TM 5-814-3/AFM 88-11, Volume III
(3) Venturi flume design formula.
in which
Q = flow through grit chamber of flume discharge;
e
= permissible divergence from design velocity (fps);
r
= ratio of minimum rate of flow to maximum rate of flow; and other symbols are as previously
stated or as indicated in figure C--1c.
b. Example. Assuming that the diameter of the inlet sewer is 2 feet, that the average rate of sewage flow
is 1.67 cfs, that the maximum rate of flow and as high as practicable over the entire range of flows. Assume
a depth in the effluent channel of 1.5 feet at maximum rate of flow. Then the required width (w) would be
5 (1.5 2) = 1 ft. 8 in. The designs for various types of control sections and grit chamber cross-sections
are as follows:
(1) Proportional weir. As the depth (h) in the chamber above the weir crest cannot exceed 2 feet
without submerging the crown of the inlet sewer, let h = 1.75 for first trial. Determine W by substitution in
equation C-1. Then,
To prevent appreciable divergence of the velocity from 1 fps when the flows are low, the depth (d) of the
rectangular section of the weir opening should be minimum practicable. As indicated by equations C-2 and
C-3, this depth is a function of b and x. The breadth (b) is limited by the width of the effluent channel (in this
case, 1 ft. 8 in.) and x is limited by the size of solids to be passed (in this case, about 3 inches, assuming that
a bar screen will be ahead of the grit chamber). Try d 0.15 and solve for b in equation 2. Then,
therefore, b = 1.53 (satisfactory). For various values of y, values of y/d are determined and the corresponding
values of x/b are taken from the table; the values of x are then determined as follows:
C-11
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