What criteria are used for sizing process piping?

Line sizing criteria balance velocity, pressure drop, and economics. Velocity limits: liquid 1-3 m/s (avoid erosion), gas 20-40 m/s (avoid noise/vibration), two-phase uses erosional velocity formula. Pressure drop: typically 0.1-0.5 psi/100ft for liquids, higher acceptable for gases. Economic optimization: larger pipe = lower operating cost (less pumping) but higher capital. Specific applications: pump suction - low velocity, minimize losses for NPSH; relief headers - API 520 back pressure limits; gravity flow - adequate slope and head. Line size affects: control valve sizing (pressure drop available), pump head, and compressor discharge pressure.