At present, there are two main types of fiber optic temperature-sensing fire detectors: the fiber Bragg grating (FBG) temperature measurement system and the distributed fiber optic temperature measurement system (DTS). The FBG was researched earlier in China with more research institutions; at present, part of the fire monitoring of domestic chemical storage tanks uses fiber Bragg grating detectors. The distributed fiber optic temperature measurement DTS is a new generation of fiber optic temperature measurement technology that has developed rapidly in recent years. The technology is becoming more and more mature and the cost is gradually declining. It has been widely used in traffic tunnels, subways, power cables, coal conveyer belts, oil and gas wells and other fields.
The FBG temperature-sensing fire detector belongs to non-electric detection technology. The temperature sensor is installed above the secondary sealing ring of the tank floating deck, and is currently the most widely used tank fire detector in China, effectively ensuring the fire safety of storage tanks. However, the FBG temperature-sensing fire detector has the following deficiencies in practical application:
(1) Poor sensor reliability: the sensor string is formed by splicing one grating sensor after another by fusion splicing. Affected by thermal expansion/contraction and water immersion, the reliability of the splicing points is poor, easily causing sensor fiber breakage faults and the loss of some or even all monitoring areas.
(2) Measurement blind zones: the FBG temperature detector belongs to point quasi-distributed measurement, which can only measure the temperature at the sensor installation position and nearby areas. The temperature at positions without grating sensors cannot be effectively monitored, so there are measurement blind zones. Note: the early sensor interval was 6m, but according to the national standards "Design Specification for Petroleum Depots GB50737-2011" and "Linear Heat Detectors GB16280-2014", the sensor interval must not exceed 3m.
(3) Heavy workload of later operation and maintenance: limited by sensor design and packaging technology, grating sensors will drift; in addition, the long-term reliability of the signal demodulation unit based on angle tuning technology is poor, sensors need to be recalibrated regularly, and the operation and maintenance workload is heavy.
The new generation of distributed fiber optic sensor DTS belongs to continuous distributed non-electric detection technology. The technology is mature and advanced, and has well overcome the drawbacks of the above fiber Bragg grating detectors, as follows: (1) The optical cable is the sensor with no splicing points in the middle — high reliability: the cable itself is the temperature sensor; the cable plays the role of the heat detector and also serves as the important role of signal transmission, combining "transmission" and "sensing" in one. Since no additional sensing elements need to be installed, there are no splicing points in the middle of the sensing cable, with high reliability. (2) Continuous distributed measurement with no measurement blind zone: DTS belongs to continuous distributed measurement, which can obtain the temperature of every point along the sensing cable. Currently the distance interval between adjacent temperature points can reach up to 0.25m, with rich temperature information and no measurement blind zone; and based on continuous temperature analysis, regional temperature-difference alarm modes can be realized. (3) Small workload of later operation and maintenance: the DTS sensing cable is designed based on outdoor communication cables, whose service life can be up to 30 years; the core components of the DTS measurement host (such as light source and beam splitter) all use communication-grade components with a design life of 100,000 hours (more than 10 years), and the host has a built-in temperature calibration unit and fault self-check module — stable and reliable temperature measurement. The application schematic diagram of distributed fiber optic temperature measurement DTS is as follows:
