Minimum welding spacing in a 36" diam. piping spool?

I need help in stablishing what is the minimum spacing between two consecutive circumferential pipe welds in a 36" diam pipe (that is, we need a short straight piece of pipe between two elbows) so as to make the straight pipe section as short as possible; because some say the rule is to allow at least one pipe diameter between welds, and others say is is possible to place the welds much closer, based on pipe thickness considerations. In any case, I need to support this based on specs or piping codes. THANX. Alfredo.