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Professional introduction to the basic method of selecting electromagnetic flowmeter

The basic method of selecting electromagnetic flowmeter
1, liquid conductivity
The premise of using EMF is that the liquid to be tested must be electrically conductive and cannot fall below the threshold (ie the lower limit). If the conductivity is lower than the threshold, measurement errors may not even be used. Even if the threshold is exceeded, the measurement may be measured, and the indication error does not change much. The threshold of the general-purpose EMF is between 10-4 and (5×10-6) S/cm. , depending on the model. It also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The length of the signal line corresponding to the conductivity is usually specified in the manufacturer's instruction manual. Non-contact capacitive coupling large-area electrodes can measure liquids with conductivity as low as 5 × 10-8 S / cm.
2, accuracy level and function
The performance of general-purpose EMF on the market is quite different, some have high precision, many functions, some have low precision and simple functions. The basic error of the instrument with high precision is (±0.5%～±1%)R, and the instrument with low precision is (±1.5%～±2.5%) FS. The price difference between the two is 1-2 times. Therefore, it is economically uneconomical to select high-precision instruments in places where measurement accuracy is not very high (for example, non-trade accounting is only for control purposes, and only requires high reliability and excellent repeatability).
Some models claim to have higher accuracy, the basic error is only (±0.2% ~ ±0.3%) R, but there are strict installation requirements and reference conditions, such as ambient temperature 20 ~ 22 ° C, the length of the straight pipe length before and after the requirements More than 10D and 3D (usually 5D and 2D), it is even suggested that the flow sensor be integrated with the front and rear straight tubes for real-flow calibration on the flow standard device to reduce the impact of the clamping. Therefore, when comparing multiple models, do not simply look at high indicators. Read the manufacturer's samples or instructions for a comprehensive analysis.
The functions of EMF on the market are also very different. The simple one is to measure the one-way flow, and only output the analog signal to drive the rear position meter; the multi-function meter has the measurement bidirectional flow, range switching, upper and lower limit flow alarm, empty tube and power cut alarm. , small signal cutoff, flow display and total calculation, automatic check and fault self-diagnosis, communication with the host computer and motion configuration. The serial digital communication function of some models can be selected from a variety of communication interfaces and dedicated chips (ASIC) to connect HART protocol system, PROFTBUS, Modbus, CONFIG, FF field bus and so on.
3, flow rate, full flow, range and caliber
The selected instrument diameter is not necessarily the same as the pipe diameter and should be determined by the flow rate. The process industry transports liquids with different viscosities such as water. The flow rate of the pipeline is generally 1.5 to 3 m/s. EMF is used on such a pipe, and the sensor diameter is the same as the pipe diameter.
When the EMF full flow rate, the liquid flow rate can be selected within the range of 1 to 10 m/s, and the range is relatively wide. The upper limit flow rate is not limited in principle, but it is generally recommended not to exceed 5 m/s unless the lining material can withstand scouring, and practical applications rarely exceed 7 m/s, and more than 10 m/s is even more rare. The minimum flow rate for full flow is typically 1 m/s, and for some models it is 0.5 m/s. For some new construction projects, the flow rate is low or the flow rate is low. From the perspective of measurement accuracy, the gauge diameter should be changed to less than the pipe diameter and connected by a different diameter pipe.
It is used for fluids with easy adhesion, deposition, scaling and other materials. The flow rate is not less than 2m/s, preferably increased to 3-4m/s or above, which can be used for self-cleaning and adhesion prevention. For fluids with high abrasiveness such as pulp, the flow rate should be less than 2~3m/s to reduce the wear on the lining and the electrode. In the measurement of low-conductivity liquid close to the threshold (ie, the lower limit), the lower flow rate (less than 0.5 to 1 m/s) is selected as much as possible, and the flow noise is increased due to the flow rate increase, and the output sloshing occurs.
The range of EMF is relatively large, usually not less than 20, and the meter with automatic range switching function can exceed 50~100. Domestically available styling products range in size from 10mm to 3000mm, although the actual application is mostly in medium and small caliber, but with most other principle flow instruments (such as volumetric, turbine, vortex or Coriolis mass, etc.) Compared to large-diameter instruments, they account for a large proportion. Among the nearly 10,000 instruments in a company, the small diameter below 50mm, the medium diameter of 65~250mm, the large diameter of 300~900mm, and the super large diameter of 1000mm or more account for 37%, 45%, 15% and 3% respectively.
4, adhesion and precipitation
When measuring the fluid that is easy to adhere to and deposit substances on the pipe wall, if the conductive material is higher than the conductivity of the liquid, the signal potential will be short-circuited and will not work. If it is a non-conductive layer, the electrode contamination should be paid attention first. Attach a pointed or hemispherical protruding electrode, a replaceable electrode, a scraper type cleaning electrode, and the like. The scraper electrode can be manually scraped off the scale manually outside the sensor. Foreign products have been equipped with ultrasonic transducers on the electrodes to remove surface scale, but it is now rare. There is also a temporary disconnection of the measuring circuit, a low-voltage large current flows during the electrode break time, and incineration removes the adhesion layer attached to the grease. The place where the adhesion is easy can be increased to achieve the purpose of self-cleaning, and a convenient and easy-to-clean pipe connection can be adopted, and the cleaning sensor can be removed. Non-contact electrode EMF adheres to the non-conductive film layer, and the meter can still work, but it does not work if it is a highly conductive layer.
5, the liquid contains mixed substances
The microbubbles mixed into the 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 covered by the gas to make the circuit instantaneously broken. On, output sloshing does not even work properly.
A solid-liquid two-phase fluid containing non-ferromagnetic particles or fibers can also measure the volumetric flow of the two phases. Fluids with higher solids content, such as drilling mud, drilling cement slurry, pulp, etc., are actually non-Newtonian fluids. Since the solids flow together in the carrier liquid, there is a slip between the two, and there is a difference in speed. The single-phase liquid calibration meter can cause additional errors for the solid-liquid two-phase fluid. Although the system test report on the influence of EMF on solids in solid-liquid two-phase fluid has not been seen, there are reports in foreign countries that the error of solid content is 14%, and the error is within 3%; China Institute of Water Resources, Yellow River Conservancy Commission According to the experimental report, the flow rate of high-sand water is measured, and the volume ratio of sediment is 17% to 40% (the median diameter of sand is 0.35 mm), and the measurement error of the meter is less than 3%.
In the slurry, there are large particles rubbing across the surface of the electrode. In the EMF of the rectangular excitation with lower frequency, spike-like serous noise is generated, which makes the flow signal unstable. It is necessary to select a higher frequency instrument or have a stronger suppression of slurry noise. The instrument of capability can also be selected by AC excitation instrument or dual frequency excitation instrument. Fluids containing ferromagnetic substances are common to EMF, and measurement errors occur because the magnetic permeability of the measuring tube is varied by the different contents of the ferromagnetic body. However, the EMF compensated by the magnetic flux detecting coil is placed in the magnetic circuit to reduce the influence of mixing into the ferromagnetic body. Shanghai Guanghua Instrument Factory said in the experimental report of the AC excitation instrument that the water contains a slurry with a liquid-solid weight ratio of about 4:1 and a particle size of ≤0.15mm. The 80mm diameter meter is used for the comparison of the flow rate of clean water and slurry. The instrument indicates a value change of 7% to 10%, and the meter equipped with the magnetic flux detecting coil has an indication error within ±2% FS.
For pulp applications containing ore particles, attention should be paid to the degree of wear on the sensor lining, which may cause additional errors in the inner diameter of the measuring tube. In this case, ceramic lining or urethane rubber lining with better wear resistance should be selected. It is also recommended that the sensor be installed on the vertical pipe to make the pipe wear evenly, and eliminate the disadvantage of partial wear of the lower half of the horizontal installation. It can also be at the inlet end of the sensor. A nozzle-shaped sheath is added to extend the service life.

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