Page 359 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.81

Selecting the Pipe Material. The pipe material selected is based on many factors. Since
no single material will meet all conditions, the material selected should be based on the
most important characteristics. Some factors to consider are:
1. Flow characteristics (friction coefficient)
2. Life expectancy
3. Resistance to scour
4. Resistance to effluent and surrounding soil
5. Ease of handling and installation
6. Physical strength to resist loading
7. Type and flexibility of joints
8. Requirements of the local authorities
9. Cost
Laterals and service connection runs for sanitary discharge are often made of extra-
heavy cast iron with compression gasket joints. Another popular material is plastic, usually
PVC with butt-fused joints.

Sizing the Sewer Pipe
The pipe size is selected after determining the pipe material, laying out the run of the sewer
to determine the slope, and calculating the peak flow rate.
Sanitary effluent has the same characteristics as storm water. Sewers are sized using the
Kutter or Manning formula. Both will yield the same results. The easiest method, rather
than using the actual equation, is to find readily available prepared charts based on the
Manning formula. Refer to Fig. 6.17, which graphically solves for the Manning equation.
Table 6.26 provides a direct reading table of the size based on pitch, gpm flow and drain-
age depth of flow, and Table 6.27 which is a simplified sizing table based only on velocity
necessary for scouring action.

SANITARY SEWER DESIGN CONSIDERATIONS

Minimum and Maximum Effluent Velocity

The minimum velocity of the effluent should be sufficient to prevent solids from being
deposited on the bottom of the pipe. This is called a scouring velocity. Tests have estab-
lished this minimum velocity at 2 fps or 0.62 meter per second (mps) when the pipe is
flowing full. Accepted practice has a recommended minimum of 2.5 fps (0.77 mps). Since
pipes do not run full most of the time, it may be necessary to flush the sewer in order to
remove accumulated sediment from time to time.
The maximum velocity for clear water in hard-surfaced pipe is quite high. Tests have
shown that storm water velocities in excess of 40 fps (12 mps) have been found harmless to
concrete channels. In practice, sanitary sewers that have continuously high velocities and
where grit is expected to be a problem should limit the highest velocity to 10 fps (3 mps).
For ordinary sewers without grit and only occasional periods flowing full, a maximum
velocity of 20 fps (7 mps) would be considered acceptable.
For large facilities, if the house sewer is sized for the peak flow rate, the velocity during
peak flow rate should be adequate to flush the pipe clean of any deposits left during periods
of lower flow, for example, during the night.

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