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| WORLD FERTILIZER |
NOVEMBER 2016
For both an integrally-geared and a single-shaft
compressor, the height of the impeller blades and the
corresponding diffuser widths become very small in the later
stages. Impeller blade height is less than 3 mm for IGC after
stage 5 and less than 2.5 mm for the SSD after stage 8.
Impellers and diffusers need to be manufactured with high
surface quality to minimise the skin friction coefficient and
optimise efficiency.
It should also not be overlooked that the diffuser outlet
diameters, which determine the housing sizes, are also larger
for the single-shaft design. This results in greater size, weight
and associated costs for the housing on all stages of the
single-shaft design.
Most significantly, while tight manufacturing tolerances
hold true for both designs, the geometrical constraints of
the single-shaft compressor have a significantly greater
impact on its efficiency. When added together, these
detailed comparisons reveal a substantial difference in
power consumption and overall isothermal efficiency
(Table 2).
Advantages and disadvantages
One of the primary benefits of integrally-geared compressors
is apparent in their gearing mechanics. By individually
selecting stage speeds, the compressors can be best matched
to aerodynamic requirements and attain significantly higher
efficiency levels. The energy savings of around 23% found in
this technical comparison are in line with (and slightly better
than) figures available from competing IGC manufacturers.
The isothermal efficiency of 0.67 also outperforms
comparable results published in technical papers and reports.
This is not to say that single-shaft designs do not offer
some advantages. Because of its simpler mechanics, a typical
single-shaft compressor requires only one pair of bearings
and shaft seals (leading to the atmosphere) per housing
section, while the integrally-geared compressor uses a pair of
shaft seals and bearings per pinion.
However, for most applications, the increased efficiency
and the option to add intercooling outweigh this
rotor-dynamic simplicity. The heightened potential for gas
leakage through the additional number of seals might also be
a concern. But this can be offset by using virtually gas-tight
dry gas seals on the later stages. These dry gas seals release
around 35 times less CO
2
into the atmosphere than the
carbon ring seals that they replace.
A comparison of the main advantages of each design is
shown Table 3.
Reliability and maintenance
Unlike older reciprocating compressors, which are quite
maintenance intensive and require service checks and
overhauls at least annually, both single-shaft centrifugal and
integrally-geared compressors can provide tens of thousands
of hours of operation before scheduled maintenance is
required.
IGCs’ lower maintenance requirements are evident in many
application areas, where one IGC typically replaces several
reciprocating compressors. The continual operation they
provide is otherwise only available through a combination of
primary and back-up reciprocating compressors.
Not only do IGCs deliver superb continuous uptime, they
also help simplify servicing. The gearbox casing allows for
easy access for bearing replacement and to monitor the
health of gears and pinions. Other than this, maintenance
does not differ greatly from standard single-shafts designs.
Issues, when they do arise, are primarily handled through
standard predictive and preventive maintenance. By routine
monitoring of variations in an IGC’s vibration characteristics,
tooth contact and gear wear can be analysed and optimised
during preventative maintenance servicing.
Implementation and outlook
The two 8-stage integrally-geared compressors that were
developed from these compressor test results were delivered
onsite at the end of 2015. Commissioning began in 3Q16 and
they will soon provide CO
2
at flow levels of up to
18 000 Nm
3
/hr (10 594 scfm) with an outlet pressure of over
200 bar(a) (2973 psia).
The compressors underwent strict pre-delivery testing
that monitored low and high CO
2
compression and were
subsequently put through full-load and speed tests. The tests
displayed smooth and unproblematic operation, including the
gas-tight properties from specially developed dry gas seals
used in the latter stages.
The design employed for these compressors can be
customised to meet other plant and application
requirements.
Due to the test results and Atlas Copco’s experience in
developing integrally-geared compressors for CO
2
and a range
of different applications, the company believes that IGCs
offer a number of advantages over reciprocating and
single-shaft centrifugal designs. Compared to reciprocating
compressors, they require significantly less maintenance, are
more efficient and much more compact. The low
maintenance requirements for integrally-geared compressors
are comparable to single-shaft designs, but they offer better
efficiency, a smaller footprint and the ability to incorporate
interstage cooling.
Table 3.
Advantages and disadvantages of both design variants
Characteristics Integrally-geared
design
Single-shaft
design
+/-
+/-
Number of bearings 2 per shaft
-
2 per housing
and shaft
+
Number of shaft
seals to atmosphere
2 for each
stage
-
2 per housing
and shaft
+
Speed optimisation Variable for
each pinion
+ Fixed
-
Intercooling option After each
stage
+ Difficult
-
Table 2.
Overall performance
Total values IGC
Single-shaft Deviation
(single-shaft
to IG)
Isothermal
efficiency
0.67
0.57
-15%
Inner power
(kW)
4290
5258
23%
