Influence factors of electromagnetic flow meter and matters needing attention in selection

2021-03-06 14:14:14

1、 Influence of various media on measurement

(1) The influence of velocity distribution is known by hydrodynamics.

When the liquid flows in the pipe, the velocity of each point on the cross section of the pipe is not equal, but whether it is laminar or turbulent, after passing through a straight pipe with a certain distance, the velocity distribution can become axisymmetric. The velocity is the largest at the center of the pipe axis and zero at the pipe wall, and the average velocity is V -, as long as the velocity distribution is relative to the measuring pipe If the central axis is symmetrical, the induced electromotive force generated on the electrode has nothing to do with the velocity distribution of each point, but is directly proportional to the average velocity of the measured liquid. Therefore, the axisymmetric velocity distribution is one of the necessary working conditions for the uniform magnetic field electromagnetic flow meter. If the velocity distribution is asymmetric with respect to the central axis of the tube, although the total flow is the same, the induced electromotive force near the electrode is large, so the measured signal is larger than the actual flow value. On the contrary, the induced electromotive force at 90 ° to the electrode is smaller than the actual flow value, resulting in measurement error. Therefore, in order to make the flow velocity distribution axisymmetric, it is necessary to add a straight pipe section in front of the flow meter.

(2) the influence of magnetic field edge effect on measurement

If it is assumed that the magnetic field along the flow direction of the fluid is always uniform, in fact, it means that the magnetic field along the tube axis is infinite, while the magnetic field of the actual flowmeter is finite. Therefore, the edge effect of the finite magnetic field on the measurement must be considered. Assuming that the tube wall is insulated, the magnetic field near the electrode is approximately uniform, and the two ends gradually weaken to form an uneven edge, and finally drop to zero. In this way, the electric field E in the liquid is not uniform, and the eddy current will be generated. The secondary flux generated by eddy current in turn changes the working flux  at the edge of the magnetic field and further destroys the uniformity of the magnetic field. At this time, the induced electromotive force measured on the electrode is different from that in the infinite magnetic field, resulting in errors. If the tube wall is conductive, the edge effect of the magnetic field will be more obvious due to the short circuit effect of the conductive tube wall. With the change of the conductivity and thickness of the tube wall, this effect will be more obvious, resulting in the increase of the loss of the induced electromotive force on the electrode. For electromagnetic flowmeter, it is very necessary to measure the insulation of pipe wall, so the pipe wall is usually coated with insulation layer. If the measured medium contains magnetic material, the edge effect of magnetic field will be more complex. Due to the existence of magnetic material, the magnetic field is seriously distorted, resulting in nonlinear measurement. Therefore, for the liquid containing liquid metal, DC excitation is generally used to reduce the edge effect of magnetic field.

(3) the influence of the conductivity of the measured medium,

The input impedance of electromagnetic flowmeter converter has been improved. When measuring the conductive liquid, the error will not be caused by the slight change of medium conductivity. However, for a certain input impedance of the converter, the conductivity of the measured medium has a lower limit, which can not be lower than the lower limit. It is not allowed that the conductivity of the measured medium is too large. For example, when the conductivity exceeds 10-1s / cm, the flow signal will be reduced and the indicated value will be changed, that is, the indicated flow value is less than the actual flow value. When the conductivity of the measured medium is large, the resistance of the external circuit is small. At this time, no matter how high the input impedance of the converter is, the result of parallel connection will depend on this part of the liquid external circuit, so as to reduce the transmission accuracy between the transmitter and the converter. Therefore, for an electromagnetic flowmeter, the measurement is not affected by the conductivity of the medium. The conductivity of the measured medium should not be too large or too small. If the conductivity of the medium is very high, a large eddy current will be generated in the edge region of the magnetic field, which will cause the secondary magnetic flux and weaken and strengthen the magnetic field on both sides of the edge region of the working magnetic field. Therefore, the medium with high conductivity should not use AC excitation, but DC excitation. With the development of electronic technology and the increase of input impedance of converter, the lower limit of conductivity of measured medium will be reduced.

2、 Selection of flow sensor

Selection of electrode materials for electromagnetic flowmeter

Improper selection of electrode material and measured medium will affect normal measurement due to chemical action or polarization phenomenon. Electrode material should be selected according to the corrosiveness of measured medium. According to the corrosion, wear and temperature of the measured medium, the lining material of electromagnetic flowmeter is selected. Try to choose electromagnetic flowmeter with lightning protection function.

3、 Installation of flow sensor

1. Requirements for installation site.

1) When measuring the mixed phase fluid, choose the place that will not cause phase separation. When measuring the two-component liquid, avoid installing it at the downstream where the mixing is not uniform. When measuring the chemical reaction pipeline, install it at the fully completed reaction section.

2) Try to avoid negative pressure in the measuring tube.

3) Choose the place with small vibration, especially for all-in-one instrument.

4) Avoid large motors and transformers nearby to avoid electromagnetic interference.

5) It is easy to realize the independent grounding of the sensor.

6) Try to avoid the corrosive gas with high concentration in the surrounding environment.

7) The ambient temperature is in the range of – 25-60 ℃ and the relative humidity is in the range of 10% – 9O%. Avoid direct sunlight as far as possible.

8)The liquid should have the conductivity required for measurement, and the conductivity distribution should be generally uniform. Therefore, the installation of the flow sensor should avoid the place where the conductivity is easy to be uneven, for example, the liquid should be added near the upstream, and the liquid adding point should preferably be located at the downstream of the sensor.

2. For 90°elbow, T-shaped tee, concentric reducer and fully opened gate valve, the straight pipe section with distance from the electrode center line and not more than 5 times the diameter of the connecting surface at the inlet end of the sensor is usually required. For valves with different opening degrees, 10d is required, and the downstream straight pipe section is 3D. When measuring the mixed liquid of different media, the distance between the mixing point and the flowmeter should be at least more than 30d

3. Installation position and flow direction, sensor installation direction can be horizontal, vertical or inclined, unlimited. However, it is better to measure the solid-liquid two-phase flow vertically and from bottom to top. This can avoid the disadvantages of serious local wear of the lower part of the lining during horizontal installation and solid precipitation at low flow rate. During horizontal installation, the electrode axis should be parallel to the horizon to prevent insulation caused by the occasional bubbles in the liquid rubbing over the surface of the electrode, and to prevent the bottom electrode from being covered by sediment. In this way, when the flow of heavy or light particles leaves the upward flow direction of the sensor, the flow can be made to flow vertically.

4.The grounding sensor must be grounded separately, and the grounding resistance is less than 100 Ω. In principle, the ground of separated type should be at one side of sensor, and the ground of converter should be at the same ground point.