1. SURGE
Compressor surge is an unstable phenomenon where the gas flow forward and backward across the machine alternatively. This phenomenon occurs at certain conditions of flow, pressure ratio and rotational speed.
As a consequence some damages can occur:
shaft vibration
instability in axial thrust
bearings seizure
seals damage

2. MAJOR PROCESS PARAMETERS DURING SURGE
FLOW
TIME (sec)
PRESSURE
TIME (sec)
Rapid flow oscillations
Rapid pressure oscillations
with process instability
TEMPERATURE
TIME (sec)
Rising temperatures
inside compressor

3. SURGE FEATURES FLOW REVERSES IN 20 TO 50 MILLISECONDS
SURGE CYCLES AT A RATE OF 0.3 sec TO 3 sec PER CYCLE
COMPRESSOR VIBRATES
TEMPERATURE RISE
“WHOOSHING” NOISE OR “CLANKING” NOISE
CONVENTIONAL INSTRUMENTS AND HUMAN OPERATORS MAY FAIL TO RECOGNIZ SURGE

5. FACTORS LEADING TO ONSET OF SURGE
DURING NORMAL OPERATION WITH:
- High discharge pressure ( Increase of plant resistance )
- Reduction of suction pressure
- Increase of suction temperature
- Gas composition changes (Decrease molecular weight)
START-UP
SHUTDOWN

8. ANTISURGE SYSTEM
An antisurge system avoids that the machine operating point falls on the surge line.
Substantially the protection consists in the opening of a control valve that put in connection the compressor discharge to the suction.

9. As a consequence the “resistance curve” become the antisurge valve resistance itself.
This causes an increase in the suction flow and so the operating point moves away from the critical condition.
On the right of the surge line a protection curve, called “antisurge line”, is defined.

11. A compressor with a single stage, or, if multistage, realizing a limited compressor ratio and where the surge is determined by the first stage, can be regulated by a law of this simple type: h
PD-PS
> K
h= Inlet flow index
PD = Pressure discharge
PS = Pressure suction
K = Set-point that corresponds to the antisurge control line

12. SLL ρ Q SLL ρ Q SLL FOR A COMPRESSOR WITH ONE IMPELLER
speed
SLL FOR A COMPRESSOR WITH ONE IMPELLER Q
SLL ρ
speed
SLL FOR A COMPRESSOR WITH
MORE THAN ONE IMPELLERS

13. CONTROLL ALGORITHM
The controller set-point is K2. It represents the deviation margin beteween SLL and SCL. The margin is expressed in percentage of flow. The algorithm takes in account the deltaP measured across the flow element (indicated by “hs” that is proportional to square suction flow (hs = Q2).
The controller algorithm compensates continuously the hs signal according to the suction pressure variation, taking in account the suction pressure measured and comparing it with the design suction pressure.

14. THE SURGE LIMIT LINE (SLL) IS NOT A FIXED LINE.
IT CHANGES DEPENDING ON THE COMPRESSOR INLET CONDITIONS:
T P MW
THE ANTISURGE CONTROLL MUST PROVIDE A DISTANCE TO SURGE CALCULATION THAT IS INDEPENDENT OF ANY CHANGE IN INLET CONDITIONS
Generalmente il compressore è regolato in modo da conservare costante la pressione di mandata. In tale maniera il punto operativo in un diagramma (P,Q) non cambia al variare delle condizioni del gas all’aspirazione, mentre le curve caratteristiche “traslano” determinando anche la traslazione della curva di pompaggio. Il risultato di questo fenomeno è un non corretto controllo del pompaggio. Risulta opportuno rappresentare il campo di funzionamento in un piano che mi conservi costante la posizione della curva di pompaggio e faccia variare il punto operativo al variare delle condizioni di aspirazione del gas.
Si possono trovare delle coordinate adimensionali che danno curve di pompaggio invarianti con le condizioni del gas in aspirazione.

15. CONTROLL CURVES
This curve is drawing at 2% on the right of SLL. When the OP reach this line the controller generate a “Quick-Opening” signal to the valve.
Pressure
Ratio
Surge Limit Line
This curve is drawing at 5% on the right of SLL. When the OP reach this line the controller generate a “Step-Opening” signal to the valve.
Safety Protection Line o Recicle trip line
Safety Correction Line
Surge Control Line
Compressor
Operating
Point
This curve is drawing at 10% on the right of SLL. When the OP reach this line the antisurge controller is activated
Speed Line
Suction Flow

17. CHOKING
When the flow rate increases beyond a certain value it may happen that in some ducts is achieved the speed of the sound.
This phenomenon determine the pressure drops due to the friction and high incidence of the flow on the fixed and mobile ducts.
As a consequence there is an enormous reduction in efficiency.

18. SOME DEFINITIONS ABOUT PERFORMANCE CURVES ACCORDING TO NORM API 617

19. FIELD OF OPERATION
COMPRESSOR RATED POINT:
The intersection on the 100 percent speed curve corresponding to the highest capacity of any specified operating point.
MAXIMUM CONTINUOUS SPEED:
The speed at least equal to 105 percent of the highest speed required by any of the specified operating condition.
NORMAL OPERATING POINT:
The point at which usual operation is expected and optimum efficiency is desired.
STABILITY: The difference in capacity (in percent of rated capacity) between the rated capacity and the surge point at rated speed.
MINIMUM ALLOWABLE SPEED:
Is the lowest speed at which the manufacturer’s design will permit continuous operation.
NORMAL SPEED :
The speed corresponding to the requirements of the normal operating point.
100 PERCENT SPEED: is the highest speed required for any specified operating point above the normal speed curve.
TURNDOWN: The percentage of change in capacity (referred to rated capacity at the rated head).