Hello,

Fr-9E DLN 1 Unit just returned back from HGPI and fail to go premix transfer on Gas with high spread around 45 Deg C during the transfer from Primary lean to premix mode.

In last 3 attempts unit goes to extended lean lean mode.

Sequence of Operation - GE controls TA at site

Unit started normally reached 40% load - did liquid fuel transfer successful with all parameters normal. Again transfer to gas increase the load to 60% - lean lean mode with all parameters normal.

Change to liquid fuel - all parameters normal checked for both primary & secondary lines leaks - no issues noted.

Transfer back to Gas - start loading further.

Unit goes to extended lean lean operation during the transfer - premix operation was not successful. Spread noted observed around 45 deg C during the transfer. Primary all flames ON. unit was brought down to primary mode and again attempt to take unit to premix but no success again went to extended lean lean.

GE decided to make IBH ON this time and unit reached premix mode successfully without any issues. At baseload noted spread was around 38 Deg C, NOx around 33ppm & CO about 5ppm. GE control TA tried adjust the split to position at best possible to lower the NOx between 78% to 81%..unit goes to extended lean lean when tried to go below 79% or above.

GE TA confirmed issue with combustion system or the accessory components for the fact Unit unable to reach premix operation with IBH off.

It was decided to make Borescope on combustion components to understand any combustion hardware issues, however no indication found during borescope for any sign of secondary/primary fuel nozzles or Liners.

After discussion with GE engineering they recommended to replace all primary/Secondary fuel nozzles - which is under progress.

Same they suggest to perform leak test on primary & Transfer purge valves VA13-1, 2, 3 & 4 along with Transfer valve. this was done with GE site team. found no issues with Purge valves and transfer valve was sent to Fisher for testing and found leakage is within their acceptable limit. Calibration of all the valves was confirmed by GE control TA at site - no issues.

Just to Add - when unit was dis-assembled we found damage of secondary/primary fuel nozzles and liners #7 & 10 can were damaged but still unit was able to go premix operation with any issues before outage and spread maxmium of 28 deg c.

Now we are going for startup and would like to know from your experience any further advice? or anything to add part of solution?

Regards,
Arun.

 

kumara,

It's not enough to just say, "It doesn't transfer into Premix mode, what could be wrong?" And I want to remind other potential posters this thread is presently about DLN-<b>I</b> combustion systems, which are not like DLN-2.n systems in operation and construction. There are few similarities between DLN-<b>I</b> fuel nozzles/liners and DLN-2.n nozzles/liners.

One needs to follow the required sequence of events that has to occur during a Lean-Lean-to-Premix transfer and say at what point the transfer was aborted by the control system (there should be a Process Alarm to indicate why) or when it failed.

Is the flame in the primary combustion zone being extinguished during the Lean-Lean-to-Premix transfer?

Is the unit actually making the transition to Premix and then flame is suddenly being detected in the primary combustion zone?

What are the exhaust temperature spreads before, during and after the Lean-Lean-to-Premix transfer?

What Process Alarms are being annunciated during the Lean-Lean-to-Premix transfer? What Process Alarms are active BEFORE the transfer is attempted?

What is the combustion reference temperature (TTRX, or possibly TTRX1) before and after the unsuccessful Lean-Lean-to-Premix transfers?

Are the operators using Pre-Selected Load Control setpoints to load the unit from Lean-Lean to Premix? If so, what is the load setpoint they are using? Have they just tried selecting Base Load and letting the unit go through the transfer and then stopping the loading by clicking on RAISE- <b>or</b> LOWER SPEED\LOAD after the transfer has completed or been aborted/failed?

The fact that burnt/damaged components were found at the beginning of the inspection is NOT a good omen. Does the unit have gas fuel heating, and, if so, was it working when the unsuccessful transfers were attempted? One of the most common causes of primary zone reignitions (unintended transfers to Extended Lean-Lean from Premix) is hydrocarbon-based liquids in the gas fuel, or unpurged, dribbling liquid fuel from the liquid fuel nozzle tips could also be the problem. If there was evidence of hydrocarbon deposits on the components which were removed that could be an indicator of entrained liquids in the fuel. Some manifold assemblies actually cause entrained liquids to exit particular nozzles, contrary to what would be expected. It has to do with where the gas fuel enters the manifold and if the manifold is continuous around the machine or not, as well as the specific gravity and amount (weight) of the entrained liquid. Even the pressure drop across a nozzle (or control valve) could be enough to result in a sufficient temperature depression to result in condensation of gaseous liquids if the gas fuel temperature is near the dewpoint of the entrained liquid (which is usually the reason for heating the gas fuel to ensure it has at least 50 deg F of superheat before it enters the turbine gas fuel system).

Unpurged liquid fuel dribbling from the fuel nozzles could also cause this problem. There are small liquid fuel flow dividers--called liquid fuel distributors on the combustor end covers that could not be working correctly and contributing to this problem.

Improper IGV LVDT calibration and/or improper Gas Splitter Valve- and/or Gas Transfer Valve LVDT calibration could cause this problem. More likely the IGVs, but possibly the Splitter or Transfer Valve. If they were refurbished and not mechanically adjusted properly, compounded by a poor LVDT calibration. one or more of these could be the problem. many times improper Gas Splitter Valve LVDT calibrations result in an inability to tune NOx/CO correctly. Many sites perform LVDT calibration without measuring actual physical position versus LVDT feedback, preferring to believe the Speedtronic is incapable of a bad calibration. An LVDT is just like any other field device--temperature switch, or pressure transmitter--they must be tested first to see if the existing calibration results in the proper feedback, and if the as-found condition is within specification then no adjustment of the calibration is necessary.

New or improperly refurbished gas fuel valves (other than the SRV) can also be the source of problems like this immediately after a maintenance outage. If they don't seal properly at either end of travel (especially the Gas Splitter Valve) or if the valve characteristics are different from the original equipment valves and the LVDT interpolation tables are not correct for the valves, this can cause this kind of problem.

If the IGVs are closed further than LVDT feedback then the fuel air mixture is going to be too rich and it won't remain in Premix mode. That's yet another reason why it's important to know at what point the Lean-to-Lean Premix transfer fails or is aborted.

A common problem with failing to meet emissions after a maintenance outage is that the dilution holes in the combustors are not the proper size, and so changing the fuel split does not result in the desired emissions reduction. If the dilution hole sizes are seriously wrong, this could negatively impact Lean-to-Lean Transfers, also. Improper re-assembly (cross-fire tubes not installed properly; etc.) are also frequent causes of problems like this immediately after a maintenance outage.

I've not experienced a fuel nozzle problem causing an inability to transfer into or remain in Premix--unless it was accompanied by high exhaust temperature spreads.

GE has developed "fishbone diagrams" for troubleshooting DLN problems--however, they are very basic and while they're sometimes helpful in quickly resolving a problem sometimes they result in a lot more mechanical work than necessary. Especially on dual fuel machines. That's why's it's vitally important to try the quick and easy stuff first, then back off and analyze exactly what the problem is--when the failure is occurring (after Premix is established, or before the transfer is complete, or ???)--and examine exactly what was done during the outage to try to narrow down the possibilities.

Troubleshooting is a logical process of elimination--especially when the first couple of attempts are unsuccessful. There's always a rush to get started after a maintenance outage, and, of course, the Mechanical Department did nothing wrong--because the Speedtronic turbine control panel is the cause of all problems! Just ask any mechanic or Plant Manager--the Speedtroinc is either the cause of every problem, or the solution to every problem. But, clear definition of the problem--when exactly the problem is occurring--is critical to any troubleshooting. Just saying the unit won't transfer isn't going to lead to an efficient and timely resolution to the problem.

And, just in case you were wondering--the Speedtronic is <b>RARELY</b> the cause of this type of problem. Even if it's a poor LVDT calibration, that's a self-inflicted Speedtronic problem--not a Speedtronic problem. Someone caused that poor LVDT calibration--either by not performing the LVDT calibration properly or not verifying it properly.

Hope this helps! Please write back to let us know what you find!

 

Hi

Sorry for my delayed response and i didn't get your response to my mail mentioned and also got busy with other plant activities.

GE tried & performed many checks -

combustion assembly / components were ok
Gas Transfer valve / GCV / Splitter valve / Primary & transfer purge valves / IBH / IGV - ok / calibrated / checked physically pressure tested - holding pressure - no leaks

Generally - normal DLN1 unit - lean lean to premix transfer operation

1.TTRF1 1077 deg C - This is the transfer temperature from lean-lean to premix. The dead band on this temperature is 28 deg C & VA13-3&4 closes

2. TTRF1 1091 deg C - This is the temperature above which the unit is in premix steady state and generates the command to close the transfer valve. The dead band on this temperature is 14 deg C. VA13-3&4 starts to open

3. TTRF1 1097 deg C - . This is the temperature above which if the unit goes to lean-lean, the unit registers this as extended lean-lean. The dead band on this temperature is 6 deg C.

Main observation -unit following above curve during transfer but the temperatures shooting high as 1115 to 1120 deg C when close command goes to transfer valve at this point gas is set 79% to Primary & 21% to secondary and VA13-3 & 4 closed. in this condition unit immediately back flashes.

GE confirmed its clear indication of Less air going to combustion system which is making combustion zone hotter, hence they adjusted to transfer set points to initiate transfer at lower TTFR1 which is now set to 1050 Deg C and unit successfully transfer to premix with all parameters normal.

However unit is still under observation and pending analysis from GE to find out where cooling air is lost which is making combustion zone hotter.

What is your opinion?

Regards,
Arun.

 

Arun/kumara,

It's extremely difficult to understand how "less air" can be going to the combustion system during initial Premix operation with IBH off. The ONLY way to limit air flow is with the IGVs--that's the only way. So, while it might be possible the IGV LVDT calibration is incorrect that doesn't make a lot of sense, either. The feedback for IGV position is exhaust temperature control for DLN-I machines, so even if the IGV LVDT calibration was off the Speedtronic is just going to make the IGVs move to the position that results in the desired exhaust temperature==even if that means the IGVs are actually, physically at 81 DGA but the "calibrated" LVDT feedback says they're at 83 DGA, or if the IGVs are physically at 83 DGA but the "calibrated" feedback says they're at 81 DGA. The Speedtronic is going to move the IGVs to the position required--regardless of the position feedback--to make the exhaust temperature equal to the exhaust temperature reference.

A 45 deg C exhaust temperature spread is pretty high--and one of the things that the Speedtronic does is if the exhaust temperature spreads are high during Premix operation is to energize the ignitors and revert to extended lean-lean. Remember--there are only four combustion cans with primary flame detectors out of 14 combustors, so it's only possible to detect primary flame in four of the 14 combustors (with a flame detector). If diffusion flame develops in the primary combustion zone of one of the combustors <b>without</b> a flame detector, then that will cause a higher than normal exhaust temperature spread and it's this spread logic when in Premix that protects against primary flame in one or more combustors <b>without</b> primary flame detectors. Unless they put primary flame detectors in all 14 combustors this is the only other way to protect against primary flame in the ten combustors without primary flame detectors.

The 1115/1120 deg C temperature you mentioned--that's TTRF1, not exhaust temperature. Has anyone verified that the TTRF1 calculation--and it's inputs--is working properly? I've seen some problems with transfers caused by incorrect TTRF1 values.

If I recall correctly, when the IBH is off and the unit is trying to transfer into Premix, the exhaust temperature, TTXM, is very close to the exhaust temperature reference limit, TTRX. This is because the IGVs are closed to keep the exhaust temperature close to TTRX--though they're not fully open yet. As the unit is loaded after transferring into Premix mode, the IGVs will have to open to prevent TTXM from exceeding TTRX. But, once the IGVs are fully open and TTXM equals TTRX then the unit is said to be at Base Load--on CPR-biased exhaust temperature control.

So, you haven't mentioned that anyone has verified the TTRF1 calculation, or that it's inputs are working correctly. This could be part of--if not the whole--problem. And, while it doesn't seem possible that "less air" could be going into the combustors unless the IGVs were not opening/working correctly, it certainly could not, and would not, hurt to confirm the IGV LVDT calibration--by physically measuring the actual IGV angle with a machinist's protractor during the verification and calibration process.

Please write back to keep us informed.

 

Arun/kumara,

I want to explain the Lean-Lean-to-Premix transfer a little better in the hopes that it will help in understanding the problem--because hopefully you can better tell us what's happening/not happening when.

When the unit reaches the L-L-to-Premix transfer temperature (1077 deg C from your post), the Speedtronic starts closing the transfer fuel system purge valves (which are directing axial compressor discharge air through the transfer nozzles of the secondary fuel nozzles to keep hot combustion gases from back-flowing into the transfer gas manifold, and to cool the transfer gas nozzles, also). The valves are supposed to close relatively slowly so as not to disturb the combustion taking place in the combustors. (The transfer nozzles are part of the secondary fuel nozzle assembly, but only have gas fuel flowing through them during a L-L-to-Premix transfer; the rest of the time they are being purged through the transfer purge valves.)

Once the transfer purge valves are closed then the gas transfer valve and the gas splitter valve start moving to: 1) to direct all of the gas fuel to the secondary fuel nozzles thereby extinguishing the flame in the primary combustion zone because there is NO fuel flowing into the primary fuel nozzles; and, 2) to direct a portion of the gas fuel from the splitter valve to the secondary fuel nozzles to the transfer nozzles of the secondary fuel nozzles.

Once the primary flame detectors sense the flame in all of the primary combustion zones (of the combustors with primary flame detectors--and there are only four of those! out of a total of 14 combustors) for a few seconds, the splitter- and transfer valves start moving again. This time the transfer valve closes, redirecting all of the fuel from the splitter valve's secondary port to the secondary fuel nozzles, and the splitter valve moves to re-admit a portion of the total gas fuel flow to the primary fuel nozzles. For most DLN-I machines, this "split" between primary- and secondary fuel nozzles when operating in Premix combustion mode is approximately 80% primary and 20% secondary.

Once the transfer purge valve has closed and the unit is successfully in Premix steady state combustion mode, then the transfer purge valves are opened. And, just like when they were closed at the beginning of the L-L-to-Premix transfer, they are supposed to open slowly so as not to introduce air too quickly and disrupt the combustion taking place in the combustors.

That's what happens when a normal Lean-Lean-to-Premix transfer takes place.

Also, during the transfer, once TTRF1 reaches a certain temperature during the transfer the Speedtronic "takes control" of the FSR and ramps it up at a pre-programmed ramp rate to ensure the unit completes the L-L-to-Premix transfer without stalling. This quite often results in the load being slightly above a Pre-Selected Load setpoint when Pre-Selected Load is being used to change turbine load. And, when the Speedtronic "releases" FSR after the transfer is complete if the load is higher than the Pre-Selected Load setpoint then the Speedtronic starts lowering FSR--which can cause TTRF1 to decrease to the point that the unit will once again transition to Lean-Lean, or sometimes into Extended Lean-Lean.

So, using Pre-Selected Load to change loads can cause problems with transfers.

The other thing about the above sequence is that there are only four flame detectors in the primary combustion zones of the 14 combustors. The presumption is that if the flame in all four of the primary combustion zones with flame detectors is extinguished then the flame in all 14 primary combustion zones is extinguished. That may or may not be true--but if it's NOT true then there will always be a high exhaust temperature spread during the transfer. About the only way that one or more primary combustion zones could have flame when the gas fuel to all 14 primary combustion zones is being reduced to near zero (more on that in a moment) is if the orifices of one of the primary fuel nozzles were significantly different from the orifices of the other primary fuel nozzles. It's actually NOT necessary for the splitter valve to go all the way to the full secondary position/zero primary position because there is SO much combustion air entering the primary combustion zones that the fuel/air mixtures will lean out to the point that diffusion flame is lost even though there might be some slight flow of gas fuel still flowing through the splitter valve into the primary fuel nozzles. Again, this is where the nozzle orifice "matching" during outage planning is very important. (Fuel nozzles are usually supplied in a set where the flow-rates are matched to each other to reduce inconsistencies in fuel flow-rates into individual nozzles.)

As was mentioned before, if there is a surface or deposit that's hot enough to ignite the fuel into a diffusion flame even if the fuel flow-rate has been reduced during the transfer then it's possible that flame in one or more of the combustors without primary flame detectors might not be extinguished--but that will be accompanied by a high exhaust temperature spread. Or, if there's a hot surface or deposit that can re-ignite the gas fuel being re-admitted to the primary combustion zones at the end of the L-L-to-Premix transfer then diffusion flame can be re-ignited, resulting in a high exhaust temperature spread.

And, as described, if the exhaust temperature spread during Premix operation is too high the Speedtronic "interprets" that as a possible primary zone re-ignition (not a flash back!) and switches to Ext. Lean-Lean combustion mode.

Lastly, there are also cross-fire tubes between the primary combustion zones of all fourteen combustors. When there is diffusion flame in one of the primary combustion zones there is a tendency for the hot combustion gases to flow into the primary combustion zones of the two combustors on either side of that combustor and ignite the diffusion flame in those combustors. HOWEVER, that usually doesn't happen at higher loads--because the air flows at higher loads are high enough to prevent sufficient cross-firing to ignite flame through the cross-fire tubes. It does happen occasionally, and it will result in high exhaust temperature spreads, but it's not typical. And, the fact that it's not happening at lower loads when IBH is on seems to indicate this isn't the problem.

This should help you to understand what's supposed to be happening--and when. You, or the GE TA, should be using the data-gathering capability of the Speedtronic to monitor exactly what's happening when during the transfer to determine what's causing the primary zones to re-ignite--or not to extinguished. To my way of thinking, that's critical here--is the flame being extinguished in the four primary combustion zones with flame detectors, or not, during the transfer? What is the exhaust temperature spread doing during the transfer--is it high to begin with at the start of the transfer? Or, does it increase quickly during the transfer indicating that possibly the flame might not be extinguished in one or more of the primary combustion zones <b>without</b> primary flame detectors? And, is the exhaust temperature spread the cause for the transfer to Ext. Lean-Lean, or is it just that the primary flame is not lost in one or more of the combustors with primary flame detectors? The latter would indicate the splitter valve is not moving as closed as it believes it is--or, that the splitter valve LVDTs might be calibrated properly. Which would also go along with the inability to achieve emissions at the normal splits as you have described.

DLN-I combustion systems are complicated--and there are lots of little things that can cause problems, and if there are enough little things not working right then that can also cause problems that are difficult to find and resolve--because there are more than one of them, and they can make troubleshooting difficult.

But troubleshooting is a logical process of elimination. And, fully understanding the Lean-Lean-to-Premix transfer sequence and how fuel is shifted ("staged") in the DLN-I combustor is critical to understanding how problems can occur.

It sounds like there are multiple things contributing to the problems there--and the accuracy of LVDT calibrations is at the top of my list. It doesn't take long to verify--but verification must be done by actually measuring stroke and physical position, not just by sitting in front of the operator interface (HMI) and watching the display.

Please write back to let us know how you progress in resolving this problem.

 

Hi CSA

Ok.. let me tell you

There were many people from GE USA & Local Team been to plant to trouble shoot as this issue got escalated on very top level.

After lots of trial runs / data gathering GE concluded only issue is less cooling air going to combustion zone as it was evident at lower TTRF1 unit is able to transfer successfully (IBH ON) with all parameters being normal.

Below following were checked by GE to understand why less cooling air going to combustion system -

1. Liners dilusion holes - they replaced all liners with another set although there was no issues with removed ones

2. External leaks / CPD air bypass - was verified no hot air leaks from bearing#2 drain / flange connections / casing joints

3. Cooling / Sealing system Orifices verified - Ok

Apart from that

entire Primary & secondary nozzles were replaced with another set. Borescope of TP's seal engagement - ok. Crossfire tubes were also found in good condition. Suspect was purge air valve leaking - so they set up transmitters to monitor purge air pressure at manifold & between the purge cavity - nothing showed up and parameter they measure is ratio between CPD to purge manifold pressure - within their limit.

To suspect any combustion hardware's issue - spread / other parameters are ok with OBH ON and only when operated with IBH Off we see spreads during transfer operation during Transfer valve is closing from 100% shoots upto 40+ Deg C and TTRF1 is going upto 1121 Deg C - primary gets re-ignited and unit back flashes.

IGV's - many exercise was done

1. Physical verification of all 64 vane angles - maxm variation noted 1.5 to 2 deg

2. Servo was replaced - although no issues like coil current / lvdt response was ok.

3. Calibration - to normal procedure - ok

4. Backlash recorded was within limits

Gas heating - We have water bath heaters to maintain the gas temperatures around 48 to 50 Deg C and the same gas going to other 2 units which has no issues with premix transfer. perhaps GE even made trails to run the unit with gas temperatures set to 38 to 40 Deg C to see any effect - no difference we could notice on unit behaviour.

Above all exercise GE did - one things they observed With Split set to 79%, IBH ON, TTRF1 - 1077 deg C - unit is able to reach premix steady state without any issues which happens somewhere at 60 to 66 MW set point, maxm spread of 28 deg C, maxm TTRF1 noted 1103 deg C when transfer valve begins to close and IGV around 50 to 54 deg variation with IBH valve closing respectively.

Same when tried with IBH OFF with 79% split, TTRF1 1077 - IGV is around 58deg, Spread during transfer 48+ deg C, TTRF1 raising up to 1115 to 1120 deg C when transfer valve begins to close at appox 98% re-ignition in primary zone and unit goes to extended lean lean with MW set point of 78 to 80MW during this period. This behaviour what GE confirms not enough cooling air going to combustion zone which is making hotter. We compared this transfers in other units maxm TTRF1 raise during transfer 1098 to 1106 deg C when transfer valve begins to close not the same phenomenon happening in this unit.

For now GE reduced the transfer TTRF1 set point from 1077 to 1050 deg C to allow us to operate the unit and analyzing the data on regular basis as we update the veiw2 files.

Actually my thinking - another place where we can loose CPD is pulsation system due to leaking / bad pilot / air diaphragm valves we are dumping continuously CPD on dirty / clean air side doing no useful work and making combustion zone hotter / improper cooling which we can corelate for the issue happening in this unit.

What you think? any comments?

Regards, Arun

 

CSA

Another things i just want be clear here... "Less air" GE referring from the Compressor discharge amountof air that is taken for cooling / dilution in the combustion zone is not enough!

So i am inclined very much towards extraction taken from CPD to air inlet air filters pulsation is being dumped continuously into inlet system clean air side due to leak / malfunction of these diaphragm operated valve which supposed to hold this CPD air and only release for pulsing based on filter dp when solenoids picks up.

Another observation i have is Air filters control panel where we have this timer giving pulse to each of these 190 solenoids giving an error signal / alarm and i check the pressure after the PCV at the outlet of pulse air tank is around 4 bar and these diaphragm valves requires 5.5 bar to hold / seal CPD air not being released for pulsing till respective solenoid pickup based on filter DP set point which is around 55dapa.

I am planning to check this in coming shutdown opportunity and confirm inlet pulsation system but i need some advice / method on how to check these diaphragm operated valve located on dirty side of inlet air filter house are working fine when unit is down. I see there are around 190 valves located in 5 columns and its big challenge to find out which valves malfunction.

let me know if you have anything to add / comment.

Thanks,
Arun

 

Arun/kumara,

Do the self-cleaning inlet air filters run continuously?

Do you hear air flowing through the air dryer between the compressor extraction and the filter house continuously?

I suggest the easiest way to determine if the "loss of air" is being caused by the self-cleaning inlet air filter solenoid valves is to first close the inlet valve to the air dryer, or the extraction valve from the combustion wrapper/compressor discharge casing and then re-try the normal Control Constants and loading with IBH off to see if the unit will transfer from Lean-Lean to Premix. That would be easier that checking all the solenoid valves--when it might not even be a solenoid valve--it could be a loose pipe connection or something similar.

I don't know what manufacturer of self-cleaning inlet air filter is in use at your site, so I can't comment on what the alarm might be. In the past (before Belfort) the alarm was pretty basic--there were two or three contacts wired in series, one from the low air pressure switch, one from the self-cleaning inlet air filter undervoltage relay, and, depending on the manufacturer, a trouble contact from the sequencer (which really wasn't more than a loss of inlet AC power, similar to the undervoltage relay signal).

As for checking the individual solenoid valves, your going to have to do that when the unit is shut down with a long ladder inside the filter compartment. You'll need to have a source of air to supply the self-cleaning inlet air pulsation system and then you're going to have to listen to each solenoid valve to determine if it's leaking. And, check all the piping system connections/joints/unions for leaks, which you can use a soapy water solution for. You could also spray the solenoid valves with a soapy water solution.

To be sure that none of the solenoid valves are getting stuck open after being pulsed, you would then have to let the system pulse each group of solenoid valves (vertical banks, usually) and then re-check each solenoid valve. This doesn't mean that one or more of them isn't getting stuck intermittently, but, it is the only way I could think of to check the valves--after you've determined there is excessive air consumption by isolating the system either at the air dryer or at the extraction valve. It seems pointless to check all the solenoid valves before knowing that the system (including the piping) is the source of the excessive flow/loss of air.

Hope this helps, and keep us informed on the progress.

 

Will keep you posted on our findings.

 

Dear CSA

In another unit GT1B, we are experiencing trouble during premix transfer operation.

Last 3 days our observation is - Druing Premix transfer when splitter closing to cut of fuel to primary we are loosing flame counts to 800 in #7 & #8 secondary flame scanners while #5 & #6 is around 1500~1900 (fluctuating) before primary flame cuts off and soon after transfer operation is successful all secondary flames count gets healthy 1900~2000 counts and maintained throughout rest of load operation.

We checked the splitter valve - calibration - found ok however we still replaced servo and its filter. Splitter was re calibrated and during unit premix transfer operation checked physically for any hunting / sticky operation - was ok.

We have informed GE on this issue.

For me if the secondary flame scanner is not reading the counts - indicate flow is not at the center of liner - getting offset / disturbance in flow thru secondary nozzle for some reason.Wanted to know what is your thoughts on this typical behaviour or what all factors / conponents we can check ?

Thanks, Arun

 

Arun/kumara,

Your troubleshooting method, while all too common, is completely, utterly wrong. The GSV (Gas Splitter Valve) feeds two manifolds: the Primary fuel nozzle manifold and the Secondary fuel nozzle manifold.

These manifolds each feed fourteen fuel nozzles, and the flow through the fuel nozzles is (designed to be) essentially equal just by virtue of the fact that all fourteen fuel fuel nozzles of each manifold have orifice areas that are nearly equal and since they are all fed by a common manifold under a common pressure the flow through the nozzles should be equal.

This is true for both the Primary, Secondary and even the Transfer fuel nozzles--they are all fed by common manifolds (separate manifolds for each set of nozzles, and the nozzle orifice areas of each of the nozzles are of the same size (within a few millimeters diameter of each other).

So, <b>what</b> would cause you to think that the flow through a single control valve upstream of the manifold would cause the flame detector intensities of one or two combustors be less than all of the other combustors? This just flies in the face of logic and understanding about how manifolds and control valves and fuel nozzles work. It simply does not make the least bit of sense that the flow through a single control valve, feeding a common manifold, supplying fuel to fourteen nozzles--each with nearly identical orifice areas--would be any different for one or two or more nozzles than any other nozzle.

Have you looked at the manifolds in the turbine compartment, and the hoses from the manifold that feed the nozzles, and the nozzles? You've had them apart several times for lots of issues; one would think you would have taken the opportunity to examine them to get as familiar with them as possible.

Also, one would think you would have looked and memorized the P&IDs for the Gas Fuel System, at a minimum, and that based on your training and experience that you would intuitively know that if one valve controls the flow to fourteen nozzles that the flow through one or more nozzles <b>CANNOT</b> be any more or any less than the flow through any other nozzle.

Unless one or more nozzles is plugged or blocked or somehow restricted more than any of the other nozzles. If there were fourteen individual control valves, each feeding a single nozzle, it is conceivable that a problem with the control valve might cause the flow through the nozzle supplied by that control valve to have more or less flow than any of the other nozzles. But, for a single control valve to restrict the flow through one or more of fourteen nozzles that it's supplying is just, well, ..., it's absurd. It's ludicrous. And, it's just plain wrong. Pure and simple wrong. Period. Full Stop. End of discussion.

So, to change the servo without first verifying there is some problem with the valve being able to control the flow through it is also similarly just plain wrong.

And, to recalibrate the LVDT feedback--because, contrary to extremely popular belief--"calibration" <b>only changes the LVDT feedback scaling--NOT the servo action or servo gain, nothing to do with the servo.</b> Period. Full stop. End of discussion.

So, let's try to figure out what might be happening. Let's just say, for fun, that the secondary fuel nozzles of the two combustors which are experiencing the low flame detector intensities during L-L to Premix transfers have some small obstruction/blockages in them, reducing the orifice area.

Now, when the unit is transferring to Premix, the GSV shuts of flow to the Primary fuel nozzles redirecting fuel flow to the Secondary fuel nozzles--and the Transfer fuel nozzles (which are also part of the Secondary fuel nozzle assembly). But, these two Secondary (and Transfer) fuel nozzle assemblies have some blockages--oh, let's just say somehow some sand or gravel or weld slag made it's way into the gas fuel system and got stuck in one or more of the nozzle orifices causing the flow through the orifices to be less than it should be. And, since we already know that all of the fuel is being directed to the Secondary fuel nozzle and Transfer fuel nozzle orifices during the L-L to Premix Transfer this blockage further restricts the flow of gas fuel to the point that the secondary flame detector intensities of the combustors drop during the transfer.

However, once the L-L to Premix transfer is complete, the Transfer Valve closes to shut off fuel to the Transfer nozzles, and the GSV moves to redirect approximately 80% of the gas fuel flow to the Primary fuel nozzles, leaving only 20% (or so) to flow through the Secondary fuel nozzles (and 0% flowing through the Transfer fuel nozzles). So, with MUCH LESS gas fuel flowing through the Secondary (and Transfer) fuel nozzles, the blockages aren't as extreme, so the flame detector intensities return to near normal.

This is all just hypothetical and in good, clean fun. Not that this could ever happen--not in a million and one years. At your site, one control valve can restrict the flow through one or two (maybe even more!) of the fourteen nozzles it supplies through a common manifold--so, the servo valve and LVDT calibration simply must be responsible for the low flame detector intensities during the L-L to Premix transfers.

But, at your site when you went through all the trouble of shutting down the unit, and changing the servo-valve, and "re-calibrating" the GSV (actually, re-calibrating the GSV LVDT feedback scaling--which you can't even say if there was any change in the scaling before and after the "calibration") there was no appreciable change in the operation, and the problem still exists. One USD5,000 servo valve, at least three or four hours of lost generation (and steam production if the unit drives an HRSG), and the time required to "re-calibrate" the GSV (LVDT feedback scaling). And, all of this was done under the "logic" that the flow through one or two fuel nozzles would be affected by a single control valve that supplies fourteen fuel nozzles.

Arun/kumara, you should not feel too bad--because this happens on a lot of other sites. But, is there no one at your site who has some understanding of how manifolds and control valves and fuel nozzles work who is willing to speak up and say, "No. This isn't right." Or, is that person (or persons) afraid to speak up.

Fuel nozzle blockages can be caused by liquids entrained in the gas fuel flow (not that's ever happened at your site, either!) which get hot and carbonize and form blockages in the nozzle orifices. But, it can also be caused by dirt, rocks, sand, gravel, weld slag, etc. In this case, the low flame detector intensities are telling you <b>exactly which fuel nozzles are experiencing low flow problems</b>--you don't have to guess which fuel nozzles to pull and inspect/replace. Instead, you choose to replace a servo valve, and "re-calibrate" the GSV (LVDT feedback scaling).

Best of luck in your endeavour. The best thing I can recommend is to get a copy of the Gas Fuel P&ID and take it out to a non-running unit and use it to "walk down" the Gas Fuel system, from strainer valve to fuel nozzle. In the course of identifying each and every component and its relation to every other component, if you find it's easier to understand the Gas Fuel System by re-drawing the P&ID in some format that you can better understand--by all means, DO SO. During the next outage, take the time to carefully and closely examine the fuel nozzles to better understand how gas fuel flows through them.

I don't care if you're the Plant Manager, the I&C technician, a Control Room operator, or an outside operator. DO IT. It's the only way you're going to learn and understand this system. It's not that complicated--unless you're refusing to learn and understand the system, and you're comfortable with making SWAGs while troubleshooting (Silly, Wild-Arsed Guesses). Which wastes time, and money, and can be especially depressing for people who are trying to learn and understand and be competent and efficient.

It's really the only way anyone--OEM field service person; mechanical engineer; controls engineer--anyone, is going to learn these systems. And, they are NOT that complicated. They use common engineering principles and components, like control valves and manifolds and nozzles. To quote a former colleague, "This ain't rocket science."

You've probably figured it out by now: I'm frustrated. I'm frustrated with this way of thinking. I'm frustrated with the way supervisors and managers and owners staff plants, and won't provide training for people who operate and maintain multi-million USD plants which are critical, in most cases, to the infrastructure of companies and nations. And, these plants have very explosive hydrocarbons flowing through them at very high flow-rates--relying on automation to protect the equipment and personnel. It's just incomprehensible that this can happen--and yet it does. At too many sites.

Best of luck. I'm going to go find a very stiff alcoholic beverage--or three or four of them.

And pray. And, I'm not a very religious person. Who needs a stiff alcoholic beverage on occasion to maintain some sense of sanity.

 

Hope you enjoyed your beverages, it's DLN transfers that have turned me into a near alcoholic !!

 

Thanks for your Post and i appreciate for your time and Patience you have to write / express your frustration.

 

We are facing the same issue in our machine. we got a exhaust temperature spread alarm in base load. so we reduce the load and the alarm gone. acceptable spread was 130, spread 1 =31 spread 2=34 spread 3 =33. we try to put in Primary again, but it didn't worked out. we got an alarm Extend L L MODE HIGH EMISSIONS ALARM. what may be the problem, is there hardware problem?

 

deepukb,

You haven't told us when this problem started: After a maintenance outage? How long has it been since the last maintenance outage? Did the problem start after a trip from load? Has anyone checked the fuel filters/strainers/knock-out drums to see if there are any entrained liquids in the gas fuel? What is the gas fuel temperature, and what is the dewpoint temperature of the gas fuel? Has the fuel supply recently changed? When was the last time the unit was tuned for emissions (DLN tuning)?

A high exhaust temperature spread during Premix combustion mode operation is usually the result of diffusion flame igniting in one of the primary combustion zones of one of the ten (10) combustors of a GE-design Frame 9E heavy duty gas turbine which does NOT have a flame detector. This causes the exhaust temperature spread to increase, and the turbine control system--programmed to recognize high exhaust temperature spreads in Premix combustion mode as a very likely sign of diffusion flame in the primary combustion zone of one of the ten (10) combustors WITHOUT a flame detector--energizes the ignitors and changes the fuel split to bring the unit into Extended Lean-Lean combustion mode (where NOx emissions are much higher than when operating in Premix combustion mode). This is to protect the combustors from damage caused by prolonged operation with diffusion flame in one or more primary combustion zones when the unit is operating in Premix combustion mode.

The fuel is divided between the primary- and secondary combustion zones when in Lean-Lean (and Extended Lean-Lean) and Premix combustion modes. When in Premix combustion mode, approximately 80% of the fuel is being admitted into the Primary combustion mode, where it is NOT supposed to be burned with a diffusion flame. Burning that much of the fuel flow-rate in a diffusion flame in the Primary combustion zone will very quickly result in damage to the combustion liner, and possible catastrophic damage to the turbine nozzles and/or -buckets. When operating in Extended Lean-Lean or Lean-Lean combustion modes, there is diffusion flame in BOTH the Primary- and Secondary Combustion zones--but there is only approximately 50% of the total fuel flow-rate going into the two combustion zones, and even at Base Load burning that much fuel in the Primary combustion zone can still be harmful to the combustion liners if left in Extended Lean-Lean for very long (more than an hour or so while troubleshooting and trying to get back into Premix combustion mode). Just because the turbine will run in Extended Lean-Lean and not trip <b>does not</b> mean the unit should be run indefinitely in Extended Lean-Lean, because combustion liner damage will eventually occur. Extended Lean-Lean combustion mode is only a temporary mode for troubleshooting purposes. If the unit will not transfer into and remain in Premix combustion mode without automatically switching into Extended Lean-Lean combustion mode, then the unit should be shut down and the problem should be resolved.

So, YES! There is most likely something mechanically wrong--a problem with a nozzle or nozzles, especially if the spread problem normalizes when in Primary or Lean-Lean (or Extended Lean-Lean). Or, there's a problem with some carbonaceous deposits on one or more of the primary fuel nozzles of one or more combustors which are so hot they are igniting the fuel in the Primary combustion zone of one or more combustors into diffusion flame.

Lastly, remember: The combustion stability of the DLN-I fuel/air mixture is extremely borderline even when properly tuned. Slight changes in air flow, fuel make-up, fuel pressure fluctuations, fuel temperature, entrained liquids in the fuel (gas fuel liquids; lubricating oil (from gas compressors/compressor seals; etc.), can all disrupt DLN-I combustion stability. As can fuel deposits which form and get very hot and act as an ignition source.

If the high exhaust temperature spread decreases significantly when operating in Primary and/or Lean-Lean (including Extended Lean-Lean) then it's a very good possiblity that something is wrong in the combustion system which is causing Primary Zone Re-ignitions (the unwanted establishment of diffusion flame in the Primary combustion zone(s) of DLN-I combustors when operating in Premix combustion mode). If the unit won't transfer from Lean-Lean to Premix and remain in Premix combustion mode without automatically switching to Extended Lean-Lean then the unit should be shut down and the combustors visually inspected for signs of problems (either initially by borescope, or by pulling the combustion can covers (the Primary fuel nozzle assemblies).

Please write back to let us know what you find!

 

there was a problem with nozzles so we changed it and its working fine now.

regards
Deepu

 

deepukb,

Thanks <b>VERY MUCH</b> for the feedback!

Was it the Speedtronic fuel nozzles? (A little attempt at humour--probably very little, but an attempt nonetheless!)

These types of problems--failures to maintain Premix Steady-State combustion mode--are rarely Speedtronic turbine control system problems. They are almost always the result of combustion hardware problems; improper fuel control valve refurbishment; fuel problems; oil quality/maintenance issues resulting in servo-valve problems; or operational problems (unsteady grid frequency; trying to operate too close to the switching temperature; etc.). If it's a control system problem, it's usually the result of some ill-advised change or improper LVDT calibration.

Anyway, thanks for the feedback--we rarely get if for this type of problem.