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FIELD OF THE INVENTION
This invention relates to measurement of metal temperature of Boiler tubes
particularly the metal temperature of steam/water tubes disposed in the hot flue
gas temperature path in boilers of power plant, HRSG (Heat Recovery Steam
Generator) of CO-GEN (co-generation) plants and CCPP (combined cycle power
plants). More particularly, the invention relates to a method of and a device for
metal temperature measurement in boiler tubes disposed in hot flue gas path of
HRSG.
BACKGROUND OF INVENTION
HRSG (Heat Recovery Steam Generator) are applicable in CO-Gen or combined
cycle plant, to generate steam from the GT (Gas Turbine) exhaust flue gas. The
hot flue gas outlet from GT, passes through heat exchanger modules like, Super
Heater, Re-heater, Economizer, evaporator, Make up water heater etc. These
modules are made using finned tubes or plain tubes.
SF (supplementary Firing) system is applicable in the HRSG where the steam
generation requirement is more than the Steam generation capacity of GT
Exhaust. The HRSG with SF system is called fired HRSG. Liquid and or gaseous
fuel is burned in HRSG, thereby increase heat input and the flue gas temperature
for generating more steam. HRSG without SF system is called unfired HRSG.
The typical arrangement of a Fired HRSG is shown in Fig.l. The heat recovery
tube modules are located in the hot flue gas (GT Exhaust) path. Both in Fired
and unfired HRSG the heat recovery tube coils / modules are exposed to the hot

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flue gases. To protect the tubes of the modules from getting over heated, metal
temperature of these tubes are measured and is called as "MTM" (Metal
Temperature Measurement). Depending upon the capacity of the HRSG the
number of MTM on various modules and its locations on the module vary.
The wall metal temperature of the tube module is subjected to the temperature
of the medium (steam / water) at inner face, and temperature of the flue gas /
GT Exhaust at outer face of the tube. The correct metal temperature is at the
middle of the wall thickness of the tube, as shown in Fig.2.
To get the correct MTM, the temperature sensor to be located at the middle of
the wall thickness of the tube, by precisely drilling a the hole in the wall
thickness of the tube, as shown in fig.2. This is called chordal drilling. Then, a
suitable temperature sensor is inserted in to the drilled hole up to the middle of
the hole. The other end of the hole is to be plugged. This method is suitable for
R&D and special applications only. This is not suitable for commercial boiler
applications, since it is very difficult to drill the chordal hole in the tube in
assembled condition. In addition, it has the risk of tube failure due to reduction
in wall thickness and chance of incorrect angle of drilling.
In the prior art method of MTM, a metal block, called Weld pad, is welded on the
boiler tube and the temperature sensor is inserted in the pre-drilled hole of the
welded pad. Measurements by this method are not reliable due to the influence
of high flue gas temperature and radiation effect of the SF flame. In addition, the
sensing tip of the temperature sensor is generally dis-located from the pad due
to material expansion which lead to erratic measurement.

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US patent.4527908 discloses an instrument, which is used for calculating the
heat flux, for boiler design and performance application, in which the metal
temperature in three locations within the cross section of the tube is measured.
The measuring locations are selected based on a) proximity to the surface
exposed to flue gas and b) proximity to surface exposed to the medium of the
tube and middle of the tube wall, at same cross section of the wall thickness.
This is not a economical and thus not suitable for commercial boilers where more
number of MTM measurements are applicable. Further, such a method needs
prefabricated tube assembly to be inserted by cut welds in the tube module,
which warrants skilled man power for the installation. Considering the problems
of tube cut and weld requirements this method is not suitable for multiple
measurements applications, required for tube over heat detection and protection
applications.
US patent 5355845 describes a method of skin Temperature measurement of the
surface of the furnace water walls. By this method the infrared radiation emitted
from the tube surfaces is detected using an optical instrument, which should be
kept out side the boiler furnace in such a location to view the tube under
measurement, in a vertical type furnace water wall. By spectrum absorption
technique, the skin temperature of the water wall tubes of boiler are measured.
This method is suitable for measuring the tube skin temperature where the
infrared radiations are involved, and needs high temperature fiber optical
instrument which is quite expensive. This method cannot be used where there is
no infrared radiation from tube surface. Thus, this method is not suitable for the
tube MTM that are not facing the burner flame or not exposed to very high
temperature of flue gas.

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OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose an improved method of
measurement of the metal temperature of boiler tubes for the protection of the
boiler.
Another object of the invention is to propose an improved method of
measurement of the metal temperature of boiler tubes, which allows easy
installation of the sensors at site at final stage of boiler commissioning.
A further object of the invention is to propose an improved method of
measurement of the metal temperature of boiler tubes which eliminates the need
for modification of the tube modules at site.
A still further object of the invention is to propose an improved device for metal
temperature measurement in boiler tubes disposed in hot flue gas path of heat-
recovery steam generators.
SUMMARY OF THE INVENTION
According to the invention, a weld pad is welded on the un-finned portion of the
tube, a shaped temperature sensor is inserted into the welded weld pad, and is
griped. A SS shield is put over the weld pad to completely cover the weld pad
with the sensor. The sensor is supported and looped in a specified way and

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taken out of the boiler. The tube metal temperature readings measured by this
method is found reliable and are closer to the mid wall temperature measured by
'Chordell drilling' arrangement (fig.2). By this method, the problem of getting
reliable MTM in HRSG is solved and thus reliable boiler tube protection is
achieved.
The disclosed method is very much cost effective and very easy for installation
as there is no cut and weld of the boiler tubes for installation.
The accessories used in the method are very simple, low cost, and with easily
available material. This method is much suitable for large number of MTM
applications, as required in fired and large size HRSGs.
BRIEF DESCRIPTION OF THE ACOMPANYING DRAWINGS
Fig 1: shows a general arrangement of boiler tubes (modules in a steam
generator (HRSG) boiler, according to prior art.
Fig 2: shows the tube mid wall metal temperature measurement details in a fin
tube and its sensor location according to the prior art.
Fig 3: shows a 'weld pad' which is to be welded on the tube at location where
the measurement is required according to the invention.
Fig 4: shows a device according to the invention.

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Fig 5: is an isometric view of the invented ' SS shield ' component, showing its
insulation and shape according to the invention.
Fig 6: is a schematic view of the component 'SS clamp', which is used to support
the sensor on the boiler tubes according to the invention.
Fig 7: shows a disposition of the device for MTM within the boiler according to
the invention
DETAILED DESCRIPTION OF THE INVENTION
Fig. 3 shows a solid SS piece (4) called 'Weld Pad' manufactured with a convex
bottom (6) to suit the OD of the tube under measurement. The weld pad (4) is
welded on the measuring tube at an un-finned portion (8) of the fin tube. The fin
tubes are having short length of un-finned portion near both end connections
with a top header (19) and a bottom header (20) as shown in fig.7.
The bottom convex surface (6) of the weld pad (4) is preciously machined to suit
the outer profile of the measuring tube, thus it seats perfectly on the tube
without any gap and to suit welding in all sides.
The weld pad (4) is welded on the tube in such away that the sensor entry
portion is towards tube fin side, as shown in fig.4 for easy insertion of the sensor
(1) in to the weld pad (4), at site.

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The weld pad (4) is made with a hole up to the middle with formed end to suit
the sensor tip for insertion and holding the sensor (1). In addition, there is a
tube portion (5) in the weld pad (4) for easy crimping of the temperature sensor
(1) and to keep the sensor tip always in touch with the weld pad (4) for effective
measurement.
The temperature sensor which could be a Cromel-Alumel (type K) thermocouple
(I) of dia 6mm size, of required length to suit the application, with compressed
mineral (magnesium oxide) insulated, of SS 310 or SS 316 sheath. An expansion
loop (10) at the measuring end has been provided with one each mounting union
(II) and termination union (12) as shown in the Fig.4, The prefabricated loop
(10) is for easy insertion of the sensor (1) in to the weld pad (4), which is
welded on the tube in the limited un-finned portion of the tube. Further it help to
avoid sensor pull out from weld pad while on expansion. The temperature sensor
(1) is inserted in to the hole of the weld pad (4) fully until its tip touches the end
of the hole. At this position, the sensor (1) is crimped with the tube portion (5)
of the weld pad. This is to hold the sensor (1) in its position in the weld pad (4)
even at hot condition.
The temperature sensor (1) is supported rigidly (13) on a straight portion of the
sensor (1) before the prefabricated loop (10) and at the weld pad (4) as shown
in fig.4. This is to help the sensor expansion movement towards away from the
weld pad (4) and thus to avoid a possible sensor pull out from weld pad (4) on
thermal expansion.

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A SS shield (9) is made up of SS 316 sheet as per the shape as shown in Fig.5
and is fitted with high-density ceramic board insulation material (15). The shape
of the insulation material (15) is such that it tightly fits over the weld pad (4)
along with the temperature sensor (1). The 'SS shield" (9) is placed over the
weld pad (4) in such a way it tightly covers the weld pad (4). The SS Shield (9) is
put over the weld pad (4) so as to cover the weld pad (4) along the sensor (1)
and tag welded with the tube to retain its position. Thus, it provides a shield for
the measuring point, tip of the sensor, from the influence of radiation due to
burner flame, and due to direct contact of flue gas.
The balance position of the temperature sensor (1) is laid inside the HRSG and
supported loosely, at suitable locations, such that it moves easily on thermal
expansion. In addition, a long expansion loop (23) is formed at the exit end in
the boiler, as shown in fig.7. The supports are provided in the form of a 'SS
clamp' made to a first shape (17) to suit the sensor size and a second shape (18)
to suit the out side diameter of the measuring pipe, as shown in Fig.6. The
sensor (1) is taken out from the HRSG through an exit tapping tube (22) filled
with an insulating material (24). This exit tapping (22) is fitted in the cross the
section of the HRSG side wall (21) as shown in fig.7. A mounting union (11)
provided in the sensor (1) is used to fix the sensor (1) with the end cover flange
of the exit tapping (22). This is to provide gas tight exit of the sensors from the
HRSG.

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The sensors (1), taken out from HRSG are connected to a junction box using the
termination union (12) provided in the sensor (1). From the junction box the
sensors (1) are connected to the monitoring / control instruments in a
conventional method using signal cables. The temperature measured by this
method, though the sensors (1) is on the outer surface of the tube, is very much
close to the mid wall temperature of the tube. By this a reliable metal
temperature measurement is achieved. By this invented method there is no
cutting and welding involved and also no critical drilling in the tube wall
thickness. This method is very much practicable and also economical for the
MTM applications in HRSG or in any boiler tubes which are in hot flue gas path.

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WE CLAIM
1. An improved method of metal temperature measurement including
installation of at least one temperature sensor in boiler tubes disposed in
hot flue gas path of HRSG, the improvement is characterized by
comprising:
- configurating a metal pad (4) having a through-hole and a tube
portion (5) for easy cramping with at least one sensor (1), the
metal pad (4) comprising a smooth convex shaped bottom (6)
corresponding to the diameter of the tube whose temperature to
be measured;
- welding the metal pad (4) on an unfinned portion of the tube at a
location close to the header connection;
- modifying a sensor (1) by forming an expansion loop (10) for
insertion of the weld pad (4) therethrough, the sensor being
provided with one each mounting union (11) and termination union
(12) for gas-tight exit of the sensor (1) from the HRSG;
- inserting the sensor (1) into the weld pad (4) till the end is loaded;
- rigidly supporting the sensor (1) before the expansion loop (10)
adjacent the weld pad (4) by means of a clamp (13);

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- covering the weld pad (4) along the sensor (1) by adapting a shield
(9) and tag-welding the shield (9) with the measuring tube to
retain the position;
- arranging a gas-tight exit of the sensor (1) through an exit-tapping
tube (22) disposed on a side wall (21) and fixing the sensor (1) by
using the mounting union (11) on the exit tapping tube (22); and
- connecting the sensor (1) to a junction box to the monitoring and
control instruments using signal cables.
2. An improved device for metal temperature measurement in boiler tubes
disposed in hot flue gas path of HRSG, the improvement is characterized
by comprising :
- a metal pad (4) having a through - hole and weldable on the un-
finned portion of a tube for which the metal temperature to be
measured, the metal pad (4) having a convex shaped bottom (6)
corresponding to the diameter to said tube;
- at least one sensor (1) having an expansion loop (10) including one
each unions (11,12) for mounting and termination, the sensor (1)
being insertable into the metal pad (4);

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- a clamp (13) for supporting the sensor (1) adjacent the metal pad
(4) before the expansion loop (10); and
- a shield (9) covering the metal pad (4) with the sensor (1), the
shield (9) being tag-weldable on the tube.

3. The device as claimed in claim 2, wherein the shield (9) comprises a high-
density ceramic board insulation material (15).
4. An improved method of metal temperature measurement including
installation of at least one temperature sensor in boilers tubes disposed in
hot flue gas path of HRSG, as substantially described and illustrated
herein with reference to the accompanying drawings.

This invention relates to an improved method of metal temperature
measurement including installation of at least one temperature sensor in boilers
tubes disposed in hot flue gas path of HRSG, the improvement is configurating a
metal pad (4) having a through-hole and a tube portion (5) for easy cramping
with at least one sensor (1), the metal pad (4) comprising a smooth convex
shaped bottom (6) corresponding to the diameter of the tube whose temperature
to be measured; welding the metal pad (4) on an unfinned portion of the tubes
at a location close to the header connection; modifying a sensor (1) by forming
an expansion loop (10) for insertion of the weld pad (1) therethrough, the sensor
being provided with one each mounting union (11) and termination union (12)
for gas-tight exit of the sensor (1) from the HRSG; inserting the sensor (1) into
the weld pad (4) till the end is loaded; rigidly supporting the sensor (1) before
the expansion loop (10) adjacent the weld pad (4) by means of a clamp (13);
converting the weld pad (4) along the sensor (1) by adapting a shield (9) and
tag-welding the shield (9) with the measuring tube to retain the position;
arranging a gas-tight exit of the sensor (1) through an exit-tapping tube (22)
disposed on a side wall (21) and fixing the sensor (1) by using the mounting
union (11) on the exit tapping tube (22); and connecting the sensor (1) to a
junction box to the monitoring and control instruments using signal cables.
The invention further relates to an improved device for carrying-out the method.