RU2559117C2 - Conductometric method to measure liquid level - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to the field of control of level of electroconductive media, preferably liquid metals in nuclear power industry. The conductometric method makes it possible to measure level of liquid metal without introduction of any elements of the level indicator structure inside the reservoir, where the liquid metal is placed. The method consists in the fact that in the area of possible position or displacement of the level of liquid sodium in the reservoir, electric field is developed on the external surface of the reservoir wall. Then on the selected local area arranged on the external wall of the reservoir with the help of two electrodes and a measurement device, they measure intensity of electric field, using which, they calculate presence of one of media on this section behind the reservoir wall, electric conductivity of which corresponds to either liquid sodium or air. Electrodes via certain gaps are installed on the entire area of the possible level position. By serial or simultaneous probing of a wall at different sections of the reservoir by a discrete-analogue method they determine a place, where there is an interface between air and liquid sodium, i.e. position of liquid metal level is determined in the reservoir. Electric field is developed with the help of current supplied to two electrodes, contacting at the outer side of the reservoir wall, besides, one of electrodes is located on the highest part of the reservoir, where the level of liquid sodium may reach, and the other electrode is on the lowest part of the reservoir. Electric field intensity on the outer surface of the reservoir wall is determined by measurement of ratio of the potential difference between two probing electrodes arranged along the vertical line on a certain selected local area of the outer surface of the reservoir, to the distance between these electrodes.

EFFECT: reliable monitoring of a liquid medium level with provision of specified metrological characteristics in wide range of temperatures, as well as continuity of control and moderate cost.

6 cl, 3 dwg

Description

The present invention relates to the field of monitoring the level of electrically conductive liquids and can be used primarily for measuring the level of liquid metals in nuclear energy.

A known conductometric method for measuring the level of an electrically conductive liquid, comprising applying an electric current to a liquid using electrodes located at different heights, measuring the potentials of the electrodes using a measuring device and calculating the position of the liquid level in a discrete-analog way [1]. The level measurement method is based on identifying the difference that exists between the electrical conductivity of air and various liquids - alkalis, acids or tap water. The electrical conductivity of the liquid or air is fixed by a measuring device. Level gauges are found in various modifications: with two, three and a large number of electrodes. Multi-electrode level gauges are used to control a level that varies over a wide range, or to control several fixed positions of the level of an electrically conductive liquid.

The disadvantage of this method is the need to introduce electrodes into the tank to ensure their contact with the measured liquid.

In the nuclear industry, the problem arose of measuring the level of liquid sodium in a tank, into which no penetration of any structural elements of the level gauge is allowed. This is due to the need to ensure particularly high reliability of the tank.

Liquid metals, including sodium, are also electrically conductive fluids, however, they differ from alkalis, acids, and tap water in that their electrical conductivity is several orders of magnitude greater. The electrical conductivity of liquid sodium is approximately 0.6 × 10 ⁷ S / m, and the electrical conductivity of tap water, acids and alkalis ranges from 10 ^-4 to 10 ^-1 S / m.

High temperatures (up to 300-525 ° C), difficult radiation conditions, and extremely high reliability requirements significantly limit the ability to choose a level measurement method.

The technical task of the invention is to measure the level of liquid sodium without penetrating into the tank any structural elements of the level gauge, if the tank is made of stainless steel.

The proposed conductometric method allows to probe through the metal wall of the tank and to determine the presence or absence in the tank of a medium with high electrical conductivity.

The proposed method consists in the fact that in the zone of a possible position or movement of the liquid sodium level in the tank, an electric field is created on the outer surface of the tank wall. While sensing the presence of liquid metal behind the wall of the tank is as follows. In a selected local region located on the outer wall of the tank, two electrodes and a measuring device measure the electric field strength, which calculates the presence of one of the media in the given section behind the tank wall, the electrical conductivity of which corresponds to either liquid sodium or air. Electrodes at certain intervals are installed on the entire area of the possible position of the level. By sequential or simultaneous sounding of the wall in different parts of the reservoir, the place where the interface between air and liquid sodium is located is determined by a discrete-analogue method, i.e. the position of the liquid metal level in the tank is determined.

An electric field is generated by the current supplied to two electrodes in contact with the outer side of the tank wall, one of the electrodes being on the very top of the tank, where the level of liquid sodium can rise, and the other electrode is on the lowest part of the tank. The electric field strength on the outer surface of the tank wall is determined by measuring the ratio of the potential difference between two probing electrodes located vertically on some selected local area of the outer surface of the tank to the distance between these electrodes.

The essence of the invention is as follows.

The current creating an electric field in the tank wall always has the same parameters provided by a stable source. If the tank is empty, i.e. there is no liquid sodium in it, then the current flows only along the wall of the tank, in this case the electric field on the outer wall is determined by the electrical conductivity of stainless steel and the wall thickness of the tank. For a given value of the input current, the thinner the wall of the tank and the lower the electrical conductivity of the steel from which the tank is made, the greater the electric field strength on its wall.

If the tank is filled with liquid sodium, then due to the shunting effect of a significant layer of liquid sodium having high electrical conductivity, the tension on the outer wall of the tank becomes significantly lower compared to when the tank was empty. The higher the electrical conductivity of the liquid metal and the thicker its layer in contact with the wall, the more striking the decrease in the electric field strength on the tank wall. The electrical conductivity of liquid sodium is approximately three times higher than the electrical conductivity of stainless steel, and the thickness of the layer adjacent to the liquid metal wall is many times larger than the wall size.

Thus, the proposed conductometric method allows to probe through the metal wall of the tank the presence or absence of a medium with high electrical conductivity behind the wall of the tank.

The determination of the level of liquid metal in a wide range of its changes is made by a well-known discrete-analogue method based on the results of measuring potentials on the outer surface of the tank using several electrodes located at different heights on the outer surface of the tank wall, and a multi-channel measuring device.

Figs. 1, 2, and 3 explain the proposed conductometric method for measuring the level. Figure 1 shows a cylindrical tank 1 with a height, for example, of the order of 6-7 meters, made of stainless steel, in which there is liquid sodium 2 at an operating temperature of 300-525 ° C. Electrodes 3 made of stainless steel are welded to the outside of the tank 1. The two extreme electrodes are current, and the rest are designed to probe the tank wall. All electrodes are located in a vertical line (along the generatrix of the cylinder of the tank) at an equal distance from each other, for example, after about 400-500 mm. A source of stable pulsed, bipolar low-frequency (0.5-5 Hz) current is supplied to two current electrodes, one of which is above all electrodes, and the other is below all electrodes. Current flows through the tank wall, creating an electric field on the stack, the intensity of which has a vertically directed component. A sufficient current is such a current at which the minimum potential difference between any two adjacent probe electrodes, provided that there is no sodium in the tank, is at least 100-150 μV. When sodium fills the reservoir to a level that completely closes the adjacent adjacent pair of probe electrodes, the potential difference of this pair of electrodes decreases several times. The electrical conductivity of the stainless steel of which the tank is made is approximately 0.2 × 10 ⁷ S / m. Sodium at temperatures above 300 ° C provides good electrical contact with stainless steel. The high electrical conductivity of sodium and its large layer (expressed in fractions of the wall thickness of the tank) have a significant shunting effect, which dramatically reduces the electric field on the outer surface of the tank, where a pair of probe electrodes is located.

The use of a pulsed electric field allows you to separate the informative component of the signal from all interference of electromagnetic origin, the change of which in time is not a multiple of the frequency of change of the electric field. The signal processing from the electrodes is as follows. During the transient period corresponding to the time of switching the polarity of the current and, consequently, the electric field, the signals from the probe electrodes are not measured, the signals from the electrodes are measured only at those times when the electric field is set constant. The summation of the signals measured with positive and negative values of the polarity of the electric field, eliminates all spurious signals whose origin is not connected with the electric field created by the current.

Thus, interference caused by industrial frequency and thermal emf are completely eliminated. Due to this, high level measurement accuracy is achieved. The power of the current source does not exceed 0.5-1.0 watts.

When the liquid metal level position is between two adjacent probing electrodes, the correction Δγ to the result of a discrete level measurement using several probe electrodes located below the pair under consideration is calculated. The correction Δγ is determined by the formula

$$\Delta y=\frac{\lambda \left(U-U_n\right)}{U_{\nu }-U_n}\left(\mathrm{one}\right)$$

where Δγ is the distance of the level to the lower of the two electrodes between which it turned out, λ is the distance between these electrodes, U is the current potential difference between these electrodes, U _ν is the potential difference between these electrodes, when there is air on the whole side of the wall the distance λ, U _n is the potential difference between these electrodes when sodium is located on the back of the wall over the entire distance λ.

The more often the electrodes are located vertically in the tank, the lower are the requirements for the accuracy of the correction calculation.

The level gauge works as follows. A source of stable pulsed, bipolar low-frequency (0.5-5 Hz) current is supplied to two current electrodes. Current flows along the wall of the tank, creating an electric field on its wall, the intensity of which has a vertically directed component. A multichannel measuring device sequentially or simultaneously measures the potentials on the probe electrodes and calculates the position of the level of liquid metal in the tank in a known discrete-analog way.

Fig. 2 shows the level gauge circuitry used if it is possible to introduce the level gauge construction elements into the tank. The level gauge has a hollow cylindrical body 4, on the inner wall of which the current and probe electrodes 3 are located vertically. The level gauge body is mounted vertically inside the tank 1, which is filled with liquid sodium 2.

A source of stable pulsed, bipolar low-frequency (0.5-5 Hz) current is supplied to two current electrodes, one of which is above all electrodes, and the other is below all electrodes. Current flows along the wall of the housing, creating an electric field on its wall, the intensity of which has a vertically directed component. A sufficient current is such a current at which the minimum potential difference between any two adjacent probe electrodes, provided that there is no sodium in the tank, is at least 100-150 μV. When sodium fills the reservoir to a level that completely closes the adjacent adjacent pair of probe electrodes, the potential difference of this pair of electrodes decreases several times. The level gauge shown in Fig. 2 works in the same way as the level gauge shown in Fig. 1.

Fig. 3 illustrates a method for level measurement in a narrow zone. Such a case usually occurs when an alarm is required to reach a level ahead of the set value. On the outer side of the tank 1, in which the liquid metal (sodium) 2 is located, two pairs of electrodes 3 are placed, welded to the outer wall of the tank at two points located vertically at some distance from each other. Moreover, at each point one current electrode and one probe electrode are welded. A source of stable pulsed, bipolar low-frequency (0.5-5 Hz) current is supplied to two current electrodes, and electric field strength is measured by two probe electrodes.

In order for the electric field strength to be sensitive to liquid metal, the minimum distance between current electrodes should be at least 8-12 tank wall thicknesses. A sufficient current is such a current at which the minimum potential difference between any two electrodes, provided that there is no sodium in the tank, is at least 100-150 μV.

The level switch operates as follows. The required value of the bipolar pulsed low-frequency current is passed through the current electrodes. Current flows through the tank wall, creating an electric field on the stack, the intensity of which has a vertically directed component. The potential difference between the probe electrodes is measured by a measuring device that calculates the electric field strength in the gap between the two points to which the electrodes are welded. If the electric field is high, then liquid sodium is below the lower electrode, if the electric field is low, then the level has reached the upper electrode. If the level is somewhere between the electrodes, then the position of the level is determined by the formula (1).

Technical result: reliable control of the level of liquid metal while ensuring the specified metrological characteristics in a wide temperature range, as well as the continuity of control and moderate cost. The proposed conductometric method for measuring the level of liquid metals will find application in the nuclear energy industry.

Information sources

1. Copyright Certificate No. 473057, G01F 23/24, Bulletin No. 21, 1975

Claims (6)

1. A conductometric method for measuring the level of an electrically conductive liquid located in a tank with a metal wall, comprising applying current to the liquid using several electrodes located at different heights, measuring the potentials of the electrodes using a multi-channel measuring device and calculating the position of the liquid level, characterized in that an external surface of the tank creates an electric field with a tension vector directed along the vertical, using the current supplied to two I have electrodes in contact with the outer side of the tank wall, one of which is above all other electrodes, and the other is below all other electrodes, and the level of conductive liquid is determined by measuring the potentials on the outer surface of the tank using the remaining electrodes and a multi-channel measuring device. 2. The method according to claim 1, characterized in that the level of the electrically conductive liquid located between the two adjacent electrodes is calculated as the distance Δγ of the level to the lower of the two electrodes according to the formula
$$\Delta \gamma =\frac{\lambda \left(U-U_n\right)}{U_{\nu }-U_n}$$

where λ is the distance between these electrodes, U is the current potential difference between these electrodes, U _ν is the potential difference between these electrodes when there is air at the entire side of the wall opposite them on the back side of the wall λ, U _n is the potential difference between these electrodes, when against them on the reverse side of the wall is an electrically conductive liquid over the entire distance λ. 3. The method according to claim 1, characterized in that the minimum distance between the two electrodes in the vertical direction, to which the current supplying the electric field is applied, must be at least 10-12 of the wall thickness of the tank. 4. The method according to claim 1, characterized in that the electric current passed through the wall of the tank is a pulse bipolar, frequency of not more than 0.5-5.0 Hz. 5. The method according to claim 1, in a tank having inside a hollow cylindrical body located vertically, characterized in that an electric field is created and potentials are measured on that surface of the body that does not touch the electrically conductive liquid. 6. The method according to claim 1 or 5, characterized in that the creation of an electric field and the measurement of potentials can be performed by combining current electrodes and electrodes that measure potentials.

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