Topic of The Day: Compressor Stall/Surge
Compressor stall or surge is not peculiar to any one particular brand or type of engine.It may occur on any turbine engine if conditions are right. Stall has been encountered on two-stage or turbo-supercharged piston engines, so there is no need to look upon
stall as some mysterious product of gas turbine engines.
Any number of mechanical defects, such as bad spark plugs, lean carb, poor timing, or sticking valves, can result in reciprocating engines backfiring. Similarly, for gas turbine engines, maintenance or flight conditions can influence the compressor stall or surge appreciably. The condition and operation of the bleed valve and fuel
system components are of vital importance in maintaining surge-free operation.
Why are engines at risk of surge? As engines are designed to meet demands for higher power or lower specific fuel consumption, the engines must accommodate:
• Increased mass airflow.
• Increased pressure (compression) ratio.
• Increased maximum allowable turbine inlet and outlet temperatures.
• Improved efficiency of the compressor and turbine sections.
Quick engine starts and rapid accelerations are also desirable. To provide higher power with low specific fuel consumption and acceptable starting and acceleration characteristics, it is necessary to operate as close to the surge region as possible.
To prevent compressor stall or surge, fuel flow must be properly metered during the start and acceleration cycle of any gas turbine engine. To accomplish this, the example engine incorporates 5th and 10th stage acceleration bleed valves. In general, there are fewer surge problems on centrifugal compressors than on axial
flow compressors. There are several reasons for the difference; the primary reason is that centrifugal flow compressors operate at somewhat lower pressure ratios than axial flow compressors.
Explanation of stall/surge
A surge from a turbine engine is the result of instability of the engine’s operating cycle. As discussed earlier, the operating cycle of the turbine engine consists of intake, compression, combustion, and exhaust, which occur simultaneously in different places
in the engine. The part of the cycle susceptible to instability is the compression phase. Compressor surge may be caused by engine deterioration, it may be the result of ingestion of birds or ice, or it may be the final sound from a “severe engine damage” type of failure.
In a turbine engine, compression is accomplished aerodynamically as the air passes through the stages of the compressor, rather than by confinement, as is the case in a piston engine. The air flowing over the compressor airfoils can stall just as the air over
the wing of an airplane can. When this airfoil stall occurs, the passage of air through the compressor becomes unstable and the compressor can no longer compress the incoming air. The high-pressure air behind the stall further back in the engine
escapes forward through the compressor and out the inlet.
This escape is sudden, rapid and often quite audible as a loud bang. Engine surge can be accompanied by visible flames forward out the inlet and rearward out the tailpipe. Instruments may show high EGT and EPR or rotor speed changes; but, in many stalls, the event is over so quickly that the instruments do not have time to
respond.
Once the air from within the engine escapes, the reason (reasons) for the instability may self-correct and the compression process may re-establish itself. A single surge and recovery will occur quite rapidly, usually within fractions of a second. Depending
on the reason for the cause of the compressor instability, an engine might experience:
1. A single self-recovering surge
2. Multiple surges prior to self-recovery
3. Multiple surges requiring pilot action in order to recover
4. A non-recoverable surge.
For complete, detailed procedures, flight crews must follow the appropriate checklists and emergency procedures detailed in their specific Airplane Flight Manual. In general, however, during a single self-recovering surge, the cockpit engine indications may fluctuate slightly and briefly. The flight crew may not notice the fluctuation.
(Some of the more recent engines may even have fuel-flow logic that helps the engine self-recover from a surge without crew intervention. The stall may go completely unnoticed, or it may be annunciated to the crew – for information only – via EICAS
messages.) Alternatively, the engine may surge two or three times before full self recovery. When this happens, there is likely to be cockpit engine instrumentation shifts of sufficient magnitude and duration to be noticed by the flight crew. If the engine does not recover automatically from the surge, it may surge continually until
the pilot takes action to stop the process. The desired pilot action is to retard the power lever until the engine recovers. The flight crew should then SLOWLY readvance the power lever. Occasionally, an engine may surge only once but still not self-recover.
The actual cause for the compressor surge is often complex and may or may not result from severe engine damage. Rarely does a single compressor surge CAUSE severe engine damage, but sustained surging will eventually over-heat the turbine, as
too much fuel is being provided for the volume of air that is reaching the combustor. Compressor blades may also be damaged and fail as a result of repeated violent
surges; this will rapidly result in an engine which cannot run at any power setting.
