Electromagnetic flowmeter is the most common flow measurement instrument in the process industry. The stability and reliability of the electromagnetic flowmeter have a huge impact on the entire system. The most important thing to ensure the normal and stable operation of the electromagnetic flowmeter is model selection: a set of equipment that is not suitable for the system will not be able to avoid frequent failures even if it fully complies with the operating specifications in daily use and undergoes frequent and meticulous maintenance. Then, in the selection of electromagnetic flowmeter, what elements related to the system need to be paid attention to? Today, Suke Instrument has sorted out the selection elements of 7 major electromagnetic flowmeters, hoping to be helpful to everyone.

1.Accuracy level and functionality
The performance of general-purpose EMFs on the market is quite different. Some have high precision and many functions, while others have low precision and simple functions. The basic error of the instrument with high precision is (±0.5%～±1%)R, while the instrument with low precision is (±1.5%～±2.5%)FS, and the price difference between the two is 1 to 2 times.

Some models of instruments claim to have higher accuracy, with a basic error of only (±0.2% ~ ±0.3%) R, but have strict installation requirements and reference conditions, such as ambient temperature, length of front and rear straight pipe sections, etc. Therefore, when choosing and comparing multiple models, don’t just look at the high indicators, but read the manufacturer’s samples or instructions in detail for a comprehensive analysis. The functions of EMF on the market are also very different. A simple one only measures one-way flow and only outputs analog signals to drive the rear instrument; multi-functional instruments can measure bidirectional flow, range switching, upper and lower limit flow alarms, empty pipes and power cutoff alarms. , Small signal removal, flow display and total calculation, automatic check and fault self-diagnosis, communication with host computer and motion configuration, etc. The serial digital communication function of some models of instruments can choose a variety of communication interfaces, such as HART protocol system, PROFTBUS, Modbus, FF field bus, etc.

2.Flow rate, full flow rate, range and diameter
The selected instrument diameter is not necessarily the same as the pipe diameter, it should depend on the flow rate. The process industry transports liquids with different viscosities such as water, and the flow rate of the pipeline is generally 1.5-3m/s at an economical rate. EMF is used on such pipelines, and the diameter of the sensor should be the same as that of the pipe. The liquid flow rate can be selected in the range of 1-10m/s when the EMF is full-scale flow, and the range is relatively wide. The upper limit flow rate is not limited in principle, but it is usually recommended not to exceed 5m/s, unless the lining material can withstand liquid flow erosion. Practical applications rarely exceed 7m/s, and exceeding 10m/s is even rarer. The lower limit of the flow velocity of the full-scale flow is generally 1m/s, and some models of instruments are 0.5m/s.

For some newly built projects with low flow or low flow velocity in the early stages of operation, from the perspective of measurement accuracy, the instrument diameter should be smaller than the pipe diameter and connected with a reducing pipe. For fluids with substances that are easy to adhere, deposit, and scale, the flow rate should be no less than 2m/s, preferably 3 to 4m/s or more, so as to self-clean and prevent adhesion and deposition. For highly abrasive fluids such as slurry, the common flow rate should be lower than 2 to 3m/s to reduce wear on the lining and electrodes. When measuring low-conductivity liquids close to the threshold, try to select a lower flow rate (less than 0.5~1m/s). As the flow rate increases, the flow noise will increase and the output will slosh.

The range of EMF is relatively large, usually not less than 20. For instruments with automatic range switching function, it can exceed 50 to 100. The diameters of the fixed products that can be provided in China range from 10mm to 3000mm. However, the actual applications are still mostly small and medium diameters, but they are different from most other principle flow meters (such as volumetric, turbine, vortex or Coriolis mass type, etc. ), large-diameter instruments occupy a larger proportion.

3.Liquid conductivity
The prerequisite for using EMF is that the liquid being measured must be conductive and cannot be lower than the threshold (i.e. lower limit). If the conductivity is lower than the threshold, measurement errors will occur until it cannot be used. If the conductivity exceeds the threshold, it can be measured even if it changes. The indication error does not change much. The threshold of the general-purpose EMF is between 10-4 ~ (5×10-6) S/cm. , depending on the model. The use also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The manufacturer’s instruction manual usually stipulates the length of the signal line corresponding to the conductivity. Non-contact capacitively coupled large-area electrode instruments 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. The conductivity of acid, alkali, and salt solutions is between 10-4 and 10-1S/cm. There is no problem in its use. Low-strength distilled water is 10-5S/cm. There is no problem with cm either. Petroleum products and organic solvents cannot be used if their conductivity is too low. Some pure liquids or aqueous solutions have low conductivity and are considered unusable. However, in actual work, there will be instances where they can be used because they contain impurities. Such impurities are beneficial to increasing conductivity.
According to experience, the actual liquid conductivity is best to be at least one order of magnitude greater than the threshold specified by the instrument manufacturer. Because the lower limit value stipulated in the manufacturer’s instrument specification is the lowest value that can be measured under various good conditions of use. It is limited by some conditions of use, such as the uniformity of conductivity, connecting signal lines, external noise, etc., otherwise there will be output shaking. We have encountered the measurement of low-grade distilled water or deionized water many times, and its conductivity is close to the threshold value of 5×10-6S/cm, and the output shakes when used.

4.The liquid contains mixed substances
Tiny bubbles 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 increases to form a bullet (block) flow, the electrode may be covered by the gas and the circuit will be interrupted instantly. On, the output shakes and even does not work properly. Solid-liquid two-phase fluid containing non-ferromagnetic particles or fibers can also measure the volume flow of the two phases. Fluids with high solid content, such as drilling mud, drilling cement slurry, paper pulp, etc., are actually non-Newtonian fluids. Since the solid flows together in the carrier liquid, there is sliding between the two, and there is a difference in speed. If the single-phase liquid calibration instrument is used for solid-liquid two-phase fluid, additional errors will occur. Although there are no systematic experimental reports on the impact of EMF on solids in solid-liquid two-phase fluids, there are reports abroad that the error is within 3% when the solid content is 14%;For the application of slurry containing ore particles, attention should be paid to the degree of wear on the sensor lining, and the expansion of the inner diameter of the measuring tube will cause additional errors. In this case, a ceramic lining or polyurethane rubber lining with better wear resistance should be used. It is also recommended that the sensor be installed on a vertical pipeline to make the pipeline wear evenly and eliminate the shortcomings of serious local wear in the lower half of the horizontal installation. A nozzle-shaped sheath can also be installed at the inlet end of the sensor to prolong the service life relatively.

5.Attachment and precipitation
When measuring fluids that tend to adhere and precipitate substances on the pipe wall, if a conductive substance with a higher conductivity than the liquid is attached, the signal potential will be short-circuited and cannot work. If it is a non-conductive layer, you should first pay attention to the contamination of the electrode. For example, if it is not easy to select Attached pointed or hemispherical protruding electrodes, replaceable electrodes, scraper-type cleaning electrodes, etc. The scraper electrode can manually scrape out the deposits outside the sensor at regular intervals.

Foreign products used to have ultrasonic transducers installed on the electrodes to remove surface scale layers, but these are now rare. There is also a method of temporarily disconnecting the measuring circuit, allowing a low voltage and high current to flow through the electrode for a short period of time to burn and remove the attached grease layer. In places where adhesion is prone to occur, the flow rate can be increased to achieve self-cleaning purposes. A more convenient and easy-to-clean pipeline connection can also be used to clean the sensor without disassembling it. Non-contact electrode EMF is attached to a non-conductive film layer, and the meter can still work, but it will also not work if it is a highly conductive layer.

6.Selection of lining materials
Commonly used lining materials for electromagnetic flowmeter linings (or measuring tubes in direct contact with the medium) include fluoroplastics, polyurethane rubber, neoprene and ceramics, etc. In recent years, high-purity alumina ceramics have been used to make linings, but only for small and medium-diameter sensors. Neoprene and fiberglass are used for non-corrosive or weakly corrosive liquids, such as industrial water, waste water and weak acids and bases, and are the cheapest. Fluoroplastics have excellent chemical resistance, but poor wear resistance and cannot be used to measure mineral slurries. The earliest application of fluoroplastics was polytetrafluoroethylene, which could not be used in negative pressure pipelines because it only relied on pressure and no adhesive force between it and the measuring tube. Later, various modified varieties were developed to achieve injection molding, which is consistent with the measuring tube. It has strong bonding force and can be used for negative pressure. Polyurethane rubber has excellent wear resistance, but has poor acid and alkali corrosion resistance. Its wear resistance is equivalent to 10 times that of natural rubber, and it is suitable for coal slurry, ore slurry, etc.; the medium temperature should be lower than 40~60/70℃. Alumina ceramics have excellent wear resistance and resistance to strong acid and alkali wear and corrosion. The wear resistance is about 10 times that of polyurethane rubber. It is suitable for corrosive slurries; however, it is brittle and cannot be installed and clamped carelessly. It is fragile and can be used at higher temperatures (120~140/180℃), but drastic temperature changes must be prevented, such as steam sterilization. Generally, the temperature change cannot exceed 100℃, and it takes 10 minutes to raise the temperature to 150℃.

7.Electrode and grounding ring material selection
The corrosion resistance of the electrode to the measurement medium is the first factor to consider when selecting materials, and the second consideration is whether surface effects such as passivation and the resulting noise will occur. A.Choose corrosion-resistant materials
The corrosion resistance of EMF electrodes is very demanding. Commonly used metal materials include molybdenum-containing acid-resistant steel, Hastelloy, etc., which can cover almost all chemical liquids. In addition, there are low-noise electrodes suitable for slurries, etc., 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 by drawing from the user’s actual and past experience with the application of the medium in other devices. Sometimes necessary experiments must be done later, such as taking liquid samples on site to conduct corrosion tests on materials to be used in the laboratory. The best experiment is on-site coupon testing, which is the corrosion test closest to actual application conditions and can draw a more reliable and applicable conclusion. B. Avoid electrode surface effects

The corrosion resistance of electromagnetic flowmeter electrodes is an important factor in material selection. However, sometimes the electrode material has good corrosion resistance to the measured medium, but it is not necessarily a suitable material. It is also necessary to avoid electrode surface effects. The electrode surface effect is divided into three aspects: surface chemical reaction, electrochemistry and polarization phenomena, and the catalytic effect of the electrode. Chemical reaction effects include the formation of a passivation film or oxide layer after the electrode surface comes into contact with the measured medium.

They may have a positive protective effect on corrosion resistance, but may also increase surface contact resistance. For example, tantalum will be oxidized when it comes in contact with water, forming an insulating layer. As for the medium to avoid or reduce the electrode surface effect–electrode material matching, there is not enough information like corrosiveness, and there is only some limited experience, which has yet to be accumulated in practice. The grounding ring is connected to both ends of the flow sensor in a plastic pipe or an insulating-lined metal pipe. Their corrosion resistance requirements are lower than those of electrodes. They are fully subject to certain corrosion and should be replaced regularly. Acid-resistant steel or Hastelloy is usually used. Due to large size and economic considerations, precious metals such as tantalum and platinum are rarely used. If the metal process pipe is in direct contact with the fluid, there is no need for a grounding ring.