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ACOUSTIC EMISSION FOR TANK BOTTOM MONITORING

CM National Forum Sydney, 15 – 17 October 2007

Dr Gary Martin MIEAust CPEng – Technical Director

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What is Acoustic Emission?

• Acoustic Emission is the process of detecting sound produced by discontinuities within a specimen when the specimen has stress applied to it.

• The sound is used to determine the specimen’s condition.

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AE Compared to other NDT methodsAE• Detects growth/movement of

discontinuity• Relates to stress field at

discontinuity• Intimate relationship to material

failure• Best suited for global monitoring• Each loading is unique• Less intrusive, easier access• Applicability, technique depend on

material• Less dependant on local geometry

Most Other Methods• Detect presence of discontinuity• Relates to size and shape of

discontinuity• Indirect relationship to material

failure• Best suited for local scanning• Inspections are easier to repeat• More intrusive, harder access• Applicability, technique depend

on material• More dependant on local

geometry

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AE and other NDT method limitationsAE• Unstressed discontinuities

will not emit• Dependence on stress

history• Sound wave Attenuation• Background Noise

Most other Methods• Close access required• Dependence on surface

condition, geometry• Some methods are limited to

surface-breaking discontinuities

• Time and money spent removing discontinuities that may not be active in service

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Acoustic Emission (AE)Transient elastic waves generated by the rapid release of energy from localised sources within a material (extract from ASTM E610.82)

Amplitude

Threshold

Duration

Rise time Decay time

Event Counts = 7

Time

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AE Sources Detects Acoustic Emission Sources such as;• Active cracks• Plastic deformation of material• Spalling of corrosion products• Leak noiseBackground noise from; • Weather • Tank internals or processes• Pressurisation

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AE Equipment & Software

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• Sensors – 30 to 150 kHz• Preamplifier Gain– 40dB• Band pass filters tuned to

sensors 10 - 400kHz• PAC DiSP AE Card• PAC AEWin Software

Sensor Preamplifier Filter

DiSP AE CardComputer

AE Equipment

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PAC AEWin software more user friendly and is Windows based, features include;

• 2D, 3D and tank bottom location, software allows for easy set up.

• Graphical representation of attenuation of AE signals.

AE Software

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AE – 3D View

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TANK BOTTOM MONITORING

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AE Test Procedure - Tank Bottom• Tank conditioned

– Isolate tank for 6-12 hours– Large crude tanks 24 hrs, Small tanks 6 hrs– Valves closed, heaters off, agitators switched off– Insulated tanks require 300 mm square holes removed to mount

sensors• Sensors mounted to tank wall around the circumference • Tank monitored for a pre-determined time• Data processed to eliminate unwanted noise• Data evaluated and sources located

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Tank Bottom Sensor Positioning• Sensors mounted 0.5 m to 2 m

above knuckle• 3 to 24 sensors used depending

on size• Cables from sensors are run

back to AE computer

Primary sensors

Guard sensors

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Sensor Mounting

• Magnetic hold downs used to fix sensor to steel structures

• Velcro or duct tape on FRP

• Duct tape or removable adhesive on non-ferrous materials

• Structure surface cleaned• Couplant applied to sensor

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Sound Reflection• Occurs at the interface between two materials

when sound waves meet a sudden change in acoustic impedance.

• The reflected energy can be calculated from((Z1-Z2)/(Z1+Z2))2 x 100%

• The % energy reflected at steel paint interface may be determined as follows:

Z1 (steel) = 46.7 (x 106Kgm-2s-1)Z2 (paint) = 2.5 (x 106Kgm-2s-1)Reflected energy = 81%

How much is transmitted? 19%

STEEL PAINT

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Sensor Calibration

• Pencil breaks (ASTM E976)

• Automatic sensor test• Can be detected up to

70 m• Variation of ±3 dB

between sensors

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AE Testing – Tank Set Up

Sensors

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AE Data Evaluation• Evaluation of AE data is done from relative

values which have been compared to follow up NDT inspections and experimental findings

• Based on number of events within a cluster (Specified distance)

• Energy of events detected

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• Identify and remove background noise• Grade overall activity • Locate noise• Separate “potential leak” data• Locate “potential leak” data

AE Data Evaluation

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Example of Good Tank

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Example of Bad Tank

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Tank Bottom Classification• Class A - Very low AE - Retest in 5 years• Class B - Low AE - Retest in 3 years• Class C - Medium AE - Retest in 1 year• Class D - High AE - Open and Inspect

internally in 1 year• Class E - Very High AE - Open and Inspect

internal immediately

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Inspection Results

52Good29E

23Good27D

313Good925C

34Good2027B

V Good3131A

Major repairs

Some Repairs

CorrelationNo Repairs

No Tanks Tested

Class

1. P.J. van de Loo - Shell, The Netherlands. B. Herrmann - Dow, Germany -1998

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Inspection Results

• Correlation of AE results with subsequent inspections • 100 % of “A” grade tanks – no repairs or damage to

floor• 74% of “B” grade floors - no repairs, 26% repairs• 52 % of “C” grade tanks – repairs• 71% of “D” grade tanks - repairs• 56 % of “E” grade tanks – floor replaced

Paper from www.ndt.net/article/ecndt98/chemical/095/095.htm

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2. Yuyama, S, et al Materials Evaluation Sept 2007

(A) High noise

Retest of new floor

Comments

GoodNo significant lossBCrude

V GoodNo significant lossAProduct 7

V Good70% loss in some areasDProduct 6

V Good>30% loss in some areas

CProduct 5

V Good20% loss in some areasCProduct 4

V GoodNo significant lossAProduct 3

V GoodNo significant lossAProduct 2

V GoodNo significant lossAProduct 1

Correlation

Floor ScanAE Class

Tank ID

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3. ATTAR – Tank test results

Classification Vs Tanks

0

2

4

6

8

10

12

14

a b c d e

Tank Classification

Num

ber o

f Tan

ks

Classification Vs Tanks

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3. ATTAR – Tank test results (Alcoa)

V GoodNo significant lossBCaustic

Rated B not A as first test

Comments

GoodSome floor lossBBayer Liquor

V GoodNo significant lossBBayer liquor

V GoodSome floor lossCCaustic

GoodSignificant floor lossCCaustic

V GoodNo significant lossADiesel

V GoodLeaks detectedEBayer liquor

V GoodNo significant lossABayer liquor

V GoodNo significant lossBBayer liquor

CorrelationFloor ScanAE ClassTank ID

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Tank Inspection EconomicsCosts should take into account:• Isolating the tank, all procedures and time spent, opening up,

entering.• Clean of sludge and disposal – environmental issues• Venting to gas free• For a 60 to 70 metre crude tank these costs amount to about

$250,000.• Floor scanning and visual inspection costs would be about

$11,000 extra.

• Tank 1 tenth that area, i.e. 20m diameter might cost $25,000.

• Cost of AE test is about $7,000, deferring tank for 12 months saves $18,000 plus cost of unnecessary repairs.

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AE Tank Bottom Inspection Limitations

• Does not determine remaining floor thickness in (mm) or even quantitative corrosion rates.

• Data used for grading tank floor condition although leaks can be detected.

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AE Tank Bottom Inspection Benefits• Identify tanks that require internal inspection and

repair.• Determine condition of damaged floors and

prioritise opening for maintenance.• Very little or no disruption to operation, no entry

required. • Quickest and most cost effective means of

assessing floor condition.• Reduces maintenance costs significantly.