Our site experiences fuel gas supply pressure fluctuations ranging from 15 bar to 21bar. The low gas pressure alarm setting is 17 bar. during low gas pressure <17 bar it is not possible to operate all the 5 nos frame6 DLN1 units at base load (premix mode) due to rapid pressure drop to <14 bar. To avoid shutting down the turbine we chose to operate 2 turbine on part load (lean lean mode) for 10 to 12 hours a day while the other 3 turbines are at base load (premix mode). once the gas pressure increases in the night the 2 units are back to base load (premix mode).

what are the consequences of operating the DLN1 unit of lean lean mode for such a long period? in this mode the diffusion flame is present in both primary & secondary zone. we had notices damages to venturi areas of the liner. please specify what is the maintenance factor for Lean lean mode of operation? GE specifies MF=10 for extended lean lean mode of operation but does not specify for lean lean mode. feed back will be of great help.

 

This is a great question for GE. There are multiple revisions of GER-3620; the last one I saw was "R"--which is Rev. #18 in GE-speak, so you may be reading from an older version.?.?.?

Primary- and Lean-Lean combustion modes are transitory modes, meaning they are just modes through which DLN-I combustor-equipped units must transition to get to Premix Steady-state combustion mode--which is the lowest emissions mode, which is the real reason for having turbines with DLN combustion systems: to reduce NOx emissions.

Anyway, GE is really still learning about the long-term effects of operation in modes other than Premix Steady-state, because they did not really envision people operating in any other state for extended periods of time. In North America if the unit can't be operated in low emissions mode for approximately 30 minutes (sometimes more; sometimes less) the unit has to be shut down until such time as it can (be operated in low emissions mode for extended periods of time).

So, they really didn't envision nor plan for long-term operation in modes other than Premix Steady-state. And they are living vicariously by gathering empirical data from units that do operate for extended periods of time in modes other than Premix Steady-state.

My personal opinion is that any prolonged operation in any combustion mode other than Premix Steady-state (for DLN-I combustor-equipped units) is equal to a MF of 10, to be on the safe side. You might decrease that a little (because of reduced fuel flows in Lean-Lean as opposed to the higher fuel flows in Extended Lean-Lean). But I wouldn't say much below 7 or 8, again to be on the safe side. And you can temper that with your own observations and experience over time.

If this is expected to be a long-term situation, it might be worth looking into adding Inlet Bleed Heat for lower load operation in Premix Steady-state. I don't know for sure if the proper fuel nozzle differential pressures can be maintained with low load operation with IBH, but it's certainly worth asking GE or a similar engineering operation.

Please write back to let us know what you learn.

 

Thanks CSA, the above question was my first in control.com, I used to read you replies and they was very useful to improve the knowledge in the past several years.

Since GE does not specify the MF for lower combustion modes (like lean-lean mode) the plant owners think that lean lean is a continuous operating mode and does not cause damage to combustion capital parts.

If there is a specific document showing the MF for lean lean mode it will be of great help.

As you mentioned the possibility of installing a IBH would increase the operating range at premix mode,this option will be explored.

 

ARI1969,

Unfortunately, I don't believe GE has documented that Lean-Lean and Primary are transitory modes--it's just implied. Again, because low NOx emissions are the reason for DLN-I combustion systems, and NOx emissions are lowest when operating in Premix Steady-state combustion mode. The intent is that the turbine will always be operated--long-term, continued operation--in Premix Steady-state, and that Primary- and Lean-Lean are just combustion modes that must be transitioned through in order to reach Premix Steady-state.

In the beginning, GE used to say that the turbine could be operated indefinitely in either Primary- or Lean-Lean combustion modes. But, historical data has shown otherwise. Diffusion flames produce very high combustion gas temperatures, and prolonged operation at these high combustion gas temperatures have caused damage--as you have noted--on combustion liners, particularly on the venturi between the Primary- and Secondary combustion zones. Short-term operation (such as when starting and stopping the gas turbine) in Primary- and Lean-Lean combustion modes does not cause the same damage as long-term operation.

One has to remember that any manufacturer doesn't have a lot of long-term operating experience with their own product(s). This is particularly true with large capital equipment, such as gas turbines. GE uses single combustors in their combustion laboratories to simulate turbine operation, and, again, DLN was developed to reduce NOx emissions without the need for water or steam (and to reduce emissions below what was possible with either water- or steam injection). Manufacturers will necessarily modify their operating instructions with data and experience from the owners and operators of their equipment.

As an owner/operator if you are seeing the damaging effects of long-term operation in Primary- or Lean-Lean combustion mode then you should take appropriate action to limit that damage to the extent possible, and try to work with the OEM (GE in this case?) to understand if there any any workarounds or new combustion hardware that can withstand prolonged operation in modes other than Premix Steady-State. If you know that when you ride your bicycle or drive your car over potholes and ruts in the pavement that it will cause unnecessary damage to your vehicle, will you continue to drive your vehicle over the potholes and ruts or will you try to avoid them, perhaps even taking another route that may be longer, in order to protect your investment? This is the same thing, only on a much larger scale.

Again, I don't know what the emissions laws are in your part of the world, but in some parts prolonged operation in any mode other the lowest emissions mode is not allowed and can result in civil- and criminal penalties for the owner and operators and their supervisors. That means people can go to jail for operating turbines for prolonged periods in modes other than Premix Steady-State.

Again, I would try to obtain a copy of the most recent version of GER-3620 (don't know what rev they're up to these days), and also ask GE if they have any new publications regarding DLN-I combustion systems and operation. They are continually publishing new documents, as well as upgrading and revising older ones, so you may be able to get new or updated information by asking GE directly. (Though finding a knowledgeable and diligent person in GE can be difficult at the best of times; there's just so much documentation. I can recommend you ask your GE contact to work with the Thermal Performance Engineering Department to possibly get the best information available. I know there's a group on Atlanta, GA, USA, and I would presume there's one in Belfort, France, as well--where the Frame 6B and Frame 9E gas turbines that use DLN-I combustion systems are built.)

Please write back with publication numbers if you learn of something new and relevant. And, good luck with the possibility of adding IBH to achieve lower loads while remaining in Premix Steady-State.

 

CSA,

Its been more than a year since your last reply on this topic,
As per your advice, few days back got a copy of the latest revision of GER3620M revision date Feb 2015.

In this latest revision page 16 there is a description about the effects of operating the DLN1 turbine on premix, lean lean & extended lean lean.

as usual GE provided the maintenance factor of 10 for ext.lean lean, 1 for Premix, but for lean lean the indication is - parts failure severity rate will be high, but did not provide with a definitive maintenance factor. However it is very clear the damage to the combustion parts will be severe during operation on lean lean.

currently the unit is operated at the following conditions due to low gas pressure.

Lean lean mode (combined cycle) - to achieve HP steam production the exhaust temperature is set almost what would be during full load at premix state. At lean lean mode the firing temperature is close to the premix transfer temperature with IGV set at 57 deg.
diffusion flame on both primary & secondary nozzle ideal to extended lean lean mode.

The inspection interval for Combustion inspection with latest hardware is set as 12000 Factored fired hours.

if the unit is operated in the above condition what maintenance factor is to be applied? it is obvious that maintenance factor cannot be 1 which is only for premix mode, since it is not perfectly extended lean lean, so maintenance factor of 10 is debatable, it is wise to have somewhere between 5 to 10 ?

As a maintenance incharge I need to convince the operator for planning the scheduled inspection.

please share your thoughts.

 

ARI1969,

"... parts failure severity rate will be high, ..." That's certainly conclusive, isn't it? (NOT!)

Actually, I don't believe there is a Simple Cycle Mode and a Combined Cycle Mode for DLN-I combustor-equipped turbines. The IGVs are modulated all the time to control the air flow into the combustor(s), and this means the exhaust temperature is high for most of the operating range.

However, when the unit is in Lean-Lean Mode there is diffusion flame in BOTH the Primary- and Secondary Combustion Zones. There is very little fuel flowing through the secondary fuel nozzle premix pegs so the flame temperatures in the primary- and secondary combustion are at their highest when the unit is at the upper end of the Lean-Lean Mode (just prior to the transfer to Premix Mode). And, the fuel split is about 50-50 in Lean-Lean Mode, meaning the flame temperatures in the primary- and secondary combustion zones are about at their highest during operation at the upper end of the Lean-Lean Mode (just prior to the transfer to Premix Mode).

I believe the 12000 hour combustion parts rating is for a FFH of 1 (Premix Mode), given that it takes some time to load and unload the unit through Primary- and Lean-Lean Modes into and out of Premix Mode).

Extended Lean-Lean Mode also has about a 50-50 fuel split and this iw when the diffusion flame temperatures in the primary- and secondary combustion zones would be at their highest (from about 80-100% of rated load). Again, diffusion flame would be present in both combustion zones, just as in Lean-Lean mode--but the flame temperatures would likely be higher.

TTRF (or TTRF1) is a calculation of the temperature exiting the first stage turbine nozzles, and when the unit is operating with diffusion flame in the primary zone or in both primary- and secondary combustion zones the flame temperature is going to be higher than the temperature of the hot combustion gases exiting the first stage turbine nozzles because of the cooling and dilution effect of axial compressor discharge air. (When the unit is operating in Premix Mode, approximately 80% of the fuel is flowing into the primary combustion zone--and there is NO diffusion flame in the primary combustion zone, so the hot gas temperature in the primary combustion zone is MUCH lower, which is why the NOx emission rate is so low.

I'm just trying to explain what's happening in the two combustion zones during the Lean-Lean and Premix and Extended Lean-Lean for a comparison. In my personal opinion, a maintenance factor of approximately 6 or even 7 should be applied to extended operation in Lean-Lean mode based on the above analysis. If the unit is operated at the upper end of Lean-Lean Mode for extended periods of time, the Maintenance Factor could be as high as 8 or 9. Again, diffusion flame temperatures are high--that's why NOx formation is high for Primary- and Lean-Lean- and Extended Lean-Lean Modes, because the fuel is burning almost exclusively in diffusion flame. And, it's the diffusion flame temperature that causes the combustion liner(s) to experience excessive thermal stress leading to premature failure.

So, because there is diffusion flame in both primary- and secondary combustion modes during Lean-Lean operation which means the flame temperatures in the two zones are as high or nearly as high as they can be during Extended Lean-Lean operation I would estimate the effect to only slightly less than the Maintenance Factor suggested for Extended Lean-Lean Mode, especially if the turbine is operated at the upper end of the Lean-Lean Mode (just prior to the transfer to Premix Mode).

The thing which is occurring when operating in Lean-Lean combustion mode is that only about 50% of the fuel flow-rate is being burned (in diffusion flame) in the primary combustion zone. Because the majority of air entering DLN-I combustors enters the primary combustion zone that means the fuel-air mixture is lean, but not as lean as when operating in Premix Mode. This probably means the diffusion flame temperature in the primary combustion zone is lower than it might otherwise be (if the fuel/air mixture were a little less lean), but since 50% of the fuel flow-rate is being burned in the secondary combustion zone where there's less cooling and dilution air flow the temperatures there are likely higher than when the unit is operating in Premix Mode (when only about 20% of the fuel flow-rate is flowing into the secondary combustion zone, and a portion of that is flowing out of the premix pegs on the body of the secondary fuel nozzle).

This is why I've estimated the Maintenance Factor for Lean-Lean combustion mode to be less than 10--but, again, if the unit is operated at high load in Lean-Lean Mode (just prior to the transfer to Premix Mode), then the Maintenance Factor should be higher than if it is operated at the lower end of Lean-Lean Mode.

Hope this helps in understanding what's happening in the various combustion modes, and how I arrived at my recommendations. You're free to think things through and come up with your own Maintenance Factor based on your analysis of operation--just as GE has allowed you to do with their "recommendation."

Most GE-design heavy duty gas turbines with DLN-I combustors are operated in Premix Mode almost exclusively to comply with local regulations regarding NOx emissions. So, GE doesn't have a lot of experience with operation in Lean-Lean mode--and has less than favourable experience with operation in Extended Lean-Lean Mode. Most sites, unlike yours, run at high loads and so will run in Extended Lean-Lean Mode when Premix Mode is not possible for some reason. That's probably why their "recommendation" for a Maintenance Factor in Lean-Lean combustion mode is so vague--most DLN-I combustor-equipped units only operate in Lean-Lean mode during start-up and shutdown, not for any extended period. (Some sites may operate longer in Primary- and Lean-Lean Modes because they have to "pause" to warm up the HRSG or wait for the steam turbine to warm, but that doesn't seem to be like your site; this would necessarily be considered "extended" operation in Lean-Lean as you seem to be describing.)

Again, hope this helps!

 

ARI1969,

Could you please tell us what did you do with the aforementioned problem? how did you handle that, eventually?

thanks in advance,

 

ARI1969,

Could you please tell us what did you do with the aforementioned problem? how did you handle that, eventually?

thanks in advance,

Do you have a similar DLN-1 operational problem or are you just curious? The present day answer is don't operate outside of Premix at all if possible or for as short a time as possible. If you haven't seen the state of combustion hardware used in Lean-Lean or Extended Lean-Lean for long periods, check the internet, it will scare you.
If you have a DLN problem, post it here and we will try and answer

 

glenmorangie,

Unfortunately yes. Actually I've read about the maintenance factor and my approach is to do anything with the machine! to make it operate in s-s premix mode. So I am curious what @AR1969 did to their machine eventually?

 

I'll leave @AR1969 (or someone from their site) to hopefully contribute. The one thing that no one discussed was the problem with the fuel gas supply pressure fluctuations at their site, if that could be corrected, they would pretty well be able to manage their problem. I don't expect that GE will ever admit that Lean-Lean is really not a good way to operate because that pretty well says that they have designed a system that doesn't work very well. You've got to understand how much pressure there was on the industry to reduce NOX in a Dry fashion and then maybe we can understand how we ended up with a not to great a system in DLN-1.