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9. What is surge-1.ppt Surging protection techniques
- 3. ©
2008
Compressor
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3
• Flow reverses in 20 to 50 milliseconds
• Surge cycles at a rate of 0.3 s to 3 s
per cycle
• Compressor vibrates
• Temperature rises
• “Whooshing” noise
• Trips may occur
• Conventional instruments and human
operators may fail to recognize surge
Surge description
- 5. ©
2008
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• From A to B…….20 - 50 ms…………….. Drop into surge
• From C to D…….20 - 120 ms…………… Jump out of surge
• A-B-C-D-A……….0.3 - 3 seconds……… Surge cycle
• Compressor reaches surge point A
• Compressor loses its ability to make
pressure
• Suddenly Pd drops and thus Pv > Pd
• Compressor surges -“Plane goes to stall”
Qs, vol
Pd
Machine shutdown
no flow, no pressure
• Electro motor is started
• Machine accelerates
to nominal speed
• Compressor reaches
performance curve
Note: Flow goes up faster
because pressure is the
integral of flow
• Pressure builds
• Resistance goes up
• Compressor “rides” the curve
• Pd = Pv + Rlosses
• Because Pv > Pd the flow reverses
• Compressor operating point goes to
point B
• Result of flow reversal is that pressure goes
down
• Pressure goes down => less negative flow
• Operating point goes to point C
• System pressure is going down
• Compressor is again able to
overcome Pv
• Compressor “jumps” back to
performance curve and goes to
point D
• Forward flow is re-established Pd = Compressor discharge pressure
Pv = Vessel pressure
Rlosses = Resistance losses over pipe
• Compressor starts to build pressure
• Compressor “rides” curve towards surge
• Point A is reached
• The surge cycle is complete
Developing the surge cycle on the
compressor curve
Pd
Pv
Rlosses
B A
C
D
- 6. ©
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Factors leading to onset of surge
• Startup
• Shutdown
• Operation at reduced throughput
• Operation at heavy throughput with:
– Trips
– Power loss
– Operator errors
– Process upsets
– Load changes
– Gas composition changes
– Cooler problems
– Filter or strainer problems
– Driver problems
• Surge is not limited to times of reduced throughput.
• Surge can occur at full operation
- 8. ©
2008
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Surge parameter based on invariant
coordinates Rc and qr
– Flow measured in suction (DPo)
– Ps and Pd transmitters used to calculate Rc
• The antisurge controller UIC-1 protects the compressor
against surge by opening the recycle valve
1
UIC
VSDS
Compressor
1
FT
1
PsT
1
PdT
Discharge
Suction
• Opening of the recycle valve lowers the resistance felt by the
compressor
• This takes the compressor away from surge
Basic Antisurge Control System
2
Rc
qr
Rprocess
Rprocess+valve
- 9. ©
2008
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9
A
Rc
B
• When the operating
point crosses the SCL,
PI control will open the
recycle valve
• PI control will give
adequate protection
for small disturbances
SLL = Surge Limit Line
SCL = Surge Control Line
qr
2
Antisurge Controller Operation Protection #1
The Surge Control Line (SCL)
• PI control will give stable control during steady
state recycle operation
• Slow disturbance example
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2008
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Antisurge Controller Operation Protection #2
The Recycle Trip® Line (RTL)
Benefits:
– Energy savings due to
smaller surge margins
needed
– Compressor has more
turndown before
recycle or blow-off
– Surge can be
prevented for virtually
any disturbance
• Disturbance arrives - the
operating point moves
towards the SCL
SLL = Surge Limit Line
RTL= Recycle Trip® Line
SCL = Surge Control Line
Output
to Valve
Time
• When the operating point
reaches the SCL, the PI
controller opens the a/s
valve based on its
proportional and integral
action.
• The operating point
overshoots the SCL
until it reaches RTL
• When the operating
point hits RTL the
conclusion is:
– We are close to
surge
– The PI controller is
too slow to catch the
disturbance
– Move the valve now!
• An open loop response
is triggered
• Operating point moves
back to the safe side of
RTL
– The Open-loop
function should be
ramped out
– PI controller
integrates to stabilize
the operating point
on the SCL
Recycle Trip® Response
PI Control Response
• Total response of the
controller is the sum of
the PI control and the
Recycle Trip® action
PI Control
Recycle Trip®
Action
+
To antisurge valve
Total Response
Rc
Q
2
OP
- 11. ©
2008
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Controls
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Additional surge margin
• Benefits of Safety On® response:
- Continuous surging is avoided
- Operators are alarmed about surge
• Compressor can
surge due to:
– Transmitter calibration
shift
– Sticky antisurge valve
or actuator
– Partially blocked
antisurge valve or
recycle line
– Unusually large
process upset
Antisurge Controller Operation Protection #3
The Safety On® Response (SOL)
• If Operating Point
crosses the Safety On®
Line the compressor is
in surge
Rc
qr
2
SLL - Surge Limit Line
RTL - Recycle Trip® Line
SCL - Surge Control Line
• The Safety On®
response shifts the
SCL and the RTL to
the right
New SCL
New RTL
• Additional safety or
surge margin is added
• PI control and
Recycle Trip® will
stabilize the machine
on the new SCL
SOL - Safety On® Line
- 13. ©
2008
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13
Disturbance comes from the
discharge side
• Pd,2 increases
• Ps,2 remains constant
• Rc,2 increases
• Section 2 moves towards surge
Disturbance
• The system is oscillating
• Slowing down the
controller tuning would
lead to:
- Increased risk of surge
• Compressor damage
• Process trips
- Bigger surge margins
• Energy waste
Interacting Antisurge Control Loops
Rc,2
qr,2
2
R
Rc,1
qr,1
2
R
R
R
Antisurge controller UIC-1 will
open the recycle valve to protect
section 1 against surge
• Pd,1 decreases
• Ps,1 increases
• Rc,1 decreases
• Section 1 moves away from surge
Opening of recycle valve on section 1
caused Pd,1 = Ps,2 to decrease
Result:
• Ps,2 decreases
• Pd,2 remains constant
• Rc,2 increases
• Section 2 moves towards surge
Antisurge controller UIC-2 will open
the recycle valve to protect section 2
against surge
• Pd,2 decreases
• Ps,2 increases
• Rc,2 decreases
• Section 2 moves away from surge
Opening of recycle valve on section 2
caused Ps,2 = Pd,1 to increase
Result:
• Pd,1 increases
• Ps,1 remains constant
• Rc,1 increases
• Section 1 moves towards surge
1
PIC
2
UIC
1
UIC
VSDS
Section 1 Section 2
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2008
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• Also called:
– Throughput control
– Capacity control
– Process control
• Matches the compressor throughput to the
load
• Can be based on controlling:
– Discharge pressure
– Suction pressure
– Net flow to the user
Compressor Performance Control
- 16. ©
2008
Compressor
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PIC - SP
• Compressor operates in
point A
Pd
qr
2
Shaft
power
qr
2
Curve 1
A
Rprocess + Rvalve
• Required power in point
A is P1
Curve 1
P1
• Pressure is controlled by
blow-off
• Point B represents the point
that would deliver the
pressure for Rprocess
Curve 2
Rprocess
B
• Required power in point
B is P2
Curve 2
P2
• Power loss is P1 - P2
• Qloss represents energy
waste
Qloss
Notes:
• Most inefficient control
method
• Regularly found in plant air
systems
• Rare in other systems
• Not recommended
• Curve 2 represents:
• Lower speed on variable
speed systems
• IGVs closed on variable
geometry compressors
• Inlet throttle valve closed on
fixed speed compressors
Performance Control
by blow-off or recycle
PT
1
PIC
1
Process
- 17. ©
2008
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• Compressor operates
in point A
Pd
qr
2
Shaft
power
qr
2
Curve 1
A
Rprocess + Rvalve
• Required power is P1
Curve 1
P1
• Pressure is
controlled by
pressure drop over
valve
PIC - SP
Pressure loss
across valve
• Opening of valve
would reduce
resistance to Rprocess
Rprocess
• Lower resistance
would require less
speed and power
Curve 2
Curve 2
P2
• Power loss is P1 - P2
Notes:
• Extremely inefficient
(consumes approximately the
same power for every load)
• Rarely used
• Not recommended
• Curve 2 represents:
• Lower speed on variable
speed systems
• IGVs closed on variable
geometry compressors
• Inlet throttle valve closed on
fixed speed compressors
Performance Control
by discharge throttling
PT
1
PIC
1
Process
- 18. ©
2008
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• Inlet valve
manipulates suction
pressure
Pd
qr
2
Shaft
power
qr
2
• Changing suction
pressure generates a
family of curves
Suction valve open
Suction valve throttled
• Pressure is controlled
by inlet valve position
PIC - SP
• Compressor operates
in point A for given
Rprocess
A
Rprocess
• Required power is P1
P1
Notes
• Common on electric
motor machines
• Much more efficient
than discharge
throttling
• Power consumed
changes proportional
to the load
• Throttle losses are
across suction valve
Performance Control
by suction throttling
PT
1
PIC
1
Process
- 19. ©
2008
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Controls
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• Change of guide vanes
angle a results in
different compressor
geometry
Pd
qr
2
Shaft
power
qr
2
• Different geometry
means different
performance curve
amin
aOP
amax
• Pressure is controlled
by inlet guide vane
position
PIC - SP
• Compressor operates
in point A for given
Rprocess
A
Rprocess
• Required power is P1
P1
P
T
1
PI
C1
Process
Notes:
• Improved turndown
• More efficient than
suction throttling
• Power consumed is
proportional to the load
• Power loss on inlet
throttling is eliminated
Performance Control
by adjustable guide vanes
- 20. ©
2008
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• Changing speed
generates a family of
curves
Pd
qr
2
Shaft
power
qr
2
Nmin
NOP
Nmax
• Pressure is controlled
by speed of rotation
PIC - SP
• Compressor operates
in point A for given
Rprocess
A
Rprocess
• Required power is P1
P1
P
T
1
PI
C
1
Process
SI
C
1
Notes
• Most efficient:
(Power f(N)3)
• Steam turbine, gas
turbine or variable
speed electric motor
• Typically capital
investment higher than
with other systems
• No throttle losses
Performance Control
by speed variation
- 21. ©
2008
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• While controlling one primary variable, constrain the
performance control on another variable
• Exceeding limits will lead to machine or process damage
• Performance controller controls one variable and can limit
two other variables.
Limiting control to keep the
machine in its stable operating zone
CONTROL BUT DO NOT EXCEED
Discharge Pressure Max. Motor Current
Suction Pressure Max. Discharge Pressure
Net Flow Min. Suction Pressure
Suction Pressure Max. Discharge Temperature
- 23. ©
2008
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Compressor networks
Control system objectives for
compressors in parallel:
• Maintain the primary performance variable
(pressure or flow)
• Optimally divide the load between the
compressors in the network, while:
– Minimizing risk of surge
– Minimizing energy consumption
– Minimizing disturbance of starting and stopping
individual compressors
• Compressors are often operated in parallel
and sometimes in series
- 24. ©
2008
Compressor
Controls
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Compressor networks
• The purposes of networks include:
– Redundancy
– Flexibility
– Incremental capacity additions
• Often each compressor is controlled, but the
network is ignored
• Compressor manufacturers often focus on
individual machines.
• A “network view” of the application is
essential to achieve good surge protection
and good performance control of the
network.
- 25. ©
2008
Compressor
Controls
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Notes
• All controllers are
coordinating
control responses
via a serial network
• Minimizes recycle
under all operating
conditions
Process
1
UIC
VSDS
Compressor 1
VSDS
Compressor 2
Suction
header
1
LSIC
2
UIC
out
RSP
Serial
network
out
RSP
2
LSIC
1
MPIC
Serial
network
Serial
network
Equidistant Loadsharing
Flow Diagram for Control Process
- 26. ©
2008
Compressor
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• Machines operate at same Rc since suction and
discharge of both machines are tied together
PIC-SP
• The DEV is a dimensionless number
representing the distance between the
operating point and the Surge Control Line
• Lines of equal DEV can be plotted on the
performance curves as shown
0.1
0.2
0.3
DEV = 0
0.1
0.2
0.3
• Machines are kept at the same relative distance
to the Surge Control Line (SCL)
• This means in practice the same DEV for both
machines
DEV1 DEV2
• Recycle will only start when all machines are
on their SCL
• Since DEV is dimensionless all sorts of
machines can be mixed: small, big, axials,
centrifugals
• The DEV will be the same for all machines
but they will operate at different speeds and
flow rates
SCL = Surge Control Line
Rc,1
qr,1
2
Rc,2
qr,2
2
Compressor 1 Compressor 2
Dev1 = Dev2
Q1 = Q2
N1 = N2
Notes:
• Maximum turndown (energy savings) without recycle or blow-off
• Minimizes the risk of surge since all machines absorb part of the
disturbance
• Automatically adapts to different size machines
• CCC patented algorithm
Equidistant Loadsharing
Parallel Compressor Control
Editor's Notes
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