Fuel Change Over Mark Vie GE Frame 9E

How is fuel changeover (from Gas to Liquid) happening in Frame 9E Gas turbines (GE) under Mark Vie control system?

I know that a signal from gas pressure switch should initiate fuel changeover from NG to liquid fuel, but how is the logic working and which signal exactly is considered as changeover signal to increase FSR1 & decrease FSR2?

Can someone explain it in detailed way please?

Regards
 
I'm sorry for the delay in responding. I was recently shamed when I tried to explain how some function worked on a unit which was built and configured by GE in Belfort, France, and learned they have completely changed how that particular worked--after decades (literally) of no problems whatsoever with that function. Instead of asking for access to the site-specific application code (the unit was controlled by a Mark VIe....) I just started explaining how it worked, and was shortly cut off by one of the site technicians who knew enough about how their specific function worked to be able to say, "He doesn't know what he's talking about!" Which, it turned out was true.

I was able to solve the problem (which was NOT with the Mark VIe, but with a device that was not even under the control of the Mark VIe) after spending some time reviewing the site-specific application code and P&IDs and walking around the unit and observing the configuration of the piping and control elements--but it was a very embarassing experience.

GE-design Frame 9E heavy duty gas turbines fall under the purview of GE-Belfort, so I am EXTREMELY reluctant to even try to provide any kind of description other than what typically happens--which may or may not be correct for the unit at YOUR site. So, I wasn't going to reply at all, but I'm going to take a swing at it--with the caveat that with units which are being configured by GE-Belfort it's just a guess as to what might really be happening at your site. The ONLY way to know for sure how things work at your site is to examine the application code running in the Mark VIe at your site, in conjunction with the relevant P&IDs.

In general, when a dual fuel unit (gas/liquid) is running on gas fuel if the Speedtronic senses low gas fuel supply pressure it will initiate a transfer to liquid fuel. Liquid fuel has to be pumped from a storage tank to the inlet to the turbine (upstream of the Liquid Fuel Stop Valve), so pumps must be usually started and some time allowed for pressure to build up in the system.

After a short time delay for pressure to build up in the liquid fuel system, the liquid fuel stop valve will open and the liquid fuel pump clutch will be energized. The Liquid Fuel Bypass Valve (LFBV) is fully open which recirculates fuel from the discharge of the high-pressure liquid fuel pump back to the pump's suction. The Atomizing Air Bypass Valve is usually opened at some point, and the Liquid Fuel Purge Air Valve is usually moved to the position to shut off the flow of purge air to the liquid fuel nozzles.

Then the Speedtronic starts ramping down the percentage of gas fuel (FSR2) as it ramps up the percentage of liquid fuel (FSR1). The percentage expressed is always a percent of liquid fuel--so when the unit is running on gas fuel, the percentage of liquid fuel is 0%. The ramping is done by an algorithmic block, usually called FSR1V1 (or FSR1Vn, where 'n' is the revision number used for a specific application). USUALLY, the time for ramping of the two fuels is approximately 30 seconds--meaning that once the ramping begins it will be completed in approximately 30 seconds, and the percentage of liquid fuel will be 100%.

Once the fuel changeover from gas to liquid is complete (the ramping of the two FSRs), after a short time delay the purge air for the gas fuel nozzles will be started.

Depending on how the unit was being operated AND how well fuel transfer tuning was done (and how close the two fuels are now (composition-wise; UHV; LHV; BTU content; etc.) the unit SHOULD end up at approximately the same load as when the transfer was initiated (presuming the gas fuel supply pressure didn't drop too far!). If the unit was operating on Pre-Selected Load Control or Base Load and the gas fuel supply pressure dropped quickly AND/OR the liquid fuel supply pressure was unstable at the start of the changeover. If the unit was operating on "straight" Droop Speed Control (meaning NOT on Pre-Selected Load Control!) the load may or may not be exactly the same when the transfer is complete. If the unit is on Pre-Selected Load Control during the transfer, and the liquid fuel supply pressure is unstable during the transfer OR the gas fuel supply pressure dropped quickly the unit might even trip. And, the same can happen if the unit was at Base Load when the transfer was initiated.

MOST people expect that a transfer from gas to liquid will be "bump-less"--but it usually isn't. And, depending on how often the unit operates on liquid fuel, it's very possible the unit will be very unstable during the changeover and will trip. It's the nature of the system that check valves, in particular, leak (liquid fuel check valves) and wear (purge air check valves and water injection purge check valves, if equipped) while running on gas fuel--so it's often that high spreads are encountered during or after a fuel transfer to liquid fuel from gas fuel. And, again--there are LOTS of liquid fuel devices that aren't under the control of the Speedtronic (liquid fuel pump pressure; forwarding system pressure (via independent regulator, usually); check valves (which operate strictly on pressure/differentials). And those devices all have to operate pretty much flawlessly for the liquid fuel system to work properly--especially when expected to do so in less than a minute or so while maintaining load and flame in the gas turbine combustors to keep load stable.

Think, for a second, about all the things that are happening during a fuel transfer--it's a LOT. And I'm only referring to what's happening while the percentage of gas fuel is being ramped down as the percentage of liquid fuel is being ramped up. If load is changing and the unit is on Pre-Selected Load Control or on Base Load and load is changing (because of air in the liquid fuel piping, or leaks of the liquid fuel purge air check valves, or poor liquid fuel atomization because of problems with the liquid fuel check valves/nozzles, etc.) and FSR is changing trying to maintain load or exhaust temperature WHILE the percentages of fuels are being ramped up/down, well, it's pretty amazing. And, then if liquid fuel supply pressure isn't stable (which means flow-rate isn't stable), and the LFBV is bouncing up and down trying to maintain a stable flow-rate (through the liquid fuel flow divider), and, possibly the gas fuel supply pressure (and flow-rate) is dropping, well, it's just a LOT of things for the Speedtronic to respond to. And, a LOT of devices that aren't directly controlled by the Speedtronic, either, that have to all be working correctly. It's a wonder it works at all, really! There aren't a lot of control systems that can do it when everything works as it should, much less when one or four or six parameters/devices aren't working properly.

If there is ANY air in the piping between the storage pumps (in the piping; in low-pressure liquid fuel canisters; in the high-pressure liquid fuel filter canister; in the liquid fuel lines between the high-pressure liquid fuel pump outlet and the liquid fuel check valves at the fuel nozzles, especially that has to pass through the liquid fuel flow divider (which is NOT controlled by the Speedtronic, either--it just receives a pulse-rate based on fuel flow-rate)--then all manner of instability can occur, which usually leads to load swings and even tripping. Air in piping is the biggest cause of instability and tripping during transfers from gas fuel to liquid fuel--presuming gas fuel supply pressure (flow) doesn't drop so quickly that flame is lost because there just isn't sufficient liquid fuel flow to make up for the loss of gas fuel and the air flowing into/through the combustors blows the flame out. (Remember: There is three to five times more air flowing through a heavy duty gas turbine than is required for combustion (fuel/air mixture). Most of that is used for cooling and diluting flame temperature, and in DLN machines its used for premixing and leaning-out the fuel/air mixture to reduce NOx formation.) The excess air is heated and that's what is expanded as the hot gases pass through the turbine nozzles and buckets, also. (In the same way combusting fuel heats water/steam in a boiler, combusting fuel heats the air that passes through a gas turbine.)

Lastly, usually when the gas fuel air valve is opened (it should open slowly, if possible!) there is usually a slight load spike caused by the combustion of the small amount of gas fuel in the gas fuel manifolds, hoses, and fuel nozzles. (Opening the purge air valves slowly helps reduce the magnitude of the spike.)

I don't know what else to say. I did look at some application code and sequencing, but, again--as with all things coming out of GE-Belfort these days there are LOTS and LOTS of changes.

Hope this helps! If the unit has DLN-I combustors (which you didn't tell us), it can be even more complicated as there are multiple gas control valves and more liquid fuel system piping/tubing and combustion modes.

Sorry I'm not able to be more specific.
 
Dear CSA,

As usual; your comment is very informative and useful. I appreciate it and thanks a lot for that.

But actually I was trying to know one specific point which is what is the signal name responsible for initiating fuel change over. I understand very well that there are differences from unit to unit, but I thought that might be some common signals which is used everywhere for automatic change over (from gas to liquid).

I will tell you Why I am asking this question, We are requested from our management to modify the logic of change over by adding some time delay (60 sec) after getting low pressure signal from pressure switch l63FG-1 which is located far away in NGPRS (natural gas pressure reduction skid), we have one more switch in turbine gas inlet which will initiate change over immediately.

I am going to put here all the signals which are linked with pressure switches and in-case you have a similar logic you may be able to identify which signal is initiating C/O process:

L43FL_PRSET: Liq Fuel Selection Preset
L83FZ: Fuel Change Permissive
L83FMX: Fuel Mix Hold Aux logic
L43FTG: Fuel Transfer To Gas

pressure switch is only used in above mentioned signals, so obviously one or more of them should be responsible to initiate change over.

My question is: if I want to add time delay, to which signal here I have to add.

I am depending here that given information may be similar to a typical logic which you may have in your site. If not, then of course it will be difficult to answer my question.

Regarding the last point mentioned in your reply, we don/t have DLN in our units.

Again and again many thanks in advance for giving a time to read my post and trying to help and explain.

Thanks & Regards
 
transporter84,

My best guess is L43FL_PRSET, because that logic signal, when it goes to a logic "1", causes the Fuel Selection Command State block to select liquid fuel and initiates the transfer to liquid fuel. It takes precedence over the other enabled PRSET inputs, because of the order of them when assigned to the Fuel Selection Command State block.

But, again--I must state that without being able to review the site-specific application code for the turbine at your site that's my best guess.

Hope this helps!
 
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