6.3.2.4 Master fuel gas valve
For LNG carriers, IMO IGC code requires a master gas fuel valve to be installed in the fuel gas
feed system. At least one master gas fuel valve is required, but it is recommended to apply
one valve for each engine compartment using fuel gas to enable independent operation.
It is always recommended to have one main shut-off valve directly outside the engine room
and valve room in any kind of installation.
6.3.2.5 Fuel gas venting
In certain situations during normal operation of a DF-engine, as well as due to possible faults,
there is a need to safely ventilate the fuel gas piping. During a stop sequence of a DF-engine
gas operation the GVU and DF-engine gas venting valves performs a ventilation sequence to
relieve pressure from gas piping. Additionally in emergency stop V02 will relief pressure from
gas piping upstream from the GVU.
This small amount of gas can be ventilated outside into the atmosphere, to a place where
there are no sources of ignition.
Alternatively to ventilating outside into the atmosphere, other means of disposal (e.g. a suitable
furnace) can also be considered. However, this kind of arrangement has to be accepted by
classification society on a case by case basis.
NOTE
All breathing and ventilation pipes that may contain fuel gas must always be built
sloping upwards, so that there is no possibility of fuel gas accumulating inside the
piping.
In case the DF-engine is stopped in gas operating mode, the ventilation valves will open
automatically and quickly reduce the gas pipe pressure to atmospheric pressure.
The pressure drop in the venting lines are to be kept at a minimum.
To prevent gas ventilation to another engine during maintenance vent lines from gas supply
or GVU of different engines cannot be interconnected. However, vent lines from the same
engine can be interconnected to a common header, which shall be lead to the atmosphere.
Connecting the engine or GVU venting lines to the LNGPac venting mast is not allowed, due
to risk for backflow of gas into the engine room when LNGPac gas is vented!
6.3.2.6 Purging by inert gas
Before beginning maintenance work, the fuel gas piping system has to be de-pressurized and
inerted with an inert gas. If maintenance work is done after the GVU and the enclosure of the
GVU hasn’t been opened, it is enough to inert the fuel gas pipe between the GVU and engine
by triggering the starting sequence from the GVU control cabinet.
If maintenance work is done on the GVU and the enclosure of the GVU need to be opened,
the fuel gas pipes before and after the GVU need to be inerted. Downstream from the GVU
including the engine built gas piping, inerting is performed by triggering the inerting sequence
from the GVU control cabinet. Regarding the engine crankcase inerting, a separate inert gas
connection exist located on the engine. Upstream from the GVU
double-block-and-bleed-valves, the inerting is performed from the gas storage system by
feeding inert gas downstream the fuel gas pipe and out from the GVU gas ventilation pipe.
In addition to maintenance, during certain alarm and emergency situations (e.g. annular space
ventilation failure and/or gas leak detection), the fuel gas piping is to be flushed with inert gas.
The following guidelines apply for flushing the engine crankcase with inert gas:
1 Max filling flow: 200l/min/cylinder
2 A sniffer is recommended to be installed in the crankcase breather pipe in
order to indicate when the crankcase have been flushed from toxic gases.
DBAD209883 6-21
6. Fuel SystemWärtsilä 46DF Product Guide
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