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2026
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01
3/5mm glass beads + 68℃ threshold: The protective code of dry sprinkler systems
In the "family" of fire sprinkler systems, dry sprinkler systems serve as the "specialized guardians" for extreme environments such as low and high temperatures. The combination of 3mm/5mm glass bulbs and a 68℃ operating threshold is the core key to the precise response of this system. Though tiny in size, they directly determine whether the protection can be effectively activated in the early stage of a fire, with rigorous engineering logic and standard specifications behind them.
Core Component: The "Thickness Secret" of 3mm/5mm Glass Balls
The glass bulb of a closed sprinkler head is the "first line of defense" for sensing fire temperature. Its thickness is directly related to sensitivity and stability. The 3mm and 5mm specifications are adapted to different scenario requirements, rather than being randomly selected.
According to the national standard GB 18428—2010 Glass Balls for Automatic Fire Extinguishing Systems, glass balls must meet strict requirements for wall thickness uniformity. Excessive wall thickness deviation will lead to uneven thermal expansion, generate internal stress, and significantly increase the risk of accidental explosion or failure. Due to its thin wall, the 3mm glass ball is more sensitive to temperature changes, capable of quickly detecting fire and breaking to activate the system. It is suitable for places where fires spread rapidly and require immediate response, such as precision instrument workshops and auxiliary areas of data centers. The 5mm glass ball, with its superior wall thickness, has stronger fatigue resistance and pressure resistance, and can resist stress impacts caused by environmental temperature fluctuations. It is mostly used in harsh working conditions such as outdoor areas and around high-temperature equipment, such as auxiliary transformer areas of power plants and under open corridors.
It should be noted that regardless of the thickness, the diameter tolerance of the glass ball is controlled within ±0.1mm to ensure precise coordination with sprinkler components and avoid affecting operational reliability due to installation gaps.
Temperature Threshold: The "Scientific Setting" and Selection Boundary of 68℃
As the most common nominal operating temperature of sprinkler heads, 68℃ is marked in red with a static operating temperature range of 65~74℃. It is not arbitrarily set but a comprehensive consideration based on the temperature characteristics of early fires and environmental adaptability.
The national standard GB 50084—2017 Code for Design of Automatic Sprinkler Fire Extinguishing Systems clearly recommends that the nominal operating temperature of closed sprinkler heads should be 30℃ higher than the maximum temperature of the service environment. This means that the ideal service environment for 68℃ sprinkler heads should not exceed 38℃ in maximum temperature. A main transformer area of a coastal power plant in southern China once experienced frequent false activations of sprinkler heads due to non-compliance with this principle. The maximum ambient temperature in this area reached 58.5℃ due to main transformer heat dissipation and sunlight exposure, which was close to the 68℃ threshold. In addition, the sudden temperature changes before and after rainfall caused the glass balls to repeatedly bear the expansion stress of the temperature-sensitive liquid, accelerating aging and leading to accidental explosion and leakage faults. The faults were completely resolved after replacing with 79℃ sprinkler heads.
For dry sprinkler systems, the adaptability of the 68℃ threshold is even more critical. Due to the inherent delay in air exhaust and water filling of dry systems, if the operating temperature of the sprinkler head is too low, it is prone to false activation due to high ambient temperature, causing unnecessary losses; if it is too high, it may miss the optimal fire extinguishing opportunity in the early stage of the fire. Therefore, 68℃ is only suitable for special environments with normal temperature fluctuations and no continuous high-temperature sources.
System Coordination: The "Cooperation Logic" Between Dry Sprinklers and Core Components
The core advantage of dry sprinkler systems is solving the problem of pipeline freezing in low-temperature environments. In the quasi-operating state, the pipelines are filled with pressurized gas instead of water. This cooperation with 3mm/5mm glass balls and the 68℃ threshold is particularly important. The entire operational process is divided into four stages:
Sensing and Activation: After a fire breaks out, when the ambient temperature rises to around 68℃, the temperature-sensitive liquid inside the glass ball expands and breaks the ball. The sprinkler head becomes a gas outlet, and the air pressure in the pipeline begins to drop;
Valve Group Operation: When the air pressure drops to the critical point, the mechanical balance of the dry alarm valve is broken, the valve clack opens, and the accelerator amplifies the pressure difference to shorten the valve activation time;
Air Exhaust and Water Filling: Water flows into the pipeline to drive out the gas, and the end quick exhaust valve actively discharges the gas to eliminate the "air lock" obstruction. This stage is an inherent delay link of dry systems, usually taking 1-3 minutes;
Water Spraying for Fire Extinguishing: After the pipeline is filled with water, water is sprayed through the broken sprinkler head, and the fire pump and alarm device are activated simultaneously.
In terms of component adaptation, dry systems preferentially use upright sprinkler heads with 3mm/5mm glass balls to avoid freezing and cracking caused by condensed water accumulation in pendant sprinkler heads. The selection of glass ball brands must also be rigorous. A power plant inspection found that some brand sprinkler heads have temperature drift problems: new models with downward drift are prone to false explosion, while old models with upward drift may fail. It is necessary to prioritize products with stable performance.
Selection and Maintenance: Avoid These Key Misunderstandings
The application of dry systems with 3mm/5mm glass balls and 68℃ sprinkler heads must adhere to the principle of "scenario adaptation" and avoid three major misunderstandings:
Misunderstanding 1: Blindly pursuing sensitivity. Using 3mm thin glass balls in high-temperature and fluctuating environments can easily accelerate aging due to stress fatigue; 5mm thick glass balls should be preferred;
Misunderstanding 2: Universal use of 68℃ sprinkler heads. For areas where the maximum ambient temperature exceeds 38℃, such as main transformers and drying workshops, sprinkler heads with an operating temperature of 79℃ or higher should be adopted;
Misunderstanding 3: Ignoring maintenance cycles. For glass balls in outdoor or high-temperature environments, regular inspections for leakage and cracks are required, and aging sprinkler heads should be replaced in a timely manner. At the same time, check the air pressure and exhaust device performance of the dry system to avoid excessive delay time.
In summary, the combination of 3mm/5mm glass balls and the 68℃ threshold is the core for the reliable operation of dry sprinkler systems in special environments. Only by accurately matching scenarios, following standard specifications, and conducting regular maintenance can this "tiny guardian" trigger precisely and provide efficient protection when a fire occurs.
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