Sir,

We have 2 Mark V frame V gas Turbines @ 20MW running on Naphtha part Load.

During each start-up after firing established from around 3000 rpm gas turbine exhaust temp increased drastically to 480 Deg C & then came to 280 Deg C for more than 5 min ( nearly 200 Deg C). Sir is it advisable.

& once during start up in diesel engine engaged mode vibration went up to 18mm/Sec for 3 min. Then after some time it came to normal mode. Now it's running smoothly @ 3 mm/sec. Sir can you clarify my doubt after that much vibration how unit will run smoothly & what may be the reason behind that?

 

During starting and acceleration, the exhaust temperature climbs very high and then as the unit reaches rated speed the exhaust temperature will decrease greatly. This is a normal characteristic of GE-design heavy duty gas turbines.

This is because axial compressors are very inefficient (low air flows and pressures) at speeds below rated, so the air flow through the machine is much less than rated.

If your machine has a Mark V, then you can trend several values during operation. Trend CPD (compressor discharge pressure), TTXM (average exhaust temperature), FSR (Fuel Stroke Reference), and TNH (actual turbine speed, in percent). You will notice that CPD is very flat until the unit approaches rated speed, and then it increases very sharply. As it increases, TTXM will decrease. FSR generally increases as speed increases, and exhaust temperature usually increases with increased fuel flow, except during acceleration with the axial compressor flow/pressure is less than normal and when the unit is operating on CPD-biased exhaust temperature control.

If the unit has modulated IGVs, then IGVs will begin to open as the unit is accelerating, and if the unit has compressor bleed valves (and most do) sometimes they will close at approximately 95% speed. Both of these actions will also increase air flow and CPD, which will help "depress" the exhaust temperature even though FSR is increasing.

If you examine the instruction books provided with the units, you should find some graphs which will also show the same behavior, that during starting and acceleration the exhaust temp will "spike" and then decrease as the unit is accelerated.

Vibration can be caused by all manner of things, though I'm not familiar with diesel starting means causing high vibration unless one is doing something abnormal with the diesel starting means (like trying to close the jaw clutch when the shaft is still rotating). You are likely relating two unrelated events.

Most GE-design heavy duty gas turbines pass through two critical vibration speeds as they accelerate to rated speed. GE nor the packagers of their turbines publish these critical speeds, but by plotting vibration vs. speed, you can usually see them as slight spikes as the unit accelerates through the speeds.

Also, L.O. temperature can have a large impact on vibrations during starting. "Cold" L.O. (less than approximately 25 deg C) usually causes high vibration.

 

Thank You Sir for your reply

Sir usually LO temp will raise only after recirculation through the bearings only. So during stating generally it will be less. What should be the temp range?

Regards

 

Most, but not all, GE-design heavy duty gas turbine L.O. tanks have electric immersion heaters to help with maintaining the L.O. temperature at a minimum level to support starting. As stated in the previous response, the desired L.O. temperature for starting a GE-design heavy duty gas turbine is approximately 26 deg C (approx. 80 deg F), to prevent high vibration due to low viscosity oil in the bearings (I think the phenomenon is called "oil whip" but I may be wrong about that).

If your unit doesn't have a heater and the only way you can increase your L.O. temperature is to circulate the L.O. by running the Aux. L.O. Pump (either manually or by selecting Cooldown ON), then you should do whatever is required to ensure the L.O. temperature is approx. 26 deg C before starting, or the unit will likely experience higher than normal vibrations during starting and acceleration.

Now, an experienced technician will note that the minimum L.O. reservoir temperature for starting (26QN) is 50 deg F, but that is the absolute minimum and it's accepted and expected that high vibration will likely occur if the unit is started with 10 deg C (50 deg F) L.O. temperature. That's there to prevent catastrophic damage from occurring by starting with colder L.O., but if it's absolutely necessary to start the unit (emergency condition) and the L.O. is at least 10 deg C (50 deg F), then the unit can be started, but, again, it is expected and accepted that there will be higher than normal vibrations if the oil temperature is less than approx. 26 deg C (80 deg F).

A really experienced technician will also note that the setting for the L.O. Tank Immersion Heater "thermostat" will drop out the heater contactor when the L.O. tank temperature reaches approximately 21 deg C (70 deg F). That's always been a little problem with the Device Summary setting for that device.

 

I think your both queries could be due to following -

1. Exhaust temperature - Going by the start up curve/profile of gas turbines, the exhaust temperature increases after firing and reaches peak of say 510 deg. cent at some 50 - 52 % of actual speed but upon further acceleration the exhaust decreases as CPD increases and then during IGV opening and finally at FSNL it reaches a much lower value, say around 275 degree centigrade

2. Vibrations - You have mentioed of vibrations while the diesel engine was still engaged which means that the speed was below the de-clutch speed. Did you re-start the machine, after a stoppage and no barring. May be a temporary bow formation on the hot shaft would have caused this. Or else the oil temperature LTTH1 could be low. Was there any change in the IGV position noticed i.e. CSGV & CSRGV although they would be following each other with a very small difference.

You need to be more specific and detailed when posing a question for proper diagnosis