Considerations for the selection of electromagnetic flowmeter

1, application overview

Large-diameter instruments are mostly used in water supply and drainage projects. Medium and small calibers are often used for solid-liquid double equal unpredictable fluids or high-demand places, such as paper pulp and black liquor in the paper industry, pulp in non-ferrous metallurgical industry, coal slurry in coal preparation plant, strong corrosive liquid in chemical industry, and blast furnace tuyere in steel industry. Cooling water control and monitoring, flow measurement and control of hydraulic conveying of long-distance pipeline coal. Small caliber and small caliber are often used in places with hygienic requirements such as the pharmaceutical industry, food industry, and bioengineering.

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, front and rear straight pipe length requirements More than 10D, 3D (usually 5D, 2D), respectively, even proposed that the flow sensor should be integrated with the front and rear straight tubes for real-flow calibration on the flow standard device to reduce the impact of poor 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.

In some pipelines with low flow rate or low flow rate at the beginning of new construction, from the perspective of measurement accuracy, the gauge diameter should be changed to less than the diameter of the pipe and connected by a reducer. It is used for fluids with easy adhesion, deposition, scaling and other materials. The flow rate is not less than 2m/s and is increased to 3~4m/s or above. It can be used for cleaning and preventing adhesion and deposition. 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. When measuring a low conductance liquid close to the threshold, select a lower flow rate (less than 0.5 to 1 m/s) as much as possible, and the flow noise will increase due to the flow rate increase, and output sloshing will occur. 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.

4, 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). Conductivity below the threshold will cause measurement error until it can not be used. If the threshold is exceeded, it can be measured even if the change is made. The indication error does not change much. The threshold of 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.

The conductivity of industrial water and its aqueous solution is greater than 10-4 S/cm, and the conductivity of acid, alkali and salt liquid is between 10-4 and 10-1 S/cm. There is no problem in use, and low-grade distilled water is 10-5 S/ There is no problem with cm. Petroleum products and organic solvents cannot be used if their conductivity is too low. Table 1 lists the conductivity of several liquids. It is found from the data that some pure liquids or aqueous solutions have low conductivity and are considered to be unusable. However, in practice, examples can be used because they contain impurities, and such impurities are advantageous for increasing conductivity. For aqueous solutions, the conductivity in the data is measured in the laboratory using pure water ratio. The actual aqueous solution may be used in industrial water ratio, and the conductivity will be higher than that obtained, which is also beneficial for flow measurement.

Based on experience, the actual conductivity of the liquid is 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 specification is the lowest value that can be measured under various conditions of good use. It is subject to some use conditions, such as conductivity uniformity, connecting signal lines, external noise, etc., otherwise output sloshing will occur. We have encountered measurement of low-grade distilled water or deionized water many times, and its conductivity is close to the threshold of 5×10-6S/cm, and output sloshing occurs when used.

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 of the effect of EMF on the solid matter in solid-liquid two-phase fluid has not been seen, it has been reported in foreign countries that the error is within 3% when the solid content is 14%;

According to the experimental report of the Institute of Hydraulic Research of the Yellow River Conservancy Commission of China, the flow rate of water with high sand content is measured. The volume ratio of sediment is 17% to 40% (the median diameter of sand is 0.35mm), and the measurement error of the instrument 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 the capability can also be selected from the mains AC excitation instrument or the 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.

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, a ceramic lining or a urethane rubber lining with better wear resistance should be used. It is also recommended that the sensor be installed on a vertical pipe to make the pipe wear evenly and eliminate the disadvantage of partial wear of the lower half of the horizontal installation. It is also possible to add a nozzle-shaped sheath at the inlet end of the sensor to extend the service life.

6, 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 scale removing electrode, and the like. The electrodes 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 in a short period of 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. The non-contact electrode EMF is attached to the non-conductive film layer, and the meter can still work, but it is also inoperable if it is a highly conductive layer.

7. Selection of materials for contact with fluids

Sensor components in contact with fluids are lined (or measuring tubes made of insulating materials), electrodes, grounding rings and gaskets. The corrosion resistance, wear resistance and upper temperature limit of the materials affect the flow of the instrument. Adaptability. Due to the small number of components, simple shape and flexible material selection, the electromagnetic flow sensor has strong adaptability to fluids.

(1) Lining materials (or measuring tubes that are in direct contact with the medium) are commonly used as lining materials such as fluoroplastics, urethane rubber, neoprene and ceramics. In recent years, it has been lined with high-purity alumina 999.7% AI2O3) ceramics, but only for small and medium-sized sensors. Neoprene and FRP are used in non-corrosive or weakly corrosive liquids, such as industrial water, waste water and weak acid and alkali, at the lowest price. Fluoroplastics have excellent chemical resistance but poor wear resistance and cannot be used to measure slurry. The earliest application of fluoroplastics is PTFE. Because it is only pressed against the measuring tube, it has no cohesive force and can not be used in negative pressure pipelines. Later, various modified varieties are developed to realize injection molding, and the measuring tube has Strong bonding force, can be used for negative pressure, urethane rubber has excellent wear resistance, but acid and alkali resistance is poor. Its wear resistance is 10 times that of natural rubber, suitable for coal slurry, pulp, etc.; the medium temperature is lower than 40 ~ 60 / 70 ° C. Alumina ceramics have excellent wear resistance and wear resistance to strong acid and alkali. The wear resistance is about 10 times that of urethane rubber. It is suitable for corrosive pulp; but it is brittle and negligent when installing and clamping. Fragile, can be used for higher temperatures (120 ~ 140 / 180 ° C) but to prevent temperature drastic changes, such as steam sterilization, the general temperature change can not be greater than 100 ° C, heating 150 ° C to have 10 min time.

(2) Electrode and grounding ring material The resistance of the electrode to the measuring medium is the first consideration of the material selection, and secondly, whether surface effects such as passivation and noise formed are generated.

1) Select corrosion resistant materials

The corrosion resistance of EMF electrodes is very high. Commonly used metal materials include molybdenum acid-resistant steel Icr18Ni12Mo2Ti, Hastelloy (corrosion-resistant nickel-based alloy) B, C, titanium, niobium and platinum-rhodium alloy, which can cover almost all chemical liquids. There are also low-noise electrodes suitable for slurry or the like, which are conductive rubber electrodes, conductive fluoroplastic electrodes and porous ceramic electrodes, or metal electrodes covering these materials. In principle, the choice of electrode materials should be determined from the actual and previous experience of the application of the medium in other equipment. Sometimes necessary experiments are required, such as taking liquid samples on site and conducting corrosive tests on the materials to be used in the laboratory. The experiment is a live coupon, which is the corrosive test closest to the actual application conditions, and can be concluded that the reliability is applicable.

2) Avoid electrode surface effects

The corrosion resistance of the electrode is an important factor in the selection of materials, but sometimes the electrode material has good corrosion resistance to the measured medium, but it is not necessarily the applicable material, and the electrode surface effect is also avoided. Electrode surface effects are divided into three aspects: surface chemical reaction, electrochemical and polarization phenomena, and the catalytic action of the electrode. The chemical reaction effect forms a passivation film or an oxide layer after the electrode surface is in contact with the measured medium. They may have a positive protective effect on corrosion resistance, but may also increase surface contact resistance. For example, when it is in contact with water, it is oxidized to form an insulating layer. For the medium to avoid or reduce the surface effect of the electrode ------ electrode material matching, there is not enough information to check as corrosive, only limited experience, still to be accumulated in practice. The grounding ring is connected at both ends of the flow sensor of the plastic pipe or the metal pipe lined with the insulating lining. Their corrosion resistance requirements are lower than the electrode, and there is sufficient corrosion and regular replacement. Acid-resistant steel or Hastelloy is usually used. Due to the large size, it is less economical to use precious metals such as ruthenium platinum. A grounding ring is not required if the metal process pipe is in direct contact with the fluid.