.
SURGE
Surge is defined as “self oscillation of pressure and flow,
often including a flow reversal”
Every centrifugal or axial comp has a characteristics
combination of max.head and and min.flow beyond
which it will surge.during surge there is actual
reversal of flow accompanied by pressure drop .
.
SURGE DESCRIPTION
. Flow reverses in 20 to 50 milisec.
.Surge cycle at a rate of .3 to 3 sec.
.Comp. Vibrates
. Temperature rises.
.Whooshing noise
.trips may occur
COMPRESSOR OPERATING CURVE
+ VE SLOPE OF OPERATING CURVE
Hp
3
MIN.STABLE FLOW POINT
.
DUE TO LOW FLOW ,IT NO LONGER
CAN ADHERE TO SUC. SIDE OF BLADE
SO STALLING OF FLOW
2
1
MAX.STABLE COMP. FLOW POINT
WHEN FLOW AT I/L REACHES SONIC
VEL.NO FURTHER FLOW INCREASE
IS POSSIBLE
1.
2.
3.
STONEWALL POINT
NORMAL POINT
SURGE POINT
Q
DEVELOPMENT OF
SURGE CYCLE
Pd
R loss
Pv
Pd comp. Disch press.
Pv vessel press.
Pd=Pv+Rloss
DEVELOPMENT OF
SURGE CYCLE
.
.
B
.pressure builds
.resistancr goes up
.comp. Rides the
curve
Pd
.electro motor is started
C
.machine acc.to normal apeed.
Comp.reaches performance curve
.flow goes up faster becoz
Press. Is integral Of flow
A
D
Qs
DEVELOPMENT OF
SURGE CYCLE
.
.
Pd
B
.COMP. REACHES
SURGE POINT A
.comp loses its ability to make
pressure
.suddenly Pd drops and Pv>Pd
A
C
D
.because Pv>Pd
The flow reverses
.comp. Operating
point goesTo B
Qs
DEVELOPMENT OF
SURGE CYCLE
.
.
Pd
B
A
C
.result of flow reversal
Is that pressure goes down
.presuure goes down ==less
negative flow
.operating pont goes to c
D
Qs
.
From A TO B
From C TO D
ABCDA
.
20-50 ms
20-120 ms
.3 TO 3 sec
B
.comp. Starts to build pressure
.comp rides curve toward surge
.point A is reached
.surge cycle is complete
Pd
C
.system pressure is going down
.comp is again able to overcome Pv
.comp jumps back to performance
curve to d
.forward flow established
A
D
Qs
EFFECTS OF SURGING
1. RAPID FLOW OSCILLATION AND PRESSURE
OSCILLATION CAUSES PROCESS IN STABILITY
2. RISING TEMP. INSIDE COMPRESSOR
3. TRIPPING OF COMP.
4. MECHANICAL DAMAGE
A. RADIAL BEARING LOAD
DURING INITIAL PHASE OF SURGING SIDE LOAD
IS PLACED ON ROTAR WHICH ACT PERPENDICULAR TO AXIS
B. THRUST BEARING LOAD
DUE TO LOADING AND UNLOADING
C. SEAL RUBBING
D.STATIONARY AND ROTATING PART CONTACT
IF THRUST BEARING OVERLOADED
BASIC ANTISURGE CONTROL SYSTEM
.ANTISURGE CONTROLLER PROTECT
THE COMP AGAINST SURGE BY
OPENING RECYCLE VALVE
.THIS REDUCES RESISTANCE
(BACK PRESS)
THIS TAKES THE COMP.AWAY
FROM SURGE
SUCT
DISCHARGE
R
R PROCESS
R PROCESS
+VALVE
Q2
SURGE LINES
STOPPING SURGE REQUIRE THE ABILITY TO KNOW
THE IMPENDING SURGE
WHICH MEANS KNOWING WHERE IS THE OPERATING POINT
RELATIVE TO SURGE POINT
SO SURGE CONTROL LINE IS DEFINED FOR SET POINT OF
ANTISURGE CONTROLLER
SLL
Hr
SCL
SLL SURGE LIMIT LINE
SCL SURGE CONTROL LINE
b
b. Is the distance between
two lines(margin of safety)
Q2
Protection 1
ANTISURGE CONTROLLER OPERATION
CLOSE LOOP CONTROL PI
WHEN OPERATING POINT CROSSES
SCL,PI CONTROLLER WILL OPEN
RECYCLE VALVE
SLL
SUITABLE FOR SMALL DISTURBANCE
AND STADY STATE
B
A
SCL
Protection 1
WHY CLOSE LOOP CONTROL IS INEFFECTIVE
. Since surging is fast so for PI controller to be fast
b shuld be high.
. But increasing b will limit comp. Operation in lower
thruput and unnecessary will recycle
SLL
b
SCL
Speed can also be increased
by adding derivative value,
but usingPID will open valve
in response to any movement
Toward surge,even if op. point
Is away from limit.
Protection 1
ANTISURGE CONTROLER OPERATION
ENHANCING THE EFFECTIVENESS OF PI CONTROLLER
BY ADAPTIVE GAIN
.WHEN THE OPERATING POINT MOVES FAST
TOWARD SCL ,(by calculating time derivative
of operating point)ADAPTIVE GAIN MOVES
THE SCL TOWARD OPERATING POINT
SLL
.THIS ALLOWS THE PI CONTROLLER TO
REACT EARLIER
.AS A RESULT SMALL B CAN BE ACHIEVED
WITHOUT SACRIFICING RELIABILITY.
b
SCL
Protection 2
ANTISURGE CONTROLER OPERATION
RECYCLE TRIP LINE
.WHEN PI CONTROLLER IS TOO SLOW
TO CATCH THE DISTURBANCE
.OPERATING POINT HITS RTL
.OPEN LOOP RESPONSE IS TRIGGERED
RTL
SLL
C
B
A
TOTAL
SCL
O/P
VALVE
PI
RTL
TIME
Protection 2
ANTISURGE CONTROLER OPERATION
RECYCLE TRIP LINE
.OPEWRATING POINT MOVES BACK
TO SAFE SIDE OF RTL
-RT FUNCTION DECAYS OUT STEP RESPONSE
- PI CONTROLLER INTEGRATE TO STABILIZE
ON SCL
RTL
SLL
C
B
.RESONSE OF CONTROLLER IS SUM OF
PI CONTROLLER AND RTL ACTION
A
TOTAL
SCL
O/P
VALVE
PI
RTL
TIME
Protection 3
ANTISURGE CONTROLER OPERATION
SAFETY ON LINE (SOL)
SOL SLL SCL
RTL
. IF OPERATING POINT CROSSES
THE SOL LINE THE COMP IS IN SURGE
.SO RESPONSE SHIFTS THE SCL AND
RTL TO RIGHT
.SO ADDITIONAL SURGE MARGIN IS
ADDED
.
BENEFITS OF SO
1.ADDITIONAL SURGING IS
AVOIDED BY LARGE SURGE
MARGIN
2.ALARM OF SURGE IS
PRODUCED(this has to reset
for normalisation)
Antisurge valve requirements
.VALVE SIZING
-must be large enough to prevent surge under all possible
operating condition
- excessive large valve will provide poor control precision
and lead to stonewall.
- 1.8Cv cal< Cv sel<2.2Cvcal
Cvcal correspond to point at intersection of mas speed
curve to SLL
.VALVE CHARACTERSTICS
- it can operate anywhere between 0% to 100%.
-linear characterstics is preffered.
Antisurge valve requirements
STROKE SPEED
-FAST STROKING SPEED IS VERY IMPORTANT
recommended full stroke time
size
close to open
1” to 4”
1 sec
6 “to 12”
2sec
16” up
3 sec
open to close
<3 sec
<5 sec
<10 sec
MISCELLENEOUS
.adequate air supply to power the valve
.tubing run shuld be minimized to reduce lag time
. One or more vol .booster are required to esure fast
response and equal opening and closing time
.fail position shuld be open
.
THANKS