Four Quadrant Centrifugal Compressor Performance .experimentally investigated for a centrifugal...
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FOUR QUADRANT CENTRIFUGAL COMPRESSOR PERFORMANCE
Elisabetta Belardini
Senior Engineer
GE Oil & Gas
Florence, Italy
Elisabetta is Senior Engineer of Radial
Turbo machinery Performance within the
Advanced Technology. She is responsible of
performance predictability and risk assessment of centrifugal
compressors, stall investigation and modeling of system
dynamics. Elisabetta, received a M.S. degree with honor in
Mechanical Engineering in 1996, a Ph.D. in Energetics in 2000
from University of Florence in the unsteady CFD simulation of
turbo machinery and joined GE in 2005.
Dante Tommaso Rubino
Engineering Manager
GE Oil & Gas
Florence, Italy
Dr. Tommaso Rubino is currently the
Manager of the Radial Turbo machinery
Aerodynamic Design and Performance
within the Advanced Technology Division.
His responsibilities include the aerodynamic design and the
performance predictability of the GE Oil & Gas turbo-
compressors and turbo-expanders product lines. Dr. Rubino
joined GE in 2006 as Design Engineer in Centrifugal
Compressor NPI team, and then he has been responsible of the
aerodynamic design of new stage families for Centrifugal
Compressors and Pumps. Mr. Rubino received a B.S. and M.S.
degree in Mechanical Engineering in 2002 and a Ph.D. degree
in Mechanical Engineering in 2006 from Politecnico of Bari,
and he graduated with honors in the Diploma Course program
at the von Karman Institute in 2002
Libero Tapinassi
Engineering Manager
GE Oil & Gas
Florence, Italy
Libero Tapinassi is currently the
Manager of Turbomachinery
Aerodynamics and Heat Transfer
department of Advanced Technology
Division. His responsibilities include the aero-thermal design
activities related to the development of GE Oil&Gas
turbomachinery product line. Mr. Tapinassi joined GE in 2003
as Design Engineer in Centrifugal Compressor NPI team, and
then he has worked as Manager of Centrifugal Compressor
Aerodynamic team supporting stage development and
performance improvement programs. Mr. Tapinassi received a
B.S. degree (Environmental Engineering, 2001) from the
University of Florence in 2001.
Marco Pelella
Engineering Manager
GE Oil & Gas
Florence, Italy
Marco is Engineering Manager for
System Operability of Centrifugal
Compressor & Turbo expander
Applications. His responsibilities include the definition of
Process Control Philosophy and Dynamic Simulations. Mr.
Pelella graduated in Mechanical Engineering at University of
Naples, Italy in 1997. He joined GE in 1999 as Design
Engineer of centrifugal compressors then moving to office
Team Leader of integrally geared and pipeline compressors,
then as centrifugal compressors design office Engineering
Manager in Le Creusot, France and axial and centrifugal
compressors design office Engineering Manager for LNG and
Down Stream applications in Florene.
ABSTRACT
The characterization of the compressor behavior in all
quadrants of performance map has acquired, in the last years,
growing attention.
Two dedicated experimental campaigns have been
performed to characterize fourth quadrant and second quadrant
operation in terms of performance, pressure fluctuations and
mechanical vibrations. Experimental campaigns allowed the
acquisition of performance curves of a centrifugal compressor
in deep choke up to reverse pressure and indicated a large
region of safe operating conditions at low peripheral Mach
number. In stable reverse flow, the trends of work coefficient
and pressure ratio were acquired for a large range of flow rates
together with the evolution of vibration and pressure fluctuation
along the speed lines. Pressure ratio and absorbed power for the
different modes of operation can be very important to simulate
dynamic scenarios far from the steady state operation and size
accordingly compressor protection equipment and predict
accurately compressor startup torque from pressurized
condition (Fourth Quadrant Operations). The main frequency
content and amplitude of pressure variations within the flow
field or the radial/axial vibrations at the bearings are important
to estimate blade loading and possible presence of excitation
frequencies in the system. In the reported experience the
operating points characterized by stable reverse flow rates close
to the nominal one in direct flow did not show features much
2
more critical than in standard conditions (Second quadrant
operation) . This result may be hopefully used to reduce BOP in
early design phase of system layout.
INTRODUCTION
The so called four quadrant operation can occur in both
axial and centrifugal compressors if the direction of flow or the
sign of the pressure difference across the compressor reverses.
In normal working conditions, the operating point stays inside
the classical portion of the performance map, with positive flow
(from impeller inlet to outlet), compression ratio higher than
unity and positive torque (rotor is absorbing power). In case of
failures of some parts of the system, emergency operations such
ESD or transient operating conditions in general flow rate,
pressure difference between inlet and outlet sections or speed
can assume opposite values. Under such conditions, both
compressor equipment (in particular impeller blades) and all
other system devices experience unusual loading levels. Shaft
torque may reverse in case of negative rotation.
The term four quadrant operation is used to indicate all the
possible operating condition of a rotating machine. Deep choke
(fourth quadrant) and deep surge with reverse flow (second
quadrant) represent important four quadrant operations. The
first is characterized by direct flow and rotation but reverse
pressure and is likely to occur during compressor start up. In
deep choke unusual high mass flow is forced across the
machine due to the high inlet pressure and axial thrust across
bearings may overcome capacity. Second quadrant is
characterized by direct speed and pressure with reverse flow:
gas is driven backward by an overwhelming discharge pressure
typical during ESD or valve failures causing unusual blade and
equipment loading.
Fourth and second quadrant characteristics have been
experimentally investigated for a centrifugal compressor stage
and, after a brief introduction about the four quadrant operating
map, the results are presented.
FOUR QUADRANT MAP
In normal working conditions the operating point stays
inside the portion of the performance map called normal
operating map delimited by the surge and choke lines.
Figure 1 Four Quadrant Compressor Map
Normal operating conditions are characterized by positive
flow (from impeller inlet to outlet), compression ratio higher
than unity and positive torque (rotor is absorbing power). Speed
rotation is ranging from about 60% up to 110% of the nominal
value (blue lines in figure 1). In transient operating conditions,
flow rate, pressure ratio between inlet and outlet sections or
speed can vary outside the normal operating map. Different
areas can be identified in the four quadrant operating map of
the compressor.
Deep Choke/Fourth quadrant: direct speed, direct flow,
reverse pressure
In Figure 1, the area located on the right of the choke line
is characterized by a pressure coefficient decreasing to zero
(deep choke) or even negative values (4th
quadrant) for
progressively increasing flow coefficients. Fourth quadrant is
characterized by a downstream pressure lower or equal to the
upstream, while speed rotation and flow are still in the same
direction of design conditions. Deep choke area is likely to be
entered by the compressor during start-up when one or more of
last stages can operate in 4th
quadrant. The proper
understanding of the basic physics at right of the choke line, in
particular for power absorption is mainly important for the
sizing of start-up devices.
Deep Surge/Second quadrant: direct speed, reverse
flow, direct pressure
The area of the performance map on the left of the surge
line in Figure 1 can be divided in two sections: the first is
characterized by positive flow with positive slope in 1st
quadrant. This region is generally unstable considering the
standard volumes involved in typical systems and the operating
points reside in this region for very small periods of time. The
second area (2nd
quadrant) is characterized by negative flow
and negative slope in the pressure coefficient versus flow plane
and is characterized by a stable behavior. This region is usually
referred to as second quadrant, characterized by positive
rotation, negative flow and positive speed. The physics of
second quadrant is used for the simulation of ESD and the
sizing of surge avoidance and control devices.
Reverse Rotation
During transient operations rotational speed can be
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