Hi,
Is anyone help me to know the reason behind the fuel change over at 10 MW from liquid to gas for GE 7FA machines.
Thanks.

 

Maha,

There's a lot of information which you have not provided, such as, when are these fuel changeovers occurring and why? Are they operator-initiated? Is the liquid fuel distillate or heavy fuel oil or crude? What is the normal operating procedure for fuel changeovers at your site? What kind of combustors does the 7FA have--conventional combustors? Multi-nozzle quiet combustors (MNQC--which are a form of conventional combustor)? DLN-2? DLN2.6? ???

Is there a compressor for the natural gas which has to be started and running to initiate a changeover from liquid to gas?

Can the unit be started on natural gas and then changed over to liquid fuel?

In general, on F-class machines there is a minimum fuel flow-rate which must be flowing during a fuel change-over. The fuel flow-rate at FSNL (Full Speed-No Load) is usually slightly unstable and so the speed of the unit, and the load at min load after synchronization (sometimes called Spinning Reserve load) is unstable and the Mark* can't hold a stable load.

Also, sometimes, the pressure drops across the fuel nozzles (gas and liquid) at low fuel flow-rates are also unstable. Since it's possible to "pause" a fuel changeover at some intermediate point insufficient pressure drops/flow rates can also result in backflows of hot combustion gases into fuel nozzle passages, manifolds and tubing which can cause damage to components. So, written in the Control Specification, usually Sect. 05, are guidelines for minimum fuel percentages and mininum fuel loads.

Also, sometimes the Atomizing Air flows at low loads (and during starting, acceleration and FSNL operation) can also be at unsafe minimums so guidelines help to prevent damage.

Finally, most Mark* turbine control systems are programmed to prevent fuel changeovers from START to FSNL, at a minimum, and some also up to some minimum load (such as 10 MW). This is so that the fuel flow-rates during prefill and transfer are at stable flows and will not result (usually) in "bumpy" load swings during fuel changeovers.

But, there can be other reasons, too, such as the normal operating instructions for the fuels at the site, the type of combustors, the configuration of the gas fuel supply system (sometimes the gas compressors are not capable of stable operation at very low fuel flows, such as during starting, acceleration and FSNL operation). So, there can be several reasons in addition to the general ones described above.

Hope this helps! It's best to follow the OEM (Original Equipment Manufacturer) guidelines for fuel changeovers to protect against unnecessary damage to fuel nozzles and liners and transition pieces and tubing and manifolds.