To ensure the measurement accuracy of the electromagnetic flowmeter, proper installation is very important.
The transmitter should be installed in a dry and ventilated place indoors. Avoid installation in places where the ambient temperature is too high, should not be subject to strong vibration, try to avoid equipment with strong magnetic fields, such as large motors, transformers, etc. Avoid installation in corrosive gases. The installation location is easy to overhaul. This is the environmental condition to ensure the normal operation of the transmitter.
In order to ensure that the measuring tube of the transmitter is filled with the measured medium, the stacker is preferably installed vertically, and the flow direction is from bottom to top. Especially for liquid-solid two-phase flow, it must be installed vertically. If the site is only allowed to be installed horizontally, it must be ensured that the two electrodes are at the same level.
The transmitter should be equipped with valves and bypass at both ends.
The millivolt AC potential measured by the electrode of the electromagnetic flow transmitter is based on the liquid potential in the transmitter. In order to stabilize the liquid potential and maintain the potential of the transmitter and fluid to ensure stable measurement, the transmitter shell and the metal tube should have good grounding at both ends, and the converter housing should also be grounded. The grounding resistance should not be greater than 10 and cannot be shared with the grounding wire of other electrical equipment. If the transmitter housing is not guaranteed to be in good contact with the metal pipe, connect them with metal wires. Re-reliable grounding.
In order to avoid interference signals, the signal between the transmitter and the converter must be transmitted with shielded wires. It is not allowed to place the signal cable and power cable in parallel in the same cable. The length of the signal cable should generally not exceed 30 m.
Converter installation site should avoid AC and DC strong magnetic field and vibration, ambient temperature is -20 to 50 ° C, does not contain corrosive gases, relative humidity is not more than 80%.
In order to avoid the influence of the flow rate on the relative measurement, the flow regulating valve should be placed downstream of the transmitter. For small-caliber transmitters, since the distance from the center of the electrode to the inlet end of the flowmeter is equivalent to several times the length of the diameter D, the upstream straight pipe may not be specified. However, for a flowmeter with a large diameter, generally there should be a straight pipe section of 5D or more in the upstream, and the downstream pipe section is generally not required.
General principles for electromagnetic flowmeter selection
(1) Whether the medium to be tested is a conductive liquid or slurry, thereby determining whether an electromagnetic flow meter is selected;
(2) The conductivity of the measured medium determines the type of electromagnetic flowmeter—whether it is high conductivity or low conductivity;
(3) The nominal diameter of the large, small and common flow process pipelines required by the process, determine whether the flow rate of the medium is at a more economical flow point, whether the pipeline needs to be reduced, and then determine the diameter of the flowmeter;
(4) Determine whether to use an integrated or split flowmeter, and the degree of protection of the flowmeter, etc., based on the layout of the process piping.
(5) Selecting the electrode type according to whether the measured medium is easy to crystallize or crusting;
(6) selecting an electrode material according to the corrosiveness of the measured medium;
(7) The corrosiveness, wear and temperature of the measured medium determine the lining material to be used;
(8) The high working pressure of the measured medium determines the nominal pressure of the flow meter;
(9) The insulation of the process piping determines the type of grounding ring.
Measuring principle of electromagnetic flowmeter
The principle of electromagnetic flowmeter measurement is based on Faraday's law of electromagnetic induction. The measuring tube of the flow meter is a non-magnetic alloy short tube lined with an insulating material. The two electrodes are fixed to the measuring tube through the tube wall in the tube diameter direction. The electrode tip is substantially flush with the inner surface of the liner. When the exciting coil is excited by the bidirectional square wave pulse, a working magnetic field having a magnetic flux density B is generated in a direction perpendicular to the axis of the measuring tube. At this time, if the fluid having a certain conductivity passes through the measuring tube, the cutting magnetic line induces the electromotive force E. The electromotive force E is proportional to the product of the magnetic flux density B, the inner diameter D of the measuring tube and the average velocity V. The electromotive force E (flow signal) is detected by the electrode and sent to the converter through the cable. After the converter amplifies the flow signal, it can display the fluid flow, and can output signals such as pulse and analog current for flow control and regulation.