ISO 16148:2006

INTERNATIONAL ISO
STANDARD 16148
First edition
2006-05-01

Gas cylinders — Refillable seamless steel
gas cylinders — Acoustic emission
testing (AT) for periodic inspection
Bouteilles à gaz — Bouteilles à gaz rechargeables sans soudure —
Essais d'émmision acoustique pour contrôle périodique




Reference number
ISO 16148:2006(E)
©
ISO 2006

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ISO 16148:2006(E)
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ISO 16148:2006(E)
Contents
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Operational principles.2
5 Personnel qualification .3
6 Special considerations to ensure valid tests.3
7 Apparatus .4
8 Calibration and equipment verification .6
9 Overall procedure .6
10 Real-time evaluation criteria.7
11 Test report .8
Annex A (normative) Instrumentation specifications.10
Annex B (informative) Alternative method for source location.12
Annex C (informative) Example instrument settings, examination methods and rejection criteria
for modal acoustic emission (MAE).15
Annex D (informative) Distance amplitude correction procedures.18
Bibliography .21
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ISO 16148:2006(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 16148 was prepared by Technical Committee ISO/TC 58, Gas cylinders, Subcommittee SC 4,
Operational requirements for gas cylinders, in collaboration with Technical Committee CEN/TC 23,
Transportable gas cylinders, of the European Committee for Standardization.
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ISO 16148:2006(E)
Introduction
In recent years, new non-destructive examination (NDE) techniques have been successfully introduced as an
alternative to the conventional re-testing procedures of gas cylinders, tubes and other cylinders.
One of the alternative NDE methods for certain applications is acoustic emission testing (AT), which has
proved to be an acceptable testing method applied during periodic inspection in some countries.
The test method requires pressurization to a level greater than the normal filling pressure.
The pressurization medium may be either gas or liquid.
Acoustic emission (AE) measurements are used to detect and locate emission sources. Other NDE methods
are needed to evaluate the significance of AE detected sources. Procedures for other NDE techniques are
beyond the scope of this International Standard. For example, shear wave, angle beam ultrasonic inspection
is commonly used to establish the exact position and dimensions of flaws that produce AE.
This International Standard includes two methods of AT and, for the purpose of differentiation, the methods
are addressed as Method A and Method B (see Clause 3).
With the agreement of the testing and certifying body approved by the competent authority of the country of
approval, the hydraulic pressure test of cylinders and tubes may be replaced by an equivalent method based
on acoustic emission.
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INTERNATIONAL STANDARD ISO 16148:2006(E)

Gas cylinders — Refillable seamless steel gas cylinders —
Acoustic emission testing (AT) for periodic inspection
1 Scope
This International Standard is a guideline for using acoustic emission testing (AT) during re-qualification of
seamless steel cylinders and tubes of water capacity up to 3 000 l used for compressed and liquefied gases.
For cylinders below 20 l additional precautions may be taken due to the potential reflections from the ends.
This examination provides indications and locations that should be evaluated by another examination for a
possible flaw in the cylinder. This International Standard covers monolithic steel cylinders.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6406, Gas cylinders — Seamless steel gas cylinders — Periodic inspection and testing
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
EN 1330-9, Non-destructive testing — Terminology — Part 9: Terms used in acoustic emission testing
EN 13477-1, Non-destructive testing — Acoustic emission — Equipment characterisation — Part 1:
Equipment description
EN 13477-2, Non-destructive testing — Acoustic emission — Equipment characterisation — Part 2:
Verification of operating characteristic
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 1330-9 and the following apply.
3.1
fracture critical flaw
defect that is large enough to exhibit unstable crack growth under certain service conditions
3.2
working pressure
settled pressure at a uniform temperature of 288 K (15 °C) for a full gas cylinder with the maximum
permissible charge of compressed gas
NOTE 1 In North America service pressure is often used to indicate a similar condition, usually at 21,1 °C (70 °F).
NOTE 2 For compressed gases, this value is usually stamped on the cylinder.
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ISO 16148:2006(E)
3.3
normal filling pressure
level to which a receptacle is pressurized during filling
NOTE This is usually greater than the marked working pressure due to the heat of compression.
3.4
acoustic emission test pressure
AE test pressure
maximum pressure at which acoustic emission testing is performed
3.5
maximum allowable pressure
maximum pressure a receptacle may experience
NOTE For liquefied gases, this is the developed pressure at the maximum service temperature (e.g. 65 °C).
3.6
acoustic emission pressure test range
range of pressure during which acoustic emission is monitored
3.7
Method A
acoustic emission testing performed during pneumatic pressurization to at least 110 % of the normal filling
pressure
3.8
Method B
acoustic emission testing performed during the hydrostatic proof pressurization to the re-test pressure
3.9
secondary AE sources
emissions other than actual crack propagation and plastic deformation
NOTE Contact between flaw surfaces as the cylinder expands, fracture or rubbing of mill scale within a flaw as the
cylinder expands are examples of secondary AE sources.
4 Operational principles
When cylinders containing flaws are pressurized, stress waves (AE) can be produced by several different
sources (e.g. secondary sources or actual propagation of cracks). These sources can produce AE at
pressures less than, equal to or greater than working pressure. The stress waves travel throughout the
structure.
Piezoelectric sensors mounted on a cylinder surface respond to stress waves. They are connected to a signal
processor, which records AE signal parameters associated with the passage of the waves under the sensor.
Stress waves travel at average speeds. With at least two sensors, one mounted at each end of a cylinder, the
approximate location of AE sources is derived from the measured arrival time of stress waves at the sensors.
If measured emissions exceed the specified levels over a linear distance on the cylinder, then such locations
shall undergo a secondary inspection (for example, ultrasonic examination) in order to verify the presence of
flaws and to measure flaw dimensions. From this secondary inspection, if the flaw depth exceeds the specified
limit (that is, a limit based on a number of factors, i.e. cylinder material, wall thickness, fatigue crack growth
estimates, fracture critical flaw depth calculations and any practical experience), then the cylinder shall be
removed from service.
If after the examination a recalibration proves negative, the relevant cylinder shall be re-examined by a non-
destructive examination (NDE) method other than AE Method A.
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ISO 16148:2006(E)
5 Personnel qualification
Properly qualified and capable personnel shall perform AT. In order to prove this qualification, the personnel
shall be certified in accordance with relevant standards as approved by the national authority (e.g. ISO 9712,
EN 473, ASNT TC 1A).
6 Special considerations to ensure valid tests
6.1 General
In order to prevent invalid AE examinations when using Method A and to overcome the Kaiser effect (see
NOTE 1), the AE test pressure shall exceed that pressure previously exerted on the receptacle during service,
i.e. normal filling pressure for compressed gases and maximum allowable pressure for liquefied gases.
After pressurization to more than the AE test pressure, Method A shall not be performed within a time period
less than one year or before a sufficient number of pressurization cycles (see NOTE 2) have occurred, since
such practice can decrease the sensitivity of the examination.
If a pressure greater than the normal filling pressure has been applied and a time period equal to or greater
than one year or a sufficient number of pressurization cycles has not elapsed, then the AE examination shall
be 10 % above this excessive pressure, but shall not exceed the design test pressure (TP) of the receptacle. If
at any stage a receptacle for liquefied gases has been overfilled, this shall be reported to the re-tester by the
receptacle owner or operator. If the AE examination would result in a pressure greater than TP, then Method
A shall not be applied. Only Method B or a conventional re-test shall be performed.
WARNING —Take appropriate measures to ensure safe operation and to contain any energy that may
be released during the hydraulic test. It should be noted that pneumatic pressure tests require more
precautions than water pressure tests since, regardless of the size of the container, any error in
carrying out this test is highly likely to lead to a rupture under gas pressure. Therefore these tests
should only be carried out after ensuring that the safety measures satisfy the safety requirements.
NOTE 1 The Kaiser effect is characterized by the absence of AE until the previous maximum applied load level has
been exceeded.
NOTE 2 A sufficient number of pressurization cycles are dependent upon the design parameters of the receptacle
undergoing periodic inspection, particularly the material composition.
6.2 Pressurization
General practice in the gas industry is to use low pressurization rates. This practice promotes safety and
reduces equipment investment. AE examinations should be performed with low enough pressurization rates to
allow cylinder deformation to be in equilibrium with the applied load. Pressurization should proceed at rates
that do not produce noise from the pressurizing medium. For Method A, typical current practices use
pressurization rates that approximate 35 bar/h (3,5 MPa/h) for tubes.
NOTE For smaller cylinders a higher pressurization rate may be suitable provided it is demonstrated that all
detrimental defects can be detected and the pressurization rate is slow enough to allow the pressurization to be stopped
before bursting of the cylinder. Pressure holds are not necessary; however, they can be useful for reasons other than
measurement of AE.
Secondary AE sources can produce emissions throughout pressurization. Flaw growth normally produces
emissions at pressures higher than the normal filling pressure.
When pressure within a vessel is low and gas is the pressurizing medium, flow velocities are relatively high.
Flowing gas (turbulence) and impact by entrained particles can produce measurable emissions. Considering
this, acquisition of AE data shall commence at some pressure greater than the starting pressure (for example,
one-half of the AE test pressure).
NOTE According to Clause 3, AE test pressure means the maximum pressure at which AT is performed.
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ISO 16148:2006(E)
Serious flaws can produce more AE from secondary sources than from flaw growth. When cylinders are
pressurized, flaws can produce emissions at pressures less than normal filling pressure. An AE test pressure
that is at least 10 % greater than normal filling pressure allows measurement of emissions from secondary
sources in flaws and from flaw growth.
Excess background noise can distort AE data or render them useless. Users shall be aware of the following
common sources of background noise:
⎯ high gas fill rate (measurable flow noise);
⎯ mechanical contact with the vessel by objects;
⎯ electromagnetic interference and radio frequency interference from nearby broadcasting facilities and
from other sources;
⎯ leaks at pipe or hose connections;
⎯ airborne sand particles, insects, rain drops or snow, etc.
AT shall not be used if background noise cannot be eliminated or sufficiently controlled.
When performing the AT (especially pneumatically), safety precautions shall be taken to protect personnel
carrying out the examination because of the considerable damage potential from the stored energy that can
be released. Additionally, since AT equipment is not explosion-proof, precautions shall be taken when the
pressurization medium is a flammable gas due to the possibility of a leakage of flammable gas.
It is essential that good, instantaneous communication exists during manual test operation between the AT
operator and the pressurization operator so pressurization can be paused or the pressure reduced if
necessary. During automatic test operations, this shall be ensured by the automatic test equipment.
7 Apparatus
Typical features of the apparatus required for this test method are provided in Figure 1. Full specifications are
in Annex A. An optional approach for source location is described in Annexes B and C.
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ISO 16148:2006(E)

Key
1 pressure transducer
2 acoustic emission sensors with integral preamplifier (two for each tube)
3 tube with sensors mounted on sidewall
4 tube with sensors mounted on end flanges
5 printer
6 video monitor
7 computer
8 acoustic emission signal processor
a
Power.
Figure 1 — Essential features of acoustic emission examination equipment
The cylinder surface at sensor places shall be cleaned (see Clause 9).
The couplant shall be used to connect sensors acoustically to the receptacle surface. Only adhesives that
have acceptable acoustic properties shall be used (see A.3). Sensors shall be held in contact with the cylinder
wall to ensure adequate acoustic coupling, e.g. with magnets, adhesive tape or other mechanical means.
A preamplifier may be enclosed in the sensor housing or in a separate enclosure. If a separate preamplifier is
used, cable characteristics are critical (see A.4 and EN 13477-1).
Power/signal cable length (that is, cable between preamplifier and signal processor) shall not exceed 150 m
(see A.5 and EN 13477-1).
Signal processors are computerized instruments with independent channels that filter, measure and convert
analogue information into digital form for display and permanent storage. A signal processor shall have speed
and capacity to process data independently from all sensors simultaneously. In addition, it shall not stop
processing and shall unambiguously identify to the operator, should the situation arise where continuous noise
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ISO 16148:2006(E)
such as from valve leakage, flow noise or high emission rate has rendered the signal permanently above the
system threshold. The signal processor shall provide capability to filter data for replay.
A video monitor should display processed test data in various formats. Display format may be selected by the
equipment operator.
A data storage device such as a compact disc may be used to provide data for replay or for archives.
Hard-copy capability shall be available from a graphics/line printer or equivalent device.
8 Calibration and equipment verification
8.1 Calibration
The pressure sensors shall be calibrated annually by personnel who are certified by calibration laboratories,
which are certified according to ISO 17025 or equivalent requirements, and the proper function of the loading
apparatus shall be checked, e.g. annually, according to its use.
The performance of the complete AE system shall be checked according to EN 13477-2 or any equivalent
standard and shall also be adjusted so it conforms to the equipment manufacturer’s specifications.
8.2 Equipment verification
Before and after the examination, the performance of the AE instrumentation shall be verified. Before and
after the examination, the response of each sensor with the adjoining measurement chain and source location
accuracy shall be verified by measuring the response according to an artificial, induced AE signal. The
preferred technique for conducting this verification check is the Hsu-Nielsen source (see EN 1330-9). The
diameter of the pencil lead, the distance to the transducers and the expected peak amplitude response are
interrelated; they shall be specified in the written test instructions.
The verification shall be performed at a distance where the obtained peak amplitude is within the dynamic
range of the measurement chain. The maximum variation allowed shall be ± 3 dB between all channels. Any
deviation outside the allowed range shall be corrected.
The use of an electronic pulser to check that there is no subsequent change in sensitivity, by comparison with
that obtained prior to the examination, is an acceptable alternative to the Hsu-Nielsen source check. If the
pulser is used, an approved procedure shall be provided that clarifies its use and calibration. For the testing of
similar cylinders, the electronic pulser can also be used for the first sensitivity check based on prior performed
examinations.
9 Overall procedure
All accessible external surfaces of the cylinders shall be visually examined. Record observations in a test
report. (See ISO 6406 or equivalent for the rejection criteria.) The procedure is as follows.
NOTE Accessible implies that the trailer need not be dismantled at time of testing when applying Method A.
a) Mechanically isolate the cylinder to prevent any contact with surface of other cylinders, hardware, etc.
When cylinders cannot be completely isolated, indicate in the test report external sources that may have
produced emissions.
b) Connect the fill hose and the pressure transducer. Eliminate any leaks at connections.
c) Place the sensor on a smooth surface, but not necessarily on bare metal. As a precaution, the coaxial
cable should be supported so its weight will not cause the sensor to become separated from the
receptacle (see Figure 1);
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ISO 16148:2006(E)
d) Adjust signal processor settings.
e) Check the system performance by breaking a pencil lead (Hsu-Nielsen source) or by using an electronic
pulser on the receptacle at a distance not less than 10 cm from the sensor. Verify that peak amplitude
exceeds 70 dB when 0,3 mm pencil lead is used. Adjust signal processor threshold above background

AE
peak noise. The dynamic range described by the difference between mean peak amplitude (response to
Hsu-Nielsen source) and the threshold setting is dependent on the method (A or B). For Method A, it is
recommended to have a threshold of 40 dB below the minimum value of the maximum peak amplitude of
the lead break at 10 cm; for Method B, the recommended threshold is 30 dB below the peak amplitude of
the lead break at 10 cm.
f) Verify that the AE system displays a correct location for the mechanical device that is used to produce
stress waves. For this purpose, lead breaks shall be performed on the receptacle wall within the axial
distance of the two sensors. The difference between the axial location displayed by the AE system and
the real location on the receptacle related to the sensor positions shall be determined for each lead break.
The accuracy shall be within ± 5 % of the sensor spacing. The inaccuracy between actual and located
positions shall not exceed ± 5 % of the sensor distance during calibration. If this accuracy cannot be
attained, more sensors should be added to reduce the sensor spacing, which may reduce the overall
inaccuracy.
g) Begin pressurizing the cylinder. Interrupt pressurization if there is an exponential increase in AE activity,
from any channel, as a function of pressure. The pressurization rate shall be low enough to ensure that
flow noise is not recorded.
h) Monitor the examination by observing displays that show plots of AE data being generated versus axial
location. If the AE indications meet the criteria of Clause 10, stop pressurization and conduct an
investigation.
In case of automated control of the system, critical signals are stored in the system under the supervision
of an authority (e.g. notified body). When during the test procedures these signals are reached or
exceeded, the system is set to immediately interrupt the test run and reduce the pressure from the
system. At the same time, an optical (lamp) and acoustic signal will alert the personnel responsible for the
testing.
Depending on the subsequent examination of the vessel by other NDE means, e.g. ultrasonic
examination, the vessel will be rejected or can remain in service.
i) Store all data on mass storage media. Stop pressurizing when pressure reaches the AE test pressure.
Pressure shall be monitored with an accuracy of ± 2 % of AE test pressure.
10 Real-time evaluation criteria
The criteria that will result in a stop of the pressurization sequence for rejection inspections or a pause for
further analysis of AE data shall be clearly defined. Supporting data for the choice of test stop criteria shall be
available from an appropriate database, standard or experience. The testing procedure can be applied in
various jurisdictions where the rejection criteria can vary but are specified by regulations.
The criteria that will result in a rejection of the tested receptacle or in a stop of the pressurization sequence
for further inspections are influenced by factors such as
⎯ type of receptacle,
⎯ material and heat treatment,
⎯ first or subsequent pressurization.
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ISO 16148:2006(E)
Rejection criteria shall be defined clearly before the examination based on an appropriate database, existing
standards or experience. Cylinders that have been rejected based on AE data shall undergo a secondary
inspection (for example, UE) before being put back into service.
The real-time evaluation criteria shall be based on at least one of the following observations:
⎯ increase in AE activity and/or energy as a function of the pressure at any channel;
⎯ number N of located burst signals with a distance-corrected peak amplitude above a ‘high’ specific value
1
A ;
1
⎯ number N of located burst signals with a distance-corrected peak amplitude above a ‘low’ specific value
2
A within an interval of size ‘X’ % of the maximum distance between sensors.
2
NOTE The value of X depends on the accuracy of the AE equipment used, number of sensors and the size of
the receptacle (e.g. diameter size).
Furthermore, for Method A the pneumatic pressurization shall be stopped immediately if
⎯ the AE energy increases in incremental steps from a defined value of energy, which means it doubles in
two consecutive pressure intervals of 5 % of the maximum test pressure, or
⎯ one of the specific predefined values for either N or N is exceeded.
1 2
NOTE Annex D gives an example of a method to measure the wave attenuation in order to calculate the distance of
the corrected peak amplitude (see EN 14584).
11 Test report
Prepare a report from each AE examination containing the following information:
a) name(s) of owner(s) of cylinders;
b) serial number(s) and manufacturer(s);
c) examination date and where examination was performed;
d) previous examination date and previous test pressure;
NOTE If the operator is aware of situations where the receptacle was subjected to pressure that exceeded normal
filling pressure, these should be described in the report.
e) normal filling pressure (to be supplied by the receptacle owner) and marked working pressure;
f) pressurization medium;
g) pressurization rate;
h) pressure at which data acquisition commenced;
i) AE test pressure;
j) location of AE sensors;
k) locations of AE sources that exceed acceptance criteria, including distance from the end of the receptacle
that bears the serial number (usually this is stamped on the receptacle);
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ISO 16148:2006(E)
l) any acceptable variation from the AE test procedure;
m) name, qualification and signature of examination operator;
n) stacking chart that shows relative locations of cylinders and associated channel number, i
...