0　Introduction LNG, as a clean and efficient energy source, is increasingly favored by people. More and more shipowners choose LNG as the power fuel for ships, especially natural gas transport ships. It is economical to use their own natural gas as fuel. An environmentally friendly choice. At the same time, the prevention and detection of natural gas leakage is an important prerequisite for ensuring the safety of ships. Currently, the commonly used measures are to use double-walled pipelines to generate negative pressure for mechanical ventilation on the outer wall, detect the leakage in time, and discharge the leaked gas to a safe area. The correct selection of the fan can make the pipeline negative pressure value and the number of air changes meet the safety requirements.

1　Ventilation system functions and requirements The natural gas supply pipeline uses a double-walled pipe. The inner pipe is used as a natural gas circulation pipeline. At the same time, the external wall mechanical exhaust generates negative pressure. The entire system consists of the following main parts: Natural gas supply pipeline (outside the engine room) ; Air supply double-wall pipe (in the engine room); ventilation inlet (with adjustable damper); generator and valve block; pressure sensor; pipeline to the fan; fan; ventilation outlet. image.png

Note: 1- Natural gas supply pipeline (outside the engine room); 2- Gas supply double-wall pipe (in the engine room); 3- Ventilation inlet (with adjustable damper); 4- Generator and valve group; 5- Pressure sensor; 6-to the fan pipeline; 7-fan; 8-ventilation outlet.

Figure 1 　 Ventilation system diagram

The specification requires that the double-walled pipe containing the fuel piping system should be equipped with an effective suction mechanical ventilation system, and its ventilation capacity should be at least 30 air changes per hour. The total volume of the system calculated in this paper is 1.65m3, that is, the selected fan displacement should not be less than 49.5m3/h. To ensure the safety of the system, a pressure sensor (serial number 5) is set. Only when the negative pressure reaches a predetermined value, the natural gas supply valve can be opened, otherwise it will lock and trigger an alarm. The system calculated in this paper uses the W?rtsil? main engine W6L20DF, and the recommended negative pressure is 20mbar (ie 2000Pa).