Abnormal sound during mode changeover in DLN2+ machines


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Ours is a GE 9FA DLN2+ machine. During mode changeovers (from diffusion to premix or during different mode changeover) abnormal sound is coming from gas turbine. What is that sound? As per GE it is a normal sound which is coming due to disturbance of combustion dynamics.
While you may consider the sound to be abnormal, it is likely not, given the way that DLN combustion systems work. During transfers, the flow through the various nozzles increases and decreases, and that in itself can cause "whistling" noises.

In addition, the combustion dynamics (pressure oscillations) inside the combustors increase and decrease as fuel is shifted from one set of nozzles to another or from one combustion zone to another (depending on the type of DLN combustion system).

So, the fact that noises other than the ones normally heard during steady-state operation are heard during transitions from one combustion mode to another is not abnormal at all; rather, it's expected. Just as you were told by the GE representative.

If you hear those noises during steady-state operation while in a particular combustion mode, that's when you should be concerned.
It is purely combustion dynamics, it is high when it is changes from PPM to PM.

During this time GT vibration & spread to be monitored

Let me share our experience. We have (Ratnagiri Gas and Power Pvt. Ltd earlier Dabhol Power) six 9FA's. Two CTG's are DLN2 and four CTG's are DLN2+ combustion systems.

The noise is very much common for DLN2+ combustion system while transferring from PPM to Premix mode. This noise can be heard from even 3km distance. Initially we also had a lot of apprehension about this but every time GE has said that it is very much common so now a days we have stopped listening to this noise.

In DLN2 system (Old system) transfer from PPM to Premix mode is noiseless.