RU2428682C1 - Method for thermal nondestructive inspection of thermal-technical state of long, non-uniform and hard-to-reach objects - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: single-mode or multimode fibre-optic line whose maximum withstand temperature is 200-250°C is laid on the heat insulating surface of an object. Temperature T_internal(t_i) of the heat-conducting fluid flowing inside the object during a time t_i is measured. Heat flux q₀ of the area of the object having quality heat insulation which is taken as reference heat insulation is measured. Insulation temperature T(x,t) is measured at discrete points on the length of the fibre-optic line. A matrix of temperature measurements is created by arranging into columns temperature values at one point of the surface of the heat-protective layer of the object measured at different times, and into rows - temperature values obtained simultaneously at different points on the length of the object. The matrix of the thickness of the heat-protective layer along the inspected object is determined from time and the technical state of the pipe and the value of loss of the transmitted energy are determined from the determined value of thickness of the heat-protective layer.

EFFECT: high reliability of results of inspecting the technical state of long, non-uniform and hard-to-reach objects.

3 cl, 24 dwg

Description

Technical field

The invention relates to the field of measuring equipment, in particular to non-destructive thermal control of objects, and can be used for thermal control (monitoring) of the technical condition of extended, complex and hard-to-reach objects (for example, pipelines and heating mains) in conditions of limited access based on Raman backscattering in standard single-mode and multimode optical fibers.

State of the art

Thermal non-destructive testing includes two main technological operations:

1. Registration of the temperature field of the surface of the controlled object.

2. Processing the temperature field in order to solve the tasks: detection of internal discontinuities, determining the quality of thermal insulation, etc.

In most cases, thermal imaging equipment is used to record the surface temperature field (O.N. Budadin, A.I. Potapov, V.I. Kolganov and others. Thermal non-destructive testing of products. - M., Nauka, 2002, 476 p.).

Methods of thermal control of the technical condition of extended objects, such as pipelines, heating mains, etc., using thermal imaging technology, include the method disclosed in the specified source. The method consists in moving a temperature field registration system - a thermal imager over the earth's surface, recording a temperature field video image and computer processing it. The processing results determine the anomalies of the temperature field characteristic of defective areas of thermal insulation. Having certain advantages, this method of recording the temperature field has significant drawbacks that significantly limit its application in practice, as well as in solving problems of long-term periodic monitoring of the technical condition of objects. These disadvantages are as follows:

- a large influence of the uneven quality of the surface from which the temperature field is recorded, the presence of interference, difficulties in moving the recording equipment (thermal imager), etc., which reduce the reliability of the control,

- low monitoring performance due to the need to move the thermal imager along the heating main,

- the high cost of control in the case of using aircraft to move the thermal imager,

- difficulties in defectometry of pipelines located in hollow boxes,

- difficulties of constant monitoring of the same object for a long time, especially with a short monitoring period.

Over the past 30 years, optical fibers (OB) have become an important element of communication systems due to the very small losses that light experiences during propagation, a large bandwidth for the information signal and insensitivity to external electromagnetic fields. For telecommunication applications, various designs of optical cables (OK) have been developed, capable of operating in a wide range of external temperatures and mechanical stresses, in conditions of high humidity in chemically active environments.

Modern fiber-optic communication lines allow the transmission of broadband signals (10 GHz or more) over a distance of tens of kilometers without repeaters. The same properties make OV and OK extremely attractive for use in control and measurement devices of various physical quantities, such as temperature, mechanical stress, pressure, etc. Such devices are called fiber optic sensors (VOD).

In the general case, water can be divided into two large classes [1]. The first class includes external or hybrid sensors. In them, the OM serves as a means of delivering a signal to various sensitive devices in which light beams are converted by external influences, undergoing modulation in intensity or polarization.

In another type of sensor, the optical fiber itself acts as a sensing element. A change in the parameters of the external environment leads to a change in the parameters of the signal propagating along the OM. Such sensors are inherently distributed, since various sections of the organic matter (all organic matter or each of its points) are exposed and, therefore, it is possible to monitor changes in the medium in a large number of points or continuously along the entire length of the fiber. In this case, one OM replaces the system of point sensors (their number in some cases can be hundreds or thousands of pieces), which must be installed to solve a specific control problem.

At present, the most common way to measure the distribution of the characteristics of the OM along its length is optical reflectometry, when the parameter is the power of the backscattering signal coming from various points of the OM. Pulse optical reflectometers (OTDRs), which use a Rayleigh backscattering signal, are widely used in fiber-optic communication for measuring attenuation and the length of optical radiation. In Rayleigh scattering, the frequency (wavelength) of the incident and scattered radiation coincide.

The temperature of the organic matter is measured by recording Brillouin or Raman scattering. The scattering spectrum in these cases consists of two components: Stokes and anti-Stokes (their wavelengths are respectively longer and shorter than the wavelength of the incident radiation).

Brillouin scattering arises as a result of the interaction between the incident radiation and acoustic waves, which are formed due to mechanical and temperature stresses in the organic matter.

Brillouin scattering has two spectral components, the frequency of which differs from the incident radiation frequency by 10.5 ... 11 GHz [2]. The temperature of the organic matter directly affects the magnitude of this frequency shift. The Brillouin reflectometry method is applicable only in single-mode OBs.

Raman scattering results from the interaction between the incident radiation and the vibrational states of the molecules of a substance. The intensity of the anti-Stokes scattering component depends on the temperature, which, on the other hand, has practically no effect on the intensity of the Stokes component. To register these components, the reflectometry method is used. As in the standard case, it allows one to obtain data on the continuous distribution of the studied parameter (in this case, temperature) along the optical fiber.

The Raman reflectometry method allows using for this purpose both standard telecommunication OBs and specialized optical fibers and cables designed for operation at high temperatures, pressures, in active media, etc. The OM simultaneously functions as a sensor and a distribution channel of measuring signals, and a Raman reflectometer - a device that registers and processes these signals.

When electromagnetic radiation passes through the physical medium, part of this radiation is scattered by the inhomogeneities and atoms (molecules) of the substance. In this case, the scattered radiation contains components with the same wavelength as the incident radiation, as well as components with different wavelengths. Figure 1 schematically shows the spectral composition of the scattered radiation in a quartz fiber.

The main parameters of Rayleigh scattering - the wavelength and intensity are independent of the temperature of the organic matter. On the other hand, its change leads to a shift in the frequency of the Brillouin components and a change in the intensity of the anti-Stokes component of Raman scattering. These effects are shown schematically in FIG. 2.

In accordance with the indicated physical phenomena, there are two methods for measuring the temperature of organic matter.

In Brillouin scattering, an interaction occurs between the propagating optical signal and acoustic waves, which arise due to mechanical and thermal stresses in the organic matter. These stresses create refractive index waves that travel at sonic speed. Due to the Doppler effect, scattered radiation appears to shift in wavelength (frequency). The speed of an acoustic wave is directly related to the density of the medium, which, in turn, depends on temperature and mechanical stress. Thus, the Brillouin frequency shift of the scattered signal carries information about the temperature and voltage of the organic matter.

This is the main problem in implementing the OB temperature meter using Brillouin scattering analysis - the inability to separate the mechanical and temperature effects on the frequency shift. In addition, it is necessary to use laser emitters with a narrow emission spectrum and high-quality fiber-optic filters in the measuring equipment, and either the heterodyne or interference reception method should be used to register Brillouin components. From this it follows that the implementation of the Brillouin scattering registration method for temperature measurement is possible only in single-mode OM, and the use of complex high-precision fiber-optic components leads to a significant increase in the cost of the instrument hardware.

In Raman scattering, a change in the OM temperature leads to a change in the intensity (power) of the anti-Stokes component. In this case, the temperature does not affect the characteristics of the Stokes component and Rayleigh scattering. The ratio of the intensities of the components of the Raman scattering is determined from the expression [3]:

where I _AS and I _S are the intensities of the anti-Stokes and Stokes components, respectively;

λ _AS and λ _S are the wavelengths of these components;

h is Planck's constant;

c is the speed of light in vacuum;

T is the absolute temperature;

ν is the magnitude of the frequency shift between the incident radiation and the Raman lines.

In quartz OM, the Raman lines are 13.2 THz or 440 cm ^-1 [3] away from the incident radiation frequency. For example, if the wavelength of the incident radiation is 1064 nm, then the peaks of the Stokes and anti-Stokes components will fall at 1116 nm and 1016 nm, respectively.

From relation (1) it follows that for measuring the temperature of the organic matter it is enough to know the ratio of intensities I _AS / I _S. Moreover, the distance along the axis of the wavelengths between the components is quite large. This allows:

- use the same methods and devices for recording Raman scattering as in ordinary Rayleigh reflectometry;

- apply standard telecommunication optical multiplexers and filters to separate the spectral scattering components;

- measure the temperature of both single-mode and multi-mode OM.

However, the intensity of the anti-Stokes component of the backscattering signal will depend not only on the OM temperature, but also on the attenuation in it. Therefore, equipment and measurement methods (techniques) should take this factor into account.

A known method of thermal control of the technical condition of extended objects, such as pipelines, heating mains, etc. (O.N. Budadin, A.I. Potapov, V.I. Kolganov and others. Thermal non-destructive testing of products. - M., Nauka, 2002, 476 p.). It includes registration of the temperature field of the surface area of the earth in the area of the pipeline and analysis of the temperature field. Due to the increased temperature of the pipeline (the coolant flowing through the pipeline has an elevated temperature compared to the environment) and broken insulation or broken continuity of the pipeline itself, anomalies of the temperature field are formed on the ground whose characteristics contain information about the technical condition of the pipeline. An analysis of the temperature field makes it possible to evaluate the characteristics of pipeline disturbances and objectively determine its technical condition.

However, this method has significant disadvantages:

- you need free access to the surface for inspection of the thermal imaging system,

- great difficulties in determining the thickness and quality of the technical state of thermal insulation due to uneven surface quality, the presence of interference, difficulties in moving registration equipment (thermal imager), etc.,

- low monitoring performance due to the need to move the thermal imager along the heating main,

- the high cost of control in the case of the use of aircraft to move the thermal imager,

- difficulties in defectometry of pipelines located in hollow boxes,

- difficulties of constant monitoring of the same object for a long time, especially with a short monitoring period.

SUMMARY OF THE INVENTION

In light of the foregoing, it becomes necessary to create a reliable method of thermal non-destructive testing (monitoring) of the technical condition of long complex and hard-to-reach objects (for example, pipelines and heating mains) in conditions of limited access based on Raman backscattering in standard single-mode and multimode optical fibers.

This will solve the following problems:

1. Improving the objectivity, reliability of the results of thermal control of the technical condition of extended, complex and inaccessible objects (for example, pipelines and heating mains) in conditions of limited access, including ensuring the reliability of control results of at least 0.98.

2. The expansion of the scope for various classes of objects and conditions for the control, including

- ensuring control without conditions of free access to the surface of the earth where the controlled object is located,

- increase noise immunity due to the exclusion of the influence of the quality of the surface of the earth with the underground location of the pipeline, etc.,

- increasing the control performance, ensuring control performance of at least 10 km of the pipeline length in 2-3 minutes for at least 15-20 years.

3. Reducing the cost of control by eliminating the use of moving nodes and blocks of the control system.

4. The ability to conduct thermal quality control and defectometry of pipelines located in hollow ducts.

Thus, it will be possible to solve practical problems of increasing the objectivity and reliability of the results of thermal control of the technical condition of extended, complex and hard-to-reach objects (for example, pipelines and heating mains) in conditions of limited access, expanding its scope, reducing the cost of monitoring and reducing losses energy due to timely repair of the facility.

The technical problem to which the present invention is directed is:

1. ensuring the reliability of the results of control, including improving the objectivity, reliability of the results of monitoring the technical condition of extended, complex and hard-to-reach objects (for example, pipelines and heating mains) in conditions of limited access,

2. expanding the scope of thermal control for various classes of objects and conditions for monitoring, including

- ensuring control without conditions of free access to the surface of the earth where the controlled object is located,

- increase noise immunity due to the exclusion of the influence of the quality of the earth’s surface with an underground location of the pipeline, etc.,

3. increasing the control performance, ensuring the control performance of at least 10 km of the pipeline length in 2-3 minutes for at least 15-20 years,

4. the possibility of conducting thermal quality control and defectometry of pipelines located in hollow ducts.

The technical result is achieved due to the fact that according to the method of thermal non-destructive testing of the thermal state of extended, complex and hard-to-reach objects, a single-mode or multi-mode fiber optic line is laid along the pipe insulation surface, the maximum allowable heating temperature of which is 200-250 ° C, and the temperature of the heat-conducting fluid flowing is measured inside the pipeline during the time T _int (t), measure the heat flux q ₀ characteristic quality section and a pipeline having a quality that is taken as a reference thermal insulation measures the temperature T (x, t) at discrete points along the length of the fiber optic line, forms a measurement matrix, arranging the temperature values in the columns at one point on the surface of the heat-insulating layer of the object, measured at different times, and in rows - temperature values obtained at different points along the length of the object at the same time:

determine the matrix of thicknesses of the heat-protective layer along the controlled object from time to time

The matrix elements are determined by the formula: δ (j, i) = λ (T _int (t _i ) -T (j, i)) / q ₀ ,

the technical condition of the pipeline and the amount of transmitted energy loss, where λ is the thermal conductivity coefficient of the insulation material, are determined by a certain value of the thickness of the heat-insulating layer.

The technical result is also achieved due to the fact that the thermal insulation temperature is measured by a device that includes a pulsed solid-state laser with a trigger circuit, an optical demultiplexer designed to separate Rayleigh and Raman scattering signals; optical receivers for detecting Rayleigh and Raman scattering signals and a synchronization signal, optical splitters for directing optical pulses into a measured optical fiber, an optical receiver of a synchronization signal, and an optical demultiplexer; an optical fiber coil for generating a backscatter reference optical signal; electronic optical fiber temperature sensor; a calculation unit for primary processing of measurement information; a control unit for controlling the measurement process and the accumulation of information; an interface unit, designed to connect the device with a personal computer, and a pulse voltage converter, designed to generate voltages to power the units of the device.

In this case, a pulsed solid-state laser is used that generates sounding pulses with a radiation wavelength of 1064 nm.

Description of the figures of the drawings

The invention will be further disclosed in more detail with reference to the accompanying drawings, where

figure 1. shows the spectral composition of scattered radiation in a quartz organic matter,

figure 2. - frequency shift of the Brillouin components and changes in the intensity of the anti-Stokes component of Raman scattering,

figure 3. the scheme of laying the fiber optic line along an extended object - heating mains,

figure 4. laying of the fiber optic line along the pipeline is shown,

figure 5. illustrates the nature of bias,

6 - the nature of the jump systematic error,

Fig.7 shows the trace of the Stokes (upper line) and anti-Stokes component of the Raman scattering,

Fig - a characteristic view of the dynamic range - the difference between the initial level of backscattering and the maximum noise level outside the trace,

Fig.9. graphs of the dependence of the amplitude of the noise on the trace from the level of the backscatter signal (trace level) for various values of the dynamic range of the reflectometer,

figure 10 - dependence of the amplitude of the noise on the trace from the error of temperature measurement on the length of the multimode OB. It is assumed that the OM attenuation at a wavelength of 1064 nm is 0.9 dB / km and the sensitivity of the anti-Stokes component to temperature changes is 0.013 dB / ° C,

11 shows a graph of the amplitude of the anti-Stokes component with respect to the Stokes component versus temperature,

12 illustrates a noise circuit of an optical receiver,

Fig.13 shows a structural diagram of the device,

Fig.14 shows the appearance of the device,

on Fig is a block diagram of a measuring algorithm,

Fig.16 illustrates examples of measurement results displayed in the form of graphs obtained using the software (software) of the device,

17 shows an optical fiber and portions whose temperature is to be analyzed,

Fig - distribution of weights,

on Fig shows a diagram of an algorithm for monitoring specified sections,

in Fig.20 shows a photograph of the device with a coil of fiber optic cable,

on Fig is a block diagram of a measuring algorithm,

on Fig - monitoring results,

23 shows an operator interface, and

Fig is a fragment of a video image of the temperature field of the earth's surface in the area of the controlled pipeline.

Description of a preferred embodiment of the invention

The following describes the use of the method for thermal non-destructive testing of the technical condition of an extended complex and difficult-to-reach pipeline in conditions of limited access based on Raman backscattering in standard single-mode and multimode optical fibers.

A single-mode or multi-mode fiber optic line is laid along the surface of the pipeline insulation. The potential heating temperature of the fiber optic line should not exceed the maximum allowable (200-250 degrees C).

The layout of the fiber optic line along the pipeline is shown in Fig.3.

Figure 4 shows photographs of the laying of a fiber optic line along the pipeline.

Measure the temperature of the heat-conducting fluid flowing inside the pipeline over time - T _int (t).

The temperature is measured either directly by the heat-conducting liquid inside the pipeline, by introducing a probe through special technological holes, or by measuring the temperature of the surface of the pipeline in the area of lack of thermal insulation.

Measurement can be carried out by a temperature sensor - Contact thermometer TK-5.09, manufacturer TekhnoAs LLC (Kolomna) with an error of not more than ± 0.5 degrees.

Measure the heat flux of a characteristic high-quality section of the pipeline (high-quality reference thermal insulation section) - q ₀ .

Measurements are carried out by the heat flow sensor - Five-channel ITP-MG4.03 "POTOK" electronic probes and heat probes and probes. Manufacturer LLC Stroypribor (Chelyabinsk).

Measure the temperature at discrete points along the length of the fiber optic line - T (x, t).

Form (have) the measurement results on the surface of the heat-shielding layer, thus obtaining a measurement matrix.

Determine the matrix of thicknesses of the heat-protective layer along the controlled object from time to time

The elements of the matrix are determined by the formula δ (j, i) = λ (T _int (t _i ) -T (j, i)) / q ₀ .

Using a certain value of the thickness of the heat-shielding layer, the technical condition of the pipeline and the amount of transmitted energy loss are determined, for example, as follows:

Here δ _min is the minimum permissible value of the insulation thickness, determined on the basis of the maximum permissible losses of the energy carrier transmitted through the pipeline.

The values of the temperature matrices T (nxk) and thickness δ (nxk) of the thermal insulation are stored in a database for a retrospective analysis of the functioning of the controlled pipeline.

Define the error of the method.

The study of the magnitude of the signals of the Raman backscattering

When determining the temperature of the optical fiber (RH), the Raman scattering components are recorded and the signal comparison procedure is performed. Since it is assumed that the characteristics of the Rayleigh and Stokes signals are temperature independent, the difference in their behavior from the anti-Stokes signal (a change in the slope of the anti-Stokes reflectogram, the appearance of large inhomogeneities on it, etc.) is interpreted as a change in temperature in the corresponding sections of the OM.

However, for a more accurate measurement of the temperature of the OM, it is necessary to take into account the differences in the signals that exist even at a constant temperature along the OM. Figure 5 shows the traces of the Stokes and anti-Stokes components of the line, consisting of two OB, connected using a welded joint. The temperature of both OMs is the same, but it can be seen that the slope of the Stokes (upper) reflectogram having a longer wavelength is less. In addition, the attenuation in the OB compound is different. If, to determine the temperature, it is easy to calculate the difference between the signals, then these differences will lead to systematic errors in the form of a slope of the temperature graph and a jump at the point of connection of the OB, as shown in FIGS. 5, 6.

When analyzing Raman signals, it must be taken into account that the wavelength of the scattered radiation differs from the wavelength of the probe pulse. It should be noted here that the backscattering coefficient B _imp-S (or V _imp-AS ) determines the fraction of the optical pulse power (with a wavelength of λ _imp ), which is converted to the corresponding component of the Raman scattering signal (with a wavelength of λ _S or λ _AS )

The parameters of the Stokes signal, as well as the Rayleigh signal, are independent of temperature, and the anti-Stokes backscattering coefficient In _{imp AS} depends both on temperature and on distance, since the temperature can be different at different points of the organic matter. This property can be expressed as follows:

In _imp-AS = In _imp-AS (T ₀ ) · In _imp-AS (ΔT (L)),

where In _{imp AS} (T ₀ ) is the value of the anti-Stokes backscattering coefficient, when the organic matter along its entire length has a certain constant temperature T ₀ ,

In _{imp AS} (ΔT (L)) is the change in this coefficient with a change in temperature.

In accordance with (12), the attenuation in the OM compound measured by an OTDR on the anti-Stokes component is determined by the expression (see (7) and (9)):

where T ₁ and T ₂ are the temperatures of the 1st and 2nd OM, respectively;

T _1.2 = T ₀ + ΔT _1.2 .

From the above equations it follows that the measured value of the attenuation in the connection depends on:

- attenuation at the wavelength of the probe optical pulse (β _{12, imp} ),

- attenuation at the wavelength of the anti-Stokes component (β _{12, AS} ),

- the ratio of the anti-Stokes backscattering coefficients of the fibers at the same temperature;

- changes in these coefficients with a change in temperature.

Information about the temperature of the organic matter is laid down precisely in the last term of the equality, therefore, for its accurate determination it is necessary to be able to take into account the values of other components. This means that it is necessary to determine by independent measurements:

- attenuation coefficients γ of each OM at the wavelengths of the optical pulse and anti-Stokes component (independent of temperature);

- values of attenuation β in organic matter compounds at these wavelengths (do not depend on temperature);

- a component of the anti-Stokes backscattering coefficient of each OM at a certain constant temperature along the OM;

- and, finally, the fraction of the power of the anti-Stokes signal at each point of the organic matter, depending on the temperature.

Factors determining the accuracy of measuring the temperature of an optical fiber during Raman scattering

Signal strength and noise amplitude.

When determining the temperature of an optical fiber by recording the intensity of the Raman scattering components, the method of pulse reflectometry is used. The backscattering signals have a small amplitude, and their analysis is carried out at a low signal-to-noise ratio (for large distances this ratio can be less than 1). Therefore, the main factors determining the error in measuring the temperature of the optical fiber are:

- the power of the radiation source, since the power of the backscattering signal (both Rayleigh and Raman) is directly proportional to the power of the probe optical pulse;

- the duration of the probe optical pulse, since the power of the backscattering signal (both Rayleigh and Raman) is directly proportional to this duration;

- intrinsic noise of the optical receiver;

- time of measurement (accumulation) of the signal.

For numerical estimates of the influence of these factors, formula (2) and the expressions are used:

ΔU _W = 5 · log (t ₁ / t ₂ ) ^1/2 ,

where ΔU _w (dB) is the change in the noise amplitude of the optical receiver when the measurement time changes (t ₁ / t ₂ ) times.

From formula (2) it follows that with an increase in the pulse power by a factor of 2, the trace level increases by

5 · log (P _imp2 / P _imp1 ) = 5 · log (2) = 1.5 dB

Similarly, from formula (16) it follows that when the measurement time is 2 times longer, the noise amplitude decreases by 0.75 dB.

The power of the backscatter signal and the intrinsic noise of the optical receiver of the OTDR determine the signal-to-noise ratio and, therefore, the amplitude of the noise in the OTDR. Since the measurement of the temperature of the OM is based on the measurement of the intensity of the anti-Stokes component of Raman scattering, the amplitude of the noise in the trace determines the minimum possible error in determining the temperature.

Figure 7 shows the reflectograms of the Stokes (upper line) and anti-Stokes components of Raman scattering.

The noise amplitude in the anti-Stokes trace is much higher; it is approximately 0.022 dB. The temperature sensitivity of the intensity of the anti-Stokes component is 0.45 ... 0.65% / ° C [6] or 0.01 ... 0.014 dB / ° C. This means that for this measurement, the minimum temperature measurement error can be 1.6 ... 2.2 ° C.

Therefore, the analysis of ways to improve the accuracy of temperature measurement, first of all, should be based on the analysis of the signal-to-noise ratio in the OTDR and ways to increase it.

Influence of the dynamic range of the OTDR on the amplitude of the noise on the OTDR

It is convenient to carry out a theoretical assessment of the noise amplitude on a trace using the concept of the dynamic range of an OTDR.

The dynamic range is the difference between the initial level of backscattering and the maximum noise level outside the trace (see Fig. 8); the specified difference is determined at a specific pulse duration and a given measurement time.

We determine the amplitude of the noise on the trace.

We denote

U _c is the voltage of the backscattering signal corresponding to a certain point of the optical wave, at the output of the optical receiver of the reflectometer, V;

U _W - noise voltage at the output of the optical receiver of the OTDR, V;

U ₀ is the maximum voltage of the backscattering signal corresponding to the starting point of the optical wave, at the output of the optical receiver of the reflectometer, V;

A is the level on the trace corresponding to the signal voltage U _c , dB.

A ₀ - the maximum trace level corresponding to the signal voltage U ₀ , dB.

Then, based on the definition, the dynamic range

D = 5 log (U ₀ / U _w ).

In addition, since the maximum level of the trace in accordance with the standard [7] is usually assigned a value of 0 dB, all lower levels will have negative values, and you can write

A = 5 log (U _c / U ₀ ).

The noise amplitude at a point on the trace corresponding to level A can be determined as follows:

It should be noted once again that the level of the trace point A is a negative value, and the dynamic range D is positive.

Figure 9 shows graphs of the dependence of the amplitude of the noise on the trace from the level of the backscatter signal (trace level) for various values of the dynamic range of the reflectometer.

In figure 10, this data is converted into a dependence of the error of temperature measurement on the length of a multimode optical waveguide. It is assumed that the attenuation of OM at a wavelength of 1064 nm is 0.9 dB / km and the sensitivity of the anti-Stokes component to temperature changes is 0.013 dB / ° C.

The obtained estimates make it possible to specify the requirements for the dynamic range of an OTDR, which determines the minimum attainable error in measuring the temperature of the OM.

Raman backscatter signal strength

The power of the Stokes P _S and anti-Stokes P _AS components of the signal scattered from the OB section of length dL located at a distance L from the beginning of the OB can be determined by the formula [8]:

dP _{S (AS)} = P (L) П _{S (AS)} Г _{S (AS)} dL,

where P (L) is the power of the incident radiation at the point L;

П _{S (AS)} - Bose-Einstein coefficients of the probability density distribution of phonons for the Stokes (S) and anti-Stokes (AS) components;

G _{S (AS)} is the conversion factor of the incident optical power into a backscatter signal

According to [8]

,

,

,

,

where ΔЕ is the energy of natural vibrations of the crystal lattice of a solid substance, for silica glass, ΔЕ = 50 meV;

k is the Boltzmann constant;

T is the absolute temperature.

The length of the scattering plot is related to the duration of the probe optical pulse by the relation [4]:

dL = (τ _imp · s) / (2 · n),

where c is the speed of light in vacuum;

n is the refractive index of the core of the OM.

Using the data presented in [8], it can be determined that in a multimode optical waveguide at room temperature (T = 300 K) with a probe wavelength of 1064 nm and an optical pulse duration of 1 ns, the power of the Stokes and anti-Stokes components reduced to the input power will be P _S = 4.67 nW / W, P _AS = 0.85 nW / W and the Stokes component is 5.5 times larger than the anti-Stokes component.

From these data it follows that for a wavelength of 1064 nm and a pulse duration of 1 ns, the backscattering coefficient for the Stokes component will be -83.3 dB; and for anti-Stokes -90.7 dB. The Rayleigh backscattering coefficient for a standard MM OM is –68 dB for a wavelength of 850 nm and –76 dB for a wavelength of 1300 nm [9].

11. The graph of the amplitude of the anti-Stokes component with respect to the Stokes component is plotted against temperature. It can be seen that in the range of 100 degrees, the ratio of intensities changes by about 2 times.

OTDR method

As indicated above, the power of the backscattering signal, and hence the dynamic range, is directly proportional to the power and duration of the probe optical pulse, i.e. his energy. In [6], it was proposed to use not single optical pulses, a pseudo-random sequence formed using complimentary correlation coding to increase the useful signal energy. The principle of using such coding in optical reflectometry was proposed in [10]. The main difference between complementary codes from others is that, theoretically, the correlation function of the sequence of such pulses does not have side lobes, all its energy is concentrated in the central peak. In [10], expressions were obtained for the signal-to-noise ratio for the usual single-pulse method and when using the encoded sequence:

In these formulas

N is the number of averaging counts;

M is the length (number of bits) of the pseudo-random sequence;

βh is the energy of a single pulse, taking into account the shape of the pulse response of the optical receiver of the reflectometer;

σ is the amplitude of the noise at the output of the optical receiver of the reflectometer.

From these equalities it can be seen that coding improves the signal-to-noise ratio in proportion to the square root of the length of the encoded sequence.

The paper [8] describes the experimental results of measuring the temperature of a multimode optical waveguide. A semiconductor laser with an external cavity was used as the radiation source of the reflectometer. The laser wavelength is 1064 nm, the radiation power is ~ 20 mW. With the help of such a device, a resolution of about 0.3 ° C was obtained when measuring organic matter with a length of 4 km for 10 minutes.

Optical receiver sensitivity

The optimization of the sensitivity of the optical receiver of an OTDR is the most important task in the design of the device.

12 shows a noise circuit of an optical receiver.

Indicated here:

- A - amplifier;

- R and C - load resistor and stray capacitance of the input circuit;

- is is the signal current source associated with the optical power P through the sensitivity of the photodiode S;

- in - noise source due to stray currents in the input circuit of the OPR, shot noise of the photodiode and thermal noise of the load resistor R;

- ea is the noise of the active amplifier element of the input circuit.

The total noise power St (A ² ) reduced to the input of the optical receiver is recorded [11]:

where z is the complex resistance of the input circuit of the optical receiver:

In further calculations, we will assume that all noise sources are “white”.

To achieve the maximum dynamic range in the optical receiver of an OTDR, an avalanche photodiode (APD) is used. Therefore, we can assume that the spectral power density Sd is due to the dark current of the APD id and shot noise caused by the incident optical signal P incident on the APD:

Sd = 2 · e · (In + M ^{2 + x} (Im + S · P)),

where e is the electron charge;

In is the non-multiplying dark current of the APD;

M is the avalanche gain coefficient of the photodiode;

x is the coefficient of excess APD noise (for germanium APDs x = 1, for APDs based on InGaAs x = 0.8);

Im is the multiplied dark current of the APD.

After integration we get:

where Ia is the input current of the amplifier;

In - bandwidth ODA at the level of 0.5.

We will calculate the threshold sensitivity value Pn of the optical receiver designed for a temperature meter with the following parameters:

R = 10 kΩ; C = 3 pF; In = 10 nA; Im = 2 nA; Ia = 750 nA; M = 30; ea = 8n V / (Hz) ^1/2 ; x = 0.8; B = 15 MHz.

By substituting these values, the magnitude of the noise power brought to the input of the optical receiver is St = 1.95 · 10 ^-16 A ² . It follows that the threshold sensitivity value Pn = 2.8 nW, i.e. with this value of optical power, the signal-to-noise ratio at the output of the optical receiver is 1.

The calculations performed above allow us to determine the optimal values of the main characteristics of the device — the dynamic range and sensitivity of the optical receiver, which will make it possible to increase the accuracy of measuring the temperature of the organic matter.

Block diagram and device design

As a result of the research, a device was developed for measuring the temperature of MM OM.

The block diagram of the device shown in Fig.13. The device contains:

- pulsed solid-state laser 1 with a trigger circuit, designed to generate probe pulses with a radiation wavelength of 1064 nm;

- optical splitters (OP-1 and OP-2) 2, 3, designed to direct optical pulses into the measured optical fiber, to an optical synchronization receiver and to an optical demultiplexer;

- optical demultiplexer 4, designed to separate the signals of Rayleigh and Raman scattering;

- optical receivers 5, 6 and 7, designed to record the signals of Rayleigh and Raman scattering (OPR-R, OPR-S and OPR-A) and the synchronization signal (OPR-sync);

- coil optical fiber OV-T 8, designed to create a reference optical signal backscattering;

- electronic temperature sensor 9 of this optical fiber;

- computing unit 10, designed for primary processing of measurement information;

- control unit 11, designed to control the measurement process and the accumulation of information;

- interface unit 12, designed to communicate the device with a personal computer;

- pulse voltage converter (IPN) 13, designed to generate the necessary voltages to power the units of the device.

The device operates as follows.

Laser 1 generates short optical pulses of a laser with a frequency that provides measurement of organic matter with a length of at least 10 km and is determined by the parameters of the trigger circuit. The main power of the laser pulses through the optical splitter OR-1 2 with a division ratio of 99: 1 is fed to the output splitter OR-2 3 with a division ratio of 50:50, and a small part of it is fed to the optical synchronization receiver OPR-sync 5. The output electrical signal of this the receiver 5 is used to synchronize the operation of the remaining blocks of the device.

An optical fiber OV-T 14 with a length of about 300 m is connected to the output of the OR-2 3 splitter. It is placed in a thermally insulated casing, which also contains an electronic temperature sensor 9. A sensor 9 measures the temperature of the OV-T 14, and this value is used for binding meter scale to absolute temperature values.

The OV-T 14 output is the output of the device, the measured OV is connected to it.

The backscattering signals of the measured OM are returned to the meter and, through the splitter OP-2 3, are fed to the input of the optical demultiplexer 4. In it, the signals of Rayleigh and Raman scattering are separated. These signals are fed to the corresponding optical receivers 5-7, converted into electrical form and amplified. Then they are digitized and further processed (averaged, logarithm, etc.) in the computing unit 10.

The processor (control unit 11) controls the operation of the meter. Data is transferred to a personal computer, stored in it and displayed on the screen.

The appearance of the device is shown in Fig.14.

On the front panel of the meter are located:

- switch 15 and indicator LED "Network" 16;

- Optical connector type FC / APC 17 for connecting the measured OV;

- LED indicator "Laser" 18.

The optical connector is covered with a protective cap 19.

On the rear panel of the meter are located:

- connector 20 for connecting a power cable "~ 230 V";

- 21 USB connector for connecting an interface cable for connecting the device with a personal computer;

- fuse socket 22;

- ground terminal 23.

To ensure the possibility of taking measurements with the device, software was developed that allows you to control the measuring module from a personal computer running Windows XP through a USB interface, based on a low-level command system. The command system allows you to set the necessary measurement parameters and read the results in real time. As a result of the execution of the algorithm according to the measurement procedure (see Fig. 15), a graph of the temperature versus distance (see Fig. 16) is plotted on the screen, and it is plotted using points with a fixed spatial resolution.

According to its purpose, the developed product should be able to diagnose a large number of potentially dangerous objects located along a multimode optical fiber. After the positive results were obtained in the temperature measurement continuously along the entire optical fiber (in an arbitrary (point) place, part 1), it was necessary to develop a methodology and software algorithm to control the parameters of discrete objects (sections) (previously known and set for temperature analysis ) - see Fig. 17.

In order for the program to display the temperature values (maximum, minimum or some average calculated value) immediately in digital form (not analog, i.e. simply in the form of a graph), it is necessary to indicate the areas affected by the temperature fields of the objects of interest. In this case, continuous monitoring of objects can be carried out in an optimal way.

Due to the fact that the influence of the temperature field of the object along the length of the plot is not uniform for calculating the weight coefficients of temperature values for each point of the plot, a Gaussian distribution is applied (see Fig. 18)

Then the temperature distribution along the length of the OM will be described by the expression

Therefore, the site monitoring algorithm may have the form shown in FIG. 19.

Main technical characteristics of the device

Operating modes:

- temperature analyzer;

- reflectometer.

The wavelength of the probe optical pulse is (1064 ± 10) nm.

The type of optical connector is FC / APC.

The maximum length of the measured OB is 10 km.

Technical specifications in temperature analyzer mode.

The range of measured temperatures is from 0 ° С to plus 250 ° С (the temperature range is determined by the intrinsic temperature strength of optical fibers and cables).

Limits of permissible absolute error of temperature measurement MM OB under operating conditions of use correspond to table 1.

Table 1 The maximum distance when measuring temperature, km Temperature measurement error, ° С one 0.5 3 0.8 10 1,5

Limits of permissible absolute error in determining the coordinate to the point of temperature measurement - not more than ± 3.0 m.

Specifications in OTDR mode.

The range of measured distances is 12 km.

The duration of the probe optical pulses is 3 ns.

The limits of permissible absolute error of distance measurement are:

ΔL = ± (dL + 3 · 10 ^-5 · L),

where dL is the resolution (sampling interval of the backscatter signal) equal to 2.5 m;

L is the measured distance, m

The dynamic range of attenuation measurement in dB at a signal-to-noise ratio of 1 (SNR = 1) and the number of averagings is 128 corresponds to table 2.

Table 2. Dynamic range, dB Rayleigh scattering signal not less than 27 Signal of the Stokes component of Raman scattering not less than 23 Signal of the anti-Stokes component of Raman scattering not less than 24

The nonlinearity of the attenuation scale should be no more than ± 0.05 dB / dB.

The meter is powered from an AC network with a voltage of (230 ± 23) V and a frequency of (50.0 ± 0.4) Hz.

The dimensions of the meter are 478 × 362 × 107 mm.

The mass of the meter is 5 kg.

In Fig.20 shows a photograph of the device with a coil of fiber optic cable.

On Fig shows a block diagram of a measuring algorithm.

Technical and operational characteristics of the developed equipment and the proposed control method are shown in table 3.

Table 3. Performance Specifications p / p Name of characteristic Value 2 3 Number of staff 1 person (operator, automatic monitoring mode is provided) The total weight of hardware Electronic unit weight up to 5 kg Sensor cable up to 15 km long, weighing no more than 30 kg / km Monitored and diagnosed objects Mines, tunnels, warehouses, pipelines, oil and gas storages, power and electrical equipment, industrial equipment, environmental and environmental facilities, construction sites, utilities and structures, potentially hazardous equipment Type of registered - Temperature distribution over the sensor cable,

p / p Name of characteristic Value 2 3 information in the field (the conditions of the actual operation of the facility without taking it out of operation) temperature fields - Locations of coolant leakage - temperature parameters in control (reference) points. Control results - the actual technical condition and residual life of potentially dangerous objects - quality (absence of continuity and structure defects) of materials, - safety and reliability of structures, - energy efficiency of structures

Experimental studies of the control method using the developed equipment were carried out on a section of a heating main (pipeline) with a length of 8 km.

In accordance with the proposed method and the developed equipment, the technical state of the pipeline thermal insulation was monitored for 3 years.

At the same time, once a year, thermal control and diagnostics of the technical condition of the pipeline were carried out in accordance with the method adopted as a prototype. A FLIR thermal imager located on a helicopter was used as measuring equipment. The results were recorded in the form of video images of temperature fields (in the video mode) in computer memory.

The results of the quality control of thermal insulation according to the claimed method in the form of a temperature profile on a 450 m pipeline section are shown in FIG. From the drawing, three defects of violation of the quality of thermal insulation, which caused a local temperature increase of 8-10 degrees, are clearly visible.

In Fig.23 shows the operator interface in the process of conducting experimental studies by the claimed method, which in the online mode allows for rapid analysis of defective areas.

On Fig, as an example, shows a fragment of a video image of the temperature field of the earth's surface in the region of the controlled pipeline, obtained as a result of thermal control by the method adopted as a prototype.

According to the results of the control, the approximate thickness of the heat-protective layer along the controlled object from time to time is determined by the formula

δ (x, t) = X (T _int (t) -T (x, t)) / q ₀

The technical condition of the pipeline and the amount of loss of transmitted energy are determined by a certain value of the thickness of the heat-shielding layer.

Here δ _min - the minimum allowable thickness of the insulation layer on the surface of the pipeline.

The magnitude of the increased (excess) heat loss is determined as follows:

Δq = λ [(T _int (t) -T (x, t)) / δ _min - (T _int (t) -T (x, t)) / δ (x, t)].

The results of a comparative analysis of the results of experimental studies of the proposed method and the method adopted as a prototype on the same object are shown in table 4.

Experimental studies have shown that the proposed method of thermal non-destructive testing (monitoring) of the technical condition of extended, complex and hard-to-reach objects (for example, pipelines and heating mains) in conditions of limited access based on backward Raman scattering in standard single-mode and multimode optical fibers provides:

1. Improving the objectivity, reliability of the results of thermal control of the technical condition of extended, complex and inaccessible objects (for example, pipelines and heating mains) in conditions of limited access, including ensuring the reliability of control results of at least 0.98.

2. The expansion of the scope for various classes of objects and conditions for the control, including

- ensuring control without conditions of free access to the surface of the earth where the controlled object is located,

- increase noise immunity due to the exclusion of the influence of the quality of the surface of the earth with the underground location of the pipeline, etc.,

- increasing the control performance, ensuring control performance of at least 10 km of the pipeline length in 2-3 minutes for at least 15-20 years.

3. Reducing the cost of control by eliminating the use of moving nodes and blocks of the control system.

4. The ability to conduct thermal quality control and defectometry of pipelines located in hollow ducts.

5. Significantly more responsiveness and control performance.

6. Ten times lower cost of control.

7. The ability to detect smaller defects (approximately 10 times).

The invention has the following advantages:

1. Improving the objectivity, reliability of the results of thermal control of the technical condition of extended, complex and inaccessible objects (for example, pipelines and heating mains) in conditions of limited access, including ensuring the reliability of control results of at least 0.98.

2. The expansion of the scope for various classes of objects and conditions for the control, including

- ensuring control without conditions of free access to the surface of the earth where the controlled object is located,

- increase noise immunity due to the exclusion of the influence of the quality of the surface of the earth with the underground location of the pipeline, etc.,

- increasing the control performance, ensuring control performance of at least 10 km of the pipeline length in 2-3 minutes for at least 15-20 years.

3. Reducing the cost of control by eliminating the use of moving nodes and blocks of the control system.

4. The ability to conduct thermal quality control and defectometry of pipelines located in hollow ducts.

5. Significantly more responsiveness and control performance.

6. Ten times lower cost of control.

7. The ability to detect smaller defects (approximately 10 times).

INFORMATION SOURCES

1. Fiber optic sensors. Introductory course for engineers and scientists: Transl. From English. / Under. ed. E. Udda. - M .: Technosphere, 2008 .-- 520 p.

2. Listvin A.V., Listvin V.N. Reflectometry of optical fibers. - M .: LESARart, 2005 .-- 208 p.

3. Wait P.C., Souza K De, Newson T.P. A theoretical comparison of spontaneous Raman and Brillouin based fiber optic distributed temperature sensors Opt. Commun. 1997, V.144 (1-3), p.17-23.

4. Grigoryants VV, Chamorovsky Yu.K. Diagnostics of fiber optical fibers and optical cables by the backscattering method // Itogi Nauki i Tekhniki, VINITI, Radiotekhnika series, 1982, v.29, p. 47-79.

5. Hartog A.H., Gold M.P. On the theory of backscattering in single-mode optical fibers - Journal of Lightwave Technology, 1984, V. LT-2, No. 2, p. 76-82.

6. Soto M.A., Sahu P.K., Faralli S. et al. High performance and highly reliable Raman-based distributed temperature sensors based on correlation-coded OTDR and multimode graded-index fibers - Proc. of SPIE, 2007, V.6619, p.66193B-1-66193B-4.

7. GR-196-Core. Generic requirements for optical time domain reflectometer (OTDR) - type equipment .: Bellcore, December 1998.

8. Farahani M.A., Gogolla T. Spontaneous Raman Scattering in Optical Fibers with Modulated Probe Light for Distributed Temperature Raman Remote Sensing - Journal of Lightwave Technologies, 1999, V.17, No. 8, p. 1339-1391.

9. Corning InfiniCor 50 µm optical fibers. Product information. http://www.corning.com/docs/opticalfiber/pil457.pdf

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Claims (3)

1. The method of thermal non-destructive testing of the thermal state of extended, complex and hard-to-reach objects, which consists in the following operations being performed:
a single-mode or multi-mode fiber optic line is laid on the surface of the thermal insulation of the object, the maximum permissible heating temperature of which is 200-250 ° C,
measure the temperature T _int (t _i ) of the heat-conducting fluid flowing inside the object for a time t _i ,
measure the heat flux q _{0 of the} site of the object having high-quality, taken as a reference thermal insulation,
measure the insulation temperature T (x, t) at discrete points along the length of the fiber optic line,
form a matrix of temperature measurements, arranging in the columns the temperature values at one point on the surface of the heat-protective layer of the object, measured at different times, and in the rows - the temperature values obtained at different points along the length of the object at the same time:

determine the matrix of thicknesses of the heat-protective layer along the controlled object from time to time

the elements of the matrix are determined in accordance with the formula
δ (j, i) = λ (T _int (t _i ) -T (j, i)) / q ₀ ,
and the technical condition of the pipeline and the amount of loss of transmitted energy, where λ is the coefficient of thermal conductivity of the insulation material, are determined from a certain value of the thickness of the heat-insulating layer. 2. The method according to claim 1, characterized in that the thermal insulation temperature is measured by a device including a pulsed solid-state laser with a trigger circuit, an optical demultiplexer, designed to separate the Rayleigh and Raman scattering signals; optical receivers for detecting Rayleigh and Raman scattering signals and a synchronization signal, optical splitters for directing optical pulses into a measured optical fiber, an optical receiver of a synchronization signal, and an optical demultiplexer; an optical fiber coil for generating a backscatter reference optical signal; electronic optical fiber temperature sensor; a calculation unit for primary processing of measurement information; a control unit for controlling the measurement process and the accumulation of information; an interface unit, designed to connect the device with a personal computer, and a pulse voltage converter, designed to generate voltages to power the units of the device. 3. The method according to claim 2, characterized in that they use a pulsed solid-state laser that generates probing pulses with a radiation wavelength of 1064 nm.

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