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© 2002 Compressor Controls Corporation
• 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
Qs, vol
Pd
Machine shutdownno flow, no pressure
• Electro motor is started• Machine accelerates
to nominal speed• Compressor reaches
performance curveNote: Flow goes up faster because pressure is the integral of flow
• Pressure builds• Resistance goes up• Compressor “rides” the curve• Pd = Pv + Rlosses
Pd = Compressor discharge pressurePv = Vessel pressureRlosses = Resistance losses over pipe
Developing the surge cycle on the compressor curve
Pd
Pv
Rlosses
B A
C
D
© 2002 Compressor Controls Corporation
• Rapid flow oscillations • Thrust reversals• Potential damage• Rapid pressure
oscillations with process instability
• Rising temperatures inside compressor
• Operators may fail to recognize surge
FLOW
TIME (sec.)
1 2 3
Major Process Parameters during Surge
PRESSURE
TIME (sec.)1 2 3
TEMPERATURE
TIME (sec.)1 2 3
© 2002 Compressor Controls Corporation
Surge description
• 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
© 2002 Compressor Controls Corporation
Some surge consequences
• Unstable flow and pressure• Damage in sequence with increasing
severity to seals, bearings, impellers, shaft
• Increased seal clearances and leakage
• Lower energy efficiency• Reduced compressor life
© 2002 Compressor Controls Corporation
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
© 2002 Compressor Controls Corporation
Objectives (user benefits)
1. Increase reliability of machinery and process• Prevent unnecessary process trips and
downtime• Minimize process disturbances • Prevent surge and surge damage • Simplify and automate startup and shutdown
2. Increase efficiency of machinery and process• Operate at lowest possible energy levels• Minimize antisurge recycle or blow-off• Minimize setpoint deviation• Maximize throughput using all available
horsepower• Optimize loadsharing of multiple units
© 2002 Compressor Controls Corporation
Calculating the distance between the SLL and the compressor operating point
The Ground Rule– The better we can measure the distance to surge, the
closer we can operate to it without taking risk
The Challenge
– The Surge Limit Line (SLL) is not a fixed line in the most commonly used coordinates. The SLL changes depending on the compressor inlet conditions: Ts, Ps,
MW, ks
Conclusion– The antisurge controller must provide a distance to surge
calculation that is invariant of any change in inlet conditions
– This will lead to safer control yet reducing the surge control margin which means:
• Bigger turndown range on the compressor
• Reduced energy consumption during low load conditions
© 2002 Compressor Controls Corporation
Coping with the high speed of approaching surge
• Increase overall system speed of response wherever feasible– Transmitters– Valves– Controllers– System Volumes
• Specialized Control Responses– Automated open loop (Recycle Trip)– Control loop decoupling– Adaptive surge control line– Adaptive gain
© 2002 Compressor Controls Corporation
Surge parameter based on invariant coordinates Rc and qr
– Flow measured in suction (Po)– Ps and Pd transmitters used to calculate Rc
1UIC
VSDS
Compressor
1FT 1
PsT1
PdT
• The antisurge controller UIC-1 protects the compressor against surge by opening the recycle valve
DischargeSuction
• Opening of the recycle valve lowers the resistance felt by the compressor
• This takes the compressor away from surge
Basic Antisurge Control System
Rc
qr2
Rprocess
Rprocess+valve
© 2002 Compressor Controls Corporation
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
qr2
Antisurge Controller Operation Protection #1 The Surge Control Line (SCL)
• PI control will give stable control during steady state recycle operation• Slow disturbance example
© 2002 Compressor Controls Corporation
Adaptive Gain Enhancing the Effectiveness of the PI Controller
A
Rc
B
• When the operating point moves quickly towards the SCL, the rate of change (dS/dT) can be used to dynamically increase the surge control margin.
• This allows the PID controller to react earlier.
• Smaller steady state surge control margins can be used w/o sacrificing reliability.
• Fast disturbance exampleQ2
© 2002 Compressor Controls Corporation
Antisurge Controller Operation Protection #2 The Recycle Trip® Line (RTL)
Benefits:– Reliably breaks the
surge cycle– 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
SLL = Surge Limit LineSCL-2 = Open Loop Line
SCL = Surge Control Line
Output to Valve
Time
Open-loop Response
PI Control Response
PI Control Step Change
+
To antisurge valve
Total Response
Rc
Q2
OP
© 2002 Compressor Controls Corporation
Improving the accuracy of Recycle Trip® open loop control
• Recycle Trip® is the most powerful method known for antisurge protection
• But, open loop control lacks the accuracy needed to precisely position the antisurge valve
• Open loop corrections of a fixed magnitude (C1) are often either too big or too small for a specific disturbance
• The rate of change (derivative) of the compressor operating point has been proven to be an excellent predictor of the strength of the disturbance and the magnitude required from the Recycle Trip® response
• Therefore, the magnitude of actual step (C) of the Recycle Trip® response is a function of the rate of change of the operating point or d(Ss)/dt
Antisurge Control
© 2002 Compressor Controls Corporation
d(Ss)dt
C = C1Td
Output to valve
Time
where:• C = Actual step to the valve• C1 = Constant - also defines
maximum step• Td = Scaling constant• d(Ss)/dt = Rate of change of the
operating point
Medium disturbance
PI ControlRecycle Trip®
Total
Large disturbanceOutput to valve
Time
PI Control
Recycle Trip®
Total
Benefits• Maximum protection
– No surge– No compressor damage
• Minimum process disturbance– No process trips
Recycle Trip®
Response calculation:
100%
0%
Recycle Trip® based on derivative of Ss
Antisurge Control
© 2002 Compressor Controls Corporation
After time delay C2 controller checks if Operating Point is back to safe side of Recycle Trip® Line- If Yes: Exponential decay of Recycle Trip® response.
Output to valve
Time
One step response
PI Control
Recycle Trip®
Total
100%
0%
C2
Multiple step responseOutput to valve
Time
PI Control
Recycle Trip®
Total
C2 C2 C2
What if one Recycle Trip® step response is not enough?
- If No: Another step is added to the Recycle Trip® response.
© 2002 Compressor Controls Corporation
Additional surge margin
Benefits of Safety On® response:Continuous surging is avoidedOperators 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 #3The Safety On® Response (SOL)
Rc
qr2
SLL - Surge Limit LineRTL - Recycle Trip® LineSCL - Surge Control Line
New SCL
New RTL
SOL - Safety On® Line
© 2002 Compressor Controls Corporation
• 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
© 2002 Compressor Controls Corporation
Pd
qr2
Shaft power
qr2
Suction valve open
Suction valve throttled
PIC - SPA
Rprocess
P1
PT1
PIC1
Process
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
© 2002 Compressor Controls Corporation
Pd
qr2
Shaft power
qr2
min
OP
max
PIC - SPA
Rprocess
P1
PT1
PIC1
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
© 2002 Compressor Controls Corporation
Pd
qr2
Shaft power
qr2
Nmin
NOP
Nmax
PIC - SPA
Rprocess
P1
PT1
PIC1
Process
SIC1
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
© 2002 Compressor Controls Corporation
• 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
© 2002 Compressor Controls Corporation
PIC-SP
Power limit
N1
A
R1
Qs, vol
Rc
B
R2
N2
R3
N4
D
N3
C
Benefits:• Maximum protection
– No machinery damage• Maximize production
– Machine can be pushed to the limits without risk of damage
Note: Same approach for othervariables (pressures,temperatures, etc.)
Power limiting with the Performance Controller
© 2002 Compressor Controls Corporation
• Interaction starts at B• Performance controller on
discharge pressure reduces performance to bring pressure back to setpoint
• Unless prevented, PIC can drive compressor to surge
• Antisurge controller starts to operate at B
• Even if surge is avoided, interaction degrades pressure control accuracy
• Results of interaction– Large pressure deviations
during disturbances– Increased risk of surge
AC
Po
PIC-SP
Rc
Ps
SLL
SC
L
B
Interacting Antisurge & Performance Loops
© 2002 Compressor Controls Corporation
Performance & Antisurge Controller’s interaction
• Both controllers manipulate the same variable - the operating point of the compressor
• The controllers have different and sometimes conflicting objectives
• The control action of each controller affects the other
• This interaction starts at the surge control line - near surge - and can cause surge
© 2002 Compressor Controls Corporation
Ways to cope with Antisurge andPerformance Loop interactions
• De-tune the loops to minimize interaction. Result is poor pressure control, large surge control margins and poor surge protection
• Put one loop on manual, so interaction is not possible. Operators will usually put the Antisurge Controller on manual. Result - no surge protection and often partially open antisurge valve
• Decouple the interactions. Result - good performance control accuracy, good surge protection and no energy wasted on recycle or blow off
© 2002 Compressor Controls Corporation
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,22
RRc,1
qr,12
R
RR
1
PIC
2UIC
1UIC
VSDS
Section 1 Section 2
© 2002 Compressor Controls Corporation
• All CCC controllers are connected on a serial network
1
PIC
2UIC
1UIC
VSDS
Section 1 Section 2
Serial network
Serial network
• This allows them to coordinate their control actions• When UIC-2 opens the recycle valve:
- Section 2 will be protected against surge- Section 1 will be driven towards surge
• How much section 1 is driven towards surge depends on how much the recycle valve on section 2 is opened
• The output of UIC-2 is send to UIC-1 to inform UIC-1 about the disturbance that is arriving
• UIC-1 anticipates the disturbance by immediately opening its valve
Note: The same applies when the antisurge valve on section 1 is opened first
Loop Decoupling between multiple Antisurge Controllers
© 2002 Compressor Controls Corporation
• Compressors are often operated in parallel and sometimes in series
• 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.
Compressor networks
© 2002 Compressor Controls Corporation
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
Compressor networks
© 2002 Compressor Controls Corporation
Process
PIC1
1UIC
VSDS
Compressor 1
2UIC
VSDS
Compressor 2
HIC1
Suction header
Swing machine
Base machine
Notes• All controllers act
independently• Transmitters are
not shown
Base Loading Flow Diagram for Control Process
© 2002 Compressor Controls Corporation
Rc,1
qr,12
Rc,2
qr,22
Compressor 1 Compressor 2
PIC-SP
Swing machine Base machine
QC,2= QP,2QC,1QP,1
where:QP = Flow to processQC= Total compressor flowQC - QP = Recycle flow
QP,1
QP,1 + QP,2 = QP,1 + QP,2
Notes:• Base loading is inefficient• Base loading increases the risk of surge since
compressor #1 will take the worst of any disturbance• Base loading requires frequent operator intervention• Base loading is NOT recommended
Base Loading Parallel Compressor Control
QP,2
© 2002 Compressor Controls Corporation
Process
PIC1
1UIC
Compressor 1
VSDS
Compressor 2
Suction header
Notes• Performance controllers
act independent of antisurge control
• Higher capital cost due to extra Flow Measurement Devices (FMD)
• Higher energy costs due to permanent pressure loss across FMD’s
1FIC
2FIC
2UIC
outout
out
RSPRSP
RSPRSP
RSPRSP
outout
RSPRSP
Equal Flow Division Loadsharing Flow Diagram for Control Process
VSDS
© 2002 Compressor Controls Corporation
Notes:• Requires additional capital investment in FMD’s• Requires additional energy due to permanent pressure loss
across FMD’s• Poor pressure control due to positive feedback in control system
(see next)• Equal flow division is NOT recommended
Rc,1
qr,12
Rc,2
qr,22
PIC-SP
QP,1 QP,2QC,2
Equal flow Equal flowQP,1 = QP,2
Equal Flow Division Loadsharing Parallel Compressor Control
Compressor 1 Compressor 2
where:QP = Flow to processQC= Total compressor flowQC - QP = Recycle flow
© 2002 Compressor Controls Corporation
Q2
Rc
N1
N3
N2
A
R1
PIC-SP
FIC-SP
R2
B
C
D
Notes• Causes instability near surge• Poor pressure control due to
positive feedback in control system
PIC1
OUTRSP
FIC1
OUTRSP
Master Slave
SIC
1
Dynamic Response / Pressure To Flow Cascade
© 2002 Compressor Controls Corporation
Notes• All controllers are
coordinating control responses via a serialnetwork
• Minimizes recycle under all operating conditions
Process
1UIC
VSDS
Compressor 1
VSDS
Compressor 2
Suction header
1
LSIC
2UIC
out
RSP
Serial network
out
RSP
2
LSIC
1
MPIC
Serial network
Serial network
Equidistant LoadsharingFlow Diagram for Control Process
© 2002 Compressor Controls Corporation
PIC-SP
0.10.2
0.3
DEV = 00.1
0.2
0.3
DEV1 DEV2
SCL = Surge Control LineRc,1
qr,12
Rc,2
qr,22
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 LoadsharingParallel Compressor Control
© 2002 Compressor Controls Corporation
Process
1AUIC
VSDS
Section 1
VSDS
Section 1
Suction Header
A
LSIC
outout
RSP
Serial network
RSP
B
LSIC
1
MPIC
Serial network
Serial network
Section 2
Section 2
2AUIC
1BUIC
1BUICSerial
network
Serial network
outout
Train B
Train A
• How to operate equidistant from the Surge Control Line (SCL) when there is more than one section per machine ???• Select per train -- in the loadsharing controller -- the section closest to the SCL• By selecting the section closest to the SCL it is guaranteed that the other section on the same train is not in recycle
Equidistant Loadsharing for multi-section compressors
• Share the load -- equal DEV’s for both trains -- on the section closest to the SCL
© 2002 Compressor Controls Corporation
Simplified P&ID for Compressors Operating in Parallel