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Sizing of cracks using the alternating current field measurement technique
International Journal of Pressure Vessels and Piping 79 (2002) 549–554 www.elsevier.com/locate/ijpvp
Sizing of cracks using the alternating current field measurement technique R. LeTessiera,*, R.W. Coadeb, B. Genevec a
Alcoa World Alumina Australia, Wagerup Refinery, WA, Australia b ETRS Pty Ltd, Mulgrave, Vic., Australia c SureSpek ISS, Perth, WA, Australia
Abstract Recently a new technique for the detection and sizing of cracks and defects has been introduced in Australia. The alternating current field measurement (ACFM) technique is an electromagnetic inspection method that uses hand-held probes, and computerised control, data acquisition and computational models. ACFM is more efficient than conventional inspection methods (e.g. UT, MT, RT) due to a reduced need for surface preparation and an ability to work through surface coatings. ACFM also has an added benefit that it is not only capable of detecting flaws, it can also size defects for length and depth. This paper presents three case studies where ACFM has been used to size defects. In each case cracking has been detected in routine inspection, but to enable recommendations whether to run, repair or replace the component, better data on defect size was required. Results from the work suggest that ACFM can be an extremely useful method to determine flaw size, but that a knowledge of the limitations of the technique must also be well understood. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Alternating current field measurement; Crack sizing; Non-destructive testing
1. ACFM technique The alternating current field measurement (ACFM) technique has been widely used in the oil, gas, mining and petrochemical industries to detect cracks and size surface breaking defects in a wide range of structural materials. ACFM is an electromagnetic inspection method that does not require electrical contact with the surface under inspection. ACFM inspection can be carried out through coatings of up to 10 mm in thickness. Generally ACFM requires little or no surface preparation prior to inspection. The technique involves the use of a hand-held probe containing two magnetic field sensors and an electric current induction system. The probe is connected directly to the ACFM system electronics, with all control and data collection/storage provided by a control PC. The detection and sizing software provides time base traces together with a ‘butterfly plot’ of the two magnetic field components plotted against each other as shown in Fig. 1 for a crack like indication. A crack is identified within a test specimen when a combination of the trace data gathered exhibits all the characteristic signals generated by a crack. The mathematical modelling of the measured magnetic field components * Corresponding author.
is directly compared to an extensive library of known cracks to provide a crack depth measurement.
2. Case studies Alcoa World Alumina Australia has used the ACFM method to size defects in various areas of plant at its three Western Australian alumina refineries over the last three years. The following case studies describe the results of some of these investigations. 2.1. Contact heater Extensive cracking was found in a 90 in. OD contact heater in the digestion train of the refinery. Initial inspection using the magnetic particle inspection (MT) confirmed that cracking had initiated at most welds, and from dents and gouges on the vessel wall that had been caused through the removal of buildup during earlier routine inspections. Cracking was predominantly in the top two strakes of the vessel (Fig. 2). Seven significant cracks were found in this vessel with three being in the fillet welds of redundant steel. These are shown in Fig. 3 and described in Table 1. Samples from the vessel were examined and stress corrosion cracking (SCC) was identified as the cracking mechanism.
0308-0161/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 8 - 0 1 6 1 ( 0 2 ) 0 0 0 8 8 - 1
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R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
† Vibrational stress from: steam injection; flashing associated with low flow conditions. This occurred after modifications to the contact heaters in 1997 and has been rectified. † Potential high temperatures (240 8C) in the upper part of vessel if the slurry levels run low. Operating conditions have been changed and high temperature excursions are avoided.
Fig. 1. Typical ACFM indication as shown on the computer screen.
The following factors are believed to have contributed to the cracking in these vessels: † Operational stresses over approximately 25 years of service. † Poor quality welding practises during the contact heater upgrade project in 1997. † Maintenance practice over earlier years, e.g. removing baffles, remnant welds from modifications still on the shell inner wall. † De-scale damage by rivet busters to the inner shell of the heater vessel.
Fig. 2. Dimensions of the 90 in. contact heater.
The crack-like indications found on the inner surface of the vessel are shown in Fig. 3. Initially, MT revealed extensive cracking that required either repair or replacement of the heater. Stress corrosion cracking, also termed caustic cracking when found in a caustic environment, is a complex phenomenon. Alloy composition, temperature, environment chemistry, electrochemical potential, metallurgical condition and stress level all influence cracking to some extent. Ferritic boiler steels are susceptible to caustic cracking but minor composition variations in these steels do not influence susceptibility markedly. The parameters of particular interest are therefore: † temperature † stress level † environment caustic concentration; aluminate and silicate concentration, aluminate/free caustic ratio. SCC was assumed to be active only for a period of two years following the modifications in 1997. As there was no cracking observed in the vessel prior to modification and the operating conditions were to be returned to those resembling the pre 1997 conditions, the driving force for SCC was considered to have been removed. Hence in conducting a defect assessment it was necessary only to assess cracks in terms of brittle fracture and fatigue mechanisms. MT had indicated that there were many cracks present and selective grinding had indicated that some of these cracks were quite deep. A technique that could not only measure the length of cracks but also the depth of cracks was then sought. Ultrasonic crack detection (UT) was a possible technique but it is slow, requires good surface preparation and is quite dependent on the orientation of the defect. ACFM with its ability to measure the length and depth of defects with little or no surface preparation was considered the appropriate technique in this instance. The results of the ACFM survey are shown in Fig. 3 and the length and depth of the defects as measured by ACFM are given in Table 1. To develop a reliable engineering assessment of vessel integrity the accuracy of the ACFM technique had to be validated. After the cracks had been surveyed, four plate sections were removed from the vessel for laboratory based ACFM
R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
551
Fig. 3. Orientation of the defects found at the top of the 90 in. contact heater.
crack sizing and subsequent sectioning to measure the depth of these cracks. The results are shown in Table 2. While the ACFM depth sizing measurements were confirmed in most cases, two measurements were found to vary significantly from the sectioned measurements. These were re-measured with ACFM which provided better results, however, for the crack indication labelled ‘1’ there was still inconsistency in the ACFM and metallurgical assessment. The ACFM operator suggested that the likely cause of the different depth sizes for crack ‘1’ was due to the complex nature of the crack, which consisted of a number of adjacent cracks joining the main crack which provided associated and misleading defect trace signals. A further aspect of the ACFM technique is that it measures the total extension of the crack through the plate as opposed to the depth that the crack extends through the plate thickness and as a result, for angled cracks, the measured crack length may exceed the plate thickness. 2.2. Condensate storage tank The 110S hot condensate storage vessel was internally inspected in December 1996. The vessel is shown in Fig. 4. After this inspection, the inspection interval for this vessel was increased from two to four years. The potential modes of failure for such vessels are internal corrosion or corrosion fatigue. The integrity of the vessel welds is critically important as surface breaking weld defects could introduce a path for corrosive liquids into the vessel wall. Such normally minor defects together with a stress concentration could then grow under corrosionfatigue conditions. The magnetic particle inspection in 2000 found cracks in the circumferential welds at the vessel roof, see Fig. 5. The
Table 1 Location and size of defects in 9000 OD contact heater shown in Fig. 3 as measured by ACFM Defect no.
Position
Defect length (mm)
DCH 1 DCH 2 DCH 3 DCH 4 DCH 5 DCH 6 DCH 7 DCH 8 DCH 9 DCH 10 DCH 11 DCH 12 DCH 13 DCH 14 DCH 15
Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Weld, redundant steelwork Weld, redundant steelwork Weld, redundant steelwork Weld, redundant steelwork Wear plate fillet weld Parent plate Parent plate de-scaling tool damage Pipe weld Wear plate fillet weld Circ. weld transverse indication Parent plate de-scaling tool damage Redundant steel fillet weld Parent plate Redundant steel fillet weld Not accessible to ACFM Seam weld Parent plate Parent plate Redundant steel fillet weld Parent plate de-scaling tool damage
Clear 9.3 Clear 17.4 Clear 8.5 11.1 Clear 35.5 28.9 30.0 8.0 28.0 112.0 36.5
3.0 14.3 1.0
13.5 9.3 32.4
4.6 1.2 4.4
7.9
3.3
13.5
2.4
DCH 16 DCH 17 DCH 18 DCH 19 DCH 20 DCH 21 DCH 22 DCH 23 DCH 24 DCH 25 DCH 26 DCH 27 DCH 28
19.7 35.5 Not accessible 27.5 9.3 14.8 12.3 Clear
Defect depth (mm)
0.9 0.9 1.5 0.8 4.8 1.0 5.4
2.1 4.8 4.5 0.7 0.8 1.2
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Table 2 ACFM crack size statistics for removed plate sections of 9000 OD contact heater Crack number
1 2 3 4 5 6 7
Crack depth (mm) Original ACFM measurement
Depth as measured by Metlabs
4.6 7.0 1.0 4.0 14.5 0.0 13.7
13 5 2.5 4 15 0 17.5
Re-evaluated ACFM measurement 8.2
17.8
cracks were inspected using the ACFM technique and found to be less than 1 mm deep. They were then successfully removed by light grinding. A crack was also detected at the vessel floor, at the circumferential weld between the torispherical head and the cylinder. The ACFM technique assessed the crack depth to be 4.6 mm (max) over a 45 mm length. The crack then became shallower over its remaining length of 70 mm. The orientation and location of the crack allowed the crack depth to be measured by UT giving a depth of 7.0 mm, significantly deeper than that from ACFM. This crack was blended out with the final depth being 7 mm (Fig. 6). This was weld repaired before the vessel was returned to service. The discrepancy between ACFM and the actual crack depth was not thoroughly investigated, it is possible that the crack was not continuous and therefore the actual surface breaking region of the crack was only 4.8 mm deep as indicated by ACFM. 2.3. Cold condensate tank
Fig. 5. Defect found in the cap of the weld at the roof of the 110S vessel.
vertically. The bottom strake of the tank is 16 mm thick plate, with the next two strakes up being 10 and 8 mm plates, respectively, and the top three strakes being 6 mm plate. The tank holds condensate that has returned from the refinery, and is stored for use in the powerhouse. The condensate returns at approximately 92 8C. The tank is covered externally with thermal insulation, and the contents are stored at atmospheric pressure. Extensive cracking was found in the weld, heat affected zone (HAZ), and parent metal using MT. Cracking was found at sites of both circumferential welds between strakes, and vertical welds within the same strake. Examples of the cracking are shown in Figs. 7 and 8. The cracks in the vicinity of the circumferential welds were normal to the weld direction, whereas the cracks in the vicinity of the vertical welds were parallel to the weld. All the welds of the bottom strake were accessible for MT, and a region
The cold condensate tanks are made of six circumferential strakes, each with four adjoining pieces, welded
Fig. 4. Schematic of the 110S condensate storage tank.
Fig. 6. Defect has been ground out. Maximum depth was 7.0 mm. The ground profile was further smoothed prior to returning the vessel to service.
R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
Fig. 7. Longitudinal crack at circumferential weld.
Fig. 8. Transverse cracks at a circumferential weld.
553
covering approximately half the plan area of the tank was inspected all the way to the tank roof. An assessment of the tank integrity was required prior to it being returned to service. It was decided that an assessment of critical crack size based on BS PD 6493 (now BS 7910) was appropriate. In addition, an analysis in accordance with AS 3788 and API 653 was conducted to assess acceptable thinning, in the event that some cracks may need to be blended out. In order for the assessment of tank integrity to be reliable an accurate measurement of crack size was required. Measuring the flaw along the surface would provide an estimate of the crack length, but crack depth was required. Initially local grinding was used. This gave some indication of crack depth, but posed the possibility of removing more material than was required to maintain minimum wall thickness. It was at this stage that the ACFM method was used. The results of the ACFM inspection are shown in Table 3. The condensate tank was of simple geometry, and the defects were surface breaking cracks. This is an ideal situation to apply the ACFM technique. Only limited verification of crack depths by grinding to the base of the crack and then blending out later was carried out. This technique was used with caution. Ultrasonic testing was not easy to use because the extent and branching of some of the cracking made interpretation of the ultrasonic signal difficult. The preliminary results of the critical crack size analysis confirmed that many cracks were close to or over the critical size. The corresponding analysis for acceptable wall thickness showed that the cracks could be ground out, but the number of cracks involved made this an extensive task. The results were based on (i) conservative fracture toughness of the weld, HAZ, and parent metal, and (ii) an
Table 3 Crack length and depth as measured using ACFM for the four lower strakes of the cold condensate tank. Results are for the east half of the tank and around the bottom. It is to be noted that only the larger cracks were measured in this assessment
Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 2 Strake 2 Strake 2 Strake 2 Strake 2 Strake 2
Weld type
Defect length (mm)
Defect depth max (mm)
CIRC VERT VERT VERT VERT VERT VERT VERT VERT VERT CIRC CIRC CIRC CIRC CIRC CIRC
31.2 29 17.2 36.9 40.8 26.7 17.2 70 25.6 26.7 13.1 13.1 23.1 23.1 36.9 17.2
2.2 1.3 1.1 1.5 1.1 2.0 4.7 1.0 2.5 2.9 0.9 1.6 1.3 2.3 0.2 1.1
Strake 2 Strake 2 Strake 2 Strake 3 Strake 3 Strake 3 Strake 3 Strake 3 Strake 3 Strake 3 Strake 4 Strake 4 Strake 4 Strake 4 Strake 4
Weld type
Defect length (mm)
Defect depth max (mm)
VERT VERT VERT CIRC CIRC CIRC CIRC CIRC CIRC VERT CIRC CIRC CIRC VERT VERT
31.2 34.6 50.5 18.4 22 31.2 26.7 34.6 20.8 34.6 31.2 23.1 13.1 25.6 24.3
1.8 2.5 4.0 1.3 1.2 2.8 1.3 3.4 1.8 1.6 1.3 3.8 3.5 0.8 1.2
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assumed value for tensile strength for the weld, HAZ, and parent metal. As the largest cracks were in the weld and HAZ, the fracture toughness of these areas were particularly important in the integrity of the vessel. The preliminary assessments used fracture toughnesses estimated from weld dimensions and hardness data, but actual testing of samples removed from the vessel revealed fracture toughness values well above the values used in the initial analysis. Using this data in the calculations, the crack sizes measured were less than 80% of critical. Surface replication of the crack tips revealed some rounding, and metallographic examination of the sample taken from the tank revealed extensive corrosion of the cracks, suggesting that they have been there for a long time. No parts of the cracks from the removed section displayed any new, uncorroded growth, nor sharp tips. This further added evidence that the cracks were no longer active. The ACFM assessment indicated the crack size was less than the critical size for unstable fracture, and the evidence suggested that continued crack growth was unlikely. It was recommended to return the tank to service with no additional repairs.
3. Conclusion The ACFM technique has been used extensively in industry to successfully size defects where traditional
techniques have been unsuccessful or unable to be used through access, location, geometry or wall thickness limitations. ACFM has significant advantages over traditional NDI techniques, these being: † Ability to test through coated materials. † Reduced requirement for pre-cleaning. † Full data records are kept, allowing data to be reviewed by another operator. † No instrument calibration is required. † Significantly faster than other conventional NDT methods. † Ability to test materials at elevated temperatures. † Detection and sizing in one instrument. However, to successfully use the ACFM technique the limitations of the technique must be very clearly understood. These limitations are: † Sensitive to surface breaking defects only. † Depth sizing models are based on isolated semi elliptical defects. † May provide misleading results when testing within areas containing multiple defects. † Probes are sensitive to gross geometry changes. † Sensitivity reduces with increasing coating thickness. † Test specimen must be of an electrically conductive material.
Sizing of cracks using the alternating current field measurement technique R. LeTessiera,*, R.W. Coadeb, B. Genevec a
Alcoa World Alumina Australia, Wagerup Refinery, WA, Australia b ETRS Pty Ltd, Mulgrave, Vic., Australia c SureSpek ISS, Perth, WA, Australia
Abstract Recently a new technique for the detection and sizing of cracks and defects has been introduced in Australia. The alternating current field measurement (ACFM) technique is an electromagnetic inspection method that uses hand-held probes, and computerised control, data acquisition and computational models. ACFM is more efficient than conventional inspection methods (e.g. UT, MT, RT) due to a reduced need for surface preparation and an ability to work through surface coatings. ACFM also has an added benefit that it is not only capable of detecting flaws, it can also size defects for length and depth. This paper presents three case studies where ACFM has been used to size defects. In each case cracking has been detected in routine inspection, but to enable recommendations whether to run, repair or replace the component, better data on defect size was required. Results from the work suggest that ACFM can be an extremely useful method to determine flaw size, but that a knowledge of the limitations of the technique must also be well understood. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Alternating current field measurement; Crack sizing; Non-destructive testing
1. ACFM technique The alternating current field measurement (ACFM) technique has been widely used in the oil, gas, mining and petrochemical industries to detect cracks and size surface breaking defects in a wide range of structural materials. ACFM is an electromagnetic inspection method that does not require electrical contact with the surface under inspection. ACFM inspection can be carried out through coatings of up to 10 mm in thickness. Generally ACFM requires little or no surface preparation prior to inspection. The technique involves the use of a hand-held probe containing two magnetic field sensors and an electric current induction system. The probe is connected directly to the ACFM system electronics, with all control and data collection/storage provided by a control PC. The detection and sizing software provides time base traces together with a ‘butterfly plot’ of the two magnetic field components plotted against each other as shown in Fig. 1 for a crack like indication. A crack is identified within a test specimen when a combination of the trace data gathered exhibits all the characteristic signals generated by a crack. The mathematical modelling of the measured magnetic field components * Corresponding author.
is directly compared to an extensive library of known cracks to provide a crack depth measurement.
2. Case studies Alcoa World Alumina Australia has used the ACFM method to size defects in various areas of plant at its three Western Australian alumina refineries over the last three years. The following case studies describe the results of some of these investigations. 2.1. Contact heater Extensive cracking was found in a 90 in. OD contact heater in the digestion train of the refinery. Initial inspection using the magnetic particle inspection (MT) confirmed that cracking had initiated at most welds, and from dents and gouges on the vessel wall that had been caused through the removal of buildup during earlier routine inspections. Cracking was predominantly in the top two strakes of the vessel (Fig. 2). Seven significant cracks were found in this vessel with three being in the fillet welds of redundant steel. These are shown in Fig. 3 and described in Table 1. Samples from the vessel were examined and stress corrosion cracking (SCC) was identified as the cracking mechanism.
0308-0161/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 8 - 0 1 6 1 ( 0 2 ) 0 0 0 8 8 - 1
550
R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
† Vibrational stress from: steam injection; flashing associated with low flow conditions. This occurred after modifications to the contact heaters in 1997 and has been rectified. † Potential high temperatures (240 8C) in the upper part of vessel if the slurry levels run low. Operating conditions have been changed and high temperature excursions are avoided.
Fig. 1. Typical ACFM indication as shown on the computer screen.
The following factors are believed to have contributed to the cracking in these vessels: † Operational stresses over approximately 25 years of service. † Poor quality welding practises during the contact heater upgrade project in 1997. † Maintenance practice over earlier years, e.g. removing baffles, remnant welds from modifications still on the shell inner wall. † De-scale damage by rivet busters to the inner shell of the heater vessel.
Fig. 2. Dimensions of the 90 in. contact heater.
The crack-like indications found on the inner surface of the vessel are shown in Fig. 3. Initially, MT revealed extensive cracking that required either repair or replacement of the heater. Stress corrosion cracking, also termed caustic cracking when found in a caustic environment, is a complex phenomenon. Alloy composition, temperature, environment chemistry, electrochemical potential, metallurgical condition and stress level all influence cracking to some extent. Ferritic boiler steels are susceptible to caustic cracking but minor composition variations in these steels do not influence susceptibility markedly. The parameters of particular interest are therefore: † temperature † stress level † environment caustic concentration; aluminate and silicate concentration, aluminate/free caustic ratio. SCC was assumed to be active only for a period of two years following the modifications in 1997. As there was no cracking observed in the vessel prior to modification and the operating conditions were to be returned to those resembling the pre 1997 conditions, the driving force for SCC was considered to have been removed. Hence in conducting a defect assessment it was necessary only to assess cracks in terms of brittle fracture and fatigue mechanisms. MT had indicated that there were many cracks present and selective grinding had indicated that some of these cracks were quite deep. A technique that could not only measure the length of cracks but also the depth of cracks was then sought. Ultrasonic crack detection (UT) was a possible technique but it is slow, requires good surface preparation and is quite dependent on the orientation of the defect. ACFM with its ability to measure the length and depth of defects with little or no surface preparation was considered the appropriate technique in this instance. The results of the ACFM survey are shown in Fig. 3 and the length and depth of the defects as measured by ACFM are given in Table 1. To develop a reliable engineering assessment of vessel integrity the accuracy of the ACFM technique had to be validated. After the cracks had been surveyed, four plate sections were removed from the vessel for laboratory based ACFM
R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
551
Fig. 3. Orientation of the defects found at the top of the 90 in. contact heater.
crack sizing and subsequent sectioning to measure the depth of these cracks. The results are shown in Table 2. While the ACFM depth sizing measurements were confirmed in most cases, two measurements were found to vary significantly from the sectioned measurements. These were re-measured with ACFM which provided better results, however, for the crack indication labelled ‘1’ there was still inconsistency in the ACFM and metallurgical assessment. The ACFM operator suggested that the likely cause of the different depth sizes for crack ‘1’ was due to the complex nature of the crack, which consisted of a number of adjacent cracks joining the main crack which provided associated and misleading defect trace signals. A further aspect of the ACFM technique is that it measures the total extension of the crack through the plate as opposed to the depth that the crack extends through the plate thickness and as a result, for angled cracks, the measured crack length may exceed the plate thickness. 2.2. Condensate storage tank The 110S hot condensate storage vessel was internally inspected in December 1996. The vessel is shown in Fig. 4. After this inspection, the inspection interval for this vessel was increased from two to four years. The potential modes of failure for such vessels are internal corrosion or corrosion fatigue. The integrity of the vessel welds is critically important as surface breaking weld defects could introduce a path for corrosive liquids into the vessel wall. Such normally minor defects together with a stress concentration could then grow under corrosionfatigue conditions. The magnetic particle inspection in 2000 found cracks in the circumferential welds at the vessel roof, see Fig. 5. The
Table 1 Location and size of defects in 9000 OD contact heater shown in Fig. 3 as measured by ACFM Defect no.
Position
Defect length (mm)
DCH 1 DCH 2 DCH 3 DCH 4 DCH 5 DCH 6 DCH 7 DCH 8 DCH 9 DCH 10 DCH 11 DCH 12 DCH 13 DCH 14 DCH 15
Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Circumferential weld Weld, redundant steelwork Weld, redundant steelwork Weld, redundant steelwork Weld, redundant steelwork Wear plate fillet weld Parent plate Parent plate de-scaling tool damage Pipe weld Wear plate fillet weld Circ. weld transverse indication Parent plate de-scaling tool damage Redundant steel fillet weld Parent plate Redundant steel fillet weld Not accessible to ACFM Seam weld Parent plate Parent plate Redundant steel fillet weld Parent plate de-scaling tool damage
Clear 9.3 Clear 17.4 Clear 8.5 11.1 Clear 35.5 28.9 30.0 8.0 28.0 112.0 36.5
3.0 14.3 1.0
13.5 9.3 32.4
4.6 1.2 4.4
7.9
3.3
13.5
2.4
DCH 16 DCH 17 DCH 18 DCH 19 DCH 20 DCH 21 DCH 22 DCH 23 DCH 24 DCH 25 DCH 26 DCH 27 DCH 28
19.7 35.5 Not accessible 27.5 9.3 14.8 12.3 Clear
Defect depth (mm)
0.9 0.9 1.5 0.8 4.8 1.0 5.4
2.1 4.8 4.5 0.7 0.8 1.2
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Table 2 ACFM crack size statistics for removed plate sections of 9000 OD contact heater Crack number
1 2 3 4 5 6 7
Crack depth (mm) Original ACFM measurement
Depth as measured by Metlabs
4.6 7.0 1.0 4.0 14.5 0.0 13.7
13 5 2.5 4 15 0 17.5
Re-evaluated ACFM measurement 8.2
17.8
cracks were inspected using the ACFM technique and found to be less than 1 mm deep. They were then successfully removed by light grinding. A crack was also detected at the vessel floor, at the circumferential weld between the torispherical head and the cylinder. The ACFM technique assessed the crack depth to be 4.6 mm (max) over a 45 mm length. The crack then became shallower over its remaining length of 70 mm. The orientation and location of the crack allowed the crack depth to be measured by UT giving a depth of 7.0 mm, significantly deeper than that from ACFM. This crack was blended out with the final depth being 7 mm (Fig. 6). This was weld repaired before the vessel was returned to service. The discrepancy between ACFM and the actual crack depth was not thoroughly investigated, it is possible that the crack was not continuous and therefore the actual surface breaking region of the crack was only 4.8 mm deep as indicated by ACFM. 2.3. Cold condensate tank
Fig. 5. Defect found in the cap of the weld at the roof of the 110S vessel.
vertically. The bottom strake of the tank is 16 mm thick plate, with the next two strakes up being 10 and 8 mm plates, respectively, and the top three strakes being 6 mm plate. The tank holds condensate that has returned from the refinery, and is stored for use in the powerhouse. The condensate returns at approximately 92 8C. The tank is covered externally with thermal insulation, and the contents are stored at atmospheric pressure. Extensive cracking was found in the weld, heat affected zone (HAZ), and parent metal using MT. Cracking was found at sites of both circumferential welds between strakes, and vertical welds within the same strake. Examples of the cracking are shown in Figs. 7 and 8. The cracks in the vicinity of the circumferential welds were normal to the weld direction, whereas the cracks in the vicinity of the vertical welds were parallel to the weld. All the welds of the bottom strake were accessible for MT, and a region
The cold condensate tanks are made of six circumferential strakes, each with four adjoining pieces, welded
Fig. 4. Schematic of the 110S condensate storage tank.
Fig. 6. Defect has been ground out. Maximum depth was 7.0 mm. The ground profile was further smoothed prior to returning the vessel to service.
R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
Fig. 7. Longitudinal crack at circumferential weld.
Fig. 8. Transverse cracks at a circumferential weld.
553
covering approximately half the plan area of the tank was inspected all the way to the tank roof. An assessment of the tank integrity was required prior to it being returned to service. It was decided that an assessment of critical crack size based on BS PD 6493 (now BS 7910) was appropriate. In addition, an analysis in accordance with AS 3788 and API 653 was conducted to assess acceptable thinning, in the event that some cracks may need to be blended out. In order for the assessment of tank integrity to be reliable an accurate measurement of crack size was required. Measuring the flaw along the surface would provide an estimate of the crack length, but crack depth was required. Initially local grinding was used. This gave some indication of crack depth, but posed the possibility of removing more material than was required to maintain minimum wall thickness. It was at this stage that the ACFM method was used. The results of the ACFM inspection are shown in Table 3. The condensate tank was of simple geometry, and the defects were surface breaking cracks. This is an ideal situation to apply the ACFM technique. Only limited verification of crack depths by grinding to the base of the crack and then blending out later was carried out. This technique was used with caution. Ultrasonic testing was not easy to use because the extent and branching of some of the cracking made interpretation of the ultrasonic signal difficult. The preliminary results of the critical crack size analysis confirmed that many cracks were close to or over the critical size. The corresponding analysis for acceptable wall thickness showed that the cracks could be ground out, but the number of cracks involved made this an extensive task. The results were based on (i) conservative fracture toughness of the weld, HAZ, and parent metal, and (ii) an
Table 3 Crack length and depth as measured using ACFM for the four lower strakes of the cold condensate tank. Results are for the east half of the tank and around the bottom. It is to be noted that only the larger cracks were measured in this assessment
Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 1 Strake 2 Strake 2 Strake 2 Strake 2 Strake 2 Strake 2
Weld type
Defect length (mm)
Defect depth max (mm)
CIRC VERT VERT VERT VERT VERT VERT VERT VERT VERT CIRC CIRC CIRC CIRC CIRC CIRC
31.2 29 17.2 36.9 40.8 26.7 17.2 70 25.6 26.7 13.1 13.1 23.1 23.1 36.9 17.2
2.2 1.3 1.1 1.5 1.1 2.0 4.7 1.0 2.5 2.9 0.9 1.6 1.3 2.3 0.2 1.1
Strake 2 Strake 2 Strake 2 Strake 3 Strake 3 Strake 3 Strake 3 Strake 3 Strake 3 Strake 3 Strake 4 Strake 4 Strake 4 Strake 4 Strake 4
Weld type
Defect length (mm)
Defect depth max (mm)
VERT VERT VERT CIRC CIRC CIRC CIRC CIRC CIRC VERT CIRC CIRC CIRC VERT VERT
31.2 34.6 50.5 18.4 22 31.2 26.7 34.6 20.8 34.6 31.2 23.1 13.1 25.6 24.3
1.8 2.5 4.0 1.3 1.2 2.8 1.3 3.4 1.8 1.6 1.3 3.8 3.5 0.8 1.2
554
R. LeTessier et al. / International Journal of Pressure Vessels and Piping 79 (2002) 549–554
assumed value for tensile strength for the weld, HAZ, and parent metal. As the largest cracks were in the weld and HAZ, the fracture toughness of these areas were particularly important in the integrity of the vessel. The preliminary assessments used fracture toughnesses estimated from weld dimensions and hardness data, but actual testing of samples removed from the vessel revealed fracture toughness values well above the values used in the initial analysis. Using this data in the calculations, the crack sizes measured were less than 80% of critical. Surface replication of the crack tips revealed some rounding, and metallographic examination of the sample taken from the tank revealed extensive corrosion of the cracks, suggesting that they have been there for a long time. No parts of the cracks from the removed section displayed any new, uncorroded growth, nor sharp tips. This further added evidence that the cracks were no longer active. The ACFM assessment indicated the crack size was less than the critical size for unstable fracture, and the evidence suggested that continued crack growth was unlikely. It was recommended to return the tank to service with no additional repairs.
3. Conclusion The ACFM technique has been used extensively in industry to successfully size defects where traditional
techniques have been unsuccessful or unable to be used through access, location, geometry or wall thickness limitations. ACFM has significant advantages over traditional NDI techniques, these being: † Ability to test through coated materials. † Reduced requirement for pre-cleaning. † Full data records are kept, allowing data to be reviewed by another operator. † No instrument calibration is required. † Significantly faster than other conventional NDT methods. † Ability to test materials at elevated temperatures. † Detection and sizing in one instrument. However, to successfully use the ACFM technique the limitations of the technique must be very clearly understood. These limitations are: † Sensitive to surface breaking defects only. † Depth sizing models are based on isolated semi elliptical defects. † May provide misleading results when testing within areas containing multiple defects. † Probes are sensitive to gross geometry changes. † Sensitivity reduces with increasing coating thickness. † Test specimen must be of an electrically conductive material.