MANAGING FURNACE INTEGRITY BY UTILIZING NON-DESTRUCTIVE TESTING (NDT)
AND MONITORING TECHNIQUES
*W. L. Ying, R. MacRosty, P. Gebski, R. Pula, A. Sadri, T. Gerritsen
Hatch Ltd.
2800 Speakman Drive
Mississauga, Canada L5K 2R7
(Corresponding author: [email protected])
ABSTRACT
Furnace run-outs can cause extended furnace downtime for repair, while leaks or more severe
failures in the cooling system can lead to brick hydration or in severe cases, steam explosions. These
incidents can result in lost production, damage to equipment, injuries and casualties. To prevent such
incidents, reliable and accurate techniques to monitor the critical components of the furnace such as
refractory lining, furnace structure, taphole integrity and main water circuits are required. This paper
describes several advanced methods that have been developed to assess the condition of furnace integrity,
refractory lining, tapholes and cooling water circuits.
KEYWORDS
furnace, integrity, monitoring, Acousto Ultrasonic-Echo (AU-E), Taphole Acoustic Monitoring (TAM)
Furnace Integrity Monitoring System (FIMS), fibre optic
INTRODUCTION
To enhance safety and reduce the risk of furnace run-outs, Hatch has developed a number of
systems to monitor the critical elements of furnaces. In this paper, five systems are presented. Among them,
the Taphole Diagnostic System (TDS); Fiber Optic Temperature Monitoring System; Taphole Acoustic
Monitoring (TAM) system; and Furnace Integrity Monitoring System (FIMS); are continuous monitoring
techniques and can be integrated into the control system. The fifth technique is the Acousto UltrasonicEcho (AU-E), which is a discrete monitoring of refractory lining condition. When a failure occurs it is
often around the tapblock and there are several mechanisms that can cause a failure:
1.
2.
3.
Loss of refractory between the copper tapblock and molten bath.
Movement of the block relative the hearth and sidewall opening a path for a leak.
Operational issues (e.g. lancing through the copper).
The Taphole Diagnostic System and Fibre Optic Temperature Monitoring System are designed to
monitor the refractory thickness in front of the tapblock and address the first failure mechanism. The
Acousto Ultrasonic Echo method can be used to periodically measure the refractory thickness. Movement
in the coolers can be detected with the Furnace Integrity Monitoring system and issues arising from
misaligned lancing when the taphole is opened can be monitored with the Taphole Acoustic Monitoring
System.
TAPHOLE DIAGNOSTIC SYSTEM
The Tapblock Diagnostic System (TDS) is an advanced real time monitoring system that
continuously monitors the tapblock throughout its life and accumulates wear events to assess the remaining
life of the tapblock. The main aim of this system is to provide an early warning of refractory wear and help
the furnace operators safely manage the operation and maintenance of furnace tapblocks. Typical
maintenance practice for the tapblocks is time or production based with regularly scheduled repairs of the
tapping channel refractory brick or replacement of the entire tapblock. Time/production-based maintenance
out of necessity is very conservative to avoid safety concerns and typically results in repairs being
performed well before required. The diagnostic system assists the operators to move to a condition-based
maintenance approach by using measurements from the instruments installed around the tapblock to
estimate the thickness of the remaining refractory.
The system uses operating data including temperature measurements, cooling water temperatures
and flow rates along with sophisticated models to assess the tapblock condition in real-time. Threedimensional thermal models are used to capture the condition of the block as it deteriorates from a new to a
fully worn condition. In cases where the existing thermocouple measurements are not sensitive enough or
do not provide sufficient coverage to adequately monitor refractory wear on the hotface, fiber optic
temperature sensors can be utilized to overcome these limitations. More information on the Fibre Optic
Monitoring System is presented in the next section.
In a typical water-cooled copper tapblock the measured temperatures from thermocouples may
only increase by several degrees Celsius as the refractory deteriorates from a new to worn condition. In
order to use these measurements to predict refractory thickness it is important to determine the tapblock
thermal resistance to a high level of accuracy. The tapblock thermal resistance is the overall thermal
resistance between the thermocouple and the cooling water, which includes the copper block resistance,
bonding resistance between the copper block and water pipe, and the cooling pipe resistance together with
the resistance of any scale build-up that may be present in the water pipes. The thermal resistance will be
unique on each tapblock and at each sensor location and can change over time, although experience
indicates that it changes very little. A bump test has been designed to extract this information while the
tapblock is in service, this procedure and the algorithms to evaluate tapblock condition are described
elsewhere (Gunnerwiek et. al, 2008; Gerritsen & Gunnewiek, 2011)
To date, the TDS has been installed and operated successfully for a number of years at two sites,
one in North America and the other in Europe. Both the sites operate Kivcet furnaces with four tapblocks
each and the diagnostic system monitors all the tapblocks simultaneously. Figure 1 shows the main
operating screen of a recent TDS installation, which provides an overview of all four tapblocks for the
operator. A simple traffic light display is implemented to advise operators whether it is safe (greyed out
light), to exercise caution (yellow light) or no further tapping should be carried out on that tapblock (red
light). The light indication is an overall condition index based on the assessment of several health indices.
In addition the number of taps on each tapblock is automatically accumulated and any temperature alarms
are highlighted to the operator. A detailed operator screen is also available to display all the temperatures
measurements and refractory wear evaluations on each tapblock.
Key features of the system include:
 continuous long term condition monitoring of tapblocks;
 provides a measure of condition as a health index (0-100%);
 automatically detects and counts tapping events;
 alarming capability to warn operators of unsafe conditions;
 includes a local historian to keep track of events for the duration it is in service.
Figure 1 – Main operator screen of the TDS showing the status of the four tapblocks
The diagnostic system installed at Teck’s Trail Operations in Canada has been in operation since
2003. Following a furnace maintenance shutdown, Hatch and Teck conducted a detailed study of a
tapblock after it had spent 30 months in service to compare the remaining refractory to the predicted
measurements from the the Taphole Diagnostic System (TDS).
The photograph in Figure 2 shows a tapblock with the refractory (green) in new condition. Figure
3 shows a tapblock removed from the furnace after 30 months of service. On this block, it was observed
that there was still 100% of the original refractory at the lower left corner. In the area to the right of the
tapping channel only 50% the original thickness of refractory remained and it was covered by black
accretion. Above the tapping channel the refractory was covered with a thick layer of accretion, which fell
off when the block was removed from the furnace. The TDS provided an accurate assessment of the
condition in these locations, with an overall health condition of this block of 50%. However, just below the
taphole, a location where there are no temperature sensors, the refractory was very thin. Without sensors in
this location, the system has no ability to predict the refractory thickness. This highlights the value of
comprehensive monitoring over the hotface of the furnace tapblock and provides the incentive to explore
fibre optic temperature sensing to provide better coverage.
Figure 2: New Tapblock
Figure 3: Tapblock removed after 30 months of
service
In the area where accretion was found, the refractory is no longer a solid homogeneous layer but
rather comprised of layers of refractory, accretion and metal. It was theorized that the layers result from a
process that starts with thermal stress cracking of the refractory which subsequently allows lead bullion
and slag to penetrate and freeze. Periodically the accretion breaks off, possibly taking off some refractory
with it, which leads to a spike in the temperature. As a new accretion layer forms the temperature readings
return to a range that is equivalent to a new tapblock.
Figure 4: Refractory Thickness Measurement for a Tapblock at thermocouple location B
This cyclic change in refractory thickness due to metal/slag penetration and accretion build-up
followed by spalling is illustrated in Figure 4. The figure shows the refractory thickness estimated for a
five-month period on a tapblock. During this period, around 250 taps were made on this tapblock or nearly
two taps a day on average. For the first month the refractory thickness remained at 100%. Then the
refractory thickness estimate is rapidly reduced to just above 60%, possibly due to a piece of
refractory/lead/accretion falling off. After a layer of accretion forms, the temperature measurements
indicate the lining thickness is back to 100% for the next two months. There are two further instances of
accretion spalling off and then in early September, the final minimum reading of about 50% is reached.
This result was somewhat surprising at the time; the expected response over the life of the block
was a monotonic increase in temperature as the wear progressed. However, the data clearly shows short
periods where the refractory is rapidly reduced (high temperature) and then the accretion rebuilds and the
lower temperatures return. In this furnace, the tapblocks operate with a very thin refractory layer relative to
other types of furnaces such as those in the Ferro-Nickel or PGM industry. The mechanisms responsible
for the rapid regeneration of the wall lining for this furnace is understood to be firstly the thin refractory
layer which will have a lower temperature at the refractory-bath interface than other furnaces making it
more capable of freezing material onto the surface. Secondly, the process is such that components (dross)
are present with a lower liquidus/solidus temperature and will readily freeze out of molten bath.
Two key findings from the investigation comparing the TDS prediction to measurements of
refractory thickness were:
1. The TDS prediction of overall tapblock health was quite accurate. However, there were areas of the
tapblock without any measurements and this raises the concern that a highly worn area could go
undetected. This led to the development and application of Fibre Optic sensors for temperature
monitoring on tapblocks to provide high spatial-resolution monitoring across the tapblock surface.
More is discussed on the Fibre Optic Temperature Monitoring System in the next section.
2. The high temperatures are only present for short periods but are important to monitor because they
provide an indication of the portion of the lining that consists of the original refractory. It is thought
that the original refractory is more robust to conditions in the furnace than an accretion layer and will
better protect the copper than accretion and therefore it is an important consideration in the assessment
of condition. With the level of monitoring that commonly exists in a typical plant control system, these
short spikes in temperature can be easily missed or forgotten once normal temperatures return. To
ensure these events are considered in the tapblock health evaluation, it is helpful to have software,
such as the TDS, to track the history over the time the block is in service and incorporate the extreme
temperatures (thinnest refractory) into the assessment.
FIBER OPTIC TEMPERATURE MONITORING FOR TAPBLOCKS
Hatch pioneered fiber optic temperature sensing technology for tapblock monitoring (MacRosty et
al, 2007; Gerritsen et al., 2009). The key benefits of the fiber optic temperature sensors are the small size of
the fiber and the ability to install many sensors along a single fiber optic strand. In addition to the benefits
of increased spatial coverage, the small diameter of the fiber allows the sensors to be located such that
sensitivity can be optimized. In the case of the tapblock, the fibers are installed across the hot face, on or
just below the surface of the copper. Figure 5 shows an example of an installation with two fiber strands (in
red) on the copper surface. Sensors spacing is customizable, but 50-100 mm intervals are typical over the
regions of interest. This will result in the use of approximately 50 fiber optic sensors per tapblock. The
traditional method for monitoring a tapblock would be eight to twelve thermocouples imbedded in the
copper; the fibre optic sensors provide a much denser measurement grid. The yellow tubes in Figure 5
show a thermowell/thermocouple arrangement for a typical tapblock.
The Fibre Optic Monitoring System is typically integrated into the existing plant control system
with a heat map display to highlight hotspots and capability to set alarm thresholds for each sensor. The
Fibre Optic Temperature Measurement System can be combined with the TDS, described in the previous
section, to augment the monitoring capability of the existing measurements. In general, the cost of fibre
optic sensors for a limited number of measurements is relatively high compared to thermocouples.
However, as the number of sensors increase to tens or hundreds of sensors, the fibre optic sensors become
very cost effective. In the last few years Hatch has developed methods to remove and re-install the sensor
arrays so they can be utilized for several campaigns, thereby greatly reducing the operating costs.
Installation of the fibre optic sensor at the copper-refractory interface provides very good
sensitivity to changes in refractory thickness. In this location the measured temperature spans a range of
over 60 °C as the tapblock refractory deteriorates from a new to a fully worn condition. In contrast,
thermocouples imbedded in the copper may only increase 2-3 °C for the same change in refractory if they
are in close proximity to the cooling water pipes. This 60oC range greatly enhances one’s ability to discern
a change in the refractory thickness and typical instrument issues like precision, accuracy and drift become
negligible on this scale. Although the fibre optic sensors do provide a high level of accuracy and stability;
an order of magnitude better than obtained with high quality thermocouples.
In certain cases where corrosion on the copper surface is an issue, tubes are cast into the copper
tapblock to house the fibre optic sensors. This has clear advantages with respect to protecting the tube from
corrosion, but there is some loss in sensitivity with respect to changes in the refractory thickness. The
thermal response to changes in the refractory thickness is highly dependent on the relative distance of the
sensor to the hotface and water pipe in the tapblock. More information on the various installation methods
is discussed by Braun et al. (2014).
Figure 5 – Tapblock showing location of thermowells and fiber optic cables
Over the life of the tapblock a general trend towards higher temperatures as the refractory wears is
expected. This is typical of furnaces in the Ferro-Nickel and PGM industry where the tapblock starts out
with a refractory lining more than hundreds of millimeters thick. Through the various cycles of the furnace
operation the temperatures move up and down but with a general trend upwards. The thermal trends
observed during tapping at several facilities are presented in MacRosty et al. (2014). In that paper, analysis
of the temperatures at a steady-state condition, approaching a state of continuous tapping, was shown to
provide a consistent basis to evaluate refractory condition. Due to different operating practices this
approach is only applicable at some facilities. In cases where there is no steady-state reached during
tapping the authors proposed using the dynamic response to determine the refractory thickness.
A key task is still to validate the predictions against actual refractory thickness measurements. A
challenge is the few opportunities that are available where it is possible to conduct a measurement survey
of the remaining refractory at the hotface of the tapblock. These shutdowns occur on a cycle of between six
months to two years depending on the facility; offering very few opportunities to inspect the refractory.
However, the refractory in the tapping channel of the tapblock is replaced on a four to eight week cycle.
Examination of the taphole refractory provides an opportunity to relate the condition of the tapblocks to the
fibre optic temperature readings in the tapping channel on a more frequent basis than is possible with
sensors on the hotface. It is thought that the relationship established between temperature and refractory
condition in the tapping channel will also apply to the hotface; this would need to be verified when the
tapblocks are eventually removed.
Figure 6 shows several months of data from a sensor located in the tapping channel at a facility in
South Africa. In this figure, the temperature spikes correspond to the temperature peaks from individual
tapping events. The saw-tooth pattern traced out by the peak temperatures during tapping is quite clearly
evident and corresponds quite closely to the brick repair schedule. The pattern generally starts out
relatively low (55oC) when the refractory is new and over time it increases, tending towards 65oC just
before the next repair. This increase in temperature is consistent with the taphole increasing in size and/or
the impregnation of the refractory with more conductive metal/matte. This saw-tooth profile is typical at
this facility; however, at another facility there is often a negligible difference in temperature readingsbefore
and after a brick repair. It is expected that the brick at that facility is still in good condition when it is
replaced. The replacement schedule is often driven by other factors at some sites.
Figure 6 – Saw-tooth temperature profile inside tapping channel between brick repairs
In summary:
1. A large number of measurements on a small diameter fiber allows good spatial resolution to be
obtained and in locations where it is not possible to install thermocouples.
2. Thermal modelling is valuable for providing context to the measured values to understand the
condition of the refractory. The three-dimensional nature of heat transfer through the tapblocks
requires sophisticated models to be developed; one-dimensional heat transfer relationships are
incapable of capturing the physics.
3. The accuracy of the prediction can be confounded by the presence of products of corrosion or air
gaps that form and modify the overall thermal resistance between the sensor and molten bath. The
impact and likelihood of such anomalies should be well understood in order to make a precise
assessment of the condition. The variable that most significantly impacts the temperature is
thickness of the refractory, a fortunate result since this is key variable in evaluating the tapblock
4.
condition. Furthermore, as the refractory thickness decreases its impact on temperature becomes
overwhelmingly dominant. At a thickness of 25mm, there is very little uncertainty associated
with the prediction. The interested reader is referred to MacRosty et al. (2014) for more details on
the work conducted to evaluate the sensitivity of factors that impact the measured temperature.
Comparing the temperature measurements with a measured refractory thickness is an important
task to establish confidence in the preditions.
TAPHOLE ACOUSTIC MONITORING (TAM)
Hatch developed the two-dimensional (2-D) TAM system to monitor taphole refractory damage
due to drilling and lancing. The TAM system works only on copper-cooled tapholes. The key advantages of
this system are that it provides immediate detection of stray lance or drill and can be installed on an
operating furnace, see Sadri, et al. (2013a) for details. TAM is based on the Acoustic Emission (AE)
monitoring principles. AE testing is a powerful method for examining the behaviour of materials
deforming under stress (Miller, 1987; Pollock, 1989). An acoustic emission can be defined as a transient
elastic wave generated by the rapid release of energy within a material. The development of TAM was
motivated by the need to monitor a particularly vulnerable tapblock that was compromised due to repeated
off-centre lancing. The system was commissioned in 2007 and was decommissioned in 2012 after the
tapblock was replaced. Several events detected by this original TAM system proved its accuracy and value
(Gebski, et al., 2013), which in turn motivated Hatch to develop the system further in cooperation with our
clients.
A typical 2-D TAM system consists of a pair of AE sensors mounted on the inlet and outlet of
each of the tapblock cooling water pipes. The illustration of this concept is presented in Figure 7, which
shows the transmission of the damage-induced elastic waves through the various media. The signal is
generated by the lance, drill or flow of metal in tappling channel and the cooling water pipes act as wave
guides for AE propagation. The elastic stress wave propagates through the refractory and copper, and then
reaches the cooling pipe where it is picked up by the AE sensors. Further signal processing is done by the
TAM software to locate the source of the acoustic emission.
Figure 7 – Schematic diagram of 2D TAM system showing one pair of sensors mounted on the
Monel pipe (different color arrows showing the AE transmission in different media)
The TAM system is valuable for detecting and reporting the upset conditions caused by off-center
drilling or lancing which can rapidly damage a tapblock. These events drastically increase the AE energy
released from the tapblock at that location and this is immediately detectable by the TAM system. The
source location algorithm, based on the input from multiple sensors, determines the particular areas of the
tapblock where the event originated. Figure 8 shows a typical screen that would be available to an operator
in the field to provide real-time feedback on lancing and drilling actions. This depicts a tapblock, from the
perspective of an operator at the cold-face looking into the furnace. The multiple zones represent the brick
inserts and the colors indicate the relative severity of the damage (the localized loss of brick thickness). For
the illustration shown, the highest damage is reported towards the hot-face of the tapblock, at the bottomright side. The color-coded damage severity (high, medium or low) relates to the proximity of the molten
material to the copper block.
Figure 8 – Visualization of a taphole brick damage detected by TAM
FURNACE INTEGRITY MONITORING SYSTEM (FIMS)
Furnace Integrity Monitoring System, or FIMS, being developed by the Hatch NDT Group for
monitoring structural integrity of furnace crucible (Gebski and Sadri, 2012; Gebski et al., 2011). Similar to
the 2D TAM technique, FIMS uses the principles of the AE monitoring technique. For the FIMS
application, AE sensors are mounted on the furnace shell to detect the lining and shell deterioration. The
sensors are installed in several rows, forming triangular or rectangular patterns. This layout allows the
entire vessel to be monitored. The highest density of sensors is focused on the most critical zones including
the skew bricks and tidal area. Ultimately, the sensor layout for a particular furnace is customized to ensure
the optimal monitoring results for that specific design. The initiation of refractory deterioration, including
wear, cracking or opening of the joints is detectable long before major failure occurs; hence FIMS can be
utilized to continuously monitor furnaces and locate defects during furnace operation.
The use of multiple sensors enables both the detection and location of deterioration-related signals
to be determined. The energy release from a local defect generates an elastic wave that propagates through
the medium. The elastic wave is detected by the piezoelectric sensors attached to the shell. Location is
determined by analysis of the differences in the time-of-arrival (TOA) of the source signal between the
sensors that measure the signal. The coordinates of the source/defect are instantly computed based on the
TOAs and the wave velocity within the medium.
Two prototype installations of FIMS were conducted in 2008 and 2009 on round electric furnaces
in Korea and in South Africa. The purpose of these installations was to validate the feasibility of this
concept and to collect initial data. The data set acquired by the system installed in South Africa included
events leading to a metal leak, as well as the readings from the run-out. This data contributed to the FIMS
development at its early stages, and it was utilized to identify patterns and a sequence of events that could
lead to a run-out.
The latest installation of the FIMS system was performed in the UK on a relatively small DC
furnace in 2014. As previously, the purpose of the installation was twofold: to provide an early warning
before a run-out and to contribute to optimizing relining schedule. The latter is achieved by evaluating the
refractory conditions in order to avoid premature relinings or failures due to refractory deterioration.
During the initial three months of monitoring several events were detected that indicated refractory damage
in critical areas of the furnace: near the tapholes and near the bottom refractory joint. The latter is
considered a severe anomaly as it opens up a pathway for metal penetration. This is also a mode of failure
previously observed at this site. The typical metal penetration pathways as well as the relevant indication
by FIMS are shown in Figure 9.
Figure 9 – Left: molten metal penetration pathways through a refractory joint
Right: A sample indication by FIMS exactly at the suspected refractory joint
Additional information about the refractory conditions based on the FIMS measurements can be
derived from the Acoustic Emission activity graph, showing the cumulative AE events over the time of
monitoring. A steep slope on the AE activity graph indicates concerning deterioration and typically occurs
shortly following the start up, partially due to the initial refractory growth and movement. It has also been
observed to occur prior to failures due to abrupt changes of the refractory conditions. An example of such
AE activity curve and sample indications of localized events is shown in Figure 10. A large release of
energy over a short period of time is often associated movement of furnace components. Such events
appear as clearly distinguishable step changes in the AE activity graph. Often, these events can be traced to
specific locations around the furnace to direct operators to the area of concern. This piece of information
greatly assists operators in assessing condition and evaluating risk when considering the maintenance
requirements and schedule.
Figure 10 – Top: Acoustic Emission activity graph
Bottom: Sample indications by FIMS localized refractory deterioration. The red dots show the location of
the AE event source (the location of the deterioration).
ACOUSTO ULTRASONIC-ECHO (AU-E)
The AU-E technique is based on stress wave propagation. A mechanical impact on the surface of
the structure is applied to generate a stress pulse, which propagates into the furnace. The pulse is partially
or fully reflected at material interfaces such as brick layers, refractory/molten metal interface or buildup/molten metal interface (Sadri et al., 2013b). The method uses both time and frequency domain analysis
of the reflected signal to determine the refractory lining and castable thickness of metallurgical furnaces.
When a furnace is newly relined and started-up, a baseline AU-E measurement of the furnace is
recommended to calibrate the material properties prior to any refractory deterioration. If the furnace has
been operated for some years, then calibration of refractory properties is done on sample bricks, or at some
known thickness regions such as recently repaired areas. Owing to the extreme conditions to which the
refractory is exposed , an annual inspection is recommended as part of the maintenance program. More
frequent inspections are often required for furnaces at the later stages of the campaign life. Such recurring
measurements allow the refraotory lining deterioration trend to be determined.
AU-E is by far the most widely used NDT technique for operating furnaces. Numerous AU-E
inspections have been conducted on electric arc furnaces to establish the lining wear, accretion build-up,
refractory impregnation, opening of gaps, refractory hydration and even lifting of the hearth due to metal
penetration. The measurements can be conducted on both the hearth and sidewalls.
Certain conditions will impact the UA-E readings and consequently an experienced technician
should always be involved when conducting surveys. For example, in cases where there is metal
penetration between the brick layers a larger than actual thickness of good brick is calculated with this
method. In cases where there is significant metal impregnation in the refractory, such as Figure 11, the
thickness of remaining refractory is determined to extend only to the point where it transitions from
original refractory to the impregnated refractory.
Figure 11 – Illustration of impregnation of refractory and signal reflection from the interface between
unaltered refractory and impregnated refractory
A typical cross-section of a worn sidewall profile is illustrated in Figure 12. At the cooler region,
the remaining good refractory profile was close to the cooler face, while at regions where the slag level
fluctuates (the tidal zone) the refractory is more susceptible to metal impregnation and wear and as a result
the remaining good refractory tends to be more highly eroded and is relatively thinner at locations between
coolers.
Figure 12 – Typical vertical cross-section of the sidewall worn profile
CONCLUSION
This paper describes various furnace condition monitoring techniques that have been developed
by Hatch over the past decade to enhance the safety of furnace operation. Hatch remains committed to the
continual improvement of furnace technologies that enhance safety and productivity and development of
these systems is ongoing. The fibre optic temperature measurement technique provides the capability to
monitor tap blocks accurately and with a fine spatial resolution. When these fibres are imbedded in a
tapblock the temperature measurements are most significantly impacted by the refractory thickness and
thus can be used to infer the condition of that tapblock. The Tapblock Diagnostic System uses thermal and
flow measurements to assess condition based on comparison with sophisticated three-dimensional thermal
models. This system has been successfully implemented on the tapblocks of Kivcet furnaces at two
smelter operations. The Tap-hole Acoustic Monitoring (TAM) system and Furnace Integrity Monitoring
System (FIMS) both use acoustic methods to monitor changes in the furnace linings. The TAM system is
capable of detecting off-centre variations in the tapping channel that result from drilling and lancing and
provide real-time feedback for tapping personnel. FIMS is useful for detecting the relative movement of
coolers and breaking refractory, both of which could have implication on the furnace wall integrity.
Acousto Ultrasonic-Echo (AU-E) is by far the most widel used NDT technique applied to operating
furnaces. Audits can be used to provide periodic measurements of refractory around the furnace. The
method uses sound waves to measure the refractory thickness from outside the furnace.
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