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MAN B&W
Gas Compressor System
The gas supply system <strong>is</strong> based on Flotech
packaged compressors:
• Low�pressure GE Oil & Gas RoFlo type gas
compressors with lubricated vanes and oil
buffered mechanical seals, which compress
the cold boil�off gas from the LNG tanks at the
temperature of –140 °C to –160 °C. The boil�off
gas pressure in the LNG tanks should normally
be kept between 1.06�1.20 bar(a). Under normal
running conditions, cooling <strong>is</strong> not necessary,
but during start up, the temperature of the
boil�off gas may have r<strong>is</strong>en to atmospheric temperature,
hence pre�heating and after�cooling <strong>is</strong>
included, to ensure stabil<strong>is</strong>ation of the cold inlet
and intermediate gas. temperature
• The high�pressure GE Oil & Gas Nuovo Pignone
SHMB type gas compressor; 4 throw,
4�stage horizontally opposed and fully balanced
crosshead type with pressure lubricated and
water�cooled cylinders & packings, compresses
the gas to approximately 250�300 bar, which <strong>is</strong>
the pressure required at the engine inlet at full
load. Only reciprocating p<strong>is</strong>ton compressors
are suitable for th<strong>is</strong> high�pressure duty; however
the unique GE fully balanced frame layout
addres�ses concerns about transmitted vibrations
and also eliminates the need for heavy
installation structure, as <strong>is</strong> required with vertical
or V�form unbalanced compressor designs. The
d<strong>is</strong>charge temperature <strong>is</strong> kept at approx. 45 °C
by the coolers.
• Buffer tank/accumulators are installed to provide
smoothing of minor gas pressure fluctuations
in the fuel supply; ± 2 bar <strong>is</strong> required.
• Gas inlet filter/separator with strainer for protection
against debr<strong>is</strong>.
• D<strong>is</strong>charge separator after the final stage gas
cooler for oil/condensate removal.
• Compressor capacity control system ensures
that the required gas pressure <strong>is</strong> in accordance
with the engine load, and that the boil�off gas
amount <strong>is</strong> regulated for cargo tank pressure
control (as described later).
MAN Diesel
7.09
Page 1 of 7
• The compressor safety system handles normal
start/stop, shutdown and emergency shutdown
commands. The compressor unit includes a
process monitoring and fault indication system.
The compressor control system exchanges signals
with the ME�GI control system.
• The compressor system evaluates the amount
of <strong>available</strong> BOG and reports to the ME�GI control
system.
Redundancy for the gas supply system <strong>is</strong> a very
important <strong>is</strong>sue. Redundancy in an extreme sense
means two of all components, but the costs are
heavy and a lot of space <strong>is</strong> required on board the
ship. We have worked out a recommendation that
reduces the costs and the requirement for space
while ensuring a fully operational ME�GI engine.
The dual fuel engine concept, in its nature, includes
redundancy. If the gas supply system falls
out, the engine will run on heavy fuel oil only.
The gas supply system illustrated in Figs. 7.09.01
and 7.09.02 are based on a 210,000 M3 LNG carrier,
a boil off rate of 0.12 and equipped with 2
dual fuel engines: 2 x 7S65ME�GI. For other sizes
of LNG carriers the setup will be the same but the
% will be changed. Fig. 7.09.03 shows the compressor
system in more detail.
Both systems compr<strong>is</strong>e a double (2 x 100%) set of
Low Pressure compressors each with the capacity
to handle 100% of the natural BOG if one falls
out (alternatively 3 x 50% may be chosen). Each
of these LP compressors can individually feed
both the High Pressure Compressor and the Gas
Combustion Unit. All compressors can run simultaneously,
which can be util<strong>is</strong>ed when the engine
<strong>is</strong> fed with both natural � and forced BOG.
The HP compressor <strong>section</strong> <strong>is</strong> chosen to be a
single unit. If th<strong>is</strong> unit falls out then the ME�GI engine
can run on Heavy Fuel Oil, and one of the LP
compressors can feed the GCU.
Typical availability of these electrically driven Flotech
/ GE Oil & Gas compressors on natural gas
(LNG) service <strong>is</strong> 98%, consequently, an extra HP
compressor <strong>is</strong> a high cost to add for the 2% extra
availability.
MAN B&W ME�GI engines ME�GI engines 198 48 85�8.4