Field of invention
The present invention relates to a thermocouple based device for temperature
measurement in a high temperature/pressure environment. The present invention
particularly relates to a thermocouple based device having enhanced life and which can
be easily connected/disconnected from the application point. Such devices are useful
for measurement of high temperature in industrial units, especially furnaces.
Background of the invention
Thermocouple based devices are widely used in industrial operations for temperature
measurement. A conventional device generally comprises : (i) a hollow assembly made
of a steel of refractory tube with a set of thermocouple wires (such as Pt-PtRh) covered
with beads of some refractory material, and (ii) a flange type fixing assembly for suitably
attaching the device to the measuring point using nut-bolt or some other means, to
prevent leakage of gases.
Conventional devices are not suitable for measurement of temperature in a high
temperature and high pressure industrial environment (for example, in the hot blast
main of blast furnace and in the dome of blast furnace stoves). The main drawbacks are
two. First, these are prone to frequent breakage due to poor resistance to thermal shock
and sudden pressure variation, and contamination of the thermocouple wire by dust and
carbon particles, resulting in: (a) loss of valuable thermocouple wire and hot gases, (b)
fire hazard due to leakage of gases in the atmosphere, (c) wear and tear of furnace
refractory lining around the device. Second, design of fixing assembly does not permit a
quick replacement of the device in case of failure. This results in large down time for the
production unit/furnace, and therefore, loss of production.
Thus there is a need for an improved thermocouple based device which has enhanced
life even in high temperature and pressure conditions and is capable of being replaced
within a short time without necessitating additional manpower.
Object of the invention
Thus the object of the invention is to provide an improved thermocouple based device
for temperature measurement in high temperature and pressure environment, which
has enhanced life.
Another object of the present invention is to provide an improved thermocouple based
device having an improved fixing assembly facilitating easy replacement of the device.
Yet another object of the present invention is to provide an improved thermocouple
based device which prevents leakage of valuable hot blast air and reduces chances of
wear and tear of refractory lining of furnaces.
A further object of the present invention is to provide an improved thermocouple based
device that ensures safe working environment around the process units in which they
are used.
Summary of the invention
Thus according to one aspect of the present invention there is provided a thermocouple
based device for temperature measurement in a high temperature/pressure
environment comprising:
a thermocouple wire, said wire having a hot junction at least part of which is
substantially surrounded with porcelain beads;
a porcelain cap enclosing said hot junction and portion of said porcelain beads, said
porcelain cap being filled with magnesia powder;
a protective tube closed at one end and enclosing said thermocouple wire, porcelain
beads and porcelain cap such that the porcelain cap is at the closed end of the
protective tube, the tube being filled at the closed part with dry cast magnesia powder
and sealed with wet cast alumina at the open end, said thermocouple wire and
porcelain beads passing through said seal and the region between the said dry cast
magnesia powder part and said sealed end being filled with dry cast alumina.
According to another aspect of the present invention there is provided a process for
manufacture of said thermocouple based device comprising the steps of:
(i) providing a thermocouple wire having a hot junction and porcelain beads;
(ii) providing a porcelain cap enclosing said hot junction and portion of said
porcelain beads and filling said porcelain cap with magnesia powder;
(iii) providing a protective tube closed at one end and enclosing said thermocouple
wire, porcelain beads and porcelain cap such that the porcelain cap is at the closed
end of the protective tube,
(iv) dry casting the region of said closed end of said protective tube using magnesia
powder;
(v) dry casting the region between the magnesia cast region of step (iv) and the
open end of said protective tube with dry alumina; and
(vi) sealing the open end of said protective tube with wet alumina.
Detailed description of the invention
The present thermocouple based device provides adequate protection to the
thermocouple wires using high purity alumina and magnesium oxide powders as fillers
in the sheath/metal protective tube. The high purity magnesium oxide powder helps in
avoiding sintering at the operating temperature and is dry cast in the front of the
thermocouple protection pipe. The high purity alumina powder is used for wet casting at
the rear portion of the protection pipe.
The magnesium oxide powder exhibits high conductivity and excellent thermal shock
resistance as well as high refractoriness and high hot strength at elevated temperature.
The alumina for wet casting exhibits permanent linear change on reheating. The linear
change is less than 1% even at temperature as high as 1800°C. It also has high thermal
shock resistance. Furthermore, the tight binding with water in the wet cast also prevents
leakage at high pressure if and when the thermocouple protection pipe gets punctured.
The device of the present invention also comprises a quick connect-disconnect type
fixing assembly which facilitates fixing of the device to the temperature measuring
position by a mere quarter-turn rotation. This assembly enables easy replacement of the
devices taking as less as five minutes to replace. A single person can perform such
replacement operation with ease and without any additional help.
Description of preferred embodiment
\ The construction of the device will now be described with reference to a preferred
\ embodiments illustrated by way of drawings in which
Figure 1 shows the thermocouple based device.
Figure 2 shows the front portion of the device fixing assembly.
Figure 3 shows the rear portion of the device fixing assembly.
The thermocouple device comprises a Pt-PtRh element wire (8) having a hot junction
(12) along with porcelain beads (6), which is enclosed into the porcelain cap (5) and
filled with magnesia powder (10). Total element (8, 13) with porcelain beads (6) and
porcelain cap (5) is enclosed into a protective tube (4) made of sillimanite. In this
construction, the total load of porcelain beads (6) is borne by the front porcelain beads,
which are approximately 1 cm embedded into magnesia powder (10) inside the
porcelain cap (5). This prevents breaking of hot junction (12) due to load of rest of the
porcelain beads (6).
The closed portion of the tube (4) is filled with dry cast magnesia powder (9). Magnesia
is preferred due to its high fusion temperature. In most applications it will not fuse over
the operating range of temperatures. Moreover, due to its high thermal conductivity, the
response time of thermocouple does not deteriorate.
The region between the magnesia cast portion (9) and around 6 inches from the open of
the tube (4) is filled with dry cast Al2 03 or alumina (7). The remaining 6 inch portion at
the open tube end is sealed with wet Al203 (11). Sealing is done so that in case of
breakage of thermocouple, air at high temperature (1100° C - 1300°C) and pressure (3
kg/cm2) does not leak and thus furnace stoppage due to failure of thermocouple is
avoided. Casting with dry MgO & Al203 helps in maintaining the thermocouple
characteristics and minimise the loss of valuable thermocouple wires in case of
breakage.
Figures 2 and 3 illustrate the front and rear parts of the device 'Fixing Assembly'. This
device is made of 40 XH steel, which can withstand high temperature.
The front portion of the device is welded to the stainless steel tube ((3) of Figure 1) at
the site. The rear portion facilitates quick installation/removal due to its unique design.
These two parts are prepared by carrying out appropriate machining steps to obtain the
shape and dimensions as shown in Figs. 2 and 3.
The process for manufacture of the device will now be described with reference to a
non-limiting example.
Example
The hot Junction (12) of the Pt-PtRh element along with porcelain beads (6) is inserted
into the porcelain cap (5) and filled with magnesia powder (10) such that approximately
1 cm at the front portion of the porcelain beads is embedded into magnesia powder (10)
inside the porcelain cap (5). Total element (8, 13) with porcelain beads (6) and porcelain
cap (5) is inserted into the protective tube (4). Then casting is done.
The first stage of casting involves dry casting using magnesia powder (9) the front part
of the protective tube. The second stage involves casting with dry Al2 o3 (7) in the
region starting from the rear end of first stage up to around 6 inches from the rear end of
the protective tube. The remaining 6 inch portion at the rear part is then sealed with wet
Al2o3(11).
We claim:
1. A thermocouple based device for temperature measurement in a high
temperature/pressure environment comprising:
a thermocouple wire, said wire having a hot junction at least part of which is
substantially surrounded with porcelain beads;
a porcelain cap enclosing said hot junction and portion of said porcelain beads, said
porcelain cap being filled with magnesia powder;
a protective tube closed at one end and enclosing said thermocouple wire, porcelain
beads and porcelain cap such that the porcelain cap is at the closed end of the
protective tube, the tube being filled at the closed part with dry cast magnesia
powder and sealed with wet cast alumina at the open end, said thermocouple wire
and porcelain beads passing through said seal and the region between the said dry
cast magnesia powder part and said sealed end being filled with dry cast alumina.
2. A thermocouple based device as claimed in claim 1, wherein said protective tube is
made of sillimanite.
3. A thermocouple paged device as claimed in claim 1, wherein said thermocouple wire
is a Pt-PtRh wire.
4. A thermocouple based device as claimed in claim 1, wherein said porcelain cap
encloses said hot junction and portion of said porcelain beads in such a way that
around 1 cm of the porcelain beads at the said hot junction is embedded into
magnesia powder.
5. A thermocouple based device as claimed in claim 1, wherein said sealing with wet
cast alumina is done till around 6 inches from the open end of said protective tube.
6. A process for manufacturing of thermocouple based device comprising the steps of:
(i) providing a thermocouple wire having a hot junction and porcelain beads;
(ii) providing a porcelain cap enclosing said hot junction and portion of said
porcelain beads and filling said porcelain cap with magnesia powder;
(iii) providing a protective tube closed at one end and enclosing said thermocouple
wire, porcelain beads and porcelain cap such that the porcelain cap is at the closed
end of the protective tube,
(iv) dry casting the region of said closed end of said protective tube using magnesia
powder;
(v) dry casting the region between the magnesia cast region of step (iv) and the
open end of said protective tube with dry alumina; and

A thermocouple based device for measurement of elevated temperatures in high
temperature environments having enhanced life and which can be easily
connected/disconnected from the application point. The device comprises a
thermocouple wire having a hot junction at least part of which is substantially
surrounded with porcelain beads, a porcelain cap enclosing the hot junction and portion
of porcelain beads, the porcelain cap being filled with magnesia powder. The
thermocouple wire, porcelain beads and porcelain cap are enclosed by a protective tube
closed at one end such that the porcelain cap is at the closed end of the protective tube.
The tube is filled at the closed part with dry cast magnesia powder and sealed with wet
cast alumina at the open end. The thermocouple wire and porcelain beads pass through
the seal and the region between the dry cast magnesia powder part and sealed end
being filled with dry cast alumina. Such devices are useful for measurement of high
temperature in industrial units especially furnaces.