Four Quadrant Centrifugal Compressor Performance .experimentally investigated for a centrifugal...

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Transcript of 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

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    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