I understand that during a compressor surge air will flow in reverse direction out of the compressor …the engine power generation cycle is basically interrupted and the engine is no longer producing power besides the very detrimental /damaging effects of the reverse flow on compressor components.

I can't not understand compressor stall phenomena, however !
what does exactly stall do to the compressor and air flow ? does it cause reverse flow ? does it cause air to be trapped inside compressor casing and just keep moving or rotating but does not exit ?
The explanations for this topic I found on the internet or in book is a bit advanced to me.

 

While they aren’t exactly the same thing, the terms are often used interchangeably. An axial compressor is a pretty unique machine in the realm of high-speed rotating equipment and compressors (such as centrifugal and reciprocating compressors). Axial compressors have very different characteristics and generally only operate properly in a specific speed range, sort of similar to centrifugal compressors but not exactly the same.

The air flows in an axial compressor are very important, and are affected by ambient temperature as well as speed. Axial compressors with variable Inlet Guide Vanes introduce another variable in the air flow situation. Too much or too little air flow can result in the pressure in the next stage of the axial compressor being too high and causing the air flow to suddenly and in some cases violently stop. This is the condition known as stall—when air flow cannot proceed from one stage to the next. The air flow from previous stages jest keeps coming—and that’s when bad things happen. I’m told that under certain conditions the air flow can actually be pushed backwards through the compressor and the inlet.

During surging, I’m told, the high pressure of a downstream stage can collapse and air can suddenly flow in the proper direction, and the pressure can suddenly increase again. When this happens there can be big bangs coming from the axial compressor, flame can be lost and suddenly reignite (not very common but I’m told it has happened on occasion under unusual and extreme circumstances).

During off-speed operation of most axial compressors—such as during start-up/acceleration and shutdown/deceleration—it is usually necessary to provide a path for air flow between certain stages to be “relieved” using bleed valves or “pop-off” valves to prevent stalling and/or surging which can cause damage to the machine.

A lot of “legacy” GE-design heavy duty gas turbine axial compressors DO NOT like off speed operation—not at all. So, grid frequency disturbances of a violent and wild nature can result in both stall and surge. Newer axial compressor designs are less susceptible to axial compressor issues during off-speed operation (during grid frequency disturbances).

There is axial compressor protection in the turbine control system of most modern-day heavy duty gas turbines, but it usually reduces or limits power output which greatly angers some owners and operators who think their expensive machine should ride through any and every grid disturbance without any disruption in output or frequency. (Which is a popular—and completely false—belief not rooted in any kind of critical thinking or science.)

Without knowing a lot more about the nature of your question and the reason for it, there’s not much more I can add. There are probably some good videos of simulations of stall and surge, but there are probably more bad ones than good ones…. I am NO mathematician and can’t offer any further explanations or knowledge. I have experienced stalling/surging ONCE during shutdown of a heavy duty gas turbine when the bleed valves didn’t open, and I can tell you I had to return to the hotel to change my underwear. And I was damn lucky the unit didn’t suffer any damage. I have heard surging of an aero derivative gas turbine once, and that was once too many times. (That unit did suffer compressor damage….)

Best of luck; hope this helped somewhat. If you learn more or can provide references to counter or correct the above information, I am always grateful for constructive criticism and clarification. If you’re asking about anything other than GE-design heavy duty gas turbine, I can’t offer any further information.

And, yes—most written literature about axial compressors is written by ivory tower, egg-head types with (possibly) lots of theoretical knowledge but very little actual hands-on experience. Lots of theses are written by theoretical types who get approved for publication by other theoretical types who are impressed by the volume of research and references—but don’t know jack about axial compressors.

And most manufacturers consider their design and testing information to be proprietary and confidential.

 

Than you so much CSA for your help, as always. Your explanation of "Stall" is easy to understand and convincing and until I find a better explanation I will stick to it.

 

Science20,

Pray tell: What sparks your need to understand axial compressor stall/surge?

 

CSA,
In our site (LNG Plant), we have lots of GE frame 5D gas turbines that drive cenrtifugal gas compressors and I am working in operation of these machines so I need to know as much as possible about them. I was learning about the CCC controller that controls centrifugal compressor recycle valve to protect against surge. And I was confused , does the anti-surge controller protect against stall as well?
That is why I was interested to learn more about surge and stall phenomenas.

 

While similar, centrifugal compressors have different operating characteristics than axial compressors, though some of the same terms are used for both (such as the infamous surge and stall). I think that multi-stage centrifugal compressors are more susceptible to surging and/or stalling.