When distribution systems are carefully planned, the pipes are usually laid out in a grid or belt system. A network of large pipes divides the community or base into areas of several blocks each (fig. 2-19). The streets within each area are served by smaller pipes connected to the larger ones. If possible, the network is planned so the whole pipe system consists of loops, and no pipes come to a dead end. In this way, water can flow to any point in the system from two or more directions. This eliminates the need to cut off the water supply for maintenance work or to repair breaks. 

Older water systems frequently were expanded without planning and developed into a treelike system. This consists of a single main that decreases in size as it leaves the source and progresses through the area originally served. Smaller pipelines branch off the main and divide again, much like the trunk and branches of a tree. A treelike system is not desirable because the size of the old main limits the expansion of the system needed to meet increasing demands. Also, there are many dead ends in the system where water remains for long periods, causing undesirable tastes and odors in nearby service lines.

MIL-HDBK-1005/7 provides specific guidance to follow when planning the location of mains. In general, mains should be located so they are clear of other structures and should be adjacent and parallel to streets but not within roadways, if possible. Mains also should be separated from other utilities to ensure the safety of potable water and to lessen interference with other utilities during maintenance.

The purpose of installing shutoff valves in water mains at various locations within the distribution system is to allow sections of the system to be taken out of service for repairs or maintenance without significantly curtailing service over large areas. Valves should be installed at intervals not greater than 5,000 feet in long supply lines and 1,500 foot in main distribution loops or feeders. All branch mains connecting to feeder mains or feeder loops should have valves installed as close to the feeders as practical. In this way, branch mains can be taken out of service without interrupting the supply to other locations. In the areas of greatest water demand or when the dependability of the distribution system is particularly important, valve spacing of 500 feet maybe appropriate.

At intersections of distribution mains, the number of valves required is normally one less than the number of radiating mains. The valve omitted from the line is usually the one that principally supplies flow to the intersection As for as practical, shutoff valves should be installed in standardized locations (that is, the northeast comer of intersections or a certain distance from the center line of streets), so they can be easily found in emergencies. All buried small- and medium-sized valves should be installed in valve boxes. For large shutoff valves (about 30 inches in diameter and larger), it may be necessary to surround the valve operator or entire valve within a vault or manhole to allow repair or replacement.

Criteria for fire hydrants are found in Fire Protection for Facilities Engineering, Design, and Construction, MIL-HDBK-1008A. Street intersections are the preferred locations for fire hydrants because fire hoses can be laid along any of the radiating streets. Hydrants should be located a minimum of 6 feet and a maximum of 7 feet from the edge of paved roadway surfaces. If they are located more than 7 feet from the edge of a road, then ground stabilizing or paving next to the hydrants may be necessary to accommodate fire-fighting equipment.