Notes on selection of electromagnetic flowmeter
的选型最好由熟悉现场工艺条件的技术人员进行，按选型资料中可测量流量范围表选择合适的口径，材料选择最好由熟悉现场工艺条件的最终用户确定。 The selection of Dongxiang electromagnetic flowmeter is best performed by a technician who is familiar with on-site process conditions. The appropriate caliber is selected according to the measurable flow range table in the selection data. The material selection is best determined by the end user who is familiar with on-site process conditions. First, select the type of flow meter: 1. The integrated type and the separated type have their own advantages. The basic principles for selection are as follows: The separated type is generally used in situations requiring range selection, batch control, frequent or long-term soaking in water and other functions. It is also used in more severe applications, such as: high temperature fluids, vibration sources and explosive environments. In most cases, the integrated and separate types can meet the requirements of use. 2. General and explosion-proof users decide to choose general or explosion-proof according to the environment in which the flowmeter is used. Second, the diameter of the sensor and the diameter of the process pipe connected to the general situation, considering the ease of installation, do not choose a reducer. But the premise is that the flow velocity in the flow meter tube should be in the range of 0.3m / s-10m / s. This choice is often suitable for newly designed projects. When selecting the flow rate, we must consider both the current working situation and the future situation of the equipment at full load. But sometimes the diameter of the sensor is not the same as the diameter of the connected process pipe. Such as: 1. The flow velocity in the pipeline is relatively low and the process flow is relatively stable. In order to meet the requirements of the flow range of the instrument, the flow rate is locally provided at the flow meter. 2. For the large-diameter electromagnetic flowmeter, the larger the diameter, the higher the price, the lower the flow velocity in the pipeline, and the stable process parameters, the smaller the diameter flowmeter can be selected, which not only makes the flowmeter operate in a better working state While reducing investment costs. III. Length of straight pipe section D = Flowmeter diameter 1. Do not install anything near the measuring tube that may interfere with the magnetic field, the voltage of the induced signal and the speed of the flowmeter. 2. No straight pipe section is required at the downstream end of the flow meter. However, if downstream valves or other joints will cause flow fluctuations, 2D to 3D straight pipe sections must be used at the downstream end. 3. It is strongly recommended to install a valve at the downstream end to avoid flow fluctuations in the flow measuring tube and to prevent the measurement from counting from the empty state of the tube. Fourth, maintain a stable fluid conductivity Avoid installing the flowmeter in a location where the fluid conductivity is not uniform. If chemicals are injected near the upstream end of the electromagnetic flowmeter, the flow rate display may be affected. To avoid this, it is recommended to change the injection of chemicals to the downstream end of the flow meter. If you must inject from the upstream end, use a long enough straight section (approximately 50D) to ensure that the fluid and chemicals are well mixed. V. Installation position: 1 The pipeline must be completely filled with liquid. It is important to ensure that the pipeline is always filled with liquid, otherwise the flow display will be affected and measurement errors will occur. The design of the circuit structure must ensure that the flow tube is always filled with fluid. It is recommended to use vertical installation when the fluid is split or contains solid particles. However, when vertical installation is used, the direction of the fluid from bottom to top must be marked to ensure that the pipeline is full of fluid. 2. Avoid air bubbles. If air bubbles enter the flow tube, the flow display will be affected, and measurement errors will occur. When the fluid contains air bubbles, the piping must be designed to prevent air bubbles from accumulating in the flow tube. If there is a valve near the flow tube, arrange the tube upstream of the valve as much as possible, this can prevent the pressure in the tube from decreasing, thereby avoiding air bubbles in the flow tube. 3. Installation direction: If the electrode is perpendicular to the ground, the air bubbles gathered on the top or bottom will cause the measurement result to be wrong. Install the junction box of the separate flow tube and the integrated converter on the top of the piping system to prevent water from entering. 4. It is recommended that the gasket used between the measuring tube and the user's flange be a compressed non-asbestos fiber gasket, PTFE gasket or a gasket with considerable elasticity. For GA, GC, and GD, use rubber gaskets or other gaskets that have considerable elasticity (such as outer Teflon rubber gaskets). Six, Yokogawa electromagnetic flowmeter selection precautions ordering information; 1, when ordering a separate measuring tube and / or a separate converter please give a specific flow range, unit, pulse equivalent, cumulative display pulse equivalent Wait for measuring tube ordering information. These parameters are set in the assembled converter before delivery. 2. If some options are set, the technical specifications related to these options need to be written when ordering. 3. Tag number Each tag number can be described by 16 characters containing letters (upper or lower case), and "-" and "." Characters. If there are special requirements, the marking number can also be written on the nameplate and marking plate (if the code SCT is selected). If the product is integrated, the tag number is also written into the converter memory. For the HART protocol, there can only be a maximum of 8 characters. If you want to change only the settings stored in the converter memory, define the software tag number. If there is no marking number, the corresponding product is not included in the delivery. 4. The value of flow range and unit flow range is limited to five digits (maximum 99999), excluding the decimal point. The integrated AXF product is set to the first range in the forward direction. The separate measuring tube is also set to the first range in the forward direction of the converter (AXFA11 or AXFA14) assembled with it. If no flow range and unit are specified, the related products are delivered set to 1m / s (3.3 ft / s). 5. If the output pulse equivalent is specified, the flow rate represented by each pulse must be set. Otherwise, the related products are set to 0 pulses / second upon delivery. 选型注意事项如果有特别指定，必须设定每个脉冲代表的流量。 6. Accumulative value display Pulse equivalent Dongxiang electromagnetic flowmeter selection considerations If specified, the flow rate represented by each pulse must be set. Otherwise, the related products are set to 0 pulses / second upon delivery.
First, the principle of electromagnetic flowmeter The electromagnetic flowmeter is a meter that measures the volumetric flow rate of conductive liquids by using Faraday's law of electromagnetic induction. In the electromagnetic flowmeter, the conductive medium in the measuring tube is equivalent to the conductive metal rod in the Faraday test, and the two electromagnetic coils at the upper and lower ends generate a constant magnetic field. When a conductive medium flows through, an induced voltage is generated, which is proportional to the flow velocity of the fluid, and the flow rate value Q is obtained.
2. Classification of electromagnetic flowmeters Electromagnetic flowmeters are classified according to the excitation current, including DC excitation, AC (power frequency or other frequency) excitation, low-frequency rectangular wave excitation, and dual-frequency rectangular wave excitation. According to the system classification of output signal connection and excitation (or power) connection, there are four-wire system and two-wire system.
According to the converter and sensor assembly methods, there are split type and integrated type. (Pressure Transmitters)
Classified by the connection method of flow sensor and pipeline, there are flange type, clamping type (clamping electromagnetic flowmeter), sanitary type (sanitary electromagnetic flowmeter), plug-in type (plug-in electromagnetic flowmeter), thread connection (oil Any connected high-pressure electromagnetic flowmeter). Note:
According to whether the flow sensor electrode is in contact with the measured liquid, there are contact type and non-contact type.
According to the structure of the flow sensor, there are short tube type and plug-in type (plug-in electromagnetic flowmeter).
Classified by purpose, there are general-purpose, explosion-proof, sanitary, anti-invasion and submersible (open channel flow meters) for open channel flow measurement.
Third, the environmental requirements of electromagnetic flowmeter installation
传感器外壳防护等级一般是IP65(国际GB4208外壳防护等级——防尘防碰水级)，安装环境的主要要求是： The protection level of the electromagnetic flowmeter sensor housing is generally IP65 (International GB4208 housing protection level-dust and water resistance). The main requirements for the installation environment are:
(1) The sensor should be installed in a dry and ventilated place, avoiding humid and easily flooded places, and avoid direct sunlight and rain directly.
(2) Avoid installing in places where the ambient temperature is too high. The integrated structure of the electromagnetic flowmeter is also subject to the ambient temperature of the electronic components, which should be lower.
(3) There should be no strong leakage current on the pipeline where the sensor is installed, and it should be as far away as possible from equipment with strong electromagnetic fields, such as large electromechanical and large transformers, so as not to cause electromagnetic field interference;
(4) The pipeline or ground on which the sensor is installed should not have strong vibration, especially the integrated instrument;
(5) The place where the sensor is installed should consider the space for the staff to repair on site.