Amongst other benefits of using electric motors
coupled with variable frequency drives is the ability to
restart the compressors while fully loaded, such that no
depressurisation or loss of refrigerant to the flare is
required.
Both the propane and the mixed refrigerant loops are
designed to manage the maximum expected settle-out
pressure (the pressure at which the suction and
discharge of the compressor equalises out upon a trip),
therefore, when the compressors trip, the pressure
across the system will not exceed the set pressure of the
relief valves, and no refrigerant will be flared.
Main refrigerant
compressors: primary
dry gas seal vent
recovery
Each of the three liquefaction
trains include three main
refrigerant compressors, of
centrifugal type, that provide
the required refrigeration duty
to liquefy the LNG. Dry gas
seals are used to keep the
process gas from migrating into
the atmosphere. These dry gas
seals are highly engineered
mechanical seals that use
sealing gas (process gas as well
as nitrogen) to maintain
sufficient pressure to achieve
the goal. Some of the sealing
gas passes through the primary
seal and is typically flared. Due
to the quantities of VOC that
this stream contains, a vent gas
seal recovery system was added
to the design to prevent these
VOC components from being
flared. This recovery network is
made out of train subheaders
and a common header, which
will collect these gases and
send them to the BOG system
for compression and transfer to
the fuel gas system at the
pretreatment facility.
Dry out and cooldown
procedures
Before starting up the process,
it is mandatory to dry out the
cold sections of the NGL
extraction plant in the PTF, as
well as the LQF. This process is
referred to as defrosting, and
most baseload liquefaction
facilities perform this using
warmed natural gas. The
Freeport LNG liquefaction
project includes the addition of an 8 in. nitrogen
pipeline that brings sufficient quantities of nitrogen to
perform the required defrosting, preventing natural gas
from being flared for the entire dry out process, which is
estimated to last approximately one week per train.
After the dry out process, the main cryogenic heat
exchanger (MCHE) from APCI needs to be cooled down
gradually from ambient temperatures to LNG
temperatures (-260˚F). This process consists of two
steps, the pre-cooldown and the final cooldown. The
pre-cooldown will lower the temperature of the MCHE
to -30˚F, and the final cooldown will bring the system to
normal LNG temperatures (-260˚F). Freeport LNG,
InterOcean Systems, Inc.
Environmental Systems Division
+1 858 565 8400 • San Diego, CA • USA
icksleuth.com
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