PIPE SYSTEM HYDRAULICS - Environmentally Conscious Materials Handling - page 252

(Spangler and Handy 1973) for details on how to perform calculations of earth
loading.
7.2 Maximum and Minimum Flow Velocities
As a general guideline, maximum and minimum pipe flow velocities should be
limited. When the flow velocity is too low, insufficient drag force is available to
carry air bubbles that collect along the crown of the pipe or at high points to air
release valves or the pipe exit for removal. If the air volumes are allowed to grow
during low-velocity conditions, the large air volumes become mobile and may
cause transient problems in the system when velocities increase. Also, suspended
sediments carried in the pipe flow are more likely to settle out of suspension in
the pipe when the velocities are low. Generally, flow velocities greater than
1 m/s (3 ft/s) are sufficient to move trapped air through the system and to keep
most sediment suspended.
Problems associated with high velocities include abrasion or erosion of the
pipe wall, valves, and fittings; cavitation at control valves and other minor loss
elements; increased friction and minor losses (energy loss increases with the
square of the velocity; decrease in efficiency of air removal at air release valves;
increased hydrodynamic torque on control valves; and increased risk of hydraulic
transients. Each of these should be considered before making the final pipe diam-
eter selection. A typical upper velocity for many applications is 6 m/s (20 ft/s).
With proper pipe design and analysis, however, higher velocities can be tolerated.
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PPI. 1980. PVC pipe design and installation (M23). Plastics Pipe Institute, Inc.