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Gas turbine Unit 9FA Alwayes tripped by "LOSS OF FLAME"
Power generation equipment control. topic
Posted by Mohamed Salem on 14 September, 2012 - 12:11 pm
Hi every one,

After greeting we have 3 units GEframe9FA in ower plant GT1,2,3. GT1 and GT2 always tripping by protection loss of flame at bas load.

i analyze the trends and trip logs for both units i found every thing ok for PM4,PM1and SRV all reference and feed back values for all gas control valves are ok up to the time of trip all the flame scanners suddenly give low intensity and unit tripped. i checked also in trip log all servo currents are ok. we try to clean Hydraulic filters for control v/vs an and also clean the trip relay for SRV and start the units it is synchronize. but after some times it is tripped again by same reason. next time i changed four flame scanners but again tripped by same reason. can any one help me what is the reason of this loss of flame? and i also regarded during startup one alarm came "Turbine can't accelerate at part speed". this come from firing speed up to 1600 rpm and unit shutdown due to this alarm. i bypass this alarm up to synch speed. any one can tell me what is the reason of this alarm?

Thanks every one,


Posted by CSA on 14 September, 2012 - 2:46 pm
Mohamed Salem,

When did this problem start?

Have you consulted with the OEM/packager about this problem?

When was the last time the unit was tuned for emissions? Before the most recent maintenance outage (CI; HGPI; Major Inspection)? Or after the most recent maintenance outage?

It has been covered MANY times before on control.com: A 'Loss of Flame' trip on a GE-design heavy duty gas turbine with a Mark V-, Mark VI- or Mark VIe Speedtronic turbine control system ONLY occurs when some condition causes flame to be lost in a majority of the flame detectors BEFORE some condition that would trip the turbine is detected. In other words, L28FD goes to logic "0" BEFORE L4T goes to logic "1" (which causes L4 to go to logic "0"). If the protective logic is written properly and is the same in all the units (which it should be--written properly and the same in all the units), something has to be causing a loss of flame before some other condition (such as loss of L.O. Pressure, or high-high vibration, or exhaust overtemperature, etc.) is tripping the turbine.

You say you have looked at trends and Trip History displays; are you certain all of the fuel valves are open and not closing prior to the trip? Is the SRV opening or closing prior to the trip? (If it's opening, then the gas fuel supply pressure is dropping or is insufficient; if it's closing then something is wrong with the P2 pressure sensing or the turbine shaft speed sensing (which drives the P2 pressure reference).)

About the only other thing which might cause this is a problem with the fuel splits caused by improper tuning or improper nozzles/liners. When a DLN machine is running in Premix Steady-State mode, the fuel/air mixture is extremely lean and can border on instability which, if there is any bobble in P2/supply pressure or air flow (IGV angle; IBH valve position; system frequency) then flame can be lost. If the wrong combustion components are installed resulting in improper air and fuel flows then any small disruption in air or fuel flow can also cause loss of flame.

Another possible problem might be some kind of liquid being entrained in the gas fuel supply, or even the gas fuel supply temperature being insufficient for the application. If liquids of some kind (water; water vapour; hydrocarbons) are entrained in the gas fuel and there is insufficient temperature to prevent condensation as the temperature drops across the various valves and nozzles in the gas fuel system then condensate can be extinguishing the flame.

You didn't mention what the exhaust temperature spreads were prior to the event; were they high, low, stable or increasing? If they are high and increasing just prior to the trip then something could be extinguishing the flame--like condensate of some kind caused by insufficient gas fuel supply temperature. GE recommends a minimum of 50 deg F of superheat to prevent moisture condensation during normal operation as gas flows through valves and nozzles (and pressure drops cause temperature drops which can cause condensation). When was the last time the gas fuel was analyzed for superheat? What is the gas fuel temperature with respect to the gas fuel dew-point temperature?

You did NOT mention what's happening to the grid frequency during these loss of flame trips....

You did NOT mention what the gas fuel supply pressure is during these loss of flame trips....

You did NOT mention what Diagnostic Alarms and Process Alarms are active (not resolved) during these loss of flame trips....

There is usually some gas fuel flow measurement system on most GE-design heavy duty gas turbines. Have you been monitoring it, or the inputs to the calculation, to see if something unusual has been happening?

What is happening to the Hydraulic System pressure immediately prior to these trips? Is it low to begin with? Are the Hydraulic Accumulators properly charged and properly valved-in?

Are you CERTAIN the IBH Control Valve is FULLY closed at Base Load? Are you certain it remains fully closed at Base Load?

Are you certain the combustion reference temperature calculation (TTRF or TTRF1) is good? Are all the inputs to the calculation correct (ambient temperature and pressure; exhaust duct back-pressure; etc., for the configuration at your site)?

What is the combustion reference temperature doing prior to the trips? Is it stable, or unstable? How does it compare to the unit which is not experiencing the trips?

Are you CERTAIN the IGV LVDTs are properly calibrated (by verifying the measured angle versus the LVDT feedback)?

Are you CERTAIN all of the servo current coils are receiving the proper current polarity?

F-class machines are accelerated by SFC (Static Frequency Converter) powering the generator which is acting as a motor to accelerate the unit based on an acceleration reference. Something is likely amiss with the acceleration signal/communication. But, without being able to see the sequencing in the Speedtronic turbine control panel at your site it's nearly impossible to say for certain what is causing the problem. If you can't troubleshoot the alarm using the CSP Cross-reference and CSP and Alarm List and Dynamic Rung Display, then have someone who is familiar with Speedtronic logic and Mark V come to site to help with understanding the alarm and the logic and to help resolve the problem.

Lastly, repeated tripping of F-class machines from Base Load is NOT good for the turbines and axial compressors. Not at all. It really degrades turbine performance and hot gas path parts life, not to mention that some turbine rotors don't take well to repeated thermal stresses from tripping (this includes axial compressor rotors as well as turbine rotors). This problem should not be allowed to continue.

Over the years your site has had a LOT of problems, and we don't get much feedback about how the problems get resolved. You ask for help but you selectively provide answers to questions, and rarely provide feedback. That's great if you're getting help from GE Controls Connect, but you're asking from help from the control.com community. As a community, we all benefit from helping individuals when the individuals provide answers and feedback so that we can all learn how problems are resolved, as well as how to troubleshoot problems.


1 out of 1 members thought this post was helpful...
Posted by CuriousOne on 16 September, 2012 - 10:45 pm
Although GE has made many improvements of the frame 7 over the years, I wish I could post the pictures of an old 7B sucking combustion cans flat after a base load trip.

The frame 5, 7, 9 folks could learn from those pics. Many people do not realize what bypassing, jumping logic during startup can do.
A base load trip is BAD on EVERYTHING.


Posted by Mohamed salem on 17 September, 2012 - 4:51 am
Dear CSA

First I need to thanks you for your fast reply I check again all data and trends for units (GT1and GT2) loss of flame problem this problem raise after last CI from march 2011 and last time for units tuned for emissions is from this date of CI (about 1 and half years ) we notes also the running hours after CI in the range 14000 Hrs after first CI .

But for another questions for GT1

1-SRV and gas control valves not opened and P2 also is stable at prior of trip .

2-IGV and IBH are stable at prior of trip.

3- Spread exhaust temperature is stable without any fluctuation.

4- fuel gas temperature is ok after coming out from Oil Bath Heater (O.B.H) it is about 120 C and the wobbe index value in this time about 46.23.

5- Frequency of grid not changing at prior of trip.

6-Gas fuel supply pressure stable it is about 31.2 bar .

7-flow of fuel gas is about 12.34 kg/s and not changed.

8- hydraulic pressure ok and not fluctuating it is about 119bar but for accumulator I didn't know it is ok or not.

9-IBH is fully close at the base load.

10- combustion reference temperature is stable it is about 1330 c.

11 last IGV calibration in last CI by GE.

But for GT2 I observe all parameters and question which you are asked is the same for GT1 except fuel gas control valve PM4 feed back was dropped from 56.67% to 37.11% at this moment P2 pressure increase from 26.81kg/cm2 to 29.104 Kg/cm2 this increasing in the P2 reflect to SRV and it is starting to close to maintain P2 this SRV start to move from 47.42 % to 44.99% this changing in parameters happened before one second of trip after one second unit tripped. In next shutdown I will try to change the servo valve.

But fore process alarm which appear in this time is only "Loss of flame"

Regarding the alarm "Turbine cant accelerate at part speed" we follow and trace the alarm. I found this alarm coming when the turbine cannot able to accelerate below minimum set point (LTNHA_LOW), when when part speed acceleration monitor enable (L83PSAM) which is coming from speed level (L14HS) which was coming from 4 speed detectors and going to I/O card VTUR as analog input signal ACCel1.

I don't know this acceleration fail due to problem in combustion or what is your opinion?

Pls any one can give help about that?

Thanks for all


Posted by CSA on 17 September, 2012 - 2:45 pm
Mohamed Salem

> But for GT2 I observe all parameters and question which you are asked is the same for GT1 except fuel gas control
> valve PM4 feed back was dropped from 56.67% to 37.11% at this moment P2 pressure increase from 26.81kg/cm2 to
> 29.104 Kg/cm2 this increasing in the P2 reflect to SRV and it is starting to close to maintain P2 this SRV start to
> move from 47.42 % to 44.99% this changing in parameters happened before one second of trip after one second unit
> tripped. In next shutdown I will try to change the servo valve.

I'm not really clear on this, even though I've read it several times. From my understanding, something is causing the PM4 reference (sorry; I don't know the signal name for this valve) to change--decrease--which would naturally cause the P2 pressure to increase and so the SRV would want to close to maintain the P2 pressure equal to the reference. So, why would you want to change a servo? Were you monitoring the PM4 reference, and did it change? Or did the PM4 feedback change? Or did the servo current to the PM4 servo change? But, why do you think it's necessary to change the servo?

From my perspective, wouldn't it make more sense to try to understand the PM4 reference, the inputs to that reference, and try to understand why it would tell PM4 to close? Isn't it more likely the servo is just doing what it's told?

That is, unless the LVDT feedback is not linear or is intermittent.

As I said before, if the tuning is borderline stable and there are fluctuations in fuel flow and/or air flow (and fuel flow fluctuations could be caused by unstable P2 and/or gas fuel pressure--and sudden changes in fuel flow caused by sudden changes in valve position) then flame can be lost in DLN combustors. You said the problems began after the CI, at which time the unit was tuned. Have the emissions changed since the tuning (which might indicate they need to be changed again--or that something is amiss with the fuel nozzles and/or liners) and all of this is contributing to a very unstable flame which is easily lost if there are any other sudden changes to flows/parameters.

I would like to understand why you feel changing servo-valves is warranted, what it's going to prove (or disprove), and what you would do if changing the servo-valve did not prevent subsequent 'Loss of Flame' trips?

Also, when flame is lost is it lost at exactly the same instant by all flame detectors? Or is the intensity decreasing on one or more flame detectors in the few seconds prior to loss of flame? You say the exhaust temperature spreads are not increasing, so it would seem the flame is stable in all combustors and then is lost pretty much instantly. If that's the case, then I'm still convinced--based on the information provided--that the DLN tuning is very questionable.

What is happening to the load just prior to these loss of flame events? Is is dropping? Slowly or quickly? Or is it stable right up to the point that flame is lost?

Lastly, I've seen a couple of sites where liquids condensed in piping/accumulators downstream of fuel heaters that caused slugs of moisture to cause loss of flame trips. This took some extensive monitoring of fuel gas piping temperatures and occasional opening of low-point piping drains to find and isolate. At one site we even found one knock-out drum (cyclone separator) was purchased and installed with no internal baffling from the manufacturer! This on one of four units all fed by the same natural gas supply, and only that unit was experiencing loss of flame trips. It was odd that the other three units knock-out drums were continually having to be drained (of hydraulic and lubricating oil entrained in the natural gas supply) and that unit's drum was never being drained.

My best guess--again, based on the information provided--is not a servo valve or the Mark V(s); it's the DLN tuning parameters and something which is causing sufficient combustor flow problems to cause loss of flame.

I did just think of one more thing which we haven't discussed: flame sensors. Does the unit use the Geiger-Mueller ("Honeywell") style of flame detectors, or the Reuter-Stokes Flame Trakkers? If it's the latter, then they use a special module to interface them to the Mark V. That module requires (nominal) 24 VDC power which is usually taken from a terminal board on <R> or <S> . If there are Diagnostic Alarms related to power supply issues that are being intermittently annunciated, or are present all the time, that could be a problem.

Also, these R-S Mark V interface modules are connected to the <P> core flame detector inputs and sometimes I have seen problems with terminal boards and ribbon cables cause one or two--but not all at the same time--flame detector inputs to be erratic. The terminations made to the <P> PTBA don't always seem to some people to be "correct" and I've seen a couple of sites modify the wiring to be what they think it should be--and in the process, cause unwanted problems.

So, let us know what kind of flame sensors are in use.

As for the acceleration issues, most F-class turbines are not self-sustaining until they are close to rated speed. This means that torque from the starting means (SFC; LCI) is required to help get the shaft up to near rated speed because fuel alone can't accelerate the unit without undue thermal stresses.

One of the reasons the starting means is used to provide most of the acceleration torque is to reduce the thermal stresses on the hot gas path components during starting by not requiring so much fuel which increases the exhaust temperature when air flows through the compressor are very low. On F-class machines the IBH control valves are usually open during starting, also, which means a portion of the axial compressor discharge is extracted and sent back to the inlet, slightly elevating the exhaust temperature, also.

I don't have a lot of experience troubleshooting SFCs/LCIs, but they should be capable of providing sufficient torque at all times during starting, presuming all the rectifier bridges are working correctly. Some sites use one or two SFCs/LCIs for two or more units with some high-voltage switches and connections to switch between them (for redundancy purposes). How many SFC/LCI units does your site have, and can more than one SFC/LCI unit be used for more than one turbine?

But, if this is only happening on one of three units, then it should be easy to compare good running/starting data to bad running/starting data and track down the problem and resolve it.


Posted by CSA on 18 September, 2012 - 12:01 pm
Mohamed Salem,

Further to this tripping issue, if I understand the original post correctly, two (2) turbines are both experiencing the same loss of flame trips. So, why would replacing a servo-valve on one unit solve the problem for both units?

Servo-valves usually "fail" when the oil quality is allowed to degrade. It has been said many times in other posts on control.com: Changes to turbine lube oil formulations by refiners have resulted in increased servo-valve problems for turbines which use lube oil as the hydraulic fluid. And, at least one refiner, BP-Castrol, if I recall correctly, has formulated a new turbine oil that has been shown to demonstrate improved hydraulic characteristics.

It was reported that the servo filters were cleaned. Sometimes, in my personal opinion, opening and removing then replacing these filters in less than ideal conditions (which describes most instrument- or mechanic shops performing this work on turbine sites) actually allows impurities to enter the extremely small passages of the servo-valve, making the problem worse instead of better.

I'm not saying that one unit may have a servo problem and the other unit has a different--or even the same (highly unlikely)--problem. Just that the likelihood is very low.

You also didn't actually indicate which servo-valve you were going to replace: the SRV or the PM4.


Posted by Mohamed salem on 20 September, 2012 - 4:59 am
Dear sir CSA,

thanks for your interest. i need to clear my brevious message which is not clear for you.

i mean by my observation for GT2 i seen the feed back for PM4 was suddenly dropped from 56.67% to 37.11 at this moment PM4 reference still 56.67% means the feed back for the PM4 not follow the reference value. i am thinking this drop is actual drop not error in feedback because the P2 pressure due to this drop in PM4 starting to increase from 26.81kglcm2 to 29.104 kglcm2. at this moment also SRV start to teak action to maintain the pressure means start to close to maintain the pressure again to 26.81 kg/cm2.

i am also observe servo current not changed at this moment. i seen the reference value signal for PM4(LFSRG3OUT) on the trend and trip log so for this reasons which i listed i need to change the servo valve for the PM4.

for emissions we have CEMS (Continuous Emission Monitoring System) this system we are not depend on it because sometimes this system not give correct or actual values.

Flame detectors intensity (4 flame detectors) was suddenly reach to zero intensity before trip (loss of flame) within one second in this period load is stable and not changed.

for our fuel gas system we have two cyclone separators (Scrubber) before and after Oil Bath Heater (O.B.H). this separators have level switch to give alarm in control system (MARK6) if there are any condensate in the separators and automatic system drain will start also.

for our machines we use flame sensors reuter - stockes flame tracker but we are not observe any diagnostic alarm related power supply.

in our site we use two (LCI) to starting units GT1, 2, 3

this is all answers for your questions which you are asked. i am very happy with your co-operation to solve this problem.

thank you very much


Posted by Irishal_0 on 20 December, 2012 - 12:57 pm
Dear Mohammed,

Did you solve your problem with Loss of Flame? What about the Fail to Accelerate problem?

I have recently had occurrences of both issues so would be interested to know your solution.

With regards the loss of flame issue at my site, we were experiencing significant grid frequency changes (-0.5hz in 2s quickly followed by +0.3Hz in 5s)which caused wobble in fuel-air ratio but it sounds like your site did not have frequency deviations.

On the Fail to accelerate issue, again our site suffered the same issue around 1600rpm (3000rpm rated speed). Solution has been found with GE which I can share if you still need assistance.

Regards
AC


0 out of 1 members thought this post was helpful...
Posted by Msalem73 on 12 January, 2013 - 3:26 pm
Dear AC,

i need to tell you our machine recently we perform Combustion inspection (CI) by GE so after that the loss of flame not happen. now the GE control engineer told the main reason for loss of flame are of all F.G control valve need to tune and calibrate and for ge turbine can't accelerate at part speed. he told also the reason is tuning because the firing can't able to accelerate the turbine shaft with LCI.

If you have any suggestion and solution for that pls tell.

regards
Mohamed

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