18.2 Water Distribution Pressure, Demand, and Storage
Key Takeaways
- Distribution-system pressure is hydraulic grade line minus node elevation, converted between ft of water and psi when needed.
- Average day, maximum day, peak hour, and fire-flow demands answer different design questions and should not be interchanged.
- Storage normally includes operational, equalization, fire, and emergency components, subject to local design criteria.
- Tanks establish or stabilize hydraulic grade, while pumps, pressure-reducing valves, and pressure zones control where that grade is usable.
- Looped networks improve reliability and reduce headloss sensitivity, but network answers still require continuity and headloss compatibility.
Distribution Systems as Hydraulic Grade Networks
The PE Civil WRE specification includes drinking water distribution systems, present and future demands, and storage. Distribution problems usually look like pipe-loss calculations, but the decision being tested is service reliability: can the system deliver the required flow while maintaining usable pressure, water age, and 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 elevation minus the ground or pipe elevation at that node. In U.S. customary units, pressure in psi is pressure head in ft divided by 2.31 for water. This conversion is one of the fastest checks on whether an answer is plausible.
Demand Terms
| Demand term | Typical meaning | PE use |
|---|---|---|
| ADD | Average day demand | Annual or monthly planning baseline |
| MDD | Maximum day demand | Source, treatment, and storage sizing check |
| PHD | Peak hour demand | Distribution pressure and pipe capacity check |
| Fire flow | Required hydrant flow for a duration | Fire storage and residual pressure check |
| Future demand | Projected demand at buildout or design year | Capacity and phasing check |
Peak hour demand is not the same as maximum day demand. Maximum day demand may govern supply and treatment, while peak hour plus fire flow may govern distribution mains, storage drawdown, and residual pressure at critical nodes. If the problem gives peaking factors, apply them to the correct base demand.
Pressure and Storage Workflow
- Convert population, unit use, industrial flow, and leakage to the requested demand basis.
- Identify the controlling demand case: average, maximum day, peak hour, fire flow, or pump-fill condition.
- Set the hydraulic grade from the tank water surface, reservoir, pump discharge head, or pressure-zone boundary.
- Subtract pipe and minor losses along the flow path to the node of interest.
- Convert remaining head above node elevation to pressure and compare with the stated criterion.
- Build storage as the sum of required components, using the units and duration in the problem.
Storage Components
| Component | Purpose | Common calculation form |
|---|---|---|
| Operational | Prevents excessive pump cycling and controls tank level range | Based on pump controls or volume between levels |
| Equalization | Covers hourly demand above supply rate | Often a percentage or mass-curve volume |
| Fire | Supports required fire flow for a duration | Flow x duration |
| Emergency | Covers outages, breaks, or source loss | Stated reserve or fraction of demand |
Elevated tanks and standpipes establish hydraulic grade directly from water surface elevation. Ground storage tanks need pumps to create service pressure unless the tank sits high enough. Pressure zones are used where topography would otherwise create too much pressure in low areas or too little pressure in high areas. Pressure-reducing valves lower downstream grade; booster pumps raise it.
Network Reasoning
A looped distribution network must satisfy both continuity at junctions and headloss compatibility around loops. On exam problems, you may not need a full Hardy Cross solution, but you should understand the logic: flow redistributes until the algebraic sum of headlosses around each closed loop is zero. Closing a valve, adding fire flow, or taking a tank out of service changes the network and may reveal a low-pressure area.
Operationally, high headloss can indicate undersized pipe, closed valves, tuberculation, high roughness, excessive velocity, or a demand assumption that is too high. Low chlorine residual or water age concerns may point to 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.
An elevated tank has a water surface elevation of 875 ft during a peak-hour condition. A junction is at elevation 730 ft, and headloss from the tank to the junction is 18 ft. What is the approximate pressure at the junction?
A community has an average day demand of 1.8 MGD. Maximum day demand is 1.8 times average day. Required equalization storage is 20 percent of maximum day demand. Fire storage is 2,500 gpm for 2 hours, and emergency storage is 0.50 MG. What total storage is required?