1

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