Marine Installation Manual
2021-08
4-42
4 Ancillary Systems
4.4 Fuel gas system
X72DF
Supplying fuel gas
The goal of the FGSS is to provide the fuel gas to the engine and gensets as re
-
quired, and this must be achieved during engine operation and bunkering. The
FGSS must be designed to handle variations in temperature and pressure. It is
important to consider the processing of excessive Boil-Off Gas (BOG)
1)
, in
-
cluding the suitability of an on-board re-liquefaction plant (see the subsection
).
Fuel gas is supplied to the main engine and gensets by the following two
methods:
Forced boil-off gas
supply
LNG is pumped from the tank by cryogenic submerged pumps to a vaporiser,
where the liquid is converted to gas at the main engine’s required pressure. As
the LNG is forced to evaporate by an external heat source, the resulting gas is re
-
ferred to as Forced Boil-Off Gas (FBOG). For WinGD’s low-pressure X-DF en
-
gines this is at a maximum pressure of 16 bar(g). The fuel gas produced from the
vaporiser can also feed the gensets by passing through a pressure reduction valve
to match the required pressure.
Natural boil-off gas
supply
The heat which passes through the tank insulation causes the LNG to “boil”,
meaning that it evaporates and collects above the LNG. This gas is therefore a
type of BOG and more specifically it is called Natural Boil-Off Gas (NBOG), as
the source of the heat of evaporation is from the natural environment and not
from any heating. In the sections that follow, the term NBOG is used to distin
-
guish from FBOG. The relative amount of liquid in relation to the total tank
volume, which evaporates from the LNG tank per day determines the Boil-Off
Rate (BOR). The BOR is provided as a percentage value.
Over time, the NBOG will accumulate and raise the pressure in the system. To
ensure that the pressure is not exceeding the safe level, the NBOG must be re
-
moved. Usually, the NBOG is used to power the main engine and the gensets.
The NBOG can also be directed to a gas-fired boiler. The NBOG that is captured
from the tank must be conditioned to meet the requirements of the main en
-
gine(s), the gensets, and the gas-fired boiler(s). This is accomplished by use of a
combination of heat exchangers and NBOG compressor(s). If the gas cannot be
consumed by the main engine(s) or the gas-fired boiler(s), then it can be released
to the GCU.
Depending on the expected amount of NBOG and the desired level of system
flexibility, the NBOG can exclusively fuel the gensets with forced boil-off of
LNG as a supplement (see
), or alternatively, the NBOG can
fuel the main engine(s) directly as well as the gensets, while having the possibility
to supplement with forced boil-off from LNG (see
). For fur
-
ther flexibility, the NBOG can be returned to the tank after a re-liquefaction pro
-
cess, or alternatively, the BOR can be controlled by cooling the LNG.
1) In general, the term BOG is used to describe the boil-off gas which is produced from
evaporation in the tank by natural heat input. As the source of the heat of evaporation
is from the natural environment and not from any other heating, BOG is also called
Natural Boil-Off Gas (NBOG). In this document, the term NBOG is used to differentiate
it from Forced Boil-Off Gas (FBOG).
