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Acoustic emission testing--defining a new standard of acoustic emission testing for pressure vessels: Part 1: quantitative and comparative performance analysis of zonal location and triangulation methods.

Introduction

Acoustic emission is especially useful in testing of pressure vessels. Indeed, AE enables global and rapid testing of large structures, significantly reducing maintenance time and shutdown of facilities. Methods have changed over the last decades, moving from very traditional and diversified methods to more standardized ones. However, some tests are still currently performed according to procedures, which have more to do with the service provider's "reputation" rather than on a proven technique. The authorities responsible for the safety of facilities in France requested "uniformization" of AE testing methods for pressure vessels: this led to the creation of the Best Practices Guideline (Guide des Bonnes Pratiques--GBP [1]), which has been officially adopted since 2004 and is used as reference for customers and service providers for application of this technology to various pressure vessels.

Several regulatory rules, codes and standards in other parts of the world define the general application rules of this technique via European standards or American ASME Boiler Codes. These rules, like GBP, authorize AE testing according to two techniques (zonal location and planar source location by triangulation or planar method, in short). However, no comparative study of their performance, thus enabling their assessment, has been carried out. Lack of quantitative comparison leads to subjective assessment of performance concerning the techniques and the most cost effective, reputed "basic" solution is often selected. We have no objective answers to the following questions:

What is the real detection capacity of an AE source using these two methods?

What is the coverage ratio of the tested structure?

How can the testing level of two different structures be compared?

By means of simple simulation calculations, this study highlights the significant differences in performance between these two techniques. The effects of other fundamental parameters in the use of AE, for example, acquisition threshold, are also quantified.

A. Analysis of Performance for the Zonal Location and Triangulation Methods

A.1. Definition of the case studied--context

The performance for both techniques used will be compared based on real cases, dealt in accordance with the recommendations from the Best Practices Guideline (GBP) used as regulation in France. It should be noted that several other European or American rules are not much different from the GBP and lead to similar testing configurations.

This analysis uses a specific application case. It represents several pressure vessels as regards attenuation values: it is a spherical storage tank with a 35-mm-thick wall; this wall is painted and coated with thermal insulating material. The AE wave attenuation curve (the frequency of the AE transducers is near 200 kHz) obtained from the Hsu-Nielsen source is shown in Fig. 1.

[FIGURE 1 OMITTED]

Using the GBP recommendations as basis, the maximum allowed distances between sensors for this case are:

--for zonal location, the maximum authorized distance between sensors is 1.5 times [Distance at the assessment threshold = 50 [dB.sub.AE] maximum]; that is, in this case, approximately 1.5 x 4 = 6 m.

--for planar location case, the maximum authorized distance between sensors of a single mesh, in the case of a maximum acquisition of 50 [dB.sub.AE], is equal to the distance to the acquisition threshold + 6 dB; that is approximately 2.5 m.

A very different number of sensors would therefore be needed for the two testing configurations: a 10-m diameter spherical tank would require approximately 50 to 60 sensors in planar location against approximately 20 sensors for zonal location.

What is the detection performance of each of these configurations and how can this performance be quantified?

A.2. Performance analysis--Calculation of source-sensor distances

In order to assess the two testing configurations, the detectability performance in each case is determined:

--for zonal location, by calculating the distance separating each point of the structure from the closest sensor,

--for planar location, by calculating the distance separating each point of the structure from the last sensor used for calculating the location (the 3rd sensor reached is used for these calculations).

[FIGURE 2a OMITTED]

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In conclusion, these various mappings show:

--For zonal location configuration, (distance between sensors = 6.0 m), the point of the structure that is hardest to detect with this method is situated at 3.4 m. For the same configuration, planar location requires distances to the 3rd sensor reached between 3.4 m and 5.8 m.

--For planar location configuration (distance between sensors = 2.5 m), the structure point that is hardest to detect with this method is situated at 1.4 m. For the same configuration, planar location requires distances to the 3rd sensor reached ranging between 1.45 m and 2.45 m.

A.3. Performance analysis--Calculation of minimum detectable amplitudes

We will later assess what is the minimum amplitude of a detectable source for each structure point for both location methods in order to better interpret these results and the performance differences that exist between these two testing configurations. The acquisition threshold must be taken into account in these calculations, as it defines the minimum measurable amplitude. In this case, the most unfavorable case authorized by the Best Practices Guideline is used; that is to say, a 50 [dB.sub.AE] acquisition threshold. Figures 3(a) to 3(d) show these results:

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[FIGURE 3b OMITTED]

[FIGURE 3c OMITTED]

[FIGURE 3d OMITTED]

Much information can be drawn from these mappings:

--Zonal testing configuration (6.0 m distance between sensors) enables detection of any AE source with equivalent source amplitude to that of a Hsu-Nielsen source (approximately 100 [dB.sub.AE] initially--98 [dB.sub.AE] used in the modeling).

--Zonal testing configuration only enables restricted application of planar location method for this type of source (see Fig. 3(b)). More specifically, planar location method is only possible on 34 % of the surface (this surface corresponds to that, for which the amplitude calculated is less than 98 [dB.sub.AE]).

--Planar testing configuration (2.5 m distance between sensors) obviously enables detection of any AE source with equivalent source amplitude to that of a Hsu-Nielsen source with the zonal location method.

--Planar testing configuration also enables application of the planar location method on the entire surface (100 %, against 34 % for zonal testing configuration).

When only these considerations are taken into account, the performance differences are therefore relatively low. Indeed, the only difference between both configurations would simply be a loss of 66 % of the planar location surface. However, two significant parameters are not considered in this initial comparison: all real AE sources do not necessarily generate as much energy as a Hsu Nielsen source; furthermore, the measuring error, that it to say, assessment of its amplitude, carried out on the source is not quantified.

A.4. Performance analysis--Consideration of variable amplitude acoustic emission sources

Only the detectability of an AE source equivalent to a Hsu-Nielsen source (0.5 mm - 2H) was considered in the previous calculations. It can be assumed that detectable AE sources in a real structure do not necessarily give off as much energy as a Hsu-Nielsen source. The detectability of a source that is X dB less than a Hsu-Nielsen source was therefore calculated for various source amplitude values. This detectability was moreover quantified in terms of detection ratio (for zonal location), and location ratio (for planar location).

Table 1 shows the performance for both types of testing configurations on detectability (zonal) and planar location. Analysis of this table enables views for given amplitude of the detection and location capability of both testing configurations. The following elements can also be reiterated:

* Amplitude source that is 2 dB less than the reference source:

For the zonal testing configuration, we observe:

** A 99% detection capability

** A 0% location capability

Whereas in the planar testing configuration, this same source can be:

** 100 % detected and

** located at 100%

* For an amplitude source that is 10 dB less than the reference source: For the zonal testing configuration, we observe:

** A 31% detection capability

** A 0% location capability.

Whereas in the planar testing configuration, this same source can be:

** 100% detected and

** located at 15%.

The performance differences for both AE testing methods, which may be carried out in compliance with current rules, can therefore be quantified and qualified by this analysis. By highlighting performance differences, it becomes obvious that AE testing carried out with zonal location method is fundamentally different from testing with the planar location method in terms of sensitivity and information quality.

Thus, when we consider for example that AE sources from the tested structure are included in an amplitude distribution centered on 85 [dB.sub.AE] (amplitudes ranging between 55 and 115 [dB.sub.AE]), for the zonal testing configuration we will note that:

** 34.6 % of the sources can be detected

** 10.6 % of the sources can be located,

[TABLE 1 OMITTED]

Whereas in the planar testing configuration:

** 59.0% of the sources can be detected

** 35.6% of the sources can be located.

A.5. Performance analysis--Assessment of error on the observed amplitude

Detecting an AE source is the first step. However, what is the relevance of the information gathered by the operator? How does the operator view the intensity (or amplitude) of this source? We are here again going to rely on the modeling previously carried out to answer these two questions so as to obtain an estimation of these two factors.

The case of an amplitude distribution of AE sources centered on 85 [dB.sub.AE] will therefore be used. We will assess the detection accuracy by calculating both factors:

** The total percentage of "good" assessment. For example, a source measured at 75 [dB.sub.AE] whereas its initial amplitude is 95 [dB.sub.AE] will give an error of 21.1 %. The total percentage is the mean calculated on a given surface for a given amplitude distribution.

** The mean error expressed in dB: This criterion is the mean of errors, expressed in dB, between the measured amplitude and the initial amplitude. Two values will be differentiated: the first incorporating errors on all AE sources, the second only incorporating errors on the detected sources.

The following results are therefore obtained:

When, for example, we consider that the AE sources generated by the tested structure are comprised of an amplitude distribution centered on 85 [dB.sub.AE] (amplitudes ranging between 55 and 115 [dB.sub.AE]) for the zonal testing configuration, we note:

** An overall detection accuracy of 20.9%

** An overall amplitude measurement error of 64.7 [dB.sub.AE].

** An amplitude measurement error on the AE sources detected of 28.6 [dB.sub.AE].

In a planar testing configuration, we obtain:

** An overall detection accuracy of 39.3%.

** An overall amplitude measurement error of 48.9 [dB.sub.AE].

** An amplitude measurement error on the AE sources detected of 26.1 [dB.sub.AE].

The difference between both configurations is clearly shown and can therefore be quantified using these different criteria.

Performance of both testing configurations for the case studied may be summarized as follows:

** Zonal testing configuration:

--65.4% of the sources are not detected

--34.6% of the sources are detected, with a mean amplitude measurement error of 28.6 [dB.sub.AE]

--10.6% of the sources can be located (included in the detected 34.6%)

** Planar testing configuration:

--41.0% of the sources are not detected

--59.0% of the sources are detected, with a mean amplitude measurement error of 26.1 [dB.sub.AE]

--35.6% of the sources can be located (included in the detected 59.0%).

[FIGURE 4a OMITTED]

[FIGURE 4b OMITTED]

A.6. Performance analysis--Consideration of information provided by planar location, enabling the correction of the measured amplitude

The current AE testing practices do not fully use information provided by planar location: indeed, the rules defining acceptability criteria, for example GBP in France, are restricted to giving the criteria based on the measured amplitude, which, as was shown in the previous section, involves a 26 [dB.sub.AE] error, on average (for the case studied).

When the information provided by planar location is used, that is to say, the specific position of the AE source, the attenuation amplitude measured can be corrected and the real amplitude of the source can be estimated. In this case, any AE source located will be measured without error (or less error). What are the overall performance gains of both testing configurations? By using the calculations carried out in Sec. A.4 (amplitude distribution centered on 85 [dB.sub.AE], amplitudes ranging between 55 and 115 [dB.sub.AE]), we therefore obtain:

For zonal testing configuration:

** An overall detection accuracy of 25 %.

** An overall amplitude measurement error of 60.5 [dB.sub.AE].

** An amplitude measurement error on the AE sources detected of 26.5 dB.

For planar testing configuration:

** An overall detection accuracy of 51.1 %.

** An overall amplitude measurement error of 37.6 [dB.sub.AE].

** An amplitude measurement error on the AE sources detected of 14.7 dB.

It can be noted that using the information connected with location makes it possible to reduce the amplitude measurement error on the detected AE sources:

--Zonal testing configuration: from 28.6 [dB.sub.AE] to 26.5 [dB.sub.AE]

--Planar testing configuration: from 26.1 [dB.sub.AE] to 14.7 [dB.sub.AE]

We can see that full use of the planar testing configuration makes it possible to obtain an information quality that is twice better than the zonal testing configuration with a lower measuring error.

B. Influence of Testing Parameters on Detection Performance

B.1. Influence of the acquisition threshold level

The acquisition threshold is a fundamental parameter influencing the results of an AE test. Indeed, this value sets the minimum detectable amplitude. The rules for determining this value are defined in the existing codes and standards and are based on the following two factors:

--The acquisition threshold value must be X dB above (for example 6 dB) the background noise, so that signals considered as non-representative are not recorded,

--The acquisition threshold value must be less than the "reference" amplitude value used to calculate the activity criteria for example.

Compliance with these two rules, in most cases, implies a certain freedom in choosing the acquisition threshold. Indeed, the most frequently encountered background noise conditions may make it possible to work with acquisition threshold levels less than the maximum authorized level. How does this impact detection of acoustic emission sources?

The effect of lowering the acquisition threshold will be quantified using the same case as that studied in previous sections (where calculations were carried out with the maximum authorized threshold, that is to say 50 [dB.sub.AE]). Figures 5a and 5b summarize these results: it can be noted that the "performance" gain obtained by lowering the threshold from 50 to 40 [dB.sub.AE] is significant as it enables:

** For the zonal testing configuration,

--the undetected source ratio decreased from 65.4 % to 33.2 %.

--the located source ratio to be increased from 10.6 % to 35 %.

** For the planar testing configuration,

--the undetected source ratio decreased from 41 % to 15.1 %.

--the located source ratio increased from 35.6 % to 70.6 %.

[FIGURE 5 OMITTED]

B.2. Influence of the meshing type used

The meshing used in the tested structure is determined by several factors including the attenuation curve and the presence of specific structural elements. Some constraints (access for example) may also prevent installation of the sensors in specific areas of the structure. The operator may have to use triangular, rectangular or other meshing depending on these elements. In order to evaluate the performance of the control, the influence of the meshing geometry on the quality of detection and location must be assessed and quantified. Here, the differences between triangular and rectangular meshing for a given case will be highlighted. Figures 6(a) and 6(b) show the mapping differences (minimum detectable amplitude): note that the topography is different. Whereas in the case of a triangular mesh the most "critical" regions are those close to sensors, they are found in the middle of segments between sensors for rectangular meshing.

[FIGURE 6a OMITTED]

[FIGURE 6b OMITTED]

B.3. Influence of the minimum number of sensors used in the location calculations

The meshing used in the tested structure (that is to say sensor coordinates) is not the only input data necessary for the location to be calculated. The number of sensors taken into account in the calculation is also an important factor influencing the result obtained as regards position and accuracy. As a rule, location algorithms (planar location) use three sensors or more by default when the information on the fourth or nth sensor is available. The operator is able to filter calculations and can select only the calculation, which used four sensors in order to obtain better location accuracy for example.

Increasing from three to four sensors minimum in the location calculation conditions significantly changes the results. The differences for a triangular meshing between location using at least 3 sensors and that using at least 4 sensors will be shown here. Figure 7 in comparison to Fig. 6a shows the mapping differences (minimum detectable amplitudes): note that the topography is different as well as the values of the minimum detectable amplitude (difference recorded in this case from 2 to 3 [dB.sub.AE]). Using an additional sensor in the location calculation causes a loss of location capability.

[FIGURE 7 OMITTED]

C. Defining a New AE Testing Assessment Methodology

C.1. Definition of an acoustic emission testing assessment methodology

The study carried out in the previous sections illustrates that current AE testing practices defined and authorized by the various regulatory rules, codes and standards whether in France or elsewhere, may result in extremely varied performance levels. As a simple example, let us compare AE testing carried out in zonal configuration and with a 50 [dB.sub.AE] acquisition threshold with testing using planar location configuration with a 40 [dB.sub.AE] threshold (amplitude distribution of the sources centered on 85 [dB.sub.AE] with amplitudes ranging between 55 and 115 [dB.sub.AE]):

** Zonal testing configuration, threshold = 50 [dB.sub.AE]:

--65.4 % of the sources are not detected

--34.6 % of the sources are detected

--10.6 % of the sources are located (included in the 34.6 % detected)

** Planar testing configuration, threshold = 40 [dB.sub.AE]:

--15.1 % of the sources are not detected

--84.9 % of the sources are detected

--70.6 % of the sources are located (included in the 84.9 % detected)

How can these significant sensitivity differences be taken into account, given that there is no possible comparison between these two testing methods!

Based on the approach developed and described in this article, we propose that any AE testing should be "assessed" in terms of location performance expressed using simple and quantitative indicators. This performance calculation will be the same as that described in Section A.5; that is, it will involve calculating the detection and location percentage for a population of AE sources, for example with amplitude centered on 85 [dB.sub.AE] (ranging between 55 and 115 [dB.sub.AE]). As in the previous example, it can be proven using both simple criteria that the first configuration (Zonal testing configuration, threshold = 50 [dB.sub.AE]) is 5 to 7 times less efficient than the second testing configuration (Planar testing configuration, threshold = 40 [dB.sub.AE]).

This assessment would enable:

--Firstly, quantitative comparison of AE testing performance. Instructing parties, users of this technique as well as organization using testing results may take into account the level of testing quality and also request a minimum level of requirements, more specifically set by these criteria. Depending on the criticality of the tested vessel, a minimum level of requirements could be required.

--Classification criteria to be adapted depending on the performance levels of the adopted testing configuration. Indeed, to date, no rule, standard or code defines classification rules incorporating the testing "coverage ratio".

C.2. Changes and perspectives

The calculations performed in this study highlight that AE testing currently conducted is not adequately controlled and is therefore not used to its full potential. There is significant room for improvement for it to be more relevant and more accurate in its diagnosis. This study moreover shows that the following factors should be used today:

--Amplitude correction: it was shown in the specific case developed, that use of amplitude correction enables the mean error as regards amplitude measurements of the detected sources to be decreased by half from 26.1 to 14.7 [dB.sub.AE]. Furthermore, this amplitude correction reinforces the benefit of implementing and using the planar testing configuration to its full potential.

--Knowledge of the less monitored or located areas: this should enable the testing configuration to be better adapted to the structure. A "critical" region may be optimally tested by installing sensors so that they are located in the optimum meshing area.

Conclusions

Acoustic emission is a unique, high potential testing technique as it enables fast and global testing of large structures thus allowing operators to reduce the shutdown times for their facilities. All regulatory rules, codes and standards, which define the general application rules for this technique such as GBP in France, authorize use of AE according to two methods (zonal location and planar location by triangulation). However, no comparative study of their performance, thus enabling their assessment, has been carried out.

From the study carried out using modeling calculations, we are able to determine that the performance differences between these two authorized techniques are significant and may reach a coefficient of 5 to 7, without being considered when analyzing the information gathered; that is, the results of the test. Furthermore, without this quantitative comparison, the "a minimal" solution is often preferred by instructing parties as it is less costly and nevertheless recognized.

We propose that any AE test should be assessed on the basis of the approach developed in this study in terms of location performance by expressing this assessment through quantitative criteria such as detection and location percentage of a defined population of AE sources. These criteria, that is, the testing coverage ratio may be taken into account in analyzing recorded information to obtain a more relevant diagnosis.

Finally, the results of this study show that use of a planar testing configuration must be preferred given that it allows the measuring error levels to be significantly decreased. The impact of the error levels on the testing result is reduced, while enabling calculation of the source amplitude.

Backed by its experience, CETIM may now use these assessment tools and carry out well-controlled AE testing. Nevertheless, the professional guides, standards and codes should change so as to allow the industry to take advantage of the real potential of acoustic emission.

Reference

[1] Guide to good practice for AE testing of pressure equipment, 1st Edition, May 2004. AFIAP (French Association of Pressure Equipment Engineers). Edited by SADAVE. ISBN 2-90631982-1

JOHANN CATTY

CETIM, 52 Avenue Felix Louat, 60304 Senlis Cedex, France
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Article Details
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Author:Catty, Johann
Publication:Journal of Acoustic Emission
Article Type:Report
Geographic Code:4EUFR
Date:Jan 1, 2009
Words:3841
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