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Efficient
retrofit
he refrigeration station of the MEK-benzene dewaxing
unit at Beijing Yan Shan Petrochemical Corporation
Refinery Business Division was built in 1969 together
with the main unit. Its main duty is to use ammonia as a
refrigerant to provide a cold source for the MEK/benzene
crystallisation section. The main equipment for the refrigeration station are five ammonia refrigeration compressors:
two for -20 ˚C evaporation system with refrigeration capacity
of 10.5 MJ/hr/unit ammonia refrigeration compressor; and
three for -42 ˚C evaporation system with refrigeration capacity of 5.6 MJ/hr/unit ammonia refrigeration compressor.
All five units were motor-driven centrifugal compressors
made by Borsig, Germany. Since the original antisurge control system for the ammonia compressors of the refrigeration station was outdated, the antisurge control system
could not realise precise automated control. To avoid compressor surge during operation at a reduced load, operators
had to open the antisurge valves to ensure sufficient recycle
flow, thus the compressors would operate away from the
surge point. This unnecessarily increased the energy consumption of the compressors. Consequently, the antisurge
control system’s electrical and auxiliary equipment had relatively high energy consumption. In order to reduce the energy consumption, to ensure the safe operation of the compressors and achieve optimum operation, the Refinery
Business Division retrofitted the antisurge control system of
the ammonia compressors in the refrigeration station.
T
The original antisurge control
Surge is a unique phenomenon of the centrifugal compressor. Any centrifugal compressor according to its geometric
size at a given speed, had a maximum operating pressure
and a corresponding minimum flow. When the flow to the
compressor is lower than this minimum value, surge will
occur.
The original antisurge control scheme (before retrofit) of
the ammonia compressor, is shown in Figure 1, (using the
-20 ˚C evaporation system’s ammonia compressor, C-501
as example).
The process control flow shows the ammonia compressor has two antisurge control valves. One is the cold flow
antisurge valve (UV-501). The ammonia gas at the final discharge of the ammonia compressor is returned to the
ammonia separation drum at the inlet of the compressor
after cooling.
The other was the hot flow antisurge valve (HV-501).
The ammonia gas at 123 ˚C from the final discharge is
returned directly to the ammonia separation drum of the
compressor inlet. The cold recycle antisurge control valve
Guan Hong Quan, SINOPEC
Beijing Yan Shan
Petrochemical Corporation
Refinery, China, discusses
the energy efficient retrofit,
based on the CCC control
system, for a methylethylketone (MEK)-benzene dewaxing unit refrigeration station.
(UV-501) used the suction pressure of the first stage inlet
pressure as a control signal. The opening of the control
valve should ensure that the inlet flow was constant and the
compressor was not surging. The remotely operated hot
flow antisurge control valve (HV-501) was to be used when
load fluctuated dramatically or during startup, when the load
had not yet been established, ensuring the normal operation
of the compressor’s units.
The above mentioned antisurge control design used a
fixed limit flow antisurge method. The principle of this control method is to ensure the flow of the compressor is always
greater than a certain pre-set flow. This ammonia compressor is at constant speed and constant inlet pressure, thus
inlet flow is also constant.
The advantage of this method is its simplicity. It uses relatively few instruments, and has high system reliability. The
disadvantages are that it acts too early at low compressor
load, has a large amount of recycle flow, and wastes
energy.
In order to protect the compressor, save as much
energy as possible, and avoid the frequent opening and
closing of the antisurge valve in actual operation, the control
of the compressor was often in manual operation, keeping
the control of the antisurge valve in manual. When the compressor load is reduced, motor power load was lower than
the rated power of 1250 kW, the antisurge valve would be
opened manually to make sure the compressor would not
operate below the rated condition. The control of the compressor would not realise automated precision control and
the compressor operation would not be stable. When the
ammonia system condensing pressure suddenly had a
large increase or the evaporation pressure suddenly
reduced due to the problem of the ammonia system, the
compressor inlet flow would be too small. It could not effectively avoid compressor surge. The compressor was not
operating safely.
Antisurge control after the retrofit
To reduce the energy lost from the entire system and to
operate the compressor safely, this retrofit replaced the
compressor’s antisurge control system. The theory of this
control system is variable constrain antisurge control. The
antisurge controller will control the compressor along the
safety operation line on the right hand side of the surge limit
line to minimise the opening of the antisurge valve and to
reduce the amount of energy lost. The control system
design is shown in Figure 2 (still using -20 ˚C evaporation
system ammonia compressor, C-501 as example).
Firstly the CCC (Compressor Controls Corporation,
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qV1 is the volumetric flow rate at compressor suction
conditions.
p2 is the absolute discharge pressure.
p1 is the absolute suction pressure.
T1 is the absolute suction temperature.
a and b are parameters determined by compressor characteristics.
If,
(
)
is less than,
compressor will not surge;
If,
is equal or higher than,
(
Figure 1. Antisurge control diagram of compressor
C-501 before retrofit.
)
compressor will go into surge state.
To further analyse the above antisurge protection equation, we have the following formula:
(2)
(3)
where:
A is the flow measuring device constant.
∆p1 is the differential pressure across flow measuring
device at compressor suction.
ρ1 is the gas density at compressor suction conditions.
M is gas molecular weight.
Z1 is the gas compressibility factor at compressor suction conditions.
R0 is the universal gas constant.
From formula (2) and (3) we get the following equation:
Figure 2. Antisurge control diagram of compressor
C-501 after retrofit.
P2/P1
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(4)
where:
is nearly a constant.
S3
S1
A
From equations (1) and (4) we get the following surge
limit equation,
S2
CCC defined antisurge control through a criteria ‘Ss’
4000
5000
6000
7000
(5)
qv(m3-h-1)
Figure 3. Surge lines for first section of
compressor C-501.
(proximity variable to surge):
where:
USA) solution is taken into consideration for antisurge control.
(6)
(1)
Rc is the compression ratio.
Suppose:
As shown in Figure 3, curve S1 is the surge limit line for
compressor C-501. Curve S2 is its surge control line. The
surge control line is on the right of the surge limit line, and
the horizontal distance between them is called the flow safety margin. Normally, the surge limit line is defined as follows:
where:
(7)
where:
K is the internal scaling parameter in CCC antisurge
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to the suction pressure as
shown in Figure 4.
P
With suction throttling, the
relationship between the disB
charge pressure of the ammonia compressor and mass flow
A
B1
will change on the straight line
between operating point A and
P1
B2
the origin. After suction throttling, the safe limit also moves
P2=0.8p
creating a larger stable operation range, reducing the possibility of the compressor operatP3=0.6p
ing in surge. With the operating
point already moved very close
to the control line (compared to
hot recycle control, cold recycle
control increases 3% over the
flow margin), and the load
needs to be reduced further.
q (kg-h-1)
v
Opening the cold recycle valve
(UV-502) recycled flow plus the
Figure 4. Regulate performance through suction throttle valve, p = discharge
flow required to meet the load
pressure.
will be slightly greater than the
flow at the surge point. This prevents the compressor from
controller.
going into surge. If the cold recycle valve is fully opened and
Substituting (7) into (6) we get:
still cannot make the operating point within the safe area, the
(8)
hot recycle valve (UV-503) will be opened for larger recycle
flow to avoid surge.
Adversely, when the load is increased, the inlet control
valve (UV-501) will be opened more. This is beneficial for antiIf Ss ≥ 1, the compressor will surge. So the line connectsurge control.
ing all the points where Ss is equal to 1 is the surge limit line.
Load balance among the parallel compressors was also
In order to allow the compressor to operate as close to the
realised through equalising individual ‘S’ values to optimise
surge point as possible, reducing energy consumption while
their performance.
ensuring the compressor will not go into surge, the CCC engiAnother significant improvement of the new control sysneer performed a surge test. During the commissioning after
tem is to automatically decrease the compressor load when
the compressor safety interlock system had been put into
motor current reaches its high limit. This limit control loop can
use, the surge test was performed by changing the compresprevent the driven motor from overload trip.
sor inlet or discharge pressure, pushing compressor operation very close to surge (or at the surge point). For the actual
compressor surge limit line (the curve S1 in Figure 3), take
The main scope of the retrofit
three surge points at different conditions and line them up
The main scope of the retrofit included:
smoothly.
Installation of CCC Series 4 antisurge control system.
Then, on the right hand side of the surge limit line by
According to the requirement of the CCC control system,
adding some flow safety margin (approximately 9%) to the
perfection of the measuring instrumentation on the comsurge limit line, we get the surge control line (the curve S2 in
pressors, including suction temperature, discharge temFigure 3). The antisurge valve will open along this line. CCC
perature at each section, pressure transmitters, and a
uses another criteria ‘S’(proximity to surge control line) to
final discharge flow measuring device. To reduce the
define the surge control line:
pressure lost at the compressor suction, the inlet flow
S = Ss + m
(9)
measuring openings were replaced with Venturi pipes.
where:
Replacement of five hot recycle antisurge valves and two
m is the safety margin applied to flow measuring signal.
cold recycle antisurge valves in the ammonia compressor
In the antisurge loop, S is the control variable and 1 is its
systems.
setpoint. When S is greater than 1, the antisurge loop will
For the purpose of ensuring the precision of the antisurge
begin to open the recycle valve. When S is less than 1, the
control and expanding the antisurge control range, the
antisurge loop will begin to close the recycle valve.
refinery discussed with CCC the final antisurge control
This control solution also uses the throttle control valve on
solution when the operating point runs closer to the surge
the suction line to regulate the discharge pressure of the sepprotection line. In this instance the cold recycle valve
aration drum (load share control).
would be opened first. If it cannot meet the antisurge
When the load is decreased, compressor suction presrequirement, the control will be on the hot recycle then
sure is decreased and the inlet control valve (UV-501) is used
inlet throttling butterfly valve. This would increase the antito throttle the flow. After suction flow is throttled, when rotasurge control range and maintain the precision of the antitional speed is constant, the characteristic of volumetric flow
surge control. This control solution cannot be realised in a
and compression ratio of ammonia compressor remains the
traditional control system.
same. As suction pressure decreases, the ammonia com Removal of five excitation panels for the synchronised
motor. Removal of five starting devices for the synchropressor mass flow and discharge pressure decreases in ratio
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nised motor. Removal of 10 control and switching panels
for the synchronised motors. Removal of five synchronised motor ventilation systems and five synchronised
motors. Removal of two auxiliary power switching panels
and original power and control cables.
Installation of five sets of induction motors, each with
1600 kW rated power. Installation of seven new power
switching panels, two operating consoles, installation of a
new lighting switching panel, installation of 6000 V induction motor power cables 500 m, and 5000 m instrumentation cables etc.
Conclusion
This retrofit was implemented in July 1999. The units were in
operation at once. It was in continuous operation for more
than one year.
The refrigeration station load optimisation and compressors antisurge control system were in fully automatic control
and the compressors and station operation were smooth.
According to the compressor operating curve display, the
compressors’ operating status can be observed very clearly
and easily. They reduced the workload of the operators, and
increased compressor reliability for safe operation and
process stability.
Great economical benefits were achieved by using a variable limiting flow control solution, effectively controlling the
recycle flow of the ammonia gas and greatly reducing energy
consumption of the compressors.
The process unit electrical power consumption was
greatly reduced:
The power consumption of October 1999 compared with
October 1998, reduced by 194 660 kWh.
The power consumption of November 1999 compared
with November 1998, reduced by 288 760 kWh.
The power consumption of December 1999 compared
with December 1998, reduced by 238 080 kWh.
The energy reduction was also due to the removal of the
original auxiliary equipment, such as the ventilator, excitation
equipment, starting devices, etc. The synchronised motors
were replaced by more efficient induction motors (the original
synchronised motor was rated 1500 kW at 179 A. The new
induction motor was 1600 kW at 182 A. Thus the rated power
increased by 100 kW, while the maximum current only
increased by 3 A.)
The MEK/Benzen dewaxing unit had an average processing capacity of 600 000 tpy. Calculating using the cost of
electricity as 0.55 Yuan/kWh. The refrigeration station can
reduce the power consumption by 1.56kWh per t of material.
The annual saving will be 1.56 x 600 000 x 0.55 = 514 800
yuan (1 US$ = 8.26 Yuan).
Another key benefit from retrofit is the avoidance of an
unscheduled shutdown. Before the retrofit, annual unscheduled shutdown ranged between 5 - 10 times. Effectively, there
has been no unscheduled shutdown after the retrofit. The
economic benefit is certainly more significant than the energy
savings alone.
References
1.
2.
3.
4.
5.
GUO QING TONG, BAI SHOU SAN, ZHAO WEI XIN, Cryogenics
Engineering Manual, Published by China Construction Industry
Publishing House, Beijing, 1985.
LU DE MIN, Petrochemical Automation Control Design Manual, 3rd
Edition, Published by Chemical Industry Publishing House, Beijing, 1993.
JIANG MAO SUN, YU QUAN SHOU, Process Control Engineering,
SINOPEC Publishing House, Beijing, 1988.
‘Using the Series 4 Antisurge Controller’, Publication UM4102 (3.3),
Compressor Controls Corporation, 2001.
‘Using the Series 4 Performance Controller’, Publication UM4104 (3.1.1),
Compressor Controls Corporation, 1999.
Compressor Controls Corporation
International Headquarters
4725 121-st Street
Des Moines IA 50323-2316 USA
Phone: 515-270-0857
e-mail: solutions@cccglobal.com
www.cccglobal.com
CCC Worldwide:
Houston, Texas USA - Rio de Janeiro, Brazil - London, UK - Amsterdam, The
Netherlands - Milan, Italy - Moscow, Russia - Singapore, Republic of Singapore
- Dammam, Saudi Arabia - Dubai, United Arab Emirates - Beijing, China
Publication #MS123
Reprinted from HYDROCARBON ENGINEERING NOVEMBER 2003