At our site we are working on a GE Frame 9E GT. the liquid fuel purge air scheme shows three valves namely 20PL, VPR-54, VA-19 which together control the flow of air into the purge air manifold. The air comes from the discharge line of the atomizing air compressor. 20PL when energized opens the stop valve VA-19 to permit air into the purge air manifold. VPR-54 allows the atomizing air system pressure to reach a lower level for energizing 20PL. On consulting the O&M manual we found that before switching to gas fuel from liquid fuel we need to purge the fuel nozzles. As we will be using only liquid fuel as of now, we feel that there is no need to use the purge air system. Question is that at site we need to fix these two valves along with the piping, if it is not required then we will not erect/commission this line. Kindly comment and advise.

 

Good on you for reading the manual!

If you "erecting" the unit, there is likely at least one GE representative at site. What does GE say about this proposal?

Liquid fuel purge air flows all the time the unit is running on gas fuel above 95% of rated speed.

Liquid fuel purge air has three functions. First, it it used to push any liquid fuel out of the fuel nozzle so the liquid fuel won't burn/carbonize in the nozzle when running on gas fuel and make future liquid fuel operation difficult if not impossible.

Second, liquid fuel purge air cools the liquid fuel nozzle passages when running on gas fuel.

Third, liquid fuel purge air prevents hot combustion gases from back-flowing into the liquid fuel lines (liquid fuel purge air pressure is slightly greater than the pressure inside the combustion liner) when running on gas fuel above 95% of rated speed.

Liquid fuel purge air flows after a transfer from liquid fuel to gas fuel when the unit is running and when the unit reaches 100% gas fuel.

When transferring from gas fuel to liquid fuel, liquid fuel purge air is stopped almost as soon as the transfer is initiated.

Lastly, most dual fuel (gas/distillate) GE-design Frame 9E heavy duty gas turbines I've worked on had a three-way valve for VA-19, and spring-operated, poppet-type, in-line check valves in the purge air lines very close to the fuel nozzle. When VA-19 was not porting air to the liquid fuel nozzles, it connects the liquid fuel purge check valves to the "Tell Tale Leak-off." If any of the liquid fuel purge air check valves leaked in the reverse direction when running on liquid fuel, the fuel is directed to the Tell Tale Leakoff. The concept is that someone monitoring the Tell Tale Leak-off could see that one or more of the liquid fuel purge check valves was leaking.

But, blinding off anything for a possibly temporary period is not advisable. It could cause unintended consequences of all of the possible effects aren't considered, and later if it's forgotten about the modifications when attempting to run on gas fuel.

 

Thanks for the reply. Our company is having a technology collaboration with GE and our design/manufacturing departments normally address field issues. So GE representative is not present. Anyway, we have contacted our GT design department and have told them to send the balance materials immediately so as we can proceed with the work, even if the unit operates on liquid fuel as of now.

Your reply has helped us understand the system better.