DlN1 Extended Lean Lean


Thread Starter


GE 9e gas turbine dln1.

A few days ago the turbine entered to extended lean lean and did not return to premix mode. I had this problem before but I was reducing the energy for a moment and then raised again and the problem began.

How does pre-selection interfere?

Is this a danger to the machine?
Dear KHE, here are my comments, short and long.

"Is this a danger to the machine?"

Yes=short answer.

Long Answer=The machine is not really designed for long periods of operation in lean-lean, or extended lean-lean operation. The assumed operation of a machine using the DLN1 style of combustion is for the machine to be started and run up in load until it enters pre-mix steady state operation. Load can then be varied from minimum load while still in pre-mix up to base load. Most operators of these machines have air permits that can only be met with the machine operating in low emissions mode where Nox emissions are low. Operating in lean-lean or extended lean-lean produces high amounts of Nox that are not desirable.

As far as the machine it is not good for the combustion hardware, mostly the combustion liner that is subjected to very high temperatures while operating at high loads with flame in the primary zone of the liner. The operator needs to understand why the unit came out of pre-mix mode and make all attempts to get the machine back in pre-mix. Some events are called primary re-ignition events where the reason for flame reestablishing in the primary zone is not known. But typically unloading the unit and then reloading and transferring back into pre-mix takes care of the problem. Other times there may be combustion problems where the control system actually relights the primary zone. This is typically due to exhaust spreads that indicate some kind of problem with combustion hardware which should be addressed as soon as possible.

"How does pre-selection interfere?" I am not sure I understand your question. All I can say without more information is that once the unit enters extended lean-lean then typically the operator has to take manual control of the load. Load must be reduced until permissives are met to reload the unit and transfer back into pre-mix. Some newer units have logic for a "High Load Pre-mix transfer" that allows the unit to perform a transfer at loads closer to base load. The units I am most familiar with still require the unit load to be around 45% to transfer from lean-lean to pre-mix.

Please write back and let us know what you are able to figure out.
>"Is this a danger to the machine?"

>Yes=short answer.

So, GE says that every hour of operation in Extended Lean-Lean is equivalent to six to ten hours of operation in Premix Steady State Combustion mode--that's how much more stressful it is on the combustion liners. Extended Lean-Lean is meant to be a temporary mode, only operated in until the unit can be successfully returned to Premix Steady State combustion mode.

I think the question about Pre-Selected Load control relates to setting a Pre-Selected Load setpoint that results an a TTRF (or TTRF1) that is very close to the temperature at which the unit will transfer out of Premix Steady State Mode. Or trying to use Pre-Selected Load Control to get the unit back into Premix Steady State.

To return a unit to Premix Steady State mode, one has to unload the unit--manually is best, but ...--until the Premix "latch" is broken (I forget the name of the signal name), and then the unit can be re-loaded and it will transfer back in to Premix Steady State on its own. Again, loading it manually is best, but once it's in Premix Steady State one can use Pre-Selected Load Control--AS LONG AS THE LOAD SETPOINT <b>IS NOT</b> set such that the unit will drop back out of Premix Steady State (usually into Lean-Lean, NOT Extended Lean-Lean).

The important thing to note here is that an operator (usually) has to manually do something to get the unit load low enough so that the Premix latch breaks, and then re-load the unit back until it transfers back to Premix. IN GENERAL, the turbine control systems are not programmed to automatically do that for the operator--it's strictly a manual operation, because the operator should be aware of the situation and what likely caused the unit to transfer out of Premix, and take appropriate action.

What isn't clear from the original post is: What was happening at the time the unit transferred from Premix to Extended Lean-Lean? Was the grid frequency unstable? Was the load unstable? Was the gas fuel supply pressure unstable? What were the exhaust temperature spreads (TTXSP1, TTXSP2, & TTXSP3)?

What alarms (Process AND Diagnostic) that were active and annunciated at the time of the transfer to Extended Lean-Lean?

Again, some machines have had the turbine control system modified to be able to transfer back into Premix without having to reduce load--but they are the exception rather than the rule. AND, it almost every case the operator has to take some action to initiate a transfer back to Premix--if it's even possible. (Sometimes, conditions are such that the unit will not successfully transfer back into Premix--but there WILL BE alarms to help a conscious operator, operations supervisor and technician understand what could be the problem.)

Pre-Selected Load Control <b>IS NOT</b> a good way to operate any heavy duty gas turbine, especially when the unit is to be operated at a load that is very close to the transfer temperature between Lean-Lean and Premix. And, operators HAVE TO BE aware of what the transfer temperatures are--because it's NOT load-related, it's machine conditions, and ambient temperature that most affect the TTRF/TTRF1 calculation, NOT load (no matter how much anyone wants transfer temperatures to be load-related!).

LOTS of things can cause a unit to transfer from Premix to Extended Lean-Lean--but an operator has to manually take action to get it transfer back to Premix. And, before doing so it should be understood what most likely caused the primary zone re-ignition that caused the transfer from Premix to Extended Lean-Lean. It's virtually impossible to say for sure in every instance, but some attempt should be made to understand what was happening at the time of the transfer before just trying to transfer back to Premix, and almost without exception the unit will NOT automatically transfer back from Extended Lean-Lean to Premix without the operator taking some kind of manual action (even when using Pre-Selected Load Control to change load, which is NOT recommended for this purpose!).

Hope this helps--MIKEVI did a very good job of explaining things, and it's hoped this will help make things a little clearer. Perhaps MIKEVI can list the <b><i>typical</b></i> transfer temperatures and deadbands for getting into or out of Premix under normal circumstances, and how far the unit has to be unloaded to break the Premix latch before the unit can be re-loaded back into Premix (again, presuming conditions will permit).
Thank you for your answer

I have read in other topics in this forum that raising or lowering the preset energy may lead to LEAN LEAN EXTENDED

I did not understand how this gets?

The incident started when we faced problems that forced us to cut energy. Knowing that the temperature is high here is 45 degrees

The IBH opens in TRANS mode to 52 percent.
>I have read in other topics in this forum that raising or
>lowering the preset energy may lead to LEAN LEAN EXTENDED

>I did not understand how this gets?

Extended Lean-Lean mode occurs when something causes the fuel in the primary combustion zone of the combustion liners to be ignited into diffusion flame when the combustion reference temperature is above the setpoint at which the unit should be in Premix Steady State combustion mode.

There are MANY possible causes--unstable grid frequency, unstable gas fuel supply pressure, hot spots in the primary combustion zone (usually fuel-borne contaminants), excessive exhaust temperature spreads, problems with combustion liner integrity, fuel nozzle problems, gas fuel control valve instability, LVDT or pressure transmitter calibration problems, (everybody's favorite) servo valve problems, IBH control valve problems, improper igniter insertion depth(s), etc.

If the grid frequency was unstable and the unit was being operated in Pre-Selected Load control then it might be possible for the unit to experience a primary zone reignition event leading to Extended Lean-Lean combustion mode.

And if the unit has IBH enabled and active then the load will have to be MUCH lower than without IBH enabled and active to break the Premix latch and get the unit into a condition where it will be possible to get the unit to transfer back into Premix while re-loading the unit.

Higher than normal ambient temperatures can also cause problems, as can lower than normal temperatures, as DLN tuning may need to be redone if previously last dome at a much different ambient temperature. While the OEM likes to say DLN-I is a mature technology--it's still a long way from "set it and forget it." A long way away.

If you want help--you MUST provide the requested information/answers, ALL of them. Not just the answers deemed (by you) relevant. We need actionable data to be of any further help, as we aren't there beside you at your site. There are too many intangibles for us to guess any more than we already have and still have a hope of being correct. If you want more help and information you have to provide the requested information. There's no other way.

We want to be of help--but we need more, and better, information. Without it, we're guessing--just like you are. Alarms and operating conditions are important keys to helping you--and us--understand what may have happened. At this point, <b> based on the information provided,</b> there's nothing else which can be said.

A LOT of GE-design Frame 9E heavy duty gas turbines are operating in parts of the world where the grid frequency is not stable and the gas fuel supply pressure is not stable and Process- and Diagnostic Alarms are not given proper attention. Just because a unit is running doesn't mean it's running properly and can or will continue to do so--ESPECIALLY when equipped with DLN-I combustors AND while being operated in Pre-selected Load Control mode during abnormal conditions. Operators need training to be able to respond appropriately to alarms and unusual situations--or they will have to learn from experience. There is a saying, "Good judgement comes from experience, and experience comes from bad judgement." That's a fancy way of saying, most people learn from mistakes; hopefully they aren't bad mistakes.

If you haven't received proper training and are being expected to properly operate a multi-million US dollar machine, some mistakes are going to occur. Turbine control systems are NOT as sophisticated as salespeople and owners and many operations supervisors think they are. Operators need training or experience--and a lot of experience comes (without training) from making, or witnessing, mistakes.

From the information provided, the operating conditions were not normal, so why should the machine be expected to operate normally? It's not logical, and especially not for a DLN combustor-equipped machine. If you would be more specific about what caused the sudden load change to be necessary, it MIGHT be possible to provide a little more specific information. But, that means providing more information about the incident, and to get a really good response we need the answers to the requested information.
It kind of looks like you don't understand how DLN1 works! Is there someone there who does? There are a lot of documents on the operation of DLN, you should have some in your manuals,do you?
Dear CSA and KHE, I will wait and see if the original poster provides more information before going too in depth with logic.

KHE, it would help us if you could try to explain how familiar you are with the operation of the gas turbine itself, and more importantly to your question the general operation of a DLN1 fuel system.

In general the combustion modes in order for a DLN1 machine with inlet bleed heat are:

Primary mode-from startup up to TTRF1 temperature of 1650 degf.

Lean-lean positive-from TTRF1 of 1650 degf to the pre-mix transfer point.

Pre-mix steady state-from TTRF1 of 1950 degf to full load exhaust temperature control rating.

There are several other sub-modes during the transfer from lean-lean positive to Pre-mix steady state but at this point I think discussing these would lead to more confusion.

KHE, if the unit is operating normally then you should never see combustion modes of extended lean-lean.

The unit can only operate in pre-mix mode in range of machine output. At low outputs there is too much air entering the combustion area to support combustion. IGV's and inlet bleed heat are way to reduce combustion air and increase the range of operation in pre-mix, but are still limited at the low end to around 45% of machine rated output.

Once the unit is in pre-mix mode then the operator needs to understand the range of operation available while still keeping the machine in pre-mix mode. Usually watching the TTRF1 temperature is the best indication although a senior operator will probably know the approximate minimum load that the unit can operate before it switches out of pre-mix and back into lean-lean. The number I have is 1950 degf, but that is for a 60hz machine in the US so your numbers may be slightly different. All of this should be in your operation manuals if you should have access to.

I hope this helps some.
Thank you for all your answers. I will do some additional analyzes to evaluate the situation more accurately.
We had a GT MS6001B with DLN-1 and IBH. can you explain to me the difference between these modes of operation.
* Positive Lean-Lean
* Negative Lean-Lean
* Extended Lean Lean

thank you

Positive- and Negative Lean-Lean are transitory combustion modes--you will only see them when transitioning from Lean-Lean to Premix (Positive Lean-Lean) and when in Lean-Lean while unloading from Premix (Negative Lean-Lean and Positive Lean-Lean).

Extended Lean-Lean has been explained MANY times on Control.com in the past. There is a 'Search' field at the top of every Control.com webpage which you can use to search previous threads to find information about Extended Lean-Lean Combustion Mode.
excellent document, thanks for sharing.

yes, i have searched and i have found very interesting and explanatory information. i ask it because in some of them they related these modes to one of the values of the exhaust temperature.

Generally speaking if the DLN gas fuel systems are working properly there won't be a problem with exhaust temperatures during any combustion mode. You wrote: "... they related these modes to one of the values of the exhaust temperature. ..."

Two things: First, can you list specific threads/responses which you feel need clarification?

Second, specifically WHICH one of the values of the exhaust temperature did the thread(s)/response(s) refer to? Are you talking about high individual exhaust temperature readings (one or more of the multiple T/C inputs from the gas turbine exhaust area)? This is typically referred to as a high exhaust temperature spread--when the difference between individual exhaust T/C readings can be very high (more than 40-50 deg C or so). Is the machine at your site experiencing high exhaust temperature spreads during combustion mode transfers or during Extended Lean-Lean operation?

Exhaust temperature spread problems have been discussed many times and in many threads on Control.com. It would be pretty unusual for a single (you wrote "...one of the values of the exhaust temperature....) exhaust T/C to be excessively high or low during one of these combustion modes. How long since the machine at your site has had a maintenance outage (Combustion Inspection (CI), or Hot Gas Path Inspection (HGPI), or Major Inspection (MI))?

But, without more information about what you're trying to understand or troubleshoot based on the limited information provided about these combustion modes and exhaust temperature value(s) we really can't be of much more help.

One has to remember that with the gas fuel systems (conventional OR DLN) of GE-design heavy duty gas turbines the amount of fuel entering the fuel nozzle(s) of each combustion chamber is strictly a function of the condition of the fuel nozzle orifices through which the gas flows into the combustion liner to be burned. EVERY fuel nozzle--and all the orifices of every fuel nozzle--get fuel from the manifold(s) around the axial compressor, and those manifolds are each supplied by a control valve. It's not possible for a control valve to cause a single exhaust temperature value to deviate from the others--especially only at certain points in the operation. There is nothing that ensures the equal flow of gas fuel into every combustion chamber--they are all supplied ("fed") by the manifold. The presumption is the orifices of every nozzle are nearly identical in diameter and therefore the flow through each similar orifice in each nozzle is the same. Now, rocks, weld slag, scarf (metal cutting from machining operations) and even large pieces of sand or other contaminants (I have seen cigarette butts and pieces of duct tape plugging nozzle orifices) as well as entrained hydrocarbon liquids (such as gas compressor lubricating oil, or diesel or gasoline, sometimes used to clean natural gas lines) will carbonize on fuel nozzle tips and block/restrict gas fuel through the orifices.

BUT, when this happens, it usually results in more than one exhaust value deviating from the other values.

As has been written MANY times on Control.com--most exhaust temperature spread issues are the result of blockages/restrictions of fuel nozzle orifices. This causes the temperature of the hot combustion gases in one or more combustion chambers to be LOWER than normal for that combustion chamber, and those affected temperatures are almost NEVER directly downstream of the affected combustion chamber (because of a phenomenon called "swirl"). There are some conditions where cracks or holes develop in combustion liners or transition pieces or the seals between them that result in excess air entering a combustion chamber and diluting the hot combustion gas temperature of that combustion chamber--again, resulting in lower-than-normal hot combustion gas temperatures for the affected combustion chamber and for more than a single exhaust temperature value. AND, these rarely occur at the combustion modes you are referring to.

There have been occasions when compressor bleed valves leak into the gas turbine exhaust area and cause one or more exhaust temperature values to be lower than normal--but again, this is not normal for the combustion modes you asked about; it usually affects the same exhaust temperature value(s) over the range of operation not just at certain times (such as during the combustion modes you asked about).

Hope this helps!

Unless you can be more specific, we can't either (be more specific).
Hello, CSA
Again thank you very much for the time spent in sharing knowledge. It is now clear to me that the modes: Positive Lean-Lean, Negative Lean-Lean and Extended Lean-Lean are transient modes between primary and pre-mix mode. My query is not because we have a problem with turbine exhaust temperatures. I am starting training to operate the machine and I find that there are many concepts that are not clear. I value very much the knowledge of my colleagues and the one found in the official GE documents. The temperature relationship that I mention is commented by my colleague mthines in message 7 but with the document that glenmorangie shared with me in message 10 it has become clear to me.

It seems to me very interesting the problems or failures that he has exposed in the answer, although they have not been presented with our machine. But at a certain moment that something like what you mention happens, I will already have an idea where to start for the solution of the problem.

As I said before, I am new to this subject and I have and will have many questions that I hope to be able to consult them through this medium to receive knowledge and guidance, which for many due to their work and academic experience will be obvious to them but not for me. I hope to count on you.

Thank you very much again
Hello CSA and friends,

having read the old forums that deal with this subject, I have the following doubts:

I have clear that the Positive Lean-Lean, Negative Lean-Lean and Extended Lean-Lean modes are transient between primary and premix, however from where each one starts and ends? it is called Positive Lean-Lean because it goes from primary to premix and Negative Lean-Lean because it goes from premix to primary? and the extended mode what condition does it present?

I remain attentive to your clarification of my questions and concerns.

thank you very much
Extended Lean-Lean is not a transient mode, you have either selected it or you are not making the transfer from Lean-Lean to
Premix properly. Check your Control Spec and see what TTRF you should make the transfer at, don't dither at least 1/2 Mw either side of that Temp. get through it quickly and never try to operate in that Temp. range or will run into Extended