The safety system of marine fuel supply system mainly includes three parts, namely: inert gas system, combustible gas detection device, and ventilation system.
(1) Inert gas system
An inert gas system is a system that delivers inert gas to compartments, containers, or pipelines containing hazardous materials, reducing the oxygen content in the surrounding atmosphere and creating a non combustible environmental condition. LNG ships mainly use a nitrogen system, which functions to inert LNG storage tanks and piping systems, as well as blow down pipelines after filling is completed. The compartment for storing nitrogen generators and nitrogen storage tanks should be equipped with an independent mechanical suction ventilation system, with a minimum of 6 air changes per hour, and a low oxygen alarm device should be installed. The nitrogen pipeline should only pass through well ventilated areas, and the nitrogen pipeline in enclosed areas should be fully welded, with only the minimum flange joints necessary for valve installation, and should be as short as possible.
(2) Combustible gas detection device
For locations containing liquid or gaseous natural gas pipelines, combustible gas detection devices should be installed, and the number of gas detection devices in each location should consider the size, layout, and ventilation conditions of the location. When the concentration of combustible gas reaches 20% of the lower explosive limit, an audible and visual alarm should be triggered. When two detectors detect that the concentration of combustible gas reaches 40% of the lower explosive limit, the safety system should be triggered. For the ventilation ducts surrounding the gas pipeline in the cabin, the alarm limit can be set at 30%. When two detectors detect a combustible gas concentration of 60%, the safety system should be triggered. At this time, the gas main valve should be able to automatically close, and the gas main valve can not only be operated locally, but also remotely closed in areas such as the cab, monitoring room, and refueling station console on the main deck. The combustible gas detection device on board should include both fixed gas detectors and portable gas detectors.
(3) Breathable system
The fuel tank compartment for storing gas and the double walled outer pipe of the gas pipeline passing through the enclosed space on the ship should be equipped with an effective negative pressure mechanical ventilation system, with a ventilation capacity of at least 30 air changes per hour, and the ventilation system should not have dead corners. The number and power of fans should ensure that when a fan powered by an independent line or a group of fans powered by a shared line stops running, the reduction in ventilation capacity will not exceed 50% of the total ventilation capacity.
