Selection of electromagnetic flowmeter

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In order to realize the accuracy, stability and reliability of the flow metering system, this article introduces many factors that should be paid attention to in the selection and application of electromagnetic flowmeter.
1. Selection of sensor size

(1) Select the sensor diameter to be the same as the diameter of pipe. This option is very convenient to install. Generally speaking, when the pipeline flow rate is 0.5-5m/s, it is an economical flow rate, and the sensor diameter is the same as the pipe diameter. Unless the fluid is less abrasive and wear-resistant materials are used, it is recommended that the long-term working flow rate should not exceed 7 m/s. For fluids that contain substances that are easy to adhere, precipitate, scale, etc., it is recommended that the flow rate is not less than 2 m/s, preferably 3-4 m/s or more, to perform self-cleaning, prevent adhesion, precipitation, etc.; For fluids with strong abrasiveness such as mineral slurry, the usual flow rate should be lower than 3m/s to reduce the wear on the lining and electrode. When measuring low-conductivity fluid with a conductivity of 5-10 us/cm, choose a low flow rate (0.5-1m/s) as much as possible, because an increase in flow rate will increase the flow noise and cause output shaking.

(2) Select the sensor diameter to be different from the diameter of pipe. The diameter of the sensor selected is not necessarily the same as the diameter of the pipeline, it should be determined according to the pipeline flow rate. It is mainly suitable for the following situations: the flow rate in the pipeline is low, and the process flow rate is relatively stable. In order to meet the requirements of the sensor for the flow range, the flow rate is increased locally, and the sensor diameter is selected to be smaller than the process pipe diameter, and a reducer is added before or after it. In terms of price, the larger the diameter, the higher the price. If the flow velocity in the pipeline is low and the process flow is relatively stable, a sensor with a smaller diameter can be selected. This can not only make the electromagnetic flowmeter run in a better working condition, but also reduce costs.


2. Selection of Materials of Electrode and Grounding Ring

The choice of electrode material should be based on the physical and chemical properties of the measured fluid, including the composition, corrosiveness and abrasiveness of the fluid. The cost of the electrode is a large part of the cost of the flowmeter. Choosing the right electrode can get a good measurement effect and reduce waste. The saturated aqueous solution of NaCl is neutral, and the selection of SS316L can meet the requirements. It is the most economical selection. The selection of Hastelloy or tantalum electrode can also meet the requirements, but the cost is much higher. When the temperature of the solution is high, electrodes such as Hastelloy and tantalum must be used. In general, the corrosion resistance of the electrode material is one level higher than that of the pipeline.

The corrosion resistance of the grounding ring is lower than that of the electrode. Acid-resistant steel or Hastelloy is usually used. From an economic point of view, precious metals such as tantalum are less used.


3. Selection of Lining Material

The lining material should be selected according to the type of fluid to be tested and the working temperature, while considering the corrosiveness and abrasiveness. The common lining materials are PTFE, FEP, PFA, polyurethane rubber, neoprene rubber and ceramics. Neoprene rubber is used for non-corrosive or weakly corrosive liquids, and the price is the cheapest. It is generally used for industrial water, waste water and weak acids and alkalis. PTFE, FEP, and PFA are all fluoroplastics, which have good corrosion resistance and are suitable for strong acid or alkali but its wear resistance is poor and cannot be used in mineral slurry. Polyurethane rubber has excellent wear resistance, but its corrosion resistance and high temperature resistance are poor. It is generally suitable for mineral slurry, and the temperature of the fluid is lower than 60℃. Ceramic has excellent corrosion resistance and wear resistance. It can be used for corrosive ore slurry, but it is brittle and fragile when installed and clamped. It can also be used for high temperature fluids, but it must prevent sudden temperature changes. Generally, the sudden change in temperature cannot be greater than 100°C. There are some similarities and differences between the materials. Some occasions can be used universally, while others cannot be used universally. The selection should be based on the physical and chemical properties of the lining material.


4. Selection of Protection Grade

Select the protection grade according to the actual working conditions of the instrument. If the instrument is in a flooded state below the ground, select IP68; if the instrument is installed above the ground and the environment is not humid, select IP65.

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