FIELD OF INVENTION
The invention relates to an apparatus for skin temperature measurement of
tubes placed in boiler furnaces. More particularly, the invention relates to an
apparatus and method for measurement of skin temperature of the boiler tubes,
both inside the flue gas path of the furnace and outside the flue gas path above
the boiler roof wall known as pent house.
BACKGROUND OF THE INVENTION
Overheating of materials in the boiler (also known as steam generators) can
shorten their life considerably. The speed at which the steam power plants can
be brought to full-load is largely dependent on the rate of change of metal
temperatures of the various plant components. So temperature measurements of
various plant components are necessary for quick start-up of the plant.
In a boiler, skin temperature measurement of superheater and reheater tubes,
headers and drum, carried out with thermocouples, are most important, because
these components are operating at high pressures and temperatures. Under
these conditions, the metallurgical constraints depend on temperature, which has
a critical influence on the life of the components vital to power generation. The
apparatus and the method used decide the accuracy and reliability of obtaining
essential operational information i.e. the skin temperature of tubes. The present

invention relates to an apparatus and method used for skin temperature
measurement of tubes of heat absorbing components (like different types of
superheaters, reheaters etc,.) placed inside boiler furnaces.
Boilers are employed world-wide by thermal power plants, refineries and many
other industries to produce steam for their operation. Many of the boilers are
designed for firing fuel oil and/or fuel gas and/or coal or other fuels. After
ensuring proper conditions for firing the fuel, the fuel is admitted into the
furnace of the boiler. The fuel gets lighted-up by ignitors, the flame gets
established and hot flue gas passes through various heat absorbing elements
inside boiler passes. The heat transfer takes place through radiation and/or
convection. The various heat absorbing elements like superheater or reheater
tubes are cooled by steam passing through the tubes. Skin temperature of these
tubes indicates the proper performance/healthiness of the total heat transfer
process.
Since the thermocouples are directly exposed to high temperature flue gas, they
need to be selected to withstand/overcome the effects of thermal stresses due to
high temperature conditions inside flue gas path and also they need to be further
protected between the point of measurement and the exit point through the
boiler casing. This is done by taking the sheathed thermocouple in carrier tubes
(also known as protection tube or sheath tube) welded to the superheater or
reheater tubes or other similar heat absorbing components. Continuous welding
of the full length of the carrier tube is essential to ensure it is always in good

thermal contact with their associated boiler tube to prevent any possible burn-
through of the carrier tubes. This ensures keeping the sheath or protection tubes
at a temperature which is approximately same as that of the superheater or
reheater tubes etc.
This provision also takes care of the situation-in boilers firing fuel oil, the
thermocouple is exposed to high-temperature oil/ash corrosion attack due to the
presence of vanadium pentoxide and sodium sulphate compounds in the flue
gases. With pulverized fuel boilers, a number of fuels contain high ash and
chlorine content and are therefore prone to slagging with a tendency for
corrosion to occur below the slag deposits.
In majority of cases, skin temperature of SH/RH tubes are measured in pent
house zone only and by computational or inferential methods, the skin
temperature of the tubes spaced inside flue gas stream is arrived at. This is no
doubt an approximate value and accuracy of measurement is not guaranateed
and so also the speed of response due to varying furnace process parameters.
The following are some of the disadvantages of the method used for the said
application.
a) In the pent house zone, where there is no flow of flue gas, the dynamic
changes, effects due to sudden fuel firing pattern would not be reflected
(viz. no continuous flue gas flow at all times but some quantity get may
entrapped in that zone).
b) Response time of measurement is also not appreciable.

c) Accuracy of measurement is not good as that of a direct measurement.
Since, Indirect measurement method is employed, errors and
approximations/assumptions are more needed.
In Patent No. US 4435092, a surface temperature measuring apparatus is
arranged to comprise means for conducting a first radiant energy generated from
an object to be measured, said energy conducting means being provided
opposite to said object, and reference radiation source means provided to mask
the influence of radiation noise from the furnace on a radio meter and to
generate a second radiation whose level is substantially determinable, whereby
the surface temperature of said object can be determined from the detected
values of said first and second radiations.
The invented design described above is different from the present invention
since it pertains to an instrument which functions based on the radiant energy
generated from an object.
Patent No. US 5785486 describe a Pyrometer with a probe beam super imposed
on its field of view for furnace temperature measurements. The pyrometer
includes a heterodyne millimeter/sub-millimeter wave or micro wave source for
probing. The receiver is adapted to receive radiation from a surface whose
temperature is to be measured. The radiation includes a surface emission portion
and a surface reflection portion which includes the probe beam energy reflected
from the surface. The surface emission portion is related to the surface

temperature and the surface reflection portion is related to the emissivity of the
surface. The simultaneous measurement of surface emissivity serves as a real
time calibration of the temperature measurement. In an alternative embodiment,
a translatable base plate and a visible laser beam allow slow mapping out of
interference patterns and obtaining peak values therefore. The invention also
includes a wave guide having a replaceable end portion, an insulating refractory
sleeve and/or a source of inert gas flow. The pyrometer may be used in
conjunction with a wave guide to form a system for temperature measurement in
a furnace.
The invented design described above is different from the present invention
since it refers to pyrometer used for flue gas temperature measurement and not
skin temperature measurement of tubes placed inside the furnace.
In Patent No. US 5355845, temperatures along the surfaces of water tubes in an
operating steam boiler are determined by detecting infrared radiation emitted
from the tube furnaces in one or more narrow pass bands founds to be available
despite the presence of hot and turbulent flue gases between the sensor and the
tube surface to be measured.

The invented design described above is different from the present invention
since it pertains to use of fibre optic cables to measure the surface temperature
of tubes based on infra red radiation emitted by the tube surfaces.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an apparatus for skin
temperature measurement of tubes placed in boiler furnaces which is capable of
achieving an accurate measurement of skin temperature of tubes exposed to hot
flue gases in boiler furnaces.
Another object of the invention is to propose an apparatus for skin temperature
measurement of tubes placed in boiler furnaces which ensures reliable
performance for many years under adverse and hostile surrounding
environments.
A still another object of the invention is to propose an apparatus for skin
temperature measurement of tubes placed in boiler furnaces which ensures
reliable performance under different fuel firing conditions and different fuel
combinations.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.l - It shows the total arrangement and also the details of various
components of the apparatus.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The invention relates to an apparatus adapted for skin temperature
measurement of tubes of heat absorbing components placed inside flue gas
streams in boiler furnaces. Figure 1 illustrates the details of components
applicable to the apparatus.
For measurement of skin temperature of superheater or reheater or other tubes
of similar heat absorbing components placed in the flue gas stream, mineral-
insulated thermocouples are used. These thermocouples are normally inserted
through the boiler casing or boiler roof panel arrangement and run down along
the side of the identified tube on which the measurement is to be made. The
exact location of measurement is decided based on design requirement. The
present invention relates to a special apparatus and method used for skin
temperature measurement of such tubes.
The apparatus according to the invention comprises a thermocouple (1) whose
hot junction is fitted into a tube insert section (2) which is welded into the

superheater or reheater element or other heat absorbing elements. The
thermocouple (1) is provided with mineral insulation. The thermocouple is
provided with an integral sheath of either 18 Cr/18Ni or 25Cr/20Ni or other
suitable material to withstand the temperature of the surrounding environment
inside boiler furnace. The size or the outer diameter of the thermocouple (1)
normally ranges from 8mm to 1.5 mm. Each thermocouple is run to a convenient
point on the outer casing with the normal provision for glanding and sealing in a
Junction box. The total length of the thermocouple (1) is decided based on the
location of the tube insert (2) in the particular assembly of the heat absorbing
elements. A metallic fused plug (7) of suitable length is provided on the hot
junction side of the thermocouple. This enables perfect contact of the hot
junction of the thermocouple with the stopper provided (8) in the tube insert.
The apparatus is made of forged steel rod or tubes having suitable wall
thickness. The apparatus permits replacement of thermocouple (1) as and when
required.
As indicated earlier, to enable hooking up of the hot junction of the
thermocouple (1) with the skin of the particular assembly of the heat absorbing
component, a tube insert (2) is used. This tube insert (2) is of suitable length
(worked out based on the design requirement-generally from 100 mm to 200
mm). This insert (2) is welded with the main tube of the assembly by suitable
means. The tube insert can be oriented directly facing or mounted at any angle
to the flue gas flow. Specified length in the central part of the tube insert (2) is

provided with eccentric diameter in such a way that the diameter is more on one
side and less in the other side of the central part of the tube. A hole or bore (9)
of suitable diameter to accommodate the thermocouple is made by appropriate
means on the higher thickness in the central part of the tube insert (2).,
ensuring the required minimum thickness immediately after the bore (i.e.
thickness of the superheater/reheater main tube). The thermocouple (1) metallic
fused plug (7) provided at the hot end is welded suitably on reaching the
farthest end.
To provide adequate protection to the thermocouple (1), a metallic tube (4) of
suitable length and diameter is used. This metallic tube (4) normally called as
protection tube or sheath tube or carrier tube is welded with the particular tube
of the heat absorbing element by suitable welding method. One end of the
sheath tube is suitably trimmed to take care of the smooth insertion of the
thermocouple (1).
To enable covering of the small length of the thermocouple (1) exposed
(between the tube insert (2) and the end of the sheath tube (4)), specially
shaped plates are used. Initially the thermocouple is covered with an inner
closing plate (5) by suitable welding method. Subsequently the inner sheathing
plate is covered by one or more outer closing sheath plates (6). The shape and
the dimensions of the covering plates are decided to ensure perfect coverage of

the thermocouple (1) so that the thermocouple outer surface is not exposed to
the hot flue gas that passes surrounding the heat absorbing component.
Salient features and advantages of the apparatus and the method are that it
• Ensures complete thermal contact between the thermocouple hot
junction and tube skin.
• It provides protection against fracture of the thermocouple due to
stressing under thermal cycling.
• It also provides protection against corrosion and slag formation one
or more specially shaped plates (5,6) of suitable dimensions covers
the thermocouple in the exposed region ensuring leak and dust proof
coverage.

WE CLAIM
1. An apparatus for skin temperature measurement of tubes placed in boiler
furnaces comprising:
a tube insert (2) of special shape, preferably of length between 100 mm
to 200 mm being welded to heat absorbing elements (3);
a thermocouple having its hot junction fitted into the tube insert (2) and
provided with a protection by a metallic tube (4) for withstanding the
surrounding temperature;
a metallic fused plug (7) provided on the hot junction side of the
thermocouple (1) for ensuring thermal contact with tube skin:
a stopper (8) being provided in the tube insert for ensuring its perfect
contact with the hot junction of the thermocouple (1);
at least one inner closing plate (5) for covering the thermocouple (1);
a plurality of outer closing sheath plates (6) covering the inner sheathing
plate (5) for protection of thermocouple (1) from hot flue gas;
Characterized in that the thermocouple hot junction is in complete thermal
contact with the tube skin.
2. An apparatus as claimed in claim 1, wherein the thermocouple (1) is
provided with mineral insulation.

3. An apparatus as claimed in claim 1, wherein the thermocouple (1) is
provided with an integral sheath of suitable material.
4. An apparatus as claimed in claim 1, wherein the outer diameter of the
thermocouple is preferably between 8 mm to 1.5 mm.
5. An apparatus as claimed in claim 1, wherein the specified length in the
central part of the tube insert (2) is provided with eccentric diameter.
6. An apparatus as claimed in claim 1, wherein the tube insert is provided
with a tube (9) of suitable diameter for accommodating the thermocouple.
7. An apparatus as claimed in claim 1, wherein one end of the metallic
sheath tube (4) is trimmed.

An apparatus for skin temperature measurement of tubes placed in boiler furnaces consists of a tube insert, preferably of length between 100 mm to 200 mm being welded to heat absorbing elements and a thermocouple (1) having its
hot junction fitted into the tube insert (2) and is provided with a metallic fused
plug (7) on its hot junction side. A stopper (8) is provided in the tube insert and
at least one inner casing plate (5) covers the thermocouple (1) and a plurality of
outer closing sheath plates (6) cover the inner sheathing plate (5) when the hot
junction of the thermocouple (1) is in complete thermal contact with the tube
skin.