GE Mark V Frame 6 Machine - IVG angle

J

Thread Starter

Johnplayers

We have GE Mark V Frame 6 machine. while reducing the load IGV angle also tends to close to keep the exhaust temperature within limits. at reaching IGV angle 57deg IBH opened to extend the premix steady sate mode. My question is mode changeover depends on TTRF1 temperature so why GE not made the opening of IBH with respect to TTRF1 temperature instead of IGV angle?

I hope my Question is clear to the experts...

Thanks & Regards
Johnplayers
 
Because the purpose of IBH on GE-design heavy duty Frame 6B turbines (not F-class; B-class) is to protect the compressor when IGV angles are less than the typical minimum operating angle (57 DGA).

Remember: The only control of air flow into the combustor is via the IGVs. And, the fuel-air mixture in a DLN-I combustor is extremely lean. As fuel is reduced during unloading if air is also not reduced then the mixture will lean out so much that combustion will not occur.

To remain in Premix Steady-State below approximately 80% of rated load it is necessary to close the IGVs below normal minimum operating angle (i.e., less than 57 DGA). However, closing the IGVs below 57 DGA at rated speed puts the compressor in danger of exceeding design limits and so a portion of the axial compressor discharge air is recirculated through the IBH control valve to the inlet of the compressor thereby heating the compressor inlet air a couple of degrees C and reducing the density of the air and protecting the compressor.

Again, all of this is necessary because to remain in Premix Steady State below approximately 80% load it's necessary to reduce the air flow to the combustors in order not to lean out the fuel-air mixture too much. The only way to do this is to close the IGVs to angles less than 57 DGA--which the GE-design compressors were never intended to do.

The axial compressor is the weakest link in gas turbines. The present designs (LMS-100 units excepted) just can't get any more air flow--limiting the power production capability. And, with DLN systems, they can't have the air flow through them restricted excessively without going into a stall/surge condition. So, the manufacturers are all working on new compressor designs to alleviate these problems--however, the new designs all increase complexity, of both the hardware and the control system.

And, IGV opening is a function of TTRF1. It's just that when the unit (non F-class units) has IBH then the IBH will open when the IGVs start to go below the axial compressor's normal design operating angle.

Hope this helps!
 
J
"However, closing the IGVs below 57 DGA at rated speed puts the compressor in danger of exceeding design limits and so a portion of the axial compressor discharge air is recirculated through the IBH control valve to the inlet of the compressor thereby heating the compressor inlet air a couple of degrees C and reducing the density of the air and protecting the compressor."

In this case why we should not reduce compressor suction head by closing the IGV further (<57DGA) to avoid surge/stall instead of re-circulating hot air (CPD air) again into the inlet of the compressor.

Forgive me CSA if the Question is silly
Best regards,
Johnplayers
 
Arun,

Axial compressors are very unique pieces of equipment and have their own unique operating characteristics and idiosyncrasies.

I have explained it as it has been explained to me--it's to protect the axial compressor from damage when the IGVs are closed below the design minimum operating angle of 57 DGA. Which for modern GE-design heavy duty B- and E-class gas turbines (including Frame 5s) only occurs when the unit is equipped with DLN-I combustors.

I believe one of the benefits of recirculating air to the suction side of the compressor is that the CPD is reduced (the back-pressure on the axial compressor). Also, not just venting the air to atmosphere there is no need for a large--and costly muffler. And, heating the air entering the axial compressor reduces the mass flow of air through the axial compressor--which definitely helps to prevent damage at IGV angles.

Wish I could be more helpful. It's basically a design choice to use IBH (a very poor name for the function it performs for DLN combustion systems).
 
J
> Thank you CSA

Once again I thank you CSA for your response,
And I have one more question regarding IBH that is "during Startup before IGV starts to modulates as per 'IGV temperature control ref' to maintain exhaust temperature IGV sticks with either 48DGA (If IBH enabled) or 54DGA (If IBH disabled).

My question is why the logic is 48DGA if IBH enabled or 54DGA if IBH disabled?

Thanks & regards,
Johnplayers
 
Johnplayers,

Once, again, you are welcome.

Well, let's see. The primary purpose of IBH is to allow DLN-I combustor-equipped units to operate in Premix Steady-State mode below approximately 80% load. That goes for reducing load, and it also goes for increasing load.

In other words, IBH is required to remain in Premix Steady-State when load is reduced below approximately 80% of rated.

And, if IBH is enabled during start-up, it <i>should</i> also allow the unit to transfer into Lean-Lean AND then into Premix Steady-State much sooner than would otherwise be possible. It should transfer into Premix Steady-State somewhere between 40-60% of rated load during loading, whereas if IBH was disabled during starting it would not transfer into Premix Steady-State until it reached approximately 80% of rated load.

So, IBH helps to get into Premix Steady-State sooner than would otherwise be possible during starting and loading, AND it helps to remain in Premix Steady-State longer than it would otherwise be possible during unloading.

I don't know where you came up with 48 DGA or 54 DGA; those are not values I'm familiar with. But, to get into--or remain in--Premix Steady-State at low fuel flows it is necessary to reduce the air flow into the combustors to prevent from excessively leaning out what is already an extremely lean fuel/air mixture, and the only way to do that is to close the IGVs. And when the IGVs on non-F-class GE-design heavy duty gas turbines are closed below their nominal minimum operating angle (which they must be to reduce air flow to prevent from leaning-out the fuel/air mixture excessively) then IBH is required.

IBH allows the IGVs to be closed to angles less than approximately 57 DGA (which has always been the nominal minimum opening for these machines; perhaps recent operating experience has shown they can be closed further without damage--I don't know). So, if you want to transfer into Premix Steady-State sooner (at a lower load) than otherwise would be possible during starting, IBH needs to be enable, and that means the IGVs will be closed below their nominal minimum operating angle to keep air flow low to maintain stoichiometric combustion. (I so rarely get to use that word: stoichiometric.)

And, if you want to remain in Premix Stead-State at lower loads than would otherwise be possible then the IGVs need to close below their nominal minimum operating opening--and that means IBH is required to protect the axial compressor.

Does this help? I hope so, because I just don't seem to be able to find the right words to help you understand that without IBH transferring into or out of Premix Steady-State is going to occur at about 80% of rated load. That's because of the design of the DLN-I combustors; they operate (in Premix Steady-State mode) at extremely lean fuel/air mixtures. If fuel were to be reduced during unloading without reducing air, then the turbine is going to lose flame and trip. And, if you want to transfer into Premix Steady-State at loads below approximately 80% of rated, it's going to be necessary to reduce air flow for the same reason--to prevent losing flame and tripping the turbine.

Next, you're going to say that there is no "flame" in Premix Steady-State. And, that's not true. There IS diffusion flame in the secondary combustion zone, and even though there's not diffusion flame in the primary combustion zone during Premix Steady-State operation, the fuel is still being combusted--there IS a temperature rise in the primary combustion zone. Just because the combustion doesn't result in diffusion flame does not mean the fuel isn't "burning", and there are still limits of fuel/air mixtures that must be observed even in the primary combustion zone during Premix Steady-State operation.

Whew! I'm gonna go find a stout ale.
 
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