B.3 - CHART 3 - Flow Straight Through Any Junction

Pressure change coefficients are presented in this chart for use in determining the elevation of the pressure line of an upstream in-line pipe relative to that of the outfall. The pipe centerlines must be parallel and not offset more than would permit the area of the smaller pipe to fall entirely within that of the larger if projected across the junction box along the pipe axis. The shape of the junction in plan is not significant in determining the pressure change. The effects of junction size and outfall pipe entrance conditions are included in the chart. Negative pressure changes occur with an upstream pipe smaller than the outfall pipe. That is, at the junction center the upstream pressure line is below the outfall pressure line for this case. No flow other than that from the upstream in-line pipe may be involved where this chart applies.

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 size ratios D _U /D _O and A/D _O - Gen. Instr. 3.

4.

Note whether the outfall pipe entrance is to be square-edged or rounded (note Gen. Instr. 6).

5.

Enter Figure B.3 Chart 3 at the pipe size ratio D _U /D _O and read K _U at the curve for the proper value of A/D _U for a square-edged entrance condition, or at the dashed curve for a rounded entrance.

6.

Calculate h _U (positive or negative) as indicated on the diagrams on the chart and by Gen. Instr. 7.

7.

Add a positive h _U to (or subtract a negative h _U from) the elevation of the outfall pressure line at the junction center to obtain the elevation of the upstream pipe pressure line at the same location.

8.

The water surface elevation in the junction corresponds to that of the upstream pipe, whether above or below the outfall pressure line.

9.

Check to be sure the water surface elevation in the junction is below the top of the junction box so that overflow may not occur.

Comments: For a square-edged entrance to the outfall pipe, values of A/D _U less than 1 do not appreciably reduce the values of K _U shown for A/D _U = 1. K _U increases for distances A/D _U greater than 3, but such values are not usual in storm drain construction. For rounded entrances, the curve shown will apply with sufficient accuracy for all values of A/D _U up to 3.

Figure B.3 Chart 3 - Flow Straight Through any Junction
(Source University of Missouri E.S.B. #41)