18.2 Water Distribution Pressure, Demand, and Storage

Distribution Systems as Hydraulic Grade Networks

The PE Civil WRE specification covers drinking-water distribution systems, present and future demands, and storage. Distribution problems look like pipe-loss calculations, but the decision being tested is service reliability: can the system deliver the required flow while holding usable pressure, acceptable water age, and an emergency reserve?

A distribution system is a connected set of sources, pumps, tanks, pressure zones, transmission mains, distribution mains, valves, hydrants, and service connections. At any node, pressure head equals the hydraulic grade line (HGL) elevation minus the ground or pipe elevation at that node. In U.S. customary units, pressure in psi equals pressure head in feet divided by 2.31 for water (because 1 psi = 2.31 ft of water). This single conversion is the fastest plausibility check on a distribution answer: 40-80 psi normal service translates to roughly 92-185 ft of head above the node.

Demand Terms

Peak hour demand is not maximum day demand. A common chain is MDD = peaking factor x ADD, then PHD = peaking factor x MDD. If ADD is 2.0 MGD with MDD factor 1.8 and PHD factor 1.5, then MDD = 3.6 MGD and PHD = 5.4 MGD. MDD usually governs supply and treatment; PHD plus fire flow usually governs distribution mains, storage drawdown, and residual pressure at critical nodes. Apply each peaking factor to the correct base.

Pressure and Storage Workflow

1. Convert population, unit use, industrial flow, and leakage to the requested demand basis.
2. Identify the controlling case: average, maximum day, peak hour, fire flow, or pump-fill.
3. Set the HGL from the tank water surface, reservoir, pump discharge head, or pressure-zone boundary.
4. Subtract pipe and minor losses along the flow path to the node of interest.
5. Convert remaining head above node elevation to pressure and compare with the criterion.
6. Build storage as the sum of required components in the problem's units and duration.

Worked pressure example. An elevated tank sits at water surface 920 ft. A critical node is at elevation 760 ft, and headloss to it under PHD is 25 ft. HGL at the node is 920 - 25 = 895 ft; pressure head is 895 - 760 = 135 ft; pressure is 135 / 2.31 = 58 psi, which clears a typical 40 psi minimum. During a coincident fire-flow event the added flow increases headloss, drops the HGL, and the same node must still hold a residual (commonly 20 psi) at the hydrant.

Storage Components

Elevated tanks and standpipes set the HGL directly from water-surface elevation. Ground storage needs pumps to create service pressure unless sited high enough. Pressure zones divide steep terrain so low areas avoid excessive pressure and high areas keep adequate pressure. Pressure-reducing valves (PRVs) lower downstream grade; booster pumps raise it.

Network Reasoning and Operations

A looped distribution network must satisfy both continuity at junctions (flow in equals flow out) and headloss compatibility around loops (the algebraic sum of headlosses around each closed loop is zero). You may not need a full Hardy Cross iteration on a six-minute item, but you should know the logic: flow redistributes until each loop balances. Closing a valve, adding fire flow, or taking a tank offline rewrites the network and can expose a low-pressure pocket.

Hardy Cross flow correction for a loop is delta-Q = -sum(hL) / [n x sum(hL/Q)], where n is the exponent on Q in the headloss law (about 1.85 for Hazen-Williams, 2 for Darcy-Weisbach). Sign convention (clockwise positive) must be consistent, and the correction is applied to every pipe in the loop until delta-Q is negligible.

Operationally, high headloss flags undersized pipe, a closed valve, tuberculation, high roughness, excessive velocity, or an overstated demand. Low chlorine residual or water-age complaints suggest oversized dead-end storage or poor turnover.

A PE answer should connect the calculation to the service goal: adequate pressure, adequate storage, and reliable flow under the specified condition, not just a number.