Q1:Why fuel gas has to enter a combustion chamber/burner of a gas turbine at a certain pressure i.e otherwise GT trips below that pessure?

Q2:What could be the effect of low furnace oil temperature on gas turines other than its clogging in lines? I mean that on our GT(Frame9 markIV) during normal operation, furnace oil temperature should be arround 110C & GT changes to HSD if furnace oil temperature drops to about 95C

 

A1: Um, I was always taught that sewage flows downhill, not uphill. In other words, the gas fuel pressure must be higher than the pressure in the combustor otherwise, it won't be able to enter the combustor. In that case, the unit would lose load as pressure and flow decreased, until, if pressure and flow dropped low enough the unit would trip on reverse power and/or lose flam.

Now, if your turbine has some kind of pressure switch or sensor that initiates a trip (emergency shutdown), that's not quite typical and would be something that's peculiar to your turbine. Many units initiate a transfer to liquid fuel (usually some kind of diesel-like distillate, sometimes called "high-speed diesel"); one could presume that if liquid fuel were not available, that the unit might trip. But, that too, would be atypical of most control sequencing.

A2: I would suspect the improper atomization of low temperature "furnace oil" (which I presume is something akin to "bunker C" and has a high viscosity) would be the reason for initiating a fuel transfer when the temperature dropped below that at which the fuel could be properly atomized. In addition to air atomization, the fuel nozzles have passages that work to mechanically atomize the fuel, and if the viscosity is too high then it probably won't atomize too well and will smoke and result in poor combustion.

 

Q1. This pressure is critical during the startup phase of a gas turbine. Being an open-loop control, startup control sequences the turbine using pre-defined fuel levels. For example, FIREFSR could be 20, WARMUPFSR, say, 18. These levels are set for certain pressure that is proportional to the speed of the turbine, TNH.

Once speed control takes over, the pressure is not as critical as it is during the startup phase. However, to ensure faster response and better control, it is advisable to regulate the fuel supply pressure.

Q2. Fuel temperature affects its viscosity, which in turn determines how well the fuel is atomised. In the case of your unit, below 95 deg. C, the furnace oil viscosity will exceed the allowable limit. That is why the unit switches to HSD.