NEW APPROACH TO ACOUSTIC EMISSION TESTING METALLIC PRESSURE VESSELS

Aim of the presentation: tti Review the basic principles i of AE examination List the main factors impacting the quality of an acoustic emission test Observe to what extent they are taken into account in conventional AET practice Offer a new approach which corrects the effects of theses factors and improve quality of AE tests on metallic pressure vessels ANVIXED sarl copyright 2015 2

Definition of Acoustic emission phenomenon energy liberation in the form of transient elastic waves, resulting from microdisplacements inside a material under stress A E source load ANVIXED sarl copyright 2015 4

Sources of acoustic emission crack growth local plastic deformation corrosion phase changes inclusion fracture stress concentration are main generating mechanisms But only when evolving with applied load ANVIXED sarl copyright 2015 5

Typical pressure diagram Pressure increase segment Pressure hold segment 2 nd sequence: Completing test up to hydrotest pressure End of 1t 1st sequence: Loading up to design pressure ANVIXED sarl copyright 2015 9

Detection and location mesh CALCULATION OF ACOUSTIC EVENT LOCATION sensor C acoustic event located at X,Y sensor A wave propagation, velocity = V sensor B hit detected by sensor A t0 hit detected by sensor B t1 hit detected by sensor C t2 X,Y = f(t0, t1, t2, V, location of sensors A, B and C) ANVIXED sarl copyright 2015 10

Acoustic Emission signals (1/2) signals emitted by the structure during testing are physical events clusters of physical events are concentrations or sources dues to defect evolution signals detected by sensors are hits a physical event releases waves which reach one or several sensors: one or several hits belong to an acoustic event ANVIXED sarl copyright 2015 13

Acoustic Emission signals (2/2) the processing system transforms hits into detected events (Ev) events for which at less 3 sensors are reached are «located events» (LE) the main characteristics of an AE event, determined on the 1st hit, are: arrival time amplitude energy location All data recorded during the test is stored in the primary file (.PRI) ANVIXED sarl copyright 2015 14

Main factors affecting AE test results & grading (1/6) Group Goup1-Vessel 1 - geometry 2 - material 3 - metal volume 4 - residual stress distribution Group 2 - Process 1 - fluid 2 - maximum previous applied pressure 3 - in-service i conditions i ANVIXED sarl copyright 2015 16

Main factors affecting AE test results & grading (2/6) Group 3- Sensors array 1 - distance between sensors 2 - type of mesh Group 4- Sensors type 1 - sensor sensitivity 2 - sensor bandwidth ANVIXED sarl copyright 2015 17

Main factors affecting AE test results & grading (3/6) Group 5- Acquisition settings 1 - acquisition threshold 2 - other channel parameters 3 - front-end filter set-up Group 6- Loading cycle 1 - delta P above max. previous pressure 2 - pressure rate 3 - hold periods duration ANVIXED sarl copyright 2015 18

Main factors affecting AE test results & grading (4/6) Group 7- Data processing 1 - primary file qualification 2 - filtering 3 - event builder adjustments 4 - data analysis method Group 8- Classification protocol 1 - activity based criteria 2 - intensity based criteria 3 - Kaiser effect criterion 4 - Felicity ratio criterion ANVIXED sarl copyright 2015 19

Main factors affecting AE test results & grading (5/6) Developping item 7.4: data analysis method 1 - Hits analysis signature 2 - Application of an amplitude evaluation threshold 3 - Mixing i acoustic analysis with classification mechanism 4 - Zonal location limitations ANVIXED sarl copyright 2015 20

Main factors affecting AE test results & grading (6/6) Developping item 7.4: data analysis method 5 - Correlation between zonal and planar locations 6 - Correction of activities versus acquisition threshold 7 - Correction of amplitudes versus distance ANVIXED sarl copyright 2015 21

How are these factors managed in current practices? There is a lot of factors conditioning conditioning performances of AE tests: numeric results, acoustic signature, and classification What is more, acquisition & processing settings can be specific to a group of channels / sensors Within the framework of this presentation, we will only browse a few points ANVIXED sarl copyright 2015 22

Item 6: AE test pressure cycle (1/2) pressure of the AE test must be in usual practice 5or 10 % over the maximum historic pressure PMA PMA is the max pressure value applied to the vessel during a specified period of time (generally 12 months) overpressure from PMA aims to provide a sufficient test sensitivity but this usual practice don t guaranteea a convenient stress value ANVIXED sarl copyright 2015 24

Item 6: AE test pressure cycle (2/2) Example from CERN helium tanks 250 m3 / DP=26 bar hydrotest carried out in fab plant up to 39 bar a month later, after installation on site: pneumatic AE examination AE test pressure to apply according to the rule: 43 bar! maximum pressure of the AE test: 26 bar results: satisfactory AE test sensitivity ANVIXED sarl copyright 2015 25

Item 7.3: Event builder parameters influence FHCDT (ms) 1,6 2,5 3 3,5 4 5 6 DT1X max (ms) 10 10 10 10 10 10 10 DTNX Max (ms) 16 1,6 16 1,6 16 1,6 16 1,6 16 1,6 16 1,6 16 1,6 cumul SIGS Σ SIGS 75000 53000 48000 46500 45000 43200 42000 cumul Ev1 cumul Ev (Ev1+) cumul Ev2 Σ Ev 45000 35500 33500 32500 31000 30000 29000 cumul Ev2+ 12000 6500 5800 5500 53000 5000 4800 cumul Ev3 cumul Ev3+ 5850 2900 2500 2400 2300 2200 2150 cumul LE Σ LE 238 131 100 80 77 75 71 ANVIXED sarl copyright 2015 26

Item 7.4.1: Hits or events analysis? (2/2) Hits are «raw» signals, and: hits number is depending on sensors array and number of sensors the hits population includes non significant data as mechanic noise, electromagnetic interference, The hit population cannot describe the global acoustic signature of the vessel The hit population has to be assembled by the «event builder» to create the events population ANVIXED sarl copyright 2015 28

Item 7.4.2: Analysis with amplitude thresholds (2/2) Green dark curve: no filter > true acoustic behavior Green clear curve: amplitude filter (threshold 50 db) > light distorsion of activity evolution versus time Red curve: amplitude filter (threshold 65 db) > strong distorsion of activity evolution versus time ANVIXED sarl copyright 2015 30

Item 7.4.4: Zonal locationboundaries (1/2) the boundary is a line drawn in the middle of the areas between zones thus sensors located on the junction line between two parts of the vessel don t permit to pick up independantly AE activity it from these parts ANVIXED sarl copyright 2015 31

Item 8: Classificationcriterias criterias (1/7) ASTM E 569: applicable to planar location Activity: trend Intensity: relative value Directly applicable Not provider or code dependant Easy to implement ANVIXED sarl copyright 2015 33

Item 8: Classificationcriterias criterias (2/7) ASME V Art 12: applicable to zonal location Activity: value beyond a limit in a given period value beyond a limit for hits with Amp> Ao trend Intensity: trend Not directly applicable Code dependant Not easy to implement ANVIXED sarl copyright 2015 34

Item 8: Classificationcriterias criterias (3/7) MONPAC: applicable tozonal location Activity: no direct criterion Intensity: Relative energy value of a pack of 200 events: H(t) Absolute value of the 10 hits with max energy: Sav Directly applicable Easy to implement ANVIXED sarl copyright 2015 35

Item 8: Classificationcriterias criterias (4/7) EN 14584: applicable to planar location Activity: activity / amplitude mixed criterias trend Intensity: trend Not directly applicable Provider dependant Not easy to apply ANVIXED sarl copyright 2015 36

Item 8: Classificationcriterias criterias (5/7) EN 15495: applicable to zonal location Activity: activity / amplitude mixed criterias Intensity: trend Not directly applicable Provider dependant Not easy to implement ANVIXED sarl copyright 2015 37

Item 8: Classification criterias (6/7) GBP AFIAP: applicable to zonal location Activity: activity / amplitude mixed criterias Kaiser effect / Felicity ratio trend Intensity: trend applicable to planar location (sources) Activity: value above a given value Intensity: no requirements Directly applicable Not easy to implement ANVIXED sarl copyright 2015 38

Item 8: Classificationcriterias criterias (7/7): discussion each protocol is dependant on numerous factors influencing the measurement of acoustic activity and energy three of them are dependant on external requirements Under these conditions, it is difficult to think they can conclude to the same diagnostic! ANVIXED sarl copyright 2015 39

What does new approach really offer? requirements for AE tests providers directions to follow by practitioners principles for specifications, procedures and reporting tools for supervisors of AE tests new concepts for standards improvements ANVIXED sarl copyright 2015 41

General AE data processing (1/2) step 1 - Qualified tests applied to the primary data file (PRI) step 2 - Qualified population extraction from the file (PRI) step 3 - Optimized events building step 4 - Events population analysis according to the different levels of acoustic signature ANVIXED sarl copyright 2015 42

General AE data processing (2/2) step 5 - Acoustic behavior analysis joining zonal and planar location methods (acoustic emission domain) step 6 - Acoustic signatures interpretation (physical domain) step 7 - Application of classification criterias step 8 - Follow-up ND T > according to damage grading step 9 - Reporting All the steps are clearly identified and separately yprocessed ANVIXED sarl copyright 2015 43

Taking into account the main influence factors: reference pressure reference pressure is more realistic than previous maximum operating pressure (MOP) the reference pressure (Pref) is the pressure beyond which the the vessel is considered as overstressed il may be lower than MOP if the vessel has been submitted to various stress situations: temperature stress, lifting, i shipping, i on-site installation i stresses, in-service cycling stress the value is determined by the AET provider and the vessel user, considering the former history of the vessel ANVIXED sarl copyright 2015 44

Taking into account the main influence factors: overloading versus pressure uncertainty pressure uncertainty t is generally around 2 % for an overstress uncertainty limited to 5 %: over pressure uncertainty is 40 %! for an overstress uncertainty limited to 10 %: overpressure uncertainty is 20 % Overloading must reach at less 20 % of the reference pressure ANVIXED sarl copyright 2015 45

Taking into account the main influence factors: Kaiser test & Felicity ratio significant the Kaiser effect test is a major test which is at the basis of AE application to non destructive examination To be valid the Kaiser test demands a sufficient pressure decrease, not compatible with a small overpressure span Relevant to the Kaiser effect is the Felicity ratio which h can be easily and accurately computed ANVIXED sarl copyright 2015 46

Taking into account the main influence factors: overloading span Activities N are dependant on the overpressure span (Pspan) applied to the vessel during the acoustic emission test To compare the activities, one must introduce a factor to «standardise» the pressure span effect Then the activities have to be expressed as the ratio of N to Pspan: K= N / Pspan ANVIXED sarl copyright 2015 47

Taking into account the main influence factors: sensor sensitivity a factor of sensor sensitivity has to be introduced, to do so, a reference sensitivity must be defined then, the sensor sensitivity is expressed as a value in db 6dB for example means that the sensor is 2 times more sensible than the reference one ANVIXED sarl copyright 2015 48

Taking into account the main influence factors: Amplitude / distance correction this point only applies to planar location of AE sources this correction allows to take into account the «mesh density» and the attenuation curve in the metal up to now, only EN 14584 considers this correction ANVIXED sarl copyright 2015 49

Taking in account the main influence factors: Acquisition threshold the acquisition threshold disturbs measurements: value of AE activity cutting of low energy hits distorsion on shape parameters of hits false location of events the amplitude distribution of signals curves allow to correct the effect of acquisition threshold on: AE activities measurements AE energy measurements ANVIXED sarl copyright 2015 50

Taking in account the main influence factors: Acquisition threshold Measured values: amplitude average value: standard deviation: activity: 43 db 10 db 80 Ev Corrected values: amplitude average value: standard deviation (inchanged): activity: 40 db 10 db 100 Ev ANVIXED sarl copyright 2015 55

Consequences: is AET a sampling method? the numerous factors that minuse activity measurement lead to wonder if AE testing is a sampling method taking only a part of indications In fact, the cutting of a part of the event population does not comply with rules of sampling the detection of indications resulting of discontinuities is in general not affected ; only the weak amplitude events are cancelled ANVIXED sarl copyright 2015 59

Consequences: Is AET a qualitative or quantitative method? the numerous factors that minuse activity measurement lead to wonder if AE testing is a sampling method taking only a part of indications In fact, the cutting of a part of the event population does not comply with rules of sampling the detection of indications resulting of discontinuities is in general not affected ; only the weak amplitude events are cancelled ANVIXED sarl copyright 2015 60

Ways to a new grading protocol use of «normalized» activities & amplitudes use of energy as intensity measurement instead if amplitudes criteria application to events and not to hits energy computation with «normalized» values enrichment of sources signatures with zonal activities contribution ANVIXED sarl copyright 2015 61

Conclusion AET is a powerfull tool applied to integrity evaluation of pressure vessels, but new approach implementation will allows: to compare the tests to improve the efficiency and universality through an advanced grading mechanism to go further in standardisation ANVIXED sarl copyright 2015 62

Thanks for your kind attention If you have any question We are also pleased to inform you of the writing of our booklet: «Contribution i to acoustic emission i testing of metallic pressure vessels» (issued in october 2015) Ordering information on request from ANVIXED ANVIXED sarl copyright 2015 63