Surge is a phenomenon seen in gas turbine engines where there is a sudden decrease in the supply pressure to the compressor inlet. This causes air to flow in the reverse direction through the compressor, creating vibrations throughout the engine. Surge can be generated by diffuser vanes retarding the airflow or rotating stall causing breakdowns of the air flow between compressor blades. Repeated surging can damage components like rotor seals, bearings, and the compressor driver through high vibrations and temperature increases. Anti-surge systems aim to prevent surge by maintaining a minimum air flow through the compressor.

2. What is surge ?
1. A phenomenon seen in the gas turbine engines.
2. Involved in the sudden decrease of supply pressure to
the compressor inlet.
3. The rate of pressure fall is slower in the downstream
of the compressor than that in the upstream of the
compressor.
4. So, air flows in the reverse direction .
5. A vibration is created throughout the machine
(engine).
6. This phenomenon is known as surge.

3. Generation of Surge:
1.By DiffuserVanes:
a. The diffuser vanes retards the airflow due to frictional force.
b. The reason behind this is that it is very difficult to split the air uniformly so that the
mass flow of air is the same in each passage.
c. As the number of diffuser increases the tendency of surge occurrence also increases.

4. 2.By Rotating Stall:
a. The non uniformity of air flow to the geometry of the channel between two blades
creates a breakdown of flow .
b. This increases angle of incidence in the upper blade (as shown in the figure 4.9) and
decreases angle of incidence in the lower blade .So, the upper blade stalls but the
middle blade pick up the flow again.
c. So, the stall transfers from one channel to another.
d. At the impeller eye the flow rotates in the direction opposite to that of the impeller.

5. Compressor characteristics
Compressor characteristic is the curve to show the
behavior of fluid like change in pressure, temperature,
entropy, flow rate etc. as it passes through the
compressor at different compressor speeds.

6. Some phenomena of compressor
 Surge process
 Surge cycle.
 Surge point
 Surge line
 Surge control line
 Surge margin
 Choke/Stone wall point
 Choke line
 Constant efficiency lines
 Maximum allowable speed
 Minimum required speed

8. .
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10. Effects of surge in centrifugal compressor

11. Occurrence of surge in centrifugal compressor:
The point at which the compressor cannot add enough energy to overcome the
system resistance or backpressure is surge.
Damages in centrifugal compressor due to surge:
 This causes a rapid flow reversal (i.e. surge).
 As a result, high vibration, temperature increases, and rapid changes in
axial thrust can occur.
 These occurrences can damage the rotor seals, rotor bearings, the
compressor driver and cycle operation.
 Most turbomachines are designed to easily withstand occasional surging.
However, if the machine is forced to surge repeatedly for a long period
of time, or if it is poorly designed, repeated surges can result in a
catastrophic failure.
 Of particular interest, is that while turbomachines may be very durable,
the cycles/processes that they are used within can be far less robust.

13. Effects of surge in axial compressor

14. In the plot of pressure-flow rate the line
separating graph between two regions- unstable and
stable is known as the surge line. This line is
formed by joining surge points at different rpms.

15. 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
• Reduced compressor output

16. Causes of surge:
The following are some of the usual causes of surge that are not
related to machine design.
 Restriction in suction or discharge of system.
 Process changes in pressure, temperatures of gas composition.
 Internal plugging of flow passage of compressor.
 Inadvertent loss of speed
 Instrument on control valve malfunction
 Malfunction of variable inlet guide vanes
 Operator errors
 Misdistribution of load in compressor
 Mispositioning of rotor

17. Compressor stall
• A compressor stall is a local disruption of the airflow in a gas turbine
or turbocharger compressor. It is related to compressor surge which is
a complete disruption of the flow through the compressor.
• Modern compressors are carefully designed and controlled to avoid or
limit stall within an engine's operating range. Stall was a common
problem on early jet engines with simple aerodynamics and manual or
mechanical fuel control units, but has been virtually eliminated by
better design and the use of hydro-mechanical and electronic control
systems such as Full Authority Digital Engine Controls(FADEC).

18. There are two types of compressor stall:
1. Rotating stall
2.Axi-symmetric stall or compressor surge

19. Notable stall occurrences
On January 15, 2009 US Airways Flight 1549, an Airbus A320, floating in
the Hudson River after bird strikes caused compressor stalls and complete
failure of both engines.

20. Prevention of surge:
Fig. Typical Anti-surge control system

21. ΔP= Ch + b
Where, P=calculated compressor
differential signal
C= Control line slop (ratio signal)
h= inlet orifice differential signal
measured by FT
b=Control Line Bias
Fig. : Surge Control Line
When the calculated ΔP is greater than
the measured ΔP, the compressor is
operating to the right of the control line.
When the calculated ΔP is equal to or less
than the measured ΔP, the compressor is
on or to the left of the control line and
surge occurs.

22. Anti surge control system
 The anti-surge control system should maintain a minimum volume of flow
through the compressor so that the surge condition is never encountered.
 This is achieved by bleeding flow from the discharge of the compressor to
maintain a minimum inlet flow.

This flow can either be dumped to atmosphere or recirculated back into the inlet
of the compressor with the help of anti-surge valve. In the recirculation case, the
flow must be cooled to the normal inlet temperature.

For most applications, a simple control based on a flow differential is adequate for
this function. However, on compressor where the speed or the gas conditions are
variable, the anti-surge control may have to be more sophisticated to insure proper
operation under all conditions. This is frequently achieved by modulating the anti-surge
control with a signal for pressure, temperature, speed, or a combination of
parameters.

23. A description of the function of each component is as follows:
FE:- The flow element is usually an orifice located in the compressor
suction, although it can be a venturi or calibrated inlet such as those used
in axial compressors. Its purpose is to cause a temporary pressure drop in
the flowing medium in order to determine the flow rate by measuring the
difference of static pressures before and after the flow-measuring
element.
FT:- The flow transmitter is a differential pressure transmitter which
measures the pressure drop across the flow element and transmits a signal
that is proportional to flow squared.
DPT:- The differential pressure transmitter measures the differential
pressure across the compressor and transmits an output signal that is
proportional to the measured pressure differential.

24. FX :—The ratio station receives the signal from the flow transmitter and
multiplies the signal by a constant. This constant is the slope of the control
line.
FZ :—The bias station receives the signal from FX, the ratio station, and biases
the surge control line.
• The ratio station must have both ratio and bias adjustment to enable the
control line to be placed as parallel to the compressor surge line as possible
(see Fig. )
ΔP= Ch + b
Where, P=calculated compressor differential signal
C= Control line slop (ratio signal)
h= inlet orifice differential signal measured by FT
b=Control Line Bias

25. Fig. : Surge Control Line

26. FIC :-The surge controller is a flow control device which compares the calculated output
of FZ to the measured ΔP output of the DPT with ΔP as defined above.
When the calculated ΔP is greater than the measured ΔP, the compressor is operating to
the right of the control line. When the calculated ΔP is equal to or less than the measured
ΔP, the compressor is on or to the left of the control line, and the surge controller
functions as a flow controller and opens the anti-surge valve as necessary to maintain
operation of the compressor on this surge control line.
For rapid flow changes, the response of the control system must be rapid to prevent
surge.
LAG :-This device functions to enable the surge controller to open the recycle valve
quickly, while providing a slow closure rate. This feature provides stability control system
and recycle valve.

27. LX:— The low signal selector is set up for two inputs and one output. The inputs are a
100% signal valve and the surge controller output signal. The output of the low selector is
sent to the recycle valve as well as back to the surge controller in the form of a feedback
signal. This prevents the surge controller from winding up. Windup of the controller
penalizes the reaction time of the anti-surge control system.
FCV:— The anti-surge recycle valve functions to prevent surge by recycling flow from the
compressor discharge back to the compressor inlet. Sizing of the anti-surge valve should be
at 1.05% of design flow at design pressure rise.

28. Anti surge control system types:
1. Pressure oriented anti surge control system
2. Flow oriented anti surge control system

29. Pressure Oriented Anti-surge System

30. Flow Oriented Anti-surge System

31. Surge VS Stall
STALL
 LOCAL PHENOMENON
 CAUSED BY FLOW SEPARATION
 TEMPORARY INSTABILITY
 ROTATING IN NATURE
 MAY LEAD TO SURGE
 MECHANICAL DAMAGE MAY OR
MAY NOT OCCUR
SURGE
 GLOBAL PHENOMENON
 CAUSED BY FLOW REVERSAL
 PERMANENT INSTABILITY
 AXIAL IN NATURE
 STALL MAY BE THE CAUSE, [TOTAL
FLOW BREAK DOWN]
 DAMAGE TO THE COMPRESSOR /
TURBNE INESCAPABLE

32. Conclusion
What we have presented so far in this presentation is that what is surge and
it’s generation. In this presentation we discussed characteristics of
compressor, phenomenon of surge like surge process, surge cycle, surge point,
surge line, surge control line, surge margin etc. We also mentioned here
causes of surge in centrifugal and axial compressor and it’s effect and
consequence in axial and centrifugal compressor and also the prevention of
surge occurrence(Anti-surge system).

33. ANY QUERY ?