pipes

Rayan, first of all, what do you need to do? Pump fuel? At which pressure and temperature?

First you must define what are your needs, regarding process operation, cleaniless and durability. Then, you should go for ASME standards and take a look in all pipe dimensioning and standard sizes. Also, check with a local business that works with fuel what are the standards for safety in your region. With basic process parameters and dimensions, go examine some materials... think about your process control and add compatible pumps and valves... there are a lot of things to do.

Fittings are just a consequence of your pipe choice.