4 • LNG journal • The World’s Leading LNG journal LIQUEFACTION Recent improvements in the manufacturing and shipping capability of Main Cryogenic Heat Exchangers (MCHE’s) together with the continued development of compressors and drivers offer the LNG industry the possibility of a new generation of LNG Trains. These synergistic developments now allow efficient single Train process designs ranging from 3 to 8 million tonnes per annum (MTPA) for the well proven propane pre-cooled mixed refrigerant (C3MR) process to over 10 MTPA for the AP-X® process. Additionally, these advancements are expected to have positive impact in realizing projects which are facing increasing capital costs, challenging environmental conditions and the need to reduce CO 2 emissions. To maximize the potential of this next generation of LNG Trains and achieve favorable project economics, it is advantageous to consider new machinery and equipment configurations rather than simply scaling existing trains. New drivers GE frame 9E (MS9001E) gas turbine drivers are becoming accepted technology for direct-drive LNG applications. Frame 9E drivers are employed for the six AP-X® Trains under construction in Qatar. Their use has already been validated in full load string testing, and the first AP-X® Train is scheduled for commissioning later this year. Frame 9E drivers offer about 50 percent more power than frame 7EA drivers with about 122 MW ISO in direct drive services. Recently, Siemens has introduced a still larger turbine for direct drive LNG services, the SGT-5 (SGT 5-2000E), which delivers about 150 MW ISO. Anticipating LNG demand for this frame, Siemens has recently constructed a facility capable of full-load testing of SGT 5 based compression strings. 1 This offers an additional option if it meets the plant owners/operators’ qualifications and requirements. The acceptance by LNG plant owners/operators of these large drivers has the potential to deliver significant improved economics, very much like the wide application of the GE frame 7EA drivers as they became the driver of choice for many recent operating plants. In addition to improved unit power cost and fuel efficiency, these large turbines offer advantages in direct drive service due to the lower design operating speed. The design operating speed of both the Frame 9E and the SGT 5 operate is 3000 rpm, which is significantly slower than the Frame 7 that operates at 3600 RPM. The slower operating speed favors the design of larger compressors that are required for larger Train sizes with minimum number of compressor casings. An important aerodynamic design constraint for centrifugal compressors is the inlet flow coefficient, Φ : where Q is the inlet volumetric flow rate, N is the speed, and D is the impeller diameter. Another important design constraint is the impeller tip speed, S: In designing larger C3MR process Trains, the specification of the propane and low pressure MR compressors are often constrained by a maximum tip speed and inlet flow coefficient considerations. For compressors designed at a maximum tip speed: Inlet flow coefficient is proportional to the speed squared. Therefore, much higher compressor volumetric flows are possible with compressors driven by 3000 RPM drivers while staying within proven compressor design constraints. This allows higher refrigerant volumetric flow rates, especially of the low-pressure mixed refrigerant and propane compressors. Higher propane volumetric flow rates can translate to more pre-cooling capability and higher overall capacity and efficiency. For environmental and governmental regulatory considerations, the increasing pressure to lower CO 2 emissions also favors the more fuel efficient driver and process designs. In cases where the plant capacity is limited by the available gas supply, reduced fuel consumption means more feed gas can be converted into LNG production. For example, GE has developed the LMS-100 which is a highly efficient aeroderivative gas turbine that may be considered for LNG plant applications. This turbine has an ISO power rating of 100 MW and an efficiency of 45 percent (KW Power/KW Fuel), much higher than the heavy frame gas turbines which range from 33 to 35 percent. This turbine is a dual-shaft design and can be configured for continuous operation at 3000 or 3600 RPM. Additionally, because this is a dual shaft machine, flaring due to compressor startups can be minimized since full pressure restart of the compression Trains is easily achievable. Of course, as always, when economically feasible, waste heat recovery from the gas turbine exhausts can be implemented to further improve the plant’s overall efficiency. Heat exchangers Recently, Air Products has upgraded its MCHE manufacturing and shipping capabilities 2 . As the market demand for larger Train capacity has developed, various improvements have been made to Air Products’ manufacturing facility. In conjunction with widening the rail transportation route from the manufacturing facility to the shipping port, exchangers can now be fabricated and shipped as a single unit at diameters up to 5 meters where the previous limit was 4.6 meters. In addition, the maximum shipping weight has increased from 310 to 430 metric tons. The increase in diameter translates to a 20 percent increase in cross sectional Equipment and machinery advances boost the next generation of LNG processes Mark J. Roberts, Adam A. Brostow, Vincent Coakley and Yu-Nan Liu Φ = Q ND 3 S = πND Φ = ∞ QN 2 Air Products 75th MCHE being erected on site at Tangguh – Photo Courtesy BP Ammended Air Products article:LNG 3 30/04/2008 19:57 Page 4
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4 • LNG journal • The World’s Leading LNG journal
LIQUEFACTION
Recent improvements in the
manufacturing and shipping capability of
Main Cryogenic Heat Exchangers
(MCHE’s) together with the continued
development of compressors and drivers
offer the LNG industry the possibility of
a new generation of LNG Trains.
These synergistic developments now
allow efficient single Train process
designs ranging from 3 to 8 million
tonnes per annum (MTPA) for the well
proven propane pre-cooled mixed
refrigerant (C3MR) process to over 10
MTPA for the AP-X® process.
Additionally, these advancements are
expected to have positive impact in
realizing projects which are facing
increasing capital costs, challenging
environmental conditions and the need to
reduce CO2 emissions.
To maximize the potential of this next
generation of LNG Trains and achieve
favorable project economics, it is
advantageous to consider new machinery
and equipment configurations rather
than simply scaling existing trains.
New driversGE frame 9E (MS9001E) gas turbine
drivers are becoming accepted technology
for direct-drive LNG applications.
Frame 9E drivers are employed for the
six AP-X® Trains under construction in
Qatar. Their use has already been
validated in full load string testing, and
the first AP-X® Train is scheduled for
commissioning later this year. Frame 9E
drivers offer about 50 percent more
power than frame 7EA drivers with
about 122 MW ISO in direct drive
services.
Recently, Siemens has introduced a
still larger turbine for direct drive LNG
services, the SGT-5 (SGT 5-2000E), which
delivers about 150 MW ISO.
Anticipating LNG demand for this
frame, Siemens has recently constructed
a facility capable of full-load testing of
SGT 5 based compression strings.1 This
offers an additional option if it meets the
plant owners/operators’ qualifications
and requirements.
The acceptance by LNG plant
owners/operators of these large drivers
has the potential to deliver significant
improved economics, very much like the
wide application of the GE frame 7EA
drivers as they became the driver of
choice for many recent operating plants.
In addition to improved unit power
cost and fuel efficiency, these large
turbines offer advantages in direct drive
service due to the lower design operating
speed. The design operating speed of
both the Frame 9E and the SGT 5
operate is 3000 rpm, which is
significantly slower than the Frame 7
that operates at 3600 RPM.
The slower operating speed favors the
design of larger compressors that are
required for larger Train sizes with
minimum number of compressor casings.
An important aerodynamic design
constraint for centrifugal compressors is
the inlet flow coefficient, Φ :
where Q is the inlet volumetric flow rate,
N is the speed, and D is the impeller
diameter.
Another important design constraint
is the impeller tip speed, S:
In designing larger C3MR process
Trains, the specification of the propane
and low pressure MR compressors are
often constrained by a maximum tip
speed and inlet flow coefficient
considerations. For compressors
designed at a maximum tip speed:
Inlet flow coefficient is proportional to
the speed squared. Therefore, much
higher compressor volumetric flows are
possible with compressors driven by
3000 RPM drivers while staying within
proven compressor design constraints.
This allows higher refrigerant
volumetric flow rates, especially of the
low-pressure mixed refrigerant and
propane compressors. Higher propane
volumetric flow rates can translate to
more pre-cooling capability and higher
overall capacity and efficiency.
For environmental and governmental
regulatory considerations, the increasing
pressure to lower CO2 emissions also
favors the more fuel efficient driver and
process designs.
In cases where the plant capacity is
limited by the available gas supply,
reduced fuel consumption means more
feed gas can be converted into LNG
production. For example, GE has
developed the LMS-100 which is a highly
efficient aeroderivative gas turbine that
may be considered for LNG plant
applications.
This turbine has an ISO power rating
of 100 MW and an efficiency of 45 percent
(KW Power/KW Fuel), much higher than
the heavy frame gas turbines which
range from 33 to 35 percent. This turbine
is a dual-shaft design and can be
configured for continuous operation at
3000 or 3600 RPM.
Additionally, because this is a dual
shaft machine, flaring due to compressor
startups can be minimized since full
pressure restart of the compression
Trains is easily achievable. Of course, as
always, when economically feasible,
waste heat recovery from the gas turbine
exhausts can be implemented to further
improve the plant’s overall efficiency.
Heat exchangersRecently, Air Products has upgraded its
MCHE manufacturing and shipping
capabilities2. As the market demand for
larger Train capacity has developed,
various improvements have been made to
Air Products’ manufacturing facility.
In conjunction with widening the rail
transportation route from the
manufacturing facility to the shipping
port, exchangers can now be fabricated
and shipped as a single unit at diameters
up to 5 meters where the previous limit
was 4.6 meters. In addition, the
maximum shipping weight has increased
from 310 to 430 metric tons.
The increase in diameter translates to
a 20 percent increase in cross sectional
Equipment and machinery advances boostthe next generation of LNG processesMark J. Roberts, Adam A. Brostow, Vincent Coakley and Yu-Nan Liu
Φ = QND3
S = πND
Φ = ∞ QN2
Air Products 75th MCHE being erected on site at Tangguh – Photo Courtesy BP
Ammended Air Products article:LNG 3 30/04/2008 19:57 Page 4
area. As will be shown in the following
examples, C3MR Trains based on this
generation of larger heat exchangers can
have LNG production capacities that are
35 percent or more greater than the
current generation of C3MR Trains based
on two GE frame 7 drivers.
This increase in Train capacity is
possible without the penalties of lowering
the overall plant thermal efficiency. The
lower compressor operating speeds of
3000rpm and new pre-cooling
arrangements allow maximum pre-
cooling of the feed and mixed refrigerant
in the propane system before entering
the MCHE.
This unloads the MCHE duties since
less refrigeration has to be supplied to
reach the desired LNG temperature
exiting the MCHE. In addition, the
operating pressure of the feed and MR
system can be increased to reduce the
frictional pressure drops in the MCHE
while staying within proven mixed
refrigerant compressor designs.
Furthermore, the design of the MCHE,
while much more complex, is similar to
the design of any other process heat
exchanger in that it involves the trade-off
of OPEX vs CAPEX, i.e., pressure drop
for heat transfer performance and cost.
These trade-offs must be carefully
considered since the MCHE has a great
impact on the overall plant performance.
For example, increasing flow causes an
increase in pressure drop resulting in a
decrease in efficiency.
Counteracting this, however, is an
increase in the heat transfer coefficient,
which tends to increase the efficiency.
For a single wound coil exchanger, within
the design regime of interests, the trade-
off of these two effects needs to be
considered to optimize the heat
exchanger design and project economics.
This is particularly the case when the
operating pressures are increased, as
higher pressures tend to minimize the
impact of increased pressure drop on
process cycle efficiency.
Taken together, the developments in
heat exchanger design, larger and slower
drivers and larger compressors allow
efficient design of the C3MR process from
the current state-of-the art 5+ MTPA
Train capacity, to as much as 8 MTPA
with the use of a single MCHE.
For the AP-X® process, similar
developments allow a single Train
capacity increase from the current 8
MTPA to more than 10 MTPA.
Some examplesA number of machinery configurations
for liquefaction Trains which use newer
gas turbines and larger MCHE’s are
described in the following examples. All
the examples use a single MCHE and the
Train capacities are based on simulation
results using a typical feed and assuming
an air cooled plant with 25 °C ambient
temperature.
In simulating the various
configurations, all of the compression
equipment was evaluated with respect to
design parameters such as flow
coefficient, mach number, tip speed,
wheel diameter and volumetric flow.
These parameters were constrained to
ranges that are generally accepted within
the industry and that fit within the
proven capability of multiple vendors.
In the examples, no distinction is made
between GE Frame 9E and Siemens SGT-
5 drivers. Total power available from
each string is based on site the rated
power of the Frame 9E plus a maximum of
30 MW of helper motor power.
If a Siemens SGT-5 were used, the
required helper power drops to about 2
MW at the same production, resulting in
potential savings in external power
generation capacity, or potentially
increased production.
In the following graphics, the
compressor strings are intended to show
only the grouping of drivers with
compressors and motors rather than the
specific order. For example, it is often
desirable to locate the starter/helper motor
between the driver and the compressor
rather than at the end of the string.
Figure 1 shows a configuration used in
a number of operating LNG plants with
the exception that Frame 7EA drivers
are replaced by Frame 9E or SGT-5
drivers. Production is 6 to 7 MTPA for
this configuration.
The SplitMR® machinery
configuration is used. By using
SplitMR® technology in which a portion
of the mixed refrigerant compression
requirement is driven by the same driver
as used for propane compression, the
power balance becomes evenly split.
This allows for full utilization of gas
turbine power and increases Train
capacity for the same number of drivers
and compressors. At the time of the
writing, there are four Trains in
operation with this technology, RasGas
Trains 3, 4, & 5 in Qatar and the Segas
LNG plant in Damietta, Egypt.
The main bottleneck for increased
production with this configuration is the
propane compressor, not the available
power. While the low speed (3000 RPM)
of the frame 9E or SGT-5 drivers helps to
increase pre-cooling capacity, it is
desirable to further debottleneck the
propane system allowing more pre-
cooling and increased Train capacity.
One option which can be used to de-
bottleneck the pre-cooling is to use a
1,4-2,3 split propane compressor casing
arrangement3 as shown in Figure 2.
Stages 1 and 4 are in the first casing and
stages 2 and 3 in the second casing.
The inlet pressures to the four stages
may be different than the single casing
compressor design and are adjusted to
maximize efficiency. The discharges from
the third and fourth stages are at the
same pressure since they are connected
to a common condenser.
Each stage would typically have
multiple impellers. This series
arrangement minimizes the complexity of
the suction piping and avoids the potential
for imbalances in compressor duties that
can occur with parallel compression.
Figure 3 shows two Frame 9E (or SGT-
5) drivers (like on Figure 1), but with
split propane. The split propane
configuration allows higher production
compared to the configuration shown
on Figure 1. Production for this
configuration is about 7.5 MTPA.
The upper machinery string in Figure
3 has three compressor casings in
addition to the starter/helper motor. The
maximum compressor casings used in
any operating LNG compression strings
is two.
While care must be taken in the
design and installation of these
machines, particularly with the rotor
dynamics analysis, the design and
LNG journal • February 2008 • 5
LIQUEFACTION
Figure 1: Two Frame 9/SGT-5 Drivers with Conventional Propane 6 to 7 MTPA Figure 2: (a) Single Casing Propane Compressor (b) 1,4-2,3 Split Casing Propane Compressor
Ammended Air Products article:LNG 3 30/04/2008 19:57 Page 5
6 • LNG journal • The World’s Leading LNG journal
LIQUEFACTION
analytical work is well within the
capability of all of the vendors typically
used for these applications.
While three compressors in three
separate casings on a single string is not
referenced in the LNG industry, there are
many examples in other applications with
even more connected to a single driver.
Figure 4 shows two 50 percent parallel
compression Trains driven by Frame 9E
(or SGT-5) gas turbines. The propane
machines are in single casings, the mixed
refrigerant machines in two additional
casings. This configuration offers a very
high production, about 8.0 MTPA from a
Train having a single MCHE.
While this configuration has more
compressor casings than either of the two
preceding examples, one advantage of
this configuration is that the machines
are all easily well within proven limits.
Due to the parallel configuration, the
machinery is equivalent capacity to a 4
MTPA C3MR Train, which is well below
the current state of the art for operating
LNG plants. Additionally, the parallel
operation gives this configuration an
inherently high on-stream factor and
availability. Loss of either compression
string results in a loss of less than 50
percent production.
Propane compressors driven by single
shaft compressors arranged in parallel is
viewed by many in the industry as a
design that has a higher risk associated
with it than other competing designs.
Since the machines operate with a
suction pressure close to atmosphere and
the prevailing industry practice is to avoid
sub-atmospheric operation, the system is
sensitive to differences that arise from
compressor manufacturing tolerances.
Additionally, piping design issues can
result in suction piping pressure drop
differences between the two parallel
machines, assuming both machines are
drawing from the same propane kettle.
Both of these factors could result in a
situation where one of the propane
compressors' performance is
compromised.
One solution would be to install a
dedicated set of propane kettles for each
machine. This is expensive and
unnecessary in light of other options.
Those options include speed adjustments,
tighter tolerances on compressor to
compressor performance, greater
attention to piping design and specifying
compressor performance characteristics
that are less sensitive to process and
physical design differences, for example
curves with a higher rise to surge.
Figure 5 shows a very simple
configuration where all the refrigeration
compression is on a single string driven
by Frame 9E (or SGT-5) gas turbines.
The configuration can offer a
production of 4.5 MTPA with a frame 9E
driver, 30 MW of helper motor power, and
a single MCHE. Significantly more
production can be achieved by using more
power, since the MCHE and other
equipment are nowhere near limits. For
example, a production of about 5 MTPA
is achievable by replacing the frame 9
with an SGT-5 driver using the same
helper power.
This configuration offers a high on-
stream factor and availability.
Production is lower than the previous
examples, however 3-5 MTPA is not a
small Train by historical standards and
not all gas reserves are large enough to
justify the investment in a larger
capacity Train.
The configuration of Figure 5 can be
employed with other smaller drivers for
situations where a smaller production
capacity is desired. For example a low cost
2.5-3.5 MTPA train could be configured
with a single frame 7EA or Siemens SGT6-
2000E turbine.
Both the configuration of Figure 4 and
of 5 have operational flexibility
advantages. All the power available from
the driver and motor can be utilized by all
the compressor casings.
This can be important as the ambient
temperature changes and the fraction of
the total power required for pre-cooling
changes. With an arrangement having
all compressors on a single string, power
inherently distributes between pre-
cooling and liquefaction compressors as
needed.
Configurations for fuelefficiency and low CO2emissionsFigure 6 shows the same configuration as
Figure 1 except that LMS 100 drivers are
used. Frame 9E drivers were replaced
with GE’s LMS-100 intercooled aero
derivative hybrid gas turbines operating
at 3,000 rpm (they are also available at
3,600 rpm). 20 MW helper motors are
assumed resulting in a production of
about 6.0 MTPA.
LMS-100s are very efficient. They can
be restarted at full load. There is no
refrigerant loss on trip so refrigerant
flaring is reduced. The LMS-100 has a
multiple shaft design; so the operating
speed range is wider than for a single
shaft turbine. This allows better
production over the temperature range.
Somewhat paradoxically, the fact that
LMS-100 driver is more fuel efficient
causes the overall thermodynamic
efficiency for liquefaction to be reduced
due to reduced end flash.
Since LMS-100 gas turbines are more
efficient, less fuel can be taken from the
flash of the LNG which exits the MCHE
into the low pressure flash drum.
Therefore, LNG product has to be
subcooled to a lower temperature which in
turn decreases the liquefaction efficiency.
Lower fuel requirement however,
means lower CO2 emissions and an
increase in LNG product of 2-3 percent
for the same feed flow. This extra
production can be a key economic driver
for projects where feed gas is limited.
LMS-100s have intercoolers and
require cooling water supply. Fuel also
has to be compressed to a higher pressure
(about 56 bar) than with other drivers, so
fuel compressor power is increased if fuel
is taken from the end flash.
Figure 7 shows a Siemens SGT-5 gasFigure 3: Two Frame 9/SGT-5 Drivers with Split Propane and SplitMR™ ~ 7.5 MTPA
Figure 4: Two Frame 9/SGT-5 Drivers Parallel Compression ~8.0 MTA
Figure 5: Single Gas Turbine Train 3-5 MTPA
Ammended Air Products article:LNG 3 30/04/2008 19:57 Page 6
LNG journal • February 2008 • 7
LIQUEFACTION
turbine driving all the mixed refrigerant
compressors, with the medium-pressure
and high-pressure stages in a single
casing. The turbine is equipped with the
heat recovery steam generator. Generated
steam is used to drive a steam turbine
that in turn, drives the propane machine.
All stages of propane are in one casing.
This configuration also uses less fuel
and, therefore, produces lower CO2emissions. As with configuration shown
on Figure 6, thermodynamic efficiency is
reduced due to reduced end flash.
Production is 6 to 6.5 MTPA.
Since the propane compressor is
driven by the steam turbine it can
operate at variable speed allowing
increased production as the ambient
temperature varies over a range. Also
due to the variable speed, the propane
compressor can be restarted at full load
without de-pressuring, reducing flaring.
At the cost of an additional casing,
production from this configuration can be
increased by splitting the propane
machine and moving the first and the
second stages from the steam turbine
driver to the gas turbine driver in a
separate casing.
The evolution of baseload LNG
technology has always been driven by
market demand and advances in
liquefaction cycles and equipment.
This will continue to be the case for
the next generation of plants. The
availability and growing acceptance of
new compressor drivers in addition to
larger MCHEs, combined with new
machinery configurations, will allow the
further advances needed to meet the
market demand for lower unit cost and
improved efficiency. �
Adam A. Brostow holds a Masters Degreein Chemical Engineering from ClemsonUniversity in South Carolina. He joinedAir Products 12 years ago after previousemployment designing air separationplants. While at Air Products he hascontributed in the areas of ProcessSynthesis and LNG Technology. He holds10 US and equivalent internationalpatents, mostly in air separation, raregas recovery, power generation and LNG.
Vincent Coakley joined Air Products in1998. He has contributed as a machineryengineer in various capacities in AirSeparation for nearly 30 years includingapplications in the electronics industry,
enhanced oil recovery, GTL and IGCC. Herecently joined the Air Products LNG teamwhere he provides machinery support forplant process design and process cycledevelopment.
Dr. Yu-Nan Liu provides technicaldirection of the process engineeringactivities for baseload LNG plants. Thiswork includes consultation with plantowners and their selected contractors inthe formulation of the Basis of Design forthe liquefaction unit, the subsequentdevelopment of Air Products’ processdesign packages, and the support of thedetail engineering design of the plant. Hehas assisted in the start-ups,performance tests, trouble-shootings,and debottlenecking studies of manyexisting LNG plants. He joined AirProducts in 1976 and has participated inall of the major LNG projects that use AirProducts’ technologies and equipment.
Ammended Air Products article:LNG 3 30/04/2008 19:57 Page 7