulfate solution mixed with the raw water to agglutinate the suspension, and the inclusion ratio is often measured by an electromagnetic flowmeter to measure the aluminum sulfate solution. The use of an acid-resistant steel electrode can achieve satisfactory results. We encountered a Hastelloy B-electrode electromagnetic flowmeter that measures 15% aluminum sulfate. During the process of application, it also showed an output swinging phenomenon. After that, the acid-resistant steel electrode was changed to the normal mission.
The platinum and rhodium electrodes have good results for most of the concentrations of sulfuric acid, nitric acid, hydrochloric acid, and hydrofluoric acid. However, the platinum electrode of each group produces a noise for hydrochloric acid with a concentration of more than 10%, and the concentration of the rhodium electrode is greater than 10%. % of hydrofluoric acid is not resistant to corrosion.
Although platinum-rhodium alloys and niobium have good corrosion resistance, they are expensive, and a couple of niobium electrodes are as high as several hundred to nearly one thousand yuan, platinum-germanium electrodes are more than twice as expensive as tantalum electrodes.
The grounding ring is connected to two ends of the flow sensor of the plastic pipe or the metal pipe lined with the insulating liner. Their corrosion resistance requirements are lower than that of the electrode, and they are promised to be eroded and replaced on schedule. The mortal uses acid-resistant steel or Hastelloy. Because of its large size, it is less economical to use precious metals such as platinum and platinum. No grounding ring is required if the metal process pipe is in direct contact with the fluid.