B.5 - CHART 5 - Rectangular Inlet With In-line Upstream Main and 90° Lateral Pipe (With or Without Grate Flow)

Pressure change coefficients are presented in this chart for use in determining the common elevation of the two upstream pipe pressure lines and the water surface in the inlet. Flow into the combination inlet and junction box is supplied by an upstream main, in-line with the outfall and flowing through the short dimension of the inlet, and a 90° lateral pipe connected at one end of the inlet box, supplemented by flow through a top grate. The main graph of Figure B.5 Chart 5 applies directly for no flow into the inlet through the grate. Increments of K _U and K _L for grate flow conditions are shown in the supplementary graphs of the upper portion of the chart.

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 ratios D _U /D _O , Q _U /Q _O , and Q _G /Q _O - Gen. Instr. 3.

4.

If no grate flow is involved, enter the lower graph on Figure B.5 Chart 5 at the pipe size ratio D _U /D _O and read K _U (or K _L ) at the curve or interpolated curve for Q _U /Q _O ; then proceed as in step (10).

5.

With grate flow, estimate a value for the water depth d.

a.

Follow Gen. Instr. 4.

b.

Estimate K = 1.5.

6.

Calculate the corresponding relative water depth d/D _O .

7.

Enter the lower graph and obtain K _U (or K _L ) as in step (4), this value applying for Q _G /Q _O = O.

8.

Enter the appropriate upper graph on Figure B.5 Chart 5, for the particular d/D _O nearest that estimated in step (6), at the given D _U ,/D _O and determine the increment of K _U (or K _L ) at the curve for Q _G /Q _O . This increment accounts for the effects of grate flow and is always a positive value, even when K _U of step (7) is negative.

9.

Add K _U from step (7) and the increment from step (8) to obtain the total value of K _U . Note that in unusual cases the total value of K _U may be negative.

10.

For a rounded outfall pipe entrance or one consisting of a pipe socket, reduce K _U , and K _L according to Gen. Instr. 6.

11.

Calculate h _U (also equal to h _L ) as indicated by the diagram on the chart and by Gen. Instr. 7.

12.

Add h _U to the elevation of the outfall pressure line at the branch point to obtain the elevation of the upstream in-line pipe pressure line at this point. The elevations of the lateral pipe pressure line and the water surface in the inlet will correspond.

13.

From this water surface elevation subtract the elevation of the inlet bottom to obtain a more precise value for the water depth d.

14.

Repeat the above procedure with the improved value of d from step (13), if necessary. Such repetition may not be necessary if the original estimated d/D _O of step (6) was reasonably accurate.

15.

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

Figure B.5 Chart 5 - Rectangular Inlet With In-line Upstream Main And 90° Lateral Pipe
(With or Without Grate Flow) (Source University of Missouri E.S.B. #41)