17. PIPE SIZE AND ACTUAL HEADLOSS
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.
3-17
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