I need some advice on the use of Liquid Fuel re-circulation on a Dual Fuel machine which will normally run on Gas. (No water injection)
Can anyone give their opinion on the use of re-circulation when running on gas? I know it is meant to give more reliable changeovers to liquid, are there any adverse problems?
I have a decent experience with liquid fuel re circulation. Our unit is 7 FA DLN2.6. The purpose of the liquid fuel re circulation system can be summarized into three points.
First, Protecting the liquid fuel divider gears from binding to each others due to non-use of liquid fuel system operation.
Second, Constantly keeping the liquid fuel lines free of air. There is a high point (Stand-pipe Vent) in the liquid fuel re circulation system.
Third, to protect The Three-Way Purge Valve internals from coking due to the heat inside the turbine compartment. Because the liquid fuel goes to the three purge valve, absorbs some heat and return back to the storage tank. by keeping the re circulation ON the liquid fuel and the Three-Way Purge Valve will be protected from overheating.
For units that don't have re circulation system, GE recommends weekly or Bi-weekly transfer. If the unit is equipped with liquid re-circulation system and the re circulation is always ON during gas operation, then GE recommends quarterly transfer. While the turbine is burning gas, the pressure of the liquid fuel lines is around 60 Psig. and flow rate is 1 lbm/s.
Regarding adverse problems, since the liquid fuel lines always pressurized during re circulation, One of the most adverse problems is liquid fuel leak from fittings inside the turbine compartment. The first sign of this leak is that, you will see smoke coming from the turbine compartment ventilation system. If immediate action is not taken, there is a possibility of fire inside the turbine compartment. More than one unit on our site caught fire due to this issue. One of the most suspected fittings to leak is the tube reducer used on the 3-Way Purge Valve and we have a TIL from GE regarding this tube reducer.
If there is a leak inside the turbine compartment and the re circulation system got isolated, then if you can't fix that fitting because the turbine is running, the liquid fuel lines has to drained and then purged with Nitrogen, otherwise the liquid inside the 3-Way Valve will coke and you will lose the reliability of the system. If the plant is not equipped with N2 plant, you will have to used portable N2 cylinders with pressure regulator for the purging process.
Hope that helps