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The corrosion resistance of the measuring tube of the electromagnetic flow meter determines its wide application in the Chlor-alkali caustic soda process, and the technical points of the flow meter selection are mainly reflected in the determination of the caliber, the material of the measuring electrode and the lining material, The choice of grounding ring or grounding electrode and the particularity of the electrolysis process.
The liquid medium in the production process of Chlor-alkali and caustic soda is mostly highly corrosive electrolytes, such as brine, hydrochloric acid, caustic soda, sodium hypochlorite, etc.; this highly corrosive production process environment and the advantages of electromagnetic flowmeters in terms of corrosion resistance determine the electromagnetic flow meters in Chlor-alkali caustic soda process is being widely used. In order to achieve accurate and reliable flow measurement and save investment costs, the correct selection of electromagnetic flowmeters is particularly important. Below, the author summarizes the experience and experience in the design, selection, and application of electromagnetic flowmeters in the production process of Chlor-alkali and caustic soda.
The measurement principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction, that is when the conductor medium cuts the magnetic field line and moves in the magnetic field medium, an induced electromotive force is generated at both ends of it. As shown in Figure 1, the conductive medium flows in a non-magnetic measuring tube perpendicular to the magnetic field, and an induced potential proportional to the flow is generated in the direction perpendicular to the flow direction. The direction of the electromotive force follows the "Fleming right-hand rule", and the induced potential E is: E=KBVD
In the formula: E - induced electromotive force (proportional to flow rate); K - proportional constant;
B——magnetic field strength (coil inductance); V——measuring the average flow velocity of the medium in the tube;
D——electrode distance (inner diameter of measuring tube)
When measuring the flow rate, the induced potential is detected by two measuring electrodes in direct contact with the medium sent to the converter through the cable for intelligent processing, and then displayed on the LCD or converted into a standard signal 4~20mA and 0~1kHz output.
Figure 1 Working principle diagram of electromagnetic flow meter
Electromagnetic flow meters can measure fluids in a wide flow (velocity) range, usually at a flow rate of 0.3 to 10.0m/s; the diameter of the flow meter is generally equal to the diameter of the processing pipeline, and can also be smaller or larger than the diameter of the processing pipeline. The determination of the caliber of the flow meter is mainly based on the volume flow rate in the process, combined with the size of the processing pipeline, taking into account the measurement accuracy, economy, and durability, to select the appropriate caliber of the flow meter, so that the average flow rate of the fluid passing through the sensor is in a suitable range. Within the range, the recommended flow velocity range is generally 1.0-5.0m/s. Within this range, the flow meter has high measurement accuracy, good linearity, and small pressure loss, and the wear of the medium on the flow meter lining and the electrode is relatively small. In addition, for fluid media containing solid suspended particles, the recommended flow rate range is 1.0-3.0m/s, which helps to avoid excessive wear of solid suspended particles on the flow meter lining and electrodes caused by excessive flow rates, and prolongs the service life, such as the flow rate of salt mud; for the fluid medium that is easy to crystallize, has a slightly higher viscosity or has sediment, the flow rate can be increased appropriately, and the recommended flow rate range is 3.0-5.0m/s, which is helpful for the self-cleaning of the measuring electrode, Prevent electrode adhesion, such as 50% caustic soda flow.
The relationship between flow velocity, flow rate, and caliber is as follows: V=354Q/D2
In the formula: V——flow velocity, m/s; Q——flow rate, m3/h; D——diameter, mm.
For example, in the caustic soda electrolysis process, the range of the brine flow meter entering the electrolytic tank is 0-50m3/h, and the diameter of the processing pipeline is DN100; They are 1.77 m/s and 2.76 m/s respectively; the calculation results are all within the recommended flow velocity range, but considering the economy, choosing a flow meter with DN80 caliber can save 1/4~1 /5 investment cost, at this time, the DN80 caliber flow meter should be selected.
The measuring electrode of the electromagnetic flow meter is the only metal material that is in direct contact with the process medium, and its function is to connect with the wire to transmit the weak induced electromotive force generated in the measuring tube to the detection unit in the converter. Since the electrode part of the electromagnetic flow meter is in the working magnetic field, in order to prevent the magnetic field lines from concentrating on the electrode, the electrode material must use a non-magnetic metal material; corroded metal thickness) to determine the choice of different metal materials for different working media. The following introduces several electrode materials commonly used in the Chlor-alkali caustic soda process and their application ranges through chart 1.
One of the basic conditions for the normal operation of the electromagnetic flow meter is that the normal current of the inner wall of the measuring tube is zero except for the electrodes. In order to meet this condition, the simplest method is to line the inner wall of the conductive metal measuring tube and the end face of the flange with an insulating lining. In layman's terms, the purpose of using an insulating lining is to prevent the induced signal voltage from being short-circuited by the metal tube. It can be seen that the insulating lining plays an important role in the application of electromagnetic flow meters. Due to the wide variety of fluids in the measured conductive medium and their different physical and chemical properties, it is impossible to use a lining of insulating material to meet the physical and chemical properties of all the measurement fluid media used in electromagnetic flow meters; therefore, in the lining In the selection of materials, the requirements of the measuring medium on the lining's temperature resistance, thermal shock, high pressure, negative pressure, wear, corrosion, bonding, and adhesion should be considered. The main performance characteristics and scope of application of common electromagnetic flow meter measuring tube lining materials are introduced below through the way of chart 2.
The flow signal potential detected by the electromagnetic flow meter in the working process is extremely small, usually, only a few millivolts; and the conductive fluid will also generate a potential when it flows in any pipeline (insulated or non-insulated), and this potential is not flowing The signal itself is an "interference signal"; especially in fluids with excellent electrical conductivity, such as salt water, acid-base solutions, etc., the magnitude of the interference signal generated during the flow process may even exceed the flow signal itself. This "interference signal" will be superimposed on the flow signal potential when flowing through the electromagnetic flow meter measuring tube, which will cause the flow signal measured by the converter to be distorted. Since the input amplifier of the electromagnetic flow meter converter is usually designed for differential input, one of the "equal potential common reference grounds" is necessary, and this "common reference ground" is the basis of measurement. For the measurement principle of the electromagnetic flow meter, the grounding ring and the grounding electrode are two forms of grounding of the flow meter, and their functions are the same. The "working ground" of the input amplifier is connected, so that the "working ground" of the amplifier and the "ground" of the signal source are connected together to form an equipotential "common reference ground"; in order to eliminate the influence of "interference signals" on flow measurement influence to ensure the accuracy of the measurement results.
The material selection of the grounding ring or the grounding electrode should, in principle, be the same material as the measuring electrode. When choosing the form of a grounding ring, in order to save investment costs, the material of grounding ring can also choose a material with a relatively low price and weak corrosion resistance; Tantalum material Ta, but the grounding ring can choose relatively cheap titanium material Ti. It should be noted that when choosing a non-repairable grounding electrode for grounding, reducing the corrosion resistance of the grounding electrode material will reduce the overall service life of the flow meter measuring tube.
As two different forms of electromagnetic flow meter grounding, both the grounding ring and the grounding electrode can achieve the purpose of equipotential "common reference ground" and eliminate interference, but the two grounding forms have their own advantages and disadvantages in the process of use. The use of grounding rings can protect the flanging of the lining from damage, but compared with consumables, especially when measuring strong corrosive media such as salt water, hydrochloric acid, and high-temperature caustic soda, the requirements for materials are very high, mostly such as Ta, Ti, For precious metals such as Hc-276, using a grounding ring to "ground" will greatly increase the investment cost. In production practice, when the medium conveying pipeline is a conductive metal pipeline and the flow meter lining does not need special protection, the "grounding support" can be welded on the metal pipeline near both sides of the electromagnetic flow meter body or on the matching flange welded with the pipeline. Ear" to achieve reliable grounding. On the contrary, it is more material-saving to choose the ground electrode form,
The investment cost is relatively low, especially in the process of Chlor-alkali and caustic soda, most of the transmission pipelines of the strong corrosive medium are non-conductive plastic-lined pipes or insulating plastic pipes, and it is impossible to achieve reliable grounding by welding "grounding lugs". However, grounding can only be done in the form of a grounding ring or a grounding electrode. In this case, the grounding electrode should generally be used for grounding; however, since the grounding electrode is generally installed and fixed at the lowest point of the electromagnetic flow meter measuring tube, it is easy to get stuck in the transmission pipeline. Under the working condition of the scale, the grounding electrode will lose the function of "grounding" due to the scale being covered, especially when the flow meter is installed horizontally, the selection of the grounding electrode should be carefully considered. In view of the particularity of the electrolytic brine process, the electromagnetic flow meter in the secondary brine refining, especially the electromagnetic flow meter closest to the electrolytic tank, such as the brine, caustic soda, and hydrochloric acid flow meter entering the electrolytic tank, must not be grounded in the form of a grounding electrode. This is because the electrolyte in the electrolytic cell usually has a DC current of 3000-14700A, and leakage occurs easily through the conductive liquid medium. The electrochemical reaction occurs on the grounding electrode, thereby electrochemically corroding the grounding electrode, resulting in damage to the grounding electrode, resulting in the failure of the electromagnetic flow meter to work normally.
In summary, although the working conditions in the production process of Chlor-alkali and caustic soda are more complicated, it has brought many difficulties to the selection and application of electromagnetic flow meters; nevertheless, since we have a good understanding of the working conditions when selecting the instrument Conditions combined with the actual situation on-site, as well as careful operation and maintenance during operation, at present, nearly 50 sets of Meacon and Supmea electromagnetic flow meters used online in the caustic soda process are in good condition, and the damage rate does not exceed 1 set per year, ensuring that Ensure the safe and stable operation of caustic soda production; and continue to improve to minimize investment costs.
