Hydrocarbon Processing - August 2021 - GP-23

SPECIAL FOCUS: SMALL-SCALE LNG
Tailored speeds and intercooling. In contrast to inline centrifugal
compressor technology, IGC stages provide more flexibility
regarding impeller aerodynamic design. While inline
compressors are sized depending on the speed of a given driver
(or a step-up gear attached to that driver), IGCs are more flexible
because they are not bound by a speed requirement. As a
result, an IGC's aerodynamics (such as impeller geometry) can
be tailored to an application's specific requirements.
By coupling a driver to a bull gear driving several pinions,
four pinions and eight compressor stages (impellers) can be
mounted onto one gearbox. To accomplish this, one or two
impellers (stages) are mounted at each end of the pinion shaft.
Two bearings support the pinion shaft, and each impeller is provided
with its own seal.
In addition to the impellers, other aero components, such
as a diffuser and volutes, are optimized in accordance with process
requirements. The required speed is then accomplished
via the gear ratio by selecting the appropriate teeth of the bull
gear and the pinion.
An inline compressor's design makes stage optimization
more challenging. IGCs are seen to be " high head " compared
to inline or single-shaft technology. By optimally selecting pinion
speeds to correspond to geometry, higher speeds enable
minimization of the size and quantity of the impellers for a given
function. By enabling optimum aerodynamic performance
at each stage, enhanced further by optional intercooling, the
best overall efficiency can be obtained.
In an IGC, all rotors are mounted on the same gearbox, and
each rotor is independently balanced. This bolsters compressor
reliability, because one rotor's imbalance does not impact
the others. The result at the Zhengtai Yida LNG plant IGC is
an availability of 96+%.
An essential pillar for the general energy efficiency of an
IGC is the possibility to install intercooling after each compressor
stage. It may be installed optionally between the stages of
IGCs, as with the SMR compressor at the Zhengtai Yida LNG
plant. By contrast, an inline solution may pose challenges for introducing
any type of intermediate cooling between the stages.
Intercooling is a necessary building block for delivering
higher overall compression efficiency. Thermodynamic principles
state that the temperature of a gas rises when it is subjected
to pressure through compression. Doing so requires
increased work and an accompanying increase in energy in the
next compression stage to achieve further compression of the
gas. This cooling between stages allows for compression that
comes closer to reflecting an ideal isentropic process due to
reduced fluid friction.
Flexibility in process control. To control mass flow and
power consumption, IGCs incorporate two control types: variable
inlet guide vanes (vIGV) or variable diffuser guide vanes
(vDGV), depending on process characteristics and specifications.
For liquefaction trains that mainly operate at 80% capacity
and higher (typically the case for water-cooled condensers),
vIGVs offer the most efficient solution. They help regulate
spikes and troughs in inlet pressure, enabling the compressor to
deliver a constant mass flow. Processes in which frequent startups
and turndowns occur, and which require high adjustability
to LNG output fluctuations, are best suited to the use of vDGVs.
To further improve process control, an SMR IGC segregates
its four stages into two groupings, which allows for the independent
control of each stage grouping with vIGVs and vDGVs.
Inlet guide vanes in the proprietarya
compressor design provide
around 25% turndown by controlling the flow to the impellers.
In addition, the vDGVs extend compressor turndown by up to
50% by controlling the discharge flow after the impeller.
By using vIGVs and vDGVs, the overall efficiency of an IGC
can regularly reach the 82%-84% range. When the inherent efficiency
gains resulting from integral-gearing technology are
taken into account, total energy savings can reach 13%-14%.
Two vIGVs are used on the Zhengtai Yida LNG plant, one at
the suction of the compressor and the second after the gas returns
from intercooling. They help improve overall machine
efficiency and reduce power consumption.
Takeaway. Increased demand for SSLNG has brought attention
to the ability of SMR IGC technology to provide a more
sustainable refrigeration performance compared to other compressor
technologies.
At 27 MW, the remote Zhengtai Yida LNG plant hosts
the world's largest integrally geared SMR compressor for the
SSLNG market. With a compact footprint, low seal leakage
rates and maximum rotor stability, Zhengtai Yida LNG demonstrates
the viability of IGCs as a solution, even in challenging
scenarios. The deployment of IGCs is also accompanied by
increased performance in terms of cost efficiency, operational
efficiency and energy efficiency benefits. GP
NOTES
a Atlas Copco Gas and Process
b Engineered, installed and commissioned by EnFlex Group
TUSHAR PATEL has held various sales and marketing positions
since joining the Atlas Copco Group in 2005, having built on
more than 25 yr of global experience in various manufacturing
industries. At present, he is responsible for global marketing
and business development for custom-engineered centrifugal
compressors and turboexpanders used in oil and gas, chemical/
petrochemical, fertilizer, industrial gases and power generation
(both conventional and renewable). Mr. Patel has authored conference papers for
the Supercritical CO2 Symposium and the Petrobras Turbomachinery Symposium,
in addition to various trade industry articles on oil and gas topics. He holds a BS
degree in mechanical engineering and an MS degree in marketing management.
MICHAEL DREWES has worked as a professional engineer
for more than a decade. In 2011, he joined Atlas Copco Gas
and Process and was responsible for individual project
proposals and framework agreements in the air separation
market. Starting in 2013, he served customers with process gas
compressor solutions for worldwide projects in the oil and gas
market. In mid-2016, he was appointed Market Manager for
fertilizer applications. Since early 2019, he has been responsible for the small-scale
LNG segment. Mr. Drewes holds a MS degree in aerospace engineering from
FH Aachen University of Applied Sciences and has authored multiple conference
papers and trade articles in the area of hydrocarbon processing.
JAMES ZHAO joined EnFlex Group in 2017 as the Vice President
of Business Development. He is responsible for technology
licensing, engineering services, marketing and project
supervision. He has more than 10 yr of experience in a variety
of engineering, project management and marketing roles,
with a special focus in the LNG, petrochemical and gas
processing market segments. Mr. Zhao earned a BS degree
in chemical engineering from the University of Texas at Austin and an MBA
degree from Rice University. He has also authored various papers and trade
articles related to the hydrocarbon processing industry.
Gas Processing & LNG | JULY/AUGUST 2021 23
Hydrocarbon Processing - August 2021

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