CCC
Compressor Controls Corporation
A Presentation
by
K.VIVEKANAND
&
P..STEEVENSON
Developing the compressor curve
process limit
Rc
adding control
margins
maximum speed
surge limit
power limit
stonewall or
choke limit
Actual available
operating zone
stable zone
of operation
minimum speed
Qs,
vol
Developing the surge cycle on the compressor curve
•
From A to B
20 - 50 ms
•
From C to D
20 - 120 ms
•
A-B-C-D-A0.3 - 3 seconds Surge cycle
•
•
•
•
B
•
•
Drop into surge
Jump out of surge
Pd
Pv
Compressor reaches surge point A
Compressor looses its ability to make
pressure
Suddenly Pd drops and thus Pv > Pd
Plane goes to stall - Compressor surges
Pd
A
•
•
•
•
Because Pv > Pd the flow reverses
Compressor operating point goes to point B
C
Rlosses
Compressor starts to build pressure
Compressor “rides” curve towards surge
Point A is reached
The surge cycle is complete
D
•
•
•
Result of flow reversal is that pressure goes down
Pressure goes down => less negative flow
Operating point goes to point C
Machine shutdown
no flow, no pressure
Qs,
= Compressor discharge
pressure
Pv
= Vessel pressure
Rlosses = Resistance losses over pipe
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
•
•
•
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
vol
•
•
•
•
Pressure builds
Resistance goes up
Compressor “rides” the curve
Pd = Pv + Rlosses
Major process parameters during surge
FLOW
1
2
TIME (sec.)
2
TIME (sec.)
2
TIME (sec.)
•
Rapid pressure oscillations
with process instability
•
Rising temperatures inside
compressor
3
TEMPERATURE
1
Rapid flow oscillations
Thrust reversals
Potential damage
3
PRESSURE
1
•
•
•
3
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
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
Factors leading to onset of surge
•
Startup
•
Shutdown
•
Operation at reduced throughput
•
Operation at heavy throughput with:
-
•
Trips
Operator errors
Load changes
Cooler problems
Driver problems
-
Power loss
Process upsets
Gas composition changes
Filter or strainer problems
Surge is not limited to times of reduced throughput. Surge
can occur at full operation
Calculating the distance between the Surge
Limit Line 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
Building the Surge Limit Line
• Any curvature of the Surge Limit Line can be characterized as a
function of the ordinate hr
• The surge parameter is defined as:
f1(hr )
Ss = 2
qr ,op
• The function f1 returns the value of qr2 on the SLL for input hr
hr
hr
2
qr,SLL
2
qr
The surge parameter Ss
2
• The function f1 returns the value of qr on the SLL for input hr
• As a result:
Ss =
q2r,SLL
q2r,op
hr
Ss > 1
• Ss < 1
:
stable operating zone
• Ss = 1
:
surge limit line (SLL)
• Ss > 1
:
surge region
OP
hr
Ss < 1
2
2
qr,SLL
2
qr,op
OP = Operating Point
qr
Introducing the distance between the operating
point and the Surge Control Line
•
Step 1:
Introduce parameter
d = 1 - Ss
•
Step 2:
Introduce parameter
DEV = d - surge margin
•
The parameter DEV is independent of the size of the compressor
and will be the same for each compressor in the plant
hr
d =0
Ss = 1
Benefits:
d <0
DEV = 0
•
One standard surge parameter
in the plant
•
No operator confusion:
Ss > 1
DEV < 0
Ss < 1
•
DEV > 0
Good
d >0
•
DEV = 0
Recycle line
DEV > 0
•
DEV < 0
Bad
2
Surge margin
qr
The approach to surge is fast
Pd
1 SEC.
100%
100%
•
Typically, performance curves are
extremely flat near surge
•
Even small changes in compressor
pressure differential cause large
flow changes.
•
The speed of approaching surge is
high. In only 0.4 seconds, DPO
dropped by 14%, with a 2% change
in DPc
ABC
D
DPo
0
100%
AB C
DPc
0
A
Qs
0
Pd
D
The approach to surge is fast - another example
100%
DPo
0
1 sec.
100%
DPc
0
For a 2% increase in pressure differential
(DPc), flow rate DPo dropped 9% in 0.3 sec.
Basic antisurge control system
•
The antisurge controller UIC-1 protects the compressor against surge by
opening the recycle valve
•
Opening of the recycle valve lowers the resistance felt by the compressor
•
This takes the compressor away from surge
Rc
Rprocess
Rprocess+valve
VSDS
Compressor
FT
1
PsT
1
PdT
1
2
qr
Discharge
Suction
UIC
1
•
Surge parameter based on invariant
coordinates Rc and qr
– Flow measured in suction (DPo)
– Ps and Pd transmitters used to
calculate Rc
Antisurge controller operation
Protection #1: The Surge Control Line (SCL)
Rc
SLL = Surge Limit Line
SCL = Surge Control Line
•
When the operating point
crosses the SCL, PI control
will open the recycle valve
•
PI control will give adequate
protection for small
disturbances
•
PI control will give stable
control during steady state
recycle operation
•
Slow disturbance example
B
A
2
qr
Adaptive Gain
Enhancing the effectiveness of the PI controller
Rc
•
When the operating point
moves fast towards the SCL,
adaptive gain moves the SCL
towards the operating point.
•
This allows the PI controller
to react earlier
•
As a result a smaller steady
state surge control margin
can be achieved without
sacrificing reliability
•
Fast disturbance example
B
A
2
qr
Antisurge controller operation
Protection #2: The Recycle Trip® Line (RTL)
•
SLL = Surge Limit Line
Rc
Disturbance arrives the Operating Point (OP) moves
towards the SCL
RTL = Recycle Trip® Line
SCL = Surge Control Line
•
Benefits:
–
–
Energy savings due to smaller surge margin
Compressor has more turndown before recycle
or blow-off
Surge can be prevented for virtually any
disturbance
–
•
Operating point Moves back to the safe side of
the RTL
–
–
•
When OP hits SCL the PI controller opens valve based
on proportional and integral action
2
Output
to Valve
qr
•
•
Total response of controller is the sum of the PI control and
Recycle Trip action
Operating point keeps moving towards surge and
hits Recycle Trip Line (RTL)
Total Response
PI Control Response •
Recycle Trip® Response
Time
The RT function decays out the step response
PI controller integrates to stabilize OP on SCL
When the operating point hits the Recycle
Trip Line (RTL) the conclusion is:
–
–
–
•
We are close to surge
The PI controller is too slow to catch the
disturbance
Get out of the dangerous zone
An open loop response is triggered
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
Recycle Trip® based on derivative of Ss
Recycle Trip®
Response calculation
C = C1Td
where:
• C
• C1
• Td
• d(Ss)/dt
Output
to valve
Benefits
•
Maximum protection
– No surge
– No compressor damage
d(Ss)
dt
•
Minimum process disturbance
– No process trips
= Actual step to the valve
= Constant - also defines maximum step
= Scaling constant
= Rate of change of the operating point
Medium disturbance
Output
to valve
Large disturbance
100%
Total
PI Control
Total
PI Control
Recycle Trip®
0%
Time
Recycle Trip®
Time
What if one Recycle Trip® step response is not enough?
•
After time delay C2 controller checks if Operating Point is back to
safe side of Recycle Trip® Line (RTL)
– If Yes:
Exponential decay of Recycle Trip® response
– If No:
Another step is added to the Recycle Trip® response
Output
to valve
Output
to valve
One step response
Multiple step response
100%
Total
PI Control
Total
PI Control
Recycle Trip®
0%
Time
C2
Recycle Trip®
C2 C2 C2
Time
Antisurge controller operation
Protection #3: The Safety On® Line (SOL)
SOL = Safety On® Line
SLL = Surge Limit Line
RTL = Recycle Trip® Line
Rc
SCL = Surge Control Line
• Additional safety or surge margin is
added
• If Operating Point crosses the Safety
On® Line the compressor is in surge
• PI control and Recycle Trip® will
stabilize the machine on the new SCL
Compressor can surge due to:
• Transmitter calibration shift
• Sticky antisurge valve or actuator
• Partially blocked antisurge valve or
recycle line
• Unusual large process upset
New SCL
New RTL
Additional surge margin
2
qr
Benefits of Safety On® response:
• Continuous surging is avoided
• Operators are alarmed about surge
• The Safety On® response shifts the
SCL and the RTL to the right
Built in surge detector
Pressure and Flow Variations
During a Typical Surge Cycle
100%
• Surge signature is recorded during
commissioning
• Rates of change for flow and pressure
during surge are determined
• Thresholds are configured slightly more
conservative than the actual rates of
change during surge
Pd
0%
1 TO 2 SECONDS
100%
DPo
0%
20 to 50
milli-seconds
• Surge is detected when the actual rates of
change exceed the configured thresholds
• The following methods can be used:
• Rapid drops in flow and pressure
• Rapid drop in flow or pressure
• Rapid drop in flow only
• Rapid drop in pressure only
• When surge is detected a Safety On®
response is triggered
• A digital output can be triggered upon a
configurable number of surge cycles
Fall-back strategies for the antisurge and
performance controller
•
Antisurge controller
– If a pressure transmitter fails, a minimum q2r algorithm is used
– If a temperature transmitter fails, hr is characterized as a function
of compression ratio
– If the speed transmitter fails, a conservative speed setting is used
– If the flow transmitter fails
• Redundant transmitter is used
• Output is driven to:
– Last value OR
– Last Value selected: If Last Value >Pre-selected fixed value OR
Pre-selected fixed value selected: If Pre-selected fixed value>Last
Value
•
•
Performance controller
– Switches to redundant transmitter upon primary transmitter
failure
– Output goes to pre-selected value if all transmitters fail or is
frozen
All transmitter failures are alarmed
Compressor performance control
•
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
Performance control by speed variation
SIC
1
Pd
Rprocess
Process
A
PIC - SP
PT
1
Nmax
NOP
PIC
1
Nmin
•
2
Shaft
power
qr
P1
2
qr
Pressure is controlled by speed of rotation
•
Changing speed generates a family of curves
•
Compressor operates in point A for given
Rprocess
Required power is P1
•
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
Power limiting in the performance controller
an example
•
Rc
Power limit
•
•
R1
•
•
R2
R3
A
D
B
PIC-SP
C
•
N4
N
N2 3
N1
•
PIC would like to speed machine up to N4 and
operate in point D
Primary variable Pd
Limiting variable Power
Process resistance changes from R1 to R2
Compressor operates in point A for R1 at N1
•
PIC will speed machine up to N2 in order to control
pressure Pd
•
•
Machine hits power limit
Compressor operates in point B for R2 at N2
•
Process resistance decreases further to R3
However power limiting loop takes
control and controls machine at speed
N3
Compressor will operate in point C for
R3 at N3
Benefits
•
Qs,
Maximum protection
–
vol
•
No machinery damage
Maximize production
–
Machine can be pushed to the limits
without risk of damage
Note: Same approach for other variables (pressures, temperatures, etc.)
Limiting Ps or Pd using the antisurge controller
VSDS
Compressor
FT
1
Suction
•
•
PsT
1
UIC
1
PdT
1
Discharge
The antisurge controller can be configured to limit:
• Maximum discharge pressure (Pd)
• Minimum suction pressure (Ps)
• Both maximum Pd and minimum Ps
This does NOT conflict with antisurge protection
Operator Interface Specs.
Alphanumeric Displays
DEV
–
Displays the normalized distance between the operating point and surge limit line.The controller
will increase its output when DEV is negative and decreases it when DEV is positive.
–
Normally Blank. Displays Safety ON surge count when DISPLAY SURGE COUNT key is pressed.
–
Displays intended valve position
–
Default display is operating state (RUN, STOP or PURGE).A variety of measured and calculated
process variable displays can be selected by pressing MENU and SCROLL.
ALT
OUT
AUX
Operator Interface Specs.
Operator Keys
AUTO
MAN
–
Toggles controller between its Automatic and Manual operating modes.Auto or Manual LED is lit
to indicate currently selected state.
–
Increments and decrements the valve position (when in manual)
Reset Safety On
– Pressing this key zeroes the surge count and cancels any increased margin of safety added by the
safety on response.
– If the surge count is not zero, this key should not be pressed unless the cause of the surges has
been corrected.
Display Surge Count
– Causes ALT window to display the number of surges detected by Safety ON response.
Display Limit
– Temporarily displays the value and set point controller’s limiting variables in DEV and ALT
windows.
– If both limiting loops are enabled, their displays will alternate each time this key is pressed.
Operator Interface Specs.
Status LEDs
AUTO
(Green)
– Lit when controller is operating in Automatic mode.
Manual
(Yellow)
– Lit when controller is operating in Manual Mode.Pressing the Up and
Down arrow keys will then open or close the Antisurge valve.
RT (Yellow)
– Lit whenever the Recycle trip response is greater than zero.
– This response rapidly increases the recycle flow rate to protect against
rapid or close approaches to the surge limit. Once the danger is passed,
this added flow is allowed to decay.
– This response allows the compressor to safely operate close to its surge
limit , a Recycle trip should not be taken as a cause for alarm or operator
intervention.
SO (Red)
– Lit whenever the safety on surge count is greater than zero.
Operator Interface Specs.
Status LEDs
Limit(Yellow)
– Lit whenever one or both limiting variables are at or beyond their set points.
Tracking(Green)
– Flashes when the low output clamp is tracking a designated analog input(i.e.., when a remote
clamp has been assigned and that signal exceeds the internal output clamp.
TranFail (Red)
– Lit when one or more transmitters are outside the alarm limits.
Fallback (Yellow)
– Lit when a fallback control strategy is being used. This occurs when a required analog input
fails.
ComErr(Red)
– Lit when controller is not receiving serially communicated information that is has been
configured to use.
Fault (Red)
– Lit when controller fails to refresh watch-dog timer or detects an internal error which might
render its operation unreliable.
–
6
How an airplane wing develops lift
v1, p1
v2, p2
Bernoulli’s law
•
pstatic + 1/2rv2 + rgH = Constant
•
The term rgH is negligible for the wing
•
Then: pstatic + 1/2rv2 = Constant
•
•
•
Lift
Due to the shape of the
wing: v2 < v1 thus p2 > p1
As a result there is Dp or
lift
And the plane can
fly
Compressors
Previous
Rew
Fwd
Help
Menu
How the airplane develops stall
Lift
Lift
Lift
Lift
•
As the wing tilts back the Dv changes and thus the Dp
•
This leads to more lift
•
When the wing is tilted too much the streaming profile
suddenly changes from laminar to turbulent
•
The air no longer “sticks” to the wing and the lift is lost
•
The plane starts to fall down
Compressors
Previous
Rew
Fwd
Help
Menu
Thankyou