CA2799824C - System and method for monitoring steam generator tube operating conditions - Google Patents

Abstract

A system for monitoring operating conditions of the steam generator tubes in a steam generator and a method for affixing a fiber optic sensing array to a steam generator tube is described. The system comprises a fiber optic sensing array disposed within a hermetical cable package, an emitter and photodetector for obtaining reflected wavelengths from the fiber optic sensors, and a central processing unit for converting the signal into the operating conditions. The method comprises cleaning a surface of a tube, affixing a guide tube to the tube surface, and threading a sensing cable package into the guide tube.

Description

SYSTEM AND METHOD FOR MONITORING STEAM GENERATOR TUBE OPERATING
CONDITIONS
FIELD
[0001] The present disclosure relates generally to steam generators. More particularly, the present disclosure relates to monitoring steam generators during operation with fiber sensing arrays.
BACKGROUND

[0002] The following background discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.

[0003] A steam generator can be used in various applications and processes including, for example, driving a turbine to create electricity, or in the steam assisted gravity drainage (SAGD) technique used for recovering heavy oil from oil sands as are found in Alberta, Canada.

[0004] A heat recovery steam generator (HRSG) is a type of steam generator that uses heat exchangers to recover heat from a hot gas stream used to generate steam.
A type of HRSG is a once-through steam generator (OTSG). OTSGs are used, for example, in some SAGD operations. Unlike HRSGs, OTSGs do not have boiler drums. A steam generator, including an HRSG or an OTSG, may comprise one or more steam generator tubes, typically made of carbon steel, that pass through one or more heating sections.
[0006] In an OTSG, the water to be heated and converted into steam is pumped in a continuous path through steam generator tubes through a radiant section and a convection section. Heat is generated by combusting fuel in a combustion chamber. The combustion chamber is located directly adjacent to the radiant section. Heat from the combustion chamber is forced through the radiant section, through the convection section and out an exhaust stack.
[0006] Cold or mild temperature water is first pumped through the convection section where heat exchange with the hot combustion flue gas pre-heats the water. To maximize heat transfer to the water, coiled carbon steel tubes in the convection section are tightly arranged next to one another in stacks or layers to maximize water surface area to water volume.
[0007] Pre-heated water or water/steam mixture exits the convection section and continues to the radiant section where it is further heated by hot gasses and by the radiation emitted from the combustion of fuel. The radiant section has a large number of steam generator tubes inside of a shell through which hot air and combusted gasses are forced.
The steam generator tubes in the radiant section are straight and arranged circumferentially around the interior of the radiant section to form a hollow cylindrical structure. No steam generator tubes are present in the centre of the cylindrical structure so as to allow combusted gasses and hot air to pass through.
[0008] The interiors of an HRSG or OTSG are harsh environments that can experience temperatures of up to 1000 degrees Celsius in the radiant section and 500-1000 degrees Celsius in the convection section.
[0009] During operation, solid deposits can accumulate in the interior of the steam generator tubes. The accumulation of deposits in the interior of the tubes is called fouling and may be caused by particles in the water, or by scaling caused by the presence of silica, carbonate, or other minerals in the water. Heat accelerates the fouling.
Fouling may reduce the performance of the HRSG and OTSG by acting as an insulator and degrading the thermal exchange efficiency of the steam generator tubes, or parts thereof. Deposits on the interior of the steam generator tubes also interfere with the flow of water through the steam generator tubes. In some cases, localized fouling creates a hot spot in a steam generator tube. The hot spot may foul more rapidly than other parts of the same steam generator tube and can lead to a burst or ruptured steam generator tube.
INTRODUCTION
[0010] The following is intended to introduce the reader to the detailed description to follow and not to limit or define the claims.
[0011] This specification describes a system for monitoring one or more operating conditions of steam generator tubes in a steam generator. The system includes a fiber optic sensing array and a hermetical cable package disposed circumferentially around the fiber optic sensing array. A light source is provided for emitting light into the fiber optic sensors and a detector is optically connected to the fiber optic sensing array for receiving refracted wavelengths of the light. A central processing unit in communication with the photodetector is configured to receive a signal from the photodetector corresponding to the refracted wavelengths of light and further configured to convert the signal into the operating conditions. A
display device may be connected to the central processing unit for displaying the operating conditions.
[0012] A method is described herein comprising a step of measuring one or more of:
average temperature of a steam generator tube temperature, local temperatures of the steam generator tube surface, static thermal strain of the steam generator tube and dynamic thermal strain of the steam generator tube. The one or more measurements may be made with a =
monitoring system as described above. The one or more measurements may be compared to threshold values to determine if maintenance or another action is required.
Optionally, multiple measurements, for example static and dynamic strain measurements, may be used in combination.
[0013] Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Embodiments of systems and methods will now be described, by way of example only, with reference to the attached Figures.
[0015] Figure 1A is an illustration of a once-through steam generator.
[0016] Figure 1B is a cross section of a circuit of the once-through steam generator depicted In Figure 1A.
[0017] Figure 2 is a perspective side view of a segment of a fiber sensing array.
[0018] Figure 3 is a cross section view of an exemplary embodiment of a fiber sensing array disposed within a hermetical cable package.
[0019] Figure 4 is a perspective side view of a sensor cable package disposed within a guide tube and affixed to a steam generator tube surface according to an embodiment of the present invention.
[0020] Figure 5 is a schematic depiction of a monitoring system and portion of a once-through steam generator in accordance with an embodiment of the present invention.
[0021] Figure 6 is a flowchart of a process for monitoring the operating conditions of steam generator tubes in an once through steam generator in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION
[0022] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments.
However, it will be apparent to one skilled in the art that these specific details are not required. In other instances, well-known electronic structures and circuits are shown in block diagram form in order to not obscure the understanding. For example, specific details are not provided as to whether the embodiments described herein are implemented as a software routine, hardware circuit, firmware, or a combination thereof.

[0023] FIG. 1A illustrates an example steam generator, in particular an OTSG 100, for use with a method and system for monitoring the operating conditions of its steam generator tubes. An HRSG is an energy recovery heat exchange system that recovers heat from a hot gas stream. The energy from the hot gas stream generates steam for electricity production or for various industrial processes. A specialized type of HRSG that does not include a boiler drum is a once-through steam generator (OTSG). An OTSG uses water to generate high pressure and high temperature steam.
[0024] In the OTSG 100, cold or pre-heated water may follow a continuous path without segmented sections for components such as economizers, evaporators, and super heaters. In the OTSG 100, preheating, evaporation, and superheating of the water may take place consecutively, within one continuous circuit 102. Water is pumped through the circuit 102, shown as arrow "A" in FIG. 1, starting from a cold end 104 of the OTSG 100. As the water flows through the OTSG 100, it is heated and changes phase as it extracts heat from the gas flow shown as arrow 106. Superheated steam flows from the hot end 108 of the OTSG 100, shown as arrow "B"
in FIG. 1. The circuit 102 includes one or more steam generator tubes that are exposed to one or more convective sections, and/or radiant sections, alternatively referred to as furnaces, together referred to as heating sections.
[0025] The temperature in the radiant section of an OTSG can reach up to 1,000 C
(degrees Celsius) The water or steam in the interior of tubes used in an OTSG
may reach 300 C and pressure of 1800 pounds per square inch guage (psig). Individual sections of the OTSG
100 may be larger or smaller based on the heat load received from the gas turbine.
[0026] FIG. 1B shows a cross-section of circuit 102 as shown in FIG. 1A.
Disposed within the circuit 102 are one or more steam generator tubes 109 which run the length of the circuit. The steam generator tubes 109 may be referred to as tubes 109 for brevity.
[0027] FIG. 2 shows an embodiment of a fiber sensing array 110. The fiber sensing array 110 comprises a strand of optical fiber 112 that reflects particular wavelengths of light and transmits all other wavelengths of light. The optical fiber 112 comprises a core 114 and a cladding 116. The cladding 116 comprises a material with a low refractive index, such as silicon dioxide, which encases the core 114, and an outer coating material, such as polyimide or metal. To achieve the desired reflective /transmission properties in the optical fiber 112, the refractive index of the core 114 is periodically varied. These variations are known as Bragg gratings (gratings) 118. Gratings 118 can be created by, for example, inscribing the core 114 with an intense ultraviolet source such as an ultraviolet laser. U.S. Patent 7,574,075 describes a fiber Bragg grating and a method of making it.

The gratings are about 5-10 millimeters in length. The distance between the gratings is about 50 millimeters.
[0028] Because of the harsh environment and extreme heat in an OTSG 100, a high-temperature fiber sensing array 110 is preferably used. An example of a high-temperature fiber sensing array 110 is a tetrahedral fiber Bragg grating sensor. U.S. Patent 8,180,185 describes a tetrahedral fiber sensing array for a harsh environment. The tetrahedral fiber sensing array comprises microcrystalline and silicon dioxide tetrahedral structure gratings which are better able to tolerate high temperatures while keeping their structural integrity and reducing thermal drift in the wavelengths of light reflected and refracted by the Bragg gratings.
[0029] FIG. 3 shows an example embodiment of a high-temperature fiber sensing array 110 encased in a hermetical cable package 120 which, together, form a sensing cable package 122. The hermetical cable package 120 comprises three concentric metal layers.
An inner metal layer 124 is disposed circumferentially about the high-temperature fiber sensing array 110. The inner metal layer 124 preferably comprises gold, nickel and aluminum. The inner layer preferably has a thickness of 10-20 microns. A middle metal layer 126 is disposed circumferentially about the inner metal layer 124. The middle metal layer 126 preferably comprises stainless steel or INCONEL. The middle metal layer 126 preferably has an outside diameter of less than 1 millimetre and an inside diameter of more than 0.25 millimetres, The outer metal layer 128 is disposed circumferentially about the middle metal layer 126. The outer metal layer preferably has an outside diameter of less than 1.5 millimetres and an inner diameter of more than 1 millimetre.
The outer metal layer 128 is preferably composed of INCONEL. The gaps between the three metal layers can be air, or thermal conductive filling material, or fluid. A
conventional pulling method is used to thread the fiber sensing array 110 through the inner metal layer 124.
[0030] FIG. 4 shows an example embodiment of the sensing cable package 122, substantially the same as shown in FIG. 3, affixed to, or integrated with, a tube 109. Prior to affixing or integrating the sensing cable package 122 to the tube 109, the surface of the tube 109 is first cleaned of all oxides. A guide tube 140 is affixed to the tube 109 by spot welding at multiple locations along the tube 109. The tube 109 and the guide tube 140 are welded together using shims 142 between them along the length of the tube 109. The shims 142 may be approximately 20 mm wide and have a curvature on one of their faces sufficient to adapt to the curvature of the tube 109 to which they are being affixed. The sensing cable package 122 is inserted or threaded into the guide tube 140. In an example embodiment, a sensing cable

- 5 -, package 122 can be from 20 to 30 feet in length. Multiple sensing cable packages 122 can be combined together, end to end, to span the entire length of a tube 109. The guide tube 140 may be sprayed with thermal sprays to mitigate potential delamination of the guide tube 140 from the shims 142, and the shims 142 from the tube 109. A first thermal spray may comprise or consist of a base coat of Metco 443. A second thermal spray may comprise or consist of alumina.
[0031] FIG. 5 shows an example embodiment of a plurality of fiber sensing cable packages 222 affixed to steam generator tubes 209 by shims 242 and connected to instrumentation 250 for monitoring the operating conditions of one or more steam generator tubes 209 in a circuit 202 of an OTSG 200. The OTSG 200, steam generator tubes 209, shims 242 and sensing cable packages 222 are substantially the same as those shown in Figures 1A, 1B, and 3. The sensing cable packages 222 run along the lengths of at least a portion of the steam generator tubes 209 within the guide tubes 240. The fiber optic sensors 210 of the sensing cable packages 222 are optically connected to a junction box 254 which transmits signals from the fiber optic sensors to a signal processing unit 256. The signal processing unit 256 may be an optical sensing interrogator, such as a sm125 from Micron Optics Inc. The optical sensing interrogator 256 comprises a broadband or tunable light source 258 and a photodetector 260. The photodetector 260 can be arranged as an array to provide a multi-channel optical spectral analysis functionality. For high accuracy spectral analysis, an optical sensing interrogator is normally integrated with a NIST standard gas calibration cell. The optical sensing interrogator 256 is connected to a central processing unit (CPU) 262 which includes a display 264. The light source 258 emits a broadband spectrum light. The spectrum of light emitted by the light source 258 can be controlled by either tuning a filter or by tuning a laser cavity. In an example embodiment the light source 258 is a tunable fiber laser that can provide 80-100 nm wide spectral range.
[0032] FIG. 6 is a flowchart of a process 300 for monitoring the operating conditions of the steam generator tubes 209 in the OTSG 200 of the system of Figure 5. The process 300 comprises the steps of emitting 302 light into a plurality of fiber optic sensors 210, detecting 304 the refracted wavelengths of the light, converting 306 the detected wavelengths multiplexed signals into individual sensor signals with a peak tracking algorithm, communicating 308 the signal to a central processing unit 262 (CPU), processing 310 the signal to determine the operating conditions of the steam generator tubes 209, and displaying 312 the operating conditions on a display.
[0033] In the step of emitting 302, the light is emitted by the light source 258 through the junction box 254 and into each of the fiber optic sensors 210. The light travels down the core

- 6 -214 of each of the fiber optic sensors 210. Upon encountering gratings 218, certain wavelengths of the light reflect and the other wavelengths refract. What wavelengths reflect and refract depends upon the properties of the grating 218, the spacing between the gratings 218, and the operating conditions of the steam generator tubes 209. The refracted wavelengths cascade through each grating 218 and travel back up the core 214 of the fiber optic sensors 210, through the junction box 254 and into the optical sensing interrogator 256.
[0034] Each grating 218, in effect, acts as an individual temperature and / or strain sensor. In an embodiment, each grating 218 is arranged to reflect slightly different wavelengths of light from the other gratings 218 that are also along the length of the fiber optic sensor 210. In this way, reflected light from a particular grating 218 (and therefore the temperature and/or strain sensed by that particular grating at a particular measurement location along the steam generator tube 209) can be differentiated from the light reflected by the other gratings 218. The range of light wavelengths each grating 218 is arranged to reflect depends upon the number of gratings 218 in the fiber optic sensor 210, the bandwidth of the light source 258, and the variance in wavelengths due to temperature and/or strain that the gratings 218 are expected to reflect.
[0035] In the step of detecting 304, the light detectors 260 in the interrogator 256 detect the refracted wavelengths of light.
[0036] In the steps of converting 306 and communicating 308, the detected wavelengths of light are converted into a digital signal and communicated to the CPU 262.
In example embodiments, communication may occur through any or all of sending and / or receiving electrical signals, optical signals, or wireless signals.
[0037] In the step of processing 310 and displaying 312, The CPU 262 processes the signal to determine the operating conditions of the steam generator tube 209 at a specific point in time and displays 209 the operating conditions on a display 264.
[0038] A grating typically has a sinusoidal refractive index variation over a defined length. The reflected wavelength AB of the pulse of light is defined by the equation = 271,A, where neis the effective refractive index of the fiber Bragg grating, and A is the grating period.
[0039] The bandwidth is defined by the equation AA, 1-28nol 2B' where 6n0 is the variation in the refractive index (i.e. n2 ¨ n1), and

- 7 -n is the fraction of power in the fiber core.
[0040] High-temperature fiber optic sensors 210 may be multi-functional. They are sensitive to both temperature and strain such that a change in either or both at any grating point along the length of the fiber optic sensor 210 causes a relative shift in the wavelength of light reflected at that grating 218. If the wavelength shift at time initial t(0) is gt(0)), then the wavelength shift of the response of the fiber optic sensors 210 to both temperature and strain at any moment, t, is defined according to the following equation:
AAB(t) = K(t) + K t AT (t), where AAB(t) = A(t) ¨ A(t(0)) and AT(t) = T (t) ¨ T (t(0)) k is the fiber sensor strain sensitivity E(t) is the thermal strain effect at time t K is the temperature sensitivity, and AT is the relative temperature variation at time t [0041] For each fiber optic sensor under strain-free condition, its wavelength shift depends upon the external temperature either linearly or nonlinearly. In general, a polynomial function up to order 3 could satisfy most of the calibration needs by AAB (t) = a + b = AT(t) + c = AT2(t) + d = A T 3 (t), where a, b,c and dare constants.
[0042] If the fiber optic sensor 210 is under a thermal strain due to the sensor package, the wavelength shift is just a function of the surface temperature of the tube 209. In such a case, the temperature sensitivity, Kt will be dominated by the coefficient of thermal expansion of the sensor package and steam generator tube. Meanwhile, the differentiation of the thermal strain and steam generator tube temperature will be realized by combing a strain-free fiber temperature sensor with fiber optic strain sensor.
[0043] A fiber optic sensor 220 can detect thermal strains and the instrumentation 250 can measure the extent to which a steam generator tube 209 deforms or ruptures.
[0044] A thermal strain due to the steam generator tube deformation at a constant temperature is described by the following equation: A(T , t) = A(T) + Kee(t).
The dependence of the wavelength shift will be similar to a slow function of time that reflects a gradual mechanical deformation or degradation trend.
[0045] A thermal strain due to a tube rupture is described by the following equation:
.1.(T , t) = A(T 0) + KEE(t) , where To is a specific steam tube operation temperature.
[0046] In this event, the observed fiber optic sensor long-term trend has suddenly been

- 8 -reverted back to strain-free status, or induces some discontinuous drop in the fiber optic sensor response.
[0047]
Preferably, both slow varied fiber optic sensor response and unexpected discontinuous response are combined for steam generator tube thermal degradation analysis.
The averaged tube temperature from all the fiber optic sensors can be used to associate with a general trend of the degree of fouling formation, while each individual fiber optic sensor in each steam generator tube surface can be used for local hot-spot detection.
[0048]
In the step of converting 306, the reflected wavelengths are multiplexed through wavelength domain signal analysis technology.
[0049] In the step of processing 310, the above-noted equations are used to determine various operating conditions of the steam generator tube 209. Operating conditions include, but are not limited to, a. the local temperatures and changes in local temperatures of a point on the steam generator tube 209 at each grating 218;
b. the local strain and changes in local strain of a point on the steam generator tube 209 at each grating 218;
c. thermal trends of a steam generator tube 209;
d. localized hot spots;
e. dynamic thermal events; and f. transient thermal events.
[0050]
The method can also include making and tracking one or more of the following measurements:
a. steam generator tube average temperature, which is useful for monitoring fouling formation or fouling trends using long term data analysis;
b. local temperatures at the steam generator tube, as determined for example by fiber optic sensors, which is useful for monitoring hot spot formation and propagation, c. static (or long term) thermal strain, or static strain trend, of the steam generator tube, which is useful for monitoring mechanical degradation of the steam generator tube over time; and, d. dynamic thermal strain of the steam generator tube, which is useful for detecting tube ruptures or potential tube ruptures.
[0051]
One or more of these measurements or trends in these measurements can be compared to threshold temperatures or trends. The threshold temperatures or trends may vary

- 9 -with the feed water or gas temperature. Measurements beyond the thresholds trigger a warning or report. Optionally or additionally, static and dynamic signals such as strain signals can be analyzed together and compared to pre-set limit values.
[0052] Prior to deploying fiber optic sensors 210 as shown in Figure 5, each fiber optic sensor 210 is preferably calibrated in a laboratory. During calibration, the calibration variables a,b,c, and d are determined through running simulations. When the fiber sensing package 210 is deployed in a steam generator, the strain on the fiber optic sensor 210 needs to be equivalent to the strain on the fiber optic sensor 210 in the laboratory during calibration so that the calibration variables a,b,c, and d are correct. After such a calibration a qualification is conducted [0053] The above-described embodiments are intended to be examples only.
Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope, which is defined solely by the claims appended hereto.

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Claims (23)

WHAT IS CLAIMED IS:

1. A system for monitoring operating conditions of steam generator tubes in a steam generator, the system comprising a fiber optic sensing array affixed to the steam generator tubes in the steam generator;
a hermetical cable package disposed circumferentially around the fiber optic sensing array;
a light source in optical communication for emitting a light into the fiber optic sensors;
a detector optically connected to the fiber optic sensing array for receiving refracted wavelengths of the light;
a central processing unit in communication with the photodetector, the central processing unit configured to receive a signal from the photodetector corresponding to the refracted wavelengths of light and further configured to convert the signal into the operating conditions; and a display device operatively connected to the central processing unit for displaying the operating conditions.

2. The system of claim 1, wherein the fiber optic sensing array consists of a plurality of fiber optic sensors.

3. The system of claim 1 or 2, wherein the fiber optic sensing array is made of tetrahedral fiber Bragg grating for high-temperature measurement.

4. The system of any one of claims 1 to 3, wherein the operating conditions comprise thermal strain and temperature measurements at multilocations along a steam generator tube.

5. The system of any one of claims 1 to 4, wherein the operating conditions comprises local and averaged temperature measurements and thermal strain measurement along a steam generator tube

6. The system of any one of claims 1 to 5, wherein the operating conditions comprises a thermal trend from a steam generator tube long-term operation performance.

7. The system of any one of claims 1 to 6, wherein the operating conditions comprise a mechanical degradation trend.

8. The system of any one of claims 1 to 7, wherein the operating conditions comprises localized hot spot(s).

9. The system of any one of claims 1 to 8, wherein the operating conditions comprise averaged steam generator tube temperature trend.

10. The system of any one of claims 1 to 9, wherein the operating conditions comprises a dynamic thermal event.

11. The system of any one of claims 1 to 10, wherein the operating conditions comprises a transient thermal rupture event.

12. The system of any one of claims 1 to 11, wherein the fiber optic sensors are disposed in a guide tube.

13. The system of any one of claims 1 to 12, wherein the hermetical cable package comprises three layers of metal disposed circumferentially.

14. The system of claim 13, wherein the inner metal layer comprises gold, nickel and aluminum, the middle metal layer comprises stainless steel and INCONEL, and the outer metal layer comprises INCONEL.

15. The system of claims 13 or 14, wherein the inner metal layer has a thickness of between 10 and 20 micrometres, the middle metal layer has an inner diameter of more than 0.25 millimetres and an outside diameter of less than 1 millimetre, and the outer metal layer has an inner diameter of more than 1 millimetre and an outer diameter of less than 1.4 millimeters.

16. A method of monitoring a steam generator tube comprising a step of using a fiber optic sensing array affixed to steam generator tubes in as steam generator and having a hermetical cable package disposed circumferentially around the fiber optic sensing array to make one or more of the following measurements:
a. steam generator tube average temperature;
b. local temperatures at the steam generator tube;
c. static strain, or strain trend, of the steam generator tube; or, d. dynamic strain of the steam generator tube.

17. A system for monitoring an operating condition of tubes in a steam generator, the system comprising:
fiber optic sensors affixed to the tubes, the sensors adapted for detecting one or more of mechanical strains, pressures, and temperatures in the tubes or sensors;
one or more computers connected to the sensors, the one or more computers adapted for receiving signals from the sensors, and monitoring the operating conditions of the tubes.

18. The system of claim 17, wherein the steam generator comprises a radiant section and a convention section, and wherein the sensors are affixed to tubes in the radiant section.

19. The system of claim 17, wherein the one or more computers are configured to identify segments of the tubes to which pertain the one or more of mechanical strains, pressures and temperatures.

20. The system of claim 17, wherein the computer is configured to monitor one or more of the following operating conditions of the tubes:
a. temperatures;
b. pressures;
c. mechanical strain;
d. thermal trends;
e. mechanical degradation;
f. localized hot spots;

g. dynamic and transient events;
h. rupture events; and i, fouled segments, based on the one or more of the mechanical strains, pressures, and temperatures.

21. The system of claim 17, wherein the fiber optic sensor comprises a tetrahedral fiber Bragg grating.

22. The system of claim 17, wherein the fiber optic sensor is encased in a hermetical cable package.

23. The system of claim 22, wherein the hermetical cable package comprises three layers of metal disposed circumferentially around each other.