RU2042123C1 - Multiprofile acoustic leak detector and process of its tuning - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: efficiency of inspection increases thanks to use in leak detector of wide-band piezoconverter, comb of retunable band-pass filters, signal compression circuit. Acoustic leak detector uses both active and passive detection of leaks with the aid of acoustic emission simulator and of original technique of electroacoustic tuning of device when receiving surface waves (Reileigh waves). EFFECT: increased efficiency of detection. 2 cl, 3 dwg, 3 tbl

Description

 A multidisciplinary acoustic leak detector is an acoustic tightness and tightness control device designed for non-destructive diagnosis of defects in pipelines, shut-off and non-return shut-off devices, sealed vessels and pressure chambers located both under excessive pressure and in normal conditions, and for determining external leaks on flanges, doors, hatches, determining the quality of welds, determining the current (leaky) tube in devices such as a heat exchanger (steam generator), d For receiving acoustic emission signals arising from cavitation and material destruction.

 Known methods of acoustic control, based on the indication of acoustic emission signals that occur during the flow of a working fluid or gas through leaks of a controlled device, are not sensitive enough due to the influence of accompanying acoustic noise and rather complicated equipment and measurement techniques on the control.

 Known circuits that use the principle of converting a mechanical signal into an electric one, in which a frequency envelope signal is generated that has the characteristics of perceived ultrasonic noise, circuits based on the detection of dependences of the envelopes of signal spectra and noise, and other solutions.

The disadvantages of these solutions include:
1. The lack of implementation of the acoustic features of ceramic and piezoelectric transducers, the unevenness of the amplitude-frequency characteristics of which reaches 30 dB in the operating frequency range when receiving Rayleigh waves.

 (See Japanese Applications N 52-10400, N 56-44367, N 57-24495, U.S. Patent 4,287,581).

 2. The presence of devices such as comparators, time-delay circuits of signals, etc. requiring stationary installation of sensors, which is problematic for portable devices and complex monitoring objects: (see ed. Certificate of the USSR N 1089442 USSR, N 1305545).

 3. The presence of test gas, the requirements for crimping devices require expensive manipulations.

 Closest to the proposed solution is an indicator of ultrasonic vibrations IKU-1, which includes a piezoelectric transducer with a waveguide, a preliminary and main amplifier with an eighth-order bandpass filter, an envelope extraction unit, and an indication device with a step automatic attenuator.

The disadvantages of this design can be attributed to: lack of work in a wide frequency range operating frequency: F _p = 105 ± 5 kHz; weak noise immunity in the presence of only one filter, preset eighth-order frequency filter with a large number of elements, strictly speaking a variable value of the resonant frequency of the piezoelectric transducer with a waveguide; (this resonant frequency depends on the method of fastening the ceramics, the accuracy of the manufacture of the waveguide, the pressing force, etc. of the values are unstable); non-linearity of performance; high power consumption (≈ 18 VA), requiring a separate power supply.

 The aim of the invention is: increasing the sensitivity of the leak detector; increased noise immunity; linearization of performance; low power consumption.

 To achieve this goal, the following are used in the multifunctional acoustic leak detector circuit: a wide-band ceramic piezoelectric transducer, a block of reconfigurable narrow-band filters, an electric signal compression scheme.

 The use of these elements in the device makes it possible to realize individual physical features of ceramic broadband piezoelectric transducers and to set optimal conditions for receiving acoustic emission signals.

These conditions are realized by appropriate electro-acoustic adjustment of the device circuit. The specified setting includes the following operations:
1. The choice of broadband piezoelectric transducer (PP).

 2. Determination of the amplitude-frequency characteristics of the PP at the installation, which includes the computer, the program of which contains the acoustic formula of the reciprocity theorem, or graphically.

 3. Pre-setting leak detector filters for resonant frequencies.

 4. The final electro-acoustic setting, correcting the distortion of the frequency response that occurs when mounting the PCB in any holder, clip, probe, etc.

 Thus, each leak detector has an individual electro-acoustic setting corresponding to the frequency response of broadband software.

 In the table. 1 shows comparative data of the minimally controlled leakage levels (MCU) of two leak detectors: according to the proposed solution and prototype. Data shown for various environments.

In FIG. 1 and 2 are graphs of the dependences of the MCU (Q l / min) on the differential pressure on the shut-off device, taken as a controlled object (Δ P kgf / cm ² ). Graphs are also provided for various work environments.

 In FIG. 3 shows a block diagram of a leak detector.

 The presence in the circuit of the device for signal compression made it possible to expand the dynamic range of the leak detector by 30-40 dB, i.e. to get a practically constant signal at the output when the input level changes by 30-40 dB and to judge the leak rate.

The leak detector block diagram contains the following elements:
1 a piezoelectric transducer narrow-band receiving for non-contact leak detection (ultrasonic microphone).

I-PP broadband receiving P piezo emitter at a fixed ultrasonic frequency
2 preamplifier
3 main amplifier
4 high-frequency envelope envelope selection unit
5 block narrow-band tunable filters
6 signal compression scheme
7 heterodyne converter
8 mixer
9 low frequency amplifier,
10 power supply
12 fixed frequency ultrasonic generator
13 battery power simulator acoustic emission signals.

 The nodes and blocks of the leak detector are made on the basis of widely used component parts according to well-known design principles.

 The most important unit of the circuit is the filter unit, which uses a double T-bridge. The maximum filter gain at the resonant frequency depends on the gain of the operational amplifier and the accuracy of the bridge settings. With an accuracy of element ratings of 0.1%, the transmission coefficient of the filter exceeds 50 dB. The filter bandwidth at the level of 0.7 of one of the settings is shown in the table. 3. In fact, the quality factor of the filters also depends on the band of peaks of extrema in the maximum conversion coefficients of the PP and is significantly improved with the correct determination of the frequency response of the PP and the corresponding electro-acoustic tuning of the leak detector.

 The number of filters, taking into account the overlap of the frequency range of 20-240 kHz at a level of 0.2, is taken equal to eight.

 The MAT indicator works as follows.

 From the output of the piezoelectric transducer (PP) 1 or acoustic emission signals are fed to the preamplifier 2. Both elements are mounted on one probe, which significantly (by 50-60 dB) suppresses the level of electromagnetic interference at the input. Next, the signals are fed to the main amplifier 3, which allows it to be amplified at the resonant frequency of one of the eight narrow-band filters selected by the mechanical switch. A constant component is allocated on the envelope extraction unit of the high-frequency signal 4, which, depending on the level of acoustic emission signals, deflects the arrow of the microammeter by a certain value. To exclude the device going off scale with high-power signals, a compression circuit 6 is provided, expanding the dynamic range of the device by 30–40 dB. To provide additional auditory control, a high-frequency signal is transferred to the normal range by a local oscillator.

 The leak detector works in three modes.

 1. Passive indication by a contact broadband sensor 1. It is produced in the presence of a pressure gradient of any working medium on the shut-off device. In this case, the acoustic emission signal is selected by the mechanical switch to the maximum of its energy and the minimum of associated acoustic noise. Leakage is recorded visually by microammeter and by ear using headphones.

 2. Passive contactless indication with an ultrasonic microphone 1a. It is produced at a fixed resonant frequency of this microphone in the presence of an air pressure gradient in the volume studied for tightness. In this case, a filter tuned to this frequency is selected. Using a probe, the volume is examined in places of possible leaks by ear and visually.

 3. Active contactless indication with an ultrasonic microphone. It is also produced at a fixed frequency, at which the acoustic emission simulator operates (12). The simulator is placed in the volume tested for tightness. With the help of a probe, this volume is examined in places of possible leaks by ear and visually.

 In the third mode, the leak detector reliably fixes leaks of the investigated volume corresponding to leaks of 1.1 lx hmk / s.

Electro-acoustic configuration of the circuit is reduced to the following actions:
1. Select a broadband ceramic piezoelectric transducer (PP).

 2. The real frequency response of this PP is measured.

 3. Preset filters, corresponding to the real frequency response, are performed using conventional equipment: a special form of signal generator, oscilloscope and frequency meter.

 4. Final adjustment is carried out on a special stand. At the same time, the frequency response distortions arising from the individual mechanical assembly of the PP in its holder, probe, clip, etc. are corrected.

 So, the electro-acoustic adjustment of the indicator is reduced to the identity of the extrema of the frequency response of the PP in the maximum conversion coefficient when receiving Rayleigh waves and resonant frequencies of the main amplifier.

 The setting option is given in table. 2, where in the first line the frequency ranges of the leak detector are indicated, in the second extremes the maximum of the conversion coefficient, in the third actually obtained resonant frequencies of the indicator. The coincidence of the first and second lines is possible in the case of mild extremes.

Below are the main technical characteristics of the MAT leak detector,
Operating frequency range, kHz 20-240
The number of narrow-band frequency filters 8
Noise level, given
ny to the input of the preliminary amplifier, no more than 10˙10 ^-6 Power consumption, W 0.96 Supply voltage from + 20% of an autonomous source, V ± 12
-10%
Uptime
from an autonomous power source, h, at least 45
Overall dimensions, mm 234х250х83 Weight, kg 4

Claims (2)

 1. A multidisciplinary acoustic leak detector containing a ceramic piezoelectric transducer, blocks of preliminary and main amplifiers, an envelope separation unit, a visual and auditory indication unit, and a signal filtering unit, electrically connected to each other, characterized in that the ceramic piezoelectric transducer is made broadband with a range of 20 - 240 kHz, the signal filtering unit is made in the form of a comb of eight tunable bandpass filters, and the leak detector is equipped with an electrically connected si compression unit drove and the heterodyne conversion unit of the high-frequency acoustic emission signal.  2. The method of tuning the leak detector, which consists in emitting Rayleigh waves using an acoustic emitter, receiving Rayleigh waves with a leak detector, indicating them in the form of an audio and / or visual signal, and adjusting the main amplifier blocks and selecting the envelope to the maximum of the received signal, characterized in that the adjustment blocks of the main amplifier and envelope separation is performed at eight extremes of the real amplitude-frequency characteristic of the piezoelectric transducer at the maximum of the conversion coefficient and resonance frequencies of the main amplifier.

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