44
LNG
INDUSTRY
APRIL
2016
and highly efficient package. Through this progression, and
the associated recognition from the American Petroleum
Institute (API), IGC compressors now see significant market
penetration and have become a dependable solution that
many industries look to for their compression needs.
1
Case study
On a recent project intended for installation in Nigeria,
compressor manufacturers were approached in 2H14
regarding their ability to supply multiple compressors for
LNG trains on an expedited basis. A 0.50 million tpy LNG
plant was at stake, which would need, among several other
major purchased components, a mixed refrigerant (MR)
gas compressor. The specifications of the project would
require four compressors to manage the total flow of gas,
and they would all need to be delivered in a critical six to
eight month window. Based on reliable and proven designs
with similar applications, it was expected that more than
one supplier would be able to develop the right solution
within the time constraints, as well as begin contract-specific
engineering work less than a month after the original inquiry.
As with many fast-tracked projects, an original equipment
manufacturer’s (OEM) ability to quickly assemble a team,
formulate a solution, and provide the necessary information
are vital to meeting the customer’s goals.
Efficient and reliable
The client was determined to meet the project schedule,
but also knew that it needed to meet the given efficiency
and reliability requirements. IGC compressors were selected
due to their proven ability to meet all of these requirements.
Equipment durability parameters, such as reliability,
availability, and mean time between failures (MTBF), were
compared and evaluated during the equipment selection.
Table 1 illustrates the relative differences in such categories.
2
Finding an LNG compression solution
Fortunately, the compressors required by this application
aligned well with a recently completed project for another
end-user. As is typical in the process gas compression
business, no two applications are ever the same, and thus
the main compressor design would need to be tweaked.
Details ranging from high level API requirements, to finer
specifics, such as superheat, rise to surge, and turndown
range, were worked through at great pace and with a high
level of detail in an effort to create a clear path forward.
The essential front-end engineering for the compressor
design needed to be worked on in an expedited basis due
to the schedule constraints. In the weeks leading up to the
contract award, direct dialogue between the aerodynamic
design engineers and the client’s process engineers was
often required to quickly work through specific questions
and to iterate the best design solution. Requested
changes in the preliminary process conditions needed to
be analysed and the individual engineers responsible for
the impeller (aero) design, rotordynamic stability, and
mechanical integrity all verified the design selection,
which was then reviewed with and approved by the client.
Project management resources responsible for the
execution of the compressors were also mobilised early in
the process to assist in the necessary meetings, as well as
with customer documentation requirements. The
compressor contract was ultimately signed by the
involved parties by the end of September 2014, with a
forecasted ship date for all four compressors in the late
spring of 2015.
Scope of design
IGC compressors are well suited and adaptable to
the varying process conditions for gas compression
applications. The compressor utilises a large helical-cut
bullgear driven at low speeds to drive multiple pinion
gears with overhung impellers. The integral gear design
allows for the rotational velocity of each pinion gear
to be designed based on the optimal aerodynamic
characteristics of the impeller. The intrinsic design
arrangement of IGC compressors naturally separates
the lubricating oil in the gear casing from the process,
ensuring oil-free process gas without the need for
additional separation systems. Some OEMs employ
enhanced measures, such as an atmospheric air gap
between the gear casing and compression volutes and/or
maintaining the gearbox at vacuum pressure to keep
lubricating oil from seeping out of the casing, to further
ensure the delivery of oil-free gas.
3
Based on the requirements of the LNG train, the MR
IGC compressor was rated for a maximum power of
9300 hp, sized accordingly for a variety of design points,
such as maximum flow, maximum ambient temperature,
minimum ambient temperature, and gas recycling. In this
case, and as with most designs, it was imperative to fully
understand and analyse each of the required operating
conditions;
specifically, changes
to the inlet suction
temperature, coolant
temperature,
molecular weight,
and required gas
flow. All of these
conditions were
known to change
from season to
season, and on a
more granular level
from day to day.
Along with the
aforementioned basic
Table 1.
Availability and reliability parameters for different compressor types
Availability (%)
Reliability (%)
Proactive
maintenance
downtime
(hr/yr per five years)
MTBF (yr)
Compressor type
Best
Average Best
Average Best
Average Best
Average
Reciprocating (lubricated)
99.5
97.3
99.8
97.8
120
240
1.5
0.5
Screw (oil-flooded)
99.2
97.7
99.8
98.8
250
300
3
1.5
Screw (oil-free)
99.6
99
99.9
99.7
150
300
10
5
Centrifugal (clean service)
99.9
99.7
100
99.8
12
30
15
8
