The document discusses challenges with ultrasonic thickness monitoring of pipes and vessels. Simple gauges can have difficulties obtaining valid readings on painted, corroded, or high-temperature surfaces. Specialized gauges use multiple echoes to ignore variations from paint, but may fail to detect pitting corrosion. When surveying large areas, contractors use flaw detectors with screen displays, allowing operators to choose echoes and adjust for valid readings on problematic surfaces. Modern instruments have automatic controls and data logging advantages over older technology. Valid thickness readings require considering factors like coated surfaces, lack of repeat echoes, temperature effects, and using equipment with advanced capabilities.

1. Corrosion monitoring
Keith McLaughlin
How valid are your ultrasonic thickness
surveys?
The ultrasonic instrumentation used should
ensure that reliable ultrasonic thickness
readings are recorded on site and later
transferred to a printer or a PC for analysis,
but how often is that true?
The current techniques
Chemical processing plants and petroleum
refineries are major users of ultrasonic
equipment to monitor the wall thickness of
pipelines and storage vessels and this
technology is widely used throughout other
industries.
Simple digital ultrasonic thickness gauges
may be used on clean, unpainted surfaces
with reasonable success, but when the
surfaces are painted, heavily corroded or at
high temperature, the simple gauges have
difficulty in obtaining valid readings.
If a surface is painted, the simplest gauges
will penetrate well bonded paint and register
it as part of the metal, but as the speed of
sound in paint is approximately one third of
the speed in steel, the error incurred is three
times the actual paint thickness. A heavily
corroded contact surface will give a similar
error as the ultrasonic couplant trapped in the
rough surface has a similar effect to paint.
Some special gauges have been introduced
to work on multiple echoes and hence ignore
the variation in readings caused by the paint
etc. If, however, there is pitting corrosion
taking place, these instruments often fail to
detect the pits and give an optimistic result by
measuring the mean backwall.
For larger surveys, most plants use outside
contractors. These people use trained
ultrasonic operators equipped with flaw
detectors to carry out the surveys. Flaw
detectors have the advantage that they have a
screen display which gives the operator more
information about the material being tested.
The operator can choose which echoes to use
for the measurement. On a clean surface he
may use the distance to the first reflection to
indicate the thickness, but on painted surfaces
he should use the distance between repeat
echoes to obtain a valid reading. The
advantage of using a screen display is that on a
heavily corroded surface, the operator can
adjust the gain to measure to the correct point
on each echo. This would be fine, except for
the fact that thickness gauging is considered to
The author
Keith McLaughlin is Managing Director of Advanced
NDT Instruments, Severn Stoke, Worcester, UK.
Keywords
Corrosion monitoring, Non-destructive testing
Abstract
Details a number of current techniques and instruments
for ultrasonic thickness surveys, including special gauges
which have been developed for use on painted, corroded,
and hot surfaces. Suggests various features to be
considered when purchasing equipment.
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Features
26
Anti-Corrosion Methods and Materials
Volume 47 . Number 1 . 2000 . pp. 26±29
# MCB University Press . ISSN 0003-5599

2. be a low technology task, given to operators
with lower qualifications and older
equipment. Modern ultrasonic instruments
with screen displays have automatic gain
control, digital measurement of the range to or
between echoes and on board data loggers to
store the results. Compare this with the older
units where the thickness reading was taken by
counting the divisions on a graticule across the
screen, gain was adjusted manually and data
logging was a scribble on the back of a
cigarette packet.
Operating problems
Coated surfaces
As already mentioned, paint coatings on the
testing surface can cause errors. The simplest
gauges add three times the paint thickness to
the actual metal thickness measured, so if
there is a means, such as a coating thickness
gauge, of measuring the coating thickness,
this may be subtracted from the ultrasonic
result to give an approximate measurement of
the metal. This is, however, a tedious and
inaccurate method.
Special gauges have been developed to
overcome the paint problem. As shown in
Figure 1, sound introduced into a vessel wall
travels through the paint layer, through the
metal, reflects from the far wall and returns to
the probe. Ultrasonic measurements are made
by accurately timing the journey time. The
first return echo has a journey time which
comprises travelling twice through the paint
and twice through the metal. Some of the
sound beam, however, does not return to the
probe but is reflected internally in the metal
(see Figure 2). When the sound from the
second reflection arrives back at the probe, a
second echo is produced on the display. On
clean, unpitted material, a whole sequence of
echoes is produced.
It will be seen that the first return echo
corresponds to:
P Â „p ‡ P Â „m
and the second echo corresponds to:
P Â „p ‡ R Â „m
The difference between the two echoes
therefore corresponds to twice the metal
thickness which is displayed in pulse echo
calibration as a single metal thickness.
(Figures 3-5 indicate measurement between
echoes.)
Figure 1 Path taken by sound introduced into a vessel wall
Figure 2 Internal reflection of some sound
Figure 3 A-Scan display with thickness reading lightly corroded plate
Figure 4 The apparent extra thickness due to a thin layer of paint
27
Corrosion monitoring
Keith McLaughlin
Anti-Corrosion Methods and Materials
Volume 47 . Number 1 . 2000 . 26±29

3. All this is fine on an uncorroded backwall but
when the surfaces are heavily corroded it is
very difficult to obtain multiple reflections.
Whilst the digital gauges designed to operate
through paint have automatic gain controls
etc. to compensate for the weaker scattered
signals, their adjustments are limited. An
operator with a screen display can see the lack
of repeat echoes and adjust the gain etc. to
obtain a valid reading (Figure 6).
When measuring the metal thickness under
paint etc., the true thickness of the metal is
measured by the time interval between repeat
echoes. In practice, it may be difficult to
obtain repeat echoes on heavily corroded
surfaces, so an instrument with a screen
display should be used to allow adjustments
to verify that the correct echoes are
measured.
High surface temperatures
Thickness readings are frequently required
whilst the plant is still operating. With care, it
is possible to obtain readings on surfaces
which are very hot but several problems are
encountered:
. The attenuation of the material being
tested increases dramatically with
temperature. A material that may be
tested quite easily at ambient
temperatures may require high power,
low frequency probes to penetrate
it at higher temperatures. This will
affect the resolution of the
measurement.
. It is difficult to maintain coupling at
elevated temperatures. Special non-
inflammable couplants are required
which can be difficult to use.
. Special high temperature probes are
required as standard probes would
rapidly be destroyed by the heat. Even the
special probes can only be in contact with
the hot surface for short periods and then
must be given a chance to cool down.
These ``high temperature'' probes use a
delay line of a heat resistant material but
even this material changes its properties
with temperature, so automatic zeroing
from the probe face or echo to echo
measurements are required to eliminate
errors.
. The velocity of sound in the material
changes with temperature. To measure
accurately, calibration must be carried
out on a test block of known dimensions
heated to the same temperature. An
alternative is to correct the erroneous
readings taken with an ambient
temperature calibration by using velocity/
temperature tables for the material being
tested.
It is possible to obtain a relatively accurate
reading even at high temperatures but not
simply by purchasing a high temperature
probe. Many other factors have to be
considered.
Equipment solutions
The problems outlined above may result in
the unwary operator obtaining totally invalid
readings. Some equipment manufacturers
have recognised this and have introduced a
new generation of ultrasonic thickness gauges
to overcome these problems (see Figure 7 ±
The Eclipse TG2 A-Scan thickness gauge).
The features to look for when purchasing
equipment are:
Figure 6 A 3mm deep pit in unpainted plate. Not detected or
measured using multiple echoes with automatic gain. The A-scan
shows the defect and allows further adjustment to measure the pit
depth
Figure 5 Correct metal thickness measured through the paint using
multiple echoes
28
Corrosion monitoring
Keith McLaughlin
Anti-Corrosion Methods and Materials
Volume 47 . Number 1 . 2000 . 26±29

4. . A large high resolution A-scan
(oscilloscope) display.
. The ability to work with a wide range of
probe types.
. On-screen thickness readout.
. Echo to echo measurement
. Automatic gain control
. Last reading, ``hold'' or echo freeze
(to retain a fast reading, e.g. on a hot
surface).
. Storage/printout of calibration data.
. On board data logger.
Finally, these features need to be packaged
in a unit which is easy and convenient to
use.
These new thickness gauges should ensure
that the operator is capable of obtaining
meaningful thickness readings which can then
be used to predict the life of the plant and
avoid downtime.
Figure 7 The Eclipse TG2 A-Scan thickness gauge
29
Corrosion monitoring
Keith McLaughlin
Anti-Corrosion Methods and Materials
Volume 47 . Number 1 . 2000 . 26±29

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