Building Design and Construction Handbook - Merritt - Ventech!

WATER-SUPPLY, SPRINKLER, AND WASTEWATER SYSTEMS 14.27
where Z � elevation, ft, of the point above some arbitrary datum
p/w � pressure head, ft
w � specific weight of water � 62.4 lb/ft 3
V 2 /2g � velocity head, ft
g � acceleration due to gravity, 32.2 ft/s 2
When water flows in a pipe, the difference in total head between any two points
in the pipe equals the friction loss h ƒ, ft, in the pipe between the points.
Any of several formulas may be used for estimating h ƒ. One often used for pipes
flowing full is the Hazen-Williams formula:
where Q � discharge, ft 3 /s
D � pipe diameter, ft
L � length of pipe, ft
C 1 � coefficient
4.727 Q1.85 hƒ � L (14.4)
4.87 D C1 The value of C 1 depends on the roughness of the pipe, which, in turn, depends
on pipe material and age. A new pipe has a larger C 1 than an older one of the same
size and material. Hence, when pipe sizes are being determined for a new installation,
a future value of C 1 should be assumed to ensure adequate flows in the
future. Design aids, such as charts (Figs. 14.5 and 14.6) or nomograms, may be
used to evaluate Eq. (14.4), but if such computations are made frequently, a computer
solution is preferable.
In addition to friction loss in pipes, there are also friction losses in meters,
valves, and fittings. These pressure drops can be expressed for convenience as
equivalent lengths of pipe of a specific diameter. Table 14.5 indicates typical allowances
for friction loss for several sizes and types of fittings and valves.
The pressure reduction caused by pipe friction depends, for a given length of
pipe and rate of flow, on pipe diameter. Hence, a pipe size can be selected to create
a pressure drop in the pipe to provide the required pressure at a plumbing fixture,
when the pressure at the water source is known. If the pipe diameter is too large,
the friction loss will be too small and the pressure at the fixture will be high. If
the pipe size is too small, the friction loss will be too large and the pressure at the
fixture will be too small.
14.9.3 Minimum Pipe Sizes
The minimum sizes for fixture-supply pipes are given for cold water and hot water
in Table 14.4.
Sizes of pipes for small buildings, such as single-family houses, can usually be
determined from the experience of the designer and applicable building-code requirements,
without extensive calculations. For short branches to individual fixtures,
for example, the minimum pipe diameters listed in Table 14.4 generally will be
satisfactory. Usually also, the following diameters can be used for the mains supplying
water to the fixture branches:
1 ⁄2 in for mains with up to three 3 ⁄4-in branches
3 ⁄4 in for mains with up to three 1 ⁄2-in or five 3 ⁄8-in branches