RU1770766C - Contact-free method of determining interface of two media - Google Patents

Abstract

The invention relates to a control and measuring technique, in particular to questions of controlling the level of liquid and granular media, and may find application in the chemical, metallurgical and other industries. The invention improves the accuracy of measurements by compensating for the effect of changes in the acoustic contact of the transducers on the results of determining the position of the interface. The invention makes it possible to expand the scope of application, allowing to determine the position of the interface on the conditions of the changing acoustic contact of the transducers. Using one electro-acoustic transducer, two bending waves with different frequencies are simultaneously emitted into the tank wall. Frequencies are selected depending on the thickness and elastic parameters of the wall material. When taken by filtration, each of these waves is isolated. The amplitude parameters of each of these waves are determined; the position of the interface between two media is judged with respect to the indicated parameters. In this case, the distance between the points of emission and reception is selected from conditions that ensure a change in the ratio of the amplitude parameters when the propagation zone of these waves contacts the liquid by an amount greater than the selected value. 1 sec and 3 s, n. flowers, 1 ill. C /) C

Description

The invention relates to a control and measuring technique, in particular, to questions of controlling the level of liquid and bulk solids, and may find application in the chemical, metallurgical, food and other industries.

A known method for controlling the liquid level, according to which there is a through passage of ultrasonic vibrations through the vessel with the liquid. In this case, the presence of liquid in the passage zone of the ultrasonic beam is determined by the amplitude of the received signal, which will be maximum when passing through the liquid and minimum when passing through air.

The disadvantage of this method is the low accuracy of measurements and the limited scope of its possible application, due to the following reasons. In actual process fluids, gas bubbles or particulate matter are typically present. In this case, the attenuation of the ultrasonic EOLNA increases and, accordingly, a strong decrease in the amplitude of the received signal Ete leads to errors in the control and, in some cases, the application of this method is impossible.

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The closest to the proposed solution in technical essence is a method of establishing an interface between a gaseous and a liquid medium or between two liquid media with a single-layer vessel, the method being that acoustic waves are excited in the vessel wall by means of a radiating electroacoustic transducer (EA). mounted on the outside of the wall, receive acoustic vibrations propagating in the wall of the tank, using the receiving EAP, also installed on the outside of the They are judged by the amplitude of the received oscillations.

The disadvantage of this method is that when the acoustic contact of the emitting and receiving EAP with the tank wall changes, the amplitude of the received vibrations can change more than from the influence of the medium interface. This can lead to errors in determining the interface between media or to a decrease in the accuracy of measurements. Long-term changes in acoustic contact in stationary installations can be compensated by periodically adjusting the amplitude of the emitted oscillations or the gain of the amplifier. However, under non-stationary conditions, when the contact change can occur almost instantly (for example, in portable level switches, when exposed to strong vibrations, etc.), the application of this method will inevitably be associated with large measurement errors, the magnitude of which will determine the region possible application of the method.

The aim of the invention is to increase the accuracy of measurements and expand the scope of the possible application of the method in conditions of a changing external acoustic field.

The goal is achieved in that in a non-contact method for determining the interface between two media in tanks, simultaneously, using one emitting EAP, two bending waves are excited in the tank wall, the frequencies of which are selected from the relations:

fa

Cf2 VTZpQ-o2) 2b E

h

where Sj is the wave velocity in the liquid;

fi.fa, respectively, the frequency of two excited bending waves;

h is the wall thickness of the tank; p is the density of the wall material; 5E - Young's modulus;

a - Poisson's ratio. When determining the amplitude parameters by filtering, the received vibrations with frequencies fi and h are separated. 10 In order to excite reception of two waves, the intrinsic resonant frequency of the emitting and receiving EAP and the frequency of the first harmonic are used (therefore, the frequency h is chosen to be less than 0.5 f i). Broadband EAPs can also be used to generate and receive two waves.

The position of the interface between two media can be determined:

- regarding the amplitude ratio of the envelopes of 20 pulses of the first and second waves;

- in relation to average values of amplitudes of several arrivals of two waves.

The proposed method differs from the known in that:

251) there is a simultaneous emission of two waves with the help of one emitting EAP;

2) excite two bending waves;

3) the frequencies of the excited waves are selected 30 from the proposed relations (forms.

1.2) ;. I ...

4) when received by filtration, each of these waves is isolated;

5) determine the amplitude parameters 35 of two waves;

6) the position of the interface between two media is judged by the difference amplitude parameters of these waves.

The scheme of measurements according to the proposed method is presented in the drawing,

The method is carried out as follows. Based on the wall thickness of the tank with the controlled fluid, and

45 also the properties of the liquid and the material wall, according to conditions (1), (2), the radiation frequencies ft and fa are chosen. As a radiating and receiving EAP (1,2; Fig. 1), they can be used as broadband converters with high quality factor. The value of the threshold value of the ratio of the amplitude parameters Kpor is selected. Electric pulses containing frequencies fi are fed to the input of the emitter (1, Fig. 1)

55 and fa. the transducers are pressed "against the wall of the tank, while in the wall into, two bending waves with the indicated frequencies are excited simultaneously. Spread in the tank wall (3, Fig. 1), bending wheels

the baths are received at the receiving EAP (2, Fig. 1) and converted into electrical signals, filtering out vibrations with frequencies fi and fa (4, Fig. 1) and determining their amplitude parameters (5, Fig. 1). If the liquid level in the tank (6, Fig. 1) is below the propagation zone of these waves, the amplitude characteristics at the frequencies fi and fa depend only on the quality of the acoustic contact, the ratio of the amplitude parameters of the two waves will be less than Kpor. If the liquid level reaches the propagation zone of the emitted and received waves, then the amplitude parameters of the oscillations with a frequency f 1 will decrease, and the choice of frequency according to formula (1) provides the maximum amount of wave attenuation under the influence of contact with the liquid. Fluctuations with the frequency fa will continue to depend mainly on the level of the acoustic signal. In this case, the ratio of the amplitude parameters of the oscillations with frequencies fi and fa will exceed the value of Kpor. Thus, by the ratio of the amplitude parameters of the oscillations with the frequencies fi and fa, one can judge the presence of liquid in the control zone,

In determining the interface by the ratio of the amplitudes of the envelopes of the vibrations with frequencies f i and fa, the practical implementation of this method is the simplest. The use of the ratio of the average values of the amplitudes as an informative parameter can significantly increase the reliability and, consequently, the accuracy of measurements, however, hardware-based implementation in this case will be more complicated.

An example of a specific embodiment of the method.

It is necessary to control the level of liquefied chlorine in steel containers with a wall thickness of 8 ± 0.5 mm using a portable level indicator.

From formula (1) we determine one of the radiated frequencies, substituting in (1) the following values of the used parameters: h 8 mm for steel E 16 1010 N / m2

, 8 103 kg / m3

(, 28

for liquefied chlorine 1.5 1.5 kg / m3

Sq 1400m / s

As a result, we obtain for fi 29.5 kHz

For the emission and reception of two frequencies, we choose a piezoceramic transducer with a fundamental resonance frequency fp 12 kHz. The first harmonic frequency for this type of transducer is defined as fr 2.5 fp 30 kHz. Thus the frequency

of the fundamental resonance will correspond to condition (2), and the frequency of the first harmonic will correspond to the condition), fh a pulser emitter electrical oscillation pulses containing frequencies fi and h, we will excite acoustic pulses of bending waves with these frequencies in the container wall.

We choose the threshold value of the ratio of amplitudes Kpor 3. By selecting the gain for each frequency, we establish the amplitudes of the received pulses, at a liquid level below the wave propagation zone, AOI Ao2 3 V. We determine the attenuation coefficients of a bending wave caused by the influence of a liquid by the formula:

, (AND)

2/1 Us2 / szh2-1

where C is the speed of the bending wave;

Sg is the speed of sound in a liquid;

РЖ, / 9 - density of liquefied chlorine and wall material, respectively;

h is the wall thickness of the container,

The speed of bending waves is determined by the formula:

-Y

T

(fh) 2

(5)

ЗРО-о2)

where E is Young's modulus;

and - Poisson's ratio;

f is the oscillation frequency.

For a steel wall with a thickness of 8 mm, we obtain Ca 906 m / s at a frequency of 12 kHz, Ca 306 m / s at a frequency of fi, and Ci 1430 m / s. For the frequency fa 12 kHz, we obtain "2 0.02 mm, for the frequency fi 30 kHz, and 0.14.

Based on the obtained data and the container design, we choose the distance between the emitter and receiver L 150 mm.

If the interface is below the zone of propagation of bending waves, the ratio of the maximum

Apo of the amplitudes of the pulses of the two waves K 1.

AOI

In the case when the bending waves propagate in contact with the liquid (the interface is higher than the location of the EAP), the amplitude of the wave with frequency will be determined from the following expression: Аа Аоа cos (lrta L) (6)

By setting the values of the parameters, we obtain Aa 2.9 V. The amplitude of the wave with frequency fi will be equal to AI Aoi e. For this frequency, we will get AI 7 V. That is, only acoustic noise will be recorded at this frequency, the amplitude of which, like

as a rule, two orders of magnitude less than AOL The ratio of the amplitudes of the two waves in this case will be

v 2 2.93 Q-r. v

To Knop

Thus, upon contact of the bending wave propagation zone with the liquid, the ratio of the wave amplitudes will become less than the threshold value chosen by us, which will make it possible to determine the position of the interface between media from the value of K. Substituting h values of 8.5 mm into formulas (4) and (5), we obtain a 0.05, ac 0.017. In this case, if the EAP are below the liquid level, K 5.0 Kpor - Similarly for h 7 mm we get a 0.06 mm; az 0.021; K 5.1 Kpor. Changes in acoustic contact at the selected frequencies will almost equally affect the amplitudes of AI and A2, and their ratio will hardly change.

Thus, the task of determining the interface between media in containers with liquefied chlorine under conditions of changing contact of the EA with the container walls is solved using the proposed method,

Claims (4)

1. A non-contact method for determining the interface between two media, in which acoustic vibrations with a fixed frequency and amplitude are excited in the tank wall from the outside, accept the vibrations propagating in the wall, determine the amplitude parameters of the received vibrations, from which the position of the boundary is determined section, characterized in that, with the aim increasing accuracy and expanding the scope of application by providing the possibility of operating under conditions of changing external acoustic field, simultaneously excite two bending waves in the tank wall, the frequencies of which are selected from the relation § where f 1 and f2 are the frequencies of the excited waves, respectively; Sj is the speed of sound in a liquid; E is the Young's modulus of the wall material; h is the wall thickness; p is the density of the wall material; a is the Poisson's ratio of the wall material, moreover, when determining the amplitude parameters by filtering, the vibrations with frequencies f 1 and f2 are separated and the difference amplitude characteristic is found, 2. The method according to claim 1, with the exception of the fact that they excite oscillations at frequencies equal to the natural resonant frequency of the emitter and the frequency of its first harmonic; 3. The method according to paragraphs. 1 and 2, characterized in that the excitation and reception of oscillations is carried out by broadband electroacoustic transducers. 4. The method according to paragraphs. 1-3, with the exception of the fact that the position of the interface between two media is judged in relation to the maximum or average values of the amplitudes. n  / / / f. / / / /: /.u