52
LNG
INDUSTRY
SEPTEMBER
2014
Compression
- the heart of
liquefaction
Large refrigeration compression
trains are the critical operating
machinery at the heart of every
LNG process. All commercial
liquefaction processes in use today
run one or more compressed
refrigerants through a heat
exchanger to chill and ultimately
liquefy the treated gas stream.
The capacity of an LNG train,
and by extension of the entire
production facility, is determined
by the size of the refrigeration
compressors. Today’s mega-plants typically operate two or
more compression trains. The uninterrupted operation of a
plant is dependent upon the reliability of this equipment.
Elliott Group has been a primary supplier of large
refrigeration compressors for LNG projects since the beginning
of commercial production. More than 100 Elliott compressors
have been installed for LNG applications worldwide, producing
in excess of 70 million tpy of LNG. Elliott has also supplied
more than 750 compressors for refrigeration applications in
petrochemical processing worldwide. The company supplied
compressors for the first large scale, US LNG production facility
at Kenai, Alaska in the 1960s. ConocoPhillips’ Kenai LNG Plant
was the largest liquefaction plant in the world when it began
operations in 1969. For four decades Kenai was the only US
LNG export facility, shipping over 1300 loads, primarily to
Japan. Elliott compressors were also installed in Libya, Algeria,
Abu Dhabi and Indonesia in the 1970s, and many of these
machines remain in service today. The company has continued
to supply ever larger and more efficient compressors for the
world’s largest LNG projects in Qatar, Russia, Yemen and
China, as well as proven and efficient compressors for
peripheral plant services such as gas boosters, boil-off gas and
end-flash gas (Figure 1).
Aerodynamic efficiency
The efficiency of the refrigeration compressors in the
liquefaction process contributes directly to the profitability
of the plant. A single compression train in a mega-plant
can produce 4.8 million tpy. A 1% increase in compressor
efficiency can result in a corresponding increase in LNG
production, or 50 000 tpy in this case.
Elliott uses state-of-the-art design and prediction tools to
optimise aerodynamic performance and increase compressor
efficiency. Impeller and matched stationary flow path
components are designed using computational fluid dynamics
(CFD) analyses. Three-dimensional blade profiles, diffuser flow
angles, crossover bend curvature, area ratio, and return channel
vane shapes are optimised for each impeller stage to provide the
best possible efficiency. Precise design of the flow distribution
channels at the inlet and discharge volutes yields additional
efficiencies (Figure 2).
The company’s abradable rotating labyrinth seals are also
carefully designed to enhance efficiency. The seal teeth attain
tighter clearance with the rotor shaft,
increasing impeller efficiency and expelling
particulates. At higher gas densities and
operating pressures, rotor stability
contributes to efficiency. Elliott’s proprietary
analytical tools have resulted in increased
rotor stiffness by increasing shaft diameter,
reducing impeller weight, and increasing
journal bearing sizes. These design
improvements increase rotor stability at
higher torque transmission and higher
speed operation. Elliott also balances every
rotor ‘at speed’ establishing first critical
within a vacuum. This is an excellent way of
balancing the rotor while confirming RDA.
Sideload optimisation
Most LNG tonnage worldwide is currently
produced using the APCI process. Elliott compressors process
approximately 35% of this production. The APCI process
includes two refrigeration cycles: a propane pre-cooling cycle
and a mixed-refrigerant (MR) cycle. The propane chiller cools
both the incoming feed stream and the MR process gas to
around -40°C. The propane compressor has multiple side
streams, which increase the thermal efficiency of the process.
Adding one or more side streams to the main flow of a
compressor increases the challenge of accurately predicting the
machine’s performance. Elliott’s research into side stream
thermal dynamics and pressure optimisation has resulted in an
understanding of the effects of sideload temperature, pressure
and flow on downstream stage performance and a compressor’s
overall efficiency. Computational flow path analyses and
proprietary models confirm the proper design of the merging
flow streams for minimum pressure loss, and contribute to the
efficiency of the company’s refrigeration compressors (Figure 3).
Advanced manufacturing in two
facilities
Elliott ensures the quality of its aerodynamic components,
rotors and casings through the use of advanced manufacturing
processes in each of its two world-class manufacturing facilities.
The factories in Jeannette, Pennsylvania, USA and Sodegaura,
Chiba, Japan are similarly equipped with advanced machine
tools and testing capabilities. The two factories work closely
together to balance their production schedules and ensure
timely project completion. The company maintains a dedicated
materials engineering laboratory and metallurgy group to study
materials such as low expansion high nickel alloys for use in
Figure 2.
3D modelling
of impellers
optimises
performance.
Figure 1.
Installation of an Elliot 60MB61 LNG mixed
refrigerant compressor in Russia.
