GE 9FA fast unload on shutdown

[stratford74]

On site we have a GE gas turbine 9FA DLN2.0 with package 4 compressor enhancement and Mark VI controller. We have noticed some times (6 out of 100 shutdown) a fast unload of GT until the breaker opening. IN more details when the plant is shutting down, GT decreases load up to 55 MW. Then waits until the ST generator breaker opens and ST is tripped, at that point takes normal shutdown command and reduces load. Normally, in the vast majority of cases, the unload is done within 2, 5-3 minutes. In some cases the GT unloaded fast, in 45-50 seconds. On all these 6 cases we have noticed no strange thing, no speed increase, no frequency increase, no strange alarm. We have only seen that the PM4 gas supply was kept until 52G breaker opening and then stopped. Usually the PM4 stops at 20-30 MW depending on combustion reference temperature TTRF1.

GE engineering was requested to ask, they said that the acceleration kicked in and unloaded the unit and the proposed to change the control constant FSKMIND4_SD from 15.7 to 17 % (Shutdown Value Of FSKMIND4[0]). Can you please share any similar experience? My understanding is that the controller works on OPEN LOOP and goes to fast unload in order to avoid loss of flame and in the same time keep a continuously decreasing firing temperature.

 

[abeltio]

There is only a handful of 9FAs with DLN 2.0 left, with Mark VI even fewer.u

I've heard about this fast unload but never actually experienced it. Do you have any high-speed data capture during shutdown to identify how the fast unload kicks in?

If GE Product Service recommended adjusting, minimally, FSKMIND4_SD you should do it. The start-up and shutdown controls are open loop. If during shutdown (L94X =1) any can flames out, the unit will abort the fired shutdown and turn off fuel. Completing the fired shutdown, helps keeping the Maintenance Factor down.







saludos.
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