Water Supply and Distribution
Water supply questions test pressure more than pipe. Three ideas anchor most of them: 0.433 psi per foot of elevation (the weight of a column of water), the minimum residual pressure fixtures need, and the maximum static pressure above which you must protect the system.
Typical design targets: about 8 psi flowing for tank-type water closets, roughly 15 to 20 psi flowing for flushometer valves, and a service that lands near 50 to 60 psi. A pressure-reducing valve is generally required when static pressure exceeds 80 psi. Once you add a PRV, a check valve, or a backflow preventer, the system becomes closed, and thermal expansion needs a tank.
Sizing uses water supply fixture units (WSFU) converted to gallons per minute through the Hunter curves, and the curve differs for flush tanks and flushometers, a classic trap. Add the fixtures, look up demand, then select pipe from the table using developed length (measured length plus a fitting allowance, often roughly 50 percent) and available pressure after subtracting elevation and meter losses.
Backflow protection is guaranteed material: air gap (best), reduced-pressure zone assembly (RPZ), double check valve assembly (DCVA), pressure vacuum breaker (PVB), and atmospheric vacuum breaker (AVB). High hazard demands an air gap or an RPZ.
Worked example: a meter reads 80 psi and the highest fixture is 30 ft above it. Static pressure there is 80 minus (30 x 0.433) = 80 minus 13 = 67 psi, before any friction loss.