Water Supply and Distribution Systems
Pressure, sizing, and the design path
Water supply design starts at the source and works toward the most remote fixture. You need the static pressure at the service, the developed length of the critical path, the elevation change, the fixture unit load (wsfu), and the pressure losses through the meter, valves, and fittings. Subtract losses and elevation (0.434 psi per foot of rise) from the available pressure; the remainder must meet the minimum residual at the fixture - commonly 8 psi flowing, with greater minimums for flushometer valves.
Components you must recognize
- Pressure reducing valve where street pressure is excessive.
- Thermal expansion tank on a closed system with a backflow preventer or check valve.
- Water hammer arrestors at quick-closing valves.
- Recirculation for hot water; properly sized and insulated lines.
- Isolation valves so sections can be serviced without shutting down the building.
Common traps
- Confusing static pressure with flowing (residual) pressure.
- Forgetting elevation loss on a tall building.
- Sizing by pipe diameter instead of by fixture units and developed length.
- Omitting an expansion tank on a closed system.
- Assuming a larger service always solves a pressure problem when the real issue is friction loss.
Rule of thumb
Each foot of height costs about 0.43 psi. A 20 ft rise costs roughly 8.7 psi before any friction is counted.
Worked example: 60 psi at the meter, 20 ft rise (8.7 psi), 10 psi of friction through meter and piping. Residual at the fixture = 60 − 8.7 − 10 = 41.3 psi, comfortably above the 8 psi minimum.