One of our GT frame 6001B running on lean lean negative due to less powee demand with others two gt to satisfy power margin.could we run run GT on lean lean negative for long time if not then why.
Thanks in advance.
ABDULLAH

 

abdullah,

Lean-Lean Negative is Lean-Lean, just slightly above the point at which the unit has to get to in order to be able to transfer back into Premix Steady State later. If you operate only in Lean-Lean Negative and then want to or need to transfer quickly into Premix Steady State, you won't be able to--the unit will go into Extended Lean-Lean.

Lean-Lean (and Primary) are transitory modes--that means they are necessary to transition through these modes to get to Premix Steady State combustion mode, which is where DLN-I units were designed to be operated. When the unit is operating in Lean-Lean, and especially when it's operated in Lean-Lean Negative (which is pretty high in the Lean-Lean load range, meaning fuel flow-rates are at about their highest for Lean-Lean operation), there is diffusion flame in BOTH the primary and secondary combustion zones. BIG diffusion flame in the primary combustion zone. Over time, the OEM has come to learn (as have many operators of units with DLN-I combustion systems) that long-term or continuous operation in Lean-Lean combustion mode has very negative effects on the combustion liner and the inner barrel of the combustion liner in which the Secondary fuel nozzle resides. The intense heat of the diffusion flame can cause localized heating of the combustion liner and the Secondary fuel nozzle "tube" (the one the secondary fuel nozzle resides in). This can cause the thermal barrier coating to bubble and break off, and can cause weaken the liner walls. It can also cause localized heating of the venturi between the primary and secondary combustion zones.

Extended Lean-Lean combustion mode is even more injurious, and can cause worsened damage. The OEM estimates that every hour of Extended Lean-Lean operation is equivalent to ten (10) hours of Premix Steady State mode. That's pretty harsh. Newer DLN-I units even have timers (accumulators) which monitor the time spent in Extended Lean-Lean because it's very important to factor that into the maintenance outage planning. I believe operation in Lean-Lean combustion mode is also estimated to be a high number of equivalent hours of Premix Steady State operation, maybe even as high as for Extended Lean-Lean.

So, the answer to your question is: If you want to keep the intervals between maintenance outages as long as possible (produce as much electricity as possible without shutting down to perform maintenance outages sooner than would otherwise be possible) and avoid replacing hot gas path components prematurely (which is expensive) you should not be operating in Lean-Lean combustion mode for long periods of time. That's not how DLN-I combustion systems were designed to be operated; they were designed to be operated in Premix Steady State--that doesn't mean they have to be operated at Base Load in Premix Steady State at all times (though that's probably the most efficient point of operation from a heat rate perspective), but if you can reduce the load on one or both of the other GTs operating in Premix Steady State enough to get the third unit into Premix Steady State also, that would be the optimal means of operating all three units: in Premix Steady State.

I can't recall the GE publication number, but GE produces a publication which, while difficult to read and interpret, does outline how units are designed to be operated and how maintenance outages are to be planned for--and that includes equivalent fired hours of operation for combustion modes OTHER THAN Premix Steady State. If I can find the GE publication number, I will post it to this thread; you will have to search the World Wide Web for it and download it for yourself. (GE has apparently seen fit in recent months to restrict access to their library of publications to the general "public." If you do business with GE for parts and/or services (such as an LTSA or Contractual Services agreement) you could ask them to provide the publication for you--if it's not in the Operations & Service Manuals provided with the units when they were new. (If my memory is correct--and it's not always correct these days...--I believe the publication number is GER-3960 GER-3620. There is usually an alpha character at the end of the publication number (for example, GER-3960L); the "L" would mean revision L (or, revision 12 (or so)) of the publication. "M" would be a higher revision number, as would "P."

If GER-3960 is not the correct publication number, I will post the correct one when I find it. GER-3620 is the correct publication number.

Hope this helps!

P.S. You can edit your post to correct the Subject (to Lean Lean negative).... (I think the subject can be edited, anyway. Or maybe one of the find moderators can do it.?.?.?)

 

Sorry; the correct publication number is: GER-3620

Here's a link to a WWW file location:

https://www.ge.com/content/dam/gepo...ine-operationg-maintenance-considerations.pdf

(Any misspelling in the link is not mine!)