Distribution System Appurtenances
Piping and valve arrangement: Water systems typically have three classifications of pipe used to transport to demand points throughout a community. These are identified as follows:
1) Primary feeders: These are large pipes, usually with diameters ranging from 12 to 36 inches, based on the size of the population served. Primary feeders transport water form the water treatment plant to corporation line of the community and/or to major water storage locations within the community.
2) Secondary feeders: These are connected to the primary feeders to transport water along the major streets of the community. Secondary feeders need to be in place to supply all commercial property, public buildings, and private sector buildings that have a needed fire flow over 1,000 gpm. Secondary feeders typically are 10 to 16 inches in diameter.
3) Distributor mains: These are used to transport water from the secondary feeders to individual streets in the areas of the community that have small businesses like convenience stores and gas stations but, more importantly, along residential streets. The minimum pipe size should be 6 inches and, based on the system design, a possible dead- end pipe may need to be 8 or even 10 inches.
The sizes of pipe associated with the three classifications of pipe in a typical water system are approximations. The needed pipe size throughout the built-upon areas of a community is based on the hydraulic gradient of the community, consumer consumption profiles through the community, needed fire flow at representative locations throughout the community, and, quite importantly, the two methods for laying and connecting pipe throughout the community. The traditional pipe system design is referred to as a Single-Point Feed System. This is illustrated in Figure 4-6. In this case, water moves from the treatment plant to the community corporation line with a single primary feeder. The primary feed, in turn, supplies the secondary feeders along the main streets of the community, and the distributor mains supply the block frontage along the residential streets. Note that any demand point on the system for either consumer consumption or fire flow through a fire hydrant, is fed from one direction only. In the single- point feed system, the pipe sizes need to meet the maximum daily consumption demand plus the needed fire flow. This results in a larger pipe than is needed under normal daily usage, without any fires. The second major weakness of this type of system is that there may be a lot of dead-end mains in the residential areas and at the end of secondary mains. This leads to the stagnation of water which rapidly reduces the quality of water.
as discussed in Chapter 3.
The more modern approach to water system design is to loop all the water mains, or cross-tie the mains, so that at any demand point the water is supplied from two directions. A basic Pipe Looped water system is depicted in Figure 4-7. This allows the designing engineer to develop a hydraulic model of the system and to determine mathematically the proper size of pipe according to flow paths to meet the consumer and needed fire flow demand points.
It is important to note that many older water systems have been updated. By laying a primary feeder around the perimeter of the community to tie in all of the dead-end mains to improve both flow distribution and water pressures through the community. A simple example of this concept is presented in Figure 4-8.
The following provisions conform to the current recommended practices of the AWWA.
In small cities, the primary feeders should form a loop about 3,000 feet in length or two-thirds of the distance from the center of the community to the outskirts. The primary feeders should have control valves not over 1 mile apart, and the mains connecting them also should be valved where they connect. This configuration is necessary so that interruptions in service will not require shutting down the feeder main.
In large cities, the primary feeders should be arranged into several interlocking loops, with the mains not over 3,000 feet apart. Looping allows continuous service through the rest of the primary mains even when one portion is shut down temporarily for repairs. Under normal conditions, looping also allows supply from two directions for large fire flows. Large feeders and long feeders should be equipped with blow-off valves at low points and air-relief valves at high points.
The secondary feeders carry large quantities of water from the primary feeders to points in the system in order to provide for normal supply and fire fighting. They form smaller loops within the loops of the primary mains by running from one primary feeder to another. Secondary feeders should be spaced only a few blocks apart. The spacing allows concentration of large amounts of water for fire fighting without excessive heat loss and resulting low pressure.
Small distribution mains for a grid over the area to be served supply water to fire hydrants and service pipes for residences and other smaller buildings. The size of these mains usually will be determined by the design flow. In residential areas, particularly where there are heavy uses for lawn watering, it may be necessary to determine the maximum consumer water demand. This network of pipes throughout all the outlying sections of a community may consist of single mains or dual mains as depicted in Figures 4-9 and 4-10. Table 4-1 presents a comparison of the repair procedures needed for single and dual main systems.
The described network of pipes throughout all but the outlying sections of a community may consist of single mains or dual mains as presented in Figure 4-9 and Figure 4-10. The AWWA generally installs valves on a community water system on mains in the following recommended manner:
four at crosses;
three at tees; and
one on each hydrant branch lines.
This is done in order to facilitate sectionalizing the water system for repairs and cleaning of the water units. In dual main systems, service headers are added on the south and west side of the streets, and piping is generally placed between the street curb and the sidewalks. In a scheme suggested by Ballou, valves are installed in the following manner: (2)
one on each main at intervals of two blocks;
one at the intersection of the service header; and
one on each end of hydrant branches that are more than 10 feet long.
Service operations and conditions in breakdowns and cleaning can then be compared, as was shown in Table 4-1.
The hydraulic advantages of dual main systems over single main systems depend more or less on local conditions, because service headers generally do not contribute to flow outside their own area. More importantly, if broken, piping in dual main systems will not impair the usefulness of hydrants or create dead-end mains.
The dual main systems frequently will compare favorably in cost with single main systems. The advantages are that repairs can be made and new services laid without interfering with traffic or damaging the pavement, repairs can be made at lower costs, leakage usually is reduced, since service lines (which are liable to have heavy leakage) are reduced in length, and service pipes need not be laid where the main passes vacant lots, thereby reducing idle investment and minimizing resident time of water in the mains. Minimum use service mains are the common source of serious water system leaks.
Distribution piping should be sized to meet design flow as determined hydraulic analysis on water system flow gradients. The minimum size of water mains, for providing fire protection and serving fire hydrants, is 6 inches in diameter. Larger size mains will be necessary to achieve required fire flow and maintain residual pressure specified for both domestic consumption of fire flow. The piping must meet the minimum standards specified in Table 4-2.