Technical introduction to non-destructive testing using APR technique

G.M.A S.r.l.

ANTI CORROSION COATING

Nº Doc. LxxxxxR0611 Pagina 1 di 9

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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

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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.



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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.



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- 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.



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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.



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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.



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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.

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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



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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).



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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

Documents

Technical introduction to non-destructive testing using APR technique