System and method for transitioning between fuel supplies for a combustion system

1. A system, comprising:

a turbine fuel controller, comprising:

a purge control logic configured to control a purge sequence of mixing a purge gas with a first fuel during a first fuel shutdown, wherein the purge sequence is configured to open a purge valve for the purge gas before fully closing a fuel valve for the first fuel.

2. The system of claim 1 , wherein the purge sequence is configured to fully open the purge valve before fully closing the fuel valve.

3. The system of claim 1 , wherein the purge sequence is configured to start opening the purge valve after starting to close the fuel valve.

4. The system of claim 3 , wherein the purge sequence is configured to start opening the purge valve after closing of the fuel valve is equal to or greater than a fuel valve closing threshold percentage.

5. The system of claim 1 , wherein the purge sequence is configured to increase a rate of opening the purge valve if a flame extinguishes in a gas turbine engine.

6. The system of claim 1 , comprising a gas turbine engine having the turbine fuel controller.

7. The system of claim 1 , wherein the turbine fuel controller comprises a fuel transition control logic configured to control a fuel transition from the first fuel to a second fuel for a gas turbine engine.

8. A system, comprising:

a fuel controller, comprising:

a fuel transition control logic configured to control a fuel transition from a first fuel to a second fuel for a combustion system; and

a purge control logic configured to control a purge sequence of mixing a purge gas with the first fuel during the fuel transition, wherein the purge sequence is configured to open a purge valve for the purge gas before fully closing a fuel valve for the first fuel.

9. The system of claim 8 , wherein the purge sequence comprises a fuel valve closing duration of progressively closing the fuel valve from a first open position to a first closed position, the purge sequence comprises a purge valve opening duration of progressively opening the purge valve from a second closed position to a second open position, and the fuel valve closing duration overlaps the purge valve opening duration.

10. The system of claim 9 , wherein the purge valve opening duration completes before the fuel valve reaches the first closed position during the fuel valve closing duration.

11. The system of claim 9 , wherein the purge valve opening duration starts after initiation of the fuel valve closing duration.

12. The system of claim 11 , wherein the purge valve opening duration starts after approximately 70 percent closure of the fuel valve during the fuel valve closing duration.

13. The system of claim 9 , wherein a rate of opening the purge valve increases during the purge valve opening duration if a flame extinguishes in the combustion system.

14. The system of claim 8 , comprising the combustion system having the fuel controller.

15. The system of claim 14 , wherein the combustion system comprises a gas turbine engine.

16. The system of claim 8 , wherein the first fuel is a gas fuel and the second fuel is a liquid fuel, or both the first and second fuels are gas fuels.

17. The system of claim 8 , wherein the purge gas comprises nitrogen, carbon dioxide, argon, helium, or steam.

18. A method, comprising:

transitioning a combustion system from a first fuel to a second fuel; and

mixing a purge gas with the first fuel during the transitioning, wherein mixing comprises opening a purge valve for the purge gas before fully closing a fuel valve for the first fuel.

19. The method of claim 18 , wherein opening the purge valve comprises fully opening the purge valve before fully closing the fuel valve.

20. The method of claim 18 , wherein opening the purge valve comprises starting to open the purge valve after closing of the fuel valve is equal to or greater than a fuel valve closing threshold percentage.