7.6 Network Layout, Pressure Zones, Demand Patterns, Fire Flow & the Hydraulic Grade Line

Laying out a system that delivers water everywhere

The WPI Water Distribution outline's largest content area, Distribution System Components at 35 percent, opens with aiding in the design of water distribution projects, assessing water production against demand, adjusting production to meet demand, and understanding schematic diagrams and flow characteristics. Those tasks all rest on system layout.

Network configurations

Looping is a water quality decision as much as a hydraulic one. Dead ends accumulate sediment, lose chlorine residual, grow biofilm, and generate the taste, odor, and discolored water complaints that dominate customer contact. Where looping is impossible, dead ends need blow-off or flushing hydrants and a routine flushing schedule.

Valving determines how much of the system must be shut down for a repair. Good practice is enough valves that any single break isolates a short segment: commonly valves at every intersection, with a target of no more than one or two blocks and a limited number of customers out of service. Valve exercising is what keeps this true; an unexercised valve that will not close turns a one-block outage into a ten-block outage.

Pressure zones

Colorado's terrain makes pressure zones unavoidable. Every 2.31 feet of elevation change is 1 psi, so a 300-foot hillside spans about 130 psi from top to bottom. A pressure zone is an area served within an acceptable pressure band, bounded by closed valves, pressure reducing valves, booster pump stations, and its own storage.

Design targets in general practice:

• Normal working pressure of roughly 40 to 80 psi at the service connection.
• Minimum 20 psi at all points during peak hour and fire flow. Falling below 20 psi is the trigger for a pressure loss event and potential backsiphonage, and it is the threshold Colorado uses in Regulation 11 for pressure-related notifications and disinfection requirements after repairs.
• Maximum around 100 psi, with individual pressure reducing valves at services above that.

Regulation 100's Step 2 distribution table converts these realities into classification: 3 to 5 pressure zones raises a system to at least Class 2, 6 or more raises it to Class 3, and normal operating pressure above 150 psi raises it to Class 3. A pressure zone must serve at least 15 service connections to be counted.

Zone boundaries are enforced by closed boundary valves, and the most common cause of a mysterious pressure or water quality problem is a boundary valve found open, cross-feeding two zones. Valve position records and periodic verification matter.

Demand

• Average day demand (ADD) — total annual volume divided by 365.
• Maximum day demand (MDD) — the highest single day, typically 1.5 to 3.0 times ADD, and higher in Colorado communities with heavy summer irrigation.
• Peak hour demand (PHD) — typically 1.5 to 2 times MDD, or 3 to 6 times ADD.
• Fire flow — added on top of maximum day demand for design.

Colorado adds a specific wrinkle: outdoor irrigation dominates summer peaks, so a mountain or Front Range system may deliver three times as much water in July as in January, and the distribution system, storage, and source capacity must all be sized on maximum day plus fire flow, not on average day. Watering restrictions are a demand-management tool that operators help enforce, and WPI lists "assess water production, including water restrictions and demand" as a job task.

Fire flow

Fire flow requirements come from the fire authority and the applicable fire code, based on building construction, area, occupancy, and sprinklering. Typical needed fire flows run from about 500 to 1,000 gpm for single-family residential up to several thousand gpm for commercial and industrial areas, delivered for a specified duration of 1 to 4 hours while maintaining at least 20 psi.

Fire flow is usually the controlling design criterion for main size in residential areas: domestic demand alone might be met by a 4-inch main, but fire flow requires 6- or 8-inch minimum, with 8-inch and larger on grid mains. That oversizing has a water quality cost — long detention time and low velocity in mains sized for a fire that may never occur — which is precisely why flushing programs exist.

Reading a system schematic

WPI lists "understand schematic diagrams" and "interpret plans, maps, and system standard specifications" as job tasks. A distribution schematic shows sources, treatment, storage, pump stations, pressure reducing stations, zone boundaries, and main sizes, with hydraulic grade elevations.

The single most useful concept for reading one is the hydraulic grade line (HGL). The HGL at any point is the elevation to which water would rise in an open tube at that point — that is, elevation plus pressure head. Key consequences:

• In a tank-fed zone with no flow, the HGL is the water surface elevation in the tank, and pressure at any point is simply (tank water surface elevation minus point elevation) divided by 2.31.
• When water flows, the HGL slopes downward in the direction of flow, and the slope is the friction loss per unit length.
• Water flows from higher HGL to lower HGL, regardless of ground elevation. This is why a customer at a high elevation can have good pressure while a customer lower down has poor pressure, if they are on different zones.

Worked example. A storage tank has a water surface elevation of 5,480 ft. A service connection sits at ground elevation 5,340 ft. With no flow, the static pressure at that service is:

(5,480 − 5,340) ÷ 2.31 = 140 ÷ 2.31 = 60.6 psi

If, during peak hour, friction losses between the tank and that service total 18 feet, the HGL at the service drops to 5,462 ft and the pressure becomes (5,462 − 5,340) ÷ 2.31 = 52.8 psi.

Matching production to demand

WPI lists "adjust the water production to meet the demand (start pumps, adjust flow valves)" and "perform operational analysis." In practice the distribution system is controlled by storage level, not by instantaneous demand:

1. Storage floats on the system, absorbing the difference between steady production and variable demand.
2. Pumps and treatment output are staged on tank level setpoints — start at a low level, stop at a high level, with a deadband to prevent short cycling.
3. Overnight, when demand is low and electricity is cheaper, the system fills storage. During the morning and evening peaks, storage supplements production.
4. During a fire, storage supplies the surge that no plant could produce.

The operator's daily analysis is straightforward: compare production, consumption, and tank level trends. If tanks are not recovering overnight, either production is inadequate, a large leak has developed, or a boundary valve is cross-feeding another zone. If tanks fill too fast and overflow, the level control or the setpoint is wrong. WPI's phrasing — "analyze operational data, meet performance objectives, document operating conditions" — describes exactly this loop.