Fundamentally it is not necessary to restrict the water
velocity, however the maximum allowable velocity shall not exceed 32 fps for
interior pipe, or 16 fps for underground pipe, when the Hazen-Williams empirical formula is used
since this formula is based on empirical
data derived using velocities below these limits (note: certain
scientific formulas have upper [e.g. turbulent] and lower [e.g. laminar] limits
to which the validity of the formula is based upon, and using data outside of
the formula’s limits invalidates the results) ~ velocities exceeding these may
use Darcy, Bernoulli, or other hydraulic equations based upon scientific (as
opposed to empirically) derived data.

Hydraulic calculations using the
Hazen-Williams formula (btw - they all do...) that include velocities exceeding
32fps are in error as do not accurately represent the friction losses. There is
no need for a code section limiting the velocity (especially since using
formulas other than the Hazen-Williams formula may substantiate the use of
higher velocities...) as the rules of mathematical is sufficient.