B.6 - CHART 6 - Rectangular Inlet With In-Line Opposed Lateral Pipes Each at 90° to Outfall (With or Without Grate Flow)

Pressure change coefficients are presented in this chart for use in determining the elevation of the pressure line of the lateral carrying the lower-velocity flow of two in-line opposed lateral pipes supplying a combination junction and inlet box. The pressure change coefficient for the higher-velocity lateral is a constant and so is not read from the chart. An inlet of this type may be used at a low point of street grade where lateral pipes supply flow from up-grade inlets in both directions, and the outfall pipe is located at right angles to the two lateral lines.

The chart may be used for cases with all probable ratios of flow rates in the two laterals, with or without grate flow. For this type of inlet and junction, the pressure changes are not modified by the depth of water in the inlet. The water surface elevation here will correspond to the pressure line of the higher-velocity lateral.

To use the chart:

1.

Determine the outfall pipe pressure line elevation - Gen. Instr. 1.

2.

Calculate the velocity head in the outfall - Gen. Instr. 2.

3.

Calculate the velocities in each of the laterals to determine which is the higher-velocity and which the lower-velocity lateral.

4.

Calculate the ratios Q _G /Q _O , Q _hv /Q _O , Q _lv /Q _O , D _hv /D _O , D _lv /D _O and D _hv /D _lv - Gen. Instr. 3.

5.

Determine H from the left-hand graph on Figure B.6 Chart 6. Enter the graph at the pipe size ratio D _hv /Do (note the two scales) and read H at the curve or interpolated curve for Q _hv /Q _O . In entering the graph, note that unequal size laterals (D _hv /D _lv , not equal to 1.0), effect an offset of the scale for D _hv /D _O . Interpolation between the two scales shown is used for intermediate values. Extrapolation beyond the scales is satisfactory.

6.

Determine L from the right-hand graph on Figure B.6 Chart 6. Enter the graph at the pipe size ratio D _lv /D _O (note only one scale is involved) and read L at the curve or interpolated curve for Q _lv /Q _O .

7.

Calculate K _lv = H - L with grate flow involved. With no grate flow, K _lv = (H - L) - 0.2.

8.

K _hv = 1.8 with grate flow involved. With no grate flow, K _hv = 1.6.

9.

Calculate 

10.

Add h _lv to the elevation of the outfall pipe pressure line at the branch point to obtain the elevation of the lower-velocity lateral pressure line at this point; similarly, add h _hv to the outfall pipe pressure line elevation to obtain the elevation of the higher-velocity lateral pressure line at the branch point.

11.

Determine the water surface elevation in the inlet, which is equal to the lower of the two lateral Pressure line elevations (that of the higher-velocity lateral).

12.

Check to be sure the inlet water surface elevation is below the top of the inlet so that inflow may be admitted.

Figure B.6 Chart 6 - Rectangular Inlet With In-line Opposed Lateral Pipes Each at 90° To Outfall (With or Without Grate Flow) (Source University of Missouri E.S.B. #41)