For our newly commissioned GE Frame 9 Fe machines recently we started schedule fuel change over test twice in a month as per the management. But it is very inconvenient to reduce the load down to 25MW for DFO to NG change back since system demand is very high. (We have to fight with Load despatch Centre for load reduction. NG is the primary fuel.) Why it is required to reduce the load while NG to DFO we are carrying out even at base load. Is there any link with TTRF? Once I tried to initiate fuel change over from DFO to NG at around 28MW and TTRF was 980 deg.C and command didn't get executed.

 

Hi,

It may have something to do with the gas mode.
When we change from NG to DFO we must reduce load so we can bring on D5 (diffusion) gas or we will flame out.

Also; once the DFO is complete the system will purge its self, this will blow any remaining fuel out off the lines and in to the combustors. This will result in a surge in the turbine and you will see a rise in load by?? % (Somebody help me out here).

Hope this helps.http://www.control.com/thread/1026245221 )

 

There could be several reasons why you need to unload before transferring. One good reference is GEK-111717; if you don't have a copy, you should.

One should never perform fuel transfers while at Base Load; there is no margin for error since the exhaust temperature is only 40 deg F below the trip setpoint, so any bobble in fuel supply (like air in the liquid fuel piping--which is very common in Liquid Fuel Systems without Liquid Fuel Recirculation systems) can cause a problem and possibly (likely) a trip. So, transfers below Base Load are recommended in all cases. One recommendation is to not attempt transfers unless the IGVs are 6-9 DGA (DegGrees Angle) below full open; on units F-class with DLN combustors, this can be well below Base Load.

DLN combustor-equipped units also have some issues with fuel transfers during certain combustion modes; we don't know which combustion system your unit has.

If the unit runs heated fuel, you should reduce load to below the point at which heated gas fuel is required. Does your unit use heated gas fuel (for performance enhancement)?

Lastly, it appears there is some sequencing above which you are prevented from attempting a fuel transfer, and you should examine the sequencing/application code of the Speedtronic turbine control system to determine exactly what that value is (or what determines the value--like a requirement to be below the point at which heated gas fuel is required, if the unit uses heated gas fuel).

Liquid fuel reliability, which is the reason for regularly exercising the Liquid Fuel System, is difficult to achieve for units which operate primarily on gas fuel and which are expected to transfer to liquid fuel "bumplessly" in the event of some interruption of gas fuel supply. There are so many components to the Liquid Fuel System, all of which must operate properly, and there is minimal instrumentation and control of many of the components. (It's been said here several times: The weakest links in a combined cycle power plant are the gas fuel compressor (if so equipped, and most F-class sites have a gas compressor), and the water treatment system. It's usually cost-prohibitive to have back-ups of both, so they are both a single point of failure.)