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Requirements and Precautions for Installation of Electromagnetic Flowmeter

Date: 2019-03-19
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As we all know, in order to make good use of instruments, we must first select the right instruments.  According to the measured object, measured medium parameters and working conditions, the type, specification and installation mode of flowmeter are selected, and the most important link in instrument selection.  Electromagnetic flowmeter is based on Faraday's law of electromagnetic induction, that is, when a conductive object cuts magnetic lines of force in a magnetic field, induced electromotive force is generated in the conductor. For electromagnetic flowmeter of the same specification, the flow rate is proportional to the induced electromotive force.  Due to the principle and structure of electromagnetic flowmeter, it has the advantages of no pressure loss, wide application range, no influence of changes in physical characteristics of medium, large range ratio, high precision and convenient maintenance and management, etc., now more and more enterprises and institutions use electromagnetic flowmeter, and its application is spread all over various industries.  Theoretically, as long as the measured fluid has a certain conductivity (conductivity greater than 5~Sem, electromagnetic flowmeter can be selected).


I. Key Points of Selection


1. Accuracy grade and function


According to the measurement requirements and use occasions, the precision grade of the instrument is selected to achieve economic benefits.  For example, when it is used for trade settlement, product handover and energy measurement, it should choose a higher accuracy level, such as 1.0, 0.5 or higher.  For process control occasions, different precision grades are selected according to control requirements;  In some situations where the process flow is only detected and precise control and measurement are not required, the lower precision level such as 1.5, 2.5 or even 4.0 can be selected, and then the low-cost plug-in electromagnetic flowmeter can be selected.


The functions of different types of electromagnetic flow meters are also very different. Simply, only one-way flow can be measured, and some only output analog signals.  Multi-functional can be bi-directional flow measurement, range switching, upper and lower limit alarm, air traffic control alarm, remote communication, etc.  The type selector shall determine and select flowmeter with different functions according to different requirements.


2. Range and caliber


When measuring the medium flow rate, measuring range and caliber of the meter, the full flow rate of the electromagnetic flowmeter can be selected within the range of 0.512ms of the measured medium flow rate, which is relatively wide.  The specification (caliber) of the selected instrument is not necessarily the same as that of the process pipeline, but should be determined according to whether the measured flow range is within the flow rate range, i.e. when the flow rate of the pipeline is too low to meet the requirements of the flow meter or the measurement accuracy cannot be guaranteed at this flow rate, the caliber of the instrument needs to be reduced, so as to improve the flow rate in the pipe and obtain satisfactory measurement results.


When measuring liquid with good conductivity, the maximum flow rate is usually not more than 5ms and the economic flow rate range is 1.5ms3ms


When measuring fluid with low conductivity, the flow rate should be chosen as low as possible, because the flow noise will increase when the flow rate increases, thus causing the output of the flow signal to shake.


When measuring the fluid that is easy to adhere, deposit and scale, the flow rate should be not less than 2ms and preferably increased to 3~4ms so as to avoid the occurrence of adhesion, deposit and scale.


If the slurry with strong abrasion resistance is measured, the flow rate shall be lower than 23ms to reduce the abrasion to the electrode and lining.  Generally, the measuring range of electromagnetic flowmeter is more than 20 times;  When necessary, instruments with automatic range switching function can be selected, and their measuring range exceeds 50 times or even 100 times.


3, electrode materials and grounding ring materials


The corrosion resistance of the electrode to the measuring medium determines which electrode material to choose, and at the same time requires that the electrode cannot have surface effect.  In other words, the corrosion resistance of the electrode is an important basis for selecting materials, but sometimes the electrode material has good corrosion resistance to the medium to be measured, but it is not necessarily a suitable material, and it depends on whether the electrode surface effect is generated.


3.1 electrode materials


The corrosion resistance of the electrodes of electromagnetic flowmeter is extremely high and corrosion is not allowed, but extremely low corrosion still exists in practical engineering applications.


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