Marine Installation Manual
2021-08
4-49
4 Ancillary Systems
4.4 Fuel gas system
X72DF
•
Option 2 (recommended by WinGD):
º
The ship owner and the shipyard consider the LHV of 36MJ/Nm
3
as
design criterion and accept a main engine power limitation of up to 92%
CMCR power in case the LHV drops to 32 MJ/Nm
3
.
º
Based on the R4 rating and the LHV of 36MJ/Nm
3
, the fuel gas supply
design pressure (at the iGPR or the GVU inlet) is selected at 10.0bar(g)
following the GTD data.
º
Considering the 0.5 bar pressure drop, the fuel gas supply design pres
-
sure from the FGSS is defined at 10.5 bar(g).
Case 2
—
Example of fuel
gas supply pressure selec
-
tion for an LNG carrier
Assumptions:
•
An engine with R4 rating is selected.
•
A combination of a low LHV (28MJ/Nm
3
) and full-load operation of the
engine is unlikely. Typically, compressed NBOG is utilised as the main fuel
gas, supplemented by FBOG, if necessary. Consequently, an FGSS layout
with an LHV of 28MJ/Nm
3
would only lead to a situation where compres
-
sors are running far below their design point during normal vessel opera
-
tion. This would result in compressor operation of a lower efficiency.
•
WinGD recommends selecting an LHV of 32 MJ/Nm
3
for normal condi
-
tion. Even if designed for this LHV, the engine can still operate with high
output if the fuel gas is supplied with an LHV of 28MJ/Nm
3
(e.g. more
than 90% CMCR power, if designed for an LHV of 32 MJ/Nm
3
).
•
A pressure drop of 0.5bar across the FGSS is considered. The real pressure
drop needs to be calculated by the shipyard or the FGSS supplier (see sec
-
).
Results:
•
A fuel gas pressure of 10.5bar(g) (at the iGPR or the GVU inlet) is sufficient
for CMCR operation.
•
Considering the assumed maximum pressure drop of 0.5 bar, the design fuel
gas supply pressure is 11.0 bar(g).
Fuel sharing operation
If the main engine is operating in fuel sharing mode, the required minimum fuel
gas supply pressure is according to the engine power, as indicated in
. However, the fuel gas flow rate will vary depending on the mixture ratio
of fuel gas and fuel oil.
NOTE
• It is recommended to consider the different fuel gas temperatures for
the different fuel gas compositions (e.g. cold fuel gas with a low LHV
at the beginning of a laden voyage and warm fuel gas with a high LHV
during a ballast voyage).
• It is recommended to consider the varying nitrogen content in relation
to the LHV. For example, approximately 22% nitrogen for an LHV of
28 MJ/Nm
3
and 11% for an LHV of 32MJ/Nm
3
in the NBOG, which
mainly contains methane.
• If the gas combustion unit is supplied by the compressor, then the
compressor flow capacity is defined by the natural boil-off rate.
