B.7 - CHART 7 - Rectangular Inlet With Offset 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 elevations of the pressure lines of each of the two horizontally offset opposed lateral pipes supplying a combination junction and inlet box. The inlet is used in the same situations as those to which Figure B.6 Chart 6 applies, but the pressure rise of the lower velocity lateral is restricted by locating the lateral pipes to enter opposite sides of the inlet box with their centerlines horizontally offset a distance not less than the sum of the two lateral pipe diameters. One lateral enters one side of the box near the outfall pipe end, and one, designated the far lateral, enters the opposite side near the other end.

This chart is used for all probable ratios of flow rates in the two laterals, with or without grate flow. For this type of inlet 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 far lateral.

To use the chart:

1.

Determine the horizontal distance between the centerlines of the opposed flow laterals at the inlet; if more than the sum of the pipe diameters, this chart will apply.

2.

Determine the outfall pipe pressure line elevation at the branch points - Gen. Instr. 1. An average elevation applicable to both is sufficiently precise.

3.

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

4.

Calculate the ratios Q _F /Q _O , Q _N /Q _O , D _F /D _O , and D _N /D _O , observing the nomenclature of Figure B.1 Chart 1- Gen. Instr. l.

5.

Calculate the factors  noting that the pipe size relations are the reciprocals of the usual ratios.

6.

For the far lateral, enter the left-hand graph of Figure B.7 Chart 7 at the abscissa value from step (5) and read K _F at the curve or interpolated curve for D _F /D _O .

7.

For the near lateral, obtain K _N , from the right hand graph by a similar procedure.

8.

For an inlet with grate flow, calculate h _F and h _N by multiplying the outfall velocity head by the corresponding coefficient K _F or K _N .

9.

For a junction without grate flow, calculate h _F and h _N by multiplying the outfall velocity head by the corresponding reduced coefficients (K _F - 0.2) or (K _N - 0.2).

10.

Add h _F and h _N to the elevation of the downstream (outfall pipe) pressure line to obtain the elevations of the pressure lines of the two laterals at their branch points.

11.

Determine the water surface elevation in the inlet, which is equal to the far lateral pressure line elevation.

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.7 Chart 7 - Rectangular Inlet With Offset Opposed Lateral Pipes each at 90° To Outfall (With Or Without Grate Flow) (Source University of Missouri E.S.B. #41)