ACOUSTIC EMISSION CONTROL
Objective. The study of the basic physical principles of acoustic emission control. Application of the method acoustic emission for inspection of tanks without decommissioning. Acquaintance with the means of collecting and processing information in the diagnosis of objects.
Acoustic emission (AE) is understood as the occurrence of elastic waves in a medium caused by a change in its state under the influence of external or internal factors. The acoustic emission method is based on the analysis of these waves. The purpose of AE control is detection, determination of coordinates and tracking (monitoring) of sources of acoustic emission.
The acoustic emission (AE) method is sensitive to any kind of structural changes over a wide frequency range of operation (typically 10 to 1000 kHz). The equipment is capable of recording not only brittle crack growth, but also the development of local plastic deformation, hardening, crystallization, friction, impacts, leaks and phase transitions.
A schematic diagram of AE control is shown in Figure 1.
Figure 1 - Scheme of AE control on the pipeline:
1 – AE transducer (receiver); 2-block amplification; 3 – filtration unit; 4 - central block for collecting and processing information based on an industrial computer; 5 - object of control; 6 – AE source; t1 is the time of signal arrival at the first receiver; t2 is the time of signal arrival at the second receiver
The main applications in which the AE method of control is used are:
Periodic control of the integrity of structures;
Monitoring the integrity of the structure during the pressure testing period;
Monitoring the performance of the object during pneumatic testing;
Monitoring (long-term control with simultaneous processing of results in real time) of the integrity of the object;
Control of the welding process;
Control of wear and contact of equipment during automatic machining;
Control of wear and lubricant losses at facilities;
Detection of lost parts and pieces of equipment;
Detection and control of leaks, cavitation and fluid flows in objects;
Control chemical reactions, including the control of corrosion processes, as well as processes of liquid-solid transition, phase transformations.
Most structural materials begin to emit acoustic vibrations in the ultrasonic part of the spectrum under loading long before failure.
The study and registration of these waves became possible with the creation of special equipment.
The registration of the signal from the AE source is carried out simultaneously with the noise of constant and variable levels (Figure 2). Noise is one of the main factors that reduce the effectiveness of AE control.
To suppress noise and extract a useful signal, two methods are usually used: amplitude and frequency.
Figure 2 - General scheme of the registered AE signal against the noise background:
1 - oscillations; 2 - floating threshold;
3 - oscillations without taking into account the floating threshold; 4 - noise
Amplitude consists in establishing a fixed and floating level of the discrimination threshold UP, below which the AE signals are not recorded by the equipment. A fixed threshold is set in the presence of noise at a constant level, a floating threshold - variable.
Frequency The noise suppression method consists in filtering the signal received by the AE receivers using low- and high-pass filters (LPF/HPF). In this case, to adjust the filters, the frequency and level of the corresponding noises are preliminarily estimated before testing.
Signals from a crack type AE source are characterized by the fact that they are emitted by one source, they are short-lived, and the time of their arrival at the acoustic emission transducers (AET) reflects the distance to the crack. The position of the AE source on the plane is found by triangulation methods. Based on the speed of wave propagation in the material and the difference in the times of signal arrival at different AETs, the location of the set of points for the AE source is calculated, which will be located on circles with radii R1, R2 and R3 from the corresponding AES (Figure 3).
Figure 3 - Scheme of AE source location on the plane
The characteristic features of the AE testing method, which determine its capabilities and scope, are the following:
The AE control method ensures the detection and registration of only developing defects, which makes it possible to classify defects not by size, but by their degree of danger;
The sensitivity of the AE control method is very high. It makes it possible to detect crack increments of the order of fractions of a millimeter under operating conditions, which significantly exceeds the sensitivity of other methods;
The property of the integrity of the AE control method provides control of the entire object using one or more AE control transducers, fixedly installed on the surface of the object;
The AE control method provides the possibility of testing objects without removing their hydro- or thermal insulation. To carry out the control, it is enough to open the insulation only in the places where the converters are installed, which greatly reduces the amount of restoration work;
The method provides the possibility of remote monitoring of inaccessible objects, such as underground and underwater pipelines, apparatuses of closed structures, etc.;
The method allows to control various technological processes and processes of changing the properties and state of materials and has fewer restrictions associated with their properties and structure;
The AE method can also be used to estimate the rate of defect development and, accordingly, to estimate the residual resource of the controlled object. Registration of AE allows to determine the formation of fistulas, through cracks, leaks in seals, plugs and flange connections.
A significant disadvantage of the method is the difficulty in separating the useful signal from noise when the defect is small. The probability of detecting an AE signal is high only with a sharp development of a defect; therefore, the AE testing method is recommended to be used in combination with other non-destructive testing methods.
Acoustic emission control of tanks
AEC is carried out to detect developing defects in welded joints and the base metal of the tank wall and bottom.
To carry out AE testing of tanks, a multichannel AE system is used, which provides registration of acoustic emission signals from defects in welded joints and the base metal of the three lower belts of the tank wall in one inspection cycle.
Before carrying out work on AE control, you should:
- maximally eliminate sources of acoustic interference;
- calibrate the AE equipment;
- determine the noise level and the radius of the sensor reception area.
When conducting AE control, continuous monitoring of incoming data is carried out. If during loading an anomalous increase in AE activity is noted - AE sources of IV (E) hazard class, then in order to prevent the occurrence of an accident (accidental leakage), the tests are suspended until the causes of the detected phenomenon are clarified.
Based on the received and processed data, AE sources in welded joints and the base metal of the tank wall are evaluated according to the degree of danger:
I - passive;
II - active;
III - critically active;
IV - catastrophically active.
On the basis of the obtained and processed bottom monitoring AE data, the signal sources are evaluated according to the degree of danger:
A - very weak corrosion;
B - early stage of corrosion development;
C - local corrosion;
D - severe corrosion of the bottom;
E - very strong corrosion of the bottom, a leak was detected.
In the case of assessing the condition of the bottom in category E, it is necessary to immediately take the tank out of operation and conduct a full technical diagnostics.
In case of detection of AE sources of II, III or IV classes or in the case when interpretation of AE sources is difficult, ultrasonic control of sections of the tank wall is carried out at the places of detection of AE sources. The final assessment of the identified AE sources is carried out based on the results of ultrasonic testing. Defects that are sources of AE III or IV classes are unacceptable.
AE control equipment
The manufactured acoustic emission devices and systems are used to control and diagnose various industrial facilities: main and process pipelines, cylinders, pressure vessels, oil product tanks, lifting equipment, etc.
Figure 4 - Acoustic emission transducers
Acoustic emission transducers, in addition to explosion-proof models, have an automatic test mode for the sensor itself, which, thanks to wave radiation, also allows checking the performance of neighboring sensors and the acoustic emission system as a whole.
The sensitive piezoelectric system is sealed with a special elastic sealant. The entire volume of the housing, including the electronic circuit, is filled with an epoxy compound with increased adhesion to stainless steel. The transducers have a wear-resistant ceramic or stainless steel protector.
Converter models differ in operating frequency band, supply voltage, gain of the preamplifier, design (usual hermetic or explosion-proof hermetic), protector material.
Acoustic emission transducers are attached to the controlled object using magnetic clamps.
Picture 5 - Magnetic clamps
System management, data collection and analysis is provided special programs. For example, the AE Studio software package that comes with the Acoustic Emission System includes:
· "Koral" - a program and technology for processing data of acoustic emission control of linear objects (linear sections of technological and main oil, gas, product pipelines, etc.);
· "Storm" - a package of programs independent from one another and a technology for processing data of acoustic emission control of volumetric objects (reservoirs, oil tanks, spherical shells, etc.).
The Burya software package is designed for complex, detailed processing of acoustic emission information obtained as a result of monitoring industrial facilities and includes the following data processing programs:
· "Bottom" - a program for processing data of acoustic emission control of flat round bottoms that do not have the ability to install acoustic emission sensors on them (VST bottoms). A feature of the program is the ability to use additional sensors that are placed on the tank wall to filter acoustic emission events from the bottom from events that occurred in the upper part of the VST volume.
Figure 6 - Data processing program "Bottom"
· "Sphere" - a program for processing data of acoustic emission control of spherical objects (spherical storages and tanks, spherical bottoms of tanks). Includes a separate program "Sphere-D", which is necessary for drawing a map of an object and creating a file of coordinates for placing sensors on a spherical surface with an ordered table of distances between receivers.
· "Cylinder" - a program for processing data of acoustic emission control of cylindrical objects (tanks, columns, walls of RVS). Includes a separate program Cylinder-D, necessary for drawing a map of an object and creating a file of coordinates for placing sensors on a cylindrical surface with an ordered table of distances between receivers.
Figure 7 - Data processing program "Sphere"
Figure 8 - Data processing program "Cylinder"
Figure 9 - Tank with characteristic defects
Similar information.
Sources of acoustic emission
When destroyed, almost all materials emit a sound (“the cry of tin”, known from mid-nineteenth centuries, the crack of breaking wood, ice, etc.), i.e. emit acoustic waves that are perceived by ear. Most structural materials (for example, many metals and composite materials) begin to emit acoustic vibrations in the ultrasonic (inaudible) part of the spectrum under loading long before failure. The study and registration of these waves became possible with the creation of special equipment. Particularly intensive work in this direction began to develop from the mid-60s of the XX century. in connection with the need to control especially critical technical objects: nuclear reactors and pipelines of nuclear power plants, missile bodies, etc.
Acoustic emission (emission - emission, generation) is understood as the occurrence of elastic waves in a medium caused by a change in its state under the influence of external or internal factors. The acoustic emission method is based on the analysis of these waves and is one of the passive methods of acoustic control. In accordance with GOST 27655-88 “Acoustic emission. Terms, definitions and designations” the mechanism of excitation of acoustic emission (AE) is a set of physical and (or) chemical processes occurring in the object of control. Depending on the type of process, AE is divided into the following types:
· AE of a material caused by dynamic local rearrangement of its structure;
· AE of friction caused by friction of surfaces of solid bodies in places of load application and in joints where compliance of mating elements takes place;
· AE of a leak caused by the interaction of a liquid or gas flowing through the leak with the walls of the leak and the surrounding air;
· AE in chemical or electrical reactions resulting from the occurrence of appropriate reactions, including those accompanying corrosion processes;
· magnetic and radiation AE arising, respectively, during the remagnetization of materials (magnetic noise) or as a result of the interaction of ionizing radiation with it;
· AE caused by phase transformations in substances and materials.
Thus, AE is a phenomenon that accompanies almost all physical processes occurring in solids and on their surface. The possibility of registering a number of types of AE due to their smallness, especially AEs that occur on molecular level, during the movement of defects (dislocations) crystal lattice, is limited by the sensitivity of the equipment, therefore, in the practice of AE control of most industrial facilities, including oil and gas industry facilities, the first three types of AE are used. It should be kept in mind that AE friction creates noise, leads to the formation of false defects, and is one of the main factors complicating the application of the AE method. In addition, only the strongest signals from developing defects are recorded from AE of the first type: during crack growth and during plastic deformation of the material. The latter circumstance makes the AE method more practical significance and determines its wide application for the purposes of technical diagnostics.
The purpose of AE testing is detection, determination of coordinates and tracking (monitoring) of sources of acoustic emission associated with discontinuities on the surface or in the volume of the wall of the test object, welded joint and manufactured parts and components. All indications caused by AE sources should, if technically possible, be evaluated by other methods of non-destructive testing.
Types of AE signals
AE recorded by industrial serial equipment is divided into continuous and discrete. Continuous AE is recorded as a continuous wave field with a high signal repetition rate, while discrete AE consists of separate distinguishable pulses with an amplitude exceeding the noise level. The continuous line corresponds to the plastic deformation (flow) of the metal or the outflow of liquid or gas through leaks, the discrete line corresponds to the jump-like growth of cracks.
The size of the radiation source of a discrete AE is small and comparable to the length of the emitted waves. It can be represented as a quasi-point source located on the surface or inside the material and emitting spherical waves or other types of waves. When waves interact with a surface (the interface between two media), they are reflected and transformed. Waves propagating inside volumes of material rapidly weaken due to attenuation. Surface waves attenuate with a distance much less than volume waves, and therefore they are predominantly recorded by AE receivers.
Registration of a signal from an AE source is carried out simultaneously with noise of a constant or variable level (Figure 10.1). Noise is one of the main factors that reduce the effectiveness of AE control. Due to the variety of causes that cause their appearance, noises are classified depending on:
generation mechanism (source of origin) - acoustic (mechanical) and electromagnetic;
type of noise signal - pulsed and continuous;
source locations - external and internal. The main sources of noise during AE control of objects are:
splashing of liquid in a container, vessel or pipeline during its filling;
· hydrodynamic turbulent phenomena at high loading speed;
friction at the points of contact of the object with supports or suspension, as well as in joints with compliance;
operation of pumps, motors and other mechanical devices;
The action of electromagnetic pickups;
impact environment(rain, wind, etc.);
· intrinsic thermal noise of the AE converter and the noise of the input stages of the amplifier (preamplifier).
To suppress noise and extract a useful signal, two methods are usually used: amplitude and frequency. Amplitude consists in setting a fixed or floating level of the discrimination threshold below which the equipment does not register AE signals. The fixed threshold is set in the presence of noise at a constant level, floating - variable. The floating threshold, which is set automatically by tracking the overall noise level, makes it possible, in contrast to the fixed one, to exclude the registration of part of the noise signals as an AE signal.
Figure 1. General scheme of the recorded AE signal against the background of noise:
1 - oscillations; 2 - floating threshold; 3 - oscillations without taking into account the floating threshold; 4 - noise
Figure 10.2. General form AE signal at the output of the amplifying path of the equipment:
1 - oscillations; 2 - envelope; - amplitude threshold value; - amplitude of the k-th pulse
The frequency method of noise suppression consists in filtering the signal received by the AE receivers using low- and high-frequency filters (LPF/HPF). In this case, to adjust the filters, the frequency and level of the corresponding noises are preliminarily estimated before testing.
After the signal passes through the filters and the amplifying path, along with the transformation of waves on the surface of the controlled product, further distortion of the initial pulses of the AE source occurs. They acquire a bipolar oscillating character, shown in Figure 10.2. The further procedure for processing signals and using them as an informative parameter is determined by computer programs for data acquisition and post-processing used in the corresponding equipment of various manufacturers. The correctness of determining the number of events and their amplitude will depend not only on the possibility of their registration (resolution of the equipment), but also on the method of registration.
For example, if you register pulses of the signal envelope above the level , then four pulses will be recorded, and if you register the number of oscillations above the same level, then nine pulses will be recorded. An impulse is understood as a train of waves with a frequency in the operating range, the envelope of which at the beginning of the impulse crosses the threshold upwards, and at the end of the impulse - downwards.
Thus, the number of registered pulses will depend on the hardware setting: the value of the end-of-event timeout. If the timeout is large enough, for example, four impulses can be registered, if it is small, then all oscillations above the level (eight in Figure 10.2) can be registered as impulses. Large errors can also be introduced by the use of the frequency bandwidth of the signals and the level of discrimination, especially when the AE signals are comparable in amplitude to the noise level.
Evaluation of the results of AE control.
After processing the received signals, the control results are presented in the form of identified (in order to exclude false defects) and classified AE sources. Classification is performed using the following main parameters of AE signals:
· total acoustic emission count - the number of registered AE pulses above the set discrimination level (threshold) for the observation time interval;
· acoustic emission activity - the number of registered AE pulses per unit of time;
· acoustic emission count rate - the ratio of the total acoustic emission count to the observation time interval;
· acoustic emission energy - the energy released by the AE source and carried by the waves arising in the material;
· amplitude of acoustic emission signals, pulse duration, AE event rise time.
The total count and AE activity during plastic deformation are proportional to the volume of the deformed material. The amplitude of the AE signals and energy during the development of a crack is directly proportional to the rate of its growth and the maximum stresses in the given zone.
When classifying AE sources, their concentration, loading parameters of the controlled object, and time are also taken into account.
Detected and identified sources of AE in accordance with PB 03-593-03 "Rules for the organization and conduct of acoustic emission control of vessels, apparatus, boilers and technological pipelines" are recommended to be divided into four classes:
· the first one is a passive source registered to analyze the dynamics of its development;
the second is an active source requiring additional control using other methods;
· the third is a critically active source requiring control over the development of the situation and taking measures to prepare for a possible load shedding;
· the fourth one is a catastrophically active source requiring an immediate decrease in the load to zero or to a value at which the source activity drops to the level of the second or third class.
Considering big number parameters characterizing the AE, the assignment of sources to the corresponding class is carried out using a number of criteria that take into account a set of parameters. The choice of criteria is carried out according to PB 03-593-03, depending on the mechanical and acoustic-emission properties of the materials of the controlled objects. The criteria include the following:
· amplitude, based on the registration of pulse amplitudes (at least three from one source) and their comparison with the value of exceeding the threshold (), which corresponds to the growth of a crack in the material. Determining , requires the study of the material on samples in preliminary experiments;
· integral, based on a comparison of the assessment of the activity of AE sources with the relative strength of these sources in each registration interval. In this case, for the determination it is required to establish the value of the coefficient in preliminary studies;
· local-dynamic, using the change in the number of AE of location events at pressure holding stages and the dynamics of changes in the energy or squared amplitude of the location event with an increase in the loading of the object. This criterion is used to assess the state of objects whose structure and material properties are not exactly known. This circumstance makes this criterion practically significant, especially when diagnosing in the field;
· integral-dynamic, which classifies the AE source depending on its type and rank. The type of source is determined by the dynamics of energy release, based on the amplitude of AE signals over the observation interval. The rank of a source is determined by calculating its concentration coefficient C and total energy . To calculate the concentration coefficient, it is necessary to determine - the average radius of the AE source. At the same time, the value of acoustic emission devices is not determined, which prevents the application of this criterion in practice;
· ASME code criteria intended for zone location and requiring knowledge of admissible values of AE parameters, which presupposes a preliminary study of the properties of controlled materials and consideration of the test object as an acoustic channel.
The MONPAC technology provides for the classification of AE sources in accordance with the value of "Force Index" and "Historical Index". The class is determined by a planar diagram depending on the value of these indices. This classification is used in the MONPAC technology using PAC (Physical Acoustics Corporation) equipment.
According to the criteria of continuous AE, usually controlled during leak detection, the situation is classified as follows:
Class 1 - no continuous AE;
· class 4 - registration of continuous AE.
For the occurrence of the AE effect, the release of energy is necessary. The regularities of AE radiation of a material caused by dynamic local rearrangement of its structure, including both plastic deformation and the formation and growth of cracks, are studied during mechanical tension of the corresponding samples.
As a rule, AE during plastic deformation is a continuous type emission, which has the form of a continuous radio signal similar to noise. To characterize the AE process, the value of acoustic emission is often used - a parameter that takes into account both the number of pulses and their amplitude, proportional to the product of the activity or count rate and the average value of the signal amplitude per unit time. For most metals during their plastic deformation, the maximum activity, counting rate, and effective AE value coincide with the yield strength.
Figure 10.3 shows the dependence of the effective value of AE () in tension of smooth samples, combined with the stress ()-strain diagram () . Dependence 1 corresponds to iron-Armco and low-carbon steel (with carbon content up to 0.015%) and is a continuous AE with a maximum in the zone of the yield tooth (platform). Dependence 2 is typical for structural carbon steel containing carbides, and, in addition to continuous AE, it includes separate high-amplitude pulses associated with the destruction of cementite plates in steel pearlite.
Figure 10.3.The dependence of the effective value of AE (U) in tension of smooth samples, combined with the diagram of stress () - strain ()
The maximum AE activity in the zone of the tooth and the yield point is explained by the mass formation and displacement of defects (dislocations) of the crystal lattice during the transition to plastic deformation and the accumulation of irreversible changes in the structure. Then the activity decreases due to the fact that the movement of newly formed dislocations is limited by the already existing ones. When reloading, the effect of "irreversibility" is manifested, called the Kaiser effect. It consists in the fact that during repeated loading after a short period of time at a fixed sensitivity level of the equipment, AE is not recorded until the previously achieved load level is exceeded. In fact, AE signals appear from the very beginning of loading, but their magnitude is so small that it is below the sensitivity level of the equipment. At the same time, upon repeated loading after a long time, AE is recorded at a load level that is lower than that previously achieved. This effect, called the Felicita effect, is explained by the reverse movement of dislocations when the load is removed.
The greatest danger is presented by crack-like defects, the development of which in most cases leads to accidents and structural failures. The formation and growth of a crack occur abruptly and are accompanied by various separate impulses of the corresponding amplitude. In materials with both natural cracks and artificial notches, stress concentration occurs at the tip of the defect when the object is loaded with working or test loads. When the local stress reaches the yield strength of the material, a plastic deformation zone is formed. The volume of this zone is proportional to the level of stresses, which are characterized by the intensity factor of these stresses To. When local stresses exceed the tensile strength, a microfracture occurs - an abrupt increment in the length of the defect, accompanied by an AE pulse. Number of pulses N grows with the increase To. Total AE dependence N on the stress intensity factor To has the form
The amplitude of AE signals during crack growth can reach 85 dB or more. For plastic deformation, the amplitude of AE signals usually does not exceed 40...50 dB. Thus, the difference in AE amplitudes is one of the main features of the difference between plastic deformation and crack growth.
The results of AE control are presented in the form of a list of registered AE sources assigned to a particular class using the accepted criterion. The location of the source is indicated on the development of the surface of the controlled object (Figure 10.4). The assessment of the state of the controlled object, in turn, is carried out by the presence of AE sources of one class or another in it.
Figure 10.4.The layout of AE sources on the vessel scan and the location of the registered defects:
1 - shell 1; 2 - shell 2; 3 - air inlet; 4 - shell 3; 5 - bottom bottom; 6 - condenser drain fitting; 7 - manhole; 8 - pressure gauge fitting; 9 - safety valve fitting; 10 - top bottom; I‑VIII - numbers of AE receivers
With a positive assessment of the technical condition of the object based on the results of AE control or the absence of registered sources of AE, the use of additional types of control is not required. When AE sources of the second and third classes are detected, additional types of non-destructive testing are used to assess the acceptability of the identified AE sources.
AE control equipment
The structure of the AE control equipment is determined by the following main tasks: receiving and identifying AE signals, their amplification and processing, determining the values of signal parameters, fixing the results and issuing information. The equipment differs in the degree of complexity, purpose, transportability, as well as the class, depending on the amount of information received. The most widespread is multichannel equipment, which, along with AE parameters, allows determining the coordinates of signal sources with simultaneous recording of test parameters (load, pressure, temperature, etc.). The functional diagram of such equipment is shown in Figure 10.5.
Figure 10.5.Functional diagram of AE control equipment
The equipment includes the following main elements connected by cable lines: 1 - acoustic emission transducers (AEC); 2 - preliminary amplifiers; 3 - frequency filters; 4 - main amplifiers; 5 - signal processing units; 6 - the main processor for processing, storing and presenting the results of control; 7 - control panel (keyboard); 8 - video monitor; 9 - sensors and cable lines of parametric channels.
Equipment elements 3 - 8, as a rule, are structurally performed in the form of a single block (shown in Figure 10.5 by a dotted line) based on a laptop computer.
The acoustic emission transducer serves to convert elastic acoustic vibrations into electrical signals and is the most important element of the AE control hardware complex. The most widespread are piezoelectric AETs, the scheme of which differs little from piezoelectric transducers (PTs) used in ultrasonic testing.
By design, the following types of PAE are distinguished:
single-pole and differential;
resonant, broadband or bandpass;
Combined with preamplifier or uncombined.
According to the level of sensitivity, AETs are divided into four classes (1-4th), according to frequency ranges - into low-frequency (up to 50 kHz), standard industrial (50 ... 200 kHz), special industrial (200 ... 500 kHz) and high-frequency (more than 500 kHz). The damping of elastic vibrations decreases with decreasing frequency; therefore, low-frequency AETs are primarily used in monitoring extended objects, such as pipelines and objects with high damping of vibrations.
Special AES are used to control small objects with a length of up to 1 m, high-frequency - in laboratory studies.
Depending on the amplitude-frequency characteristic, resonant AETs are distinguished (bandwidth is 0.2, where is the operating frequency of AET), bandpass (bandwidth is 0.2 ... 0.8) and broadband (bandwidth is more than 0.8).
The main difference between AET and direct PETs lies in the features of the damping necessary to dampen the free natural oscillations of the piezoelectric plate, as well as in the thickness of the piezoelectric plate itself. The back side of the PAE piezoelectric plate can remain free or partially or completely damped.
One of the main characteristics of the AET is the conversion coefficient k, determined from the expression
where is the maximum electrical voltage on a piezoelectric plate, V; - maximum elastic displacement of particles of the controlled object directly under the AET, m.
The conversion coefficient has the dimension V/m and determines the sensitivity of the AET. The maximum value of k takes place in narrow-band resonant AETs, the back side of which is not damped. Mechanical damping leads to equalization of the AET sensitivity in a wider range, however, the absolute sensitivity (conversion coefficient k) is significantly reduced in this case.
The AET is fixed on the surface of the test object in various ways: with the help of glue, clamps, clamps, magnetic holders, with the help of permanently installed brackets, etc. In the practice of industrial AE testing, mainly resonant AETs are used, since their sensitivity is much higher. The design of one of these converters is shown in Figure 10.6.
Figure 10.6.Diagram of a resonant AET designed by CJSC Eltest:
1 - leaf spring;
2 - permanent magnet of the magnetic holder;
3 - body; 4 - clamping cap;
5 - self-aligning spherical bracket;
6 - electrical connector; 7 - piezoelectric element;
8 - ceramic protector
The fastening of the PAE is carried out using a magnetic clamp. To ensure maximum sensitivity, the back side of the plate is made free, and the side surface is damped only by 30% with a compound.
The acoustic emission transducer is connected by a short cable (not longer than 30 cm) to the preamplifier (see Figure 10.5). Along with amplification (usually up to 40 dB), the preamplifier improves the signal-to-noise ratio when transmitting a signal over a cable line to the main equipment unit (3 - 8), remote at a distance of up to 150 ... 200 m.
The filter sets the frequency pass spectrum. The filter is adjusted in such a way as to cut off the noise of various frequencies as much as possible.
The main amplifier is designed to amplify the signal attenuated after passing through the cable line. It has a uniform frequency response with a gain of 60...80 dB.
To suppress electromagnetic interference, the entire channel, including the PAE, preamplifier, main unit and connecting cable lines, is shielded. Often, a differential method of suppressing electromagnetic interference is also used, based on the fact that the PAE piezoelectric plate is cut into two parts and one half is turned over, thus changing its polarization. Further, the signals from each half are amplified separately, the phase of the signals on one of the halves is changed by l, and both signals are added. As a result, electromagnetic interference is out of phase and suppressed.
The signal processing unit fixes the time of their arrival, registers signals above the set discrimination level, converts the signals into digital form and stores them. The final processing of AE signals recorded through different channels is carried out using the main processor, which also determines the location (location) of the AE signal source. When monitoring a linear object (for example, a pipeline), it is enough to have two AETs; for planar objects with comparable overall dimensions and a large surface area, at least three AETs surrounding the source.
Signals from a crack type AE source are characterized by the fact that they are emitted by one source, they are short-lived, and the time of their arrival at the AET reflects the distance to the crack. The position of the AE source on the plane is found by triangulation methods. Based on the speed of wave propagation in the material and the difference in the times of signal arrival at different AETs, the location of the set of points for the AE source is calculated, which will be located on circles with radii , and from the corresponding AETs (Figure 10.7, a). The only true position of the AE source is determined by solving triangles for which all three sides are known. To do this, the AET coordinates on the product are fixed with the highest possible accuracy and are entered into block 6 on the surface scan before testing (see Figure 10.5).
Figure 10.7.AE sources location schemes:
a - planar (on a plane); b - linear
The linear location scheme is shown in Figure 10.7, b. If the AE source is not located in the middle between the AET, then the signal at the far AET will arrive later than at the near one. Having fixed the distance between the AET and the time difference of the signal arrival time, the coordinates of the location of the defect are calculated using the formulas
The AE method allows you to control the entire surface of the test object. To carry out the control, direct access to the surface areas of the control object for the installation of the AET should be provided. If this is not possible, for example, when carrying out periodic or continuous monitoring of underground main pipelines without releasing them from soil and isolation, waveguides permanently fixed on the controlled object can be used.
The location accuracy must be at least equal to two wall thicknesses or 5% of the distance between AETs, whichever is greater. The errors in the calculation of coordinates are determined by the errors in measuring the time the signal arrives at the transducers. Sources of errors are:
· measurement error of time intervals;
difference between real ways of propagation and theoretically accepted ones;
the presence of anisotropy in the speed of signal propagation;
change in the shape of the signal as a result of propagation through the structure;
Overlay of signals in time, as well as the action of several sources;
registration by wave converters various types;
· error in measuring (setting) the speed of sound;
· the error in setting the AET coordinates and the use of waveguides.
Before loading the object, the operability of the equipment is checked and the error in determining the coordinates using a simulator is estimated. It is installed at the selected point of the object and the readings of the coordinate system are compared with the real coordinates of the simulator. A piezoelectric transducer excited by electric pulses from a generator is used as a simulator. For the same purpose, the so-called Su-Nielsen source (a fracture of a graphite rod with a diameter of 0.3 ... 0.5 mm, hardness 2T (2H)) can be used.
Visualization of the location of AE sources is carried out using a video monitor, on which the sources are displayed in the appropriate place on the scan of the controlled object (see Figure 10.4) in the form of luminous dots of different brightness, color or shape (depending on the used software). Documentation of control results is carried out with the help of appropriate peripheral devices connected to the main processor.
The above method for determining the location of AE sources, based on measuring the difference in signal arrival times, can only be used for discrete AE. In the case of continuous AE, it becomes impossible to determine the signal delay time. In this case, the coordinates of the AE source can be determined using the so-called amplitude method based on measuring the signal amplitude by different AETs. In the practice of diagnostics, this method is used to detect leaks through through holes of the controlled product. It consists in constructing a bar graph of the amplitude of the source signal received by various AETs (Figure 10.8). Analysis of such a histogram makes it possible to identify the area of the leak location. Convenient for diagnosing such linear objects as oil and gas pipelines.
Diagnostic monitoring systems based on the AE method are the most versatile. The hardware solution of such a system usually includes:
Figure 10.8. Illustration of the amplitude method for determining AE sources: 1-7 - numbers of AE receivers
· typical blocks of acoustic emission equipment;
· Blocks for coordination and switching of all types of primary converters of additional types of non-destructive testing, the composition of which is determined by the type of controlled object;
blocks of control and decision-making based on the results of diagnostic information about current state controlled object.
Figure 10.8.Illustration of the amplitude method for determining AE sources: 1-7 - numbers of AE receivers
Procedure and scope of AE control
An appropriate control technology is developed for each object. AE control work begins with the installation of AET on the object. Installation is carried out directly on the cleaned surface of the object, or an appropriate waveguide must be used. To locate AE sources on a bulk object with a large surface area, AES are placed in the form of groups (antennas), each of which uses at least three transducers. On a linear object, two AES are used in each group. The placement of the AET and the number of antenna groups is determined by the configuration of the object and the optimal placement of the AET associated with the signal attenuation and the accuracy of determining the coordinates of the AE source.
Depending on the configuration, the object is divided into separate elementary sections: linear, flat, cylindrical, spherical. For each section, select the appropriate layout of the transducers. The distance between AETs is chosen in such a way that the signal of the AE simulator (a break in the graphic rod) located anywhere in the controlled area is detected by the minimum number of transducers required to calculate the coordinates.
The location of the AET should, as a rule, ensure control of the entire surface of the object. However, in some cases, especially when monitoring large-sized objects, it is allowed to place AETs only in those areas of the object that are considered the most important.
After installing the AET on a controlled object, the AE system is checked for operability using an AE simulator located at a certain distance from each AET. The deviation of the registered AE signal amplitude should not exceed ±3dB average value for all channels. The channel gain and the amplitude discrimination threshold are selected taking into account the expected range of AE signal amplitudes. Perform other checks provided for by the control technology of this object.
AE control of the technical condition of the examined objects is carried out only when a stress state is created in the structure, which initiates the work of AE sources in the material of the object. To do this, after completing the preparatory and adjustment work, the object is subjected to loading by force, pressure, temperature field, etc. The choice of the type of load is determined by the design of the object and the conditions of its operation, the nature of the tests and is given in the technology of AE control of a particular object.
The acoustic emission method refers to diagnostics and is aimed at identifying the state of pipeline pre-fracture by determining and analyzing the noise that accompanies the process of formation and growth of cracks.
To register acoustic emission waves, equipment is used that operates in a wide frequency range - from kHz to MHz.
When testing, the application of a load leads to the appearance of an acoustic signal in the pre-fracture zone. Information about signal propagation time, its amplitude, frequency spectrum etc. perceived by piezoelectric acoustic sensors. The processing of the received information serves as the basis for the conclusion about the nature, location and growth of the defect.
Sources of acoustic emission. AE signal control
When destroyed, almost all materials emit sound, that is, they emit acoustic waves that are perceived by ear. Most structural materials (for example, many metals and composite materials) begin to emit acoustic vibrations in the ultrasonic (inaudible) part of the spectrum under loading long before failure. The study and registration of these waves became possible with the creation of special equipment.
Acoustic emission (emission - emission, generation) is understood as the occurrence in a medium of elastic waves caused by a change in its state under the influence of external or internal factors. The acoustic emission method is based on the analysis of these waves and is one of the passive methods of acoustic control. In accordance with GOST 27655--88 “Acoustic emission. Terms, definitions and designations” the mechanism of excitation of acoustic emission (AE) is a set of physical and (or) chemical processes occurring in the object of control. Depending on the type of process, AE is divided into the following types:
* AE material caused by dynamic local rearrangement of its structure;
*AE of friction caused by the friction of surfaces of solids in places where the load is applied and in joints where compliance of the mating elements takes place;
* AE of a leak caused by the interaction of a liquid or gas flowing through the leak with the walls of the leak and the surrounding air;
* AE in chemical or electrical reactions resulting from the occurrence of appropriate reactions, including those accompanying corrosion processes;
* magnetic and radiation AE arising, respectively, during the remagnetization of materials (magnetic noise) or as a result of the interaction of ionizing radiation with it;
* AE caused by phase transformations in substances and materials.
Thus, AE is a phenomenon that accompanies almost all physical processes occurring in solids and on their surfaces. The ability to detect a number of types of AE due to their smallness, especially AEs that occur at the molecular level, during the movement of defects (dislocations) of the crystal lattice, is limited by the sensitivity of the equipment, therefore, in the practice of AE control of most industrial facilities, including oil and gas industry facilities, the first three types are used. AE. It should be kept in mind that AE friction creates noise, leads to the formation of false defects, and is one of the main factors complicating the application of the AE method. In addition, only the strongest signals from developing defects are recorded from AE of the first type: during crack growth and during plastic deformation of the material. The latter circumstance makes the AE method of great practical importance and determines its wide application for the purposes of technical diagnostics. The purpose of AE testing is detection, determination of coordinates and tracking (monitoring) of sources of acoustic emission associated with discontinuities on the surface or in the volume of the wall of the test object, welded joint and manufactured parts and components. All indications caused by AE sources should, if technically possible, be evaluated by other methods of non-destructive testing.
The registration of the signal from the AE source is carried out simultaneously with the noise of a constant or variable level. Noise is one of the main factors that reduce the effectiveness of AE control. Due to the variety of causes that cause their appearance, noises are classified depending on:
* generation mechanism (source of origin) - acoustic (mechanical) and electromagnetic;
* type of noise signal -- pulse and continuous;
* source locations -- external and internal.
The main sources of noise during AE control of objects are:
* splashing of liquid in a container, vessel or pipeline when it is filled;
* hydrodynamic turbulent phenomena at high loading speed;
* friction at the points of contact of the object with supports or suspension, as well as in joints that have compliance;
* operation of pumps, motors and other mechanical devices;
* the action of electromagnetic pickups;
* environmental impact (rain, wind, etc.);
* own thermal noise of the AE converter and the noise of the input stages of the amplifier (preamplifier).
To suppress noise and extract a useful signal, two methods are usually used: amplitude and frequency. Amplitude consists in setting a fixed or floating level of the discrimination threshold U n , below which the equipment does not register AE signals. A fixed threshold is set in the presence of noise at a constant level, a floating threshold - variable. The floating threshold U n , which is set automatically by tracking the overall noise level, makes it possible, in contrast to the fixed one, to exclude the registration of part of the noise signals as an AE signal.
The frequency method of noise suppression consists in filtering the signal received by the AE receivers using low- and high-frequency filters (LPF/HPF). In this case, to adjust the filters, the frequency and level of the corresponding noises are preliminarily estimated before testing.
After the signal passes through the filters and the amplifying path, along with the transformation of waves on the surface of the controlled product, further distortion of the initial pulses of the AE source occurs. They acquire a bipolar oscillating character. The further procedure for processing signals and using them as an informative parameter is determined by computer programs for data acquisition and post-processing used in the corresponding equipment of various manufacturers. The correctness of determining the number of events and their amplitude will depend not only on the possibility of their registration (resolution of the equipment), but also on the method of registration.
After processing the received signals, the control results are presented in the form of identified (in order to exclude false defects) and classified AE sources.
The revealed and identified sources of AE are recommended to be divided into four classes:
* the first one is a passive source registered to analyze the dynamics of its development;
* the second is an active source requiring additional control using other methods;
* the third is a critically active source that requires monitoring the development of the situation and taking measures to prepare for a possible load shedding;
* the fourth is a catastrophically active source, requiring an immediate reduction in load to zero or to a value at which the source activity drops to the level of the second or third class.
Given the large number of parameters characterizing AE, the assignment of sources to the corresponding class is carried out using a number of criteria that take into account a set of parameters. The choice of criteria is carried out according to PB 03-593-03, depending on the mechanical and acoustic-emission properties of the materials of the controlled objects. The criteria include the following:
* amplitude, based on the registration of pulse amplitudes (at least three from one source) and their comparison with the value of exceeding the threshold (A,), which corresponds to the growth of a crack in the material.
* integral, based on a comparison of the assessment of the activity of AE sources F with the relative strength of these sources J k in each registration interval.
* locally dynamic, using the change in the number of AE of location events at pressure holding stages and the dynamics of changes in the energy or squared amplitude of the location event with an increase in the load of the object. This criterion is used to assess the state of objects whose structure and material properties are not exactly known.
* integral-dynamic, which classifies the AE source depending on its type and rank. The type of source is determined by the dynamics of energy release, based on the amplitude of AE signals over the observation interval. The source rank is determined by calculating its concentration coefficient C and total energy E.
* ASME code criteria intended for zone location and requiring knowledge of the permissible values of the AE parameters, which involves a preliminary study of the properties of the controlled materials and taking into account the test object as an acoustic channel.
The AE method allows you to control the entire surface of the test object. To carry out the control, direct access to the surface areas of the control object for the installation of the AET should be provided. If this is not possible, for example, when carrying out periodic or continuous monitoring of underground main pipelines without releasing them from soil and isolation, waveguides permanently fixed on the controlled object can be used.
Before loading the object, the operability of the equipment is checked and the error in determining the coordinates using a simulator is estimated. It is installed at the selected point of the object and the readings of the coordinate system are compared with the real coordinates of the simulator. A piezoelectric transducer excited by electric pulses from a generator is used as a simulator.
Visualization of the location of AE sources is carried out using a video monitor, on which the sources are displayed in the appropriate place on the scan of the controlled object (see Fig. 1) in the form of luminous dots of different brightness, color or shape (depending on the software used). Documentation of control results is carried out with the help of appropriate peripheral devices connected to the main processor.
In the case of continuous AE, it becomes impossible to determine the signal delay time. In this case, the coordinates of the AE source can be determined using the so-called amplitude method based on measuring the signal amplitude by different AETs. In the practice of diagnostics, this method is used to detect leaks through through holes of the controlled product. It consists in constructing a bar graph of the amplitude of the source signal received by various AETs. Analysis of such a histogram makes it possible to identify the area of the leak location. Convenient for diagnosing such linear objects as oil and gas pipelines.
Diagnostic monitoring systems based on the AE method are the most versatile. The hardware solution of such a system usually includes:
* typical blocks of acoustic emission equipment;
* Units for coordination and switching of all types of primary converters of additional types of non-destructive testing, the composition of which is determined by the type of controlled object;
* blocks of control and decision-making based on the results of diagnostic information about the current state of the controlled object.
An appropriate control technology is developed for each object. AE control work begins with the installation of AET on the object. Installation is carried out directly on the cleaned surface of the object, or an appropriate waveguide must be used. To locate AE sources on a bulk object with a large surface area, AES are placed in the form of groups (antennas), each of which uses at least three transducers. On a linear object, two AES are used in each group.
Control is carried out only when a stress state is created in the structure, which initiates the work of AE sources in the material of the object. To do this, the object is subjected to loading by force, pressure, temperature field, etc.
Supervision and control should be carried out at all stages of testing. Some types of defects manifest themselves during the release of pressure. Thus, when the pressure decreases, signals arise from the friction of the edges of the cracks when they close. Defects such as bulges, which most often occur when the metal is hydrogenated and manifested in the delamination of the metal in thickness, are also detected at the pressure release stage (bulges are well detected visually under oblique illumination, sometimes they are felt when pressed by hand). To confirm their presence, ultrasound methods are usually used.
During the loading process, it is recommended to continuously monitor the overview picture of the AE radiation of the test object on the monitor screen. Tests are terminated ahead of schedule in cases where the registered AE source belongs to the fourth class. The object must be unloaded, the test is either terminated, or the source of the AE is clarified and the safety of continuing the tests is assessed. A fast (exponential) increase in the total count, pulse amplitude, energy, or MARSE can serve as an indicator of accelerated crack growth leading to failure.
The characteristic features of the AE testing method, which determine its capabilities and scope, are the following:
* AE control method provides detection and registration of only developing defects, which allows classifying defects not by size, but by their degree of danger. At the same time, large-sized defects can fall into the class of non-hazardous ones, which significantly reduces losses due to rejection. At the same time, with the development of a dangerous growing defect, when its dimensions approach a critical value, the amplitude of AE signals and the rate of their generation increase sharply, which leads to a significant increase in the probability of detecting such an AE source and increases the reliability of the equipment in operation;
* The sensitivity of the AE control method is very high. It makes it possible to detect crack increments of the order of fractions of a millimeter under operating conditions, which significantly exceeds the sensitivity of other methods. The position and orientation of the object do not affect the detectability of defects;
* the integral property of the AE control method provides control of the entire object using one or more AE control transducers fixedly installed on the surface of the object;
* AE control method provides the possibility of testing objects without removing their hydro- or thermal insulation. To carry out the control, it is enough to open the insulation only in the places where the converters are installed, which greatly reduces the amount of restoration work;
* the method provides the possibility of carrying out remote monitoring of inaccessible objects, such as underground and underwater pipelines, devices of closed structures, etc.;
* the method allows for the control of various technological processes and processes of changing the properties and state of materials and has fewer restrictions associated with their properties and structure;
* in the control of industrial facilities, the method in many cases has the maximum value of the efficiency/cost ratio.
A significant disadvantage of the method is the difficulty in separating the useful signal from noise when the defect is small. Another significant drawback of the method, along with the high cost of the equipment, is the need highly qualified AE control operator.
The structure of the AE control equipment is determined by the following main tasks: reception and identification of AE signals, their amplification and processing, determination of the values of signal parameters, fixation of results and output of information. The equipment differs in the degree of complexity, purpose, transportability, as well as the class, depending on the amount of information received.
The most widespread is multichannel equipment, which, along with AE parameters, allows determining the coordinates of signal sources with simultaneous recording of test parameters (load, pressure, temperature, etc.).
The AET is fixed on the surface of the test object in various ways: with the help of glue, clamps, clamps, magnetic holders, with the help of permanently installed brackets, etc. In the practice of industrial AE testing, mainly resonant AETs are used, since their sensitivity is much higher.
The fastening of the PAE is carried out using a magnetic clamp. To ensure maximum sensitivity, the back side of the plate is made free, and the side surface is damped only by 30% with a compound.
Figure 2 - Scheme of the location of AE sources on the development of the vessel and the location of the registered defects: 1 - shell 1; 2 -- shell 2; 3 - air inlet; 4 -- shell 3; 5 -- bottom bottom; 6 - condenser drain fitting; 7 -- manhole; 8 - pressure gauge fitting; 9 - safety valve fitting; 10 -- top bottom; I--VIII -- numbers of AE receivers
Currently, a number of systems are operated on pipelines, the operation of which is based on various physical principles.
Acoustic systems register in the acoustic frequency range the waves generated by leaks. These systems include: SNKGN-1, SNKGN-2 (Introscopy Research Institute at Tomsk Polytechnic University); "LeakWave" (firm "Energoavtomatika", Moscow); "Kapkan" (LLC "Project-resource", Nizhny Novgorod); "WaveAlert Acoustic Leak Detection System" (Acoustic Systems Incorporated, USA); "Leak and Impact / Shock Detection System L.D.S." (France).
Parametric systems are based on the measurement of pressure and flow of the pumped product. Systems are also proposed that operate on other physical principles, among which, in particular, it should be noted a vibroacoustic monitoring system based on a fiber-optic cable; fiber-optic sensor (cable) for detecting oil and oil products leaks; operational remote control of leaks, based on the measurement of the conductivity of the insulating coating of the pipeline.
Acoustic and parametric systems have advantages over others due to higher technical characteristics and economic indicators. When comparing systems, a significant indicator is the cost of equipment, its installation and ongoing maintenance per 1 km of pipeline length. And if the characteristics of the two systems are comparable, then preference is given, of course, to an economically more attractive development.
An analysis of economic indicators allows us to conditionally divide the listed systems into two cost groups (distributed and extended systems), which differ in the way the equipment is installed on the pipeline:
in distributed systems, recording modules are installed on the pipeline, as a rule, at a considerable distance from each other and use available communication channels - radio channel, satellite, telemechanical, fiber optic. This group includes acoustic and parametric systems;
in extended systems, the installed equipment requires laying an additional communication channel along the pipeline.
For distributed systems, the cost of equipment, installation and ongoing maintenance per 1 km is about 10 times lower compared to extended systems.
At the same time, analysis specifications of these systems shows that they provide registration of large leaks, accompanied by a pressure drop, and have a sensitivity limit, which is about 1% of the pipeline capacity. At the same time, leaks with a low intensity (less than 1%) are not recorded by such systems. So, for example, at a capacity of 2000 m 3 /h, a system with a sensitivity of 1% is only able to detect a leak with an intensity of 333.3 l/min or more.
The sensitivity of the considered systems is limited by the "noise" of the measured parameters. AT recent times the productivity of main pipelines is growing, which leads to an increase in "noise" and a decrease in the sensitivity of systems. The implementation of only one function of monitoring the technical condition in acoustic systems is their significant drawback.
To provide several functions, such as leak detection, pipeline protection, tracking (location control) of inline devices, it is necessary to install 3 different systems, which leads to a decrease and reliability in the implementation of individual functions and an increase in overall costs.
