We have switched over to Natural Gas in 2009. Earlier we were burning Naphtha in our Frame 6 GT with Kerosene as a startup fuel.

Now as per control logic, in case of Natural gas failure, the fuel supply changes to Kerosene.

The question is why I can't changeover from Natural Gas to Naphtha directly?

 

Without being able to see the sequencing or application code running in the turbine control system at your site, and without understanding how the unit was originally configured or may have been modified to also burn Natural Gas, it's very difficult for us to say why it's behaving the way you describe. A Frame 6 turbine is basically a Frame 6, but the auxiliaries (including the fuel systems) and the packaging and the control philosophies can be very different for any Frame 6.

To make a specific recommendation or explain precisely how your machine is operating or could be operated, it would first be necessary to examine the sequencing or application running in the turbine control system at your site. There may also be some other issues we can't be aware of that preclude the direct changeover from Natural Gas to naptha when the unit is running.

It sounds like when the unit was being operated on liquid fuel (kerosene/naptha) that it was a requirement the unit be started and stopped on kerosene. So, kerosene was a "primary" fuel of sorts, possibly, and naptha was burned when available, sort of a "secondary" fuel of sorts.

And it seems someone didn't communicate the desired operating modes during the original configuration or when the switch to Natural Gas was made (we don't know if the unit was shipped with dual fuel (gas/liquid) capability or if gas fuel was added after the original installation).

If your turbine control system is programmable, it could probably be programmed to do what you would like after checking to see that the unit was at load, with no STOP in progress.

We also don't know how the transfer from kerosene to naptha is made (through a three-way valve?). So, there may be some additional sequencing modifications required.

But, there is a reason it's behaving the way you describe, we just don't enough information to be able to say for certain what that reason is. But, again, it's probably likely the programming of the turbine control system could be changed to make the fuel transfer directly to naptha from Natural Gas under load and with no active STOP sequence in progress.

 

Sir, as you said, previously Kerosene was startup fuel and then it was switching over to Naphtha as main fuel. The fuel changeover is by a 3 way valve.

The machines were designed for gas firing, but we have recently started firing gas. My question is wrt feasibility of switching to Naphtha from Natural Gas considering issues like flash point of Naphtha etc.

 

Great question for the packager of the turbine, or for GE, or for a turbine service company.

We seem to have a lot of readers at control.com that burn naptha at their sites; perhaps one or more of them has experience with dual fuel operation with Natural Gas as the other fuel and would share their experiences with us.

 

I've been mulling this over, and, yes, the desired mode of operation would be to have the better fuel in the fuel system during shutdown. But, if the unit is running on liquid fuel and trips, then there's going to be naptha in the system.

I wouldn't think there would be any reason why the unit couldn't transfer directly from Natural Gas to naptha while running, which is what I think the originator is asking about. But, during a normal, fired shutdown from operation on naptha it would probably be best, as you suggest, otised, to have the unit automatically switch to kerosene when STOP was initiated and then continue the shutdown on kerosene, leaving kerosene in the liquid fuel system after shutdown. Leaving it ready to start on kerosene if a liquid fuel start was initiated.

If the unit had started on Natural Gas, and transferred to naptha, and then tripped, it would be the same as starting on kerosene, switching to naptha and tripping. Can't get around that. Whatever precautions they would normally take for such an event would have to be followed.

But, the temperatures in the combustor when operating on Natural Gas or kerosene shouldn't be that much different when operating on naptha (I'm not a combustion engineer, either).

But, I'm still betting on the fact that something wasn't made clear during the requisition phase or during the initial gas fuel commissioning, and that if the unit control was programmable it could be made to transfer from Natural Gas to naptha while running. The load swings might be interesting, and the whole three-way valve thing might also be a challenge.

Of course, then they are going to want to transfer from naptha to Natural Gas, which is a question I forgot to ask: Is it possible to transfer directly from naptha to Natural Gas, or can you only transfer from kerosene to Natural Gas? If you could transfer directly from naptha to Natural Gas, that would leave naptha in the liquid fuel system, and that's probably not desirable.

So, maybe there's something to your theory, otised. But, there are still questions, aren't there?

I still say the originator should contact the turbine packager for help with determining what's possible and what's not and for help with achieving what's possible if it meets their needs. But, there's still hope that someone here has experience with this naptha/Natural Gas operational question.

 

The reason could be Naptha is highly corrosive at lower temperatures may this could be the reason they didn't naptha, this the reason b4 also your using kerosene to start and then changing to naptha. If any got more Info please post