Page 309 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.31

13. When the first junction with another contributing source has been reached, add
together the entire area of all contributing sources up to this first junction. Calculate
the new time of concentration from the most remote inlet and the time in pipe to the
design point of the storm sewer. Entering the intensity-duration-frequency chart
to find the rainfall rate will now give all necessary information required for inser-
tion into the rational formula [Eq. (6.5)] to find total CFS. The resultant flow into
the pipe along with the slope of the pipe is used to size that particular segment
downstream from the design point. This is continued in succession to the end of
the system, requiring a longer time of concentration each time a new design point
is reached.
14. As progress is made along the route of the sewer, all branches to the main sewer line
should be sized individually as their connection is reached. The branch is sized using
the time to concentration required only for that branch. To size the main, add all the
areas contributing to the flow at any particular junction and use the longer time of con-
centration determined from the initial overland flow time from the critical inlet added
to the flow time of the water in the pipe.

SYSTEM DESIGN CONSIDERATIONS

1. The velocity in the piping system should not be less than 2 fps (0.6 m/s), to allow for
the movement of sediment and any other solids in the pipe.
2. The maximum velocity should be limited to approximately 30 fps to prevent erosion of
the pipe interior. If large quantities of sand are present in the water, the scouring action
will increase and so the velocity should be reduced for this condition.
3. To limit velocity, the slope of the piping system should be decreased rather than using
oversized pipe. There are other methods available, also. If the slope of the ground is
steep, a drop manhole should be used where the difference in elevation between inlet
and outlet inverts is more than 2 ft (0.6 m).
4. A manhole should be located at every change in pipe size, slope, or direction. In many
cases, a drainage structure can be used for this purpose if such placement is practical.
5. The minimum size pipe should be 12 in (DN 300) to reduce the possibility of stoppage
by debris. A smaller size for runout from a building roof storm water system is generally
acceptable.
6. When a change in pipe size occurs, the pipe should be installed crown to crown to elimi-
nate a surcharge of the upstream portion of the pipe.
7. Factors such as snowmelt may add additional unexpected quantities of water.
Snowmelt does not usually add a significant amount of inflow, but it may if the
depth of snow is large. However, increasing one pipe size may lead to a problem by
reducing velocity too much. In that case, it is advisable to increase the slope of the
sewer line. As a general rule, one should be a little generous with pipe sizes. The
slight increase in cost will be more than made up by the safety factor of additional
capacity provided.
8. Exfiltration occurs when water leaks out of a pipe through bad joints or cracks into
the surrounding ground. When a pipe passes under a road or railroad, a joint should be
chosen that will permit very little or no leakage to avoid washing away the subgrade.
9. Piping should be installed roughly parallel to the slope of the ground. Standard percent-
ages of slopes, if possible, should be used to permit contractors to easily install the
pipe.

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