Buffer Volume for Fuel Changeover (From Natural Gas to Oil)

Well this seems like a basic physics problem to me that requires you to research several critical parameters before even trying to calculate anything. First you must know the minimum and maximum fuel feed pressure for the gas turbine to properly operate. Then you need to know the fuel consumption rate of the gas turbine under expected and maximum load. Now you need to know the switch over time you need to get your oil fuel system operational. (How do you burn oil in a gas turbine?)

Once you have this information, you can make an approximation of your requirements. The change over time and the gas fuel consumption rate will tell you how much fuel you will likely burn during the change over. Now if your feed line is a fixed volume (no pressure regulating expansion tank) you can then use the universal gas law (PV=nRT) and the acceptable pressure window to tell you how large of a volume you need. I would ignore temperature changes in a first calculation iteration and just add a capricious fudge factor of some slightly larger volume to handle this and other uncertainties.

You will notice that I've not given you a formula other than the universal gas law to just plug in numbers. The units of the values you find should tell you how to construct the equations you need. For an example, once you know how large of a volume you need and you identify your pipeline inner diameter, you can then determine the desired length so you can place the cut off valve. You will likely find that units must be converted so that units will properly cancel.

If this doesn't make sense, then maybe you need a tutor or to hire an engineer.

Oh, as far as Q1... I suspect that you will find significant problems just relying on your upstream pipeline just by doing your calculations. But since I don't know anything about your system, I could just as easily be wrong.