"Never value the advantages derived from anything involving breach of faith, loss of self-respect, hatred, suspicion, or execration of others, of insincerity or the desire for something which has to be veiled or hidden." - Marcus Aurelius
Solid advice for our leaders, right there.
Today it's time to discuss compressor stall. Readers may remember the stall scene in the movie Top Gun. In that scene, one engine of the aircraft suffers a compressor stall and flame-out as it passes through another aircraft's jet wash, then the other engine dies, and the plane goes into a flat spin. The flat spin was caused by loss of thrust on one side of the aircraft, while the other engine was still operating at full thrust, causing the aircraft to pinwheel.
Compressor stall was a known issue with the F-14, and it could be deadly. This was in the days before high speed data networks and computers existed to assist with control of a malfunctioning aircraft - or to rapidly adjust the engines to keep them functioning properly. That compressor stall, coupled with a bad ejection, cost "Goose" his life - in the movie.
Below is a cutaway of a simplified turbojet engine - these are at the heart of every turbofan engine in modern aircraft. A black arrow marks the location of the compressor discharge, where these events originate.
In the video below, a pilot explains the basics of compressor stall in aircraft engines.
Below is a passenger video that captures several compressor stalls during climb-out, followed by an emergency landing. There are a series of loud bangs as the airflow through the engine is interrupted several times. This banging is the result of a severe form of stall known as "surging". Surging will destroy an engine in short order, so best practice is to land the aircraft ASAP after experiencing surging, and have the engine inspected for damage.
On aircraft, compressor surging or stalling is most common when the compressor discharge pressure is at its highest. Under what conditions does this occur?
- When the aircraft is at or near ground level - the surrounding air is most dense at ground level, which means it is pulling in more mass than when flying at altitude.
- When there is low ambient air temperature - this increases air density, so again, for a given compressor speed, more mass will flow into the compressor.
- When the engines are at Full Power - the the compressor is operating at very high RPM, shoveling large quantities of air into the engine. This increases compressor discharge pressure.
- When the aircraft has significant speed combined with the above items - forward motion of the aircraft causes additional airflow into the compressor, again increasing discharge pressure.
Stalling and surging are typically seen near the end of the take-off roll, while at take-off thrust. They are also seen during the climb-out, and during full-power engine reversal upon landing. These are the times when most of the conditions above are met.
One characteristic of all combustion engines is that they produce more power the colder the inlet air temperature is. This is due to a couple of effects: Cold air is more dense than warm air, so for a given compressor RPM, more molecules of air will be pulled into the engine. If more air flows in, more fuel must be added - you need to burn more fuel to balance combustion, so you get more power.
The other reason that thrust-producing combustion engines produce more power with cold air is that they are "mass flow" engines. This means that their power output is proportional to the mass drawn into them. The more material you can fling out the back of the engine at high velocity, the more thrust it delivers. If the engine is able to draw in large quantities of really dense 20 degree air, it will deliver maybe 20% more thrust than if it were taking off from Phoenix in 110 degree heat
Because these are "mass flow" engines, they are designed to maximize the quantity of air they pull in. They compress the air to very high pressure (500-600 psi) before injecting fuel, burning it, and expanding the super-heated combustion gas through the turbine. This optimization for maximum power and efficiency creates very high pressure at the compressor discharge.
When compressor discharge pressure gets above design limits, we start to see stalling and surging. As stated before, this occurs with high air density at high power settings, or when a lot of water or snow enter the engine and vaporizes - as this adds flow, and therefore increases compressor discharge pressure. At that point, compressor discharge pressure becomes so high that additional air cannot be forced in. The normal airflow movement from the front to the rear "stalls". Typically airflow resumes after a brief and minor hiccup, and the condition is difficult to even notice. Instrumentation is necessary to spot mild compressor stalling, and it's not a big deal. You don't want to continue operating that close to the limit, but you are not causing any damage.
However if the stall becomes a full-on surge, airflow briefly reverses - with a loud bang, and some flame. It's hard on the engine, and it's alarming to the passengers and crew. See the video below for an example of surging.
Aircraft engine stalls are rare, because these engines are equipped with Variable Stator Vanes (VSVs) and modern engine management systems. The system constantly monitors and adjusts the amount of air entering the engine in real-time. The variable stator vanes are necessary to choke off airflow to prevent a flame-out at low power, but they can also rotate to reduce excessive airflow when the compressor discharge pressure gets above the allowable pressure. Nothing is perfect though, so we still occasionally see stalling. Pilots are taught to reduce engine power slightly until the stalling abates.
Below: A VSV actuator opens and closes the compressor vanes on an aircraft engine. These VSVs will open automatically as the pilot advances the throttle and the engine spools up.
Now we have a sense of the causes surging and stalling, using aircraft engines for reference material. I used aircraft engines to introduce the topic, because there are plenty of examples and videos of those. Time to talk about the big stuff.
Power plant industrial (or Frame) turbines differ significantly from aircraft engines in design, although they both operate on the same principle. Both types of engines pull in air, compress it, add fuel and ignite it, and use the hot expanding air to spin a turbine.
The two types of engines differ considerably in design though, because industrial turbines don't need to fly. They are built like tanks, with very thick casings, and they are made from steel, not lightweight alloys. The volume of air a that a GE Frame 7 engine pulls in will be six or seven times that of an aircraft engine at full thrust, and it can run at full power for months on end.
The industrial turbine does not spin at the high RPM of the aero turbine - it rumbles along at 3600 RPM, roughly 1/3 of the speed of the aircraft engine. 3600 is the synchronous speed for a two-pole 60 Hz generator, which is why this speed is chosen. In Europe and Asia, which use a 50 Hz electrical grid, the machines run a bit slower, at 3000 RPM.
Compared to aircraft engines, these turbines are simple and robust. Aircraft engines can be easily removed from the wing and replaced with one from a crate. Industrial engines are bolted into concrete and stay there for their useful life. On a majority of industrial turbines there is only one set of vanes to restrict airflow, and these are called Inlet Guide Vanes (IGVs). Below: Stroking IGVs to verify their position.
Industrial turbines don't have to operate up in the thin air, so they are optimized for, and have guaranteed performance standards for conditions at sea level and at 55 degree inlet air temperature. Actual power output will deviate - for better or for worse - when operating under other conditions.
Both types of engines use a significant amount of intake air for cooling the hot section of the turbine. The blades of each type of turbine are riddled with tiny cooling air passages. Cooler air constantly flows through each hollow blade and out tiny orifices. This keeps the blade from melting, and forms a thin film of air to prevent the hot exhaust gas from corroding the blades.
Since there are no weight considerations on industrial turbines, they are equipped with large inlet filters, to remove this debris before it can enter the engine.
These inlet filters are like the air filter on your car, although they are quite a bit larger due to the volume of air being filtered. Over time, these inlet filters become fouled. The filters at my current plant are fouled with soot from last summer's wildfires, and they will be replaced during the next scheduled maintenance outage.
And this is how you can run into a compressor stall situation on an industrial gas turbine - insufficient air due to filter fouling. Air is being sucked into the compressor, and the compressor discharge pressure is at the normal value. However with a fouled inlet filter, there is now an airflow restriction at the inlet of the compressor, and so the inlet duct will begin to experience negative pressure. This condition worsens over time, as the filter accumulates even more foreign material, or when fog saturates the filter media. Stalling can result, because airflow through the compressor is interrupted. In this case, the stall is caused by low compressor inlet pressure, rather than high high compressor discharge pressure.
Fortunately, these industrial engines have a very simple safety device - "old-school" pressure switches with electrical contacts at the inlet filter. If the air pressure in the inlet duct drops below a certain value, below which compressor stalling is likely, the turbine will automatically reduce power. As the turbine unloads, the Inlet Guide Vanes will also begin to close, reducing airflow. Once the low compressor inlet pressure condition clears, the power reduction stops. The term for this automatic load reduction is called a "Runback".
Other models of gas turbine have blow-in doors on the bottom side of the inlet duct. They are placed underneath so that accumulated dust and bird poop don't flow into the engine in the event the blow-in door gets pulled open by the vacuum in the inlet duct.
Now you know!
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