above or below sea level.

Except in

than water, the pressure becomes P =

requiring

extreme

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).

Any

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.

In

fluid considered in this course is

water

distribution,

atmospheric

water (specific gravity of 1), the

pressure is used as a datum so that

equations become:

all pressure values given are the

value

above

or

below

atmospheric

pressure.

Any pressure value given

with atmospheric pressure as a base is

called

a

gage

pressure.

All

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.

Similarly a

c. Head.

A pressure exists at the

pressure of 20 psi is equivalent to 20

service

connection

(outside

water

x 2.31 = 46.2 feet of water.

This

connection at the building) in a

amount of pressure is available only

gravity

water

distribution

system

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

due to friction.

The design problem

tank above the building the greater

then becomes the choosing of a pipe

the pressure will be. Thus it can be

large

enough

so

that

after

the

seen that there is a relationship

losses

are

removed

the

between difference in elevation and

pressure

left

will

be

enough

to

pressure.

From a above, pressure is

service the building.

defined as the force per unit area.

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 =

or

62.4

pounds

per

square

foot.

c. Determine peak demand.

Converting to pounds per square inch

d. Calculate allowable headloss.

gives

psi.

Next

consider

a

right

circular

cylinder

of

cross

e. Determine

pipe

size

and

actual

sectional area "A" (square feet) and

headloss.

height "h" (feet), and a fluid of

specific

weight

(weight

per

unit

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

expressed as

pounds

per

square

considered as one step.

foot.

Converting this to psi gives P

=

psi.

For water, P =

h=

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

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