SEPTEMBER
2014
LNG
INDUSTRY
53
very low temperature applications. Elliott also has a dedicated
facility to produce API-614 lube oil and buffer gas consoles, a
capability that significantly reduces the risk of delay for project
completion.
Advanced five-axis, CNC machining centres ensure the
quality of Elliott’s impeller designs. Impellers are stress
relieved, machine finished, balanced statically and
dynamically, spin tested, and mounted onto the shaft with an
interference fit to improve performance under off-design
conditions. Diaphragms are fabricated from steel plate or in
combination with cast steel where thickness precludes using
steel plate alone. Precision machining ensures dimensional
accuracy and a precise finish on gas path surfaces for
enhanced performance.
On midsized compressor casings, the horizontal flanges are
milled by giant CNC master head machining centres rather than
welded on. End walls are made from a single solid plate. The
resulting casings have fewer sealing surfaces and increased
pressure capabilities. Larger horizontally-split casings have
rolled barrel sections with welded horizontal flanges and
endplates. Elliott’s vertically-split barrel compressors feature a
complete inner casing assembly bundle that can be inserted or
removed from the outer casing as a single piece. The inner
bundle casings are thicker to more effectively handle off-design
conditions. Barrel compressor designs include innovative
cradle tooling for simpler installation and removal of large
bundles, and reduced turnaround time (Figure 4).
Driver experience
Large amounts of energy are required to chill natural gas to
its liquefaction point at -260°F (-162°C). The power efficiency
of the refrigeration process directly affects the economics of
LNG production. In mega LNG plants, a single MR compression
train can require 65 MW of power. The cost of energy to drive
the compression trains, whether BTUs burned in a turbine or
kilowatts to power a motor, reduces operating margins. Many
factors influence the selection of compression drivers for an
LNG plant. With a history of LNG production extending over
more than 50 years, Elliott has direct experience with every
type of driver application.
Early LNG plants used steam turbine drivers. Steam
turbines are efficient, can deliver precisely the power specified
for the process, and provide high reliability and high availability,
operating for years without interruption or overhaul. But steam
turbine installations are large, complex, and capital intensive,
comprising boilers, condensers, desalination and water
conditioning, feed pumps, and other auxiliaries.
The ready availability of natural gas at an LNG plant led to
the general replacement of steam turbines by increasingly
larger frame gas turbines. Gas turbines offer several
advantages over steam turbines including uncomplicated
design, lower capital expense, and a smaller footprint. On the
other side of the equation, gas turbines produce higher CO
2
,
and they require frequent and extensive on-site maintenance,
reducing plant availability. A gas turbine might require a steam
turbine or electric motor as a helper at start-up. A gas turbine’s
fixed optimal speed also dictates that the compressors and
refrigeration process must be designed around the turbine. The
associated design compromises might not make full use of the
turbine’s power.
Aeroderivative gas turbines offer greater thermal efficiency
and a smaller footprint than industrial gas turbines. They might
not need a starter motor, and they can be swapped out for
simpler maintenance. However, they produce higher NO
x
, and
their design complexity and higher operating pressures and
temperatures create additional maintenance issues. Fuel
quality is critical to aeroderivative turbines, and their power
output is more directly affected by ambient temperatures.
Electric motors are popular as LNG compression train
drivers in locations where the availability of 400 - 500 MW of
power can be ensured. Motors offer higher availability than
turbines and require little auxiliary infrastructure and fewer
support personnel. Variable speed electric motors can be
closely matched to process and compressor design, optimising
the overall process efficiency and eliminating the need for
gearboxes with large flow compressors.
Elliott’s experience includes 3.2 million hp in installed
electric LNG compression.
Full-load, full-speed testing
An LNG refrigeration compressor string is a complex system.
Two large MR compressors might be driven by an electric
motor or a gas turbine with an electric starter motor or
steam turbine helper, all from different manufacturers.
To ensure maximum efficiency of the entire train, Elliott
design engineers conduct performance simulations that
Figure 3.
Sideload mixing in a propane compressor.
Figure 4.
Removing the inner barrel of an Elliot 88MB mixed
refrigerant compressor.
