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August
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HYDROCARBON
ENGINEERING
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map for gas mix compositions different for
design/reference ones.
These calculations are closely coupled with
thermodynamic gas compression and thermodynamic real
gas mixtures, so that a single variation in any one of the
gas mixture components can have an effect. The main
purpose of this study is to numerically investigate the
consequence of the variation of each one of the gas mix
components on the overall compressor performance. To
develop the investigation, several cases with different gas
mix composition have been considered and calculated. All
considered cases have been set to the same overall mix
molecular weight, to observe the effect on the machine
performance of gas mixes with the same molecular weight
but different gas mix composition.
For all cases the inlet conditions have been set
constant.
With CMap it is possible to calculate compressor
performance in three off-design conditions. These three
conditions feature the same inlet pressure and
temperature, and the same gas molecular weight but,
importantly, a different gas mix composition (Figure 1).
Effect of single gas mix components of
performance
As mentioned, the gas mix composition is changed in
such a way that the molecular weight remains constant.
Figure 2 shows how performance changes when there
is a variation in gas mix composition, but with constant
molecular weight.
In these curves, the shift of the surge line, the shift
of the stonewall and the change of the slope curve can
be noted.
The numerical analysis presented in this article
illustrates how it is possible, using advanced
computational tools, to determine the centrifugal
compressor performance accurately under varying inlet
conditions.
The investigation shows the effect on performance
caused by the modification of the gas mix composition
while maintaining the same overall molecular weight, and
highlights how the impact on the performance can be
considerable even when this molecular weight remains
constant.
The experiences with real machinery showed that
compressor performance predictions obtained with
CMap software are in alignment with field
measurement and OEM predictions for machines in
good condition.
Conclusion
In summary the methods proposed and described in this
article make it possible to:
Predict the performance map of a centrifugal
compressor in off-design conditions. The prediction
of compressor performances is accurate even at high
pressures, where the ideal gas theory commonly
used introduces considerable errors.
Predict the modification of surge points in actual
operative conditions, with different inlet pressure
and temperatures, and different operative gas. It is
also possible to implement advanced protection
from surge.
Detect deviation from expected performance and
therefore identify possible machinery trouble before
other indicators, such as vibrations.
Detect trends in parameter deviation which may
provide valuable support for planning machinery
outage and set up effective and money-saving
maintenance plans.
Figure 3.
Screenshot from CMap 2.0.