Water Hydraulics - Wastewater Operators - page 80

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figure 2.18 Pipe in parallel.
When making calculations involving pipes in series, remember
these two important basic operational tenets:
1. The same flow passes through all pipes connected in series.
2. The total head loss is the sum of the head losses of all of the com-
ponent pipes.
In some operations involving series networks where the flow is given
and the total head loss is unknown, we can use the Hazen-Williams
equation to solve for the slope and the head loss of each pipe as if they
were separate pipes. Adding up the head losses to get the total head loss
is then a simple matter.
Other series network calculations may not be as simple to solve
using the Hazen-Williams equation. For example, one problem we may
be faced with is what diameter to use with varying sized pipes con-
nected together in a series combination. Moreover, head loss is applied
to both pipes (or other multiples), and it is not known how much loss
originates from each one; thus, determining slope would be difficult but
not impossible.
In such cases the equivalent pipe theory , as mentioned earlier, can
be used. Again, one single “equivalent pipe” is created that will carry
the correct flow. This is practical because the head loss through it is
the same as that in the actual system. The equivalent pipe can have any
C factor and diameter, just as long as those same dimensions are main-
tained all the way through to the end. Keep in mind that the equivalent
pipe must have the correct length so it will allow the correct flow and
yield the correct head loss (the given head loss). *
2.11.1.3 Pipes in Parallel
Two or more pipes connected (as in Figure 2.18) so flow is first
divided among the pipes and is then rejoined comprise a parallel pipe
system. A parallel pipe system is a common method for increasing the
* For more information on how to use the equivalent pipe theory in making computations
involving series or parallel pipe combinations, refer to Lindeburg, M.R., Civil engineering
reference manual , 4th ed., Professional Publications, San Carlos, CA, 1986.