At our site we have GE series-9E gas turbine with MARK VIe control system (ISO rating 106 MW). In these machines compressor's IGV (inlet guide vanes) position is between 34 to 84 degree and minimum opening for synchronized machine is 54 degree.

Now, in case when ambient air temp goes below 16 degree C, minimum opening of IGV increases up to 57 degree from 54 degrees.

What is the logic behind this increased opening by 3 degrees in low ambient temp. conditions?

 

Do these units have DLN-I combustion systems, and if so, do they have Inlet Bleed Heat and is the Inlet Bleed Heat system enabled and in service?

If the units do not have DLN-I combustors, do the units have IGV Exhaust Temperature Control and if so, is IGV Exhaust Temperature Control active?

If the units are not operating with IGV Exhaust Temperature Control active, what load are the units being operated at when the IGVs open and what is the exhaust temperature before the IGVs open and what is the exhaust temperature after the IGVs open?

The IGV operating reference (CSRGV) is the output of a Minimum Select block, so you should be able to look at the application code using ToolboxST to see all of the inputs to the Minimum Select Block to see which one is the lowest value and then work backwards from that point to see the logic and control algorithms acting to cause the IGVs to open from their minimum operating angle.

Basic non-DLN GE-design heavy duty gas turbines with modulated IGVs and without IGV Exhaust Temperature Control enabled and in operation open the IGVs when the exhaust temperature reaches a particular value (700 deg F or 900 deg F--I can't remember which). As exhaust temperature increases as the turbine is loaded the IGVs open from their minimum operating position to maintain that exhaust temperature until the IGVs are at their maximum operating angle, at which point the exhaust temperature will increase as the turbine is loaded until it reaches CPD- (or CPR-) biased exhaust temperature control.

Lastly, what is the turbine speed/grid frequency when this phenomenon is occurring? Is it higher or lower than normal? Much higher or much lower?

And one more question, is the IGV opening causing some operational issue or is this just a curiosity question?

 

Mr. CSA

Thanks for replying to my post.

here are some operational specifics of our machine:
ours is a combined cycle power plant and our GT's don't have DLN combustion system. they have IGV temp. control function, which we use always as our GTs run in combined cycle mode and we require minimum exhaust to support HRSG.

In our machine IGV close position is 34 degrees and from 34 it starts to open at 80% speed (50 Hz, 3000 RPM machine) and opens up to 54 degree. then after machine synchronization it will open further from 54 to 84 at 25 MW if IGV temp control is in OFF Mode. (this is normally for open cycle mode)

And for combine cycle operation we use IGV temp control ON and it will start opening from 54 degree (up to 84) at around 82-83 MW, depending upon load setpoint. (IGV full open at 84 Degree for base load)

hence for load below 80 MW in combined cycle IGV will be at 54 degree to control exhaust temp. and it is so normally except for when ambient temp. goes below 16 degree. In that case IGV opens to 57 degree from 54 degree,and this results in low exhaust temp in GT and reflects as low parameters in HRSG. (HP Steam temp.)

Now this is precisely our concern as why IGV should open further from 54 degree below 80 MW load in low ambient condition as this results in low exhaust temp in GT. clearly it is not a exhaust temp control exercise as it is working adversely in this situation. (bringing down the exhaust temp)

I believe it has something to do with gas turbine compressor protection then. I hope and believe some of the members here at control.com with their vast experience might have come across this phenomena in GE-9E series machines will be able to explain this.

Thanks again!

 

Mr. Om Choudhary,

I have not seen any Compressor Operating Limit protection operate at ambient temperatures as high as 16 deg C, but I have seen some Compressor Operating Limit protection operate at ambient temperatures closer to 0 deg C. And, this only for units equipped with DLN combustors operating.

Again, if you want to know <b>exactly</b> what is causing the IGVs to change position <b>at your site</b> you need to view the application code running in your turbine to find which of several references feeding the IGV position reference is "taking control" when this issue is occurring. Many times there are "notes" in the application code. Also, I would expect that if there was some kind of Compressor Operating Limit protection it would be documented in the Control Specification document provided with every Speedtronic turbine control system (possibly in Sect. 06).

IGV angle is usually a function of exhaust temperature. When a non-DLN unit with IGV Exhaust Temperature Control is enabled and active (when operating in "combined cycle" mode) if TTXM exceeds TTRX then the IGVs will open to maintain TTXM equal to TTRX.

A non-DLN unit operating in "simple cycle" mode with IGV exhaust temperature control disabled and OFF if the exhaust temperature reaches either 700 deg F or 900 deg F (I can't recall which) then the IGVs will begin to open to maintain [700 deg F or 900 deg F] as the load increases.

You have NOT responded to what the exhaust temperature is when this "problem" is occurring. It would also be helpful if you told us what the actual Exhaust Temperature (TTXM) is and what the exhaust temperature control reference (TTRX) is. I believe there is another exhaust temperature reference for IGV Exhaust Temperature Control (TTRXGV) which you could also provide for more information.

Having said all of the above, GE Belfort (which now have responsibility for GE-design heavy duty Frame 9E gas turbines) is known to implement many elaborate control schemes--but they are usually documented in the Control Specification (sometimes the documentation is better than others).

Please write back to let us know what you find. (It is noted this same question has been posted in more than one Discussion Group on www.LinkedIn.com.)....

 

Also, what is the turbine speed when this phenomenon is occurring?