Hydrocarbon Processing - May 2021 - 41

Maintenance and Reliability

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flow is diffused. In this space, primary and secondary
flows mix, which results in friction loss and other losses
referred to as vaneless diffuser loss.
Vaned diffuser loss depends on diffuser shape,
blade loading factors, incidence angle and surface
friction of the vane.
Mixing loss is also defined as slip of the flow field against
the direction of rotation from pressure to suction side.
After the flow has left the impeller, the primary and
secondary flows mix. This is not ideal and energy loss
and more recirculation will take place.
Wake mixing loss due to mixing with jet flow right after
impeller exit.
Super critical mach number loss due to shock wave loss.
Chocking loss due to the relative mach number at the throat.
Incidence loss due to the difference between the inlet
blade angle and flow angle.
Entrance diffusion loss due to the diffusion inlet to throat.
Mechanical losses include power dissipated through
bearings, seals, shaft-driven lube pumps and gear boxes.

The role of OEMs. OEMs play a critical role in the restaging/

rewheeling of compressors. Normally, all mechanical, rotary
modifications, rewheeling, internals modification and restaging are carried out by OEMs due to their expertise, experience,
facilities and tools. OEMs will conduct the thermodynamic,
mechanical, rotary, aerodynamic and vibration studies, and will
establish the safe operating envelop and provide performance
guarantees. An engineering contractor or in-house engineering
team can study equipment and processes upstream and downstream of the compressor unit.
Performance test. A centrifugal compressor's performance
estimation can be done by three methods: the adiabatic method, N-method and Mollier method. In the adiabatic method,
the compression process is completely reversible (isentropic),
meaning that the input power is completely converted into
pressure energy, which is not the practical case. The N-method
is named based on a polytropic exponent (n) that is used to estimate discharge conditions and is independent of the state of
compressed gas, and the polytropic head is the sum of each stage
head, which are not true in adiabatic processes. Mollier diagrams
are generally preferred for pure gases.
A performance test run of the rerated/restaged compressor
can be carried out as prescribed in ASME PTC 10 and API 617.
However, an agreement with the client and the OEM on PTC 10
test class and methodology is precedent. A complete unit test-
including casing, gear, driver and auxiliary units-for evaluating
performance is always recommended. Shop performance test
limitations must be considered.
Decision-making process. A payback period of up to 5 yr can

be considered as a potential opportunity for improvement. If the
payback period is less than 2 yr, it requires urgent rerating/restaging. However, TABLE 5 details some of the major factors considered during the decision-making process.
Takeaway. Gas molecular weight, gas property, suction operating conditions, upstream and downstream equipment limitations,

and existing compressor data are playing a vital role in the analysis
of rerating/restaging. Deep analysis on compressor stability is often required during the rerating study. However, rerating/restaging is an attractive and cost-effective solution to maximize the
utilization of available resources (optimization) and address the
changes in process and plant requirements. Rerating is often used
to reduce the capital cost and improve the viability of a project.
Rerating or restaging often calls for multi-disciplinary input
and knowledge and should, in general, be carried out in consultation with the OEM. The ripple effect on upstream and downstream equipment and units should be a part of any study. The
outlined guidelines and limitations will help designers to carry
out a feasibility study of rerating/restaging.
LITERATURE CITED
API Standard 617, " Axial and centrifugal compressors and expander-compressors, " 8th Ed., September 2014.
2
Simmons, P., B. Nesbitt and D. Searle, Guide to European compressors and their
applications: The complete practical reference guide to compressors design, operation
and applications, Vol. 2, Professional Engineering Publishing, London, U.K., 2003.
3
Khan, M. O., " Basic practices in compressors selection, " International Compressor
Engineering Conference, Purdue University, West Lafayette, Indiana, 1984.
4
Ludtke, K., " Rerate of centrifugal process compressors-Wider impellers or higher speed or suction side boosting? " 26th Turbomachinery Symposium, College
Station, Texas, 1997.
5
Blahovec, J. F., et al., " Guidelines for specifying and evaluating new and rerated
multistage centrifugal compressors, " 27th Turbomachinery Symposium, College
Station, Texas, 1998.
6
Ludtke, K., " Twenty years of experience with a modular design system for centrifugal process compressors, " 21st Turbomachinery Symposium, College Station,
Texas, 1992.
7
Sorokes, J. M., E. A. Memmott and S. T. Kaulius, " Revamp/rerate design considerations, " 42nd Turbomachinery & 29th Pump Symposium, Houston, Texas, 2013.
8
Garcia, D., et al., " Restage of centrifugal gas compressors for changing pipeline
landscapes, " IAGT 2015 Symposium, Banff, Alberta, Canada, 2015.
9
Gutierrez Velasquez, E. I., " Determination of a suitable set of loss models for
centrifugal compressor performance prediction, " Chinese Journal of Aeronautics,
October 2017.
10
Li, P.-Y, C-W Gu and S. Yin, " A new optimization method for centrifugal compressors based on 1D calculations and analyses, " Energies, Department of Thermal
Engineering, Tsinghua University, Beijing, China, 2015.
11
Albusaidi, W. and P. Pilidis, " An iterative method to drive the equivalent
centrifugal compressor performance at various operating conditions: Part II
Modelling of gas properties impact, " Energies, School of Aerospace, Transport and
Manufacturing, Cranfield University, Bedfordshire, U.K., 2015.
12
Botha, B. W. and A. Moolman, " Determining the impact of the different losses on
centrifugal compressor design, " R & D Journal, 2005.
1

MURALITHARAN RAMALINGAM works as Group Manager,
Process Engineering, at Wood Plc. India (previously Amec Foster
Wheeler India Private Ltd.) and leads a team of more than
100 process engineers. He has more than 30 yr of post-graduate
experience in oil and gas, FPSO, upstream and downstream
projects execution, plant operation and commissioning. He has
successfully handled numerous projects ranging from conceptual
studies to commissioning, feasibility studies, optimization, troubleshooting and
revamp/retrofitting engineering over the past two decades. He holds BS and MS
degrees in chemical engineering from Anna University, India, has mentored numerous
junior engineers and trainees, and has presented numerous design concepts and
studies in in-house training. Mr. Ramalingam is an Associate of Chemical Engineers
Association, India. He can be contacted at m.ramalingam@woodplc.com.
CHANDRAGUPTHAN BAHUBALI works as a Senior Principal
Process Engineer at Wood Plc. India (previously Amec Foster
Wheeler India Private Ltd.). He has more than 15 yr of postgraduate experience in oil and gas projects. He holds an MS
degree in refining and petrochemical engineering from the
University of Petroleum & Energy Studies and a BS degree in
chemical engineering from Madras University, India. Mr. Bahubali
has successfully completed many AIChE certificate courses in process safety as well
as many international certification courses. He has also authored numerous papers
on flow assurance, gas hydrates, fixed-bed reactors and economics. He can be
contacted at c.bahubali@woodplc.com or b.chandragupthan@gmail.com.
Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - May 2021