Criteria, tools, and practices for high-purity water distribution systems for semiconductor manufacturing plants include water quality, pressure, flow, velocity, piping pressure losses, computer-aided design, hydraulic modeling, model maintenance, models of 3-loop concepts, direct return loop, reverse return loop, serpentine loop, process control concepts, back-pressure regulating valve on end of the return, pump variable frequency drive control based on flow to the loop, back-pressure regulating valve or pressure control valve before the loop, manual valve and rotameter at ends of laterals, back-pressure regulating valve or pressure control valve mid-main, back-pressure regulating valve or pressure control valve at end of main, and joining (for example, IR welding PVDF piping).