B.1 - General Instructions for use of Design Charts

Several operations are common to use of the design charts for various types of junctions. Instructions for performing these recurring procedures are consolidated in the following General Instructions. In the detailed instructions for use of the individual charts, references to these General Instructions are made by number (Gen. Instr. 1, etc.). The General Instructions follow.

1.

Determine and tabulate the elevation of the outfall pipe pressure line at the branch point or inlet center (refer to Figure B.1 Chart 1).

This elevation is obtained by adding to the elevation of the pressure line at the preceding structure downstream the pipe friction loss

(Equation B.1a)

h _f = LS _f

h _f = friction loss (ft.)
L = length from center to center of structures (ft.)
S _f = friction (or resistance) loss per ft. at the given rate of flow for the given pipe flowing full.

2.

Calculate the mean velocity head of the flow in the outfall pipe.

(Equation B.1b)

h _v = velocity head (ft.)
g = acceleration of gravity = 32.2 ft./sec .
Q = rate of flow in pipe flowing full (cfs)
Area = Area of pipe = 0.7854 D (ft )
D = pipe diameter (ft.)

3.

Calculate the required flow rate and size ratios.

Examples: Q _U /Q _O , Q _L /Q _O , Q _G /Q _O , etc.
D _U /D _O , D _L /D _O , B/D _O , etc.

4.

Estimate the depth of water in a rectangular inlet with flow into the inlet from a top grate, either alone or combining with flow from an upstream pipe.

d = total depth of water (ft.)

  = (outfall pressure line elevation minus inlet bottom elevation) 

K = the pressure change coefficient for the inlet water depth (This is estimated as detailed for each type of inlet. Such estimates are not necessary for inlets with in-line or off-set opposed laterals.)

5.

Use the coefficients K from the charts for inlets and junctions with square-edged entrance to the outfall pipe (entrance flush with box side, with sharp edges).

6.

Use reduced coefficients K, where applicable, for a rounded entrance to the outfall pipe (rounded on ¼ circle arc of approximate radius / ₈ D _O ) or for an entrance formed by the socket end of a standard tongue-and-groove concrete pipe.

Figure B.2 Chart 2-insignificant effect; make no reduction.

Figure B.3 Chart 3-read directly from chart.

Figure B.4 Chart 4-reduce K _U by 0.1 for usual proportions of grate flow; by 0.2 for Q _G about 0.5 Q _o .

Figure B.5 Chart 5-reduce K _U and K _L in same manner as Chart 4.
Figure B.6 Chart 6-insignificant effect; make no reduction.
Figure B.7 Chart 7-insignificant effect; make no reduction.
Figure B.8 Chart 8 , Figure B.9 Chart 9, and Figure B.10 Chart 10-see specific instructions for each case.

7.

Calculate pressure change.

To calculate the change of pressure at a junction, working upstream from the outfall pipe to an upstream pipe, the design chart applying to the type of junction involved is selected. The pressure change coefficient for a specific upstream pipe is read from the chart for the particular flow rate and size ratios already calculated. The pressure change is calculated from

(Equation B.1c)

The coefficient is a dimensionless number, and therefore, the change of pressure will be in feet.

8.

Apply the pressure change.

The pressure change, in feet, for each upstream pipe is added to the outfall pipe pressure line elevation at the branch point to obtain the elevation of each pressure line for further calculations upstream along that pipe. In some cases the upstream pressure line at the branch point will be at a lower elevation than the downstream pressure line. Where this less common situation may occur with a particular type of junction, it is mentioned in the instructions for use of the specific chart.

9.

Determine the elevation of the water surface.

The elevation of the water surface in a junction or an inlet (with or without grate flow) receiving flow from a pipe or pipes will correspond to that of the upstream in-line pipe pressure line. At a junction with offset opposed laterals, the water surface will correspond to the elevation of the far lateral pipe pressure line. At a junction with in-line opposed laterals, the water surface will correspond to the elevation of the pressure line of the higher-velocity lateral pipe.

Each of the inlet and junction types for which design charts were derived from the analytical and experimental investigation are now listed separately the chart number appropriate for each is stated, and detailed instructions are given for determination of the change of pressure through use of the chart.

Figure B.1 Chart 1 - Manhole Junction Types & Nomenclature (Source University of Missouri E.S.B. #41)