Good morning everybody,

We recently carried out CI on one of our 9Es.During startup the unit was always tripping on loss of flame during the transition from warmup to acceleration.

Liquid fuel bypass valve was checked,flow divider flow was trended, fuel check valves were changed but the problem was still persisting. A trend of TNHA and TNHAR showed that TNHA was going higher than TNHAR during the transition so FSR was cutting back to FSRMIN causing the flame out.

We suspected fuel nozzle differences so we changed FSRWU Constant from 12.3 to 11 and FSRMINU values were changed as follows:<pre>
Initial Value Final Value

FSKMINU1{0} 6.65 To 9.00

FSKMINU1{1} 6.65 To 9.00

FSKMINU2{0} 10.99 To 13.00

FSKMINU2{1} 10.99 To 13.00</pre>
The unit was started and synchronized successfully and has been running for the past two weeks.

We decided to shut the unit down for job to be done on the LCO skid and during the normal shutdown,the unit tripped on loss of flame.

When the unit was started again and as soon as the cranking motor and the atomizing booster drops out, the unit vibrates and trips on loss of flame.

A trend taken showed that after the cranking motor and the atomizing air boster drops out, TNH Remains almost the same whiles FSR continues to increase. The trend also shows that atomizing air differential pressure during this period is 0.7 bar.

What could be the possible cause of this?

Control system is Mark VIe, fuel is distillate and crude oil. No gas yet.

Thank you.

 

Kwaku,

When a turbine trips, the GE "standard" is there should always be a Process Alarm to indicate precisely what tripped the turbine. Now, unfortunately, that isn't always adhered to by factory or field personnel--but whenever a turbine trips the operators should be capable of reviewing the Alarm Log (printout or electronic file) to determine precisely what tripped the turbine.

Even the cryptic "Loss of Flame Trip" alarm tells an operator or technician something: Flame was lost in multiple combustors with flame detectors installed on them BEFORE any condition which would trip the turbine was detected.

So, before we can do too much to help you, it is necessary to know PRECISELY what alarms are annunciated when the turbine is "tripped", and when the turbine is "tripped" (at what speed and/or load).

As for the changes you have made, we don't know how well the unit started before the CI. Did it start well, without any problems like flickering flame detectors (common on liquid-fueled machines) and exhaust temperature spreads (common on liquid-fueled machines)? What did previous trends of start-ups look like, particularly the TNHA vs. TNHAR relationship?

And we don't know what was changed during the CI. Does the LFBV (Liquid Fuel Bypass Valve) have LVDTs? Were the LVDTs "calibrated" during the outage? Were the fuel nozzles changed during the outage? Were any Control Constants changed during the outage? Was the Torque Adjustor mechanism adjusted or worked on during the outage?

You say the unit is "dual fuel"--distillate/heavy fuel oil. Usually, such machines must start on distillate fuel, switch to heavy fuel oil at some point after reaching rated speed (sometimes at FSNL (Full Speed-No Load), or at some low load on the generator), and then during a normal fired shutdown the unit must return to distillate fuel before the fired shutdown sequence (after the generator breaker is opened) is started. This is done to "purge" heavy fuel oil out of the liquid fuel system and to get lighter, thinner distillate fuel into the liquid fuel system so that starting can be done much easier. Usually, heavy fuel oil must be heated before it can be run through the liquid fuel system (flow divider; liquid fuel check valves; liquid fuel nozzles), and that's not easily done until there is a higher flow--higher than experienced during starting and acceleration. So, starting and acceleration is done with distillate fuel.

However, sometimes, when the turbine trips while operating on heavy fuel oil there is heavy fuel oil left in the liquid fuel system which must be purged out of the system before it can be re-started (using distillate fuel oil). Sometimes, if this isn't done quickly enough the heavy fuel oil can carbonize in the liquid fuel nozzles and cause problems during starting. And, when that happens what usually is done to resolve the problem (instead of removing and cleaning the fuel nozzles) is that Control Constants are "modified" to get the turbine to start and accelerate and they are never returned to normal after the fuel nozzles are replaced or cleaned, and that's when problems like this start.

So, I'm suggesting that some "original" Control Constants were modified after commissioning, and then when the fuel nozzles and check valves were replaced and or cleaned that these problems began. That's the usual likely cause.

But we don't have enough information to be able to say what might have happened because we don't know enough about what did happen: What Process Alarms were annunciated to tell the operator why the turbine tripped, and exactly when (at what speed/load) the turbine tripped.

Why are you suspecting fuel nozzle differences? Were the fuel nozzles changed during the CI? If not, why are you suspecting fuel nozzle differences?

Help us help you by providing more information.

 

Perhaps the atomizing air compressor on the accessory gear has broken a quill shaft. When the startup atomizing air compressor shuts down, atomizing air pressure is lost.

White smoke from exhaust as turbine nears L14HA, Big turbine shake, and a trip.

 

CuriousOne brings up a possibility, but the quill shaft would have had to have broken after the successful run because the unit would not have run at load without the main atomizing air compressor.

Most people never look at the exhaust stack for signs of smoking, but, if there was insufficient atomizing air then white smoke (unburned fuel vapors) might be visible coming from the exhaust stack. Usually there is a small delay in the time between when an insufficient atomizing air problem starts and when white smoke can be seen coming from the stack, just because of the volumes involved and the distances (in some cases the unburned vapors have to travel through an HRSG).

After re-reading the original post I see the unit was ostensibly tripped on loss of flame a couple of times. Again, a loss of flame trip says, basically, the unit ran out of fuel or didn't get enough fuel for the conditions in the combustors and flame was lost before any trip condition was detected.

I would also like to know specifically at what speeds the starting motor and the booster atomizing air compressor are being dropped out. For most Frame 9Es, the starting motor is "de-coupled" (by de-pressurizing the variable torque converter) at 60% of rated speed (1800 RPM for a 50 Hz unit). I've seen the booster atomizing air compressor usually shut down at 95% speed, sometimes, on Frame 7E machines I've seen it shut down at 50% speed.

I would suggest something other than fuel nozzles has changed. If the unit uses the typical variable torque adjustor mechanism on the torque converter it may be that it's not working correctly or was incorrectly adjusted during the outage.

Many times when problem like this occur, the turbine trips on what's called 'Starting Device Bogged Down' which means that instead of increasing in speed the turbine actually starts decreasing in speed during the time it should be accelerating. This is usually due to insufficient torque being transmitted from the starting means through the torque converter or from too little fuel, or a combination of the two.

I would also suggest the problem is not necessarily the Mark VIe control system parameters (Control Constant) because if the unit was working before the CI and now it's not, unless someone changed some parameters (Control Constants or Liq Fuel Flow Divider feedback scaling), it's not likely there is a problem with the control system.

The "vibration"/shuddering is what's kind of got me confused. It almost sounds like flame is being lost in several combustors and then being re-established all at one and then suddenly being lost again, and so on. When I've encountered this problem before it was usually because of unstable liquid fuel supply pressure upstream of the liquid fuel stop valve, wildly fluctuating. And, air trapped in the low pressure liquid fuel filters can also exacerbate the problem. People don't realize the Speedtronic doesn't control a lot of the really important parameters of the liquid fuel system--such as the liquid fuel supply pressure. There is a regulator to control liquid fuel supply pressure that the Speedtronic assumes is set correctly and is maintaining stable pressure. If the regulator is not, then the Speedtronic may be having a difficult time trying to control the liquid fuel bypass valve to limit fuel flow because the supply pressure--and flow-rate--is jumping all around.

But, we need more, actionable information--information that can be used to understand when and what happened more precisely.

 

Dear all,

Thank you all for your response.

The trend was studied carefully and i found that after the cranking motor and the atomizing air booster drops out at 60% speed. the atomizing air pressure continues to stay at 0.7 bar whiles TNH increases gradually to 65% with FSRACC increases to 28.

I suspected atomizing air because of the drastic increase in FSR at 65% speed which causes the violent shaking of the unit.

The atomizing air check valves were removed and serviced. The unit was started successfully.

The unit has since been normally shutdown twice and started again perfectly.

Thank you all for your response.

 

Kwaku,

Thank you for your response!

Again, I don't have access at this time to any GE P&IDs, but I'm not recalling any check valves in the atomizing air supply lines to the fuel nozzles of a Frame 7E/EA with conventional combustors (i.e, without DLN combustors). And, I'm not even recalling any check valves in the atomizing air supply lines to the fuel nozzles of DLN combustors.

Perhaps you mean the liquid fuel purge air check valves? The liquid fuel purge air check valves use atomizing air for purging the liquid fuel nozzles when running on gas fuel, and are supposed to block the flow of liquid fuel in the reverse direction when running on liquid fuel. There should be a three-way, pneumatically-operated valve controlled by a solenoid that controls the flow of purge air to the check valves/fuel nozzles. The liquid fuel purge check vavles might cause the problems you reported.

However, if liquid fuel was getting past the liquid fuel purge air check valves then it should be flowing out of the tell-tale leak-off outside the turbine compartment and into a gas turbine drains tank. And, unless all of the liquid fuel purge air check valves were leaking then the machine should have likely had a lot of difficulty firing and reaching even 60% speed with the assist of the starting means.

Were the liquid fuel purge air check valves installed backwards after the CI?

The air for the Main Atomizing Air Compressor (and even for the Booster Atomizing Air Compressor) is compressor discharge air. Compressor discharge pressure doesn't really start to increase until the unit gets to approximately 80% speed or more, the IGVs open to the minimum modulating position, but still doesn't approach operating pressure until near rated speed. If you look at a trend of CPD during start-up, it is pretty flat until the time the IGVs move from the closed position and still doesn't approach normal until near rated speed. So, the Atomizing Air Compressors aren't going to produce much pressure--especially the Main Atomizing Air Compressor if it's Accessory Gear-driven, because it isn't at rated speed, either when the turbine is at 60-65% speed. There will be a slight pressure increase by the Main Atomizing Air Compressor as the unit is increasing in speed, but more than anything it is just producing air flow to help with atomizing the liquid fuel.

At any rate, thanks again for the feedback! It's the most important contribution to the threads here at control.com.

 

CSA,

I totally agree with you because i was also thinking the same thing that 0.7 bar was enough for 60% to 65% speed but strange things happen sometimes because even after servicing the atomizing air compressors Non return valves (NV 101 and NV 201), a trend taken still showed that the atomizing air pressure at 65% speed still remained at 0.7 bar but surprisingly FSR stayed at approximately 20.5.

But two other things were done though:

1. All the four compressor bleed valves were stroked with external compressed air.

2. The atomizing air bypass valve VA18-1 (which is normally closed when using liquid fuel)was also stroked with external compressed air.

So one of these may have done the trick.

 

kwaku,

Thank you. I'm more confused, though; sorry.

If the compressor bleed valves weren't 100% open (which is when the open-position limit switches are supposed to be set to actuate) then it should hot have been possible to start the unit without forcing logic. And, if the bleed valve limit switches were sticking, it would seem there would have been alarms to that effect, as well. And, the compressor bleed valves should not close until the unit is at or near rated speed.

Now, if 20CB-1 had stuck in the energized state it might have been possible for the bleed valves to start closing during acceleration, but I think it would take more than 0.7 barg to actuate the bleed valve actuators.

GE-design heavy duty gas turbines are kind of difficult to generalize during acceleration. The newer Speedtronic turbine control systems actually use an acceleration rate reference (TNHAR) compared against TNHA (the actual acceleration rate). Some Speedtronic panels seems to use FSRN to change the fuel flow-rate during acceleration with FSRACC set as a back-up to limit acceleration if it goes too fast (which is what FSRACC does during rated speed operation--it serves to limit FSR if acceleration is too fast, such as when load decreases very quickly), while some Speedtronic panels use FSRACC during acceleration with FSRN set higher and doesn't actually come into control until about 95-100% speed. The Mark IV panels were particularly bad at accelerating the unit on a preset schedule; in fact, they really didn't even though that was the intent.

I suggest you look closely at some other trends (past and future) and observe how your unit accelerates when it is behaving normally--whether it accelerates on FSRACC or FSRN after warm-up is complete and up to approximately 100% TNH. And, then compare those trends to the ones you cited in this thread. That information could be very helpful in the future.

I'm not saying that it's a control system problem; I'm just saying it's not likely a problem with the compressor bleed valves or the two atomizing air check valves (unless the limit switches or the solenoid (20CB-1) are sticking)--in which case there should be alarms, probably).

The shuddering/vibrating you describe could have been a compressor surge or stall problem which could have caused a loss of flame trip, but that would suggest there was a problem with compressor bleed valves and if they weren't fully open at 60-65% speed then it would seem there would have been an alarm of some kind. I don't have any sequencing to look at, and I've heard some of the newer units now have both open- and closed-position compressor bleed valve limit switches (I'm only referring to the open position limit switches in this thread) so I don't know how the sequencing would look with both sets of switches. I would expect the sequencing would have some kind of alarm if both the open and the closed position limit switches were not actuated for some period of time, but that's just a guess on my part because the bleed valves should be either 100% open or 100% closed, and there is only a limited time when, while having been told to move, then should be not at one end of travel or the other.

You didn't say if there was a Starting Means Bogged Down trip when the shuddering occurred, which would have happened if the speed was decreasing when it should have been increasing--before a loss of flame trip.

It would be interesting to know how much air pressure it takes to start closing the compressor bleed valves at your site. Just for information's sake.

Anyway, best of luck! And, thanks again for the feedback!