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Technical introduction to non-destructive testing using APR technique
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G.M.A S.r.l.
ANTI CORROSION COATING
Nº Doc. LxxxxxR0611 Pagina 1 di 9
Redazione AGQ – R&S
Firma
Nº Allegati
Revision Date Verification
DGE Approval DGE Modifications
0 16/09/2011 1° edizione
1 19/09/2011 2° edizione
Technical introduction
to non-destructive testing
using APR technique
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
2
1. Introduction: hardware
One of the techniques used by GMA to
perform non-destructive testing (NdT) is the
Acoustic Pulse Reflectometry (APR)
consisting in the generation of an acoustic
wave at one end of the tube and the
registration of the echoes reflected. Originally
designed for performing NdT in the
aeronautic field, the instrument has been
subsequently adopted in the heavy industry
for inspections in heat exchangers and similar
devices.
The greatest innovation in this technique is
the probe, which, in its original “dolphin
nose” G1 model, (see Fig. 1) includes both
loudspeaker and microphone units and is
capable of producing and receiving the
acoustic signals, thus relieving operators from
the need to access both extremities of the
inspected item.
This early probe has been recently replaced
by the improved and compact third-generation
G3 model, resulting in many advantages in
terms of hardware toughness, maneuverability
in tight spaces and signal quality (the distance
between loudspeaker and microphone was
responsible for the generation of echoes from
tube inlets and major defects, which needed to
be filtered during data analysis).
The acoustic wave generated by the
loudspeaker travels in a single direction along
the tube and is reflected by the possible
defects, producing a return signal which is
collected by the microphone. Considering that
the acoustic signal travels at the speed of
sound, data collection is quite fast, being
about ten seconds per tube.
It is not necessary to create a sample tube
before the analysis, because the software is
able to create a reference signal from the
measurements performed.
Each type of defect causing a variation of the
inner diameter of the tube produces a distinct
signal, allowing for its identification. The
intensity of the reflected signal is proportional
to the severity of the defect, while the time
elapsed between the production and reflection
of the signal gives the defect’s position.
Figure 1. Left: first generation APR probe; right: third generation APR probe, together with its data acquiring and
processing unit. The physical separation between loudspeaker and microphone which determined the length of the
G1 probe, is not present anymore in the G3.
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
3
2. Software
The instrument comes with its own
processing unit, which performs both data
collection and analysis; figure 2 shows four
screenshots from the PTS data processing
software, with the different types of defects
identified by the instrument. Generally, an
upward peak in the signal means a cross-
section due to fouling or impingements; a
downward peak means an enlargement of the
cross-section due to corrosion or bulging. As
the cross-section returns to its former value,
the peak inverts in a more or less symmetrical
manner (see Fig.2).
Signal coming from leaks (holes) have a
peculiar shape, consisting in an abrupt fall in
the signal followed by a shallow, wide
positive peak.
These characteristics make them visually
identifiable by a trained operator, even in a
fast preliminary screening of the signals.
For a more accurate evaluation of the severity
of other defect types (blockages and
corrosion) the software provides an automated
data analysis. This requires a variable time
due to several factors (tube length and
number, presence of generalized fouling
and/or corrosion) and generates a table of
possible defects, which the operator examines
following a criterion of
acceptance/declination of each single defect.
Once the analysis is completed, a report is
generated which contains:
- A 2D tubesheet map with an
indication of the most severe defects
detected for each tube (see Fig. 3);
- A table reporting all the defects
detected for each tube, divided by type
(leaks, blockages, erosions, pittings)
Figure 2. Screenshots from the PTS data analysis program showing examples of the identifiable types of defect: in
reading order, a) blockage (positive peak, inverting), b) pitting (negative peak, inverting) c) erosion proceeding for a
given tract (negative peak, not inverting), d) leak (negative peak with distinctive characteristics followed by shallow
positive peak). Superposed to the hole signal, in red, is the theoretical “signature” for a hole of given diameter in
that position. Signals from adjacent defects add in an algebraical manner.
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
4
- Charted signals from defects
considered significant for their
severity or characteristics;
- For any peculiar findings resulting by
the analysis, a 3D representation of the
tube bundle with a localization of
defects, or any other significant
characteristic of the signals or groups
of signals (see Fig. 4).
Software detection limits are 2% of the inner
diameter for blockages and 10% of the tube
wall thickness for erosion and pitting. It is
possible to set up higher thresholds in order to
only point out the most severe defects, in
particular for machinery already in service
and if the aim of the inspection is to plug the
tubes most likely to fail soon.
3. Requirements, advantages, disadvantages
As discussed, availability of a sample tube is
not strictly necessary to perform an APR test.
This allow testing even with minimal
forewarning. Before measurements it is
advisable to have a scheme of the tubesheet
which can be a technical drawing or even a
photograph of the exchanger; the software has
a feature designed for rapid identification and
numbering of the tubes. Furthermore it is
necessary to know the exchanger’s technical
data (tube length and diameter and wall
thickness). Lacking these data, it is possible to
perform the analysis, but quantification of the
defects detected will not be accurate.
Figure 3. Screenshot from the PTS program with the 2D scheme of a tubesheet undergoing partial inspection (upper
left quarter). Tubes in good shape are marked in green; the ones in yellow are blocked to a lesser or greater degree.
The darker the hue, the more severe is the obstruction.
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
5
Figure 4. Screenshot from the PTS program. Visualization of signals (500 in all) from the inlets of a single-pass
exchanger. There are two groups of markedly different signals: the upper and lower half of the exchanger seem to
have a different design of the inlets, although visually no difference could be detected.
Overall, the features of the APR system allow
for the rapid analysis of heterogeneous groups
of exchangers, differing in tube diameter,
thickness and material, in a simple and
straightforward manner.
Since the measurement is carried out, in
practice, on the air within the tubes, the APR
technique can be applied to machineries of
every material both metallic and organic,
without any modifications to the instrument,
and its accuracy is not affected by the material
tested as is the case, for instance, with the
eddy current testing. It is therefore possible to
analyze, in a single measurement session, the
tube bundle of a condenser with the in-
condensable gas extraction zones composed
of Cu-Ni or superalloy tubes.
The main disadvantage of the system lies in
its inability to detect defects located on the
outer surface of the tubes, unless they are
through-wall leakages. On the other hand,
since the presence of protrusion on the outer
surface has no effect on the signal, it is
possible to test finned or studded tubes and
the accuracy is not hindered in the zone of the
baffle plates. Furthermore, since the probe
does not have to travel within the tube, it is
possible to test U-tubes with any bend radius,
with internal walls and grooves, spirals, with
a square or elliptical cross-section, et cetera.
The presence of inserts or corrugations,
although complicating data interpretation,
does not forbid testing.
Minimum and maximum diameter are ⅜”
(9,525 mm) and 4” (101,6 mm) respectively,
whereas a theoretical maximum length does
not exist: in practice, the signal undergoes a
progressive dampening which occurs more
rapidly in narrow tubes and when the inner
surface is dirty or corroded.
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
6
Figure 5. Some types of tubes which can be successfully analyzed with the APR technique. In reading order: finned
tubes from an air cooler; finned tubes from the air conditioning system of a motor yacht; tubesheet of a small heat
exchanger with star inserts; “snail” tube from a nuclear plant in Russia; U-tube exchanger from a refinery; detail of
the tube bundle of a steam dryer/reheater from a nuclear plant in Belgium; inner surface of a spiral tube; tubes
with a helical inner wall, from a power plant; graphite exchanger from a chemical plant.
As with all non destructive testing, a thorough
clearing of the tube is necessary. The degree
of cleanliness depends on the purpose of the
inspection: if the exchanger has just been
cleaned and an assessment of the cleaning
efficiency is needed, it is necessary and
sufficient that the tubes be dry; if the purpose
of the inspection is to detect the loss of wall
thickness due to corrosion, the tube surface
must be free from all mud and oxides which
could fill the pits and hinder their detection.
Buildups of dirt will hide all types of
corrosion and may even fill holes.
It is to be noticed, however, that in the
presence of massive, irremovable blockages
caused by massive obstructions, foreign
bodies, corrosion tubercles etc., the
instrument is always able to give information
about the state of the tube downstream of the
blockage. A resume of the APR method
characteristics, compared with other
widespread methods for nondestructive
testing, is shown in table 1.
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
7
4. References
The reliability of the method has been
demonstrated both by laboratory testing
(detection of known defects artificially
introduced to new tubes) and in-the-field
comparative analyses between the findings of
the APR and those detected by other NdT
methods (endoscopy, eddy current). Given the
novelty of the technique, there is no EN473
qualification regarding the technicians
conducting the inspection; GMA personnel is
trained by the instrument manufacturers, who
issue a qualification certificate. The G3
system is currently undergoing qualification
at the Southwest Research Institute, an
American independent institute for applied
research.
Method Ed
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curr
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X-r
ays
Ult
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Ther
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ag
ing
Pre
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test
ing
Aco
ust
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n
AP
R
Needs probe insertion Yes Yes Yes Yes No Yes No
Needs access to tubesheet Yes Yes Yes Yes Yes Yes No
Cost-effective No No No No Yes No Yes
Indicates defect position Yes Yes Yes Yes No Yes Yes
Time-consuming data collection Yes Yes Yes Yes Yes Yes No
Generates ionizing radiation No Yes No No No No No
Detects generalized corrosion Yes Yes Yes No No No Yes
Quantifies total or partial blockages No No No No No No Yes Table 1. Resume of the chracteristics of the main methods of nodestructive testing applicable to heat exchangers.
Inspections performer by GMA using the APR technique include:
å Steel mill, Italy: general control before recoating of three air-coolers; the same intervention is to
be performer on other 30 similar units.
Ltubes: 1,95 metres Øe: 17.8 mm thickness: 1.4 mm bundle: 126 steel tubes, finned.
Ltubes: 1.95 metres Øe: 16.8 mm thickness: 1.4 mm bundle: 140 steel tubes, finned.
Ltubes: 2.08 metres Øe: 18 mm thickness: 1.25 mm bundle: 112 steel tubes, finned.
å Nuclear power plant: inspection of one steam dryer/reheater in group 2. Two leaks detected
(confirmed from pressure testing) and recommended plugging of additional tubes; an urgent
inspection of 3 identical dryers was required by the customer and performed within the following
week. Testing repeated after 18 months, to assess the bundle corrosion rate.
Ltubes: 24.84 to 26.85 metres Øe: 16.5 mm thickness: 2.05 mm bundle: 647 steel “U” tubes.
å Nuclear power plant: inspection of 4 steam dryers/reheaters in group 3. Some blockages
detected maybe due to mechanical damage (impingement) localized on one side of the bundle, at 11
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
8
and 13 metres, on two of the dryers.
Ltubes: 25 to 27 metres Øe: 15 mm thickness: 1.2 mm bundle: 2,048 steel “U” tubes.
å Power plant, Italy: “zero-point” inspection on 4 newly built air-coolers, before coating, to
check for construction defects.
Ltubes: 3.55 metres Øe: 18.875 mm thickness: 1 mm bundle: 369 steel tubes, finned.
å Power plant, Italy: inspection on 4 air-cooler already in service, before coating. Some corrosion
(pitting) detected.
Ltubes: 3.47 metres Øe: 18.875 mm thickness: 1 mm bundle: 369 steel tubes, finned.
å Power plant, Italy. Assessment of the steam condenser residual fouling, after hydromechanical
cleaning with scrapers: inspections were repeated on samples of 500/1,000 tubes, to determine the
scale buildup rate and optimize the cleaning strategy.
Ltubes: 10.95 metres Øe: 22.25 mm thickness: 0.508 mm bundle: 12,978 titanium tubes.
å Power plant, Italy: inspection of a 1,000 tubes sample of the steam condenser. Assessment of
the steam condenser residual fouling, after hydromechanical cleaning with scrapers.
Ltubes: 7.89 metres Øe: 22.25 mm thickness: 0.6 mm material: titanium.
å Private customer, Italy: search of leakages in an exchanger. Two leaking tubes and an
extremely corroded third one were recommended for plugging.
Ltubes: 4.840 metres Øe: 20.17 mm thickness: 2.9 mm material: unknown.
å Private customer, Italy: general inspection of two exchangers from the air conditioning system,
undergoing refurbishing.
Ltubes: 1.99 metres Øe: 19.15 mm thickness: 1.15 mm bundle: 48 spiral tubes, finned.
Ltubes: 0.97 metres Øe: 19.15 mm thickness: 1.15 mm bundle: 60 spiral tubes, finned.
å Refinery, Italy: general control of cleaning efficiency and corrosion state of 56 heat exchangers
from several departments within the plant, for a total of 13,102 tubes. In all, 32 leakages were
identified on 11 exchangers; the efficiency of the preliminary clearing operations was found to be
fair.
Ltubes: 3.05 to 12.13 metres Øe: 25.4 to 19.05 mm thickness: 3.4 to 1.24 mm bundles: Al-brass, SAF
2507 steel, carbon steel.
å Polyolefins plant, Italy: post-cleaning control on 2 ethylene gas exchangers. A number of small
erosions, and some scale residues which were not removed during cleaning, were detected.
Ltubes: 4.88 metres Øe: 16 mm thickness: 1.17 mm bundle: 500 steel tubes.
Ltubes: 4.88 metres Øe: 16 mm thickness: 1.17 mm bundle: 467 steel tubes.
å Petrochemical plant, Italy: general control of exchangers provided for refurbishing
interventions, with identification of leakages and tubes to be plugged. Three exchangers inspected
so far for a total of about 1,500 tubes.
Ltubes: 4.880 metres Øe: 19.05 mm thickness: 1.65 mm bundle: 1.001 Al-brass tubes.
Ltubes: 6.10 metres Øe: 19.05 mm thickness: 1.65 mm bundle: 288 Al-brass tubes (of which 6
already plugged).
- Research & Development Department www.gma-tech.com
LxxxxxxR0611 – Nondestructive testing with APR technique
9
Ltubes: 6.10 metres Øe: 25.4 mm thickness: 2.85 mm bundle: 230 tubes (of which 20 already
plugged).
å Waste treatment facility, Italy: general control of the fume reheater. A sample of 1,160 tubes
was analyzed, resulting in detection of one leak and widespread corrosion problems; during the
subsequent outage, the analysis was repeated for the twin reheater.
Ltubes: 9.55 metres Øe: 33.3 mm thickness: 1.4 mm material: Saekaphen-coated steel.
å Waste treatment facility, Italy: general control of the steam condenser: erosion-corrosion was
found at the inlets and generalized corrosion in the uppermost of the bundle, subjected to a higher
thermal stress: a strategy for the recuperation was suggested.
Ltubes6.34 metres Øe: 22.25 mm thickness: 1/1.25 mm bundle: 4,390 Al-brass tubes, 270 Cu-Ni 70-
30 tubes.
Document compiled by:
Chiara Martellossi, PhD
Research & Development Assistant
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