RU2558010C2 - Inductive level gauge of liquid-metal coolant - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to control of level of liquid-metal coolants of reactor plants of nuclear power stations and test benches. A level gauge includes an excitation winding fed by alternating current of sound frequency and a measurement winding, which are enclosed in a tight protective cover submerged into controlled medium. The excitation winding is made of two longitudinal seat-shaped inductance coils wound on a metal tubular housing with length of not less than the level measurement range. The measurement winding is single-turn and short-circuited and represents a wall of the tubular housing, on which excitation coils are located. On the wall of the frame with the specified discreteness as to height equal to absolute measurement error there installed diametrically opposite to each other in gaps between the excitation coals are potential electrodes (current collectors) that are electrically connected to the wall of the tubular frame and form measuring pairs for output of differences of potentials induced by the excitation field in the wall of the frame, which are a measure of submersion of the level gauge in liquid-metal coolant. Potential electrodes are connected to the input of the multichannel secondary instrument.

EFFECT: simpler design; improved operating reliability and lower manufacturing cost of the level gauge.

2 cl, 4 dwg

Description

The present invention relates to instruments for monitoring the level of liquid metals and can be used to measure this parameter in equipment and tanks of nuclear power plants with reactors on fast and intermediate neutrons.

The inductive level gauge UID-1M according to the patent of the Russian Federation No. 2252397 is known, which is used to solve this problem in buffer tanks of the second circuit of the BN-800 reactor unit (4th unit of the Beloyarsk NPP).

The level gauge according to the specified patent contains excitation windings evenly distributed over the height level, measuring and compensation windings fixed inside the protective cover at a fixed distance from each other. The field winding is connected to a sound frequency alternating current generator, and the measuring and compensation windings included in the opposite direction are connected to a voltage meter. At a zero level, the coefficient of mutual induction of the windings, and hence the magnitude of the EMF of the measuring and compensation windings, is maximum. With an increase in the level and “immersion” of the windings in liquid metal, the mutual induction coefficient of the windings decreases due to eddy currents induced in the controlled medium by the alternating electromagnetic field of the field winding, and the EMF of the measuring winding decreases accordingly. The degree of EMF reduction is proportional to the height of the “flooded” part of the windings, i.e. level value.

Compensation winding is used to reduce the temperature error of the level gauge, it is located above the range of level changes. The level gauge according to RF patent No. 2252397 is analog, its output voltage varies linearly and continuously in accordance with the level, but its accuracy is limited by the temperature error, which although it decreases due to the compensation winding, but cannot be reduced to zero due to the geometric asymmetry of the measurement and compensation windings and the presence of a temperature gradient between the liquid metal and the gas volume in the zone of which the compensation winding is located.

The closest in technical essence to the proposed device is a discrete-analog level gauge according to the patent of the Russian Federation №2328704.

In this level gauge, the sensitive element is made in the form of a series of field coils and a series of measuring coils located between them. As the coils are “flooded” with an increase in the EMF level of the measuring coils, it decreases due to a decrease in inductive coupling with neighboring excitation coils, and the logic device connected to the measuring coils determines the number of completely “flooded” coils and the degree of flooding of the coil closest to the level surface.

In this level gauge, the EMF of the “dry” coil is 400–600% higher than the EMF of the flooded coil, i.e. 5–7 times, which is many times higher than the temperature change in the signal; accordingly, the absolute reliability of fixing the “flooding” of the next coil is ensured. In addition, due to the peculiarities of the mutual arrangement of the excitation coils and measuring coils, the EMF of the latter monotonically decreases with a change in level from the downstream to the upstream excitation coils, i.e. in addition to a discrete signal, an analog signal is received from the measuring coil, and by the time the measuring coil is completely "flooded" and its EMF is reduced to the minimum value, the level reaches the sensitivity zone next to the height of the measuring coil, and, therefore, the level gauge has no "dead" zones. This level gauge meets all metrological requirements for the level gauges of reactor plants and will be used at nuclear power plants in tanks with small and medium ranges of level changes, however, its implementation to large measurement limits meets serious structural difficulties. For example, when controlling the level in the range from 0 to 5300 mm and ensuring an error of not more than 1% of the measuring range, up to 60 pairs of coils will need to be placed inside the protective cover, i.e. 240 conclusions of the ends of coils. When using high-temperature cables in a steel sheath for winding coils, such a number of leads cannot be placed inside the protective cover, since most of its internal section is occupied by the coils themselves. The manufacture of one such level gauge requires several kilometers of cable; the coils are wound manually, i.e. to implement this design requires large material and labor costs, the device becomes extremely expensive (from 10 to 14 million rubles per unit).

In addition, the complexity of the design of the primary Converter reduces its reliability. It is enough during operation to fail one measuring coil, as an uncontrolled interval is formed in the measured range. In the process of polling coils, the multiplexer reaches a substandard coil, the signal of which is zero, passes it to a computing device, which "rejects" the zero signal, stops polling the remaining coils and gives a malfunction signal to external circuits. Such cases were during the intake of sodium into the drain tanks of the BN-800 reactor unit (4th unit of the Beloyarsk NPP). The only way to return the instrument to working in this situation is to exclude the damaged coil from the number of respondents, which means knowingly allowing a “gap” in the measurement range.

The aim of the invention is to simplify the design of the level gauge, increase the reliability of its operation and reduce the cost of its manufacture.

This goal is achieved by the fact that the measuring winding is made of a single-turn short-circuited, the role of which is a metal tube frame of the field winding with a length not less than the level measurement level, on the wall of which with a given discreteness in height equal to the absolute error of level measurement, diametrically opposite to one another are installed electrically potential electrodes (current collectors) connected to the frame wall, forming measuring pairs for the output of field-induced excitation of potential differences in the wall of the tube cage, which is a measure of immersion of the level gauge in the liquid metal coolant, at the input of a multichannel secondary device.

Recalculation of the electrical signals entering the secondary device into level units is based on the calibration of each measuring pair of potential electrodes by the simulation method or on a stand with a natural controlled environment.

The device of the proposed level meter is illustrated in FIGS. 1 and 2 by a simplified construction of its primary transducer and a simplified electrical circuit, respectively.

The primary transmitter of the level gauge contains:

1 - technological (protective) cover;

2 - tube frame of the sensing element;

3 - wall of the tube frame;

4, 4 ′ - longitudinal saddle-shaped field windings;

5 - potential electrodes (current collectors), ℓ - step between adjacent electrodes;

6 - mounting-spacing shapers of the field windings;

7, 7 ′ - conclusions of field windings.

On the simplified electrical diagram, only those elements are indicated that allow you to understand the principle of measurement:

L ₁ , L ₂ - field windings;

R ₁ , R ₂ , R ₃ , ..., R _n - the resistance of the wall of the tube frame, concluded at the step between the points of attachment of adjacent electrodes. The device of the secondary device is traditional: it includes a generator 8, a multiplexer 9, an amplifier-standard converter 10, a rectifier 11, a microprocessor 12, an analog-to-digital converter 13, and a digital-to-analog converter 14.

Figure 3 illustrates a circuit for connecting potential electrodes to each other. Each measuring pair is formed by electrodes located diametrically opposite on the tube frame: 1-1 ′, 1′-2, 2-2 ′…, (n-1) ′ - n. The total number of electrodes installed in the measuring range of the level gauge H is 2n.

The work of the proposed level gauge is as follows.

An alternating voltage of sound frequency of 6-8 kHz is supplied to the excitation windings L1, L2 (Fig. 2) from the generator 8. The wall of the tube frame is a short-circuited coil of the secondary winding of the transformer, the primary winding of which is L1 and L2. According to the law of mutual induction, eddy currents of the same frequency are generated in the secondary winding (wall of the frame 3 (Fig. 1), they create a secondary electromagnetic field acting opposite to the electromagnetic field of the field windings. The mutual induction EMF induced in the wall of the tube frame, on which the longitudinal field windings are wound , puts the wall of the frame 3 under a certain electric potential φ ₀ with respect to physical zero ("earth"). When the protective cover 1 (figure 1) is immersed in liquid metal, eddy currents begin to be generated in it, weakening the field of the field windings L1, L2 to a much greater extent than the currents in the wall of the sheath (due to the greater electrical conductivity of the liquid metal and the greater thickness of the layer permeable by the electromagnetic field of the field windings). Due to this, the potential of the frame wall immersed in the liquid metal φ _к , becomes significantly less than the initial potential φ _0. By measuring the potential difference (φ ₀ -φ _к ) = u _{ii ′} along the wall of the tube frame 3 using the electrodes 5 in contact with it, it is possible to determine the boundary of the rise of liquid metal from the protective cover 1. The sign of a level rise to the installation mark of the i-th electrode on the frame wall is a decrease in the initial potential difference between the electrodes forming a measuring pair in the order shown in Fig. 3. Such inclusion of diametrically oppositely mounted electrodes makes it possible to track the level with discreteness ℓ / 2 over the measurement range N. The nature of the change in the potential difference u _{ii ′} obtained on the model of the level gauge with a length of 1200 mm and a pitch between the opposite electrodes ℓ / 2 = 40 mm is shown in FIG. .four. The drop-down section of each curve u _{1-1 ′} , u _1′-2 ... u _{(n-1) ′ - n} allows you to continuously monitor the level across the measurement range, moreover, across several pairs of electrodes, since the curves u = f (h) have overlapping plots. This is very important from the point of view of reliability of level control, since failure of one electrode (for example, a break) does not lead to loss of level information in this range and does not create a “gap” in the conversion characteristic of the level gauge. The calibration characteristics of each pair of electrodes were obtained by moving a duralumin level simulator (thick-walled duralumin bushings) along a protective cover (Fig. 1).

The computing device of the secondary device calculates the level of liquid metal according to the formula:

k _{ii ′} is the conversion coefficient of the measuring pair formed by diametrically opposite electrodes with serial numbers i and i ′. The criteria for "flooding" of the electrodes are selected based on a comparison of the measured potential difference u _{ii ′} with its initial value (in the absence of a level between the electrodes (ii ′) of the pair). For example, to reliably count the number of "flooded" electrodes, the beginning of "flooding" can be considered a decrease in the potential difference by 20% from the initial value, the end of the "flooding" is the reduction of the potential difference to 20% from the initial value. Compared with the solutions closest in technical essence — inductive level gauges according to RF patents No. 22252397 and RF No. 2238704 — the proposed device is much simpler in design, since the role of the measuring winding in it is played by the wall of the mounting tube frame for attaching longitudinal field windings, and to obtain information Levels require only current collectors that are electrically connected to the wall. The cost of an expensive winding wire in the proposed level gauge is two orders of magnitude less, the labor costs for its manufacture are several times less than with known devices. In addition, the reliability of level measurements of the proposed device is significantly higher due to self-reservation - overlapping measurement ranges of adjacent measuring pairs of potential electrodes.

Claims (2)

1. An inductive level meter of a liquid metal coolant containing an excitation winding, powered by an alternating current of sound frequency, and a measuring winding enclosed in a sealed protective cover immersed in a controlled environment, characterized in that the measuring winding is made of a single-turn short-circuited, in which the metal tube frame of the winding acts excitation with a length of not less than the level measurement range, on the wall of which with a given discreteness in height equal to the absolute error Measurements level, diametrically opposite one another are set electrically connected to the wall of the carcass potential electrodes (current collectors), forming a measuring pair of output field-induced excitation of potential differences in the wall of the tubular frame, which are a measure of dipping the transmitter in the liquid metal coolant, the input multi-channel secondary device. 2. The inductive level gauge according to claim 1, characterized in that the field winding is made in the form of two longitudinal saddle-shaped inductors with a length of at least a measurement range.

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