1、summary

Most of the oil products stored and transported in the tank area belong to Class A and Class A fire hazard media, usually stored in liquid form under increased pressure conditions at room temperature, with gas-liquid two-phase properties. The fire hazard is mainly manifested in the following aspects：

（1）Easy to evaporate. After storing and releasing pressure in a nocturnal form, it immediately evaporates into gas, expands by 250-300 times in volume after gasification, and rapidly diffuses and spreads。

（2）High relative density (1.5-2 times that of air). Heavier than air, prone to stagnation and accumulation in low-lying areas such as cable trenches and sewers, and prone to forming explosive mixtures with air. Once the explosion limit is reached, it can ignite and explode when encountering a fire source。

（3）Flammable and explosive. Low flash point, ignition temperature lower than general combustible gas temperature (about 400-530 ℃), high danger, forms explosive mixed gas after contact with air, explosion limit is 2.1% -9.5% (volume ratio), can be ignited by small sparks, explosion speed is 2000-3000m/s。

（4）High combustion heat value. The calorific value is greater than 15605.5KJ/KG (91272KJ/m3), the flame temperature is as high as 2120 ℃, the radiation heat is strong, and it is extremely easy to ignite and ignite surrounding flammable and explosive substances, causing the fire to expand。

（5）Easy to expand. A storage tank belongs to a pressure vessel, and the oil stored in the container is in a gas-liquid coexistence equilibrium state at a certain rated temperature and saturated vapor pressure. As the temperature increases, the liquid volume will continue to expand, the gas pressure will also continue to increase, and the possibility of gas leakage will also increase。

（6）It is corrosive. Internal corrosion can continuously thin the walls of containers, leading to their compressive strength, shortening their service life, causing perforation, gas leakage, or explosion, resulting in fire and explosion accidents。

（7）Easy to generate static electricity. When oil is sprayed at high speed from the nozzle, nozzle or damaged area, static electricity can be generated, and the static voltage can reach thousands or even tens of thousands of volts. According to the measurement, when the electrostatic voltage is between 350-450V, the discharge spark generated can cause combustible gas combustion or explosion。

2、Significant advantages of fiber optic grating temperature sensing fire alarm system

The ZZ-FBG10 series fiber optic grating linear temperature sensing fire detection system is a new type of online temperature sensing fire detection system specially developed by Nanjing Zhunzhi Sensing for users in the petrochemical fire protection industry. The design concept is that the system adopts decentralized on-site installation of front-end equipment (such as oil tank temperature measurement devices), and these front-end equipment are connected by multiple optical cables to form an oil tank fire monitoring network; The oil depot fire monitoring and management system is formed by connecting the fire alarm through the switch interface. The advantages of using this system are: on-site installation of equipment, omitting many intermediate links, and avoiding potential accident hazards; By comprehensively utilizing the comprehensive advantages of networked petrochemical systems, we can accelerate the speed of fire alarm and avoid losses in the petrochemical industry。

The significant advantages of the PI-8F24AC series fiber optic grating linear temperature sensing fire detection system include：

Ø  Based on fiber optic grating sensing technology, it has achieved no electricity detection, intrinsic safety and explosion-proof, and is suitable for various flammable and explosive occasions；

Ø  Fiber Bragg Grating has good resistance to strong electromagnetic interference, lightning strikes, corrosion, and strong environmental adaptability；

Ø  Distributed measurement methods can be used to form a fiber optic sensing network, with flexible and convenient temperature arrangements；

Ø  Fiber Bragg Grating can measure temperature in real-time on-site, with high sensitivity, short response time, and high temperature accuracy；

Ø  Adopting advanced and mature patented technology, the product has stable and reliable performance, compact system structure, simple installation, convenient maintenance, and long service life。。

Ø  Automatically conduct real-time inspections of the area where the grating sensor is located, detect abnormal temperature fluctuations on site, and achieve early fire alarm。

Ø  After the fiber optic grating alarm, it can automatically recover and reuse, and the sensors on the monitoring site are maintenance free；

Ø  The alarm threshold of the fiber optic grating temperature fire detector can be set on site according to the actual situation to meet the needs of the engineering site; Adopting a two-level alarm method, the alarm temperature can be remotely set, effectively preventing false alarms；

Ø  Real time display of temperature values and temperature change curves at corresponding monitoring points in the form of graphics and text. The monitoring location and temperature can be displayed, and alarm monitoring is intuitive and convenient for management personnel to operate and maintain。

Ø  The temperature and alarm information of each monitoring point are saved in the large capacity storage of the fiber optic grating temperature fire detector, and the historical temperature change curves of each monitoring point can be viewed, providing data support for decision-making and maintenance。

3、Fire monitoring methods for different types of storage tank areas in petrochemical industry

The fire monitoring methods vary for different types of storage tanks in the petrochemical industry, as follows：

【Fire monitoring of dome roof oil tanks】

The application of fiber optic grating linear temperature sensing fire detection system in dome roof tanks. Fix the fiber optic grating temperature sensing unit link directly at the junction of the tank top and tank wall as an alarm area, and set the alarm area in the same way as other dome top tanks,. Install a fixed clamp every 1m to fix the monitoring optical cable. Use two high-strength magnets to attach the monitoring optical cable and the fixed clamp to the surface of the tank. If there are suitable bolts, M10 nuts can also be used to fix the fixed clamp to the bolts。

【Fire monitoring of spherical oil tanks】

The application of fiber optic grating linear temperature sensing fire detection system in spherical oil tanks. Divide the fiber optic grating temperature sensing unit into multiple circles and arrange it around the spherical tank, with a spacing of about 3 meters between each circle. It can also be arranged along the fire protection pipeline on the spherical tank, with two alarm areas divided by the upper and lower parts of the tank. The spectrophotometer is installed on the support column of the spherical tank. Fixed with Ω - shaped clips (as shown in Figure 3), one fixed clip is installed every 1 meter interval。

【Fire monitoring of external floating roof tanks】

The application of fiber optic grating linear temperature sensing fire detection system on external floating roof tanks. Fix the fiber optic grating temperature probe on the edge of the outer floating plate or (secondary) sealing ring using a special fixing card or thermal conductive adhesive as an alarm area. The transmission optical cable is led up to the top of the tank through a galvanized steel pipe with a diameter of 20mm, and then laid along a movable staircase to the fiber optic junction box. The fiber optic junction box is installed at the top of the floating roof tank, and a single-mode fiber optic cable is led out from the fiber optic junction box to connect the temperature measurement grating. It should be noted that the length of the transmission optical cable must ensure that the floating roof reaches the highest position from the lowest position, and sufficient expansion and contraction allowance should be reserved to coil on the floating plate。

【Fire monitoring of internal floating roof tanks】

The application of fiber optic grating linear temperature sensing fire detection system on inner floating roof storage tanks. Fix the fiber optic grating temperature probe at the junction of the floating cabin and the sealing ring on the inner wall of the tank using specialized fixing cards or thermal conductive adhesive as an alarm area. The junction box for connecting the optical cable is fixed outside the ventilation hole, and the length of the single-mode optical cable entering the ventilation hole must ensure that the floating roof can reach the highest position from the lowest position. The method of connecting the optical cable externally is the same as that of the external floating roof tank。

4、Introduction to Fiber Bragg Grating Linear Temperature Sensing Fire Detection System

The ZZ-FBG10 series fiber optic grating linear temperature sensing fire detection system consists of a fiber optic grating temperature sensing unit, a signal processing unit (fiber optic grating demodulator), an optical cable splice box, and a transmission optical cable. This system provides real-time temperature monitoring within the detected area. The system can partition the monitored object according to user requirements and monitor all temperature points in real time。

In order to better connect with other systems, the fiber optic grating linear temperature fire detection system adopts standard communication interfaces and protocols: RS-485, RS-232, and ETHERNET IEEE802.3 specifications, supporting IPX and TCP/IP protocols. Due to the use of ETHERNET standards, the system can be interconnected with the management network。

The fiber optic grating linear temperature sensing fire detection system scheme includes the following interrelated subsystems：

(1)Sensors and transmission systems：

The system applies advanced quasi distributed fiber optic grating sensing technology and converts the measured temperature values into optical signals that are easy to record and reprocess through fiber optic grating temperature fire detectors. Due to the fact that the signal output from the fiber optic grating thermal fire detector is an optical signal, it can be directly transmitted over long distances through optical cables. The excellent environmental resistance of fiber optic gratings also provides reliable guarantees for long-term safety monitoring of tunnels。

(2)data acquisition system ：

The function of the system: The optical signal generated by the fiber optic grating temperature fire detector is remotely transmitted through optical cables, and then directly recognized and converted into physical quantities through the fiber optic grating temperature fire detection signal processor placed in the monitoring room. The recorded data can be displayed, recorded, saved, or directly entered into the monitoring database on the computer terminal。

(3)Monitoring and management system;

The purpose of this system is to effectively store various types of data through appropriate organization based on the characteristics of each monitoring project. Minimize data redundancy while ensuring necessary information storage. This management system can easily obtain data from the data volume system, and the data can be easily accessed by different users。

The fiber optic grating linear temperature fire detection system developed by Nanjing Zhunzhi Sensing uses fiber optic as the temperature sensing element and signal transmission medium. In order to improve the sensitivity and accurate positioning ability of the fiber optic to temperature, a series of temperature sensitive areas - fiber optic gratings - are made on ordinary single-mode optical fibers using the most advanced international fiber local processing technology. These sensitive areas can accurately and sensitively detect subtle changes in the surrounding temperature, while the other parts of the fiber optic are only used for signal transmission and are not sensitive to mechanical stress and environmental interference, thereby ensuring the high sensitivity and low false alarm rate of the entire fiber optic grating temperature detection system. The basic principle is to utilize the temperature sensitive characteristics of the optical spectrum reflected by the fiber optic grating, which is an internal sensitive component of the grating temperature sensing probe. Through the display of various functional modules inside the fiber optic grating temperature alarm instrument, the input light source excitation/output optical spectrum analysis and physical quantity conversion of the fiber optic grating sensor are completed. The temperature information of each monitoring point is provided in digital form, and real-time fire warning and alarm signals are given based on the pre-set alarm temperature setting value and alarm temperature rise rate。

5、Main components and introduction of the system

The fiber optic grating linear temperature fire detector ZZ-FBG10 consists of a fiber optic grating temperature sensing unit, a signal processor, and a connecting part of the fiber optic (cable)。

Fiber Bragg Grating Temperature Sensing Unit

The temperature sensitive unit includes three parts: fiber optic grating, sealing shell, and outgoing optical cable. Its working principle is to selectively reflect the incident broadband ASE light source (total power less than 20mW) transmitted remotely along the optical cable, convert the detected temperature into a narrowband reflected light signal with linearly changing wavelength, and return along the original path of the incident optical cable. The fiber optic grating temperature sensing unit is installed and deployed in hazardous areas such as oil tanks and tunnels that require fire monitoring. It is connected to signal processors placed in safe areas such as monitoring rooms through optical cables to achieve real-time detection of fires. The sensitive components of each channel are composed of multiple (up to 24) temperature sensitive units connected in series. The basic structure of the temperature sensitive unit is shown in the following figure。

The signal processor (fiber Bragg grating demodulator) is the core unit of the ZZ-FBG10 series fiber Bragg grating linear temperature sensing fire detector when connected to the network. On the one hand, it is responsible for processing and analyzing the monitoring data on site, judging whether to send corresponding sound and light alarm signals based on user settings, and transmitting the alarm information to the alarm controller. When needed, it sends relevant information to the remote server to complete remote signal telemetry; On the other hand, it provides the necessary broadband light for on-site monitoring. At the same time, it is also equipped with a serial interface for communication with other intelligent devices in the factory station。