Integrating Refrigerating Compressor Trains For Large LNG Plants

RP_CT225.
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■ Low BTU fuel gas compressors driven after a 100 MW generator by a Frame 9E gas turbine at the Ilva steel mill in Taranto, Italy. Three similar
INTEGRATING REFRIGERATING COMPRESSOR TRAINS

units are installed in this steel mill.
FOR LARGE LNG PLANTS

Machinery Now Designed to Serve Unitary Plants of 8 Million T/Y and Above
By Roberto Chellini
LNG plant size has been growing drive the refrigerating compressors. In nominal speed (3000 rpm). FEA and
considerably in the past 30 years parallel with the increase in size and CFD calculations were carried out to
which has lowered the production absorbed power of the compressors, verify the possibility of operating the
cost of LNG for the operator, helping it has been necessary to identify the machine at different speeds for long
make it more and more competitive in most suitable gas turbine for the re- periods of time, as it can be required
the marketplace. In the ’80s, it was quired power output. for mechanical drive applications.
common practice to produce 2-3 mil- Up until now, GE Frame 6 and 7, Finally, the Frame 9E was qualified
lion tons per steam per year. Present originally designed to drive generators for steady operation within the 96 to
units produce 4-5 million tons per at constant speed, were also available 102% speed range.
year, and engineering companies are for variable speed, mechanical drive GE Nuovo Pignone had already
now planning plants with unit capaci- applications, as required in LNG proc- gained field experience with a hybrid
ties in the order of 7-8 million tons ess plants. mechanical drive/power generation ap-
per year. It is believed that such sizes Larger LNG units require more plication with Frame 9Es in a steel mill
will allow savings up to 30% in the power. In order to be used in the where a large fuel gas compressor train
cost per ton of LNG plant capacity Qatargas II train, GE Energy Oil & is driven on the same shaft as the elec-
over the present plant size. Gas has submitted its Frame 9E model tric generator. These gas turbines are
GE Energy Oil & Gas, a specialist in (rated 126 MW) to a special qualifica- fueled with low BTU gas from the mill
this sector, is manufacturing the tion and testing program carried out and produce 100 MW of electric power
equipment needed for the new off- by the Florence engineering team in while driving a 30 MW compressor.
shore Qatargas II plant, an approx. synergy with the GE Energy team in Their ISO rating is 126 MW, but in real-
7.8 million t/y (tons per year) plant Greenville, South Carolina, U.S.A., ity they deliver around 140 MW. In
based on the Air Products refrigerat- and Belfort, France, where this model spite of the load and the type of fuel,
ing process AP-X in Qatar, Ras Laffan is manufactured. which is not of the cleanest quality,
Industrial City. This machine was originally quali- their availability has been recorded in
In LNG plants, it is common prac- fied for transients in the power gener- the 95 to 96% range. Because of this,
tice to use industrial gas turbines to ation mode between 95 and 105% of continued on page 66
MARCH 2005 COMPRESSORTechTwo

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chine compressor casings with internal

diameters up to 12.4 ft. (3800 mm),
while new 5-axis milling machines can
produce 3-D shrouded impellers hav-
ing diameters exceeding 6.5 ft. (2000
mm) from a single forging.
GE Nuovo Pignone has used this
technology to produce over 300 im-
pellers, with dimensions at the begin-
ning up to 2.9 ft. (900 mm) diameter,
then up to 5.11 ft. (1.5 m) in a second
step, and now up to 6.5 ft. (2 m). This
innovative production system elimi-
nates any welding and consequent
stress-relieving heat treatment. In addi-
tion, the gas channels feature exact di-
mensions (no distortion) and have a
very smooth surface finishing.
A bearing span in the order of 16.4 ft.
(5 m) requires a shaft of suitable diame-
ter on the bearings and end seals. Dry
■ Rotor of the MCL 1402 nitrogen compressor for the 7.8 million t/y Qatargas II LNG plant. gas seals with internal diameters of 1.14
ft. (350 mm) needed to be qualified.
the customer has decided to move their operations on the outboard compres- In order to limit the weight to be
major overhaul originally planned after sor casing. lifted on-site for maintenance pur-
the first 48,000 hours to 56,000 hours. While the Frame 7 has a 3600 rpm ro- poses, the upper half of the diaphragm
The two turbines installed at the Ilva tating speed, the Frame 9E is a 3000 rpm bundle is no longer fixed to the upper
Taranto steel mill have operated on machine. In order to directly couple the half of the horizontally split casing, as
low BTU fuel gas since their installation compressor train, it was necessary to in- is common for smaller size machines.
in 1995, validating these machines for crease the size of the compressors not The top part of the outer shell is first
this type of fuel. only because of larger capacity, but also lifted (which weighs about 80 tons)
The Qatargas II units are equipped to adapt them to the lower speed. and then the upper part of the di-
with a DLN combustion system to Compression of large flows of gas aphragm bundle, which is kept in po-
maintain NOx emission below 25 ppm can be carried out, starting with an ax- sition by special keys, is lifted sepa-
and can burn low BTU fuel with high ial compressor on the low pressure rately to give access to the rotor.
nitrogen content. DLN experience side followed by a centrifugal compres- The compressor models presently in
comes from Frame 7 models with sor casing or using only centrifugals ac- the production stage for the 7.8 million
DLN systems in operation in other cording to customer preferences. t/y Qatargas II plant at Ras Laffan are
LNG plants where they are fueled Qatargas II plant selected an all- propane cycle 3MCL 1403 + MCL 1402;
with gas having very high nitrogen centrifugal solution with the use of mixed refrigerant cycle MCL 1805 +
percentage. The Frame 9 machine is, unprecedented casing sizes. 2BCL 1006; and nitrogen cycle MCL
in fact, a scale-up of the Frame 7 fea- The large capacity and the reduction 1402 + BCL 1003
turing the same combustors apart of the rotating speed from 3600 (FR7) A full load testing program will be
from their number — 10 on the Frame to 3000 rpm (FR9), imposed an in- carried out before delivery of each in-
7 versus 14 on the Frame 9E. crease of impeller diameters from a dividual compression string, which is
Compression power required by the typical size of 1300-1400 mm to 1600- scheduled before May 2006. ■
various refrigerating cycles serving a 8 1800 mm. The Florence shop of GE SEE DIRECTLINK AT
million t/y LNG plant is in the range Nuovo Pignone is equipped to ma- WWW.COMPRESSORTECH2.COM
of 300 MW. It is clear that the Frame
9E turbine becomes the optimum dri-
ver capable of delivering the required
power, minimizing the number of
units.
To overcome the start-up inertia of
the turbine rotor coupled to the com-
pressor rotor in its pressurized casing,
it is necessary to dispose of a 45 MW
starter-helper motor. An ASI Robicon,
4-pole motor was selected because of
its compact size and drive technology.
The ASI Robicon PWM (Pulse With
Modulation) drive delivers a continu-
ously approximate sinusoid of current
which avoids pulsations on the turbine-
compressor rotor couplings system,
minimizing the torque ripple effect.
The starter-helper motor has been
placed between the turbine and the
compressor for easier maintenance ■ Shrouded impeller milled on a 5-axis machine from a single forging.
COMPRESSORTech
Two
REPRINTED FROM MARCH 2005 Copyright Diesel & Gas Turbine Publications
Printed in U.S.A.

Integrating Refrigerating Compressor Trains For Large LNG Plants

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RP_CT225.

qxp 3/2/05 2:44 PM Page 1

INTEGRATING REFRIGERATING COMPRESSOR TRAINS

FOR LARGE LNG PLANTS

MARCH 2005 COMPRESSORTechTwo

chine compressor casings with internal

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