Please tell me the reason behind IGV degree angles being limited to 34-86dgA range? is it related to blade design and shape?
>Please tell me the reason behind IGV degree angles being
>limited to 34-86dgA range? is it related to blade design and
The axial compressors were designed to have minimum flow occur at 34 DGA, and maximum flow at 86 DGA. During starting (purging and firing and acceleration) and shutdown air flow through the axial compressor needs to be low because of the possibility of compressor stall or surge (which the compressor bleed valves also help to reduce during starting and shutdown). Excessive air flow at low axial compressor speeds can cause serious axial compressor damage.
GE and its packagers have been offering performance upgrades which allow the maximum operating angle to be increases to 88 DGA, and there are anecdotal reports of even 89 DGA. HOWEVER, changing the maximum operating angle should only be done if it is determined the axial compressor blading (stationary and rotating) can tolerate the increased air flow, AND the hot gas path section (combustors, transition pieces, turbine nozzles and turbine buckets) can withstand the increased air flow and firing. AND, the IGV ring stops would also have to be appropriately changed to prevent the IGVs from hitting the stops at the increased operating angle.
The answer is basically: Because that's how the axial compressor was designed and has been operated for decades. New axial compressor designs are operating at different minimum- and maximum operating angles, some with fewer stages AND some with multiple variable stator vane sections.
Hope this helps!!!
thanks for fast response as usual.
Means in shutdown condition i.e. IGV being at 34degA, there is still some amount of air flow to the compressor? Will whole of this air (at 34degA in shutdown) be bypassed to exhaust through bleed valves or we will be getting some compressor discharge?
The statement/qualifier about shutdown is confusing. There must be air flow through the combustors in order to establish flame during start-up even though the compressor bleed valves are open. Only a portion of the air flow through the forward part of the compressor is redirected-directed through the compressor bleed valves. This is during start-up and acceleration.
During a fired shutdown after the compressor bleed valves open and the IGVs are at minimum operating angle (commonly known as "closed"), there is still air flow through the combustors even though a portion of the air flow through the forward part of the compressor is redirected to the exhaust through the compressor bleed valves.
A review of the P&IDs will show there is no difference in start-up or shutdown in valving or air flow. This includes even if the unit is tripped from rated speed. None. Zero. Zippo. Nada. Niente. Full stop.
A look inside the axial compressor bellmouth with the IGVs even at the "full closed" mechanical stop will show the IGVs are NOT fully
closed and will allow air to enter the compressor. Full closed and full open refer to the furthest closed and open the IGVs can travel--not that they are actually closed or open.
And the compressor bleed valves cannot redirect the full air flow from the forward part of the compressor.
And if they could there would be no air to mix with the fuel in the combustors during start-up or shutdown.
P&IDs are key to understanding how a GE-design heavy duty gas turbine. The P&IDs and the Operation& Service Manuals. Learn how to read and interpret the P&IDs or repeat asking questions such as this.
A great work you are doing here sir. Please permit me to quote your statement below:
"During a fired shutdown after the compressor bleed valves
>open and the IGVs are at minimum operating angle (commonly
>known as "closed"), there is still air flow through the
>combustors even though a portion of the air flow through the
>forward part of the compressor is redirected to the exhaust
>through the compressor bleed valves"
This is my query to your explanation as per Bleed valves function with regards to the IGV during shutdown. I am with GE frame 6B P&ID, AND what I am seeing here with me is that the Compressor bleed valve VM15-1 and VA20-1 are only taking compressor discharge air flow to the Bleed heating at the air intake filter. The only valve I see taking compressor air flow to the exhaust duct is VA2-1, and that is taken from the stage 11 of the axial compressor (GE Frame 6B). Please I need you to shed more light about the philosophy of the Compressor bleed valves during the shutdown condition.
>This is my query to your explanation as per Bleed valves
>function with regards to the IGV during shutdown. I am with
>GE frame 6B P&ID, AND what I am seeing here with me is that
>the Compressor bleed valve VM15-1 and VA20-1 are only taking
>compressor discharge air flow to the Bleed heating at the
>air intake filter. The only valve I see taking compressor
>air flow to the exhaust duct is VA2-1, and that is taken
>from the stage 11 of the axial compressor (GE Frame 6B).
The original poster didn't say anything about Inlet Bleed Heating or Inlet Air Heating or Anti-icing, which would be VM15-1 and VA20-1.
I don't have any P&IDs in front of me, but I believe the Compressor Bleed Valves are VA2-1 and VA2-2. And, yes, when open they redirect a portion of the air flow from the 11th stage of a GE-design Frame 6B heavy duty gas turbine to the exhaust. They are only to be open during starting and acceleration to near rated speed (sometimes they are closed at 95% speed; sometimes they are closed at 100% speed; sometimes they are closed at a slight positive power, say, 2-3 MW). And, they are to be open any time the unit is operating below approximately 95% speed as the unit coasts down--whether there is flame in the machine or not.
VA20-1 is re-directing a portion of the axial compressor discharge air back to the inlet of the unit for some reason (IBH for DLN combustors; and/or Inlet Air Heating (sometimes also called Anti-Icing)). IBH is to protect the compressor only when the IGV angles are below design minimum (less than 57 DGA, usually) on DLN combustor-equipped GE-design Frame 6B heavy duty gas turbines. This is also to protect the axial compressor, though not from a low CTIM, it's just from angles less than 57 DGA (actually, usually less than 62 DGA, which is where VA20-1 opens during unloading, or closes during loading).
When used to prevent ice formation on the IGVs at very low ambient temperatures VA20-1 will open at or near full load to recirculate a portion of axial compressor discharge (which is near 700 deg F) back to the inlet to try to warm the air to prevent ice formation on the IGVs. When this happens, the unit power output decreases--which most plant managers do not like.
Don't confuse VA20-1 and VA2-1 & -2.
Hope this helps!