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Thermography for iron and steel plant
Thermography for iron and steel plant H. B. Phillips, K. F. Williams Iron and steelmaking use a series of processes in which heat is introduced, evolved or contained. Studies of the thermal patterns from such processes provide useful information on changes in performance and service conditions of containment vessels, machinery and components. Infra-red sensing and real-time data presentation provided by thermal cameras make possible rapid studies of thermal patterns. Several systems have already undergone industrial trial and it has been concluded that reliability, reasonably fast scanning and compilation and rapid data recording were required. The authors report applications of thermal imaging to steelworks plant. They describe the procedures adopted. Finally they consider the cost and ease of operation of thermography compared with other methods of temperature surveying.
Using the thermographic technique it is possible by remote observation to present on a screen, real-time pictures showing the variation in the surface heat emission of an object. A large number of the steel-making processes involve the emission or control of heat. In a number of these, thermal imaging or thermal scanning cameras could be employed usefully for maintenance, control and process studies. The thermal imaging and profiling technique was developed as a natural consequence of the availability of fast-response and sensitive temperature sensors. To date, the temperature sensors utilized in most commercial instruments are the cryogenic type and have a typical time constant of 1/~s. The equipment
The equipment used to present these thermographic pictures consists usually of a camera unit and an oscilloscope display screen. Generally the camera unit includes a lens system focussing on the liquid nitrogen cooled detector and a very fast mechanical scanning system placed between the lens and the detector enabling the detector to see only a very small part of the field of view at any one instant (Fig.l). The oscilloscope image is built up by a beam of electroi;s scanning in sympathy with the camera scanning system, the intensity of the beam being controlled by the output of the thermal detector in the camera. The picture on the screen is therefore of varying intensities representing the different surface temperatures of the objects in the field of view of the camera. It is possible to calibrate the system and to record the picture on film. The latest thermographic equipment utilize a colour recording or even a real-time colour display scheme where each colour represents a given temperature range. Using these systems it is possible to display the complete range of surface temperatures in a H. B. Phillips and K. F. Williams are with the British Steel Corporation.
152
Fig.1 The AGA Thermovision 680 infrared camera unit has a mechanical optical scanning system
number of isotherm bands on one colour picture for ease of interpretation (Fig.2). Alternatively the detectors can be used in line-scan equipment that uses the same basic technique, that is, they have a fast scanning mechanism in the camera coupled with a liquid nitrogen cooled detector. On the oscilloscope display the position on the scan is indicated on the x-axis and the detector output, the temperature, displayed on the y-axis, (Fig.3). Some profiling equipments include a vertical scan enabling a number of temperature proffdes from different positions to be displayed on the screen. After trials over a few years it is only very recently that thermography has been applied in earnest in the British Steel Industry. The equipment has been used successfully in the fields of medicine, defence and effluent tracing for a number of years.
NON-DESTRUCTIVE TESTING . JUNE 1974
from the structures, this is especially important when high temperatures are involved. Real-time observation allows varying parameters to be observed and recorded without the need to disturb the processes involved, for instance temperature measurements can be made on fast moving hot roiled steel strip. Permanent records can be made very easily either on film or on electromagnetic tape. For instance, using film, the thermal parameters of an iron or steel plant structure can be monitored at periods throughout its working life and by careful comparative analysis it should be possible to suggest design improvements for any future structures. This is especially true for refractory lined vessels. Savings in refractory could be made in certain areas and the life of the vessel could be increased by using thicker skin in other parts of the vessel. Careful recordings with the thermal imaging camera should firmly establish all these parameters over a period of time. Isotherms allow changes in temperature with position to be observed rapidly. This is very useful, for instance, in ingot moulds which heat up non-uniformly relatively quickly. Problems
Fig.2 The surface temperatures of a blast furnace stove are depicted in isothermal bands on this Tharmogram; the original colours are seen here as different shades of grey
Fig.3 The temperature across a hot steel strip can be shown by a line scan of temperature; the ordinate shows temperature and the abscissa shows position
Relative
advantages
of thermal
imaging
cameras
The thermal imaging camera system is a fairly complex means of measuring temperatures, thus in all cases it is sensible to consider whether a two-dimensional image or a rapid line profile is essential or whether other means of measuring temperatures are satisfactory, before proceeding with the use of the camera. The major advantages of thermal imaging camera systems are: remote observation; real-time observation; permanent records; and isotherms. Remote observation is required because the majority of the fixed structures in the iron and steel plants are large and therefore any contact temperature measuring system would mean the costly and time-consuming erection of scaffolding or lift systems. Measurements can be made at safe distances
N O N - D E S T R U C T I V E T E S T I N G . JUNE 1974
of applications
and some solutions
The majority of the presently available equipments are fairly bulky and together with accessories can weigh 50 Kg. Ideally purpose-built vehicles have to be used for the transport and setting up of the equipment. This is very satisfactory for work out of doors but indoors thermography can still present manhandling problems. The manufacturers themselves have the ultimate answer to the size problem and more recently very versatile portable thermal imaging cameras have become available. When accurate temperature measurements are required the values obtained from the system are dependent on an accurately known reference temperature in the field of view. If it is operated out-of-doors one can in most cases take the ambient temperature as a reference. Indoors, thermocouple type instruments are usually used to establish an accurate reference temperature. Small hand-held infra-red sensitive thermometers can be used, but these also are dependent on knowing the emissivity value of the objects. It is possible to purchase standard reference temperature sources; these are quite satisfactory for laboratory use, but their use on site is strictly limited because of accessibility and safety problems. The emissivity of materials can have a large effect on the temperature readings. This does not present too great a difficulty when all the objects in the field of view have the same emissivity. Usually, if the absolute temperature of the object is not of prime importance and it is of fairly uniform appearance, (for instance, corroded steel) it is acceptable to assume the mean emissivity. Otherwise, if an important section of the field is at a different emissivity from that of the bulk it is necessary to make a separate calculation to obtain the temperature of this particular section. It is always necessary to have available a supply of liquid nitrogen, and care has to be taken in the filling of the detector reservoir. Unless set up for a certain application and calibrated suitably, temperature calculations are fairly time consuming. The instrument scale is in 'isotherm units' and the value of an isotherm unit varies with temperature because of the non-
153
linear output of the detector, hence reference has to be made to calibration charts.
The iron and steelmaking processes Most processes in the steel industry utilize the principle of heat transmission or retention in one way or another. The most easily recognisable structure in the iron and steel works is the blast furnace. A mixture of iron ore, coke and limestone is fed into the top of the furnace and hot air is blown in at high velocities at the bottom. The air combines with the carbon in the coke to form carbon monoxide which, at high temperatures reduces the iron ore to iron. These processes inside the furnace produce a large amount of heat and movement of the contents. A modern blast furnace is about 40 m high and about 8 m in diameter at its base or hearth. It consists of a steel shell, enclosing a refractory lining which is designed to withstand the rigours of the reduction process for up to 4 years while 2 - 3 million t of iron are produced and perhaps twice this in the future (Fig.4). The external shell temperature of the blast furnace will depend on the internal temperature, on the lining material and thickness and on the efficiency of any cooling system. Thus the use of a thermal imaging camera to monitor the external shell temperatures will be an indication of the furnace conditions. The heat of combustion produced inside the blast furnace is recycled. The hot gases evolved at the top of the stack are passed through the blast furnace stoves. These are cylindrical structures about 20 m high and about 10 m diameter which contain large amounts of refractory in a labryinth and are refractory lined. When one stove is saturated with heat from the blast furnace, the furnace exhaust gases are then directed to the next stove. Each blast furnace usually has 3 or 4 stoves to allow enough continuous heat re-cycling capacity. The air supply to the furnace is of course drawn through each hot stove in turn. The high velocities of the gases together with entrained dust, in time, erode the interior wall of the stoves. The exact principle of the stoves differs but in any case careful monitoring of the external wall temperatures yield useful information on the state of the lining. The next process in the steelworks is the conversion of the iron into steel. This process is carried out in a different part of the works. The molten iron from the blast furnace therefore has to be transported from the blast furnace area to the steel plant. This is usually done in large refractory lined ladles. The largest of such ladles are torpedo ladles which are long cigar-shaped vessels. Erosion of the lining occurs during filling and by movement of the molten iron during transportation of the vessel. Here again, therefore, external shell temperature monitoring can be of great assistance in determining the extent of shell lining wear. In some steelworks hot iron is stored in mixer vessels until it is needed in the steelmaking plant, these again are large refractory lined vessels where the thermal imaging method of measuring external shell temperatures has obvious advantages. The iron is commonly converted into steel in the steelmaking vessels. These are large vessels capable of holding 150 to 300 t of steel. Firstly scrap and molten iron are fed into the vessel then oxygen at high pressure is blown onto the surface of the mix for a period of about 20 min oxidizing the carbon in the iron resulting in low carbon
154
Fig.4 A blast furnace is basically a large steel vessel lined with refractory material and designed to contain and withstand great heat
steel. This turbulent reaction causes erosion of the vessel lining which can be detected by external temperature monitoring. The steel thus obtained is then cast into ingots which are subsequently rolled in various stages ending up in the form of a steel coil ready for further processing. These finishing processes involve a considerable amount of heat emission where continuous thermal monitoring can be of great use.
Applications To illustrate the operational advantages and difficulties, the use of the thermal imaging system within the blast furnace area will be described in more detail and reference will be made to its applications within the other areas of the steelworks. The blast furnace is by far the largest heat containing structure in the steelworks. To date, practical experience, point contact temperature measurement, cooling water temperature rise, and occassionally radio-tracer techniques have been used to indicate furnace condition and have helped indicate the type and thickness of future furnace linings. Point contact temperature measurement has obvious disadvantages. Access to the side of the furnace is both difficult and dangerous, also, it is not practical due to the sheer physical size of the furnace to cover all the surface area. However, using a thermal imaging camera it was possible to examine the upper regions of the furnace from the open, about 50 m away using a 10 ° x 10 ° lens. From this distance the outline shape of the furnace was seen in the thermogram enabling any areas of interest to be pin pointed quickly and accurately.
N O N - D E S T R U C T I V E T E S T I N G . J U N E 1974
When an excessively hot patch was seen to be present on one blast furnace it was immediately reported to the management (Fig.5). Their reaction was to water spray the area in question to lower its temperature and thus reduce the rate of erosion until a more permanent repair could be affected. A large part of the blast furnace is under cover, thus any thermal imaging of these areas has to be done with the camera much closer to the furnace. This necessitates the use of a wider angle camera lens; 25 ° x 25 ° was found to be suitable for this work. Thermography indoors at the blast furnaces presented more difficulties than the work out of doors. All the equipment had to be manhandled up narrow flights of stairs and along uneven floor areas to obtain the best viewing positions. For safety, a portable generator was usually used as a power supply. Having found a suitable viewing area the camera team usually had to wait for suitable viewing conditions. Within the cast house there seems to be an abundance of overhead crane operation, also, during the tapping of the furnace, great clouds of dust emerge, these, together with an excessive amount of reflected heat originating from the molten iron cause delays in producing a good thermographic survey. From one chosen position it is usually possible to carry out a fairly comprehensive thermal study of one side of the furnace. From the same position it is also usual to examine the hot gas main pipe to the furnace, the bustle main. The discovery of a hot area on the furnace is usually followed by a blast furnace management enquiry into the causes. Sometimes it is due to a failing water cooler, if this is the case the cooler can usually be put right. If the hot patch is considered to be due tO a localized thinning of the refractory furnace lining the immediate action usually taken is to water spray, and then, in suitable positions, during the next blast furnace down period quick-setting refractory material can be pumped into the lining. It is usually fairly easy to locate
1
ii
Fig.5
i
ii
ii
I
I
A thermogram of the upper half of a blast furnace reveals
a severe hot spot
N O N - D E S T R U C T I V E T E S T I N G . J U N E 1974
Fig.6 The temperature profile and thermogram (top left) are of a used torpedo ladle similar to the one shown
1
50
2
100
5
2o0
10
500
20
1.000
0
Fig.7
'
.2
,
.
,ll
~
/,
.
THV 1.0
A thermogram of an ingot mould helps in design evaluation
a specific area from the thermal picture, as railings and pipes etc which are at different temperatures to the furnace itself, show up clearly. The policy for the future is to examine the blast furnaces periodically with the thermal imaging system. It is now thought that periods of between three and six months will be suitable. In the longer term it is expected that this practice will assist in the design of future blast furnaces having special regard to refractory and lining thickness parameters. Similar detailed work has been carried out on the blast furnace stoves. As the stoves usually outlive the blast furnaces the results obtained here coupled with future periodic examination are of long term interest. The other three major areas where work has been carried out to date are the torpedo ladles, steelmaking vessels and ingot moulds. The torpedo ladle and steelmaking vessel exercises were similar to the blast furnace work in that the prime interest was refractory lining thickness (Fig.6). However, the effect of increased temperature on the creep properties of the steel shell of torpedo ladles is an important consideration. The ingot mould work is different in that refractories are not the main considerations. Very important thermal and mechanical facets have to be considered in t h e design of the ingot mould: thermal, because non.uniform heating and cooling in the walls will tend to cause fractures because of the non-linear expansion in the walls (Fig.7); mechanical, because in the stripping bay the ingots are pushed out of the moulds by a hydraulic press which places great mechanical stress on the mould. It is found in practice that if a mould is designed for optimum thermal p r o -
155
perties its mechanical strength is not sufficient. Further programmes of work are now being prepared following the purchase of a thermal imaging system, the aim is to optimize ingot mould design.
Corporation aiding the production personnel towards better thermal control of the mills and their products.
At the roiling mills, line scanning equipment is used. The aim is to measure accurately the steel strip temperature both longitudinally and in the transverse direction during the various rolling stages. It is also possible to measure roll temperatures. For the research trials the equipment was mounted on a mobile hydraulic lift so that it could be positioned above and at a safe distance from the moving hot steel strip. For longer term trials of course the equipment could be rigidly f'Lxedto a suitable structure on the mill.
There are definite advantages in the use of thermal imaging systems on refractory lined heat retaining structures in the works. The main advantages being the speed at which measurements can be made and the complete data presentation. It has been estimated that if thermal imaging was used as a tool for the examination of torpedo ladles, thus avoiding the practice of taking the ladles out of service to allow them to cool sufficiently for internal examination, at one works the utilization could be improved by one torpedo ladle in twelve.
It is well known that the physical properties of steel, mechanical strength etc vary with temperature and thus one aim of the exercises with the thermoprot'tle equipment was to attempt to correlate the temperature distribution across the steel strip with its physical shape. Some degree of success has been achieved. In an exercise carried out at one works it was shown by using the thermoprofding equipment that the strip tended to be hotter on the operating side of the mill. It was also shown by accurate physical sampling and measurement that if the mill did turn out wedge-shaped material the thin end of the wedge was always on the operating side. It was thought that the reason for this uneven temperature distribution was the slight bias of the descaling sprays towards the drive side of the mill. The descaling sprays are high pressure water sprays designed to remove traces of scale from the strip. Measurements have also been carried out on the work rolls immediately after their removal from the mill. A matt black line was painted quickly onto the shiny roll to increase the emissivity to a measurable value. Not enough work has been carried out to-date to quantify the effect of roll temperature on strip temperature shape. Even at this early stage of development it has been established beyond reasonable doubt that the thermal profiling method does have great advantages both for research and production use. Thermoprofile systems have or are being used at a number of the mills of the British Steel
156
Conclusions
Potential applications include monitoring refractory lining conditions in blast furnaces, stoves or heat exchangers, hot molten iron transporting ladles, hot metal mixers and steel making converters. In the engineering maintenance fields the equipment can be used for early warning of bearing failure, locating electrical faults and showing faults in many important components. Many processes involving heat emission such as hot-dipped coatings, annealing, ingot, slab or bloom reheating may be studied by this technique. The thermal profiling equipment has also established itself as a useful tool, mainly in hot mill temperature measurement and control. Further work is in progress with both types of equipment to realize their full potential within the iron and steel works.
Acknowledgements The authors wish to express thanks to the Management of the Research Department of the Strip Mills Division of the British Steel Corporation for permission to publish this paper. It is appreciated that none of the work described would be possible without the co-operation of plant Management, Engineers and Research colleagues and the active assistance provided by many others at the works visited. Thanks are due also to the equipment suppliers for extensive help during the trials.
N O N - D E S T R U C T I V E TESTI NG . JUN E 1974
Using the thermographic technique it is possible by remote observation to present on a screen, real-time pictures showing the variation in the surface heat emission of an object. A large number of the steel-making processes involve the emission or control of heat. In a number of these, thermal imaging or thermal scanning cameras could be employed usefully for maintenance, control and process studies. The thermal imaging and profiling technique was developed as a natural consequence of the availability of fast-response and sensitive temperature sensors. To date, the temperature sensors utilized in most commercial instruments are the cryogenic type and have a typical time constant of 1/~s. The equipment
The equipment used to present these thermographic pictures consists usually of a camera unit and an oscilloscope display screen. Generally the camera unit includes a lens system focussing on the liquid nitrogen cooled detector and a very fast mechanical scanning system placed between the lens and the detector enabling the detector to see only a very small part of the field of view at any one instant (Fig.l). The oscilloscope image is built up by a beam of electroi;s scanning in sympathy with the camera scanning system, the intensity of the beam being controlled by the output of the thermal detector in the camera. The picture on the screen is therefore of varying intensities representing the different surface temperatures of the objects in the field of view of the camera. It is possible to calibrate the system and to record the picture on film. The latest thermographic equipment utilize a colour recording or even a real-time colour display scheme where each colour represents a given temperature range. Using these systems it is possible to display the complete range of surface temperatures in a H. B. Phillips and K. F. Williams are with the British Steel Corporation.
152
Fig.1 The AGA Thermovision 680 infrared camera unit has a mechanical optical scanning system
number of isotherm bands on one colour picture for ease of interpretation (Fig.2). Alternatively the detectors can be used in line-scan equipment that uses the same basic technique, that is, they have a fast scanning mechanism in the camera coupled with a liquid nitrogen cooled detector. On the oscilloscope display the position on the scan is indicated on the x-axis and the detector output, the temperature, displayed on the y-axis, (Fig.3). Some profiling equipments include a vertical scan enabling a number of temperature proffdes from different positions to be displayed on the screen. After trials over a few years it is only very recently that thermography has been applied in earnest in the British Steel Industry. The equipment has been used successfully in the fields of medicine, defence and effluent tracing for a number of years.
NON-DESTRUCTIVE TESTING . JUNE 1974
from the structures, this is especially important when high temperatures are involved. Real-time observation allows varying parameters to be observed and recorded without the need to disturb the processes involved, for instance temperature measurements can be made on fast moving hot roiled steel strip. Permanent records can be made very easily either on film or on electromagnetic tape. For instance, using film, the thermal parameters of an iron or steel plant structure can be monitored at periods throughout its working life and by careful comparative analysis it should be possible to suggest design improvements for any future structures. This is especially true for refractory lined vessels. Savings in refractory could be made in certain areas and the life of the vessel could be increased by using thicker skin in other parts of the vessel. Careful recordings with the thermal imaging camera should firmly establish all these parameters over a period of time. Isotherms allow changes in temperature with position to be observed rapidly. This is very useful, for instance, in ingot moulds which heat up non-uniformly relatively quickly. Problems
Fig.2 The surface temperatures of a blast furnace stove are depicted in isothermal bands on this Tharmogram; the original colours are seen here as different shades of grey
Fig.3 The temperature across a hot steel strip can be shown by a line scan of temperature; the ordinate shows temperature and the abscissa shows position
Relative
advantages
of thermal
imaging
cameras
The thermal imaging camera system is a fairly complex means of measuring temperatures, thus in all cases it is sensible to consider whether a two-dimensional image or a rapid line profile is essential or whether other means of measuring temperatures are satisfactory, before proceeding with the use of the camera. The major advantages of thermal imaging camera systems are: remote observation; real-time observation; permanent records; and isotherms. Remote observation is required because the majority of the fixed structures in the iron and steel plants are large and therefore any contact temperature measuring system would mean the costly and time-consuming erection of scaffolding or lift systems. Measurements can be made at safe distances
N O N - D E S T R U C T I V E T E S T I N G . JUNE 1974
of applications
and some solutions
The majority of the presently available equipments are fairly bulky and together with accessories can weigh 50 Kg. Ideally purpose-built vehicles have to be used for the transport and setting up of the equipment. This is very satisfactory for work out of doors but indoors thermography can still present manhandling problems. The manufacturers themselves have the ultimate answer to the size problem and more recently very versatile portable thermal imaging cameras have become available. When accurate temperature measurements are required the values obtained from the system are dependent on an accurately known reference temperature in the field of view. If it is operated out-of-doors one can in most cases take the ambient temperature as a reference. Indoors, thermocouple type instruments are usually used to establish an accurate reference temperature. Small hand-held infra-red sensitive thermometers can be used, but these also are dependent on knowing the emissivity value of the objects. It is possible to purchase standard reference temperature sources; these are quite satisfactory for laboratory use, but their use on site is strictly limited because of accessibility and safety problems. The emissivity of materials can have a large effect on the temperature readings. This does not present too great a difficulty when all the objects in the field of view have the same emissivity. Usually, if the absolute temperature of the object is not of prime importance and it is of fairly uniform appearance, (for instance, corroded steel) it is acceptable to assume the mean emissivity. Otherwise, if an important section of the field is at a different emissivity from that of the bulk it is necessary to make a separate calculation to obtain the temperature of this particular section. It is always necessary to have available a supply of liquid nitrogen, and care has to be taken in the filling of the detector reservoir. Unless set up for a certain application and calibrated suitably, temperature calculations are fairly time consuming. The instrument scale is in 'isotherm units' and the value of an isotherm unit varies with temperature because of the non-
153
linear output of the detector, hence reference has to be made to calibration charts.
The iron and steelmaking processes Most processes in the steel industry utilize the principle of heat transmission or retention in one way or another. The most easily recognisable structure in the iron and steel works is the blast furnace. A mixture of iron ore, coke and limestone is fed into the top of the furnace and hot air is blown in at high velocities at the bottom. The air combines with the carbon in the coke to form carbon monoxide which, at high temperatures reduces the iron ore to iron. These processes inside the furnace produce a large amount of heat and movement of the contents. A modern blast furnace is about 40 m high and about 8 m in diameter at its base or hearth. It consists of a steel shell, enclosing a refractory lining which is designed to withstand the rigours of the reduction process for up to 4 years while 2 - 3 million t of iron are produced and perhaps twice this in the future (Fig.4). The external shell temperature of the blast furnace will depend on the internal temperature, on the lining material and thickness and on the efficiency of any cooling system. Thus the use of a thermal imaging camera to monitor the external shell temperatures will be an indication of the furnace conditions. The heat of combustion produced inside the blast furnace is recycled. The hot gases evolved at the top of the stack are passed through the blast furnace stoves. These are cylindrical structures about 20 m high and about 10 m diameter which contain large amounts of refractory in a labryinth and are refractory lined. When one stove is saturated with heat from the blast furnace, the furnace exhaust gases are then directed to the next stove. Each blast furnace usually has 3 or 4 stoves to allow enough continuous heat re-cycling capacity. The air supply to the furnace is of course drawn through each hot stove in turn. The high velocities of the gases together with entrained dust, in time, erode the interior wall of the stoves. The exact principle of the stoves differs but in any case careful monitoring of the external wall temperatures yield useful information on the state of the lining. The next process in the steelworks is the conversion of the iron into steel. This process is carried out in a different part of the works. The molten iron from the blast furnace therefore has to be transported from the blast furnace area to the steel plant. This is usually done in large refractory lined ladles. The largest of such ladles are torpedo ladles which are long cigar-shaped vessels. Erosion of the lining occurs during filling and by movement of the molten iron during transportation of the vessel. Here again, therefore, external shell temperature monitoring can be of great assistance in determining the extent of shell lining wear. In some steelworks hot iron is stored in mixer vessels until it is needed in the steelmaking plant, these again are large refractory lined vessels where the thermal imaging method of measuring external shell temperatures has obvious advantages. The iron is commonly converted into steel in the steelmaking vessels. These are large vessels capable of holding 150 to 300 t of steel. Firstly scrap and molten iron are fed into the vessel then oxygen at high pressure is blown onto the surface of the mix for a period of about 20 min oxidizing the carbon in the iron resulting in low carbon
154
Fig.4 A blast furnace is basically a large steel vessel lined with refractory material and designed to contain and withstand great heat
steel. This turbulent reaction causes erosion of the vessel lining which can be detected by external temperature monitoring. The steel thus obtained is then cast into ingots which are subsequently rolled in various stages ending up in the form of a steel coil ready for further processing. These finishing processes involve a considerable amount of heat emission where continuous thermal monitoring can be of great use.
Applications To illustrate the operational advantages and difficulties, the use of the thermal imaging system within the blast furnace area will be described in more detail and reference will be made to its applications within the other areas of the steelworks. The blast furnace is by far the largest heat containing structure in the steelworks. To date, practical experience, point contact temperature measurement, cooling water temperature rise, and occassionally radio-tracer techniques have been used to indicate furnace condition and have helped indicate the type and thickness of future furnace linings. Point contact temperature measurement has obvious disadvantages. Access to the side of the furnace is both difficult and dangerous, also, it is not practical due to the sheer physical size of the furnace to cover all the surface area. However, using a thermal imaging camera it was possible to examine the upper regions of the furnace from the open, about 50 m away using a 10 ° x 10 ° lens. From this distance the outline shape of the furnace was seen in the thermogram enabling any areas of interest to be pin pointed quickly and accurately.
N O N - D E S T R U C T I V E T E S T I N G . J U N E 1974
When an excessively hot patch was seen to be present on one blast furnace it was immediately reported to the management (Fig.5). Their reaction was to water spray the area in question to lower its temperature and thus reduce the rate of erosion until a more permanent repair could be affected. A large part of the blast furnace is under cover, thus any thermal imaging of these areas has to be done with the camera much closer to the furnace. This necessitates the use of a wider angle camera lens; 25 ° x 25 ° was found to be suitable for this work. Thermography indoors at the blast furnaces presented more difficulties than the work out of doors. All the equipment had to be manhandled up narrow flights of stairs and along uneven floor areas to obtain the best viewing positions. For safety, a portable generator was usually used as a power supply. Having found a suitable viewing area the camera team usually had to wait for suitable viewing conditions. Within the cast house there seems to be an abundance of overhead crane operation, also, during the tapping of the furnace, great clouds of dust emerge, these, together with an excessive amount of reflected heat originating from the molten iron cause delays in producing a good thermographic survey. From one chosen position it is usually possible to carry out a fairly comprehensive thermal study of one side of the furnace. From the same position it is also usual to examine the hot gas main pipe to the furnace, the bustle main. The discovery of a hot area on the furnace is usually followed by a blast furnace management enquiry into the causes. Sometimes it is due to a failing water cooler, if this is the case the cooler can usually be put right. If the hot patch is considered to be due tO a localized thinning of the refractory furnace lining the immediate action usually taken is to water spray, and then, in suitable positions, during the next blast furnace down period quick-setting refractory material can be pumped into the lining. It is usually fairly easy to locate
1
ii
Fig.5
i
ii
ii
I
I
A thermogram of the upper half of a blast furnace reveals
a severe hot spot
N O N - D E S T R U C T I V E T E S T I N G . J U N E 1974
Fig.6 The temperature profile and thermogram (top left) are of a used torpedo ladle similar to the one shown
1
50
2
100
5
2o0
10
500
20
1.000
0
Fig.7
'
.2
,
.
,ll
~
/,
.
THV 1.0
A thermogram of an ingot mould helps in design evaluation
a specific area from the thermal picture, as railings and pipes etc which are at different temperatures to the furnace itself, show up clearly. The policy for the future is to examine the blast furnaces periodically with the thermal imaging system. It is now thought that periods of between three and six months will be suitable. In the longer term it is expected that this practice will assist in the design of future blast furnaces having special regard to refractory and lining thickness parameters. Similar detailed work has been carried out on the blast furnace stoves. As the stoves usually outlive the blast furnaces the results obtained here coupled with future periodic examination are of long term interest. The other three major areas where work has been carried out to date are the torpedo ladles, steelmaking vessels and ingot moulds. The torpedo ladle and steelmaking vessel exercises were similar to the blast furnace work in that the prime interest was refractory lining thickness (Fig.6). However, the effect of increased temperature on the creep properties of the steel shell of torpedo ladles is an important consideration. The ingot mould work is different in that refractories are not the main considerations. Very important thermal and mechanical facets have to be considered in t h e design of the ingot mould: thermal, because non.uniform heating and cooling in the walls will tend to cause fractures because of the non-linear expansion in the walls (Fig.7); mechanical, because in the stripping bay the ingots are pushed out of the moulds by a hydraulic press which places great mechanical stress on the mould. It is found in practice that if a mould is designed for optimum thermal p r o -
155
perties its mechanical strength is not sufficient. Further programmes of work are now being prepared following the purchase of a thermal imaging system, the aim is to optimize ingot mould design.
Corporation aiding the production personnel towards better thermal control of the mills and their products.
At the roiling mills, line scanning equipment is used. The aim is to measure accurately the steel strip temperature both longitudinally and in the transverse direction during the various rolling stages. It is also possible to measure roll temperatures. For the research trials the equipment was mounted on a mobile hydraulic lift so that it could be positioned above and at a safe distance from the moving hot steel strip. For longer term trials of course the equipment could be rigidly f'Lxedto a suitable structure on the mill.
There are definite advantages in the use of thermal imaging systems on refractory lined heat retaining structures in the works. The main advantages being the speed at which measurements can be made and the complete data presentation. It has been estimated that if thermal imaging was used as a tool for the examination of torpedo ladles, thus avoiding the practice of taking the ladles out of service to allow them to cool sufficiently for internal examination, at one works the utilization could be improved by one torpedo ladle in twelve.
It is well known that the physical properties of steel, mechanical strength etc vary with temperature and thus one aim of the exercises with the thermoprot'tle equipment was to attempt to correlate the temperature distribution across the steel strip with its physical shape. Some degree of success has been achieved. In an exercise carried out at one works it was shown by using the thermoprofding equipment that the strip tended to be hotter on the operating side of the mill. It was also shown by accurate physical sampling and measurement that if the mill did turn out wedge-shaped material the thin end of the wedge was always on the operating side. It was thought that the reason for this uneven temperature distribution was the slight bias of the descaling sprays towards the drive side of the mill. The descaling sprays are high pressure water sprays designed to remove traces of scale from the strip. Measurements have also been carried out on the work rolls immediately after their removal from the mill. A matt black line was painted quickly onto the shiny roll to increase the emissivity to a measurable value. Not enough work has been carried out to-date to quantify the effect of roll temperature on strip temperature shape. Even at this early stage of development it has been established beyond reasonable doubt that the thermal profiling method does have great advantages both for research and production use. Thermoprofile systems have or are being used at a number of the mills of the British Steel
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Conclusions
Potential applications include monitoring refractory lining conditions in blast furnaces, stoves or heat exchangers, hot molten iron transporting ladles, hot metal mixers and steel making converters. In the engineering maintenance fields the equipment can be used for early warning of bearing failure, locating electrical faults and showing faults in many important components. Many processes involving heat emission such as hot-dipped coatings, annealing, ingot, slab or bloom reheating may be studied by this technique. The thermal profiling equipment has also established itself as a useful tool, mainly in hot mill temperature measurement and control. Further work is in progress with both types of equipment to realize their full potential within the iron and steel works.
Acknowledgements The authors wish to express thanks to the Management of the Research Department of the Strip Mills Division of the British Steel Corporation for permission to publish this paper. It is appreciated that none of the work described would be possible without the co-operation of plant Management, Engineers and Research colleagues and the active assistance provided by many others at the works visited. Thanks are due also to the equipment suppliers for extensive help during the trials.
N O N - D E S T R U C T I V E TESTI NG . JUN E 1974