JPH049627A - Method for detecting abnormal low temperature and abnormal low temperature detector utilizing the same - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to cracks and holes in low-temperature parts, particularly storage facilities such as tanks for liquefied gases such as liquefied natural gas, and transport devices such as vibrine for the liquefied gases. The present invention relates to an abnormal low temperature detection method and an abnormal low temperature detection device for detecting an extremely low temperature region caused by the above.

[Prior Art] A block diagram of a distributed optical fiber temperature sensor using the conventional 0TDR method is shown in FIG. The laser pulse oscillated from the l/-Zerbars 10 in the light source section is
The backward Raman scattered light that is incident on the optical fiber 12 to be measured, generated in the optical fiber 12, and returned to the input end.)
It's coming. The subsequent Calaman scattered light is guided to a measurement device by a light directional coupler 11, and first, a filter 13 separates and detects the stray light and anti-Stokes light in the Raman scattered light, and converts them into photoelectric converters 14 and 4 respectively. ' is converted into an electrical signal proportional to its intensity. The electrical signals are each sent to a preamplifier 1.
5. The averager 16 performs averaging processing a predetermined number of times. The averaged signal is transmitted to the signal processing unit 17, where the ratio of the signals of Stokes light and anti-Stokes light is calculated, and processing such as conversion to temperature distribution is performed.

It has been proposed that such measuring means be installed with optical fibers around liquefied natural gas tanks and vibrate lines, and be used in L7 or low-temperature site detection means for the purpose of preserving them. That is, when liquefied gas leaks due to cracks in the tank or vibrating line, the surrounding light soaver is cooled by the heat of vaporization.First, the crack location is identified by measuring the temperature distribution.

[Problems to be Solved by the Invention (2)] When the Bunkarin optical fiber temperature sensor is applied to detect a low temperature site, the following problems occur.

(1) A special optical fiber with low loss at low temperatures is required.

(2) Since such optical fibers cannot be coated with a protective coating like conventional ones, they are very numerous and require special installation techniques.

(3) The above results in an increase in cost.

(4) When using conventional optical fibers that have large losses at low temperatures, the signal attenuation rate [dB/
kml increases nonlinearly as the temperature decreases, making accurate temperature measurement impossible.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes a light source section that injects a laser pulse into an optical fiber to be measured and a back Raman scattered light from the optical fiber to be measured. A light directional coupler that guides the Raman scattered light to the measuring device, a filter that separates Stokes light and anti-Stokes light in the backward Raman scattered light, and a filter that separates the Stokes light and anti-Stokes light into intensities. A photoelectric conversion unit that performs photoelectric conversion into a proportional electric signal, an averaging processing unit that averages the electric signal from the photoelectric conversion unit a predetermined number of times, and a signal of Stokes light and anti-Stokes light that has been averaged over the number of steps. Using a distributed optical fiber temperature sensor consisting of a signal processing section that takes the ratio and converts it into a temperature distribution, a signal loss of less than a threshold value occurs at a certain distance from the light source in the temperature distribution obtained in the signal processing section. abnormal low temperature detection method i4, characterized in that the low temperature part is detected by comparing and contrasting the attenuation rate of the signal of the loss part with the attenuation rate of the signal at each temperature of the optical fiber to be measured stored in advance; This is what we provide.

The block diagram of the distributed optical fiber temperature sensor of the present invention is the same as the example shown in FIG. May be connected.

The steps of the detection method are described below.

The temperature distribution of the optical fiber to be measured is measured using the a9 distributed optical fiber temperature sensor.

bl, a portion with a sudden loss in the temperature distribution is focused on, and if the loss is a threshold amount "f", it is determined that an abnormal low temperature has not occurred.

Similarly to b2 and bl, if the loss is greater than or equal to the threshold value, it is determined that an abnormal low temperature has occurred, and the following processing is performed.

Calculate the signal attenuation rate [dB/km] of the C1 loss part.

The attenuation factors of d and C are compared and contrasted with pre-stored attenuation factors at each temperature of the optical fiber to be measured, and the temperature of the loss portion is determined.

[Function] FIG. 2 shows the attenuation rate temperature characteristics of the optical fiber used in the present invention. Although it exhibits nonlinearity in this way, if the detection temperature is selected appropriately, it is possible to detect a sufficiently reliable low temperature region.

That is, the threshold upper limit temperature is T1 and the lower limit temperature is T.
2. The average temperature of the room temperature section is 70%. The attenuation rate at this time is DI.
D2, Do [dB/km], fluctuation of the attenuation rate in the room temperature section △D [dB/km], OT D
Assuming that the dynamic range of the R section is P [dB] and the measurement length is L [km], T and Ta can be detected if P>L-D0 and ΔD<DI-01.

Conversely, it is also possible to freely control this temperature characteristic by making the coating material or the structure of the optical fiber sensitive to low temperatures, such as a plastic clad fiber.

[Example] Figure 3 shows the attenuation rate temperature characteristics of the optical fiber used in the present invention. It is possible to distinguish between 173"K and 153"K using this optical fiber and an 0TDR unit (distributed optical fiber temperature sensor) with a one-way dynamic range of 10dB. The attenuation rate at 173° is approximately 10 dB/km, the attenuation rate at 153 brushes is approximately 20 dB/km, the attenuation rate at room temperature 300° is approximately 5 dB/km, the measurement length is 1 km, and the fluctuation of the attenuation rate is 0.
5 dB/km.

The following method is effective in identifying the low temperature section from the loss waveform 1 of the 0TDR section as shown in FIG. Divide the distance measurement range into multiple blocks and compare the loss. At this point, a comparison is made with past loss data as shown in Figure 3, and if no loss increase above a certain threshold (for example, 0.5 dB) is observed, it is assumed that no low-temperature region has occurred and 0TDR is determined. Return to measurement mode. This threshold value is determined experimentally or by considering the installation state of the optical fiber.

Next, the block with the largest loss is divided into smaller pieces and the losses of each block are compared. This operation is repeated thereafter, and ends when the required positional accuracy is satisfied (successive approximation method). By optimizing the number of comparison steps and the number of divisions for each step, the reliability of position detection can be increased and the processing time can be shortened.

In addition, there is little variation in loss per unit length at room temperature.
If the S/N ratio is good, position detection is also possible by frequency separation using simple differentiation.

[Effects of the Invention] The present invention is capable of accurately detecting the temperature and position of a low-temperature area caused by damage to a liquefied gas tank or vibrator without using special optical fibers that have low loss at low temperatures. produce an effect.

[Brief Description of the Drawings] Figures 1 to 3 show examples of the present invention, Figure 1 is a graph of an example of temperature distribution measurement, and Figure 2 is a graph showing nonlinearity of the attenuation rate depending on temperature. FIG. 3 is a graph showing actual data of the attenuation rate, and FIG. 4 is a block diagram of a conventional distributed optical fiber temperature sensor. Figure ↑

Claims (3)

[Claims] (1) A light source unit that injects a laser pulse into the optical fiber to be measured, a light directional coupler that guides the backward Raman scattered light from the optical fiber to be measured to the measurement device, and Stokes light and anti-Stokes light in the backward Raman scattered light. a filter that separates light; a photoelectric conversion section that photoelectrically converts Stokes light and anti-Stokes light into electrical signals proportional to their respective intensities; and an averaging processing section that averages the electrical signals from the photoelectric conversion section a predetermined number of times. Temperature distribution obtained in the signal processing section using a distributed optical fiber temperature sensor consisting of a signal processing section that takes the ratio of Stokes light and anti-Stokes light signals that have been averaged a predetermined number of times and converts it into a temperature distribution. If a signal loss of less than a threshold occurs at a certain distance from the light source, the signal attenuation rate of the lost portion is compared and contrasted with the pre-stored signal attenuation rate at each temperature of the optical fiber under test. An abnormal low temperature detection method characterized by detecting a low temperature part. (2) An abnormal low temperature detection device using the detection method according to claim 1. (3) A liquefied gas outflow detection device installed in a liquefied gas storage facility or transport device using the detection method according to claim 1.