World Academy of Science, Engineering and Technology

International Journal of Mechanical and Mechatronics Engineering

Vol:1, No:10, 2007

Industrial Compressor Anti-Surge Computer

Control
Ventzas Dimitrios, and Petropoulos George

The input signals are the suction flow differential pressure,

Abstract—The paper presents a compressor anti-surge control suction pressure and discharge pressure, see Fig. 2. A
system, that results in maximizing compressor throughput with temperature measurement corrects flow and speed at the
pressure standard deviation reduction, increased safety margin compressor suction.
between design point and surge limit line and avoiding possible Surge detection devices are static and dynamic ones. The
machine surge. Alternative control strategies are presented.
International Science Index, Mechanical and Mechatronics Engineering Vol:1, No:10, 2007 waset.org/Publication/327

pressure transmitter monitors the pressure and controls the

blow-off valve to avoid stall and surge. Surge cycles occur
Keywords—Anti-surge, control, compressor, PID control, safety, fast and can even go undetected if the applied instrumentation
fault tolerance, start-up, ESD.
and controls are too slow.
I. INTRODUCTION III. COMPRESSOR SURGE

T HE compressor is a multi-stage type; the incoming air is

compressed in the first stage and then passed through a
heat exchanger to be cooled off before being injected into the
Compressor surge occurs when the developed compressor
head (H) drops below the network resistance (discharge
pressure). Surge is a fluctuation of flow and pressure and it
next stage, see Fig. 1. causes overheating and damages; it may be violent enough to
Anti-surge control system design practices encompass the damage a compressor in a few cycles. Surging gives cyclic
proper sizing, selection, and location of piping of the recycle flow, back-flow, high vibrations, pressure shocks and rapid
loops, the recycle valves, volumes of vessels in the recycle temperature increase. Process design minimizes the piping
path, check valves, etc.. volume in the recycle and forward path [1].

Fig. 1 Compressor details

II. INSTRUMENTATION AND ACTUATORS

Fig. 3 Momentary flow reversal

Fig. 2 Required input signals

Ventzas Dimitrios is with Department of Informatics & Communications,

An increase in pressure across the compressor, results in a
TEI of Larissa, Larissa 41 100, Greece (e-mail: dventzas@uth.gr). momentary flow reversal, see Fig. 3. This surge cycle goes
Petropoulos George, Associate Professor is with Department of until a change is made in the process conditions. The
Mechanical Engineering, University of Thessaly, Greece (e-mail: controller operates a surge valve to maintain sufficient
gpetrop@uth.gr).

International Scholarly and Scientific Research & Innovation 1(10) 2007 554 scholar.waset.org/1307-6892/327
 World Academy of Science, Engineering and Technology
International Journal of Mechanical and Mechatronics Engineering
Vol:1, No:10, 2007

forward flow to prevent surge, see Fig. 4; the external causes-

effects are:
¾ bad machine design, system damage
¾ improper assembly, mispositioned rotor
¾ restriction in suction or discharge
¾ internal plugging of flow passages (fouling)
¾ process conditions upsets, sudden load change
¾ inadvertent loss of speed
¾ changes pressure,temperature,gas composition Fig. 5 Anti-surge controller
¾ instrument or control malfunction
¾ hardware malfunction (variable inlet vanes)
¾ de-tuned controller
¾ operator error
¾ the inability of the cold recycle valve to quickly and
effectively de-pressurise the large volume in the cold
recycle flow loop, etc
International Science Index, Mechanical and Mechatronics Engineering Vol:1, No:10, 2007 waset.org/Publication/327

Surge is detected by
1. the boundary layer reversal
2. the compressor 2nd harmonic excitation
The model features are inlet flow, gas parameters, molecular
weight, pressure and temperature, compression ratio, shaft Fig. 6a Pressure oriented anti-surge control system
power in steady-state and transient conditions.

Fig. 6b Flow oriented anti-surge control system

In the surge control map the compressor flow set point is

provided by the control line positioned to the right of the
Fig. 4 Actual surge test results of opening / closing the recycle /
suction valve
surge line which provides a safety margin for the surge PI
controller output to the surge valve to prevent the flow from
decreasing below the control line, resulting in a safe
IV. ANTI-SURGE COMPUTER CONTROL relationship between the control line and the surge line, see
The methods for accomplishing surge control are a blow-off Fig. 8. Surge control action is initiated at the control line by
to atmosphere or recirculation from the compressor outlet to opening the surge valve as required to maintain forward flow
the inlet, see Figs. 5, 6, 7. It tries to keep the flow and pressure through the compressor; this prevents a further shift of the
within the area under the curve, see Fig. 8. Look-ahead operating point to the left towards surge [4], [5].
setpoint, adaptive gain, open-loop response techniques are The method automatically compensates for changes in
incorporated. In case of imminent surge, monitored by ΔP(F), molecular weight, temperature, compressibility, pressure and
the anti-surge value opens, recirculates gradually the gas and compressor rotor speed. It utilizes a characterization of
returns it to non-surge region. The anti-surge control is compression ratio (Pd/Ps) vs compensated inlet flow function
capable of coping with rapid flow fluctuations and process (hs / Ps)½ as control parameters. The algorithm allows
gas variations. Surge controller in worst-case operating minimized recycle or blow-off flow. This method reduces the
conditions can result in inefficient compressor operation and initial cost and simplifies engineering, testing, operation, and
wasted energy [2]. maintenance [6].
Different control algorithms can be configured such as:
1. standard PID-type algorithms
2. generic antisurge controller
3. purpose-built anti-surge controller
4. advanced controller
5. predictive controller

International Scholarly and Scientific Research & Innovation 1(10) 2007 555 scholar.waset.org/1307-6892/327
 World Academy of Science, Engineering and Technology
International Journal of Mechanical and Mechatronics Engineering
Vol:1, No:10, 2007

6. a multivariable control system The software uses specialized turbine and compressor
7. DVP (Dynamic Valve Positioning) control functions. Software development for the control
8. Event-controlled an expected valve position system primarily involved configuring and testing proven
9. response line adaptation (up to 10 lines) software function modules. Transitions from “auto” to
10. temperature and/or speed correction “manual” or conversely “manual” to “auto,” are bumpless [3].
11. optimum control trimming of each actuator
12. Partial Opening: activated by the first surging.
13. Full opening: activated by "surge protection "
14. Event-controlled -change-over
15. ESD compressor protection
16. effectiveness of the hot and cold recycle loops
17. retardation-free pressure-limiting control
18. minimization of the dead-time
19. surge spike counter and alarm functions
International Science Index, Mechanical and Mechatronics Engineering Vol:1, No:10, 2007 waset.org/Publication/327

Fig. 9 Operating parameter trends for suction valve discharge (SDV)

- cold recycle (ASV) - suction valve (SDV)

Simulation extends data available, field experience and

engineering common-sense in:
—ESD while operating in steady-state
—Full closure of the compressor discharge valve
—Full closure of the compressor suction valve
Fig. 7 Control Strategy
A “proximity to surge” variable, SS [7], describes the
location of the compressor’s operating point as compared to
its surge limit; if SS < = >1, the compressor is operating in the
safe zone, at the surge limit. in the non-stable zone, see Fig. 9.

V. PID CONTROL
The controller needs to be slow for normal operating
conditions, but fast when needed to protect the compressor
from surge. Derivative action makes the system unstable.
Tuned PID algorithm might prove to be totally ineffective for
some disturbances varying in size, speed, etc. The look ahead
setpoint positioning (hover) is based on the rate of change
toward the surge line. Adaptive set point control prevents
surge until the cause of the instability can be corrected.
Process compressor capacity or load control comes from a
Fig. 8 Compressor performance map cascade loop which determines the speed setpoint.
The performance curves of compressors are flat near the
surge, i.e. a small pressure increase can drive quickly toward
Initial surge curves are derived from manufacturer data, see surge. The adaptive gain is a special PI anti-surge control
Fig. 8. Suction and discharge valve closure tests are algorithm that increases the speed of response without
performed at constant compressor speed. The operating curve distortion in normal conditions. The controller gain is
of a compressor is divided by the surge line into a stable and increased when the operating point is less than the anti-surge
an instable area caused by a decrease of the flow quantity or setpoint (2.5% < the surge control line).
an increase of the discharge pressure. Surge curve verification With larger and/or faster disturbances, it may be necessary
is needed, since the actual compressor surge limits differ to manipulate both the hot and cold recycle valves
significantly from the surge limits indicated by the compressor simultaneously. In such cases, the hot recycle valve will open
map and approximations and inaccuracies lead to non-reliable for a very short time, arresting the movement of the operating
control. point towards the surge limit. Thus, the control strategy
employed is to use the cold recycle loop exclusively to protect

International Scholarly and Scientific Research & Innovation 1(10) 2007 556 scholar.waset.org/1307-6892/327
 World Academy of Science, Engineering and Technology
International Journal of Mechanical and Mechatronics Engineering
Vol:1, No:10, 2007

against small disturbances and the hot recycle loop to protect hardware (IEC-65108) is safety integrity level certified to SIL
against large disturbances. Once the disturbance is 3, making it suitable for mission critical machines, can be
sufficiently arrested, the hot recycle valve will close to replaced on-line and maintains compliance to several
prevent overheating the gas at the compressor suction, while international standards. Process limit overrides, high
the recycle flow required for safe operation is slowly discharge temperature, high discharge pressure and low
transferred to the cold recycle loop. The two controllers (the suction pressure conditions have priority over other speed or
hot-recycle anti-surge controller and the cold-recycle anti- flow demands.
surge controller) collaborate to provide effective and efficient
anti-surge protection. VII. CONCLUSION
Initial tests revealed that the cold recycle valve, should be Anti-surge control results in standard deviation of master
adequate to protect against slow disturbances, while both the pressure fluctuation compared to pre-shutdown operation
hot and cold recycle valves may be required for protection reduced from 0.4 barg to 0.1 barg. Anti-surge curves fitted on
against fast disturbances. compressor controller increased safety margin between design
The PID controller acts on the error between the point of compressor and surge limit line by about 10%,
compressor’s operating point and the surge control line, and maximizing throughput from compressor and avoiding
no action is taken until the operating point moves to the left of possible machine surge with a wider margin from surge
International Science Index, Mechanical and Mechatronics Engineering Vol:1, No:10, 2007 waset.org/Publication/327

the control line. The purpose-built controller uses a control line.

combination of closed-loop, open-loop, and anticipatory
control responses, each with different tuning and control REFERENCES
points; these algorithms allow the controller to begin moving
[1] Gaston, J.R. "Turbocompressor Antisurge Control, New Solution for an
the recycle valve even before the operating point crosses the Old Problem", The American Society of Mechanical Engineers 92-GT-
surge control line resulting in higher effectiveness. 428, presented at the International Gas Turbine And Aeroengine
The controller modulates the cold recycle valve in an effort Congress and Exposition, Germany (June 1-4, 1992).
to move back into the stable operating zone; this results in [2] Rigoni, K.B. "Anti-Surge Control Algorithm", Dresser-Rand Control
Systems, 1999.
aggressive valve movement and unstable operation, see Fig. 9. [3] Keyur. G. Vora, Krishnan Narayanan, Interrupted production leads to
Anti-Surge Control & Surge Protector are available with many integrated turbo revamp, ends manual intervention during load changes,
PLCs, without the need for additional external hardware. (PTA/PX) Reliance Industries Ltd India, Advanced Application &
Research Compressor Controls Corp, Iowa, 2005.
[4] Boyce, M. P, Managing Power Plant Life Cycle Costs, International
VI. SAFETY, FAULT TOLERANCE, START-UP, ESD Power Generation, pp. 21-23, July 1999.
In order to improve responsiveness the system surge [5] Nakajima, S, Total Productive Maintenance, Productive Press, Inc, 2000.
protection features include: [6] Kmo Software Surge Protector and kmo Anti-Surge Control, Atlas
Copco, Borsig, Demag, Escher Wyss, GHH, Ingersoll Rand, Joy, KKK,
• Surge characterization MAN Turbo, Nuovo Pignone, PGW, Siemens PGI, Sulzer, 1995.
• Choice of Δp vs h or Pd/Ps-1 vs hc algorithms [7] Johncock, Allan, W. and Gaston, John R., TS3000 Surge Control,
• If a surge occurs, the surge safety margin readjusts Triconex Systems, Inc., 1994, La Marque, Texas.
• Active control line “hovers” to actual operating point
• Non-symmetrical opening and closing D. E. Ventzas (1956) is Electronic Engineer and Professor of
• Equal percentage valve linearization Technological Institute of Larissa, Greece. He owns an MSc
• Recycle valve “dumps” or opens immediately in Control Eng and a PhD in Microprocessor based
• Independent P term forces recycle valve open Instrumentation from Bradford University, Yorkshire, UK.
He was Instrument and System engineer in Hellenic
• speed monitor Aspropyrgos Refinery, Athens. His research interests are
• autonomous over-speed protection Control and Instrumentation, Biomedical engineering and
• sequence-of-event reporting (1 ms) Computer Tools for Instrumentation. He is SMISA, MTEE.
• fault-tolerant strategy and configuration
The startup of the turbine is accomplished by initially G. Petropoulos (1959) graduated from the Department of
setting the maximum limit for the inlet valve. The extraction Physics, School of Applied Sciences at the Aristoteles
University of Thessaloniki, Greece, in 1983. He was awarded
valve, is adjusted to satisfy the initial “horsepower demand.” a PhD in Mechanical Engineering from the School of
On ESD, the anti-surge valves are opened and go full open Engineering, Department of Mechanical Engineering, in 1991.
based on their stroke times. Shaft deceleration rate on ESD is From 1984 until 1993 he worked as a research assistant and as
4% per second. Fail-safe fallback strategies are used in a research associate in the Laboratory for Machine Tools and
Machine Dynamics in the Department of Mechanical Engineering. From 1992
transmitter failures.
until 1995 he was a visiting Professor of Tribology in the Technological
The master discharge pressure controller is not reliable, Educational Institute of Piraeus. Since 1995 he is Lecturer and Director of the
should be very fast and designed for continuous override Laboratory of Manufacturing Processes in Dynamics of Manufacturing
control and safely implemented by engineers with little Processes in the Department of Mechanical and Industrial Engineering at the
University of Thessaly. The research interests and activities of G. Petropoulos
compressor control experience. Two out of Three (2oo3)
involve Theory and Technology of Machining Processes, Tribology,
voting architecture, maximise system availability without Metrology, Machine Vibrations. He is a member of the Union of Greek
compromising safety. The application (IEC-61511) software Physicists, EEEE (Hellenic Operational Society, a member of IFORS), Balkan
(IEC-1131) offers on-line programme changes, on-line Tribological Association, ASME (American Society of Mechanical
diagnostics with 100% system emulation capabilities. The Engineers), Greek Metallurgical Society. He is reviewer in the Journal of
Industrial Lubrication.

International Scholarly and Scientific Research & Innovation 1(10) 2007 557 scholar.waset.org/1307-6892/327