RU2308685C1 - Method of measuring flow rate - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: method comprises passing current through the pairs of the electrodes mounted diametrically opposite on the outer wall of the steel nonmagnetic pipe through which a liquid metal flows. The value of the flow rate of the liquid heat-transfer agent indicates the value of current when the zero potential difference between the second pair of electrodes that are in a contact with the pipe wall is set at the same points as for the first pair.

EFFECT: enhanced precision.

1 cl, 2 dwg

Description

The invention relates to instrumentation, and in particular to a technique for measuring the flow of liquid metals using an electromagnetic method, i.e. a method based on the interaction of a moving fluid with a magnetic field. This interaction obeys the law of electromagnetic induction, according to which in a fluid crossing a magnetic field, an EMF is proportional to the velocity of the fluid.

A known method of measuring the flow rate of liquid metal flowing in a magnetic field through a steel non-magnetic pipe [1]. Since the measurement is carried out, as a rule, at a high temperature of liquid metal, do not use an electrical insulation coating on the inner surface of the pipe.

The method consists in determining the flow rate of liquid metal by the potential difference between two electrodes touching the outer surface of the pipe at points diametrically opposed along a line perpendicular to the direction of the magnetic field. A disadvantage of the known measurement method is the shunting action of the pipe walls, leading to a partial loss of signal and low accuracy in determining the flow rate.

A known electromagnetic method for measuring the flow rate of liquid metal, providing in addition to the method discussed above, passing through the electrodes of the external current of such a magnitude that the potential difference between the electrodes becomes equal to the EMF, induced by the movement of the liquid metal in a magnetic field [2]. In this method, the current through the pipe wall is completely determined by an external current, there is no shunting effect of the walls, and the potential difference between the electrodes is equal to the EMF induced by the interaction of the measured medium flow with the magnetic field. Moreover, as a measure of volumetric flow rate, the value of the external current is taken.

For example, in [3] a balanced electric bridge circuit was described, in which one of the bridge arms is the resistance between the electrodes of the empty flowmeter tube, i.e. resistance determined only by the pipe wall. Such a bridge (with an empty pipe) is electrically balanced at any voltage applied to one of the diagonals. When the liquid metal flow moves through the pipe, the electric bridge becomes unbalanced, the bridge is balanced only when a voltage is strictly defined to the diagonal, at which the potential difference between the electrodes becomes equal to the EMF induced by the movement of the liquid metal pipe in the magnetic field. In this case, the voltage or current supply to the bridge can be a measure of consumption.

The disadvantage of this method is the low accuracy of flow measurement if, during operation of the device, for some reason, the resistance of the pipe wall changes, because external current is set strictly in accordance with the previously determined shunt resistance formed by the pipe wall.

The measurement of the flow rate of liquid metals is necessary, for example, in the operation of power plants, where liquid metal is used as a coolant.

During long-term operation of the device, changes in the wall thickness of the pipe are possible, deposits and oxides are adhering to the inner wall surface and any components are washed out of the metal wall, and these effects change the wall resistance. Since with all known methods of measurement [1], [2], [3] a current flows through the pipe wall, any deviation from the wall resistance set during graduation affects the flowmeter readings.

The present invention eliminates this disadvantage.

It proposes to supply an external current to the electrodes of such a magnitude that the potential difference between the electrodes becomes equal to zero. Those. a measurement mode is provided in which there is no current in the pipe wall. At all points of the wall, the electric potentials become the same and equal to zero, although the flow of liquid metal flows through the pipe and there is its interaction with the magnetic field. As a measure of consumption, the value of the external current is taken.

Figure 1 and figure 2 explain the method of measuring the flow rate of molten metal, in which the external current can be set using an adjustable constant current source, and the zero potential difference between the electrodes is controlled using a zero indicator. To increase the accuracy of measurements, it is desirable to have potential and current electrodes touching the same points on the outer wall of the pipeline.

In Fig. 1, the following notation is adopted: 1 - a permanent magnet, 2 - a zero indicator, 3 - a tube with electrodes - potential electrodes, connected to a zero indicator 2, and current electrodes connected to a current meter 4 and an adjustable source of external current 5. Moreover, each pair of electrodes, one of which is potential and the other current, touch the same point on the outer surface of the pipeline.

Figure 2 shows a simplified electrical equivalent circuit node of the pipeline flowmeter. The following notation is used in the diagram: E is the EMF induced in the channel as a result of the interaction of the flow velocity with the magnetic field, r is the internal resistance of the EMF source (resistance of liquid metal), R is the resistance of the pipe wall, U is the voltage between the electrodes, j is the current from third-party source.

The voltage between the electrodes U is added by the expression:

Moreover, the EMF is proportional to the volumetric flow rate of the liquid metal Q.

It follows from expression (1) that when the external current j passed through the wall is j = E / r, then the potential difference between the electrodes is U = 0. Electrical potentials anywhere inside the wall become the same with each other and do not depend on the resistance value of the wall R.

In this case, the current j carries information only about the EMF E, i.e. about the volume flow Q, and the resistance of the coolant r and does not depend on the resistance of the wall. The current value is measured with a milliammeter 4.

It is assumed that the internal resistance of the measured medium r does not change during the operation of the device, except for its dependence on temperature.

The temperature of the coolant is controlled and taken into account when calculating the error of the device. If the temperature coefficient of resistance of the liquid metal and its operating temperature are known, then the correction for the change in the temperature of the liquid metal from the calibration is calculated by the formula

where j _x is the value of the external current characterizing the flow of liquid metal at the operating temperature t, j is the measured value of the external current at the operating temperature t; α is the temperature coefficient of the liquid metal, t ₀ is the temperature of the liquid metal during calibration of the flow meter.

Calculations show that E varies from 0 to several mV, the resistance r is several mOhm, and the current j varies from 0 to 0.4-0.5 A.

The advantage of the proposed measurement method is that it allows you to determine the flow rate in the absence of current in the wall, and therefore, in the absence of the influence of changes in the resistance of the pipeline wall.

Sources of invention

1. P.P. Kremlevsky. "Measurement of multiphase flow rate." L .: Engineering, 1982.

2. Welt, I.D. Instruments and control systems, No. 10, 1972, p. 17.

3. USSR Copyright Certificate No. 200199, Bulletin No. 16, 1967.

Claims (2)

1. The electromagnetic method of measuring the flow rate of liquid metal flowing in a magnetic field through a non-magnetic steel pipe, on the outer wall of which, along a line perpendicular to the direction of the magnetic field, two electrodes are installed diametrically opposite, forming the first pair, including passing through a side current electrode of a certain magnitude, which is taken as a measure of the volumetric flow rate of liquid metal, characterized in that the second pair of electrical contacts with the pipe wall at the same points as the first pair of the rods on which the potential is measured, while the measure of the flow of liquid metal at the operating temperature is the current j _x calculated by the formula j _x = j [1 + α (tt ₀ )], where j is the measured value of the external current at the operating temperature t; α is the temperature coefficient of resistance of liquid metal; t ₀ is the temperature of the liquid metal during calibration of the flow meter, t is the working temperature of the liquid metal, and the measured value of the external current is such that the potential difference between the electrodes of the second pair is zero. 2. The method of measuring the flow rate according to claim 1, characterized in that the zero potential difference between the electrodes is controlled using a zero indicator, and the value of the external current is set using an adjustable constant current source.

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