Application of Distributed Fiber Optic Temperature System in Tunnel Fire Alarms

Senzor temperature iz optičnih vlaken, Inteligentni nadzorni sistem, Porazdeljen proizvajalec optičnih vlaken na Kitajskem

Distributed Fiber Optic Temperature Systems Principles

Distributed fiber optic temperature sensing systems use two main temperature measurement principles; Raman scattering effect and optical time-domain reflection technology. According to the temperature measurement principle of fiber optic temperature sensing system, the detection system consists of optical modules connected to the sensing fiber optic cables, through photoelectric conversion into the application software, so as to realize the detection of temperature.

Positioning principle of temperature measurement

(1) Raman scattering effect with fiber optic temperature measurement principle.
When the light propagates in the fiber, the laser pulse and the fiber molecules and impurities collide with each other, resulting in various types of reflected light, such as; Brillouin scattering, Rayleigh scattering, Mie scattering and Raman scattering. The principle of temperature measurement in a distributed fiber optic sensing system (DTS) is the Raman scattering effect. When a photon propagates in an optical fiber, the photon interacts with the molecules in the fiber, resulting in two types of light; Stokes light (Stokes) and Anti-Stokes light (Anti-Stokes). Stokes light intensity is independent of temperature changes, anti-Stokes light intensity is related to temperature changes. The temperature of any point in the optical waveguide can be obtained from the ratio of the intensity of the non-Stokes light signal to that of the Stokes light signal.

According to the Raman scattering temperature measurement technology and optical time-domain reflection spatial positioning technology, the distributed fiber optic sensing system structure is divided into four modules; optical circuit module, sensing cable, circuit module and application software.
The optical module is the core structure of the system, in which the semiconductor laser generates effective laser pulses, which are coupled through the spectral path, reach the temperature sensing fiber, and propagate along the temperature sensing fiber to the measured area.
During the propagation of the laser in the fiber optic cable, Raman scattering phenomenon occurs, resulting in Stokes and non-Stokes light, which continues to propagate backward along the temperature-sensitive fiber, and then enters the optical filter through the beam splitter again. The optical filter filters out the other interfering light, removes the Stokes and non-Stokes light, and extracts the two types of light separately. The detector absorbs the extracted Stokes and non-Stokes light, and a photoelectric converter converts the optical signal into an electrical signal. Circuit module will receive the electrical signal processing, through the dual high-speed amplifier, the signal is collected by the high-speed data acquisition system, converted to digital signals, the temperature and positioning information into the host computer, and then the fire and faults to the fire alarm controller, to achieve the alarm function.

(1) Distribute continuous temperature measurement along the line. Distributed fiber-optic temperature sensing system sensors are distributed along the fiber-optic cable, which determines that distributed fiber-optic temperature sensing technology is suitable for alarming at any temperature point in long tunnels, with large measurement data, high positioning accuracy, real-time monitoring of temperature changes of the sensors in each circuit at 1 m intervals, and division of the tunnel into fire protection zones in accordance with the needs of the tunnel.

(2) Stable and accurate measurement.
Based on the principle of optical temperature measurement, the distributed fiber-optic temperature sensing system is free from electromagnetic interference and open fire hazards, and has obvious advantages over other technologies. The distributed fiber-optic temperature sensing technology is able to maintain stable performance in the flammable and explosive measurement environment of highway tunnels. The laser emits tens of thousands of effective laser pulses per second and outputs the average value of the measured temperature to the system, which largely reduces the error, and then manually determines and verifies the alarm after the pre-alarm, which effectively reduces the false alarms and makes the system measure accurately.

(3) Advanced technology.
In recent years, many developed countries (regions) outside the distributed fiber-optic temperature sensing system has replaced the point fire detectors and traditional linear fixed-temperature fire detectors, and the technology development has become mature. The application of distributed fiber-optic temperature sensing system to the monitoring system of road tunnels not only does not affect the stability of the data emergency plan of the tunnel comprehensive monitoring system, but also realizes the linkage between the distributed fiber-optic temperature sensing system and other electromechanical systems.

Comparison between fiber optic temperature sensing system and traditional temperature sensing system The detectors applied to tunnel fire are divided into point detectors and wire distributed fiber optic temperature sensing system.

Main features

Distributed fiber-optic relative to the traditional point-type temperature sensors have significant advantages, more adapted to the harsh monitoring environment of the tunnel fire, more and more widely used in the current tunnel fire monitoring detector of two types.

(1) Point-type detector mainly has dual-wavelength or three-wavelength flame automatic detector, the principle is to detect the specific wavelength and spectral range of the open flame, the system set up two or three sensors for receiving different wavelengths, to identify the spectrum and frequency of the flame combustion, to achieve the detection of the fire. Point detectors have fast response time, but they are susceptible to interference from smoke and open flames, as well as omission or false alarms caused by objects blocking the fire source. Point detectors do not provide a continuous measurement of the temperature inside the tunnel and cannot monitor the complete temperature distribution inside the tunnel. The sensors are easily dirty, resulting in lower detection accuracy and affecting the normal operation of the system.

(2) Linear fire detectors mainly include: air pipe temperature difference detectors, temperature sensing cables, thermal alloy wire temperature difference detectors, and fiber optic temperature sensors. Air pipe temperature difference detectors and temperature sensing cables are early detectors, which have been eliminated from the market due to their low detection reliability, unstable system, and inability to meet the current detection needs of tunnels. Thermal alloy cable temperature detectors are susceptible to electromagnetic wave interference and have a high frequency of false alarms, and are not capable of monitoring medium-length and extra-long tunnels on highways.

Distributed fiber optic temperature sensing system is one kind of wire-type fire detector. Compared with point-type detector and traditional wire-type detector, distributed fiber optic temperature sensing system can accurately determine the location of fire and fire temperature, with high sensitivity, stable performance, strong anti-interference ability, and realize long-distance monitoring.

Comparison between distributed fiber optic temperature sensing system and traditional temperature sensing system

Design of Tunnel Fire Monitoring System

Due to the difficulties in fighting tunnel fires and serious losses, it is of great significance to set up tunnel fire monitoring and early warning fire fighting system.

The company has independently developed a complete tunnel fire monitoring and early warning system with video fire detectors assisted by distributed fiber-optic fire detectors and fire control linkage.

Video surveillance system The video surveillance system is used to review and confirm the event and provide additional information in case of fire in the tunnel. The video surveillance system consists of a front-end system, a transmission system, a control system and a display system. It distributes, stores and restores the monitored image signals.
When detecting the alarm signal from the fiber optic sensor, the monitor can obtain the specific situation of the fire scene (the location of the fire point, the direction and speed of the fire and smoke spreading, and the situation of the people in the tunnel) more clearly and quickly, which is conducive to the rapid judgment of the video monitoring personnel, effective control of the development of the fire, and facilitates the fire fighting and rescue of the fire.

Fire fighting system The fire fighting system is an emergency response to tunnel fires and consists of 4 major systems: ventilation control system, lighting control system, broadcasting system and emergency telephone system. The ventilation control system can effectively control the wind speed, wind direction, carbon monoxide concentration and smoke concentration inside the tunnel, while the lighting control system is used to minimize the visual problems caused by the large brightness difference between the tunnel and the outside world. The broadcasting system is used to disseminate information to the tunnel personnel and to facilitate the evacuation of vehicles and people outside the tunnel in the event of a fire. The emergency telephone system is used in the event of a fire to enable the drivers of vehicles in the tunnel to call for help directly from the outside world.