The document discusses compressor surge in centrifugal compressors. It defines surge as a condition where the flow of gas reverses direction in the compressor due to an imbalance between the compressor's delivery pressure and the downstream system pressure. This causes surging of gas flow at frequencies of 0.3 to 3 seconds per cycle. The document outlines factors that can lead to surge, provides a theoretical compressor characteristics curve to illustrate stable and unstable operating regions, describes the surge cycle in detail, and explains that the purpose of an antisurge control system is to prevent the operating point from reaching the surge limit line through controlled opening of an antisurge valve.

1. By
K Pavan Kumar
INTRODUCTION TO SURGE
IN CENTRIFUGAL COMPRESSOR.

2. INDEX:
• Effects of Compressor Surge.
• Factors Leading to Onset of Surge.
• Theoretical characteristic curve of compressor.
• Surge Description.
• Developing Surge Cycle on Performance curves.
• Purpose of Antisurge control system.

3. EFFECTS OF COMPRESSOR SURGE:
• 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.

4. 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

5. THEORETICAL CHARACTERISTICS CURVE XYZ:
Theoretical characteristics curve XYZ for a constant speed:
22-Jun-19 5
Mass flow
PressureRatio
0
1
X
Y
Z
S
O
X-Y: Unstable
Y-O; Stable
Y Maximum
Efficiency and
Pressure Ratio
X, Pressure head produced by the action of the
impeller on the gas trapped between the vanes
Onset of
Surge
Stonewall point
Maximum flow

6. THEORETICAL CHARACTERISTICS CURVE XYZ:
CONTINUED
Point X:
The centrifugal pressure head produced by the action of the impeller on the gas
trapped between the vanes. As flow is increased diffuser begins to influence the
pressure rise, for which the pressure ratio increases upto point B.
Point Y:
Efficiency approaches its maximum and the pressure ratio also reaches its
maximum. Further increase of mass flow will result in a fall of pressure ratio.
Point Z:
For mass flows greatly in excess of that corresponding to the design mass flow,
the gas angles will be widely different from the vane angles and breakaway of
the gas will occur. In this hypothetical case, the pressure ratio drops to unity
at ‘Z' and all the power is absorbed in overcoming internal frictional resistances.

7. SURGE DESCRIPTION:
• If we suppose that the compressor is operating at a point ‘S' on the part
of characteristics curve having a positive slope, then a decrease in mass
flow will be accompanied by a fall in pressure at diffuser outlet.
• If the pressure of the gas downstream of the compressor does not fall
quickly enough, the gas will tend to reverse its direction and will flow
back in the direction of the resulting pressure gradient.
• When this occurs, the pressure ratio drops rapidly causing a further drop
in mass flow until the point ‘X' is reached, where the mass flow is zero.
• When the pressure downstream of the compressor has reduced
sufficiently due to reduced mass flow rate, the positive flow becomes
established again and the compressor picks up to repeat the cycle of
events which occurs at high frequency.

8. • This surging of gas may not happen immediately when the operating
point moves to the left of ‘Y' because the pressure downstream of the
compressor may at first fall at a greater rate than the delivery pressure.
• As the mass flow is reduced further, the flow reversal may occur and the
conditions are unstable between ‘X' and ‘Y'.
• As long as the operating point is on the part of the characteristics having
a negative slope (Y to Z), however, decrease in mass flow is
accompanied by a rise in delivery pressure and the operation is stable.

9. Developing the surge cycle on the compressor curve
Qs, vol
Pv / Pd
Machine shutdown
no flow, no pressure
Flow rate decreased then,
• Pressure builds
• Resistance goes up
• Compressor “rides” the curve
A
• 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
To Vent
Surge Cycle diagram
Pv
Diffuser outlet pressure = Pd
Discharge header pressure = Pv

10. A-B-C (Pv ) and A-C-B-C (Pd )
Qs, vol
Pv / Pd
A
B
• Because Pv > Pd the flow reverses
• Compressor operating point goes to point B
C
D
Pd
To Vent
Surge Cycle diagram
Pv
Diffuser outlet pressure = Pd
Discharge header pressure = Pv
• Diffuser outlet pressure decreases with flow rate as it
follows theoretical characteristic curve
• Due to pressure difference reverse flow occurs.
• Diffuser outlet pressure increases due to reverse flow until both are equal.
• At this presssure compressor is not able to develop forward flow.
• Further Pd follows Pv to reach point C.

11. C-D-A
Qs, vol
Pv / Pd
A
B
C
C:
• Discharge header pressure (Pv)= Diffuser outlet pressure (Pd ) due to reduce in
input and flow reversal.
• At this pressure compressor is able to pick forward flow
• Pressure goes down => less negative flow
D
D
• Compressor “jumps” back to
performance curve and goes to point D
• Forward flow is re-established
Pd
To Vent
A
• Compressor starts to build pressure
• Compressor “rides” curve towards surge
• Point A is reached
• The surge cycle is complete
• 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
Surge Cycle diagram
Pv
Diffuser outlet pressure = Pd
Discharge header pressure = Pv

13. PURPOSE OF ANTISURGE CONTROL SYSTEM:
• The purpose of the “ANTISURGE CONTROL SYSTEM” is to avoid the
operating point “A” reaches the “Surge Limit Line”.
• To achieve this objective, it’s defined, on the right of the “SLL”, a
protection line where the control system will operate opening the
antisurge valve.
• This line is called “SURGE CONTROL LINE” (“SCL”). Usually 10 % of
flow margin.
• The opening of the antisurge valve increases the suction flow moving the
operating point along the speed characteristic curve, from the critical
condition to the stable operating area.

14. THANK YOU