Hydrocarbon Processing - October 2021 - GP-16

SPECIAL FOCUS: LNG TECHNOLOGY
Liquefaction train
Pretreatment train
Refrigeration compressor string
Refrigeration compressor string
Number of
refrigeration
compressor strings
per train
conditions. The gas turbine must have
completed 25,000 hr for aeroderivative
gas turbines/32,000 hr for heavy-duty gas
turbines of continuous satisfactory operation
in a continuous process plant without
major problems or modifications.
Gas turbines with references coming
Pretreatment train
Number of turbines in series per string
FIG. 1. LNG plant screening of architecture.
FIG. 2. Gas turbine typical de-rating curve-
power vs. ambient temperature.b
Several types of drivers can be considered
to drive LNG compressors (gas turbines,
steam turbines, motors), but once
the choice is made it will usually apply to
all the main refrigerant compressors for
reasons of interchangeability, flexibility
and maintainability. The selection should
be reached in parallel to defining the
compressor machinery arrangement to
obtain the required balance of power and
to select the standardized driver model
for all shaftlines. Steam turbines and electric
motors could be custom made to the
required shaftline power; however, gas
turbines come in discrete sizes and, therefore,
the selection of optimum gas turbine
depends on matching the available gas
turbine models and versions.
At the early stages of concept definition,
as many as 15 architectures or more
can be screened and quantitatively ranked
to support the owner's selection when including
the liquefaction process, machinery
arrangement and other key selections.
The criteria defined with the owner for the
ranking and the selection are, at this stage,
crucial to best match the owner's objective
and to ensure the success of the project.
Gas turbine as a prime mover. Both
heavy-duty gas turbines and aeroderivative
gas turbines have been applied in
LNG plants to drive main refrigerant
compressors. The heavy-duty gas turbines
have been adapted to mechanical
drive from the versions built especially
for the power generation industry and
proven through decades of use. The
power generation industry requires large
power at a fixed speed, typically 3,000
rpm for a 50-Hz electric grid network and
3,600 rpm for a 60-Hz network. As a result
of these constraints, large, heavy-duty,
gas turbines have evolved (more than
500 MW in the largest sizes) with a single
drive shaft. Startup torque for an offline
generator is very low; however, the adaptation
to mechanical drive in LNG plants
implies the addition of large starter motors
to overcome the compressor start
from standstill torque.
Aeroderivative gas turbines were initially
developed for aircraft engines, but
in most cases have been first used in the
power generation industry before being
adopted for mechanical drive. Adaptation
includes the addition of a free power turbine
and the optimization of inspection
and maintenance cycles that are obviously
much longer for LNG trains than for
commercial aircraft. The LM9000, which
is now in the manufacturing stage for Novatek's
Arctic 2 project, is an aeroderivative
gas turbine derived from the world's
most powerful civil aircraft engine and
specifically developed for mechanical
drive service in LNG.
Irrespective of the model, a referenced
gas turbine must have field-proven design,
meaning that it must have a proven and
successful track record of performance in
equivalent service, operating and process
16 SEPTEMBER/OCTOBER 2021 | GasProcessingNews.com
only from the power generation industry
cannot be considered as suitably referenced
for mechanical drive application, as
described. To qualify as a mechanical driver,
they must be verified for the following:
* The rotor dynamics of the shaftline
in the mechanical drive application
must account for the gas density
change that will occur during
the operation of the coupled
compressor, impacting the axial
stability of the shaftline.
* The much higher startup torque
for the mechanical drive should, if
possible, allow startup from settleout
pressure to avoid flaring.
* In high-power gas turbines, the
coupling and/or gearbox might not
be referenced considering that the
service factor will be higher in the
mechanical drive than in power
generation applications.
Once the required gas turbine site
output power is estimated, then the ISO
power can be determined by considering
the gas turbine de-rating at the most demanding
site condition and the worst operating
condition. Gas turbine de-rating
means that the gas turbine will deliver
to the site less power out than the ISO
power that is declared on the nameplate.
Several factors adversely affect the
output power of gas turbines, including
ambient temperature, pressure drop at
the inlet and outlet, and factors relevant
to fouling and aging. Roughly speaking,
with an increase of 1°C in ambient temperature,
the output power decreases by
0.7%-0.8%. Aeroderivative gas turbines
are more sensitive to variations in ambient
conditions compared to heavy-duty
gas turbines.
On the other hand, fouling is a condition
caused by the ingress of contaminants
from the ambient air sticking on the
axial air compressor blades and reducing
their aerodynamic performance. Aging is
related to the deterioration of gas turbine
components due to extended operation,
especially for the hot section.
De-rating extents are unique to each
gas turbine model and version, and the
Number of liquefaction trains
Number of pretreatment trains
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Hydrocarbon Processing - October 2021

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