SURGE

Surge is defined as “self oscillation of pressure and flow,often including a flow reversal”

Every centrifugal or axial comp has a characteristicscombination 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

Hp

Q

1

2

3

2. STONEWALL POINT3. NORMAL POINT3. SURGE POINT

.

+ VE SLOPE OF OPERATING CURVE

MAX.STABLE COMP. FLOW POINT

MIN.STABLE FLOW POINT

WHEN FLOW AT I/L REACHES SONIC VEL.NO FURTHER FLOW INCREASE IS POSSIBLE

DUE TO LOW FLOW ,IT NO LONGER CAN ADHERE TO SUC. SIDE OF BLADESO STALLING OF FLOW

Pv

Pd

R loss

DEVELOPMENT OF SURGE CYCLE

Pd comp. Disch press.Pv vessel press.Pd=Pv+Rloss

.

.AB

C D

Pd

Qs

DEVELOPMENT OF SURGE CYCLE

.electro motor is started

.machine acc.to normal apeed.Comp.reaches performance curve.flow goes up faster becozPress. Is integral Of flow

.pressure builds

.resistancr goes up

.comp. Rides the curve

.

.AB

C D

Pd

Qs

.COMP. REACHES SURGE POINT A.comp loses its ability to make pressure.suddenly Pd drops and Pv>Pd

.because Pv>Pd The flow reverses.comp. Operating point goesTo B

DEVELOPMENT OF SURGE CYCLE

.

.AB

C D

Pd

Qs

.result of flow reversal Is that pressure goes down.presuure goes down ==less negative flow.operating pont goes to c

DEVELOPMENT OF SURGE CYCLE

.

.AB

C D

Pd

Qs

.system pressure is going down

.comp is again able to overcome Pv

.comp jumps back to performance curve to d.forward flow established

.comp. Starts to build pressure

.comp rides curve toward surge

.point A is reached

.surge cycle is complete

From A TO B 20-50 msFrom C TO D 20-120 msABCDA .3 TO 3 sec

EFFECTS OF SURGING

1. RAPID FLOW OSCILLATION AND PRESSURE OSCILLATION CAUSES PROCESS IN STABILITY2. RISING TEMP. INSIDE COMPRESSOR3. 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

DISCHARGESUCT

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

R

Q2

R PROCESS

R PROCESS +VALVE

SURGE LINES

STOPPING SURGE REQUIRE THE ABILITY TO KNOW THE IMPENDING SURGEWHICH MEANS KNOWING WHERE IS THE OPERATING POINTRELATIVE TO SURGE POINTSO SURGE CONTROL LINE IS DEFINED FOR SET POINT OF ANTISURGE CONTROLLER

Q2

HrSLL

SCL SLL SURGE LIMIT LINESCL SURGE CONTROL LINE

b b. Is the distance between two lines(margin of safety)

ANTISURGE CONTROLLER OPERATION

SLL

SCL

AB

CLOSE LOOP CONTROL PI

WHEN OPERATING POINT CROSSES SCL,PI CONTROLLER WILL OPEN RECYCLE VALVE

SUITABLE FOR SMALL DISTURBANCEAND STADY STATE

Protection 1

WHY CLOSE LOOP CONTROL IS INEFFECTIVE

b

. Since surging is fast so for PI controller to be fastb shuld be high.. But increasing b will limit comp. Operation in lower thruput and unnecessary will recycle

Speed can also be increased by adding derivative value,but usingPID will open valvein response to any movementToward surge,even if op. pointIs away from limit.

SLL

SCL

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

.THIS ALLOWS THE PI CONTROLLER TO REACT EARLIER

.AS A RESULT SMALL B CAN BE ACHIEVED WITHOUT SACRIFICING RELIABILITY.

SLL

SCL

b

Protection 1

Protection 2 ANTISURGE CONTROLER OPERATION

RECYCLE TRIP LINE

SLL

SCL

RTL

ABC

.WHEN PI CONTROLLER IS TOO SLOW TO CATCH THE DISTURBANCE.OPERATING POINT HITS RTL.OPEN LOOP RESPONSE IS TRIGGERED

O/PVALVE

TIME

TOTAL

RTL

PI

Protection 2 ANTISURGE CONTROLER OPERATION

RECYCLE TRIP LINE

SLL

SCL

RTL

ABC

.OPEWRATING POINT MOVES BACKTO SAFE SIDE OF RTL -RT FUNCTION DECAYS OUT STEP RESPONSE - PI CONTROLLER INTEGRATE TO STABILIZE ON SCL

.RESONSE OF CONTROLLER IS SUM OF PI CONTROLLER AND RTL ACTION

O/PVALVE

TIME

TOTAL

RTL

PI

Protection 3 ANTISURGE CONTROLER OPERATION

SAFETY ON LINE (SOL)

SOL SLL

RTL

SCL

. 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 SO1.ADDITIONAL SURGING ISAVOIDED BY LARGE SURGE MARGIN2.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 open to close 1” to 4” 1 sec <3 sec 6 “to 12” 2sec <5 sec 16” up 3 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