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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 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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Equipment and machinery advances boost the next … frame 9E (MS9001E) gas turbine drivers are becoming accepted technology for direct-drive LNG applications. Frame 9E drivers are

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Page 1: Equipment and machinery advances boost the next … frame 9E (MS9001E) gas turbine drivers are becoming accepted technology for direct-drive LNG applications. Frame 9E drivers are

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

Page 2: Equipment and machinery advances boost the next … frame 9E (MS9001E) gas turbine drivers are becoming accepted technology for direct-drive LNG applications. Frame 9E drivers are

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

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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

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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