Hi to all,

I have a question. When a GE design gas turbine (for example frame9) is in service (rated speed), is there any probability to occur surge?

How is that possible?

 

dontknowman,

Yes. Today's heavy duty gas turbines could produce more power than they currently do (I'm referring to non-F-class machines)--but the ability of the axial compressor to push more air through the machine is the limiting factor. For a long time, the limit was hot gas path materials (turbine nozzles; turbine buckets (blades); combustion liners and transition pieces), but that's changed in recent years. And, most current-generation GE-design heavy duty gas turbines, especially those equipped with DLN combustion systems, have protective algorithms to prevent the axial compressor from operating outside their limits of safety. This generally involves reducing load or reduced load--which most Operations- and Plant Managers are very unhappy about, but that's what happens.

The two causes of potential axial compressor surging/stalling are ambient temperature extremes (which affects mass flow through the axial compressor at rated speed) and IGV angle (which also affects mass flow through the compressor at rated speed). Most GE-design heavy duty gas turbines with DLN combustors use Inlet Bleed Heat (which is a way of protecting the axial compressor when the IGVs are closed below approximately 57 DGA (which is where most of the current design axial compressor were expected to operate above (at rated speed)) to help protect the axial compressor at low IGV angles when at rated speed. But, ambient conditions and machine conditions can also cause the axial compressor to exceed its margins of safety and cause the Compressor Operating Limit Protection algorithm to become active (along with a Process Alarm to alert the operator of the condition).

The instrumentation (CPD (Compressor-Pressure, Discharge transmitters) play a huge role in detecting potential operating limit excursions. This is just another reason they are so important to keep calibrated and working correctly, and to ensure the tubing (sensing) lines remain free of condensate and rust and that the isolation valve(s) is(are) in the proper position(s) prior to starting and during operation.

So, yes--it is possible to have axial compressor surging/stalling at rated speed under some conditions and/or if the instrumentation/sensing lines are not well-maintained.

Now, having said that I believe that surging and stalling are similar, if not the same, phenomena--caused by the inability of air to move from one stage to the next stage in the axial compressor. As the rotating axial compressor blades push the air from one stage to the next it's important to remember that the air in downstream stages is at a higher pressure and must be able to flow through the machine. Otherwise, there will be a stalling of air flow--which when overcome, results in a surge, and then a stall, and so forth. If these activities are allowed to reach an extreme situation where the forces on the axial compressor blades (both rotating and stationary) are opposite of what they should be, or the resultant oscillations cause unnatural harmonic frequencies of the blades, then serious and catastrophic damage can result.

Yes; under normal conditions the risk of surging/stalling is higher at axial compressor speeds less than rated (such as during starting or shutdown), and the axial compressor bleed valves, and variable IGVs are used to help reduce that possiblity during starting and shutdown. But, even at rated speed axial compressors can still experience stalling/surging and many current-generation control systems monitor for and try to protect the equipment by various means (most of which result in lower output while the protection is operating).

Hope this helps!

 

Thank U dear CSA for your answer. So I need some more clarification.

Under which conditions it's probable to take place a surge in nominal RPM? Suppose a F9 is at service in base load.

for example high ambient temperature and high IGV degree? Or some problem in some sensors?
And did U see or hear such a condition?
I've found that when there is overflow in axial compressor and no way for exit, there is more possibility for surge or stall.

again thank U

 

dontknowman,

I've never experienced it myself.

What have you experienced? Does the machine trip? F-class turbines have protective sequencing to reduce the risk of damage caused by sudden changes in compressor discharge pressure--which would likely happen when surging/stalling occurred.

I would have to see a Compressor Operating Limit algorithm with Control Constants to tell you for sure, but the one condition I already mentioned is when the IGVs are closed below 57 DGA on DLN combustion systems (I presume you're referring to a Frame 9E). That's what IBH is for--to protect the compressor during rated speed operation when the IGVs are closed less than 57 DGA.

Very cold ambients cause the air to be much more dense--so when the axial compressor is running at rated speed it will push much more air for the same ambient and IGV angle. Some axial compressors require a means of reducing the air flow under these conditions (very low ambient temperatures), usually by closing the IGVs--which will limit power output.

Again, to comment on a particular machine I would have to see Constants and sequencing (for a Mark V).

Sensor problems can cause erroneous problems, since sensors are used to detect problems.

I have seen dirty inlet air filters cause the vacuum in the inlet duct work to cause the duct work to "pulse" as it vibrates from high vacuum. I can envision that the beginning of axial compressor "surging" might sound similar. Most duct work metal (vertical sides and roof) aren't as think as some people imagine them to be, and will flex quite a bit, which will make the problem of dirty inlet filters resulting in duct work flexing/pulsation even worse.

There are typically implosion doors on inlets to protect against excessive vacuum in the duct caused by dirty (or iced) inlet air filters (an excessive inlet filter differential), which can result in flexing of inlet duct sheet metal, eventual implosion of inlet duct sheet metal (which I've seen pictures of), or ruptured inlet air filters which can cause large particles of filter elements and trapped dirt to enter the axial compressor. However, most sites never check the operation of the implosion doors--which can rust close and not operate properly if not maintained and manually operated from time to time. (There are supposed to be limit switches on the implosion doors to generate an alarm if they open, which does allow unfiltered air to enter the axial compressor--better that, though, than filter pieces and concentrations of dirt from the ruptured filters, or, worse, implosion of inlet duct work.

>I've found that when there is overflow in axial compressor
>and no way for exit, there is more possibility for surge or
>stall.

I've seen some crude axial flow measurement systems, but none used for surge/stall detection. How do you create an "exit" for axial compressor overflow? The IGVs on "F9"s are typically only open/close, not modulated, and they would cause a huge change in CPD/load/exhaust temperature.

Please provide details of your experiences.

 

Hi CSA,

Honestly I"ve never seen such a phenomena too.
That was only a question and imagination in my mind for probability of surge in normal condition of machine.

Once again I have to mention my gratefully to your answers. Not just for me for all the knowledge seeking in this site.

Regards