above or below sea level.
than water, the pressure becomes P =
0.433ah where "a" is the specific
accuracy it is assumed to be 14.7
gravity of the fluid (ratio of its
pounds per square inch (psi).
weight to the weight of an equal
time a pressure value is given, some
volume of water).
Since the only
datum value is stated or implied.
fluid considered in this course is
water (specific gravity of 1), the
pressure is used as a datum so that
all pressure values given are the
Any pressure value given
with atmospheric pressure as a base is
references to pressure in this course
Thus a column of water 20 feet high
are to gage pressure.
exerts a pressure at the base of (20 x
0.433) = 8.66 psi.
A pressure exists at the
pressure of 20 psi is equivalent to 20
x 2.31 = 46.2 feet of water.
connection at the building) in a
amount of pressure is available only
if the water is not flowing, since if
because the storage tank is elevated
the water flows some head will be lost
above the building.
The higher the
The design problem
tank above the building the greater
then becomes the choosing of a pipe
the pressure will be. Thus it can be
seen that there is a relationship
between difference in elevation and
From a above, pressure is
service the building.
defined as the force per unit area.
12. DISTRIBUTION SYSTEM DESIGN
For water under static conditions (no
flow) this force is the weight of the
water and the resultant pressure is
There are generally five steps in the
due to this weight acting on an area.
design of a distribution system:
Since 1 cubic foot of water weighs
62.4 pounds (density = 62.4 pounds per
a. Locate the lines.
cubic foot) the pressure exerted by a
b. Check for excess pressure.
1-foot cube of water is P =
c. Determine peak demand.
Converting to pounds per square inch
d. Calculate allowable headloss.
sectional area "A" (square feet) and
height "h" (feet), and a fluid of
The last step might be considered to
volume) "w" (pounds per cubic foot).
be two separate steps but they are
The pressure on the base can be
done at the same time and so will be
considered as one step.
Converting this to psi gives P
13. LOCATION OF LINES
For water, P =
Topographic maps of the physical location
0.433h, which of course agrees with
of the supply lines and the buildings
the pressure computation for the 1-
foot cube above.
For fluids other