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Ultrasonic Probes and System for Tube Inspection Abstract

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					               Ultrasonic Probes and System for Tube Inspection




By

Gilles Rousseau
Walter Matulewicz
Jean-François Bureau
R/D Tech Québec, Canada




Abstract
Inspection of tubing can be done with several NDT techniques: eddy current, remote field eddy
current, flux leakage and ultrasonic methods. Each of these techniques has its merits and
limitations. Electromagnetic techniques are very useful to locate areas of concern but sizing is
hard because of the difficult interpretation of an electric signature. On the other hand, ultrasonic
methods are very accurate in measuring wall loss damage, and are reliable for detecting cracks.
Ultrasonic data is also easier to analyze since the data displayed is generally the remaining wall
thickness. It should be emphasized that ultrasound is an important tool for sizing defects in tubing.
In addition, it can be used in situations where eddy current or remote field eddy current is not
reliable, or as a flaw assessment tool to supplement the electromagnetic data.

The need to develop specialized ultrasonic tools for tubing inspection was necessary considering
the limitations of electromagnetic techniques to some common inspection problems. These
problems the sizing of wall loss in carbon steel tubes near the tubesheet or support plate, sizing
internal erosion damage, and crack detection.

This paper will present an ultrasonic tube inspection system with sample applications.
Introduction
Why ultrasonic (UT) tubing inspection? Ultrasonics can be used for the inspection of a wide range
of materials including ferrous, nonferrous, and nonmetallic tubing for detection of wall loss as the
result of corrosion, erosion, wear, pitting, cracking, and baffle cutting. Using an angled-beam probe,
ultrasonics is the only reliable crack detection method because ultrasound is totally reflected at the
crack interface.

Compared to the widely used electromagnetic tube testing techniques, ultrasonics offers several
advantages:

   •   Ultrasound is not affected by support plates or tubesheets
   •   Ultrasound is not affected by variation of electrical conductivity or permeability
   •   Ultrasound can accurately measure tube inner diameter (ID) and wall thickness
   •   Ultrasound can provide an accurate mapping of wall thickness or cracking
   •   Ultrasound interacts strongly with material interfaces such as those produced by cracks
Ultrasonic tube testing is not new and several systems are commercially available. Such systems
focus on a narrow range of applications:

   •   Internal rotary inspection system (IRIS): ultrasonic system for petrochemical heat
       exchanger. The probe and apparatus was developed in the late 50’s by Shell Oil for
       inspection of fin fan coolers (aluminum fin carbon steel tubes). IRIS has found others
       application such as in boiler tubes.
   •   Motorized probes are used for specific applications such as steam generator tubing
       inspections. These probes can perform wall thickness measurement, and detect and size
       cracking.
Several goals were set for the development of R/D Tech’s UT system:

   •   Produce a high speed tube inspection system for real-time wall loss measurement;
   •   Allow real-time B-scan and C-scan displays of wall thickness
   •   Improve the existing IRIS system
   •   Be able to use IRIS, motorized, and multielement probes
   •   Provide an economical UT method to do crack detection in tubing

IRIS Based UT Inspection System
The operating principle is based on the pulse-echo detection (Figure 1). A transducer excited by a
high frequency pulse produces an ultrasonic wave that propagates into water. A mirror deflects the
wave to produce a normal incidence beam on the ID tube. Echoes, reflected back from each metal-
water interface, are digitized and processed to extract the time of flight and amplitude of the
frontwall echo and back wall echoes. Further processing is applied to calculate the tube ID, outer
diameter (OD), and wall thickness (WT).


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Figure 1 : principle of pulse-echo for wall thickness measurement (left) and crack detection (rigth) with
an immersion probe.

Complete tube inspection is obtained by rotating the mirror. A hydraulic turbine (IRIS) or an
electric motor produces the driving force. Synchronization of the rotation can be obtained by various
methods: stepper motors, encoders, or ultrasonic target. The TC5700 can accommodate these
synchronization modes in order to display, in real time, the data either on a cross-section-thickness
display (B-scan), or as a surface area-thickness map (C-scan) (Figure 2).



                   C-scan                                    B-scan




                                                             Wall
                                                             loss
                   Flaws




                   D-scan




Figure 2: TC5700 ultrasonic display. Surface area-thickness map (C-scan) and cross-section-thickness
displays (B-scan and D-scan). During inspection, the C-scan and B-scan are in real-time.

During the inspection, the real-time B-scan and C-scan displays of ID, OD, or WT provide the
inspector with through thickness information that makes data analysis and damage assessment
straightforward. C-scan data can also be recorded in real time or a user-selected defective area can
be saved for offline analysis, reporting, and archiving.


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Setting up the equipment is the most critical step in tubing inspection. The hardware must be set to
detect the front wall and back wall echoes. Delay and velocity must be set properly to ensure a
precise measurement of the ID, OD, and WT. To achieve the best settings, an A-scan display is used
to make all the adjustments on the digitized waveform (Figure 3). This feature provides an optimum
inspection setup, which results in reliable tube size measurements.




Figure 3: A-scan setup mode. Tube size and meterial velocity are must be setup properly for a
precise measurement of tube's ID, OD and wall thickness.

The tube measurement is function of the accuracy of the couplant velocity and the tube material.
These two parameters are user-adjustable in order to set the proper couplant and material velocity. In
applications where velocity may change, the system has provisions for monitoring the couplant or
the material velocity, and automatically compensates for any variation during inspection. Such
feature is required for the inspection of component, such as pressure tube, where the water
temperature may vary along the tube.

Inspection is performed using at high pulse repetition frequency (PRF). The system PRF is
programmable from 1 to 18,000 pulses/second. Typically, an IRIS turbine can spin up to 3600
RPM. Full coverage of the tube circumference requires that the transducer is pulsed up to 12 kHz.
This provides a circumferential resolution of 1.8° or 0.016 in. for a 1-in.tube and an axial resolution
of 0.040 in. at a pulling speed of 2.4 inches/second.



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Ultrasonic Probes
The system can operate several types of probes:

   •   IRIS probe for wall loss measurement
   •   Shear wave IRIS probe for circumferential crack detection
   •   Motorized probes for wall thickness measurement or crack detection
   •   Multielement probe for high-speed wall thickness measurement
   •   Multielement probe for corrosion mapping of waterwall tubes, pipes, tank floor, and
       pressure vessels




Probe lead                              Centering unit                            Turbine
Figure 4: the IRIS probe is made of a hydraulic turbine, a transducer, a centering unit, and a probe
lead. The probe illustrated is for 1 inch tube.

The IRIS probe is made of a hydraulic turbine mounted with a 45-degree mirror (Figure 4). The
mirror reflects the ultrasound beam on the tube ID surface and multiple interface echoes are
reflected. Water is fed through the cable and acts as the driving force to the turbine. A target pin
supplies the rotation synchronization. The ultrasound bounces off the pin at every rotation and this
signal is monitored to start a new B-scan. Probe centering is critical, and replaceable centering units
permit inspections of tube IDs from 0.480 in. (12 mm) up to 3 inches (75 mm). The IRIS is a
technique well suited for petrochemical and BOP tube inspections. The TC5700 equipped with an
IRIS probe can produce high-resolution wall thickness maps. In Figure 5, the smallest set of 3 holes
is 0.040 in. in diameter, spaced by 0.080 in. and 0.008in. deep.




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Figure 5: Resolution of an IRIS test is demonstrated with a calibration tube. The tube is 1 inch OD X
0.095 WT, and has 18 flat bottom OD hole. The smallest set of holes are 0.040" in diameter, 0.080"
apart, and 0.010" deep.

The IRIS probe can be modified to produce a 45-degree shear wave in the tube wall. The shear wave
propagates in the tube wall, and any discontinuity with an edge normal to the tube wall will reflect an
echo. Typical discontinuities that can be found with this probe are circumferential cracking at the
tubesheet and baffle cutting. In shear wave testing, a flawless tube does not produce any echoes and
thus the C-scan display shows no indication. It should be noted that a shear wave IRIS probe will
not work on a conventional IRIS system because of insufficient data processing and display
capabilities (Figure 6).




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Figure 6: Result obtained from a calibration tube containing 5 EDM notches from 20% to 100% deep,
and with 3 grooves from 20 to 60 %. All defect were detected with an angle-beam IRIS probe.

A motorized probe allows high-resolution tube inspection. This, results in a more accurate wall
thickness map than an IRIS probe can provide. The use of a motor makes it possible to get position
feedback from encoders or stepper motor signals. A motorized probe may also be used in
applications were circumferential or axial cracking must be detected. Various kinds of motorized
probes can be used with the TC5700. An external motor controller is required to power the motor
and generate encoder signals. Furthermore, with a motorized probe, local immersion can be applied
to minimize the amount of couplant required. Eddy current coils can be easily added into the design
for detection of support plate, or bobbin inspection.

High-speed tube scanning can be performed using multielement probes. A 16-element probe
provides a measurement of wall thickness every 22.5 degrees. With an inspection speed of 1.5 ft/s
(500 mm/s), 0.040” (1 mm) axial resolution is achieved. A multielement probe has been designed
for the inspection of pressure tubes to accurately measure the tube ID and WT. For this application,
an 8-element probe is used to measure the wall thickness with an accuracy of ±0.002 in. (0.05 mm).
The tube ID is measured by pairing two channels firing in opposite direction. Channel pairing
eliminates the effect of probe centering on ID measurements. Temperature from the bottom to the
top of the tube may vary by several degrees and can introduce a large error on the ID measurement.
In order to meet the ID measurement accuracy (±0.008 in.), monitoring of the water velocity is
performed with an additional ultrasonic channel. This channel monitors the time of flight to a
stationary target, and any variation in the travel time is used to correct the water velocity.

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The use of multielement probes has been extended to surface inspection of plate, vessels, and
tubing. One application is water wall tube inspection with a handheld transducer probe (Figure 7).
The method consists of performing a thickness scan from the fireside. Four dual-crystal transducer
probes scan the OD surface. It is a very effective method for detecting isolated pitting that results
from hydrogen attack or caustic corrosion. Such pits are difficult to find with spot check thickness
gauging.




Figure 7: Waterwall tube inspection from the fireside using a handheld probe. The probe is made of 4
transducers in order to take thickness reading at various location around the circumference. The
operator slides the probe along a tube for detection of localized attack.

Sample Applications

    Carbon steel tube inspection
Inspection of ferritic and carbon steel tubes may be performed by remote field testing (RFT). RFT
is a fast and reliable method for scanning tubes in order to find damage caused by corrosion,
erosion, and wear. However, RFT data is more difficult to analyze because of some important
limitations. RFT cannot distinguish between ID and OD defects because of double wall
transmission. The double wall transmission causes the remote field signal from ID and OD defect
(with the same depth) to undergo the same phase shift because the signal has diffused to the same
thickness of metal. The double wall transmission does not allow separation of ID or OD
discontinuities based on phase analysis. Another limitation of RFT is the difficulty to size defect
under or near support plates because the remote field is absorbed by the support plate. With such
limitations, sizing of discontinuities found by RFT is function of the defect location, and detection is
difficult near support plates.

Using an ultrasonic method as a backup technique can compensate for the limitations of RFT. In the
example shown in Figure 8, a defect was found next to the third support plate. RFT sizing, using the

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voltage plane analysis method gave a maximum depth of 50% and a circumferential extension of
140 degrees. A through cross-section of the tube can be obtained with an IRIS probe. The IRIS
shows the tube cross section and confirms that maximum depth of the indication is 50% and the
circumferential extent is approximately 150 degrees. Analyzing the IRIS data in the C-scan display,
the defect can be sized along the circumference (B-scan) or along the axis (D-scan) of the tube. The
C-scan shows a color map of the remaining wall thickness and thus provides three views of the
defect. RFT, IRIS, and the C-scan data are complementary and provide a precise description of the
defective area.




         RFT defect signal                                        Defect as seen by IRIS
Figure 8: remote field and IRIS data from the same defect. The defect was detected using RFT.The
flaw is next to a support plate and was size as 51% short wall losses with a circumferential extend of
135 degrees. The sizing of the flaw was confirmed with an IRIS test. Shown to the right is the B-scan
of the defective area.

An inspection approach that provides both speed of inspection and accurate flaw assessment is to
combine RFEC and IRIS. RFEC is a fast method for scanning ferrous tubing and locating any
defective area. IRIS is a very accurate sizing technique and can be adapted for wall thickness
measurement or crack detection.

    Inspection of air cooler tubes for ID pits
Air cooler tubes are made of carbon steel tubes wrapped with aluminum fins to enhance convection
efficiency. Air cooler tubes are very difficult to inspect using remote field testing because the
aluminum fins disturb the diffusion of the electromagnetic field.

IRIS is the only technique capable of detecting and sizing damage in a carbon steel tube wrapped
with fins. Most of the damage is found inside the tube and is caused by internal pitting. For such
inspection, the analysis will focus mainly on the internal by displaying an ID C-scan. The pits are
small and the C-scan improves the ability to detect small changes of thickness by the use of the
color variations. Any abrupt change in color generally indicates an internal pit. Once pits are
detected, the sizing is done on the B-scan for an accurate depth measurement (Figure 9).

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Figure 9

    Sizing of internal wall losses in condenser tubes
Internal defects are often found in condenser tubes. This type of defect is difficult to size using
eddy current phase or amplitude information. An alternative method that can provide an exact
measurement of wall losses and tube ID is to use an ultrasonic probe. The transducer can be
selected to focus on the tube ID and then produce a highly accurate measurement of the tube ID,
OD, and WT as illustrated in Figure 10. Since recorded data can display B-, C-, and D-scan views, a
volumetric assessment of the flaw is presented.




Figure 10




                                                                                          Page 10
    Outside Corrosion in double coaxial tubes
In the last example, the problem is the internal corrosion of a tube-to-tube weld. The corroded area
is about 0.5 inch from the bottom of the tubesheet. Location of the defective area makes it
particularly difficult to inspect because of the length of the IRIS probe and the need to flood the
tube. A probe support was built in order to center the probe and two inspection windows were cut
out to permit the ultrasonic wave to reach the tube wall (Figure 11). The inspected area is
approximately 1.5 inches long. To cover the full circumferences, the tube is inspected in two passes
with the probe support rotated by 90 degrees between each pass. Figure 11 displays data from a
defective tube with 50% wall loss at the weld.




Figure 11




                                                                                            Page 11
Conclusions
The TC5700 ultrasonic system has been successfully used for two years on several applications:

  •    Carbon steel tube inspection in petrochemical and power generation
  •    Backup technique for sizing defects found by remote field testing
  •    Sizing of internal wall losses detected by eddy current
  •    Inspection of pressure tubes with multielement probes
  •    Corrosion mapping of water wall tubes
The use of ultrasonics has been limited due to the lack of a flexible system that can perform wall
thickness mapping, and can interface to various probe types. The inspection system developed has
overcome several limitations of currently available systems, and has provided new solutions to
ultrasonic tube testing.

Some inspection problems cannot be dealt with either eddy current or RFT techniques. In such
cases, an ultrasonic approach may be the only practical inspection solution.

Using ultrasonics as a sizing tool to supplement eddy current or RFT is desirable in situations
where the flaw depth and extent detection is unreliable by electromagnetic techniques. An ultrasonic
wall-thickness mapping is a non-invasive approach that provides information on flaws with accuracy
comparable to tube pulling.




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