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This research aims to assess the causes of inefficient and unstable operation of
centrifugal compressors and turboexpanders in process gas applications in order to
provide a solution for performance restoration and enhancement. It encompasses
thermodynamic and flow evaluations to examine the efficiency and operating range
improvement options of new units. Besides, this work is complemented by a technoeconomic
analysis to provide a rounded outcome from these studies. In order to achieve
the desired objectives, a novel integrated approach has been developed to assess the
design and performance of multi-stage centrifugal compressors. The proposed
systematic methodology involves five basic elements including evaluation of
compressor selection, compressor sizing and casing structure, performance prediction
at the design and off-design conditions, modelling of efficiency and head deterioration
causes; and stage design evaluation. This will contribute towards evaluating the
geometrical parameters of the new units’ designs at the early preliminary design phase,
and thus, will be useful to identify the options for efficiency and operating range
enhancements. For installed units, this approach can be implemented to assess the cause
of inefficient and unstable operation by assessing the available operation data.
A method was developed to predict the performance curve of multi-stage centrifugal
compressor based on a stage stacking technique. This approach considers the
advantages of Lüdtke and Casey-Robinson methods with an incorporation of a
methodology for compressor selection and sizing to generate more accurate results. To
emphasize the validity of the developed model, it has been evaluated for both low and
high flow coefficient applications. The obtained results show a significant improvement
in the estimated efficiency, pressure ratio, shaft power and operating range as compared
with the existing methods.
The centrifugal compressor is designed to run under various operating conditions and
different gas compositions with the primary objective of high efficiency and reliability.
Therefore, a new iterative method has been developed to predict the equivalent
compressor performance at off-design conditions. This technique uses the performance
parameters at design conditions as a reference point to derive the corresponding
performance characteristics at numerous suction conditions with less dependency on
the geometrical features. Through a case study on a gas transport centrifugal
compressor, it was found that the developed approach can be applied for design
evaluation on the expected variation of working conditions, and for the operation
diagnosis of installed units as well. Furthermore, a parametric study has been conducted
to investigate the effect of gas properties on the stage efficiency, surge margin, and
compressor structure. The obtained results support the need for considering the gas
properties variation when the off-design performance is derived.
To evaluate the impact of internal blockage on the performance parameters, this study
proposed an approach to model the effect of non-reactive deposits, which has been
qualified using four operation cases and the obtained results are compared with the
internal inspection findings from the stage overhauling process. This also covers the
influential aspects of flow blockage on the technical and economic values. Since the
main challenge here is to analyze the process gas composition in real time, the
influences of the non-reactive deposits have been compared with the effect of the
unanticipated gas composition change. Subsequently, it has turned out that the pressureratio parameter is not enough to assess the possibility of flow blockage and unexpected
gas properties change. Moreover, it was observed that the stage discharge pressure was
more sensitive to the fouled aftercooler comparing with suction and internal blockage.
However, the effect of contaminated aftercooler on the surge point and discharge
pressure and temperature of the upstream stage was found greater than its impact on the
shaft power. Thus, a substantial surge margin reduction was detected when the first
stage was operating with a fouled aftercooler comparing with the measured reduction
as a result of unanticipated gas properties change. Furthermore, a larger pressure ratio
drop was measured in the case of liquid carryover which revealed a more significant
impact of the two phases densities difference comparing with the gas volume fraction
(GVF) effect. The possibility of hydrate formation has been assessed using hydrate
formation temperature (HFT) criteria.
Additionally, this research highlights a number of challenges facing the selection of
typical centrifugal stage design by assessing the contribution of design characteristics
on the operating efficiency and stable flow range. Besides, an empirical-based-model
was established to select the optimum impeller and diffuser configurations in order to
make a compromise decision based on technical and economic perspective. It was
concluded that there is no absolute answer to the question of optimum rotor and stator
configuration. The preliminary aerothermodynamic evaluation exposed that the
selection of the optimum impeller structure is governed by several variables: stage
efficiency, pressure loss coefficient, manufacturing cost, required power cost,
resonance frequency and stable operating range. Hence, an evaluation is required to
compromise between these parameters to ensure better performance. Furthermore, it
was argued throughout this study that the decision-making process of the typical stage
geometrical features has to be based upon the long-term economic performance
optimization. Thus, for higher long-term economic performance, it is not sufficient to
select the characteristics of the impeller and diffuser geometry based on the low
manufacturing cost or efficiency improvement criterion only.
For turboexpanders, a simple and low cost tool has been developed to determine the
optimum turboexpander characteristics by analysing the generated design alternatives.
This approach was used in designing a turboexpander for hydrocarbon liquefaction
process. Moreover, since the turboexpanders are expected to run continuously at severe
gas conditions, the performance of the selected turboexpander was evaluated at
different inlet flow rates and gas temperatures. It has turned out that designing a
turboexpander with the maximum isentropic efficiency is not always possible due to
the limitations of the aerodynamic parameters for each component. Therefore, it is
necessary to assess the stage geometrical features prior the construction process to
compromise between the high capital cost and the high energetic efficiency. |
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