Temperature profile monitoring method and apparatus

The invention provides a method and apparatus for determining the approximate temperature profile of a large curved surface such as the outer surface of a chemical or hydrocarbon conversion reactor. At least two arrays of wire having a temperature dependent electrical resistance are placed over the surface to be monitored, with the arrays being aligned in different directions to cover the surface with a grid of overlapping wires. By measuring the resistance of each wire and comparing this resistance to predetermine reference values, wires which pass through regions of elevated temperature compared to the remainder of the surface may be located. By attributing the elevated temperatures to the area covered by the intersecting wires which show an increased resistance the areas of high temperature on the surface may be located. The system requires fewer connections and is simpler than a temperature monitoring system which requires a thermocouple or other temperature measuring means to be located at each corresponding intersecting grid point.

FIELD OF THE INVENTION 
The invention relates to a method of locating high temperature areas or 
measuring approximate temperatures at a number of points on a surface. The 
invention also relates to a system or apparatus for measuring temperature 
which employs electrical resistance measurements. The invention is more 
specifically related to a method and system for mapping a temperature 
profile over a large curved surface using immobile elements which are in 
contact with the surface. The invention is directly related to a method 
and apparatus for determining the approximate temperature profile of the 
outer surface of a process vessel used in the petroleum refining, 
petrochemical and chemical industries which is maintained at a relatively 
high temperature. The invention may also be used at the extremely low 
temperatures such as monitoring the surface temperature of cryogenic 
storage containers. 
DESCRIPTION OF THE PRIOR ART 
Many different types of temperature measuring apparatus have been developed 
including various types of bimetallic strip thermometers which could be 
used to monitor the temperature of a high temperature surface. However, 
only two types of temperature measuring devices are known to have received 
widespread commercial acceptance in the petroleum refining and 
petrochemical industries. The common means of measuring temperature is 
through the use of a large number of thermocouples having their junctions 
located at the various points at which it is desired to monitor a 
temperature. The thermocouples may be placed into thermowells to monitor 
temperatures within a process vessel or within the wall of the process 
vessel and would be placed on the outer surface of the vessel to monitor 
the skin temperature of the vessel. 
A second method which is used to locate high temperature regions on a 
process vessel is the use of an infrared scanning device or similar 
apparatus which is capable of translating the radiation of a specific wave 
length emitted from the vessel into a temperature determination. Several 
of these scanning devices would be mounted at a significant distance from 
the vessel such that they view large sections of the outer surface of the 
vessel, with different devices scanning different sections of the vessel. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides a method and apparatus for locating high temperature 
areas or obtaining an approximate temperature profile over a large surface 
area. Compared to the use of thermocouples, the invention reduces the 
number of electrical connections and the complexity of the wiring which is 
required to monitor the same number of points. One broad embodiment of the 
invention may be characterized as a method of detecting temperature 
profile irregularities which comprises the steps of establishing a first 
array of elongated electrically conductive elements having a temperature 
dependent electrical resistance across a surface which it is desired to 
monitor, with the elements of the first array being oriented such that a 
major portion of the elements are substantially parallel and are aligned 
in a first direction; establishing a second array of similar elongated 
electrically conductive elements across the surface, with a major portion 
of the elements of the second array being substantially parallel and 
aligned in a second direction such that the first and the second arrays 
intersect at a plurality of known points distributed across the surface; 
measuring the electrical resistance of each element of each array and 
comparing the instantaneous measured electrical resistance of each 
specific element with a value representative of the resistance of that 
element when the element or the surface is at a known temperature; and 
locating points on the surface at which the temperature differs 
significantly from said known temperature by attributing any above average 
increase in the electrical resistance in intersecting elements of the 
arrays to temperature increases occurring at the point or points of 
intersection of these elements. This method may be used in both extremely 
high temperature applications and in cryogenic applications. 
A second embodiment of the invention may be characterized as comprising a 
system for locating relatively high temperature regions on the outer 
surface of a process vessel which comprises a first array of spaced-apart 
wires which have a temperature dependent electrical resistance located 
adjacent to the outer surface of a process vessel and aligned in a first 
direction; a second array of spaced-apart wires which have a temperature 
dependent electrical resistance located adjacent to the outer surface of 
the process vessel and aligned in a second direction, with each wire of 
the second array intersecting with a wire of the first array at least once 
at a known location on the surface of the process vessel; means to 
periodically measure the electrical resistance of each individual wire of 
the first array and of the second array; and means to compare a first 
value corresponding to the instantaneous measurement of the electrical 
resistance of each wire to a second value corresponding to a predetermined 
electrical resistance of that particular wire based on a calibration 
performed with the surface of the process vessel at a known temperature(s) 
and to determine the location of the intersection of any two wires from 
different arrays which have significantly above average increases in 
measured electrical resistance. 
DETAILED DESCRIPTION 
In many industrial situations it is desired to periodically measure the 
exact or approximate temperatures of various points on the outer surface 
of a container, enclosure or process vessel. These temperature 
measurements or profiles may be desired over surfaces which are flat, 
curved or box-like in configuration. For instance, it may be desired to 
monitor the temperature at various points on the inner surface or the 
outer surface of a furnace or similar high temperature heater to determine 
temperature distributions within the furnace or to determine the 
effectiveness of insulation applied to the exterior or interior surface of 
the furnace enclosure. Space vehicles and solar radiation collection 
panels also have surfaces which it may be desired to monitor. In these 
applications it may only be required to locate high temperature areas 
rather than measure temperatures. At the opposite end of the temperature 
spectrum, it may be desired to monitor the skin temperature of large 
cryogenic storage tanks which may be stationary or on board a ship. A very 
specific application for which the subject invention is believed 
especially well suited is the monitoring of the temperature of various 
process vessels used in the chemical, petrochemical and petroleum refining 
industries. These vessels will typically be reactors but could also be 
used for other purposes. 
High temperature conditions are used in processing many hydrocarbonaceous 
liquids and vapors including petroleum-derived fractions or 
hydrocarbonaceous materials derived from coal or tar sand. One example of 
a high temperature processing operation is the hydrotreating (which 
includes both hydrocracking and hydrodesulfurization) of petroleum 
fractions. This hydrogen consuming process may be very exothermic, and 
these reactors operate at elevated temperatures normally above 300.degree. 
C. Other reactors, such as those used in methanation processes may operate 
at higher temperatures, and other process vessels such as the regeneration 
zone of a fluidized catalytic cracking unit in which carbon deposited on a 
cracking catalyst is combusted with oxygen charged to the vessel operate 
at yet higher temperatures which may exceed 700.degree. C. These vessels 
must withstand the temperatures associated with the planned conversion 
process, and are occasionally exposed to unplanned temperature escalations 
which may be caused by temporary misoperation, power failure, flow 
interruptions or various side reactions including very exothermic 
demethylation reactions. Temperature excursions may also occur within a 
process vessel due to the presence of specific reaction fronts. These 
reaction fronts may occur when carbon is being removed from the catalyst 
particles making up a fixed bed of catalyst held within the vessel or when 
the reaction catalyzed by the catalyst occurs in a very narrow band in the 
catalyst bed at a point at which fresh reactants contact catalyst having a 
high activity. 
In many of these instances it is desired to monitor the temperature of the 
outer surface of the process vessel at a large number of points for the 
purpose of monitoring the reaction which is occurring within the vessel. A 
second reason to monitor temperatures is to ensure that the temperature at 
any point in the vessel does not reach or exceed the temperature limit 
imposed by the metallurgy of the vessel. With a vessel maintained at 
cryogenic conditions, a temperature profile may be desired to locate 
points of insulation failure or other causes of excessive heat transfer 
into the vessel. 
If insulation is applied to the outside of a vessel which will be subjected 
to high temperature conditions the insulation will hinder the removal of 
heat from the outer surface and thereby prevent both the cooling of the 
vessel and the detection of "hot spots." Some safety and vessel design 
codes therefore require that insulation is applied to only the internal 
surface of vessels to prevent the possible propagation or concealment of 
hot spots by external insulation. This internal insulation may be 
subjected to extreme temperature, erosion and abrasion when placed within 
the vessel and is subject to localized failure and removal. The provision 
of an adequate temperature profile measuring system on the external 
surface of the vessel would reduce or eliminate the dangers associated 
with insulating the outer surface of a vessel subject to hot spots. This 
in turn would eliminate the problems associated with internally insulated 
vessels. It is therefore an objective of the subject invention to provide 
a low cost, highly reliable system for monitoring a temperature profile 
across an extended surface. It is another objective of the subject 
invention to provide a method of locating points of localized temperature 
excursions on the surface of an externally insulated process vessel. It is 
a specific objective of the subject invention to provide a method and 
apparatus for monitoring the approximate temperature at a large number of 
points on the outer surface of a process vessel or similar apparatus 
subjected to extreme temperatures.

DETAILED DESCRIPTION OF THE DRAWINGS 
The application of the inventive concept to the measurement of the surface 
temperature profile of a large process vessel is illustrated in the 
Drawing. Referring now to the Drawing, a large vertical process vessel 1 
having a diameter in excess of 2 meters and a height in excess of 4 meters 
receives a charge stream from line 2 which comprises an admixture of heavy 
hydrocarbons and hydrogen. The charge stream passes downward through the 
vessel and through one or more beds of highly active hydrotreating 
catalysts maintained at hydrotreating conditions which include a 
temperature between 300.degree. C. and 600.degree. C. An effluent stream 
comprising the refined hydrocarbons, by-products and any unconsumed 
hydrogen is removed from the bottom of the process vessel through line 3 
and passed to the appropriate product recovery facilities. 
Two arrays of wire having a temperature dependent electrical resistance 
form a square grid which covers the outer surface of the vessel. The first 
array peels off of the large bundle of lead-in wires 4 into the individual 
wires a-j shown in the Drawing. It has not been attempted to show in the 
Drawing the equivalent vertical wires which would be placed on the reverse 
side of the vessel. Each of the individual sensing wires a-j of the bundle 
4 extend upward to the top of the vessel. As shown in the Drawing these 
wires are preferably evenly spaced apart and substantially parallel while 
they are in contact with the cylindrical portion of the vessel, which is 
actinomorphic in structure. Near the top of the vessel the vertical wires 
extend away from the surface of the vessel and are collected into a bundle 
5 which completes the connection to a resistance measurement means 8. 
A second array of wires, which are attached to the bundle 6 of lead-in 
wires, is formed in a horizontal direction by the individual sensing wires 
a-h which are placed at different vertical elevations on the outer surface 
of the vessel. These wires are also substantially parallel and uniformly 
spaced apart. The wires of this array extend completely around the 
circumference of the vessel in contrast to the vertical wires which only 
extend from the top to the bottom of the vessel. As the wires emerge from 
completing a loop around the vessel, they are connected to a fourth bundle 
of wires 7 which completes the circuit by connecting the second end of 
this array to the resistance measurement means 8. 
The resistance measurement means generates a signal identifying each 
specific wire which is being monitored and a signal representative of the 
result of a resistance measurement of that specific wire. These two 
signals are then transmitted to a temperature determination means 15 
through means 9 and 10 for each wire of each array. The temperature 
determination means, preferably in conjunction with the receipt through 
means 13 and 14 of signals transmitted by thermocouples 11 and 12 or any 
other conventional temperature determination means are placed on the outer 
surface of the vessel, compares the instantaneous measured resistance of 
each wire to predetermined or calculated values for resistances through 
that wire and then generates a composite temperature profile for the outer 
surface of the vessel. This temperature profile, an alarm signal or any 
other desired signal is then transmitted to an output means through line 
16. 
The electrically conductive elements which make up the two or more arrays 
which intersect on the surface which is to be monitored may be formed from 
a wide variety of materials. Preferably these elements are metallic, and 
most preferably they are comprised of small gauge wire such as 20 gauge 
nichrome wire. Each of these elements should be electrically insulated 
from the vessel if the vessel is metallic and from any other potential 
electrical conductor including the conductive elements of the same and 
other arrays. This electrical insulation may consist of a ribbon of 
fiberglass or a woven ceramic composite. The electrical insulation should 
provide a minimum amount of thermal insulation, and the electrical 
conducting elements should be placed in as close proximity as possible to 
the surface which is to be monitored. That is, the wires or other 
conductors should be placed directly against the outer surface of the 
process vessel or separated from this outer surface only by the minimal 
required amount of electrical insulation. The electrically conductive 
elements (sensing wires) may extend to the resistance measurement means. 
However, it is preferred that a different conductor having a lower 
resistance and a resistance which is affected to a lesser degree by 
temperature changes is used to complete the connection. Thus, the sensing 
elements or wires may be connected to a bundle of copper lead-in or 
collector wires. 
The electrically conductive elements may be placed on the surface in two, 
three, four or more arrays. The important criteria in the placement of the 
arrays are that the individual elements extend over all areas which it is 
desired to monitor, that the elements of the different arrays intersect, 
and that the locations of these intersections are known. As used herein, 
the term "intersect" is intended to indicate that two elements, each from 
a different array, come into extremely close proximity, with the preferred 
distance between the two elements being less than 6 inches and more 
preferably less than 2 inches. It is not necessary for the two elements to 
physically overlap at the point on the surface being monitored to be 
within sufficiently close proximity to fulfill this definition and for the 
subject invention to function. However, it is preferred that the elements 
of different arrays do cross each other to form a checkerboard-type grid 
over the majority of the surface. The shape of the grid which is formed by 
the overlapping elements will of course be dependent on the angle of 
intersection and the shape of the surface. With the spherical surface, the 
grid formed by the overlapping elements may be much like the latitude and 
longitude markings commonly found on a globe. 
Although the same two elements may intersect at more than one point, it is 
preferred that each pair of elements (one from each array) intersects at 
only one point. The shape of the areas outlined by crossing electrically 
conductive elements may vary between squares, diamonds or rectangles 
depending on how the elements are placed on the surface. It is preferred 
that the elements are uniformly spaced apart over the majority of the 
surface of the vessel, and that the individual elements of each array are 
substantially parallel over a majority of their length if the shape of the 
surface allows such placement. Additional elements may be placed in 
certain critical areas which it is desired to monitor more closely for the 
purpose of providing more detailed temperature profiles. It is preferred 
that each array contains between 5 and 20 elements. However, there is no 
upper limit on the number of elements which may be present in any of the 
arrays and the arrays may therefore contain up to 200 or more elements. 
Each array may contain a different number of elements. The arrays need not 
cover the entire surface of a vessel or enclosure if it is not desired to 
monitor the temperatures over some portion of the vessel. It is also 
possible to utilize different pairs of arrays over different parts of the 
same vessel rather than to attempt to cover an extremely large vessel with 
just two arrays. 
The subject invention cannot measure the exact temperature of a particular 
point on the surface being monitored. This is because the resistance 
through the conductive element which is being monitored is a composite 
value dependent on the temperatures present at all points along the length 
of the sensing wire and any wires connecting the sensing wire to the 
resistance measurement means. An increase in the electrical resistance of 
an element may therefore be the result of an extremely high temperature at 
one point, moderately high temperatures at two or more points, or a 
relatively low temperature increase along the entire length of the sensing 
wire. The subject system is however able to locate regions or areas of 
above average temperature by attributing any substantial increase in 
resistance to the points at which those conductive elements showing an 
above average increase in resistance intersect. As used herein, a term 
such as "a significant increase in electrical resistance" and similar 
terms such as "significantly elevate electrical resistance" is intended to 
inidcate an increase in the measured electrical resistance which is at 
least 5% greater, and preferably 10% greater than the average increase in 
electrical resistance of all of the elements of an array which are being 
used as compared to predetermined resistance measurements chosen as a 
reference standard. 
To increase the accuracy of any temperature profile or temperature 
measurement generated according to the inventive concept the system should 
be calibrated prior to use. This calibration will preferably comprise a 
series of resistance measurements for each conductive element which is 
used while the surface which is to be monitored and the electrical 
connections leading to the monitoring means are at known temperatures. One 
set of resistance readings is preferably taken at an ambient temperature 
in the range of between 10.degree. C. and 30.degree. C. and at least one 
resistance calibration measurement is taken at a temperature close to the 
normal operating temperature of the surface being monitored. Calibrations 
are preferably performed at three and more preferably at four to seven 
known temperatures. This will allow the determination of the 
proportionality constant of the temperature-resistance relationship and a 
temperature estimating algorithm. 
To obtain non-ambient surface temperatures during calibration it is 
necessary to supply the requisite heating or cooling to the surface. With 
a cryogenic container, this could be performed as part of a normal cooling 
procedure carried out prior to the use of the vessel. The vessel could be 
cooled though normal refrigeration-type means during this time or by the 
use of small amounts of the cryogenic material which is to be stored. In 
the case of a process vessel which is to be used at an elevated 
temperature, it is preferred that the temperature calibrations are 
performed while a high temperature fluid is passed through the vessel and 
with no reaction occurring within the vessel. A sizable rate of fluid 
transfer through the vessel should bring the outer surface of the vessel 
to a substantially uniform temperature, with temperature variances being 
primarily the result of differences in heat loss to the configuration of 
the vessel or the effectiveness of the insulation applied to the vessel. 
It is especially preferred that the actual temperature at several points on 
the outer surface of the vessel is monitored during the calibration 
procedure by thermocouples or other point temperature measuring means 
which are applied in close proximity to the electrically conductive 
elements on the subject system. The calibration resistance measurements 
may be combined with these calibration temperature measurements to produce 
an algorithm or similar means to convert the instantaneous resistance 
measurements into a calculated average temperature for the surface of the 
vessel along each element or at specific points on the vessel. The 
algorithm may include values based upon the length of the sensing element 
and any connecting wires and the electrical resistances of these two 
different electrical conductors. Any calculated temperature or resistance 
preferably is adjusted for any temperature dependent effects. That is, the 
monitoring means preferably adjusts calculated values based on preliminary 
calculations and on actual temperature measurements. 
The instrumentation necessary for the practice of the subject invention is 
available commercially. The resistance measurement means may be separate 
from or integrated with the temperature determination means or other 
monitoring apparatus which is being used. The instrumentation will 
preferably include a mini- or microcomputer which may be part of the 
resistance measurement means or the temperature determination means. The 
complexity of the operation of the subject system may be expanded to the 
extent allowed by the computational and memory capacity of the computer. A 
very simplistic system would not require means to convert resistance 
measurements into temperature readings and would operate only by locating 
the conductive elements having the significantly above average increase in 
electrical resistance. The next step of advancement would be either the 
conversion of the instantaneous resistance measurements into corresponding 
average temperatures or the determination of specific areas having an 
abnormally high temperature. As part of the program used to determine the 
areas of localized high temperatures, the computer may compare the 
resistance measurements with spot temperature readings taken over a 
plurality of points on the surface. 
A particularly preferred capbility for the resistance measurement and 
temperature determination means would include comparing the indicated 
average temperatures of intersecting wires of two arrays and of allocating 
significant resistance increases to small localized lengths of these 
intersecting wires. The increases in resistance may then be attributed to 
those sections of the element which are at the above average temperature 
and the temperature required to produce this increased resistance over a 
small section of the wire may be more closely calculated. For instance, 
the system may detect a significant increase in resistance in one element 
of a first array and somewhat smaller increases in resistance in only four 
elements of a second array which intersects the element of the first 
array. From this information, it may be determined that the majority of 
the increase in resistance of the element of the first array may be 
attributed to a length equal to approximately the distance along the 
overlap with the four elements of the second array. In a similar manner, 
the increase in the resistance of each of the four elements of the second 
array may be attributed to a rather short length approximately equal to 
the distance between elements of the first array. Based on these 
approximate lengths, it is possible to estimate an average temperature 
over the small high temperature area which will produce the observed 
resistance increase. 
The preferred embodiment of the inventive concept may be characterized as a 
method of detecting high temperature regions on the surface of an 
insulated process vessel which comprises the steps of placing a 
spaced-apart first array of wires having a temperature dependent 
electrical resistance across the surface of the process vessel, with the 
first array of wires being aligned in a first direction around the vessel; 
placing a spaced-apart second array of wires having a temperature 
dependent electrical resistance across the surface of the process vessel, 
with the second array being aligned in a second direction and intersecting 
the first array at a plurality of known points across the surface of the 
process vessel; measuring reference values representative of the 
electrical resistance of the individual wires of the first array and the 
second array while the surface of the process vessel is maintained at a 
known elevated temperature which is within 150 Centigrade degrees of the 
expected on-stream average temperature of the surface of the process 
vessel; comparing a value representative of the instantaneous on-stream 
electrical resistance of each wire of the first array and the second array 
with the previously measured reference value for the same wire; and 
locating high temperature regions on the surface of the process vessel by 
attributing significant above average increases in the electrical 
resistance of intersecting wires of different arrays to an increased 
temperature at the point of intersection.