Designing Water & Hydrant Systems; Part 3
© 2000 Capt. Willis Lamm, Water Supply Officer, Moraga-Orinda (CA) Fire District
Background    Criteria    Storage    Distribution    Hyd. Design    Placement    Installation    Codes & Markings    Applications    Codes & Standards

Getting water distributed adequately and reliably to points of use is a product of sound engineering practices and an understanding of water distribution principles to support fire fighting operations.

Grid Systems

The most reliable means to provide water for fire fighting is by designing redundancy into the system. There are several advantages gained by laying out water mains in a grid, with feeder and distributor mains interconnecting at roadway intersections and other regular intervals.

  • Water will still be distributed through the system if a single section fails. the damaged section can be isolated and the remainderof the system will still carry water.

  • Water supplied to fire hydrants will feed from multiple directions. Thus during periods of peak fire flow demand, there will be less impact from "friction loss" in water mains as the velocity within any given section of main will be less since several mains will be sharing the supply.

  • Hydrants will not be supplied by "dead end" mains so that discharges will remain more stable when multiple hydrants are in use simultaneously.

Water system map book
Water system details
Each geographic point of the distribution system should be supplied by primary feeders which are typically 16 in. (400 mm) in diameter. Secondary feeders which are typically 12 in. (300 mm) in diameter would be appropriate to supply specific neighborhoods. 8 in. (200 mm) diameter distributor mains would typically be used to supply water to individual streets and to the fire hydrants on those streets. Hydrant laterals should not be less than 6 in. (150 mm).

Street valves should be provided at every junction for all mains branching from those junctions. In this manner any single section of main can be taken out of service in the event of breakage and for repair without disrupting water service beyond the affected section of pipe. By having the system completely interconnected, even the loss of a section of primary feeder would not result in a complete loss of water delivery as the other components of the system would provide a "bypass" around the outage.

Specific pipe sizes used may vary from these standards based on local conditions and projected demands upon the system. In any event, no distributor mains should be smaller than 6 in. (150 mm) and unless they are cross-connected into the grid at least every 600 ft. (180m), they should be no less than 8 in. (200mm) in diameter.

Construction Materials

Sleection of pipeline construction materials may vary based on a number of factors including geological conditions, operating pressures and locally available. Concrete lined steel is most adequate for feeder mains while PVC provides reliable service for distributor mains. Additionally, PVC is easy to work with with respect to adding service and hydrant connections after the mains have been installed.

Regulator Systems

It may be necessary to regulate pressure to small, low lying areas where installation of separate reservoirs is not feasible. These regulator systems are reliable when properly designed.

To prevent upsystem damage from "water hammer," pressure regulation on water mains should be accomplished by means of a parallel array of regulators known as a regulator set. Ranging in size from 2 in. (51mm) to 6 in (150mm), the sets should be configured so that the smaller regulators open first, then the larger regulators open in sequence as demand increases, and then the regulators close in sequence as demand decreases in the portion of the system being regulated.

Typical regulator vault
(Two fire hydrants are installed,
one on either side of the regulator set,
to provide controlled "bypass" capability
in the event the set fails to operate.)

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