Water Distribution: Pressure, Storage, and Main Work
What the distribution system must do
A distribution system delivers adequate pressure and quantity while keeping water safe from the plant to the tap. Candidates should understand the components and how they interact: transmission and distribution mains, valves (gate, butterfly, check, pressure-reducing, air-release), hydrants, meters, service lines, booster pumps, and storage (elevated tanks, ground storage, and hydropneumatic tanks). Storage provides fire flow, demand equalization, and emergency reserve, but it also creates stagnation and water-age problems, so turnover and tank mixing matter. Pressure zones, hydraulic grade lines, and pressure-reducing valves control where pressure is highest and where it can be lost.
Traps
Unit conversion between pressure and head appears constantly: 1 psi = 2.31 ft of water, and 1 ft of water = 0.433 psi. Read whether a question asks for static pressure (no flow, from elevation difference only) or residual pressure (while flowing, reduced by friction loss). Flow conversion errors are common (1 MGD ≈ 694 gpm; 1 cfs ≈ 448.8 gpm). New mains are not just pressure-tested; they are chlorinated, flushed, and bacteriologically sampled before service. Backflow prevention is a distribution issue, not only a customer issue: cross-connections, unprotected hose bibs, and irrigation lines are classic exam scenarios.
Worked example
An elevated tank's high-water level is at elevation 120 ft and a tap sits at elevation 20 ft. Static pressure at the tap = (120 − 20) ft × 0.433 psi/ft ≈ 43.3 psi. If the tap must stay above 20 psi under fire flow, the allowable friction loss in the path is about 23 psi.