be chosen so that the total headloss

in the mains, branches, and hose

connection feeding a facility is less

The

last

step

in

the

water

than the allowable.

To calculate the

distribution system design is the

allowable headloss the elevation of

choosing of pipe sizes.

They must be

the storage tank, the elevation of the

chosen

so

that

the

allowable

service connection, and the amount of

headlosses are not exceeded by the

pressure (required head) desired at

losses

in

the

pipe

and

the service connection must be known.

fittings. If the length of the system

These elevations can be determined

exceeds 1,000 feet, the effect of

from existing drawings or a survey can

fittings becomes negligible.

If the

be run to establish them.

They will

line is not that long, the headloss in

be expressed in feet above a common

the

fittings

must

be

considered.

datum.

For a storage tank, the floor

Fittings are considered by use of

elevation is used, so that when the

figures which give a conversion from a

tank is close to empty the design

particular fitting to a length of pipe

pressures will be still available.

which has the same resistance as that

The amount of pressure or required

fitting.

The pipe is then considered

head will be specified in the job

to be longer than it actually is, and

directive for a water system, or, if

this extra length exactly makes up for

not specified, will be 20 psi for

losses in the fittings. These figures

theater of operations construction.

will not be considered further in this

This pressure of 20 psi at the service

course.

The first step in pipe

connection

will

allow

a

working

selection is the choice of pipe sizes

pressure of at least 5 psi at outlets

for mains, which must be assumed.

within the facility being served.

Normally three or four sizes will be

tried for each section of main and a

b. Calculation.

Allowable headloss

combination of sizes will be selected

can be found by use of the following

so that main headlosses will be low

enough to allow additional losses in

the runs from points on the mains to

Allowable Hf = ElTk -(ElSc + Hreq)

the service connections.

The actual

headlosses can be determined in three

ways: (1) the equation of continuity

where Hf is the headloss.

and the Hazen-Williams formula, (2)

figure 14, (3) figure 15.

ElTk is the elevation of the tank in

feet.

a. Equations.

The actual headloss

in a pipe can be calculated quite

E1Sc is the elevation of the service

accurately by the use of the equation

connection of the facility in feet.

of continuity and the Hazen-Williams

formula.

Their use is beyond the

Hreq is the required

head

at

the

scope of this course.

service connection.

b. Headloss by figure 14.

Figure

The required head is the required

14

is

a

nomograph

from

which

pressure at the service converted into

the headloss per 1,000 feet of pipe

feet of head of water.

It can be

may be read.

Figure 14 applies only

found

by

use

of

the

following

for a coefficient of friction (c) of

relationship:

100, the value normally used for

design.

The value of c runs from 60

Hreq = Preq x 2.31

for old, tuberculated pipe to 140

for new cast iron pipe.

The left

The

allowable

headloss

must

be

column is the flow rate, the second

calculated for each facility rather

column the nominal pipe diameter,

than be determined for a point on the

main or branches.