Tunnel Fire Suppression Expert: Suspended Ultra-Fine Dry Powder

　　A utility tunnel, or underground city pipeline corridor, is a tunnel-like space constructed beneath urban areas, integrating various engineering pipelines such as power, communications, gas, heating, and drainage systems. It serves as a critical infrastructure and lifeline that ensures the smooth operation of cities.

　　However, the utility tunnels are intricately complex, making it difficult to arrange firefighting equipment—yet they remain a critically important part of the urban infrastructure, causing considerable headaches for many fire equipment purchasers.

　　The utility tunnel contains numerous combustible materials, primarily cables, optical fibers, and pipelines. Notably, cables are composed of protective layers, insulators, and conductors, with polyvinyl chloride and rubber being the most commonly used materials—both of which are highly flammable and prone to burning or igniting easily.

　　Inter-phase short circuits, ground faults, poor contact, and cable overloading—among other factors—are all highly likely to trigger cable fires. When densely packed cables ignite within a tunnel gallery, the intense heat generated by the fire cannot dissipate quickly, causing the temperature inside the gallery to rise rapidly. Moreover, since the electrical cables in the gallery are arranged in tightly stacked layers, a fire on an upper-level cable could easily lead to molten materials like PVC and rubber dripping down at high temperatures, swiftly spreading the flames to the cables, optical fibers, and wires below. Conversely, if a lower-level cable catches fire, the resulting hot gases and thermal radiation would rapidly escalate upward, making it exceptionally challenging to control the blaze and leading to severe damage.

　　Pipe corridor fires have the following characteristics:

　　The fire was fierce and spread rapidly. Due to the structural design of the cable tunnel—running continuously from front to back—and the dense arrangement of cables and optical fibers, once a fire broke out, it would quickly engulf nearby areas, spreading swiftly along the path of the cables.

　　Firefighting is challenging. The burning cables release large amounts of toxic gases, the space inside the cable tunnel is extremely confined, and various communication devices are affected by signal shielding, making it difficult to maintain clear communication—further complicating the firefighters' efforts to extinguish the blaze from within.

　　There is a risk of electric shock. In cable tunnels where wires and cables are densely arranged, high-voltage cables may still retain residual pressure even after being de-energized, posing an electric shock hazard.

　　Meanwhile, the internal structure of the utility tunnel is complex, making the installation of fire protection systems particularly challenging. It cannot accommodate certain large-scale firefighting devices, and the installation process is both time-consuming and labor-intensive. For electrical fires in enclosed environments, automatic fire suppression systems are typically employed. Currently, the most commonly used fire-extinguishing methods include ultra-fine dry powder systems, gas-based extinguishing systems, aerosol-based systems, and water spray systems. Among these, gas-based systems—such as carbon dioxide (CO₂) systems and piped-type heptafluoropropane systems—are widely adopted. However, due to pressure drops caused by long-distance delivery and significant evaporation losses, the effective discharge volume of these systems is reduced. To ensure comprehensive fire protection throughout the entire tunnel, it often becomes necessary to install multiple CO₂ units or dedicated heptafluoropropane storage stations. Unfortunately, this approach not only requires substantial space but also incurs high costs and presents considerable construction challenges, making it less practical for widespread implementation.

　　Considering the challenges posed by tunnel fires, the following feasible and effective firefighting methods are listed for comparison:

　　Automatic Water Spray Fire Suppression System

　　An automatic water spray system is a fire suppression system composed of containers, water, deluge valve assemblies, filters, and water mist nozzles. When using an automatic water spray fire suppression system, the integrated utility tunnel project must include a fire pump room, along with associated fire protection piping, certain electrical equipment, and parts of the automated control system.

　　Advantages: Real-time monitoring and effective reduction of temperatures at fire scenes.

　　Drawback: The provision of the main fire protection pipe within the utility tunnel, along with dedicated fire pipes for each fire zone, increases the overall cross-sectional size of the tunnel.

　　Aerosol Automatic Fire Suppression System

　　The aerosol automatic fire suppression system is a fixed-type extinguishing system that uses strontium nitrate as the primary oxidizer, with potassium nitrate serving as a secondary oxidizer. Strontium nitrate decomposes into strontium oxide, strontium hydroxide, and strontium carbonate upon heating.

　　Advantage: These three substances do not absorb moisture from the air, preventing the formation of conductive and corrosive electrolytes and electrolyte liquid films that could damage equipment—but they come with high costs and maintenance expenses.

　　Drawbacks: The product is a gas-solid two-phase flow, making it impossible to design a combined distribution piping network. Additionally, the system must be connected to the power grid to start, and the inrush current occupies excessive space. This can easily lead to contamination and damage of electrical equipment.

　　Suspension-type Heptafluoropropane System

　　The suspended-type heptafluoropropane system comes in three control options: temperature-controlled, electrically controlled, and electro-thermal dual-controlled. Similar to the fire-extinguishing mechanism of halon-based extinguishing agents, it falls under the category of chemical firefighting. By thermally decomposing the extinguishing agent, it generates fluorine-containing free radicals, which then react in the gas phase with H, OH, and O₂⁻ active free radicals—species that typically trigger chain reactions during combustion—thereby effectively inhibiting the chemical reactions responsible for sustaining the fire and ultimately achieving fire suppression.

　　Advantages: Unlike pipeline-based systems, suspended Heptafluoropropane systems don’t require a dedicated storage station or lengthy piping connections, thus avoiding pressure drops over long distances. At the same time, they sidestep the issue of uneven spray distribution often seen in cabinet-style Heptafluoropropane systems due to insufficient pressure. Moreover, suspended Heptafluoropropane systems inherit the key benefits of this fire suppression agent—low toxicity and zero environmental pollution—while delivering rapid fire extinguishing with exceptionally high efficiency. Compact in design, easy to maintain, and readily available for supply, these systems offer both performance and practicality.

　　Drawbacks: Fluorinated halocarbon fire suppressants, when exposed to high temperatures at the fire scene, release significant amounts of hydrogen fluoride (HF) gas. This gas combines with atmospheric moisture to form hydrofluoric acid—a highly corrosive substance that severely damages skin, leather, paper, glass, and sensitive electronic equipment. Moreover, as the scale of the fire increases and the discharge duration extends, the FM200 fire suppression system generates higher concentrations of HF, intensifying its corrosive effects on precision instruments and equipment, as well as increasing its irritant impact on human skin. Additionally, since FM200 produces a "white mist" during discharge—also known for its corrosive properties—the risk escalates further during the evacuation of the gaseous byproducts, posing greater hazards to both people and property.

　　As for the ultra-fine dry powder fire suppression system:

　　Ultra-fine dry powder particles are finer than conventional dry powders, making them non-toxic, harmless, non-irritating to human skin, and non-corrosive to protected materials. Under normal conditions, they remain stable—neither decomposing, absorbing moisture, nor clumping—and exhibit excellent flowability, dispersibility, and electrical insulation properties. With an average particle size of less than 10 microns, this new type of fire suppressant boasts a fire-extinguishing efficiency 6 to 10 times greater than that of ordinary dry powder. Ultra-fine dry powder fire suppression systems are driven by nitrogen gas and offer multiple activation methods, including temperature-sensitive and electrically controlled options, ensuring double safety for rapid response and effective full-room coverage. This innovative approach delivers outstanding fire-suppression performance, demonstrating significantly faster extinguishing speeds and higher efficiency compared to any other fire-extinguishing agent currently identified worldwide.

　　Advantages:

　　(1) No need to lay out extensive pipelines—low cost;

　　(2) Made of steel, featuring a beautifully designed exterior, it employs a quick-connect structure for easy installation and straightforward maintenance, requiring no upkeep for 5 years.

　　(3) It is a waterless, non-pressure firefighting method that causes no secondary damage to protected objects or the site;

　　(4) Offers diverse control methods, enabling temperature-controlled start-up, on-site manual start-up, and fire alarm-triggered联动 start-up;

　　(5) Vertical jetting—space-saving, easy to control, and highly effective in firefighting;

　　(6) Quick start, extinguish fires instantly.