Environmental Engineering FOURTH EDITION

120 ENVIRONMENTAL ENGINEERING

constraints:

0 hourly fluctuations in water consumption within the service area,

0 short-term shutdown of the supply network for servicing, and

0 back-up water requirements to control fires.

These reservoirs are most often constructed as open or covered basins, elevated tanks,
or, as in the past, standpipes. Adequately designed distribution or service reservoirs
require supply conduits feeding them to carry about 50% excess water over the average
daily demand of the system or subsystem.

Elements of Closed-Conduit Hydraulics

Before discussing flow, some important properties of fluids are reviewed.

The density of a fluid is its mass per unit volume. Density is expressed in terms of

g/cm3 or kg/L or kg/m3, and in slugdft3 or lb-s2/ft4 in the English system. The density

of water at 20°C is 1 g/cm3 or 1.94 slugs/@.

Specific weight is the force exerted by gravity on a unit volume of fluid. Specific

weight is related to density as

where

w = specific weight (in kg/m3 or lb/ft3),

p = density (in kg/m3 or lb-s2/ft!), and

g = gravitational constant (in m/s2 or ft/s2).

Kilograms (or grams) are the units for both mass and weight in the metric system.

The specific weight of water at 20°C is loo0 kg/m3 in the metric system and 62.4 lb/ft3

in the English system. SpeciJic gravity of a liquid is the ratio of its density to that of pure

water at a standard temperature. The specific gravity of water is 1 .OO. The viscosity of

a fluid is a measure of its resistance to shear or angular deformation, and is defined as

the proportionality constant relating the shear stress t to the rate of deformation duldy,

and is usually written as

t = ,U duldy.

This expression assumes that shear stress is directly proportional to the rate of defor-

mation, and holds for fluids known as Newtonian fluids. For non-Newtonian fluids,

like biological sludge, shear stress is not proportional to the deformation rate. Thus

the term viscosity, when applied to non-Newtonian fluids such as biological sludge, is

significant only if either the deformation rate or the shear stress is also specified. The

unit of viscosity in the metric system is the poise, .or gdcm-s. The viscosity of water

at 20°C (68.4"F) is 0.01 poise, or 1 centipoise. In the English system unit of viscosity

is lb-s/ft2. Therefore, 1 lb-dft2 equals 479 poise.