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Industry Information
The function of the electromagnetic flowmeter is very different. Measuring unidirectional flow only outputs analog signals.
2012-3-16 13:25:52

6.3 Velocity, full-scale flow, range, and caliber The selected instrument caliber may not necessarily be the same as the pipe diameter, but it should depend on the flow rate. The process industry conveys liquids with different viscosities, such as water, and the flow velocity of the pipeline is generally 1.5 to 3 m / s. LDE is used on such pipes, and the sensor diameter can be the same as the pipe diameter.
满度流量时液体流速可在1~10m/s范围内选用,范围是比较宽的。 The liquid flow velocity of the electromagnetic flowmeter can be selected in the range of 1 ~ 10m / s when the full-scale flow rate is reached, and the range is relatively wide. The upper limit flow rate is not limited in principle, however, it is generally not recommended to exceed 5m / s, unless the lining material can withstand the erosion of the liquid flow. Practical applications rarely exceed 7m / s, and more than 10m / s is more rare. The lower limit of full-flow velocity is generally 1m / s, and some models are 0.5m / s. For some new construction projects, the flow rate is low or the flow rate is low at the initial stage. From the perspective of measurement accuracy, the diameter of the instrument should be changed to be smaller than the diameter of the pipe, and connected with a reducer.
For fluids with substances that are easy to adhere, deposit, scale, etc., select a flow rate of not less than 2m / s, and preferably increase to 3 to 4m / s or more, to perform self-cleaning and prevent adhesion and deposition. For abrasive fluids such as slurry, the usual flow rate should be less than 2 ~ 3m / s to reduce the wear on the lining and the electrode.
When measuring low-conductivity liquids close to the threshold, choose a lower flow rate (less than 0.5 to 1 m / s) as much as possible. As the flow rate increases, the flow noise will increase, and the output will slosh.
The range of LDE is relatively large, usually not lower than 20, and the instrument with automatic range switching function can exceed 50 to 100.
The calibers that can be provided in China are from 10mm to 3000mm. Even in practical applications, they are mostly small and medium calibers, but they are similar to most other flow meters (such as volumetric, turbine, vortex, or Coriolis mass type). ) Compared with large-caliber instruments, they occupy a larger proportion. Among nearly 10,000 meters of an enterprise, small diameters below 50mm, medium diameters from 65 to 250mm, large diameters from 300 to 900mm, and super large diameters above 1000mm account for 37%, 45%, 15%, and 3%, respectively.
6.4 Liquid conductivity The premise of using LDE is that the measured liquid must be conductive and cannot be lower than the threshold (ie, the lower limit). If the conductivity is lower than the threshold value, a measurement error will occur until it cannot be used. It can be measured even if it exceeds the threshold value. The indication error does not change much. The threshold value of the general-purpose LDE is between 10-4 to (5 × 10-6) S / cm , Depending on the model. It also depends on the length of the flow signal line and its distributed capacitance between the sensor and the converter. The manufacturer's instruction manual usually specifies the length of the signal line corresponding to the conductivity. Non-contact capacitively coupled large-area electrodes can measure liquids with conductivity as low as 5 × 10-8S / cm.
The conductivity of industrial water and its aqueous solution is greater than 10-4S / cm, and the conductivity of acid, alkali and saline solution is between 10-4 ~ 10-1S / cm. There is no problem in use. Low-distilled water is 10-5S / cm is not a problem. Petroleum products and organic solvents are too low to be used. Table 1 lists the conductivity of several liquids. From the information, we found that some pure liquids or aqueous solutions have low electrical conductivity and are considered unusable. However, in practice, we will encounter examples that can be used because they contain impurities. Such impurities are beneficial to increase the conductivity. For the aqueous solution, the conductivity in the data is measured in the laboratory with pure water ratio. The actual aqueous solution may be used with industrial water ratio. The conductivity will be higher than that found, which is also beneficial to flow measurement.
Electrical conductivity of several liquids at 20 ° C Liquid name Electrical conductivity Liquid name Electrical conductivity Liquid name Electrical conductivity Oil (3 ~ 5) × 10-13
Acetone (2 ~ 6) × 10-8
Pure water, highly distilled water 4 × 10-8
Benzene 7.6 × 10-8
Liquid ammonia 1.3 × 10-7
Methanol (4.4 ~ 7.2) × 10-7
Drinking water ≈10-4
Seawater ≈ 4 × 10-2
Sulfuric acid (5% to 99.4%) (2.1 × 10-1) to (8.5 × 10-3)
Ammonia water (4% ~ 30%) (1 × 10-3) ~ (2 × 10-4)
Sodium hydroxide (4% ~ 50%) (1.6 × 10-1) ~ (8 × 10-2)
Saline solution (2.5%) 2 × 10-1
Liquid name conductivity Liquid name conductivity Liquid name conductivity Oil (3 ~ 5) × 10-13
Acetone (2 ~ 6) × 10-8
Pure water, highly distilled water 4 × 10-8
Benzene 7.6 × 10-8
Liquid ammonia 1.3 × 10-7
Methanol (4.4 ~ 7.2) × 10-7
Drinking water ≈10-4
Seawater ≈ 4 × 10-2
Sulfuric acid (5% to 99.4%) (2.1 × 10-1) to (8.5 × 10-3)
Ammonia water (4% ~ 30%) (1 × 10-3) ~ (2 × 10-4)
Sodium hydroxide (4% ~ 50%) (1.6 × 10-1) ~ (8 × 10-2)
Saline solution (2.5%) 2 × 10-1
According to experience, the liquid conductivity of the practical application is preferably at least an order of magnitude greater than the threshold specified by the instrument manufacturer. Because the lower limit specified by the manufacturer's instrument specifications is the lowest value that can be measured under various conditions of good use. It is limited by some use conditions, such as conductivity uniformity, connection of signal lines, external noise, etc. Otherwise, output jitter will occur. We have repeatedly encountered the measurement of low-grade distilled water or deionized water, whose conductivity is close to the threshold of 5 × 10-6S / cm, and the output is shaken during use.
6.5 The micro bubbles in the liquid containing the mixture mixed into a bubble flow can still work normally, but the mixed volume flow rate containing the bubble volume is measured; if the gas content is increased to form a bomb (block) flow, the electrode may be gas Covering makes the circuit momentarily open, and output wobble may not even work properly.
Solid-liquid dual-phase fluids containing non-ferromagnetic particles or fibers can also measure the two-phase volume flow. Fluids with a high solid content, such as drilling mud, drilling cementing slurry, and pulp, are actually non-Newtonian fluids. Because the solid flows together in the carrier liquid, there is sliding between the two, and there is a difference in speed. The single-phase liquid calibration instrument for solid-liquid dual-phase fluid will cause additional errors. Although no systematic experimental report on the effect of EMF on solids in solid-liquid dual-phase fluids has been seen, there are reports abroad that the error is within 3% when the solids content is 14%; The experimental report states that when measuring the flow of water with high sand content, the volume ratio of sand content is 17% to 40% (median diameter of sand is 0.35mm), and the measurement error of the instrument is less than 3%.
Large particles in the slurry rub across the electrode surface. Spike-shaped slurry noise will be generated in the low-frequency rectangular-excitation LDE, which will cause the flow signal to be unstable. It is necessary to use a higher-frequency instrument or have a strong suppression of slurry noise. Capable instruments can also be selected from AC mains or dual frequency instruments.
For fluids containing ferromagnetic substances, the normal LDE has a measurement error due to the change in the permeability of the measuring tube due to the different content of the ferromagnet. However, the LDE with magnetic flux detection coil compensation in the magnetic circuit can reduce the influence of mixing into the ferromagnet. In the experimental report of the AC excitation meter, it is stated that the slurry containing iron-ore concentrate with a liquid-solid weight ratio of about 4: 1 in the water and a particle size of ≤0.15mm is used for a comparative flow test of water and slurry with an 80mm caliber meter. The value of the usual meter changes 7% ~ 10%, the instrument equipped with a magnetic flux detection coil, the error of the displayed value is within ± 2% FS.
For slurry applications containing ore particles, attention should be paid to the degree of abrasion of the sensor lining. Enlarging the inner diameter of the measuring tube will cause additional errors. In this case, ceramic lining or polyurethane rubber lining with better abrasion resistance should be selected. At the same time, it is recommended that the sensor be installed on the vertical pipeline to make the pipeline wear uniform and eliminate the disadvantage of serious local wear in the lower half of the horizontal installation. A nozzle-shaped sheath can also be installed at the inlet of the sensor to prolong the service life.
6.6 Adhesion and Precipitation Measurement When the fluid that is easy to attach and deposit substances on the pipe wall, if a conductive substance with higher conductivity than the liquid is attached, the signal potential will be short-circuited and cannot work. If it is a non-conductive layer, the electrode pollution should be paid attention to first , For example, it is not easy to attach pointed or hemispherical protruding electrodes, replaceable electrodes, scraper-type cleaning electrodes, etc. Scraper-type electrodes can be used to manually scrape out dirt regularly outside the sensor. Foreign products used to have ultrasonic transducers on the electrodes to remove surface dirt layers, but these are now rare. There is also a temporary disconnection of the measurement circuit, a low voltage and large current flows in the electrode for a short time, and the oil and grease adhesion layer is removed by incineration. Places prone to adhesion can increase the flow rate to achieve the purpose of self-cleaning. It can also adopt a more convenient and easy-to-clean pipe connection, and the sensor can be cleaned without disassembly. The non-contact electrode LDE is attached with a non-conductive film layer, and the instrument can still work, but it also cannot work if it is a highly conductive layer.
6.7 Material selection of components in contact with fluid Sensor components in contact with fluid have linings (or measuring tubes made of insulating materials), electrodes, grounding rings, and gaskets, and their materials are resistant to corrosion, abrasion, and The use of the upper temperature limit affects the adaptability of the instrument to fluids. Because there are few parts, simple shapes, flexible material selection, the electromagnetic flow sensor has strong adaptability to fluids.
(1) Lining material (or measuring tube in direct contact with the medium)
Common lining materials include fluoroplastics, polyurethane rubber, neoprene, and ceramics. In recent years, there have been linings made of high-purity alumina (999.7% AI2O3) ceramics, but only for small and medium diameter sensors.
Neoprene and FRP are used for non-corrosive or weakly corrosive liquids, such as industrial water, waste water, and weak acids and bases, and are the least expensive. Fluoroplastics have excellent chemical resistance, but poor abrasion resistance, and cannot be used for measuring slurry. The earliest application of fluoroplastics is polytetrafluoroethylene. Because it is only pressed against the measuring tube and has no adhesive force, it cannot be used in negative pressure pipes. Later, various modified varieties were developed to achieve injection molding and have Strong binding force can be used for negative pressure. Polyurethane rubber has excellent abrasion resistance, but poor acid and alkali corrosion resistance. Its abrasion resistance is equivalent to 10 times of natural rubber, suitable for coal slurry, ore slurry, etc .; the medium temperature should be lower than 40 ~ 60/70 ° C. Alumina ceramic has excellent abrasion resistance and abrasion and corrosion resistance to strong acids and bases. The abrasion resistance is about 10 times that of polyurethane rubber. It is suitable for corrosive mineral pulp; but it is brittle and negligent during installation and clamping. It is fragile and can be used at higher temperature (120 ~ 140/180 ℃). However, to prevent sudden temperature changes, such as steam sterilization, the sudden temperature change should not be greater than 100 ℃, and it takes 10min to increase the temperature to 150 ℃.
几种材料的压力温度大体适用范围可参阅。 The general application range of pressure and temperature of several materials for general-purpose electromagnetic flowmeter can refer to.
(2) Electrode and ground ring material The corrosion resistance of the electrode to the measurement medium is the first factor to consider when selecting a material, and the second consideration is whether surface effects such as passivation and the resulting noise will occur.
1) Select corrosion-resistant materials. The requirements for the corrosion resistance of LDE electrodes are very high. Commonly used metal materials include molybdenum-containing acid-resistant steel Icr18Ni12Mo2Ti, Hastelloy (corrosion-resistant nickel-based alloy) B, C, titanium, tantalum, and platinum-iridium alloy, which can almost Cover all chemical fluids. In addition, there are low-noise electrodes suitable for slurry, which are conductive rubber electrodes, conductive fluoroplastic electrodes and porous ceramic electrodes, or metal electrodes coated with these materials. In principle, the choice of electrode material should be determined from the user's practical application and other past experience of the medium in other equipment. Sometimes necessary tests are performed later, such as taking liquid samples on site and doing corrosiveness tests of the materials in the laboratory. The best experiment is to hang the film on site. This is the corrosive test closest to the actual application conditions. It can be concluded whether it is reliable or not.
2) Avoiding electrode surface effects The electrode's corrosion resistance is an important factor in selecting materials, but sometimes electrode materials have good corrosion resistance to the measured medium, but they are not necessarily suitable materials, and electrode surface effects must be avoided. .
Electrode surface effects are divided into three aspects: surface chemical reactions, electrochemical and polarization phenomena, and the catalytic effect of electrodes.
Chemical reaction effects such as the formation of a passivation film or an oxide layer after the electrode surface contacts the measured medium. They may play a positive role in protecting against corrosion, but they may also increase surface contact resistance. For example, tantalum is oxidized in contact with water, forming an insulating layer.
Regarding the medium for avoiding or mitigating the effect of the electrode surface ----------- electrode material matching, there is not enough data to be searched as corrosive, only some limited experience has yet to be accumulated in practice.
The grounding ring is connected to both ends of the flow sensor of the plastic pipe or the metal pipe lined with insulation and lining. Their corrosion resistance requirements are lower than those of the electrodes, and there is a certain amount of corrosion, and they should be replaced regularly. Usually choose acid-resistant steel or Hastelloy. Because of its large size, it is economically considered to use less precious metals such as tantalum and platinum. If the metal process pipe is in direct contact with the fluid, a ground ring is not required.
7. Precautions for installation and use 7.1 General matters to be paid attention to during use The liquid shall have the conductivity required for measurement, and the conductivity distribution shall be substantially uniform. Therefore, the flow sensor should be installed to avoid the place where the conductivity is likely to be uneven. For example, if the liquid is added near the upstream, the liquid adding point is preferably located downstream of the sensor.
The sensor measuring tube must be filled with liquid during use (except for non-full tube type). When mixed, its distribution should be substantially uniform.
The liquid should be at the same potential as the ground and must be grounded. For example, when insulating materials such as plastics are used for process pipelines, frictional static electricity is generated during transportation of the liquid, which causes a potential difference between the liquid and the ground.
7.2 Flow sensor installation (1) The installation site usually has an IP65 (dust-proof and water-spray protection level specified in GB 4208) such as the electromagnetic flow sensor housing protection, etc., which has all the following requirements for the installation site.
1) When measuring mixed phase fluids, choose a place that will not cause phase separation; when measuring two-component liquids, avoid installing downstream where the mixing is not uniform; when measuring chemical reaction pipelines, install downstream of the fully completed reaction section;
2) Avoid negative pressure in the measuring tube as much as possible;
3) Choose a place with small vibration, especially for integrated meters;
4) Avoid having large motors, transformers, etc. nearby to avoid electromagnetic field interference;
5) A place where it is easy to achieve separate grounding of the sensor;
6) Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;
7) The ambient temperature is within the range of -25 / -10 to 50/60 ° C. The temperature of the integrated structure is also limited by the electronic components, and the range should be narrower;
8) Environmental relative humidity is in the range of 10% to 90%;
9) Avoid direct sunlight as much as possible;
10) Avoid rain and water.
If the protection level is IP67 (dust-proof and water-proof) or IP68 (dust-proof and diving-proof), the above 8) and 10) requirements are not required.
(2) Straight pipe length requirements In order to obtain normal measurement accuracy, there must be a certain length of straight pipe upstream of the electromagnetic flow sensor, but its length is lower than most other flow meters. 90? Elbow, T-shaped pipe, concentric reducer, fully open gate valve is generally considered as long as it is 5 times the diameter (5D) length of straight pipe section from the electrode centerline (not the sensor inlet connection surface), valves with different opening degrees Requires 10D; downstream straight pipe section is (2 ~ 3) D or no requirement; but it is necessary to prevent the butterfly valve disc from protruding into the sensor measuring tube. The lengths of the upstream and downstream straight pipe sections proposed by each standard or verification procedure are also inconsistent. As shown in Table 2, the requirements are higher than usual. This is because it is required to ensure that it meets the current level 0.5 accuracy instrumentation.
Spoiler name standard or verification procedure number ISO 6817
ISO 9104
JIS B7554
ZBN 12007
JJG 198
Upstream elbow, shaped pipe, fully open gate valve, progressive pipe 10D or manufacturer specified 10D
5D
5D
10D
Reducer can be regarded as straight pipe
Various downstream requirements 5D
2D not mentioned
2D
If the valve can be opened and used, the valve should be installed at an angle of 45 ° with the electrode shaft in the direction of valve shutoff.
(3) Installation position and flow direction The installation direction of the sensor can be horizontal, vertical, or inclined, without restrictions. However, it is best to measure the solid-liquid two-phase fluid vertically and flow from bottom to top. This can avoid the disadvantages such as severe local wear of the lower half of the lining during horizontal installation and solid phase precipitation at low flow rates.
When installing horizontally, make the electrode axis parallel to the horizon, do not be perpendicular to the horizon, because the electrodes at the bottom are easy to be covered by the sediment, and the top electrode is easy to be rubbed by the air bubbles in the liquid to cover the electrode surface, making the output signal fluctuate . In the piping system shown in Figure 5, c and d are suitable positions; a, b, and e are unsuitable positions; b may not be filled with liquid, and a and e are likely to accumulate gas; and the short pipe section at e may not be full. The discharge port is best shown as f. It is not suitable for solid-liquid two-phase flow c.
(4) Bypass pipe, easy to clean connection and preset inlet hole to facilitate checking and adjustment of zero point when the process pipeline continues to flow and the sensor stops flowing, a bypass pipe should be installed. However, large-diameter piping systems are often difficult to achieve due to investment and space constraints. It is difficult to correct the measured value according to the degree of electrode contamination, or to determine a judgment standard for the degree of pollution that does not affect the measured value. In addition to the above, using non-contact electrodes or instruments with scraper removal device electrodes can solve some problems, and sometimes it is necessary to remove the attachments on the inner wall and remove them without removing the sensor.
For piping with a diameter greater than 1.5-1.6m on the pipeline near the EMF, preset holes are provided to clean the inner wall of the sensor measuring tube when the piping stops.
(5) The installation of fluoroplastic-lined sensors for negative pressure piping systems must be applied with caution to negative pressure piping systems; positive pressure piping systems should prevent the generation of negative pressure. After operation, the fluid will shrink to form a negative pressure. A negative pressure prevention valve should be installed near the sensor. The manufacturer has specified that PTFE and PFA plastic linings can be used in negative pressure piping. The absolute pressure can be used at 200C, 1000C, 1300C. Must be greater than 27, 40, 50KPa, respectively.
(6) The grounding sensor must be grounded separately (grounding resistance below 100Ω). In principle, the separation type should be on the sensor side, and the converter ground should be at the same ground point. If the sensor is installed on a pipeline with cathodic corrosion protection, in addition to grounding the sensor and the grounding ring together, a thicker copper wire (16mm2) should be used to bypass the two connection flanges of the sensor crossover pipeline to make the cathodic protection current between the sensors isolation.
Sometimes the back stray current is too large. If the leakage current of the electrolytic cell along the electrolyte affects the normal measurement of EMF, you can take the method of electrical isolation between the flow sensor and the process to which it is connected. This method can also be used when the cathodic protection current affects EMF measurement on the same cathodic protection pipeline.
无单独安装转换器;分离型转换器安装在传感器附近或仪表室,场所选择余地较大,环境条件比传感器好些,其防护等级是IP65 或IP64 (防尘防溅级)。 7.3 Converter installation and connection cable The integrated electromagnetic flowmeter does not have a separate installation converter. The separate converter is installed near the sensor or the instrument room. The place to choose is larger and the environmental conditions are better than the sensor. Its protection level is IP65 or IP64 ( Dust and splash level). The requirements for the installation site are the same as those in 3), 4), 6), 8), 9), and 10) in Section (1) of Section 7.2. The ambient temperature is limited by the electronic components. Narrower.
The distance between the converter and the sensor is limited by the conductivity of the measured medium and the type of signal cable, that is, the distributed capacitance of the cable, the cross-section of the wire, and the number of shielding layers. Use the signal cable supplied with the instrument (or specified model) by the manufacturer. For liquids with low conductivity and long transmission distances, three-layer shielded cables are also required. The “Instruction Manual” of the general instrument gives the corresponding transmission distance range for liquids with different conductivity. Single-layer shielded cable can be used for industrial water or acid-alkaline transmission distance of 100m.
In order to avoid interfering with the signal, the signal cable must be routed separately in a grounded protective steel pipe, and the signal cable and power line cannot be installed in the same steel pipe. One