Gas Turbines - Compressors — Page 232, Lesson 308

Let's talk about compressor stall and surge — two of the most critical things you'll ever need to understand about a gas turbine engine. I want you to think of the compressor as the heart of the engine; if it stops working properly, everything downstream suffers. First, let's establish what determines the angle of attack of a compressor blade. The angle of attack is the result of two velocities: the axial velocity of the air passing across the blade, and the rotational speed of the blade. These two velocities combine to form a vector — a single resultant direction — which gives the actual angle of attack of the airflow over the blade. So the blade's angle of attack isn't fixed; it's a product of how fast air is moving along the engine axis and how fast the blade is spinning. Now, a compressor stall is an imbalance between these two velocities. It can occur through various causes, and I want to walk you through each one because they're all distinct failure modes. First, excessive fuel flow caused by abrupt engine acceleration. When you slam the throttle forward, you dump fuel in, and the combustion chamber back pressure increases. That increased back pressure reduces the axial velocity of the air. So the air slows down axially, and the angle of attack changes — leading to stall. Second, engine operation above or below the engine design rpm parameters. This increases or decreases the rotational speed of the compressor blade. If you're running outside the design rpm, the blade speed is wrong relative to the airflow, and again you get an imbalance. Third, turbulent or disrupted airflow to the engine intake. This reduces the axial velocity directly — the air coming in is disturbed, so it doesn't flow smoothly along the axis. Fourth, contaminated or damaged compressor components. This causes decreased axial velocity because of decreased compression ratio. If the compressor blades are dirty or damaged, they can't do their job of compressing the air as effectively, so the compression ratio drops and the axial velocity drops with it. Fifth, contaminated or damaged turbine. This causes a loss of power to the compressor, which again leads to decreased axial velocity because of decreased compression ratio. The turbine drives the compressor; if the turbine is damaged, it can't drive the compressor as hard, so compression suffers. And sixth, excessively lean fuel/air mixture caused by abrupt engine deceleration. This is the opposite of the first case — when you chop the throttle, the combustion chamber back pressure decreases, and the axial velocity is increased. So here the air speeds up axially, and again the angle of attack goes out of balance. Any of these conditions can cause compressor stall to commence. And as soon as it does, there is a partial breakdown of airflow through the engine. The airflow through the compressor starts to separate and break down. Now, the indications of compressor stall are two-fold: an increase in the vibration level of the engine, and an increase in the Exhaust Gas Temperature — EGT. That EGT increase is caused by the fact that there is less air going to the combustion chambers. With less air, there is less air to cool the products of combustion — the exhaust gases. So the exhaust gases run hotter because there's less cooling air mixing with them. Compressor stall is a progressive phenomenon. It could initially, in theory, occur at just one blade. Then it worsens to encompass the whole of one stage — that's one row of rotor blades and the associated stator. And then, if nothing is done to prevent it, it affects the whole engine. So it starts small and spreads. Now, surge. The progressive deterioration of the situation will eventually cause a complete breakdown of airflow through the engine — and that complete breakdown is called a surge. In severe cases, this could cause an instantaneous reversal of the gases in the engine, with air being expelled through the engine intake with a loud bang. So instead of air flowing in through the intake, it's blasted back out. If surge does occur, the throttle of the affected engine must be closed slowly. That's a critical operating instruction — close the throttle slowly, not abruptly. This situation is most commonly caused by fuel system malfunction or mishandling. And in extreme cases, surge could inflict such large bending stresses on the compressor rotor blades that they contact the stator blades, with potentially catastrophic results. So the rotor blades get bent so badly they physically touch the stator blades — that's blade contact, and that's catastrophic. Let me summarize the key distinction for you. Stall is a partial breakdown of airflow — it starts at one blade and spreads. Surge is the complete breakdown of airflow — the ultimate progression of an untreated stall, with possible flow reversal and a loud bang. The indications of stall are increased vibration and increased EGT. And the response to surge is to close the throttle slowly.