Advanced Anti surge Control System for Turbine Driven Centrifugal Compressors

Sci.Int.(Lahore),27(3),1845-1854,,2015 ISSN 1013-5316; CODEN: SINTE 8 1845
May-June
ADVANCED ANTI-SURGE CONTROL ALGORITHM FOR TURBINE DRIVEN
CENTRIFUGAL COMPRESSORS
Arslan Ahmed Amin, Khalid Mahmood-Ul-Hasan
Department of Electrical Engineering, University of Engineering and Technology, Lahore
Corresponding Author: arslan_engineer61@yahoo.com
ABSTRACT: Centrifugal compressors are widely used for natural gas transportation by increasing the pressure of the gas.
Surge is an inherent phenomenon in a centrifugal compressor, defined as reversal of fluid flow which can possibly damage the
machine. Anti-surge control is an important control for safety of the compressor and it can keep the compressor always
running in the safe area right to the surge line. In this paper, simulation results of the surge phenomenon occurring in a
centrifugal compressor in various conditions such as high header pressure, low suction pressure, start-up and emergency
shutdown of the unit are presented and their control strategies are discussed. Dynamic simulation of the centrifugal
compressor is carried out in HYSYS. Conventional PID and Advanced (PI plus Recycle Trip) controllers are implemented to
check their performance. Advanced controller prove to be superior to simple PID controller in protecting the compressor in
severe surge case. Start-up and shut down surge was prevented by fully opening the anti-surge control valve.
Key Words: Surge Limit Line, Surge Control Line, Anti-Surge Controller, Deviation, Advanced PI plus RTL Response,
Surge Parameter, Emergency Shutdown
INTRODUCTION
High pressure is required for the gas to be transported to
commercial and industrial areas from remote well head
places through pipelines. Compressors are used for this
application to raise the pressure of the gas. A complete
compression process consists of turbo-compressor unit,
scrubbers, piping, control valves and coolers. [1] The
incoming gas is first filtered to remove the foreign and dust
particles, then it is passed to scrubbers where liquid droplets
are removed to protect compressor from liquid entry damage.
The compressed gas is then passed through coolers to allow
the high temperature to decrease.
Surge
Surge is an inherent phenomenon in centrifugal compressors
which can possibly damage the machine. It is defined as
reversal of fluid flow. At this point, compressor peak head
capability is reached and this is the point of minimum flow.
Below it, the severe oscillations in flow and discharge
pressure are created producing huge noise, large vibrations
and consequently costly damage to the machine making the
entire system unstable. At surge point, flow separation
occurs inside the impellers making it unsteady and changes
its direction. In order to avoid the problems associated with
surge, an anti-surge control system is used to maintain a safe
operating volumetric flow through the compressor. [2]
As shown in Fig. 1, the lower limit of the flow is termed as
surge limit and the higher limit of flow is termed as choke
limit (also stonewall limit). Above choke limit, the fluid flow
reaches the speed of sound at a given speed and no further
increase in flow becomes possible. The stable flow of the
compressor is between these two limits. [3]
Figure 1. Compressor Characteristic Curve [3]
Surge is a cyclic phenomenon i.e., surge repeats unless a
control action is taken to bring compressor out of surge. A
complete surge cycle is shown in Fig. 2 in which the flow
reversal and recovery is shown. It is a high speed
phenomenon i.e., flow reversals can occur in less than 150
milliseconds. The intensity of surge varies from application
to application and is proportional to the density of the fluid.
Higher pressure and higher molecular weight applications
can result in greater mechanical damage. [1-3]
Figure 2. Surge Cycle [3]

1846 ISSN 1013-5316; CODEN: SINTE 8 Sci.Int.(Lahore),27(3),1845-1854,,2015
May-June
The time domain profile of the surge is shown in Fig. 3
which shows that during surge condition flow reverses
periodically and discharge pressure fluctuates.
Figure 3. Time Domain Profile of Surge [5]
Causes of Surge
In the operation process of the compressor, surge mostly
occurs during abnormal operating conditions such as
pressure building in main header [P(header) > P(Discharge)],
cut off / lower demand of flow, low suction pressure, start-up
of unit and emergency shutdown of unit. The other possible
causes of surge can be inlet valve failure that result in low
flow, blockage of inlet filter, high pressure that is caused by
the failure of outlet valve, failure or blockage of anti-surge
control valve and human failure. [1-3]
Surge Prevention Techniques
The occurrence of surge can be prevented effectively by
maintaining a certain minimum flow above a specified
margin from the surge limit to keep the compressor running
in stable zone of operation. This minimum flow is generally
set at 1.1 times of the surge line flow in the same pressure
ratio. This can be achieved by opening a blow-off valve at
discharge line of compressor or by operating a recycle valve
in the discharge process system. Blow off causes waste of
expensive process fluid, therefore, recycling is preferred and
mostly carried out through electronic 4-20mA operated
control valve which is termed as anti-surge control valve. [1-
3]
Anti-Surge Control System
Anti-surge control system is designed to determine the surge
condition in compressor and to operate the anti-surge control
valve in efficient and speedy manner to protect centrifugal
compressor from surge. Fig. 4 represents a complete anti-
surge control system as implemented in local gas compressor
station.
are as follows:
Surge Detection and Control Algorithm: The main
algorithm governing the protection of the compressor by
taking input from suction and discharge transmitters,
calculating surge parameter and deviation and generating
output to anti-surge control valve for flow recycle.
Surge Limit Model: The surge limit model represents the
surge pints at various speeds of the compressor. These are
usually provided by vendor in datasheets and are also
determined at site during commissioning of anti-surge
controllers.
Figure 4. Anti-Surge Control System [3]
The components required in a complete surge control system
Actuation System: The anti-surge control valve or recycle
valve receives input in terms of 4-20mA from anti-surge
controller and protects the compressor. The selection of the
right type of valve and its size is very important for effective
surge control system.
System Instrumentation: The instrumentation includes
suction pressure transmitter, suction temperature transmitter,
suction flow orifice meter, discharge pressure transmitter and
discharge temperature transmitter. Theses transmitters
provide electronic 4-20mA signals to anti-surge controller
according to their measuring scales. The selection of proper
type of instrument is also very important for effective anti-
surge control system.
Piping System: The compressor system piping determines
the response time requirement for the recycle valve. Piping
volume also influence the process control operation
(precision of control and speed of response). [3]
Advanced Anti-Surge Control Algorithm
Avoidance control is the most commonly used surge control
strategy for centrifugal compressors. In this control strategy,
a control line termed as Surge Control Line (SCL) is defined
at some distance from surge line called the surge margin and
the operating point is restricted to the right of this control
line. The distance between surge line and surge control line
should not be too close, normally 10%-20%, because
actuator‟s response time is not fast enough and fluctuations
in operating point can drive the compressor into surge easily
when operating conditions change. However, the compressor
efficiency is maximum near the surge limit line. Hence in
order to increase the compression efficiency, compressor‟s
operating point should be close to surge line as far as
possible. [1, 2]
In more advanced control system, a backup line closer to the
surge line can also be defined which when crossed generates
more aggressive action from the controller. Additionally a
safety line can be added at surging conditions that if it is
crossed it will further increase the surge margin in order to
avoid surges in the future. These lines are shown in Fig. 5
below. [3]

May-June
Figure 5. Safety and Control Lines in Control Algorithm [3]
The surge point is dependent on multiple parameters such as
the molecular weight and flow into the compressor. In order
to describe the surge line it is beneficial to use a coordinate
system that is invariant or nearly invariant of the inlet
conditions. There are several possible coordinate systems
that satisfy the required conditions and one of the mostly
used is reduced polytrophic head versus the reduced suction
flow rate squared. [4]
Compressor pressure ratio „Rc‟ is defined as [5]:
s
d
C
P
P
R  …….………………. 1
Where „Pd‟ is discharge pressure of compressor and „Ps‟ is
suction pressure of compressor. Parameter polytrophic head
exponent „ ‟ is defined as[5]:













s
d
s
d
P
P
log
T
T
log
 ………………. 2
Where „Td‟ is discharge temperature of compressor and „Ts‟
is suction temperature of compressor. Reduced pressure head
„hr‟ which incorporates both temperature and pressure effects
is defined as [5]:
 


1R
h c
r

 …………………. 3
Mostly, orifice type flow meter is used in industry for flow
measurement applications due to its accuracy and simplicity.
In this algorithm, reduced flow „qr2‟ is used which is the
ratio of differential pressure across orifice plate „∆Pos „ as
given by flow transmitter and the suction pressure „Ps‟ as
given by suction pressure transmitter [5].
Ps
Pos
2qr ……………………..4
Surge Parameter (Ss) is the ratio of SLL flow value
corresponding to current OP flow value and the current OP
flow value itself as shown in Fig. 6. It will be less than one
for stable operation and greater than one for unstable
operation. [5]
oprq ,
2
sll,r
2
q
Ss  ……….……………5
Figure 6. Surge Parameter ‘Ss’ Calculation [5]
Ss < 1 Stable operating zone
Ss= 1 Surge Limit Line (SLL)
Ss > 1 Surge region
Deviation parameter (DEV) is defined as [5]:
d = 1 – Ss…………….……………6
DEV = d – surge margin = 1 – Ss – surge margin……7
Where surge margin is normally 10% additional margin for
safety purpose. Its corresponding calculation is depicted in
Fig. 7.
Figure 7. Deviation Parameter ‘DEV’ Calculation [5]
DEV > 0 Good
DEV = 0 Surge Control Line
DEV < 0 Bad
Hence, the main purpose of the algorithm is to determine the
DEV parameter and to keep it positive.
PID Controller
The general control equation for a PID controller is given by
[6]:
where:
OP(t) = controller output at time t
E(t) = error at time t
Kc= proportional gain
Ti= integral (reset) time
Td= derivative (rate) time
The stability of a system is a very important aspect to
consider when designing control schemes. Improper tuning
parameters can cause the oscillatory or even unstable
response of a system. [6]
"Tuning" a control loop is a term used for optimum
adjustment of PID controller parameters proportional gain
Kc, Reset Time Ti, derivative time Td for the desired control
……...8

May-June
response. There are several methods for tuning a PID loop
e.g., "Ziegler-Nichols method", “Cohen- Coon Method” and
“Hit and Trial Method” etc. Now-a-days, manual methods of
tuning a PID controller are no longer used due to
advancement of software technology. Many industrial
softwares are now available which accurately model the
process and then provide optimum values of these parameters
for the user defined response. HYSYS has built-in PID
controller tuning feature which is very beneficial for getting
the proper tuning parameters with the facility of Online
changing the parameters. [6]
Advanced Controller
In Advanced Controller, the control action is split into two
actions: PI action and Recycle Trip action. [5]
PI action will deal with small and steady state disturbances
while Recycle Trip Action will cope with sudden and quick
process disturbances as depicted in Fig. 8. [5]
Figure 8. PI Plus Recycle Trip Response for Anti-surge Control
Valve [5]
Two lines are introduced right to the SLL for this control
algorithm: Surge Control Line (SCL) and Recycle Trip Line
(RTL) as shown in Fig. 9.
PI action will be initiated when OP will touch the SCL while
RTL-action will be activated by controller when OP will
touch the RTL. RTL will generate an open loop response i.e.,
step opening response until the OP returns to the safe area.
The magnitude of RTL response will be equal to the
derivative of the surge parameter „Ss‟ i.e., greater the rate of
change, the greater will be the step response magnitude. [5]
Figure 9. Surge Control Line (SCL) and Recycle Trip Line
(RTL) [5]
Total response will be the sum of PI control response and
Recycle Trip Response as shown in Fig. 10. [5]
Figure 10. Output to Anti-surge Control Valve [5]
Please note that this derivative action (RTL) is separate from
derivative action of a normal PID controller as this D-action
is only concerned with opening of control valve whereas in a
PID controller, derivative action takes part in both opening
and closing of the valve. [5]
Methodology
Dynamic simulation of a centrifugal compressor was carried
out in HYSYS. Surge conditions were simulated for high
header pressure, low suction pressure, start-up and
emergency shutdown of the unit. Conventional PID and
Advanced (PI plus Recycle Trip) controllers were
implemented to check their performance to protect the
compressor from surge in different scenarios. The model of
compression system implemented in HSYSY is shown in
Fig. 11.
Figure 11. Model of Centrifugal Compressor Implemented in
HYSYS
Model Validation:
The parameters for the centrifugal compressor, scrubbers,
heat exchanger, control valves and piping were taken from
Gas Compressor Station. Comparison between values
obtained from HYSYS and actual values obtained from gas
compressor station was carried out to check the validity of
the model and great similarity was found. Table 1 shows the
comparison of some important values obtained from field
with those obtained from the model and are quite similar
which validates our HYSYS model.

May-June
Table 1: Validation of HSYSY Model of Centrifugal
Compressor Station
Parameter
Practical
Value
HYSYS
Value
Units
Suction Pressure, Ps 1538 1528 kPag
Suction Temperature, Ts 33.6 33.81 degC
Suction Flow, DeltaP 13.65 13.37 kPag
Discharge Pressure, Pd 2973 2925 kPag
Discharge
Temperature, Td
98.9 88.27 degC
Compressor Speed 7746 7746 rpm
Suction Throttle 99.66 100 %
ASV Position 0 0 %
RESULTS AND DISCUSSIONS
Simulation of Surge Scenarios
System Parameters:
Rated (100 %) Speed of Turbo-compressor Unit = 9500 rpm
70 % (Minimum Governor) Speed of Turbo-compressor
Unit= 6650 rpm
Surge limit Flow Level at 70% Speed = 5000 act_m3/hr
Surge Control Line Level (1.1 times Surge Limit Level) =
5500 act_m3/hr
RTL Response Level (6% from Surge Limit Level) = 5300
act_m3/hr
Note:
In this simulation, gas composition is assumed constant and
simulation is carried out with fixed speed of centrifugal
compressor which is the minimum governor speed i.e., 6650
rpm. Therefore, for simplicity, a flow based controller is
implemented as surge controller taking input PV from
compressor inlet and output is the anti-surge control valve.
The controller maintains the flow at 1.1 times the surge flow
at the given compressor speed through two ways discussed
separately: simple PID control action and Advanced PI plus
RTL action.
Surge Parameter at SLL = 1
Surge Parameter at SCL = 0.826
Deviation, DEV at SLL= -0.173
Deviation, DEV at SCL = 0
Hence, the safe range of surge parameter is less than 1 and
safe range for DEV parameter is greater than -0.173.
The parameters incorporated for controllers are mentioned as
under:
PID Controller Parameters:
Set Point = 5500 act_m3/hr
Gains: Kc = 0.3, Ti = 0.5 min / rep, Td = 0.083 min
In tuning the PID controller, assistance from HSYSY built-in
PID Auto Tuning feature was taken for optimum, fast and
stable response.
Advanced Controller Parameters:
Set Point = 5500 act_m3/hr
Gains: P = 0.5, Ti = 0.1 min /rep
RTL = Depends upon rate of change of Ss (10% optimum set
in our simulation case. Greater than 10 % causes unstable
response with hunting in ASV and less than 10 % causes
slow response making system ineffective for fast surge
protection.)
Case 1: Header Pressure > Discharge Pressure
Surge condition is simulated by closing the Discharge Valve
VLV-100 in a quick manner to increase the header pressure
for surge creation as shown in Fig. 12. As the header
pressure is increased from 2447 kPag to 2720 kPag, the
compressor goes into surge and flow starts fluctuating.
Figure 12. Surge Condition when Header Pressure > Discharge
Pressure
Case 2: Low Suction Pressure
The surge condition for low suction pressure was simulated
by closing the suction throttle valve in a quick manner as
show in Fig. 13. As the suction pressure is reduced from
1520 kPag to 1200 kPag, the surge occurs and flow starts
fluctuating first peak goes from 8564 m3/hr to -8107 m3/hr
near 1500th
second.
Figure 13. Surge Condition when Suction Pressure Decreases
Case 3: Start-Up of Unit
Surge condition during start-up of unit was simulated as
shown in Fig. 14. Speed of turbo compressor unit is
increased from 0 to 6650 rpm with closed ASV, surge occurs
and flow fluctuates from -6258 to 8480 and further continues
fluctuating making the system unstable.
Figure 14. Surge Condition during Start-up of Unit

May-June
Case 4: Emergency Shutdown (ESD) of Unit
Surge condition during ESD of unit is shown in Fig. 15. As
the unit shutdown was initiated, Suction throttle valve and
Discharge valve Val-101 were fully closed with closed ASV
and speed of the turbine was ramped down. When the speed
decreases from 6650 rpm to 3325 rpm, flow reversal takes
place from 8627 m3/hr to -3646 m3/hr. Thus the system
experiences surge.
Figure 15. Surge Condition during ESD of Unit
Anti-Surge Controller Simulation Results
Case 1: Response of ASC when Surge Occurs due to High
Header Pressure
Conventional PID Controller
When the header pressure is increased from 3281 kPag to
3548 kPag at 3100th
second, the flow reduces to 5049 m3/hr,
ASC generates output to open ASV to increase flow to the
set point 5500 m3/hr as shown in Fig.16. The system does
not experience surge in this case.
Surge parameter goes from 0.6 to 1.0 and DEV goes from
0.2 to -0.175 during low flow situation and restores to
normal values after control action by ASV.
Limitations of PID Control Action
As the header pressure is increased from 3282 kPag to 3600
kPag at 5860th
second in a quick manner to bring quick sever
surge, ASC generates 25% output) to open ASV to restore
flow but the flow reduces from 6235 m3/hr to 3299 m3/hr
and the compressor goes into surge as shown in Fig. 17.
After 02 cycles of surge, the system restores to the set point
with 33 % output to ASV.
0.18 to -1.47 in this case. The compressor experiences surge.
Advanced PI Plus RTL Controller
When the header pressure is increased from 3284 kPag to
3619 kPag at 4920th
second in a quick manner to bring sever
surge, ASC generates 27% output (17% PI + 10% RTL )in a
speedy manner to open ASV to restore flow and the flow
reduces from 6250 m3/hr to 5213 m3/hr, then restores to
5500 m3/hr as shown in Fig. 18. The compressor is protected
from surge.
Figure 16. Conventional PID Controller Response for Surge
Protection due to High Header Pressure
0.18 to -0.05, not crossing the limits, thus the system remains
stable. Advanced PI plus RTL response is superior in
performance and protection of centrifugal compressor.
Case 2: Response of ASC when Surge Occurs due to Low
Suction Pressure
Conventional PID Controller
As the suction pressure is decreased from 1827 kPag to 1686
kPag at 9140th
second, the flow reduces from 6279 m3/hr to
5406 m3/hr, ASC generates 7% output to open ASV to
restore flow to the set point 5500 m3/hr as shown in Fig. 19.

May-June
Figure 17. Limitation of Conventional PID Controller for Sever
Surge Case due to High Header Pressure
0.19 to -0.02 as shown in Fig. 19.
Limitation of Conventional PID Controller
kPag at 9500th
second in a quick manner to bring sever surge,
ASC generates 30% output to open ASV to restore flow but
the flow reduces from 6319 m3/hr to 3580 m3/hr and the
compressor goes into surge as shown in Fig. 20. After 01
cycle of surge the system restores to the set point with 33 %
output to ASV.
Surge parameter goes from 0.625 to 1.97 and DEV goes
from 0.2 to -1.15 as shown in Fig. 20.
Advanced PI Plus RTL Controller
kPag at 7520th
second in a quick manner to bring sever surge,
ASC generates 35% output (25% PI + 10% RTL ) in a
speedy manner to open ASV to restore flow and the flow
Figure 18: Advanced PI + Recycle Trip Controller Response for
Sever Surge Case due to High Header Pressure
reduces from 6320 m3/hr to 5071 m3/hr, then restores to the
set point 5500 m3/hr as shown in Fig. 21. The compressor is
protected from surge.
Response for Sever Surge Case due to Low Suction Pressure
for Sever Surge Case due to Low Suction Pressure
Surge parameter goes from 0.625 to 0.97 and DEV goes
from 0.2 to -0.15, then restores to the normal values and the
system remains stable as shown in Fig
Case-3: Protection from Surge during Start-up
ASV remain fully open as the speed of the turbine
compressor unit is increase from 0 rpm to 6650 rpm in
60seconds, the flow increases to maximum value 8681 m3/hr
and then becomes smooth at 8627 m3/hr as shown in Fig. 22.
No surge occurs in this case.

May-June
Figure 19. Conventional PID Controller Response for Low
Suction Pressure Surge
Case-4: Protection from Surge during Shut-down of Unit
As ESD command initiated, ASV fully opens and the speed
of the turbine compressor unit decrease to 0 rpm from 6650
rpm to 0 rpm, the flow decreases in line with the speed as
shown in fig 23. No surge occurs in this case.
CONCLUSION:
Anti-surge valve and Anti surge controller plays an
important role in the protection of centrifugal compressor
from surge. Surge is a speedy phenomenon, can occur during
start-up, emergency shutdown, high header pressure and low
suction pressure as explored in this study. Properly tuned
conventional PID controllers become ineffective when severe
surge occurs in a very less time, thus advanced control
becomes necessary. Advanced controller generates PI
response and a quick opening open loop response called as
recycle trip response which gives sudden opening to ASV
when RTL threshold is crossed. Advanced controller works
well in sever surge case and protects the compressor from
surge effectively. ASV should be opened fully during start-
up and ESD of the unit to protect from surge
Figure 20. Limitation of Conventional PID Controller
. 21.

May-June
Figure 21. Advanced PI + Recycle Trip Controller Response
Figure 22. Protection from Surge during Start-up of Unit
Figure 23. Protection from surge during ESD of Unit
.
REFERENCES
[1] Wen L., Gao L., and Dai Y., “Research on System
Modeling and Control of Turbine-Driven Centrifugal
Compressor”, 6th IEEE Conference on Industrial
Electronics and Applications, pp. 2090-2095, 2011.
[2] Wang C., Shao C., and Han Y., “Centrifugal Compressor
Surge Control Using Nonlinear Model Predictive
Control Based on LS-SVM”, IEEE Transaction on
Control Systems.
[3] Brun K., and Nored G.M., “Application Guideline for
Centrifugal Compressor Surge Control Systems”, Gas
Machinery Research Counsel Southwest Research
Institute, Release Version 4.3 April 2008.
[4] Bloch, H. P., “Compressors and Modern Process
Applications”, Hoboken, New Jersey: John Wiley &
Sons, 2006.
[5] Mirsky S., McWhirter J., Jacobson W., Zaghloul M., and
Tiscornia D., “Development and Design of Anti-surge
and Performance Control Systems for Centrifugal
Compressors” Proceedings of the Forty-Second Turbo
machinery Symposium October 1-3, 2012.
[6] Aspen HYSYS, “Dynamic Modeling Guide”, Aspen
Technology, Inc., Version 7.3, Burlington,
Massachusetts, 2011.
[7] Cooper D. J., “Practical Process Control Using LOOP-
PRO Software”, Control Station, 2005.
[8] Dimitrios V., and George P., “Industrial Compressor
Anti-Surge Computer Control”, World Academy of
Science, Engineering and Technology International
Journal of Mechanical, Aerospace, Industrial and
Mechatronics Engineering, 1,10, 2007
[9] Chen H., and Jiang L., “A New Anti-surge Study Based
on Fuzzy Self-Adaptation PID Controller”, Seventh
International Conference on Fuzzy Systems and
Knowledge Discovery (FSKD 2010), 1147-1151.
[10] Ren Y., Zhang L., Ye Y., Liang W., and Yang H.,
“Reliability Assessment of Anti-surge Control System
in Centrifugal Compressor”, Fourth International
Conference on Computational and Information
Sciences, 1240-1243, 2012.

May-June
[11] White R.C., and Kurz R., “Surge Avoidance for
Compressor Systems”, Proceedings of the Thirty Fifth
turbo machinery symposium, 2006.
[12] Aspen HYSYS, “Operations Guide”, Aspen
Technology, Inc., Burlington, Massachusetts, 2005.
[13] Control Station, “Fundamentals of Instrumentation and
Process Control”, Practical Process Control, 2005.
[14] Compressor Control Corporations (CCC), “Series 3 Plus
Anti-surge Controller for Axial and Centrifugal
Compressors”, Publication IM301 (6.2.1), Product
Version: 756-005, May 2009.

Advanced Anti surge Control System for Turbine Driven Centrifugal Compressors

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Advanced Anti surge Control System for Turbine Driven Centrifugal Compressors (20)

More from Arslan Ahmed Amin (7)

Recently uploaded (20)

Advanced Anti surge Control System for Turbine Driven Centrifugal Compressors