Marine Installation Manual
2021-08
4-47
4 Ancillary Systems
4.4 Fuel gas system
X72DF
4.4.5
Fuel gas supply pressure
Layout of the
fuel gas supply system
The engine and the FGSS are laid out such that unrestricted engine power output
is ensured for all gas qualities down to a lower heating value of 28 MJ/Nm
3
. This
is typically the lowest value of LNG’s natural boil-off gas composition with high
nitrogen content (approximately 22 %), as it can be found on LNG carriers at the
beginning of laden voyages.
The design limit of gas pressure regulating equipment for both the iGPR (see sec
-
) and the GVU (see section
) is 16 bar(g).
For the specific project, the gas piping class can be defined according to the de
-
sign fuel gas supply pressure specified in the following paragraphs, with the con
-
sideration of pressure fluctuation and a safety margin for pressure losses.
Design fuel gas supply
pressure selection
indicates the required minimum design fuel gas
supply pressure (at the iGPR or the GVU inlet) for R1
—
R3 and R2
—
R4 rated
engines as a function of the fuel gas’ LHV and the actual engine power output.
The design fuel gas supply pressure must be selected according to the 100%
CMCR engine output and the selected LHV. The maximum pressure drop of the
FGSS must be added. In addition, a margin must be added to consider the
FGSS’s ability to compensate for pressure fluctuations caused by variations of
flow rates.
A fuel gas with a lower LHV than the specified LHV can be used. However, the
main engine may have power limitations at certain rating levels. Please refer to
Option 2 of Case 1 for more information.
NOTE
The ship owner and the shipyard have the right and responsibility to de
-
fine the main engine rating (CMCR) and the LHV for 100% CMCR en
-
gine output in the ship building specification.
