LNG Fuel Gas Supply system
- The gas supply system for LNG carriers comprises:
- A gas compressor or compressor system, which compresses the cold boil off gas from the LNG tanks at the temperature of −140°C to −160°C. The boil-off gas pressure in the LNG tanks should normally be kept within 1.03-1.25 bar. Several compressor types can be used to compress the cold gasses. But for the high pressure, only the reciprocating compressors are suitable to generate the required inlet engine pressure at full load, which is 250-300 bar. At 30% load, the pressure is reduced to 130-180 bar. The discharge pressure set points must vary within ±5%, and coolers keep the discharge temperature at approx. 45°C.
- A buffer tank/accumulator must be installed. Their purpose is to provide smoothing of minor gas pressure fluctuations in the fuel supply, ± 2 bar is required.
- An inlet filtration system to remove particle matter and entrained liquids from the gas stream. A two-stage filter solution comprising a pre-filter and a coagulating filter.
- A compressor control system ensuring that the required gas pressure is in accordance with the engine load, and that the amount is acceptable for constant pressure control. The minimum requirement for the regulation is down to 30% of max. flow, or the shipowner’s requirement. The controlsystem should be able to operate on normal start/stop, shutdown and emergency shutdown commands. The compressor unit should include a process monitoring and fault indication system. The control system unit should be able to exchange signals with the ME-GI control system.
- A cargo monitoring system keeping track of the amount of available boil off gas. The unit should be able to report to the ME-GI control system, this could be made as a part of the compressor control system. Redundancy for the gas supply system is a very important issue. Redundancy in an extreme sense means two of all components, but the costs are heavy and a lot of space is required on board the ship. We have worked out a recommendation that reduces the costs and the requirement for space while ensuring a fully operational ME-GI engine. The dual fuel engine concept, obviously, includes redundancy. If the gas supply system falls out, the engine will run on heavy fuel oil only. Several solutions to the redundancy issues are being evaluated, and we recommend the reader to seek additional information from makers. In addition, the external systems comprise safety systems, which should include a hydrocarbon analyser for checking the hydrocarbon content of the air, both
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