FIELD OF INVENTION
The present invention generally relates to a measurement system
which uses optical media with Bragg gratings. In particular, the
invention relates to measurements of metal temperature and stress.
More particularly, the invention relates to an improved optical fibre
based apparatus and a method to determine the temperature and
stress developed in heat exchanging structure of a steam generator.
BACKGROUND OF THE INVENTION
Power Plants, Industries and Refineries deploy water tube steam
generating units for producing steam. The furnace in these steam
generators is designed for firing fuel oil, fuel gas or coal. The high
temperature flue gas from the furnace passes through the various
heat absorbing elements disposed in different locations of the steam
generator.

Monitoring the condition of heat exchanging surface is crucial in the
steam generator plant operating at a relatively high temperature and
pressure. In particular, the critical parameters for monitoring the heat
exchanging surface conditions, are thermal gradient across the heat
exchanging surface and stress developed due to the thermal
gradient.
Existing state-of-art widely practiced utilizes one or more
temperature monitoring devices operating by electric or mechanical
means for measurement of the surface temperature of the heat
exchanger metal tube, in a steam generator.
Conventional device for monitoring the heat exchanger tube metal
temperature, is the thermocouple, which is a device based on
thermoelectric effect, and which may be deployed throughout the
furnace and the heat transfer surface. The thermocouples are placed
in the steam generator in such a way that protective sheaths
separate them. The protective sheaths are adapted to protect the
relatively fragile thermocouple junctions from the hostile environment
inside the steam generator. Consequently, the thermocouples do not
sense the reaction temperature directly, instead they respond to the
transmitted heat.

As a result, there may be a substantial delay in thermocouple
response to the changes in temperature within the combustion
chamber, due to the lag time inherent in conductive heat transfer.
Such phenomenon in particular is true during system startup when
combustion reaction initiation results in a rapid temperature rise
which must be detected in order to confirm combustion initiation. In
addition, the heat transfer lag times further affect thermocouple
response to changes in operating conditions during normal system
operation. Moreover, the thermocouples are generally spot
measurement devices.
Another device adapted for temperature measurement in a furnace,
is an acoustic pyrometer. Physically, the acoustic pyrometer is
mounted external to the furnace. It is connected to the furnace via a
purgeable lance tube which normally extends from the pyrometer
into the furnace internals. The medium temperature is deduced from
the time of flight it takes for a calibrated sound signal to travel a path
between a transmitter and a receiver of the device which are
separated across the width of the furnace. A major limitation of the
pyrometer type temperature monitor arises from the difficulty

encountered in keeping the lance tube free of obstructions. Further,
the signals arising from the auxiliary components of the furnace, for
example, blowers, and further from the steam leaks in the furnace
also influence the measurand to a considerable extent. Acoustic
pyrometer can only provide an averaged value of the measurand in
the path between its receiver and transmitter.
There are already known various constructions of sensing
arrangements, for example, U.S. Pat. No. 4,806,012, entitled
"Distributed, Spatially Resolving Optical Fiber Strain Gauge", discloses
a sensing device capable of sensing stresses within a structure which
comprises an optical fiber containing a plurality of periodic Bragg
gratings of different original periodicities. Thus, each reflecting light
in a narrow range around a central wavelength can be determined by
the respective periodicity. Such bragg gratings are disposed at
different regions of the structure so as to be subjected to different
stresses, temperatures and strains depending on their locations in
the structure, and undergoing strain-related changes in their
periodicities including in their central wavelengths of reflection.
During the use of this known sensing arrangement, light is launched

into the optical fiber in a wavelength range, such that, the
wavelengths of interest with respect to all of the bragg gratings
under all conditions can be embraced. Then, the light returned back
to the launching end of the fiber is examined , or the light reaching
the other end is examined, respectively for the presence or the
absence or diminishing or otherwise of intensity, in respect of the
altered light around each central wavelengths of the gratings, the
alteration being taking place due to the stresses existing at the
respective locations of the structure, thereby the magnitude of such
stresses is determined.
Prior to installation of the optical fibre core in the structure,the
individual Bragg gratings are provided in the optical fiber core, by
exposing the optical fiber core through the cladding to an
interference pattern of two ultraviolet light beams , the light
frequency and/or orientation of the light beams relative to a
longitudinal axis of the optical fiber for each of the gratings, being
such that, the interference pattern maxima and minima extend
through the fiber in directions normal to the longitudinal axis, and
that the periodicity (e.g. the distance between two consecutive
maxima) matching to that desired for the particular grating.

However, this approach is particularly advantageous, in the applications in which the
number of locations along the optical fiber to be monitored for stresses in the
structure is relatively small, because the device entails an important drawback in
that each of the gratings has to have assigned to it a considerable amount of the
available spectrum (i.e. not only its relatively narrow wavelength or frequency band
but also the separation fro the adjacent wavelength or frequency band assigned to
another grating by an amount sufficient to avoid overlapping or crosstalk between
the adjacent channels under all circumstances, that is, even when the central
wavelengths of the adjacent channels have moved, as a result of the stresses
applied at the locations of the associated gratings, toward one another to the
maximum extent). Such allocation of larger spectrum to each of the gratings,
severely limit the number of grating sensors that can be employed within each
sensing optical fiber. For example, if a solid state device, such as an edge-emitting
diode or a laser diode is sued as the light source, which would be highly desirable
because of the relatively low cost and reliability of such device, it would only cover a
wavelength range of few tens of nm. On the other hand, each of the optical sensors
of the type disclosed in the above-cited reference, would require up to 5 nm of
bandwidth to cover the entire measurement bandwidth. Thus, only a limited
number of sensors can be associated with each optical source disclosed in the US
Patent under discussion.

WO 02095349 (Al) describes a method of and apparatus for determining the
spatial distribution of polarization properties of an optical fibre. Pulses of light are
transmitted along the optical fibre and the polarization state of light
backscattered from portions and elements of the optical fibre detected. A spatiall
distribution of linear retardance delta, orientation of linear retardance axes and
circular retardance axes can be accurately determined. This has application in
the analysis of Polarisation Mode Dispension in telecommunications as well as,
inter alia, strain, stress, temperature and electric current and voltage
measurement using optical fibres.
In the field of Fiber optic based instrumentation, various techniques are deployed
to measure the different parameters of interest. Rayleigh backscatter based
measurement and FBG based measurement are two of the available techniques.
Though both these methods utilize fiber optic based sensors, they are principally
different in that
a) The backscatter is a diffuse reflection phenomenon whereas FBG based
measurement is a specular reflection (like in a mirror), It may be noted
that scattering in itself is a diffuse phenomenon and backscatter is that
miniscule part of the scatter that is available in the direction of the
incident light. The FBG based measurement do not rely upon the back-
scatter component but instead focus on measuring the peak reflected
component avaiJable from the FBG sensor.

b) In a backscatter based fiber sensor, the entire fiber acts as a sensing
device - each atom that forms the internal structure of the fiber
contributes to the backscatter. The weak backscatter is then optically
analyzed to infer on the various parameters of interest along the entire
length of the fiber.
c) In contrast, in a FBG based sensor array, well defined finite number
of FBG sensors (created using laser etching techniques - out of scope
of the present application) are present at distinct locations along the
length of fiber optic cable. Each FBG sensor is designed to reflect back a
component of the incident light wave that has a certain frequency. (In
fact the each FBG sensor can be visualized to behave like a frequency
sensitive mirror that can only reflect light of one colour shade - for ease
of understanding).
d) The present application disclosed utilizing peak shift in the wavelength
of the reflected component available from the FBG sensor and not
the polarization properties available from the backscatter component
as disclosed in the cited articles. Hence it is principally different from
the referred articles.

OBJECTS OF THE INVENTION
It is therefore, an object of the invention, to propose an improved
method to determine the temperature and stress developed in heat
exchanging structure of a steam generator by adapting an optical
sensing means.
Another object of the invention is to propose an improved optical
fibre-based apparatus to determine the temperature and stress
developed in heat exchanging structure of a steam generator.
A still another object of the invention is to propose an improved
optical fibre-based apparatus to determine the temperature and
stress developed in heat exchanging structure of a steam generator
which uses data relating to the proportional change in a physical
property of an optical sensing means of the apparatus.
A further object of the invention is to propose an improved optical
fibre-based apparatus to determine the temperature and stress
developed in heat exchanging structure of a steam generator which
uses an array of multi point sensors to form the optical sensing
means.

A still further object of the invention is to propose an improved
optical fibre-based apparatus to determine the temperature and
stress developed in heat exchanging structure of a steam generator,
atleast a single optical means of which apparatus being embedded
into the heat exchanger structure.
An yet another object of the invention is to propose an improved
optical fibre-based apparatus to determine the temperature and
stress developed in heat exchanging structure of a steam generator
which is capable of directly generating a temperature and stress
profile utilising the data captured from the optical sensing means.
SUMMARY OF THE INVENTION
The present invention in one aspect provides an improved optical
fiber-based apparatus, for measuring the temperature and stress
developed in a steam generator heat exchanger tube. In a second
aspect, the invention proposes a method of using the improved
optical fiber-based apparatus for such measurement. More
particularly and in accordance with an exemplary embodiment, the
improved optical fiber-based apparatus comprises an optical fiber-
based sensor including an optical fiber which has a plurality of fiber
gratings disposed thereon at selected locations along the longitudinal
axis of the optical fiber. The gratings exhibit exceptional thermal

stability and do not degrade at conditions prevailing on the steam
generator heat exchanger metal tube, thereby enabling the optical
fiber-based apparatus to be adapted for measuring tempeature and
stress developed in a heat exchanger metal tube in a steam
generator. An interrogator is provided to acquire data relating to the
changed characteristics of a light beam spectrum launched from a
light source when reflected via the changed optical geometry of the
sensors due to parametric value fluctuations of the metal tube. A
processor is provided to directly generate a planar profile from the
measured data derived from the multipoint optical sensing array
means.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
Figure: 1 shows schematic diagram of an optical fibre-based
apparatus according to the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION:
As shown in fig 1, the apparatus comprises a quasi distributed optical
sensing array means (1). The quasi distributed optical sensing array
means (1), is disposed within the heat exchanging structure (2) of

the steam generator which is a metal tube. The fiber sensing array
(1) which is embedded into the metal tube heat exchanger receives
the light originating from a light source when launched into it. The
received light undergoes reflection within the fiber at the sensor
locations. As the sensor array means (1) undergoes geometric
deformation due to effect of temperature and stress developed on
the tube of the heat exchanger, the extent of reflection
correspondingly changes its characteristics. Thus, when the light
passes through the optical sensor array means (1), the characteristic
reflected spectrum undergoes a change. An interrogator (3) disposed
at the end of the optical array (^determines the heat exchanger
temperature/stress developed at the sensor points as a function of
the shift in the reflected peaks of the received interrogating light
signals. A signals processor (4) deduces the temperature/stress
profile of heat exchanger structure from the point temperature values
of the sensor elements in the array embedded within the heat
exchanger metal tube.

WE CLAIM :
1. An improved optical fibre-based apparatus to determine the
temperature and stress developed in a heat exchanger of a steam
generator, the apparatus comprising :
- at least one optical fibre-based sensor having an optical fibre with a
plurality of fibre gratings disposed along the longitudinal axis of the
optical fibre, a sensor array means (1) thus formed being disposed
within the heat exchange (2);
- a light-source for launching light to the sensor array means (1), the
light undergoing reflection within the optical fibre at sensor
locations;
- the sensor array means (1) undergoing geometric deformation
corresponding to fluctuations in the temperature including
development of stress on the structure (2) which cause the
reflected spectrum of light to correspondingly change its initial
characteristics;

- an interrogator (3) disposed downstream of the optical
sensing array means (1) to acquire the data in respect of
temperature and stress developed at the sensor points,
the data representing a function of the shift generated in
the reflected peaks of the received interrogating light
signals; and
characterized in that a signal processor is provided in
downstream with the interrogator such that a planar profile
is directly generated in respect of the temperature and
stress developed in the structure from the point of
temperature data continuously processed from starting point
to elevated temperature wherein the temperature and stress
is measured from the shifting of the reflected peaks of the
received interrogating light signals from the built-up planar
profile.
2. The apparatus as claimed in claim 1, wherein the sensing array
means (1) constitutes a quasi distributed optical sensing
means.

3. A method in an improved optical fibre-based apparatus as
claimed in claim 1, to determine the temperature and stress
developed in heat exchange structure of a steam generator, the
method comprising the steps of:
- providing a light source convertable from heat energy,
capable of launching light in multi directions;
- providing at least one optical fibre-based sensor having a
plurality of fibre gratings to form an optical sensing array
means (1) and disposing the array means inside the
structure;
- launching light from the light source so as to cause the
light to undergo reflection at sensor locations;
- capturing initial characteristic data relating to the
reflected light beam spectrum;

- acquiring characteristic data in respect of a changed
reflected spectrum of light in an interrogator, the changes
occurring corresponding to fluctuations in the metal
temperature in the structure including stress developed
therein; and
- generating directly a planar profile respecting the
temperature and stress developed in the structure by
adapting a signal processor based on the data
transmitted from the interrogator.
4. An improved optical fibre-based apparatus to determine the
temperature and stress developed in heat exchange structure
of a steam generator as substantially described herein and
illustrated with reference to the accompanying drawing.

ABSTRACT

AN IMPROVED OPTICAL FIBRE-BASED APPARATUS TO
DETERMINE THE TEMPERATURE AND STRESS IN SOLID
HEAT-EXCHANGE STRUCTURE
An improved optical fibre-based apparatus for measuring the
temperature and stress developed in a solid heat exchange structure
provided with an optical fibre-based sensor including an optical fibre
which has a plurality of fibre gratings diposed thereon at selected
locations along the longitudinal axis of the optical fibre, and a light
energy convertable from heat energy incident on/stress developed in
the heat exchange structure to light energy and this light waves are
propagated though a plurality of fibre cables disposed axially and the
peak planar profile measure the temperature and stress developed in
the heat exchange structure by calculating the shifting of reflected
peak characteristic from the initial condition to elevated temperature.