Hello to all.

Introduction: The unit is Dual-Fuel 7FA turbine, DLN2.6. Primary Fuel is Natural Gas.The liquid fuel system have 3-way valve and re-circulation system. The unit runs on liquid for testing each outage period. We mainly suffer from two issues related to 3-way valve.

The first is liquid fuel coking inside the 3-way valves which causes high exhaust temperature spreads. The reason we have the spread issue is that in Summer, when the operations see smoke coming from the turbine compartment (indicating there is external liquid fuel leak) they isolate the liquid fuel re-circulation system without purging it with N2. But I believe this doesn't have anything to do with the problem in hand because this problem we suffer from long time.

Second issue we face is that while the unit on turning gear, we force the re-circulation system, and the 3-way valve passes amount of liquid into the Cans where it shouldn't. This is caused by the failure of the seals. Also when you run the unit on liquid. first the turbine will go into purge for some time, during purge time the liquid re circulation will start to get rid of any air in the lines. during this purge time and re circulation ON maybe some liquid will leak into the Cans and go to the false start drain. Although that we have these issue, but I think they are not the cause of the issue I will talk about. Because We have been experiencing these issue since long time but the issue I will share is new one.

As regular, we do borescope for all units during outage. One of the units showed fuel nozzle Carbonization nearly in all Cans. We have GE office for consulting. according to them this is the first time this happens in our Turbines. They have sent the picture for Engineering waiting for their reply. In the meantime, I wanted to share the pictures with you and benefit for the good experience you have, and I am ready for any more details you want me to provide.

I don't remember there was any alarm before this problem occurred

Next week we will do scheduled PMs for the same unit, like changing the liquid fuel filter (I think they don't have high delta P), cleaning the LF Strainer, etc.

The Borescope pictures are in the following links

Note: We have started the re-circulation while the unit on turning gear before doing the Borescope. Since some 3-way valves are passing, you will see liquid at the liquid fuel passage (the hole in the center)

https://drive.google.com/file/d/0B9R-rponHkn5eTN2NnUtM0Z6TDg/view?usp=sharing

https://drive.google.com/file/d/0B9R-rponHkn5cVB3QTdfTVJHZ2s/view?usp=sharing

https://drive.google.com/file/d/0B9R-rponHkn5dlRSSlcxWkhMbXc/view?usp=sharing

https://drive.google.com/file/d/0B9R-rponHkn5cjAwTlRGbkRsamM/view?usp=sharing

https://drive.google.com/file/d/0B9R-rponHkn5SGgzcjM4QXQ1ZVU/view?usp=sharing

https://drive.google.com/file/d/0B9R-rponHkn5V2RlalhGdlR4TlU/view?usp=sharing

https://drive.google.com/file/d/0B9R-rponHkn5dW1EdGhvVkMyWVk/view?usp=sharing

 

Aptx4869,

Thanks for sharing these photos.

In my personal opinion the amount of coking (carbonization) is extremely minimal; I've seen much worse. MUCH worse. I suppose, though, that the liquid fuel internals of the fuel nozzles may have blockages that resulted from the carbonization of fuel in the passages. And, that's probably worse from a reliability standpoint that what is visible on the fuel nozzle tips.

I do have a comment about your statement that when transferring to liquid fuel from gas fuel when the unit is running that any leakage of liquid fuel past the three-way valves into the combustors goes into the false start drains. I think it's going to be vaporized/combusted long before that. It's not likely that with the air flows through the fuel nozzle passages that the fuel could dribble out and fall to the false start drains (which are only on the lower cans and the combustion wrapper).

Look, it's been said many times before on control.com and on other similar World Wide Web forums: Liquid fuel check valves (three-way valves in your case) are the Achilles heel (weak link) in dual fuel systems. They just are. There's not any nice way to say it. It just is. If you know the seals of the three-way valves leak and you "force" recirculation when the unit is on turning gear then who's at fault? GE? The valve manufacturer? The operators? Supervision? The Maintenance Department? (These are rhetorical questions.)

There are water-cooled three-way valves; are the three-way valve at your site water-cooled? Have you considered insulating or otherwise trying to cool the three-way valves at your site--on at least one turbine? It's likely that either heat is causing the seals to fail prematurely, or the wrong material is being used for the seals (I presume o-rings). But, even if the seals were probably correct it's still likely heat is the culprit here. Especially if these three-way valves have been in service on this particular unit for a while and you're just finding this carbonizaion evidence.

Can you provide pictures of the location of the three-way valves?

There really are four options here (four, if you count doing nothing as one option). First, you can work with GE to try to understand the cause of the issue, and why the three-way valve seals are passing (presumably prematurely). Second, you can work with valve suppliers/manufacturers to source, modify or design new valves which could be tested at your site. Third, you can modify or design new valves yourselves or source them without the aid of a supplier/manufacturer. Fourth, you can try solutions (such as trying to reduce exposure to heat either by shielding the valves, or providing more cooling air flow, or by water-cooling them, etc.) to try to mitigate the problem.

But, it seems if you know the valve seals are leaking (passing) and you continue to force recirculation when the unit is on turning gear, well, then that doesn't really seem to be any kind of solution or mitigation. And, if the departments can't all get together and decide on a proper response to external liquid fuel leaks when the unit is running then that's not really helping the problem either.

Your candor and honesty is appreciated, but, can you seriously say that the actions you describe are not contributing the carbonization you have found? Sure; the valve seals are leaking/passing, but it would seem, wouldn't it, that if you're aware of this issue that you would take steps to minimize leakage until such time as the seals can be repaired or the valves replaced? And, in the interim begin taking action on one of the other four options--and there's NOTHING preventing parallel paths of troubleshooting and resolution.

A lot of people but these machines and think they bought a perfect machine with no issues--and that the packager/OEM will resolve every issue that arises. I'm willing to bet that if GE knows about the operations actions you have described they are going to say, "We don't condone the methods and practices, and you have caused your own problems. We've never seen this before."

If you buy an automobile or a bicycle with tires that don't last as long as desired, or brake pads that don't last as long as desired, are you going to keep using OEM parts? Or are you going to source other parts in an effort to improve reliability?

The OEM is working hard on this problem--trust me when I say this. There is at least one group tasked with developing more reliable solutions. And, since there are multiple locations with responsibility for different Frame sizes, there is probably at least one group in each location working on the problem, if not a multi-national (world-wide) group also working on this issue. It's funny, because, 25 years ago, banks and investors forced owners to buy gas turbine with dual fuel capability. And they all hated it (the owners). As soon as they could, they got rid of the liquid fuel capability. Now, many of the current owners of those plants see dollar signs for having dual fuel capability and they are requesting companies to restore liquid fuel capability. And, those that have it are experiencing component problems, such as yours. Things have come full circle. As they usually do

As an owner/operator there's nothing that prevents one from working on their own solution. I know lots of problems that the OEM did not acknowledge that were solved by some very smart people at the sites. Sometimes, the OEM adopted those solutions, other times they tried to re-invent/"improve" them, with mixed results. Work with them, or work on your own--but waiting for them to solve the problem, and continuing to operate as described, and you're going to continue to have problems.

And be frustrated.

Sorry; wish the news were better. It's a quandary. You could also hire a consultancy to analyze the problem and propose solutions--a fifth option.

 

CSA

Thank you for the fast reply. When running the unit on gas and the re-circulation is ON, usually we suffer from liquid passing into the False Start Drain Tank. The source that may cause the passing are several points in the Liquid Fuel System including the Liquid Fuel Filters Thermal Relief Valves. So, the On-Site GE Control Engineer Adviser gave us a procedure to pinpoint the cause of the passing. The Engineer didn't specify wither the test should be carried out while the unit is on-service or not. So we do it while the unit is on turning gear for easiness (WE force it only for half an hour for testing), and the GE Engineer knows about it. during the test, and if the 3-Way Valve O-Ring Seal is damaged, it will pass small amount of liquid into the Cans.

The 3-Way valve is conventional one, not water-cooled and I believe one of the cause of the premature failure of the O-Ring is that the liquid fuel re-circulation will be isolated during Summer if there is external leakage of liquid inside the Turbine Compartment. Just like the Jug Kettle in home, when you run it without any water inside, it will be overheated. The same goes for the O-Ring, when there is no continuous flow of liquid through the 3-Way Valve, the valve will overheat and the O-Ring will damage prematurely.

I don't think the cause of the present issue is the forcing the re-circulation while on turning gear, because one of the units experienced the same issue "same amount of carbonization in the fuel nozzle" without forcing the re-circulation while on turning gear. but that unit had carbonization in 4 Cans only.

So there must be a cause of the problem in hand, and I am trying to identify the cause before the problem gets bigger.

The three-way valve is on the end cover of the can inside the turbine compartment, here is one picture:

https://drive.google.com/file/d/0B9R-rponHkn5N1otbjhsdHpUSjQ/view?usp=sharing