Ultrasonic Testing Field Report 1979 - 1998 · testing of compressed gas cylinders) also contains...

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Field report ultrasonic testing

1979 - 1998

1 Introduction

Some 30 years ago, a lethal accident occurred in Glarus, CH through the bursting of a 50 litre oxygen cylinder without any obvious external influence, even though this cylinder had undergone the statutory recurring water pressure test.

This was important evidence that the water pressure test yields no reliable statement on any incipient cracks or on crack growth on the internal surface.

This accident prompted the search for an improvement to routine testing, to permit the localising and evaluation of large-scale material separations.

As ultrasonic testing has now been successfully used in practice for over 20 years and has been granted European recognition according to the international agreement ADR/RID since 1.1.97, an in depth consideration of the currently available experience with US testing is justified.

1.1 The start of ultrasonic testing of steel cylinders

Due to the accident at Glarus in 1969, there was a need to find a new, more suitable procedure. After detailed studies, the first automatic US testing installation was placed in operation in 1974 and a start made with the routine testing of seamless steel cylinders of the older type.

The initial US tests on selected series showed clearly that ultrasonic testing is capable of detecting safety-relevant flaws in gas cylinders which would remain undetected with an internal pressure test.

Already at the start of the US testing, container series from the years 1909 – 1920 were found to have base cracks and were subsequently eliminated by the industry itself, without further tests.

Supported by experience gained with this type of testing, and following application by the test authorities, ultrasonic testing was incorporated into the Swiss national regulations in 1990 in the sense of a possible alternative to the statutory water pressure test.

As the Switzerland example shows, the decision led the Swiss gas industry wherever possible to procure only US tested cylinders, and to make the test compulsory in the case of hydrogen cylinders, so that manufacturers found themselves compelled to include this new testing method into their production monitoring.


Subsequently US testing gained admission to the European harmonisation efforts where it was accepted into the international agreement ADR/RID from 1.1.97 as an alternative to internal testing with water pressure.

Now, after more than 15 years successful application of US testing, a major proportion of cylinders in use in Switzerland is already undergoing US testing for the second time, in which it is noticeable that the percentage scrapped on the second test is already strikingly reduced.

2 US testing technique

2.1 Description of a system

Mobile ultrasonic gas cylinder test rig

Requirements

The requirements imposed on US testing, on the basis of international agreement ADR/RID (2) 216, are contained in the following standard:

- prEN 1968, IGC TN 26/81/D - TÜV/Swiss TS- Standard of March 1998 (draft)

Testing procedure

Testing takes place by passing the test head holder along the rotating cylinder. The test range comprises 100% scanning of the cylindrical part of the bottle, including the base transition zone.

The test is carried out using the immersion technique with water inlet section in the pulse reflection procedure. The five test heads are arranged so as to simultaneously check for transverse and longitudinal flaws, inside and outside, and to measure wall thickness. The EDP evaluation of the measured values permits determination of eccentricity, ovality and other


aspects. Further channels are available for the random sample type testing of the base.

Test sensitivity

Test sensitivity is rated using cylinder sections (calibration pieces) with incorporated reference faults at a test error depth of 5% of the design wall thickness.

Mechanical part, test head holder

Non-rusting rack with loading device. The feed of the test head holder and rotation rate of the test object is steplessly adjustable.

Electronic part

USIP 20 GP-8 for detecting longitudinal and transverse flaws

WDMU 1 wall thickness checking AS 500 optical display for defect location

Control and evaluation and logging of the measured values are performed on a Notebook with a specially developed software.

Area of application

In addition to the checking of seamless aluminium/steel cylinders of varied provenance, with large and small diameters and lengths, the test rig is also suitable for tests on tubes and sleeves.

Ultra small cylinders such as e.g. CO2 cartridges, for which water pressure testing is very costly can be tested very efficiently thanks to a specially developed test head device.

3. The present situation from the view of the manufacturer

Although in many national regulations, e.g. the German ”Technical regulations for gases”, there is presently still no requirement for the ultrasonic testing of compressed gas cylinders, as a result of advancing technical development and the determined demands of the gas industry in their specifications, all Western European gas producers have meanwhile installed ultrasonic testing systems.

The IGC directive IGC-TN 26/81 published by the gas industry at the beginning to the mid 80s, which as a result of damage in use to hydrogen cylinders, stipulated ultrasonic testing, at least for hydrogen cylinders, promoted the construction of US test installations at the cylinder manufacturers.

Since, as already mentioned, there still exist no harmonised regulation requirements for ultrasonic testing, there are regrettably also no binding test


specifications. The basis for testing is therefore usually the specifications of the IGC-TN 26/81 or national specifications for the ultrasonic testing of tubular hollow bodies. In Germany for instance these are the Steel-Iron test data sheets SEP 1915 (longitudinal flaw test), SEP 1918 (transverse flaw test) and SEP 1919 (laminations).

However, for the first time, appendices have appeared for future international and European standards for the performance of ultrasonic testing of compressed gas cylinders with detailed requirements for test rig calibration and test performance. Accordingly, ISO DIS 9809 and prEN 1964 contain the corresponding appendices. The prEN 1968 (for the recurring testing of compressed gas cylinders) also contains the relevant requirements for testing gas cylinders in the framework of recurring testing.

Today, with new cylinder testing, ultrasonic testing is predominantly carried out on the finished cylinder. According to definition, this means the cylinder after completion of the final heat treatment, after which no further defects can occur in production.

Test sensitivity is still defined as 5% (test fault depth) with reference to the design wall thickness of the cylinder. This means that all flaws deeper than 5% of the wall thickness are detected.

Resulting from the standard wall thickness tolerances for gas cylinders, which can be up to 30%, the actual US flaw sensitivity in the testing for fault depths is increased by ca, 4%.

Due to the sometimes low level of knowledge of different users on these discussion points on the one hand, and on the other hand the increased use of ultrasonic tests for recurring testing based on national special licenses, the results of ultrasonic testing in the recurring test or in a repeat test, e.g. in reception tests, are not always comparable with the test results of the manufacturer.

The reasons are varied:

• The application of test yardsticks to cylinder which have not yet been ultrasonically tested on manufacture (higher failure rate)

• The setting of different test parameters to the manufacturer.

• Non-conforming or different configuration of the calibration pieces.

Users in the gas industry continue frequently to use different quantities of cylinders of various manufacturers. This can for instance be due to strong links between a gas producer and a particular supplier. Combined with frequently inadequate statistical evaluations, this leads to unilateral and incorrect data on the failure frequency of gas cylinders with the use of ultrasonic testing in recurring testing.

A comparison of the failures in ultrasonic testing is in no case comparable with corresponding data from conventional testing (water pressure testing,


visual inspection, weighing). This is a comparison of two completely different testing philosophies, which however have a single aim; to guarantee the cylinder as safe for further use in the future utilisation period.

In the following therefore, the specific testing benefits of the presently competing techniques of ultrasonic testing and conventional recurring testing are compared, their specific strengths and weaknesses are derived and the scrap quotas, respectively fault types are analysed in detail.

4 Discussion and fault evaluation

4.1 Which faults occur in principle on compressed gas cylinders?

The faults expected to be identified in the recurring testing are basically those due to manufacture, plus damage caused in use, handling and by the filling gas itself.

Such faults can be grouped as follows:

1 Corrosion 1.1 Surface corrosion 1.2 Deep/shallow pitting corrosion 1.3 Stress crack corrosion

2 Thread damage

3 Thermal damage 3.1 Welds 3.2 Action of heat/heat treatment

4 Mechanical damage 4.1 Notches, impressions, dents 4.2 Scoring

5 Wall thickness reduction

6 Leaks

7 Forming defects 7.1 Laminations 7.2 Back finning 7.3 Form defects (ovality, eccentricity) 7.4 Cracks

8 Surface defects

4.2 Which faults are detectable using the conventional recurring test method?

Internal pressure test using water


The water pressure test is principally a strength test, in which it is verified that the cylinder withstands 1.5 times the operating pressure at the time of testing. Major leaks can be found.

- Leaks

Visual inspection

The main findings with the internal and external visual inspection are:

- Corrosion 1.1, 1.2; thread damage 2; thermal/mechanical damage 3, 3.1/4.1, 4.2; Leaks 6; forming faults 7.2; surface defects 8

Weighing the cylinder

The weighing method of determining weight loss as a result of rusting is a highly disputed test, at least with regard to the credibility of the findings.

- Advanced rusting 1.1

Hammer/sound test

For the finding of material defects, this test has only historical significance; it was last used with welded cylinders on the occasion of the leak test.

4.3 Which defects can be detected by ultrasonic testing?

The US test serves mainly for locating critical material defects in the cylindrical part of the cylinder, especially in the casing/base region.

- Corrosion 1.1, 1.2, 1.3; mechanical damage 4.1, 4.2; wall thickness reduction 5; forming defects 7.1, 7.2, 7.3, 7.4; surface defects 8

4.4 Which faults are to be regarded as critical?

To be assessed as critical in all cases are material separations (cracks) in the base transition zone, coarse back-finning in the casing zone plus massive reductions in wall thickness. These can lead to the failure type ”burst before leak” with steel cylinders of the lower tenacity level.

Leaking in connection with toxic or flammable gases is problematic, normally however, it is not detected with the recurring testing but during filling.

- Stress crack corrosion 1.3 - Action of heat/heat treatment 3.2 - Scoring 4.2 - Reduction in wall thickness 5 - Forming defects (back-finning/cracks) 7.2, 7.4


4.4 Fault classification

Which faults are operator-specific?

Operator-specific is mainly all damage resulting from improper handling in use or during maintenance, also damage as a result of reactions, initiated by the filling product itself or its impurities.

- Corrosion 1 Surface corrosion 1.1 Shallow/deep pitting corrosion 1.2 Stress crack corrosion 1.3 - Thread damage 2 Which defects are manufacturer-specific?

The following types of fault can occur according to production method:

- Forming defects 7 Laminations 7.1 Back-finning 7.2 Form deviations (ovality, eccentricity) 7.3 Cracks 7.4 - Surface defects 8

Plus damage caused in use/during production

Individual types of fault can be caused by the particular manufacturing process or can also be user-specific. Without closer investigation, individual defects cannot be unambiguously classified.

- Thermal damage Welds 3.1 Action of heat/heat treatment 3.2 - Mechanical damage Notches, impressions, dents 4.1 scoring 4.2 - Reduction of wall thickness 5 - Leaks 6

4.6 Comparability between US / water pressure testing

Faults detected with US testing and with water pressure testing differ fundamentally. With the classic water pressure testing, in particular with the visual inspection, this is primarily corrosion damage which can normally lead to a leak (leak before burst), should it not be detected in good time.

With US testing on the other hand, these are fault types which, with the traditional recurring test were previously not found or in some cases not fully recognised. Furthermore, the proportion of critical cylinders is relatively large; critical cylinders being those for which it must be reckoned that failure can occur under normal operating conditions without prior indication.


4.7 Advantages/disadvantages of US testing

In principle the test methods are based on different philosophies (water pressure test = strength test; US test = fault-free material), whereby both aim to satisfy the demands of container safety.

With water pressure testing, relatively costly container preparation/after treatment is necessary (filling with water, draining, drying etc.). Moreover, apart from a few exceptions, they practically never supply any findings.

US testing has the decisive advantage that material flaws which are not externally visible, can be identified and where necessary recorded.

The problem with US testing is the assessment of the measuring results. It demands very experienced testing personnel, capable of assessing the individual indications quickly, safely and competently.

Assessment with recurring testing is moreover more difficult than with initial testing and assumes a comprehensive knowledge of the manufacturing process. Special attention must be paid to the casing/base transition zone, in particular with automated testing.

Summarised below are the main advantages and disadvantages with respect to US testing methods.

Advantages:

• Depending on the system, better fault recognition and reproducibility • No impurities inside the cylinders (ultra-clean gases, corrosive gases) • No further damage in the strength test (opening up of existing material

defects) • In special cases, testing is possible on ready-to-use, i.e. filled containers

(checking of ”critical containers” before filling) • Environment-friendly test method (no contaminated test water, drying

etc.) • Testing of C02 cartridges, small dimensions (water pressure test not

economically feasible).

Disadvantages:

• No ”integral” testing of the complete cylinder • No strength test at 1.5 times operating pressure • Testing range restricted to cylindrical part, transition zone and base • Small leaks (hose pores, thread cracks) not reliably detected • Highly qualified testing personnel necessary


5 Statistical fault evaluation by Swiss TS

After more than 20 years experience with ultrasonic testing, Swiss TS Wallisellen now possesses sufficient empirical data to make safe statements on faults found with US testing, respectively on anticipated faults and their frequency, with the aid of a statistical evaluation.

The statistics are based on the annual checking of ca. 40 000 cylinders, carried out by different inspectors, yielding a total of well over half a million tested cylinders.

With respect to the proportion of eliminated cylinders, it should be noted that for reasons of difficulty or inconvenience, individual cylinders and sometimes whole series were eliminated prior to testing.

5.1 Comparison of the scrap percentage US/H20

0%

2%

4%

6%

8%

10%

79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

H 2O -Test

U S -T est

Due to the fact that ultrasonic testing is capable of finding safety-relevant flaws in gas cylinders which would remain undetected with internal pressure testing and the associated visual internal and external inspections, there is bound to be a larger percentage of defective cylinders.

The relatively high scrap percentage of the early years 79 – 82 results from the fact that at the start of US testing, only older series cylinders and in particular conspicuous series were selected for testing.

A slight increase after the introduction in wall thickness measurement in 85: US testing combined with the cylinder database additionally permitted the automated checking of the cylinder wall thickness.

The quality assurance measures, improved mastery of the process at the manufacturer, plus the fact that newer cylinders had already undergone a factory US test indicate a clear reduction in the percentage of defective cylinders.

5.2 First/second test

It should be noted in principle that the scrap quota with one and the same cylinders is higher with the first than with the second test. The reason is that defects of group 7, once eliminated are naturally not repeated and prior damage in use accounts for a smaller proportion.


Since US testing has been introduced to manufacturers as standard practice since the mid 80s, cylinders of more recent date show an essentially smaller scrap percentage, as the graph shows.

Year of manufacture before/after

1980

4.9 %

0.8 %

0%

2%

4%

6%

First test Secondtest

5.3 Period of use, type of gas etc.

Corresponding to the type of use of the cylinders, these are diverse requirements resulting from being subjected to more or less wear; for instance the carefully kept argon cylinder in the laboratory and the anonymous travelling transport cylinders used for corrosive gases and gas mixtures. Hence it is hardly surprising that scrap percentages are decisively determined by the use of the cylinders.

In this connection it appears important that the type of use of the cylinders and their use over their total life should be monitored and taken into account with the assessment of scrap in the recurring testing.

4.0%2.0%

0.8%

14.9%

0%

4%

8%

12%

16%

Age > 40years

Corrosivegases

Inertgases

Mfr. >1980


5.4 Comparability of the type of fault US/H20

Scrap percentage with water pressure testing

6.96 - 9.98: 0.73 % scrap

27%

23%

13%

5%

4%

3%

3%

3%

19%

Cylinder corrosion 25.6 %

Shallow pitting corrosion 23.1 %

Electrode ignition point 13.2 %

Deep pitting corrosion 5.0 %

Local corrosion 4.0 %

Notch 3.4 %

Action of heat 3.2 %

Damaged thread 3.2 %

other defects 19.3 %

Scrap percentage with US-testing 1992 - 1998: 2.8 % scrap

22%

13%28%

7%

1%2%2%

15%

10%

Longitudinal flaw, internal 23.0 %

Longitudinal flaw, external 9.6 %

Transverse flaw 12.6 %

Wall thickness reduction 28.7 %

Crack, base/casing transition 6.6 %

Crack, cylinder 1.2 %

Corrrosion internal1.6 %

Corrosion, external 1.8 %

Miscellaneous (not US) 14.9 %

Defects with classical water pressure testing leading to elimination are almost always found in the visual inspection and not, as is generally assumed, with the water pressure test.

Defects detected with „water pressure testing“ are mostly corrosion (> 57 %) and mechanical damages of the surface.

With US testing the major proportion is manufacturing flaws such as transverse and longitudinal flaws, material separations and cracks which endanger the safety of the cylinder.

Statistics also indicate that to a certain extent the detected faults are determined by the test method itself, i.e. that cracks in the base transition can only be found after introduction of the transverse flaw in the transition zone. This applies likewise to the wall thickness reduction (28.7%).

Critical defects (burst before leak), remaining undetected with the water pressure test!

Detached base resulting from forming faults in the base transition zone

Detail: Crack in the transition zone


5.5 Defect frequency for different manufacturers:

With the different makes it is primarily the quality and care of the cylinder manufacturer which influence the possible scrap percentages. The production process (block, blank, tube) and the processed material itself (hardened and tempered steel, aluminium) are of secondary importance.

3.5%

0.9%0.5%

4.1%

0.2%

2.2%2.2%2.7%

7.5%

1.6%

4.9%

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

Mfr. A Mfr. B Mfr. C Mfr. D Mfr. E Mfr. F Mfr. G Mfr. H Mfr. I Mfr. K Mfr. A-K

Statistics: Swiss TS-98

In evaluating the defect frequency of different makes it should be noted that the table likewise contains values determined on potentially endangered cylinder series.

6 Concluding remarks

Although US testing has proved its worth over many years, and the value of the results/improved expressiveness is generally accepted, it is only being applied hesitantly.

One reason is the aforementioned different philosophies of the testing methods. The equipment for US testing differs fundamentally from that for water pressure testing and the testing personnel must have higher qualifications.To apply US testing more widely, the know-how must first be structured on a broader basis, and inspectors and operating personnel comprehensively trained.

Although US testing requires more costly test equipment compared to water pressure testing, it is not imperative for test costs per cylinder to be higher. Practical experience clearly shows that pure testing costs must be calculated against the costly preliminary and after-operations, such as filling with water, draining and drying etc. With the efficient use of ultrasonic testing, the test costs are lower than for water pressure testing.

The enhanced proven safety of the tested cylinders must be the yardstick for the testing technique with recurring testing. The failure of a cylinder, also when it has previously been water pressure tested is difficult to explain to the public. To say nothing of the loss of image of the operator.


It is to be anticipated that thanks to US testing, which has long been used in manufacture and is now also being introduced for the recurring testing, the percentage of defective cylinders will continue to fall and safety will be improved.

The development of gas storage technology, the use of increasingly higher pressures, changes in cylinder materials and production processes will ensure that if we want to maintain safety in the future, we must continue to seek more refined testing methods.

Initially higher failure rates with ultrasonic testing compared to water pressure testing can be relativised by the experience described in this report, and in the sense of gas cylinder operating safety, should not lead to a rejection of ultrasonic testing.

Finally it can be stated that with the testing of far more than 500 000 cylinders which forms the basis of this report, some ”supercritical” cylinders were able to be found, which sooner or later would have very probably led to damage had they not been taken out of service.

In addition we would mention that, as latest investigations show, it is now possible to carry out cost-effective US testing of small dimension cylinders such as C02 cartridges (water pressure testing would be too costly). Also possible is the US testing of critical containers immediately prior to filling, in the sense of a check.

Léon Kaelin Head Testing Systems Swiss TS Technical Services AG Richtistrasse 15

CH-8304 Wallisellen/Switzerland

e-mail: [email protected]

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