Gas turbine: Mark-V<I> Frame-5

Gas turbine was shut down for maintenance and start up was preceded. Hunting in SRV was obvious initially from 2-5kg/cm2. Gas turbine was in Firing mode and no further signal was give.
Firing FSR is 25%
Warm-up FSR is 18%
Acceleration FSR is 26%

But gas turbine ramped to 3200 RPM in firing mode. Why gas turbine is automatically ramping up to 3200 or even beyond on Firing mode. In which direction we should proceed.

 

FIRE mode is meant only for testing purposes, and it is also used to dry out a turbine after an off-line water wash if it's not going to be put back on line within 24 hours of the water wash. It's <b>NOT</b> meant for extended periods of operation, and there <b>IS NO</b> speed control during FIRE mode operation.

Again, when a GE-design heavy duty gas turbine is being operated in FIRE mode there <b>IS NO</b> speed control.

FSR is held constant when FIRE mode is selected, meaning that the GCV position is held constant.

However, the unit will be accelerated partly by the starting means and also just by virtue of the fact that the unit is extracting some work from the combustion gases, and is the turbine warms up the longer it remains in FIRE mode it will also become a little efficient at converting the energy of the combustion gases to torque. Increased torque will mean increased speed.

Because the reference for the P2 pressure upstream of the GCV (controlled the SRV) is actual turbine speed, as the turbine speed increases <b>so will the P2 pressure reference</b>.

As the actual P2 pressure increases upstream of the GCV the flow through the GCV will also increase--even though the GCV is being held at a constant position--which means more fuel will flow into the turbine which means more torque will be produced which means more speed which means increased P2 pressure reference which means increased actual P2 pressure which means increased fuel flow which means increased torque which means increased speed which means.....

Eventually everything will stabilize as you have noted--if you wait long enough--but during the time you are waiting for this to happen the starting means is still putting out a LOT of torque. For a Frame 5 with a jaw clutch, sometimes the jaw clutch will open at around 60% speed and disengage the starting means from the turbine, but sometimes it won't when the FSR is being held constant. If it doesn't disengage and the starting means is still supplying torque while you are holding the unit in FIRE mode for a long period of time, well, you ain't doing the starting means or torque converter any good.


None.

Zero.

So, what you are seeing is to be expected (sort of, because it's not normal to run in FIRE mode until the turbine reaches approximately 60% of speed).

 

If everything on SRV is OK, servo, P2 pressure transmitters,etc. you maybe want to look at the speed feedback to the SRV control. Checkout speed-pickups, speed signal,etc. Do you have history of the event that you can show us. (Is this the same event that was posted on Linkedin?)

 

Thanks everyone for your answers. I have s simple question that what if due to some unknown reason gas turbine keep speeding when in Fire mode then why acceleration control FSR is not limiting the fuel going inside? It seems that due to rise in speed from 1200 to 3500 rpm (Selector switch in FIRE mode) SRV is opening which seems ok but GCV is not closing fully as i think it is stuck in MINFSR. So net flow inside gas turbine is increasing and which in the end results in over speed trip. Warm-up FSR is 19% and Acceleration FSR is 26%.

How does Acceleration control FSR works?

Best Regards.

 

Acceleration Control FSR is a type of speed control.

There is no speed control--of any type--when FIRE mode is selected.

When FIRE mode is selected, FSR is held at FSKSU_WU for as long as FIRE mode is selected, whether that be for five minutes or fifteen minutes or sixty minutes.

Unless there is a serious problem with P2 pressure control (as in the Speedtronic isn't properly controlling P2 pressure) it would be very, Very, VERY unlikely that the unit would trip on overspeed if left in FIRE mode "indefinitely".

Acceleration control is only active when AUTO mode is selected.

 

Adele Imtiaz,

Please tell us what you think should be happening when FIRE mode is selected.

Why are you using FIRE mode?

 

First of all i was reading your answer a similar sort of problem. Well we replaced servo of SRV valve which was hunting 3-5 Kg/cm2. In one of out simulation on trainer I found FSRMIN to be 15.83 and it did not change much as we shifted to speed control and temperature control using Frequency and temperature sources on trainer setup. We too have atomizing air circuit which is inactive from many years and recently during this start up we found traces of oil from VA-17. The oil supply was completely removed and plugged now.

Fire mode was just a test to see if speed stabilizes after 2100 rpm and to rule out oil taking part in ignition. Heavy oil was found in combustors when opened later. So mostly we are assuming that over speed was due to oil firing. Gas turbine also tripped on over-temperature during one of the start up immediately after firing and temperature were found to be about 280degc. Any further opinion?

Thanks & Best Regards,
Adeel