Dear Sir,

We have GE PG9171E with DLN 1.0 technology gas turbine.

The specified design Modified Wobbe Index for the gas turbine is 47 with 5% deviation limit off the design value (upper limit: 49.35, lower limit: 44.65).

Due to LNG injection in the natural gas stream by the Natural Gas supplier, the gas turbine is experiencing a Modified Wobbe Index of upto 53 (13% off design) with an LHV of 870 to 985 Btu/scf, SG of 0.57 to 0.67 and constant gas temperature 45 degree celsius (with no performance heating).

The gas turbine has been operating under these conditions for 4 months and no abnormalities have occurred so far.

What effect would this have on Combustion dynamics and gas turbine hardware?

 

Dear Sir,

This is question best posed to GE or one of its authorized packagers.

GE also have several publications out regarding Modified Wobbe Index control system option which may have some information that you would find helpful.

The thing I would most worry about is if the deviation is too high then the sonic/sub-sonic/super-sonic fuel nozzle characteristics can be affected which would mean that dynamic pressures in the combustor may be adversely affected, along with other characteristics which could impact fuel nozzles and combustion liners as well as "flame" stability (yes, GE uses that term for DLN combustion systems which they also so "don't burn") and emissions.

I believe when the Modified Wobbe Index (MWI) control option is used on a GE-design heavy duty gas turbine that, among other things, the P2 pressure reference is varied in order to maintain the desired fuel control valve and fuel nozzle operating conditions as fuel make-up varies. I think the later versions of MWI also use an on-line, real-time gas chromatograph to provide information the turbine control system needs for correcting operating conditions for a variable gas make-up.

But, try to find the GE publications (sorry; I don't have the publication numbers) and possibly ask this question of your GE or GE-authorized packager. Of course, they're probably going to try to sell you the MWI option (if you don't already have it) or some modification to the existing MWI you already have.

Lastly, many times people operating GE machines have a better knowledge of real operating considerations and their effects than GE does (primarily, they're just a manufacturer and don't have a lot of real-time operating experience--though they're collecting terabytes of operating data from machines these days that they haven't even begun to mine for information yet, part of their IoT (Internet of Things) initiative). Generally, this is true, and there are some experienced GE field service people who have seen years and decades of machines during maintenance inspections who probably have some tales they can share. So, asking World Wide Web forums like this is a good idea.

Unfortunately, I don't think there is a Frame 9E Users' Group like there are for Frame 7E/EA machines, and 7F machines and 9F machines. The 9Es are very similar to 7E/EA machines, and I had heard at one point that the 7E/EA Users' Group was going to start accepting 9E users, as well as 7B users (actually, I know they are acceting 7B users at this time).

Find and read the appropriate GE publications to get a better idea of the design intentions and to ask better questions if they don't provide you with the information you are looking for--and then try to find an individual in GE who can help you with your well-defined questions (after reading the appropriate publications). Finding someone can be a real challenge, but there are people who can answer you question. It's just being able to "get through" those who don't know and who don't know who can answer the questions--or who just want to sell you something which may not even be appropriate.

Hope this helps!