What criteria are considered when sizing process piping?

Criteria include: pressure drop limit (available pressure, pump sizing), velocity limits (erosion, noise, vibration), economic optimization (pipe cost vs energy cost), code requirements (minimum wall thickness for pressure), two-phase flow considerations (avoid slug flow regime), control valve pressure drop allocation (typically 25-50% of system dP), and special requirements (gravity flow, self-draining, slope for two-phase). Standard pipe sizes are selected from calculated requirements, often rounding up for margin.