Water Supply and Distribution Systems
Design Fundamentals
Water supply work is about pressure and velocity. The available pressure at the street must overcome elevation (about 0.433 psi per foot of rise), friction loss through pipe and fittings, and the pressure required at the most remote fixture. Velocity should be kept moderate, roughly 4 to 8 feet per second, to limit noise, erosion, and water hammer. Sizing starts with water supply fixture units for each fixture, totals them, and converts the total to demand in gallons per minute using the code chart for the type of system (flush tank versus flushometer).
Traps in Questions
- Forgetting that a closed system with a backflow preventer or pressure-reducing valve needs a thermal expansion tank; a check valve traps heated water and the relief valve starts dripping.
- Installing a temperature and pressure relief valve discharge with a threaded end, reduced size, or a trap; it must discharge full size to a safe location.
- Sizing by pipe diameter alone without checking developed length, which includes fittings and elevation.
- Assuming a private well system meets code without a check valve, pressure switch, and potable-water protection.
- Ignoring minimum fixture pressure, roughly 20 psi at the fixture, and maximum static pressure of about 80 psi before a regulator is required.
Worked Example
A hose bibb is 20 feet above the meter, and the meter reads 65 psi. Subtract 20 x 0.433 = 8.7 psi for elevation, leaving 56.3 psi. Then subtract friction loss through the meter, valves, and 60 feet of piping, perhaps 8 psi, for 48 psi static at the bibb. That comfortably exceeds the 20 psi minimum, so the run can likely stay at 3/4 inch, subject to the sizing table.