Hi,
What is the main uses of fire mode?

Fire mode is one the operation modes available in our screen (Fr9E/simple cycle) in addition to others (OFF, Cool-down, Crank, and Auto).

I would like to know why there is a Fire-Mode?
i know that the fuel is approx. remains constant and the prime mover still in business during that mode, but for what?

thanks,
Maythem

 

Maythem,

FIRE mode is basically a test mode. When selected prior to initiating a START, the unit will go through a normal purge and firing sequence, but when flame is detected and fuel is cut back to the warm-up value it will be held there as long as FIRE mode remains active.

The starting means (an electric motor on GE-design Frame 9E heavy duty gas turbines) will still be supplying torque (150-160% of rated current if the Torque Adjustor limit switches are adjusted normally and the sequencing/application code in the Speedtronic turbine control panel is typical). So, one doesn't want to run in FIRE mode for long periods of time. When the torque output is set to 150-160% of rated, that means the current being drawn by the starting motor is 150-160% of rated so long periods of operation at this high current flow is NOT recommended. (This is NORMAL operation for the motor--which does NOT run continuously at this current draw--but only during acceleration, for about 10 minutes or so. The motor insulation is supposed to be capable of periodic operation at these high current draws, and, typically, the starting motor is run for a few minutes, unloaded, after the acceleration is complete to allow the motor rotor fans to circulate air through the motor with current at about 40% of rated--a "cooldown" for the starting (cranking) motor.

So, this mode is useful for testing after an outage to be sure the unit will start but the unit won't be re-synchronized for hours or days later. To shut the unit down, just click on STOP, and the unit will coast back down to cooldown mode (turning gear; slowroll; whatever is the normal cooldown method for the unit).

FIRE mode is also useful for drying out a turbine after an off-line water wash--again, when it's not going to be synchronized and loaded for hours or days.

Lastly, FIRE mode is very useful for helping to re-start a unit after some problem occurred during cooldown operation when the rotor was hot. So, for example, let's say the unit tripped from load because the plant was separated from the grid and went "black." No oil pumps (other than the DC pump) and no cooldown (slow shaft rotation to prevent bowing). The turbine reaches zero speed, but can't be turned for 30 minutes or more. After waiting 24 hours to try to turn the turbine shaft (that's right--wait at least 24 hours when something like the above occurs if the shaft was hot when it reached zero speed and couldn't be rotated in about 30 minutes!) and all power has been restored, select FIRE and START and let the unit establish flame and warm-up slowly without accelerating to FSNL. Again, this can last about 15-20 minutes on warm-up fuel with FIRE selected as the shaft warms up and straightens out during low-speed rotation with the heat from the combustors.

Then, just select AUTO and the unit will begin accelerating as fuel is increased. Under the scenario above, there will likely be some high vibrations, but not too high, as the unit accelerates to FSNL, at which time the unit should be run for a few minutes while monitoring vibrations before synchronization.

So, when operating on FIRE with flame in the machine, if one wants to shut the unit down, just click on STOP; if one wants to continue accelerating to FSNL, just select AUTO. That's all that's required.

There is some thought that when FIRE Mode is selected the unit speed will stay around firing speed (usually around 15-20%)--but that's just NOT true. As the machine warms up from the fuel that is being burned, the speed will increase slightly. Also, there will be the assist from the starting means--which is sometimes increased to maximum after flame is detected (even if FIRE Mode is selected!). AND, if the unit is being started on gas fuel, as the speed increases so will the P2 pressure reference (the pressure between the SRV and the GCV)--and even though the GCV position is being held constant because the pressure upstream of the GCV is increasing the flow through the GCV will increase! Which will increase P2 pressure reference, and the P2 pressure, and the fuel flow-rate through the GCV, and so on, and so on, and so on. So the speed will not stay constant, but it will increase slowly, but usually not to more than 30-40% or so. (This is because gas fuel flow-rate is neither the reference nor the feedback during starting and firing--GCV valve position (and P2 pressure) are calculated to provide the desired gas fuel flow-rate, but since speed changes during firing and warm-up which affects P2 pressure, and actual gas fuel flow-rate isn't used in the equation for GCV position, the speed will increase slowly as P2 pressure increases.)

The same thing happens on liquid fuel--but not to the same extent because unlike gas fuel, actual liquid fuel flow-rate is compared to a liquid fuel flow-rate reference but as the liquid fuel pump speed increases so does the liquid fuel flow-rate (to a small degree).

Those are the most common usages of FIRE Mode. And, the common misconceptions about FIRE Mode and turbine speed (and the reason why speed doesn't stay constant at firing speed during FIRE Mode operation). I know of people who have left FIRE Mode selected for more than a couple of hours (usually trying to dry out an exhaust duct or -stack or HRSG) and were shocked to see the turbine speed had increased to more than 50-60% and were CERTAIN the Speedtronic wasn't working correctly! They were also unaware of the damage they had caused to the starting motor, and the Starting Motor Lock-out Alarm which had been annunciated when the starting motor protective relay tripped because of high motor stator temperatures....)

Hope this helps!

 

Thank you so much CSA for this valuable information; I have one more question regarding this topic:

Is there any chances for the exhaust over temp. trip to occur during FIRE-Mode if the unit stay on that mode for along time due to low air flow and pressure (IGV angle is minimum, slow compressor speed as the shaft speed is around 1200~1400 rpm) but with an accumulated heat coming from the combustion and continuously( although slowly) building up the heat inside the turbine.

Thanks,
Maythem

 

Remember Exhaust Temperature Control is still functioning all the time and will prevent any high exhaust temperature from happening.

 

Maythem,

Here are <i>examples</i> of firing- and warm-up FSRs:<pre>FSKSU_FI (Firing FSR): 20.0% FSR
FSKSU_WU (Warm-up FSR): 16.5% FSR</pre>
And, a typical firing speed for a GE-design Frame 9E heavy duty gas turbine is 10.0-12.0% TNH.

So, with FIRE Mode selected during a unit START the unit would fire with the GCV at 20% FSR (approximately--it's usually biased by ambient temperature so it can be a few tenths of a percent TNH higher or lower), and shortly after flame was detected, the FSR is cut back to (approximately) 16.5% FSR. (FSR, Fuel Stroke Reference, is the GCV position reference ("stroke" in GE-speak) when running on gas fuel, and with properly calibrated LVDT feedback, the GCV, when commanded to go to 20% FSR should go to 20% stroke; with an FSR of 16.5%, the GCV should go to 16.5% stroke.)

And as long as FIRE Mode is active the FSR will remain at approximately 16.5% FSR--no matter what the speed.

And as long as the P2 pressure being maintained by the SRV between the SRV and the GCV is correct (not excessive)--meaning the SRV is properly controlling pressure--then, no, the unit should not trip on exhaust over-temp if left running in FIRE Mode for an "extended" period of time. Even if the turbine speed slowly increased to 50% TNH or even 60% TNH (when the torque converter usually de-pressurizes and the starting motor goes on cooldown and is no longer supplying torque to the turbine-generator shaft) the exhaust temperature should NOT increase to the trip level.

In the only example I know of where a GE-design Frame 9E heavy duty gas turbine was allowed to reach approximately 70% TNH while in FIRE Mode, the unit did not trip, but remained at that speed as everything leveled out (meaning that no more work could be extracted from the fuel being combusted so speed did not increase which caused P2 pressure to stabilize which cause gas fuel flow-rate to stabilize, which caused turbine speed to stabilize--and the unit did not trip on exhaust overtemperature. But it did make some people very angry because they could NOT keep the unit running at approximately 20% speed (and the reason for maintaining 20% speed was never detailed)).

So, in my opinion, when left in FIRE mode for a long period of time a GE-design Frame 9E heavy duty gas turbine should not trip on exhaust overtemperature. But, neither should a GE-design Frame 9E heavy duty gas turbine be left in FIRE mode for more than about 20 minutes, maximum, because of the high current being drawn by the starting motor which is NOT good for the motor windings (which is normal--but not for periods of more than about 20 minutes).

Hope this helps!

 

glenmorangie,

I don't have any elementaries to refer to at this writing, but I believe when either Firing or Warm-up FSR is active they are both like minimums--and even if exhaust temperature control becomes active and tries to limit (reduce) FSR it can't go any lower.i say this because I know that some units that accelerate quickly during warm-up such that the actual acceleration rate is higher than the acceleration rate reference will not reduce FSR to a value less than Warm-up FSR <i>until the warm-up timer expires.</i> And then, sometimes it cuts FSR back so much the flame goes out.

But, again, I don't have any elementaries to refer to just now. But I think when either firing or warm-up FSR is active FSR can't be reduced for any reason. It's only after the warm-up timer expires that FSR can be reduced for any reason. And that seems logical because warm-up FSR is already very low and cutting it back too much can result in loss of flame.

But, I've been wrong before, and I'll be wrong again soon enough; eh?

 

CSA,

You are absolutely right. There is no chance to reach over exhaust temperature as FSR remains in low value. I am adding a reference document of our GE 9FA:

http://www.mediafire.com/view/qrqm5t2yru3urby/FSR_SU.pdf

 

niazbibiyana,

As you noted, that document refers to the firing and warm-up FSR for a GE-design Frame 9FA heavy duty gas turbine. Those machines--unlike the Frame 9E the original poster asked about--use the generator as a starting motor, which supplies torque during acceleration to as high as 95% TNH. So, if FIRE Mode was selected during a START fuel is really only used to warm the machine during nearly the entire run from firing to near rated speed. Flame is established at low speed to reduce thermal stresses on the internal components and the Speedtronic turbine control panel is really only trying to maintain flame as the starting means does most of the work during nearly the entire start sequence. Also, it's difficult to establish flame when the axial compressor is at or very near rated speed. So, there's a difference in control philosophy even though the sequences are nearly identical.

By the way, thank you for taking the time to obtain written approval from GE to post that snippet (the Copyright statement says it is required). They're not usually so agreeable, and (per the Copyright statement) they consider the document to be "internal" and proprietary, which means if you hadn't obtained written permission to post that page your company (presuming it's not GE or one of its subsidiaries--and the number of subsidiaries keeps growing and growing!) could be sued not only for Copyright infringement but also for intellectual property theft.

And their intellectual property lawyers are almost as tenacious and supportive of the Company's knowledge as their tax lawyers are of the Company's profits. An argument could be made that an internal rivalry exists between the two groups of lawyers.

 

niazbibiyana,

I miswrote when I wrote that fuel was used to warm the unit in FIRE Mode to as high as 95% speed. Even on a GE-design Frame 9F-class heavy duty gas turbine when FIRE Mode is selected during a START, the speed will remain close to firing speed--but the Speedtronic will adjust the static starter speed reference to maintain speed, because there still isn't enough energy in the combustion gases to accelerate the unit.

Flame is used on F-class turbines during an AUTO Mode start to warm the turbine as its being accelerated to as high as approximately 95% speed.

 

Thanks CSA,

The situation i asked about (exhaust over temp. trip) actually happened with me when we were doing commissioning for a Fr.5 unit back in 2006 but it was during acceleration not in fire mode but in Auto mode (acceleration stage). the prime mover was a diesel (an old lazy diesel) so the speed of turbine rotor was increasing very slowly and even sometimes stays at the same reading for some time, eventually the unit tripped with exhaust over temp.

From the above i was convincing myself that temp of exhaust could be building up if the fuel flow continues but with no observable rotor speed increment for a certain time (fire-mode specs)

regards,