Building Design and Construction Handbook - Merritt - Ventech!

WATER-SUPPLY, SPRINKLER, AND WASTEWATER SYSTEMS 14.21
the ratings for any part or all of a system is a measure of the load the combination
of fixtures would impose if all were operating. The probable maximum water demand,
gal/min, can be determined from the total number of fixture units served by
any part of a system by use of graphs shown in Fig. 14.4.
The demand obtained from these curves applies to fixtures that are used intermittently.
If the system serves fixtures, such as air-conditioning units, lawn sprinklers,
or hose bibs, that are used continuously, the demand of these fixtures should
be added to the intermittent demand. For a continuous or semicontinuous flow into
a drainage system, such as from a pump, pump ejector, air-conditioning system, or
similar device, two fixture units should be used for each gallon per minute of flow.
When additional fixtures are to be installed in the future, pipe and drain sizes should
be based on the ultimate load, not on the present load.
14.9 WATER-PIPE SIZING
The required domestic-water pipe sizes should be determined by application of the
principles of hydraulics. While economy dictates use of the smallest sizes of pipe
permitted by building-code requirements, other factors often make larger sizes advisable.
These factors include:
1. Pressure at the water-supply source, usually the public main, psi
2. Pressure required at the outlets of each fixture, psi
3. Loss of pressure because of height of outlets above the source, pressure loss due
to friction caused by the flow of water through water meters and backflow preventers,
and friction from water flow in the piping
4. Limitations on velocity of water flow, ft/s, to prevent noise and erosion
5. Additional capacity for future expansion (normally 10% minimum)
14.9.1 Method for Determining Pipe Sizes
1. Sketch all the proposed risers, horizontal mains, and branch lines, indicating the
number and the type of fixtures served, together with the required flow
2. Compute the demand weights of the fixtures, in fixture units, using Table 14.4
3. From Fig. 14.4 and the total number of fixture units, determine the water demand,
gal/mm
4. Compute the equivalent length of pipe for each stack in the system, starting
from the street main
5. Obtain by test or from the water company the average minimum pressure in the
street main. Determine the minimum pressure needed for the highest fixture in
the system
6. Compute the pressure loss in the piping with the use of the equivalent length
found in item 4
7. Choose the pipe sizes from a chart like that in Fig. 14.5 or 14.6, or from the
charts given in the plumbing code being used