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IBH Valve Operating Mode at Part Load
How does IBH valve works?
1 out of 1 members thought this post was helpful...

Hello everybody,

forgive my poor english, I'm an italian student and need of your help.
I'm not sure I understood working conditions of IBH valve.
Correct me if I'm wrong plz...

IBH valve allows a re-circulation of inlet air flow. This is bleeded from one of the last steps of compressor. So this hotter mass flow rate avoids ice formation and protects the compressor by stall.

Is it?

Now, I have a very HUGE doubt.
In addition to these functions, in CCGT power plant, IBH often is used to optimize operating modes at part load.

I understand that opening IBH valve increases air inlet temperature, so the temperature of flue gases exiting gas turbine raises as well as HP steam generation rate in HRSG.

But how is it possible?

Indeed I think that if the air inlet temperature raises, its mass flow rate decreases and at the same way exhaust gas flow decreases. Can higher temperature of flue gases leads to increase HP stream generation although exhaust gas flow decreases?

I hope I make myself clear enough.
Thx for reading...

If you can, suggest me some reference about IBH operating mode at partial load.

1 out of 1 members thought this post was helpful...

fhabbio,

Inlet Bleed Heating, at least as GE applies it for their heavy duty gas turbines, is used to protect the axial compressor when the IGVs (Inlet Guide Vanes) are closed below their normal minimum operating angle. The IGVs are closed below their normal minimum operating angle in order to allow the turbine combustors to remain in the lowest NOx formation mode over as wide a range of operation (load) as possible. For example, many GE-design heavy duty gas turbines can only operate in the lowest NOx emissions mode possible from approximately 80% of rated load to 100% of rated load. Using Inlet Bleed Heat and allowing the IGVs to close below their normal minimum operating angle allows the load range to increase to approximately 40%-100% while remaining in low NOx emissions mode. This can be a great economic benefit to many owner/operators. BUT, it's only possible when the IGVs can be closed below normal minimum operating angle.

On older turbines with non-DLN combustors it was common to open the IGVs during loading to maximum operating angle to keep the heat rate of the turbine as high as possible. This had the effect of making the gas turbine exhaust temperature relatively cool during part load operation--because the IGVs were open for a large portion of the load range.

Then, it was discovered that the gas turbine exhaust could be used to produce steam by flowing it into a waster heat recovery boiler, or an HRSG (Heat Recovery Steam Generator). And, then it discovered that by closing the IGVs as much as possible during part load operation that the gas turbine exhaust temperature would be higher than it would otherwise be which increases the steam production. Now, this has the disadvantage of worsening the heat rate of the gas turbine, BUT it increases the overall heat rate of the plant--which is good.

Then when DLN combustors came around, it was necessary to use the IGVs to more tightly control air flow through the turbine by limiting the air flow at part load. And this is done by closing the IGVs. And this has the effect of raising the gas turbine exhaust temperature--which is okay when the gas turbine exhaust is being used to produce steam.

AND, when its necessary or desirable to increase the load range of a DLN combustor-equipped unit while remaining in low NOx emissions mode it's necessary to close the IGVs below their normal minimum operating angle--which has the effect of raising the exhaust temperature. And, to close those IGVs below their normal minimum operating angle it's necessary to use IBH to protect the axial compressor.

IBH, by extracting some of the axial compressor discharge air flow and recirculating it back to the axial compressor inlet does two things. First, it reduces the air flow into the turbine (combustors and exhaust), which has the effect of slightly elevating the gas turbine exhaust temperature even further, but not by a lot. Second, it increases the axial compressor inlet temperature which is the primary desired effect because it protects the axial compressor. (Increasing the axial compressor inlet temperature reduces the density of the air flowing through the compressor--which is how the axial compressor is protected when the IGVs are closed below normal minimum operating angle.)

So, it's not really the IBH that increases gas turbine exhaust temperature. It's using the IGVs to limit air flow into the DLN combustors that causes the gas turbine exhaust temperature to be higher than it would otherwise be (if the IGVs were left to operate at their original design control scheme--which was to open them to their maximum operating angle earlier in the loading process). And, then to allow DLN combustor-equipped units to operate at low loads in the low NOx emissions mode it is necessary to close the IGVs below their normal minimum operating angle.

It's really all about the IGV angle--and how that affects the gas turbine exhaust. Closing the IGVs to maximize exhaust temperature helps to improve overall plant thermal efficiency (even though it slightly reduces gas turbine thermal efficiency). And when the IGVs have to be closed below their normal minimum operating angle it's necessary to use IBH to protect the axial compressor.

Hope this helps! Keep studying!

The prevention of ice on the inlet guide vanes when they are at their maximum operating angle is accomplished using much the same hardware (exactly the same in some installations). BUT, GE calls it Inlet Air Heating (not Inlet BLEED Heat), or Anti-Icing Control. And, because it reduces the air flow through the unit it has the effect of raising the exhaust temperature. EXCEPT that it's usually only used when the IGVs are fully open and the unit is already producing high exhaust temperature. BUT the undesirable effect of Inlet Air Heating/Anti-Icing Control is that it reduces the power output of the unit--which most owner-operators just absolutely hate!

So, it's really important to understand what the axial compressor discharge extraction air/re-circulation scheme is being used for--and when.

Again, hope this helps!