What is the basic philosophy of fuel gas supply system in an LNG dual fuelled vessel?
The fundamental philosophy of a fuel gas supply system in an LNG fual fuelled vessel powered by (excluding LNG carriers) is to manage the pressure of boil off gas from the LNG fuel tanks and supply gas fuel (warmed-up LNG) to the main engine, auxiliary engines, and boilers. These fuel gas supply systems are designed and constructed in compliance with the IGF Code and Class requirements.
The design and equipment philosophy of the gas fuel handling system depends on the choice of the dual-fueled main engine by the shipowner. The main engine may operate on the “Diesel” cycle, requiring high pressure gas injection (around 300 bar), or the “Otto” cycle, requiring low pressure gas admission (around 15 bar). Dual-fueled auxiliary engines and boilers typically require low pressure gas admission (less than 5 bar), and auxiliary boilers may also consume boil off gas at very low pressures in free flow from the LNG fuel tanks.
The fuel gas supply system typically includes the following equipment:
- Glycol water system: A closed-loop system with a steam heater and two supply pumps that circulate glycol water solution as a heating medium in the high and low-pressure vaporizers.
- High-pressure and low-pressure vaporizers: Heat exchangers that warm up the LNG using glycol water solution, as needed.
- Low-pressure LNG fuel supply pumps: Usually submerged in the LNG fuel tanks, these pumps are used to supply LNG at low pressure to the fuel handling equipment or low-pressure gas consumers such as auxiliary engines or boilers.
- High-pressure LNG fuel supply pumps: Used to pressurize the LNG up to 300 bar in case of a main engine operating on the “Diesel” cycle with high-pressure gas injection.
- Boil off gas compressor: Used to consume boil off gas in the main engine and auxiliaries. The boil off gas compressor can operate in high-pressure and low-pressure modes.
- Centralized control system.
- Safety devices and automation in compliance with the IGF Code.
- Nitrogen generators for nitrogen purging (inerting) of the LNG fuel tanks, components and piping in order to reach a safe gas concentration when the fuel gas supply is stopped
The above-mentioned equipment is typically housed in an independent fuel gas preparation room (except from the nitrogen generators) which should be designed and constructed in accordance with the IGF Code and Class requirements."